Sorption mechanisms of antibiotic sulfamethazine (SMT) on magnetite-coated biochar: pH-dependence and redox transformation

Sorption of sulfonamides (SAs) on magnetite-coated biochar (MBC) is a promising approach for the remediation of antibiotic contaminants, due to its extended adsorption capacity and irreversibility. However, the actual sorption mechanisms of SAs on MBC remain unclear and the gap in knowledge hinders understanding of the fate of SAs in soils or sediments. In this study, various MBCs were prepared under different pyrolysis temperatures, with batch sorption experiments conducted using SMT as the model pollutant. Results of a two-compartment kinetic model demonstrated that aromatic components of MBCs dominated slow-sorption mechanisms, whereas the embedded magnetite further accelerated fast-sorption due to H-bonding. Modification of BC with magnetite improved the distribution coefficient (Kd) and isotherm linearity of SMT. Multi-parameter model results indicated that the pH-dependence of SMT sorption on BCs and MBCs occurred via a dominant mechanism of π-bond assisted H-bonding. Compared to pristine BCs, the change in pH-dependent sorption characteristics of SMT on MBC results from the regulation of π-bonding and proton configuration. Simultaneous transformation of SMT to sulfate ions on BCs or MBCs was also observed. The degradation of SMT occurred because of persistent free radicals (PFRs) on BCs or the inherent redox of iron minerals on MBCs. However, the small fraction of SMT transformed on BCs or MBCs was not found to result in overestimation of SMT sorption. This study presents the critical mechanisms of SMT sorption on pyrochars and provides novel understanding of the fate of SMT on carbonaceous materials during practical remediation applications.