Equilibrium and kinetics studies on As(V) and Sb(V) removal by Fe²⁺-doped Mg-Al layered double hydroxides

Mg-Al layered double hydroxides (Mg-Al LDHs) doped with Fe²⁺ adsorbed As(V) (AsO₄^3-) and Sb(V) (Sb(OH)₆^-) from an aqueous solution through anion exchange with Cl^- intercalated in the LDH interlayer. Fe²⁺-doped Mg-Al LDH exhibited superior As(V) removal compared with Mg-Al LDH. The oxidation of Fe²⁺ doped in the Mg-Al LDH host layer to Fe³⁺ increased the positive layer charge of the LDH, thus increasing the anion-uptake capacity owing to stronger electrostatic attractive force between the positively charged layer and the anion. However, Fe²⁺-doped Mg-Al LDH was not superior to Mg-Al LDH in terms of Sb(V) removal. This was attributed to the preferential intercalation of OH^- over Sb(OH)₆^-. The As(V) and Sb(V) removal by LDH followed Langmuir-type adsorption, which proceeded via a pseudo-first-order reaction. The equilibrium and kinetics studies confirm that the adsorption of As(V) and Sb(V) by Fe²⁺-doped Mg-Al LDH was the result of chemical adsorption, involving the anion exchange of AsO₄^3- and Sb(OH)₆^- with the intercalated Cl^-.