Simultaneous removal of Pb2+, Cu2+ and Cd2+ ions from wastewater using hierarchical porous polyacrylic acid grafted with lignin.

In PAA-g-lignin, phase separation, caused by the difference in expansion properties between lignin and polyacrylic acid, is used to build a porous hydrogel. In this study, PAA-g-APL was produced by grafting polyacrylic acid with acid-pretreated alkali lignin. Acid-pretreated alkali lignin acts as a hierarchical pore-forming agent that enhances the simultaneous adsorption capacities for Pb2+, Cu2+ and Cd2+ ions from wastewater. Notably, PAA-g-APL acted as a selective adsorbent for Pb2+ ions has an excellent selective removal coefficient α (20.22) in contaminated wastewater contained Cu2+ ions. Its molar partition coefficient for Pb2+ ions (68 %) is higher than that for either Cu2+ ions (28.6 %) or Cd2+ ions (3.4 %). At equilibrium, the total adsorption capacities of PAA-g-APL for Pb2+, Cu2+ and Cd2+ were 1.076 mmol g-1, 0.3233 mmol g-1 and 0.059 mmol g-1, respectively. The experimental kinetic data fitted well to a pseudo-second order model and to an intra-particle-diffusion model. The Freundlich isotherm model gave the best fit with the experimental equilibrium data. The ΔG° for PAA-g-APL is < 0, indicating that the adsorption of heavy metal ions is a spontaneous process. This study provides a highly promising candidate for the treatment of wastewater contaminated with a mixture of heavy metals.