Mobilization Of Different Species Of As(?)during Reductive Dissolution And Catalytic Transformation Of Fe Minerals

Fe minerals have been extensively studied to immobilize soluble arsenic in aqueous environments due to their strong affinities to arsenic.However,how can the speciation of As(?)[adsorbed or structurally-incorporated] and the crystalline structure of Fe minerals containing As(?)affect the reactivity of Fe minerals during dissolution or transformation processes are still poorly understood.In this study,reductive dissolution of different As(?)-Schwertmannites by S(-?)was investigated,and catalytic transformation of different As(?)-Schwertmannites as well as amorphous ferric arsenate and Scorodite by Fe(?)was compared.By means of XRD,FTIR,Raman,TEM and SEM analyses,effect of secondary minerals formed during these processes on the mobilization of As(?)was also elucidated.The reactivity of the three schwertmannites depends on whether As(?)is adsorbed or structurally incorporated and follows the order of Schwertmannite [Sch-free] > As(?)-incorporated Schwertmannite [Sch-As(?)] > As(?)-adsorbed Schwertmannite [Sch*-As(?)].Differences in As(?)release can also be observed between Sch*-As(?)and Sch-As(?)during dissolution.As(?)releases from both Sch*-As(?)and Sch-As(?)with time,and re-adsorption of aqueous As(?)is only observed in the Sch-As(?)system.During the catalytic trans-formation of As(?)-Schwertmannites by the adsorbed Fe(?),secondary minerals such as magnetite and/or goethite,detected in the Sch-As(?)and Sch*-As(?)systems,contribute to the immobilization of the released As(?).Both of the adsorbed and structurally-incorporated As(?)inhibit the transformation of Schwertmannite by Fe(?),while the adsorbed As(?)shows a more significant inhibition.At pH 6.5,As(?)-Schwertmannite is mainly transformed to Goethite and Lepidocrocite,whereas at pH 8.8,Magnetite is also found to be an important secondary mineral.During the catalytic transformation,formation of secondary minerals is accelerated by Fe(?),resulting in the immobilization of the released As(?).In addition,degeneration is found in both Sch-As(?)and Sch*-As(?)systems during the transformation.Degeneration is more significant in Sch-As(?)at pH 6.5 and Sch*-As(?)at pH 8.8.During the catalytic transformation by Fe(?),AFA is firstly transformed to Lepidocrocite and then Goethite by Fe(?)at pH 6.5,while Scorodite is only transformed to Lepidocrocite.At pH 8.8,transformation of both AFA and Scorodite is significantly inhibited,and AFA with a low degree of crystalline structure is transformed to a more crystalline Scorodite in the presence of Fe(?),whereas degeneration is found in the Scorodite system.Fe(?)inhibits the original dissolution of AFA and Scorodite,thus improving the immobilization of the released As(?).In addition,formation of secondary minerals and reformation of AFA crystalline structure also contributed to the immobilization of the released As(?).The significance of this study is to reveal the mobilization mechanisms of As(?)in the reductive dissolution of AsFeOx by S(-?)and in the catalytic transformation of As-FeOx by Fe(?)in the anoxic environments,and provides a theoretical basis for effectively controlling arsenic contamination in aqueous environments.