Transforming Waste into Value: A Sustainable Zero-Waste Biorefinery for Biochar Production and Gallic Acid Adsorption from Apple Pomace

The zero-waste biorefinery maximizes the use of biomass and reduces environmental impacts, transforming waste into high value-added products. In this study, the apple pomace underwent three rounds of extraction of phenolic compounds, sugar, and pectin recovery. Considering the biorefinery concept, the solid residue from both processes was used for biochar synthesis. The evaluation of the adsorptive efficiency of biochar and its characterization occurred by three synthesis routes: (1) biochar from the residue after the extraction of phenolic compounds and sugar (CPB); (2) from the residue after the extraction of phenolic compounds, sugar, and pectin (CPPB); and (3) from dry apple pomace (APB). The sequential extraction yields of total phenolic compounds were 4.6 ± 0.2, 1.2 ± 0.1, and 0.33 ± 0.01 mg GAE g^-1, respectively, for the first, second, and third extraction rounds. The pectin yield was 13.74 ± 0.25% with a degree of esterification of 66.38%. The gallic acid adsorption assay at 10 mg L^-1 provided an adsorption efficiency of 89.93 ± 0.87% for the CPPB sample. The Avrami model with a theoretical equilibrium adsorption of 7.7 ± 0.1 mg g^-1 biochar represented the adsorption kinetics of gallic acid by the CPPB sample with <i>R</i> ² = 0.9953. The intraparticle diffusion model presented multilinearity with two stages. Finally, the Path2green sustainable extraction metric scored 0.665, demonstrating strong adherence to green chemistry principles and new perspectives for industrial processes applied to the pharmaceutical, food, and cosmetic sectors.