Removal of various aqueous heavy metals by polyethylene glycol modified MgAl-LDH: Adsorption mechanisms and vital role of precipitation

The adsorption mechanisms of layered double hydroxides (LDHs) for aqueous heavy metals have been studied extensively, but few prior quantitative investigations of each mechanism were conducted. In this work, we prepared polyethylene glycol modified MgAl-LDH with molecular weight of 200 and 4000 (LDH-200 and LDH-4000) to investigate the adsorption mechanisms for Cu(II), Pb(II), and Cd(II) qualitatively and quantitatively. The interaction mechanisms were conducted by a number of characterization methods and quantitatively analyzed by a series of extraction procedures. The results indicated that the leading mechanisms of LDH-200 and LDH-4000 for Cu(II), Pb(II), and Cd(II) were precipitation with contributions of 44.20%-72.12%. The percentages of complexation, isomorphous substitution, and electrostatic attraction to the total adsorption amounts were 12.43%-37.89%, 6.35%-18.81%, and < 3.5%, respectively. For MgAl-LDH, the precipitation and complexation mechanisms accounted for 28.68%-52.71% and 18.12%-66.13%. After polyethylene glycol modification, the contributions of precipitation increased and the complexation decreased. The material characterizations and quantitative analyses of adsorption mechanisms provide theoretical guidance and foundation to further understand the interfacial sorption process, which is conducive to a deeper understanding of the removal mechanism of heavy metals.