Synthesis, characterization, and utilization of poly-amino acid-functionalized lignin for efficient and selective removal of lead ion from aqueous solution

Effectively removing heavy metals from the wastewater using a green low-cost natural polymer-based adsorbent received a great deal of attention. In this study, natural γ-polyglutamic acid (γ-PGA) and ε-poly-L-lysine (ε-PL) functionalized lignin (DLGS and DLLS) were synthesized through demethylation/phenolation and amination to increase its intrinsic reactivity and adsorption sites, which were used to remove heavy metals from aqueous solutions. DLGS and DLLS were comprehensively characterized by 31P, 2D HSQC NMR, and elemental analysis (EA). The adsorption property of DLGS and DLLS for Pb2+ have been systematically investigated. The adsorption kinetics and isotherms were compliant with the pseudo-second-order model and Langmuir isotherm model, respectively. The maximum adsorption capacity of DLGS and DLLS for Pb2+ was 275.9 and 231.8 mg g−1 at 298 K, respectively, which is higher than that of most of the lignin-based adsorbent reported. Furthermore, it is confirmed that DLGS and DLLS exhibit excellent selectivity for removing Pb2+ through competitive adsorption experiments. Based on the Fourier transform infrared spectrum (FTIR) and X-ray photoelectron spectroscopy (XPS) and the density functional theory (DFT) calculations, the hydroxyl, carboxyl, amino, methoxyl, and dithiocarbamate groups are contributed to the binding with Pb2+, and density functional theory calculation further suggested dithiocarbamate and the carboxyl group are the primary active sites.