The mechanism for the photocatalytic inactivation of highly resistant microorganisms (i.e., Bacillus subtilis spores) in water was studied using a kinetic approach. This required characterizing the basic processes that occur within the photoreactor. The radiative intensity that entered the photocatalytic system was estimated using the ferrioxalate actinometrical process, the amount of hydroxyl radical produced under a specific photo-assisted Fenton reaction was measured, and a kinetic model to predict the hydroxyl radical generation was proposed to fit the experimental values. These results were then used to suggest new assessment related to the spore inactivation mechanism under controlled photo Fenton reaction conditions. The kinetic model was found to fit the experimental data fairly well (r 2 >0.99) and hydroxyl radical generation was determined to significantly affect the inactivation process. It was determined that a specific amount of hydroxyl radical is required to overwhelm the self-repairing mechanisms of the cell and cause cell death. The amount of hydroxyl radicals generated was found to be a function of radiative intensity and reagent concentration, as previously reported. The proposed relationship between the amount of hydroxyl radical and the inactivation process was supported by adding chloride ions to acting as radical scavengers. It was observed that even the lowest chloride ion concentration was capable of producing a significant delay in the inactivation process by scavenging hydroxyl radicals and generating low reactive species at the pH conditions tested.

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