Application of Mg–Al layered double hydroxide for treating acidic mine wastewater: a novel approach to sludge reduction

To reduce sludge volume, Mg–Al layered double hydroxide (LDH) was used to remove pollutants from mine wastewater. Mg–Al LDH can remove both cations and anions from mine wastewater via co-precipitation and anion exchange mechanisms, respectively. We herein investigated the difference in removal efficiencies between Mg–Al LDH and Ca(OH) ₂ . Initial As and Fe concentrations of 8.4 and 308.6 mg/L led to final concentrations of <0.001 and 15.4 mg/L, respectively, when 5 g/L LDH with an Mg/Al molar ratio of 2:1 was employed. This suggests that As was removed in the form of H ₂ AsO ₄ ⁻ , which exchanged with intercalated NO ₃ ⁻ in the Mg–Al LDH structure, while Fe was removed by the formation of Fe–Al LDH via co-precipitation. In contrast, treatment with 5 g/L Ca(OH) ₂ gave As and Fe concentrations of 0.1 and <0.01 mg/L, respectively. Thus, under Ca(OH) ₂ treatment, As is removed by the co-precipitation of arsenate with the formation of ferric oxyhydroxide (FeO(OH)), while Fe is removed by co-precipitation of the formed Fe(OH) ₃ . The sludge volume obtained using Mg–Al LDH was approximately ten times lower than that obtained when using Ca(OH) ₂ .