Combined Washing Of Organic Phosphonic Acids And Auxiliaries For The Removal Of Heavy Metals From Soils

Soil contamination with heavy metals has become one of the most concerned environmental problems all over the world.Soil washing is an alternative method because of its simple operation,fast speed,and high efficiency.The screening of environment-friendly washing agents has become the focus of this technology.In this study,two contaminated soils,including polluted farmland soil（soil A）and mine soil（soil B）,were tested.Three washing agents including amino trimethylene phosphonic acid（ATMP）,hydroxy ethylidene diphosphonic acid（HEDP）and diethylene triamine pentamethylene phosphonic acid（DTPMP）,were used to explore the effects of different concentrations,p H,and washing time on Pb and Cd removal efficiencies.Meanwhile,the combined washing of organic phosphonic acids and auxiliaries（ascorbic acid,phytic acid,and sodium isoascorbate）was also explored to enhance the removal rates of heavy metals in contaminated soils.The main results are as follows:（1）ATMP,HEDP and DTPMP could effectively remove Pb and Cd from polluted soils,and their concentrations,p H values,and washing time had significant influence on metal removals（P<0.05）.The removal rates of soil Pb and Cd changed in the logarithmic function with the increasing concentrations（P<0.05）.However,with the increase of solution p H values,the washing rate of Pb increased at first and then decreased,while the removal rate of Cd decreased rapidly at first（P<0.05）and then tended to be stable（P>0.05）.The removal rates of Pb and Cd increased significantly at first（P<0.05）and then stabilized（P>0.05）with the extension of washing time.Compared with the quasi-first-order kinetic equation,the removal kinetics of Pb and Cd by organic phosphonic acids were more consistent with the quasi-second-order kinetic equation.（2）The addition of ascorbic acid,phytic acid,and sodium isoascorbate could effectively enhance the removal efficiencies of Pb and Cd by ATMP,HEDP and DTPMP from the polluted soils.With the increase of auxiliaries concentrations,soil Pb and Cd removal efficiencies increased gradually or increased at first and then tended to keep stable.Compared with single organic phosphonic acids treatments,soil Pb and Cd removal efficiencies increased by 2.37～19.58%（soil A）and 1.52～34.56%（soil B）,respectively.The better enhancement of heavy metals removal rates could be found by the mixed solutions of0.1 mol L^-1 ascorbic acid with organic phosphonic acids for the removal of Pb in soil B,and0.1 mol L^-1 phytic acid with organic phosphonic acids for the removal of Cd in soil A.（3）The heavy metals removal efficiencies were closely related to the washing amount of mineral elements（P<0.05）.A significant positive correlation between the removal rates of Pb²⁺and Cd²⁺with the washing amount of Fe³⁺was found（P<0.05）.However,the removal rates of Pb²⁺and Cd²⁺had a significant negative correlation with the amount of Ca²⁺dissolution,with pearson correlation coefficient of-0.994～-0.772（P<0.05）.（4）After washing by organic phosphonic acids alone and its combination with auxiliaries,the contents of exchangeable,reducible,oxidizable and residual Pb and Cd in the two soils decreased significantly（P<0.05）.Compared with organic phosphonic acids washing alone,the contents of Pb and Cd in soil decreased further after combined washing,which was significantly better than that of single washing（P<0.05）.Among them,the removal effects of reducible and residual Pb were better,which could continue to increase by 2.63～53.89%and 9.09～39.75%,respectively.And the contents of reducible and oxidizable Cd also decreased significantly（P<0.05）.Combined washing further reduced the potential ecological risks of residual heavy metals in soils.（5）The mixed solutions of organic phosphonic acids and auxiliary agents washing could mitigate the loss of soil nutrients,compared with single washing by organic phosphonic acids.The contents of soil organic matter increased by 2.85-8.53%and the concentrations of soil total phosphorus and available phosphorus also increased after wahing with three kinds of organic phosphonic acids.The contents of p H values,total nitrogen,total potassium,alkali-hydrolyzable nitrogen and available potassium in the two soils decreased by 0.37-0.57 values,11.76-16.61%,12.31-18.13%,17.83-33.33%and 11.27-27.16%,respectively.Compared with single organic phosphonic acids washing,the addition of ascorbic acid,phytic acid and sodium isoascorbate increased soil p H value slightly,and the loss rates of soil total nitrogen,total potassium,alkali-hydrolyzable nitrogen and available potassium decreased by 1.00～13.15%.