Sorption of Tetracyclines in Desert-Soil Environment

The broad-spectrum antibiotic tetracycline is used extensively for human and animal health, but causes unintended environmental consequences. Significant amounts of tetracycline are excreted with animal manures and can pollute soil, surface water, and groundwater. The overall objective of this dissertation is to combine experimental and modeling work to determine the environmental chemistry of tetracyclines in desert soils.;Some previous studied observed that a) when Ca2+-to-tetracycline ratios were large, then Ca2H(Tec)2+ species dominate in the solution above pH 5, and b) at low concentration of cation and high pH, Ca2+- and Mg2+-clays sorbed significantly large amounts of oxytetracycline. Since Saudi Arabian topsoils can have pH near 7.5 along with large Ca2+ concentrations, these conditions together may enable relatively strong sorption of cationic Ca2+-tetracycline complexes by cation exchange capacities (CEC) of clay minerals. Thus, the objective of current study was to test this hypothesis. To do so, soil samples were collected from the Agricultural and Veterinary Training and Research Station at King Faisal University, Al-Ahsa, Saudi Arabia. Three soils with relatively high clay contents were selected, and sorption isotherms were measured by using liquid chromatography/tandem mass spectrometry (LC-MS/MS) instrument to quantify oxytetracycline (OTC) concentrations. The results of this study showed that oxytetracycline sorption was significant at pH 7.5 for all three soils, and the hypothesis of this study - that desert soils at pH 7.5 may adsorb reasonably large amounts of oxytetracycline - was supported.;The speciation of tetracycline is complicated by several ionic species that form complexes with aqueous cations and also with mineral surfaces, so computational tools are needed to understand and predict partitioning of tetracycline into its various species. The objective of this study was to use many experimental data sets to create new thermodynamic parameters using Phreeqc for modeling the sorption speciation of tetracyclines. Since clay minerals are important sorbents, cation exchange parameters were developed for tetracycline and its K+- and Ca2+-complexes for better understanding of that very complicated system. A self-consistent set of parameters was derived that enabled tetracycline cation-exchange to be modeled in both K+- and Ca2+-systems over a range in pH from 4 to 8.