Impact of concentration and species of sulfamethoxazole and ofloxacin on their adsorption kinetics on sediments

- The sorption kinetics of SMX is concentration and pH-independent.
- The sorption kinetics of OFL is dependent on the concentration and pH.
- The neutralization effect of adsorbed OFL+ slows down subsequent OFL+ sorption.
- The sorption rate of antibiotics is concentration-dependent of adsorbable species.

Antibiotics are used widely in human and veterinary medicine and are ubiquitous in environmental matrices worldwide. The influence of the concentration of antibiotics on adsorption kinetics is still unclear. This study used sulfamethoxazole (SMX) and ofloxacin (OFL) as adsorbates to investigate the adsorption kinetics on sediment affected by varying concentrations of antibiotics adsorbable species. At the experimental pH values, the major adsorbed species of SMX and OFL on sediment were SMX0 and OFL+ by hydrophobic interaction and electrostatic attraction, respectively. The apparent adsorption rate of SMX was not affected by the initial concentration and the pH values because the hydrophobic interactions were concentration-independent and charge-independent. However, the apparent adsorption rate of OFL significantly slowed down as the initial concentration increased. The adsorbed OFL+ effectively neutralized the negative charges of the sediment, leading to a reduced adsorption rate of subsequent OFL+. The neutralization effect was greatly enhanced due to the increased OFL+ with the increasing OFL concentration. Additionally, the apparent adsorption rate of OFL significantly increased at higher pH due to the reduced neutralization effect that resulted from the decreased OFL+ and increased negative charges of the sediment surface. This study implied that the adsorption kinetics of antibiotics was greatly dominated by the concentration of adsorbable species rather than apparent overall concentration.