Thiol–Ene Synthesis of Cysteine-Functionalized Lignin for the Enhanced Adsorption of Cu(II) and Pb(II)

N-Acetyl-l-cysteine-functionalized lignin (CFL) was synthesized via UV-initiated thiol–ene click reaction. The structure and morphology of CFL were characterized by FT-IR, 1H NMR, and SEM. Influences of the pH, dosage, contact time, concentration, and temperature on metal ions adsorption onto CFL were analyzed. Because of the introduction of more adsorption sites, the maximum adsorption capacities of CFL for Cu(II) and Pb(II) could reach 68.7 and 55.5 mg/g, which were 12.5 and 7.6 times that of raw lignin, respectively. The results from adsorption kinetics and isotherms suggest that the metal ions were adsorbed onto CFL by a monolayer chemisorption mechanism. The chemical interactions between CFL and metal ions through chelation were studied and confirmed by FT-IR and XPS analysis. The feasible construction of CFL through UV-initiated reaction has proven to be a promising "clickable" method in synthesizing lignin-based functional materials with designed modules and enhanced adsorption abilities for metal ions removal.