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.