Oxidation of bioreduced iron-bearing clay mineral triggers arsenic immobilization

Iron-bearing clay minerals and arsenic commonly coexist in soils and sediments. Redox oscillation from anoxic to oxic conditions can result in structural Fe(II) oxidation in clay minerals. However, the role of structural Fe(II) oxidation in clay minerals on arsenic immobilization is still unclear. In this study, we found that oxidation of structural Fe(II) in bioreduced clay mineral nontronite (NAu-2) triggered As(III) adsorption onto NAu-2. As(III) was adsorbed onto NAu-2 through ligand exchange with hydroxyl groups which were generated by the oxidation of structural Fe(II) in NAu-2. In addition, oxidation of structural Fe(II) led to the oxidation of As(III) to As(V), which further enhanced the adsorption of dissolved As(III) on NAu-2. Therefore, the adsorption capacity of As(III) onto oxidized NAu-2 was 1.6 times higher than that of native NAu-2. Oxidation of structural Fe(II) was a two-stage process that proceeded from exterior sites to interior sites, and the immobilization and oxidation of As(III) occurred predominantly at the rapid exterior structural Fe(II) oxidation stage. Our findings highlight that the oxidation of structural Fe(II) in iron-bearing clay minerals may play an important role in arsenic immobilization and transformation in the subsurface environment.