Glutathione crosslinked network of pine needles and carrageenan as an efficient adsorbent for methylene blue dye

To promote sustainability through the circular economy, we synthesized a crosslinked network of lignocellulosic biomass, pine needles (PNs), and a biopolymer, carrageenan (Carr), using glutathione (Glu) as a crosslinker. Glu, which contains two terminal carboxyl groups, underwent esterification with the hydroxyl groups of the components of PNs (cellulose, hemicellulose, and lignin) at one end and the hydroxyl groups of Carr at the other end, forming crosslinked PNs-cl-Carr. We characterized the as-synthesized PNs-cl-Carr by FTIR, XRD, FESEM, EDX, surface charge analysis, TGA, BET, and ¹³C NMR and evaluated the network for the removal of methylene blue (MB) dye from aqueous solution under varying adsorption conditions, viz., time, temperature, pH, and initial dye concentration. The network achieved the highest removal of MB (93.04%) within 30 min at 35 °C and pH 7.0 using an initial MB concentration of 50 ppm. We applied non-linear kinetic models, namely pseudo-first-order, pseudo-second-order (PSO), Elovich, and intraparticle diffusion, and isotherm models, namely Langmuir, Freundlich, and Temkin, to investigate the adsorption mechanism. The adsorption followed the PSO kinetic model and the Langmuir isotherm with a maximum adsorption capacity of 247.9 mg g^-1. Furthermore, reusability studies of PNs-cl-Carr showed a cumulative adsorption capacity of 684.57 mg g^-1 after nine cycles, demonstrating its regeneration ability and potential for prospective wastewater treatment applications.