Carbon, Hydrogen, and Nitrogen Isotope Fractionation Trends in <i>N</i>‑Nitrosodimethylamine Reflect the Formation Pathway during Chloramination of Tertiary Amines

Assessing the precursors and reactions leading to the carcinogenic <i>N</i>-nitrosodimethylamine (NDMA) during drinking water disinfection is a major challenge. Here, we investigate whether changes of ¹³C/¹²C, ²H/¹H, and ¹⁵N/¹⁴N ratios of NDMA give rise to isotope fractionation trends that can be used to infer NDMA formation pathways. We carried out compound-specific isotope analysis (CSIA) of NDMA during chloramination of four tertiary amines that produce NDMA at high yields, namely ranitidine, 5-(dimethylaminomethyl)furfuryl alcohol, <i>N,N</i>-dimethylthiophene-2-methylamine, and <i>N,N</i>-dimethylbenzylamine. Carbon and hydrogen isotope ratios of NDMA function as fingerprints of the N­(CH₃)₂ moiety and exhibit only minor isotope fractionation during the disinfection process. Nitrogen isotope ratios showed that NH₂Cl is the source of the N atom of the nitroso group. The large enrichment of ¹⁵N in NDMA was indicative of the isotope effects pertinent to bond-cleavage and bond-formation reactions during chloramination of the tertiary amines. Correlation of δ¹⁵N versus δ¹³C values of NDMA resulted in trend lines that were not affected by the type of tertiary amine and treatment conditions, suggesting that the observed C and N isotope fractionation in NDMA may be diagnostic for NDMA precursors and formation pathways during chloramination

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<i>N</i>-Nitrosodimethylamine (NDMA) is a carcinogenic disinfection byproduct from water chloramination. Despite the identification of numerous NDMA precursors, essential parts of the reaction mechanism such as the incorporation of molecular O₂ are poorly understood. In laboratory model systems for the chloramination of secondary and tertiary amines, we investigated the kinetics of precursor disappearance and NDMA formation, quantified the stoichiometries of monochloramine (NH₂Cl) and aqueous O₂ consumption, derived ¹⁸O-kinetic isotope effects (¹⁸O-KIE) for the reactions of aqueous O₂, and studied the impact of radical scavengers on NDMA formation. Although the molar NDMA yields from five <i>N</i>,<i>N</i>-dimethylamine-containing precursors varied between 1.4% and 90%, we observed the stoichiometric removal of one O₂ per <i>N</i>,<i>N</i>-dimethylamine group of the precursor indicating that the oxygenation of N atoms did not determine the molar NDMA yield. Small ¹⁸O-KIEs between 1.0026 ± 0.0003 and 1.0092 ± 0.0009 found for all precursors as well as completely inhibited NDMA formation in the presence of radical scavengers (ABTS and trolox) imply that O₂ reacted with radical species. Our study suggests that aminyl radicals from the oxidation of organic amines by NH₂Cl and <i>N</i>-peroxyl radicals from the reaction of aminyl radicals with aqueous O₂ are part of the NDMA formation mechanism

Biotransformation of Benzotriazoles: Insights from Transformation Product Identification and Compound-Specific Isotope Analysis

Benzotriazoles are widely used domestic and industrial corrosion inhibitors and have become omnipresent organic micropollutants in the aquatic environment. Here, the range of aerobic biological degradation mechanisms of benzotriazoles in activated sludge was investigated. Degradation pathways were elucidated by identifying transient and persistent transformation products in batch experiments using liquid chromatography–high-resolution tandem mass spectrometry (LC-HR-MS/MS). In addition, initial reactions were studied using compound-specific isotope analysis (CSIA). Biodegradation half-lives of 1.0 days for 1H-benzotriazole, 8.5 days for 4-methyl-1H-benzotriazole, and 0.9 days for 5-methyl-1H-benzotriazole with activated sludge confirmed their known partial persistence in conventional wastewater treatment. Major transformation products were identified as 4- and 5-hydroxy-1H-benzotriazole for the degradation of 1H-benzotriazole, and 1H-benzotriazole-5-carboxylic acid for the degradation of 5-methyl-1H-benzotriazole. These transformation products were found in wastewater effluents, showing their environmental relevance. Many other candidate transformation products, tentatively identified by interpretation of HR-MS/MS spectra, showed the broad range of possible reaction pathways including oxidation, alkylation, hydroxylation and indicate the significance of cometabolic processes for micropollutant degradation in biological wastewater treatment in general. The combination of evidence from product analysis with the significant carbon and nitrogen isotope fractionation suggests that aromatic monohydroxylation is the predominant step during the biotransformation of 1H-benzotriazole

Compound-Specific Carbon, Nitrogen, and Hydrogen Isotope Analysis of <i>N</i>‑Nitrosodimethylamine in Aqueous Solutions

Mitigation of <i>N</i>-nitrosodimethylamine (NDMA) and other hazardous water disinfection byproducts (DBP) is currently hampered by a limited understanding of DBP formation mechanisms. Because variations of the stable isotope composition of NDMA can potentially reveal reaction pathways and precursor compounds, we developed a method for the compound-specific isotope analysis (CSIA) of ¹³C/¹²C, ¹⁵N/¹⁴N, and ²H/¹H ratios of NDMA by gas chromatography coupled to isotope ratio mass spectrometry (GC/IRMS). Method quantification limits for the accurate isotope analysis of NDMA, <i>N</i>-nitrosodiethyl-, -dipropyl-, and -dibutylamine as well as <i>N</i>-nitrosopyrrolidine were between 0.18 to 0.60 nmol C, 0.40 to 0.80 nmol N, and 2.2 to 5.8 nmol H injected on column. Coupling solid phase extraction (SPE) to GC/IRMS enabled the precise quantification of C, N, and H isotope ratios of NDMA in aqueous samples at concentrations of 0.6 μM (45 μg L^–1). We validated the proposed method with a laboratory experiment, in which NDMA was formed with stoichiometric yield (97 ± 4%) through chloramination of the pharmaceutical ranitidine (3 μM). δ¹³C and δ²H values of NDMA remained constant during NDMA formation while its δ¹⁵N increased due to a reaction at a N atom in the rate-limiting step of NDMA formation. The δ²H value of NDMA determined by SPE-GC/IRMS also corresponded well to the δ²H value of the N­(CH₃)₂-group of ranitidine measured by quantitative deuterium nuclear magnetic resonance spectroscopy. This observation implies that the N­(CH₃)₂-moiety of ranitidine is transferred to NDMA without being chemically altered and illustrates the accuracy of the proposed method

Black Carbon-Amended Engineered Media Filters for Improved Treatment of Stormwater Runoff

Urban stormwater runoff is a significant driver of surface water quality impairment. Recently, attention has been drawn to potential beneficial use of urban stormwater runoff, including augmenting drinking water supply in water-stressed areas. However, beneficial use relies on improved treatment of stormwater runoff to remove mobile dissolved metals and trace organic contaminants (TrOCs). This study assesses six engineered media mixtures consisting of sand, zeolite, high-temperature gasification biochar, and regenerated activated carbon (RAC) for removing a suite of co-contaminants comprising five metals, three herbicides, four pesticides, a corrosion inhibitor, six per- and polyfluoroalkyl substances (PFASs), five polychlorinated biphenyls (PCBs), and six polycyclic aromatic hydrocarbons (PAHs). This long-term laboratory-scale column study uses a novel approach to generate reproducible synthetic stormwater that incorporates catch basin material and straw-derived dissolved organic carbon. Higher flow conditions (20 cm hr–1), larger sized media (0.42–1.68 mm), and downflow configuration with outlet control increase the relevance of this study to better enable implementation in the field. Biochar- and RAC-amended engineered media filters removed nearly all of the TrOCs in the effluent over the course of three months of continuous flow (480 empty bed volumes), while sample ports spaced at 25% and 50% along the column depth provide windows to observe contaminant transport. Biochar provided greater benefit to TrOC removal than RAC on a mass basis. This study used relatively high concentrations of contaminants and low biochar and RAC content to observe contaminant transport. Performance in the field is likely to be significantly better with higher biochar- and RAC-content filters and lower ambient stormwater contaminant concentrations. This study provides proof-of-concept for biochar- and RAC-amended engineered media filters operated at a flow rate of 20 cm hr–1 for removing dissolved TrOCs and metals and offers insights on the performance of biochar and RAC for improved stormwater treatment and field trials