Stability study of the As(V)-Fe(III) oxyhydroxide coprecipitate over a broad pH range: Characteristics and mechanism

Abstract

Fe(III)-oxyhydroxide composites are efficient adsorption materials, however, their detailed geometrical structures and adsorption mechanisms in a broad pH range are still unclear. In this study, the structures of arsenic(V) adsorbed on Fe(III)-oxyhydroxide adsorption complexes were investigated using hybrid density functional theory calculations together with experimental Fourier transform infrared (FT–IR) spectroscopy. Adsorption experiments showed higher arsenic(V) removal rates in the range of pH 5.0–10.0. FT–IR and X-ray diffraction (XRD) revealed that arsenic(V) is stably adsorbed by Fe(III)-oxyhydroxide complexes. The formation of monomeric As(V) appears to be favored in the 1.0–4.0 pH range, while dimeric and trimeric As(V)-Fe(III) oxyhydroxide complexes are preferably formed at higher pH (4.0–10.0). Namely, dimeric and trimeric Fe(III)-oxyhydroxides exhibited good removal efficiencies for arsenic(V) over a wide pH range. Interestingly, the stabilities of the complexes formed between As(V) and monomeric, dimeric and trimeric Fe(III)-oxyhydroxides follow the orders: As(V) trimer > As(V) dimer > As(V) monomer; these complexes are also interconvertible.