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.