Treatment Performance Evaluation Of Electro Fenton Reactors

Treatment Performance Evaluation of Conventional Fenton Process and Electro-Fenton Process Using Two Different Electro-Fenton Reactors for Aniline Degradation

Eric D. Pagaling BS Chemical Engineering

A Thesis presented to the Graduate Division College of Engineering University of the Philippines

In Partial Fulfillment of the Requirements For the Degree of Master of Science in Environmental Engineering

College of Engineering University of the Philippines Diliman, Quezon City

2006

ACKNOWLEDGEMENT

This thesis could not have been accomplished without Dr. Gene Peralta and Dr. Ming-Chun Lu, who not only served as advisers but also challenged me throughout the duration of my research and never accepting less than my best efforts. I also thank them for giving me the opportunity not only on acquiring incalculable quantities of technical knowledge but also on learning and appreciating other Asian cultures as well.

I also thank Dr. Ernie De la Cruz and Dr. Marilou Dalida for their persistence and valuable contributions in refining this manuscript.

I am also thankful for the financial support given by the National Science Council of Taiwan, Republic of China through Grant No. NSC94-2211-E-04-002 and the U.P. Chemical Engineering Alumni Foundation Inc. (UP ChEAFI) through the efforts of Ms. Louernie F. De Sales-Papa.

I am also grateful for the support given to me throughout my research and academic program by Laarni Panopio, Lorenzo Lim, Nolan Tolosa, Ate Babes Ali, Jobert Papa, Prof. Jun Ballesteros and Gemma Seminiano of the Environmental Engineering Graduate Program of UP Diliman. I also thank Kuan-Shien Ho, Yi-Chun Tsai, Wan-Ping Ting, Wan-Shu Hsu, Shin-Yi Tu, Chun-Yi Li and Su-Yi Wang of Chia Nan University of Pharmacy and Science; Massakul Kitmongkonsak and Somboon Chintitanun of Chulalongkorn University; Nonglak Boonrattanakij, Aroon Khongnou, Witchuda Auiaree, Praphat Siriruang, Wichidtra Sudjarid, Rosenee Sama-ae of King Mongkut’s University of Thonburi Thailand; Nguyen Duy Hung, Pham Minh Tu and Tran Phuong Ha of Hanoi University of Technology.

Lastly I thank my family for the ceaseless support and encouragement in this research and in all of my endeavors. This thesis is sincerely dedicated to all of you.

ABSTRACT

Aniline degradation was investigated using the conventional Fenton (CF) process and electro-Fenton (EF) process employing the plate type electrode and rod type electrode EF reactors with IrO2/RuO2 coated titanium metal anodes and stainless steel cathodes. The treatment performance evaluation of the two EF reactors was conducted by determining the effects H2O2, Fe2+, electric current and initial aniline concentration on the aniline and COD removal efficiencies, initial aniline degradation rates, kinetic models, H2O2 efficiency and power consumption for aniline and COD removal. Aside from this the change in the BOD, toxicity and oxalic acid production of a high initial aniline concentration having 10,000ppm COD was also studied using the two EF reactors.

The EF process was superior to the CF process due to its higher treatment efficiency at low Fe2+ concentrations, low inorganic sludge production and low residual H2O2 after treatment. Both of the EF reactors have the same aniline and COD removal efficiencies at high H2O2 concentrations however at H2O2 concentrations lower than 58mM, the plate type electrode EF reactor was superior to the rod type electrode EF reactor. An increase in H2O2 and Fe2+ concentrations lowered power consumptions for aniline and COD removal for both of the EF reactors. However increase in H2O2 greater than 58mM and Fe2+ greater than 1.61 mM had no further effect on power consumption due to decreased aniline and COD removal efficiencies. The rod type electrode EF reactor consumes twice as much power for aniline and COD removal than the plate type electrode EF reactor and that COD removal requires twice as much power than aniline removal. Increase in applied electric current increases the difference in the power consumption between the rod type electrode and plate type electrode EF reactors from 1.7 to 3.3 times. Treatment of high concentration aniline solution resulted into an increased BOD and lowered toxicity values of 0.18 and 0.25 for the plate type electrode and rod type electrode EF reactors respectively. The plate type electrode EF reactor was found to be more versatile than the rod type electrode EF reactor since it can be operated at different Fe2+ concentrations and at higher electric current applications.

TABLE OF CONTENTS

I. Introduction…………………………………………………………………...

1

1.1 Objective of the study…………………………………………………

2

1.2 Scope and limitations………………………………………………….

3

II. Review of Related Literature……………………………………………….

4

2.1 Advanced oxidation processes………………………………………...

4

2.2 Conventional Fenton process………………………………………….

6

2.3 Photo-Fenton, Fenton-like, electro-Fenton and photo electro-Fenton processes................................................................................................

9

2.4 Uses and properties of aniline…………………………………………

11

2.5 Aniline degradation using different Fenton processes………………...

12

III. Methodology………………………………………………………………...

14

3.1 Electro-Fenton reactors………………………………………………..

14

3.2 Materials and reagents.………………………………………………...

14

3.3 Experimental procedures………………………………………………

16

3.4 Sampling and laboratory analysis……………………………………..

17

3.5 Statistical analyses methods…………………………………………...

18

3.6 Kinetic modeling method……………………………………………...

18

IV. Results and Discussion……………………………………………………

19

4.1 Aniline degradation using conventional Fenton and electro-Fenton Processes……………………………………………………………….

19

2+

4.2 Fe regeneration efficiency of the plate type electrode and rod type electrode EF reactors……………………………………………...

23

4.3 Effect of H2O2, Fe2+, electric current and initial aniline concentration on the aniline degradation efficiency using the plate type electrode and rod type electrode EF reactors…………………….

i

24

4.4 Effect of H2O2, Fe2+, electric current and initial aniline concentration on the initial aniline degradation rate using the plate type electrode and rod type electrode EF reactors…………………….

32

2+

4.5 Effect of H2O2, Fe , electric current, initial aniline degradation rate on the second order aniline degradation rate constant using the plate type electrode and rod type electrode EF reactors…………………….

36

4.6 Effect of H2O2, Fe2+, electric current, initial aniline concentration on the COD removal efficiency using the plate type electrode and rod type electrode EF reactors……………………………………………..

40

2+

4.7 Effect of H2O2, Fe , electric current, initial aniline concentration on the H2O2 efficiency for aniline and COD removal using the plate type electrode and rod type electrode EF reactors…………………….

44

4.8 Effect of H2O2, Fe2+, electric current and initial aniline concentration on the power consumption for aniline and COD removal using the plate type electrode and rod type electrode EF reactors………………………………………………………………...

49

4.9 Degradation of high initial aniline concentration………………………

54

V. Conclusions…………………………………………………………………..

57

VI. Recommendations for Future Work………………………………………

60

References……………………………………………………………………….

61

Appendices……………………………………………………………………....

66

Appendix A. Aniline degradation rate constant calculations……………...

67

Appendix B. Raw data…………………………………………………….

75

Appendix C. Sample statistical analyses calculations…………………….

83

Appendix D. Photographs of the electro-Fenton reactors…………………

86

ii

LIST OF TABLES

Table

Title

Page

2-1

List of reactive compounds with their respective oxidation power

7

4-1

Effect of H2O2 on the aniline degradation efficiency using the plate

27

type electrode and rod type electrode EF reactors

4-2

Effect of Fe2+ on the aniline degradation efficiency using the plate

29

type electrode and rod type electrode EF reactors

4-3

Effect of electric current on the aniline degradation efficiency

30

using the plate type electrode and rod type electrode EF reactors

4-4

Effect of initial aniline concentration on the aniline degradation

31

efficiency using the plate type electrode and rod type electrode EF reactors

4-5

Effect of H2O2 on initial aniline degradation rate using the plate

32

type electrode and rod type electrode EF reactors

4-6

Effect of Fe2+ on the initial aniline degradation rate using the plate

33

type electrode and rod type electrode EF reactors

4-7

Effect of electric current on the initial aniline degradation rate using the plate type electrode and rod type electrode EF reactors

iii

34

4-8

Effect of initial aniline concentration on initial aniline degradation

35

rate and aniline removal efficiency using plate type electrode and rod type electrode EF reactors

4-9

Effect of H2O2 on the COD removal efficiency using the plate type

40

electrode and rod type electrode EF reactors

4-10

Effect of Fe2+ on the COD removal efficiency using plate type

41

electrode and rod type electrode EF reactors

4-11

Effect of electric current on the COD removal efficiency using

42

plate type electrode and rod type electrode EF reactors

4-12

Effect of initial aniline concentration on the COD removal

43

efficiency using the plate type electrode and rod type electrode EF reactors

4-13

Effect of H2O2 on the H2O2 efficiencies for aniline and COD

45

removal using the plate type electrode and rod type electrode EF reactors

4-14

Effect of Fe2+ on the H2O2 efficiency for aniline and COD removal

46

using the plate type electrode and rod type electrode EF reactors

4-15

Effect of electric current on the H2O2 efficiency for aniline and

47

COD removal using the plate type electrode and rod type electrode EF reactors

4-16

Effect of initial aniline concentration on the H2O2 efficiency for aniline and COD removal using the plate type electrode and rod type electrode EF reactors

iv

48

LIST OF FIGURES

Figure

Title

Page

2-1

Aniline degradation pathway using the Fenton process

13

3-1

Schematic diagrams of the plate type electrode and rod type

15

electrode electro-Fenton reactors

3-2

Experimental procedure summary flowchart

17

4-1

Effect of conventional Fenton and electro-Fenton processes on

19

aniline degradation and COD removal using [Fe2+] = 0.27mM and [H2O2] = 58mM 4-2

Effect of conventional Fenton and electro-Fenton processes for

20

aniline degradation and COD removal using [Fe2+] = 1.07mM and [H2O2] = 58mM 4-3

Residual H2O2 from aniline degradation using conventional

22

Fenton and electro-Fenton processes

4-4

Fe2+ regeneration efficiency of the plate type electrode and rod

23

type electrode EF reactors

4-5

Effect of H2O2 on aniline degradation using plate type electrode

24

and rod type electrode EF reactors

4-6

Effect of Fe2+ on aniline degradation using the plate type electrode and rod type electrode EF reactors

v

25

4-7

Effect of electric current on aniline degradation using the plate

25

type electrode and rod type electrode EF reactors

4-8

Effect of initial aniline concentration on aniline degradation using

26

the plate type electrode and rod type electrode EF reactors

4-9

Effect of H2O2 on the second order aniline degradation rate

36

constant using the plate type electrode and rod type electrode EF reactors

4-10

Effect of Fe2+ on the second order aniline degradation rate

37

constant using the plate type electrode and rod type electrode EF reactors

4-11

Effect of electric current on the second order aniline degradation

38

rate constant using the plate type electrode and rod type electrode EF reactors

4-12

Effect of initial aniline concentration on the second order aniline

39

degradation rate constant using the plate type electrode and rod type electrode EF reactors

4-13

Effect of H2O2 on the power consumption for aniline and COD

50

removal using the plate type electrode and rod type electrode EF reactors

4-14

Effect of Fe2+ on the power consumption for aniline and COD removal using the plate type electrode and rod type electrode EF reactors

vi

51

4-15

Effect of electric current on the power consumption for aniline and

52

COD removal using the plate type electrode and rod type electrode EF reactors

4-16

Effect of initial aniline concentration on the power consumption

53

for aniline and COD removal using the plate type electrode and rod type electrode EF reactors

4-17

Aniline and COD remaining ratios from degradation of

54

10,000ppm COD of aniline using the plate type electrode and rod type electrode EF reactors

4-18

BOD and toxicity tests on treated high concentration aniline

55

solution using the plate type electrode and rod type electrode EF reactors

4-19

Oxalic acid content of the treated high concentration aniline solution using the plate type electrode and rod type electrode EF reactors

vii

55

I. INTRODUCTION

Treatment of wastewater contaminated with recalcitrant and toxic compounds using chemical oxidation technologies has been an interest in recent studies because of the increasing generation of such wastes and the inability of existing technologies for treatment and remediation.

Application of Advanced Oxidation Processes (AOPs) for treatment has been studied extensively and has been found to be highly efficient. AOPs generate radical intermediate compounds that are highly oxidative species. These radical intermediate compounds oxidize toxic pollutants into harmless species and are generated by systems that utilize technologies like the Fenton process, photocatalysis and cavitation.

Among the current AOP technologies, the Fenton process is the most cost effective and has already been applied in full scale wastewater treatment operations. The Fenton process produces highly oxidative hydroxyl radical species from the reaction of hydrogen peroxide (H2O2) and Ferrous ions (Fe2+). Since its inception, the Fenton process has been modified to increase its efficacy and solve operational problems.

Fenton

technologies include the conventional Fenton (CF) process, electro-Fenton (EF) process, photo-Fenton process, Fenton-like process and the photo electro-Fenton process.

One of the potential Fenton technologies is the EF process. The EF process utilizes H2O2 and Fe2+ to produce hydroxyl radicals inside an electrochemical system. The EF process was designed to solve excessive inorganic sludge production of the CF process by electro regeneration of Fe2+ from Ferric ions (Fe3+).

The electro-Fenton process employing a rod type electrode was first used for full scale applications due to its advantage over the conventional Fenton process. However the rod type electrode EF reactor also had several disadvantages because of limitations in its design. These problems include high power consumption and difficult maintenance

1

procedures during electrode fouling problems. Because of this, the plate type electrode EF reactor was proposed as an alternative. However the difference in the treatment performance between the two EF reactors was not known.

In this regard the treatment performance of the CF process and EF process employing the plate type electrode and rod type electrode EF reactors, having the same materials of construction, was studied in order to determine their respective optimum operating conditions using aniline as the target organic pollutant.

Aniline was chosen as the target organic pollutant since aniline degradation was previously studied using different Fenton processes including EF reactors with different designs and experimental conditions. In addition to this, aniline is also a recalcitrant organic compound and has been found to be toxic to human health and aquatic life.

1.1 Objective of the Study

The main objective of the study was to evaluate the treatment performance of the conventional Fenton process and the electro-Fenton process employing the plate type electrode and rod type electrode electro-Fenton reactors for aniline degradation. The specific objectives of the study were to determine and compare the: 1. Treatment performance between the CF and EF process 2. Fe2+ regeneration efficiency of the plate type electrode and rod type electrode EF reactors 3. Effects of H2O2, Fe2+, electric current and initial aniline concentration using the two EF reactors on the following parameters: 3.1 Aniline removal efficiency 3.2 Initial aniline degradation rate 3.3 Aniline degradation rate constant 3.4 COD removal efficiency

2

3.5 H2O2 efficiency for aniline and COD removal 3.6 Power consumption for aniline and COD removal 4. Change in the BOD and toxicity of a 10,000ppm COD aniline solution using the two EF reactors.

1.2 Scope and Limitations

The study focused on the use of the conventional Fenton process and electroFenton process using the plate type electrode and rod type electrode EF reactors for the degradation of aniline.

Treatment performance evaluation was limited to the results of laboratory scale experiments using synthetic wastewater containing aniline. Actual application of the conventional Fenton and electro-Fenton processes on real wastewaters was not conducted in the study.

3

II. REVIEW OF RELATED LITERATURE

2.1 Advanced Oxidation Processes

Advanced oxidation processes (AOPs) generate radical intermediate compounds that are highly oxidative species. These radical intermediate compounds degrade toxic and recalcitrant organic pollutants to harmless species and are generated through specially designed equipments or reaction of certain chemicals. Examples of AOP technologies include cavitation, photocatalytic oxidation and the Fenton’s process.

In cavitation, radical intermediate compounds are formed from the very fast production and sudden collapse of microbubbles or cavities. Cavitation is being done through the use of ultrasonic irradiation equipments or constrictions in hydraulic devices (Gogate 2004). Radical intermediate compounds are formed from acoustic cavitation through the use of high frequency sound waves or ultrasounds. Acoustic cavitation has been applied on the degradation of different compounds like propionic acid (Yoo 1997) and 3-chloroaniline (David 1998). Hydrodynamic cavitation produces radical intermediate compounds by passing the liquid into a constriction.

Hydrodynamic

cavitation has been applied for the treatment of compounds like ethyl benzene, xylenes, cyanide and phenol (Gogate 2004).

Radical intermediate compounds are formed in photocatalytic oxidation by using ultraviolet radiation or near UV light on wastewaters containing photochemically degradable compounds like H2O2 or ozone. Photochemical oxidation has also been conducted with the use of a semiconductor catalyst like titanium dioxide to enhance the generation rates of radical intermediate compounds (Gogate 2004). Photochemical oxidation has been applied for treatment of compounds like chlorophenols (Trapido 1997) and perchloroethylene (Gupta 1995).

4

The Fenton process produces radical intermediate compounds by the reaction of H2O2 and Fe2+. The Fenton process has been applied in wastewater treatment processes and is known to be very effective in the removal of many hazardous organic pollutants (Neyens 2003). Radical intermediate compounds produced from the Fenton process is composed mostly of hydroxyl radicals. Hydroxyl radicals exhibit faster rates of oxidation reactions as compared to those using conventional oxidants like hydrogen peroxide or permanganate (Gogate 2002).

Application of the Fenton process on oxidation of different refractory pollutants in soil media and aqueous solutions has been done extensively since its conception. For aqueous solutions, refractory pollutants that were investigated includes phenols and alkyl hydrocarbons (Gao 2004), polyphenols and aromatic compounds (Peres 2004), pesticides (Rivas 2004; Chan 2003), ethylenediaminetetraacetic acid (EDTA) (Ghiselli 2004), surfactants (Gaca 2005), explosives (Liou 2003), phenol (Kavitha 2004), dyes (Meric 2003; Dutta 2003; Muruganandham 2004), MTBE (Xu 2004), microcystins (Bandala 2004), nitroaromatic compounds (Trapido 2003), p-nitrophenol (Khan 2005) and dichlorvos (Lu 1999).

The Fenton process was also used in the treatment of actual wastewater with mixtures of different refractory pollutants like pharmaceutical wastewater (Martinez 2003), sanitary landfill leachate (Rivas 2003) and (Lopez 2004), Oilfield wastewater (Gao 2004), Table olive processing wastewater (Kotsou 2004) and cork cooking wastewater (Guedes 2003).

The Fenton process has also been applied for contaminated soil remediation. Examples of refractory pollutants investigated in studies on soil media include n-alcohols (Quan 2003), trichloroethylene (TCE) and tetrachloroethylene (PCE) (Go 2003; Baciocchi 2004) and petroleum (Millioli 2003).

5

Applications of the Fenton process other than toxic and recalcitrant compound degradation in aqueous solutions and soil matrices includes treatment of a bacteria containing solution (Barbusinski 2005), removal of harmful disinfection by-products by treatment of natural organic matter prior to chlorination (Murray 2004), sludge conditioning (Buyukkamaci 2004), regeneration of activated carbon saturated with organochloro compounds (Toledo 2003), polycyclic aromatic hydrocarbon degradation in municipal sewage sludge (Flotro 2003) and activated sludge dewatering (Lu 2003)

The Fenton process has been modified in recent years in order to increase its efficacy and solve operational problems. Fenton technologies now include the electroFenton process, photo Fenton process, photo electro-Fenton process and Fenton-like process. In order to differentiate the original Fenton process from the new and emerging Fenton technologies, it is referred in this study as the conventional Fenton process.

2.2 Conventional Fenton Process

The conventional Fenton process was first discovered and used by H. J. H. Fenton in 1894 when he observed that the rate of oxidation of tartaric acid increased dramatically when dilute hydrogen peroxide was added with the solution containing dissolved Fe2+ ions. Fenton’s chemistry is a reaction between hydrogen peroxide and Fe2+ forming hydroxyl radicals, which is the main oxidizing agent. However the hydroxyl radical mechanism of the Fenton’s reaction was only identified 40 years after its discovery (Watts 2005).

Hydroxyl radicals (OH·) are short-lived reactive oxygen species with a high oxidation potential that can rapidly destroy many biorefractory contaminants (Watts 2005). It is one of the most reactive chemical species, second only to elemental fluorine in its relative oxidation power as listed in Table 2.1 (Barbusinski 2005).

6

Table 2-1: List of reactive compounds with their respective oxidation power. Reactive Species

Relative Oxidation Power

Flourine

2.23

Hydroxyl radical

2.06

Atomic oxygen

1.78

Hydrogen peroxide

1.31

Perhydroxyl radical

1.25

Permanganate

1.24

Chlorine dioxide

1.15

Hypochlorous acid

1.10

Chlorine

1.00

Bromine

0.80

Iodine

0.54

The hydroxyl radical is considered to be an electrophile due to a deficit of one electron in its valence orbit. The most common reactions of hydroxyl radicals are electrophilic substitution in aromatic compounds and addition to alkenes. Other chemical reactions involving hydroxyl radicals include hydrogen abstraction forming an organic free radical and water (Watts 2005).

The conventional Fenton process generates hydroxyl radicals through the reaction of hydrogen peroxide with ferrous ions according to reaction (2.1) and with organic radical compounds through reaction (2.2) (Watts 2005; Quan 2003). Fe2+ + H2O2 Æ Fe3+ + OH- + OH·

(2.1)

R· + H2O2 Æ ROH + OH·

(2.2)

The organic radical compounds (R·) from reaction (2.2) are formed from the reaction of organic compounds with excess hydroxyl radicals as shown in reaction (2.3) (Meric 2003; Gao 2004).

7

RH + OH· Æ R· + H2O

(2.3)

The conventional Fenton process also yields other reactive oxygen species through propagation reactions. These reactive oxygen species includes perhydroxyl radical (HO2·) which is a relatively weak oxidant, superoxide radical anion (O2·-) which is a weak reductant and is a nucleophile, and hydroperoxide anion (HO2-) which is a strong nucleophile (Watts 2005). These non-hydroxyl radicals are formed according to the reactions (2.4), to (2.8) (Watts 2005; Quan 2003; Trapido 2003; Guedes 2003; Gao 2004; Meric 2003). OH· + H2O2 Æ HO2· + H2O

(2.4)

HO2· ÅÆ O2·- + H+

(2.5)

HO2· + Fe2+ Æ HO2- + Fe3+

(2.6)

HO2· + O2·- Æ HO2- + O2

(2.7)

2+

Fe-OOH

Æ HO2· + Fe

2+

(2.8)

However scavenging reactions of hydroxyl radicals by H2O2 and Fe2+ and non hydroxyl radicals by Fe3+ are also present in the conventional Fenton process like reaction (2.4), (2.9) and (2.10) (Trapido 2003; Martinez 2003; Guedes 2003; Dutta 2003). Fe2+ + OH· Æ Fe3+ + OHFe3+ + HO2· Æ Fe2+ + H+ + O2

(2.9) (2.10)

Fe3+ produced from the Fenton process can react with excess H2O2, perhydroxyl radicals, and organic radical compounds in the so-called Fenton-like reaction, which leads to regeneration of Fe2+ according to reactions (2.11) to (2.14) (Trapido 2003; Guedes 2003; Gao 2004; Meric 2003; Martinez 2003; Dutta 2003). Fe3+ + H2O2 ÅÆ H+ + Fe-OOH2+

(2.11)

Fe-OOH2+ Æ HO2· + Fe2+

(2.12)

R· + Fe3+ Æ R+ + Fe2+

(2.13)

8

Fe3+ + HO2· Æ Fe2+ + H+ + O2

(2.14)

Reactions (2.11) to (2.14) have been simplified by other studies into reaction (2.15) (Martinez 2003; Dutta 2003; Peres 2004). Fe3+ + H2O2 Æ Fe2+ + HO2· + H+

(2.15)

Inhibitions to the Fenton process have also been investigated in recent studies. Anions like H2PO4, Cl-, NO3 and ClO4- was found to inhibit the Fenton reaction therefore reducing its efficiency. Among the anions, H2PO4 was found to inhibit the reaction the most since the phosphate ions will produce a complex reaction with ferrous and ferric ions (Lu 1997). The inhibition of low concentration chloride ions was found to be controlled by extending the reaction time. However inhibition is significant if the ratio of chloride to ferrous ions is greater than 200. Likewise, inhibition by chloride ions was controlled by increasing the initial pH near to 5 and increasing the amount of ferrous ions (Lu 2005; Sajiki 2004). Presence of chloride ions in the Fenton process also produces chloroorganic compounds as byproducts (Gaca 2005). The effect of temperature on the rate of reaction of the Fenton process was also studied and was found to increase as the solution temperature was increased. However the effect of temperature was only obvious at temperatures lower than 20 0C. In addition to this application of temperatures greater than 40 0C, the treatment efficiency declines due to decomposition of H2O2 into oxygen and water. Application of the Fenton process has been normally conducted at temperatures of 20 to 40 0C (Watts 2005). The optimal pH range for the application of the Fenton process was also determined to be at pH 3 and pH 6. Application of the Fenton process at high pH values will result into inhibition of the Fenton reaction since the Fe2+ ions will form colloidal Fe3+ ions. Likewise application of the Fenton process at very low pH values would result into the decomposition of H2O2 into oxygen and water by iron without forming hydroxyl radicals (Neyens 2003).

9

2.3 Photo-Fenton, Fenton-like, electro-Fenton and Photo electro-Fenton processes

The conventional Fenton process has been modified such that the treatment efficiency was increased with reduced inorganic slugged production, and prevention of inhibition reactions of some ions. Among the emerging Fenton technology are the photo Fenton, Fenton-like, electro-Fenton and Photo electro-Fenton processes.

The photo Fenton process utilizes UV to enhance the treatment efficiency by regenerating ferrous ions produced from reaction (2.1). The efficiency was enhanced when additional hydroxyl radicals are produced when the regenerated Fe2+ reacts with excess H2O2. Studies have been conducted using photo Fenton on aqueous solutions containing bisphenol A (BPA) (Katsumata 2004), explosives like 2,4,6- trinitrophenol (PA),

ammonium

picronitrate

(AP),

2,4-dinitrotoluene

(DNT),

methyl-2,4,6-

trinitrophenylnitramine (Tetryl) and 2,4,6-Trinitrotoluene (TNT), hexahydro-1,3,5trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7- tetranitro-1,3,5,7-tetrazocine (HMX) (Liou 2003), microcystin-LR toxin (Bandala 2004), natural organic matter (Murray 2004), chlorotriazine reactive azo dye specifically Reactive Orange 4 (RO4) (Muruganandham 2004), phenol (Will 2004), dimethyl phthalate (DMP) (Zhao 2004) and ethylenediaminetetraacetic acid (EDTA) (Ghiselli 2004). Fenton-like process uses other transition metal catalyst other than Fe2+. This modification reduces or completely eliminates the production of inorganic sludge. Some studies investigated the use of Fe-containing zeolites (Kuznetsova, 2004), soluble manganese (II) and amorphous and crystalline manganese (IV) oxides (Watts 2005), soluble iron (III) and pyrolusite Sb-MnO2, (Watts 2005), heterogeneous catalyst (swarf) (Barbusinski 2005) , Fe3+, Cu2+ and mixtures of Fe2+/Cu2+ and Fe3+/Cu2+ (Ghiselli 2004), suspended iron powder (Liao 2003), laponite clay-based Fe nanocomposite (Fe-Lap-RD) (Feng 2003), zero-valent iron (Bergendahl 2004) and strongly acidic ion exchange resin (SAIER) (Feng 2004).

10

The electro-Fenton process involves the use of the Fenton reagents in an electrochemical system. This modification reduces sludge production by electro regenerating Fe2+ ions from Fe3+ at the cathodes and Fenton reagent utilization by directly producing H2O2 or Fe2+ or both through the electrochemical system. The electro-Fenton technology has been modified in some studies to reduce the use of Fenton reagents. Different electro-Fenton configurations included the production of H2O2 or both hydrogen peroxide and ferrous ions using an electrochemical system (Lu 2004). Examples of cathodes that are used for the production of hydrogen peroxide include O2 diffusion PTFE (Brillas 2002), graphite, reticulated vitreous carbon, mercury pool and carbon felt (Brillas 2003). Anodes used for these systems are Ti/Pt mesh (Brillas 2002), Pt, boron doped diamond (Brillas 2004) and sacrificial iron (Lu 2004). Another electro-Fenton technology only involves the electro regeneration of Fe2+ from Fe3+ according to reaction (2.16). With this technology, addition of Fe2+ and H2O2 into the solution is required. Different anodes used in this technology include Ti/Pt mesh anode or DSA plate (Brillas 2002), Ti/Pt special net anodes (Anotai 2006) and IrO2/RuO2 coated Ti rod anode (Lu 2003). Fe3+ Æ Fe2+ + e-

(2.16)

Pollutants treated by this technology includes aniline (Brillas 2002; Lu 2003; Anotai 2006), chlorophenoxy herbicides (Brillas 2004), 3,6-dichloro-2-methoxybenzoic acid herbicide (Brillas 2003). The electro-Fenton process has also been used for the treatment of non-biodegradable wastewater (Chang 2004).

The electro-Fenton has been further modified through the added use of UV to further enhance its efficiency by destroying Fe3+ complexes with degradation by products such as oxalic acids. This new technology was called the photo electro-Fenton and was applied for the degradation of aqueous solutions of aniline, nitrobenzene, and 4chlorophenol (Casado 2005).

11

2.4 Uses and Chemical Properties of Aniline

Aniline or aminobenzene is an oily, flammable liquid, with a clear to slightly yellow color and a distinctive odor. Aniline has one amino group attached to a benzene ring as the structure. This compound is slightly soluble in water and does not evaporate at room temperature (ATSDR-TOXFAQS). This compound is mainly used as a raw material for the production of other chemicals like isocyanates, polyurethane foams, pesticides, dyes, rubber, drugs, photographic chemicals, varnishes and explosives. In addition to this, the United States has considered aniline to be a High Production Volume (HPV) compound because of an annual production of more than 1 billion pounds with an increase in demand of 3% to 4% per year in the United States alone (EPA-HPV Chemical List).

However aniline is a recognized carcinogen, gastrointestinal toxicant, kidney

toxicant, neurotoxicant, respiratory toxicant, sense organ toxicant and is found to be highly toxic to aquatic life (EPA-OPPT Chemical Fact Sheet 1994).

2.5 Aniline degradation using different Fenton processes

Aniline degradation studies using the Fenton process includes the conventional Fenton and the electro-Fenton processes. Aniline degradation was investigated using the conventional Fenton and electro-Fenton processes through a 4-step and 1-step addition of H2O2 for a total reaction time of two hours. The study was conducted using an initial aniline concentration of 10mM, 7,700ppm of H2O2 and 327ppm of Fe2+ for the conventional Fenton process. The electro-Fenton process was conducted using the same experimental condition but with the addition of 2A of electric current. The electro-Fenton reactor used in the study had a titanium rod coated with RuO2/IrO2 as the anode and a stainless steel as the cathode. It was determined in the study that the electro-Fenton process was superior to the conventional Fenton process due to a COD removal of 90% compared to an increased COD value for the conventional Fenton process after treatment. It was also determined in the study that the 4-step addition of H2O2 remove slightly the COD by 10% (Lu 2003).

12

Another study was conducted on aniline degradation using the conventional Fenton process and the electro-Fenton process. The electro-Fenton reactor used in the study had an anode and cathode of Ti/Pt special net and stainless steel nets respectively. The H2O2 was added into the reactor in six equal parts every twenty minutes to ensure the presence of H2O2. It was also determined from the study that the electro-Fenton process was superior to the conventional Fenton process due to a difference of 1.2 to 3.1 times for aniline removal efficiencies (Anotai 2006). An aniline degradation pathway was proposed from another study and is shown in Figure 2-1 (Brillas 1998).

Nitrobenzene Aniline Æ Phenol Æ Hydroquinone Æ Benzenetriol Æ Maleic acid Æ Oxalic acid ÆCO2 Benzoquinonimine Æ Benzoquinone

Figure 2-1: Aniline degradation pathway using the Fenton process.

Aniline degradation was also investigated using an electro-Fenton reactor with a Ti/Pt mesh anode or DSA plate and a H2O2 producing cathode. The experimental condition used in the study includes a 1,000ppm initial aniline concentration at an initial pH of 3. It was determined in the study that a 61% TOC degradation was obtained at a total reaction time of two hours using 20 A of electric current (Brillas 2002).

Another study on aniline degradation was also conducted using a plate type electrode electro-Fenton reactor with an anode made of a Ti coated with RuO2/IrO2 and a stainless steel cathode. It was determined from the study that the optimum initial pH was at 3.2 using an initial aniline concentration of 10mM, 58mM of H2O2 and 1mM of Fe2+. It was also determined from the study that H2PO4 inhibits the Fenton process more than chlorides and that recycling of electro regenerated Fe2+ rich solution had no detrimental effect on the treatment performance of the electro-Fenton reactor (Tsai 2006).

13

III. METHODOLOGY

3.1 Electro-Fenton Reactors Aniline degradation experiments were carried out in two electro-Fenton (EF) reactors with different electrode configurations, which were the plate type electrode and rod type electrode. Both of the anodes and cathodes of the EF reactors were titanium metals coated with IrO2/RuO2 and stainless steels, respectively. The plate type electrode EF reactor had a plexiglas rectangular vessel with a 5L volume capacity (15 (W) x 21 (L) x 20 (H) cm3). On the other hand the rod type electrode EF reactor had a stainless steel cylindrical vessel that also serves as the cathode with a 1L volume capacity. Both of the EF reactors were operated at a constant electric current of 0.8A/L and were provided with mixers for appropriate agitation and connected to a Topward 33010D DC power supply. The schematic diagrams of the plate type electrode and rod type electrode EF reactors are shown in Figures 3-1 (a) and (b), respectively.

3.2 Materials and Reagents The synthetic wastewater was prepared by using reagent grade aniline (≥ 99.5%) from MERCK that was diluted with deionized water, from a Millipore system with a resistivity of 18.2 MΩcm-1, to the desired aniline concentration. The prepared solutions were adjusted to pH 3.2 using 25% H2SO4 that was prepared using reagent grade H2SO4 (acidimetric assay of 95-97%) from MERCK. pH was measured using a SUNTEX TS-1 portable pH/MV meter. Reagent grade 35% H2O2 (manganometric assay of 35-36.5%) was supplied from MERCK and the Fe2+ was obtained from reagent grade FeSO4·7H2O (manganometric assay of 99.5-102%) that was also obtained from MERCK. For the Fe3+ reduction experiments, reagent grade Fe2(SO4)3·nH2O from KANTO CHEMICAL CO., INC. was

14

diluted with deionized water to the desired concentration of 500ppm Fe3+ at an initial pH of 2.

(a)

(b)

Figures 3-1: Schematic diagrams of the (a) plate type electrode and (b) rod type electrode EF reactors

15

3.3 Experimental Procedures The treatment performance evaluation using the conventional Fenton and electroFenton processes were conducted using 10mM of aniline and 58mM of H2O2. Fe2+ concentration applied was at 0.27mM and 1.07mM. For both of the experimental conditions, the electro-Fenton process was conducted using a constant electric current of 0.8A/L. The effect of H2O2 was determined by varying the H2O2 concentration added into the EF reactors from 14.5mM, 29mM, 58mM, 87mM and 130mM. All experimental conditions used a constant Fe2+ concentration of 1.07mM, 10mM of aniline and an applied electric current of 0.8A/L. The effect of Fe2+ on aniline degradation was determined by varying the concentration of the Fe2+ from 0.27mM, 0.54mM, 1.07mM, 1.61mM and 2.41mM using a constant H2O2 concentration of 58mM, 10mM of aniline and 0.8A/L of applied electric current. However the rod type electrode EF reactor was only operated at Fe2+ concentration greater then 1.07mM since the required 0.80A/L of electric current could not be achieved due to decreased solution conductivity. The effect of electric current on aniline degradation was determined by varying the applied electric current from 0.2A/L, 0.4A/L, 0.8A/L, 1.2A/L and 1.8A/L using 10mM of aniline, 58mM of H2O2 and 1.07mM of Fe2+. However the rod type electrode EF reactor could not be operated at electric currents greater than 0.8A/L due to the limited voltage that the power supply can generate. The effect of initial aniline concentration on aniline degradation was determined by varying the aniline concentration from 2.5mM, 5mM, 10mM, 15mM and 22.5mM using 58mM of H2O2, 1.07mM of Fe2+ and a constant applied electric current of 0.8A/L. Degradation experiments of a high initial aniline concentration with a 10,000ppm COD was conducted using 640mM of H2O2, 11.85mM of Fe2+ and 0.8A/L of electric

16

current. Fe2+ regeneration experiments were conducted using 500ppm of Fe3+ at an initial pH of 2. Figure 3-2 shows the summary procedure for the conduct of the experiments. Prepare aniline solution in the reactor vessel

Assemble electrode and power supply connections

Adjust pH to 3.2 using sulfuric acid solution

Add determined amount of Fe2+ and dissolve

Get samples for H2O2, Fe2+, aniline and organic acid analysis at the determined time intervals

Add determined amount of H2O2 while simultaneously turning on the power supply

Figure 3-2: Experimental procedure summary flowchart

3.4 Sampling and Laboratory Analyses Samples were taken at determined time intervals of every 0, 2, 5, 10, 30, and 60 minutes for laboratory analysis. Samples for aniline analyses were first added into a 0.1N NaOH solution to stop the Fenton reaction and for dilution. The samples were first filtered using Whatman glass filter with pore size of 0.45 µm before injection into the Gas Chromatograph equipment.

17

Laboratory analysis conducted for the samples followed the standard methods. H2O2 analysis was conducted using iodometric titration method, closed reflux titrimetric method for COD, permanganate titration method and phenanthroline method for Fe2+ analyses and azide modification method-dilution technique for BOD. Aniline was analyzed using an HP 4890 Gas Chromatograph (GC) that was equipped with a flame ionization detector and HP-5 column (0.53mm inside diameter and 15 m long). The GC equipment used had injector and detector temperatures of 250 oC. Organic acids were detected using DIONEX Ion Chromatograph equipped with an Ion Pac AS 11 analytical column, 4 mm. ASRS-ULTRA suppressor using KOH as eluent at 1.0mL/min.

3.5 Statistical Analyses Methods Statistical analyses used for calculating significant differences were the PairedSamples T-test and the Single Sample T-test using SPSS version 10.0 of SPSS Inc. 19891999. Significant differences were concluded when the significance level value obtained was less than 0.05 using 95% level of confidence. The Paired Samples T-test was used when the significant difference between the two EF reactors at the same experimental conditions was determined. Likewise the Single Sample T-test was used when the significant difference between two parameters or indexes was determined at the same experimental conditions.

3.6 Kinetic Modeling Method Kinetic modeling was conducted by calculating the rate constants using the aniline degradation data with respect to time. The data was fitted using the first order and second order rate equations. The rate constant and coefficient of linearity was obtained by plotting the data using Microcal(TM) Origin® Version 6.0 software of Microcal Software Inc. The best fit was chosen when the coefficient of linearity was nearly equal to the value of 1.

18

IV. RESULTS AND DISCUSSIONS

4.1 Aniline degradation using conventional Fenton and electro-Fenton processes Aniline degradation using the conventional Fenton (CF) and electro-Fenton (EF) processes was conducted to determine the difference in the treatment performance between the two AOPs. Figure 4.1 (a) and (b) shows the aniline and COD remaining ratios respectively using the CF and EF processes at 58mM of H2O2 and 0.27 mM of Fe2+. At this experimental condition, the CF process had an aniline and COD removal efficiency of 23.4% and 21.1% respectively. Aniline degradation from the CF process stopped after five minutes of the reaction time as shown in Figure 4.1 (a). This phenomenon was due to the competition of aniline degradation by products with hydroxyl radicals as shown by the decreasing trend of COD after five minutes of the

1.0

1.0

0.8

0.8 COD remaining (C/Co)

Aniline remaining (C/Co)

reaction time.

0.6 0.4 0.2

0.6 0.4 0.2 Electro-Fenton Conventional Fenton

Electro-Fenton Conventional Fenton

0.0

0.0 0

10

20

30

40

time (min)

(a)

50

60

0

10

20

30

40

50

60

time (min)

(b)

Figure 4-1: Effect of conventional Fenton and electro-Fenton processes on (a) aniline degradation and (b) COD removal using [Fe2+] = 0.27mM; [H2O2] = 58mM. Experimental condition: [aniline] = 10mM; IEF = 0.8A/L; initial pH = 3.2

19

Aniline degradation using the EF process was conducted by applying a constant electric current of 0.8A/L at the same experimental condition. The EF process had an aniline degradation efficiency and COD removal efficiency of 65.8% and 30.7% respectively. Aniline degradation slowed down at time 30 minutes due to the competition of aniline degradation by products with hydroxyl radicals as shown by the reduction of COD in Figure 4.1 (b).

The electro-Fenton process had a significantly higher aniline

degradation and COD removal efficiencies than the conventional Fenton process due to the production of more hydroxyl radicals when excess H2O2 reacted with the electro regenerated Fe2+. Aniline degradation using a different experimental condition was also investigated and is shown in Figure 4-2. Both of the CF and EF processes were operated using 58mM of H2O2 and 1.07mM of Fe2+. The CF process had an aniline degradation and COD removal efficiencies of 94.4% and 40.1% respectively. For the EF process, increase in the Fe2+ resulted to aniline degradation and COD removal efficiencies of 99.5% and 45.9% respectively.

1.0

1.0 Electro-Fenton Conventional Fenton

0.8 COD remaining (C/Co)

Aniline remaining (C/Co)

0.8 0.6 0.4 0.2 0.0

0.6 0.4 0.2

Electro-Fenton Conventional Fenton

0.0 0

10

20

30

40

time (min)

(a)

50

60

0

10

20

30

40

50

60

time (min)

(b)

Figure 4-2: Effect of conventional Fenton and electro-Fenton processes for (a) aniline degradation and (b) COD removal using [Fe2+] = 1.07mM; [H2O2] = 58mM. Experimental condition: [aniline] = 10mM; IEF = 0.8A/L; initial pH = 3.2

20

As shown in Figures 4-1 and 4-2, the treatment efficiencies of the CF and EF processes were significantly different if the Fe2+ was low. At this operating condition the EF process was obviously the best choice for aniline degradation because of its higher aniline degradation and COD removal efficiencies. In addition to this, a study was previously conducted on aniline degradation using 10mM of aniline, 230mM of H2O2 and 1.17mM of Fe2+. It was determined from the study that the EF process was more effective than the CF process due to a 90% COD removal efficiency using step wise addition of H2O2 (Lu 2003). However if high Fe2+ concentration was applied, the treatment efficiency of the CF process approaches that of the EF process. This phenomenon was also observed by another study where the CF and EF processes were used for aniline degradation using an initial aniline concentration of 10mM and 300mM of H2O2. Based on the study, the aniline removal efficiency for the EF process decreased from 2.6 to 1.2 times of that of the CF process when the Fe2+ was increased from 4mM to 18mM (Anotai 2006). Because of this, choosing the best technology for aniline degradation at high Fe2+ concentrations is now dependent on other operating factors like high inorganic sludge production and presence of high residual H2O2 in the solution after treatment. One advantage of the EF process is its ability to electro regenerate Fe2+ from Fe3+ at the cathode of the electrolytic system according to reaction (2.17). This ability reduces sludge production since the high Fe2+ content solution from the EF process can be recycled to treat another batch of aniline containing wastewater. Based on a previous study, it was also determined that the treatment efficiency of the EF process using recycled Fe2+ solution for five times had no detrimental effect on its treatment efficiency (Tsai 2006). On the contrary, sludge production from the CF process is very high since the Fe3+ rich solution would have to be disposed of after every treatment. The CF process had a higher residual H2O2 at the experimental conditions used as shown in Figure 4-3 (a) and (b). When 0.27mM of Fe2+ was used, the residual H2O2 for the CF process was 1,440 mg/L while the EF process only had 450mg/L. Likewise at

21

1.07mM of Fe2+, the residual H2O2 for the CF process was 461mg/L while the EF process had 18.6mg/L. Presence of high unreacted H2O2 in the treated solution from the CF process was not economical due to wastage of a highly expensive chemical. Aside from this, further treatment of the treated wastewater using biological processes will not be

2000

2000

1750

1750

1500

1500 H2O2 residual (mg/L)

H2O2 residual (mg/L)

feasible due to toxicity of high H2O2 concentration with microorganisms (Lu 1999).

1250 1000 750 500 Electro-Fenton Conventional Fenton

250

Electro-Fenton Conventional Fenton

1250 1000 750 500 250

0

0 0

10

20

30

40

50

time (min)

(a)

60

0

10

20

30

40

50

60

time (min)

(b)

Figure 4-3: Residual H2O2 from aniline degradation using conventional Fenton and electro-Fenton processes (a) [aniline] = 10mM; [Fe2+] = 0.27mM; H2O2 = 58mM; IEF = 0.8A/L; initial pH = 3.2 and (b) [aniline] = 10mM; [Fe2+] = 1.07mM; H2O2 = 58mM; IEF = 0.8A/L; initial pH = 3.2

22

4.2 Fe2+ regeneration efficiency of the plate type electrode and rod type electrode EF reactors The Fe2+ regeneration efficiency of the two EF reactors was determined in the study by conducting Fe3+ reduction experiments using 500ppm of Fe3+, an initial pH of 2 and a constant electric current of 0.8 A/L. Fe2+ regeneration efficiencies of the EF reactors are shown in Figure 4-4. The rod type electrode EF reactor had a higher Fe2+ regeneration efficiency of 86% while the plate type electrode EF reactor had 51%. This difference in the Fe2+ regeneration efficiencies between the two EF reactors was due to the difference in the distance between the electrodes of the EF reactors. The plate type electrode EF reactor had a shorter distance between the electrodes compared with the rod type electrode EF reactor. Distance between the electrodes of an EF reactor affects Fe2+ regeneration due to different simultaneous reactions at the electrodes. Fe2+ would be regenerated at the cathodes of the electrolytic system according to reaction (2.17). However if the electrodes are placed too short, the electro regenerated Fe2+ would immediately be oxidized at the anode according to reaction (4.1) (Zhang 2006). Fe2+ Æ Fe3+ + e-

(4.1)

2+

Fe regeneration efficiency (%)

100 plate type rod type

80 60 40 20 0 0

10

20

30

40

50

60

time (min)

Figure 4-4: Fe2+ regeneration efficiency of the plate type electrode and rod type electrode EF reactors. Experimental condition: [Fe3+] = 500ppm; I = 0.8A/L; pH = 2

23

4.3 Effect of H2O2, Fe2+, electric current and initial aniline concentration on the aniline degradation efficiency using the plate type electrode and rod type electrode EF reactors The effect of H2O2, Fe2+, electric current and initial aniline concentration on aniline degradation using the plate type electrode and rod type electrode EF reactors are shown in Figures 4-5, 4-6, 4-7, and 4-8, respectively. The results showed that for both of the EF reactors, aniline degradation rate was fast during the first 2 minutes of the reaction then slowed down until the end of the reaction time. This high initial aniline degradation rate was also observed when aniline degradation was conducted at different concentrations of H2O2, Fe2+, electric current and initial aniline concentration. This high initial aniline degradation rate was attributed to the reaction of aniline with hydroxyl radicals that were formed during the first 2 minutes of the total reaction time according to reaction (2.1). 1.0

1.0 2+

Aniline remaining (C/Co)

0.6 0.4 0.2 0.0

0.8 Aniline remaining (C/Co)

H2O2 = 0 mM; Fe = 0 mM H2O2 = 14.5 mM H2O2 = 29.0 mM H2O2 = 58.0 mM H2O2 = 87.0 mM H2O2 = 130 mM

0.8

2+

H2O2 = 0 mM; Fe = 0 mM H2O2 = 14.5 mM H2O2 = 29.0 mM H2O2 = 58.0 mM H2O2 = 87.0 mM H2O2 = 130 mM

0.6 0.4 0.2 0.0

0

10

20

30

40

50

60

time (min)

(a)

0

10

20

30

40

50

60

time (min)

(b)

Figure 4-5: Effect of H2O2 on aniline degradation using (a) plate type electrode and (b) rod type electrode EF reactors. Experimental condition: [aniline] = 10mM; [Fe2+] = 1.07mM; I = 0.8A/L; initial pH = 3.2

24

1.0

1.0

2+

Fe = 1.07 mM 2+ Fe = 1.61 mM 2+ Fe = 2.41 mM

2+

Fe = 0.27 mM 2+ Fe = 0.53 mM 2+ Fe = 1.07 mM 2+ Fe = 1.61 mM 2+ Fe = 2.41 mM

0.6

0.8 Aniline remaining (C/Co)

Aniline remaining (C/Co)

0.8

0.4 0.2

0.6 0.4 0.2 0.0

0.0 0

10

20

30

40

50

0

60

10

20

30

40

50

60

time (min)

time (min)

(a)

(b)

Figure 4-6: Effect of Fe2+ on aniline degradation using the (a) plate type electrode and (b) rod type electrode EF reactors. Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; initial pH = 3.2

1.0

1.0

Aniline remaining (C/Co)

0.8 0.6 0.4 0.2

I = 0.20 A/L I = 0.40 A/L I = 0.80 A/L

0.8 Aniline remaining (Co/C)

I = 0.20 A/L I = 0.40 A/L I = 0.80 A/L I = 1.20 A/L I = 1.80 A/L

0.6 0.4 0.2 0.0

0.0 0

10

20

30

40

50

60

0

10

time (min)

(a)

20

30

40

50

60

time (min)

(b)

Figure 4-7: Effect of electric current on aniline degradation using the (a) plate type electrode and (b) rod type electrode EF reactors. Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; [Fe2+] = 1.07mM; initial pH = 3.2

25

1.0 Aniline = 2.50 mM Aniline = 5.00 mM Aniline = 10.0 mM Aniline = 15.0 mM Aniline = 22.5 mM

0.8 0.6 0.4 0.2

Aniline = 2.50 mM Aniline = 5.00 mM Aniline = 10.0 mM Aniline = 15.0 mM Aniline = 22.5 mM

0.8 Aniline remaining (C/Co)

Aniline remaining (C/Co)

1.0

0.6 0.4 0.2 0.0

0.0 0

10

20

30

40

time (min)

50

60

0

10

20

30

40

50

60

time (min)

(a)

(b)

Figure 4-8: Effect of initial aniline concentrations on aniline degradation using the plate type electrode and rod type electrode EF reactors. Experimental condition: [Fe2+] = 1.07mM; [H2O2] = 58mM; [Fe2+] = 1.07mM; initial pH = 3.2 Low aniline degradation rate after two minutes of the reaction time was due to H2O2 reduction and competition of aniline degradation by products with hydroxyl radicals. Aside from this, low aniline degradation after two minutes of the reaction time was attributed to the reaction of aniline with non hydroxyl radical species and direct anodic oxidation when no residual H2O2 was present in the reactor. Non hydroxyl radical species like perhydroxyl radicals (HO2·), superoxide (O2·-) and hydroperoxide anions (HO2-) were formed due to the presence of excess H2O2 and Fe2+ in the solution. These non hydroxyl radicals were relatively weaker oxidants and were formed according to reactions (2.4) to (2.8). Degradation of aniline through direct anodic oxidation was attributed to the production of adsorbed hydroxyl radicals at the anodes. These adsorbed hydroxyl radicals were formed from the oxidation of water according to reaction (4.2) (Brillas 2004). H2O Æ OHads· + H+ + e-

(4.2)

26

Aniline degradation through direct anodic oxidation was also conducted and the results are shown in Figure 4-5. Direct anodic oxidation was conducted using 10mM of aniline at a constant electric current of 0.8A/L and without the addition of Fenton’s reagents. Direct anodic oxidation resulted into an aniline degradation efficiency of 13% for the plate type electrode and 18% for the rod type electrode EF reactors. Aniline degradation efficiencies at different concentrations of H2O2, Fe2+, electric current and initial aniline concentration using the plate type electrode and rod type electrode EF reactors are shown in Tables 4-1, 4-2, 4-3, and 4-4 respectively. As shown in Table 4-1, the aniline degradation efficiencies increased from 62.3% to 100% for the plate type electrode EF reactor and 72.3% to 100% for the rod type electrode EF reactor as the H2O2 concentration was increased from 14.5 mM to 130mM. This increase in the aniline degradation efficiencies was due to the increase in the available H2O2 for the production of more hydroxyl radicals when reacted with the electro regenerated Fe2+. This phenomenon resulted into 100% aniline degradation efficiency for both of the EF reactors at H2O2 concentrations greater than 58mM.

Table 4-1: Effect of H2O2 on the aniline degradation efficiency using the plate type electrode and rod type electrode EF reactors Aniline removal efficiency

H2O2 (mM)

(%) Plate type electrode

Rod type electrode

14.5

62.3

72.3

29.0

92.1

88.3

58.0

99.5

98.6

87.0

100

100

130

100

100

Experimental condition: [aniline] = 10mM; [Fe2+] = 1.07mM; I = 0.8A/L; pH = 3.2 However at 14.5 mM of H2O2, the rod type electrode EF reactor had an aniline degradation efficiency of 72.3%, which was higher than the plate type electrode EF

27

reactor. This difference was attributed to the predominant effect of direct anodic oxidation when the rod type electrode EF reactor was used at very low H2O2 concentrations as shown in Figure 4-5 On the contrary, at H2O2 concentrations of 29 mM and 58mM, the plate type electrode had higher aniline degradation efficiencies of 92.1% and 99.5% respectively than the rod type EF reactor, which were only 88.3% and 98.9% respectively. This difference in the aniline removal efficiencies between the two EF reactors was due to the different Fe2+ regeneration efficiencies of the EF reactors as shown in Figure 4-4. The rod type electrode EF reactor had a higher Fe2+ regeneration efficiency. However presence of an excess amount of electro regenerated Fe2+ from the rod type electrode EF reactor could be inhibiting Fenton’s reaction due to the scavenging reactions with hydroxyl radicals according to reaction (2.9), which would then result into lower aniline removal efficiencies. Hydroxyl radical scavenging reactions of excess Fe2+ associated with Fenton processes was also found in other studies. This phenomenon resulted into optimum Fe2+ concentrations for a specific experimental condition (Dutta 2003; Trapido 2003). Aniline removal efficiencies using 58mM of H2O2 and 10mM of aniline at different Fe2+ concentrations are shown in Table 4-2. Results showed that as the Fe2+ concentration was increased from 0.27mM to 2.41mM for the plate type electrode EF reactor, the aniline degradation efficiency increased from 65.8% to 100%. While the aniline degradation efficiency for the rod type electrode EF reactor also increased from 98.6% to 100% as the Fe2+ was increased from 1.07mM to 2.41mM. This phenomenon was due to the increase in the available Fe2+ at the start of the reaction time that would produce more hydroxyl radicals when reacted with H2O2. Both of the EF reactors had an aniline degradation efficiency of 100% at Fe2+ concentrations greater than 1.61mM. However at 1.07mM of Fe2+, the rod type electrode EF reactor had lower aniline degradation efficiency which could be attributed to the hydroxyl radical scavenging reactions of excess electro regenerated Fe2+ with hydroxyl radicals according to reaction (2.9).

28

Table 4-2: Effect of Fe2+ on the aniline degradation efficiency using the plate type electrode and rod type electrode EF reactors Aniline removal efficiency

Fe2+ (mM)

(%) Rod type electrode

Plate type electrode

0.27

---

65.8

0.53

---

91.0

1.07

98.6

99.5

1.61

100

100

2.41

100

100

Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; pH = 3.2 Aniline degradation using the rod type EF reactor was not carried out at Fe2+ concentrations lower than 1.07mM because the required electric current of 0.8A/L could not be achieved due to the decrease in the conductivity of the solution. Because of this different electric current application was also investigated in the study to determine its effect on aniline degradation. The electric current was varied from 0.20 A/L to 1.8 A/L for the plate type electrode EF reactor and 0.20A/L to 0.80A/L for the rod type electrode EF reactor. The rod type electrode EF reactor cannot be operated at electric currents higher than 0.8A/L due to the limitations of the power supply used. Aniline degradation efficiencies at different electric currents using the pate type electrode and rod type electrode EF reactors are shown in Table 4-3. The results showed that as the electric current was increased from 0.20A/L to 1.80A/L for the plate type electrode EF reactor, the aniline degradation efficiency increased from 94.7% to 99.8%. Likewise as the electric current was increased from 0.20A/L to 0.40A/L for the rod type electrode EF reactor, the aniline degradation efficiency also increased from 97.94% to 99.4%. This increase in the aniline degradation efficiencies was due to the increase in the production of electro regenerated Fe2+ at the cathode as the electric current was increased (Zhang 2006). However as the electric current was increased to 0.8A/L for the rod type electrode EF reactor, no further effect

29

was observed on the aniline degradation efficiency. This was due to the presence of excess electro regenerated Fe2+. As also shown in Table 4-3, the rod type electrode EF reactor had higher aniline degradation efficiencies at electric currents of 0.20A/L to 0.40A/L. This phenomenon was due to lower Fe2+ regeneration efficiency of the plate type electrode EF reactor at these applied electric currents that resulted to an insufficient amount of Fe2+ for the production of more hydroxyl radicals.

Table 4-3: Effect of electric current on the aniline degradation efficiency using the plate type electrode and rod type electrode EF reactors Aniline removal efficiency

Electric current (A/L)

(%) Plate type electrode

Rod type electrode

0.20

94.7

97.9

0.40

96.9

99.4

0.80

99.4

98.6

1.20

99.5

---

1.80

99.8

---

Experimental condition:[aniline] = 10mM; [Fe2+] = 1.07mM; [H2O2] = 58mM;pH = 3.2

The aniline degradation efficiencies at different initial aniline concentrations are shown in Table 4-4. Degradation of different initial aniline concentrations were conducted using 58mM of H2O2 and 1.07mM of Fe2+ at a constant electric current of 0.8A/L. Initial aniline concentrations were varied from 2.50mM to22.5mM for both of the EF reactors. As shown in Table 4-4, aniline degradation efficiencies decreased from 100% to 87.2% for the plate type electrode EF reactor and 100% to 77.2% for the rod type electrode EF reactor as the aniline concentration was increased from 2.50mM to 22.5mM.

30

This decrease in the initial aniline degradation rates and aniline removal efficiencies was due to the insufficient amount of the Fenton reagents for aniline degradation. As also shown in Table 4-4, the rod type electrode EF reactor had lower aniline degradation efficiencies. This was due to the scavenging reactions of excess electro regenerated Fe2+ with hydroxyl radical according to reaction (2.9).

Table 4-4: Effect of initial aniline concentration on the aniline degradation efficiency using the plate type electrode and rod type electrode EF reactors Aniline removal efficiency

Aniline (mM)

(%) Plate type electrode

Rod type electrode

2.50

100

100

5.00

100

100

10.0

99.5

98.6

15.0

98.6

97.7

22.5

87.2

77.2

Experimental condition: [H2O2] = 58mM; [Fe2+] = 1.07mM; I = 0.8A/L; pH = 3.2

31

4.4 Effect of H2O2, Fe2+, electric current and initial aniline concentration on the initial aniline degradation rate using the plate type electrode and rod type electrode EF reactors. Information on the effects of H2O2, Fe2+, electric current and initial aniline concentrations on the initial aniline degradation rate was important since this rate was attributed to the reaction of aniline with hydroxyl radicals and would therefore aid on choosing possible options of achieving higher treatment efficiencies. The initial aniline degradation rate was calculated using the amount of aniline degraded during the first two minutes of the reaction time. The effect of H2O2, Fe2+, electric current and initial aniline concentration on the initial aniline degradation rates using the plate type and rod type electrode EF reactors are shown in Tables 4-5, 4-6, 4-7 and 4-8. When the H2O2 concentration was increased from 14.5mM to 87mM, the initial aniline degradation rate increased from 2.69 mM min-1 to 4.19 mM min-1 for the plate type electrode EF reactor and 3.09 mM min-1 to 4.08 mM min-1 for the rod type electrode EF reactor. This increase in initial aniline degradation rate was due to the increase in the available H2O2 for the production of hydroxyl radicals when reacted with Fe2+ according to reaction (2.1).

Table 4-5: Effect of H2O2 on initial aniline degradation rate using the plate type electrode and rod type electrode EF reactors H2O2 (mM)

Initial aniline degradation rate (mM min-1) Plate type electrode

Rod type electrode

14.5

2.69

3.09

29.0

3.16

3.42

58.0

3.20

3.82

87.0

4.19

4.08

130

4.17

3.97

Experimental condition: [aniline] = 10mM, [Fe2+] = 1.07mM, I = 0.8A/L, pH = 3.2

32

However the change in the initial aniline degradation rate was not significant when the H2O2 concentration was further increased to 130 mM. This phenomenon was due to scavenging reactions of excess H2O2 with hydroxyl radicals according to reaction (2.4) and Fe3+ according to reaction (2.11). The difference in initial aniline degradation rates between the two EF reactors was determined to be not significant. The effect of Fe2+ on the initial aniline degradation rate is shown in Table 4-6. The initial aniline degradation rate increased from 1.44 mM min-1 to 4.2 mM min-1 as the Fe2+ concentration was increased from 0.27mM to 1.61mM for the plate type electrode EF reactor and 3.82 mM min-1 to 4.25 mM min-1 for the rod type electrode EF reactor as the Fe2+ concentration was increased from 1.07mM to 2.41mM. This increase in initial aniline degradation rates was due to the increase in the available initial Fe2+ needed for the production of more hydroxyl radicals when reacted with H2O2. However further increase of Fe2+ to 2.41mM had no further significant effect on the initial degradation rate due to the scavenging reactions of excess Fe2+ with hydroxyl radicals according to reaction (2.9). Table 4-6: Effect of Fe2+ on the initial aniline degradation rate using the plate type electrode and rod type electrode EF reactors Fe2+ (mM)

Initial aniline degradation rate (mM min-1) Plate type electrode

Rod type electrode

0.27

1.44

---

0.53

1.47

---

1.07

3.2

3.82

1.61

4.2

4.25

2.41

4.64

4.46

Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L, pH = 3.2

33

The effect of electric current on the initial aniline degradation rate is shown in Table 4-7. The initial aniline degradation rate increased from 2.98 mM min-1 to 3.74 mM min-1 as the electric current was increased from 0.20A/L to 1.80A/L for the plate type electrode EF reactor. For the rod type electrode EF reactor, the change in the initial aniline degradation was not significant due to scavenging reactions of excess electro regenerated Fe2+ according to reaction (2.9). Results also showed that the difference in the initial aniline degradation rates between the two EF reactors at different electric current applications were not significant.

Table 4-7: Effect of electric current on the initial aniline degradation rate using the plate type electrode and rod type electrode EF reactors Electric current (A/L)

Initial aniline degradation rate (mM min-1) Plate type electrode

Rod type electrode

0.20

2.98

3.33

0.40

3.14

3.11

0.80

3.2

3.82

1.20

3.66

---

1.80

3.74

---

Experimental condition: [aniline]=10mM, [H2O2]=58mM, [Fe2+]=1.07mM, pH = 3.2 The effect of different initial aniline concentrations on the initial aniline degradation rate is shown in Table 4-8. The initial aniline degradation rate decreased from 5.00 mM min-1 to 1.68 mM min-1 for the plate type electrode EF reactor and 5.00mM min-1 to 0.68mM min-1 for the rod type electrode EF reactor as the initial aniline concentration was increased from 2.50mM to 22.5 mM. The decrease in the initial aniline degradation rates was attributed to the insufficient amount of Fenton’s reagents for the production of hydroxyl radicals during the first two minutes of the total reaction time. For both of the EF reactors, the difference in the initial aniline degradation rates was also not significant.

34

Table 4-8: Effect of initial aniline concentration on initial aniline degradation rate and aniline removal efficiency using plate type electrode and rod type electrode EF reactors Aniline

Initial aniline degradation rate (mM min-1)

(mM)

Plate type electrode

Rod type electrode

2.50

5.00

5.00

5.00

4.50

3.70

10.0

3.20

3.52

15.0

3.00

3.32

22.5

1.68

0.68

Experimental condition: I = 0.8A/L; [H2O2] = 58mM; [Fe2+] = 1.07mM; pH = 3.2

35

4.5 Effect of H2O2, Fe2+, electric current, initial aniline degradation concentration on the second order aniline degradation rate constant using the plate type electrode and rod type electrode EF reactors The kinetic model for aniline degradation and its response to different concentrations of H2O2, Fe2+, electric current and initial aniline concentrations were also determined in the study. Aniline degradation for both of the EF reactors followed the second order behavior and the effect of H2O2, Fe2+, electric current and initial aniline concentrations on aniline degradation rate constants are shown in Figures 4-9, 4-10, 4-11 and 4-12 respectively. Data on aniline degradation with respect to time on the effects of H2O2, Fe2+, electric current and initial aniline concentration from Figures 4-5 to 4-8 were used to calculate the rate constants as shown in Appendix A. The effect of H2O2 on the aniline degradation rate constant is shown in Figure 49. The rate constants increased from 14.1x10-2 M-1 min-1 to 304 x10-2 M-1 min-1 and 21.5 x10-2 M-1 min-1 to 219 x10-2 M-1 min-1 for the plate type and rod type electrode EF reactors respectively as the H2O2 concentration was increased from 14.5mM to 130mM.

plate type rod type

300

-2

-1

-1

rate constant, k (x10 M min )

350

250 200 150 100 50 0 0

20

40

60

80

100

120

140

H2O2 (mM)

Figure 4-9: Effect of H2O2 on the second order aniline degradation rate constant using the plate type electrode and rod type electrode EF reactors. Experimental condition: [aniline] = 10mM; [Fe2+] = 1.07mM; I = 0.8A/L; initial pH = 3.2

36

This increase in rate constants was due to the presence of excess H2O2 forming more hydroxyl radicals when reacted with the electro regenerated Fe2+. As also shown in Figure 4-9, the rod type electrode EF reactor had lower rate constants at H2O2 concentrations higher than 29mM. This difference was due to higher hydroxyl radical competition of aniline degradation by products when the rod type electrode EF reactor was used. At 14.5mM and 29mM of H2O2, the rod type and plate type electrode EF reactors have the same rate constants due to the insufficient amount of H2O2 for further aniline degradation. The kinetic model for aniline degradation at different Fe2+ concentrations using the plate type electrode and rod type electrode EF reactors is shown in Figure 4-10. The second order rate constant increased from 21.5 x 10-2 M-1 min-1 to 429 x 10-2 M-1 min-1 as the Fe2+ concentration was increased from 0.27mM to 2.41mM for the plate type electrode EF reactor. Likewise for the rod type electrode EF reactor, the second order rate constant also increased from 50.7 x 10-2 M-1 min-1 to 318 x 10-2 M-1 min-1 as the Fe2+ concentration was increased from 1.07mM to 2.41mM.

400

plate type rod type

-2

-1

-1

rate constant, k (x10 M min )

500

300 200 100 0 0.0

0.5

1.0

1.5

2.0

2.5

2+

Fe (mM)

Figure 4-10: Effect of Fe2+ on the second order aniline degradation rate constant using the plate type electrode and rod type electrode EF reactors. Experimental conditions: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; initial pH = 3.2

37

This increase in the rate constants was attributed to the increase in the production of hydroxyl radicals when the initial Fe2+ for the reaction with H2O2 was increased. As also shown in Figure 4-10, the rod type electrode EF reactor had lower aniline degradation rate constants at all Fe2+ concentrations due to higher presence of scavenging reactions of excess electro regenerated Fe2+. The effect of electric current on the aniline degradation rate constant is shown in Figure 4-11. For the plate type electrode EF reactor, the rate constant increased from 21.5 x 10-2 M-1 min-1 to 299 x 10-2 M-1 min-1 as the electric current was increased from 0.20A/L to 1.80A/L. Likewise, the rate constant for the rod type electrode EF reactor increased from 51.9 x 10-2 M-1 min-1 to 101 x 10-2 M-1 min-1 as the electric current was increased from 0.20A/L to 0.40A/L. However as the electric current was further increased to 1.8A/L and 0.8A/L for the plate type electrode and rod type electrode EF reactors respectively, the aniline degradation rate constants decreased to 253 x 10-2 M-1 min-1 and 68.2 x 10-2 M-1 min-1 respectively.

300

-2

-1

-1

rate constant, k (x10 M min )

350 plate type rod type

250 200 150 100 50 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 electric current (A/L)

Figure 4-11: Effect of electric current on the second order aniline degradation rate constant using the plate type electrode and rod type electrode EF reactors. Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; [Fe2+] = 1.07mM; initial pH = 3.2

38

This decrease in the aniline degradation rate constants was due to the scavenging reactions of excess electro regenerated Fe2+. The decrease was also attributed to increased competitive reactions at the electrodes like production of O2 and H2 at the anode and cathode respectively according to reactions (4.3) and (4.4) (Zhang 2006). 2H2O2 Æ 4H+ + O2 + 4e-

(4.3)

2H+ + e- Æ H2

(4.4)

The effect of different initial aniline concentrations on the aniline degradation rate constant is shown in Figure 4-12. The aniline degradation rate constants for both of the EF reactors decreased as the initial aniline concentration was increased from 2.5mM to 22.5mM. However the rod type electrode EF reactor had lower rate constants at initial aniline concentrations of 2.5mM and 5mM due to the scavenging reactions of excess electro regenerated Fe2+. However at higher aniline concentrations both EF reactors have the same aniline degradation rate constants.

plate type rod type

400

-2

-1

-1

rate constant, k (x10 M min )

500

300 200 100 0 0

5

10

15

20

25

Aniline (mM)

(a)

(b)

Figure 4-12: Effect of initial aniline concentration on the second order aniline degradation rate constant using the plate type electrode and rod type electrode EF reactors. Experimental condition: I = 0.8A/L; [H2O2] = 58mM; [Fe2+] = 1.07mM; initial pH = 3.2

39

4.6 Effect of H2O2, Fe2+, electric current, initial aniline concentration on the COD removal efficiency using the plate type electrode and rod type electrode EF reactors The effects of H2O2, Fe2+, electric current and initial aniline concentrations on COD removal efficiencies were also determined in the study and are shown in Tables 49, 4-10, 4-11 and 4-12, respectively. The effect of H2O2 on the COD removal is shown in Table 4-9. COD removal efficiencies increased from 31.1% to 60.2% and 18.8% to 61.9% for the plate type electrode and rod type electrode EF reactors respectively as the H2O2 concentration was increased from 14.5mM to 130mM. This increase in COD removal efficiencies was due to the increase in available H2O2 for the production of hydroxyl radicals. As also shown in Table 4-9, the rod type electrode EF reactor had higher COD removal efficiencies at H2O2 concentrations of 87 mM and 130mM which was 53.3% and 61.9% respectively. This difference in COD removal efficiencies was due to the difference in the Fe2+ regeneration abilities between the two EF reactors and the higher competition of aniline degradation by products with the hydroxyl radicals as determined from the effect of H2O2 on the second order aniline degradation rate constant.

Table 4-9: Effect of H2O2 on the COD removal efficiency using the plate type electrode and rod type electrode EF reactors H2O2 (mM)

COD removal efficiency (%) Plate type electrode

Rod type electrode

14.5

31.1

18.8

29.0

33.5

22.3

58.0

45.9

40.9

87.0

47.9

53.2

130

60.2

61.9 2+

Experimental condition: [aniline] = 10mM, [Fe ] = 1.07mM, I = 0.8A/L, pH = 3.2

40

Therefore at higher H2O2 concentrations, excess H2O2 are available for the regenerated Fe2+ to produce more hydroxyl radicals. However, the high Fe2+ regeneration capacity of the rod type electrode EF reactor will yield lower COD removal efficiencies at H2O2 concentrations lower than 87 mM due to scavenging reactions of regenerated Fe2+ with hydroxyl radicals according to reaction (2.9). COD removal efficiencies at different Fe2+ concentrations using the plate type electrode and rod type electrode EF reactors are shown in Table 4-10. As the Fe2+ concentration was increased from 0.27mM to 1.61mM for the plate type electrode EF reactor, the COD removal efficiency increased from 30.7% to 46.7%. Likewise as the Fe2+ concentration was increased from 1.07mM to 1.61mM for the rod type electrode EF reactor, the COD removal efficiency slightly increased from 40.9% to 43.6%. Table 4-10: Effect of Fe2+ on the COD removal efficiency using the plate type electrode and rod type electrode EF reactors COD removal efficiency

Fe2+ (mM)

(%) Plate type electrode

Rod type electrode

0.27

30.7

---

0.53

39.9

---

1.07

45.9

40.9

1.61

46.7

43.6

2.41

43.9

40.9

Experimental condition: [aniline] = 10mM, [H2O2] = 58mM, I = 0.8A/L, pH = 3.2 However for both of the EF reactors, further increase of Fe2+ to 2.41mM resulted into a decrease of the COD removal efficiency to 43.9% and 40.9% for the plate type electrode and rod type electrode EF reactors respectively. This decrease was due to the scavenging reactions of excess Fe2+ with hydroxyl radicals according to reaction (2.9). Aside from this, the rod type electrode EF reactor had lower COD removal efficiencies due to presence of excess electro regenerated Fe2+.

41

COD removal efficiencies at different applied electric current are shown in Table 4-11. COD removal efficiencies increased from 37.3% to 44.8% for the plate type electrode EF reactor as the electric current was increased from 0.20A/L to 1.20A/L. For the rod type electrode EF reactor, the COD removal efficiency also increased from 42.2% to 43.28% as the electric current was increased from 0.20A/L to 0.80A/L. However for the plate type electrode EF reactor, further increase of the electric current to 1.8A/L had no further effect on the COD removal efficiency. This was due to the increase in the scavenging reactions of the excess electro regenerated Fe2+ and the increase in the competitive reactions at the electrodes according to reaction (4.3) and (4.4).

Table 4-11: Effect of electric current on the COD removal efficiency using the plate type electrode and rod type electrode EF reactors Electric Current (A/L)

COD removal efficiency (%) Plate type electrode

Rod type electrode

0.20

37.3

42.2

0.40

43.8

43.3

0.80

45.9

40.9

1.20

44.8

---

1.80

43.4

---

Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; pH = 3.2 The effect of different initial aniline concentrations on the COD removal efficiencies is shown in Table 4-12. The COD removal efficiency decreased from 57% to 33.5% for the plate type electrode EF reactor and 76.1% to 30.5% for the rod type electrode EF reactor as the initial aniline concentration was increased from 2.50mM to 22.5mM. As also shown in Table 4-12, the rod type electrode EF reactor had higher competitions of aniline degradation by product with hydroxyl radicals at initial aniline concentrations lower than 10mM. This was due to higher COD removal efficiency than

42

the plate type electrode EF reactor but had lower aniline degradation efficiency as determined previously from the effect of initial aniline concentrations on the aniline degradation efficiencies. At initial aniline concentrations greater than 10mM, the difference in the COD removal efficiencies was not significant between the two EF reactors.

Table 4-12: Effect of initial aniline concentration on the COD removal efficiency using the plate type electrode and rod type electrode EF reactors COD removal efficiency Aniline (mM)

(%) Plate type electrode

Rod type electrode

2.50

57.0

76.1

5.00

56.7

62.9

10.0

45.9

40.9

15.0

34.5

32.1

22.5

33.5

30.5

Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; pH = 3.2

43

4.7 Effect of H2O2, Fe2+, electric current, initial aniline concentration on the H2O2 efficiency for aniline and COD removal using the plate type electrode and rod type electrode EF reactors. The H2O2 efficiency for aniline and COD removal was determined in the study since it was an important index that would aid in achieving the optimum usage of H2O2 for the production of hydroxyl radicals. Obtaining the optimum usage of H2O2 was important since H2O2 was the most expensive compound used in the EF process. In addition to this, H2O2 efficiency was also an index that would determine presence of scavenging reactions of different species with hydroxyl radicals. The H2O2 efficiency for aniline removal was calculated using the amount of H2O2 consumed for the amount of aniline or COD removed. Theoretically, the highest H2O2 efficiency of 100% for aniline removal would mean that 1 mol of aniline was removed by 1 mol of H2O2. The H2O2 efficiency for COD removal was calculated using equation (1) (Zhang 2006)

η =

∆COD (mg/L)

x 100

(1)

available O2 (mg/L) where the available O2 is the theoretical reactive oxygen in the amount of H2O2 consumed with respect to the amount of COD removed. The effect of H2O2, Fe2+, electric current and initial aniline concentrations on the H2O2 efficiency on aniline and COD removal are shown in Tables 4-13, 4-14, 4-15, and 4-16 respectively. The effect of H2O2 on the H2O2 efficiency for aniline and COD removal is shown in Table 4-13. The H2O2 efficiency for aniline removal at 14.5mM of H2O2 was 42.9% and 49.9% for the plate type electrode and rod type electrode EF reactors, respectively. This means that not all of the H2O2 consumed were used to degrade aniline only. The

44

H2O2 consumed could be used for the degradation of intermediates or by scavenging reactions. However the H2O2 efficiencies for COD removal at 14.5mM of H2O2 were high, which was 307% and 186% for the plate type electrode and rod type electrode EF reactors respectively. H2O2 efficiencies higher than 100% were possible in this study since aniline degradation and COD removal was not only attributed to Fenton’s reactions. As the H2O2 was increased to 130mM, the H2O2 efficiencies for aniline and COD removal further decreased for both of the plate type and rod type electrode EF reactors. This decrease in the H2O2 efficiency was due to the scavenging reactions of excess H2O2 with hydroxyl radicals according to reaction (2.4). As also shown in Table 4-13, both of the EF reactors had the same H2O2 efficiencies for aniline removal. However at low H2O2 concentration of 14.5mM, the rod type electrode EF reactor had a slightly higher H2O2 efficiency for aniline removal due to the higher aniline degradation efficiency caused by the added effect of direct anodic oxidation.

Table 4-13: Effect of H2O2 on the H2O2 efficiencies for aniline and COD removal using the plate type electrode and rod type electrode EF reactors H2O2 efficiency for aniline

H2O2 efficiency for COD

removal

removal

(%)

(%)

H2O2 (mM)

Plate type

Rod type

Plate type

Rod type

electrode

electrode

electrode

electrode

14.5

42.9

49.9

307

186

29.0

31.7

30.5

165

110

58.0

17.3

17.0

114

101

87.0

12.0

12.8

82.2

98.3

130

9.93

10.6

85.6

94.6

2+

Experimental condition: [aniline] = 10mM, [Fe ] = 1.07mM, I = 0.8A/L, pH = 3.2

45

In addition to this, the rod type electrode EF reactor had lower H2O2 efficiency for COD removal at low H2O2 concentrations due to the presence of scavenging reactions of excess electro regenerated Fe2+. The effect of Fe2+ on the H2O2 efficiency for aniline and COD removal are shown in Table 4-14. The H2O2 efficiency for aniline and COD removal decreased from 58.7% to 7.69% and 191% to 23.6% respectively as the Fe2+ concentrations was increased from 0.27mM to 2.41mM for the plate type electrode EF reactor. Likewise for the rod type electrode EF reactor, the H2O2 efficiency for aniline and COD removal also decreased from 17.0% to 7.69% and 49.3% to 22.0% respectively. This decrease in the H2O2 efficiency was due to the scavenging reactions of excess Fe2+ with hydroxyl radicals according to reaction (2.9) as the Fe2+ concentration was increased. As also shown in Table 4-14, the difference in the H2O2 efficiencies for aniline and COD removal between the two EF reactors was not significant at different Fe2+ concentrations. Table 4-14: Effect of Fe2+ on the H2O2 efficiency for aniline and COD removal using the plate type electrode and rod type electrode EF reactors H2O2 efficiency for aniline

H2O2 efficiency for COD

removal

removal

(%)

(%)

Fe2+ (mM)

Plate type

Rod type

Plate type

Rod type

electrode

electrode

electrode

electrode

0.27

58.7

---

191

---

0.53

36.3

---

111

---

1.07

17.3

17.0

55.9

49.3

1.61

11.5

11.5

37.6

35.1

2.41

7.69

7.69

23.6

22.0

Experimental condition: [aniline] = 10mM, [H2O2] = 58mM, I = 0.8A/L, pH = 3.2

46

The H2O2 efficiency for aniline and COD removal at different electric current applications are shown in Table 4-15. The H2O2 efficiency for aniline and COD removal decreased from 73.1% to 7.71% and 412% to 48% respectively as the electric current was increased from 0.20A/L to 1.8A/L for the plate type electrode EF reactor. Likewise for the rod type electrode EF reactor, the H2O2 efficiency for aniline and COD removal also decreased from 71.6% to 17.0% and 441% to 100% respectively when the electric current was increased from 0.20A/L to 0.80A/L. This decrease in the H2O2 efficiency was due to the presence of excess electro regenerated Fe2+ that was inhibiting Fenton reactions by scavenging the hydroxyl radicals according to reaction (2.9). This decrease in degradation efficiencies was also attributed to the increase in competitive reactions at the electrodes such as the production of O2 and H2 at the anode and cathode respectively. As also shown in Table 4.15, both of the EF reactors had nearly the same H2O2 efficiencies for aniline and COD removal.

Table 4.-15: Effect of electric current on the H2O2 efficiency for aniline and COD removal using the plate type electrode and rod type electrode EF reactors H2O2 efficiency for aniline

H2O2 efficiency for COD

Electric

removal

removal

Current

(%)

(%)

Plate type

Rod type

Plate type

Rod type

electrode

electrode

electrode

electrode

0.20

73.1

71.6

412

441

0.40

35.5

36.0

229

226

0.80

17.3

17.0

114

100

1.20

11.5

---

73.8

---

1.80

7.31

---

48.0

---

(A/L)

Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; pH = 3.2

47

The effect of initial aniline concentration on the H2O2 efficiency for aniline and COD removal are shown in Table 4-16. The H2O2 efficiency for aniline and COD removal for the plate type electrode EF reactor decreased from 69.0% to 6.71% and 563% to 36.9% respectively as the initial aniline concentration was increased from 2.50mM to 22.5mM. Likewise for the rod type electrode EF reactor, the H2O2 efficiency for aniline and COD removal also decreased from 71.4% to 6.04% and 777% to 34.2% respectively. This decrease in the H2O2 efficiencies was due to the decrease in the aniline and COD removal efficiencies as determined from the effect of initial aniline concentration on aniline and COD removal efficiencies. As also shown in Table 4-16, the difference on the H2O2 efficiencies for aniline and COD removal between the two EF reactors was not significant.

Table 4-16: Effect of initial aniline concentration on the H2O2 efficiency for aniline and COD removal using the plate type electrode and rod type electrode EF reactors H2O2 efficiency for aniline

H2O2 efficiency for COD

removal

removal

(%)

(%)

Aniline (mM)

Plate type

Rod type

Plate type

Rod type

electrode

electrode

electrode

electrode

2.50

69.0

71.4

563

777

5.00

34.5

36.6

279

329

10.0

17.3

17.1

114

100

15.0

10.5

11.2

56.8

52.8

22.5

6.71

6.04

36.9

34.2

Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; pH = 3.2

48

4.8 Effect of H2O2, Fe2+, electric current and initial aniline concentration on the power consumption for aniline and COD removal using the plate type electrode and rod type electrode EF reactors The effect of H2O2, Fe2+, electric current and initial aniline concentration on the power consumption for aniline and COD removal using the plate type electrode and rod type electrode EF reactors were also determined in the study and are shown in Figures 413, 4-14, 4-15 and 4-16 respectively. Power consumption for aniline and COD removal was calculated using equation (2) (Brillas 2002). Power (kWh/L) = V (V) * I (A) * t (hr) vol (L)

x

[aniline]initial (or [COD]initial) ∆aniline (or ∆COD)

(2)

where V is the average voltage for the total reaction time t and vol is the total volume of solution treated. The effect of H2O2 on power consumption for aniline and COD removal is shown in Figure 4-13. The power consumption for aniline degradation and COD removal decreased from 0.009kWh/L to 0.005kWh/L and 0.018kWh/L to 0.010kWh/L respectively for the plate type electrode EF reactor as the H2O2 concentration was increased from 14.5mM to 58mM. For the rod type electrode EF reactor the power consumption for aniline degradation and COD removal decreased from 0.025kWh/L to 0.014kWh/L and 0.10kWh/L to 0.030kWh/L respectively when the H2O2 concentration was increased from 14.5mM to 58mM. This decrease in power consumption was due to the increase in aniline and COD removal efficiencies as the H2O2 concentration was increased from 14.5mM to 58mM. However further increase of H2O2 to 87mM and 130mM had no further effect on power consumption since aniline and COD removal efficiencies did not change significantly. As also shown in Figure 4-13, the power consumption for aniline and COD removal for the rod type electrode EF reactor was twice as much as the plate type electrode EF reactor. Likewise, COD removal requires twice as much power than aniline removal.

49

plate type rod type

0.025 0.020 0.015 0.010 0.005 0.000 0

20

40

60

80

H2O2 (mM)

(a)

100

120

140

Power consumption for COD removal (KWh/L)

Power consumption for aniline removal (KWh/L)

0.030

0.10

plate type rod type

0.08 0.06 0.04 0.02 0.00 0

20

40

60

80

100

120

140

H2O2(mM)

(b)

Figure 4-13: Effect of H2O2 on the power consumption for aniline and COD removal using the plate type electrode and rod type electrode EF reactors. Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; initial pH = 3.2 The power consumption for aniline and COD removal at different Fe2+ concentrations using the plate type electrode and rod type electrode EF reactors are shown in Figure 4-14 (a) and (b) respectively. The power consumption for aniline degradation and COD removal decreased from 0.009kWh/L to 0.005kWh/L and 0.019kWh/L to 0.009kWh/L respectively for the plate type electrode EF reactor as the Fe2+ concentration was increased from 0.27mM to 1.07mM. For the rod type electrode EF reactor the power consumption for aniline degradation and COD removal decreased from 0.014kWh/L to 0.012kWh/L and 0.034kWh/L to 0.029kWh/L respectively when the Fe2+ concentration was increased from 1.07mM to 1.61mM. This decrease in power consumption was due to the increase in the conductivity of the solution when more Fe2+ ions were added. An increase in conductivity resulted into lower voltage requirements at constant electric current therefore lowering power consumption. Aside from this, power consumption also decreased when aniline and COD removal efficiencies increased based on equation (2). Because of this, increasing the Fe2+ concentration greater than 1.07mM

50

for the plate type EF reactor had no further effect on power consumption for aniline and COD removal since the aniline and COD removal efficiencies decreased at these Fe2+

0.014 0.012

plate rod

0.010 0.008 0.006 0.004 0.002 0.000 0.0

0.5

1.0

1.5

2.0

2.5

3.0

2+

Fe (mM)

(a) 2+

Figure 4-14: Effect of Fe

Power requirement for COD removal (KWh/L)

Power requirement for aniline removal (KWh/L)

concentrations. 0.040 0.035

plate rod

0.030 0.025 0.020 0.015 0.010 0.005 0.000 0.0

0.5

1.0

1.5

2.0

2.5

3.0

2+

Fe (mM)

(b) on the power consumption for aniline and COD removal

using the plate type electrode and rod type electrode EF reactors. Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; I = 0.8A/L; initial pH = 3.2 The effect of electric current on the power consumption for aniline and COD removal using the plate type electrode and rod type electrode EF reactors are shown in Figure 4-15 (a) and (b) respectively. The power consumption for aniline degradation and COD removal increased from 0.003kWh/L to 0.007kWh/L and 0.006kWh/L to 0.017kWh/L respectively for the plate type electrode EF reactor as the electric current was increased from 0.20A/L to 1.8A/L For the rod type electrode EF reactor, the power consumption for aniline degradation and COD removal also increased from 0.005kWh/L to 0.014kWh/L and 0.011kWh/L to 0.034kWh/L respectively when the electric current was increased from 0.20A/L to 0.80A/L. Increase in power consumption was significantly affected by the increase in the applied electric current rather than the aniline degradation and COD removal efficiencies.

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As also shown in Figure 4-15, the difference in the power consumption for the rod type electrode and plate type electrode EF reactors also significantly increased from 1.7 times at 0.2A/L to 3.3 times at 0.8A/L. This increase in the difference in the power consumption between the rod type electrode and plate type electrode EF reactors was due to the higher decrease in the aniline and COD removal efficiencies for the rod type

0.016 plate rod

0.014 0.012 0.010 0.008 0.006 0.004 0.002 0.000 0.0

0.3

0.6

0.9

1.2

electric current (A/L)

(a)

1.5

1.8

Power consumption for COD removal (kWh/L)

Power consumption for aniline removal (kWh/L)

electrode EF reactor at high electric current applications.

0.040 plate rod

0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0.0

0.3

0.6

0.9

1.2

1.5

1.8

electric current (A/L)

(b)

Figure 4-15: Effect of electric current on the power consumption for aniline and COD removal using the plate type electrode and rod type electrode EF reactors. Experimental condition: [aniline] = 10mM; [H2O2] = 58mM; [Fe2+] = 1.07mM; initial pH = 3.2 The effect of different initial aniline concentration on power consumption for aniline and COD removal is shown in Figure 4-16 (a) and (b) respectively. The power consumption for aniline degradation slightly decreased from 0.006kWh/L to 0.005kWh/L and 0.023kWh/L to 0.013kWh/L for the plate type electrode and rod type electrode EF reactor respectively as the initial aniline concentration was increased from 2.50 mM to 22.5 mM. Decrease in power consumption for aniline removal was due to the increase in the conductivity of the solution as the aniline concentration was increased. However this

52

phenomenon slightly decreases the power consumption for aniline removal for the plate type EF reactor since the aniline removal efficiencies were low In contrast, the power consumption for COD removal slightly increased from 0.010kWh/L to 0.130kWh/L for the plate type electrode EF reactor and 0.030kWh/L to 0.040kWh/L for the rod type electrode EF reactor. Power consumption for COD removal slightly increases for both of the EF reactors due to the predominant effect of the

0.028 plate type rod type

0.024 0.020 0.016 0.012 0.008 0.004 0.000 0

5

10

15

Aniline (mM)

(a)

20

25

Power consumption for COD removal (kWh/L)

Power consumption for aniline removal (kWh/L)

decrease in the COD removal efficiencies.

0.049 plate type rod type

0.042 0.035 0.028 0.021 0.014 0.007 0.000 0

5

10

15

20

25

Aniline (mM)

(b)

Figure 4-16: Effect of initial aniline concentration on the power consumption for aniline and COD removal using the plate type electrode and rod type electrode EF reactors. Experimental condition: I = 0.80A/L; [H2O2] = 58mM; [Fe2+] = 1.07mM; initial pH = 3.2

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4.9 Degradation of high initial aniline concentration Aniline degradation using a very high initial aniline concentration having a COD value of 10,000ppm was also investigated in the study. The aniline remaining and COD remaining ratios from the experiment are shown in Figure 4-17 (a) and (b), respectively. For both of the EF reactors, 100% of the aniline was removed at time 2 minutes of the reaction. Likewise, the COD removal efficiency for both of the EF reactors was 86% at the end of the reaction time. Both of the EF reactors had the same aniline and COD removal efficiencies at this initial aniline concentration since it was shown in the results from the effect of initial aniline concentration on the aniline and COD removal efficiencies that both of the EF reactors had the same treatment performance at higher initial aniline concentrations.

1.0

1.0 plate type rod type COD remaining (C/Co)

Aniline remaining (C/Co)

0.8 0.6 0.4 0.2

plate type rod type

0.8 0.6 0.4 0.2 0.0

0.0 0

10

20

30

40

50

60

0

10

20

30

40

time (min)

time (min)

(a)

(b)

50

60

Figure 4-17: Aniline and COD remaining ratios from degradation of 10,000ppm COD of aniline using the plate type electrode and rod type electrode EF reactors. Experimental condition: [H2O2] = 640mM; [Fe2+] = 11.9mM; initial pH = 3.2 The change in the BOD and toxicity of the aniline solution was also determined in the study and is shown in Figure 4-18 (a) and (b) respectively. The initial BOD and toxicity of the aniline solution was determined to be both at 0mg/L. This means that the

54

10,000ppm COD of aniline was initially not biodegradable and was highly toxic to microorganisms.

0.25

300 0.20

200

BOD/COD

BOD (mg/L)

250

150

0.15

0.10

100 0.05

50 0.00

0 rod type EF

plate type EF

rod type EF

(a)

plate type EF

(b)

Figure 4-18: BOD and toxicity tests on treated high concentration aniline solution using the plate type electrode and rod type electrode EF reactors. Experimental condition: [H2O2] = 640mM; [Fe2+] = 11.9mM; initial pH = 3.2

1800 1600

Oxalic acid (mg/L)

1400 1200 1000 800 600 400 200 0 plate type

rod type

Figure 4-19: Oxalic acid content of the treated high concentration aniline solution using the plate type electrode and rod type electrode EF reactors. Experimental condition: H2O2] = 640mM; [Fe2+] = 11.9mM; initial pH = 3.2

55

However after treatment of the aniline solution using the plate type electrode and rod type electrode EF reactors, the solution BOD significantly increased to 240ppm and 323ppm for the plate type electrode and rod type electrode EF reactors respectively. Likewise, the toxicity also decreased and had a value of 0.18 and 0.25 for the plate type electrode and rod type electrode EF reactors respectively. The main biodegradable component of the treated aniline solution was also determined in the study and the oxalic acid concentration after treatment is shown in Figure 4-19. The oxalic acid content of the treated solution was 1,607ppm for the plate type electrode EF reactor and 1,675ppm for the rod type electrode EF reactor. These results showed that further treatment of the aniline solution using existing technologies like biological treatment systems is possible due to production of biodegradable compounds like oxalic acid. However further treatment is also necessary in order to achieve typical governmental standards for wastewater effluents. Therefore preliminary treatment of high concentration aniline solution using the plate type electrode or rod type electrode EF reactors would greatly enhance its biodegradability.

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V. CONCLUSIONS

Based on the results obtained within the scope and limitations of the study, the following conclusions can be made:

1. The EF process is more superior to the CF process due to its higher treatment efficiency at low Fe2+ concentrations and low residual H2O2 after treatment. In addition to this, previous studies concluded that the EF process produces low inorganic sludge than the CF process. 2. The rod type electrode EF reactor has a higher Fe2+ regeneration ability than the plate type electrode EF reactor.

3. At H2O2 concentrations greater than 87mM, both of the EF reactors have 100% aniline removal efficiency. However, at H2O2 concentrations lower than 58mM, the plate type electrode EF reactor is superior to the rod type electrode EF reactor because of the inhibition of the excess electro regenerated Fe2+ with Fenton’s reaction. Aside from this, application of electric current greater than 0.8A/L for both of the EF reactors also resulted to a 100% aniline removal efficiency. However increasing the initial aniline concentration at constant H2O2 and Fe2+ concentrations significantly decreases the treatment efficiency of both of the EF reactors. In addition to this, the change in H2O2, Fe2+ and initial aniline concentration affects the aniline removal efficiency significantly than the change in electric current. 4. Increase of H2O2, Fe2+ and electric current increases the initial aniline degradation rate. However further increase of the Fe2+ concentration greater than 1.07mM had no further effect on the initial aniline degradation rate. An increase in the initial aniline concentration significantly decreases the initial aniline degradation rate. Both of the EF reactors have nearly the same initial aniline degradation rates at all experimental conditions used. In addition to this, the change in Fe2+ and initial aniline concentration

57

affects the initial aniline degradation rate significantly than the change in H2O2 and electric current.

5. The kinetic model for aniline degradation using the EF reactors followed the second order behavior. The rate constants were determined to increase as the concentrations of H2O2, Fe2+ and electric current were also increased. However further increase of the applied electric current to 0.8A/L for the rod type electrode EF reactor and 1.8A/L for the plate type electrode EF reactor resulted into a significant decrease in the rate constant. At H2O2 concentrations lower than 29mM, both of the EF reactors had the same rate constants. However at high H2O2 concentrations, the rod type electrode EF reactor have lower rate constants due to the higher competition of aniline degradation by products with hydroxyl radical. At all Fe2+ concentrations used in the study the plate type electrode EF reactor have higher rate constants. However at low applied electric currents, it had lower rate constants due to insufficient amounts of electro regenerated Fe2+. In addition to this, both of the EF reactors have the same rate constants at an initial aniline concentration greater than 15mM. Fe2+ and initial aniline concentration had a greater effect on the rate constant than the change in H2O2 and electric current. 6. An increase in the H2O2 and Fe2+ concentrations was determined to increase the COD removal efficiency of both of the EF reactors. However further increase in Fe2+ and electric current greater than 1.07mM and 0.4A/L had no further effect on the COD removal efficiency. At low H2O2 concentrations, the rod type electrode EF reactor had lower COD removal efficiencies due to inhibition of excess electro regenerated Fe2+. Both of the EF reactors have nearly the same COD removal efficiencies at the Fe2+ concentrations and electric currents used in the study. However an increase in the initial aniline concentration significantly decreases the COD removal efficiencies. The change in Fe2+, H2O2 and initial aniline concentration significantly affects the COD removal efficiency than electric current. 7. For both of the EF reactors, an increase in the concentrations of H2O2, Fe2+, electric current and initial aniline concentrations significantly decreases the H2O2

58

efficiency for aniline and COD removal. Both of the EF reactors have the same H2O2 efficiencies at all concentrations of Fe2+, electric currents, and initial aniline concentrations used in the study. However at very low H2O2 concentration, the rod type electrode EF reactor had a higher H2O2 efficiency for aniline removal. However at H2O2 concentrations lower than 58mM, the rod type electrode EF reactor had lower H2O2 efficiencies for COD removal. 8. Aside from this, an increase in H2O2 and Fe2+ concentration lowered power consumptions for aniline and COD removal for both of the EF reactors. However further increase of H2O2 greater than 58mM and Fe2+ greater than 1.61 mM had no further effect on power consumption due to decreased aniline and COD removal efficiencies. The rod type electrode EF reactor consumes twice as much power for aniline and COD removal than the plate type electrode EF reactor. In addition to this, COD removal requires twice as much power than aniline removal due to its lower COD removal efficiencies. Increase in applied electric current increases the power consumption for both of the EF reactors. However the difference in the power consumption between the rod type electrode and plate type electrode EF reactors also increases by 1.7 to 3.3 times. An increase in the initial aniline concentration for plate type electrode EF reactor had no significant effect on the power consumption for aniline and COD removal.

9. Both of the EF reactors have the same aniline and COD removal efficiencies when a 10,000ppm COD of aniline solution was treated. Treatment of the high concentration aniline solution resulted into an increased BOD and lowered toxicity of the synthetic wastewater for both of the EF reactors. Therefore further treatment of the wastewater through biological treatment systems is feasible.

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VI. RECOMMENDATIONS FOR FUTURE WORK

The areas for future work identified during the conduct of this study are the following:

1. Analyses of Total Organic Carbon (TOC) will investigate the aniline degradation pathway by using the Average Oxidation States (AOS) index.

2. Scale up of the rod type electrode and plate type electrode EF reactors will determine feasibility of full scale wastewater treatment applications.

3. Use of step wise addition of H2O2 and determine its effect on the H2O2 efficiency for aniline and COD removal using the rod type electrode and plate type electrode EF reactors will determine optimum H2O2 usage for pollutant degradation. 4. Investigating the effect of anions and cations typically present in a real wastewater stream using the laboratory scale rod type electrode and plate type electrode EF reactors will establish the feasibility of real wastewater treatment applications.

5. Investigating the use of time delay in the electric current application for the electro-Fenton reactors and its effects on the power consumption for aniline and COD removal will determine optimum power usage and conservation.

6. A study on an electro-Fenton system coupled with a biological treatment system will verify the feasibility of further degradation of byproducts through biological treatment systems.

7. Research on the use of UV in an electro-Fenton system will determine its effect on the treatment performance efficiency.

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APPENDIX D Photographs of the electro-Fenton Reactors

D-1: Plate type electrode electro-Fenton Reactor.

The plexiglas rectangular vessel with the electrodes and mixers attached.

The electro-Fenton reactor attached to the power supply.

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D-2: Rod type electrode electro-Fenton reactor.

The cylindrical, stainless steel vessel with the electrodes, mixer and power supply.

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