Hydrothermal Carbonization of Sucrose into Hydrochar: Insights into Surface Chemistry for Efficient Removal of Cationic Dyes

Abstract Synthetic dyes, which are extensively used in textile and other industries, produce wastewater that is toxic, recalcitrant, and non-biodegradable. As such, environmental concerns are drawing attention to sustainable adsorbents of the dyes. In this study, hydrochar was green-synthesized using table sugar (sucrose) via hydrothermal carbonization (HTC) at 200 °C for 12 h without toxic additives. The hydrochar was analyzed in terms of elemental, thermal stability, and point of zero charge (pHPZC ≈ 6). Adsorption studies were performed using cationic dyes, methylene blue (MB), rhodamine B (RhB), and rhodamine 6G (R6G), as well as anionic dyes, methyl orange and eosin blue, for comparative purposes. The hydrochar showed high adsorption for MB (2.0747 mg/g), RhB (3.5063 mg/g), and R6G (1.8268 mg/g), while nearly no anionic dyes were adsorbed. The kinetics data were well described by a pseudo-second-order model, and the equilibrium data were well described by a Langmuir isotherm, indicating that monolayer adsorption was occurring on homogeneous surface sites. The reusability evaluations revealed stable MB adsorption and moderate desorption (in the case of RhB and R6G) through several cycles, indicating practical utility. Moreover, the high adsorption capacity and good recyclability of the sucrose-derived hydrochar demonstrated its potential as an effective, low-cost, and environmentally friendly adsorbent for cationic dye removal, contributing to the application of sustainable water treatment in the context of a circular economy.