Chlorinated solvents, such as trichloroethylene (TCE) or perchloroethylene (PCE), are among the most common groundwater pollutants. TCE and PCE often exist in the subsurface as dense non-aqueous phase liquids (DNAPLs), which serve as a source of long-term contamination. The inefficiency and high cost associated with the conventional remediation methods to remove DNAPLs led to seek for alternative ways of remediation, which accelerate the rate at which contaminated sites are restored back to an acceptable condition, and thereby reduce the cost of remediation. Probably the most attention in recent years is focused on the in situ chemical reduction using nanoscale metallic iron (Fe0), also referred to nanoscale zero valent iron (nZVI). The technology involves corrosion of Fe0 which provides the electrons necessary for the reduction of compounds such as chlorinated solvents. In aqueous solution, nZVI reacts with water and oxygen to form outer layer consisted of iron oxide/hydroxide. The accumulation of the corrosion products on Fe0 surface can affect reduction either by inhibiting contaminant access to the metal surface or by forming new sites for contaminant adsorption, reaction, and catalysis to occur.
Iron has long been recognized as physiological requirement for life of many microorganisms that persist in water, soils and sediments. Several iron reducing bacteria use Fe(III) oxyhydroxide and iron oxides as terminal electron acceptors under anaerobic conditions. These dissimilatory iron reducing bacteria (DIRB) that are widely distributed in the subsurface may play an important role in enhancement reductive treatment by nZVI. Hydrogen which is catholically produced during anaerobic corrosion of Fe0 can stimulate anaerobic bioremediation by serving as electron donor for the biotransformation of reducible contaminants. Moreover, DIBR could enhance the reactivity of nZVI by reductive dissolution of Fe(III)-oxide layers and formation of reactive minerals such as green rust and magnetite.
The aim of this study was to investigate whether DIRBs are able to utilize Fe(III) on the surface of nZVI and thus enhance its reactivity. Furthermore TCE removal by three strains of DIRBs was examined using different sources of Fe(III) including nZVI. Batch experiments using 120 mL serum bottles containing artificial groundwater, real groundwater and soil were used. The batch experiments were conducted in dark, at 12°C, and bottles were shaken using horizontal shaker. The bacterial concentrations were obtained using the most probable number method. Fe(II) and Fe(III) were analyzed using Ferrozine test, and head space analysis of TCE was performed on gas chromatograph/ mass spectrometer. Furthermore pilot test on a site contaminated by chlorinated ethylenes was conducted.
Experiments showed that tested DIRBs had sufficient microbial activity to reduce Fe(III) on the surface of nZVI. The experimental results also showed that DIRBs were able to degrade TCE in the condition of natural groundwater, however, did not improve the reactivity of nZVI.
ACKNOWLEDGEMENTS
This work was financially supported by Technology Agency of the Czech Republic, project number TA02020654.
Innovative Nature of the Topic.
Both enhanced reductive dechlorination (ERD) and in situ chemical reduction (ISCR) have emerged as cost-effective remedial approaches for groundwater with elevated concentrations of chlorinated solvents or heavy metals. ERD involves the addition to groundwater of an organic electron donor that can promote the activity of bacteria that mediate reductive dechlorination reactions. The electron donors can be augmented with a bacterial culture or consortium with proven ability to fully degrade common chlorinated solvents. ISCR treatment combines an organic electron donor addition with a chemical reducing agent, such as zero valent iron (ZVI) or divalent iron (DVI). In general, ERD is seen as the simplest technical approach for many chlorinated solvent sites, while ISCR is viewed as a more robust technology capable of dealing with more challenging groundwater conditions (e.g., wide range of pH, high sulfate levels, persistent contaminant sources, combined organics and metals).
Objectives.
Several factors should be taken into account when selecting an organic electron donor for use in ERD or ISCR applications, including cost, ease of use, and longevity. A wide variety of carbon substrates, including lactate, molasses, and vegetable oil, have been used in ERD and ISCR applications in the past. Recently, lecithin has been identified as a potentially advantageous organic electron donor based on its physical, chemical, and nutritional properties. This work involved testing of lecithin alone, and lecithin supplemented with DVI, under bench-scale, pilot-scale, and full-scale conditions. The full-scale applications were conducted at military sites in Texas and California, USA.
Approach/Activities: Bench-scale studies determined the influence of lecithin, and lecithin supplemented with DVI, on groundwater ORP, pH, TOC, and TCE degradation, including production and destruction of daughter products. Pilot-scale demonstrations focused on delivery of the lecithin substrate to the subsurface and evaluation of its influence on TCE degradation. Full-scale applications involved treatment of a TCE plumes and included monitoring of recognized ERD parameters including target compound degradation, metabolite generation and removal, and cost analysis.
Conclusions.
Bench-scale studies indicated that both lecithin and lecithin supplemented with DVI generated reducing conditions more rapidly than alternative organic carbon substrates, and supported TCE removal for more than 15 months. The pilot-scale demonstration enabled estimation of the zone of influence and addressed the issues of large-scale emulsion preparation, dilution, and injection methodology. Full-scale application focused on scale-up issues of substrate preparation, delivery, injection, distribution, TCE and metabolite degradation, and impacts on aquifer geochemistry. Cost information will also be presented.
Keywords: ERD, ISCR, DVI, Lecithin, Bio-Chemical Treatment, Metabolite Degradation, Groundwater Remediation, Chlorinated Solvents
A recent release of approximately 7,600 Kilograms of trichloroethylene (TCE) that occurred in the mid-90s resulted in an 18-meter thick impacted vadose zone and a 6-hectare dissolved groundwater plume. Early testing conducted in 2001 and 2003 identified TCE concentrations in the source area as high as 81,000 mg/Kg in soil and 1,200 mg/L in groundwater, near its water solubility limit. The owner has set ambitious clean-up objectives to reach the allowable regulatory TCE concentration of 5 ug/L throughout the entire plume by the year 2025. Multiple interim and final remedial approaches were implemented toward achieving this goal. Traditional and innovative technologies were applied, two of which are not usually considered in consort for the same site. This presentation will discuss remediation performance following a full scale injection of In-Situ Chemical Oxidation (ISCO) and In-Situ Chemical Reduction (ISCR) reagents that were pilot tested at the site, the results of which were presented at this conference in 2013.
Meeting the site clean-up goal required an aggressive approach of intensive interim remedial measures while completing the Remedial Investigation, Risk Assessment, and Feasibility Study, followed by a comprehensive final Remedial Action. A remediation strategy was developed to meet the various technical and schedule requirements of this particularly challenging site characterized by relatively high source area concentrations, low permeability saprolite overlying fractured bedrock, low natural attenuation rate, large plume area with limited accessibility, and a very aggressive remediation timeframe.
Approximately 89% of the contaminant mass was removed by the interim remedies, which consisted of excavation, soil vapor extraction (SVE) and In-Situ Thermal Desorption (ISTD) in the source area, and a Pump & Treat system along the property boundary. In the final remedy, these were replaced by SVE in the vadose zone, aggressive ISCO injection for rapid and complete contaminant mass removal in the source area aquifer, and a series of passive, long-lasting permeable reactive barriers (PRBs) using ISCR to address long-term contaminant advection and diffusion in the plume area. The latter two technologies are rarely applied at the same site because of their antagonistic redox environment and must be given careful consideration during design and implementation. However, their application as parts of the same combined remedy can be greatly beneficial to challenging sites. The efficacy of combining the antagonistic ISCO/ISCR remedial approaches was pilot tested at the same site prior to the full scale implementation. Modeling and monitoring were conducted during design and implementation as a basis for reagent requirements, injection point spacing, scale-up for future expansion of the treatment, and to ensure reagents do not interact and destroy each other. The injection wells were installed using sonic drilling methods to allow for a close examination of the overburden and bedrock core in the target injection intervals. Due to the low-permeability saprolite and random fracturing patterns in bedrock, traditional injection methods have proven unsuccessful at this site. Hence, both reagents were emplaced as slurry using a hydraulic injection technique in the form of fractures at a 1.2-meter vertical spacing.
For the ISCO source area treatment, 75 metric tons of potassium permanganate (KMnO4) and sand blend (50% each) were injected in three stages between 2011 and 2014, including the pilot study. Seventeen injection wells were used covering an area of approximately 390 square meters and extending 24 meters into the saprolite overburden and 4 meters into the underlying fractured granitic gneiss. The ISCR treatment consisted of three zero-valent iron (ZVI) PRBs that effectively divide the plume into four segments, thereby drastically shortening its lifespan. A total of 658 metric tons of granular ZVI were injected into 62 injections wells with 6 meters of separation between wells and PRB lengths ranging from 73 meters to 161 meters. Vertically, the PRBs extended up to 26 meters into the saprolite and up to 13 meters into the underlying granitic schist and gneiss.
Performance monitoring has been conducted at least quarterly. As of the fourth quarter of 2014, approximately 80 Kgs of TCE have been removed from the vadose zone by the SVE system. A similar amount (~74 Kgs) has been removed by the ISCO injection. It is estimated the clean-up goal in the source area (<5.0 ug/L) should be met after the removal of an additional 2 Kgs. Of the 15 monitoring wells in the source area, 11 wells have experienced complete TCE concentration reduction (>99.99%). TCE concentrations in the remaining four wells dropped by 92% to 99.5% from the ISCO baseline levels. Permanganate was not observed in any of the wells located outside the source area, and reducing conditions have been sustained in the immediately downgradient ZVI injection wells; therefore, the permanganate did not adversely affect the PRBs.
The ZVI performance monitoring wells have exhibited significant reductions in TCE concentrations ranging between 90.5% and 99.8% at the most upgradient PRB, but at lower rates in the mid-PRB (69.7% - 100%) and most downgradient PRB (34% - 59%), where groundwater flow rates are relatively lower.
Dual-phase extraction (DPE) or pump-and-treat (P&T) systems are widely used for the remediation of high concentrations of hydrocarbon nonaqueous phase liquid (NAPL) at contaminated sites. While the initial phase of DPE system operation typically achieves rapid reduction of NAPL the long-term effectiveness diminishes and the system often reaches an asymptote. Further operation of a system in asymptote conditions would provide little incremental benefit in treating soil or groundwater contamination thus negatively impacting both project costs and time.
The leveling off of DPE effectiveness typically arises as a result of hydrocarbon distribution through zones of differential matrix permeability, the presence of slowly dissolving smeared or sorbed hydrocarbon contamination, or a combination of both of these factors. For many remediation practitioners the next logical choice for remediation when DPE operation is asymptotic is to use In Situ Chemical Oxidation (ISCO). While the use of ISCO can be successful in many instances there are still two main limitations to ISCO to treat heavy sorbed phase contamination. The first being that DPE systems are often used in low permeability sites where they achieve greater treatment radii because of the beneficial use of high vacuum flow. These very same soils may prevent efficient distribution and contact of a chemical oxidant. The second point is that while a DPE system may have reached an asymptote the corresponding soil and groundwater concentrations may still be quite high meaning that the number of injections and volume of reagent required would be costly.
The use of surfactants to enhance recovery of sorbed-phase or smeared hydrocarbon is another option, but applications are rare owing to perceptions of cost, pore-blockage, trapping of residual hydrocarbons by sub-CMC residual surfactant, and high residual surfactant biological oxygen demand (BOD) that inhibit follow-on biodegradation or natural / enhanced attenuation of residual hydrocarbon.
This presentation will provide information on a reagent-based approach which systematically addresses the above issues in order to increase the efficiency and expedite the closure of physical extraction-based clean-up projects (DPE, pump-and-treat, etc.). This technology is entirely inorganic and presents no BOD yet provides combined ISCO and enhanced desorption at contaminated sites to treat bound hydrocarbon and NAPL. This approach can also be used to increase efficiency at failing DPE installations for fast and cost-effective mass reduction. An overview of the results from laboratory and field studies will be presented and the potential modes of usage and anticipated benefits to common remediation projects explored.
Chlorinated solvents are one of the most abundant contaminants of groundwater due to its frequent historical industrial exploitation. Chlorinated solvents are accompanied very often by hexavalent chromium Cr(VI) as a result of improper handling these chemicals during degreasing and subsequent metal plating processes. Despite of approximately 25 years of remediation technology development, achieving typical concentration-based cleanup goals in soil and groundwater for these constituents remains technically challenging at many contaminated sites. This study combines in-situ chemical reduction by nanoscale zero-valent iron (nZVI) and subsequent biological reductive treatment, supported by addition of whey as an organic substrate in order to treat aquifer impacted by Cr(VI) and chlorinated ethenes. Added substrate as electron donor supports microbial reduction of Cr(VI) to non-toxic and significantly less mobile Cr(III), in addition, hydrogen and acetate as products of substrate fermentation serve as the electron donors in reductive dechlorination of chlorinated ethenes.
Combining these two remedial approaches takes advantage of features from both – fast nZVI-mediated decrease of Cr(VI) concentrations in source area groundwater to prevent the further spread of the contamination followed by more economical treatment of chlorinated ethenes and the lower Cr(VI) concentrations in the plume by microorganisms.
The combined technology was tested in the field at a pilot site where the initial Cr(VI) concentration in groundwater ranged from 4.4 to 57 mg/l. The total concentration of chlorinated ethenes ranged from 400 to 6526 µg/l. Trichloroethene (TCE) and cis-1,2-dichloroethene (cis-DCE) were dominant chlorinated contaminants (TCE formed 45% up to 93% and cis-DCE formed 5% up to 53% of total chlorinated ethenes on a molar basis). At the pilot test site the aquifer lies in Quaternary sands and gravels with a saturated thickness of 4 m. nZVI was injected twice with a 4-month interval. 20 kg of pure nZVI in suspension was used for each injection using a direct push technology. Two months after the second nZVI injection, whey was added in order to achieve 60 mg TOC/l in groundwater.
During the pilot test, the applications of nZVI rapidly pretreated the aquifer with regards to toxic Cr(VI) and to some extend also to chlorinated ethenes without any negative impact on the composition or abundance of indigenous bacteria. Stimulation of biological reductive treatment using whey resulted in a further decrease in Cr(VI) concentrations in the groundwater below a detection limit (<0.05 mg/l) throughout the treated areas without any rebound of Cr(VI) concentrations after substrate depletion. Addition of whey resulted in a temporal increase of concentration of chlorinated ethenes as a consequence of the desorption/solubilisation effect of whey and its metabolites in the groundwater followed by intensive subsequent dechlorination of chlorinated ethenes up to ethene and ethane.
The chlorine number (average number of Cl atoms per ethane in the groundwater sample from down-gradient monitoring wells) decreased from initial 2.6 – 2.8 to 0.1 – 0.9 approximately 3.5 months after addition of whey. Within the same time the total concentration of chlorinated ethenes decreased to the range from 69 to 747 µg/l (the pilot test is on-going).
The results of PLFA analyses clearly indicated positive effect of nZVI injection on the abundance of indigenous microorganisms. The effect of whey application was rather complex and will be evaluated in a paper in detail. Dechlorinating bacteria belonging to genus Dehalobacter, Dehalococcoides and Sulfurospirillium were detected by polymerase chain reaction (PCR) or quantitative PCR (qPCR) in groundwater during the test. Cultivation tests showed positive effect of both nZVI and whey injection on psychrophilic bacteria.
No adverse effects to hydrochemical composition of groundwater were observed. Depletion of nitrate, temporary elevated concentrations of iron and manganese and a decrease in the content of sulphate are (together with a drop of EH) common indicators of the created thermodynamically favorable conditions for reduction of both contaminants.
In sum, the successive combination of the two in-situ methods – chemical reduction by nZVI and biological reductive treatment seems to be an efficient and sustainable remedial approach for treatment of the mixture of the rather frequent contaminants - Cr(VI) and chlorinated ethenes.