C & N isotope analysis of diclofenac to distinguish oxidative and reductive transformation and to track commercial products

Nitrogen stable isotope analysis of sulfonamides by derivatization-gas chromatography-isotope ratio mass spectrometry

The continuous introduction of micropollutants into the environment through livestock farming, agricultural practices, and wastewater treatment is a major concern. Among these pollutants are synthetic sulfonamide antibiotics such as sulfamethoxazole, which are not always fully degraded and pose a risk of fostering antimicrobial resistance. It is challenging to assess the degradation of sulfonamides with conventional concentration measurements. This study introduces compound-specific isotope analysis of nitrogen isotope ratios at natural abundances by derivatization-gas chromatography hyphenated with isotope ratio mass spectrometry (derivatization-GC-IRMS) as a new and more precise method for tracing the origin and degradation of sulfonamides. Here, sulfamethoxazole was used as a model compound to develop and optimize the derivatization conditions using (trimethylsilyl)diazomethane as a derivatization reagent. With the optimized conditions, accurate and reproducible δ15N analysis of sulfamethoxazole by derivatization-GC-IRMS was achieved in two different laboratories with a limit for precise isotope analysis of 3 nmol N on column, corresponding to 0.253 µg non-derivatized SMX. Application of the method to four further sulfonamides, sulfadiazine, sulfadimethoxine, sulfadimidine, and sulfathiazole, shows the versatility of the developed method. Its benefit was demonstrated in a first application, highlighting the possibility of distinguishing sulfamethoxazole from different suppliers and pharmaceutical products.

Carbon and Nitrogen Isotope Fractionations During Biotic and Abiotic Transformations of 2,4-Dinitroanisole (DNAN)

2,4-Dinitroanisole (DNAN) is a main constituent in various new insensitive munition formulations. Although DNAN is susceptible to biotic and abiotic transformations, in many environmental instances, transformation mechanisms are difficult to resolve, distinguish, or apportion on the basis solely of analysis of concentrations. We used compound-specific isotope analysis (CSIA) to investigate the characteristic isotope fractionations of the biotic (by three microbial consortia and three pure cultures) and abiotic (by 9,10-anthrahydroquinone-2-sulfonic acid [AHQS]) transformations of DNAN. The correlations of isotope enrichment factors (ΛN/C) for biotic transformations had a range of values from 4.93 ± 0.53 to 12.19 ± 1.23, which is entirely distinct from ΛN/C values reported previously for alkaline hydrolysis, enzymatic hydrolysis, reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, and UV-driven phototransformations. The ΛN/C value associated with the abiotic reduction by AHQS was 38.76 ± 2.23, within the range of previously reported values for DNAN reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, albeit the mean ΛN/C was lower. These results enhance the database of isotope effects accompanying DNAN transformations under environmentally relevant conditions, allowing better evaluation of the extents of biotic and abiotic transformations of DNAN that occur in soils, groundwaters, surface waters, and the marine environment.

Kinetic isotope effects of C and N indicate different transformation mechanisms between atzA- and trzN-harboring strains in dechlorination of atrazine

Compound-specific stable isotope analysis provides an alternative method to insight into the biotransformation mechanisms of diffuse organic pollutants in the environment, e.g., the endocrine disruptor herbicide atrazine. Biotic hydrolysis process catalyzed by chlorohydrolase AtzA and TrzN plays an important role in the detoxification of atrazine, while the catalytic mechanism of AtzA is still speculative. To investigate the catalytic mechanism of AtzA and answer whether both enzymes catalyze hydrolytic dechlorination of atrazine by the same mechanism, in this study, apparent kinetic isotope effects (AKIE) for carbon and nitrogen were observed by three atzA-harboring bacterial isolates and their membrane-free extracts. The AKIEs obtained from atzA-harboring bacterial isolates (AKIEC = 1.021 ± 0.010, AKIEN = 0.992 ± 0.003) were statistically different from that of trzN-harboring strains (AKIEC = 1.040 ± 0.006, AKIEN = 0.983 ± 0.006), confirming the different activation mechanisms of atrazine preceding to nucleophilic aromatic substitution of Cl atom in actual enzymatic reaction catalyzed by AtzA and TrzN, despite the limitation of variable dual-element isotope plots. The lower degree of normal carbon and inverse nitrogen isotope fractionation observed from atzA-harboring strains, suggesting AtzA catalyzing hydrolytic dechlorination of atrazine by coordination of Cl and one aromatic N to the Fe2+ drawing electron density from carbon-chlorine bond that facilitating the nucleophilic attack, rather than in TrzN case that protonation of aromatic N increasing nucleophilic substitution of Cl atom. This study suggests considering the potential influences of phylogenetic diversity of bacterial isolates and evolution of enzymes on the applications of CSIA method in future study.

Linking reaction rate constants and isotope fractionation of ozonation reactions using phenols as probes

Ozonation is nowadays a widely used method in drinking water treatment for disinfection and pollutant control. However, transformation products of ozonation can be more toxic than their parent compounds. Therefore, the knowledge of the reaction mechanisms and product formation is essential for a safe application. Different analytical methods such as high-resolution mass spectrometry (HRMS) and compound-specific isotope analysis (CSIA) can be applied to elucidate products and primary attack positions of oxidation agents such as ozone. During the investigation of the ozonation of phenolic compounds in water by CSIA, a reaction rate depending carbon isotope fractionation was observed. The fractionation strongly depends on the phenol speciation. With decreasing pH values and reaction rates <10⁵ M^-1 s^-1, the isotope enrichment factor ε increases (ε is between -5.2 and -1.0‰). For faster reactions (>10⁵ M^-1 s^-1), the carbon isotope enrichment was not significant anymore (ε is between -1.0 and 0‰). Based on these data a concept to correlate isotope enrichment factors with kinetic data for aromatic compounds is proposed. The additional investigation of aliphatic double and triple bonds did not fit this correlation suggesting different rate-limiting steps. However, double and triple bond showed a similar enrichment factor, which implies the same rate-limiting step in the reaction with ozone, the monodentate addition of ozone.

Biodegradation and photooxidation of phenolic compounds in soil-A compound-specific stable isotope approach

Phenolic compounds occur in a variety of plants and can be used as model compounds for investigating the fate of organic wastewater, lignin, or soil organic matter in the environment. The aim of this study was to better understand and differentiate mechanisms associated with photo- and biodegradation of tyrosol, vanillin, vanillic acid, and coumaric acid in soil. In a 29 d incubation experiment, soil spiked with these phenolic compounds was either subjected to UV irradiation under sterile conditions or to the native soil microbial community in the dark. Changes in the isotopic composition (δ¹³C) of phenolic compounds were determined by gas chromatography-isotope ratio mass spectrometry and complemented by concentration measurements. Phospholipid-derived fatty acid and ergosterol biomarkers together with soil water repellency measurements provided information on soil microbial and physical properties. Biodegradation followed pseudo-first-order dissipation kinetics, enriched remaining phenolic compounds in ¹³C, and was associated with increased fungal rather than bacterial biomarkers. Growing mycelia rendered the soil slightly water repellent. High sample variation limited the reliable estimation of apparent kinetic isotope effects (AKIEs) to tyrosol. The AKIE of tyrosol biodegradation was 1.007 ± 0.002. Photooxidation kinetics were of pseudo-zero- or first-order with an AKIE of 1.02 ± 0.01 for tyrosol, suggesting a hydroxyl-radical mediated degradation process. Further research needs to address δ¹³C variation among sample replicates potentially originating from heterogeneous reaction spaces in soil. Here, nuclear magnetic resonance or nanoscopic imaging could help to better understand the distribution of organic compounds and their transformation in the soil matrix.

13C- and 15N-Isotope Analysis of Desphenylchloridazon by Liquid Chromatography-Isotope-Ratio Mass Spectrometry and Derivatization Gas Chromatography-Isotope-Ratio Mass Spectrometry

The widespread application of herbicides impacts surface water and groundwater. Metabolites (e.g., desphenylchloridazon from chloridazon) may be persistent and even more polar than the parent herbicide, which increases the risk of groundwater contamination. When parent herbicides are still applied, metabolites are constantly formed and may also be degraded. Evaluating their degradation on the basis of concentration measurements is, therefore, difficult. This study presents compound-specific stable-isotope analysis (CSIA) of nitrogen- and carbon-isotope ratios at natural abundances as an alternative analytical approach to track the origin, formation, and degradation of desphenylchloridazon (DPC), the major degradation product of the herbicide chloridazon. Methods were developed and validated for carbon- and nitrogen-isotope analysis (δ¹³C and δ¹⁵N) of DPC by liquid chromatography-isotope-ratio mass spectrometry (LC-IRMS) and derivatization gas chromatography-IRMS (GC-IRMS), respectively. Injecting standards directly onto an Atlantis LC-column resulted in reproducible δ¹³C-isotope analysis (standard deviation <0.5‰) by LC-IRMS with a limit of precise analysis of 996 ng of DPC on-column. Accurate and reproducible δ¹⁵N analysis with a standard deviation of <0.4‰ was achieved by GC-IRMS after derivatization of >100 ng of DPC with 160-fold excess of (trimethylsilyl)diazomethane. Application of the method to environmental-seepage water indicated that newly formed DPC could be distinguished from "old" DPC by the different isotopic signatures of the two DPC sources.

Isotope Fractionation Pinpoints Membrane Permeability as a Barrier to Atrazine Biodegradation in Gram-negative Polaromonas sp. Nea-C

Biodegradation of persistent pesticides like atrazine often stalls at low concentrations in the environment. While mass transfer does not limit atrazine degradation by the Gram-positive Arthrobacter aurescens TC1 at high concentrations (>1 mg/L), evidence of bioavailability limitations is emerging at trace concentrations (<0.1 mg/L). To assess the bioavailability constraints on biodegradation, the roles of cell wall physiology and transporters remain imperfectly understood. Here, compound-specific isotope analysis (CSIA) demonstrates that cell wall physiology (i.e., the difference between Gram-negative and Gram-positive bacteria) imposes mass transfer limitations in atrazine biodegradation even at high concentrations. Atrazine biodegradation by Gram-negative Polaromonas sp. Nea-C caused significantly less isotope fractionation (ε(C) = -3.5 ‰) than expected for hydrolysis by the enzyme TrzN (ε(C) = -5.0 ‰) and observed in Gram-positive Arthrobacter aurescens TC1 (ε(C) = -5.4 ‰). Isotope fractionation was recovered in cell-free extracts (ε(C) = -5.3 ‰) where no cell envelope restricted pollutant uptake. When active transport was inhibited with cyanide, atrazine degradation rates remained constant demonstrating that atrazine mass transfer across the cell envelope does not depend on active transport but is a consequence of passive cell wall permeation. Taken together, our results identify the cell envelope of the Gram-negative bacterium Polaromonas sp. Nea-C as a relevant barrier for atrazine biodegradation.

Exploring Trends of C and N Isotope Fractionation to Trace Transformation Reactions of Diclofenac in Natural and Engineered Systems

Although diclofenac ranks among the most frequently detected pharmaceuticals in the urban water cycle, its environmental transformation reactions remain imperfectly understood. Biodegradation-induced changes in ¹⁵N/¹⁴N ratios (ε_N = -7.1‰ ± 0.4‰) have indicated that compound-specific isotope analysis (CSIA) may detect diclofenac degradation. This singular observation warrants exploration for further transformation reactions. The present study surveys carbon and nitrogen isotope fractionation in other environmental and engineered transformation reactions of diclofenac. While carbon isotope fractionation was generally small, observed nitrogen isotope fractionation in degradation by MnO₂ (ε_N = -7.3‰ ± 0.3‰), photolysis (ε_N = +1.9‰ ± 0.1‰), and ozonation (ε_N = +1.5‰ ± 0.2‰) revealed distinct trends for different oxidative transformation reactions. The small, secondary isotope effect associated with ozonation suggests an attack of O₃ in a molecular position distant from the N atom. Model reactants for outer-sphere single electron transfer generated large inverse nitrogen isotope fractionation (ε_N = +5.7‰ ± 0.3‰), ruling out this mechanism for biodegradation and transformation by MnO₂. In a river model, isotope fractionation-derived degradation estimates agreed well with concentration mass balances, providing a proof-of-principle validation for assessing micropollutant degradation in river sediment. Our study highlights the prospect of combining CSIA with transformation product analysis for a better assessment of transformation reactions within the environmental life of diclofenac.

First comprehensive screening of lipophilic organic contaminants in surface waters of the megacity Jakarta, Indonesia

Jakarta is an Indonesian coastal megacity with over 10 million inhabitants. The rivers flowing through the city receive enormous amounts of untreated wastewaters and discharge their pollutant loads into Jakarta Bay. We utilized a screening approach to identify those site-specific compounds that represent the major contamination of the cities' water resources, and detected a total number of 71 organic contaminants in Jakarta river water samples. Especially contaminants originating from municipal wastewater discharges were detected in high concentrations, including flame retardants, personal care products and pharmaceutical drugs. A flame retardant, a synthetic fragrance and caffeine were used as marker compounds to trace the riverine transport of municipal wastewaters into Jakarta Bay. These markers are also appropriate to trace municipal wastewater discharges to other tropical coastal ecosystems. This application is in particular useful to evaluate wastewater inputs from land-based sources to habitats which are sensitive to changing water quality, like coral reefs.

Compound-specific isotope analysis (CSIA) of micropollutants in the environment - current developments and future challenges

Over the last decade, the occurrence of micropollutants in the environment has become a worldwide issue of increasing concern. Compound-specific stable-isotope analysis (CSIA) of natural isotopic abundance may greatly enhance the evaluation of sources and transformation processes of micropollutants, such as pesticides, personal care products or pharmaceuticals. We summarize recent advances from laboratory studies, review current limitations and analytical challenges associated with low concentrations and high polarity of micropollutants, and delineate the potential of micropolluant CSIA for field applications. We highlight future challenges and prospects regarding source apportionment, identification of biotic and abiotic transformation reactions on a mechanistic level, as well as integrative evaluation of degradation hot spots on the catchment scale. Such advances may feed into a framework for risk assessment of micropollutants that includes CSIA.

Compound-Specific Stable Isotope Fractionation of Pesticides and Pharmaceuticals in a Mesoscale Aquifer Model

Compound-specific isotope analysis (CSIA) receives increasing interest for its ability to detect natural degradation of pesticides and pharmaceuticals. Despite recent laboratory studies, CSIA investigations of such micropollutants in the environment are still rare. To explore the certainty of information obtainable by CSIA in a near-environmental setting, a pulse of the pesticide bentazone, the pesticide metabolite 2,6-dichlorobenzamide (BAM), and the pharmaceuticals diclofenac and ibuprofen was released into a mesoscale aquifer with quasi-two-dimensional flow. Concentration breakthrough curves (BTC) of BAM and ibuprofen demonstrated neither degradation nor sorption. Bentazone was transformed but did not sorb significantly, whereas diclofenac showed both degradation and sorption. Carbon and nitrogen CSIA could be accomplished in similar concentrations as for "traditional" priority pollutants (low μg/L range), however, at the cost of uncertainties (0.4-0.5‰ (carbon), 1‰ (nitrogen)). Nonetheless, invariant carbon and nitrogen isotope values confirmed that BAM was neither degraded nor sorbed, while significant enrichment of (13)C and in particular (15)N corroborated transformation of diclofenac and bentazone. Retardation of diclofenac was reflected in additional (15)N sorption isotope effects, whereas isotope fractionation of transverse dispersion could not be identified. These results provide a benchmark on the performance of CSIA to monitor the reactivity of micropollutants in aquifers and may guide future efforts to accomplish CSIA at even lower concentrations (ng/L range).

Position-specific isotope modeling of organic micropollutants transformation through different reaction pathways

The degradation of organic micropollutants occurs via different reaction pathways. Compound specific isotope analysis is a valuable tool to identify such degradation pathways in different environmental systems. We propose a mechanism-based modeling approach that provides a quantitative framework to simultaneously evaluate concentration as well as bulk and position-specific multi-element isotope evolution during the transformation of organic micropollutants. The model explicitly simulates position-specific isotopologues for those atoms that experience isotope effects and, thereby, provides a mechanistic description of isotope fractionation occurring at different molecular positions. To demonstrate specific features of the modeling approach, we simulated the degradation of three selected organic micropollutants: dichlorobenzamide (BAM), isoproturon (IPU) and diclofenac (DCF). The model accurately reproduces the multi-element isotope data observed in previous experimental studies. Furthermore, it precisely captures the dual element isotope trends characteristic of different reaction pathways as well as their range of variation consistent with observed bulk isotope fractionation. It was also possible to directly validate the model capability to predict the evolution of position-specific isotope ratios with available experimental data. Therefore, the approach is useful both for a mechanism-based evaluation of experimental results and as a tool to explore transformation pathways in scenarios for which position-specific isotope data are not yet available.

Isotope Fractionation Associated with the Indirect Photolysis of Substituted Anilines in Aqueous Solution

Organic micropollutants containing aniline substructures are susceptible to different light-induced transformation processes in aquatic environments and water treatment operations. Here, we investigated the magnitude and variability of C and N isotope fractionation during the indirect phototransformation of four para-substituted anilines in aerated aqueous solutions. The model photosensitizers, namely 9,10-anthraquinone-1,5-disulfonate and methylene blue, were used as surrogates for dissolved organic matter chromophores generating excited triplet states in sunlit surface waters. The transformation of aniline, 4-CH3-, 4-OCH3-, and 4-Cl-aniline by excited triplet states of the photosensitizers was associated with inverse and normal N isotope fractionation, whereas C isotope fractionation was negligible. The apparent 15N kinetic isotope effects (AKIE) were almost identical for both photosensitizers, increased from 0.9958±0.0013 for 4-OCH3-aniline to 1.0035±0.0006 for 4-Cl-aniline, and correlated well with the electron donating properties of the substituent. N isotope fractionation is pH-dependent in that H+ exchange reactions dominate below and N atom oxidation processes above the pKa value of the substituted aniline's conjugate acid. Correlations of C and N isotope fractionation for indirect phototransformation were different from those determined previously for the direct photolysis of chloroanilines and offer new opportunities to distinguish between abiotic degradation pathways.

Rapid analysis of diclofenac in freshwater and wastewater by a monoclonal antibody-based highly sensitive ELISA

The non-steroidal anti-inflammatory drug (NSAID) diclofenac (DCF) is found worldwide in the aqueous environment. Therefore, it has raised increased public concern on potential long-term impact on human health and wildlife. The importance of DCF has been emphasized by the European Union recently by including this pharmaceutical in the first watch list of priority hazardous substances in order to gather Union-wide monitoring data. Rapid and cheap methods of analysis are therefore required for fresh and wastewater monitoring with high sample load. Here, for the first time, well-characterized monoclonal antibodies (mAbs) against DCF were generated and a highly sensitive ELISA developed. The best antibody (mAb 12G5) is highly affine (KD = 1.5 × 10(-10) M), stable to potential matrix interferences such as pH value (pH range 5.2-9.2), calcium ion concentration (up to 75 mg/L), and humic acid content (up to 20 mg/L). The limit of detection (LOD, S/N = 3) and IC50 of the ELISA calibration curve were 7.8 and 44 ng/L, respectively. The working range was defined between 11 and 180 ng/L. On average, about 10 % cross-reactivity (CR) was found for DCF metabolites 5-OH-DCF, 4'-OH-DCF, and DCF-acyl glucuronide, but other structurally related NSAIDs showed binding <1 % compared to the parent compound. While DCF concentrations at the low ppt range were measured in river and lake water, higher values of 2.9 and 2.1 μg/L were found in wastewater influents and effluents, respectively. These results could be confirmed by solid phase extraction combined with LC-MS.

Carbon Stable Isotope Fractionation of Sulfamethoxazole during Biodegradation by Microbacterium sp. Strain BR1 and upon Direct Photolysis

Carbon isotope fractionation of sulfamethoxazole (SMX) during biodegradation by Microbacterium sp. strain BR1 (ipso-hydroxylation) and upon direct photolysis was investigated. Carbon isotope signatures (δ(13)C) of SMX were measured by LC-IRMS (liquid chromatography coupled to isotope ratio mass spectrometry). A new LC-IRMS method for the SMX metabolite, 3-amino-5-methylisoxazole (3A5MI), was established. Carbon isotope enrichment factors for SMX (ε(C)) were -0.6 ± 0.1‰ for biodegradation and -2.0 ± 0.1‰ and -3.0 ± 0.2‰ for direct photolysis, at pH 7.4 and pH 5, respectively. The corresponding apparent kinetic isotope effects (AKIE) for ipso-hydroxylation were 1.006 ± 0.001; these fall in the same range as AKIE in previously studied hydroxylation reactions. The differences in SMX and 3A5MI fractionation upon biotic and abiotic degradation suggest that compound specific stable isotope analysis (CSIA) is a suitable method to distinguish SMX reaction pathways. In addition, the study revealed that the extent of isotope fractionation during SMX photolytic cleavage is pH-dependent.

Flume experiments to investigate the environmental fate of pharmaceuticals and their transformation products in streams

The hyporheic zone—the transition region beneath and alongside the stream bed—is a central compartment for attenuation of organic micropollutants in rivers. It provides abundant sorption sites and excellent conditions for biotransformation. We used a bench-scale flume to study the fate of 19 parent pharmaceuticals (PPs) and the formation of 11 characteristic transformation products (TPs) under boundary conditions similar to those in hyporheic zones. The persistence of PPs ranged from readily degradable with a dissipation half-life (DT50) as short as 1.8 days (acetaminophen, ibuprofen) to not degradable (chlorthalidone, fluconazole). The temporal and spatial patterns of PP and TP concentrations in pore water were heterogeneous, reflecting the complex hydraulic and biogeochemical conditions in hyporheic zones. Four TPs (carbamazepine-10,11-epoxide, metoprolol acid, 1-naphthol, and saluamine) were exclusively formed in the sediment compartment and released to surface water, highlighting their potential to be used as indicators for characterizing hyporheic transformation of micropollutants in streams. The accumulation of certain TPs over the experimental period illustrates that we might face a peak of secondary contamination by TPs far from the point of release of the original contaminants into a stream. Such TPs should be considered as priority candidates for a higher-tier environmental risk assessment.

Isotope fractionation associated with the direct photolysis of 4-chloroaniline

Compound-specific isotope analysis is a useful approach to track transformations of many organic soil and water pollutants. Applications of CSIA to characterize photochemical processes, however, have hardly been explored. In this work, we systematically studied C and N isotope fractionation associated with the direct photolysis of 4-Cl-aniline used as a model compound for organic micropollutants that are known to degrade via photochemical processes. Laboratory experiments were carried out at an irradiation wavelength of 254 nm over the pH range 2.0 to 9.0 as well as in the presence of Cs(+) as a quencher of excited singlet 4-Cl-aniline at pH 7.0 and 9.0. We observed considerable variation of C and N isotope enrichment factors, ϵC and ϵN, between -1.2 ± 0.2‰ to -2.7 ± 0.2‰ for C and -0.6 ± 0.2‰ to -9.1 ± 1.6‰ for N, respectively, which could not be explained by the speciation of 4-Cl-aniline alone. In the presence of 1 M Cs(+), we found a marked increase of apparent (13)C-kinetic isotope effects ((13)C-AKIE) and decrease of 4-Cl-aniline fluorescence lifetimes. Our data suggest that variations of C and N isotope fractionation originate from heterolytic dechlorination of excited triplet and singlet states of 4-Cl-aniline. Linear correlations of (13)C-AKIE vs (15)N-AKIE were distinctly different for these two reaction pathways and may be explored further for the identification of photolytic aromatic dechlorination reactions.