Predicting the sorption of azithromycin and levofloxacin to sediments from mineral and organic components.

Despite the strong association of azithromycin (AZM), a macrolide antibiotic, and levofloxacin (LVF), a quinolone antibiotic, to sediment, sorption data are scarce. We conducted sorption experiments with eight river sediments, their major clay minerals (illite and chlorite), a highly negatively charged clay mineral (montmorillonite), and an organic-matter-rich soil (Andosol). The sorption of AZM and LVF to the sediments was influenced by the concentration and type of coexisting inorganic cations as much as by reported organic cations. In addition, their linear sorption coefficients (Kd) to sediments were correlated with cation exchange capacity (CEC) but not organic carbon content, so cation exchange is the dominant sorption mechanism. Multiple linear regression analysis showed improved prediction of sediment Kd from CEC contributed by minerals and organic matter for AZM, but not for LVF. Kcec (= Kd/CEC) values of AZM were 2-3 orders of magnitude higher on minerals than on Andosol, but those of LVF ranged within a factor of 4. Therefore, mineral and organic components need to be separated in estimating AZM sorption to sediments. Sediment Kd values of AZM and LVF were satisfactorily predicted by a cation-exchange-based model using individual Kcec values on illite, chlorite, and Andosol (mean absolute error of 0.57 and 0.22 log units, respectively). Kcec values on montmorillonite and chlorite ranged within a factor of about 3 from those of illite for both antibiotics, and Kcec differences by mineral type would generally be negligible in model estimation. Because AZM was sorbed mostly to minerals in sediments, the model and sorption data can be applicable to various soils or sediments. Overall, the trend of LVF sorption corresponds to reported sorption of other organic cations, whereas remarkably higher AZM Kcec to minerals than to Andosol is attributable to its large lactone ring, higher molecular weight, or two charged amines.