Rhodamine B sequestration using acid-precipitated and microwave-treated softwood lignin: Comparative isotherm, kinetics and thermodynamic studies

Sustainable development goals have emphasized the need to focus on water treatment to mitigate the ever-rising demands for clean water. High costs and energy requirements have been the Achilles’ heel of most treatment technologies. Herein, we develop cheap lignin-derived sorbents prepared by microwave (MW) and acid precipitation (AP) treatments of raw lignin (RL) for Rhodamine-B (RhB) adsorption in aqueous solution. Our findings evidence micro- and mesoporous structures, with irregular grain size. X-ray diffraction revealed an amorphous material, while Fourier-transform infrared analysis points to the presence of carboxyl, hydroxyl and sulfite functional groups, which may facilitate the adsorption of the dye. Adsorption isotherm and kinetic data describe complex and pore-driven interactions, based on the Sips, D-R and intraparticle diffusion models, considering the sum of squared error values obtained from nonlinear regression analysis. Adsorption efficiency of ∼97% is obtainable at optimal conditions for treated lignin (3 g/L sorbent dose, 720 min contact time and under acidic pH conditions). Thermodynamic studies revealed that RL-RhB and MW-RhB are driven by physisorption considering ∆H° values, while stronger interactions occurred for AP-RhB, considering the high enthalpy (∼49 kJ/mol) and adsorption capacity (∼2766 mg/g). Van der Waals attraction, π-π stacking, and pore-filling mechanisms are among the several interactions that may occur between RhB and the sorbents. Our findings offer a way to valorize lignin as an alternative pathway to economically viable and sustainable water purification.