The environmental photochemistry of pharmaceutical compounds in aqueous solution and on a clay surface

There is growing concern over the introduction of pharmaceutical pollutants to the environment. Loss processes such as photolysis are expected to be important in predicting the environmental fate of a given pharmaceutical. Environmental photochemical kinetics were investigated for several pharmaceutical compounds. Direct photolysis quantum yield values were determined for mefenamic acid, tylosin, tetracycline, and chlortetracycline. The photolysis of mefenamic acid was sensitized by a reaction with excited triplet-state natural organic matter. Tylosin photolysis occurred in two steps: a reversible, efficient photoisomerization equilibrium via rotation about a carbon-carbon double bond, followed by a slower pseudo-first-order loss of the isomer mixture. Aqueous tetracycline species were found to be predominantly bound to calcium and magnesium in many natural waters. The species-dependent photochemical parameters for the calcium/magnesium/tetracycline system were deconvoluted from a series of kinetic experiments. The photolysis rate constant for tetracycline loss under simulated sunlight was observed to vary by up to an order of magnitude depending on the pH and water hardness. Photolysis of chlortetracycline was investigated in aqueous solution and on a kaolinite clay surface. The quantum yield for photolysis of chlortetracycline on a clay surface was found to be an order of magnitude lower than in aqueous solution.