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 (¹O₂) produced via sunlight-mediated excitation of natural organic matter (NOM) in surface waters. The ¹O₂ concentration within a NOM macromolecule or supramolecular assembly ([¹O₂]_internal) is orders of magnitude higher than in the bulk solution ([¹O₂]_bulk). In close proximity of NOM, MS2 is thus exposed to an elevated ¹O₂ concentration ([¹O₂]_NOM), and inactivation is likely to be enhanced as compared to the bulk solution. In experiments using a solar simulator, we determined [¹O₂]_bulk, [¹O₂]_internal, 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, k_obs, over a range of 1−25 mg NOM/L. The k_obs 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 k_obs. Thus, even for the less adsorbing NOM, inactivation was dominated by [¹O₂]_NOM rather than [¹O₂]_bulk. Changes in solution chemistry to promote closer interactions between MS2 and NOM also enhanced k_obs. Addition of Mg²⁺ to neutralize the negative surface charge of MS2 and NOM increased k_obs up to 4.1-fold. Similarly, lowering the solution pH closer to the isoelectric point of MS2 (pI = 3.9) enhanced k_obs 51-fold in 5 mg/L AHA.