SEQUENTIAL USE OF ACTIVATED PERSULFATE OXIDATION AND SULFATE-REDUCTION FOR IN-SITU REMEDIATION
INTRODUCTION
Metropolitan has developed and applied an innovative approach to the use of sodium persulfate for the sequential in-situ treatment of subsurface contaminants through chemical oxidation followed by enhanced biological degradation through sulfate reduction. This approach has broad applicability to a wide range of contaminants, and shows strong cost-saving benefits through reducing the initial volume of chemical oxidant necessary and enhancing the in-situ biological degradation of contaminants. Through proper subsurface geochemical characterization and chemical dosing design, the approach focuses on utilizing the oxidant for immediate mass reduction at the source area, followed by degradation or polishing of the residual contamination using sulfate reducing bacteria.
Depending upon the oxidant activation method, this approach is applicable to petroleum hydrocarbons including both volatiles and PAHs, chlorinated volatile organic compounds (CVOCs) including chlorinated ethene, ethane and methane groups, as well as PCBs. As discussed further below, enhanced sulfate reduction is also conducive to metals precipitation, and therefore, potentially useful for metals remediation and addressing metals mobilization concerns typical of in-situ chemical oxidation. Although still developing, we believe this sequential oxidation and enhanced biodegradation approach is also applicable for the remediation of explosives or accelerants such as TNT, TNB, RDX and HMX.
Metropolitan is currently seeking and pursuing opportunities for pilot or full-scale demonstration projects to further document and evaluate the timing and robustness of these two processes for treatment of any or a combination of these contaminant or chemical mixtures. Further benefits and supporting details of this innovative approach are provided in the following discussion. For additional information, please contact one of the Metropolitan remediation professionals provided below.
Depending upon the oxidant activation method, this approach is applicable to petroleum hydrocarbons including both volatiles and PAHs, chlorinated volatile organic compounds (CVOCs) including chlorinated ethene, ethane and methane groups, as well as PCBs. As discussed further below, enhanced sulfate reduction is also conducive to metals precipitation, and therefore, potentially useful for metals remediation and addressing metals mobilization concerns typical of in-situ chemical oxidation. Although still developing, we believe this sequential oxidation and enhanced biodegradation approach is also applicable for the remediation of explosives or accelerants such as TNT, TNB, RDX and HMX.
Metropolitan is currently seeking and pursuing opportunities for pilot or full-scale demonstration projects to further document and evaluate the timing and robustness of these two processes for treatment of any or a combination of these contaminant or chemical mixtures. Further benefits and supporting details of this innovative approach are provided in the following discussion. For additional information, please contact one of the Metropolitan remediation professionals provided below.
IN-SITU CHEMICAL OXIDATION (ISCO)
Persulfate Oxidation
Persulfates are strong oxidants that have been widely used in many industries for initiating emulsion polymerization reactions, clarifying swimming pools, hair bleaching, micro-etching of copper printed circuit boards, and TOC analysis. In the last few years there has been increasing interest in sodium persulfate as an oxidant for the destruction of a broad range of soil and groundwater contaminants. The persulfate anion is the most powerful oxidant of the peroxygen family of compounds and one of the strongest oxidants used in remediation. The standard oxidation – reduction potential for the reaction
is 2.1 V, as compared to 1.8 V for hydrogen peroxide (H2O2). This potential is higher than the redox potential for the permanganate anion (MnO4-) at 1.7 V, but slightly lower than that of ozone at 2.2 V.
In addition to direct oxidation, sodium persulfate can be induced to form sulfate radicals, thereby providing free radical reaction mechanisms similar to the hydroxyl radical pathways generated by Fenton’s chemistry. The generation of sulfate radicals is
The sulfate radical is one of the strongest aqueous oxidizing species with a redox potential estimated to be 2.6 V, similar to that of the hydroxyl radical, 2.7 V.
In addition to its oxidizing strength, persulfate and sulfate radical oxidation has several advantages over other oxidant systems. First, it is kinetically fast. Second, the sulfate radical is more stable than the hydroxyl radical and thus able to transport greater distances in the sub-surface. Third, persulfate has less affinity for natural soil organics than does the permanganate ion and is thus more efficient in high organic soils. Finally, the persulfate releases sulfate anions in the subsurface that can be used by sulfate-reducing bacteria to biologically degrade or stabilize a large number of contaminants. These attributes combine to make persulfate an effective and economic option for the chemical oxidation of a broad range of contaminants.
Source Area Mass Reduction/Ability to Treat a Wide-Range of Contaminants
ISCO has proven extremely useful for source area mass reduction to destroy mobile and recalcitrant contaminants. Important advantages of ISCO include the speed of reaction, that there is no need for permanent above-ground installations and its cost benefit. Activated Sodium Persulfate is an emerging oxidant of choice for chemical oxidation because of its ability to treat a wide range of contaminants, safer handling characteristics and longer staying-power in the subsurface. When properly activated, persulfate provides an excellent combination of oxidative power and control that can be delivered both safely and cost efficiently. Successful pilot and full-scale field applications of Activated Sodium Persulfate have demonstrated it’s ability to treat a wide range of contaminants including; chlorinated ethenes (TCE, PCE, DCE, and vinyl chloride), chlorinated ethanes (1,1,1-TCA, DCA, vinyl chloride), chlorinated methanes (carbon tetrachloride, chloroform), polyaromatic hydrocarbons (PAHs), petroleum hydrocarbons, BTEX, MTBE and 1,4-dioxane.
The Metropolitan Technology Results in Reduced Amount of Oxidant and thus Saving Time and Money
Chemically, the stoichiometric oxidant demand for sodium persulfate varies between 20 and 45 pounds of oxidant per pound of oxidizable compound. This high demand results in increased time in the field, increased number of mobilizations and resulting higher overall project costs. One primary challenge of any ISCO project is developing a thorough characterization and dosing design for the contaminants present. No less challenging is the delivery of the oxidant to the subsurface and achieving contact with all contaminant mass necessary to meet remedial goals or performance criteria (ISCO is a “contact sport”). These chemical and physical challenges, while well understood and being further developed / optimized continuously, lead to relatively high demand for chemical oxidant quantities and incentive for conservative “safety factors” in the dosing design. However, the stoichiometry and other design challenges do not account for any subsequent reduction of the contaminant mass by sulfate-reducing bacteria that would be stimulated by the addition of the sulfate.
Thus, following initial oxidation, the persulfate can play a role in enhancing the natural attenuation of contaminants. In fact, enhanced sulfate reduction shows robust capabilities for biological degradation of a wide range of possible contaminants. While not well studied or understood as a sequential approach, Metropolitan believes the destruction of contaminants using both the oxidation and anaerobic degradation processes can be demonstrated and quantified, resulting in reduced chemical and project costs, and improved contaminant destruction beyond the initial, and potentially limited ISCO contact treatment. By taking into consideration the contaminant degradation by the sulfate-reducing bacteria, will reduce the amount of persulfate that needs to be injected at the source zone. For example, while 1,000 pounds of persulfate can oxidize 22 pounds of BTEX, the SRB bacteria can utilize the injected sulfate to destroy an additional 160 pounds of BTEX. At the same time, metals would be immobilized due to the production of metal sulfides that represent among the most stable compounds on earth.
Thus, following initial oxidation, the persulfate can play a role in enhancing the natural attenuation of contaminants. In fact, enhanced sulfate reduction shows robust capabilities for biological degradation of a wide range of possible contaminants. While not well studied or understood as a sequential approach, Metropolitan believes the destruction of contaminants using both the oxidation and anaerobic degradation processes can be demonstrated and quantified, resulting in reduced chemical and project costs, and improved contaminant destruction beyond the initial, and potentially limited ISCO contact treatment. By taking into consideration the contaminant degradation by the sulfate-reducing bacteria, will reduce the amount of persulfate that needs to be injected at the source zone. For example, while 1,000 pounds of persulfate can oxidize 22 pounds of BTEX, the SRB bacteria can utilize the injected sulfate to destroy an additional 160 pounds of BTEX. At the same time, metals would be immobilized due to the production of metal sulfides that represent among the most stable compounds on earth.
SULFATE REDUCTION FOLLOWING THE CHEMICAL RELEASE OF SULFATE
At many source areas, the natural bioremediation of contaminants is hindered by the presence of biotoxic concentrations and/or the lack of electron acceptors. Sulfate is a well known electron acceptor utilized in anaerobic biodegradation. This process is termed sulfate reduction and results in the production of sulfide. Chemically, sulfate consists of one sulfur atom surrounded by four oxygen atoms. Sulfate-reduction strips away the four oxygen atoms leaving the sulfur atom in a form known chemically as sulfide.
The four oxygen atoms are used by the sulfate-reducing bacteria (SRB) to change carbon containing "foods" or "fuels" (i.e. contaminants) into carbon dioxide and water. It is estimated that once natural or enhanced sulfate-reduction takes hold, approximately five (5) pounds of sulfate per pound of petroleum hydrocarbons is needed to achieve anaerobic degradation of the contaminants. This is substantially lower than the 20-45 pounds of persulfate needed to oxidize one pound of contaminants. Evidence suggests that the growth and development of naturally occurring sulfate-reducing bacteria can happen relatively quickly (e.g., within thirty days or so), and that these populations are robust and aggressive, and show substantial contaminant degradation capabilities.
The four oxygen atoms are used by the sulfate-reducing bacteria (SRB) to change carbon containing "foods" or "fuels" (i.e. contaminants) into carbon dioxide and water. It is estimated that once natural or enhanced sulfate-reduction takes hold, approximately five (5) pounds of sulfate per pound of petroleum hydrocarbons is needed to achieve anaerobic degradation of the contaminants. This is substantially lower than the 20-45 pounds of persulfate needed to oxidize one pound of contaminants. Evidence suggests that the growth and development of naturally occurring sulfate-reducing bacteria can happen relatively quickly (e.g., within thirty days or so), and that these populations are robust and aggressive, and show substantial contaminant degradation capabilities.
Further, when provided with an organic carbon source (such as BTEX, other petroleum hydrocarbons, etc.), SRB will reduce sulfate to soluble sulfide; bicarbonate ions are also produced, helping to stabilize the water pH. The soluble sulfide reacts with metals in the groundwater to form insoluble metal sulfide precipitates. Therefore, this approach is more appropriate in situations where there are potential concerns with metal mobilization, as with a conventional oxidation method. Using this sequential remediation method, the SRB-created sulfides can precipitate metals and minimize displacement or mobilization.
SRB CAPABLE OF REDUCTIVE DECHLORINATION
Although the sulfate-reduction process or phase of this sequential persulfate application technology is not well understood or studied, studies and literature provide strong evidence of the capability of SRB to treat many contaminants including chlorinated solvents through reductive dechlorination. In general, SRB produce sulfide as a waste product while obtaining electrons from molecules such as alcohols or organic acids. Several SRB species can also be useful in the co-metabolic reduction of CAHs. Studies have shown that SRB Desulfitobacterium frappieri and Desulfomonile tiedjei are capable of degrading PCE to cis-DCE (Gerritise et al., 1996; Townsend and Suflita, 1996). Desulfitobacterium chlororespirans has been shown to degrade other CAHs such as 3-chloro-4-hydroxybenzoate (Gerritise et al., 1996).
Scientists have sequenced the genome of a sulfate-breathing bacterium that can damage oil and natural gas pipelines and corrode oilfield equipment. The microbe, Desulfovibrio vulgaris, plays a role in a process called microbial-influenced corrosion (MIC). The analysis of the microbe's genes is expected to help find better ways to minimize such damage as well as to develop methods to use such microbes to help remediate metallic pollutants such as chromium, arsenic, etc.
SRB SPECIES CAPABLE OF METAL REDUCTION
Desulfovibrio is a model for the study of sulfate-reducing bacteria, which use hydrogen, organic acid, or alcohols as electron donors to "reduce" (that is, add electrons to) certain metals, including chromium, arsenic, uranium, etc. Other sequenced microbes that are capable of such reduction include Shewanella oneidensis and Geobacter sulfurreducens, In their analysis of the D. vulgaris genome, scientists found a network of c-type cytochromes – proteins which facilitate electron transfers and metal reduction during the organism's energy metabolism. The presence of those c-type cytochrome genes are thought to give D. vulgaris a significant capacity and flexibility to reduce metals.
RDX REMEDIATION
Although even less studied at the current time, some work has been done that provides further evidence that SRB are capable of degrading explosives or accelerant contaminants prevalent at many military sites. The metabolism of TNB (not TNT), RDX, and HMX by a sulfate-reducing bacterial consortium, Desulfovibrio spp., was studied by Boopathy, R., Gurgas, M., Ullian, J., and Manning, J.F. (1998). Metabolisms of Explosive Compounds by Sulfate-Reducing Bacteria. Current Microbiology 37(2): 127-131.
The results indicated that the Desulfovibrio spp. used all of the explosive compounds studied as their sole source of nitrogen for growth. The concentrations of TNB, RDX, and HMX in the culture media dropped to below the detection limit (less than 0.5 parts per million [ppm]) within 18 days of incubation. The sulfate reducing bacteria may be useful in the anaerobic treatment of explosives-contaminated soil.
The results indicated that the Desulfovibrio spp. used all of the explosive compounds studied as their sole source of nitrogen for growth. The concentrations of TNB, RDX, and HMX in the culture media dropped to below the detection limit (less than 0.5 parts per million [ppm]) within 18 days of incubation. The sulfate reducing bacteria may be useful in the anaerobic treatment of explosives-contaminated soil.
COMPETITION BETWEEN METHANOGENS AND SULFATE-REDUCING BACTERIA
It has long been known that methane production in marine sediments occurs only after sulfate has been depleted from the pore water (Martens and Berner 1974; Winfrey and Zeikus 1977). Subsequently, it was found that this is due to the competition between methanogens and SRB for some electron donors (Banat et al. 1983; Winfrey and Ward 1983). Based on thermodynamic considerations, the utilization of H2 or acetate by SRB yields more energy than the utilization of these two substrates by methanogens. Thus, SRB obtain more energy for a given substrate than do methanogens, and they out-compete the methanogens for that substrate if sulfate is sufficiently abundant in the habitat.
When sulfate is depleted, methanogens carry out the terminal steps of decomposition in the anaerobic environment. In addition, the sulfate reducer Desulfovibrio vulgaris (Marburg) has a lower apparent Ks (half-velocity constant) for H2 than does the methanogen Methanobrevibacter arboriphilus (Kristjansson et al. 1982), indicating that the former outcompetes the methanogen for H2 when the concentration of this electron donor is low. Similarly, the Ks value for acetate is lower for the acetate-utilizing sulfate reducer Desulfobacter postgatei than for the methanogen Methanosarcina barkeri (Schönheit et al. 1982). Thus, SRB have both thermodynamic and kinetic advantages over methanogens.
When sulfate is depleted, methanogens carry out the terminal steps of decomposition in the anaerobic environment. In addition, the sulfate reducer Desulfovibrio vulgaris (Marburg) has a lower apparent Ks (half-velocity constant) for H2 than does the methanogen Methanobrevibacter arboriphilus (Kristjansson et al. 1982), indicating that the former outcompetes the methanogen for H2 when the concentration of this electron donor is low. Similarly, the Ks value for acetate is lower for the acetate-utilizing sulfate reducer Desulfobacter postgatei than for the methanogen Methanosarcina barkeri (Schönheit et al. 1982). Thus, SRB have both thermodynamic and kinetic advantages over methanogens.
THE BOTTOM LINE
The use of Metropolitan’s hybrid chemical oxidation/anaerobic biodegradation approach has the potential to substantially reduce the in-situ treatment costs for a large number of contaminants and at complex sites where a mixture of chemicals are present. Metropolitan’s method will ensure that matrix issues are dealt with reliably (minimize the rebounding effects), while reducing the amount of field work with minimum disruptions to the property.
REFERENCES
1) Behrman, E.J. and J.O. Edwards. Reviews in Inorganic Chemistry, 2, p 179 (1980) Brown, R.A., D. Robinson and G. Skladany. “Response to Naturally Occurring Organic Material: Permanganate versus Persulfate”, ConSoil 2003, Ghent Belgium, (2003) Bruell, C. J. “Kinetics of Thermally Activated Persulfate Oxidation of Trichloroethylene (TCE) and 1,1,1- Trichloroethane (TCA),” The First International Conference on Oxidation and Reduction Technologies for In-Situ Treatment of Soil and Groundwater, Niagara Falls, Ontario, Canada, June 25-29, 2001
2) Beller, H.R., Reinhard, M., and Grbic-Galic, D., 1992, Metabolic byproducts of anaerobic toluene degradation by sulfate-reducing enrichment cultures: Appl. Environ. Microbiol., v. 58, p. 3192-3195.
3) Balazs, G.B., J.F. Cooper, P.R. Lewis and G.M. Adamson. Emerging Technologies in Hazardous Waste Management 8, ed. Tedder and Pohland, Kluwer Academic / Plenum Publishers, New York, 2000.
4) Beller, H. R., D. Grbic-Galic, and M. Reinhard. 1992b. Microbial degradation of toluene under sulfate-reducing conditions and the influence of iron on the process. Appl. Environ. Microbiol. 58:786-793
5) Coates, J.D., R.T. Anderson, and D.R. Lovley. 1996. “Oxidation of Polycyclic Aromatic Hydrocarbons under Sulfate-Reducing Conditions.” Appl. Environ. Microbiol.62:1099-1101.
6) Coates, J.D., J. Woodward, J. Allen, P. Philip, and D.R. Lovley. 1997. “Anaerobic Degradation of Polycyclic Aromatic Hydrocarbons and Alkanes in Petroleum-Contaminated Marine Harbor Sediments.” Appl. Environ. Microbiol., 63:3589-3593.
7) Elmendorf, C., F. Sessa. Poster at the 4th Annual Battelle Conference On the Remediation of chlorinated and Recalcitrant Compounds, Panther Technologies and FMC Corporation (2004).
8) FMC Corporation , activation of persulfate using peroxide, patent pending technology (2002)
9) FMC Corporation, activation of persulfate using high pH, patent pending technology (2003)
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