Ii Treatment

Chemosphere 46 (2002) 49±57

www.elsevier.com/locate/chemosphere

Treatment system for solid matrix contaminated with ¯uoranthene. II±±Recirculating photodegradation technique Abdellah Rababah *, Sadao Matsuzawa Atmospheric Environmental Protection Department, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 11 December 2000; received in revised form 15 March 2001; accepted 15 March 2001

Abstract A laboratory-scale solar reactor and photodegradation technique were developed to enhance the degradation process of ¯uoranthene. Fluoranthene was used in this study to represent toxic polycyclic aromatic hydrocarbons (PAHs) that are persistent in the environment. The extracted ¯uoranthene from soil in organic solvent (EFOS) and hydrogen peroxide (H2 O2 ) were pumped from a 100 ml vessel into a solar glass cell coated with titanium dioxide …TiO2 † at 80 ll min 1 . This work compares the eciency of the developed photocatalytic degradation technique with the conventional batch process. The degradation eciency of the developed technique was assessed at di€erent initial concentrations of ¯uoranthene and percentages of H2 O2 in the extract using di€erent ¯ow rates. Preliminary results indicated that the developed technique degraded 99% of ¯uoranthene from EFOS in the presence of H2 O2 and 83% without H2 O2 . There was no signi®cant di€erence between ¯uoranthene degradation rates by the developed technique and the batch method. The developed technique however, treated double the volume of solution that was treated by the batch reactor method which was time consuming and required continuous attention. Ó 2001 Published by Elsevier Science Ltd. Keywords: Soil; Titanium dioxide; Hydrogen peroxide; Solar degradation; PAH; Fluoranthene

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) adsorption onto organic particles of sediments can cause carcinogenic PAHs to reach much higher concentrations in the bottom±sediment compartment than in the upper water column (Vigan, 2000). PAHs are introduced to animals and humans from the food chain through the digestion system (Boese et al., 1998; Nilsen et al., 1999; Sram et al., 1999; Wang et al., 1999). Clarke et al. (2000) determined high risk posed by consumption of seafood/

*

Corresponding author. Tel.: +81-298-61-8265; fax: +81298-61-8258. E-mail address: [email protected] (A. Rababah).

marine prey species contaminated with ¯uoranthene to humans and to the Chinese White Dolphin (Sousa Chinensis). Such toxicity was enhanced with UV exposure (Hatch and Burton, 1999; Monson et al., 1999). For example, Cole et al. (2000) reported that the toxicity of ¯uoranthene, at concentration below 50 lg g 1 (dry weight) in sediments to marine infaunal amphipod Rhepoxynius abronius was enhanced by UV irradiation during 27±83 days of exposure. Monson et al. (1999) correlated an increase in death of Rana pipiens larvae (96±118 h old frogs) with ¯uoranthene concentration and light exposure duration. Other researchers (Driscoll et al., 1998; Lotufo, 1998; Raszyk et al., 1998; Clement et al., 1999; Hellou et al., 1999) reported that ¯uoranthene accumulated in the environment at concentration of potential risk on aquatic and land life along with risk on human health.

0045-6535/02/$ - see front matter Ó 2001 Published by Elsevier Science Ltd. PII: S 0 0 4 5 - 6 5 3 5 ( 0 1 ) 0 0 0 9 0 - X

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The identi®cation of ¯uoranthene as a priority pollutant of potential carcinogenic risks, by the United States Environmental Protection Agency (USEPA, 1985) and the European Communities (European Communities, 1996) plan to regulate environmental levels for PAH, necessitate the need for developing new degradation techniques and improving existing methods. Di€erent degradation techniques may be used for the remediation of PAH-contaminated matrices. Suitable degradation technique however, depends on many factors, such as required duration of treatment, cost, type of environmental matrix, site sensitivity, climate, PAH molecular weight and concentration. The most common degradation techniques that have been widely investigated are hydrothermal, biodegradation, phytoremediation and photodegradation. In hydrothermal treatment, the contaminated matrix is treated with water or CO2 under supercritical (>350°C) conditions. Shanableh (2000) has shown that hydrothermal technique removed more than 95% of sewage sludge organic matter. This technique however, may be limited by the high cost of maintaining supercritical conditions and reactor's material that must withstand the high pressure and temperature. Phytoremediation technique utilizes plants for contaminants removal. Liste and Alexander (2000) have shown that plants promoted the removal of 74% of pyrene from vegetated soil within eight weeks compared to less than 40% removal from unplanted soil. It is generally accepted that marine sediments and soils contain microorganisms able to biodegrade a wide range of contaminants by using these contaminants as a source of carbon and energy, therefore preventing accumulation of pollutants in the environment. Biodegradation may be achieved under aerobic or/and anaerobic conditions. Aerobic biodegradation is more favorable, because it is faster than anaerobic biodegradation. Joshi and Lee (1996) have shown that Acinetobacter sp. biodegraded more than 80% of PAHs form contaminated soil aerobically, after seven days in controlled laboratory conditions. Rockne and Strand (1998) reported ecient anaerobic biodegradation of naphthalene, phenanthrene, and biphenyl in a ¯uidized bed reactor (FBR) enrichment after 100±200 days. Nonetheless, microorganisms may be at risk from the high levels of PAHs in soils and sediments under certain environmental conditions (e.g., UV exposure). Moreover, PAHs are quite resistant to degradation. Their half-lives in soils may reach 28 years depending on the compound (Trapido, 1999). The fact that ¯uoranthene is a high molecular weight PAH which is dominant and persistent in sediments and soils (Zhou et al., 1998; Hellou et al., 1999; Trapido, 1999) indicates that biodegradation may not be an ideal treatment option in some cases. Titanium dioxide …TiO2 † has been demonstrated to be photocatalytically active in the presence of sunlight

for a large variety of chemicals. Moreover, TiO2 is stable in acidic and basic media and non-toxic. Therefore, it is the most widely used photocatalyst for organic photodestruction despite the fact that it utilizes only 3±4% of the energy in the solar UV spectrum (300±400 nm) (Vidal, 1998). Das et al. (1994) have shown ecient photocatalytic oxidation of acenaphthene, anthracene, ¯uorene and naphthalene in aqueous suspensions of TiO2 on irradiation with a 500 W superhigh pressure mercury lamp in one experiment and sunlight in another experiment. Nogueira and Jardim (1996) irradiated dichloroacetic acid-contaminated water (5 mmol l 1 ) ¯owing on an inclined TiO2 -coated glass plates at 2±6 l h 1 with solar light and achieved 2 mmol min 1 degradation rate. Processes combining UV radiation and hydrogen peroxide show in most cases high ecacy to eliminate the parent compound and to form simpler oxygenated molecules that are easier to degrade because of the generation of the non-selective hydroxyl radical, the principal oxidizing agent in this type of treatment (Rivas et al., 2000). Furthermore, advanced oxidation processes based on hydroxyl radical chemistry and photocatalysis, can eciently oxidize many organic compounds (Engwall et al., 1999). Hydrogen peroxide has the ability to vigorously oxidize organic compounds, even those with an aromatic moiety. The basic oxidation reaction is described in Eq. (1) (Kawahara et al., 1995). H2 O2 ‡ Cn Hx ! xH2 O ‡ nCO2

…1†

Hence, this work focuses on developing recirculatingtype photocatalytic technique assisted by the oxidizing agent H2 O2 for solar light utilization in ¯uoranthene degradation at low operational costs. The degradation eciency of the developed technique at di€erent operational conditions was evaluated here in comparison to the batch reactor method.

2. Experimental 2.1. Standards and reagents Fluoranthene standard solutions were prepared by diluting ¯uoranthene crystals (98%) (Aldrich, USA) in acetonitrile (99.8%) (Wako Pure Industries, Japan). Nitric acid solution 61%, (speci®c gravity (d) ˆ 1.38; Wako Pure Industries, Japan) was used for dispensing and coating glass with titanium dioxide. Ethanol 99.5% and cyclohexane 99.7% (Wako Pure Industries, Japan). Cyclohexane:ethanol (3:1) (CH:ETOH) mixture was used for extraction. The reasons for selecting CH:ETOH mixture are outlined in an accompanying paper (Rababah and Matsuzawa, 2001). Titanium dioxide (TiO2 ) (P-25, Japan Aerosil) was used as a photocatalyst. Aqueous

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solution of H2 O2 30% (Wako Pure Industries, Japan) which yields OH radicals by photolysis was used to accelerate the photocatalytic degradation process. 2.2. Extract preparation Soil sample extracts were prepared and treated as described in (Rababah and Matsuzawa, 2001). Brie¯y, the collected soil was cleaned then spiked with ¯uoranthene 19:4 lg g 1 dry weight. About 200 mg of this spiked soil sample, 22.5 ml of cyclohexane and 7.5 ml of ethanol were mixed and ultrasonicated at 40°C for 20 min by a 200 W/20 kHz Branson Soni®er in the extraction chamber. Then, the extract was collected into a 100 ml glass container (bu€er tank) after 10 min of settling. This procedure was repeated three times. 2.3. Recirculating-type photodegradation reactor The photodegradation system consists of bu€er tank, solar cell, solar reactor, pump, Te¯on tubes and a recirculating tank as can be seen in Fig. 1. The 50 mm in diameter solar cell was made from transparent Pyrex. The inner surfaces of the bottom and the wall of the cell were mechanically roughened and dried at 400°C for 2 h to clean the roughened surfaces. The dispensed TiO2 in 0.01% nitric acid solution (10 mg ml 1 ) was pipetted on the rough surfaces of the cell. The coated surfaces were annealed at 400°C for 2 h. This procedure was repeated three times. The coated solar cell was left to cool down to room temperature. Cyclohexane, ethanol, dichloromethane, methanol, methyl ethyl ketone, n-propyl acetate and toluene were pumped over the coated TiO2 layer each at time for 2 h. The TiO2 layer was not washed by the dynamic solvents and was well ®xed onto the rough glass surfaces. The solar reactor (Bunko Keiki, Japan) is equipped with a 300 W xenon lamp (98 mW cm 2 ). The solar cell was placed inside the solar cell chamber where the incoming light intensity of AM 1.5 is ®ltered by a water cell situated between the xenon lamp and the solar cell.

Fig. 1. Experimental recirculating-type photodegradation system (not to scale).

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Water at 20°C was recirculating around the solar cell to minimize heat e€ect (Fig. 2). The ¯ow rate of the Roller pump (Furue Science, Japan) ranges between 0.018 and 25 ml min 1 . The pump was selected so that the extract mix does not come in contact with its mechanical parts. The Te¯on tubes delivered the solvent from the extraction chamber to the bu€er tank, solar cell and recirculating tank.

2.4. Operational principles of the photodegradation reactor 2.4.1. Sample preparations for irradiation trials The extracted ¯uoranthene from soil in organic solvent (EFOS) was transferred into two groups of vials. The EFOS samples in one group of the vials were mixed with 30% H2 O2 . The ®nal solution contained 3% H2 O2 . The EFOS samples in the second group of the vials were not mixed with H2 O2 .

2.4.2. Irradiation trials The two prepared EFOS samples were irradiated on a TiO2 -coated solar cell in one experiment and on a not

Fig. 2. Solar cell and solar cell chamber inside the developed solar reactor (not to scale).

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coated solar cell in another experiment for comparison. The irradiation trials were conducted in the solar reactor using batch and continuous techniques as follows: 1. Batch technique. The xenon lamp of the solar reactor was switched on and stabilized while the solar cell was loaded with one of the prepared EFOS samples. Each loaded cell was placed in the cell chamber (Fig. 2) of the solar reactor and irradiated for 6 h. The depth of the 10 ml mixture inside the cell varied between 2 and 3 mm. The percentage degraded from ¯uoranthene was calculated using the concentration in solution before and after irradiation. 2. Recirculating technique. The xenon lamp of the solar reactor was switched on and stabilized. Each of the prepared EFOS samples was recirculated through the solar glass cell at 80 ll min 1 for 6 h. The cell was located in the cell chamber of the reactor as in the batch technique. The selected depth for the ¯ow inside the solar cell ranged between 2 and 3 mm. In this developed technique, the contact between ¯uoranthene molecules and the TiO2 layer was optimized by the shallow depths inside the solar cell. The percentage of degraded ¯uoranthene was calculated based on ¯uoranthene concentration in solution before and after irradiation. 2.4.3. Fluoranthene measurement by HPLC Fluoranthene concentrations were measured as described in Rababah and Matsuzawa (2001). Following standards solution injection, 2 ll of the irradiated samples was injected. Each sample was injected ®ve times.

3. Results and discussion Fig. 3 shows degradation eciencies of ¯uoranthene obtained by using the recirculating-type solar degradation system under di€ered operational conditions. Based on the null hypothesis test (Student's t-test, Miller and Miller, 1993) there was no signi®cant di€erence (P ˆ 0:05) between ¯uoranthene degradation eciencies of the batch and recirculating techniques under similar H2 O2 and/or TiO2 -utilization conditions (Fig. 3). The recirculating-type solar system however, treated 20 ml of the EFOS while the batch-type solar system treated 10 ml during 6 h of irradiation. The single factor ANOVA test (Miller and Miller, 1993) showed that the photodegradation eciencies of ¯uoranthene in the presence of TiO2 , H2 O2 , TiO2 =H2 O2 and without adding any agent di€ered signi®cantly (P ˆ 0:05). There was no signi®cant di€erence (P ˆ 0:05) between ¯uoranthene degradation eciencies for the TiO2 -coated solar cell and the non-coated cell, when the H2 O2 -EFOS mix was irradiated. The photodegradation eciency in the presence of H2 O2 however, was signi®cantly (P ˆ 0:05) higher than the photodegradation ef®ciency when H2 O2 was not mixed with the EFOS. H2 O2 poorly absorbs light of wavelength …k† > 300 nm compared to light of shorter wavelength (Vidal, 1998). Nonetheless, this study showed 99% ¯uoranthene degradation eciency when irradiated with light of k > 290 nm in the presence TiO2 and H2 O2 , 97% in the presence H2 O2 and 83% in the presence of TiO2 (Fig. 3). Fluoranthene did not degrade when mixed with H2 O2 (3% in the ®nal solution) and kept in the dark for 10

Fig. 3. Fluoranthene degradation eciencies by the developed recirculating solar reactor and batch reactor in the EFOS under different conditions (n ˆ 5, error bar ˆ 1 standard deviation).

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days. It can be concluded from these results that irradiation is necessary for ¯uoranthene degradation. However, the mechanism of ¯uoranthene photodegradation under the laboratory conditions of this study is not clear. A charge separation process takes place when irradiating TiO2 with solar light as described in Eq. (2) (Vidal, 1998). TiO2 ! h‡ vb ‡ ecb ;

…2†

where h‡ vb and ecb are valence band holes and conduction band electrons, respectively. The h‡ vb can be used to oxidize H2 O molecules to hydroxyl radicals (OH) which are short lived, extremely potent, very reactive and non-speci®c oxidizing agents capable of oxidizing organic compounds (Kawahara et al., 1995; Vidal, 1998). Furthermore, direct photolysis of H2 O2 by UV irradiation (254 nm) produces hydroxyl radicals (OH) and hydroperoxy radicals (HO2 ) according to the following Eqs. (3)±(5) (Rivas et al., 2000): H2 O2 ‡ hm ! 2  OH H2 O2 ‡ OH ! HO2  ‡ H2 O HO2 ‡ OH ! HO2  ‡ OH

…3† …4† …5†

The generation of radicals (OH or/and HO2 ) in suf®cient quantities enhances ¯uoranthene degradation. The eciency of the xenon lamp utilized in this work to generate radicals may be low, because the wavelength of the emitted light was >290 nm. Nonetheless, there is clear evidence that reaction of hydrogen peroxide and ferrous ions under solar light or >300 nm irradiation (photo±Fenton reaction) plays an important role in the degradation of toxic chemicals (Kong et al., 1998; Fukushima et al., 2000). The average concentration of iron in the model soil used here was 57 mg g 1 and most likely in the form of Fe3‡ because soil's pH is more than 6 (Kawahara et al., 1995). Iron in soils may react with H2 O2 to form ferrous ion (Fe2‡ ) according to Eq. (6) (Fukushima et al., 2000). Fe3‡ ‡ H2 O2 ! Fe2‡ ‡ HO2  ‡ H‡

…6†

2‡

The Fe ions interact with hydrogen peroxide to generate hydroxyl radicals according to the Fenton reaction that is described by Eq. (7) (Kawahara et al., 1995). Fe2‡ ‡ H2 O2 ! Fe3‡ ‡ OH ‡ HO

…7†

In addition to the above reactions, a complex set of further interactions may have taken place in the modi®ed solar reactor to generate active species that may have contributed to the enhancement of ¯uoranthene photocatalytic degradation, including the partial degradation without adding H2 O2 or TiO2 .

Fig. 4. Variation in concentrations of ¯uoranthene and byproducts with time. Fluoranthene initial concentration ˆ 10 5 M. HPLC chromatograms were measured at 288 and 462 nm excitation and emission wavelengths, respectively.

3.1. By-products of ¯uoranthene degradation Fig. 4 shows chromatographic peaks of ¯uoranthene along with those of unidenti®ed by-products before and throughout the irradiation process. The peak intensities of the detected by-products at 1.5 h irradiation time gradually decreased throughout the degradation process as can be seen in Fig. 4. This decrease in the intensities indicates that the by-products were being removed from the irradiated solution. They may have been photodegraded or transformed into gaseous products. It was dicult to collect gaseous products using the utilized solar reactor.

3.2. E€ect of ¯uoranthene initial concentration on degradation Fig. 5 shows the e€ect of the initial concentration of ¯uoranthene in the EFOS on the photodegradation ef®ciency of the developed technique. There was no signi®cant di€erence (P ˆ 0:05) among the degradation eciencies of the samples after 6 h of irradiation in the developed recirculating type reactor as can be seen in Fig. 5. This agrees with the results obtained by Vidal (1998) where almost all ethylbenzene photodegraded after 40 min of UV irradiation regardless of ethylbenzene initial concentration in water.

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Fig. 5. Fluoranthene degradation eciencies at di€erent initial concentrations by the developed recirculating-type solar reactor.

Fig. 6. The eciencies of ¯uoranthene degradation in the developed recirculating-type solar reactor with di€erent percentages of H2 O2 in the EFOS.

3.3. E€ect of H2 O2 concentration on ¯uoranthene degradation Fig. 6 shows the e€ect of H2 O2 percentage in the EFOS on ¯uoranthene degradation eciency. Fluoranthene degradation eciencies when 3% and 6% of H2 O2 dissolved in the EFOS were signi®cantly (P ˆ 0:05) higher than the degradation eciencies when the percentage of H2 O2 was 1.5% or 9% (Fig. 6). There was no signi®cant (P ˆ 0:05) di€erence between the degra-

dation eciencies of 3% and 6%. Hence 3% of H2 O2 in the EFOS was employed for the developed technique. Fig. 7 shows e€ect of irradiation time on ¯uoranthene degradation eciency. Rate of ¯uoranthene degradation eciency increased signi®cantly (P ˆ 0:05) after 3 h of irradiation as can be seen in Fig. 7. This tendency is applied to batch and recirculating techniques. Fig. 8 shows the e€ect of the EFOS ¯ow rate in the developed recirculating-type solar reactor on ¯uoranthene photodegradation eciency.

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Fig. 7. E€ect of irradiation time on degradation eciencies of ¯uoranthene.

Fig. 8. Fluoranthene degradation eciencies of the developed recirculating-type solar reactor at di€erent ¯ow rates of the EFOS.

About 55% of ¯uoranthene was degraded when the extract was recirculating at a relatively low rate through the developed solar reactor for 6 h (Fig. 8). The low degradation eciency at a slow ¯ow rate may be due to the relatively long retention time inside the pipe system where the solution was not exposed to solar radiation. Fluoranthene degradation percentage however, reached more than 99% when the ¯ow rate increased to 80 ll min 1 . There was no signi®cant di€erence (P ˆ 0:05) between the ¯uoranthene degradation eciencies at 80 and 100 ll min 1 .

4. Conclusions A recirculating-type photocatalytic technique was developed and its eciency to degrade ¯uoranthene from EFOS was evaluated as compared to batch solar methods. The mass of ¯uoranthene degraded by the developed recirculating-type system was larger than that degraded by the batch system for the same size of solar cell and irradiation duration. The batch technique needed continuous labor attention. The optimum operation conditions for the laboratory-scale recirculating-type

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solar system were observed at 80 ll min 1 ¯ow rate and 3% of H2 O2 in the EFOS. Nonetheless, the degradation eciency was not signi®cantly (P ˆ 0:05) a€ected by the initial concentration of ¯uoranthene when the EFOS was irradiated for 6 h. H2 O2 enhanced the ¯uoranthene degradation eciency (99%) when the EFOS ¯owed gently over a rough glass surface coated with TiO2 . The developed technique would be automated for ®eld applications to photodegrade ¯uoranthene without extensive labor attention. Further investigations are continuing to examine the application of the developed method for the detoxi®cation of PAHs in contaminated soils and sediments.

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