Effective distribution of emulsified edible oil for enhanced anaerobic bioremediation

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.

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.

Mechanisms of biostimulant-enhanced biodegradation of PAHs and BTEX mixed contaminants in soil by native microbial consortium

Despite the co-occurrence of polycyclic aromatic hydrocarbons (PAHs) and benzene, toluene, ethylbenzene, and xylene (BTEX) in the field, to date, knowledge on the bioremediation of benzene and benzo[a]pyrene (BaP) mixed contaminants is limited. In this study, the mechanisms underlying the biodegradation of benzene and BaP under individual and co-contaminated conditions followed by the enhanced biodegradation using methanol, ethanol, and vegetable oil as biostimulants were investigated. The results demonstrated that the benzene biodegradation was highly reduced under the co-contaminated condition compared to the individual benzene contamination, whereas the BaP biodegradation was slightly enhanced with the co-contamination of benzene. Moreover, biostimulation significantly improved the biodegradation of both contaminants under co-contaminated conditions. A trend of significant reduction in the bioavailable BaP contents was observed in all biostimulant-enhanced groups, implying that the bioavailable BaP was the preferred biodegradable BaP fraction. Furthermore, the enzymatic activity analysis revealed a significant increase in lipase and dehydrogenase (DHA) activities, as well as a reduction in the catalase and polyphenol oxidase, suggesting that the increased hydrolysis of fats and proton transfer, as well as the reduced oxidative stress, contributed to the enhanced benzene and BaP biodegradation in the vegetable oil treatment. In addition, the microbial composition analysis results demonstrated that the enriched functional genera contributed to the increased biodegradation efficiency, and the functional genera in the microbial consortium responded differently to different biostimulants, and competitive growth was observed in the biostimulant-enhanced treatments. In addition, the enrichment of Pseudomonas and Rhodococcus species was noticed during the biostimulation of benzene and BaP co-contamination soil, and was positively correlated with the DHA enzyme activities, indicating that these species encode DHA genes which contributed to the higher biodegradation. In conclusion, multiple lines of evidence were provided to shed light on the mechanisms of biostimulant-enhanced biodegradation of PAHs and BTEX co-contamination with native microbial consortiums.

Impacts of co-contaminants and dilution on perchlorate biodegradation using various carbon sources

This research investigates the biodegradation of perchlorate in the presence of the co-contaminants nitrate and chlorate using soluble and slow-release carbon sources. In addition, the impact of bio-augmentation and dilution, which results in lower total dissolved salts (TDS) and contaminant levels, is examined. Laboratory microcosms were conducted using actual groundwater and soils from a contaminated aquifer. The results revealed that both soluble and slow-release carbon sources support biodegradation of contaminants in the sequence nitrate > chlorate > perchlorate. Degradation rates, including and excluding lag times, revealed that the overall impact of the presence of co-contaminants depends on degradation kinetics and the relative concentrations of the contaminants. When the lag time caused by the presence of the co-contaminants is considered, the degradation rates for chlorate and perchlorate were two to three times slower. The results also show that dilution causes lower initial contaminant concentrations, and consequently, slower degradation rates, which is not desirable. On the other hand, the dilution resulting from the injection of amendments to support remediation promotes desirably lower salinity levels. However, the salinity associated with the presence of sulfate does not inhibit biodegradation. The naturally occurring bacteria were able to support the degradation of all contaminants. Bio-augmentation was effective only in diluted microcosms. Proteobacteria and Firmicutes were the dominant phyla identified in the microcosms.

Biotic and Abiotic Biostimulation for the Reduction of Hexavalent Chromium in Contaminated Aquifers

Hexavalent chromium is a carcinogenic heavy metal that needs to be removed effectively from polluted aquifers in order to protect public health and the environment. This work aims to evaluate the reduction of Cr(VI) to Cr(III) in a contaminated aquifer through the stimulation of indigenous microbial communities with the addition of reductive agents. Soil-column experiments were conducted in the absence of oxygen and at hexavalent chromium (Cr(VI)) groundwater concentrations in the 1000–2000 μg/L range. Two carbon sources (molasses and EVO) and one iron electron donor (FeSO4·7H2O) were used as ways to stimulate the metabolism and proliferation of Cr(VI) reducing bacteria in-situ. The obtained results indicate that microbial anaerobic respiration and electron transfer can be fundamental to alleviate polluted groundwater from hazardous Cr(VI). The addition of organic electron donors increased significantly Cr(VI) reduction rates in comparison to natural soil attenuation rates. Furthermore, a combination of organic carbon and iron electron donors led to a longer life span of the remediation process and thus increased total Cr(VI) removal. This is the first study to investigate biotic and abiotic Cr(VI) removal by conducting experiments with natural soil and by applying biostimulation to modify the natural existing microbial communities.

Influence of aquifer heterogeneity on Cr(VI) diffusion and removal from groundwater

Previous studies have indicated aquifer heterogeneity has an important influence on the removal of Cr(VI) in groundwater, but little attention is paid to the effects of aquifer heterogeneity during the process especially under conditions like actual groundwater temperature and hydraulic gradient in the field. Thus, in this study, in situ remediation of Cr(VI)-contaminated shallow groundwater in a sandbox was conducted, and the influences of the heterogeneous aquifer composed of coarse, medium, and fine sand on Cr(VI) diffusion and removal before and after emulsified vegetable oil (EVO) injection were investigated, under the conditions of 19±0.5 °C and hydraulic gradient 3‰. The results show that Cr(VI) diffused consistently with groundwater from top left to bottom right; Cr(VI) spread faster in the horizontal direction than in vertical direction, and the horizontal diffusion of Cr(VI) in coarse, medium, and fine sand was 0.054 m/day, 0.036 m/day, and 0.018 m/day, respectively; a high performance of EVO toward Cr(VI) removal by over 95% was mainly because different concentrations of microorganisms migrated among heterogeneous aquifers vertically and horizontally; compared with coarse and medium sand, fine sand, with a better adsorption capacity and a lower permeability, retained relatively more microorganisms, providing favorable conditions during the remediation; a stable and unified effective removal zone, similar to the shape of Ʃ (approximately 1357.87 cm2), was ultimately formed downstream of the injection well.

Application of Combined In Situ Chemical Reduction and Enhanced Bioremediation to Accelerate TCE Treatment in Groundwater

Groundwater at trichloroethylene (TCE)-contaminated sites lacks electron donors, which prolongs TCE’s natural attenuation process and delays treatment. Although adding electron donors, such as emulsified oil, accelerates TCE degradation, it also causes the accumulation of hazardous metabolites such as dichloroethylene (DCE) and vinyl chloride (VC). This study combined in situ chemical reduction using organo-iron compounds with enhanced in situ bioremediation using emulsified oil to accelerate TCE removal and minimize the accumulation of DCE and VC in groundwater. A self-made soybean oil emulsion (SOE) was used as the electron donor and was added to liquid ferrous lactate (FL), the chemical reductant. The combined in situ chemical reduction and enhanced in situ bioremediation achieved favorable results in a laboratory microcosm test and in an in situ biological field pilot test. Both tests revealed that SOE+FL accelerated TCE degradation and minimized the accumulation of DCE and VC to a greater extent than SOE alone after 160 days of observation. When FL was added in the microcosm test, the pH value decreased from 6.0 to 5.5; however, during the in situ biological pilot test, the on-site groundwater pH value did not exhibit obvious changes. Given the geology of the in situ pilot test site, the SOE+FL solution that was injected underground continued to be released for at least 90 days, suggesting that the solution’s radius of influence was at least 5 m.

Transport and release of electron donors and alkalinity during reductive dechlorination by combined emulsified vegetable oil and colloidal Mg(OH)2: Laboratory sand column and microcosm tests

Emulsified vegetable oil combined with colloidal Mg(OH)₂ (EVO-CM) can slowly release electron donors and OH^- into groundwater and is therefore regarded as a promising amendment for enhanced in situ treatment of chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE). However, its migration ability in different porous media and the simultaneous release of electron donor and pH buffer during enhanced reductive dechlorination (ERD) have never been evaluated in detail. In this study, EVO-CM with uniform drop size and desirable stability was prepared. Laboratory-scale column tests were conducted to investigate the transport and spatial distribution of the EVO-CM in different porous media. Batch microcosm experiments were carried out to study the dechlorination efficiency under different EVO:Mg(OH)₂ ratios. Experimental results indicate that prepared EVO-CM emulsions can be transported effectively through different porous media with grain size ranging from 0.1-1.0 mm. The emulsified vegetable oil (EVO) and colloidal Mg(OH)₂ showed synchronous movement through the porous media column systems, providing both electron donor and alkalinity at the same location. The retention degree of EVO-CM is greater in the finer grained media and decreases with increasing distance from the inlet. The injection of EVO-CM lead to a significant mobilization of TCE in the column. The reductive dechlorination of TCE in the microcosms was remarkably enhanced in the presence of EVO-CM. The coaddition of sufficient colloidal Mg(OH)₂ effectively limits the deleterious pH decline caused by acid release. The microcosm achieves an optimum dechlorination efficiency when the EVO:Mg(OH)₂ ratio is 1:1.

Enhanced reductive dechlorination of trichloroethene in an acidic DNAPL impacted aquifer

A field pilot test was conducted using an emulsified vegetable oil (EVO) and colloidal magnesium hydroxide [Mg(OH)₂] formulation to enhance reductive dechlorination of dense non-aqueous phase liquid (DNAPL) trichloroethene (TCE) in an acidic (pH < 4), heterogeneous aquifer. The field test consisted of i) a single well injection test to evaluate Mg(OH)₂ distribution and ii) installation of two EVO-Mg(OH)₂ permeable reactive barriers (PRBs; PRB-1 & PRB-2) at varying distances downgradient of the DNAPL source area. Distribution of Mg(OH)₂ was observed up to 2.3 m away from the injection point within a permeable coarse sand layer; however, Mg(OH)₂ transport in the overlying clayey-silty sand was minimal. Downgradient of the PRBs, colloidal Mg(OH)₂ increased the pH of the coarse sand to levels appropriate for biological reductive dechlorination (pH >∼5); however, some settling of Mg(OH)₂ in the injection wells generated persistent high pH (∼9-10) within the PRBs. A redesigned suspension of colloidal Mg(OH)₂ was tested and proved to be more effective at raising aquifer pH without an excessive rise in pH within the PRBs. At PRB-1 (located closest to the DNAPL source area), limited TCE biodegradation was observed due to the influx of high TCE concentrations (up to 400 mg/L) and inhibition of dechlorinating bacteria. At PRB-2 (located 25 m downgradient of the DNAPL source area), TCE concentrations were much lower (13-26 mg/L) and production of cis-1,2-dichloroethene (cDCE) and some vinyl chloride (VC) was observed. Subsequent bioaugmentation with a commercial dechlorinating culture at PRB-2 improved conversion of cDCE to VC and ethene at downgradient monitoring wells over the duration of the study. These results emphasize the importance of PRB location (relative to the DNAPL source), base selection for pH adjustment, source strength, and local heterogeneities for the design and long-term performance of ERD in acidic DNAPL-impacted aquifers.

One-dimensional experimental investigation and simulation on the transport characteristics of heterogeneous colloidal Mg(OH)2 in saturated porous media

Recent laboratory studies have shown the injection of colloidal Mg(OH)₂ could provide an effective and low cost alternative as a long term pH buffering system. In this study, Mg(OH)₂ was modified by Tween 80 and SDS and the modified suspension had the properties of high stability, small particle size (the average particle diameter D₅₀ was smaller than 1 μm) and negative charge (zeta potential = -14.26 mV at pH = 10.54). All of these properties demonstrated that colloidal Mg(OH)₂ may have satisfactory transport performance in porous media. However, colloidal Mg(OH)₂ is heterogeneous colloids with a high concentration, the transport performance in porous media is significantly different from homogeneous colloids, and the model simulation is relatively complex. To solve these problems, method of calculus combined with colloid filtration theory (CFT), T-E equation and modified Maxwell theory was used to simulate the transport performance of high concentration of Mg(OH)₂ colloids. Results indicated that the observed experimental results matched well with the model simulations. Hydrodynamic force, DLVO attractive force and colloid diffusion are the major factors controlling the migration of colloidal Mg(OH)₂ in porous media and could quantitatively describe the influence of injection velocity, porous media size and ionic strength on colloidal Mg(OH)₂ transport properties by model calculation.

Bioremediation in Fractured Rock: 1. Modeling to Inform Design, Monitoring, and Expectations

Field characterization of a trichloroethene (TCE) source area in fractured mudstones produced a detailed understanding of the geology, contaminant distribution in fractures and the rock matrix, and hydraulic and transport properties. Groundwater flow and chemical transport modeling that synthesized the field characterization information proved critical for designing bioremediation of the source area. The planned bioremediation involved injecting emulsified vegetable oil and bacteria to enhance the naturally occurring biodegradation of TCE. The flow and transport modeling showed that injection will spread amendments widely over a zone of lower-permeability fractures, with long residence times expected because of small velocities after injection and sorption of emulsified vegetable oil onto solids. Amendments transported out of this zone will be diluted by groundwater flux from other areas, limiting bioremediation effectiveness downgradient. At nearby pumping wells, further dilution is expected to make bioremediation effects undetectable in the pumped water. The results emphasize that in fracture-dominated flow regimes, the extent of injected amendments cannot be conceptualized using simple homogeneous models of groundwater flow commonly adopted to design injections in unconsolidated porous media (e.g., radial diverging or dipole flow regimes). Instead, it is important to synthesize site characterization information using a groundwater flow model that includes discrete features representing high- and low-permeability fractures. This type of model accounts for the highly heterogeneous hydraulic conductivity and groundwater fluxes in fractured-rock aquifers, and facilitates designing injection strategies that target specific volumes of the aquifer and maximize the distribution of amendments over these volumes.

Evaluation of liquid aerosol transport through porous media

Application of remediation methods in contaminated vadose zones has been hindered by an inability to effectively distribute liquid- or solid-phase amendments. Injection as aerosols in a carrier gas could be a viable method for achieving useful distributions of amendments in unsaturated materials. The objectives of this work were to characterize radial transport of aerosols in unsaturated porous media, and to develop capabilities for predicting results of aerosol injection scenarios at the field-scale. Transport processes were investigated by conducting lab-scale injection experiments with radial flow geometry, and predictive capabilities were obtained by developing and validating a numerical model for simulating coupled aerosol transport, deposition, and multi-phase flow in porous media. Soybean oil was transported more than 2m through sand by injecting it as micron-scale aerosol droplets. Oil saturation in the sand increased with time to a maximum of 0.25, and decreased with radial distance in the experiments. The numerical analysis predicted the distribution of oil saturation with only minor calibration. The results indicated that evolution of oil saturation was controlled by aerosol deposition and subsequent flow of the liquid oil, and simulation requires including these two coupled processes. The calibrated model was used to evaluate field applications. The results suggest that amendments can be delivered to the vadose zone as aerosols, and that gas injection rate and aerosol particle size will be important controls on the process.

The change of microbial community from chlorinated solvent-contaminated groundwater after biostimulation using the metagenome analysis

The compositions of bacterial community in one site contaminated with PCE/TCE after the slow polycolloid-releasing substrate (SPRS) (contained vegetable oil, cane molasses, and surfactants) addition were analyzed. Results show that SPRS caused a rapid enhancement of reductive dechlorination of TCE. The transformation of PCE/TCE into ethene was observed after 20 days of operation. To compare the change of bacterial communities before and after SPRS addition, 16S rRNA amplicon sequencing using the metagenome analysis was performed. Results demonstrated the detection of the increased amounts of Dehalogenimonas by 2.2-fold, Pseudomonas by 3.4-fold and Sulfuricurvum by 4-fold with the analysis of the ribosomal database project (RDP). Metagenomic DNA was extracted from PCE/TCE-contaminated groundwater after SPRS addition, and subjected to sequencing. Results obtained from metagenomic sequencing indicate that genes from Dehalococcoides mccartyi was ranked as the second abundant bacteria among all of the detected bacteria via the analysis of the lowest common ancestor (LCA). Abundance of these bacterial groups, as shown above suggests their role in TCE biodegradation. Functional analysis of the metagenome, with the specific reference to chloroalkane and chloroalkene degradation, revealed the presence of some genes responsible for TCE biodegradation. Overall, results of this study provided new insights for a better understanding of the potential of biostimulation on TCE-contaminated sites.

Delivery of vegetable oil suspensions in a shear thinning fluid for enhanced bioremediation

In situ anaerobic biological processes are widely applied for dechlorination of chlorinated solvents in groundwater. A wide range of organic substrates have been tested and applied to support the dechlorination processes. Vegetable oils are a promising type of substrate and have been shown to induce effective dechlorination, have limited geochemical impacts, and maintain good longevity. Because they are non-aqueous phase liquids, distribution of vegetable oils in the subsurface has typically been approached by creating emulsified oil solutions for injection into the aquifer. In this study, inexpensive waste vegetable oils were suspended in a shear-thinning xanthan gum solution as an alternative approach for delivery of vegetable oil to the subsurface. The stability, oil droplet size distribution, and rheological behavior of the oil suspensions that are created in the xanthan solutions were studied in batch experiments. The injectability of the suspensions and the oil distribution in a porous medium were evaluated in column tests. Numerical modeling of oil droplet transport and distribution in porous media was conducted to help interpret the column-test data. Batch studies showed that simple mixing of vegetable oil with xanthan solution produced stable suspensions of the oil as micron-size droplets. The mixture rheology retains shear-thinning properties that facilitate improved uniformity of substrate distribution in heterogeneous aquifers. Column tests demonstrated successful injection of the vegetable oil suspension into a porous medium. This study provides evidence that vegetable oil suspensions in xanthan gum solutions have favorable injection properties and are a potential substrate for in situ anaerobic bioremediation.

Remediation of trichloroethylene-contaminated soils by star technology using vegetable oil smoldering

Self-sustaining treatment for active remediation (STAR) is an innovative soil remediation approach based on smoldering combustion that has been demonstrated to effectively destroy complex hydrocarbon nonaqueous phase liquids (NAPLs) with minimal energy input. This is the first study to explore the smoldering remediation of sand contaminated by a volatile NAPL (trichloroethylene, TCE) and the first to consider utilizing vegetable oil as supplemental fuel for STAR. Thirty laboratory-scale experiments were conducted to evaluate the relationship between key outcomes (TCE destruction, rate of remediation) to initial conditions (vegetable oil type, oil: TCE mass ratio, neat versus emulsified oils). Several vegetable oils and emulsified vegetable oil formulations were shown to support remediation of TCE via self-sustaining smoldering. A minimum concentration of 14,000 mg/kg canola oil was found to treat sand exhibiting up to 80,000 mg/kg TCE. On average, 75% of the TCE mass was removed due to volatilization. This proof-of-concept study suggests that injection and smoldering of vegetable oil may provide a new alternative for driving volatile contaminants to traditional vapour extraction systems without supplying substantial external energy.

Integration of stable carbon isotope, microbial community, dissolved hydrogen gas, and ²HH₂O tracer data to assess bioaugmentation for chlorinated ethene degradation in fractured rocks

An in situ bioaugmentation (BA) experiment was conducted to understand processes controlling microbial dechlorination of trichloroethene (TCE) in groundwater at the Naval Air Warfare Center (NAWC), West Trenton, NJ. In the BA experiment, an electron donor (emulsified vegetable oil and sodium lactate) and a chloro-respiring microbial consortium were injected into a well in fractured mudstone of Triassic age. Water enriched in ²H was also injected as a tracer of the BA solution, to monitor advective transport processes. The changes in concentration and the δ¹³C of TCE, cis-dichloroethene (cis-DCE), and vinyl chloride (VC); the δ²H of water; changes in the abundance of the microbial communities; and the concentration of dissolved H₂ gas compared to pre- test conditions, provided multiple lines of evidence that enhanced biodegradation occurred in the injection well and in two downgradient wells. For those wells where the biodegradation was stimulated intensively, the sum of the molar chlorinated ethene (CE) concentrations in post-BA water was higher than that of the sum of the pre-BA background molar CE concentrations. The concentration ratios of TCE/(cis-DCE+VC) indicated that the increase in molar CE concentration may result from additional TCE mobilized from the rock matrix in response to the oil injection or due to desorption/diffusion. The stable carbon isotope mass-balance calculations show that the weighted average ¹³C isotope of the CEs was enriched for around a year compared to the background value in a two year monitoring period, an effective indication that dechlorination of VC was occurring. Insights gained from this study can be applied to efforts to use BA in other fractured rock systems. The study demonstrates that a BA approach can substantially enhance in situ bioremediation not only in fractures connected to the injection well, but also in the rock matrix around the well due to processes such as diffusion and desorption. Because the effect of the BA was intensive only in wells where an amendment was distributed during injection, it is necessary to adequately distribute the amendments throughout the fractured rock to achieve substantial bioremediation. The slowdown in BA effect after a year is due to some extend to the decrease abundant of appropriate microbes, but more likely the decreased concentration of electron donor.

A dual flowing continuum approach to model denitrification experiments in porous media colonized by biofilms

We present a modeling exercise of solute transport and biodegradation in a coarse porous medium widely colonized by a biofilm phase. Tracer tests in large laboratory columns using both conservative (fluorescein) and biodegradable (nitrate) solutes are simulated by means of a dual flowing continuum approach. The latter clearly distinguishes concentrations in a flowing porous phase from concentrations conveyed in the biofilm. With this conceptual setting, it becomes possible to simulate the sharp front of concentrations at early times and the flat tail of low concentrations at late times observed on the experimental breakthrough curves. Thanks to the separation of flow in two phases at different velocities, dispersion coefficients in both flowing phases keep reasonable values with some physical meaning. This is not the case with simpler models based on a single continuum (eventually concealing dead-ends), for which inferred dispersivity may reach the unphysical value of twice the size of the columns. We also show that the behavior of the dual flowing continuum is mainly controlled by the relative fractions of flow passing in each phase and the rate of mass transfer between phases. These parameters also condition the efficiency of nitrate degradation, the degradation rate in a well-seeded medium being a weakly sensitive parameter. Even though the concept of dual flowing continuum appears promising for simulating transport in complex porous media, its inversion onto experimental data really benefits from attempts with simpler models providing a rough pre-evaluation of parameters such as porosity and mean fluid velocity in the system.

In situ bioremediation of uranium with emulsified vegetable oil as the electron donor

A field test with a one-time emulsified vegetable oil (EVO) injection was conducted to assess the capacity of EVO to sustain uranium bioreduction in a high-permeability gravel layer with groundwater concentrations of (mM) U, 0.0055; Ca, 2.98; NO3(-), 0.11; HCO3(-), 5.07; and SO4(2-), 1.23. Comparison of bromide and EVO migration and distribution indicated that a majority of the injected EVO was retained in the subsurface from the injection wells to 50 m downgradient. Nitrate, uranium, and sulfate were sequentially removed from the groundwater within 1-2 weeks, accompanied by an increase in acetate, Mn, Fe, and methane concentrations. Due to the slow release and degradation of EVO with time, reducing conditions were sustained for approximately one year, and daily U discharge to a creek, located approximately 50 m from the injection wells, decreased by 80% within 100 days. Total U discharge was reduced by 50% over the one-year period. Reduction of U(VI) to U(IV) was confirmed by synchrotron analysis of recovered aquifer solids. Oxidants (e.g., dissolved oxygen, nitrate) flowing in from upgradient appeared to reoxidize and remobilize uranium after the EVO was exhausted as evidenced by a transient increase of U concentration above ambient values. Occasional (e.g., annual) EVO injection into a permeable Ca and bicarbonate-containing aquifer can sustain uranium bioreduction/immobilization and decrease U migration/discharge.

U(VI) bioreduction with emulsified vegetable oil as the electron donor--model application to a field test

We amended a shallow fast-flowing uranium (U) contaminated aquifer with emulsified vegetable oil (EVO) and subsequently monitored the biogeochemical responses for over a year. Using a biogeochemical model developed in a companion article (Tang et al., Environ. Sci. Technol.2013, doi: 10.1021/es304641b) based on microcosm tests, we simulated geochemical and microbial dynamics in the field test during and after the 2-h EVO injection. When the lab-determined parameters were applied in the field-scale simulation, the estimated rate coefficient for EVO hydrolysis in the field was about 1 order of magnitude greater than that in the microcosms. Model results suggested that precipitation of long-chain fatty acids, produced from EVO hydrolysis, with Ca in the aquifer created a secondary long-term electron donor source. The model predicted substantial accumulation of denitrifying and sulfate-reducing bacteria, and U(IV) precipitates. The accumulation was greatest near the injection wells and along the lateral boundaries of the treatment zone where electron donors mixed with electron acceptors in the groundwater. While electron acceptors such as sulfate were generally considered to compete with U(VI) for electrons, this work highlighted their role in providing electron acceptors for microorganisms to degrade complex substrates thereby enhancing U(VI) reduction and immobilization.

Nanoscale zero-valent iron: future prospects for an emerging water treatment technology

For the past 15 years, nanoscale metallic iron (nZVI) has been investigated as a new tool for the treatment of contaminated water and soil. The technology has reached commercial status in many countries worldwide, however is yet to gain universal acceptance. This review summarises our contemporary knowledge of nZVI aqueous corrosion, manufacture and deployment, along with methods to enhance particle reactivity, stability and subsurface mobility. Reasons for a lack of universal acceptance are also explored. Key factors include: concerns over the long-term fate, transformation and ecotoxicity of nZVI in environmental systems and, a lack of comparable studies for different nZVI materials and deployment strategies. It is highlighted that few investigations to date have examined systems directly analogous to the chemistry, biology and architecture of the terrestrial environment. Such emerging studies have highlighted new concerns, including the prospect for remobilisation of heavy metals and radionuclides over extended periods. The fundamental importance of being able to accurately predict the long-term physical, chemical and biological fate of contaminated sites following nZVI treatment is emphasised and, as part of this, a universal empirical testing framework for nZVI is suggested.

Bioaugmentation of a historically contaminated soil by polychlorinated biphenyls with Lentinus tigrinus

BACKGROUND

Several species belonging to the ecological group of white-rot basidiomycetes are able to bring about the remediation of matrices contaminated by a large variety of anthropic organic pollutants. Among them, polychlorobiphenyls (PCBs) are characterized by a high recalcitrance due to both their low bioavailability and the inability of natural microbial communities to degrade them at significant rates and extents. Objective of this study was to assess the impact of a maize stalk-immobilized Lentinus tigrinus CBS 577.79 inoculant combined with soybean oil (SO), as a possible PCB-mobilizing agent, on the bioremediation and resident microbiota of an actual Aroclor 1260 historically contaminated soil under unsaturated solid-phase conditions.

RESULTS

Best overall PCB depletions (33.6 ± 0.3%) and dechlorination (23.2 ± 1.3%) were found after 60 d incubation in the absence of SO where, however, the fungus appeared to exert adverse effects on both the growth of biphenyl- and chlorobenzoate-degrading bacteria and the abundance of genes coding for both biphenyl dioxygenase (bph) and catechol-2,3-dioxygenase. A significant (P < 0.001) linear inverse relationship between depletion yields and degree of chlorination was observed in both augmented and control microcosms in the absence of SO; conversely, this negative correlation was not evident in SO-amended microcosms where the additive inhibited the biodegradation of low chlorinated congeners. The presence of SO, in fact, resulted in lower abundances of both biphenyl-degrading bacteria and bph.

CONCLUSIONS

The PCB depletion extents obtained in the presence of L. tigrinus are by far higher than those reported in other remediation studies conducted under unsaturated solid phase conditions on actual site soils historically contaminated by Aroclor 1260. These results suggest that the bioaugmentation strategy with the maize stalk-immobilized mycelium of this species might be promising in the reclamation of PCB-contaminated soils. The addition of SO to matrices contaminated by technical PCB mixtures, such as Aroclor 1242 and Delor 103 and characterized by a large preponderance of low chlorinated congeners, might not be advisable.

Uranium reduction and microbial community development in response to stimulation with different electron donors

Stimulating microbial reduction of soluble U(VI) to less soluble U(IV) shows promise as an in situ bioremediation strategy for uranium contaminated groundwater, but the optimal electron donors for promoting this process have yet to be identified. The purpose of this study was to better understand how the addition of various electron donors to uranium-contaminated subsurface sediments affected U(VI) reduction and the composition of the microbial community. The simple electron donors, acetate or lactate, or the more complex donors, hydrogen-release compound (HRC) or vegetable oil, were added to the sediments incubated in flow-through columns. The composition of the microbial communities was evaluated with quantitative PCR probing specific 16S rRNA genes and functional genes, phospholipid fatty acid analysis, and clone libraries. All the electron donors promoted U(VI) removal, even though the composition of the microbial communities was different with each donor. In general, the overall biomass, rather than the specific bacterial species, was the factor most related to U(VI) removal. Vegetable oil and HRC were more effective in stimulating U(VI) removal than acetate. These results suggest that the addition of more complex organic electron donors could be an excellent option for in situ bioremediation of uranium-contaminated groundwater.

Enhanced transport of colloidal oil droplets in saturated and unsaturated sand columns

Colloidal-sized triacylglycerol droplets demonstrated enhanced transport compared to ideal latex colloid spheres in both saturated and unsaturated quartz sand columns. Oil droplets (mean diameter 0.74 ± 0.03 μm, density 0.92 g cm(-3), ζ-potential -34 ± 1 mV) were injected simultaneously with latex microsphere colloids (FluoSpheres; density 1.055 g cm(-3), diameters 0.02, 0.2, and 1.0 μm, ζ-potentials -16 ± 1, -30 ± 2, and -49 ± 1, respectively) and bromide into natural quartz sand (ζ-potential -63 ± 2 mV) via short-pulse column breakthrough experiments. Tests were conducted under both saturated and unsaturated conditions. Breakthrough of oil droplets preceded bromide and FluoSpheres. Recovery of oil droplets was 20% greater than similarly sized FluoSpheres in the saturated column, and 16% greater in the 0.18 ± 0.01 volumetric water content (VWC) unsaturated column. Higher variability was observed in the 0.14 ± 0.01 VWC column experiments with oil droplet recovery only slightly greater than similarly sized FluoSpheres. The research presents for the first time the direct comparison of colloidal oil droplet transport in porous media with that of other colloids, and demonstrates transport under unsaturated conditions. Based on experimental results and theoretical analyses, we discuss possible mechanisms that lead to the observed enhanced mobility of oil droplets compared to FluoSpheres with similar size and electrostatic properties.

Encapsulation of nZVI particles using a Gum Arabic stabilized oil-in-water emulsion

Stabilization of reactive iron particles against aggregation and sedimentation is a critical engineering aspect for successful application of nZVI (nanoscale zero valent iron) within the contaminated subsurface environment. In this work we explore the stability and reactivity of a new encapsulation approach that relies upon Gum Arabic to stabilize high quantities of nZVI (∼ 12 g/L) in the dispersed phase of a soybean oil-in-water emulsion. The emulsion is kinetically stable due to substantial repulsive barriers to droplet-droplet induced deformation and subsequent coalescence. Sedimentation time scales were found to be on the order of hours (τ=4.77 ± 0.02 h). Thus, the use of Gum Arabic represents an advance in stabilizing nZVI-in-oil-in-water emulsions. nZVI within the emulsion was shown to be reactive with both TCE degradation and H(2) production observed. Degradation rates were observed to be on the same order of magnitude as those reported for less stable, aqueous suspensions of nZVI. TCE consumption within the emulsion was described with an equivalent aqueous phase rate coefficient of ∼ 5 × 10(-4)L(aq)/m(2)h.

Oil-in-water emulsions for encapsulated delivery of reactive iron particles

Treatment of dense nonaqueous phase liquid (DNAPL) source zones using suspensions of reactive iron particles relies upon effective transport of the nano- to submicrometer scale iron particles within the subsurface. Recognition that poor subsurface transport of iron particles results from particle-particle and particle-soil interactions permits development of strategies which increase transport. In this work, experiments were conducted to assess a novel approach for encapsulated delivery of iron particles within porous media using oil-in-water emulsions. Objectives of this study included feasibility demonstration of producing kinetically stable, iron-containing, oil-in-water emulsions and evaluating the transport of these iron-containing, oil-in-water emulsions within water-saturated porous media. Emulsions developed in this study have mean droplet diameters between 1 and 2 microm, remain kinetically stable for > 1.5 h, and possess densities (0.996-1.00 g/mL at 22 degrees C) and dynamic viscosities (2.4-9.3 mPa x s at 22 degrees C and 20 s(-1)) that are favorable to transport within DNAPL source zones. Breakthrough curves and post-experiment extractions from column experiments conducted with medium and fine sands suggest little emulsion retention (< 0.20% wt) at a Darcy velocity of 0.4 m/day. These findings demonstrate that emulsion encapsulation is a promising method for delivery of iron particles and warrants further investigation.

Numerical modeling of emulsified oil distribution in heterogeneous aquifers

In situ anaerobic bioremediation using edible oil emulsions will be most effective if the oil droplets can be brought into close contact with the contaminant to be treated. However, uniformly distributing oil in heterogeneous aquifers can be difficult. The impact of injection conditions on emulsion distribution in a three-dimensional heterogeneous aquifer is examined using MODFLOW and RT3D. Emulsion retention is simulated using a rate-limited Langmuir isotherm. Volume and flow contact efficiency are shown to be functions of mass of oil injected, injection fluid volume, well spacing, and injection sequence. Regression equations are developed relating dimensionless scaling factors to expected contact efficiency for area treatment and barriers. Cleanup time for uncontacted zones is estimated using a mobile-immobile zone modeling approach.

Concurrent bioremediation of perchlorate and 1,1,1-trichloroethane in an emulsified oil barrier

A detailed field pilot test was conducted to evaluate the use of edible oil emulsions for enhanced in situ biodegradation of perchlorate and chlorinated solvents in groundwater. Edible oil substrate (EOS) was injected into a line of ten direct push injection wells over a 2-day period to form a 15-m-long biologically active permeable reactive barrier (bio-barrier). Field monitoring results over a 2.5-year period indicate the oil injection generated strongly reducing conditions in the oil-treated zone with depletion of dissolved oxygen, nitrate, and sulfate, and increases in dissolved iron, manganese and methane. Perchlorate was degraded from 3100 to 20,000 microg/L to below detection (<4 microg/L) in the injection and nearby monitor wells within 5 days following the injection. Two years after the single emulsion injection, perchlorate was less than 6 microg/L in every downgradient well compared to an average upgradient concentration of 13,100 microg/L. Immediately after emulsion injection, there were large shifts in concentrations of chlorinated solvents and degradation products due to injection of clean water, sorption to the oil and adaptation of the in situ microbial community. Approximately 4 months after emulsion injection, concentrations of 1,1,1-trichloroethane (TCA), perchloroethene (PCE), trichloroethene (TCE) and their degradation products appeared to reach a quasi steady-state condition. During the period from 4 to 18 months, TCA was reduced from 30-70 microM to 0.2-4 microM during passage through the bio-barrier. However, 1-9 microM 1,1-dichloroethane (DCA) and 8-14 microM of chloroethane (CA) remained indicating significant amounts of incompletely degraded TCA were discharging from the oil-treated zone. During this same period, PCE and TCE were reduced with concurrent production of 1,2-cis-dichloroethene (cis-DCE). However, very little VC or ethene was produced indicating reductive dechlorination slowed or stopped at cis-DCE. The incomplete removal of TCA, PCE and TCE is likely associated with the short (5-20 days) hydraulic retention time of contaminants in the oil-treated zone. The permeability of the injection wells declined by 39-91% (average=68%) presumably due to biomass growth and/or gas production. However, non-reactive tracer tests and detailed monitoring of the perchlorate plume demonstrated that the permeability loss did not result in excessive flow bypassing around the bio-barrier. Contaminant transport and degradation within the bio-barrier was simulated using an advection-dispersion-reaction model where biodegradation rate was assumed to be linearly proportional to the residual oil concentration (Soil) and the contaminant concentration. Using this approach, the calibrated model was able to closely match the observed contaminant distribution. The calibrated model was then used to design a full-scale barrier to treat both ClO4 and chlorinated solvents.