Coating magnetite alters the mechanisms and site energy for sulfonamide antibiotic sorption on biochar.

Magnetite-coating biochar (MBC) is a promising remediator for antibiotic contamination. Accurate models describing the sorption affinity are required to better understand the role of minerals. In this study, the presence of magnetite led to the improvements of oxygen-containing groups (i.e. C˭O) and regulation of π-systems within BC. Based on Dubinin-Ashtakhov (DA) model, the differences of site energy (E) and sorption heterogeneity (σ*) led to the variances between sorption capacities of sulfonamides (SAs). The positive correlations between E and the oxygen content or pore volume of MBCs indicated that π-π interactions, H-bonding, and pore-filling may act as the high energy sites. Moreover, σ* was related to the distribution of magnetite on BC and their porosities. These results suggested that compared to BCs, the coating minerals improved the π-interaction assisted H-bonding and proton configuration of antibiotic when sorbing on MBC. The negative correlations between the E of different SAs with their molecular sizes and solubilities resulted from steric effects and competition with water, which further confirmed the proposed high energy sites on MBCs. This study provided the insightful information of site energy distribution and understanding of fate and transport of organic pollutants on BC when the iron minerals were embedded or coated.