Elodie Passeport

Compound-specific isotope analysis (CSIA) is widely used to verify in situ biodegradation of organic contaminants. Chloroanilines are persistent groundwater pollutants known to undergo reductive dechlorination under environmental conditions, yet their isotope fractionation remains unclear. This study reports multi-element isotopic fractionation (ε) associated with reductive dechlorination of 2,3-dichloroaniline (2,3-DCA) and 2-chloroaniline (2-CA) using a Dehalobacter-dominated enrichment culture. During 2,3-DCA dechlorination, significant C (εC = −8.1 ± 0.3‰) and Cl (εCl = −7.5 ± 0.1‰) isotope fractionation was observed (ΛC–Cl = 1.12 ± 0.09), while N isotope fractionation was insignificant (Δδ15N < 1‰). Similarly, reductive dechlorination of 2-CA yielded εC = −8.9 ± 0.3‰ and εCl = −18.1 ± 0.4‰ (ΛC–Cl = 0.48 ± 0.03), with negligible changes in δ15N. The large and comparable apparent kinetic isotope effects for C and Cl may suggest that both reactions share similar transition-state characteristics, with C–Cl bond cleavage as the rate-limiting step. In contrast to isotope fractionation reported for aerobic 2,3-DCA biodegradation, the distinct isotope patterns observed here enable differentiation between aerobic and anaerobic pathways. Using groundwater samples from a chloroaniline-impacted aquifer, we show that multi-element CSIA can help distinguish reductive dechlorination from aerobic biodegradation under spatially and temporally variable redox conditions.

Enhancing green stormwater infrastructure for contaminant removal.

2,3-dichloroaniline (2,3-DCA) has widespread use in chemical manufacturing and remains a persistent groundwater contaminant. To better understand the pathway and kinetics of its reductive dechlorination, we conducted a laboratory kinetic experiment using an anaerobic enrichment culture dominated by Dehalobacter. At an initial field-relevant concentration of 40 mg/L, complete and stoichiometric dechlorination of 2,3-DCA to aniline via 2-chloroaniline (2-CA) was achieved. The intermediates, 2-CA and 3-chloroaniline, were transiently formed in a ratio of 8:1. The growth yields of Dehalobacter on 2,3-DCA and 2-CA were 1.2 ± 0.1 × 10⁸ and 1.3 ± 0.1 × 10⁸ 16S rRNA gene copies/μmol chloride released, respectively. The maximum specific growth rate for 2,3-DCA, μ_max, was 0.18 ± 0.03 day^-1 with a half-saturation constant, K_s, at 45 ± 16 mg/L. The first-order decay constant for Dehalobacter when starved of chlorinated electron acceptors was estimated at 0.017 ± 0.001 day^-1. Lactate fermentation, acetogenesis from ethanol, syntrophic propionate oxidation, and hydrogenotrophic methanogenesis were observed during dechlorination. This work provides insights into the organohalide respiration of 2,3-DCA to aniline and advances the understanding of microbial interactions during anaerobic dechlorination. These results offer guidance for developing stable dechlorinating microbial ecosystems and key kinetic parameters for predictive modeling of groundwater 2,3-DCA fate and transport.

2,3-dichloroaniline (2,3-DCA) has widespread use in chemical manufacturing and remains a persistent groundwater contaminant. To better understand the mechanism and kinetics of its anaerobic biotransformation, we conducted a laboratory kinetic experiment using an anaerobic enrichment culture dominated by Dehalobacter . At an initial field-relevant concentration of 40 mg/L, complete and stoichiometric dechlorination of 2,3-DCA to aniline was achieved. The intermediates, 2-chloroaniline (2-CA) and 3-chloroaniline, were transiently formed in a ratio of 8:1. The growth yields of Dehalobacter on 2,3-DCA and 2-CA were 1.2 ± 0.1 × 10 ⁸ and 1.3 ± 0.1 × 10 ⁸ 16S rRNA gene copies/μmol chloride, respectively. The maximum specific growth rate for 2,3-DCA, μ _max , was 0.18 ± 0.03 day ^-1 with a half-saturation constant, K _s , at 45 ± 16 mg /L. The first-order decay constant for Dehalobacter when starved of chlorinated electron acceptors was estimated at 0.017 ± 0.001 day ^-1 . Lactate fermentation, acetogenesis from ethanol, syntrophic propionate oxidation, and hydrogenotrophic methanogenesis were observed during dechlorination. This work provides insights into the biotransformation pathway of 2,3-DCA to aniline and advances the understanding of microbial interactions during anaerobic dechlorination. These results offer guidance for developing stable dechlorinating microbial ecosystems and key kinetic parameters for predictive modeling of groundwater 2,3-DCA fate and transport.

Urban stormwater is both a major source and a mode of transport for microplastics in the environment. Black-box field studies have found that bioretention cells, a type of low impact development (LID), consisting mainly of engineered porous media, are effective systems for capturing microplastics. However, the mechanisms of how microplastics are transported, removed from stormwater, and fragmented within bioretention cells are unclear. Additionally, the impacts of microplastics on the hydrology and microplastic removal performance of bioretention cells remain unclear. This study assesses tools to model microplastic removal using LID and reviews the literature on the mechanisms of microplastic filtration in porous media. None of the evaluated stormwater tools were found to be well-suited to model microplastic removal via bioretention. We identified 74 studies that, at times, misinterpreted “all microplastics” as colloids. We recommend using a combination of two models to evaluate the full spectrum of microplastic sizes. Currently, the best-suited models are HYDRUS and the cake-layer model described in Li and Davis (2008a), which can be adapted for this purpose. More column studies are needed to parametrize these models that use the full range of polymer types and morphologies of urban stormwater microplastics.

Compound-specific isotope analysis (CSIA) is an established tool to track the in situ transformation of organic chemicals at contaminated sites. In this work, we evaluated the potential of multi-element CSIA to assess biodegradation of 2,3-dichloroaniline (2,3-DCA), which is a major industrial feedstock. Using controlled laboratory experiments, we determined, for the first time, negligible carbon (in situ 2,3-DCA biodegradation at a contaminated site where the carbon and nitrogen isotope signatures from field samples suggested similar aerobic processes by native microorganisms. Under the assumption of the applicability of the Rayleigh model in a pilot wetland treating contaminated groundwater, the extent of biodegradation was estimated to be up to 80-90%. This study proposes multi-element CSIA as a novel application to study 2,3-DCA fate in groundwater and surface water and provides insights into biodegradation pathways.

Constructed wetlands can be a suitable remediation technique for the treatment of industrial contaminants via transfer (i.e., non-destructive) and transformation (i.e., destructive) processes. Providing direct evidence of in situ transformation using concentrations and biogeochemical parameters alone is challenging. Compound specific isotope analysis (CSIA) is a widely used tool to assess in situ transformation of contaminants based on changes in their stable isotope signatures. In this work, we evaluated the potential of CSIA to identify and possibly quantify the in situ transformation of six NO2- and NH2-chlorobenzenes in complex aqueous samples from a pilot constructed wetland system. No significant changes in δ13C, i.e., ≤2‰ were observed for any of the target compounds despite the contaminant concentration decreased by more than 99% between the inlet and outlet of the system. Using multi-element CSIA of carbon, hydrogen, and nitrogen and laboratory-derived isotope enrichment factors, we successfully identified and quantified the extent of in situ transformation of 2,3-dichloroaniline (2,3-DCA) in the pilot constructed wetlands. The isotopic trends provide evidence for aerobic biotransformation as a dominant pathway in the surface flow planted wetlands; whereas sorption was identified as the likely process in planted and unplanted upflow gravel bed wetlands during the initial wetland operation periods. Another major contaminant from the NO2-chlorobenzene group, i.e., 2-chloronitrobenzene (2-CNB), showed negligible δ13C, and small δ2H (±20‰) and δ15N (±2‰) isotope fractionation. No laboratory-controlled CSIA studies are yet available for 2-CNB biotransformation to characterize transfer and transformation processes. This study highlights the applicability of CSIA as a quantitative tool for 2,3-DCA in dynamic environmental conditions of wetlands and the need for pathway-specific isotope enrichment factors for the successful CSIA application of other target compounds.