Enhanced Inactivation of <i>Bacillus subtilis</i> Spores during Solar Photolysis of Free Available Chlorine

Aqueous free available chlorine (FAC) can be photolyzed by sunlight and/or artificial UV light to generate various reactive oxygen species, including HO^• and O­(³P). The influence of this chemistry on inactivation of chlorine-resistant microorganisms was investigated using <i>Bacillus subtilis</i> endospores as model microbial agents and simulated and natural solar radiation as light sources. Irradiation of FAC solutions markedly enhanced inactivation of <i>B. subtilis</i> spores in 10 mM phosphate buffer; increasing inactivation rate constants by as much as 600%, shortening inactivation curve lag phase by up to 73% and lowering <i>CT</i>s required for 2 log₁₀ inactivation by as much as 71% at pH 8.0 and 10 °C. Similar results were observed at pH 7.4 and 10 °C in two drinking water samples with respective DOC concentrations and alkalinities of 0.6 and 1.2 mg C/L and 81.8 and 17.1 mg/L as CaCO₃. Solar radiation alone did not inactivate <i>B. subtilis</i> spores under the conditions investigated. A variety of experimental data indicate that the observed enhancements in spore inactivation can be attributed to the concomitant attack of spores by HO^• and O₃, the latter of which was found to accumulate to micromolar concentrations during simulated solar irradiation of 10 mM phosphate buffer (pH 8, 10 °C) containing [FAC]₀ = 8 mg/L as Cl₂