10.1021/es070295h.s001 Tamar Kohn Tamar Kohn Matthew Grandbois Matthew Grandbois Kristopher McNeill Kristopher McNeill Kara L. Nelson Kara L. Nelson Association with Natural Organic Matter Enhances the Sunlight-Mediated Inactivation of MS2 Coliphage by Singlet Oxygen American Chemical Society 2007 inactivation rate constants Suwannee river humic acid Aldrich humic acid SRHA AHA Pony lake fulvic acid bulk solution PLFA Natural Organic Matter Enhances MS 2 Singlet OxygenMS 2 coliphage NOM MS 2. FHA MS 2 Coliphage kob mg Fluka humic acid 1 O 2 concentration MS 2 inactivation rate adsorption equilibrium constants 2007-07-01 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Association_with_Natural_Organic_Matter_Enhances_the_Sunlight_Mediated_Inactivation_of_MS2_Coliphage_by_Singlet_Oxygen/2999377 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.