Development of L-Met functionalized cellulose nanofiber for an efficient removal of Pb²⁺ from aqueous solution: Characterization, adsorption behavior, DFT calculations and physiochemical mechanism

Lead-containing wastewater has been a significant challenge in the field of wastewater treatment. Cellulose surface has a large number of active sites, which is conducive to load modification. And amino acids have rich functional groups, which is a good choice for cellulose modification. In this study, the non-toxic L-methionine modified cellulose nanofiber (M-CNF) adsorbent was developed to adsorb Pb²⁺ in aqueous solution. M-CNF exhibited a maximum adsorption efficiency of 98.24 ± 0.04 %. The theoretical adsorption capacity could reach 113.3 ± 13.4 mg g^-1. After five cycles of adsorption-desorption process, the removal efficiency of Pb²⁺ was still >80 %. The pseudo-second-order kinetic model (R² = 0.9913) and Brunauer-Emmet-Teller adsorption isotherm model (R² = 0.9916) can be used to describe the adsorption behavior of M-CNF on Pb²⁺. The Gibbs free energy (ΔG) and enthalpy (ΔH) of the reaction are negative, indicating that the reaction was a spontaneous exothermic process. Density functional theory calculation indicated that -COOH and -C-S-C- groups played a role in the adsorption process. The adsorption mechanism of M-CNF might be the interaction of ion exchange, electrostatic adsorption, complexation and van der Waals force. This work suggested that M-CNF played a definitive role in the adsorption and removal of Pb²⁺ in aqueous solutions.