Valorization of the Organic Fraction of Municipal Solid Waste (OFMSW): Biochar Production, Activation, and Characterization for Hexavalent Chromium Adsorption

The valorization of the organic fraction of municipal solid waste (OFMSW), an underexplored lignocellulosic waste, represents a sustainable strategy for the production of adsorbents aimed at wastewater remediation, particularly against hexavalent chromium Cr(VI) from tannery effluents, a persistent environmental hazard in regions such as Tungurahua, Ecuador, where concentrations widely exceed regulatory limits. This study investigates the valorization of OFMSW pyrolyzed at 575 °C and subsequently activated by physical treatment with CO2 to produce an activated biochar (BCA-OFMSW) for remediating aqueous solutions containing Cr(VI). The structural, ultimate, and textural properties were characterized before and after activation, and batch adsorption experiments evaluated the effects of pH, adsorbent dosage, initial concentration, and contact time, followed by equilibrium and kinetic modeling. Activation increased the surface area by more than two orders of magnitude (NLDFT = 1159.58 m2 g−1). Maximum removal occurred at pH 2, consistent with the alkaline point of zero charge of the material (pHPZC = 11.47). The Freundlich model (R2 = 0.9590) fitted the equilibrium data better than the Langmuir model (R2 = 0.8453; qm = 58.86 mg g−1), and the Elovich model gave the highest R2 among the kinetic models tested (0.9654). Adsorption increased with temperature (ΔH° = +9.08 kJ mol−1, an approximate estimate derived from the Freundlich constant). The pH dependence suggests a contribution of electrostatic attraction; post-adsorption XRD and EDS show chromium at the surface but do not identify its oxidation state, so Cr(VI) reduction could not be assessed. These results indicate that OFMSW-derived activated biochar is a promising adsorbent for Cr(VI)-contaminated effluents.