N, S-Functionalized Magnetic Rice Husk Biochar for Flucloxacillin Sodium Adsorption: Regeneration and Water-Matrix Effects

Rice husk-derived biochars with progressively modified surface properties were investigated for adsorbing flucloxacillin sodium (FLX-Na) from aqueous solution. Four materials—rice husk biochar (RHB), nitrogen-functionalized RHB (N-RHB), magnetic nitrogen-functionalized RHB (MN-RHB), and magnetic nitrogen/sulfur-functionalized RHB (MN,S-RHB)—were prepared and characterised using morphological, structural, textural, and surface-chemical techniques. The sequential preparation routes produced materials with different surface areas, pore structures, and surface functionalities. FLX-Na adsorption increased in the order RHB < MN-RHB < N-RHB < MN, S-RHB, with the highest equilibrium adsorption capacity obtained for MN,S-RHB (118.6 mg g−1). Because chemical activation and a higher carbonisation temperature were introduced only for MN,S-RHB, the higher uptake is interpreted as an effect of the combined modification route rather than being attributed specifically to N/S co-doping. Kinetic and equilibrium data were described using several empirical models, with the pseudo-second-order and Sips models giving the highest coefficients of determination among the tested fits. The adsorption capacity of MN,S-RHB decreased from 84.60 ± 1.30 mg g−1 in deionised water to 60.30 ± 1.80 mg g−1 in a mixed inorganic/humic-acid matrix, corresponding to 71.3% capacity retention. Spectroscopic changes after adsorption were consistent with changes in the surface chemical environment, while the matrix experiments indicated that coexisting inorganic ions and dissolved organic matter influenced adsorption performance. These results show that the combined physicochemical modification route altered the adsorption behaviour of rice husk-derived biochar under the investigated conditions.