Biodegradation of chlorobenzene, 1,2-dichlorobenzene, and 1,4-dichlorobenzene in the vadose zone

Anaerobic biodegradation of perfluorooctane sulfonate (PFOS) and microbial community composition in soil amended with a dechlorinating culture and chlorinated solvents

Perfluorooctane sulfonate (PFOS), one of the most frequently detected per- and polyfluoroalkyl substances (PFAS) occurring in soil, surface water, and groundwater near sites contaminated with aqueous film-forming foam (AFFF), has proven to be recalcitrant to many destructive remedies, including chemical oxidation. We investigated the potential to utilize microbially mediated reduction (bioreduction) to degrade PFOS and other PFAS through addition of a known dehalogenating culture, WBC-2, to soil obtained from an AFFF-contaminated site. A substantial decrease in total mass of PFOS (soil and water) was observed in microcosms amended with WBC-2 and chlorinated volatile organic compound (cVOC) co-contaminants - 46.4 ± 11.0 % removal of PFOS over the 45-day experiment. In contrast, perfluorooctanoate (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS) concentrations did not decrease in the same microcosms. The low or non-detectable concentrations of potential metabolites in full PFAS analyses, including after application of the total oxidizable precursor assay, indicated that defluorination occurred to non-fluorinated compounds or ultrashort-chain PFAS. Nevertheless, additional research on the metabolites and degradation pathways is needed. Population abundances of known dehalorespirers did not change with PFOS removal during the experiment, making their association with PFOS removal unclear. An increased abundance of sulfate reducers in the genus Desulfosporosinus (Firmicutes) and Sulfurospirillum (Campilobacterota) was observed with PFOS removal, most likely linked to initiation of biodegradation by desulfonation. These results have important implications for development of in situ bioremediation methods for PFAS and advancing knowledge of natural attenuation processes.

The effect of Tween 80 on monochlorobenzene migration in bentonite

Several studies conducted at industrial sites have documented the infiltration of dense non-aqueous phase liquids (DNAPLs) into clay layers, a phenomenon potentially influenced by the coexistence of chemicals like surfactants in some common pollutants. Bentonite (Ben), monochlorobenzene (MCB), and Tween 80 (T80) were selected as reference components to investigate the influences of nonionic surfactants on DNAPLs migration in clays. Results showed that T80 promotes MCB dissolution and encourages MCB adsorption on Ben. This process reduces the hydrophilicity of Ben, resulting in water loss and shrinkage, which creates cracks and facilitates the migration of MCB within the clay. Tw80 notably enhances MCB solubility, as indicated by a molar solubilization ratio of 7.80. The MCB adsorption on Ben (QMCB) displays a linear increase with raising the T80 adsorption on Ben (QT80), especially when QT80 are below the thresholds, e.g., 408.4 mg/g at pH 3 and 339.3 mg/g at pH 7; however, QMCB is decreased with increasing adsorbed T80 further. The average fracture ratio, crack length, and crack width of cracked samples in the cracking experiments were 0.794%, 11.29 mm, and 0.209 mm, respectively. The findings here contribute to understanding the role of surfactants in VOC transport in contaminated sites.

Estimate of the C-Cl photoionization cross section and absolute photoionization cross sections of chlorinated organic compounds

Chlorinated organic compounds are prominently used for industrial production, but their vapors and emission byproducts can cause detrimental effects to human health and the environment. To accurately quantify organochlorine compounds, the absolute photoionization cross section of tetrachloroethylene, chlorobenzene, 1,2-dichlorobenzene, and chloroacetone are measured using multiplexed synchrotron photoionization mass spectrometry at the Advanced Light Source at Lawrence Berkeley National Laboratory. These measurements allow for the estimation of the C-Cl photoionization cross section, increasing quantification accuracy of chlorinated emissions for kinetic modeling and pollutant mitigation. CBS-QB3 calculations of adiabatic ionization energies and thermochemical appearance energies are also presented and agree well with the experimental results.

Reduction characteristic of chlorobenzene by a newly isolated Paenarthrobacter ureafaciens LY from a pharmaceutical wastewater treatment plant

A highly efficient chlorobenzene-degrading strain was isolated from the sludge of a sewage treatment plant associated with a pharmaceutical company. The strain exhibited a similarity of over 99.9% with multiple strains of Paenarthrobacter ureafaciens. Therefore, the strain was suggested to be P. ureafaciens LY. This novel strain exhibited a broad spectrum of pollutant degradation capabilities, effectively degrading chlorobenzene and other organic pollutants, such as 1, 2, 4-trichlorobenzene, phenol, and xylene. Moreover, P. ureafaciens LY co-metabolized mixtures of chlorobenzene with 1, 2, 4-trichlorobenzene or phenol. Evaluation of its degradation efficiency showed that it achieved an impressive degradation rate of 94.78% for chlorobenzene within 8 h. The Haldane-Andrews model was used to describe the growth of P. ureafaciens LY under specific pollutants and its concentrations, revealing a maximum specific growth rate (μmax ) of 0.33 h-1 . The isolation and characterization of P. ureafaciens LY, along with its ability to degrade chlorobenzene, provides valuable insights for the development of efficient and eco-friendly approaches to mitigate chlorobenzene contamination. Additionally, investigation of the degradation performance of the strain in the presence of other pollutants offers important information for understanding the complexities of co-metabolism in mixed-pollutant environments.

Assessing HCH isomer uptake in Alnus glutinosa: implications for phytoremediation and microbial response

Although the pesticide hexachlorocyclohexane (HCH) and its isomers have long been banned, their presence in the environment is still reported worldwide. In this study, we investigated the bioaccumulation potential of α, β, and δ hexachlorocyclohexane (HCH) isomers in black alder saplings (Alnus glutinosa) to assess their environmental impact. Each isomer, at a concentration of 50 mg/kg, was individually mixed with soil, and triplicate setups, including a control without HCH, were monitored for three months with access to water. Gas chromatography-mass spectrometry revealed the highest concentrations of HCH isomers in roots, decreasing towards branches and leaves, with δ-HCH exhibiting the highest uptake (roots-14.7 µg/g, trunk-7.2 µg/g, branches-1.53 µg/g, leaves-1.88 µg/g). Interestingly, α-HCH was detected in high concentrations in β-HCH polluted soil. Phytohormone analysis indicated altered cytokinin, jasmonate, abscisate, and gibberellin levels in A. glutinosa in response to HCH contamination. In addition, amplicon 16S rRNA sequencing was used to study the rhizosphere and soil microbial community. While rhizosphere microbial populations were generally similar in all HCH isomer samples, Pseudomonas spp. decreased across all HCH-amended samples, and Tomentella dominated in β-HCH and control rhizosphere samples but was lowest in δ-HCH samples.

Current status and future challenges of chlorobenzenes pollution in soil and groundwater (CBsPSG) in the twenty-first century: a bibliometric analysis

The global industrial structure had undertaken significant changes since the twenty-first century, making a severe problem of chlorobenzene pollution in soil and groundwater (CBsPSG). CBsPSG receives increasing attention due to the high toxicity, persistence, and bioaccumulation of chlorobenzenes. To date, despite the gravity of this issue, no bibliometric analysis (BA) of CBsPSG does exist. This study fills up the gap by conducting a BA of 395 articles related to CBsPSG from the Web of Science Core Collection database using CiteSpace. Based on a comprehensive analysis of various aspects, including time-related, related disciplines, keywords, journal contribution, author productivity, and institute and country distribution, the status, development, and hotspots of research in the field were shown visually and statistically. Moreover, this study has also delved into the environmental behavior and remediation techniques of CBsPSG. In addition, four challenges (unequal research development, insufficient cooperation, deeply mechanism research, and developing new technologies) have been identified, and corresponding suggestions have been proposed for the future development of research in the field. Afterwards, the limitations of BA were discussed. This work provides a powerful insight into CBsPSG, enabling to quickly identify the hotspot and direction of future studies by relevant researchers.

Complex behavior of petroleum hydrocarbons in vadose zone: A holistic analysis using unsaturated soil columns

The migration of petroleum hydrocarbons in vadose zone involves complex coupled processes such as downward displacement and natural attenuation. Despite its significance in determining groundwater vulnerability to petroleum contamination and optimizing the remedial strategy, it has not been comprehensively studied in terms of overall processes under field-relevant conditions. In this study, a series of unsaturated soil column experiments were conducted by simulating subsurface diesel contamination within a vadose zone using different soil textures at different soil bulk densities and initial diesel concentrations, while partly exposing them to simulated precipitation. The results showed that the soil column with less fine fraction was favorable for the downward migration of diesel but unfavorable for its natural degradation. However, precipitation complicated the relative conductivities of multiple fluids (water, air, and diesel) through the pore network, therby decreasing diesel migration and degradation. For example, the downward migration of diesel in the SL column decreased by 8.4% under precipitation, while the overall attenuation rate dropped to almost 0.24% of its original state. Lowering bulk density (from 1.5 to 1.23 g/cm3), however, could enhance the attenuation rate presumably due to the secured void space for the incoming fluids. A high initial concentration of diesel (2%; w/w) inhibited its natural attenuation, while its influence on its vertical propagation after the precipitation was not significant. The present findings provide a mechanistic basis for approximating the behavior of petroleum hydrocarbons in a random vadose zone.

Response relationship of environmental factors caused by toluene concentration during leaching of capillary zone

Due to the leaching of capillary water, the petroleum pollutants initially trapped in vadose zone may migrate to lower aquifer, thus increasing the risk of groundwater pollution. In order to explore the effect of capillary leaching on toluene-contaminated soil and the relationship between toluene concentration (TC) and environmental factors (EFs) during the leaching process, the sterilized and non-sterilized soil column experiments were designed. The EFs were used to estimate TC. The results showed that the difference between leaching and volatilization rates directly determined the changing trend of toluene concentration in capillary water. The toluene concentration in the medium always showed decreasing trend due to leaching. The indigenous microbial community structure of the non-sterilized soil column was analyzed by 16S rRNA sequencing. It was found that indigenous microorganisms could degrade toluene after 33.0 days of acclimatation. The microbial population was dominated by bacteria, among them the Ellin6055 strain and Pseudomonas, Pseudoxanthomonas, Cupriavidus, Bdellovibrio, Sphingobium, Phenylobacterium, Ramlibacter, Bradyrhizobium, Shinella genera. The Pseudomonas was the most crucial bacterial genus that degraded toluene. Indigenous microbial degradation was the fundamental reason for strong response relationship. Furthermore, we suggested a relationship of function between environmental factors (pH, DO, ORP) and time (t) for toluene attenuation: C₀+ln(e^Atα^Bγ^Cβ)=C_Toluene, (α, β, γ represent the pH, DO, and ORP in leaching capillary water, respectively; A, B, and C represent undetermined coefficients), and the fitting coefficient R² > 0.950. This relationship can only characterize the attenuation process of capillary zone leaching on toluene. However, it may still be utilized to give a theoretical foundation for understanding the dynamic of pollutant concentration change processes under specific environmental factors.

Effect of site hydrological conditions and soil aggregate sizes on the stabilization of heavy metals (Cu, Ni, Pb, Zn) by biochar

Biochar is a popular material that would effectively immobilize heavy metals in soil, which can greatly decrease the health risk of heavy metals. Although many previous studies have studied the immobilization of heavy metals by biochar, the influence of hydrological conditions on the immobilization effect is still not clear. This paper carried out column experiments to study the effect of fluctuating groundwater table on Cu, Ni, Pb, Zn distribution and speciation with the addition of biochar from pyrolysis of swine manure. Experimental results showed that biochar could significantly decrease the leaching toxicity of Cu and Ni by 24.4% and 44.7% respectively, while the immobilization effect of Pb and Zn was relatively insignificant. The average reduction percentage of bioavailable Cu was 14.5%, 39.5% and 33.3% in the unsaturated zone, fluctuating zone and saturated zone respectively, showing the better immobilization effect in the fluctuating zone and saturated zone. The residual fraction of heavy metals increased significantly after the addition of biochar, and the increase of residual fraction was larger in small soil aggregates. This study helped illustrate the influence of hydrological conditions and soil aggregate sizes on the stabilization effect of heavy metals by biochar, which could be used to guide the remediation process of sites contaminated by heavy metals.

Multi-element isotopic evidence for monochlorobenzene and benzene degradation under anaerobic conditions in contaminated sediments

Industrial chemicals are frequently detected in sediments due to a legacy of chemical spills. Globally, site remedies for groundwater and sediment decontamination include natural attenuation by in situ abiotic and biotic processes. Compound-specific isotope analysis (CSIA) is a diagnostic tool to identify, quantify, and characterize degradation processes in situ, and in some cases can differentiate between abiotic degradation and biodegradation. This study reports high-resolution carbon, chlorine, and hydrogen stable isotope profiles for monochlorobenzene (MCB), and carbon and hydrogen stable isotope profiles for benzene, coupled with measurements of pore water concentrations in contaminated sediments. Multi-element isotopic analysis of δ¹³C and δ³⁷Cl for MCB were used to generate dual-isotope plots, which for 2 locations at the study site resulted in Λ_C/Cl(130) values of 1.42 ± 0.19 and Λ_C/Cl(131) values of 1.70 ± 0.15, consistent with theoretical calculations for carbon-chlorine bond cleavage (Λ_T = 1.80 ± 0.31) via microbial reductive dechlorination. For benzene, significant δ²H (122‰) and δ¹³C (6‰) depletion trends, followed by enrichment trends in δ¹³C (1.6‰) in the upper part of the sediment, were observed at the same location, indicating not only production of benzene due to biodegradation of MCB, but subsequent biotransformation of benzene itself to nontoxic end-products. Degradation rate constants calculated independently using chlorine isotopic data and carbon isotopic data, respectively, agreed within uncertainty thus providing multiple lines of evidence for in situ contaminant degradation via reductive dechlorination and providing the foundation for a novel approach to determine site-specific in situ rate estimates essential for the prediction of remediation outcomes and timelines.

The role of soils in the disposition, sequestration and decontamination of environmental contaminants

Soil serves as both a 'source' and 'sink' for contaminants. As a source, contaminants are derived from both 'geogenic' and 'anthropogenic' origins. Typically, while some of the inorganic contaminants including potentially toxic elements are derived from geogenic origin (e.g. arsenic and selenium) through weathering of parent materials, the majority of organic (e.g. pesticides and microplastics) as well as inorganic (e.g. lead, cadmium) contaminants are derived from anthropogenic origin. As a sink, soil plays a critical role in the transformation of these contaminants and their subsequent transfer to environmental compartments, including groundwater (e.g. pesticides), surface water (phosphate and nitrate), ocean (e.g. microplastics) and atmosphere (e.g. nitrous oxide emission). A complex transformation process of contaminants in soil involving adsorption, precipitation, redox reactions and biodegradation control the mobility, bioavailability and environmental toxicity of these contaminants. Soil also plays a major role in the decontamination of contaminants, and the 'cleaning' action of soil is controlled primarily by the physico-chemical interactions of contaminants with various soil components, and the biochemical transformations facilitated by soil microorganisms. In this article, we examine the geogenic and anthropogenic sources of contaminants reaching the soil, and discuss the role of soil in the sequestration and decontamination of contaminants in relation to various physico-chemical and microbial transformation reactions of contaminants with various soil components. Finally, we propose future actions that would help to maintain the role of soils in protecting the environment from contaminants and delivering sustainable development goals. This article is part of the theme issue 'The role of soils in delivering Nature's Contributions to People'.

Soil redox dynamics under dynamic hydrologic regimes - A review

Electron transfer (redox) reactions, mediated by soil microbiota, modulate elemental cycling and, in part, establish the redox poise of soil systems. Understanding soil redox processes significantly improves our ability to characterize coupled biogeochemical cycling in soils and aids in soil health management. Redox-sensitive species exhibit different reactivity, mobility, and toxicity subjected to their redox state. Thus, it is crucial to quantify the redox potential (E_h) in soils and to characterize the dominant redox couples therein. Several, often coupled, external drivers, can influence E_h. Among these factors, soil hydrology dominates. It controls soil physical properties that in turn further regulates E_h. Soil spatial heterogeneity and temporally dynamic hydrologic regimes yield complex distributions of E_h. Soil redox processes have been studied under various environmental conditions, including relatively static and dynamic hydrologic regimes. Our focus here is on dynamic, variably water-saturated environments. Herein, we review previous studies on soil redox dynamics, with a specific focus on dynamic hydrologic regimes, provide recommendations on knowledge gaps, and targeted future research needs and directions. We review (1) the role of soil redox conditions on the soil chemical-species cycling of organic carbon, nitrogen, phosphorus, redox-active metals, and organic contaminants; (2) interactions between microbial activity and redox state in the near-surface and deep subsurface soil, and biomolecular methods to reveal the role of microbes in the redox processes; (3) the effects of dynamic hydrologic regimes on chemical-species cycling and microbial dynamics; (4) the experimental setups for mimicking different hydrologic regimes at both laboratory and field scales. Finally, we identify the current knowledge gaps related to the study of soil redox dynamics under different hydrologic regimes: (1) fluctuating conditions in the deep subsurface; (2) the use of biomolecular tools to understand soil biogeochemical processes beyond nitrogen; (3) limited current field measurements and potential alternative experimental setups.

Vertical distribution of microbial communities and their response to metal(loid)s along the vadose zone-aquifer sediments

AIMS

This study attempted to demonstrate the vertical shift in bacterial, archaeal and fungal communities along the vadose zone-aquifer sediments and their respective responses to environmental factors.

METHODS AND RESULTS

We collected samples from the vadose zone and three aquifer sediments along a 42·5 m bore of a typical agricultural land. The results showed that the bacterial community shifted greatly with depth. The classes of Actinobacteria (19·5%) and NC10 (11·0%) were abundant in the vadose zone while Alphaproteobacteria (22·3%) and Gammaproteobacteria (20·1%) were enriched in the aquifer. Archaeal and fungal communities were relatively more homogeneous with no significant trend as a function of depth. Process analyses further indicated that selection dominated in the bacterial community, whereas stochastic processes governed archaeal and fungal communities. Moreover environment-bacteria interaction analysis showed that metal(loid)s, especially alkali metal, had a closer correlation with the bacterial community than physicochemical variables.

CONCLUSIONS

Depth strongly affected bacterial rather than archaeal and fungal communities. Metal(loid)s prevailed over physicochemical variables in shaping the bacterial community in the vadose zone-aquifer continuum.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our study provides a new perspective on the structure of microbial communities from the vadose zone to the deep aquifers.

Radiolytic degradation of chlorobenzene in aerated and deoxygenated aqueous solutions

Radiation-induced degradation of chlorobenzene was conducted at 0.1, 0.4, 0.5, 0.7, and 1.0 mmol/dm³ concentrations in aerated environment and at 1.0 mmol/dm³ in oxygen-free and N₂O-saturated solutions. The results demonstrated that the elimination of chloride is important when the solution is oxygen free, because the [Formula: see text] attacks at the ipso position of the chloro group produces hydrochloric acid. The degradation was affected to a large extent by the concentration and to a lesser extent by the presence or absence of oxygen in the solutions which were irradiated. Thereby, the degradation occurred faster in the solutions with air and without oxygen and more slowly in the saturated solution with N₂O. Some by-products were identified using an HPLC-UV-mass system. In addition, it was found that there is a linear correlation between the ln C/C₀ and the dose, indicating that the radiolytic degradation followed pseudo-first-order reaction kinetics. The radiolytic oxidation was followed by the chemical oxygen demand (COD) test. The COD decreases when the solute concentration increases. The COD results were for a 0.47 mmol/dm³ of 5.94 mg O₂ dm^-3 kGy^-1 and for 0.09 mmol/dm³ of 7.45 mg O₂ dm^-3 kGy^-1.

Pd NP-Loaded and Covalently Cross-Linked COF Membrane Microreactor for Aqueous CBs Dechlorination at Room Temperature

We report an allyl-decorated and hydrazine-connected covalent organic framework (COF-AO, 1), which could support Pd nanoparticles (Pd NPs) to generate Pd@COF-AO, 2. The incorporation of 2 with thiol-functionalized polysiloxane (termed as PSI-SH) via thiol-ene click reaction provided the stand-alone and elastic membrane (3). The obtained COF-involved and Pd NP-loaded covalently linked membrane of 3 is robust, permanently porous, uniform, processable, and water permeable. Moreover, it can be used to construct highly efficient membrane-based microreactor for continuous-flow operation to catalyze chlorobenzenes (CBs) dechlorination in water at room temperature. The provided approach herein allows the processability and practical application of the powdered COF materials to be feasible.

Phytoremediation removal rates of benzene, toluene, and chlorobenzene

Phytoremediation is a sustainable remedial approach, although performance efficacy is rarely reported. In this study, we assessed a phytoremediation plot treating benzene, toluene, and chlorobenzene. A comparison of the calculated phytoremediation removal rate with estimates of onsite contaminant mass was used to forecast cleanup periods. The investigation demonstrated that substantial microbial degradation was occurring in the subsurface. Estimates of transpiration indicated that the trees planted were removing approximately 240,000 L of water per year. This large quantity of water removal implies substantial removal of contaminant due to large amounts of contaminants in the groundwater; however, these contaminants extensively sorb to the soil, resulting in large quantities of contaminant mass in the subsurface. The total estimate of subsurface contaminant mass was also complicated by the presence of non-aqueous phase liquids (NAPL), additional contaminant masses that were difficult to quantify. These uncertainties of initial contaminant mass at the site result in large uncertainty in the cleanup period, although mean estimates are on the order of decades. Collectively, the model indicates contaminant removal rates on the order of 10^-2-10⁰ kg/tree/year. The benefit of the phytoremediation system is relatively sustainable cleanup over the long periods necessary due to the presence of NAPL.

37Cl-compound specific isotope analysis and assessment of functional genes for monitoring monochlorobenzene (MCB) biodegradation under aerobic conditions

A laboratory approach was adopted in this study to explore the potential of ³⁷Cl-CSIA in combination with ¹³C-CSIA and Biological Molecular Tools (BMTs) to estimate the occurrence of monochloroenzene (MCB) aerobic biodegradation. A new analytical method for ³⁷Cl-CSIA of MCB was developed in this study. This methodology using a GC-IRMS allowed to determine δ³⁷Cl values within an internal error of ±0.3‰. Samples from a heavily MCB contaminated site were collected and MCB aerobic biodegradation microcosms with indigenous cultures in natural and enhanced conditions were set up. The microcosms data show a negligible fractionation for ¹³C associated to MCB mass decrease of >95% over the incubation time. Conversely, an enrichment factor of -0.6±0.1‰ was estimated for ³⁷Cl, which is a reflection of a secondary isotope effect. Moreover, the dual isotope approach showed a pattern for aerobic degradation which differ from the theoretical trend for reductive dehalogenation. Quantitative Polymerase Chain Reaction (qPCR) results showed a significant increase in todC gene copy number with respect to its initial levels for both natural attenuation and biostimulated microcosms, suggesting its involvement in the MCB aerobic degradation, whereas phe gene copy number increased only in the biostimulated ones. Indeed, ³⁷Cl fractionation in combination with the dual carbon‑chlorine isotope approach and the todC gene copy number represent valuable indicators for a qualitative assessment of MCB aerobic biodegradation in the field.

Natural Attenuation and Anaerobic Benzene Detoxification Processes at a Chlorobenzene-Contaminated Industrial Site Inferred from Field Investigations and Microcosm Studies

A five-year site investigation was conducted at a former chemical plant in Nanjing, China. The main contaminants were 1,2,4-trichlorobenzene (TCB) reaching concentrations up to 7300 μg/L, dichlorobenzene (DCB) isomers, monochlorobenzene (MCB), and benzene. Over time, these contaminants naturally attenuated to below regulatory levels under anaerobic conditions. To confirm the transformation processes and to explore the mechanisms, a corresponding laboratory microcosm study was completed demonstrating that 1,2,4-TCB was dechlorinated to 1,2-DCB, 1,3-DCB, and 1,4-DCB in approximately 2%/10%/88% molar proportions. The DCB isomers were dechlorinated via MCB to benzene, and, finally, benzene was degraded under prevailing sulfate-reducing conditions. Dechlorination could not be attributed to known dechlorinators Dehalobacter or Dehalococcoides, while anaerobic benzene degradation was mediated by microbes affiliated to a Deltaproteobacterium ORM2, previously associated with this activity. Unidentified organic compounds, possibly aromatic compounds related to past on-site production processes, were fueling the dechlorination reactions in situ. The microcosm study confirmed transformation processes inferred from field data and provided needed assurance for natural attenuation. Activity-based microcosm studies are often omitted from site characterization in favor of rapid and less expensive molecular surveys. However, the value of microcosm studies for confirming transformation processes, establishing electron balances, assessing cocontaminant inhibition, and validating appropriate monitoring tools is clear. At complex sites impacted by multiple compounds with poorly characterized transformation mechanisms, activity assays provide valuable data to incorporate into the conceptual site model to most effectively inform remediation alternatives.

Bubble-Facilitated VOC Transport from LNAPL Smear Zones and Its Potential Effect on Vapor Intrusion

Most conceptual and mathematical models of soil vapor intrusion assume that the transport of volatile organic compounds (VOCs) from a source toward a building is limited by diffusion through the soil gas. Under conditions where advection occurs, transport rates are higher and can lead to higher indoor air concentrations. Advection-dominated conditions can be created by gas bubble flow in the saturated zone. A series of laboratory column experiments were conducted to measure mass flux due to bubble-facilitated VOC transport from light nonaqueous phase liquid (LNAPL) smear zones. Smear zones that contained both LNAPL residual and trapped gas, as well as those that contained only LNAPL residual, were investigated. Results showed that the VOC mass flux due to bubble-facilitated transport was orders-of-magnitude higher than under diffusion-limited conditions. Results also showed that the mass flux due to bubble-facilitated transport was intermittent, and increased with an increased supply of dissolved gases.

A feasible approach to dispose of soil washing wastes: adsorptive removal of chlorobenzene compounds in aqueous solutions using humic acid modified with monoolein (HA–M)

Humic acid (HA), one of most ubiquitous natural resources in the world, was modified with monoolein to prepare a novel hydrophobic adsorbent (HA–M) for removal of chlorobenzene compounds (CBs) from aqueous solutions.

Sediment Monitored Natural Recovery Evidenced by Compound Specific Isotope Analysis and High-Resolution Pore Water Sampling

Monitoring natural recovery of contaminated sediments requires the use of techniques that can provide definitive evidence of in situ contaminant degradation. In this study, a passive diffusion sampler, called "peeper", was combined with Compound Specific Isotope Analysis to determine benzene and monochlorobenzene (MCB) stable carbon isotope values at a fine vertical resolution (3 cm) across the sediment water interface at a contaminated site. Results indicated significant decrease in concentrations of MCB from the bottom to the top layers of the sediment over 25 cm, and a 3.5 ‰ enrichment in δ¹³C values of MCB over that distance. Benzene was always at lower concentrations than MCB, with consistently more depleted δ¹³C values than MCB. The redox conditions were dominated by iron reduction along most of the sediment profile. These results provide multiple lines of evidence for in situ reductive dechlorination of MCB to benzene. Stable isotope analysis of contaminants in pore water is a valuable method to demonstrate in situ natural recovery of contaminated sediments. This novel high-resolution approach is critical to deciphering the combined effects of parent contaminant (e.g., MCB) degradation and both production and simultaneous degradation of daughter products, especially benzene.

Natural Attenuation of Nonvolatile Contaminants in the Capillary Fringe

When anoxic polluted groundwater encounters the overlying vadose zone an oxic/anoxic interface is created, often near the capillary fringe. Biodegradation of volatile contaminants in the capillary fringe can prevent vapor migration. In contrast, the biodegradation of nonvolatile contaminants in the vadose zone has received comparatively little attention. Nonvolatile compounds do not cause vapor intrusion, but they still move with the groundwater and are major contaminants. Aniline (AN) and diphenylamine (DPA) are examples of toxic nonvolatile contaminants found often at dye and munitions manufacturing sites. In this study, we tested the hypothesis that bacteria can aerobically biodegrade AN and DPA in the capillary fringe and decrease the contaminant concentrations in the anoxic plume beneath the vadose zone. Laboratory multiport columns that represented the unsaturated zone were used to evaluate degradation of AN or DPA in contaminated water. The biodegradation fluxes of the contaminants were estimated to be 113 ± 26 mg AN·m(-2)·h(-1) and 76 ± 18 mg DPA·m(-2)·h(-1) in the presence of bacteria known to degrade AN and DPA. Oxygen and contaminant profiles along with enumeration of bacterial populations indicated that most of the biodegradation took place within the lower part of the capillary fringe. The results indicate that bacteria capable of contaminant biodegradation in the capillary fringe can create a sink for nonvolatile contaminants.

Effects of Rate-Limited Mass Transfer on Modeling Vapor Intrusion with Aerobic Biodegradation

Most of the models for simulating vapor intrusion accept the local equilibrium assumption for multiphase concentration distributions, that is, concentrations in solid, liquid and vapor phases are in equilibrium. For simulating vapor transport with aerobic biodegradation controlled by counter-diffusion processes, the local equilibrium assumption combined with dual-Monod kinetics and biomass decay may yield near-instantaneous behavior at steady state. The present research investigates how predicted concentration profiles and fluxes change as interphase mass transfer resistances are increased for vapor intrusion with aerobic biodegradation. Our modeling results indicate that the attenuation coefficients for cases with and without mass transfer limitations can be significantly different by orders of magnitude. Rate-limited mass transfer may lead to larger overlaps of contaminant vapor and oxygen concentrations, which cannot be simulated by instantaneous reaction models with local equilibrium mass transfer. In addition, the contaminant flux with rate-limited mass transfer is much smaller than that with local equilibrium mass transfer, indicating that local equilibrium mass transfer assumption may significantly overestimate the biodegradation rate and capacity for mitigating vapor intrusion through the unsaturated zone. Our results indicate a strong research need for field tests to examine the validity of local equilibrium mass transfer, a widely accepted assumption in modeling vapor intrusion.

Biodegradation of vapor-phase toluene in unsaturated porous media: Column experiments

Biodegradation of organic chemicals in the vapor phase of soils and vertical flow filters has gained attention as promising approach to clean up volatile organic compounds (VOC). The drivers of VOC biodegradation in unsaturated systems however still remain poorly understood. Here, we analyzed the processes controlling aerobic VOC biodegradation in a laboratory setup mimicking the unsaturated zone above a shallow aquifer. The setup allowed for diffusive vapor-phase transport and biodegradation of three VOC: non-deuterated and deuterated toluene as two compounds of highly differing biodegradability but (nearly) identical physical and chemical properties, and MTBE as (at the applied experimental conditions) non-biodegradable tracer and internal control. Our results showed for toluene an effective microbial degradation within centimeter VOC transport distances despite high gas-phase diffusivity. Degradation rates were controlled by the reactivity of the compounds while oxic conditions were found everywhere in the system. This confirms hypotheses that vadose zone biodegradation rates can be extremely high and are able to prevent the outgassing of VOC to the atmosphere within a centimeter range if compound properties and site conditions allow for sufficiently high degradation rates.

Identification of Anaerobic Aniline-Degrading Bacteria at a Contaminated Industrial Site

Anaerobic aniline biodegradation was investigated under different electron-accepting conditions using contaminated canal and groundwater aquifer sediments from an industrial site. Aniline loss was observed in nitrate- and sulfate-amended microcosms and in microcosms established to promote methanogenic conditions. Lag times of 37 days (sulfate amended) to more than 100 days (methanogenic) were observed prior to activity. Time-series DNA-stable isotope probing (SIP) was used to identify bacteria that incorporated (13)C-labeled aniline in the microcosms established to promote methanogenic conditions. In microcosms from heavily contaminated aquifer sediments, a phylotype with 92.7% sequence similarity to Ignavibacterium album was identified as a dominant aniline degrader as indicated by incorporation of (13)C-aniline into its DNA. In microcosms from contaminated canal sediments, a bacterial phylotype within the family Anaerolineaceae, but without a match to any known genus, demonstrated the assimilation of (13)C-aniline. Acidovorax spp. were also identified as putative aniline degraders in both of these two treatments, indicating that these species were present and active in both the canal and aquifer sediments. There were multiple bacterial phylotypes associated with anaerobic degradation of aniline at this complex industrial site, which suggests that anaerobic transformation of aniline is an important process at the site. Furthermore, the aniline degrading phylotypes identified in the current study are not related to any known aniline-degrading bacteria. The identification of novel putative aniline degraders expands current knowledge regarding the potential fate of aniline under anaerobic conditions.

Modeling aerobic biodegradation in the capillary fringe

Vapor intrusion from volatile subsurface contaminants can be mitigated by aerobic biodegradation. Laboratory column studies with contaminant sources of chlorobenzene and a mixture of chlorobenzene, 1,2-dichlorobenzene, and 1,4-dichlorobenzene showed that contaminants were rapidly degraded in thin reactive zones with high biomass and low substrate concentrations in the vicinity of the capillary fringe. Such behavior was well characterized by a model that includes oxygen-, substrate-, and biomass-dependent biodegradation kinetics along with diffusive transport processes. An analytical solution was derived to provide theoretical support for the simplification of reaction kinetics and the approximation of reactive zone location and mass flux relationships at steady state. Results demonstrate the potential of aerobic natural attenuation in the capillary fringe for preventing contaminant migration in the unsaturated zone. The solution indicates that increasing contaminant mass flux into the column creates a thinner reactive zone and pushes it toward the oxygen boundary, resulting in a shorter distance to the oxygen source and a larger oxygen mass flux that balances the contaminant mass flux. As a consequence, the aerobic biodegradation can reduce high contaminant concentrations to low levels within the capillary fringe and unsaturated zone. The results are consistent with the observations of thin reactive layers at the interface in unsaturated zones. The model considers biomass while including biodegradation in the capillary fringe and unsaturated zone and clearly demonstrates that microbial communities capable of using the contaminants as electron donors may lead to instantaneous degradation kinetics in the capillary fringe and unsaturated zone.

Biodegradation of cis-dichloroethene and vinyl chloride in the capillary fringe

Volatile chlorinated compounds are major pollutants in groundwater, and they pose a risk of vapor intrusion into buildings. Vapor intrusion can be prevented by natural attenuation in the vadose zone if biodegradation mechanisms can be established. In this study, we tested the hypothesis that bacteria can use cis-dichloroethene (cis-DCE) or vinyl chloride (VC) as an electron donor in the vadose zone. Anoxic water containing cis-DCE or VC was pumped continuously beneath laboratory columns that represented the vadose zone. Columns were inoculated with Polaromonas sp. strain JS666, which grows aerobically on cis-DCE, or with Mycobacterium sp. JS60 and Nocardiodes sp. JS614 that grow on VC. Complete biodegradation with fluxes of 84 ± 15 μmol of cis-DCE · m(-2) · hr(-1) and 218 ± 25 μmole VC·m(-2) · h(-1) within the 23 cm column indicated that microbial activities can prevent the migration of cis-DCE and VC vapors. Oxygen and volatile compound profiles along with enumeration of bacterial populations indicated that most of the biodegradation took place in the first 10 cm above the saturated zone within the capillary fringe. The results revealed that cis-DCE and VC can be biodegraded readily at the oxic/anoxic interfaces in the vadose zone if appropriate microbes are present.

Diffusion sampler for compound specific carbon isotope analysis of dissolved hydrocarbon contaminants

Compound Specific Isotope Analysis (CSIA) is widely utilized to study the fate of organic contaminants in groundwater. To date, however, no method is available to obtain CSIA samples at a fine (cm) spatial scale across the sediment-surface water interface (SWI), a key boundary for discharge of contaminated groundwater to surface water. Dissolved contaminants in such discharged zones undergo rapid temporal and spatial changes due to heterogeneity in redox conditions and microbial populations. The compatibility of a passive sediment pore water sampler ("peeper") to collect 40 mL samples for CSIA of benzene, toluene, monochlorobenzene, and 1,2-dichlorobenzene at field-relevant concentrations (0.1-5 mg L(-1)) was evaluated in laboratory experiments. Results demonstrate that physical diffusion across the polysulfone membrane does not alter the carbon isotope values (±0.5‰). Measured δ(13)C values also remain invariant despite significant adsorption of the compounds on the peeper material, an effect which increased with higher numbers of chlorine atoms and sorption coefficient (Koc) values. In addition, isotope equilibrium between the peeper chamber and the sediment pore water occurred in less than a day, indicating the peeper method can be used to provide samples for CSIA analysis at fine spatial and temporal sampling resolutions in contaminated sediments.

In situ vadose zone bioremediation

Contamination of the vadose zone with various pollutants is a world-wide problem, and often technical or economic constraints impose remediation without excavation. In situ bioremediation in the vadose zone by bioventing has become a standard remediation technology for light spilled petroleum products. In this review, focus is given on new in situ bioremediation strategies in the vadose zone targeting a variety of other pollutants such as perchlorate, nitrate, uranium, chromium, halogenated solvents, explosives and pesticides. The techniques for biostimulation of either oxidative or reductive degradation pathways are presented, and biotransformations to immobile pollutants are discussed in cases of non-degradable pollutants. Furthermore, research on natural attenuation in the vadose zone is presented.