Sorption of As(III) and As(V) to siderite, green rust (fougerite) and magnetite: Implications for arsenic release in anoxic groundwaters

The formation of secondary Fe(II) or mixed Fe(II)/Fe(III) minerals such as siderite, green rust (fougerite) and magnetite has been suggested to reduce the aqueous concentration of Fe while leaving the concentration of As high in the Bangladesh and West Bengal aquifers. To test this hypothesis we have studied the adsorption of arsenite and arsenate to siderite, fougerite and magnetite using batch sorption experiments and EXAFS spectroscopy. We find that As(V) sorbs to fougerite, magnetite, and siderite by forming inner-sphere surface complexes resulting from corner sharing between AsO4 groups and FeO6 octahedra. No evidence of As(V) reduction is found. As(III) also forms inner-sphere surface complexes on magnetite and fougerite but only a (presumably) weak outer-sphere complex on siderite. No evidence for As(III) oxidation is found. As(V) desorbs from magnetite, fougerite and siderite at pH > 8; however, As(III) sorption to all three phases is enhanced with increasing pH. Hence, dissimilatory reduction of iron oxides, followed by precipitation of fougerite, magnetite or siderite will released dissolved As(V) at pH > 8; however, if As(V) is also reduced to As(III), sorption of As will be enhanced.