Unravelling long-term source removal effects and chlorinated methanes natural attenuation processes by C and Cl stable isotopic patterns at a complex field site

Trichloromethane dechlorination by a novel Dehalobacter sp. strain 8M reveals a third contrasting C and Cl isotope fractionation pattern within this genus

Trichloromethane (TCM) is a pollutant frequently detected in contaminated aquifers, and only four bacterial strains are known to respire it. Here, we obtained a novel Dehalobacter strain capable of transforming TCM to dichloromethane, which was denominated Dehalobacter sp. strain 8M. Besides TCM, strain 8M also completely transformed 1,1,2-trichloroethane to vinyl chloride and 1,2-dichloroethane. Quantitative PCR analysis for the 16S rRNA genes confirmed growth of Dehalobacter with TCM and 1,1,2-trichloroethane as electron acceptors. Carbon and chlorine isotope fractionation during TCM transformation was studied in cultured cells and in enzymatic assays with cell suspensions and crude protein extracts. TCM transformation in the three studied systems resulted in small but significant carbon (ε_C = -2.7 ± 0.1‰ for respiring cells, -3.1 ± 0.1‰ for cell suspensions, and - 4.1 ± 0.5‰ for crude protein extracts) and chlorine (ε_Cl = -0.9 ± 0.1‰, -1.1 ± 0.1‰, and - 1.2 ± 0.2‰, respectively) isotope fractionation. A characteristic and consistent dual CCl isotope fractionation pattern was observed for the three systems (combined Λ^C/Cl = 2.8 ± 0.3). This Λ^C/Cl differed significantly from previously reported values for anaerobic dechlorination of TCM by the corrinoid cofactor vitamin B12 and other Dehalobacter strains. These findings widen our knowledge on the existence of different enzyme binding mechanisms underlying TCM-dechlorination within the genus Dehalobacter and demonstrates that dual isotope analysis could be a feasible tool to differentiate TCM degraders at field studies.

A modified method to calculate dual-isotope slopes for the natural attenuation of organic pollutants in the environment

Two-dimensional compound-specific isotope analysis has become a powerful tool for distinguishing reaction mechanisms. Lambda (Λ), an essential and important parameter for processing two-dimensional isotope fractionation data, exhibits values specific to a reaction mechanism. In the present article, we modified the existing algorithms for calculation of lambdas based on a review of current methods. Specifically, by regressing [(1000+δE_0,2)*(n₁*x₂)*ΔδE_bulk,1] versus [(1000+δE_0,1)*(n₂*x₁)*ΔδE_bulk,2] by the York method, a novel method was developed to calculate Λs. The improved method eliminates both the influence of the nonreacting position and the initial isotope signatures. Furthermore, this method retains the advantages of a two-dimensional isotope plot, which eliminates contributions from commitment to catalysis, does not require determination of the fraction of remaining substrate, and can be constructed even from field data. Additionally, the one-sample t test is applied to generate a 95% confidence interval of the dataset of Λ_ris for various reaction mechanisms. The ranges of 5.67-24.8, 8.54-9.80, 0.51-8.35, 25.2-36.8, and 7.09-21.9 are applicable for the oxidation of C-H bonds (Z_C=1, Z_H=3; Z_C and Z_H are the number of indistinguishable carbon and hydrogen atoms in intramolecular competition, respectively), oxidation of C-H bonds (Z_C=1, Z_H=4), aerobic biodegradation of benzene (Z_C=6, Z_H=6), methanogenic or sulfate-reducing biodegradation of benzene (Z_C=6, Z_H=6), and nitrate-reducing biodegradation of benzene (Z_C=6, Z_H=6). The accumulation and correction of these values will make the data measured in the field easier to interpret.

Groundwater quality

Groundwater is a vital component of water supply for residential, industrial, and agricultural purposes. However, many groundwater basins are being used unsustainably and groundwater contamination is a growing water quality problem. Although anthropogenic activities and natural processes have been increasing the contamination in this valuable water resource, several remediation techniques have been developed in the last few decades to reduce these contamination levels. This review paper focuses on the recent studies developed on groundwater pollutions, remediation practices, and groundwater quality management. PRACTITIONER POINTS: Groundwater pollution is mainly due to anthropogenic activities and it is considered as a growing water quality problem. Groundwater bioremediation is one of the sustainable long-term solutions that uses the microorganisms to degrade the complex environmental pollutants. Groundwater quality management techniques play a significant role to restore or maintain water quality, which is critical for the sustainable development.

Multi-element (C, H, Cl, Br) stable isotope fractionation as a tool to investigate transformation processes for halogenated hydrocarbons

Compound-specific isotope analysis (CSIA) is a powerful tool to evaluate transformation processes of halogenated compounds. Many halogenated hydrocarbons allow for multiple stable isotopic systems (C, H, Cl, Br) to be measured for a single compound. This has led to a large body of literature describing abiotic and biotic transformation pathways and reaction mechanisms for contaminants such as chlorinated alkenes and alkanes as well as brominated hydrocarbons. Here, the current literature is reviewed and a new compilation of Λ values for multi-isotopic systems for halogenated hydrocarbons is presented. Case studies of each compound class are discussed and thereby the current strengths of multi-element isotope analysis, continuing challenges, and gaps in our current knowledge are identified for practitioners of multi-element CSIA to address in the near future.

Integrative isotopic and molecular approach for the diagnosis and implementation of an efficient in-situ enhanced biological reductive dechlorination of chlorinated ethenes

Based on the previously observed intrinsic bioremediation potential of a site originally contaminated with perchloroethene (PCE), field-derived lactate-amended microcosms were performed to test different lactate isomers and concentrations, and find clearer isotopic and molecular parameters proving the feasibility of an in-situ enhanced reductive dechlorination (ERD) from PCE-to-ethene (ETH). According to these laboratory results, which confirmed the presence of Dehalococcoides sp. and the vcrA gene, an in-situ ERD pilot test consisting of a single injection of lactate in a monitoring well was performed and monitored for 190 days. The parameters used to follow the performance of the ERD comprised the analysis of i) hydrochemistry, including redox potential (Eh), and the concentrations of redox sensitive species, chlorinated ethenes (CEs), lactate, and acetate; ii) stable isotope composition of carbon of CEs, and sulphur and oxygen of sulphate; and iii) 16S rRNA gene sequencing from groundwater samples. Thus, it was proved that the injection of lactate promoted sulphate-reducing conditions, with the subsequent decrease in Eh, which allowed for the full reductive dechlorination of PCE to ETH in the injection well. The biodegradation of CEs was also confirmed by the enrichment in ¹³C and carbon isotopic mass balances. The metagenomic results evidenced the shift in the composition of the microbial population towards the predominance of fermentative bacteria. Given the success of the in-situ pilot test, a full-scale ERD with lactate was then implemented at the site. After one year of treatment, PCE and trichloroethene were mostly depleted, whereas vinyl chloride (VC) and ETH were the predominant metabolites. Most importantly, the shift of the carbon isotopic mass balances towards more positive values confirmed the complete reductive dechlorination, including the VC-to-ETH reaction step. The combination of techniques used here provides complementary lines of evidence for the diagnosis of the intrinsic biodegradation potential of a polluted site, but also to monitor the progress, identify potential difficulties, and evaluate the success of ERD at the field scale.

Bioelectrochemical Systems for Groundwater Remediation: The Development Trend and Research Front Revealed by Bibliometric Analysis

: Due to the deficiency of fresh water resources and the deterioration of groundwater quality worldwide, groundwater remedial technologies are especially crucial for preventing groundwater pollution and protecting the precious groundwater resource. Among the remedial alternatives, bioelectrochemical systems have unique advantages on both economic and technological aspects. However, it is rare to see a deep study focused on the information mining and visualization of the publications in this field, and research that can reveal and visualize the development trajectory and trends is scarce. Therefore, this study summarizes the published information in this field from the Web of Science Core Collection of the last two decades (1999–2018) and uses Citespace to quantitatively visualize the relationship of authors, published countries, organizations, funding sources, and journals and detect the research front by analyzing keywords and burst terms. The results indicate that the studies focused on bioelectrochemical systems for groundwater remediation have had a significant increase during the last two decades, especially in China, Germany and Italy. The national research institutes and universities of the USA and the countries mentioned above dominate the research. Environmental Science & Technology, Applied and Environmental Microbiology, and Water Research are the most published journals in this field. The network maps of the keywords and burst terms suggest that reductive microbial diversity, electron transfer, microbial fuel cell, etc., are the research hotspots in recent years, and studies focused on microbial enrichment culture, energy supply/recovery, combined pollution remediation, etc., should be enhanced in future.