Evaluation of adsorption performance and applicability of agricultural by-product biochar derived from molten salt-assisted pyrolysis

Molten salt-assisted pyrolysis has been proposed to address the limited heat transfer of conventional gas-phase pyrolysis. Beyond enhancing heat transfer, molten salt could also modify the physicochemical characteristics of the resulting biochar and thereby affect its adsorption performance. In particular, alkaline molten salts have been reported to facilitate the modification of biomass-derived carbon materials; therefore, NaOH-Na 2 CO 3 was selected as a molten salt medium in this study due to its strong alkalinity, relatively low cost, and suitable eutectic characteristics for the pyrolysis conditions. However, the catalytic effects of NaOH-Na 2 CO 3 molten salt have not been systematically elucidated across different agricultural by-products, and the feasibility of molten salt recovery remains insufficiently explored. Therefore, this study systematically investigated the physicochemical properties and adsorption performance of biochar prepared from different agricultural by-products using a binary NaOH-Na 2 CO 3 molten salt and evaluated the feasibility of salt recovery. Molten salt-assisted pyrolysis significantly increased the BET surface area (119.12 to 430.98 m 2 /g), enhanced aromatic condensation, and enriched the oxygen-containing functional groups. The adsorption capacity also increased for methylene blue (449.50 -874.79 mg/g) and for sulfamethazine (173.00-456.67 mg/g). In addition, more than 98% of the sodium was recovered after pyrolysis, and the recovered Na 2 CO 3 exhibited high thermal stability. These findings demonstrate the potential of NaOH-Na 2 CO 3 molten salt-assisted pyrolysis for modifying the physicochemical and adsorption properties of agricultural by-products biochar. Also, the effective salt recovery system should support the development of a sustainable molten salt-assisted pyrolysis process.