Environ. Sci. Technol. 2007, 41, 4626-4632
Association with Natural Organic Matter Enhances the Sunlight-Mediated Inactivation of MS2 Coliphage by Singlet Oxygen T A M A R K O H N , * ,†,§ MATTHEW GRANDBOIS,‡ KRISTOPHER MCNEILL,‡ AND KARA L. NELSON† Department of Civil and Environmental Engineering, Univeristy of California, Berkeley, California 94720, and Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455
MS2 coliphage, a surrogate for human enteric viruses, is inactivated by singlet oxygen (1O2) produced via sunlightmediated 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, kobs, over a range of 1-25 mg NOM/L. The kobs 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 kobs. 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 kobs. Addition of Mg2+ to neutralize the negative surface charge of MS2 and NOM increased kobs up to 4.1-fold. Similarly, lowering the solution pH closer to the isoelectric point of MS2 (pI ) 3.9) enhanced kobs 51fold in 5 mg/L AHA.
Introduction Sunlight irradiation contributes significantly to the inactivation of viral and bacterial pathogens in sewage-impacted surface waters (1-5). This process is also essential to some * Corresponding author phone: +41 21 693 0891; e-mail:
[email protected]. † Univeristy of California. ‡ University of Minnesota. § Current address: E Ä cole Polytechnique Fe´de´rale de Lausanne, Faculte´ Environnement Naturel, Architectural et Construit, CM 1 118, Station 2, 1015 Lausanne, Switzerland. 4626
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low-cost water and wastewater treatment systems, such as solar disinfection (SODIS; 6) and waste stabilization ponds (WSP; 7, 8). Photoinactivation can occur directly, via damage to nucleic acids by UVB light, or indirectly, via damage to cell components by reactive oxygen species (ROS) that may potentially be formed by UV and visible light (9). ROS are formed when internal (endogenous) sensitizers (e.g., flavins (10)) or sensitizers present in the surrounding water sample (exogenous sensitizers, e.g., natural organic matter (NOM) (11)) are excited by sunlight, followed by energy transfer and redox reactions with oxygen in the solution. It was recently demonstrated that indirect photoinactivation of MS2 coliphage occurred in solutions of commercial NOM and in WSP water (12). In the presence of these external sensitizers, singlet oxygen (1O2) was the most important ROS inactivating MS2. It was also recently shown that the 1O2 concentration within NOM ([1O2]internal) was orders of magnitude higher than the steady-state concentration in the aqueous bulk, [1O2]bulk, and that [1O2] rapidly decreased with increasing distance from the molecule of NOM (13). Therefore, we hypothesized that the photoinactivation of MS2 proceeds more rapidly when the MS2 particles are in close proximity to the sensitizer (NOM or WSP constituents), and thereby exposed to elevated 1O2 concentrations ([1O2]NOM) compared to [1O2]bulk. The degree of association between viruses and NOM is likely to be important for many transport and treatment processes. However, very little is known about their interactions, in part because free and NOM-associated viruses are difficult to separate. MS2 and enteric viruses have been postulated to associate with small (
