Removal Of Heavy Metals From Contaminated Soil By Single Plant Washing Agents And Their Combination With HPMA And PBTCA

Soil contamination with heavy metals derived from various anthropogenic activities has been one of the major environmental problems throughout the world.Several remediation techniques including stabilization,electrodialysis,soil washing and phytoremediation have been widely applied to repair heavy metal polluted soils.Among them,soil washing,which can permanently separate metals from soils in a short period,is considered as one of the most promising methods to remove soil heavy metals because of its time-saving,cost-efficient and applicable to multiple contaminants.In this study,four plant washing agents,which extracted by water from Coriaria nepalensis?CN?,Clematis brevicaudata?CB?,Pistacia weinmannifolia?PW?and Ricinus communis?RC?,were used to explore their single application and their combination with hydrolytic polymaleic anhydride?HPMA?and 2-phosphonobutane-1,2,4-tricarboxylic acid?PBTCA?respectively,for the removal of heavy metals from contaminated soil under different influencing factors.Then,soil chemical properties,metal fractions and potential ecological risk index?E_rⁱ?were investigated to evaluate the ecological risks by these agents after washing.The main results of this study were:?1?CN,CB,PW and RC could be feasible washing agents for the removal of soil heavy metals.Among them,CN was the best washing agent generally,and it has the highest removal efficiencies for soil Pb?62.02%?and Zn?29.18%?,respectively,but only21.60%for soil Cd at its concentrations of 80 g L^-1,which was lower than CB and RC for the removal of soil Cd?25.89%and 25.24%,respectively?.However,PW had the lowest washing efficiencies for soil heavy metals.?2?The concentrations of plant washing agents,pH and washing time significantly affected the Cd,Pb and Zn removal?p<0.05?.The removal efficiencies of heavy metals show a logarithmic growth trend with the increasing concentrations,and presented different trends with the increasing time.Besides,the removal efficiencies decreased with the increasing pH.?3?The metal contents of exchangeable fractions for soil Cd,Pb and Zn were mainly reduced after washing with the four washing agents,while the residual fractions were removed minimally.The potential ecological risk index of soil Cd decreased significantly after single washing,while it decreased insignificantly of soil Pb and Zn,except CN and RC for the removal of soil Pb,which also decreased significantly.The potential ecological risk index of heavy metals showed_rE^Cd>_rE^Pb>_rE^Zn after washing.The concentrations of soil total nitrogen,phosphorus and potassium,available nitrogen,phosphorus and potassium,as well as soil urease,acid phosphatase and?-glucosidase were significantly reduced after washing by plant agents,but the extent of the decreases were significantly lower than those by EDTA?p<0.05?,while soil organic carbon increased after washing by plant agents.?4?FTIR analysis for plant washing agents indicated that the abilities of plant washing agents for the removal of soil heavy metals could be from the functional groups,such as–OH,–NH₂,–COOH,in the plant chemical compositions,which may remove soil heavy metals by ion exchange,chelation or other chemical reactions through reacting with metal ions on the soil surface sites.?5?The addition of HPMA or PBTCA could greatly enhance the removal efficiencies of soil heavy metals by plant washing agents?p<0.05?.Compared with single plant agents washing alone,the enhancements of metal removals by the addition of HPMA with plant agents were higher than those by the addition of PBTCA under acidic conditions.In this condition,the maximum improvements of soil Cd,Pb and Zn were 18.69%,12.89%and18.00%by PW+HPMA,CN+HPMA and PW+HPMA,respectively.However,under neutral and alkaline conditions,the improvements of metal removal by combining HPMA with plant agents were lower than those by combining PBTCA with plant agents,and the maximum enhancements of Cd,Pb and Zn removal were 24.18%,54.38%and 25.47%for by PW+PBTCA,respectively.?6?The concentration of washing agents,pH and washing time significantly affected the removal of soil metals by the combinations.The washing efficiencies increased with the concentration of plant washing agents and washing time increasing,while show different trends with pH increasing.When the pH of the composite agent>7.0,the metal removal efficiencies showed a slightly decreasing trend by the addition of HPMA,while presented significantly increasing trend with the addition of PBTCA.?7?The exchangeable fractions of soil Cd,Pb and Zn decreased significantly after composited washing,and the reduced percentages were 43.22-55.28%,50.60-95.19%and63.09-83.11%,respectively,which were the largest reduction among four metal fractions.Compared with single washing agents,the composited agents can effectively decrease the potential ecological risk of heavy metals,and the extent of decrease by the addition of HPMA is greater than that by the addition of PBTCA.It is worth mentioning that the soil _rE^{C d} is still high,while the ecological risk of soil Pb changed from moderate risk to low risk,especially_rE^{P b}decreased from 44.82 to 15.27 and 19.70 after washing by HPMA+CN and PBTCA+CN,respectively.The contents of soil nutrients?except for organic carbon?,and enzymes decreased after composited washing,but the degree of reduction was still significantly lower than that by 50 mmol L^-1 EDTA?p<0.05?,while organic carbon was still improved.Compared with single washing by plant agents,the contents of soil nutrients and enzymes decreased insignificantly by composited washing.?8?FTIR analysis for composited washing agents signified that the peak intensity of the C=O double bond at 1570-1614 cm^-1 decreased significantly after washing at pH 4.0,but increased after washing at pH 7.0 and 9.0?except for PW+PBTCA?,which may be the reason that the removal efficiencies of heavy metals by composited washing agents under acidic conditions were higher than those under neutral and alkaline conditions.On the contrary,after washing under different pH conditions,all peaks intensities of PW+PBTCA reduced to different extent,which may be one of the reasons that PW+PBTCA has high metal removal efficiency under both acidic and alkaline conditions.