Sustainable remediation of mixed heavy metals using hydrothermal and in-situ synthesis of Mg/Al-LDHs

This study explores the comparative efficacy of magnesium-aluminum/layered double hydroxides (Mg/Al-LDHs) synthesized via hydrothermal and in situ methods for the simultaneous removal of Cu(II), Pb(II), and Cr(VI) from aqueous solutions. The LDHs before and after adsorption were thoroughly characterized using SEM, XRD, FTIR, BET surface area analysis, XPS, and zeta potential measurements. The results demonstrated that hydrothermally synthesized Mg/Al-LDHs exhibited enhanced structural ordering and superior crystallinity, as evidenced by sharp and intense XRD peaks and increased BET surface area, and higher adsorption capacities, 240 mg/g for Cu(II), 512 mg/g for Pb(II), and 484 mg/g for Cr(VI), achieving equilibrium within 60 min. Adsorption kinetics followed a pseudo-second-order model, and equilibrium data were best described by the Langmuir isotherm, indicating a monolayer chemisorption process. In contrast, the in situ synthesis method proved advantageous in operational simplicity and reagent efficiency, achieving 100 % Pb(II) and >80 % Cu(II) and Cr(VI) removal under alkaline conditions. Mechanistic analysis suggested that adsorption involved surface complexation, ion exchange, and surface precipitation. Furthermore, the adsorption of Cu(II) was only marginally affected by changes in pH, with a slight increase in capacity observed at higher pH levels. In contrast, Pb(II) showed a pronounced rise in adsorption with increasing pH, while Cr(VI) displayed the opposite behavior, with its adsorption decreasing as pH increased. These results demonstrate that optimizing LDH mineral structure enhances heavy metal uptake through improved stability and surface reactivity, supporting the development of high-performance, reusable adsorbents for sustainable wastewater remediation.