Reactivity-directed analysis - a novel approach for the identification of toxic organic electrophiles in drinking water

Thiol Reactome: A Nontargeted Strategy to Precisely Identify Thiol Reactive Drinking Water Disinfection Byproducts

The precise identification of predominant toxic disinfection byproducts (DBPs) from disinfected water is a longstanding challenge. We propose a new acellular analytical strategy, the 'Thiol Reactome', to identify thiol-reactive DBPs by employing a thiol probe and nontargeted mass spectrometry (MS) analysis. Disinfected/oxidized water samples had reduced cellular oxidative stress responses of 46 ± 23% in Nrf2 reporter cells when preincubated with glutathione (GSH). This supports thiol-reactive DBPs as the predominant drivers of oxidative stress. This method was benchmarked using seven classes of DBPs including haloacetonitriles, which preferentially reacted with GSH via substitution or addition depending on the number of halogens present. The method was then applied to chemically disinfected/oxidized waters, and 181 tentative DBP-GSH reaction products were detected. The formulas of 24 high abundance DBP-GSH adducts were predicted, among which nitrogenous-DBPs (11) and unsaturated carbonyls (4) were the predominant compound classes. Two major unsaturated carbonyl-GSH adducts, GSH-acrolein and GSH-acrylic acid, were confirmed by their authentic standards. These two adducts were unexpectedly formed from larger native DBPs when reacting with GSH. This study demonstrated the "Thiol Reactome" as an effective acellular assay to precisely identify and broadly capture toxic DBPs from water mixtures.

Amino Chemoassay Profiling of Aromatic Aldehydes-Unraveling Drivers of Their Skin Sensitization Potency

Aromatic aldehydes are ubiquitous in humans' everyday life. As aldehydes, they can form imines (Schiff bases) with amino groups of skin proteins, leading to immune response-triggered allergic contact dermatitis. Many known aromatic aldehydes are considered as weak or nonsensitizers, but others like atranol and chloratranol, two components of the fragrance oak moss absolute, show strong sensitization potency. This large discrepancy in potency and, in particular, the underlying reaction mechanisms are only little understood so far. To reduce this knowledge gap, our chemoassay employing glycine-para-nitroanilide (Gly-pNA) as an amino model nucleophile was applied to 23 aromatic aldehydes. The determined Gly-pNA second-order rate constants for imine formation (k₁ ≤ 2.85 L·mol^-1·min^-1) and the imine stability constant (K ≤ 333 L·mol^-1) are on the lower end of the known amino reactivity scale for aldehydes, confirming many aromatic aldehydes as less potent sensitizers in line with animal and human data. The substantially higher sensitization potency of atranol and chloratranol, in turn, is reflected by their unique reaction chemistry profiles, inter alia, identifying them as cross-linkers able to form thermodynamically more stable epitopes with skin proteins (despite low formation kinetics, k₁). The discussion further includes a comparison of experimentally determined k₁ values with computed reactivity data (Taft σ*), the impact of the substitution pattern of the aryl ring on the reactivity with Gly-pNA, and analytically determined adduct patterns. Overall, this work provides new insights into the reaction of aromatic aldehydes with amino groups under aqueous conditions and fosters a better understanding of the chemistry underlying skin sensitization.

Ozonation of lake water and wastewater: Identification of carbonous and nitrogenous carbonyl-containing oxidation byproducts by non-target screening

Ozonation of drinking water and wastewater is accompanied by the formation of disinfection byproducts (DBPs) such as low molecular weight aldehydes and ketones from the reactions of ozone with dissolved organic matter (DOM). By applying a recently developed non-target workflow, 178 carbonous and nitrogenous carbonyl compounds were detected during bench-scale ozonation of two lake waters and three secondary wastewater effluent samples and full-scale ozonation of secondary treated wastewater effluent. An overlapping subset of carbonyl compounds (20%) was detected in all water types. Moreover, wastewater effluents showed a significantly higher fraction of N-containing carbonyl compounds (30%) compared to lake water (17%). All carbonyl compounds can be classified in 5 main formation trends as a function of increasing specific ozone doses. Formation trends upon ozonation and comparison of results in presence and absence of the ^•OH radical scavenger DMSO in combination with kinetic and mechanistic information allowed to elucidate potential carbonyl structures. A link between the detected carbonyl compounds and their precursors was established by ozonating six model compounds (phenol, 4-ethylphenol, 4-methoxyphenol, sorbic acid, 3-buten-2-ol and acetylacetone). About one third of the detected carbonous carbonyl compounds detected in real waters was also detected by ozonating model compounds. Evaluation of the non-target analysis data revealed the identity of 15 carbonyl compounds, including hydroxylated aldehydes and ketones (e.g. hydroxyacetone, confidence level (CL) = 1), unsaturated dicarbonyls (e.g. acrolein, CL = 1; 2-butene-1,4-dial, CL = 1; 4-oxobut-2-enoic acid, CL = 2) and also a nitrogen-containing carbonyl compound (2-oxo-propanamide, CL =1). Overall, this study shows the formation of versatile carbonous and nitrogenous carbonyl compounds upon ozonation involving ozone and ^•OH reactions. Carbonyl compounds with unknown toxicity might be formed, and it could be demonstrated that acrolein, malondialdehyde, methyl glyoxal, 2-butene-1,4-dial and 4-oxo-pentenal are degraded during biological post-treatment.

Formation of carbonyl compounds during ozonation of lake water and wastewater: Development of a non-target screening method and quantification of target compounds

Ozonation of natural waters is typically associated with the formation of carbonyl compounds (aldehydes, ketones and ketoacids), a main class of organic disinfection byproducts (DBPs). However, the detection of carbonyl compounds in water and wastewater is challenged by multiple difficulties inherent to their physicochemical properties. A non-target screening method involving the derivatisation of carbonyl compounds with p-toluenesulfonylhydrazine (TSH) followed by their analysis using liquid chromatography coupled to electrospray ionisation high-resolution mass spectrometry (LC-ESI-HRMS) and an advanced non-target screening and data processing workflow was developed. The workflow was applied to investigate the formation of carbonyl compounds during ozonation of different water types including lake water, aqueous solutions containing Suwannee River Fulvic acid (SRFA), and wastewater. A higher sensitivity for most target carbonyl compounds was achieved compared to previous derivatisation methods. Moreover, the method allowed the identification of known and unknown carbonyl compounds. 8 out of 17 target carbonyl compounds were consistently detected above limits of quantification (LOQs) in most ozonated samples. Generally, the concentrations of the 8 detected target compounds decreased in the order: formaldehyde > acetaldehyde > glyoxylic acid > pyruvic acid > glutaraldehyde > 2,3-butanedione > glyoxal > 1-acetyl-1-cyclohexene. The DOC concentration-normalised formation of carbonyl compounds during ozonation was higher in wastewater and SRFA-containing water than in lake water. The specific ozone doses and the type of the dissolved organic matter (DOM) played a predominant role for the extent of formation of carbonyl compounds. Five formation trends were distinguished for different carbonyl compounds. Some compounds were produced continuously upon ozonation even at high ozone doses, while others reached a maximum concentration at a certain ozone dose above which they decreased. Concentrations of target and peak areas of non-target carbonyl compounds during full-scale ozonation at a wastewater treatment plant showed an increase as a function of the specific ozone dose (sum of 8 target compounds ∼ 280 µg/L at 1 mgO₃/mgC), followed by a significant decrease after biological sand filtration (> 64-94% abatement for the different compounds). This highlights the biodegradability of target and non-target carbonyl compounds and the importance of biological post-treatment.

Chlorination of para-substituted phenols: Formation of α, β-unsaturated C4-dialdehydes and C4-dicarboxylic acids

Despite the widespread occurrence of phenols in anthropogenic and natural compounds, their fate in reactions with hypochlorous acid (HOCl), one of the most common water treatment disinfectants, remains incompletely understood. To close this knowledge gap, this study investigated the formation of disinfection by-products (DBPs) in the reaction of free chlorine with seven para-substituted phenols. Based on the chemical structures of the DBPs and the reaction mechanisms leading to their formation, the DBPs were categorized into four groups: chlorophenols, coupling products, substituent reaction products, and ring cleavage products. In contrast to previous studies that investigated the formation of early-stage chlorophenols, the primary focus of this study was on the elucidation of novel ring cleavage products, in particular α, β-unsaturated C₄-dialdehydes, and C₄-dicarboxylic acids, which, for the first time, were identified and quantified in this study. The molar yields of 2-butene-1,4-dial (BDA), one of the identified α, β-unsaturated C₄-dialdehydes, varied among the different phenolic compounds, reaching a maximum value of 10.4% for bisphenol S. Molar yields of 2-chloromaleic acid (Cl-MA), one of the identified C₄-dicarboxylic acids, reached a maximum value of 30.5% for 4-hydroxy-phenylacetic acid under given conditions. 2,4,6-trichlorophenol (TCP) was shown to be an important intermediate of the parent phenols and the C₄-ring cleavage products. Based on the temporal trends of α, β-unsaturated C₄-dialdehydes and C₄-dicarboxylic acids, their formation is likely attributable to two separate ring cleavage pathways. Based on the obtained results, an overall transformation pathway for the reaction of para-substituted phenols with free chlorine leading to the formation of novel C₄ ring cleavage products was proposed.