Session: Aquatic Photochemistry:

Previous research into the toxic response of photolyzed aqueous solutions of nitroguanidine (NQ) have utilized laboratory photolyzed solutions and tested nearly immediately after degradation was complete (i.e., complete photolysis). In this study, we sought to elucidate any differences in the toxic response of Daphnia pulex and product distribution due to irradiation conditions between laboratory-based and natural sunlight photolysis. Furthermore, experiments were designed to investigate the effect of increasing percent NQ photolysis on toxicity threshold, and the effect of aging the photolyzed solutions to determine if the reaction products causing enhance toxicity were transient. Toxicity increased dramatically with minor proportional degradation of the parent NQ (~10%) where toxicity increased with increasing proportional degradation. Only minor changes in toxicity were observed during the aging of the solutions (8 and 13 days) indicating the photolysis products causing increased toxicity are long-lived. The most dramatic changes in photolytic product distributions across all experiments were found to be in the nitrate (NO₃^-) and nitrite (NO₂^-) concentrations but continue to fail to explain the increased toxicity of the photolytic product mixtures. It was found that while solutions of NQ photolyzed under natural light resulted in 2-order of magnitude increases in toxicity relative to the parent NQ, the toxicity of the natural light photolyzed solution was not as great as the laboratory photo-reactor solution which elicited 3-order of magnitude toxicity increases. The differences in toxicity among the naturally photolyzed sample may be due to the different degradation product distributions when compared to the laboratory-based degradation experiments. Overall, the differences UV-light of laboratory photoreactors and natural sunlight may provide overly conservative hazard estimates of natural sunlight photolysis. None-the-less, we have demonstrated that dramatic increases in toxicity are still plausible when NQ is degraded in natural sunlight.

Sarah Zetterholm

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