Removal Of Heavy Metals From Soil By EDTMP And PAA

Soil heavy metal pollution has become one of the environmental problems concerned by the whole world.Soil washing is a fast and effective remediation technology for soil contaminated by heavy metals.This research adopts the ethylenediamine tetramethylene phosphonic acid（EDTMP）and polyacrylic acid（PAA）which are two different types of chelating agents to remove the heavy metal in polluted soil.We explore and compare the two kinds of chelating agents under the different conditions of soil heavy metal removal efficiency.And through the quadratic saturation-D optimal design（QSDD）we further study of the mechanism of washing,and optimize the washing process.At the same time,the effects of washing on soil were evaluated from the basic physical and chemical properties of soil and the morphology of residual heavy metals.At last,the different treatment methods of washing liquid waste were compared and studied.The main results are as follows:（1）EDTMP and PAA as chelating agents have good washing effects on soil heavy metals.Among them,the washing efficiency of EDTMP for Pb and Cd is good.When the concentration is 2%,the washing efficiency of Pb and Zn is 89.80%and 46.66%,respectively,and the washing efficiency of Cd is 65.63%when the concentration is 2.5%.In contrast,PAA has the highest washing efficiency for Cd,reaching 74.29%at a concentration of 2%,and for Pb and Zn are 60.69%and 34.25%,respectively.（2）The concentration,p H and time of the two chelating agents had significant influence on the washing efficiency of heavy metals in soil.With the increase of EDTMP and PAA concentration,the removal rate of the three heavy metals increased as a power function（p<0.05）.With the increase of eluent p H,PAA for Cd and Pb washing rate at p H 3.00 9.00between lower after rising first,therefore PAA is more suitable for washing under the conditions of weak acid and neutral,for EDTMP,with the increase of eluent p H,the washing rate of Pb are first decreases after rising trend,therefore EDTMP may be more suitable for the washing in strong acid,strong alkali environment.With the increase of washing time,the washing rates of heavy metals of the two chelating agents showed a significant increase at first（p<0.05）and a gradual gradual trend after 2.0 h.（3）PAA may play a role in washing through three mechanisms.First,hydrogen ions promote the desorption and dissolution of metal ions.Secondly,carboxyl polyions bind to metal ions through chelation and electrostatic adsorption.Finally,electrostatic repulsion will disperse the soil and increase the contact area between PAA and soil particles.In EDTMP,methylene phosphonic acid groups are attached to nitrogen atoms,and four phosphonic carboxyl groups[-（HO）₂OP]are dissociated from each EDTMP molecule in solution,which are combined with metal ions to form cyclic chelates.Negative charge O atoms in molecules and N atoms in phosphine groups have coulomb effects on metal ions.（4）Through the optimal design of secondary saturation-D,various washing effects of EDTMP and PAA in washing were explored.And the optimal washing conditions were obtained when three influencing factors were changed at the same time.The optimal removal rates of EDTMP for Cd,Pb and Zn were 92.74%,96.14%and 50.76%,respectively,and the optimal removal rates of PAA for Cd,Pb and Zn were 84.62,79.24 and 41.66%,respectively.（5）Washing had no obvious effect on the physical and chemical properties of soil.After washing with EDTMP and PAA,soil total nitrogen,total potassium,available nitrogen,available potassium and cation exchange capacity did not change significantly compared with that before washing（p<0.05）.Soil organic matter decreased less than 5.58%,p H basically remained unchanged.In addition,the total phosphorus and available phosphorus levels increased slightly after EDTMP washing（p<0.05）because the phosphonic acid groups of EDTMP remained in the soil after washing.（6）EDTMP and PAA washing had great influence on the distribution of Cd,Pb and Zn in soil.The removal effect of PAA on exchangeable state,carbonate binding state and organic binding state is relatively good.Compared with PAA,EDTMP has a stronger removal effect for the residual state,while the removal effect for other components is weaker than PAA.In general,washing of EDTMP and PAA significantly reduced the exchangeable and carbonate-bound components of Cd,Pb and Zn（p<0.05）,thus reducing the environmental risk of soil pollution.（7）The heavy metals in EDTMP and PAA washing solution can be basically removed by chemical precipitation and adsorption.Attention should be paid to the dosage of precipitant during the use of chemical precipitation method.The reduction method is obviously poor in the treatment of the effluent.The maximum removal rates of Cd,Pb and Zn were 34.71%,32.34%and 79.95%,respectively,and PAA were 74.18%,55.81%and 89.38%,respectively.Since the reduction method can only react with ionic heavy metals,this result further confirms the speculation on the washing mechanism of two different chelating agents in the washing mechanism part.The washing effect of EDTMP is mainly chelating,while for PAA,the reaction mechanism is mainly acid desorption and electrostatic adsorption.