| In the environment, antibiotics are a group of new organic contaminants. In thevadose zone, their adsorption on a kind of common iron mineral goethite is veryimportant. In this study, effects of pH, Ca2+, phosphate, and natural organic matter(NOM) on levofloxacin (LEV) adsorption to goethite were investigated, and theinteractions between NOM and goethite were also studied. The results from our studyindicated that:(1) The adsorbed amount of LEV to goethite reached its maximum (0.034μmol/m2)at pH values around6, and then decreased when pH was lower or higher than thisrange. The LEV adsorption envelope was asymmetric on the two sides of themaximum adsorption around pH6. This was because to a large extent of theasymmetric charge change on goethite surface and the consumption of protons duringthe adsorption of LEV to goethite(2) In the presence of Ca2+, a decrease in adsorption was observed, because theCa-LEV complexes (CaH2L3+) was formed in solutions, which would cause theelectrostatic interactions and the decrease in free LEV in solutions.(3) Adsorption of LEV (or phosphate) to goethite in single systems was wellpredicted using the CD-MUSIC (Charge Distribution Multi-Site Complexation)model. In the binary systems, the presence of phosphate decreased LEV adsorption togoethite significantly over the whole pH range. Electrostatic competition was themain reason for the competition of LEV and phosphate to goethite surface in thebinary systems.(4) The amount of humic acid (HA) adsorbed to goethite was near its equilibriumvalue (0.51mg C/m2) in5min. HA with high molecular weight was mainly adsorbed.More HA was adsorbed to goethite at low pH. Specific adsorption and hydrophobicinteraction might also involved during the interactions between HA and goethitebesides electrostatic attraction.(5) The presence of nine types of NOM samples increased the adsorption of LEV over the whole pH range. The adsorption envelops were well fitted to the linearadditive model which applies the CD-MUSIC model to simulate LEV adsorption togoethite and the Langmuir model to simulate LEV adsorption to NOM. The optimizedstability constants (log K) for LEV adsorption to NOM were significantly andpositively correlated with Specific UV absorbance (SUVA) values of NOM, andnegatively correlated with E2/E3values, carboxyl contents, and the polarity of NOM.H-bond and cationic complexation are probably not that important in the LEVadsorption process to NOM. Hydrophobic interactions and-interactions wereproposed as the major mechanisms in the adsorption of LEV to (mineral-complexed)NOM. |
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