Elucidating the coupling of deep eutectic solvents to the hydrothermal carbonization of palm kernel shell towards selective pollutant removal

Palm kernel shell (PKS) residues are typically discarded or utilized in co-combustion with other organic and fossil fuels, releasing harmful gases. This study aims to significantly advance the coupling of deep eutectic solvents (DES) to hydrothermal carbonization (HTC) for the valorization of PKS. This strategy enhances the adsorption capacity of hydrochars for challenging contaminants that are typically resistant to adsorption without using energy-intensive or chemically demanding treatments. Therefore, it expands the application of DES-modified hydrochars on selective pollutant adsorption. Material characterization via SEM–EDS, FTIR, XRD, TGA, and BET indicates how the DES treatment (i.e., before or after HTC) significantly affected the materials’ morphology, surface modification, and texture. Remarkably, applying DES treatment after HTC appears less effective, indicating that the order and combination of treatments are critical in optimizing adsorption performance. The DES treatment alone is the most effective method for enhancing cationic pollutant adsorption capacity, achieving over 80% adsorption, probably due to increased surface area and the introduction of functional groups. Conversely, the combination of HTC and a DES treatment shows superior performance for adsorbing an anionic dye (nearly 60% adsorption) and diclofenac (close to 90% adsorption). In comparison, untreated materials show negligible adsorption for the anionic dye and less than 40% adsorption for diclofenac. The treatments complement each other by modifying the surface chemistry and promoting pollutant-material interactions.