Methylene blue removal from water using chaya-derived biochar: Effect of metabolite extraction on adsorption

This study evaluated biochars derived from Cnidoscolus aconitifolius (chaya) leaf residues for methylene blue (MB) removal from water. Two materials were produced under identical thermal conditions from untreated biomass (CCP-A) and post-extraction biomass (CCP-B), allowing direct assessment of the effect of metabolite extraction. Characterization by SEM/EDS, XRD, FTIR, Raman spectroscopy, XPS, and N2 adsorption-desorption showed that extraction altered surface chemistry and textural properties, increasing oxygen-containing functionalities and BET surface area from 1.92 to 6.10 m2 g-1 and decreasing mean pore diameter from 14.1 to 8.4 nm. These differences were associated with improved MB removal by CCP-B under dark and irradiation conditions. Kinetic analysis was consistent with multiple transport contributions. Among the equilibrium models, Langmuir yielded the highest linearized R2 and a model-estimated qmax of 155.3 mg g-1, whereas degrees-of-freedom-adjusted RMSE analysis on the original qe scale did not identify a single unequivocally superior model. Despite its relatively low BET surface area, the MB uptake of CCP-B suggests that adsorption cannot be explained solely by the dry surface accessible to N2 at 77 K. Oxygen-containing functionalities may contribute to electrostatic attraction and hydrogen bonding, whereas aromatic domains may support π-π interactions. Under UV and simulated solar irradiation, CCP-B reached overall removal efficiencies of 98.6% and 93.8%, respectively. Because adsorption and possible light-induced transformations were not quantified independently, these values represent overall photo-assisted removal. Inhibition by isopropanol was consistent with possible participation of hydroxyl radicals or related reactive oxygen species; however, this evidence was indirect, the species were not detected directly, and degradation products and mineralization were not evaluated. CCP-B removal efficiency decreased from approximately 92% to 31% over five adsorption-drying reuse cycles, indicating limited reuse performance without effective regeneration. Under the conditions evaluated, metabolite extraction before carbonization modified the surface properties of chaya-derived biochar and was associated with improved MB removal.