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

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.

Current trends and challenges in the analysis of marine environmental contaminants by isotope ratio mass spectrometry

An increasing number of organic and inorganic pollutants are being detected in the marine environment, posing a severe threat to the ecosystem and human health, even in trace concentrations. Isotope ratio mass spectrometry (IRMS) is one of the critical methods for determining the origin and fate of environmental pollutants and characterising their transformation processes. It has been used for a relatively long time for ecological monitoring of some well-studied industrial hydrocarbons at contaminated sites. However, the method still faces many analytical challenges. This review provides a comprehensive overview of recent technical advances concerning IRMS analysis of various contaminants and discusses typical pitfalls encountered in marine environment analysis. Particular attention is given to the study of sampling techniques and sample preparation for examination, often the keys to successful research given the complexity of marine matrices and the diverse and numerous nature of contaminants. Prospects for developing IRMS to monitor pollution sources and pollutant transformation in the marine environment are outlined.

Reactive transport of micropollutants in laboratory aquifers undergoing transient exposure periods

Groundwater quality is of increasing concern due to the ubiquitous occurrence of micropollutant mixtures. Stream-groundwater interactions near agricultural and urban areas represent an important entry pathway of micropollutants into shallow aquifers. Here, we evaluated the biotransformation of a micropollutant mixture (i.e., caffeine, metformin, atrazine, terbutryn, S-metolachlor and metalaxyl) during lateral stream water flow to adjacent groundwater. We used an integrative approach combining concentrations and transformation products (TPs) of the micropollutants, compound-specific isotope analysis (δ¹³C and δ¹⁵N), sequencing of 16S rRNA gene amplicons and reactive transport modeling. Duplicate laboratory aquifers (160 cm × 80 cm × 7 cm) were fed with stream water and subjected over 140 d to three successive periods of micropollutant exposures as pulse-like (6000 μg L^-1) and constant (600 μg L^-1) injections under steady-state conditions. Atrazine, terbutryn, S-metolachlor and metalaxyl persisted in both aquifers during all periods (<10 % attenuation). Metformin attenuation (up to 14 %) was only observed from 90 d onwards, suggesting enhanced degradation over time. In contrast, caffeine dissipated during all injection periods (>90 %), agreeing with fast degradation rates (t_1/2 < 3 d) in parallel microcosm experiments and detection of TPs (theobromine and xanthine). Significant stable carbon isotope fractionation (Δδ¹³C ≥ 6.6 ‰) was observed for caffeine in both aquifers, whereas no enrichment in ¹⁵N occurred. A concentration dependence of caffeine biotransformation in the aquifers was further suggested by model simulations following Michaelis-Menten kinetics. Changes in bacterial community composition reflected long-term bacterial adaptation to micropollutant exposures. Altogether, the use of an integrative approach can help to understand the interplay of subsurface hydrochemistry, bacterial adaptations and micropollutants biotransformation during stream-groundwater interactions.

Isotope Effects on the Vaporization of Organic Compounds from an Aqueous Solution-Insight from Experiment and Computations

An isotope fractionation analysis of organic groundwater pollutants can assess the remediation at contaminated sites yet needs to consider physical processes as potentially confounding factors. This study explores the predictability of water–air partitioning isotope effects from experiments and computational predictions for benzene and trimethylamine (both H-bond acceptors) as well as chloroform (H-bond donor). A small, but significant, isotope fractionation of different direction and magnitude was measured with ε = −0.12‰ ± 0.07‰ (benzene), ε_C = 0.49‰ ± 0.23‰ (triethylamine), and ε_H = 1.79‰ ± 0.54‰ (chloroform) demonstrating that effects do not correlate with expected hydrogen-bond functionalities. Computations revealed that the overall isotope effect arises from contributions of different nature and extent: a weakening of intramolecular vibrations in the condensed phase plus additional vibrational modes from a complexation with surrounding water molecules. Subtle changes in benzene contrast with a stronger coupling between intra- and intermolecular modes in the chloroform–water system and a very local vibrational response with few atoms involved in a specific mode of triethylamine. An energy decomposition analysis revealed that each system was affected differently by electrostatics and dispersion, where dispersion was dominant for benzene and electrostatics dominated for chloroform and triethylamine. Interestingly, overall stabilization patterns in all studied systems originated from contributions of dispersion rather than other energy terms.

Water table fluctuations affect dichloromethane biodegradation in lab-scale aquifers contaminated with organohalides

Dichloromethane (DCM) is a toxic industrial solvent frequently detected in multi-contaminated aquifers. It can be degraded biotically or abiotically, and under oxic or anoxic conditions. The extent and pathways of DCM degradation in aquifers may thus depend on water table fluctuations and microbial responses to hydrochemical variations. Here, we examined the effect of water table fluctuations on DCM biodegradation in two laboratory aquifers fed with O₂-depleted DCM-spiked groundwater from a well-characterized former industrial site. Hydrochemistry, stable isotopes of DCM (δ¹³C and δ³⁷Cl), and bacterial community composition were examined to determine DCM mass removal and degradation pathways under steady-state (static water table) and transient (fluctuating water table) conditions. DCM mass removal was more pronounced under transient (95%) than under steady-state conditions (42%). C and Cl isotopic fractionation values were larger under steady-state (ε_bulk^C = -23.6 ± 3.2‰, and ε_bulk^Cl= -8.7 ± 1.6‰) than under transient conditions (ε_bulk^C = -11.8 ± 2.0‰, and ε_bulk^Cl = -3.1 ± 0.6‰). Dual C-Cl isotope analysis suggested the prevalence of distinct anaerobic DCM degradation pathways, with Λ^C/Cl values of 1.92 ± 0.30 and 3.58 ± 0.42 under steady-state and transient conditions, respectively. Water table fluctuations caused changes in redox conditions and oxygen levels, resulting in a higher relative abundance of Desulfosporosinus (Peptococcaceae family). Taken together, our results show that water table fluctuations enhanced DCM biodegradation, and correlated with bacterial taxa associated with anaerobic DCM degradation. Our integrative approach allows to evaluate anaerobic DCM degradation under dynamic hydrogeological conditions, and may help improving bioremediation strategies at DCM contaminated sites.

Application of Compound-Specific Isotope Analysis in Environmental Forensic and Strategic Management Avenue for Pesticide Residues

Unintended pesticide pollution in soil, crops, and adjacent environments has caused several issues for both pesticide users and consumers. For users, pesticides utilized should provide higher yield and lower persistence while considering both the environment and agricultural products. Most people are concerned that agricultural products expose humans to pesticides accumulating in vegetation. Thus, many countries have guidelines for assessing and managing pesticide pollution, for farming in diverse environments, as all life forms in soil are untargeted to these pesticides. The stable isotope approach has been a useful technique to find the source of organic matter in studies relating to aquatic ecology and environmental sciences since the 1980s. In this study, we discuss commonly used analytical methods using liquid and gas chromatography coupled with isotopic ratio mass spectrometry, as well as the advanced compound-specific isotope analysis (CSIA). CSIA applications are discussed for tracing organic pollutants and understanding chemical reactions (mechanisms) in natural environments. It shows great applicability for the issues on unintended pesticide pollution in several environments with the progress history of isotope application in agricultural and environmental studies. We also suggest future study directions based on the forensic applications of stable isotope analysis to trace pesticides in the environment and crops.

Photodegradation of pesticides using compound-specific isotope analysis (CSIA): a review

Pesticides are commonly applied in agriculture to protect crops from pests, weeds, and harmful pathogens. However, chronic, low-level exposure to pesticides can be toxic to humans. Photochemical degradation of pesticides in water, soil, and other environmental media can alter their environmental fate and toxicity. Compound-specific isotope analysis (CSIA) is an advanced diagnostic tool to quantify the degradation of organic pollutants and provide insight into reaction mechanisms without the need to identify transformation products. CSIA allows for the direct quantification of organic degradation, including pesticides. This review summarizes the recent developments observed in photodegradation studies on different categories of pesticides using CSIA technology. Only seven pesticides have been studied using photodegradation, and these studies have mostly occurred in the last five years. Knowledge gaps in the current literature, as well as potential approaches for CSIA technology for pesticide monitoring, are discussed in this review. Furthermore, the CSIA analytical method is challenged by chemical element types, the accuracy of instrument analysis, reaction conditions, and the stability of degradation products. Finally, future research applications and the operability of this method are also discussed.

Magnitude of Diffusion- and Transverse Dispersion-Induced Isotope Fractionation of Organic Compounds in Aqueous Systems

Determining whether aqueous diffusion and dispersion lead to significant isotope fractionation is important for interpreting the isotope ratios of organic contaminants in groundwater. We performed diffusion experiments with modified Stokes diaphragm cells and transverse-dispersion experiments in quasi-two-dimensional flow-through sediment tank systems to explore isotope fractionation for benzene, toluene, ethylbenzene, 2,6-dichlorobenzamide, and metolachlor at natural isotopic abundance. We observed very small to negligible diffusion- and transverse-dispersion-induced isotope enrichment factors (ε < -0.4 ‰), with changes in carbon and nitrogen isotope values within ±0.5‰ and ±1‰, respectively. Isotope effects of diffusion did not show a clear correlation with isotopologue mass with calculated power-law exponents β close to zero (0.007 < β < 0.1). In comparison to ions, noble gases, and labeled compounds, three aspects stand out. (i) If a mass dependence is derived from collision theory, then isotopologue masses of polyatomic molecules would be affected by isotopes of multiple elements resulting in very small expected effects. (ii) However, collisions do not necessarily lead to translational movement but can excite molecular vibrations or rotations minimizing the mass dependence. (iii) Solute-solvent interactions like H-bonds can further minimize the effect of collisions. Modeling scenarios showed that an inadequate model choice, or erroneous choice of β, can greatly overestimate the isotope fractionation by diffusion and, consequently, transverse dispersion. In contrast, available data for chlorinated solvent and gasoline contaminants at natural isotopic abundance suggest that in field scenarios, a potential additional uncertainty from aqueous diffusion or dispersion would add to current instrumental uncertainties on carbon or nitrogen isotope values (±1‰) with an additional ±1‰ at most.

Quantifying biodegradation rate constants of o-xylene by combining compound-specific isotope analysis and groundwater dating

The objective of this study is to estimate hydraulic conductivities and biodegradation rate constants in a coal-tar contaminated aquifer by compound-specific isotope analysis (CSIA) and tracer-based (³H-³He) groundwater dating (TGD). In two observation wells downgradient from the contaminant source in situ biodegradation of o-xylene, toluene and naphthalene under sulfate-reducing redox conditions could be demonstrated using CSIA. Median biodegradation rate constants for o-xylene ranging between 0.08 and 0.22 a^-1 were estimated. By using tracer-based groundwater dating in these two wells, hydraulic conductivities could be also estimated, which are in a similar range as k-values derived from sieve analysis, a pumping test and a calibrated groundwater flow model. These results clearly demonstrate the applicability of tracer-based groundwater dating for the determination of in situ hydraulic conductivities in aquifers without pumping contaminated groundwater. Finally, a sensitivity analysis is performed using a Monte Carlo simulation. These results indicate high sensitivities of the assumed effective porosity for the estimation of the hydraulic conductivity and the selected isotope enrichment factor for the biodegradation rate constant, respectively. Conversely, the outcome also evidently demonstrates the main limitations of the novel combined isotope approach for a successful implementation of monitored natural attenuation (MNA) at such field sites.

Isotope fractionation in atrazine degradation reveals rate-limiting, energy-dependent transport across the cell membrane of gram-negative rhizobium sp. CX-Z

In the biological mass transfer of organic contaminants like atrazine, the cellular membrane limits bioavailability of pesticides. We aimed to illustrate the roles of cellular membrane physiology and substrate uptake (e.g., passive diffusion and energy-dependent transport) on the limitations of bioavailability in atrazine biodegradation by Gram-negative strain Rhizobium sp. CX-Z. Compound-specific stable isotope analysis revealed energy-dependent transport across cellular membrane led to bioavailability limitations in atrazine biotransformation. Carbon isotope fractionation (ε_(C) = -1.8 ± 0.3‰) was observed and significantly smaller in atrazine biodegradation by Rhizobium sp. CX-Z than that expected in acid hydrolysis (ε_(C) = -4.8 ± 0.4‰) and hydrolysis by the pure enzyme TrzN (ε_(C) = -5.0 ± 0.2‰). However, isotope fractionation was restored in membrane-free cells of Rhizobium sp. CX-Z (ε_(C) = -5.4 ± 0.2‰) where no cellular membrane limits substrate uptake. When respiratory chain was inhibited by rotenone, the pseudo-first order kinetic rate constants (0.08 ± 0.03 h^-1, 0.09 ± 0.03 h^-1) was observed to be statistically less than in the control group (0.23 ± 0.02 h^-1, 0.33 ± 0.02 h^-1), demonstrating that energy-dependent transport dominated atrazine transfer across the cellular membrane. Therefore, our results revealed energy-dependent transport across cellular membrane existing in Gram-negative strain Rhizobium sp. CX-Z determines bioavailability of atrazine in biotransformation process even at high concentration.

Solid-phase extraction method for stable isotope analysis of pesticides from large volume environmental water samples

Compound-specific isotope analysis (CSIA) is a valuable tool for assessing the fate of organic pollutants in the environment. However, the requirement of sufficient analyte mass for precise isotope ratio mass spectrometry combined with prevailing low environmental concentrations currently limits comprehensive applications to many micropollutants. Here, we evaluate the upscaling of solid-phase extraction (SPE) approaches for routine CSIA of herbicides. To cover a wide range of polarity, a SPE method with two sorbents (a hydrophobic hypercrosslinked sorbent and a hydrophilic sorbent) was developed. Extraction conditions, including the nature and volume of the elution solvent, the amount of sorbent and the solution pH, were optimized. Extractions of up to 10 L of agricultural drainage water (corresponding to up to 200 000-fold pre-concentration) were successfully performed for precise and sensitive carbon and nitrogen CSIA of the target herbicides atrazine, acetochlor, metolachlor and chloridazon, and metabolites desethylatrazine, desphenylchloridazon and 2,6-dichlorobenzamide in the sub-μg L^-1-range. ¹³C/¹²C and ¹⁵N/¹⁴N ratios were measured by gas chromatography-isotope ratio mass spectrometry (GC/IRMS), except for desphenylchloridazon, for which liquid chromatography (LC/IRMS) and derivatization-GC/IRMS were used, respectively. The method validated in this study is an important step towards analyzing isotope ratios of pesticide mixtures in aquatic systems and holds great potential for multi-element CSIA applications to trace pesticide degradation in complex environments.

Enantiomer-specific stable carbon isotope analysis (ESIA) to evaluate degradation of the chiral fungicide Metalaxyl in soils

Chiral pesticides are often degraded enantioselectively in soils, leading to disparity among enantiomers that may display different toxicity levels. Monitoring pesticide degradation extents and processes remains out of reach in the field using conventional bulk and enantiomer concentration analyses. Enantioselective stable carbon isotope analysis (ESIA) combines compound specific isotope analysis (CSIA) and enantioselective analysis, and bears potential to distinguish enantiomer degradation from non-destructive dissipation. We developed ESIA of the fungicide Metalaxyl, providing the ¹³C/¹²C ratios for S-Metalaxyl and R-Metalaxyl separately, and applied it to follow degradation in soil incubation experiments. Significant enantioselective degradation (k_S-MTY = 0.007-0.011 day^-1 < k_R-MTY = 0.03-0.07 day^-1) was associated with isotope fractionation (Δδ¹³C_S-MTY ranging from 2 to 6‰). While R-Metalaxyl degradation was rapid (T_1/2≈10 days), concomitant enrichment in heavy isotopes of the persistent S-Metalaxyl occurred after 200 days of incubation (ε_S-Metalaxyl ranging from -1.3 to -2.7‰). In contrast, initial racemic ratios and isotopic compositions were conserved in abiotic experiments, which indicates the predominance of microbial degradation in soils. Degradation products analysis and apparent kinetic isotope effect (AKIE) suggested hydroxylation as a major enantioselective degradation pathway in our soils. Altogether, our study underscores the potential of ESIA to evaluate the degradation extent and mechanisms of chiral micropollutants in soils.

Transformation of chlorpyrifos in integrated recirculating constructed wetlands (IRCWs) as revealed by compound-specific stable isotope (CSIA) and microbial community structure analysis

Carbon isotope analysis and 454 pyrosequencing methods were used to investigate in situ biodegradation of chlorpyrifos during its transport through three model integrated recirculating constructed wetlands (IRCWs). Results show that plant and Fe-impregnated biochar promoted degradation of chlorpyrifos and its metabolite 3,5,6-trichloro-2-pyridinol (TCP). Carbon isotope ratios in the IRCWs shifted to -31.24±0.58‰ (IRCW1, plant free), -26.82±0.60‰ (IRCW2, with plant) and -24.76±0.94‰ (IRCW3, with plant and Fe-biochar). The enrichment factors (Ɛ_bulk,c) were determined as -0.69±0.06‰ (IRCW1), -0.91±0.07‰ (IRCW2) and -1.03±0.09‰ (IRCW3). Microbial community analysis showed that IRCW3 was dominated by members of Bacillus, which can utilize and degrade chlorpyrifos. These results reveal that plant and Fe-biochar can induce carbon isotope fractionation and have a positive impact on the chlorpyrifos degradation efficiency by influencing the development of beneficial microbial communities.

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.