Enhanced Electrokinetic Remediation Of Multi-metals Contaminated Soil

Soils are contaminated by heavy metals with a range of anthropogenic activities including mining,improper disposal/discharge of chemicals and wastes,and the application of sewage sludge in arable land,resulting in a serious environment problem.Especially,most of anthropogenic activities produced a serious soil contaminated with multi-heavy metals,such as cadmium(Cd),copper(Cu),nickel(Ni),lead(Pb)and zinc(Zn),which is frequently found as complex mixtures.Heavy metals in the soil would cause a serious ecological environment threat and potential health risks through direct or indirect(e.g.food chains)means.Therefore,it is important to carry out research on heavy metals contaminated soil remediation.In this study,the electrokinetic(EK)remediation of multi-metals(Cd,Cu,Ni,Pb and Zn)contaminated soil are conducted.Combining with material characterization techniques,Visual MINTEQ and energy prediction model,the infulence of electrode materials,the type and concentration of assisting agents,and power supply type on the removal efficiency of heavy metals,energy consumption and environmental hazards were investigated.The details are outlined as following:(1)Carbon nanotube(CNT)covered polyethylene terephthalate yarns(PET-CNT)electrode has been investigated as a novel cathode material for the EK remediation of multi-metals contaminated kaolin.The results of scanning electron microscopy(SEM)and thermogravimetric analysis(TGA)showed that CNT was successfully covered on the surface of PET.The weight percentage of CNT in PET-CNT samples was approximately 29%.The results obtained from EK process showed that PET-CNT as a cathode obviously elevated electric current and electro-osmotic flow(EOF),significantly decreased kaolin pH,and enhanced heavy metals removal efficiencies.The removal efficiencies of Cd,Cu,Ni,Pb,and Zn in PET-CNT treatment were 89.7%,63.6%,90.7%,19.2%,and 88.7%,respectively.In comparison with the Pt/Ti and graphite treatments,the removal efficiencies of Cd,Ni,and Zn were improved at least about 30%,Cu and Pb were improved at least 16.6% and 6.9%,respectively.Our results demonstrated the PET-CNT was a good alternative cathode material for enhancing efficiency of EK remediation.(2)A novel assisting agent(citric acid+calcium chloride,CA+CaCl2),combined the advantages of two diffenrent compositions,has been investigated for the EK remediation of multi-metals contaminated kaolin simultaneously.The result shows that CA and CaCl2 concentration significant affect the removal efficiency of heavy metals.The optimum concentration is obtained at the 0.2 mol/L CA mixed with 0.05 mol/L CaCl2 in this study,and the removal efficiencies of Cd,Cu,Ni,Pb,and Zn were 98.19%,95.24%,98.95%,86.21%,and 99.01%,respectively,with the consuming of 124.8 KWh/m3 electric energy.Compared to traditional CA and HCl treatment,the CA+CaCl2 poses the advantages of cost-efficient,eco-friendly,high removel efficiency for multi-heavy metals,may promote the progress toward commercial application.(3)A specific energy concept was developed based on the assumption that all the metals were removed by electromigration.The model was used to predict the energy consumption for heavy metals in the EK remediation heavy metals contaminated black soil.The energy consumption for heavy metals in black soil treatment was compared with the energy consumption in the EK remediation heavy metals contaminated kaolin treatment.The kaolin treatment was conducted with the similar treatment condition(except for treatment time)of black soil treatment.After the EK treatment,the similar removal amount of heavy metals were obtained.The result showed that the total energy consumption in kaolin treatment seem to be in agreement with the predicted energy consumption that was used to remove the heavy metals in black soil treatment.Thus,we can conjecture that the specific energy concept can effectively predict the energy consumption for each metal.(4)Two periodic power(12 h ON/12 h OFF and 18 h ON/6 h OFF)EK treatments were conducted to investigate their influence on the removal of heavy metals,depletion of soil macroelements(K,Na,Ca,Mg,Fe and Mn)and electric energy consumption.The results indicated that the removal efficiencies of heavy metals were higher than 80% in all the tested EK treatments.Periodic power EK treatment reduced the depletion of Fe and the electrical energy consumption compared with the constant power EK treatment.The lowest energy consumption and depletion of Fe were obtained in the periodic power of 12 h ON/12 h OFF EK treatment.The predicted energy consumption for each metal results showed that more than 40% of electric energy was consumed to remove Fe in all the tested EK treatments.In comparison with the constant power EK treatment,the energy consumption for Fe reduced by 1264 kWh/m3 in the periodic power(12 h ON/12 h OFF)EK treatment,which accounted for 54.74% of the total reduction energy consumption in this EK treatment.Our results demonstrated that the reduction of energy consumption for Fe was one of the main contributor to save the total energy consumption.