Hydrothermal carbonization of spent coffee grounds: Hydrochar preparation, characterization, and adsorptive removal of rhodamine 6G

Hydrochars synthesized under different hydrothermal conditions were investigated to evaluate their structural properties and adsorption performance toward Rhodamine 6 G (Rh 6 G). Increasing hydrothermal temperature and residence time promoted the formation of a condensed carbon framework, consistent with progressive dehydration and aromatization. FTIR analysis revealed oxygen containing functional groups (-OH, C=O, and C=C), while EDX showed increasing carbon content and decreasing oxygen content with increasing hydrothermal severity. XRD patterns confirmed an amorphous structure, and nitrogen adsorption-desorption analysis indicated low BET surface areas of 2.0-4.53 m 2 g -1 . Batch adsorption experiments showed rapid initial uptake of Rh 6 G followed by equilibrium, with removal efficiencies of 72-96%. Adsorption kinetics followed the pseudo-second-order model (R 2 = 0.991-0.9998), indicating surface-controlled adsorption. Equilibrium data fitted the Langmuir isotherm, suggesting monolayer adsorption, with maximum adsorption capacities of 299.21 ± 2.97 to 633.12 ± 12.17 mg g -1 . Thermodynamic analysis indicated spontaneous and endothermic adsorption, with enthalpy values of 20.81-27.71 kJ mol -1 and positive entropy changes. Adsorption efficiency increased with increasing solution pH and adsorbent dosage. Post-adsorption FTIR spectra showed peak shifts and new bands, confirming surface functional group involvement in dye binding. Despite the low surface area, high adsorption performance was attributed to electrostatic attraction, π-π interactions, and hydrogen bonding. Reusability tests showed that the hydrochars retained approximately 85% of their initial adsorption capacity after four cycles, demonstrating their potential as reusable adsorbents for dye-contaminated wastewater. • Hydrothermal conditions strongly influence hydrochar structure and surface chemistry • Hydrochars show 72-96% Rhodamine 6 G removal despite low BET surface areas • Adsorption follows pseudo-second-order kinetics and Langmuir monolayer behavior • Solution pH influences adsorption via multiple dye-surface interaction mechanisms • Hydrochars retain about 85% adsorption capacity after four reuse cycles