Wavelength dependent photoreactivity of mirex in Lake Ontario

Mirex is a toxic, hydrophobic pollutant in Lake Ontario that undergoes an indirect photoreaction mediated by natural dissolved organic matter. The monohydro product photomirex further photolyzes to form a dihydro product. The efficiency of mirex photolysis was determined as wavelength-dependent reaction quantum yield coefficients. Quantum yield coefficients were determined for 14 lake samples collected from multiple locations and overthe course of several weeks from one location. Values declined with increasing wavelength, with reactivity extending above 436 nm. Individual samples showed no seasonal trend. Pooled quantum yield coefficients and 95% confidence intervals were calculated by combining the results for all samples. Using the quantum yield coefficients within this confidence interval, mirex photolysis rates were accurately predicted during a sunlight exposure study for an independently collected water sample. In a relative rate experiment, the rate constants for mirex photolysis and subsequent photomirex photolysis were not significantly different from each other at the 95% confidence level, and this finding was further supported by the sunlight exposure results. Using the results from this study, it was estimated that, on a cloudless June day, the photolysis half-life of mirex in the epilimnion of Lake Ontario is 19 days.