The antioxidant capacity and formation of photochemically produced reactive intermediates (RI) was studied for water samples collected from the Florida Everglades with different spatial (marsh versus estuarine) and temporal (wet versus dry season) characteristics. Measured RI included triplet excited states of dissolved organic matter (DOM*), singlet oxygen (O), and the hydroxyl radical (OH). Single and multiple linear regression modeling were performed using a broad range of extrinsic (to predict RI formation rates, R) and intrinsic (to predict RI quantum yields, Φ) parameters. Multiple linear regression models consistently led to better predictions of R and Φ for our data set but poor prediction of Φ for a previously published data set,1 probably because the predictors are intercorrelated (Pearson's r > 0.5). Single linear regression models were built with data compiled from previously published studies (n ≈ 120) in which E2:E3, S, and Φ values were measured, which revealed a high degree of similarity between RI-optical property relationships across DOM samples of diverse sources. This study reveals that OH formation is, in general, decoupled fromDOM* and O formation, providing supporting evidence that DOM* is not aOH precursor. Finally, Φ for O and DOM* correlated negatively with antioxidant activity (a surrogate for electron donating capacity) for the collected samples, which is consistent with intramolecular oxidation of DOM moieties byDOM*.
We studied the formation of photochemically produced reactive intermediates (RI) from dissolved organic matter (DOM). Specifically, we focused on the effects of variable molecular weight and chemical reduction on the optical properties of DOM (absorbance and fluorescence) and the formation of singlet oxygen ((1)O2), DOM triplet excited states ((3)DOM*), and the hydroxyl radical ((•)OH). The data are largely evaluated in terms of a charge-transfer (CT) model, but deficiencies in the model to explain the data are pointed out when evident. A total of two sets of samples were studied that were subjected to different treatments; the first set included secondary-treated wastewaters and a wastewater-impacted stream, and the second was a DOM isolate. Treatments included size fractionation and chemical reduction using sodium borohydride. Taken as a whole, the results demonstrate that decreasing molecular weight and borohydride reduction work in opposition regarding quantum efficiencies for (1)O2 and (3)DOM* production but in concert for fluorescence and (•)OH production. The optical and photochemical data provide evidence for a limited role of CT interactions occurring in lower-molecular-weight DOM molecules. In addition, the data suggest that the observed optical and photochemical properties of DOM are a result of multiple populations of chromophores and that their relative contribution is changed by molecular-weight fractionation and borohydride reduction.
The optical properties of dissolved organic matter influence chemical and biological processes in all aquatic ecosystems. Dissolved organic matter optical properties have been attributed to a charge-transfer model in which donor-acceptor complexes play a primary role. This model was evaluated by measuring the absorbance and fluorescence response of organic matter isolates to changes in solvent temperature, viscosity, and polarity, which affect the position and intensity of spectra for known donor-acceptor complexes of organic molecules. Absorbance and fluorescence spectral shape were largely unaffected by these changes, indicating that the distribution of absorbing and emitting species was unchanged. Overall, these results call into question the wide applicability of the charge-transfer model for explaining organic matter optical properties and suggest that future research should explore other models for dissolved organic matter photophysics.
This
study focused on the effects of ozonation on the photochemical
and photophysical properties of dissolved organic matter (DOM). Upon
ozonation, a decrease in DOM absorbance was observed in parallel with
an increase in singlet oxygen (1O2) and fluorescence
quantum yields (Φ1O2 and ΦF). The
increase in Φ1O2 was attributed to the formation
of quinone-like moieties during ozonation of the phenolic moieties
of DOM, while the increase in ΦF can be explained
by a significant decrease in the internal conversion rate of the first
excited singlet state of the DOM (1DOM*). It is a consequence
of an increase in the average energy of the first electronic transition
(S1 → S0) that was assessed using the
wavelength of maximum fluorescence emission (λF,max). Furthermore, ozonation did not affect the ratio of the apparent
steady-state concentrations of excited triplet DOM (3DOM*)
and 1O2, indicating that ozonation does not
affect the efficiency of 1O2 production from 3DOM*. The consequences of these changes for the phototransformation
rates of micropollutants in surface waters were examined using photochemical
model calculations. The decrease in DOM absorbance caused by ozonation
leads to an enhancement of direct photolysis rates due to the increased
transparency of the water. Rates of indirect photooxidation induced
by 1O2 and 3DOM* slightly decrease
after ozonation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.