Enhanced removal of ammonium nitrogen from aqueous solutions using a novel biochar derived from millet shells through both static adsorption and dynamic column experiments

Ammonia pollution in the water environment poses significant risks and hazards to the environment and human health. This study explored efficient removal of ammonia nitrogen (NH4+-N) from water using millet shell biochar (MS-400) through static and dynamic column experiments. In static experiments, three factors were investigated using Box-Behnken design (BBD) method and it was found that MS-400 removed 95.07 % of NH4+-N under optimal adsorption conditions. Equilibrium data fitted well with the Freundlich isotherm (R2 = 0.990), while the pseudo-second-order model (R2 = 0.955) better described NH4+-N adsorption behavior, indicating chemisorption involving electron sharing and transfer between MS-400 and NH4+-N ions. In the dynamic column experiment, BBD was also applied, achieving a maximum NH4+-N adsorption rate of 78.02 % under optimal conditions. Moreover both the Thomas model and Yoon-Nelson model demonstrated excellent fitting performance, offering practical insights for industrial scale-up and production applications. Furthermore significant reductions in intensities for C-C/C-H, C=O, and -OH peaks were observed upon NH4+-N adsorption and direct hydrogen bonding between NH4+-N and oxygen-functional groups enhanced negatively charged sites for NH4+-N adsorption, suggesting electrostatic attraction, ion exchange and hydrogen bonding in the adsorption process. This study introduced MS-400 as a promising biosorbent for NH4+-N removal, providing a comprehensive analysis of NH4+-N adsorption behavior through static and dynamic column experiments. The findings offer practical guidance for operational applications.