Remediation Of Cd-Zn-Contaminated Soil By Two Organic Acid And Recovery Of Washing Reagents By Electrochemical

Soil washing technology is widely used in the remediation of heavy metal contaminated soil because of its high removal efficiency and thorough remediation.The purpose of leaching is to fundamentally solve the environmental pollution and harm caused by heavy metals,so it is necessary to find more suitable environment-friendly new leaching agents.In this study,diethylenetriamine pentamethylphosphonic acid（DTPMPA）and carboxyacid-sulfonic acid-acrylate terpolymer（AA/AMPS/HPA）were used as leaching agents to investigate the removal effects of these two reagents on cadmium（Cd）and zinc（Zn）in soil under different leaching conditions.The Box-Behnken design（BBD）was used to optimize the leaching process,and the effects of leaching on soil environment were evaluated by the changes of soil physicochemical properties and heavy metal forms before and after leaching.Finally,electrochemical technology was used to treat the leaching waste liquid,explore the reusability of the eluent,and evaluate its reuse effect and economic benefits.The main research results are as follows:（1）Both DTPMPA and AA/AMPS/HPA can effectively remove heavy metals from soil.The removal efficiency of Cd and Zn is affected by the concentration of eluent,p H of eluent and leaching time.Under the leaching conditions of 5%leaching concentration,p H=3 and leaching time of 120 min,the removal efficiency of Cd and Zn by DTPMPA,AA/AMPS/HPA were 69.72%and 30.9%,65.92%and 26.16%,respectively.The leaching mechanism of DTPMPA for heavy metals is mainly the strong chelation of phosphonic acid groups.AA/AMPS/HPA uses the dispersion of polymer to form complexes with metal cations through electrostatic adsorption.DTPMPA and AA/AMPS/HPA are two feasible leaching agents for Cd and Zn contaminated soil.（2）Through the optimal design of BBD,the influence of various factors on the removal of heavy metals in the two organic acid leaching was explored.The degree of influence from large to small was p H,reaction time,leaching agent concentration,and p H had strong interaction effects with the other two factors respectively.Under the optimal leaching conditions,the best removal rates of Cd and Zn by DTPMPA were74.57%and 35.41%,and the best removal rates of AA/AMPS/HPA were 69.92%Cd and30.64%Zn.（3）Electrochemical method can effectively recover two kinds of leaching reagent,AA/AMPS/HPA and heavy metal ions electrostatic adsorption（non-bonding）,the performance of the electrochemical recovery stage is better than DTPMPA.Compared with the initial extraction capacity,the leaching capacity of Cd and Zn of the recovered AA/AMPS/HPA remained at 88%and 53%.EDS and XPS results showed that Cd²⁺and Zn²⁺migrated to the cathode and were removed by deposition during the electrochemical process.FT-IR results showed that the functional groups of DTPMPA were partially separated after electrochemical treatment,while the functional groups of AA/AMPS/HPA were relatively complete.The two organic acids had different effects on the physical and chemical properties of soil after leaching.After DTPMPA and AA/AMPS/HPA leaching,soil p H decreased and organic carbon content increased.Part of available nutrients were taken away after leaching,which had no significant effect on total nitrogen,total phosphorus and total potassium in soil.The leaching of DTPMPA and AA/AMPS/HPA significantly reduced the exchangeable and reducible components of Cd and Zn,thus reducing the environmental risk of heavy metal pollution in soil.Of the two reagents,DTPMPA was more effective in slowing the loss of available nutrients.（4）The leaching of two organic acids had different effects on soil physicochemical properties,and DTPMPA could slow down the loss of available nutrients.After DTPMPA and AA/AMPS/HPA leaching,soil p H decreased and organic carbon content increased.Some available nutrients were taken away after leaching,and there was no significant effect on total nitrogen,total phosphorus and total potassium of soil（P>0.05）.DTPMPA and AA/AMPS/HPA leaching can significantly remove the exchangeable and reducible acid states of Cd and Zn in soil,reducing the environmental risk of heavy metal pollution of soil.（5）According to the economic benefit evaluation results,AA/AMPS/HPA is more economical in the whole process from leaching to recovery due to the lower leaching cost and excellent recovery performance.After five consecutive cycles of leaching,recycling and re-leaching,the extraction capacity of Cd and Zn by DTPMPA is less than 10%,while AA/AMPS/HPA can still reach about 40%and 27%,which has good sustainability and can be reused for 4-5 times under actual conditions.In conclusion,both organic acids can be used as environmentally friendly reagents for further study.Considering from the leaching process,DTPMPA as washing reagent can realize rapid and efficient remediation of heavy metal contaminated soil.From the perspective of sustainability,AA/AMPS/HPA with a better heavy metal removal rate is more economical and sustainable.