Integral valorization of banana peel through pectin and biochar production for methylene blue adsorption from aqueous solutions

This investigation developed a sustainable approach for banana peel valorization through sequential extraction and thermal conversion processes. Pectin was initially isolated from banana peel biomass using phosphoric acid-assisted acid hydrolysis. The optimal extraction yield of 12.00% was achieved at a phosphoric acid concentration of 0.5 M. The acid-impregnated solid residue remaining after extraction was subsequently valorized for biochar synthesis via controlled pyrolysis at multiple thermochemical conversion temperatures (250–550 °C). Notably, the residual phosphoric acid in the substrate served as an intrinsic activating agent during carbonization, eliminating the need for external chemical activators and thereby reducing the production cost of biochar. The synthesized biochars were comprehensively characterized using a series of analytical techniques, including N₂ adsorption, scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR). Subsequently, the engineered biochars were systematically assessed for their adsorption performance of methylene blue in aqueous solutions. The biochar produced at the optimal pyrolysis temperature of 350 °C (BC350) exhibited superior textural properties, attaining the maximum BET surface area and adsorption capacity. A statistical physics-based multilayer adsorption model provided mechanistic insights, suggesting multi-molecular occupancy at heterogeneous adsorption sites (n = 2.20 molecules/site). According to this model, the saturated adsorption capacity of BC350 for methylene blue reached 173.4 mg g⁻¹. Thermodynamic analysis indicated physisorption dominance with endothermic characteristics. pH-dependent adsorption behavior was observed, suggesting the involvement of electrostatic interaction mechanisms.