Arsenite removal by using ZnAlFe mixed metal oxides derived from layered double hydroxides

ZnAlFe mixed metal oxides (ZnAlFe-MMOs) were synthesized from layered double hydroxides (LDHs) prepared by the coprecipitation method at pH 9 using an initial weight composition of Zn²⁺ = 75%, Al³⁺ = 15% and Fe³⁺ = 10%, with or without the addition of citric or oxalic acid. The solids were calcined at 400 °C to obtain the respective MMOs, which exhibited relatively high specific surface areas (165.3-63.8 m² g^-1) and semiconductor properties active in the visible region (bandgap values (E_g) of 2.42-1.77 eV). The synthesized materials were tested for the removal of trivalent arsenic by adsorption and by photocatalysis under visible light irradiation (λ ≥ 420 nm). The best removal of As(III) by adsorption (65.9%) and by photocatalysis (99.9%) was obtained with the ZnAlFe-MMOs prepared in the absence of organic acids. The XPS results indicate the coexistence of As³⁺ and As⁵⁺ over ZnAlFe-MMOs after the photocatalytic reaction and also confirm the formation of Fe²⁺ sites on the hematite surface that enhances the removal of As(III). Raman measurements confirmed that, in the photocatalytic experiments, As is largely retained as As(V) on ZnAlFe-MMOs, bound to Fe. The results of fluorescence of 7-hydroxycoumarin suggest that the photocatalyst produces HO^•, which can be the main species for As(III) oxidation under UV-Vis irradiation. Moreover, ZnAlFe-MMOs exhibited a good reusability after regeneration making ZnAlFe-MMOs a promising material for arsenic decontamination in polluted water.