An in Situ ATR-FTIR Study of Sulfate / Phosphate, and Oxytetracycline Surface Complexation on Kaolinite in a Competitive Adsorption Scenario.

Fate of antibiotics in the environment is an emerging issue due to antimicrobial resistance and its harmful effects on plants, animals and ecosystems. Veterinary antibiotics are one of the major concerns for their heavy use in livestock production systems. Fate of these veterinary antibiotics in the environment is mainly controlled by their retention in the soil minerals. Although a significant body of literature exists on the sorption studies of antibiotics on various soil minerals, detailed molecular level in situ spectroscopic studies are lacking. Owing to complexity of antibiotic structure and their variable sorption interactions on variety of soil minerals, thorough in situ spectroscopic investigation of specific antibiotics on specific soil minerals is necessary. Thus, here we propose to investigate sorption mechanisms of oxytetracycline (OTC), an important veterinary antibiotic, on kaolinite using in situ ATR-FTIR spectroscopic probes. In addition, we would like to investigate the effects of environmentally relevant co-occurring oxyanions such as sulfate (S) and phosphate (P) on the sorption mechanism of OTC on kaolinite. Three types of in situ ATR-FTIR experiments were carried out to study competitive sorption of OTC and P/ S on kaolinite: 1) sorption of P / S alone on kaolinite, 2) sorption of OTC alone on kaolinite, and 3) sorption of OTC + P / S on kaolinite. The preliminary results indicated that at an environmental conc. range of P (upto 100 μM), growth of OTC surface complex on kaolinite was not hindered at pH = 5.5-7.5. However, for competitive sorption of OTC and sulfate (S) on kaolinite, at higher pH (6.5), some desorption of S was noticed. Further study is ongoing to isolate important mechanisms. These results have important implications on the fate of OTC in the environment as well as in the biogeochemical cycling of S, P in agricultural land.