Aqueous singlet oxygen reaction kinetics of furfuryl alcohol: effect of temperature, pH, and salt content.

The rate constant for the reaction between furfuryl alcohol (FFA) and singlet oxygen (1O2) in aqueous solution was measured as a function of temperature, pH and salt content employing both steady-state photolysis (ß value determination) and time-resolved singlet oxygen phosphorescence methods. The latter provided more precise and reproducible data. The reaction rate constant, krxn,FFA, had a relatively small temperature dependence, no pH dependence and showed a small increase in the presence of high salt concentrations (+19% with 1 M NaCl). A critical review of the available literature suggested that the widely used value of 1.2 × 108 M-1 s-1 is likely overestimated. Therefore, we recommend the use of 1.00 × 108 M-1 s-1 for reactions performed in low ionic strength aqueous solutions (freshwater) at 22 °C. Furthermore, corrections are provided that should be applied when working at higher or lower temperatures, and/or at high salt concentrations (seawater).

Photochemical production of singlet oxygen from particulate organic matter.

Dissolved organic matter is established as one of the most relevant photosensitizers in aquatic environments, producing singlet oxygen (1O2) alongside other photochemically produced reactive intermediates. While the production of 1O2 from DOM has been well studied, the relative importance of particulate organic matter (POM) to the overall 1O2 production is less well understood. POM is known to play an important role in pollutant fate through the sorption and transport of hydrophobic pollutants. If POM is directly involved in 1O2 production, sorbed molecules would be expected to undergo enhanced photodegradation. In this work, synthetic POM was prepared by coating silica particles with commercial humic acid. The photochemical behavior of these POM samples was compared to dissolved commercial humic acids (DOM). Suspended natural sediment was also studied to test the environmental relevance of the synthetic POM model. Synthetic POM particles appear to simulate well the 1O2-production of suspended sediment. The 1O2 concentrations experienced by POM-sorbed probe molecules was up to 30% higher than experienced by DOM-sorbed ones, even though the aqueous concentration of 1O2 in irradiated POM suspensions was much lower than the analogous DOM solutions. These results were interpreted with a reaction-diffusion model, which suggested that the production rate of 1O2 by POM is lower than DOM, but the loss of 1O2 from the POM-phase is also lower than DOM. Based on the experimental results of this study, calculations were conducted to estimate the impact of removing POM on 1O2-mediated processes. These calculations indicate that compounds with a log Koc value near 4 will be most affected by removal of POM and that the magnitude of the effect is proportional to the fraction of the total organic matter represented by POM. This study demonstrates that particles can play an important role in the degradation of organic compounds via aquatic photochemistry.

On the use of hydroxyl radical kinetics to assess the number-average molecular weight of dissolved organic matter.

Dissolved organic matter (DOM) is involved in numerous environmental processes, and its molecular size is important in many of these processes, such as DOM bioavailability, DOM sorptive capacity, and the formation of disinfection byproducts during water treatment. The size and size distribution of the molecules composing DOM remains an open question. In this contribution, an indirect method to assess the average size of DOM is described, which is based on the reaction of hydroxyl radical (HO(•)) quenching by DOM. HO(•) is often assumed to be relatively unselective, reacting with nearly all organic molecules with similar rate constants. Literature values for HO(•) reaction with organic molecules were surveyed to assess the unselectivity of DOM and to determine a representative quenching rate constant (k(rep) = 5.6 × 10(9) M(-1) s(-1)). This value was used to assess the average molecular weight of various humic and fulvic acid isolates as model DOM, using literature HO(•) quenching constants, kC,DOM. The results obtained by this method were compared with previous estimates of average molecular weight. The average molecular weight (Mn) values obtained with this approach are lower than the Mn measured by other techniques such as size exclusion chromatography (SEC), vapor pressure osmometry (VPO), and flow field fractionation (FFF). This suggests that DOM is an especially good quencher for HO(•), reacting at rates close to the diffusion-control limit. It was further observed that humic acids generally react faster than fulvic acids. The high reactivity of humic acids toward HO(•) is in line with the antioxidant properties of DOM. The benefit of this method is that it provides a firm upper bound on the average molecular weight of DOM, based on the kinetic limits of the HO(•) reaction. The results indicate low average molecular weight values, which is most consistent with the recent understanding of DOM. A possible DOM size distribution is discussed to reconcile the small nature of DOM with the large-molecule behavior observed in other studies.