Research On The Effects Of Chemical Washing For Remediating Heavy-metal Contaminated Agricultural Soil

In order to develop a more effective and friendly chemical washing technology for heavy-metal contaminated agricultural soil,it is necessary to study and evaluate further the applicability of various washing technologies in treating heavy-metal contaminated soils.This research studied the effects of five commonly used washing agents(citric acid,EDTA,CaCl2,FeCl3 and HCl)to remove Cd,Pb and Zn from a Cd-Pb-Zn contaminated soil by oscillating washing in the laboratory,and the release of Cd,Pb and Zn from washed soils and the changes of some nutrients during incubation under flooding and 70%field water capacity,respectively.The resilience of soil microbial communities in the washed soils during the incubation were determined using high-throughput sequencing.A comprehensive evaluation on the applicability of the washing technologies for heavy-metal contaminated agricultural soils was conducted based on the above studies.The main results were as follows:1.Cadmium,Pb and Zn can be effectively removed from the contaminated soil by washing with the solutions of citric acid,EDTA,FeC;3 and HCl.CaCl2 solution could remove some Cd from the soil but it was difficult to remove Pb and Zn.Inert heavy metals in the soil was partially activated by the washings.As a result,the availability,the mobility and the potential risk index of the heavy metals,especially Pb and Zn,in the washed soils increased.2.Lead transferred between different fractions in the soil more easily than Cd and Zn during the washing process.All the washings increased weak acid extractable Pb(F1),which was attributed to the mobilization and transformation of reducible Pb(F2).One washing with FeC13 also increased weak acid extractable Zn whereas the the washings with other extractants reduced the weak acid extractable Cd or Zn.3.After 180 days of incubation under flooding or 70%field water capacity incubation,the DTPA-extractable Cd in the washed soil did not change obviously while the DTPA-extractable Pb significantly increased.The DTPA-extractable Pb in citric acid washed soil was higher than the unwashed soil.The availability of Zn also changed apparently during the incubation.Under flooding condition,reducible Pb was significantly transformed into weak acid extractable Pb accompanied by the activation of Fe-Mn oxides,which was the main cause for the increase in Pb availability during the incubation.4.Available soil Cu and readily available P were slightly reduced,and exchangeable Ca and Mg were significantly reduced by washing with citric acid.The available Cu,exchangeable Mg and exchangeable K in the soil were significantly lost by the washing with DTPA,CaC12,FeC13 and HCl.Readily available soil P was decreased by washing with EDTA and CaCl2.Exchangeable Ca was significantly reduced by all the washings except CaCl2.Available soil Mn was only reduced by washing with FeCl3.It was generally different for the lost soil nutrients to recover to the levels before washing,except for the available Mn in the FeCl3-washed soil that increased apparently during incubation.5.The richness and diversity indices of soil microbes in FeCl3-washed soil increased while those in EDTA-washed soil decreased with increasing time.Under flooding incubation,the microbial community structure in citric acid-washed soil recovered apparently whereas that in the soils washed with other extractants recovered slowly.The soil microbial community composition was influenced differently when the washed soils were incubated.After 180 days of flooding incubation,Bacteroidetes and Acidobacteria,Firmicutes and Acidobacteria and Chloroflexi,Firmicutes and Acidobacteria became the dominant phyla in citric acid-,EDTA-,and FeCl3-washed soils,respectively.After 180 days of 70%field water capacity incubation,Acidobacteria and Chloroflexi,Proteobacteria and Gemmatimonadetes,Firmicutes and Chloroflexi were the dominant phyla in citric acid-,EDTA-and FeCl3-washed soils.6.Soil pH,Eh and exchangeable K showed significant effects on soil microbial community in citric acid-washed soil,implying that increasing soil pH and applying P and Ca fertilizers could enhance the restoration of the microbial community in the soil.The soil microbial community in EDTA-washed soil under flooding and 70%field water capacity incubation were significantly affected by exchangeable Mg and readily available P,respectively.Therefor application of Mg and P fertilizers could enhance the restoration of the microbial community in EDTA-washed soil.As for FeCl3-washed soil,the microbial community was significantly influenced by soil pH,Hence,the restoration of the microbial community in FeCl3-washed soil could be enhanced by increasing soil pH.Based on the above results,it is concluded that citric acid could be used to wash Cd and/or Zn contaminated agricultural soil.CaCl2 could act as the washing reagent for treating Cd-contaminated agricultural soil.FeCl3 and HCl are suitable to washing Pb-or Zn-contaminated agricultural soils,respectively.EDTA is not a suitable washing reagent for treating agricultural soil.