The Study On The Stability Of Ferrihydrite In Soil And Its Effect On The Chemical Behavior Of Arsenic In Soils

Ferrihydrite,as its high affinity of arsenic(As)adsorption is widely used for the in-situ chemical remediation of arsenic contaminated soils.However,its poorly crystalline phase is vulnerable and maybe affected by the environment and sometimes transformed to more crystalline iron oxides.This process may lead to the arsenic release and enhance its bioavailability in the environment.In this study,firstly,synthetic ferrihydrite was added to three typical red soils sampled in three towns in Shimen Region(Baiyun Town(BY),XinGuan Town(XG)and MengQuan(MQ))and then started the indoor soil incubation experiment.Sequential extraction and Diffusive Gradients in the Thin-films Technique(DGT)was employed in this research to study the effect of two varied soil moistures(30%and 70%SWHC)and physicochemical properties on the stability of ferrihydrite in soils.Secondly,three uncontaminated soils were spiked with arsenic to prepare contaminated soil.Until the concentration of available arsenic tended to be stable,ferrihydrite was spiked into the soils and then studied the stability of ferrihydrite and the effect of its transformation process on the behavior of arsenic in soils.Finally,we studied the stability of ferrihydrite and the effect of its transformation process on the mobility of arsenic within the soil DryWet Cycle water management.The obtained main results are as following:1.Transformation/dissolution of ferrihydrite was observed in three uncontaminated or contaminated soils at two varied SWHCs followed by the formation of new crystalline iron oxides.Ferrihydrite transformed significantly faster at 70%SWHC than 30%SWHC.However,ferrihydrite transformation process was retarded in arsenic contaminated soil compared with uncontaminated soils.2.The variation of soil properties among three soils can significantly affect the transformation rate of ferrihydrite.The transformation rate of ferrihydrite in uncontaminated at 30%SWHC followed the order BY>XG>MQ while at 70%SWHC this order changed to XG>BY>MQ.The transformation rate of ferrihydrite in arsenic contaminated soils at 30%SWHC followed the order MQ>XG>MQ while at 70%SWHC this order varied to XG>MQ>BY.3.Different doses of ferrihydrite performed significantly effect on adsorption of arsenic at both two SWHCs.1%w/w ferrihydrite addition had an better effect on arsenic adsorption than 0.1%w/w ferrihydrite addition.The concentration of labile As decreased 59-67%and 71-76%at 30%and 70%SWHC with 0.1%ferrihydrite addition respectively.Almost no labile As could be detected at two SWHCs in varied soils with 1%w/w ferrihydrite addition.However,the concentration of labile As presented an increasing trend during 15-60 d as a result of ferrihydrite transformation/dissolution.After ferrihydrite addition,the content of poorly crystalline iron oxide combined As(F1-As)increased.The content of well crystalline iron oxide combined As(F2-As)also slightly increased indicating the F1-As transformed to F2-As together with ferrihydrite transformation.4.Transformation rate of ferrihydrite(0.1%w/w)in three soils(one uncontaminated soil and two contaminated soils)at three soil water management modes(100%SWHC,Dry/Wet Cycle,DWC and 30%SWHC)followed the order 100%SWHC>DWC>30%SWHC.The concentration of labile As was significantly decreased within all the treatments of two contaminated soils,the reduction order of three soil water management modes as followed:100%SWHC>DWC>30%SWHC.At 100%SWHC and DWC modes,the concentration of labile As increased as a result of ferrihydrite transformation/dissolution while no increasing trend was observed at 30%SWHC.After ferrihydrite addition,the content of arsenic in F1 and F2 both increased at 100%SWHC and DWC whereas at 30%SWHC,the content of arsenic incrementwas only measured in F1.