Microscopic insight into precipitation and adsorption of As(V) species by Fe-based materials in aqueous phase

The mechanism of As(V) removal from the drinking water and industrial effluents by iron materials remains unclear at the molecular level. In this work, the association of Fe-based materials with As(V) species was explored using density functional theory and ab initio calculations. Solvent separated ion pair structures of [FeH₂AsO₄]²⁺_aq species may be dominant in an acidic solution of FeAs complex. The association trend of H₂AsO₄^- species by Fe³⁺_aq is found to be quite weak in the aqueous solution, which may be attributed to the strong hydration of Fe³⁺_aq and [FeH₂AsO₄]²⁺ species. However, the association of H₂AsO₄^- species by colloidal clusters is quite strong, due to the weakened hydration of Fe(III) in colloidal structures. The hydrophobicity of Fe-based materials may be one of the key factors for their As(V) removal efficiency in an aqueous phase. When the number of OH^- coordinated with Fe(III) increases, the association trend of As(V) by colloidal ferric hydroxides weakens accordingly. This study provides insights into understanding the coprecipitation and adsorption mechanisms of arsenate removal and revealing the high efficiency of arsenate removal by colloidal ferric hydroxides or iron salts under moderate pH conditions.