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 (similar to 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 (k(2)) 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.