The Effect Of AS(III) On The Oxidative Dissolution Process Of FES And The Mechanism Of AS(III) Transformation And Migration

Oxidative dissolution of iron sulfides is not only important for the formation of acid mine drainage(AMD),but also an important geochemical process to control arsenic mobilization in the aquatic environments.However,little is known regarding the presence of As(?)on the oxidative dissolution of FeS and the speciation of secondary Fe minerals,and less attention has been paid to the generation of oxidative radicals during FeS oxidation.In this study,the presence of As(?)on the oxidative dissolution of FeS and the subsequent formation of secondary Fe minerals under pH-neutral and alkaline conditions will be investigated.By means of kinetic experiments and XRD,Raman,FTIR,EPR,XPS,SEM and TEM analyses,we will also investigate mechanisms involved in the transformation and mobilization of arsenic during this process.The results show that the dissolution rates of Fe(?)from FeS increase with increasing As(?)concentrations(<4.7 mM)at pH 7.0 and 9.0.Moreover,more than 98.5%of 6.7 and 4.7 mM As(?)can be immobilized by 2 g L-1 FeS within 1.5 h at initial pHs of 7.0 and 9.0,respectively,demonstrating the high efficiency of amorphous FeS to immobilize concentrated As(?)under oxic conditions.By contrast,6.7 mM soluble As(?)cannot be effectively removed after 18 h oxidation,with 1.7 mM As(?)left in solution when FeS is replaced by equivalent FeCl2.XPS results further reveal that,irrespective of pH and initial As(?)concentrations,more than 60%As(?)has been oxidized to As(V)before it is immobilized in the secondary Fe(?)(oxy)hydroxides.TEM and SEM results demonstrate that the plate Lepidocrocite is the primary oxidation product at pH 7.0 and 9.0 in the absence of As(?),but shows some degeneration with increasing As(?)concentrations,and completely changes to ferric arsenate particles at 6.7 mM As(?),revealing the inhibition of As(?)and/or As(V)on the development of Lepidocrocite.This also,from the other side,suggests that,apart from adsorption,coprecipitation of Ferric arsenate is another important pathway to immobilize soluble arsenic at high concentrations of As(?)during oxidative dissolution of FeS.EPR and scavenger experiments suggest the generation of ·OH and O2·-radicals,whereas the singlet oxygen has not been detected during the oxidative dissolution of Mackinawite.No As(?)detected in the HA system indicates the reducing environment caused by HA which inhibits further oxidation of As(?)during the oxidative dissolution of Mackinawite.On the contrary,the presence of FFA and EDTA enhances As(?)oxidation,and the maximum As(?)concentrations are 0.13 and 0.3 mM,respectively.In the presence of 1.3 mM As(?),scavengers(such as FFA)and Fe(?)-complexing ligands(such as EDTA)suppress the formation of Lepidocrocite and Ferric arsenate during the oxidative dissolution of Mackinawite,but favor sulfate production.The results of this study enrich our understanding on the effect of As(?)on the oxidative dissolution of FeS and mackinawite,as well as on the speciation of the oxidative radicals during this process.Moreover,it provides new insights on how to mediate transformation and mobilization of arsenic during this oxidation process.