Influence of iron (II) and acetic acid on arsenite and arsenate sorption by magnetite under oxic conditions

Magnetite is widely recognized as an excellent adsorbent for mitigating aqueous arsenic (As) contamination; however, the influence of aqueous Fe(II) and organic acids on As retention under oxic conditions remains poorly understood. Here, we examined the effects of 10 mM Fe(II) and 10 mM acetic acid on the adsorption kinetics of As(III) and As(V). Magnetite preferentially adsorbed As(V) over As(III), with partial structural Fe dissolution, contributing to enhance As adsorption. Adsorption kinetics differed markedly among treatments (control, Fe(II), acetic acid, and Fe(II) + acetic acid). In the As(V) system, the adsorption was rapid and appeared to reach a near steady state by the first sampling point at 24 h, with Fe(II) addition—either alone or with acetic acid—promoting nearly complete As(V) adsorption (100% of added As) compared with the control (35% of added As). Conversely, acetic acid alone decreased As(V) sorption (29% of added As). In the As(III) system, the adsorption pattern was more complex due to concurrent As(III) oxidation. Fe(II) addition enhanced As(III) adsorption but decreased As(V) adsorption, whereas combined Fe(II) + acetic acid decreased As(III) but increased As(V) adsorption. Acetic acid promoted As(III) oxidation and reduced overall As adsorption. Overall, Fe(II) enhanced both As(V) and As(III) immobilization by magnetite under oxic conditions, whereas acetic acid increased As(III) oxidation but reduced total As retention. Our findings highlight the interactive effects of Fe(II) and organic acids on As–magnetite interactions, relevant to As mobility at the soil–rice root interface in flooded rice systems.