Valorization of Olive Pomace into Functional Hydrochars for Dye Removal from Water: Effects of Hydrothermal Carbonization and Soft Alkaline Activation

Olive pomace (OP), an abundant byproduct of the olive oil industry, was investigated as a low-cost precursor for the production of hydrochar-based adsorbents for methylene blue (MB) removal from water. Hydrothermal carbonization (HTC) was performed at 180 and 220 °C with different residence times, followed by soft alkaline activation (SAA), consisting of alkaline treatment and carbonization at 300 °C, and post-washing to improve the adsorption performance. The materials were characterized by ATR-FTIR, Raman spectroscopy, XRD, SEM-EDX, BET, and mercury intrusion porosimetry. HTC increased the degree of carbonization and surface area, but non-activated hydrochars showed lower adsorption capacity than untreated OP because of the loss of oxygenated functional groups. Among non-activated hydrochars, OP_220_1 showed the highest adsorption capacity (20.7 mg g−1). SAA increased the adsorption capacity of the hydrochar by 63%, while washing further enhanced performance by removing carbonate precipitates and restoring pore accessibility. The washed activated hydrochar derived from OP_220_1 achieved the highest adsorption capacity (69.9 mg g−1 at 250 mg L−1 MB) and BET surface area (40.0 m2 g−1). Kinetics followed the pseudo-second-order model, while washing shifted the isotherm behavior from Langmuir to Freundlich. A performance-based assessment showed that the washed activated hydrochar reduced adsorbent demand by 49% compared with untreated OP.