Enhanced Inactivation of <i>Bacillus subtilis</i> Spores during Solar Photolysis of Free Available Chlorine
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Abstract
Aqueous free available chlorine (FAC)
can be photolyzed by sunlight
and/or artificial UV light to generate various reactive oxygen species,
including HO<sup>•</sup> and O(<sup>3</sup>P). 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 log<sub>10</sub> 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 CaCO<sub>3</sub>. 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<sup>•</sup> and O<sub>3</sub>, 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]<sub>0</sub> = 8 mg/L as Cl<sub>2</sub>