Natural attenuation of chlorinated ethenes in hyporheic zones: A review of key biogeochemical processes and in-situ transformation potential

Acetochlor degradation in anaerobic microcosms with hyporheic sediments: Insights from biogeochemical data, transformation products, and isotope analysis

Steep redox gradients and diverse microbial communities in the anaerobic hyporheic zone create complex pathways for the degradation of herbicides, often linked to various terminal electron-accepting processes (TEAPs). Identifying the degradation pathways and their controlling factors under various TEAPs is of great significance for understanding mechanisms of water purification in the hyporheic zone. However, current research on herbicides in this area remains insufficient. Acetochlor, a commonly detected herbicide in aquatic environments, was the target contaminant in this study. Biogeochemical data, transformation products examination, and compound-specific isotope analysis (CSIA) were used to elucidate the degradation mechanisms of acetochlor under various TEAPs in anaerobic microcosms with hyporheic sediments. Results showed that carbon isotope fractionation of acetochlor during abiotic reduction by reduced sulfur species (εbulk,C = -16.4 ± 0.4‰), such as HS- and Sn2-, was significantly larger than that observed during anaerobic biodegradation (εbulk,C = -3.7 ± 0.4‰). This suggested the utility of CSIA in identifying biotic/abiotic degradation pathways of acetochlor in anaerobic environments. CSIA and transformation products examination revealed that biodegradation under Fe(III) reducing conditions and abiotic reduction by reduced sulfur species under SO42- reducing conditions were the main pathways for acetochlor degradation in anaerobic hyporheic sediments. TEAPs controlled the abilities and mechanisms of acetochlor degradation in different hyporheic sediments, which were highly associated with terminal electron acceptors (Fe(III) and SO42-), Fe(III) reducing bacteria (Geobacter and Anaerolinea), SO42- reducing bacteria (Bacteroidetes_vadinHA17), and tryptophan-like substances. This study provides important insights into the mechanisms of herbicides degradation in the hyporheic zone.

Control of dissolved H2 concentration enhances electron generation, transport and TCE reduction by indigenous microbial community

Electrokinetic enhanced bioremediation (EK-Bio) is practical for trichloroethene (TCE) dechlorination because the cathode can produce a wide range of dissolved H2 (DH) concentrations of 1.3-0 mg/L from the electrode to the aquifer. In this study, TCE dechlorination was investigated under different DH concentrations. The mechanisms were discussed by analyzing the microbial community structure and abundance of organohalide-respiring bacteria (OHRB) using 16S rRNA, and the gene abundances of key enzymes in the TCE electron transport chain using metagenomic analysis. The results showed that the moderate DH concentration of 0.19-0.53 mg/L exhibited the most pronounced TCE dechlorination, even better than the higher DH concentrations, due to the optimal redox environment, the enrichments of OHRB, reductive dehalogenase (rdhA) genes and key enzyme genes in the electron generation and transport chain. More electrons were obtained from H2 metabolism by Dehalobacter by promoting the formation of [NiFe] hydrogenase (HupS/L/C) or from glycolysis by versatile OHRB by stimulating the formation of formate and enriching formate dehydrogenase (FDH) under moderate DH conditions. In addition, the enhanced amino acid metabolism improved the vitamin K cycle for electron transport and enriched the reductive dechlorinating enzyme (RDase) genes. This study identifies the optimal DH concentration that facilitates bioremediation efficiency, provides insights into microbial community shifts and key enzymatic pathways in EK-Bio remediation.

Sediment grain size regulates the biogeochemical processes of nitrate in the riparian zone by influencing nutrient concentrations and microbial abundance

Riparian zones play a crucial role in reducing nitrate pollution in both terrestrial and aquatic environments. Complex deposition action and dynamic hydrological processes will change the grain size distribution of riparian sediments, affect the residence time of substances, and have a cascade effect on the biogeochemical process of nitrate nitrogen (NO3--N). However, simultaneous studies on NO3--N transformation and the potential drivers in riparian zones are still lacking, especially neglecting the effect of sediment grain size (SGS). To fill this knowledge gap, we first systematically identified and quantified NO3--N biogeochemical processes in the riparian zone by integrating molecular biotechnology, 15N stable isotope tracing, and microcosmic incubation experiments. We then evaluated the combined effects of environmental variables (including pH, dissolved organic carbon (DOC), oxidation reduction potential, SGS, etc.) on NO3--N transformation through Random Forest and Structural Equation Models. The results demonstrated that NO3--N underwent five microbial-mediated processes, with denitrification, dissimilatory nitrate reduction to ammonium (DNRA) dominated the NO3--N attenuation (69.4 % and 20.1 %, respectively), followed by anaerobic ammonia oxidation (anammox) and nitrate-dependent ferric oxidation (NDFO) (8.4 % and 2.1 %, respectively), while nitrification dominated the NO3--N production. SGS emerged as the most critical factor influencing NO3--N transformation (24.96 %, p < 0.01), followed by functional genes (nirS, nrfA) abundance, DOC, and ammonia concentrations (14.12 %, 16.40 %, 13.08 %, p < 0.01). SGS influenced NO3--N transformation by regulating microbial abundance and nutrient concentrations. RF predicted that a 5 % increase in the proportion of fine grains (diameter < 50 μm) may increase the NO3--N transformation rate by 3.8 %. This work highlights the significance of integrating machine learning and geochemical analysis for a comprehensive understanding of nitrate biogeochemical processes in riparian zones, contributing valuable references for future nitrogen management strategies.

Biofilm formation on MgFe-LDH@quartz sand as novel wetland substrate under varied C/N ratios for BDE-47 removal

Layered double hydroxide (LDH)-coated substrates could enhance the removal of various wastewater-born pollutants. However, research on biofilms attached to LDH-coatings and their synergistic purification effects on strongly hydrophobic persistent organic pollutants (POPs) remains limited. This study aims to investigate biofilm formation on MgFe-LDH@quartz sand and its effectiveness in removing tetrabromodiphenyl ether (BDE-47), an emerging halogenated POP in municipal wastewater. Under different C/N ratios (3, 5, and 10), BDE-47 removal rates ranged from 28.0% to 41.6% after 72 h. The optimal performance was achieved with LDH coating at C/N = 5, when substrate biofilm reached its highest extracelluar polymer substances (EPS) content, dehydrogenase activity and relative hydrophobicity. Moreover, distinct distribution patterns of EPS components' fluorescence peaks were observed in the LDH-coating treatment using three dimensional excitation-emission matrix (3D-EEM). While substrate adsorption was the primary mechanism for BDE-47 removal, accounting for 59.6%-83.4% of the total, biofilm adsorption and degradation contributed a relatively lower amount, ranging from 11.5% to 21.4%, and were more dependent on the C/N ratio. Notably, the maximum carrying capacity of protein predicted by the logistic growth model exhibited a strong positive correlation with the total BDE-47 removal rate (R2 = 0.82, p < 0.05), highlighting the importance of biofilm extracelluar proteins.

Simultaneous natural attenuation of Cr(VI) and nitrate in the hyporheic zone sediments from an upstream tributary of the Jinsha River in the Sichuan Basin

The coexistence of hexavalent chromium (Cr(VI)) and nitrate (NO3-) in groundwater and surface water presents a considerable challenge for the natural attenuation of these two contaminants because their interactions in nature remain contentious. This study investigated the interplay between Cr(VI) and NO3- in hyporheic zone (HZ) sediments by integrating Cr(VI) reduction kinetics, NO3- transformation, microbial community structure, and a three-rate model. The concurrent natural attenuation of Cr(VI) and NO3- in the sediments was significantly influenced by their initial concentrations and redox conditions. The reduction of low concentrations of Cr(VI) (37.1 and 96.2 μM) was slightly enhanced by NO3-, while inhibitory effects were observed at high concentrations of Cr(VI) (200.0 μM). However, except for an initial low concentration of Cr(VI) (37.1 μM) and NO3- (450 μM), the reduction of NO3- was adversely affected by Cr(VI). The reduction rates and efficiencies of Cr(VI) and NO3- were noticeably lower under aerobic conditions than under anaerobic conditions. This phenomenon can be attributed to the presence of O2, which decreased the selectivity of sediments-associated Fe(II) towards Cr(VI) and NO3- and induced alterations in the microbial community structure, leading to subsequent changes in NO3- transformation. Furthermore, the three-rate model represents a robust approach for elucidating the reduction of Cr(VI) in the presence of co-contaminants, such as NO3- contamination under diverse redox conditions. This study provides further insights into the interaction mechanism between Cr(VI) and NO3- within the HZ, necessitating the consideration of the microbial toxicity of Cr(VI) and electron competition among Cr(VI), NO3-, and O2.

The response of C/N/S cycling functional microbial communities to redox conditions in shallow aquifers using in-situ sediment as bio-trap matrix

Microbial communities are fundamental components driving critical biogeochemical carbon (C), nitrogen (N) and sulfur (S) cycles in groundwater ecosystems. The reduction-oxidation (redox) potential is one important environmental factor influencing the microbial community composition. Here, we developed a bio-trap method using in-situ sediment as a matrix to collect aquifer sediment samples and evaluate the response of microbial composition and C/N/S cycling functions to redox variations created by providing sole O2, joint O2 and H2, and sole H2 to three wells. Illumina sequencing analyses showed that the microbial communities in the bio-trap sediment could respond quickly to redox changes in the wells, demonstrating that this bio-trap method is promising for detecting microbial variation in the aquifer sediment. The microbial metabolic functions related to C, N and S cyclings and organic pollutants degradation were predicted by the Kyoto Encyclopedia of Genes and Genomes (KEGG) approach. It was found that the joint O2 and H2 injection produced medium oxidation-reduction potential (ORP -346 and -614 mV) and enhanced more microbial functions than sole O2 or H2, which mainly include oxidative phosphorylation, most carbon source metabolism, various pollutants degradation, and nitrogen and sulfur metabolism. Moreover, the functional genes encoding phenol monooxygenase, dioxygenase, nitrogen fixation, nitrification, aerobic and anaerobic nitrate reductase, nitrite reductase, nitric oxide reductase, and sulfur oxidation increased. These findings tell us the contaminant bioremediation and N, S metabolism can be promoted by adjusting ORP realised by injecting joint O2 and H2.

Unveiling complete natural reductive dechlorination mechanisms of chlorinated ethenes in groundwater: Insights from functional gene analysis

Monitored natural attenuation (MNA) of chlorinated ethenes (CEs) has proven to be a cost-effective and environment-friendly approach for groundwater remediation. In this study, the complete dechlorination of CEs with formation of ethene under natural conditions, were observed at two CE-contaminated sites, including a pesticide manufacturing facility (PMF) and a fluorochemical plant (FCP), particularly in the deeply weathered bedrock aquifer at the FCP site. Additionally, a higher abundance of CE-degrading bacteria was identified with heightened dechlorination activities at the PMF site, compared to the FCP site. The reductive dehalogenase genes and Dhc 16 S rRNA gene were prevalent at both sites, even in groundwater where no CE dechlorination was observed. vcrA and bvcA was responsible for the complete dechlorination at the PMF and FCP site, respectively, indicating the distinct contributions of functional microbial species at each site. The correlation analyses suggested that Sediminibacterium has the potential to achieve the complete dechlorination at the FCP site. Moreover, the profiles of CE-degrading bacteria suggested that dechlorination occurred under Fe3+/sulfate-reducing and nitrate-reducing conditions at the PMF and FCP site, respectively. Overall these findings provided multi-lines of evidence on the diverse mechanisms of CE-dechlorination under natural conditions, which can provide valuable guidance for MNA strategies implementation.

Response of TCE biodegradation to elevated H2 and O2: Implication for electrokinetic-enhanced bioremediation

The study investigated the influences of pure H2 and O2 introduction, simulating gases produced from the electrokinetic-enhanced bioremediation (EK-Bio), on TCE degradation, and the dynamic changes of the indigenous microbial communities. The dissolved hydrogen (DH) and oxygen (DO) concentrations ranged from 0.2 to 0.7 mg/L and 2.6 to 6.6 mg/L, respectively. The biological analysis was conducted by 16S rRNA sequencing and functional gene analyses. The results showed that the H2 introduction enhanced TCE degradation, causing a 90.4% TCE removal in the first 4 weeks, and 131.1 μM was reduced eventually. Accordingly, cis-dichloroethylene (cis-DCE) was produced as the only product. The following three ways should be responsible for this promoted TCE degradation. Firstly, the high DH rapidly reduced the oxidation-reduction potential (ORP) value to around -500 mV, beneficial to TCE microbial dechlorination. Secondly, the high DH significantly changed the community and promoted the enrichment of TCE anaerobic dechlorinators, such as Sulfuricurvum, Sulfurospirillum, Shewanella, Geobacter, and Desulfitobacterium, and increased the abundance of dechlorination gene pceA. Thirdly, the high DH promoted preferential TCE dechlorination and subsequent sulfate reduction. However, TCE bio-remediation did not occur in a high DO environment due to the reduced aerobic function or lack of functional bacteria or co-metabolic substrate. The competitive dissolved organic carbon (DOC) consumption and unfriendly microbe-microbe interactions also interpreted the non-degradation of TCE in the high DO environment. These results provided evidence for the mechanism of EK-Bio. Providing anaerobic obligate dechlorinators, and aerobic metabolic bacteria around the electrochemical cathodes and anodes, respectively, or co-metabolic substrates to the anode can be feasible methods to promote remediation of TCE-contaminated shallow aquifer under EK-Bio technology.

Natural attenuation of sulfonamides and metabolites in contaminated groundwater - Review, advantages and challenges of current documentation techniques

Sulfonamides are applied worldwide as antibiotics. They are emerging contaminants of concern, as their presence in the environment may lead to the spread of antibiotic resistance genes. Sulfonamides are present in groundwater systems, which suggest their persistence under certain conditions, highlighting the importance of understanding natural attenuation processes in groundwater. Biodegradation is an essential process, as degradation of sulfonamides reduces the risk of antibiotic resistance spreading. In this review, natural attenuation, and in particular assessment of biodegradation, is evaluated for sulfonamides in groundwater systems. The current knowledge level on biodegradation is reviewed, and a scientific foundation is built based on sulfonamide degradation processes, pathways, metabolites and toxicity. An overview of bacterial species and related metabolites is provided. The main research effort has focused on aerobic conditions while investigations under anaerobic conditions are lacking. The level of implementation in research is laboratory scale; here we strived to bridge towards field application and assessment, by assessing approaches commonly used in monitored natural attenuation. Methods to document contaminant mass loss are assessed to be applicable for sulfonamides, while the approach is limited by a lack of reference standards for metabolites. Furthermore, additional information is required on relevant metabolites in order to improve risk assessments. Based on the current knowledge on biodegradation, it is suggested to use the presence of substituent-containing metabolites from breakage of the sulfonamide bridge as specific indicators of degradation. Microbial approaches are currently available for assessment of microbial community's capacities, however, more knowledge is required on indigenous bacteria capable of degrading sulfonamides and on the impact of environmental conditions on biodegradation. Compound specific stable isotope analysis shows great potential as an additional in situ method, but further developments are required to analyse for sulfonamides at environmentally relevant levels. Finally, in a monitored natural attenuation scheme it is assessed that approaches are available that can uncover some processes related to the fate of sulfonamides in groundwater systems. Nevertheless, there are still unknowns related to relevant bacteria and metabolites for risk assessment as well as the effect of environmental settings such as redox conditions. Alongside, uncovering the fate of sulfonamides in future research, the applicability of the natural attenuation documentation approaches will advance, and provide a step towards in situ remedial concepts for the frequently detected sulfonamides.

Unveiling the inhibitory mechanisms of chromium exposure on microbial reductive dechlorination: Kinetics and microbial responses

Chromium and organochlorine solvents, particularly trichloroethene (TCE), are pervasive co-existing contaminants in subsurface aquifers due to their extensive industrial use and improper disposal practices. In this study, we investigated the microbial dechlorination kinetics under different TCE-Cr(Ⅲ/VI) composite pollution conditions and elucidated microbial response mechanisms based on community shift patterns and metagenomic analysis. Our results revealed that the reductive dechlorinating consortium had high resistance to Cr(III) but extreme sensitivity to Cr(VI) disturbance, resulting in a persistent inhibitory effect on subsequent dechlorination. Interestingly, the vinyl chloride-respiring organohalide-respiring bacteria (OHRB) was notably more susceptible to Cr(III/VI) exposure than the trichloroethene-respiring one, possibly due to inferior competition for growth substrates, such as electron donors. In terms of synergistic non-OHRB populations, Cr(III/VI) exposure had limited impacts on lactate fermentation but significantly interfered with H2-producing acetogenesis, leading to inhibited microbial dechlorination due to electron donor deficiencies. However, this inhibition can be effectively mitigated by the amendment of exogenous H2 supply. Furthermore, being the predominant OHRB, Dehalococcoides have inherent Cr(VI) resistance defects and collaborate with synergistic non-OHRB populations to achieve concurrent bio-detoxication of Cr(VI) and TCE. Our findings expand the understanding of the response patterns of different functional populations towards Cr(III/VI) stress, and provide valuable insights for the development of in situ bioremediation strategies for sites co-contaminated with chloroethene and chromium.

Groundwater chlorinated solvent plumes remediation from the past to the future: a scientometric and visualization analysis

Contamination of groundwater with chlorinated hydrocarbons has serious adverse effects on human health. As research efforts in this area have expanded, a large body of literature has accumulated. However, traditional review writing suffers from limitations regarding efficiency, quantity, and timeliness, making it difficult to achieve a comprehensive and up-to-date understanding of developments in the field. There is a critical need for new tools to address emerging research challenges. This study evaluated 1619 publications related to this field using VOSviewer and CiteSpace visual tools. An extensive quantitative analysis and global overview of current research hotspots, as well as potential future research directions, were performed by reviewing publications from 2000 to 2022. Over the last 22 years, the USA has produced the most articles, making it the central country in the international collaboration network, with active cooperation with the other 7 most productive countries. Additionally, institutions have played a positive role in promoting the publication of science and technology research. In analyzing the distribution of institutions, it was found that the University of Waterloo conducted the majority of research in this field. This paper also identified the most productive journals, Environmental Science & Technology and Applied and Environmental Microbiology, which published 11,988 and 3253 scientific articles over the past 22 years, respectively. The main technologies are bioremediation and chemical reduction, which have garnered growing attention in academic publishing. Our findings offer a useful resource and a worldwide perspective for scientists engaged in this field, highlighting both the challenges and the possibilities associated with addressing groundwater chlorinated solvent plumes remediation.

Application of dissimilatory iron-reducing bacteria for the remediation of soil and water polluted with chlorinated organic compounds: Progress, mechanisms, and directions

Chlorinated organic compounds are widely used as solvents, but they are pollutants that can have adverse effects on the environment and human health. Dissimilatory iron-reducing bacteria (DIRB) such as Shewanella and Geobacter have been applied to treat a wide range of halogenated organic compounds due to their specific biological properties. Until now, there has been no systematic review on the mechanisms of direct or indirect degradation of halogenated organic compounds by DIRB. This work summarizes the discussion of DIRB's ability to enhance the dechlorination of reaction systems through different pathways, both biological and biochemical. For biological dechlorination, some DIRB have self-dechlorination capabilities that directly dechlorinate by hydrolysis. Adjustment of dechlorination genes through genetic engineering can improve the dechlorination capabilities of DIRB. DIRB can also adjust the capacity for the microbial community to dechlorinate and provide nutrients to enhance the expression of dechlorination genes in other bacteria. In biochemical dechlorination, DIRB bioconverts Fe(III) to Fe(II), which is capable of dichlorination. On this basis, the DIRB-driven Fenton reaction can efficiently degrade chlorinated organics by continuously maintaining anoxic conditions to generate Fe(II) and oxic conditions to generate H2O2. DIRB can drive microbial fuel cells due to their electroactivity and have a good dechlorination capacity at low levels of energy consumption. The contribution of DIRB to the removal of pesticides, antibiotics and POPs is summarized. Then the DIRB electron transfer mechanism is discussed, which is core to their ability to dechlorinate. Finally, the prospect of future work on the removal of chlorine-containing organic pollutants by DIRB is presented, and the main challenges and further research directions are suggested.

Site-scale groundwater pollution risk assessment using surrogate models and statistical analysis

Petroleum pollution in soil and groundwater has emerged as a significant environmental concern worldwide. As a sustainable and cost-effective in-situ remediation technique, Monitored Natural Attenuation (MNA) exhibits significant promise in addressing sites contaminated by petrochemicals. This study specifically targets a typical petrochemical-contaminated site in northern China and employs GMS software to establish a comprehensive physical model. The model relies on time-series monitoring data of phenol concentrations spanning from 2018 to 2020, effectively simulating both the leakage and natural attenuation of phenol. Within this study, the adsorption coefficient and maximum adsorption capacity emerge as the foremost influential factors shaping the outcomes of the model. Given the inherent heterogeneity of the site and the variability of hydrochemical conditions, parameters such as dispersion, porosity, and adsorption coefficient exhibit significant uncertainties. Consequently, relying on traditional deterministic models to predict the feasibility of MNA technology is not reliable. Therefore, this study employs machine learning (ML) methods to construct stochastic parameter models based on physical processes. The Random Forest Regression (RFR) algorithm, after trained, demonstrates strong alignment with numerical model output, exhibiting an average Nash-Sutcliffe Efficiency (NSE) >0.96. Using a stochastic approach, RFR iteratively computes phenol concentration across 6000 sets of parameters. Applying probability statistics, the model shows a notable reduction in the likelihood of phenol concentrations exceeding a threshold, dropping from 64.0% to 15.7% before and after natural attenuation. In parameter uncertainty, the stochastic model emphasizes natural attenuation's efficacy in mitigating phenol pollution risk (porosity being the most influential factor). This case study proposed a novel method to quickly assess the pollution risks at petrochemical sites under the influence of the uncertainty of pollutant transport and reaction parameters. The results can provide a reference for the pollution risk assessment at petrochemical sites, especially in sites with high stratigraphic heterogeneity or insufficient transport parameter data.

Bioremediation of chlorinated ethenes contaminated groundwater and the reactive transport modeling - A review

Improper disposal of chlorinated ethenes (CEs), a class of widely used solvents in chemical manufacturing and cleaning industries, often leads to severe groundwater contamination. In situ bioremediation of CE-contaminated groundwater has received continuous attention in recent years. The reactive transport simulation is a valuable tool for planning and designing in situ bioremediation systems. This paper presents a detailed and comprehensive review on the main biotransformation pathways of CEs in aquifers, the mathematical modeling of bioremediation processes, and the available software developed for the simulation of reactive transport of CEs over past three decades. The aim of this research is to provide guidance on the selection of appropriate models and software suitable for systems of varying scales, and to discern prevailing research trends while identifying areas worthy of further study. This paper provides a detailed summary of the equations, parameters, and applications of existing biotransformation models from literature studies, highlighting the operation, benefits, and limitations of software available for CEs reactive transport simulations. Lastly, the support of reactive transport simulation programs for the design of full-scale in situ bioremediation systems was elucidated. Further research is needed for incorporating the effects of key subsurface environmental factors on biodegradation processes into models and balancing model complexity with computer data processing power to better support the development and application of reactive transport modeling software.

Co-metabolic biodegradation of chlorinated ethene in an oxygen- and ethane-based membrane biofilm reactor

Groundwater contamination by chlorinated ethenes is an urgent concern worldwide. One approach for detoxifying chlorinated ethenes is aerobic co-metabilims using ethane (C2H6) as the primary substrate. This study evaluated long-term continuous biodegradation of three chlorinated alkenes in a membrane biofilm reactor (MBfR) that delivered C2H6 and O2 via gas-transfer membranes. During 133 days of continuous operation, removals of dichloroethane (DCE), trichloroethene (TCE), and tetrachloroethene (PCE) were as high as 94 % and with effluent concentrations below 5 μM. In situ batch tests showed that the co-metabolic kinetics were faster with more chlorination. C2H6-oxidizing Comamonadaceae and "others," such as Methylococcaceae, oxidized C2H6 via monooxyenation reactions. The abundant non-ethane monooxygenases, particularly propane monooxygenase, appears to have been responsible for C2H6 aerobic metabolism and co-metabolism of chlorinated ethenes. This work proves that the C2H6 + O2 MBfR is a platform for ex-situ bioremediation of chlorinated ethenes, and the generalized action of the monooxygenases may make it applicable for other chlorinated organic contaminants.

Machine Learning Modeling Based on Microbial Community for Prediction of Natural Attenuation in Groundwater

Natural attenuation is widely adopted as a remediation strategy, and the attenuation potential is crucial to evaluate whether remediation goals can be achieved within the specified time. In this work, long-term monitoring of indigenous microbial communities as well as benzene, toluene, ethylbenzene, and xylene (BTEX) and chlorinated aliphatic hydrocarbons (CAHs) in groundwater was conducted at a historic pesticide manufacturing site. A machine learning approach for natural attenuation prediction was developed with random forest classification (RFC) followed by either random forest regression (RFR) or artificial neural networks (ANNs), utilizing microbiological information and contaminant attenuation rates for model training and cross-validation. Results showed that the RFC could accurately predict the feasibility of natural attenuation for both BTEX and CAHs, and it could successfully identify the key genera. The RFR model was sufficient for the BTEX natural attenuation rate prediction but unreliable for CAHs. The ANN model showed better performance in the prediction of the attenuation rates for both BTEX and CAHs. Based on the assessments, a composite modeling method of RFC and ANN was proposed, which could reduce the mean absolute percentage errors. This study reveals that the combined machine learning approach under the synergistic use of field microbial data has promising potential for predicting natural attenuation.

Improve Niche Colonization and Microbial Interactions for Organohalide-Respiring-Bacteria-Mediated Remediation of Chloroethene-Contaminated Sites

Organohalide-respiring bacteria (OHRB)-mediated reductive dehalogenation is promising in in situ bioremediation of chloroethene-contaminated sites. The bioremediation efficiency of this approach is largely determined by the successful colonization of fastidious OHRB, which is highly dependent on the presence of proper growth niches and microbial interactions. In this study, based on two ecological principles (i.e., Priority Effects and Coexistence Theory), three strategies were developed to enhance niche colonization of OHRB, which were tested both in laboratory experiments and field applications: (i) preinoculation of a niche-preparing culture (NPC, being mainly constituted of fermenting bacteria and methanogens); (ii) staggered fermentation; and (iii) increased inoculation of CE40 (a Dehalococcoides-containing tetrachloroethene-to-ethene dechlorinating enrichment culture). Batch experimental results show significantly higher dechlorination efficiencies, as well as lower concentrations of volatile fatty acids (VFAs) and methane, in experimental sets with staggered fermentation and niche-preconditioning with NPC for 4 days (CE40_NPC-4) relative to control sets. Accordingly, a comparatively higher abundance of Dehalococcoides as major OHRB, together with a lower abundance of fermenting bacteria and methanogens, was observed in CE40_NPC-4 with staggered fermentation, which indicated the balanced syntrophic and competitive interactions between OHRB and other populations for the efficient dechlorination. Further experiments with microbial source tracking analyses suggested enhanced colonization of OHRB by increasing the inoculation ratio of CE40. The optimized conditions for enhanced colonization of OHRB were successfully employed for field bioremediation of trichloroethene (TCE, 0.3-1.4 mM)- and vinyl chloride (VC, ∼0.04 mM)-contaminated sites, resulting in 96.6% TCE and 99.7% VC dechlorination to ethene within 5 and 3 months, respectively. This study provides ecological principles-guided strategies for efficient bioremediation of chloroethene-contaminated sites, which may be also employed for removal of other emerging organohalide pollutants.

Genome-Resolved Metagenomics and Metatranscriptomics Reveal Insights into the Ecology and Metabolism of Anaerobic Microbial Communities in PCB-Contaminated Sediments

Growth of organohalide-respiring bacteria such as Dehalococcoides mccartyi on halogenated organics (e.g., polychlorinated biphenyls (PCBs)) at contaminated sites or in enrichment culture requires interaction and support from other microbial community members. To evaluate naturally occurring interactions between Dehalococcoides and key supporting microorganisms (e.g., production of H2, acetate, and corrinoids) in PCB-contaminated sediments, metagenomic and metatranscriptomic sequencing was conducted on DNA and RNA extracted from sediment microcosms, showing evidence of both Dehalococcoides growth and PCB dechlorination. Using a genome-resolved approach, 160 metagenome-assembled genomes (MAGs), including three Dehalococcoides MAGs, were recovered. A novel reductive dehalogenase gene, distantly related to the chlorophenol dehalogenase gene cprA (pairwise amino acid identity: 23.75%), was significantly expressed. Using MAG gene expression data, 112 MAGs were assigned functional roles (e.g., corrinoid producers, acetate/H2 producers, etc.). A network coexpression analysis of all 160 MAGs revealed correlations between 39 MAGs and the Dehalococcoides MAGs. The network analysis also showed that MAGs assigned with functional roles that support Dehalococcoides growth (e.g., corrinoid assembly, and production of intermediates required for corrinoid synthesis) displayed significant coexpression correlations with Dehalococcoides MAGs. This work demonstrates the power of genome-resolved metagenomic and metatranscriptomic analyses, which unify taxonomy and function, in investigating the ecology of dehalogenating microbial communities.

Zinc and copper supplements enhance trichloroethylene removal by Pseudomonas plecoglossicida in water

The effects of two microelements, zinc and copper, on the aerobic co-metabolic removal of trichloroethylene (10 mg/L) by the isolate Pseudomonas plecoglossicida were investigated. The strain was previously isolated from a petroleum-contaminated site using toluene (150 mg/L) as substrate. Different concentrations (1, 10 and 100 mg/L) of microelements provided with SO42- and Cl- were tested. The results showed the supplement of Zn2+ and Cu2+ at the low concentration (1 mg/L) significantly enhanced cell growth. The removal efficiencies for toluene and trichloroethylene were also enhanced at the low concentration (1 mg/L) of Zn2+ and Cu2+. Compared to the control without zinc supplement, higher concentrations of zinc (10 and 100 mg/L) enhanced the removal efficiencies for both toluene and trichloroethylene in the first three days but showed some inhibitory effect afterward. However, the higher concentrations of Cu2+ (10 and 100 mg/L) always showed inhibitory to the toluene removal while showing inhibitory to the TCE removal after three days. For both Zn2+ and Cu2+, the anions SO42- and Cl- did not show significant difference in their effects on the toluene removal. A possible mechanism for Zn2+ and Cu2+ to enhance the removal of toluene and trichloroethylene would be their involvement in toluene oxygenase-based transformation processes. In addition, high concentrations of Zn2+ and Cu2+ ions could be removed from the liquid by the cells accordingly. The results imply a potential of supplementing low concentrations of zinc and copper to enhance bioremediation of the sites co-contaminated with toluene and trichloroethylene.

Prediction models and major controlling factors of antibiotics bioavailability in hyporheic zone

Recently, antibiotics have been frequently detected in the hyporheic zone (HZ) as a novel contaminant. Bioavailability assessment has gradually attracted more attention in order to provide a more realistic assessment of human health risks. In this study, two typical antibiotics, oxytetracycline (OTC) and sulfamethoxazole (SMZ), were used as target pollutants in the HZ of the Zaohe-Weihe River, and the polar organics integrated sampler was used to analyze the variation of antibiotics bioavailability. According to the characteristics of the HZ, the total concentration of pollutants, pH, and dissolved oxygen (DO) were selected as major predictive factors to analyze their correlation with the antibiotics bioavailability. Then the predictive antibiotic bioavailability models were constructed by stepwise multiple linear regression method. The results showed that there was a highly significant negative correlation between OTC bioavailability and DO (P < 0.001), while SMZ bioavailability showed a highly significant negative correlation with total concentration of pollutants (P < 0.001) and a significant negative correlation with DO (P < 0.01). The results of correlation analysis were further verified by Principal Component Analysis. Based on the experimental data, we constructed eight prediction models for the bioavailability of two antibiotics and verified them. The data points of the six prediction models were distributed in the 95% prediction band, indicating that the models were more reliable and accurate. The prediction models in this study provide reference for the accurate ecological risk assessment of the bioavailability of pollutants in the HZ, and also provide a new idea for predicting the bioavailability of pollutants in practical applications.

Distribution and risk assessment of antibiotics under water level fluctuation in the riparian zone of the Hanjiang River

The riparian zone (RZ) is an important region connecting surface water and groundwater, and it has widely been acknowledged for its pollutant buffering capacity. However, the decontaminating effect of RZ on trace organic compounds such as antibiotics has received little attention. This study explored the distribution of 21 antibiotics and 4 sulfonamide metabolites in river water and groundwater in the lower reaches of the Hanjiang River. The diffusion and exchange of contaminants between the river and riverbanks under the influence of water conservancy projects (Xinglong Dam and the Yangtze-Hanjiang Water Diversion Project) were investigated. Macrolide antibiotics were prevalent in river water (62.5-100%) and groundwater samples (42.9-80.4%). Ofloxacin and chlortetracycline were detected with the highest concentrations in river water (12.2 ng L^-1) and groundwater (9.3 ng L^-1) respectively. Higher levels of antibiotics were observed in spring and winter than in other seasons. The river-groundwater interaction has a certain interception effect on antibiotics, especially near riverbanks. Redox sensitive element Fe²⁺ showed significantly positive correlations with some tetracycline and macrolide antibiotics (p < 0.05), and thus the migration mechanism between Fe²⁺ and antibiotics under the condition of redox change should be investigated further. Environmental risks posed by antibiotics were assessed for algae, daphnids, and fish in surface water and groundwater. Only clarithromycin and chlortetracycline presented a medium risk to algae (0.1 < RQ < 1), and the rest presented low risk (RQ < 0.1). Nevertheless, the risk range may be further extended by interactions between groundwater and surface water. Accurate understanding of antibiotic transport in RZ is critical for developing management strategies aimed at reducing the pollution load on the watershed.

Degradation of chlorinated solvents with reactive iron minerals in subsurface sediments from redox transition zones

Reactive iron (Fe) mineral coatings found in subsurface reduction-oxidation transition zones (RTZs) contribute to the attenuation of contaminants. An 18.3-m anoxic core was collected from the site, where constituents of concern (COCs) in groundwater included chlorinated solvents. Reactive Fe mineral coatings were found to be abundant in the RTZs. This research focused on evaluating reaction kinetics with anoxic sediments bearing ferrous mineral nano-coatings spiked with either tetrachloroethylene (PCE), trichloroethylene (TCE), or 1,4-dichlorobenzene (1,4-DCB). Reaction kinetics with RTZ sediments followed pseudo-first-order reactions for the three contaminants with 90% degradation achieved in less than 39 days. The second-order rate constants for the three COCs ranged from 6.20 × 10^-4 to 1.73 × 10^-3 Lg^-1h^-1 with pyrite (FeS₂), 4.97 × 10^-5 to 1.24 × 10^-3 Lg^-1h^-1with mackinawite (FeS), 1.25 × 10^-4 to 1.89 × 10^-4 Lg^-1h^-1 with siderite (FeCO₃), and 1.79 × 10^-4 to 1.10 × 10^-3 Lg^-1h^-1 with magnetite (Fe₃O₄). For these three chlorinated solvents, the trend for the rate constants followed: Fe(II) sulfide minerals > magnetite > siderite. The high reactivity of Fe mineral coatings is hypothesized to be due to the large surface areas of the nano-mineral coatings. As a result, these surfaces are expected to play an important role in the attenuation of chlorinated solvents in contaminated subsurface environments.

Improved Darcian streambed measurements to quantify flux and mass discharge of volatile organic compounds from a contaminated aquifer to an urban stream

Quantifying VOC transport from contaminated groundwater to streams is challenging and important for understanding off-site migration of VOCs, cross-media contamination (groundwater to surface water and eventually air), and potential impacts on downstream ecosystems and human populations. A streambed point sampling approach was used to quantify fluxes of water and 14 VOCs from groundwater to an urban stream in North Carolina, USA, during summer (June 2015) and winter (January 2016). The approach is unique in coupling measurements of vertical hydraulic conductivity, vertical hydraulic head gradient, and groundwater VOC concentration at each individual sampling point, reducing or eliminating some potential concerns with other Darcian methods for quantifying VOC inputs to streams. Most results were consistent with discharge of two main VOC plumes on opposite sides of the stream. Plume 1 from the west side was dominated by cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) at mean concentrations of 19 and 11 μg L^-1, respectively. Plume 2 from the east side was dominated by benzene (mean concentration 56 μg L^-1). Plume 2 was not previously known, and the improved sampling approach allowed VOC discharge from both plumes to be quantified simultaneously. For 13 of the 14 detected VOCs, the mean VOC flux from groundwater to the stream (f_VOC) was higher in January 2016 than in June 2015, mainly because groundwater flux was higher in January. The only exception was cDCE, the most abundant VOC in Plume 1, which had mean f_VOC values of 9.8 and 9.5 mg m^-2 d^-1 in June 2015 and January 2016, respectively. Benzene was the most abundant VOC in Plume 2 and had mean f_VOC values of 11 and 37 mg m^-2 d^-1 in June 2015 and January 2016, respectively. High groundwater flux drove almost all the occurrences of high VOC flux. For a given VOC, the flow-weighted mean concentration (with each VOC concentration weighted by the upward groundwater flux at the VOC sampling point) was generally larger than the unweighted mean concentration. Thus, flow-weighting of concentrations gave a more accurate indication of the average VOC concentration in net groundwater discharge to the stream. An estimate of total VOC mass discharge from groundwater to the study reach of the stream, 3.6 kg of VOC per year, was based on the f_VOC results and streambed area in the reach. The bulk of this discharge was due to benzene, cDCE, and VC, with individual mass discharges of 2.1, 0.83, and 0.40 kg yr^-1, respectively. Estimates of maximum potential VOC degradation in the streambed suggest that the 3.6 kg yr^-1 estimate of mass discharge was not sensitive to potential degradation of VOCs in the streambed sediments above the groundwater sampling depth.

Versatile mechanisms and enhanced strategies of pollutants removal mediated by Shewanella oneidensis: A review

The removal of environmental pollutants is important for a sustainable ecosystem and human health. Shewanella oneidensis (S. oneidensis) has diverse electron transfer pathways and can use a variety of contaminants as electron acceptors or electron donors. This paper reviews S. oneidensis's function in removing environmental pollutants, including heavy metals, inorganic non-metallic ions (INMIs), and toxic organic pollutants. S. oneidensis can mineralize o-xylene (OX), phenanthrene (PHE), and pyridine (Py) as electron donors, and also reduce azo dyes, nitro aromatic compounds (NACs), heavy metals, and iodate by extracellular electron transfer (EET). For azo dyes, NACs, Cr(VI), nitrite, nitrate, thiosulfate, and sulfite that can cross the membrane, S. oneidensis transfers electrons to intracellular reductases to catalyze their reduction. However, most organic pollutants cannot be directly degraded by S. oneidensis, but S. oneidensis can remove these pollutants by self-synthesizing catalysts or photocatalysts, constructing bio-photocatalytic systems, driving Fenton reactions, forming microbial consortia, and genetic engineering. However, the industrial-scale application of S. oneidensis is insufficient. Future research on the metabolism of S. oneidensis and interfacial reactions with other materials needs to be deepened, and large-scale reactors should be developed that can be used for practical engineering applications.

A new insight into the influencing factors of natural attenuation of chlorinated hydrocarbons contaminated groundwater: A long-term field study of a retired pesticide site

Natural attenuation of contaminants has been increasingly applied as a strategy to manage the retired pesticide manufacturing sites due to the increasing restrictions on the reuse of contaminated sites in China. However, the influencing factors to enhance natural attenuation for chlorinated hydrocarbons in retired pesticide sites were not well studied. In this paper, monitoring of pollutants, environmental factors and microbial community was conducted from 2016 to 2021 in a retired pesticide site in Jiangsu Province undergoing natural attenuation, where the groundwater was severely contaminated with chlorinated hydrocarbons. The spatial variation of main pollutants, including chlorinated ethenes and ethanes, indicated that the site could be divided into the source area, diffusion area, and the end of diffusion area, where organohalide-respiring bacteria (OHRB) were detected. Pollutants and environmental factors influenced the OHRB community structure, which explained 7.6% and 33.2% of the variation, respectively. The abundances of obligate and facultative OHRB were affected in opposite ways by pollutants and environmental factors. Dehalococcoides and Dehalogenimonas in obligate OHRB were significantly inhibited by sulfate (r = -0.448, p < 0.05). The spatial-temporal characteristics of pollutants and the reveal of microbial community structure and its restricting factors in different areas make the foundation for strengthening the implementation of natural attenuation.

Resilience of organohalide-detoxifying microbial community to oxygen stress in sewage sludge

Organohalide pollutants are prevalent in the environment, causing harms to wildlife and human. Organohalide-respiring bacteria (OHRB) could detoxify these pollutants in anaerobic environments, but the most competent OHRB (i.e., Dehalococcoides) is susceptible to oxygen. This study reports exceptional resistance and resilience of sewage sludge microbial communities to oxygen stress for attenuation of structurally distinct organohalide pollutants, including tetrachloroethene, tetrabromobisphenol A, and polybrominated diphenyl ethers. The dehalogenation rate constant of these organohalide pollutants in oxygen-exposed sludge microcosms was maintained as 74-120% as that in the control without oxygen exposure. Subsequent top-down experiments clarified that sludge flocs and non-OHRB contributed to alleviating oxygen stress on OHRB. In the dehalogenating microcosms, multiple OHRB (Dehahlococcoides, Dehalogenimonas, and Sulfurospirillum) harboring distinct reductive dehalogenase genes (pceA, pteA, tceA, vcrA, and bdeA) collaborated to detoxify organohalide pollutants but responded differentially to oxygen stress. Comprehensive microbial community analyses (taxonomy, diversity, and structure) demonstrated certain resilience of the sludge-derived dehalogenating microbial communities to oxygen stress. Additionally, microbial co-occurrence networks were intensified by oxygen stress in most microcosms, as a possible stress mitigation strategy. Altogether the mechanistic and ecological findings in this study contribute to remediation of organohalide-contaminated sites encountering oxygen disturbance.

Efficient and Complete Detoxification of Polybrominated Diphenyl Ethers in Sediments Achieved by Bioaugmentation with Dehalococcoides and Microbial Ecological Insights

Polybrominated diphenyl ethers (PBDEs) are prevalent environmental pollutants, but bioremediation of PBDEs remains to be reported. Here we report accelerated remediation of a penta-BDE mixture in sediments by bioaugmentation with Dehalococcoides mccartyi strains CG1 and TZ50. Bioaugmentation with different amounts of each Dehalococcoides strain enhanced debromination of penta-BDEs compared with the controls. The sediment microcosm spiked with 6.8 × 10⁶ cells/mL strain CG1 showed the highest penta-BDEs removal (89.9 ± 7.3%) to diphenyl ether within 60 days. Interestingly, co-contaminant tetrachloroethene (PCE) improved bioaugmentation performance, resulting in faster and more extensive penta-BDEs debromination using less bioinoculants, which was also completely dechlorinated to ethene by introducing D. mccartyi strain 11a. The better bioaugmentation performance in sediments with PCE could be attributed to the boosted growth of the augmented Dehalococcoides and capability of the PCE-induced reductive dehalogenases to debrominate penta-BDEs. Finally, ecological analyses showed that bioaugmentation resulted in more deterministic microbial communities, where the augmented Dehalococcoides established linkages with indigenous microorganisms but without causing obvious alterations of the overall community diversity and structure. Collectively, this study demonstrates that bioaugmentation with Dehalococcoides is a feasible strategy to completely remove PBDEs in sediments.

Effects of colloids on ammonia nitrogen release under different ion conditions in natural sediments of Lake Taihu, China

Colloids act as vectors for accelerating contaminant movement in natural porous media such as lake sediments. Releasing characteristics of colloids and colloid-adsorbed ammonia nitrogen from lake sediments with the presence of monovalent and divalent cations were studied by an indoor anaerobic flooding incubation experiment. Results show that release of colloids was influenced by valence state and strength of the cation. In the presence of Na⁺ and Ca²⁺, the concentrations of the colloids in the overlying water were 11-163 mg/L and 13-88 mg/L during the entire incubation process and their values increased by 12.7%-122.3% and 1.5%-29.1%, respectively, compared to the control group, which indicated that the promoting effect of monovalent cations on release of colloids was more obvious than that of divalent cations. However, the total mass of colloids release reduced with the increasing ionic strength. Colloid-adsorbed ammonia nitrogen in the overlying water reached 0.15-1.72 mg/L and 0.15-1.12 mg/L with the presence of Na⁺ and Ca²⁺ and was higher 61.7%-161.7% and 21.3%-80.9%, respectively, than in the control group, indicating a consequent effect of ion conditions on the release of ammonium nitrogen from sediments. A significant positive correlation between colloids and ammonia nitrogen concentration further shows that colloidal activity determinately resulted in the increase or decrease in the ammonia nitrogen concentration in the overlying water, which could adsorb ammonia nitrogen and act as vehicles to carry ammonia nitrogen together into the aqueous medium or sink into the sediment. The release of ammonia nitrogen is possibly enhanced by colloidal behavior and varies with spatiotemporal ionic conditions in natural sediments. These findings are essential for improving the understanding of the geological fate of environmental colloids and associated nutritive salts, which provide scientific basis and technical support for the control of endogenous pollution and the comprehensive treatment of water bodies in Lake Taihu.

Combining reverse osmosis and microbial degradation for remediation of drinking water contaminated with recalcitrant pesticide residue

Groundwater contamination by recalcitrant organic micropollutants such as pesticide residues poses a great threat to the quality of drinking water. One way to remediate drinking water containing micropollutants is to bioaugment with specific pollutant degrading bacteria. Previous attempts to augment sand filters with the 2,6-dichlorobenzamide (BAM) degrading bacterium Aminobacter niigataensis MSH1 to remediate BAM-polluted drinking water initially worked well, but the efficiency rapidly decreased due to loss of degrader bacteria. Here, we use pilot-scale augmented sand filters to treat retentate of reverse osmosis treatment, thus increasing residence time in the biofilters and potentially nutrient availability. In a first pilot-scale experiment, BAM and most of the measured nutrients were concentrated 5-10 times in the retentate. This did not adversely affect the abundances of inoculated bacteria and the general prokaryotic community of the sand filter presented only minor differences. On the other hand, the high degradation activity was not prolonged compared to the filter receiving non-concentrated water at the same residence time. Using laboratory columns, it was shown that efficient BAM degradation could be achieved for >100 days by increasing the residence time in the sand filter. A slower flow may have practical implications for the treatment of large volumes of water, however this can be circumvented when treating only the retentate water equalling 10-15% of the volume of inlet water. We therefore conducted a second pilot-scale experiment with two inoculated sand filters receiving membrane retentate operated with different residence times (22 versus 133 min) for 65 days. While the number of MSH1 in the biofilters was not affected, the effect on degradation was significant. In the filter with short residence time, BAM degradation decreased from 86% to a stable level of 10-30% degradation within the first two weeks. The filter with the long residence time initially showed >97% BAM degradation, which only slightly decreased with time (88% at day 65). Our study demonstrates the advantage of combining membrane filtration with bioaugmented filters in cases where flow rate is of high importance.

Molecular Structure and Sulfur Content Affect Reductive Dechlorination of Chlorinated Ethenes by Sulfidized Nanoscale Zerovalent Iron

Sulfidized nanoscale zerovalent iron (SNZVI) with desirable properties and reactivity has recently emerged as a promising groundwater remediation agent. However, little information is available on how the molecular structure of chlorinated ethenes (CEs) affects their dechlorination by SNZVI or whether the sulfur content of SNZVI can alter their dechlorination pathway and reactivity. Here, we show that the reactivity (up to 30-fold) and selectivity (up to 70-fold) improvements of SNZVI (compared to NZVI) toward CEs depended on the chlorine number, chlorine position, and sulfur content. Low CEs (i.e., vinyl chloride and cis-1,2-dichloroethene) and high CEs (perchloroethene) tended to be dechlorinated by SNZVI primarily via atomic H and direct electron transfer, respectively, while SNZVI could efficiently and selectively dechlorinate trichloroethene and trans-1,2-dichloroethene via both pathways. Increasing the sulfidation degree of SNZVI suppressed its ability to produce atomic H but promoted electron transfer and thus altered the relative contributions of atomic H and electron transfer to the CE dechlorination, resulting in different reactivities and selectivities. These were indicated by the correlations of CE dechlorination rates and improvements with CE molecular descriptors, H₂ evolution rates, and electron transfer indicators of SNZVI. These mechanistic insights indicate the importance of determining the structure-specific properties and reactivity of both SNZVI materials and their target contaminants and can lead to a more rational design of SNZVI for in situ groundwater remediation of various CEs.

A Polyhydroxybutyrate (PHB)-Biochar Reactor for the Adsorption and Biodegradation of Trichloroethylene: Design and Startup Phase

In this work, polyhydroxy butyrate (PHB) and biochar from pine wood (PWB) are used in a mini-pilot scale biological reactor (11.3 L of geometric volume) for trichloroethylene (TCE) removal (80 mgTCE/day and 6 L/day of flow rate). The PHB-biochar reactor was realized with two sequential reactive areas to simulate a multi-reactive permeable barrier. The PHB acts as an electron donor source in the first “fermentative” area. First, the thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses were performed. The PHB-powder and pellets have different purity (96% and 93% w/w) and thermal properties. These characteristics may affect the biodegradability of the biopolymer. In the second reactive zone, the PWB works as a Dehalococcoides support and adsorption material since its affinity for chlorinated compounds and the positive effect of the “coupled adsorption and biodegradation” process has been already verified. A specific dechlorinating enriched culture has been inoculated in the PWB zone to realize a coupled adsorption and biodegradation process. Organic acids were revealed since the beginning of the test, and during the monitoring period the reductive dichlorination anaerobic pathway was observed in the first zone; no chlorinated compounds were detected in the effluent thanks to the PWB adsorption capacity.

Reactive Transport of NH4+ in the Hyporheic Zone from the Ground Water to the Surface Water

Nowadays, ammonia nitrogen (NH4+) pollution gets more and more attention in drinking water sources. This study investigated the main behavior of biogeochemical NH4+ from groundwater to surface water in a hyporheic zone (HZ) sediment from a reservoir. The experiments were conducted using synthetic groundwater to investigate ammonium transformation. The results indicated that ammonium concentration decreased, apparently resulting from the influence of microbial oxidation and ion exchange with Ca2+, Mg2+, K+, and Na+. However, all the ammonium in the sediment was oxidized, then the adsorbed NH4+ became bioavailable by being released back when NH4+ concentration decreased in the aqueous phase. The results showed NH4+ behavior in a HZ where the aerobic and anaerobic environments frequently exchange, with different hydrological conditions controlled by a strong coupling between microbial activities, geochemistry, hydrology, and ion exchange.

Biotic and abiotic reductive dechlorination of chloroethenes in aquitards

Chlorinated solvents occur as dense nonaqueous phase liquid (DNAPL) or as solutes when dissolved in water. They are present in many pollution sites in urban and industrial areas. They are toxic, carcinogenic, and highly recalcitrant in aquifers and aquitards. In the latter case, they migrate by molecular diffusion into the matrix. When aquitards are fractured, chlorinated solvents also penetrate as a free phase through the fractures. The main objective of this study was to analyze the biogeochemical processes occurring inside the matrix surrounding fractures and in the joint-points zones. The broader implications of this objective derive from the fact that, incomplete natural degradation of contaminants in aquitards generates accumulation of daughter products. This causes steep concentration gradients and back-diffusion fluxes between aquitards and high hydraulic conductivity layers. This offers opportunities to develop remediation strategies based, for example, on the coupling of biotic and reactive abiotic processes. The main results showed: 1) Degradation occurred especially in the matrix adjacent to the orthogonal network of fractures and textural heterogeneities, where texture contrasts favored microbial development because these zones constituted ecotones. 2) A dechlorinating bacterium not belonging to the Dehalococcoides genus, namely Propionibacterium acnes, survived under the high concentrations of dissolved perchloroethene (PCE) in contact with the PCE-DNAPL and was able to degrade it to trichloroethene (TCE). Dehalococcoides genus was able to conduct PCE reductive dechlorination at least up to cis-1,2-dichloroethene (cDCE), which shows again the potential of the medium to degrade chloroethenes in aquitards. 3) Degradation of PCE in the matrix resulted from the coupling of reactive abiotic and biotic processes-in the first case, promoted by Fe²⁺ sorbed to iron oxides, and in the latter case, related to dechlorinating microorganisms. The dechlorination resulting from these coupling processes is slow and limited by the need for an adequate supply of electron donors.

Direct aerobic oxidation (DAO) of chlorinated aliphatic hydrocarbons: A review of key DAO bacteria, biometabolic pathways and in-situ bioremediation potential

Contamination of aquifers and vadose zones with chlorinated aliphatic hydrocarbons (CAH) is a world-wide issue. Unlike other reactions, direct aerobic oxidation (DAO) of CAHs does not require growth substrates and avoids the generation of toxic by-products. Here, we critically review the current understanding of chlorinated aliphatic hydrocarbons-DAO and its application in bioreactors and at the field scale. According to reports on chlorinated aliphatic hydrocarbons-DAO bacteria, isolates mainly consisted of Methylobacterium and Proteobacterium. Chlorinated aliphatic hydrocarbons-DAO bacteria are characterized by tolerance to a high concentration of CAHs and highly efficient removal of CAHs. Trans-1,2-dichloroethylene (t-DCE) is easily transformed biomass for bacteria, followed by 1,2-dichloroethane (1,2-DCA), dichloromethane (DCM), vinyl chloride (VC) and cis-1,2-dichloroethylene (c-DCE). Significant differences in the maximum specific growth rates were observed with different CAHs and biometabolic pathways for DCM, 1,2-DCA, VC and c-DCE degradation have been successfully parsed. Detection of the functional genes etnC and etnE is useful for the determination of active VC DAO bacteria. Additionally, DAO bacteria have been successfully applied to CAHs in new types of bioreactors with satisfactory results. To the best of the authors' knowledge, only one study on DAO-CAHs was conducted in-situ and resulted in 99% CAH removal. Lastly, we put forward future development prospect of chlorinated aliphatic hydrocarbons-DAO.

Cometabolic degradation mechanism and microbial network response of methanotrophic consortia to chlorinated hydrocarbon solvents

The cometabolism mechanism of chlorinated hydrocarbon solvents (CHSs) in mixed consortia remains largely unknown. CHS biodegradation characteristics and microbial networks in methanotrophic consortia were studied for the first time. The results showed that all CHSs can efficiently be degraded via cometabolism with a maximum degradation rate of 4.8 mg/(h·gcell). Chloroalkane and chloroethylene were more easily degraded than chlorobenzenes by methanotrophic consortia, especially nonfully chlorinated aliphatic hydrocarbons, which were converted to Cl- with a production rate of 0.29-0.36 mg/(h·gcell). In addition, the microecological response results indicated that Methylocystaceae (49.0%), Methylomonas (65.3%) and Methylosarcina (41.9%) may be the major functional degraders in methanotrophic consortia. Furthermore, the results of the microbial correlation network suggested that interactive relationships constructed by type I methanotrophs and heterotrophs determined biodegradability. Additionally, PICRUSt analysis showed that CHSs could increase the relative abundance of CHS degradation genes and reduce the relative abundance of methane oxidation genes, which was in good agreement with the experimental results.

Fabrication of chitin nanofiber-PDMS composite aerogels from Pickering emulsion templates with potential application in hydrophobic organic contaminant removal

Natural polymers have aroused increasing attention in water treatment but their application in removing hydrophobic organic contaminants (HOCs) was limited due to their hydrophilicity. Herein, hydrophobic aerogels were successfully fabricated from Pickering emulsions stabilized by chitin nanofibers (ChNF) with polydimethylsiloxane (PDMS) as dispersed phase and glutaraldehyde as a crosslinking agent, and their performance in HOCs removal were evaluated. The Pickering emulsions with PDMS ratios of 2.5-20% v/v showed high stability, demonstrating great potential as aerogel templates. The solidified PDMS droplets were evenly distributed within the matrix, contributing to homogeneous and permanent hydrophobicity. The composite aerogels with water contact angles of over 130° could selectively remove non-aqueous phase HOCs from water. The CCl₄ adsorption capacity was 521-2820 wt%, depending on PDMS contents. Meanwhile, the mechanical resilience of the composite aerogels was significantly improved, facilitating the adsorbent regeneration by simple mechanical squeezing. The adsorption capacity remained above 85% for 24 cycles. Moreover, the aerogels could also remove dissolved HOCs from water with a maximum adsorption capacity of 1.34 mg/g for 10 mg/L TCE. This work reveals the potential of Pickering emulsions in the fabrication of composite hydrophobic materials from natural biopolymers with promising application in HOCs related water treatment.

Regulating the dechlorination and methanogenesis synchronously to achieve a win-win remediation solution for γ-hexachlorocyclohexane polluted anaerobic environment

The wish for rapid degradation of chlorinated organic pollutants along with the increase concern with respect to greenhouse effect and bioenergy methane production have created urgent needs to explore synchronous regulation approach. Microbial electrolysis cell was established under four degressive cathode potential settings (from -0.15V to -0.60V) to regulate γ-hexachlorocyclohexane (γ-HCH) reduction while CH₄ cumulation in this study. The synchronous facilitation of γ-HCH reduction and CH₄ cumulation was occurred in -0.15V treatment while the facilitation of γ-HCH reductive removal together with the inhibition of CH₄ cumulation was showed in -0.30V treatment. Electrochemical patterns via cyclic voltammetry and morphological performances via scanning electron microscopy illustrated bioelectrostimulation promoted redox reactions and helped to construct mature biofilms located on bioelectrodes. Also, bioelectrostimulated regulation pronouncedly affected the bacteria and archaeal communities and subsequently assembled distinctly core sensitive responders across bioanode, biocathode and plankton. Clostridum, Longilinea and Methanothrix relatively accumulated in the plankton, and Cupriavidus and Methanospirillum, and Perimonas and Nonoarcheaum in biocathode and bioanode, respectively; while Pseudomonas, Stenotrophomonas, Methanoculleus and Methanosarcina were diffusely enriched. Microbial interactions in the ecological network were more complicated in -0.15V and -0.30V cathodic potential treatments, coincident with the increasement of γ-HCH reduction. The co-existence between putative dechlorinators and methanogens was less significant in -0.30V treatment when compared to that in -0.15V treatment, relevant with the variations of CH₄ cumulation. In all, this study firstly corroborated the availability to synchronously regulate γ-HCH reductive removal and methanogenesis. Besides, it paves an advanced approach controlling γ-HCH reduction in cooperation with CH₄ cumulation, of which to achieve γ-HCH degradation facilitation along with biogas (CH₄) production promotion with -0.15V cathode potential during anaerobic γ-HCH contaminated wastewater digestion, or to realize γ-HCH degradation facilitation with the inhibition of CH₄ emission with -0.30V cathode potential for an all-win remediation in γ-HCH polluted anaerobic environment such as paddy soil.

Combined Strategies to Prompt the Biological Reduction of Chlorinated Aliphatic Hydrocarbons: New Sustainable Options for Bioremediation Application

Groundwater remediation is one of the main objectives to minimize environmental impacts and health risks. Chlorinated aliphatic hydrocarbons contamination is prevalent and presents particularly challenging scenarios to manage with a single strategy. Different technologies can manage contamination sources and plumes, although they are usually energy-intensive processes. Interesting alternatives involve in-situ bioremediation strategies, which allow the chlorinated contaminant to be converted into non-toxic compounds by indigenous microbial activity. Despite several advantages offered by the bioremediation approaches, some limitations, like the relatively low reaction rates and the difficulty in the management and control of the microbial activity, can affect the effectiveness of a bioremediation approach. However, those issues can be addressed through coupling different strategies to increase the efficiency of the bioremediation strategy. This mini review describes different strategies to induce the reduction dechlorination reaction by the utilization of innovative strategies, which include the increase or the reduction of contaminant mobility as well as the use of innovative strategies of the reductive power supply. Subsequently, three future approaches for a greener and more sustainable intervention are proposed. In particular, two bio-based materials from renewable resources are intended as alternative, long-lasting electron-donor sources (e.g., polyhydroxyalkanoates from mixed microbial cultures) and a low-cost adsorbent (e.g., biochar from bio-waste). Finally, attention is drawn to novel bio-electrochemical systems that use electric current to stimulate biological reactions.

Application of nanofiber carriers for sampling of microbial biomass from contaminated groundwater

Sampling of microbial biomass is crucial for understanding and controlling remediation processes ongoing at contaminated sites in general, particularly when molecular genetic analyses are employed. In this study, fiber-based carriers with a nanofiber layer were developed and tested as a method to sample microbial biomass in groundwater for molecular genetic analysis. Nanofiber carriers, varying in the shape and the linear density of nanofibers, were examined throughout a 27-month monitoring period in groundwater contaminated with benzene, toluene, ethylbenzene and xylene isomers (BTEX), and chlorinated ethenes. The effect of carrier shape and nanofiber layer density on the microbial surface colonization and composition of the microbial biofilm was determined using real-time PCR and next-generation sequencing (NGS) analysis. Differences in microbial community composition between nanofiber carriers, groundwater, and soil samples were also analyzed to assess the applicability of carriers for biomass sampling at contaminated sites. The nanofiber carriers showed their applicability as a sampling tool, particularly because of their easy manipulation that facilitates DNA isolation. The majority of taxa (Proteobacteria, Firmicutes, and Bacteroidetes) present on the carrier surfaces were also detected in the groundwater. Moreover, the microbial community on all nanofiber carriers reflected the changes in the chemical composition of groundwater. Although the carrier characteristics (shape, nanofiber layer) did not substantially influence the microbial community on the carrier surface, the circular and planar carriers with a nanofiber layer displayed faster microbial surface colonization. However, the circular carrier was the most suitable for biomass sampling in groundwater because of its high contact area and because it does not require pre-treatment prior to DNA extraction.

Waste activated sludge stimulates in situ microbial reductive dehalogenation of organohalide-contaminated soil

Due to its enriched organic matter, nutrients and growth cofactors, as well as a diverse range of microorganisms, waste activated sludge (WAS) might be an ideal additive to stimulate organohalide respiration for in situ bioremediation of organohalide-contaminated sites. In this study, we investigated the biostimulation and bioaugmentation impacts of WAS-amendment on the performance and microbiome in tetrachloroethene (PCE) and polychlorinated biphenyls (PCBs) dechlorinating microcosms. Results demonstrated that WAS-amendment increased PCE- and PCBs-dechlorination rate as much as 6.06 and 10.67 folds, respectively. The presence of WAS provided a favorable growth niche for organohalide-respiring bacteria (OHRB), including redox mediation and generation of electron donors and carbon sources. Particularly for the PCE dechlorination, indigenous Geobacter and WAS-derived Dehalococcoides were identified to play key roles in PCE-to-dichloroethene (DCE) and DCE-to-ethene dechlorination, respectively. Similar biostimulation and bioaugmentation effects of WAS-amendment were observed on both PCE- and PCBs-dechlorination in three different soils, i.e., laterite, brown loam and paddy soil. Risk assessment suggested low potential ecological risk of WAS amendment in remediation of organohalide-contaminated soil. Overall, this study provided an economic and efficient strategy to stimulate the organohalide respiration-based bioremediation in field applications.

Increasing electron donor concentration does not accelerate complete microbial reductive dechlorination in contaminated sediment with native organic carbon

Experiments with Fe(III)-rich, chloroethene-contaminated sediment demonstrated that trichloroethylene (TCE) and vinyl chloride (VC) were completely reduced to ethene regardless of whether electron donor(s) were added at 1 × stoichiometry or 10 × stoichiometry relative to all-electron acceptors. Unamended controls uniformly reduced TCE to ethene with a mean time to complete dechlorination (operationally defined as the presence of stoichiometric ethene production) of 79 days. Adding 1 × and 10 × acetate hindered the rate and extent of TCE and VC reduction relative to unamended controls, with several only partially reduced when the experiments were terminated. Adding high molecular mass (soybean oil derivative) substrates did not increase microbial reductive dechlorination relative to unamended incubations, and in many cases, hindered microbial dechlorination in favor of methanogenesis. The mean time to complete dechlorination was comparable between low (× 1) and high (× 10) electron donor concentration for all lipid-based electron donors tested. Those tested included Newman Zone® Standard without sodium lactate (96 vs. 75 days, respectively), CAP 18 ME (85 vs. 94 days, respectively), EOS 598B42 (68 vs. 72 days, respectively), and acetate (134 vs. 125 days, respectively). These data suggest that the addition of an electron donor does not always increase the rate and extent of reductive dechlorination but will increase costs. In particular, increasing the concentration of electron donors higher than the stoichiometric demand only decreased complete microbial reductive dechlorination, which is the opposite of most standard "more time and more electrons" approaches. These data argue that site-specific electron donor demands must be evaluated, and in some cases, a monitored natural attenuation (MNA) approach is most favorable.

Microbial debromination of hexabromocyclododecanes

Hexabromocyclododecanes (HBCDs), a new sort of brominated flame retardants (BFRs), are globally prevalent and recalcitrant toxic environmental pollutants. HBCDs have been found in many environmental media and even in the human body, leading to serious health concerns. HBCDs are biodegradable in the environment. By now, dozens of bacteria have been discovered with the ability to transform HBCDs. Microbial debromination of HBCDs is via HBr-elimination, HBr-dihaloelimination, and hydrolytic debromination. Biotic transformation of HBCDs yields many hydroxylated and lower brominated compounds which lack assessment of ecological toxicity. Bioremediation of HBCD pollution has only been applied in the laboratory. Here, we review the current knowledge about microbial debromination of HBCDs, aiming to promote the bioremediation applied in HBCD contaminated sites. KEY POINTS: • Microbial debromination of HBCDs is via hydrolytic debromination, HBr-elimination, and HBr-dihaloelimination. • Newly occurred halogenated contaminants such as HBCDs hitch the degradation pathway tamed by previously discharged anthropogenic organohalides. • Strategy that combines bioaugmentation with phytoremediation for bioremediation of HBCD pollution is promising.

The role of ecotones in the dehalogenation of chloroethenes in alluvial fan aquifers

The presence of ecotones in transition zones between geological strata (e.g. layers of gravel and sand interbedded with layers of silt in distal alluvial fan deposits) in aquifers plays a significant role in regulating the flux of matter and energy between compartments. Ecotones are characterised by steep physicochemical and biological gradients and considerable biological diversity. However, the link between organic pollutants and degradation potential in ecotones has scarcely been studied. The aim of this study is to relate the presence of ecotones with the dehalogenation of chloroethenes. A field site was selected where chloroethene contamination occurs in a granular aquifer with geological heterogeneities. The site is monitored by multilevel and conventional wells. Groundwater samples were analysed by chemical, isotopic, and molecular techniques. The main results were as follows: (1) two ecotones were characterised in the source area, one in the upper part of the aquifer and the second in the transition zone to the bottom aquitard, where the aged pool is located; (2) the ecotone located in the transition zone to the bottom aquitard has greater microbial diversity, due to higher geological heterogeneities; (3) both ecotones show the reductive dehalogenation of perchloroethylene and trichloroethylene; and (4) these ecotones are the main zones of the reductive dehalogenation of the pollutants, given the more reductive conditions at the centre of the plume. These findings suggest that ecotones are responsible for natural attenuation, where oxic conditions prevailed at the aquifer and bioremediation strategies could be applied more effectively in these zones to promote complete reductive dehalogenation.

Invertebrate and Microbial Response to Hyporheic Restoration of an Urban Stream

All cities face complex challenges managing urban stormwater while also protecting urban water bodies. Green stormwater infrastructure and process-based restoration offer alternative strategies that prioritize watershed connectivity. We report on a new urban floodplain restoration technique being tested in the City of Seattle, USA: an engineered hyporheic zone. The hyporheic zone has long been an overlooked component in floodplain restoration. Yet this subsurface area offers enormous potential for stormwater amelioration and is a critical component of healthy streams. From 2014 to 2017, we measured hyporheic temperature, nutrients, and microbial and invertebrate communities at three paired stream reaches with and without hyporheic restoration. At two of the three pairs, water temperature was significantly lower at the restored reach, while dissolved organic carbon and microbial metabolism were higher. Hyporheic invertebrate density and taxa richness were significantly higher across all three restored reaches. These are some of the first quantified responses of hyporheic biological communities to restoration. Our results complement earlier reports of enhanced hydrologic and chemical functioning of the engineered hyporheic zone. Together, this research demonstrates that incorporation of hyporheic design elements in floodplain restoration can enhance temperature moderation, habitat diversity, contaminant filtration, and the biological health of urban streams.

The Significance of Vertical and Lateral Groundwater–Surface Water Exchange Fluxes in Riverbeds and Riverbanks: Comparing 1D Analytical Flux Estimates with 3D Groundwater Modelling

Riverbed temperature profiles are frequently used to estimate vertical river–aquifer exchange fluxes. Often in this approach, strictly vertical flow is assumed. However, riverbeds are heterogeneous structures often characterised by complex flow fields, possibly violating this assumption. We characterise the meter-scale variability of river–aquifer interaction at two sections of the Aa River, Belgium, and compare vertical flux estimates obtained with a 1D analytical solution to the heat transport equation with fluxes simulated with a 3D groundwater model (MODFLOW) using spatially distributed fields of riverbed hydraulic conductivity. Based on 115 point-in-time riverbed temperature profiles, vertical flux estimates that are obtained with the 1D solution are found to be higher near the banks than in the center of the river. The total exchange flux estimated with the 3D groundwater model is around twice as high as the estimate based on the 1D solution, while vertical flux estimates from both methods are within a 10% margin. This is due to an important contribution of non-vertical flows, especially through the riverbanks. Quasi-vertical flow is only found near the center of the river. This quantitative underestimation should be considered when interpreting exchange fluxes based on 1D solutions. More research is necessary to assess conditions for which using a 1D analytical approach is justified to more accurately characterise river–aquifer exchange fluxes.

Anaerobic Dehalogenation by Reduced Aqueous Biochars

Dehalogenation is one of the most important reactions for eliminating trace organic pollutants in natural and engineering systems. This study investigated the dehalogenation of a model organohalogen compound, triclosan (TCS), by aqueous biochars (a-BCs) (<450 nm). We found that TCS can be anaerobically degraded by reduced a-BCs with a pseudo first-order degradation rate constant of 0.0011-0.011 h^-1. The 288 h degradation fraction of TCS correlated significantly with the amount of a-BC-bound electrons (0.055 ± 0.00024 to 0.11 ± 0.0016 mol e^-/mol C) available for donation after 24 h of pre-reduction by Shewanella putrefaciens CN32. Within the reduction period, the recovery of chlorine based on residual TCS and generated Cl^- ranged from 73.6 to 85.2%, implying that a major fraction of TCS was fully dechlorinated, together with mass spectroscopic analysis of possible degradation byproducts. Least-squares numerical fitting, accounting for the reactions of hydroquinones/semiquinones in a-BCs with TCS and byproducts, can simulate the reaction kinetics well (R² > 0.76) and suggest the first-step dechlorination as the rate-limiting step among the possible pathways. These results showcased that the reduced a-BCs can reductively degrade organohalogens with potential applications for wastewater treatment and groundwater remediation. While TCS was used as a model compound in this study, a-BC-based degradation can be likely applied to a range of redox-sensitive trace organic compounds.

Natural attenuation of a chlorinated ethene plume discharging to a stream: Integrated assessment of hydrogeological, chemical and microbial interactions

Attenuation processes of chlorinated ethenes in complex near-stream systems result in site-specific outcomes of great importance for risk assessment of contaminated sites. Additional interdisciplinary and comprehensive field research is required to enhance process understanding in these systems. In this study, several methods were combined in a multi-scale interdisciplinary in-situ approach to assess and quantify the near-stream attenuation of a chlorinated ethene plume, mainly consisting of cis-dichloroethene (cis-DCE) and vinyl chloride (VC), discharging to a lowland stream (Grindsted stream, Denmark) over a monitoring period of seven years. The approach included: hydrogeological characterisation, reach scale contaminant mass balance analysis, quantification of contaminant mass discharge, streambed fluxes of chlorinated ethenes quantified using Sediment Bed Passive Flux Meters (SBPFMs), assessment of redox conditions, temporal assessment of contaminant concentrations, microbial analysis, and compound-specific isotope analysis (CSIA). This study site exhibits a special attenuation behaviour not commonly encountered in field studies: the conversion from an initially limited degradation case (2012-16), despite seemingly optimal conditions, to one presenting notable levels of degradation (2019). Hence, this study site provides a new piece to the puzzle, as sites with different attenuation behaviours are required in order to acquire the full picture of the role groundwater-surface water interfaces have in risk mitigation. In spite of the increased degradation in the near-stream plume core, the contaminant attenuation was still incomplete in the discharging plume. A conceptualization of flow, transport and processes clarified that hydrogeology was the main control on the natural attenuation, as short residence times of 0.5-37 days restricted the time in which dechlorination could occur. This study reveals the importance of: taking an integrated approach to understand the influence of all attenuation processes in groundwater - surface water interactions; considering the scale and domain of interest when determining the main processes; and monitoring sufficiently both spatially and temporally to cover the transient conditions.

Natural attenuation method for contaminant remediation reagent delivery assessment for in situ chemical oxidation using aqueous ozone

A Monitored Natural Attenuation (MNA) assessment approach typically used for contaminant remediation feasibility assessment was developed here for remediation-reagent delivery assessment. Subsurface delivery of oxidants, such as aqueous ozone (O₃) for in situ chemical oxidation (ISCO) of groundwater contaminants, is naturally attenuated by oxidant demand and reactivity. We compared mixed reactor kinetic experiments, sand column tracer transport experiments, and reactive transport modeling and assessment methods to quantify natural attenuation kinetics, aqueous O₃ solute transport, oxidant demand kinetics, and ISCO reagent delivery limitations. Sorption of aqueous O₃ to quartz sand was observed during transport of O₃ through water-saturated porous media. Pseudo 1st order decomposition rate constants of O₃ bulk attenuation with transport were comparable to mixed reactor experiments without transport, and reactive transport modeling of miscible-displacement column experiments was used to quantify each attenuation process. Aqueous ionic strength was correlated with O₃ decomposition rate constants, which was the dominant reagent delivery attenuation process. These results suggest that aqueous O₃ decomposition and oxidant delivery attenuation can be predictable upon characterization of the sediment oxidant demand and dispersion, and increasing groundwater velocity during aqueous O₃ injection can maximize transport distance for reagent delivery.

Natural attenuation of pools and plumes of carbon tetrachloride and chloroform in the transition zone to bottom aquitards and the microorganisms involved in their degradation

In the transition zone between aquifers and aquitards, DNAPL pools of carbon tetrachloride and chloroform accumulate because of heterogeneity in this zone. Natural attenuation occurs at pools and plumes, indicating that remediation might be possible. The aims of the study were: i) to assess the role of heterogeneity in the natural attenuation of these compounds, ii) determine degradation processes within this zone, and iii) identify dechlorinating microorganisms. For this, groundwater concentrations, redox-sensitive parameters, CSIA isotopic and DGGE molecular techniques were used. The main findings at depth of the transition zone were: (1) the important key control played by heterogeneity on natural attenuation of contaminants. (2) Heterogeneity caused the highly anoxic environment and dominant sulfate-reducing conditions, which accounts for more efficient natural attenuation. (3) Heterogeneity also explains that the transition zone constitutes an ecotone. (4) The bacteria size exclusion is governed by the pore throat threshold and determines the penetration of dechlorinating microorganisms into the finest sediments, which is relevant, since it implies the need to verify whether microorganisms proposed for bioremediation can penetrate these materials. (5) Reductive dechlorination caused the natural attenuation of contaminants in groundwater and porewater of fine sediments. In the case of carbon tetrachloride, it was an abiotic process biogenically mediated by A. suillum, a bacterium capable of penetrating the finest sediments. In the case of chloroform, it was a biotic process performed by a Clostridiales bacterium, which is unable to penetrate the finest materials. (6) Both microorganisms have potential to be biostimulated to dechlorinate contaminants in the source and the plume in the transition zone. These outcomes are particularly relevant given the longevity of DNAPL sources and have considerable environmental implications as many supply wells in industrial areas exploit aquifers contaminated by chlorinated solvents emerging from DNAPL pools accumulated on the low-conductivity layers in transition zones.

In situ groundwater remediation with bioelectrochemical systems: A critical review and future perspectives

Groundwater contamination is an ever-growing environmental issue that has attracted much and undiminished attention for the past half century. Groundwater contamination may originate from both anthropogenic (e.g., hydrocarbons) and natural compounds (e.g., nitrate and arsenic); to tackle the removal of these contaminants, different technologies have been developed and implemented. Recently, bioelectrochemical systems (BES) have emerged as a potential treatment for groundwater contamination, with reported in situ applications that showed promising results. Nitrate and hydrocarbons (toluene, phenanthrene, benzene, BTEX and light PAHs) have been successfully removed, due to the interaction of microbial metabolism with poised electrodes, in addition to physical migration due to the electric field generated in a BES. The selection of proper BESs relies on several factors and problems, such as the complexity of groundwater and subsoil environment, scale-up issues, and energy requirements that need to be accounted for. Modeling efforts could help predict case scenarios and select a proper design and approach, while BES-based biosensing could help monitoring remediation processes. In this review, we critically analyze in situ BES applications for groundwater remediation, focusing in particular on different proposed setups, and we identify and discuss the existing research gaps in the field.