DOM-mediated membrane retention of fluoroquinolone as revealed by fluorescence quenching properties

In this study, membrane filtration tests showed that the membrane rejection degree of difloxacin hydrochloride (DFHC) increased significantly in the presence of Suwannee River DOM or Aldrich humic acid (2–10 mg-C/L). Titration experiments showed that the excitation and emission of Peak R belonging to DFHC exhibited blue shifts by 5 nm and 10 nm, respectively, in the presence of DOM. The presence of DFHC can, in turn, lead to more significant overlapping of the fluorescence peaks of the Suwannee River DOM and Aldrich humic acid. The parallel factor analysis (PARAFAC) of the excitation-emission matrix (EEM) spectra can well decompose the components belonging to DFHC from the DOM + DFHC mixtures. The maximum fluorescence intensity (FI (max)) of the antibiotic-like component (C1) sharply decreased upon the initial addition of DOM. More specifically, the Aldrich humic acid showed a larger quenching effect on DFHC than the Suwannee River DOM. The stability constants (K (M)) obtained by the Ryan and Weber model also corroborated that the Aldrich humic acid had a much higher binding stability (K (M) = 4.07 L/mg) than the Suwannee River DOM (K (M) = 0.86 L/mg). These results have great implications for our understanding of the membrane filtration behavior of trace contaminants in natural waters.