A review on the recent advances in the use of hydrochar for adsorption of methylene blue dye from aqueous systems

This study reviewed the use of hydrochars in the adsorption of methylene blue, a toxic dye, from aqueous phases. Journal articles published predominantly between 2020 and 2025 were assessed. The articles were mainly identified through keyword searches on ScienceDirect, Scopus and Web of Science. The studies reported a high efficacy of hydrochars against the dye. Organic wastes were primarily used as a feedstock in the hydrothermal carbonization processes, aligning to the waste to resource principle, an efficient and cost effective route of synthesis of adsorbents. The pseudo-first order, pseudo-second order and the Elovich model best described the kinetic data while the Freundlich, Sips and Langmuir isotherms best fitted the experimental data, with model predicted adsorption capacities in the range of 30-1060 mg g− 1. All the reviewed studies reported a spontaneous and feasible adsorption process (negative ΔG), while the entropy (ΔS) and enthalpy (ΔH) values varied with feedstock used, the specific modification routes and modification conditions. Several systems retained between 65 and 95% capacity over five cycles with eluents such as HCl, ethanol and methanol. The mechanism of adsorption was reported by most of the studies to be through electrostatic interactions, hydrogen bonding and π-π interactions.