Optimization of Aqueous Chlorine Photochemistry for Enhanced Inactivation of Chlorine-resistant Microorganisms

Abstract

<italic>Bacillus subtilis</italic> spores, which are highly resistant to chlorine disinfection, have frequently been used as models for such chlorine-resistant, waterborne pathogens as <italic>G. lamblia</italic> and <italic>C. parvum</italic>. The goal of this research was to investigate the use of simulated sunlight to produce hydroxyl radical via HOCl/OCl- photolysis during free chlorine disinfection to synergistically enhance <italic>B. subtilis</italic> spore inactivation. Reactors were irradiated indoors using a Newport solar simulator equipped with a 450-W Xenon lamp (O₃ free) or outdoors with natural sunlight. The solar simulator emits light primarily at non-germicidal wavelengths between 300 and 400nm. At starting conditions, reactors contained phosphate buffer (0.01 M, pH 6-8) or natural NOM-containing water sampled from nearby municipal treatment facilities (pH 7.4), 10<super>4</super>-10<super>5</super> CFU/ml <italic>B. subtilis</italic> spores, and para-chlorobenzoic acid (pCBA) as a hydroxyl radical probe. The inactivation of <italic>B. subtilis</italic> spores at 10, 25 (indoor) and 33 °C (outdoor) was monitored after adding between 1-8 mg L-1 as Cl₂ of free available chlorine (FAC) and either keeping reactors in the dark or exposing them to simulated or natural sunlight for a brief period of time. After removing irradiated reactors from the light source, continued inactivation by FAC was monitored in the dark. <italic>B. subtilis</italic> spores were enumerated using the spot titer culture assay method, allowing for at least 2-log10-unit reductions to be measured in all cases. Inactivation of <italic>B. subtilis</italic> spores was modeled using the delayed Chick-Watson model. Inactivation was primarily characterized by pseudo-first order rate constants (<italic>k</italic>, in L(min-mg)-1), lag phase length (in Ct units), and Ct₉₉ values. The Ct₉₉ refers to the Ct (in (mg-min) L<super>-1</super>) needed to achieve a 2-log10 (99%) reduction. To understand the effect of hydroxyl radical exposure on enhancement of inactivation by FAC, the rate enhancement factor (ratio of light:dark) and lag phase or Ct₉₉ reduction factors (ratio of dark:light) were calculated. Neither simulated sunlight nor natural sunlight inactivated <italic>B. subtilis</italic> spores on their own. With FAC alone, inactivation efficiency was negatively related to pH and positively related to temperature. For phosphate-buffered solutions, pH was the most important determinant of the enhancement effect from irradiation with FAC: reducing Ct₉₉ values more than two-fold at pH 8. Overall, the Ct₉₉ reduction factor showed a positive linear relationship with pH that was independent of temperature (including 10, 25 and 33 °C data). In natural waters with low NOM levels, the enhancing effect of simulated solar radiation was only slightly lower than would be expected based on pH despite much slower dark inactivation with FAC only. The enhancing effect of irradiation is hypothesized to be due primarily to the generation of hydroxyl radical during FAC photolysis. Accordingly, when a hydroxyl radical scavenger, tert-butanol (50 mM), was added to FAC-containing reactors during irradiation, kinetics were reduced to the level observed with FAC alone. Previous studies have indicated that hydroxyl radical effectively destroys chlorine-recalcitrant components of the <italic>B. subtilis</italic> spore coat and/or cortex. Thus, in this system, initial attack by hydroxyl radical may make the spores more vulnerable to subsequent attack by free chlorine, resulting in a synergistic effect. These findings highlight the possibility of augmenting chlorination strategies for inactivation of otherwise chlorine-recalcitrant organisms through the use of low-energy photochemical energy sources (i.e., sunlight, fluorescent lamps), as well as more energy-intensive germicidal UV light, in centralized and decentralized water treatment applications.