Study On Treatment Of Heavy Metal Contaminated Soil By Coupled Soil Washing And Electrochemical Reduction

In this study,we proposed a heavy metal contaminated soil remediation process coupling chelating agent washing and electrochemical reduction.The removal performance and mechanisms of Cd and Pb under different types of chelating agent,chelating agent concentrations and different current conditions(current value and current mode)were investigated based on the combination technique.On the one hand,compared the effects of aminopolycarboxylic acid chelating agents and the low molecular organic acid chelating agents on soil pH,electrical conductivity,inter-electrode voltage and the removal efficiency of Cd and Pb under separate washing and combination technique.The combination technique system with better heavy metal removal efficiency and lower operating cost was screened out,and the migration process of soil heavy metals and the regeneration process of chelating agent were clarified.On the other hand explored the effects of EDTA concentration,current value and current mode on the heavy metal transport process,electrodepostion efficiency and fractionation of Cd and Pb in the soil based on the EDTA-combination technique system.The effects of different operating conditions on soil properties,heavy metal removal efficiency,heavy metal forms on the cathode and cost under the combination technique were investigated,and the operating parameters with the highest economic benefit were selected.The main conclusions of the study are as follows:(1)The effects of chelating agent types on the removal efficiency of heavy metalsUnder the combination technique,the aminopolycarboxylic acids could increase the removal efficiency of Cd and Pb by 15.24%and 27.05%and the re-washing ability reached 60.47%～96.24%and 73.48%～80.39%of the original washing solution.Low-molecule organic acids could increase the removal efficiency of Cd and Pb by 5.31%and 10.24%and the re-washing ability reached 14.39%～16.78%and 21.12%～39.81%of the initial washing.The combination technique could effectively improve the removal efficiency of heavy metals,and aminopolycarboxylic acids were more suitable for the combination system.(2)The effects of operating parameters of EDTA-combination technique on the removal efficiency and fractionation of heavy metalsThe combination technique could further remove heavy metals in soil,and the removal efficiency of heavy metals varied with different operating parameters(EDTA concentration,current value,current mode).Overall,the removal efficiency of Pb was higher than that of Cd.The higher the concentration of EDTA and the higher current value,the higher the removal efficieny of heavy metals.The current mode hardly affected the removal efficiency(p>0.05).The Cd in the initial soil was mainly exchangeable and residual,and Pb was mainly reducible and exchangeable.EDTA washing individually could effectively remove exchangeable heavy metals in the soil,and the removal efficiency increased with increasing concentration.Low-concentration EDTA washing had limited ability to remove metals in the reducible state,oxidizable state and residual state heavy metals.Under the EDTA-combination technique,the exchangeable and reducible heavy metal content of soil heavy metals was greatly reduced,and the proportion of oxidizable and residual heavy metals increased.Coupling the electrochemical reduction enabled the reducible heavy metals in the soil to be effectively removed and the removal efficiency increased with the increasing current value.(3)The effects of different current mode of EDTA combination technique on the mass transfer processCompared with continuous current(CC)mode,the Cd content in the soil suspension supernatant decreased from 0.186 mg to 0.059～0.090 mg,and the Pb content decreased from 5.163 mg to 1.226～1.991 mg under the pulse current(PC)mode process.The average mass of cathode electrodeposited Cd and Pb in PC mode was 1.44 and 1.37 times that in CC mode,respectively.Different current modes could affect the mass transfer process between the supernatant of the soil suspension and the cathode.Long-term electron transport in the CC mode would accumulate a large number of electrons at the cathode and promote the reduction of H+in the soil suspension.The resulting H2 barrier is the main factor hindering the mass transfer process.In contrast,the PC system could break the mass transfer barrier in the traditional CC system,promote the mass transfer process between the soil suspension supernatant and the cathode surface,improve the grain structure of the electrodeposition coating,and reduce the electrical energy consumption of the operating system.(4)Synergistic enhancement effect mechanism of combination techniqueOn the one hand,the synergistic enhancement effect of the combination technique was reflected in the regeneration cycle of the chelating agent in the cathode,thereby further leaching the heavy metals in the soil;on the other hand,the combination technique could promote the deep removal of the reducible heavy metals in the soil.Different types of chelating agents have different synergistic enhancement effects.The main factors affecting the synergistic enhancement effect include the chelating ability of the chelating agent itself,the pH value of the environment and the electrodeposition rate of the chelating agent on the cathode.(5)Cost-benefit analysis of the combination techniqueAminopolycarboxylic acids and low-molecule organic acids had little difference in electrical energy consumption.In terms of pharmaceutical costs,aminopolycarboxylic acids were generally higher than low-molecule organic acids.Although GLDA and IDS could achieve higher removal efficiency,the high chemical cost was not suitable for large-scale promotion.The cost of CA and OA was relatively low,but the removal efficiency often did not reach the ideal level under the combination technique.In contrast,the EDTA-combination system was the most cost-effective.Comprehensive consideration,the most economical and effective operating parameters were:EDTA-10mM—PC—0.032A—32 min on/16 min off,of which 47.56%of Cd and 77.00%of Pb were removed from the soil,and the electricity consumption was 8.24 Wh.