Isotope Enrichment Factors Research Articles

Vitamin B12 as a source of variability in isotope effects for chloroform biotransformation by Dehalobacter.

Carbon and chlorine isotope effects for biotransformation of chloroform by different microbes show significant variability. Reductive dehalogenases (RDase) enzymes contain different cobamides, affecting substrate preferences, growth yields, and dechlorination rates and extent. We investigate the role of cobamide type on carbon and chlorine isotopic signals observed during reductive dechlorination of chloroform by the RDase CfrA. Microcosm experiments with two subcultures of a Dehalobacter-containing culture expressing CfrA-one with exogenous cobamide (Vitamin B12, B12+) and one without (to drive native cobamide production)-resulted in a markedly smaller carbon isotope enrichment factor (εC, bulk) for B12- (-22.1 ± 1.9‰) compared to B12+ (-26.8 ± 3.2‰). Both cultures exhibited significant chlorine isotope fractionation, and although a lower εCl, bulk was observed for B12- (-6.17 ± 0.72‰) compared to B12+ (-6.86 ± 0.77‰) cultures, these values are not statistically different. Importantly, dual-isotope plots produced identical slopes of ΛCl/C (ΛCl/C, B12+ = 3.41 ± 0.15, ΛCl/C, B12- = 3.39 ± 0.15), suggesting the same reaction mechanism is involved in both experiments, independent of the lower cobamide bases. A nonisotopically fractionating masking effect may explain the smaller fractionations observed for the B12- containing culture.

Unraveling the mechanism of assimilatory nitrate reduction and methane oxidation by Methylobacter sp. YHQ through dual N-O isotope analysis and kinetic modeling

Assimilatory nitrate reduction and methane (CH4) oxidation by bacteria play important roles in carbon (C) and nitrogen (N) biogeochemical cycles. Here, an investigation of enzymatic assimilatory nitrate reduction and CH4 oxidation by Methylobacter sp. YHQ from the wetlands is presented, specifically concentrating on N and oxygen (O) isotope fractionation with various initial nitrate and oxygen concentrations. The N enrichment factors (15εassimilation) increased from 4.2 ± 0.7‰ to 6.9 ±1.3‰ and the O isotope enrichment factors (18εassimilation) increased from 2.7 ± 0.9‰ to 4.7 ± 0.8‰ during nitrate assimilation when initial nitrate concentrations increased from 0.9 mM to 2 mM. Similar 18ε and 15ε values were observed at different oxygen concentrations. The values of 18ε and 15ε provided vital parameters for the assessment of assimilatory nitrate reduction via the Rayleigh equation approach. The ratios of O and N isotope enrichment factors (18ε:15ε)assimilation ranged from 0.64 ± 0.15 to 0.74 ± 0.18 during nitrate assimilation by Methylobacter sp. YHQ with Nas, which were different from (18ε:15ε)assimilation for assimilatory eukaryotic nitrate reductase (eukNR) from literature data. Thus, N and O isotope fractionation could be useful tools to distinguish eukNR from Nas during nitrate assimilation. Additionally, the rates of CH4 oxidation and nitrate reduction were evaluated with a reaction-based kinetic model, and it quantitatively described the enzymatic reactions of nitrate assimilation. Combining dual N-O isotope analysis with kinetic modeling provides new insights into the microbially driven C-N interactions.Graphical

Antimony Isotope Fractionation during Kinetic Sb(III) Oxidation by Antimony-Oxidizing Bacteria Pseudomonas sp. J1.

Antimony (Sb) isotopic fractionation is frequently used as a proxy for biogeochemical processes in nature. However, to date, little is known about Sb isotope fractionation in biologically driven reactions. In this study, Pseudomonas sp. J1 was selected for Sb isotope fractionation experiments with varying initial Sb concentration gradients (50-200 μM) at pH 7.2 and 30 °C. Compared to the initial Sb(III) reservoir (δ123Sb = 0.03 ± 0.01 ∼ 0.06 ± 0.01‰), lighter isotopes were preferentially oxidized to Sb(V). Relatively constant isotope enrichment factors (ε) of -0.62 ± 0.06 and -0.58 ± 0.02‰ were observed for the initial Sb concentrations ranging between 50 and 200 μM during the first 22 days. Therefore, the Sb concentration has a limited influence on Sb isotope fractionation during Sb(III) oxidation that can be described by a kinetically dominated Rayleigh fractionation model. Due to the decrease in the Sb-oxidation rate by Pseudomonas sp. J1, observed for the initial Sb concentration of 200 μM, Sb isotope fractionation shifted toward isotopic equilibrium after 22 days, with slightly heavy Sb(V) after 68 days. These findings provide the prospect of using Sb isotopes as an environmental tracer in the Sb biogeochemical cycle.

Impact of organic carbon on sulfide-driven autotrophic denitrification: Insights from isotope fractionation and functional genes

Additional organics are generally supplemented in the sulfide-driven autotrophic denitrification system to accelerate the denitrification rate and reduce sulfate production. In this study, different concentrations of sodium acetate (NaAc) were added to the sulfide-driven autotrophic denitrification reactor, and the S0 accumulation increased from 7.8% to 100% over a 120-day operation period. Batch experiments revealed a threefold increase in total nitrogen (TN) removal rate at an Ac−-C/N ratio of 2.8 compared to a ratio of 0.5. Addition of organic carbon accelerated denitrification rate and nitrite consumption, which shortened the emission time of N2O, but increased the N2O production rate. The lowest N2O emissions were achieved at the Ac−-C/N ratio of 1.3. Stable isotope fractionation is a powerful tool for evaluating different reaction pathways, with the 18ε/15ε values in nitrate reduction ranging from 0.5 to 1.0. This study further confirmed that isotope fractionation can reveal denitrifying nutrient types, with the 18ε (isotopic enrichment factor of oxygen)/15ε (isotopic enrichment factor of nitrogen) value approaching 1.0 for autotrophic denitrification and 0.5 for heterotrophic denitrification. Additionally, the 18ε/15ε values can indicate changes in nitrate reductase. There is a positive correlation between the 18ε/15ε values and the abundance of the functional gene napA, and a negative correlation with the abundance of the gene narG. Moreover, 18ε and 15ε were associated with changes in kinetic parameters during nitrate reduction. In summary, the combination of functional gene analysis and isotope fractionation effectively revealed the complexities of mixotrophic denitrification systems, providing insights for optimizing denitrification processes.

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.

Exploring the potential of stable isotope methods for identifying the origin of CO2 in the carbonation process of cementitious materials within the carbon capture and storage environment

This study investigates the potential of stable isotope (δ13C) methods for exploring the carbonation process of cementitious materials, including well cement paste, portlandite (CH), and calcium silicate hydrate (C–S–H). Through comprehensive chemical and isotopic characterization, along with extensive data analysis, several key conclusions emerge. Firstly, the pH of the solution, as well as the solubility and pH characteristics of mineral phases (CH and C–S–H), are identified as influential factors impacting the kinetics, thermodynamics, and distribution of carbon isotopes (12C and 13C) during the reaction process. Secondly, the study uncovered that higher final pH values correlated with a DIC pool enriched with 13C and a higher εDIC-CO2 isotopic enrichment factor (10.7–12.6 ‰). Furthermore, higher levels of DIC content, solution electrical conductivity, and lower pH lead to higher δ13C–CaCO3 and greater εCaCO3-DIC isotope fractionation (−12.9 to −7.5 ‰). These conditions favor the formation of carbonate minerals with higher isotopic signature (δ13C–CaCO3), thus better preserving the δ13C-DIC isotope ratio. It was identified that the carbonate mineral (δ13C–CaCO3) formed through the carbonation of cementitious materials (class G cement, CH, and C–S–H) appears to preserve the isotopic signature of the δ13C–CO2 source. In conclusion, stable isotope methods could be a valuable tool for assessing and monitoring the cement carbonation process in Carbon Capture and Storage (CCS) wells, particularly when it is crucial to distinguish between injected and endogenous CO2.

Fractionation of stable carbon isotopes during formate consumption in anoxic rice paddy soils and lake sediments

Abstract. Formate is energetically equivalent to hydrogen and thus is an important intermediate during the breakdown of organic matter in anoxic rice paddy soils and lake sediments. Formate is a common substrate for methanogenesis, homoacetogenesis and sulfate reduction. However, how much these processes contribute to formate degradation and fractionate carbon stable isotopes is largely unknown. Therefore, we measured the conversion of formate to acetate, CH4 and CO2 and the δ13C of these compounds in samples of paddy soils from Vercelli, Italy, and the International Rice Research Institute (IRRI) in the Philippines and of sediments from the NE and SW basins of Lake Fuchskuhle, Germany. The samples were suspended in a phosphate buffer (pH 7.0) in both the absence and presence of sulfate (gypsum) and of methyl fluoride (CH3F), an inhibitor of aceticlastic methanogenesis. In the paddy soils, formate was mainly converted to acetate under both methanogenic and sulfidogenic conditions. Methane was only a minor product and was mainly formed from the acetate. In the lake sediments, the product spectrum was similar but only under methanogenic conditions. In the presence of sulfate, however, acetate and CH4 were only minor products. The isotopic enrichment factors (εform) of formate consumption, determined by Mariotti plots, were in the low range of −8 ‰ to −2.5 ‰ when sulfate was absent, and formate was mainly converted to acetate and CH4. However, no enrichment factor was detectable when formate was degraded with sulfate to mainly CO2. The δ13C of acetate was by about 25 ‰–50 ‰ more negative than that of formate, indicating acetate production by chemolithotrophic homoacetogenesis. Hence, formate seems to be an excellent substrate for homoacetogenesis in anoxic soils and sediments, so that this process is competing well with methanogenesis and sulfate reduction.

Disclosing phototransformation mechanisms of decabromodiphenyl ether (BDE-209) in different media by simulated sunlight: Implication by compound-specific stable isotope analysis.

As one of the typical brominated flame retardants, decabromodiphenyl ether (BDE-209) has been widely detected in environment. However, scarce information was available on BDE-209 phototransformation mechanisms in various media. In this study, compound-specific stable isotope analysis was first applied to investigate BDE-209 phototransformation in n-hexane, MeOH:H2O (v:v, 8:2), and simulated seawater by simulated sunlight. BDE-209 transformation followed pseudo-first-order kinetic, with degradation rate in the following of n-hexane (2.66 × 10-3min-1) > simulated seawater (1.83 × 10-3min-1) > MeOH:H2O (1.41 × 10-3min-1). Pronounced carbon isotope fractionation was first observed for BDE-209 phototransformation, with carbon isotope enrichment factors (εC) of -1.01 ± 0.14‰, -1.77 ± 0.26‰, -2.94 ± 0.38‰ in n-hexane, MeOH:H2O and simulated seawater, respectively. Combination analysis of products and stable carbon isotope, debromination with cleavage of C-Br bonds as rate-limiting step was the main mechanism for BDE-209 phototransformation in n-hexane, debromination and hydroxylation with cleavage of C-Br bonds as rate-limiting steps in MeOH:H2O, and debromination, hydroxylation and chlorination in simulated seawater. This present study confirmed that stable carbon isotope analysis was a robust method to discovery the underlying phototransformation mechanisms of BDE-209 in various solutions.

Identification of nitrate accumulation mechanism of surface water in a mining-rural-urban agglomeration area based on multiple isotopic evidence

The accumulation of nitrate (NO3−) in surface waters resulting from mining activities and rapid urbanization has raised widespread concerns. Therefore, it is crucial to develop a nitrate transformation information system to elucidate the nitrogen cycle and ensure sustainable water quality management. In this study, we focused on the main river and subsidence area of the Huaibei mining region to monitor the temporal and spatial variations in the NO3− content. Multiple isotopes (δD, δ18O-H2O, δ15N-NO3−, δ18O-NO3−, and δ15N-NH4+) along with water chemistry indicators were employed to identify the key mechanisms responsible for nitrate accumulation (e.g., nitrification and denitrification). The NO3− concentrations in surface water ranged from 0.28 to 7.50 mg/L, with NO3− being the predominant form of nitrogen pollution. Moreover, the average NO3− levels were higher during the dry season than during the wet season. Nitrification was identified as the primary process driving NO3− accumulation in rivers and subsidence areas, which was further supported by the linear relationship between δ15N-NO3− and δ15N-NH4+. The redox conditions and the relationship between δ15N-NO3− and δ18O-NO3−, and lower isotope enrichment factor of denitrification indicated that denitrification was weakened. Phytoplankton preferentially utilized available NH4+ sources while inhibiting NO3− assimilation because of their abundance. These findings provide direct evidence regarding the mechanism underlying nitrate accumulation in mining areas, while aiding in formulating improved measures for effectively managing water environments to prevent further deterioration.

Determination of Stable Hydrogen Isotopic Composition and Isotope Enrichment Factor at Low Hydrogen Concentration.

Determination of stable hydrogen isotopic compositions (δ2H) is currently challenged to achieve a high detection limit for reaching the linear range where δ2H values are independent of concentration. Therefore, it is difficult to assess precise δ2H values for calculating the hydrogen isotope enrichment factor (εH) and for field application where the concentrations of contaminants are relatively low. In this study, a data treatment approach was developed to obtain accurate δ2H values below the linear range. The core concept was to use a logarithmic function to fit the δ2H values below the linear range and then adjust the δ2H values below the linear range into the linear range by using the fitted logarithmic equation. Moreover, the adjusted δ2H values were calibrated by using laboratory reference materials, e.g., n-alkanes. Tris(2-chloroethyl) phosphate (TCEP) and hexachlorocyclohexane (HCH) isomers were selected as examples of complex heteroatom-bearing compounds to develop the data treatment approach. This data treatment approach was then tested using δ2H values from a TCEP transformation experiment with OH radicals. Comparable δ2H values and εH between the low-concentration experiment and the reference experiment were obtained using the developed approach. Therefore, the developed data treatment approach enables a possibility of determining the hydrogen isotopic compositions of organic components in low concentrations. It is especially valuable for determining organic contaminants in environmental samples, which are usually present in low concentrations.

Mercury Isotope Fractionation during Dark Abiotic Reduction of Hg(II) by Dissolved, Surface-Bound, and Structural Fe(II).

Stable mercury (Hg) isotope ratios are an emerging tracer for biogeochemical transformations in environmental systems, but their application requires knowledge of isotopic enrichment factors for individual processes. We investigated Hg isotope fractionation during dark, abiotic reduction of Hg(II) by dissolved iron(Fe)(II), magnetite, and Fe(II) sorbed to boehmite or goethite by analyzing both the reactants and products of laboratory experiments. For homogeneous reduction of Hg(II) by dissolved Fe(II) in continuously purged reactors, the results followed a Rayleigh distillation model with enrichment factors of -2.20 ± 0.16‰ (ε202Hg) and 0.21 ± 0.02‰ (E199Hg). In closed system experiments, allowing reequilibration, the initial kinetic fractionation was overprinted by isotope exchange and followed a linear equilibrium model with -2.44 ± 0.17‰ (ε202Hg) and 0.34 ± 0.02‰ (E199Hg). Heterogeneous Hg(II) reduction by magnetite caused a smaller isotopic fractionation (-1.38 ± 0.07 and 0.13 ± 0.01‰), whereas the extent of isotopic fractionation of the sorbed Fe(II) experiments was similar to the kinetic homogeneous case. Small mass-independent fractionation of even-mass Hg isotopes with 0.02 ± 0.003‰ (E200Hg) and ≈ -0.02 ± 0.01‰ (E204Hg) was consistent with theoretical predictions for the nuclear volume effect. This study contributes significantly to the database of Hg isotope enrichment factors for specific processes. Our findings show that Hg(II) reduction by dissolved Fe(II) in open systems results in a kinetic MDF with a larger ε compared to other abiotic reduction pathways, and combining MDF with the observed MIF allows the distinction from photochemical or microbial Hg(II) reduction pathways.

Carbon isotope fractionation of di-(2-ethylhexyl)-phthalate during photosensitized degradation by •OH and [formula omitted] for characterization of reaction mechanisms

Compound specific isotope analysis (CSIA) has been used to explore the degradation pathways of di-(2-ethylhexyl)-phthalate (DEHP) in photosensitization experiments with H2O2, TiO2, Fe(III) and PMS. The reaction kinetics could be described by first order rate constants [k (h−1)]. The highest degradation rates of DEHP were observed in UV/TiO2 catalyzed reactions.Carbon isotope fractionation factors (εC) and metabolite patterns were examined to analyze the radical reaction. Similar carbon isotope fractionation factors have been found in experiments dominated by OH radical reactions (UV/H2O2, −1.2 ± 0.3‰; UV/TiO2, −1.2 ± 0.5‰). An inverse carbon isotope fractionation has been found for UV/Fe(III) (0.8 ± 0.2‰) and SO4− (0.6 ± 0.2‰ at pH 3) catalyzed reactions indicating that these reactions preferentially taking place at the H atom bond at tert-butyl carbon of the side chain. This hypothesis is further supported by an experiment with isobutyl acetate (εC = 1.8 ± 0.2‰ for UV/Fe(III) at pH 7; εC = -1.1 ± 0.2‰ for UV/PMS at pH 6 and εC = 0.9 ± 0.3‰ for UV/PMS at pH 3).Quenching experiments in the UV/PMS catalyzed reaction indicated a coexistence of SO4− and OH at pH 7. SO4− was the predominant radical at pH 3. OH was found to be the predominant radical in UV/H2O2 and UV/TiO2 reactions. CSIA indicated that the addition of OH to the aromatic ring of DEHP by the RAF pathway is the main reaction mechanism in UV/H2O2 and UV/TiO2 experiments. The SO4− attacks preferentially the C-H bond at the tert-butyl carbon position of the side chain of DEHP which causes an inverse carbon isotope effect. Thus, carbon isotope enrichment factors can be used to distinguish different radical reaction mechanisms with DEHP as a substrate.

Novel Insight into Microbially Mediated Nitrate-Reducing Fe(II) Oxidation by Acidovorax sp. Strain BoFeN1 Using Dual N-O Isotope Fractionation.

Microbially mediated nitrate reduction coupled with Fe(II) oxidation (NRFO) plays an important role in the Fe/N interactions in pH-neutral anoxic environments. However, the relative contributions of the chemical and microbial processes to NRFO are still unclear. In this study, N-O isotope fractionation during NRFO was investigated. The ratios of O and N isotope enrichment factors (18ε:15ε)-NO3- indicated that the main nitrate reductase functioning in Acidovorax sp. strain BoFeN1 was membrane-bound dissimilatory nitrate reductase (Nar). N-O isotope fractionation during chemodenitrification [Fe(II) + NO2-], microbial nitrite reduction (cells + NO2-), and the coupled process [cells + NO2- + Fe(II)] was explored. The ratios of (18ε:15ε)-NO2- were 0.58 ± 0.05 during chemodenitrification and -0.41 ± 0.11 during microbial nitrite reduction, indicating that N-O isotopes can be used to distinguish chemical from biological reactions. The (18ε:15ε)-NO2- of 0.70 ± 0.05 during the coupled process was close to that obtained for chemodenitrification, indicating that chemodenitrification played a more important role than biological reactions during the coupled process. The results of kinetic modeling showed that the relative contribution of chemodenitrification was 99.3% during the coupled process, which was consistent with that of isotope fractionation. This study provides a better understanding of chemical and biological mechanisms of NRFO using N-O isotopes and kinetic modeling.

Denitrification fractionates N and O isotopes of nitrate following a ratio independent of carbon sources in freshwaters.

The stable isotope technique has been used in tracking nitrogen cycling processes, but the isotopic characteristics are influenced by environmental conditions. To better understand the variability of nitrate isotopes in nature, we investigated the influence of organic carbon sources on isotope fractionation characteristics during microbial denitrification. Denitrifying cultures were inoculated with freshwater samples and enriched with five forms of organic compounds, that is, acetate, citrate, glucose, cellobiose, and leucine. Though the isotope enrichment factors of nitrogen and oxygen (15 ε and 18 ε) changed with carbon sources, 18 ε/15 ε always followed a proportionality near 1. Genome-centred metagenomics revealed the enrichment of a few populations, such as Pseudomonas, Enterobacter, and Atlantibacter, most of which contained both NapA- and NarG-type nitrate reductases. Metatranscriptome showed that both NapA and NarG were expressed but to different extents in the enrichments. Furthermore, isotopic data collected from a deep reservoir was analysed. The results showed δ18 O- and δ15 N-nitrate did not correlate in the surface water where nitrification was active, but 18 ε/15 ε followed a proportionality of 1.05 ± 011 in deeper waters (≥ 12 m) where denitrification controlled the nitrate isotope. The independence of 18 ε/15 ε from carbon sources provides an opportunity to determine heterotrophic denitrification and helps the interpretation of nitrate isotopes in freshwaters.

Degradation pathways of atrazine by electrochemical oxidation at different current densities: Identifications from compound-specific isotope analysis and DFT calculation

Current density was the key factor that impacted pollutant degradation by electrochemical oxidation, and reaction contributions at various current densities were non-negligible for the cost-effective treatments of organic pollutants. This research introduced compound specific isotope analysis (CSIA) into atrazine (ATZ) degradation by boron doped diamond (BDD) with current density of 2.5–20 mA/cm2, in order to provide “in-situ” and “fingerprint” analysis of reaction contributions with changed current densities. As results, the increased current density displayed a positive impact on ATZ removal. The ɅC/H values (correlations of Δδ13C and Δδ2H) were 24.58, 9.18 and 8.74 when current densities were 20, 4, and 2.5 mA/cm2, with ·OH contribution of 93.5%, 77.2% and 80.35%, respectively. While DET process favored lower current density with contribution rates up to ∼20%. What's more interesting, though the carbon and hydrogen isotope enrichment factors (εC and εH) were fluctuate, the ɅC/H linearly increased accompanied with applied current densities. Therefore, increasing current density was effective due to the larger ·OH contribution even though side reactions may occur. DFT calculations proved the increase of C–Cl bond length and the delocalization of Cl atom, confirming dechlorination reaction mainly occurred in the direct electron transfer process. While ·OH radical mainly attack the C–N bond on the side chain, which was more benefit to the fast decomposition of ATZ molecule and intermediates. It was forceful to discuss pollutant degradation mechanism by combining CSIA and DFT calculations. Target bond cleavage (i.e., dehalogenation reaction) can be conducted by changing reaction conditions like current density due to the significantly different isotope fractionation and bond cleavage.

Isotopic evidence (δ13C, δ37Cl, δ2H) for distinct transformation mechanisms of chloroform: Catalyzed H2-water system vs. zero-valent iron (ZVI)

Catalytic hydrodechlorination is an efficient technology for degrading organochlorinated compounds, such as chloroform (CF), into harmless products. Compound-specific stable isotope analysis (CSIA) of multiple elements is widely used for the investigation of degradation mechanisms. Yet, its application in the study of catalytic hydrodechlorination is still limited. We have applied CSIA to derive chlorine, carbon, and hydrogen isotope enrichment factors (ε) during the degradation of CF over Pd/Al2O3 and over Rh/Al2O3. In addition, the enrichment factors for the same isotopes were derived for the reaction of CF with zero-valent iron (ZVI) for comparison. For the reactions of CF over Pd/Al2O3 and Rh/Al2O3, εC values (−21.9 ± 0.25 ‰ and −23.4 ± 2.3 ‰) and εCl values (−12.1 ± 1.7 ‰ and −10.3 ± 0.6 ‰) were determined, respectively. The corresponding εC and εCl values, for the dechlorination of CF with ZVI were −22.2 ± 2.8 ‰ and −4.7 ± 0.45 ‰, respectively. The apparent kinetic isotope effects (AKIE) of Cl suggest that the transformation mechanism in the catalyzed hydrogen-water system is a non-concerted reaction, unlike the known reductive dechlorination of CF with ZVI. Moreover, dual-isotope slopes (ΛC/Cl) for both catalyzed reactions (ΛPd = 1.8 ± 0.13 and ΛRh = 2.1 ± 0.14) are markedly different than for the ZVI (ΛZVI = 5.8 ± 0.41), reflecting that the reactions proceed in different mechanisms. For hydrogen isotopes, while there was no clear trend for the catalyzed reactions, an inverse secondary hydrogen isotope effect was observed for the reaction of CF with ZVI.

Insight of microbial degradation of n-hexadecane and n-heneicosane in soil during natural attenuation and bioaugmentation by Compound-specific Stable Isotope Analysis (CSIA)

Microbial degradation of n-hexadecane (n-C16) and n-heneicosane (n-C21) in soil during natural attenuation (NA) and bioaugmentation (BA) were investigated by compound-specific stable isotope analysis (CSIA), microbial community and key enzyme genes analysis. In both NA and BA treatments, more than 95% of n-C16 and n-C21 were degraded within 3 and 6 days, respectively, with a slight faster degradation in BA than NA. According to CSIA, n-C16 showed a reverse hydrogen isotope fractionation effect, while n-C21 had a normal hydrogen isotope fractionation effect, and the degree of hydrogen isotope fractionation in BA were greater than NA. Isotope enrichment factors (εH) of n-C16 and n-C21were 3.50 ± 0.60‰ and − 2.11 ± 0.57‰ in NA, and 5.18 ± 0.46‰ and − 2.73 ± 0.39‰ in BA, respectively. The dominant genera in BA and NA were significantly different for n-C16 and n-C21 degradation and few strains added during BA remained active, such as Rhodococcus in both n-C16 and n-C21, and Brevundimonas only in n-C21, which showing high application potential in alkanes contamination remediation. Both alkB and almA genes played more key roles than Cytochromes P450 in n-C16 and n-C21 biodegradation, but alkB was much more promoted by n-C16 while almA was more motivated by n-C21. The different types of alkane-degrading bacteria with different enzyme reaction kinetics might be the reason of opposite hydrogen isotope fractionation of n-C16 and n-C21 during their biodegradation. Above all, CSIA can be a proper technology to quantitatively evaluate the biodegradation of different alkane during NA and BA bioremediation process in contamination sites.

Oxygen and chlorine isotope fractionation during microbial perchlorate reduction in static and flow through systems

Perchlorate (ClO4−) is an oxyanion with both anthropogenic and natural sources that is widespread on earth and throughout the solar system. Stable isotope ratio measurements are a powerful tool to assess the origin and predict the fate of perchlorate in the environment. The aim of this study was to perform an experimental survey of parameters that influence microbial O and Cl isotope fractionation during microbial perchlorate reduction, covering microbiological (genetic background and community context) and environmental (temperature, salinity, electron donor type and limitation) factors. A new method for the purification of perchlorate for isotope ratio mass spectrometry (IRMS) was developed to allow for more efficient sample preparation and suitable for samples containing small amounts of perchlorate (0.4–2 mg). Although most experiments, regardless of treatment, yielded a consistent isotope enrichment factor ratio (Λ = ε18O/ε37Cl ∼ 2.5), a greater variability was observed of individual isotope enrichment factors (ε18O and ε37Cl). Although most experiments yielded isotope enrichment factors (epsilon) in the larger negative range of previously published values, smaller negative values of ε18O and ε37Cl were observed for a halophilic isolate, Haloarcula marismortui, (ε18O = −15 to −22‰ and ε37Cl = −5 to −7‰), and in sediment column experiments (ε18O = −4 to −15‰ and ε37Cl = −1 to −6‰). Given the complexity of isolating individual parameters from our proposed model, we view our dataset as a survey that points to future opportunities where a particular parameter could be isolated to better understand controls on O and Cl isotope fractionation during microbial perchlorate reduction.

Effect of increased carbon load on denitrification efficiency and nitrate isotope enrichment factors in subsurface wastewater infiltration system.

Denitrification plays a dominant role in nitrate removal in subsurface wastewater infiltration system (SWIS). However, the effect of increased carbon (C) load on denitrification efficiency in the SWIS remain unclear. In this study, we used analyses of stable isotopes of nitrogen (N) and oxygen (O) in nitrate to investigate the N and O isotope enrichment factors (15 ε and 18 ε) and quantified N losses via denitrification in SWIS. The results demonstrated that an increase in C loads positively affected the pollutant removal performance of SWIS. The natural abundance of 15 N and 18 O increased with decreasing nitrate concentration from 12.5 to 7.3 mg/L, accompanied by increased 15 ε and 18 ε from -8.7‰ to -10.6‰ and -5.9‰ to -8.2‰, respectively, as the C load increased from 18 to 36 g/(m2 d). The contribution of denitrification to nitrate removal was 62%, 71%, and 77% when C loads were 18, 27, and 36 g/(m2 d), respectively, indicating that increased C loads could improve the nitrate removal through denitrification in SWIS. PRACTITIONER POINTS: Increasing C loads positively affected the nitrate removal performance of SWIS. N and O isotope enrichment factors of nitrate increased with the enhancement of influent C load. A C load of 36 g/(m2 d) is recommended in SWIS to improve the N removal performance and denitrification efficiency.

Mechanistic characterization of anaerobic microbial degradation of BTBPE in coastal wetland soils: Implication by compound-specific stable isotope analysis

As a novel brominate flame retardants, 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) has been extensively used in various consumer products, and frequently detected in various environmental matrices. However, the microbial degradation of BTBPE remains unclear in the environment. This study comprehensively investigated the anaerobic microbial degradation of BTBPE and therein stable carbon isotope effect in the wetland soils. BTBPE degradation followed the pseudo-first-order kinetic, with degradation rate of 0.0085 ± 0.0008 day−1. Based on identification of degradation products, stepwise reductive debromination was the main transformation pathway of BTBPE, and tended to keep the stable of 2,4,6-tribromophenoxy group during the microbial degradation. The pronounced carbon isotope fractionation was observed for BTBPE microbial degradation, and carbon isotope enrichment factor (εC) was determined to be −4.81 ± 0.37‰, indicating cleavage of C–Br bond as the rate-limiting step. Compared to previously reported isotope effects, carbon apparent kinetic isotope effect (AKIEC = 1.072 ± 0.004) suggested that the nucleophilic substitution (SN2 reaction) was the potential reaction mechanism for reductive debromination of BTBPE in the anaerobic microbial degradation. These findings demonstrated that BTBPE could be degraded by the anaerobic microbes in wetland soils, and the compound-specific stable isotope analysis was a robust method to discover the underlying reaction mechanisms.