Influence of humate on the degradation of chloramphenicol by sulfidated ferrihydrite under dynamic anoxic/oxic environments: A combined DFT calculation and experimental study

Abstract

Sulfidation of ferrihydrite is known to affect the degradation of contaminants, but little was known about the role of natural organic matter (NOM) in antibiotics degradation by sufidated ferrihydrite under redox-dynamic conditions. Here, a typical antibiotic (i.e., chloramphenicol (CAP)) was chosen to investigate how it redistributed when ferrihydrite reacted with reductive dissolved sulfide (S(-II)dis) in the presence of humate (HA) under dynamic anoxic/oxic environments. In anoxic environments, HA enhanced CAP reduction via dichlorination or decarboxylation by sufidated ferrihydrite in the low concentration of S(-II), while HA inhibited CAP reduction in the high concentration of S(-II) by the contribution of S(-II) and surface-bound Fe(II) (Fe(II)adsorbed). When the conditions transited from anoxic to oxic, remaining CAP molecules in solutions continued undergoing oxidative degradation to form the succinic acid, hexanedioic acid, CO2, and H2O by the attack of ·OH. Meantime, HA was adsorbed to ferrihydrite to block autocatalytic Fe(II) oxidation, which inhibited the generation of ·OH under oxic conditions. Additionally, from the density function theory (DFT) calculation and intermediate products analysis obtained from HPLC-MS/MS, two oxidative degradation pathways of CAP during the oxidation of sulfidated ferrihydrite have been proposed. Collectively, the framework elucidated different roles of HA in CAP elimination and environmental behavior of ferrihydrite when exposed to the S(-II) under the dynamic redox conditions.