Hydrothermal Carbonization of Palm Kernel Shell: Surface Area and Oxygenated Functional Groups with Performance in Sodium Adsorption

In this work, we explore the use of oil palm kernel shells (PKS) to produce hydrochar (HC), which promotes sodium adsorption in saline soils. The aim was to identify a PKS-derived hydrochar with a high specific surface area and presence of oxygenated functional groups for sodium adsorption. We applied a 2² factorial design, varying temperature and time, with a randomized center point. The hydrochar were porous and superficially characterized by gas physisorption of nitrogen at 77 K to obtain the specific surface area (ABET), XPS for elemental composition, FTIR for functional groups, elemental analysis, and TGA for weight changes, and DTG for rate of weight change. Simple ANOVA results showed that temperature has a significant effect on ABET: ABET,180 °C < ABET,205 °C ≈ ABET,230 °C. Time did not have a significant effect, as the ABET ranges (4 h, 22 h, 40 h) overlapped. Thus, 205 °C and 22 h were selected as the operational conditions to produce the hydrochar (HC-205-22) for sodium adsorption analysis, due to the high ABET and oxygen/carbon ratio of 39.1 m² g^-1 and 0.35, respectively. The surface contained oxygenated groups of HC-205-22, showing mainly ether oxygen in anhydrides and lactones (C-O-C), alcohol-phenol bonds (C–OH), carbonyl (C=O), and carboxyl (O=C–O) groups. The Langmuir isotherm estimated a maximum sodium adsorption capacity of 62.57 mg g^-1 and an adsorption intensity (n) of 1.292, indicating that this material is suitable for sodium adsorption.