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.

β-lactam antibiotics are frequently investigated as emerging contaminants in aquatic environments because of their widespread use and incomplete removal in conventional treatment systems. In this study, sulfur- and nitrogen-co-doped activated carbon (S,N-AC) was prepared from rice husk through carbonisation, KOH activation, and thiourea-assisted heteroatom functionalisation. Pristine rice husk (PRH), rice husk biochar (RHB), and S,N-AC were characterised using nitrogen adsorption-desorption, SEM-EDX, FTIR, XRD, TGA, and zeta-potential measurements. S,N-AC exhibited a BET surface area of 1126 m² g⁻¹ and a developed micro–mesoporous structure. Its adsorption performance was evaluated for cephalexin, cefprozil, and cefixime in single-solute and ternary systems. The Langmuir model provided the highest correlation for S,N-AC, with fitted qmax values of 285, 260, and 295 mg g⁻¹ for cephalexin, cefprozil, and cefixime, respectively. Kinetic data were described using pseudo-first-order, pseudo-second-order, and intraparticle-diffusion models. Thermodynamic parameters indicated spontaneous and exothermic adsorption over the investigated temperature range. In the ternary system, adsorption capacities decreased relative to the corresponding single-solute experiments, consistent with competition for adsorption sites. XPS and surface-charge measurements were used together with textural and kinetic results to discuss possible adsorption interactions. The results indicate that S,N-AC has potential for further investigation as a rice-husk-derived adsorbent for antibiotic-contaminated aqueous systems.