Association with Natural Organic Matter Enhances the Sunlight-Mediated Inactivation of MS2 Coliphage by Singlet Oxygen

MS2 coliphage, a surrogate for human enteric viruses, is inactivated by singlet oxygen (<sup>1</sup>O<sub>2</sub>) produced via sunlight-mediated excitation of natural organic matter (NOM) in surface waters. The <sup>1</sup>O<sub>2</sub> concentration within a NOM macromolecule or supramolecular assembly ([<sup>1</sup>O<sub>2</sub>]<sub>internal</sub>) is orders of magnitude higher than in the bulk solution ([<sup>1</sup>O<sub>2</sub>]<sub>bulk</sub>). In close proximity of NOM, MS2 is thus exposed to an elevated <sup>1</sup>O<sub>2</sub> concentration ([<sup>1</sup>O<sub>2</sub>]<sub>NOM</sub>), and inactivation is likely to be enhanced as compared to the bulk solution. In experiments using a solar simulator, we determined [<sup>1</sup>O<sub>2</sub>]<sub>bulk</sub>, [<sup>1</sup>O<sub>2</sub>]<sub>internal</sub>, as well as the association of MS2 with four NOMs (Fluka humic acid, FHA; Suwannee river humic acid, SRHA; Aldrich humic acid, AHA; Pony lake fulvic acid, PLFA), and studied their effect on the MS2 inactivation rate constant, <i>k</i><sub>obs,</sub> over a range of 1−25 mg NOM/L. The <i>k</i><sub>obs</sub> values were modeled as the sum of the inactivation rate constants in close proximity to the NOM and in the bulk solution, assuming Langmuir-type adsorption of NOM onto MS2. FHA and SRHA exhibited 13−22 fold greater adsorption equilibrium constants than AHA and PLFA. Inactivation in the bulk solution contributed between 2% (20 mg/L FHA) and 39% (5 mg/L AHA) toward the overall <i>k</i><sub>obs</sub>. Thus, even for the less adsorbing NOM, inactivation was dominated by [<sup>1</sup>O<sub>2</sub>]<sub>NOM</sub> rather than [<sup>1</sup>O<sub>2</sub>]<sub>bulk</sub>. Changes in solution chemistry to promote closer interactions between MS2 and NOM also enhanced <i>k</i><sub>obs</sub>. Addition of Mg<sup>2+</sup> to neutralize the negative surface charge of MS2 and NOM increased <i>k</i><sub>obs</sub> up to 4.1-fold. Similarly, lowering the solution pH closer to the isoelectric point of MS2 (pI = 3.9) enhanced <i>k</i><sub>obs</sub> 51-fold in 5 mg/L AHA.