Phosphate Removal From Aqueous Solution

PHOSPHATE REMOVAL FROM AQUEOUS SOLUTION USING FLY ASH,BLAST FURNACE SLAG AND ELECTRO COAGULATION

Introduction 

Present in low concentration.

 Essential for the growth of Micro organisms.  Excess amount causes Eutrophication.  Harmful to aquatic life.  Depletion of dissolved oxygen.

Conventional methods 

Chemical treatment method  Reaction between aluminium and phosphate  Reaction between calcium and phosphate  Reaction between iron and phosphate



Biological treatment method

Case Studies 1) Lu S.G, et. al (2008) 3) Ensar Oguz (2004) 5) Nihal Bekthas et. al (2003)

Removal of phosphate from Aqueous solution using fly ash (source: Lu S.G, et .al 2008)

Materials and method High ,medium and low Calcium content. Coded as FA1,FA2,FA3. The pH is determined using pH meter. Particle size distribution is calculated by using sieving and laser beam technique.

Chemical composition of fly ash (Source Lu. S,G. et. al ,2008)

Parameter

FA 1

FA 2

FA 3

pH

10.9

10.9

12.1

SiO2(%)

49.51

47.23

43.06

Electric conductivity

1.14

1.06

5.64

Al2O3(%)

31.12

34.23

28

Fe2O3(%)

5.36

6.32

8.63

Ca O(%)

5.44

5.54

10.82

Density(gm/cm3)

1.93

2.20

2.29

Scanning electron microscopy of FA1 (Source Lu. S,G .et. al ,2008)

Scanning electron microscopy of FA2 (Source Lu. S,G. et. al ,2008)

Scanning electron microscopy of FA3 (Source Lu. S,G. et. al ,2008)

Batch adsorption experiment  0.5gm of fly ashes and acid treated fly ashes were weighed and equilibrated in 50ml centrifuge tubes containing 25ml of 100mg/l phosphate solution at room temperature.  The tubes were stopped and continuously shaken at a spinning speed of 120 cycles /min .  The shaking was interrupted momentarily at predetermined time interval for the tube to be removed.  The suspension was filtered to determine the concentration of phosphate

Results and Discussion

Mechanism of Phosphate removal  Adsorption and Precipitation.  High pH and calcium concentration are required.  Major form of precipitation is hydroxy apetite.

Variation of pH with phosphate added (Source Lu. S,G. et. al ,2008)

Variation of Calcium concentration with phosphate added (Source Lu. S,G et. al ,2008)

Phosphorus adsorption kinetics Freundlich model, q = k C1/n Langmuir model, C/q = C /Q m+1/ bQm

(5) (6)

Where C= Phosphate concentration in equilibrium (mg/l) Q= Amount of phosphate adsorbed/unit of sorbent Qm = Langmuir sorption maximum k, n = constants related to adsorption capacity b = constants related to binding strength of phosphate

Characteristic parameters and determination of coefficients of the experimental data according to Freundlich and Langmuir equations Adsorbent

Freundlich equation Langmuir equation 1/n k R2 Qm b R2

FA1

0.483

6.17

0.974 90.09 0.040 0.775

FA2

0.450 8.96

0.967 90.91 0.078 0.877

FA3

0.584 13.47 0.962 107.53 0.207 0.941

Variation of pH with time for FA1 (Source Lu. S,G. et. al ,2008)

Variation of pH with time for FA2 (Source Lu. S,G .et. al ,2008)

Variation of pH with time for FA3 (Source Lu. S,G. et. al ,2008)

Variation of Percentage of Removal with Time (Source Lu. S,G et. al ,2008)

Removal of Phosphate From Aqueous Solution Using Blast Furnace Slag (Source Ensar Oguz ,2004)

Materials and Methods  Iskenderun blast furnace slag is used.  Adsorption studies are carried at phosphate concentrations of 180 ppm.  Minimum equilibrium time of removal is 20 min.

(contd…)  Experiments were carried out at different temperature and pressure.  Phosphate is measured using calorimetric method.  Precipitated as phosphates of aluminium, calcium and iron.

Chemical composition of Iskenderun blast furnace slag (Source Ensar Oguz ,2004) Parameter SiO2 Fe2O3 Al2O3 CaO MgO SO3 Chloride undetermined

Component (%) 39.56 0.33 10.82 37.68 6.79 0.33 0.125 3.99

Results and Discussion

Variation of phosphate concentration and phosphate dose per kg blast furnace slag (Source Ensar Oguz ,2004)

Relation between Phosphate concentration and Phosphate dose absorbed per kg blast furnace slag (Source Ensar Oguz ,2004)

Effect of agitation rate on adsorption (Source Ensar Oguz ,2004)

Effect of temperature on adsorption (Source Ensar Oguz ,2004)

Effect of pH on adsorption (Source Ensar Oguz ,2004)

Nature of Slag  Before and after X-Ray Diffraction  Before and after Scanning Electron Microscopy

Scanning electron microscopy of blast furnace slag before adsorption (Source Ensar Oguz ,2004)

Scanning electron microscopy of blast furnace slag after adsorption (Source Ensar Oguz ,2004)

Removal of Phosphate from Aqueous Solution using Electro Coagulation (Source Nihal Bekthas et. al ,2003)

Materials and Methods  Phosphate removal using electrochemical reactor  No other chemicals are used.  Precipitated as phosphates of aluminium.

Electrochemical Reactor (Source Nihal Bekthas et. al ,2003)

Results and Discussion

Variation of Phosphate removal with initial pH (Source Nihal Bekthas et. al ,2003)

Variation of Phosphate removal with time and initial concentration (Source Nihal Bekthas et. al ,2003)

Variation of charge loading with time and initial concentration (Source Nihal Bekthas et. al ,2003)

Variation of percentage of phosphate removal with current density and initial concentration (Source Nihal Bekthas et. al ,2003)

Comparison

Parameters

Phosphate removal using fly ash

Phosphate removal using blast furnace slag

Phosphate removal using electro coagulation

pH

High (9.4-11.6)

Low(3-4.5)

Medium(6-6.9)

Adsorption isotherm

Freundlich isotherm

Freundlich isotherm Not mentioned

Time for maximum removal

Within 5minutes (95.6%)

Within 60 minutes (99%)

Depends upon concentration and current density

Grain size of adsorbent

Small (>150micro meter)

Large (0.020.03mm)

Not mentioned

Form of precipitate

Phosphates of calcium

Phosphates of Phosphates of aluminium, calcium aluminium and iron

Removal efficiency

95.6% (within 5 minutes)

99% (within60 minutes)

Depends upon time, concentration and current density)

Temp

Not mentioned

Different temperature(25-65)

21-22 degree celcious

Conclusion Using fly ash maximum removal efficiency obtained within 5 minutes. Removal of phosphate occurs mainly by the formation of calcium phosphate. Removal efficiency depends upon the calcium ion concentration in the fly ash. Availability of fly ash makes this method more economic. Availability and low cost are the main advantages by using blast furnace slag. In the method of blast furnace, slag removal efficiency increases with temperature, time and concentration

(contd….) Maximum efficiency occurs within 60 minute, it is a main drawback compared to other methods In electro coagulation removal is carried out by using electro chemical reactor. Phosphate is removed as precipitates of aluminum As the three methods of phosphate removal have some limitations, fly ash could be a promising solution to the removal of phosphate in the wastewater treatment and pollution control.