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International Journal of ChemTech Research CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.9, No.10 pp 166-176, 2016
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Removal of lead ions (pb+2) from a synthetic wastewater
by electrocoagulation using aluminum (Al) as a rotating electrode
Rawaa Zahd Jafat1 and Sata Kathum. Ajjam2*
Chemical Engineering Department, University of Babylon, Babylon, Iraq.
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Abstract : The performance of the lead electrocoagulation process was tested for five major factors that affected the process which were: different initial lead concentration (200,300,400,500)mg/different applied voltage(2.5,5,7.5,and10)V, and distance between the electrode (2.3,2.8)cm. different rotational velocity of anode (0, 50,100, and 150) rpm, variable pH (5, 7, 9, and 11) and time(5,10,15,20,25,30).The results showed that the removal rate of lead (removal efficiency) decreased with increasing concentration at concentrations (200, 300,400and 500) mg/l. Also removal efficiency, increased with increased applied voltage and reach to a maximum efficiency value at 10V, but decreased with increasing distance ,while rotating anode velocity doesn't fix that is a stat with increased with low velocities (0-50)rpm and reached to higher efficiency at (100)rpm, while at high velocity(150-200)rpm the removal efficiency start in decrease gradually due to the destabilization of flocks that is formed While pH shows peak performance curve. It shall be the highest removal efficiency rate of acidic (7). The optimum removal efficiency of 99 % was achieved at concentration 200mg/l and at a voltage of 10V with 2.3cm spacing between the electrodes and rotational velocity= 100rpm and pH = 7,using (Al/St. St. ) electrodes, within 20 min of operating time. The lead removal data has been used to find adsorption isotherm.
Keywords: Electrocoagulation, lead removal efficiency, Wastewater, rotating anode.
Introduction
The major challenges for the 21st century are water and energy. Due to increased pollution from point and non-point sources quality of the water become a crucial problem, particular for the Third-World Countries1. Wastewater can be defined as the flow of used water discharged from homes, businesses, industries, commercial activities and institutions which are directed to treatment plants by a carefully designed and engineered network of pipes. There are many sources of water pollution, but two general categories exist: direct and indirect contaminant sources. Direct sources include effluent outfalls from industries, refineries, waste treatment plant, etc. Indirect sources include contaminants that enter water supply from soil/groundwater system and form atmosphere via rain water2. Due to the discharge of large amounts of metal-contaminated wastewater, industries bearing heavy metals, such as Cd, Cr, Cu, Ni, As, Pb, and Zn, are the most hazardous among the chemical intensive industries3.Heavy metals are elements having atomic weights between 63.5 and 200.6, and a specific gravity greater than 5.0. Heavy metals are the environmental priority pollutants and are becoming one of the most serious environmental problems. So these toxic heavy metals should be removed
from the wastewater to protect the people and the environment4.Water pollution by heavy metal ions has become one of the world wide environmental problems due to population explosion, urbanization and industrialization5.
Electrocoagulation Mechanism
Electrocoagulation involves three major mechanisms6,7
The destabilization mechanism of the contaminant, particulate suspension, and breaking of emulsion may be summarized as follows6.
Reactions that occurs at the electrodes surface
Electrocoagulation electrochemically introduces metal cations in situ, usually aluminum9.
At the anode the reaction is
( ) ( ) ( )
At the cathode side, hydrogen gas (H₂) and the hydroxyl ion (OH⁻) are generated by reducing the water.
( ) ( ) ( ) ( )
II.Experimental
Electrolytic Cell Design (Electrochemical Cell)
The experiments were performed in transparent Pyrex glass of 1000ml in volume, the electrolytic cell consisting of the following parts. See Figure (1) for electro-coagulation process.
The detail of the electrochemical technique consists of two electrodes:
Counter Electrode (rotational anode)
The anode was a rotating cylinder made of aluminum .
Working Electrode (cathode)
The two cathode made of stainless steel was used.
Batch mode of electrocoagulation operations.
The associated schematic diagram of the equipment is shown in Figure 1
Fig.1: Schematic diagram of electrocoagulation (EC) rotating anode
III. Results and Discussion
The effect of initial pH on the removal efficiency of Lead Ions
The pH of solution plays an important role in electrochemical and chemical coagulation process10.The influence of pH on electrocoagulation of lead is shown in figure (2– 5) .The solutions were adjusted to the desired pH for all experiments using sulfuric acid or sodium hydroxide. Fig 6, this figure shows lead removal efficiency as a function of pH and various rotating velocity of anode, the maximum lead removal efficiency was obtained at optimum time of 20min, within pH of 7, and 100rpm, this it can be concluded that the majority of Al+3 coagulants are formed at this pH. So PH of 7 was used in other experiments. However, there is no significant effect of time after 20 min for pH of 7, so it is not feasible to use long time period of treatment because it leads to high power consumption. It can be also showed that for all concentration the rotational velocity not stable, and was seen that it is once low rotating velocity is best and once other that high velocity is preferred, and notes that 100rpm is the best velocity for all time, and all concentration.
Fig 2: Effect of pH on the removal efficiency of lead at different Rotational (time, 5min)
Fig 3: Effect of pH on the removal efficiency of lead at different Rotational velocity (time, 10min).
Fig 4: Effect of pH on the removal efficiency of lead at different Rotational velocity (time, 15min).
Fig 5: Effect of pH on the removal efficiency of lead at different Rotational velocity (time, 20min)
The effect of applied voltage on the removal efficiency of lead ions
The influence of applied voltage on the removal efficiency of lead is shown in figure [ 6 to 9] for 200mg/l. The Current density increase with the voltage increase, Current density directly determines both coagulant dosage and bubble generation rates and strongly influences both solution mixing and mass transfer at the electrodes. From Fig.8, it can be seen that removals are increased when voltages applied increased; this is attributed with research of11 that reported increase the voltage may increase the removal of heavy metal. When applied potential rate is increased from (7.5- 10)volt, lead removal efficiency increased from (97 - 99)% and then decreased from (5 - 2.5)volt about (92 - 88) % ,of the optimum value is 10Volt.
Fig 6 :Effect of applied voltage on the removal efficiency of lead at different time (rotating anode velocity, 0rpm).
Fig 7: Effect of applied voltage on the removal efficiency of lead at different time (rotating anode velocity, 50rpm).
Fig 8 :Effect of applied voltage on the removal efficiency of lead at different time (rotating anode velocity,100rpm).
Fig 9 :Effect of applied voltage on the removal efficiency of lead at different time (rotating anode velocity,150rpm).
The effect of the rotational anode velocity on the removal efficiency of lead ions
The mixing speed is an important operating factor influencing the performance of electrocoagulation process. To examine its effect on the removal of lead, the rotating anode speed was varied in the range of (0–200) rpm, were studied at a constant voltage for 10V for pH 7 and 2.3cm , different time .The influence of rotating anode velocity on the removal efficiency of lead is shown in Fig.10.
Fig 10. Effect of rotating velocity on removal efficiency of lead ion.
It is noted from this figure that increasing rotating anode velocity of the solution (0-50) rpm leads to the removal efficiency increased this speed did not supply a homogeneous mixture in the cell .And shows a maximum for rotating anode speed (50-100) rpm and then it decreases smoothly with increasing rotating anode velocity (100-150) rpm, coagulant matter formed of aluminum ions, attached together and disperse in the cell making the content of the cell homogenous. However, decrease in the efficiency at higher stirring rate (200) rpm may be due to the destabilization of flocks formed in the cell. So the best rotating anode velocity was obtained at 100 rpm. Graphical results shown the best rotating velocity is 100rpm for all concentrations, and this also showed that the flocks deposited between electrodes, because the flocks couldn’t mix homogeneously and this deposition caused to the increment of cell resistance at low stirring speed. The increase in the cell resistance causes the increase of potential value in the systems where constant current density and this causes the increase of the amount of energy consumption per unit volume. The increased in rotating speed from (0-50-100), this confirms the fact that the removal efficiency is diffusion controlled, and the increase in stirring speed leads to increase in the intensity of turbulence and reduces the diffusion layer thickness at the electrode surface and Improves the mixing conditions in the electrolyte bulk ,This is attributed with the work of12.
Fig 11: Effect of rotating anode velocity on the removal efficiency of lead with different times
The effect of initial lead ions concentration on the removal efficiency
The effect of initial concentration on electrocoagulation of lead ions Shown graphically in Fig.12 represented optimum condition for concentration effect. It is clear from this Fig, higher concentrations greater time is needed for the removal of lead ions at different rotational anode velocity, but higher initial concentrations of lead were reduced significantly in relatively less time compared to lower concentrations. This can be explained as follows: (1) from faraday‘s law (W=I*e*t/n*F), where a constant amount of Al+3 passed into solution at the same current density and time for all lead concentrations, Al+3 was insufficient for solutions including higher lead concentration ,therefore the same amount of flocks would be produced in the solution.(2) Although the same amount of the coagulant produced in the electrocoagulation cell at the same current density for different lead concentrations, this amount of coagulant species. This result came in attributed with other researchers13, because of increasing lead concentration, the applied potential for the solution and the consumed energy was decreased as well.
Fig 12 :Effect of initial concentration on removal efficiency of lead (rotating anode velocity, 100rpm)
The effect of distance between electrodes on the RE (removal efficiency)
Fig.13 represented the preferred operating condition (100rpm, 200 mg/l, pH 7,and 10volt of applied voltage) for the effect of distance.
Fig 13: Effect of the distance on removal efficiency of at different time (rotating anode velocity=100rpm).
In the EC operation, the ions produced from the cathode and anode collects to form flocks. The travelling time of the participating ions in the EC reaction decreases with a decrease in the inter electrode distance, resulting in a higher removal efficiency for a lower duration electrolysis14.According to ohmʼs law, the value of the electric current during a metal conductor in a circuit is directly proportional to the voltage impressed across it, for any given temperature.
The effect of treatment time
It was confirmed that the best condition for lead removal was at the sample original pH 7.0 with voltage 10volt , initial lead ions concentration 200 mg/l with distance between electrodes 2.3 cm and rotating anode velocity 100rpm form (5-20) min of operating time, Fig .14 illustrate Pb+2 removal efficiency versus time for these conditions. It shows that when treatment time is increased, removal efficiency also increases.
Fig 14 :Effect of time on the removal efficiency of lead.
Studies of Adsorption isotherms modeling
The effect of pH, on the adsorption of lead ions by electrocoagulation technique. This was done at pH values, (5-11), for a particular study, a parameter was varied while the others were kept constant.
Adsorption dependence on PH
Table (1) Isotherm Constant and correlation coefficients at different pH for lead ion removal.
pH |
Langmuir Isotherm |
Freundlich Isotherm |
||||
Qm (mg/g) |
kL (L/mg) |
R2 |
KF(mg/g)(L/mg)1 /n |
n |
R2 |
|
5 |
312.5 |
0.0095 |
0.9607 |
10.8 |
1.76 |
0.9472 |
7 |
769.39
|
0.0016 |
0.9345 |
1.303 |
1.02 |
1 |
9 |
416.6
|
0.0202 |
0.9345 |
22.9 |
1.9 |
0.9793 |
11 |
175.43 |
0.0867 |
0.9913 |
3.109 |
1.28 |
0.9552 |
The R2 values indicating that the experimental data can be better explained by the Freundlich Isotherm than the langmuir. The KF value as calculated from the Freundlich isotherm.Table (2) shows the RL values against the initial lead concentration.
Table (2) Essential Characteristics of Langmuir Isotherm at different pH.
Co(mg/l) |
RL |
|||
pH |
||||
5 |
7 |
9 |
11 |
|
200 |
0.3448 |
0.7496 |
0.1984 |
0.0545 |
300 |
0.2597 |
0.6662 |
0.1416 |
0.0370 |
400 |
0.2083 |
0.5995 |
0.1101 |
0.0280 |
500 |
0.1739 |
0.5449 |
0.0900 |
0.02254 |
RL values between o and 1 indicate that the sorption of lead ion is highly favorable, moreover ,the sorption is favorable for both low and high initial Concentration as the values of RL are very close to zero.
The initial pH of a solution is a very important factor to be considered in adsorption studies as it has been observed to play a major role in the adsorption of metal ions by various adsorbents15.The result illustrating the dependence of pH on adsorption of lead is presented in Fig.14 Maximum adsorption was obtained in the pH range of (5-11) and its increased as pH values increased from 2 to 7. Indeed, the Maximum adsorption, qmax, of Pb (II) ions was attained at a pH value of 7Indicating very strong capacity for lead ions removal at pH equal to 7 that is alkali solution as compared to 9 and 11.This can be explained that at low pH values the solution is highly acidic. This led to the competition between lead ions and protons for the active sites resulting in a low adsorption capacity. Subsequently, as the pH of the solution increased, the number of protons decreased which reduced the competition between lead ions and protons for the active sites, hence more lead ions were adsorbed from solution16.
Fig.15: Effect of pH on adsorption of lead (rotating velocity, 100rpm, applied voltage, 10volt).
V. Conclusion
Experiments have been carried out to determine the best operating conditions
VI- References
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