Ball-milling tailored corn straw biochar: A dramatic shift from physical to chemical adsorption dominance for enhanced ammonia nitrogen removal in source-separated urine

Enhancing the functional groups on carbonized straw pores can boost adsorption, though mechanisms need study. This study used corn straw pyrolysis (500 °C) to produce biochar (B), and prepared ball milled biochar (BMB) with abundant O-containing functional groups on the surface. Batch tests measured NH₄⁺-N adsorption from synthetic urine (containing 0.5 mol/L NH 4 + -N). Combined characterization (SEM/XPS/EPR) and adsorption models revealed surface functionalities, •OH radicals, and oxygen vacancies roles in enhanced chemisorption. Results show that ball milling significantly increased surface oxygen-containing functional groups (such as -OH, -COOH), hydroxyl radicals, and oxygen vacancies, thereby enhancing chemical adsorption sites. Dynamics studies and microscopic characterization jointly indicate that the dominant adsorption mechanism of biochar after ball milling has shifted from physical adsorption (B) to chemical adsorption (BMB) with pore diffusion. The adsorption capacity of NH 4 + -N increased by 28 % (from 32 mg/g to 41 mg/g). These results confirm that ball milling technology effectively overcomes the physical adsorption limitations of traditional straw biochar and improves its equilibrium adsorption capacity. Oxygen-rich BMB provides an efficient and low-cost solution for nitrogen recovery from urine wastewater, providing critical technical support for sustainable agricultural waste utilization and decentralized sanitation management. • Ball milling increased the ammonium nitrogen adsorption capacity of biochar by 28 %. • The main adsorption mechanism shifted from physical to chemical, particularly in BMB. • Pore size and free radicals can affect the initial adsorption efficiency. • The increase in oxygen-containing functional groups due to the increase in specific surface area determines the adsorption upper limit.