Enhanced Inactivation of Bacillus subtilis 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 center dot and O(P-3). The influence of this chemistry on inactivation of chlorine-resistant microorganisms was investigated using Bacillus subtilis endospores as model microbial agents and simulated and natural solar radiation as light sources. Irradiation of FAC solutions markedly enhanced inactivation of B. subtilis spores in 10 inM phosphate buffer; increasing inactivation rate constants by as much as 600%, shortening inactivation curve lag phase by up to 73% and lowering CTs required for 2 log(10) inactivation by as much as 71% at pH 8.0 and 10 degrees C. Similar results were observed at pH 7.4 and 10 degrees 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 B. subtilis 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 center dot and O-3, the latter of which was found to accumulate to micromolar concentrations during simulated solar irradiation of 10 in.M phosphate buffer (pH 8, 10 degrees C) containing [FAC](0) = 8 mg/L as Cl-2.