Characterization of protonated amine modified lotus (Nelumbo nucifera) stem powder and its application in the removal of textile (Reactive Red 120) dye from liquid phase

The protonated amine modified-lotus stem powder (PAM-LSP) can be used to eliminate Reactive Red 120 (RR-120) from an aqueous solution as a potential adsorbent. Different operating variables like pH, agitation speed, adsorbent dosage, contact time, temperature, and initial RR-120 concentration were highly dependent on the adsorption process. FE-SEM (morphological), EDX (elemental), FTIR (functional), and N2 adsorption/desorption isotherms (texture properties) have been conducted to detect the adsorptive characteristics of LSP and PAM-LSP. To analyze the equilibrium data, non-linear isothermal models of Langmuir, Temkin, D-R (Dubinin-Radushkevich), and Freundlich were used. Among the four isothermal models, as shown by lower χ2 value (19.85), higher R2 (0.9956), the Langmuir matches the experimental data better than others. The experimental data were examined using non-linearized kinetic models of PFO (pseudo-first-order) and PSO (pseudo-second-order) and were better represented in the PSO model (R2 > 0.9902). It is noticeable that the adsorption of the RR-120 on PAM-LSP was a feasible, spontaneous, and endothermic physisorption process based on the calculated thermodynamic parameters like ΔHo (10.07 kJ/mol), ΔSo (33.8 J/mol K), ΔGo (-0.0539 to −1.3816 kJ/mol), Ea (5.9 kJ/mol), and S* (0.0043). Desorption experiments were performed with some selected eluents and the best desorbing eluent was found between NaOH, NaHCO3, NaCl, HCl, HNO3, CH3COOH, and DIW. The PAM-LSP was used without significant adsorptive performance loss for five adsorption/desorption cycles. Electrostatic interactions between the RR-120 dye molecules and the PAM-LSP are responsible for the primary adsorption mechanism. To remove RR-120 from wastewater, the eco-friendly prepared PAM-LSP was considered a highly recyclable and efficient adsorbent.