Influence of Ionic Strength on Triplet-State Natural Organic Matter Loss by Energy Transfer and Electron Transfer Pathways

Triplet state excited natural organic matter chromophores ((3)NOM*) are important reactive intermediates in indirect photochemical processes, yet the impact of salt concentrations relevant to estuarine and marine environments on (3)NOM* is poorly understood. The formation rates, pseudo-first-order loss rate constants, and steady-state concentration of (3)NOM* were monitored using the sorbate probe method in synthetic matrices with increasing ionic strength (IS) to seawater values using seawater halides or other salts. The steady-state concentration of (3)NOM* approximately doubled at seawater IS, regardless of the salt used, due to a decrease in the (3)NOM* decay rate constant. The electron transfer-mediated degradation of 2,4,6-trimethylphenol (TMP) by (3)NOM* was significantly slowed at higher IS. A model is proposed wherein high IS slows intra-organic matter electron transfer pathways, an important (3)NOM* loss pathway, leading to longer (3)NOM* lifetimes. Although IS did not appear to impact energy transfer pathways directly, the higher (3)NOM* steady-state concentrations promote energy transfer interactions. The observed decrease in decay rate constant, increase in steady-state concentration of (3)NOM* at high IS, and the inhibition of electron transfer pathways should be considered when determining the fate of organic pollutants in estuarine and marine environments.