The opposite impacts of Cu and Mg cations on dissolved organic matter-ofloxacin interaction

Dialysis equilibrium system was applied to investigate the roles of Cu(II) and Mg(II) on DOM-ofloxacin (OFL) interaction. The binding behavior of both cations and OFL were studied. The introduction of Cu(II) increased DOM-OFL interaction, while Mg(II) decreased DOM-OFL binding. Cu(II) binding to DOM was also increased by OFL, while Mg(II) binding was decreased by OFL. The change in OFL binding amount in the absence and presence of cations (ΔCb) was calculated and compared with cation binding (Cbm). ΔCb/Cbm was in the range of 1–3 for Cu(II) depending on the applied Cu concentration. Two ternary complexes of DOM-OFL-Cu and DOM-Cu-OFL were proposed. For Mg(II), ΔCb/Cbm was around −1 at Mg(II) concentrations lower than 1 mM, but decreased up to −5 with increasing Mg(II) concentration. The competitive effect of Mg(II) to OFL was thus proposed. FTIR spectra were collected for mechanistic discussion.

► Cu increased DOM-OFL binding while Mg inhibited DOM-OFL binding.
► OFL promoted DOM-Cu interaction but decreased DOM-Mg complexation.
► Ternary complexes of DOM-OFL-Cu and DOM-Cu-OFL increase Cu and OFL binding in DOM.
► Mg competes or shields binding sites for OFL in DOM.