Single-step removal of arsenite ions from water through oxidation-coupled adsorption using Mn/Mg/Fe layered double...

This study developed a layered double hydroxides (Mn/Mg/Fe-LDH) material through a simple co-precipitation method. The Mn/Mg/Fe-LDH oxidized arsenite [As(III)] ions into arsenate [As(V)] anions. The As(III) and oxidized As(V) were then adsorbed onto Mn/Mg/Fe-LDH. The adsorption process of arseniate [As(V)] oxyanions by Mn/Mg/Fe-LDH was simultaneously conducted for comparison. Characterization results indicated that (i) the best Mg/Mn/Fe molar ratio was 1/1/1, (ii) Mn/Mg/Fe-LDH structure was similar to that of hydrotalcite, (iii) Mn/Mg/Fe-LDH possessed a positively charged surface (pH_IEP of 10.15) and low Brunauer-Emmett-Teller surface area (S_BET = 75.2 m²/g), and (iv) Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺/Mn⁴⁺ coexisted in Mn/Mg/Fe-LDH. The As(III) adsorption process by Mn/Mg/Fe-LDH was similar to that of As(V) under different experimental conditions (initial solutions pH, coexisting foreign anions, contact times, initial As concentrations, temperatures, and desorbing agents). The Langmuir maximum adsorption capacity of Mn/Mg/Fe-LDH to As(III) (56.1 mg/g) was higher than that of As(V) (32.2 mg/g) at pH 7.0 and 25 °C. X-ray photoelectron spectroscopy was applied to identify the oxidation states of As in laden Mn/Mg/Fe-LDH. The key removal mechanism of As(III) by Mn/Mg/Fe-LDH was oxidation-coupled adsorption, and that of As(V) was reduction-coupled adsorption. The As(V) mechanism adsorption mainly involved: (1) the inner-sphere and outer-sphere complexation with OH groups of Mn/Mg/Fe-LDH and (2) anion exchange with host anions (NO₃^-) in its interlayer. The primary mechanism adsorption of As(III) was the inner-sphere complexation. The redox reactions made Mn/Mg/Fe-LDH lose its original layer structure after adsorbing As(V) or As(III). The adsorption process was highly irreversible. Mn/Mg/Fe-LDH can decontaminate As from real groundwater samples from 45-92 ppb to 0.35-7.9 ppb (using 1.0 g/L). Therefore, Mn/Mg/Fe-LDH has great potential as a material for removing As.