Investigation of olive stone‐derived hydrochar produced via hydrothermal carbonization for methylene blue removal

Abstract BACKGROUND Hydrochar produced from olive ( Olea europaea ) stones via hydrothermal carbonization at 200 °C for 4 h was investigated as a sustainable adsorbent for methylene blue (MB) removal from aqueous solutions. RESULTS The hydrochar exhibited an amorphous carbon structure with abundant oxygen‐containing functional groups, a low Brunauer–Emmett–Teller surface area (1.143 m 2 g −1 ) and a zeta potential of −19.4 mV, confirming the predominance of negatively charged sites. Batch adsorption experiments were conducted at the natural pH of dye solution to examine the effects of contact time, initial concentration and temperature on MB adsorption. The maximum adsorption capacity estimated by the Langmuir model was 46.3 mg g −1 . The equilibrium data fitted best to the Temkin model ( R 2 = 0.987), indicating physisorption controlled by surface functional groups, while kinetic analysis showed that the process followed the pseudo‐second‐order model ( R 2 = 0.99), implying surface‐controlled interactions. Thermodynamic parameters (Δ G ° = −0.17 to −3.27 kJ mol −1 ; Δ H ° = +72.23 kJ mol −1 ; Δ S ° = +245.76 J mol −1 K −1 ) revealed that the adsorption was spontaneous, endothermic and entropy‐driven. CONCLUSIONS Olive stone‐derived hydrochar is a promising, low‐cost and sustainable adsorbent for the removal of MB from dye‐containing wastewater. Adsorption proceeded mainly via surface interactions and was dominated by physisorption, as supported by kinetic, isotherm and thermodynamic analyses. The adsorption capacity increased with temperature within the investigated range, indicating an endothermic and spontaneous process. © 2026 Society of Chemical Industry (SCI).