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Lack of Influence of Extracellular Polymeric Substances (EPS) Level on Hydroxyl Radical Mediated Disinfection of <i>Escherichia coli</i>
Photolysis of nitrate, a prevalent constituent in agriculturally impacted waters, may influence pathogen attenuation in such systems through production of hydroxyl radical (<sup>•</sup>OH). This study focuses on the efficacy of <sup>•</sup>OH generated during nitrate photolysis in promoting <i>E. coli</i> die-off as a function of extracellular polymeric substances (EPS) coverage. EPS levels of four <i>E. coli</i> isolates were systematically altered through a sonication extraction method and photochemical batch experiments with a solar simulator examined isolate viability loss as a function of time in nitrate solutions. <i>E. coli</i> viability loss over time exhibited two regimes: an initial induction time, <i>t</i><sub>s</sub>, with little decay was followed by rapid exponential decay characterized by a first-order disinfection rate constant, <i>k</i>. Increasing steady-state <sup>•</sup>OH concentrations enhanced <i>E. coli</i> viability loss, increasing values of <i>k</i> and decreasing <i>t</i><sub>s</sub> values, both of which were quantified with a multitarget bacterial disinfection model. Notably, at a given steady-state <sup>•</sup>OH concentration, values of <i>t</i><sub>s</sub> and <i>k</i> were independent of EPS levels, nor did they vary among the different <i>E. coli</i> strains considered. Results herein show that while <sup>•</sup>OH generated via nitrate photolysis enhances rates of disinfection in surface water, the mechanism by which <sup>•</sup>OH kills <i>E. coli</i> is relatively insensitive to common bacterial variables