Co-pyrolyzed and valorized municipal solid wastes and coconut husks into biochar adsorbing methylene blue in aqueous solution

The adsorption potential of biochar derived from municipal solid waste (MSW) and coconut husk (CH) for methylene blue (MB) removal was investigated in this study. Both produced biochars exhibited a pH_PZC of 8.5 with no observed significant compositional changes (consistent with literature values). The integration of CH in the co-pyrolysis process altered the surface morphology of MSW-BC. Moreover, the post-adsorption images indicated MB deposition on the biochar surfaces. The MCH-BC had 559 m² g⁻¹ specific surface area and about 29 mg g⁻¹ adsorption capacity, and practically indicated superior adsorbent quality than MSW-BC (with corresponding lower values). Furthermore, the adsorption behaviors suggested that the phenomena were (1) potentially influenced by chemisorption from kinetic studies, (2) best fitted the Freundlich isotherm model, denoting a heterogeneous adsorption mechanism, and (3) endothermic and spontaneous from the thermodynamic analysis. The proposed adsorption mechanism was feasibly dominated by physisorption, as indicated by low ΔG° values (−1.234 kJ mol⁻¹ to −4.531 kJ mol⁻¹) and functional groups, likely facilitating hydrogen bonding, which perhaps followed by initially identified chemisorption from kinetic studies. Finally, the study highlighted the potential of co-pyrolyzed MSW and CH biochar as a competitive low-energy synthesis, producing an adsorbent for MB removal.