Département Ressources aquatiques et eau potable

The carbonate radical CO₃^•- and the excited triplet states of chromophoric dissolved organic matter play an important role in the photodegradation of some easily oxidized pollutants in surface waters, such as the aromatic amines. Anilines and sulfadiazine are known to undergo back-reduction processes when their degradation is mediated by the excited triplet states of photosensitizers (triplet sensitization). Back-reduction, which inhibits photodegradation, means that phenols or the antioxidant (mostly phenolic) moieties occurring in the natural dissolved organic matter of surface waters reduce, back to the parent compounds, the radical species derived from the mono-electronic oxidation of anilines and sulfadiazine. Here we show that a similar process takes place as well in the case of substrate oxidation by CO₃^•−. The carbonate radical was here produced upon oxidation of HCO₃^-/CO₃^2- by either HO^•, generated by nitrate photolysis, or SO₄^•-, obtained by photolysis of persulfate. Back-reduction was observed in both cases in the presence of phenols, but at different extents as far as the details of reaction kinetics are concerned, and the occurrence of additional reductants might affect the efficacy by which phenols carry out the reduction process. In particular, when the carbonate radicals were produced by NO₃^– photolysis in the presence of HCO₃^-/CO₃^2-, the numerical values of [PhOH]_1/2 (the phenol concentration that halves the photodegradation rate of the substrate) were 2.19 ± 0.23 µM for aniline, 1.15 ± 0.25 µM for 3-chloroaniline, 1.18 ± 0.26 µM for 4-chloroaniline, and 1.18 ± 0.22 µM for 3,4-dichloroaniline. In contrast, when CO₃^•- was produced by photolysis of persulfate in the presence of HCO₃^-/CO₃^2-, the corresponding values were 0.28 ± 0.02 µM for aniline and 0.79 ± 0.10 µM for sulfadiazine. Back-reduction has the potential to significantly inhibit photodegradation by CO₃^•− and excited triplet states in natural waters, and to comparatively increase the importance of HO^•-mediated degradation that is not affected by the same phenomenon.

The composition of oxidant-reactive dissolved organic nitrogen (DON) is poorly characterized, although its ozonation is likely to form a great variety of disinfection by-products containing a nitrogen-oxygen bond (N-DBPs). In this study, two chemical oxidation procedures were developed: continuous ozonation at pH 7.0 and free available chlorine (FAC) titrations at pH 9.2. The formation of two oxidation products (nitrate (NO₃^-) and chloramines, respectively) was used to quantify and characterize oxidant-reactive nitrogenous moieties in DON. In addition, batch experiments were conducted to study the NO₃^- yields of 30 selected nitrogenous model compounds upon ozonation. The NO₃^- yields of 12 primary and secondary amines were highly variable (17-100%, specific ozone dose of 20 mol_O3/mol_N), 7 amino acids had high NO₃^- yields (≥90%), and tertiary amines as well as pyrrole, acetamide and urea had low NO₃^- yields (≤15%). The mechanisms of NO₃^- formation were further examined with benzylamine and N-methylbenzylamine as model compounds. Our results show that nitroalkanes are the last intermediate products before the formation of NO₃^-, both for primary and secondary amines. The presence of an electron-withdrawing group in the vicinity of the N-atom facilitates the formation of NO₃^- from nitroalkanes. Therefore, the formation of NO₃^- is attributed to amino acids and activated primary and secondary amines. In contrast, all primary and secondary amines were transformed to chloramines upon chlorination, which was determined by a novel oxidative titration with chlorine. To further support the selectivity of this assay, it was demonstrated by derivatization of amine moieties that chloramine formation could be inhibited. 13-45% of the DON of 4 dissolved organic matter isolates and 2 wastewater effluents formed NO₃^- and 0-39% formed chloramines, indicating that the potential for N-DBP formation is high (µM_N/mg_C-level). From differences in the formation of NO₃^- and chloramines the nature of the precursors can be hypothesized (e.g., activated or non-activated primary and secondary amines, partially oxidized nitrogenous compounds). This study highlights the capacity of two novel methods to characterize the oxidant-reactive DON fraction. Our results suggest that this fraction is significant and could form a variety of potentially toxic N-DBPs.

Surfactants are high-production-volume chemicals that are among the most abundant organic pollutants in municipal wastewater. In this study, sewage sludge samples of 36 Swiss wastewater treatment plants (WWTPs), serving 32% of the country's population, were analyzed for major surfactant classes by liquid chromatography mass spectrometry (LC-MS). The analyses required a variety of complementary approaches due to different analytical challenges, including matrix effects (which can affect adduct ion formation) and the lack of reference standards. The most abundant contaminants were linear alkylbenzene sulfonates (LAS; weighted mean [WM] concentration of 3700 μg g^-1 dry weight), followed by secondary alkane sulfonates (SAS; 190 μg g^-1). Alcohol polyethoxylates (AEO; 8.3 μg g^-1), nonylphenol polyethoxylates (NPEO; 16 μg g^-1), nonylphenol (NP; 3.1 μg g^-1), nonylphenol ethoxy carboxylates (NPEC; 0.35 μg g^-1) and tert-octylphenol (tert-OP, 1.8 μg g^-1) were present at much lower concentrations. This concentration pattern agrees with the production volumes of the surfactants and their fates in WWTPs. Branched AEO homologues dominated over linear homologues, probably due to higher persistence. Sludge concentrations of LAS, SAS, and NP were positively correlated with the residence time in the anaerobic digester. Derivation of the per capita loads successfully revealed potential industrial/commercial emission sources. Comparison of recent versus historic data showed a decrease in NPEO and NP levels by one or two orders of magnitude since their ban in the 1980s. By contrast, LAS still exhibit similar concentrations compared to 30 years ago.