Comparative transformation of antibiotics and their salts by birnessite under low moisture condition: Moisture-dependent kinetics and the promoting role of exchangeable Fe(III)

The water solubility of antibiotics and their salts affect their environmental behavior. However, the influence of moisture on their interfacial transformation on mineral surfaces remain poorly understood. This study investigates the transformation of sulfonamide and macrolide antibiotics, along with their sodium salts, mediated by Fe(III)-exchanged birnessite (Fe(III)δ-MnO₂) under low moisture condition, with a focus on the critical roles of interfacial processes and moisture. Fe(III) exchange significantly enhanced the transformation rate constant ( k ) of sulfadiazine (SDZ) by 5.96-fold from 0.0100 to 0.0596   min⁻¹. Antibiotics transformation by δ-MnO₂ was inhibited as water content (WC) increased ( k for SDZ decreased from 0.0100   min⁻¹ at 6.1% WC 0.0013   min⁻¹ at 60% WC. In contrast, the transformation of antibiotic salts was less inhibited by moisture. Notably, k values for sulfadiazine sodium salt (SDZ-Na) were increased from 0.0086 to 0.0100   min⁻¹ as WC rose from 6.1% to 60%). Fe(III) exchange markedly reduced the sensitivity of antibiotic transformation to moisture. For 0.1Fe(III)δ-MnO₂, k value for SDZ at 60% WC (0.0554   min⁻¹) showed no significant difference from that under dry conditions (0.0596   min⁻¹)). The alteration of the mineral-water-pollutant interface by Fe(III) was demonstrated through experimental and theoretical calculations. HAADF-STEM and XRD revealed that water molecules competed with antibiotic molecules for binding sites on δ-MnO₂. Fe(III) not only expanded the mineral's interlayer spacing but also acted as a bridge, forming strong connections with the antibiotic and their salts. Meanwhile, Fe(III)-δ-MnO 2 characterization suggested that Fe(III) increased specific surface area, surface acid-site density, redox activity, and free radical (•OH and O 2 •− ) yield. Product identification suggested that antibiotics were transformed into less toxic molecules via non-radical/radical pathways. This work provides a mechanistic insight into the role of exchangeable Fe(III), forming a basis for designing remediation materials effective across a range of water content.