Adsorption isotherms and kinetic modeling of methylene blue dye onto a carbonaceous hydrochar adsorbent derived from coffee husk waste

In this study, activated carbon in the form of carbonaceous hydrochar adsorbents with highly functionalized surface-active sites were produced from coffee husk waste via hydrothermal carbonization under low-temperature conditions (180 °C) and subsequent chemical activation. Thereafter, the hydrochars were characterized using diverse analytical techniques, and batch experiments of methylene blue (MB) adsorption were performed under various operating conditions. The results indicated that the activated hydrochar (AH) had a larger specific surface area (862.2 m2 g−1) compared to that of its carbonaceous precursor (33.7 m2 g−1). The maximum MB sorption capacity of the hydrochar activated with potassium hydroxide was extremely high (415.8 mg g−1 at 30 °C). In addition, adsorption isotherms and kinetics were studied using experimental data fitting to further understand and describe the dynamic equilibrium, dynamic kinetics, and mechanism of MB adsorption onto the prepared hydrochars. As compared to the Freundlich isotherm model, the Langmuir isotherm model provided a better fit with the experimental data exhibiting a maximum monolayer adsorption capacity of 418.78 mg g−1. The linear pseudo-second-order kinetic model was found to be suitable for describing the adsorptive kinetics of the hydrochar. The results demonstrated the immense potential of coffee husk waste to produce activated carbon as an alternative green hydrochar that can be applied to dye removal from wastewater as well as improvement of waste management.