Hydrothermal carbonization (HTC) of rice husk was optimized in terms of the adsorption capacity at equilibrium (q_e) and hydrochar mass yield (MY). The studied variables were reaction temperature, residence time, and biomass-to-water ratio by means of response surface methodology. In both cases, reaction temperature resulted the most significant parameter promoting high q_e values at higher temperatures when treating methylene blue (MB) as the target pollutant. Nevertheless, MY was low (~40%) when focusing on a possible industrial application. Hence, maximizing q_e and MY simultaneously by optimization of multiple responses emerges as a promising solution to improve MY values (>60%) with no significant differences regarding the q_e response. Furthermore, additional activation was conducted on optimal hydrochars to further investigate the enhancement of q_e. As a result, no statistical differences between non-modified and activated hydrochars were observed for q_e; however, the pseudo-second-order constant (k₂) seemed to be increased after alkali activation, mainly due to a larger surface area. Non-modified and activated hydrochars were characterized via SEM, FTIR, XRD, and BET, resulting in two significant effects contributing to MB adsorption: increased surface area and functionalized hydrochar surface. Consequently, this work provides valuable insights on subsequent application of this HTC optimization scheme at an industrial scale.

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 (pH_PZC = 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.