The broad-spectrum antibiotic tetracycline is used extensively for human and animal health, but as usual, causes unintended environmental consequences. Since large fractions of tetracycline are not metabolized in animals, massive amounts of tetracycline are excreted with animal manures and pollute soil, surface water, and groundwater. Among other risks, tetracycline induces antibiotic resistance in environmental bacteria. In order to understand and quantify this risk, the objective of this study was to model bacterial uptake of tetracycline in complex media. 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. This study used many experimental data sets to create thermodynamic parameters for use in the chemical speciation model Phreeqc. Experimental work has shown that the neutral species of tetracycline dominates the uptake by bacteria, so a new method for modeling the distribution of organic chemicals using Phreeqc was employed to successfully model bacterial uptake in complex solutions of $Ca^{2+}$, $Mg^{2+}$, and five organic acids. Since clay minerals are important sorbents, cation exchange parameters were developed for tetracycline and its K- and Ca-complexes. While experimental data for K- or Ca-systems could be successfully fit separately, no parameter set could be found that fit both together over a range of four ionic strengths. This calls into question the literature values for $Ca^{2+}$/$K^{+}$ exchange itself, which provides a path forward for future research.
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