Photochemical processes in aquatic systems are important for maintaining the natural balance of these systems. Singlet oxygen (¹O ₂) is a reactive oxygen species produced in natural waters as a result of the energy transfer from photochemically-excited dissolved organic matter (DOM) to molecular oxygen. It has a steady-state concentration of 10 ^-12-10^-14 M in natural waters and can participate in select oxidation reactions. This research has examined aspects of both the production of ¹O₂ and subsequent reactions in natural water. Singlet oxygen degrades both naturally occurring substrates and organic pollutants.

In order to predict the photochemical fate of these compounds in natural water, it is necessary to have a good understanding of the concentration of ¹O₂ that the substrate will likely encounter. This concentration will change throughout a water body based on depth as a result of light attenuation within the water column. As the depth increases, the production of ¹O₂ decreases, which can be predicted based on the absorbance of the DOM in the sample. The model used to predict these values was examined by measuring the steady-state concentrations of ¹O₂ at various depths throughout a black anodized tube with a collimated light source and comparing the observed results with theoretically predicted values. While the two sets of data did not agree, it was possible to correct the predicted values utilizing a known actinometer to account for light field discrepancies as a result of the experimental setup. Also, the potential effects on the depth dependence by the addition of suspended sediment particles or algae are discussed.

Naturally occurring amino acids are an important component of dissolved organic matter. Four common amino acids, histidine, methionine, tyrosine, and tryptophan, are known to react with ¹O₂ and their degradation was studied in a range of natural waters. The percentage of sensitized degradation due to reaction with ¹O₂ varied from 3% for tyrosine in Lake Superior water to 110% for histidine in Suwannee River humic acid solutions. The photodegradation of histidine, specifically, is known to be caused exclusively by reaction with ¹O ₂, as opposed to other reactive intermediates. A mechanism for this reaction and implications of the observed production of ammonia are presented. In addition to free amino acids, the reactivity of two synthetic histidine-containing peptides, as dissolved combined amino acid models, were examined. One in particular was shown, through quenching and enhancement experiments, to degrade as a result of reaction with ¹O₂ and a rate of physical quenching of ¹O₂ was measured to be very similar to free histidine.

The ¹O₂-mediated degradation of a class of organic pollutants known as the nitrofuran antibiotics was also investigated. These compounds were expected to degrade, at least in part, through reaction with ¹O₂ due to their ¹O₂-reactive furan ring. In fact, degradation was found to be due solely to direct photolysis, with half-lives for the compounds ranging from 5-26 minutes in mid-summer at 45° latitude.