Effects of natural organic matter with different properties on levofloxacin adsorption to goethite: Experiments and modeling

Adsorption of levofloxacin (LEV) to goethite in the pH range of 3-10, and in the absence or presence of natural organic matter (NOM) represented by nine types of humic acid (HA) and fulvic acid (FA), was studied using batch experiments. The adsorption of LEV to goethite was weak and showed a maximum around pH 5.8. Adding NOM to goethite strongly increased LEV adsorption to goethite, but hardly affected its pH dependency. The adsorption envelopes were well fitted to a linear additive model, in which LEV adsorption to goethite was simulated with the Charge Distribution Multi-Site Complexation (CD-MUSIC) model, and LEV adsorption to NOM was simulated with the Langmuir model. The fitted affinity constants (log K) for LEV adsorption to NOM were significantly and positively correlated with the SUVA (specific ultraviolet absorbance at 280 nm) values of NOM, and negatively correlated with E-2/E-3 (absorbance ratio at 250 nm and 365 nm) values, carboxyl contents, and the polarity of NOM. The results indicated that aromatic moieties of NOM play a key role in the interactions between LEV and NOM, and hydrophobic interactions and p-p interactions were the major mechanisms for LEV adsorption to NOM, whereas H-bond or surface complexation might not play an important role. Results show that both the concentrations and properties of NOM have a significant effect on the distribution or treatment of antibiotics in soils and waters, which will eventually affect the influence of antibiotics on microorganisms in the environmental systems.