Macroscopic and Spectroscopic Assessment of the Cosorption of Fe(II) with As(III) and As(V) on Al-Oxide

The cosorption of Fe­(II) with As­(III) and As­(V) in anoxic suspensions of γ-Al₂O₃ at pH 7.5 was investigated with batch kinetic experiments and synchrotron EXAFS analyses. Single-sorbate results showed that Fe­(II) formed secondary Fe­(II)–Al­(III)-layered double hydroxide (LDH) phases during reaction with the Al-oxide sorbent, whereas As­(III) and As­(V) formed inner-sphere surface complexes. The kinetics and mechanisms of Fe­(II) and As­(III) sorption were identical in dual-sorbate and single-sorbate experiments, indicating that the processes involved operate independently. In contrast, As­(V) and Fe­(II) interacted strongly during cosorption. Fe­(II) enhanced the rate and extent of As­(V) removal from solution, but did not affect the mechanism of As­(V) adsorption. Conversely, As­(V) hindered the formation of Fe­(II)–Al­(III)–LDH, slowing down precipitation at low As­(V) concentrations and preventing it at high concentrations. This was attributed to interference of adsorbed As­(V) with the Al supply needed for Fe­(II)–Al­(III)–LDH precipitation, possibly combined with enhanced surface complexation of Fe­(II) cations promoted by anionic As­(V) surface species. No evidence was found for redox reactions between Fe­(II) and As­(V) or As­(III), or for precipitation of Fe-arsenic phases. These results improve our understanding of the geochemistry of Fe­(II) and arsenic in reducing environments, and demonstrate the utility of mechanistic studies on geochemically complex model systems