Preparation of Adsorbent Materials from Rice Husk via Hydrothermal Carbonization: Optimization of Operating Conditions and Alkali Activation

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.