Mechanistic insights to sorptive removal of four sulfonamide antibiotics from water using magnetite-functionalized biochar

Magnetite-functionalized biochar(MBC)is a promising engineered material for remediation of antibiotic-contaminated fields.However,sorption mechanisms of ionizable organic compounds such as sulfonamide antibiotics(SAs)on MBC are still unclear.This study employed four representative SAs including sulfamethazine(SMT),sulfamerazine(SMR),sulfadiazine(SDZ),and sulfamethoxazole(SMX),to compare the difference in sorption on MBC.Results showed that the sorption capacities and affinities of the four SAs varied with their substituents,hydrophobic properties,and dissociation constants(pKa).Synergistic effect during co-pyrolysis with Fe^(3+)enhanced the sorption performance of MBC towards SAs compared to original BC.Spectral methods confirmed structural changes of MBC such as the variance in oxygen-containing groups and defective/graphitized phases.Results of modeling pH-dependent sorption revealed that H-bonding orπ-bond assisted H-bonding determined the sorption affinities and capacities of SAs.In particular,the SAs with lower pKa were thermodynamically favorable to form H-bonding with MBC via proton exchange with water molecules.Quantum calculation results quantified the contributions of H-bonding strengths and found that the energies of H-bonding were correlated with affinities of SAs.Moreover,contributions of oxygencontaining groups instead of minerals dominated the H-bonding energies.Mechanistic insights from this study can be valuable in exploring engineered BC composites for practical application in field remediation.