Valorization of Opuntia dillenii biomass into a reusable bio-based composite for sustainable and efficient methyl orange adsorption.

The discharge of dye-laden effluents from textile industries poses serious environmental and health risks, necessitating the development of efficient and sustainable adsorbents. In this study, a novel bio-based composite adsorbent (SPCA) was rationally designed and fabricated via the valorization of Opuntia dillenii-derived pectin combined with sodium alginate, chitosan, activated carbon, and eco-friendly additives. The composite was synthesized under mild conditions through dual crosslinking, involving Zn2+-mediated ionic coordination and glutaraldehyde-induced covalent bonding, while sodium bicarbonate served as a green pore-forming agent to generate a hierarchical porous structure. Structural characterization (FT-IR, PXRD, SEM) confirmed the formation of an amorphous, interconnected polymeric network enriched with abundant oxygen- and nitrogen-containing functional groups. BET analysis revealed a well-developed mesoporous architecture with high surface area, and thermal analyses (TGA-DSC) demonstrated enhanced structural stability resulting from synergistic intermolecular interactions. Under optimal conditions (pH 4 and 4 h contact time), SPCA exhibited a maximum adsorption capacity of 18.55 mg g-1 at an initial MO concentration of 200 mg L-1, while achieving 96.0% removal efficiency with a capacity of 14.38 mg g-1 at 150 mg L-1. Kinetic studies indicated that adsorption follows a pseudo-second-order model (R 2 = 0.9985), suggesting chemisorption-dominated interactions, and equilibrium data were best described by the Freundlich isotherm (R 2 = 0.9945), confirming heterogeneous multilayer adsorption. Notably, SPCA retained 83.62% of its initial adsorption efficiency after six regeneration cycles using ethanol as a benign desorbing solvent. Overall, this work demonstrates an effective strategy for transforming underutilized cactus biomass into a reusable, environmentally benign adsorbent for dye-contaminated wastewater treatment.