| The major goal of this study was to determine the effect of solution chemistry variables on the sorption of commonly used antibiotics to environmental particles. Interaction of selected antibiotics, ciprofloxacin and tetracycline, with the hydrous oxides of Al (HAO) and Fe (HFO), soil humic acid, and humic-hydrous Al oxide complexes was investigated. A combination of macroscopic batch, quantitative modeling, and spectroscopic techniques were employed to determine the dominant interactions for the above systems.; Both tetracycline and ciprofloxacin formed strong complexes on the surface of HAO and HFO, and ligand (antibiotic)-promoted dissolution of both the hydrous oxides was observed. The surface interactions appeared to occur primarily on the edge Al/Fe atoms of the minerals, which are the sites with singly-coordinated surface hydroxyl groups and are often considered as active sorption sites. Due to the similarity in sorption trends for both these antibiotics with HAO and HFO, subsequent experiments with humic substances (HS) and humic-HAO complexes were performed only on tetracycline. The interaction of tetracycline with a model HS (Elliott Soil Humic Acid, ESHA) showed a strong pH and ionic strength (I) dependence. Cation exchange was proposed as the major mechanism for tetracycline complexation with ESHA. The presence of a divalent metal cation (Ca2+) increased tetracycline sorption suggesting that ternary complex formation (cation bridging) could be important, especially at higher bridging ion concentrations. Aggregation of ESHA was observed in the presence of tetracycline, which could result in different microscopic pathways during sorption and desorption steps leading to an apparent hysteretic behavior. Strong affinities for HS exhibited by HAO resulted in a significant decrease in the sorption of tetracycline to HAO in the presence of ESHA. HS could influence tetracycline sorption by directly competing for potential sorption sites and/or by altering the surface charge properties of inorganic minerals.; The results from this study indicate that soil components can strongly influence the fate, transformation and reactivity of antibiotics in subsurface environments. This research work provides a better understanding of the sorption processes and should help in evaluating the ability of soils to act as a potential "sinks" for antibiotic compounds. |
