Drinking Water

Selenium (mainly in the forms of selenite (Se(IV)) and selenate (Se(VI)) is a regulated drinking water contaminant, but there is little information on the kinetics and mechanisms of Se(IV) oxidation during water treatment. Species-specific and apparent second-order rate constants for the oxidation of Se(IV) at pH 7.0 were determined in buffered solutions and they decrease in the order bromine (5.8 ± 0.3 × 10³ M⁻¹ s⁻¹) > ozone (O₃, 513.4 ± 10.0 M⁻¹ s⁻¹) > chlorine (61.0 ± 3.6 M⁻¹ s⁻¹) > permanganate (2.1 ± 0.1 M⁻¹ s⁻¹), monochloramine (NH₂Cl, (1.3 ± 0.1) × 10⁻³ M⁻¹ s⁻¹), and hydrogen peroxide (H₂O₂, (2.3 ± 0.1) × 10⁻⁵ M⁻¹ s⁻¹). The reaction stoichiometries for the reactions of Se(IV) with bromine, O₃, chlorine, NH₂Cl, and H₂O₂ are 1:1. For Mn(VII), the stoichiometries varied with pH and were 5:2, 3:2, and 1:2 for acidic, neutral, and alkaline conditions, respectively. Based on the reaction orders and stoichiometries, the corresponding Se(IV) oxidation mechanisms for various oxidants are discussed. The role of bromide for Se(IV) oxidation was also investigated during chlorination and ozonation of Se(IV)-containing water. During chlorination, bromide-catalysis enhances the rate of the oxidation of Se(IV) to Se(VI) from 50% to nearly 90% with bromide concentrations of 50 μg L⁻¹ and 200 μg L⁻¹, respectively, at pH 7.0 and a chlorine dose of 2.0 mg L⁻¹ (within 15 min). During ozonation, bromide had no effect on Se(IV) oxidation. Based on the determined second order rate constants, the oxidation of Se(IV) by chlorine and ozone were successfully predicted in a natural water by a kinetic model. The second order rate constants for the same oxidants were also investigated and/or evaluated for other related anions, such as arsenite (As(III)) and sulfite (S(IV)). They decreased in the order S(IV) > As(III) > Se(IV).

Dissolved organic matter (DOM) has been shown to inhibit the oxidation of aromatic amines initiated by excited triplet states, an effect that was attributed to the reduction of oxidation intermediates back to their parent compounds. The present study focuses on the quantification of an analogous inhibitory effect of DOM on aqueous oxidations induced by the sulfate radical (SO₄^·–). Second-order rate constants for the SO₄^·–-induced transformation of selected anilines and sulfonamide antibiotics were determined by competition kinetics in the presence and absence of DOM from three different isolates at pH 8. In the presence of 1 mg_C L^–1 of DOM, a significant reduction in the rate constant was observed for most of the compounds compared to DOM-free solutions, but for two electron-rich anilines, increases in the rate constant were measured. For 4-cyanoaniline and sulfamethoxazole, the DOM concentration dependence of the rate constant consisted of a sharp decrease up to ∼1.0 mg_C L^–1 of DOM followed by a region of slight changes or even increases for higher DOM concentrations (up to 5 mg_C L^–1). This behavior was attributed to the occurrence of the aforementioned inhibitory effect and a counteracting accelerated transformation of the contaminants due to reactions with secondary radical oxidants resulting from DOM oxidation by SO₄^·–. Both effects of inhibition and secondary oxidants should be considered when assessing the abatement of aromatic amines in SO₄^·–-based advanced oxidation processes.

The essential elements selenium (Se) and iodine (I) are often present in low levels in terrestrial diets, leading to potential deficiencies. Marine I and Se emissions and subsequent atmospheric wet deposition has been suggested to be an important source of I and Se to soils and terrestrial food chains. However, the contribution of recycled moisture of continental origin to I and Se to precipitation has never been analyzed. Here we report concentrations and speciation of I and Se, as well as of bromine (Br), sulfur (S), and DOC-δ¹³C signatures for weekly collected precipitation samples (in the period of April 2015 to September 2016) at two high altitude sites, i.e., Jungfraujoch (JFJ; Switzerland) and Pic du Midi (PDM; France). Analysis of precipitation chemistry and moisture sources indicate combined marine and continental sources of precipitation and Se, I, Br, and S at both sites. At JFJ, concentrations of I and Se were highest when continental moisture sources were dominant, indicating important terrestrial sources for these elements. Furthermore, correlations between investigated elements and DOC-δ¹³C, particularly when continental moisture source contributions were high, indicate a link between these elements and the source of dissolved organic matter, especially for I (JFJ and PDM) and Se (JFJ).

Photodegradation processes play an important role in releasing elements tied up in biologically refractory forms in the environment, and are increasingly recognized as important contributors to biogeochemical cycles. While complete photooxidation of dissolved organic carbon (to CO₂), and dissolved organic phosphorous (to PO₄^3-) has been documented, the analogous photoproduction of sulfate from dissolved organic sulfur (DOS) has not yet been reported. Recent high-resolution mass spectrometry studies showed a selective loss of organic sulfur during photodegradation of dissolved organic matter, which was hypothesized to result in the production of sulfate. Here, we provide evidence of ubiquitous production of sulfate, methanesulfonic acid (MSA) and methanesulfinic acid (MSIA) during photodegradation of DOM samples from a wide range of natural terrestrial environments. We show that photochemical production of sulfate is generally more efficient than the production of MSA and MSIA, as well as volatile S-containing compounds (i.e., CS₂ and COS). We also identify possible molecular precursors for sulfate and MSA, and we demonstrate that a wide range of relevant classes of DOS compounds (in terms of S oxidation state and molecular structure) can liberate sulfate upon photosensitized degradation. This work suggests that photochemistry may play a more significant role in the aquatic and atmospheric fate of DOS than currently believed.

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 (Φ_1O2 and Φ_F). The increase in Φ_1O2 was attributed to the formation of quinone-like moieties during ozonation of the phenolic moieties of DOM, while the increase in Φ_F 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 (λ_F,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.