Evaluation of chemically activated hydrochar from filter coffee waste for ibuprofen removal: process optimisation using response surface methodology

In this study, hydrochars prepared by hydrothermal carbonization from filter coffee waste, which is released in large quantities from the coffee brewing and processing industries and is considered an industrial byproduct due to its organic content, were activated with three different chemical agents, and the activated hydrochar with the highest surface area (1591.89 m2/g) was used as an adsorbent for ibuprofen removal from water. For this purpose, the activated hydrochar was comprehensively characterized using SEM, FT-IR, TGA, and BET analyses. The adsorption process was systematically optimized using response surface methodology, ensuring accurate prediction and optimization of maximum ibuprofen removal efficiency under varying operational parameters (pH, temperature, and contact time). ANOVA results showed that achieving maximum removal efficiency (93.52%) at an initial concentration of 50 mg/L and an adsorbent amount of 0.1 g/50 mL solution required a contact time of 91 min under ambient conditions of pH = 2.35 and 28 °C. Experimental results showed that the adsorption process was best described by the pseudo second order model (R2 = 0.985) and the Freundlich isotherm (R2 = 0.982). Thermodynamic analysis revealed that the adsorption process was spontaneous and predominantly endothermic. Gibbs free energy (ΔG°) values decreased from −5.03 kJ/mol to −16.17 kJ/mol, confirming the favorable nature of ibuprofen removal. Enthalpy (ΔH°) was determined as 16.99 kJ/mol, while entropy (ΔS°) took a value of 68.77 J/mol.K, indicating increased randomness at the solid-liquid interface. This study demonstrates that activated hydrochars, which can be regenerated and reused for up to four additional cycles while maintaining a high ibuprofen removal efficiency of 80.45%, have considerable potential as effective and sustainable adsorbents for the treatment of aqueous solutions. Moreover, by demonstrating the conversion of waste into value-added products, this approach offers a promising pathway for reducing pharmaceutical-related ecotoxicological risks, conserving water resources, and advancing the development of low-cost, and environmentally friendly water treatment technologies.