Water hardness as a photochemical parameter: Tetracycline photolysis as a function of calcium concentration, magnesium concentration, and pH

The environmental photochemical kinetics of the antibiotic compound tetracycline were investigated. The aqueous speciation of tetracycline over a range of natural pH and water hardness values is dominated by association with Ca²⁺ and Mg²⁺ ions. The association constants necessary to calculate tetracycline aqueous speciation given knowledge of pH, [Ca ²⁺], and [Mg²⁺] were measured by spectrophotometric titrations and matrix deconvolution of a series of UV-vis absorption spectra into individual component species. A series of photolysis experiments was performed under simulated sunlight, and quantum yields for the solar photolysis of each environmentally relevant species were calculated. The results indicate that the pseudo-first-order rate constant for tetracycline photolysis at varied Mg²⁺ and Ca²⁺ concentrations relevant to natural conditions can vary by up to an order of magnitude. A self-sensitization effect was observed and was accounted for by varying the initial tetracycline concentration under each set of photolysis conditions.