| Abandoned industrial and mining sites polluted by heavy metals have become one of the major environmental problems restricting the city sustainable development.Soil washing is a feasible technology for rapid remediation and reuse of contaminated sites.However,the washing efficiency of new leaching agents and environmental risk are still unclear.In this study,Cd and Pb were removed from soil by washing with polyamino polyether methylene phosphonate(PPMP)and bis(hexamethylene triamine penta(methylenephosphonic acid))(BHMT).And the washing efficiency and residual metal migration risk(risk assessment code,RAC)of the two chelators were investigated.Then the washing process was optimized by a two-objective optimization method to determine the optimum washing conditions and mechanisms.Finally,ferrite precipitation was applied to remove lead and cadmium from practical soil-washing wastewater,and the effect and mechanism were evaluated.Meanwhile,the reusability of PPMP and BHMT were analyzed.The main results are as follows:(1)PPMP and BHMT containing phosphonic acid functional groups are two feasible remediation reagents for heavy metal contaminated waste industrial soil.In addition,d-optimal mixture design could be used for optimizing and modeling of the two objectives during soil washing.Among the three factors(concentration,solution pH,or contact time),concentration is the main factor affecting washing efficiencies of PPMP and BHMT.For RAC,soil Cd and Pb washing by BHMT was significantly affected by the concentration and pH of solution(P <0.05).However,washing time was the predominant factor only in soil Cd washing with PPMP.It is worth noting that RAC decreased with the increase of solution pH or contact time.(2)At the same concentration,the metal extraction performance of PPMP was superior to that of BHMT(P < 0.05).And BHMT is more effective than PPMP in reducing the metal ecological risk at the same removal efficiency(P < 0.05).The chelation,acid dissolution effect and complexation mechanism of PPMP or BHMT were discussed from dynamic changes of efficiency and ecological risk.PPMP as chelating reagent react with Cd and Pb through chelation at strong acid in soil,whereas acid dissolution effect would convert the stable form of soil Pb into unstable form.BHMT could rapidly remove more acid-soluble fractions of Cd and Pb through the electric double layer adsorption,and is favorable under neutral conditions.The two-objective dynamic change models showed that the removal efficiency and RAC value of soil Cd and Pb were optimized at PPMP concentration of 7–10%,pH of 3–5,washing time of 105–130 min.In BHMT washing soil Cd and Pb,the concentration,pH and contact time were optimized at 9–10%,3–5,and 15–30 min respectively.(3)The practical soil-washing wastewater was treated by ferrite precipitation,and the eluent could reach the standard of cadmium and lead while forming the product which is expected to be reused.With the increase of initial pH of PPMP-washing wastewater,the removal effect of Cd and Pb by ferrite precipitation method is better.When the pH value was lower,there was a higher proportion of unstable phase in BHMT-washing wastewater,which created more favorable conditions for the removal of Cd and Pb.In addition,the concentrations of Cd and Pb were lower than the maximum allowable discharge concentration after treatment(integrated wastewater discharge standard GB8978-1996),and the ferrite prepared by doping cadmium and lead was generated.(4)The separation of cadmium-lead ferrite precipitation from eluent by magnetic adsorption was beneficial to the recovery of chelating reagent.Since a recyclable and practical washing repair process could form,this is beneficial to realize engineering application.The results showed that the recovered PPMP and BHMT still had a good leaching efficiency of soil Cd,and the removal rate of Cd and Pb were about 80 and 30%.The recycling PPMP and BHMT could combine with heavy metals through coordination. |
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