Adsorption properties of lignin-g-polyacrylic acid hydrogel on Pb²⁺, Cu²⁺, Cd²⁺

A new lignin-g-polyacrylic acid hydrogel (FRL-g-PAA hydrogel) was made from furfural residue lignin and polyacrylic acid, and it can effectively remove Pb²⁺, Cu²⁺ and Cd²⁺ from water. The absorption behavior of prepared FRL-g-PAA hydrogel was characterized by various methods, including effects of pH value, temperature and K⁺ interfering ions on its equilibrium adsorption capacity. Adsorption kinetics, isotherm, thermodynamics and XPS spectra were used to analyze the adsorption mechanism of FRL-g-PAA. The results showed that grafting of FRL could help the composite hydrogel to enhance adsorption capacity of Pb²⁺, Cu²⁺ and Cd²⁺ ions. When adjusting pH value to 5, temperature reduction was beneficial for rapid and efficient adsorption on heavy metal ions. Under the interference of K⁺, the adsorption capability of FRL-g-PAA hydrogel was Pb²⁺>Cu²⁺>Cd²⁺. The adsorption process followed the pseudo-second-order model and the spontaneous exothermic reaction of Freundlich model. XPS spectra further indicated that Pb²⁺, Cu²⁺ and Cd²⁺ were adsorbed mainly by the hydroxyl group and carboxyl group of FRL-g-PAA hydrogel. The complexation between heavy metal ion and FRL-g-PAA hydrogel is synergistic effect of physical and chemical adsorptions.