Preparation and characterization of hydrochars and CO₂-activated hydrochars from date and olive stones

Agriculture and food processing waste valorization is a sustainable process to protect the environment and to produce high added–value products. In this context, this study focuses on the conversion of two abundant agrifood residues largely available in Algeria, namely date pits and olive stones into carbonaceous materials. These biomasses were hydrothermally carbonized at 220 °C for various residence times ranging between 10 and 48 h. Then, the derived hydrochars were activated at 750 °C for 1 and 3 h under a carbon dioxide (CO₂) atmosphere. Several techniques were used to characterize the obtained carbonaceous materials including gas adsorption, scanning electron microscopy (SEM), Fourier transform infrared (FTIR), Raman, and X-ray diffraction spectroscopy. Results showed that the obtained hydrochars present high mass yields, important amounts of oxygen groups, relatively low porosities, and low specific surface area (less than 24 m²/g for all hydrochars). However, the derived activated carbons (after 1 h of CO₂ activation) show an increase of specific surface areas to 449 and 562 m²/g for date pits and olive stones, respectively. Moreover, the hydrochar activation process leads also to the development of both narrow and large micropores. The increase in the residence time favors the formation of carbon microspheres and increases the microcrystallites (aromatic rings). This spherical structure development is significantly promoted after the activation process. Therefore, the produced activated carbons have interesting textural, structural, and chemical properties indicating their potential for pollutants removal from aqueous and gaseous effluents. Graphical Abstract: [Figure not available: see fulltext.]