| Soil contamination with heavy metals has become one of the most serious environmental concerns in recent decades.In order to enhance the removal efficiencies of cadmium?Cd?,lead?Pb?and zinc?Zn?by polyepoxysuccinic acid?PESA?,two polluted soil,collected from Lead–zinc mines of Ya'an?soil A?and Panzhihua?soil B?,respectively,were chosen to explore the effects of the PESA concentrations,pH of washing solution,and contact time on the metal removals followed by optimizing,the selecting washing parameters by quadratic regression-orthogonal rotation design?QRRD?.Meanwhile,three reducing agents including scorbic acid?AA?,sodium dithionite?SD?and thiourea dioxide?TD?were used to enhance the removal efficiencies of Cd,Pb and Zn by PESA.The main conclusions showed as follows:?1?PESA was an effective chelating agent to remove Cd,Pb and Zn from contaminated soils.The concentration of PESA,pH and washing time have a significantly influences on their removals.In the single factor experiments,removal efficiencies of soil Cd,Pb and Zn showed power,linear or logarithmic changes?P<0.05?,respectively,with increasing PESA concentration or washing time,or decreasing pH.?2?QRRD with three factors and five levels was an effective and reliable tool for optimization washing parameters.The predicted removal efficiencies of Cd,Pb and Zn for soil A were 79.44,64.86 and 46.64%at the optimal parameters with PESA concentration of39.08 mmol L-1,pH of 3.16,and washing time of 2.08 h.Similarly,their removal efficiencies for soil B could obtain 63.84,66.68 and 40.85%at PESA concentration of38.07 mmol L-1,pH of 3.16,and washing time of 2.42 h.The error of measured and predicted values for soil Cd,Pb and Zn removals were 1.05-5.59%by the QRRD models.?3?AA,SD,and TD could effectively enhance the removal efficiencies of Cd,Pb and Zn by PESA from the polluted soils?P<0.05?.With increasing the concentrations of reducing agents,the removal efficiencies of Cd for soil A,and Cd and Pb for soil B increased steadily with PESA+AA,and the similar to Pb of soil A with PESA+TD,and the other treatments increase significantly?P<0.05?and followed by keep constant.Compared with the single PESA treatment,3.51-9.73%,4.30-17.53%,5.11-15.00%for soil A and 4.30-9.83%,4.10-16.03%,2.71-15.83%for soil B of Cd,Pb and Zn removal effieceies could be impoved,and the optimal treatment could be found by the mixed solutions of 0.03 mol L-11 sodium dithionite and PESA except the Pb for soil B.?4?The all fractions of soil Cd,Pb and Zn decreased after PESA washing.Compared with washing by single PESA,the metal fractions were lower than by the mixed solutions of PESA and reducing agent(0.02 mol L-1),except for PESA+SD,PESA+TD for organic-bound Cd of soil A,PESA+AA,PESA+TD for exchangeable fraction Cd of soil B,PESA+TD for residual Zn of soil A,reducing agent+PESA for exchangeable fraction Pb of soil A and Zn of soil B.?5?The soil fertility was influenced after washing by combining of PESA and reducing agent to some extent.Total nitrogen,total phosphorus and total potassium have no obviously change?P>0.05?.The mixed soiltions of PESA and reducing agent washing could buffer the lost of organic matter,available nitrogen,available phosphorus and available potassium compared with washing by single PESA,except for available phosphorus of soil B by PESA and SD treatment. |
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