Thioarsenate immobilization by ferrous sulfides for geogenic contaminated groundwater rehabilitation: Mechanisms and kinetic constraints.

Thioarsenic prevails in sulfidic environments and exhibits higher mobility than arsenate (As(V)) and arsenite (As(III)), posing sustained challenge to the effectiveness of geogenic As-contaminated groundwater rehabilitation. Ferrous sulfide (FeS) minerals are proposed scavengers of monothioarsenate (MTA) in sulfidic groundwater, but the underlying mechanisms and kinetic processes remain elusive. This research presents new key evidence that MTA retention by FeS is constrained by a multi-reaction process: initial rapid adsorption, and subsequently slow binding to kinetics-controlled sites and irreversibly-retained phase. Polysulfides critically strengthen MTA retention by transforming MTA to less mobile As(V) and facilitating arsenopyrite (FeAsS) production. Simulating MTA isothermal adsorption edge with modified dual mode model confirms that MTA removal involves surface chemisorption and mineral transformation. While MTA removal rates were higher under weakly acidic conditions (pH 6-7), more stable FeAsS formed under alkaline conditions (pH 8-9) can strengthen As immobilization against re-desorption. Further column experiments to mimic in-situ FeS coating for MTA immobilization indicate that the simultaneously formed FeS, which induces MTA co-precipitation and production of pyrite and FeAsS, exhibited greater retention capacity than the pre-loaded FeS. These findings manifest important implications in scheming modes of in-situ Fe coating and transformation for managed rehabilitation of As-contaminated groundwater.