Controlled Release Electron Donors: Hydrogen Release Compound (HRC)—An Overview of a Decade of Case Studies

Hydrogen Release Compound (HRC) has been a commercially available product for engineered bioremediation of anaerobically biodegradable contaminants since 1999. HRC is a polylactate ester that, upon hydration or microbial cleavage of its ester bonds, slowly releases lactic acid. Lactic acid serves as an electron donor for microbial reductive biodegradation, while also providing hydrogen and carbon where required. HRC is a viscous amber-colored liquid that is typically injected into a contaminated aquifer using direct push technology or backfill injection into boreholes created by traditional drilling methods. Once in place, HRC creates a plume of lactic acid and its fermentation products (other organic acids and hydrogen) downgradient of the injection area and serves to accelerate anaerobic bioremediation processes. In this review of HRC field application results, the authors summarize application types, contaminants treated, site types, application locations, injection methods, site lithology and hydrology, and concentration ranges of geochemical species. The source of this information is a database of more than 850 HRC field applications, a series of 80 HRC publications that are publicly available, and 44 detailed site case histories that are available electronically.     

Modeling Chlorinated Solvent Bioremediation Using Hydrogen Release Compound (HRC)

Chlorinated solvents such as tetrachloroethene (PCE) and trichloroethene (TCE) are common groundwater contaminants. One approach that has been used to manage these contaminants is in situ bioremediation, where an electron donor is added to contaminated groundwater to stimulate indigenous bacteria to degrade the chlorinated compounds. A technique that is increasingly being used to supply electron donor to the subsurface involves application of a commercial product with the trade name Hydrogen Release Compound (HRC). HRC is a viscous fluid that releases lactic acid, which subsequently is metabolized to provide molecular hydrogen as an electron donor. This study investigates application of HRC to remediate a site contaminated with TCE. A user-defined dual-Monod biodegradation reaction module was developed for the RT3D-reactive transport code to simulate in situ biodegradation of TCE by reductive dehalogenation stimulated by release of molecular hydrogen in the subsurface as a result of HRC injection. The model was used to show how a remediation system using HRC to stimulate reductive dehalogenation could be designed, and how mixing, as quantified by hydraulic conductivity and dispersivity, impacts the system design.     

Elevated nitrate enriches microbial functional genes for potential bioremediation of complexly contaminated sediments

Nitrate is an important nutrient and electron acceptor for microorganisms, having a key role in nitrogen (N) cycling and electron transfer in anoxic sediments. High-nitrate inputs into sediments could have a significant effect on N cycling and its associated microbial processes. However, few studies have been focused on the effect of nitrate addition on the functional diversity, composition, structure and dynamics of sediment microbial communities in contaminated aquatic ecosystems with persistent organic pollutants (POPs). Here we analyzed sediment microbial communities from a field-scale in situ bioremediation site, a creek in Pearl River Delta containing a variety of contaminants including polybrominated diphenyl ethers (PBDEs) and polycyclic aromatic hydrocarbons (PAHs), before and after nitrate injection using a comprehensive functional gene array (GeoChip 4.0). Our results showed that the sediment microbial community functional composition and structure were markedly altered, and that functional genes involved in N-, carbon (C)-, sulfur (S)-and phosphorus (P)- cycling processes were highly enriched after nitrate injection, especially those microorganisms with diverse metabolic capabilities, leading to potential in situ bioremediation of the contaminated sediment, such as PBDE and PAH reduction/degradation. This study provides new insights into our understanding of sediment microbial community responses to nitrate addition, suggesting that indigenous microorganisms could be successfully stimulated for in situ bioremediation of POPs in contaminated sediments with nitrate addition.     

A Long-Term Field Study of In Situ Bioremediation in a Fractured Conglomerate Trichloroethene Source Zone

ABSTRACT An 8-year bioremediation field study was conducted in a trichloroethene (TCE)-contaminated, highly indurated (i.e., hard), recharge-limited (i.e., contains little water) conglomerate where common remediation strategies, such as groundwater recirculation and direct push installation of a large well network, could not be used. A tracer test using isotopically distinct water from the Hetch Hetchy Reservoir indicated that remediation fluids mainly flowed through fractures and sand lenses in the conglomerate. This was confirmed during in situ bioremediation of the site, in which Dehalococcoides (from a bioaugmentation culture) and volatile fatty acids (from injection of lactate) were the most accurate indicators of transport between wells. Some contaminants were also displaced out of the area due to injection of tracer water. Despite these difficulties, dissolved contaminant mass decreased by an estimated 80% by the end of the test, reaching the lowest values ever recorded at this site. Furthermore, the persistence of ethene 4 years after bioaugmentation suggests that the dechlorinating capacity of the remaining microbial community is comparable to the matrix diffusion of TCE into the dissolved phase.     

Nucleic-acid-based methods for monitoring the performance of in situ bioremediation

Over the last decade, major advancements have occurred in the application of nucleic-acid-based methods to detect and determine the levels of catabolic genes in environmental samples. Studies have focused on validating methods in microcosms, studying changes in the structure and expression of microbial communities in response to contaminants, and improving the sensitivity of the methods. Only in the last few years have these methods transitioned from development and validation to efforts to apply these methods for monitoring in situ bioremediation. Methods that analyse nucleic acids extracted from environmental samples are of value to bioremediation because they allow analysis independent of the artefacts that can arise from laboratory biodegradative potential assays and laboratory culture-based enumerations and from the inability to culture a large proportion of the micro-organisms in the environment In theory, these methods enable a more comprehensive perspective, and a more defensible interpretation, of the microbial community response to intrinsic and engineered bioremediation processes. Results from the first studies applying nucleic-acid-based methods to intrinsic or engineered bioremediation indicate that these methods have both potential and limitations. The rapidly increasing number of cloned and sequenced catabolic genes, methodological advancements such as the ability to track specific micro-organisms without prior sequence data, and the potential use of bioaugmentation in the field suggest that the utility of these methods for in situ bioremediation will increase in the coming years.     

Measurement of Oxygen Demand of Petroleum Hydrocarbon-Contaminated Groundwater

The standard biological oxygen demand (BOD) test was modified for application to petroleum hydrocarbon-contaminated groundwater. The goal was to assess the potential oxygen demand of plume constituents as part of a field trial investigating oxygen-enhanced in situ bioremediation. Modifications to standard BOD protocol included the use of an adapted microbial population developed from site groundwater and methods to minimize both the loss of volatile compounds and the exposure of samples to air. Results from this study indicated that the measured oxygen demand was significantly greater than the oxygen demand estimated solely by stoichiometric calculations from the concentrations of the analytes of typical regulatory concern, that is, benzene, toluene, ethylbenzene, and xylenes (BTEX). This is not surprising, because the petroleum hydrocarbon sources typically contain many organic contaminants other than BTEX, as well as potentially oxidizable natural dissolved organic matter and inorganic species typically present in hydrocarbon plumes. However, in practice, estimation of the total oxygen demand of a contaminated groundwater by exhaustive analyses of all oxidizable or aerobically degradable species typically will be infeasible. The modified BOD test may be a simple, low-cost, useful tool when assessing the potential for natural attenuation by aerobic biodegradation or designing methods to supply oxygen for enhanced aerobic bioremediation.     

Enhancement of metal bioremediation by use of microbial surfactants

Metal pollution all around the globe, especially in the mining and plating areas of the world, has been found to have grave consequences. An excellent option for enhanced metal contaminated site bioremediation is the use of microbial products viz. microbial surfactants and extracellular polymers which would increase the efficiency of metal reducing/sequestering organisms for field bioremediation. Important here is the advantage of such compounds at metal and organic compound co-contaminated site since microorganisms have long been found to produce surface-active compounds when grown on hydrocarbons. Other options capable of proving efficient enhancers include exploiting the chemotactic potential and biofilm forming ability of the relevant microorganisms. Chemotaxis towards environmental pollutants has excellent potential to enhance the biodegradation of many contaminants and biofilm offers them a better survival niche even in the presence of high levels of toxic compounds.     

Field applications of genetically engineered microorganisms for bioremediation processes

Genetically engineered microorganisms (GEMs) have shown potential for bioremediation applications in soil, groundwater, and activated sludge environments, exhibiting enhanced degradative capabilities encompassing a wide range of chemical contaminants. However, the vast majority of studies pertaining to genetically engineered microbial bioremediation are supported by laboratory-based experimental data. In general, relatively few examples of GEM applications in environmental ecosystems exist. Unfortunately, the only manner in which to fully address the competence of GEMs in bioremediation efforts is through long-term field release studies. It is therefore essential that field studies be performed to acquire the requisite information for determining the overall effectiveness and risks associated with GEM introduction into natural ecosystems.     

In situ hydrogen consumption kinetics as an indicator of subsurface microbial activity

There are few methods available for broadly assessing microbial community metabolism directly within a groundwater environment. In this study, hydrogen consumption rates were estimated from in situ injection/withdrawal tests conducted in two geochemically varying, contaminated aquifers as an approach towards developing such a method. The hydrogen consumption first-order rates varied from 0.002 nM h(-1) for an uncontaminated, aerobic site to 2.5 nM h(-1) for a contaminated site where sulfate reduction was a predominant process. The method could accommodate the over three orders of magnitude range in rates that existed between subsurface sites. In a denitrifying zone, the hydrogen consumption rate (0.02 nM h(-1)) was immediately abolished in the presence of air or an antibiotic mixture, suggesting that such measurements may also be sensitive to the effects of environmental perturbations on field microbial activities. Comparable laboratory determinations with sediment slurries exhibited hydrogen consumption kinetics that differed substantially from the field estimates. Because anaerobic degradation of organic matter relies on the rapid consumption of hydrogen and subsequent maintenance at low levels, such in situ measures of hydrogen turnover can serve as a key indicator of the functioning of microbial food webs and may be more reliable than laboratory determinations.     

Fundamentals and Application Potential of Arsenic-Resistant Bacteria for Bioremediation in Rhizosphere: A Review

As a hazardous environmental metalloid toxicant, arsenic (As)—at elevated levels in water and soil—has created a major public health concern through its entry into the food chain by accumulation in crops. Among the various methods reported thus far for reclamation of As-contaminated crop fields, bioremediation using bacteria with plant-growth-promoting traits has been found to be a most promising solution. There is every possibility that bacterial isolates with the ability to remove or immobilize As could be used for successful bioremediation. However, bioremediation needs to define its boundaries between promise and field application, as most studies have been restricted to laboratory results only. Rhizosphere interactions play a critical role in monitoring As bioavailability to crop plants, thus a better understanding of it might improve rhizoremediation technologies. The challenges rely on the application of these novel approaches under field conditions. Despite some limitations, the prospect for successful stimulation and exploitation of microbial metabolism for As rhizoremediation appears to be very promising.     

A comprehensive overview of elements in bioremediation

Sustainable development requires the development and promotion of environmental management and a constant search for green technologies to treat a wide range of aquatic and terrestrial habitats contaminated by increasing anthropogenic activities. Bioremediation is an increasingly popular alternative to conventional methods for treating waste compounds and media with the possibility to degrade contaminants using natural microbial activity mediated by different consortia of microbial strains. Many studies about bioremediation have been reported and the scientific literature has revealed the progressive emergence of various bioremediation techniques. In this review, we discuss the various in situ and ex situ bioremediation techniques and elaborate on the anaerobic digestion technology, phytoremediation, hyperaccumulation, composting and biosorption for their effectiveness in the biotreatment, stabilization and eventually overall remediation of contaminated strata and environments. The review ends with a note on the recent advances genetic engineering and nanotechnology have had in improving bioremediation. Case studies have also been extensively revisited to support the discussions on biosorption of heavy metals, gene probes used in molecular diagnostics, bioremediation studies of contaminants in vadose soils, bioremediation of oil contaminated soils, bioremediation of contaminants from mining sites, air sparging, slurry phase bioremediation, phytoremediation studies for pollutants and heavy metal hyperaccumulators, and vermicomposting.     

Petroleum hydrocarbon bioremediation: sampling and analytical techniques, in situ treatments and commercial microorganisms currently used

The sampling and analytical methods, along with available microorganisms, used for in situ hydrocarbon bioremediation are reviewed. Each treatment method is briefly described and its advantages and limitations pertaining to potential applications are evaluated. Bioremediation provides cost-effective, contaminant- and substrate-specific treatments equally successful in reducing the concentrations of single compounds or mixtures of biodegradable materials. In situ treatments rarely yield undesirable byproducts, but precautions and preliminary baseline tests are always recommended. Sampling methods should adhere to good laboratory and field practices and usually do not require highly trained personnel. Analytical methods vary in sensitivity, cost, duration of sample analysis and personnel training required. Voucher specimens of bacterial strains used in bioremediation exist in various repositories (e.g. ATCC, DSM, etc.) or are commercially available, and are usually covered by patent rights. Each one of these strains may yield spectacular results in vitro for specific target compounds. However, the overall success of such strains in treating a wide range of contaminants in situ remains limited. The reintroduction of indigenous microorganisms isolated from the contaminated site after culturing seems to be a highly effective bioremediation method, especially when microorganism growth is supplemented by oxygen and fertilizers. Received: 10 June 1997 / Received revision: 14 August 1997 / Accepted: 25 August 1997     

海洋石油污染物的微生物降解与生物修复

石油是海洋环境的主要污染物 ,已经对海洋及近岸环境造成了严重的危害。微生物降解是海洋石油污染去除的主要途径。海洋石油污染物的微生物降解受石油组分与理化性质、环境条件以及微生物群落组成等多方面因素的制约 ,N和P营养的缺乏是海洋石油污染物生物降解的主要限制因子。在生物降解研究基础上发展起来的生物修复技术在海洋石油污染治理中发展潜力巨大 ,并且取得了一系列成果。介绍了海洋中石油污染物的来源、转化过程、降解机理、影响生物降解因素及生物修复技术等方面内容 ,强调了生物修复技术在治理海洋石油污染环境中的优势和重要性 ,指出目前生物修复技术存在的问题。     

Perchlorate Remediation by Electrokinetic Extraction and Electrokinetic Injection of Substrates

Perchlorate (ClO4?) contamination of groundwater has recently become a major concern across the nation. Electrokinetic (EK) extraction with the simultaneous EK injection of organic material to promote degradation could allow for the efficient removal of perchlorate while simultaneously promoting degradation of perchlorate. Column experiments were conducted to evaluate the technology. Lactate and glycine served as organic substrates to promote degradation after injection into the columns as well as maintaining the pH near neutral. Removal of perchlorate from contaminated materials kaolin, sand, and a natural soil historically contaminated by perchlorate was controlled by the ionic flux of perchlorate and not by transport from the osmotic flux which was only significant for kaolin experiments. Perchlorate was removed from contaminated sand and clay below our detection limits (5 ppb). Both lactic acid and glycine were successfully injected into clay and a sand matrix. Results from a contaminated site soil indicate that the Chemical Oxygen Demand was increased after electrokinetic injection of glycine and lactate. Experiments using soil from a contaminated site confirmed that EK can be used to both remove perchlorate and stimulate bioremediation by the injection of lactate or glycine. The use of EK technology to both remove and provide for continued source removal by bioremediation offers a potential new tool to treat low permeability systems.     

Perchlorate Remediation by Electrokinetic Extraction and Electrokinetic Injection of Substrates

Perchlorate (ClO₄^-) contamination of groundwater has recently become a major concern across the nation. Electrokinetic (EK) extraction with the simultaneous EK injection of organic material to promote degradation could allow for the efficient removal of perchlorate while simultaneously promoting degradation of perchlorate. Column experiments were conducted to evaluate the technology. Lactate and glycine served as organic substrates to promote degradation after injection into the columns as well as maintaining the pH near neutral. Removal of perchlorate from contaminated materials kaolin, sand, and a natural soil historically contaminated by perchlorate was controlled by the ionic flux of perchlorate and not by transport from the osmotic flux which was only significant for kaolin experiments. Perchlorate was removed from contaminated sand and clay below our detection limits (5 ppb). Both lactic acid and glycine were successfully injected into clay and a sand matrix. Results from a contaminated site soil indicate that the Chemical Oxygen Demand was increased after electrokinetic injection of glycine and lactate. Experiments using soil from a contaminated site confirmed that EK can be used to both remove perchlorate and stimulate bioremediation by the injection of lactate or glycine. The use of EK technology to both remove and provide for continued source removal by bioremediation offers a potential new tool to treat low permeability systems.     

Lactic acid bacteria of meat and meat products

When the growth of aerobic spoilage bacteria is inhibited, lactic acid bacteria may become the dominant component of the microbial flora of meats. This occurs with cured meats and with meats packaged in films of low gas permeability. The presence of a flora of psychrotrophic lactic acid bacteria on vacuum-packaged fresh chilled meats usually ensures that shelf-life is maximal. When these organisms spoil meats it is generally by causing souring, however other specific types of spoilage do occur. Some strains cause slime formation and greening of cured meats, and others may produce hydrogen sulphide during growth on vacuum-packaged beef. The safety and stability of fermented sausages depends upon fermentation caused by lactic acid bacteria. Overall the presence on meats of lactic acid bacteria is more desirable than that of the types of bacteria they have replaced.     

In situ bioremediation of contaminated aquifers and subsurface soils

Bioremediation, the use of microorganisms to detoxify and degrade hazardous wastes, is an emerging in situ treatment technology for the remediation of contaminated aquifers and subsurface soils. This technology depends upon the alteration of the physical/chemical conditions in the subsurface environment to optimize microbiological activity. As such, successful bioremediation depends not only upon an understanding of microbial degradation processes, but also upon an understanding of the complex interactions that occur between the contaminants, the subsurface environment, and the indigenous microbial populations at each site. At present, these interactions are poorly understood. Site‐specific evaluation and design therefore are essential for bioremediation. In this paper, we review microbiological, hydrological, and geochemical factors that should be considered in evaluating the appropriateness of bioremediation for hazardous waste‐contaminated aquifers and subsurface soils.     

Fate of Pesticides in the Environment and its Bioremediation

The present paper is an overview of the presence and fate of pesticides as persistent organic pollutants in the environment as well as of the potential for their detoxification, also combined with chemical and physical treatment. It contains information gathered from a range of currently available sources. The fate of pesticides in the environment is analyzed considering the processes that determine their persistence and mobility, grouped into transport, transfer and transformation processes. Few pesticide characteristics such as persistence, mobility and biodegradability are emphasized. The fate of a pesticide and the potential for its persistence and mobility from the site of application are considered to be affected by the chemical and physical properties of the pesticide, site characteristics such as soil and groundwater individuality, climate and local weather conditions, biological population, and the handling practices of the pesticide user. Bioremediation, as one of the most environmentally‐sound and cost‐effective methods for the decontamination and detoxification of a pesticide‐contaminated environment is discussed especially considering the factors affecting the biodegradability of pesticides such as biological factors and the characteristics of the chemical compounds. In situ and ex situ bioremediation as possible types of bioremediation activities are weighted up. Also, the paper includes some considerations for developing strategies regarding the choice of bioremediation technology, as well as advantages and disadvantages of the bioremediation of environmental components polluted with pesticides.     

A Field Application of Hydrogen-Releasing Compound (HRCTM) for the Enhanced Bioremediation of Methyl Tertiary Butyl Ether (MTBE)

Due to a greater understanding of the behavior of the fuel oxygenate Methyl Tertiary Butyl Ether (MTBE) in groundwater, the United States Environmental Protection Agency (EPA) and the American Petroleum Institute (API) recently have acknowledged the need for the development and application of additional remedial strategies to address the more extensive, longer lived, and faster moving dissolved MTBE plumes often associated with oxygenated fuel releases (API, 2000 and USEPA, 2000a). The need for alternative methods for managing dissolved MTBE plumes is particularly evident in the case of the Upper Glacial aquifer of Long Island, New York. Hydrogeologic conditions in the this water table aquifer (i. e., high hydraulic conductivity, high average pore velocities, low organic carbon, and high rates of recharge) have been found to contribute to the formation of extensive, long, narrow, and three-dimensional dissolved MTBE plumes that plunge into the aquifer in response to recharge (Weaver et. al. 1999). The characteristics of MTBE plumes in the Upper Glacial aquifer in combination with abundant sensitive receptors (mainly drinking water supply wells), often renders monitored natural attenuation (MNA) plume management strategies inappropriate, resulting in the need for plume control, frequently via pumping and treating (NYSDEC, 2000). In such cases, remedial costs can rise well beyond those associated with similar fuel releases that did not contain MTBE (USEPA, 1998a). Consequently, the application of remedial technologies for MTBE other than MNA, or pumping and treating, are of great interest to those responsible for the management of dissolved MTBE plumes on Long Island or in similar hydrogeologic settings. An alternative strategy for the remediation of dissolved MTBE plumes was recently field tested at an oxygenated fuel spill site on Long Island. The strategy was enhanced biodegradation via the application of Hydrogen Release Compound (HRC^TM). HRC^TM is a form of polylactate ester that slowly releases biodegradation stimulating constituents into the aquifer and has been shown in other studies to foster methanogenic conditions that advance the reductive dechlorina-tion of perchloroethene (PCE) and trichloroethene (TCE) (Koenigsberg, 1998). Numerous reports have been written that discuss the biodegradation of MTBE under aerobic conditions, as well as microcosm studies in which MTBE biodegradation was observed under anaerobic conditions. However, there are limited reports that document the natural anaerobic biodegradation of dissolved MTBE (McLoughlin, 2000). Despite the lack of documented natural anaerobic biodegradation of MTBE, it has been observed that MTBE transport often occurs under anoxic conditions at oxygenated fuel releases as the result of the biodegradation of other fuel constituents, such as benzene, toluene, ethylbenzene and xylene (BTEX), which deplete the available dissolved oxygen as well as other electron acceptors (nitrate, ferric iron, manganese, etc.) (USEPA, 2000c and API, 1996). Therefore, an anaerobic biodegradation strategy is attractive due to its synergy with the existing geochemical conditions. Consequently, the study was conceived and designed to test the ability of HRC(tm) to foster the anaerobic bio-degradation of MTBE under methano-genic conditions (McLoughlin, 2000). The application of HRC(tm) did result in the formation of a large area of enhanced reducing conditions in the vicinity and down gradient of the application zone. However, under these site conditions, the HRC(tm) application did not induce measurable methanogenic conditions with the associated elevated dissolved hydrogen concentrations required for significant MTBE anaerobic biodegradation. The high hydraulic conductivity and high average pore velocity at the site were likely responsible. Despite this, the study can be viewed as a success since much was learned that can be used in future studies of anaerobic biodegradation of MTBE and the application of HRC(tm).     

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.