Mobilization Of Arsenic During Reductive Dissolution And Transformation Of Arsenic Contained Iron (Hydr)Oxides

The significance of this study was trying to investigate mobilization mechanismof arsenate during the dissolution and transformation of iron (hydr)oxides and proposerate expressions to model kinetics for the mobilization, which provided a theoreticalbasis for controlling arsenic pollution in nature. In this study, mobilization of arsenatewas investigated on schwertmannite, ferrihydrite and hematite in reductive dissolutionand Fe(II)-catalyzed transformation process. We compared mobilization of the surface-loaded and the incorporated arsenate, and investigated the presence of arsenate on theformation of iron oxides and compared the reactivities of the iron oxides duringreductive dissolution and Fe(II)-catalyzed transformation.Arsenate showed an inhibitory effect on the formation of schwertmannite, andstructure-incorporated arsenate could result in changes of morphology and crystallinestructure of the formed iron hydroxides. Schwertmannite was transformed into FeAsO4with increasing molar ratio of As/Fe, as arsneate was incorporated into the crystallinestructure of iron oxides. However, surface-loaded arsenate did not make any changeson the morphology and crystalline structure of schwertmannite.Both of the surface-loaded and the incorporated arsenate would be released duringreductive dissolution of iron oxides. More reactive sites exposed during dissolutioncould contribute to the re-adsorption of the released arsenate on ferrihydrite andschwertmannite, while the insignificant re-adsorption of arsenate on hematite could beexplained by the low reactivity of hematite and the inhibitory adsorption of oxalate onarsenate. Although dissolution rates of iron oxides decreased with the increase ofsurface-loaded arsenate on iron oxides, dissolution rate was mainly determined by thereactivity of iron oxides. However, the incorporated arsenate could enhance reactivityof iron oxides, which resulted in higher dissolution rate and more release of arsenate.Besides, mathematic model proposed in our study could accurately describe themobilization and re-adsorption of arsenate during the dissolution process.Schwertmannite could be transformed into lepidocrocite and then goethitecatalyzed by Fe(II) under anaerobic conditions. The surface-loaded As(V)-schwertmannite was less than1%after24h in the transformation process, while the incorporated As(V)-schwertmannite decreased from67%at24h to1%at96h duringthe transformation. The structure-incorporated arsenate showed an inhibitory effect onthe transformation process. Only0.6%of the surface-loaded arsenate and0.7%of theincorporated arsenate could be released in the transformation process. Over99%ofarsenate was still immobilized on the primary and secondary iron oxides.