Bio-adsorbent for wastewater treatment: amination of green coconut husk waste-derived lignin for the removal of Congo red dye

Aminated lignin (AmL) has significant potential as a superior bio-adsorbent due to its unique structure and properties. In this study, AmL was synthesized by a Mannich reaction from coconut fiber alkali lignin (CFAL). The structure of CFAL and AmL was confirmed with the help of FT-MIR, ¹H-NMR spectroscopy, and CHNS elemental analysis. The crystallinity, morphology, and thermal properties were measured using XRD, SEM, and TGA, respectively. Finally, AmL was used to remove the anionic Congo red (CR) dye from an aqueous solution to investigate the performance of AmL compared to CFAL. The effect of pH, dose of adsorbent, and concentration of dye solution was investigated. 95.5% of the CR dye was removed within 80 minutes at pH 5 from a 20 ppm solution using 0.025 g of AmL. The dye removal efficiency of AmL samples was better than that of CFAL. This work demonstrates that AmL shows greater promise as a bio-adsorbent and can be further utilized as a green alternative in wastewater treatment. Thermodynamic analysis revealed that the adsorption process is spontaneous and exothermic, with negative Gibbs free energy (Δ<i>G</i>°) and enthalpy (Δ<i>H</i>°) values; AmL (Δ<i>H</i>° = -52.06 kJ mol^-1) displayed a stronger adsorption affinity than CFAL. In addition, reusability studies demonstrated that AmL retained 83.4% of its adsorption efficiency after four adsorption-desorption cycles, highlighting strong operational stability compared with CFAL. To complement the experimental findings, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out at the B3LYP/6-31G(d) basis set level. The results revealed that amination slightly reduces the HOMO-LUMO band gap (4.761 → 4.747 eV), enhances charge delocalization, and stabilizes excited states through partial n → π* transitions introduced by nitrogen functionalities. Electrostatic potential (ESP) mapping further demonstrated moderated charge localization and more uniform surface polarity in AmL, providing additional active sites for electrostatic interactions and hydrogen bonding with CR dyes. The theoretical insights are consistent with the experimental findings, indicating that electronic modification <i>via</i> amination enhances both adsorption affinity and charge-transfer efficiency.