Electrokinetic Extraction of Heavy Metals from Contaminated Soils Using Ammonium Citrate
ELECTROKINETIC EXTRACTION OF HEAVY METALS FROM CONTAMINATED SOILS USING AMMONIUM CITRATE
P. Bala Ramudu
Lecturer, Department of Civil Engineering, Institute of Technology, Banaras Hindu University, Varanasi–221 005, India.
E-mail:
R.K. Srivastava
Professor, Department of Civil Engineering, Motilal Nehru National Institute of Technology, Allahabad–211004, India.
ABSTRACT: This paper presents the results of electrokinetic (EK) remediation experiments grouped for the individual as well as mixed-form of both cadmium and copper which were dosed at their maximum sorption on typical expansive clayey soil. Ammonium citrate (AC) as chelating agent was used to enhance the EK removal efficiency of cadmium and copper from artificially spiked contaminated soil at applied voltage of 35 V DC for a duration of 120 h. Results show that the current, electroosmotic flow, migration of Cd(II) and Cu(II) have been influenced by pH. The higher removal efficiency for cadmium and copper was observed in case of experiments run by ammonium citrate when it was used as both washing and flushing solution. The removal efficiency was found to be 48.9% & 28.9% for cadmium for individual as well as mixed cases and 30.0% & 29.8% in case of copper for individual and mixed cases respectively when ammonium citrate was used as both washing as well as flushing solution.
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Electrokinetic Extraction of Heavy Metals from Contaminated Soils Using Ammonium Citrate
1. INTRODUCTION
Contamination of soils due to unavoidable circumstances of industrial growth as well as environmental conditions is of increasing concern now a days. Cadmium and copper may enter water and soil from industries or waste disposal plants or from leaching from landfill sites. Soil contamination by cadmium has been occurring due to waste from Ni-Cd batteries, pigment coating and plating and plastic manufacturing industries. Similarly Copper and its compound are widely used by modern industries resulting in large quantities of copper being discharge into the environment. The primary sources of copper compounds are leather tanning and finishing, metal finishing and corrosion control, textile dyes, pigments and primer paints, fungicides and wood treatment. Remediation of cadmium and copper contaminated soils is necessary to reduce public health risk. Few remediation technologies are available for the removal of heavy metals from low hydraulic permeability soils i.e. clay. Electrokinetic remediation is one of the developing techniques that have significant potential for in-situ remediation of fine grained soils (Reddy et al., 2002). The EK process has received increasing attention across the globe (Acar & Alshawabkeh 1993, 1996; Reddy et al. 2001). The process involves the application of low level dc electric field across a porous medium. In this process, the major mechanisms leading to the removal of contaminants from soil include the elctroosmosis, electropherosis, electromigration and diffusion etc (Ottosen et al., 2001). The electroosmosis flow driven under an electric field causes the movement of H+ ions generated from H2O electrolysis at the anode advancing through soil toward the cathode, and the migration of charged ions towards the opposite electrodes. Cadmium and combination of Copper-Cadmium contamination of soil was done artificially.
2. MATERIALS AND METHODOLOGY
2.1 Materials
The materials used in this study are locally available soil, cadmium nitrate (Cd(NO3)2 4H2O) and copper sulphate (CuSO4.5H2O). For the present study, field soil was obtained from unpolluted agricultural field site near by Allahabad city (Uttar Pradesh, India) at a depth of 1-2 m. The sample was then hand sorted to remove any pebbles and vegetative matters present in it and was further washed through 0.425 mm sieve to obtain fine grained soil. Various physico-chemical characterization tests were carried out. Tested geotechnical parameters and their values are presented in Table 1. The major constituents of soil are quartz, feldspar, dolomite, calcite, kaolinite and illite (Bala Ramudu et al., 2008).
Initial Cd and Cu concentrations for both individual (Cd, Cu) and mixed (Cd+Cu) cases for EK remediation tests have been determined as 10.044 mg/g, 15.88 mg/g and 8.013 mg/g + 12.680 mg/g respectively which was assessed by soil digestion process conducted on soils already contaminated artificially for their sorption values. Figure 1 shows the scanning electron microscope (SEM) of Cd+Cu contaminated soil.
Table 1: Geotechnical Properties of Soil
Property / ValueLiquid Limit (%) / 45.8
Plastic Limit (%) / 17.0
Plasticity Index (%) / 28.8
Shrinkage Limit (%) / 12.8
Cation exchange capacity (USEPA) (meq/100 g) / 22
pH / 8.36
Organic Content (%) / 6.25
Loss on Ignition (LOI) (%) / 9.93
CaCO3 (%) / 2.5
Soil classification (USCS) / CL
Cd (mgg–1) / Not traceable
Cu (mg g–1) / 0.15
Fig. 1: Scanning Electron Image of Cd+Cu Contaminated Spiked Soil
2.2 Electrokinetic Setup
A schematic test setup used in this experiment is presented in Figure 2. The test set up consists of cylindrical cell made up of perspex material, having two electrode compartments, an anode reservoir and cathode reservoir, dc power supply, multi meters etc. The central cell diameter was 69 mm and 350 mm in length. The cork with nylon net is used as reinforcement which is immediately provided next to filter paper to prevent spreading of fine soil particles from entering into the reservoir. Gas vents were provided in both the electrode compartment to pass gases generated from the electrolysis process.
Fig. 2: Schematic Diagram of Electrokinetic Test Set-up
2.3 Methodology
The basic information of the experimental program and parameters used in each test is summarized and a total of six experiments were conducted, their detailed information is mentioned in the Table 2.
Table 2: Basic Information of Experiments
Test / Washing solution / Flushing solutionEKCdAC / Deionized water / AC(1M) (pH 10)
EKCdAA / AC (1M) (pH 5) / AC(1M) (pH 10)
EKCuAC / Deionized water / AC(1M) (pH10)
EKCuAA / AC (1M) (pH 5) / AC(1M) (pH 10)
EKCdCuAC / Deionized water / AC(1M) (pH 10)
EKCdCuAA / AC (1M) (pH 5) / AC(1M) (pH 10)
EKCdAC : Electrokinetic remediation of cadmium contaminated soil using ammonium citrate.
EKCdAA : Electrokinetic remediation of cadmium contaminated soil saturated with ammonium citrate and same used as flushing agent.
EKCuAC : Electrokinetic remediation of copper contaminated soil using ammonium citrate.
EKCuAA : Electrokinetic remediation of copper contaminated soil saturated with ammonium citrate and same used as flushing agent.
EKCdCuAC : Electrokinetic remediation of copper contaminated soil using ammonium citrate.
EKCdCuAA : Electrokinetic remediation of copper contaminated soil saturated with ammonium citrate and same used as flushing agent.
3. RESULTS AND DISCUSSION
Figure 3 shows the plot for cumulative volume of effluent versus time during the electrokinetic process. It has been observed that the direction of fluid flow was from the anode to cathode (zeta potential < 0) in all the EK tests.
Fig. 3: Cumulative Electroosmotic Flow Measured During Electrokinetic Remediation
The sequence of the EO flow in all tests was EKCdAC>EKCdAA>EKCuAC>EKCuAA>EKCdCuAC>EKCdCuAA (Table 3).
Table 3: Electroosmotic Permeability Values
Test / EO permeability (ke) (cm2/V-s)EKCdAC / 1.44 × 10–04
EKCdAA / 1.32 × 10–04
EKCuAC / 6.09 × 10–05
EKCuAA / 6.31 × 10–05
EKCdCuAC / 6.65 × 10–05
EKCdCuAA / 3.03 × 10–05
Moisture content of the soil was measured and is presented in (Figure 4). The overall moisture content was observed decrease in EKCdCuAC, EKCdAC, and EKCdAA (38.0% to 39.0%) which indicates that more volume changes occurred due to use of ammonium citrate as washing as well as flushing agent. As a result, consolidation had occurred and crack formation was observed in EKCdAA. The highest average moisture was found in case of EKCuAC. The moisture content of EKCuAC, EKCuAA and EKCdCuAA was observed to be high with respect to initial moisture content (40%).
Fig. 4: Variation of Moisture Content with Distance after Treatment of Cd & Cu Contaminated Soil
Distribution of cadmium and copper concentrations in the soil after EK process is shown in Figures 5 & 6. For all the values of the EK treated soils of individual contamination are of c/c0 were below 1, which implied that cadmium was partly removed from soil. In comparison with EKCdAC (848 mL of EOF), EKCdAA (657 mL of EOF) achieved less cumulative EO flow and more significant enhancement on the EK movement of cadmium. The removal efficiency of both cadmium and copper individual contamination as well as mixed contamination in the sub-samples after EK treatment are shown in the Table 4. It has been observed that both cadmium and copper removal was high in case of tests run with individual contaminated soil samples as compared to mixed contaminated soils.
Fig. 5: Cadmium and Copper Residue for Individual Cases after the Electrokinetic Treatment
Fig. 6: Cadmium and Copper Residue for Mixed Cases after the Electrokinetic Treatment
Table 4: Removal Efficiency
Designation / Removal efficiency (%)Individual / Cd+Cu
Cd / Cu / Cd / Cu
EKCdAC / 36.3 / ---
EKCdAA / 48.9 / ---
EKCuAC / --- / 25.3
EKCuAA / --- / 30.0
EKCdCuAC / 23.7 / 9.1
EKCdCuAA / 28.9 / 29.8
4. CONCLUSIONS
The findings of heavy metals extraction from artificially contaminated soil by use of an alkaline reagent, ammonium citrate during electrokinetic process is to help to understand the removal of both Cd and Cu in their individual and mixed form. This is expected to be useful for the development in the future of combined innovative processes for the remediation of polluted soils, contaminated with multiple contaminants.
REFERENCES
Acar, Y.B. and Alshawabkeh, A. (1993). “Principles of Electrokinetic Remediation”, Environmental Science and Technolology, 27(13): 2638–2647.
Acar, Y.B. and Alshawabkeh, A.N. (1996). “Electrokinetic Remediation: I. Pilot-scale Tests with Lead-spiked Kaolinite”, ASCE, Journal of Geotechnical Engineering, 122(3): 173–185.
Bala Ramudu, P.R.P. Tiwari and R.K. Srivastava (2008). “Electrokinetic Remediation of Copper Contaminated Expansive Soil by Surfactant & Chelating Agents”, Proc. Indian Geotechnical Conference, 2008, Indian Institute of Sciences, Bangalore.
Reddy, K.R., Chinthamreddy, S. and Al-Hamdan. A. (2001). “Synergistic Effects of Multiple Metal Contaminants on Electrokinetic Remediation of Soils”, Remediation, Summer: 85–109.
Reddy, K.R., Saichek, R.E., Maturi. K. and Ala. P. (2002). “Effects of Soil Moisture and Heavy Metal Concentrations on Electrokinetic Remediation”, Indian Geotechnical Journal, 32(2): 258–287.
Ottosen, L.M., Hansen, H.K., Ribeiro A.B and Villumsen, A. (2001). “Removal of Cu, Pb and Zn in an Applied Electric Field in Calcareous and Non-calcareous Soils”, Jl. of Haz. Mat, B(85): 291–299.
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Electrokinetic Extraction of Heavy Metals from Contaminated Soils Using Ammonium Citrate
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