Influence of the natural colloids on the multi-phase distributions of antibiotics in the surface water from the largest lake in North China.

Understanding antibiotic adsorption on natural colloids is crucial for prediction of the behavior, bioavailability and toxicity of antibiotics in natural waters. In the present study, the filtered water (dissolved phase, <0.7μm) was further separated into colloidal phase (1kDa-0.7μm) and soluble phase (<1kDa) by cross-flow ultrafiltration (CFUF), and the spatial-temporal variation and distribution of six antibiotics in multi-phases were investigated in Baiyangdian Lake. Results indicated that antibiotic concentrations differed significantly with sampling location and time. The mean concentrations of antibiotics ranged between 13.65 and 320.44ngL-1 in the dissolved phase, and the colloidal phase accounted for 4.7-49.8% of all antibiotics, suggesting that natural colloids play an important role as carriers of antibiotics in aquatic environments. Because of the influence of colloids, the partition coefficients of antibiotics between suspended particulate matter (SPM) and soluble phase (intrinsic partition coefficients, Kpint) were found to be 6.18-109.60% higher than corresponding observed partition coefficients (Kpobs, between SPM and dissolved phase). The mean partition coefficients between colloidal and soluble phase (Kcol.) ranged between 6218 and 117,374Lkg-1, which were 1-2 orders of magnitude greater than Kpint values. In order to explore the adsorption mechanism of antibiotics on colloids, Pearson's correlations were performed. The results showed that log Kcol. were negatively correlated with cations in natural colloids; especially with Mg (r, -0.643, P<0.01) for oxytetracycline (OTC), and with both Ca (-0.595, P<0.01) and Mg (-0.593, P<0.01) in the case of ofloxacin (OFL). This result revealed that the competitive effect between cations and antibiotics was the main factor influencing the adsorption behavior of antibiotics on natural colloids in the lake.