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 (O-1(2)) produced via sunlightmediated excitation of natural organic matter (NOM) in surface waters. The O-1(2) concentration within a NOM macromolecule or supramolecular assembly ([O-1(2)](intemal)) is orders of magnitude higher than in the bulk solution ([O-1(2)](bulk)). In close proximity of NOM, MS2 is thus exposed to an elevated O-1(2) concentration ([O-1(2)](NOM)), and inactivation is likely to be enhanced as compared to the bulk solution. In experiments using a solar simulator, we determined [O-1(2)](bulk), [O-1(2)](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 [O-1(2)](NOM) rather than [O-1(2)](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 (pl = 3.9) enhanced k(obs) 51fold in 5 mg/L AHA.