Facile method for treating Zn, Cd, and Pb in mining wastewater by the formation of Mg–Al layered double hydroxide

Heavy metal pollution threatens aquatic systems worldwide, and mining activities are an important pollution source. Currently, the treatment of polluted water using a cost-effective technology that can purify multiple pollutants and is sustainable, environmentally friendly, and simple is a major challenge. The in situ preparation of Mg–Al layered double hydroxides (LDHs) and the concurrent treatment of Zn, Cd, and Pb from mining wastewater are important for preventing multiple steps and increasing adsorption sites. This study focused on mining wastewater containing high concentrations of Mg²⁺ and Al³⁺, with anion chemistry controlled by SO₄ ²⁻, which facilitates the formation of LDHs. The required amounts of Mg²⁺ and Al³⁺ ions were added to the wastewater, and the conditions for the creation of Mg–Al LDHs were controlled. The heavy metals in the experimental wastewater were effectively removed after the treatment via Mg–Al LDH formation. The X-ray diffraction of the post-treatment products suggested the formation of Mg–Al LDHs. The Mg/Al molar ratio (2.3:1) in the product approached the initial ratio (2:1), which meets the general limits of Mg–Al LDH formation. Scanning electron microscopy revealed that the products had a sheetlike stacked morphology, providing evidence for the formation of Mg–Al LDHs. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated that SO₄ ²⁻ might be the intercalated anion in the Mg–Al LDH layers. Consequently, SO₄ ²⁻ was removed from the mining wastewater, as it was captured between the LDH layers during the formation reaction of Mg–Al LDHs.