Influence of Carbonization and Steam Activation Temperatures on the Physicochemical and Sorption Properties of Activated Carbons Derived from Agricultural and Agri-Food Biowaste

This study investigated the influence of combined carbonization and steam activation temperatures on the physicochemical and adsorption-related properties of activated carbons derived from five agricultural and agri-food biowastes: sugar beet pulp, sunflower husk pellets, apple pomace, straw, and corn cobs. The precursors were carbonized at 400, 500, or 600 °C and subsequently activated with steam at 600, 700, or 850 °C, respectively. Apple pomace, straw, and corn cobs were pelletized using sugar beet molasses as a binder. The resulting materials were characterized in terms of process yield, iodine number, BET specific surface area, ash content, and other physicochemical parameters. Selected activated carbons were additionally examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Increasing thermal treatment severity generally improved the adsorption-related properties, although the response was strongly precursor-dependent. Sugar beet pulp exhibited the highest BET specific surface area (459 m2/g) and iodine number (477 mg/g), while corn cob-derived activated carbon reached 449 m2/g and 467 mg/g, respectively, after the 600/850 °C treatment sequence. SEM observations revealed limited morphological differences between sugar beet pulp samples treated at 500/700 and 600/850 °C, whereas corn cob-derived carbons exhibited more pronounced surface fragmentation and visible cavities at higher treatment severity. EDS analysis revealed distinct inorganic elemental signatures, particularly Ca in sugar beet pulp-derived carbon and K in corn cob-derived carbon. Improved adsorption-related properties were accompanied by reduced material yields, while straw exhibited limited thermal stability under the most severe treatment conditions. The findings demonstrate the importance of precursor-specific optimization of thermal processing conditions and the combined evaluation of physicochemical, morphological, and material yield characteristics.