Dissolved Organic Matter Photoreactivity Is Determined by Its Optical Properties, Redox Activity, and Molecular Composition

Abstract

Predicting the formation of photochemically produced reactive intermediates (PPRI) during the irradiation of dissolved organic matter (DOM) has remained challenging given the complex nature of this material and differences in PPRI formation mechanisms. We investigate the role of DOM composition in photoreactivity using 48 samples that span the range of DOM in freshwater systems and wastewater. We relate quantum yields for excited triplet-state organic matter (f TMP ), singlet oxygen (Φ 1 Od 2 ), and hydroxylating species (Φ • OH ) to DOM composition determined using spectroscopy, Fourier-transform ion cyclotron resonance mass spectrometry, and electron-donating capacity (EDC). f TMP and Φ 1 Od 2 follow similar trends and are correlated with bulk properties derived from UV−vis spectra and EDC. In contrast, no individual bulk property can be used to predict Φ • OH . At the molecular level, the subset of DOM that is positively correlated to both Φ • OH and EDC is distinct from DOM formulas related to Φ 1 Od 2 , demonstrating that • OH and 1 O 2 are formed from different DOM fractions. Multiple linear regressions are used to relate quantum yields of each PPRI to DOM composition parameters derived from multiple techniques, demonstrating that complementary methods are ideal for characterizing DOM because each technique only samples a subset of DOM.

Formation of Targeted and Novel Disinfection Byproducts during Chlorine Photolysis in the Presence of Bromide

“…Furthermore, Spearman rank analyses demonstrate that each oxidative process targets different DOM fractions. The highly aromatic, reduced formulas that react during dark chlorination (Figure a) are located in the same region as novel chlorinated DBPs (H:C ∼ 0.5–1.3; O:C ∼ 0.3–0.8; Figure c), in agreement with observations of selective reactivity of chlorine. , Similar formulas react with UV (Figure e); these highly aromatic formulas are correlated with specific UV absorbance and therefore absorb more light. ,,,, Chlorine photolysis reacts with formulas that range widely in H:C (Figure b), while quenching with t -BuOH and nitrogen sparging partially protects aliphatic formulas (Figures S24a and S24b). These high H:C formulas are highly similar to formulas that react directly with • OH .…”

“…In addition to formation of organic DBPs, chlorine photolysis can alter the composition of DOM. Understanding how DOM changes is important because DOM composition determines both its photolytic and chemical reactivity. ,,, For example, direct photolysis of DOM and reaction with • OH can alter DOM so that it is more reactive with chlorine . DOM remaining after chlorine photolysis is likely to form DBPs during reaction with residual disinfectant in the distribution system.…”

Dissolved Organic Matter Composition Determines Its Susceptibility to Complete and Partial Photooxidation within Lakes

“…The deviation between formulas associated with carbon-normalized gas concentrations and quantum yields is attributable to several factors. Highly aromatic formulas absorb more light and are more susceptible to oxidation because they are electron-rich . However, on a per photon basis, saturated formulas are more efficient at consuming O 2 and producing DIC.…”

“…Highly aromatic formulas absorb more light and are more susceptible to oxidation because they are electron-rich. 84 However, on a per photon basis, saturated formulas are more efficient at consuming O 2 and producing DIC. Previous work has shown that high MW, aromatic DOM (i.e., formulas associated with carbon-normalized gas concentrations; Figure 1b,c) is able to quench 3 DOM efficiently.…”

Copper Safeguards Dissolved Organic Matter from Sunlight-Driven Photooxidation

“…The photooxidation-induced changes in DOM composition have important implications for the photosensitizing capacity of DOM in natural aquatic systems and the performance of engineered aquatic systems. , The prolonged persistence of phenolic moieties within DOM in the presence of Cu may decrease the quantum yields of 3 DOM* and 1 O 2 due to the quenching of 3 DOM* by these phenolic moieties in sunlit waters. , Additionally, in drinking water treatment systems that receive these irradiated Cu-rich source waters with high EDM concentrations, the consumption of disinfectants (e.g., chlorine, chlorine dioxide, and ozone) would increase and lead to higher formation potentials of harmful disinfection byproducts. ,,, …”