Fabrication of ZnAl layered double hydroxide films on recycled aluminum sheets and phosphate removal

Phosphate removal is a process essentially required for the treatment of natural and wastewater. It is widely known that such removal process exploiting adsorption mechanism is one of the most effective methods. A type of anionic clay named zinc aluminum layered double hydroxide (ZnAl-LDH) with a few numbers of its series was synthesized on an aluminum substrate using a microwave-assisted method at a microwave power of 300 W, 450 W, and 600 W. The as-received LDHs were examined to confirm the layered structure and intercalating ions through X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transformed infrared spectroscopy (FT-IR). Their BET surface areas were also determined. Experiments on phosphate adsorption were conducted by varying the LDHs dosage, contact time, and the initial pH of the phosphate solution. Point of zero charge (pzc) and FT-IR analyses indicated that phosphate abstraction by LDHs involved electrostatic attraction, ligand exchange, and ion exchange. Of all studied LDH samples, the ZnAl-LDH prepared at 600 W exhibited the highest phosphate removal efficiency mainly due to its larger surface area. Langmuir isotherm also represents the equilibrium adsorption onto ZnAl-LDHs-600W. The calculated maximum adsorption capacity of ZnAl-LDHs-600W was 658 mg/g. In addition, the phosphate removal efficiency of the prepared ZnAl-LDHs was not affected by interference of other anions (Cl⁻, NO₃⁻, BrO₄⁻, CO₃²⁻, SO₄²⁻, and I⁻) during adsorption.