The broad-spectrum antibiotic tetracycline is used extensively for human and animalhealth, 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 todetermine 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 ofoxytetracycline. Since Saudi Arabian topsoils can have pH near 7.5 along with large Ca2+concentrations, these conditions together may enable relatively strong sorption of cationicCa2+-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 FaisalUniversity, Al-Ahsa, Saudi Arabia. Three soils with relatively high clay contents were selected, and sorption isotherms were measured by using liquid chromatography/tandem massspectrometry (LC-MS/MS) instrument to quantify oxytetracycline (OTC) concentrations.The results of this study showed that oxytetracycline sorption was significant at pH 7.5 forall three soils, and the hypothesis of this study - that desert soils at pH 7.5 may adsorbreasonably large amounts of oxytetracycline - was supported.The speciation of tetracycline is complicated by several ionic species that form complexeswith aqueous cations and also with mineral surfaces, so computational tools are needed tounderstand and predict partitioning of tetracycline into its various species. The objective ofthis study was to use many experimental data sets to create new thermodynamic parametersusing Phreeqc for modeling the sorption speciation of tetracyclines. Since clay minerals areimportant sorbents, cation exchange parameters were developed for tetracycline and itsK+- and Ca2+-complexes for better understanding of that very complicated system. Aself-consistent set of parameters was derived that enabled tetracycline cation-exchange to bemodeled in both K+- and Ca2+-systems over a range in pH from 4 to 8.
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