Collaborative Research: Quantifying the electron reduction potential distribution of excited triplet states within Dissolved Organic Matter

Contaminants such as pesticides, pharmaceuticals, and personal care products often enter rivers and lakes. They typically originate from wastewater treatment plants and agricultural runoff. Because these pollutants can threaten environmental and public health, understanding how they break down in surface waters is important. Photochemistry stimulated by sunlight is a key pollutant degradation pathway. This reaction often involves high-energy, reactive forms of dissolved organic matter (DOM) that are in so-called "excited triplet states." This project will evaluate how triplet state reactivity varies across complex DOM from different sources. To achieve this, the team will develop a standardized method to assess triplet DOM reactivity. The team will then apply this method to DOM from diverse environments. These results will help scientists predict triplet state reactivity in environmental photochemistry. Ultimately, this will help understand how organic contaminants break down in sunlit waters. The project will help train future researchers and provide public tutorial publications on triplet DOM reactivity. Finally, the principal investigators will integrate research outcomes into their courses. Triplet excited states of dissolved organic matter (3DOM*) are one of the important photochemically produced reactive species upon dissolved organic matter sunlight absorption. Unlike other reactive species such as the hydroxyl radical and singlet oxygen, 3DOM* is comprised of a mixture of species all with varying reactivities and associated quantum yields. 3DOM* mainly react with organic contaminants via one-electron oxidation. For well-characterized, pure solutions of triplet states, the triplet-contaminant reactivity is governed by the one electron reduction potential of the excited triplet states. Given the complexity of 3DOM*, these one electron reduction potentials are not well-characterized and, thus, predicting the reactivity of 3DOM* in environmental photochemistry is not generally possible, leaving researchers to assess contaminants degradation via 3DOM* on a case-by-case basis. This project will characterize the distribution of triplet state one-electron reduction potentials in dissolved organic matter and the subsequent impacts on environmental photochemistry. The project will focus on three aims: Aim 1 will address developing the method to measure 3DOM* one electron reduction potential distributions; Aim 2 will assess the dependence of 3DOM* reactivity on DOM source and use targeted experiments (borohydride reduction) to elucidate the structure of DOM triplets; Aim 3 will examine the impact that the differences in reduction potential can have on the observed (and modeled) phototransformation of contaminants. The project will alleviate the need for case-by-case studies of contaminant reactivity with 3DOM*, thus improving modeling approaches for predicting the photochemical fate of contaminants in surface waters. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.