WO2003076092A1 - In situ microbial stabilization of metals using phosphate amended bioventing/biosparging - Google Patents

Abstract

A method is disclosed for stabilizing metal contaminants within a contaminant zone. Bioventing or biosparging is augmented with a phosphate amendment in the form of an organophosphorous compound. Metabolic action of microorganisms within the contaminant zone produce a concentration of inorganic phosphate permeating the contaminant zone. Reaction of the phosphate with metal contaminants stabilizes the metal contaminants. The method may be used alone or in conjunction with other bioremediation techniques.

Description

IN SITU MICROBIAL STABILIZATION OF METALS USING

PHOSPHATE AMENDED BIOVENTING/BIOSPARGING

Cross Reference to Related Applications

This application has the same inventors and relates to the same subject matter as disclosed in provisional application, Serial No.60/361 ,600; filed March 4, 2002, entitled In Situ Microbial Stabilization of Metals Using

Phosphate Amended Bioventing/Biosparging." Priority is clamed from said provisional application.

Statement of Government Rights

The United States Government has rights in this invention pursuant to Contract No. DE-AC09-96SR18500 between the U.S. Department of Energy and Westinghouse Savannah River Company.

Field of the Invention

This invention relates to methods for remediation and reclamation of contaminated soil and groundwater. More particularly, the invention relates to a method of utilizing gas phase phosphate esters for bioremediation of contaminants such as metals, including radionuclides, by creating increased levels of inorganic phosphate to stabilize contaminant metals present in the contaminated soil and/or groundwater. Background of the Invention

Contamination of soil and groundwater poses serious ecological and health problems in industrialized nations. As knowledge and awareness of the seriousness and extent of contamination grow, so does the awareness of the need for large-scale, cost-efficient, remediation of the contamination. Contaminants threaten the environment by toxically eliminating and/or changing the natural balance of microbes in soil. The microbes perform important functions such as nitrogen-fixation. Contaminants pose a health threat through direct or indirect ingestion by humans and animals, such as by contamination of drinking water or damage to or uptake by food crops. It is, for example, estimated that over 15% of drinking water supplies in the United States are contaminated with chlorinated hydrocarbons.

Several types of remediation methods have been proposed and used to eliminate or control contaminants in soil and groundwater. The method considered to be the baseline method for remediation of soil is excavation and disposal. While this methodology is effective to remove contamination from the original site, it presents several problems. The exceedingly high cost of physical removal of the soil is one such problem. At the same time, such massive excavation as is needed to completely decontaminate the site has an obviously detrimental effect on the natural ecology of the site. Another problem with this method is that it simply relocates the contaminants. Areas for dumping contaminated soil that were once considered remote enough for safety are increasingly becoming populated, leading anew to the need for remediation. Also, as awareness of the problem increases, simply relocating the problem becomes a less acceptable solution. Therefore, methods have been attempted whereby the soil is treated to remove or stabilize the contaminants.

In a similar manner, contaminated groundwater can be removed through extraction wells or along with the excavation of contaminated soil. Contaminated groundwater cannot simply be relocated, inasmuch as the contaminants will again disperse into the environment. Methods have been utilized whereby the water is separated from the soil, or pumped therefrom in the first instance, and then subjected to a variety of treatment methods. These methods almost uniformly require the creation of fairly complex treatment facilities.

Another significant problem with removal and treatment of soil or water is the sheer volume of material that must be excavated and treated. A relatively small amount of a contaminant can affect a very large volume of soil and water. In the case of contaminated groundwater, plumes of contaminated water moving through an aquifer may extend laterally many hundreds of meters in each direction, and extend many meters vertically. Like volumes of soil are contaminated as the plume passes through the soil. While excavation may be effective, the costs in materials, energy, and time to excavate and treat these large quantities is prohibitive. An alternative to excavation and treatment is in situ remediation. In situ remediation can also take many forms. Techniques are known for stripping volatile components, as described for example in U.S. Patent No. 4,832,122 to Corey et al. Other techniques involve injecting reagents to react with the contaminants to produce less toxic or nontoxic reaction products, or to precipitate contaminants to thus immobilize them.

Another type of in situ remediation is bioremediation. In this type of remediation, living organisms are utilized to treat the contaminants within the soil or water. The metabolic processes of the organisms, specifically or in conjunction with natural geochemical processes, can be effective to break down, immobilize, or detoxify contaminants in the soil or water. These processes can be highly effective for certain types of contaminants, and have the advantage of utilizing existing resources. The process can also be slow and inefficient, however, in comparison with other decontamination processes.

Bioremediation can rely on the indigenous organisms naturally existing at the contaminated site, or particular organisms can be added to the site. The added organisms may be naturally-occurring or genetically altered exogenous microorganisms. In either case, increasing the supply of critical nutrients can enhance the remediation rates achieved by the microorganisms. The biomass population can be increased, for example, by addition of oxygen, nitrogen, and phosphorous at the site. Inorganic trace elements such as iron, magnesium, and manganese can also be used to increase the biomass. The amount and kind of elements needed will depend on known factors such as the types of microorganisms constituting the biomass, the geochemistry of the site, and other factors, and the mixture of nutrients to be added can be accordingly tailored as needed. When it is determined through sampling that the contaminant level has decreased to acceptable levels, the nutrient supply is terminated and the microbial population returns to its normal pretreatment level.

One example of a bioremediation method using ex situ techniques is shown in U.S. Patent No. 4,401,569 to Jhaveri et al. According to that disclosure, contaminated water is pumped to the surface. Microorganisms and nutrients such as oxygen, nitrogen, and carbon dioxide are added to the water and mixed. The mixture is then pumped back to the contaminated site, where it is supposed to leach out and help biodegrade the contaminants.

Other known methods rely on adding nutrients to the contaminated site via injection wells. Examples of these techniques can be found in U.S. Patent No. 4,765,902 to Ely et al.; U.S. Patent No. 3,946,290 to Raymond; U.S. Patent No.4,849,360 to Norris et al.; and U.S. Patent No.4,850,745 to Hater et al. Generally, these techniques consist of pumping water, oxygen, phosphates, nitrates, alkali metals, and other nutrients into the contaminated site through injection wells. The injected mixture may also contain selected cultures of microorganisms to create or augment the necessary biomass. These injections are supposed to increase the metabolism of contamination hydrocarbons. Other techniques utilizing bioremediation have been disclosed. In U.S. Patent No. 5,384,048 to Hazen et al., a cyclical treatment is used to enhance degradation of contaminants. The method disclosed in this patent consists of injecting oxygen and nutrients to stimulate the growth of microbes capable of aerobic degradation of the particular contaminant. This injection is followed by injection of oxygen alone, such that the absence of nutrients forces the enhanced population to metabolize the contaminant. This cycle is continued until the contaminant level is acceptably reduced. In U.S. Patent No. 5,326,703 to Hazen et al., an amount of tetrachloroethylene (also known as perchloroethylene (PCE)) is injected near the contaminated site to attract indigenous microbes to the site, which thereafter also metabolize the contaminants.

While these techniques, and others, hold some promise, there remain several drawbacks. A contaminated site may extend for hundreds of meters laterally and many meters vertically. Injection of solid or liquid mixtures of nutrients and/or microorganisms is not successful in treating the entire mass.

Liquid mixtures adsorb to the soil in the immediate vicinity of the injection site. The nutrients do not disperse and in fact may so stimulate the growth of biomass near the injection site that the injection well becomes plugged. Providing enough injection wells to infuse the entire contaminant field is impractical due to costs, the presence of surface (e.g., buildings) or subsurface (e.g., pipelines) structures, and other impediments. Solid materials, of course, disperse even less than liquid materials and thus are less mobile causing similar problems and having even greater limitations on use than liquid materials.

Attempts have been made to supply the nutrients in a gaseous form. In this form, the nutrients disperse much more readily and widely. Examples of such nutrients include gas phase oxygen, nitrogen, carbon dioxide, ammonia, and methane. Other nutrients, however, are not available in a gas phase, or are available only as toxic gases. There are, for example, few nontoxic compounds of phosphorus that, at environmental temperatures and pressures, are gaseous or have high vapor pressures. Another drawback to existing methods of bioremediation relates to the treatment of contaminating metals. The bioremediation techniques known in the art are largely limited to the treatment of organic contaminants such as hydrocarbons. Many contaminated sites, however, also contain metals such as lead or cadmium as contaminants. These types of contaminants are typically present throughout extensive areas in low concentrations. They may be found in areas also contaminated with organic contaminants, or may constitute the major contaminants of the site. Certain sites, such as but not limited to mine tailings and certain industrial sites, are contaminated by metals including radionuclides, but have little or no organic contaminants. Known techniques to remediate contamination by metals are both expensive and largely ineffective. Excavation and treatment, or excavation and confinement, are expensive, very limited in applicability, and resource intensive. Excavation and treatment requires a great expenditure of money and material to excavate and treat the contaminated soil or water. Excavation and confinement simply relocates the problem. Chemical treatment includes mixing solid material into the soil or water, where the solid material contains reactants that will precipitate or adsorb the metals. Due to costs and other factors, such in situ treatment is limited to very shallow surface treatment. Also, this technique cannot be used where surface or subsurface structures, or land use, prevents introduction of the treating material. Many of the compounds needed to treat the metal contamination are themselves toxic and therefore unusable. There is therefore a need for methodologies that will permit the in situ remediation of contaminants including metals that avoid or eliminate the foregoing problems. The methods must be capable of utilizing nontoxic substances, and must be capable of application throughout the contaminated area. Finally, of course, the methods must be capable of adequately treating the contaminants.

Summary of the Invention

It is an object of this invention to provide a method for in situ remediation of contaminants in soil and water. It is another object of this invention to provide a method for in situ remediation of metal contaminants in soil and water. It is a further object of this invention to provide a method of supplying nutrients to microorganisms capable of utilizing gas phase phosphorous compounds as a nutrient and producing phosphate as a metabolic product. It is also an object of this invention to provide a method of controlling the growth of indigenous or exogenous microorganisms in a subsurface area by providing a controlled amount of macronutrients such that the microorganisms treat contaminants by metabolic processes.

It is also a further object of this invention to provide a method of controlling the growth of subsurface microorganisms by providing a controlled amount of macronutrients such that the microorganisms supply a controlled amount of phosphate within a contaminant zone to treat metal contaminants.

It is also an object of this invention to provide a method for controlling a population of microorganisms in soil and water by adding selected nutrients whereby phosphorous compounds are converted to phosphate for the precipitation and/or immobilization of metals in the soil or water.

It is a further object of this invention to provide a method for precipitating, co-precipitating , and/or otherwise immobilizing metal contaminants in soil or water by providing selected nutrients to microorganisms in the soil or water such that phosphorous compounds are produced, the phosphorous compounds participating in reactions causing the precipitation, co-precipitation, and/or other immobilization of metal contaminants. It is another object of this invention to provide a method for treating inorganic contaminants in soil or water in situ by providing macronutrients to a biomass of microorganisms to control the population of microorganisms such that the microorganisms metabolically produce a relative excess of phosphate to treat contaminants.

These and other objects of the invention are achieved by providing a method for treating contamination in a contaminant zone, the method comprising the steps of (1 ) assaying the contaminant zone to determine the type and amount of contamination therein; (2) assaying the contaminant zone to determine (i) the type and amount of microorganisms therein and (ii) the nutrient requirements of said microorganisms needed to achieve a selected population or growth rate of the microorganisms; (3) contacting at least one organophosphorous nutrient with a gas such that at least a portion of said nutrient is vaporized and mixes with said gas to form a nutrient mixture; (4) delivering said nutrient mixture to said contaminant zone in an amount at least equal to said nutrient requirements; and (5) continuing said delivering for a period of time sufficient to cause the metabolic production by said microorganisms of an amount of phosphates from said organophosphorous nutrient, said amount of phosphates being sufficient to treat certain types of said contamination. The method can include the step of introducing into the soil or water an exogenous population of microorganisms. The method can also include providing nutrients in addition to the organophosphorous nutrient. The method can be used solely for the remediation of inorganic contaminants, such as metals, and may be used in conjunction with other types of bioremediation.

These and other objects of the invention are also achieved by providing a method for treating contamination in a contaminant zone, the contaminants comprising metals including but not limited to radionuclides, the method comprising the steps of (1) assaying the contaminant zone to determine the type and amount of contamination therein; (2) assaying the contaminant zone to determine (i) the type and amount of microorganisms therein and (ii) the nutrient requirements of said microorganisms needed to provide said microorganisms a ratio or nutrients such that the phosphorous component of said nutrients is in excess of the bulk C:N:P ratio required to at least sustain said microorganisms; (3) contacting at least one organophosphorous nutrient with a gas such that at least a portion of said nutrient is vaporized and mixes with said gas to form a nutrient mixture; (4) delivering said nutrient mixture to said contaminant zone in an amount at least equal to said ratio of nutrients; and (5) continuing said delivering for a period of time sufficient to cause the metabolic production by said microorganisms of an amount of phosphates from said organophosphorous nutrient, said amount of phosphates being sufficient to treat certain types of said contamination.

Brief Description of the Drawings Fig. 1 is a schematic illustration of the nutrient cycle created by a preferred embodiment of the current invention.

Fig. 2 is a schematic illustration of an example configuration for carrying out a preferred embodiment of the current invention. Fig.3 is a schematic illustration of a second example configuration for carrying out a preferred embodiment of the current invention.

Detailed Description of the Invention

Contamination may arise from a variety of sources, and may take a variety of forms. Contaminants may be released as a result of an industrial process, leakage from storage facilities or during transportation of materials, from sites such as mines or other operations, or from accidental releases, e.g., accidents during transportation of hazardous materials.

The release of such hazardous materials, or contaminants, forms what is referred to herein for convenience as a contaminant zone. A "contaminant zone" is, simply, the area or formation of soil and/or water that contains contaminants. A well-known type of contaminant zone is a plume of contaminants within an aquifer. Thus, the contaminant zone may be, and usually is, mobile. The contaminant zone may consist of only contaminated water, only contaminated soil, or both. The term "contaminants" as used herein is intended to have a broad meaning. Some contaminants are compounds that are known to be toxic even at very low concentrations, while other contaminants may be harmless at low concentrations but toxic at higher concentrations. Contaminants include organic compounds, such as hydrocarbons exemplified by benzene and other petroleum products and manufactured or synthetic organics such as perchloroethylene (PCE); inorganic compounds such as metal sulfates; or elements, such as lead or cadmium. The term "metal contaminants," as used herein, refers to metal compounds and complexes of any kind; elemental metals; and radionuclides such as uranium.

There are many methods of remediation of contaminant zones, as noted, one of which is bioremediation. In essence, bioremediation is the use of microorganisms to metabolize the contaminants into nontoxic compounds.

The microorganisms constituting the biomass may be indigenous or exogenous to the contaminant zone, and may be naturally occurring or genetically engineered. Bioremediation has been proposed for ex situ remediation, where the contaminated soil and/or water is excavated to a treatment site. Microorganisms and nutrients are mixed with the excavated material under conditions favoring the metabolic breakdown of the contaminants. Fairly precise control of the remediation process is possible because the mixture is effectively isolated in a reaction chamber. This methodology can be effective, but is still subject to the costs and considerations of excavating the contaminated material.

In situ bioremediation techniques eliminate the need for excavation, but present other challenges. In situ bioremediation requires the controlled and uniform delivery of amendments, such as basic nutrients, to the contaminant zone. To effectively treat all of the contaminant material within the contaminant zone requires that the biomass of microorganisms, and hence the necessary added amendments, permeate the entire zone.

Two common bioremediation methods are known as bioventing and biosparging. Examples of these methodologies can be found in U.S. Patent Nos. 5,480,549 and 5,753,109, currently assigned to the U.S. Department of Energy. These methods rely on the engineered movement of gas through the soil and/or water comprising the contaminant zone. The gas is the means of delivery of the nutrients required by the microorganisms. Essentially, these methods require first that the nutrient amendments be created in the form of a gas mixture, the constituents of which are further discussed below. The gas mixture can be introduced to the contaminant zone through one or more injection wells, which can be vertical and/or horizontal wells of various forms and configurations. The injection wells are so placed with respect to the contaminant zone that the injected gas will permeate the zone.

Movement of the gas through the contaminant zone can be further controlled through the use of one or more extraction wells. By extracting material from the contaminant zone, or from a selected point outside the zone, the injected material may be drawn through the zone as desired. Variables such as the amount of injected material drawn through the zone, or the speed with which the material moves, may be controlled by the respective injection and extraction rates.

In a contaminant zone having hydrocarbon contaminants, the primary element limiting aerobic biodegradation of the contaminants is oxygen. Nitrogen and phosphorous, respectively, are the next two limiting elements. Bioventing and biosparging can provide the necessary oxygen in the form of normal or oxygen-enhanced air. The oxygen is provided to the microorganisms as the air moves through the zone. Nitrogen also can be provided as a constituent of air, or in the form of ammonia or similar nitrogen compounds. Data using fluorescent antibody markers has indicated that nitrogen-fixing microbes are stimulated early in the process. Thus, even if nitrogen compounds are required to begin a process of enhancing microbial growth, as the process proceeds, nitrogen is available from air, and the use of compounds can be eliminated.

The remaining critical amendment, phosphorous, must also be delivered to enable the enhancement and control of the microbial population. In the past, attempts have been made to introduce phosphorous compounds in solid form. Adequate dispersion of phosphorous in such form throughout a contaminant zone, however, was not possible. Solid phosphorous sources in situ will not permeate the contaminant zone and will, in fact, relatively quickly become effectively sealed off by clogging caused by organic growth or inorganic reactions. Until recently, while phosphates in vapor form were known, they were typically toxic themselves and therefore could not be used.

In U.S. Patent Nos. 5,480,549 and 5,753,109, there is disclosed a method by which gas phase phosphorous compounds may be used in bioventing and biosparging. The method, referred to as PHOSter™ (Westinghouse Savannah River Company, Aiken, South Carolina), utilizes a nutrient mixture including organic phosphates, of which triethyl phosphate (TEP: (C₂H₅)₃PO₄), tripropyl phosphate (TPP: (C₃H₇)₃PO ), and tributylphophate (TBP: (C₄H₉)₃PO₄) are examples. Such phosphates have sufficiently high vapor pressure to be used in a bioventing or biosparging process, but are substantially nontoxic.

Stoichiometric calculations can be made to determine the amount of amendment necessary to form cell mass (biomass), that is, to create and control the microbial population at a level useful for biodegradation of contaminants. Using as an exemplary hydrocarbon ethylbenzene and as the phosphate compound TEP, it is possible to write the following equations for the production of biomass:

Eq. I hydrocarbon + oxygen + nitrogen + phosphorous - cell mass + carbon dioxide + water

Eq. II C₈H₁₀ + 2.86O₂ + 1.7143TEP → 0.143C₆₀H₈₇O₂₃N₁₂P + 0.286CO₂ +

2.43H₂O

Cell mass formation (growth) requires carbon:nitrogen:phosphorous in the elemental ratio of approximately 60:12:1. Additionally, it requires utilization of a small amount of the contaminant hydrocarbon or recycle of the microbial biomass. There may be some instances or some environments in which the addition of a carbon source is necessary, at least in the preliminary stages of population growth. The resulting cell mass (biomass) produces energy, utilizing the contaminant hydrocarbon and oxygen. The contaminant hydrocarbon is removed from the environment through metabolic biodegradation. The process of the resultant biodegradation can be monitored through known techniques, most commonly through measures of oxygen and carbon dioxide levels before and during the venting or sparging process. Direct sampling for levels of contaminants also provides useful information. The biomass can be controlled through selective addition of the necessary nutrients, and the microbial population is maintained at the desired levels until the contaminant concentration has reached acceptable levels. It is noted that control of the biomass means a positive control to increase the total biomass, a negative control to decrease the biomass, a "neutral" control to maintain a population at a desired overall level, or a combination of these. Careful control of added nutrients can be used effectively to force a biomass to utilize alternate sources, such as organic contaminants, for energy.

Bioventing and biosparging techniques have been successfully used to remediate contaminant zones having hydrocarbon and other organic contaminants. Other contaminants exist, however, that are not susceptible of treatment through biodegradation. It is therefore necessary to have a means for treating other contaminants which may also be present in a hydrocarbon contaminant zone or which are found in the absence of other contaminants.

Metal contaminants present special challenges to developing and implementing cost effective remediation. As noted, excavation and treatment or excavation and storage are very costly and inefficient. Also, due to geology existing surface or subsurface structures, or other factors, excavation is not always an option even if the process could be made more efficient. Metal contamination is most often very extensive in terms of the volume of soil or water contaminated thereby and is most often found in low concentrations. This means that a very large contamination zone must be thoroughly treated. Access to the contaminant zone is usually limited due to surface structures or existing land use. There are also limitations, both practical and political, in achieving regulatory acceptance of treatment techniques.

Elemental contaminant metals and metal compounds are not subject to direct biodegradation techniques. Other than removal, the most effective means of treating metal contaminants is to immobilize the metals or metal compounds such that they are prevented from having an adverse environmental impact. That is, through immobilization, the metal contaminants can be prevented from entering water supplies or otherwise affecting either wildlife or food crops.

Metals can be immobilized by precipitation, i.e., conversion to an insoluble compound; by co-precipitation with other compounds; and through adsorption to a variety of minerals. There is a wide variety of chemicals that will undergo chemical reactions with metals such as lead or cadmium resulting in the precipitation or co-precipitation of such metals. Most of these compounds, however, are themselves toxic and therefore cannot be used for remediation purposes. Also, because of the large areas of contamination and the low concentrations of metals therein (on a weight per volume basis), any chemical amendment would be required to completely permeate the entire contaminant zone, often at very high concentrations. Even if the selected reactant is not necessarily toxic in low concentrations, the high concentrations needed to effectively treat the metal contaminants become toxic. The large amounts of reactants needed also means high costs, as does the amount of equipment needed to achieve the necessary permeation. Reactant compounds that are not themselves soluble also cannot be used because they cannot be effectively used to permeate the contaminant zone. It has been found, however, as an aspect of the current invention, that the bioremediation techniques of bioventing and biosparging, can be used to also effectively treat metal contaminants. The method of the invention eliminates or overcomes each of the existing problems outlined above. The method is capable of treating very extensive contaminant zones at a relatively low cost. Only limited numbers of injection and/or extraction sites are needed, thus avoiding problems with geographical or land use limitations. Also, the reactant or amendment added to the contaminant zone is not itself a contaminant and poses relatively little harm to the environment.

In its broadest aspect, the method provides a means of effectively treating metal contamination through the controlled addition of phosphate compounds, with or without other macronutrients, to the contaminant zone. Through application of the method, the phosphate compounds will permeate the entire contaminant zone. Indigenous bacteria, or injected bacteria, will likewise permeate the zone, and this biomass can be controlled though the selected use of macronutrients. Metabolism of the phosphate compounds added as an amendment through the method of the current invention will result in the production of inorganic phosphates. The phosphates will in turn react with contaminant metals to form precipitants or co-precipitants thereof, and significantly increase contaminant sorption by minerals existing in the contaminant zone and/or created by the addition of phosphates. It has also been found, as an aspect of this invention, that selection of an appropriate organophosphate compound achieves at least two hitherto unattained goals. First, by appropriate selection of the added organophosphate, a primary or sole carbon source is provided for the indigenous microbes in the contaminant zone. This eliminates the need for an existing carbon source and the need to add additional amendments to provide carbon. Second, there is provided by metabolic means inorganic phosphate throughout the contaminant zone for use in remediating metal contaminants. Bioremediation methods known to the art rely on manipulating the microbial population, through population growth or otherwise, to a point at which the community utilizes organic contaminants as a food source. Macronutrients such as oxygen and nitrogen are added, effectively forcing the microbes to utilize the organic contaminants as food sources. The contaminants are metabolized to nontoxic compounds.

Fig. 1 is a diagrammatic illustration of a nutrient cycle enabled by practice of a preferred embodiment of the current invention. A biomass comprising microorganisms utilizes nutrients including carbon, oxygen, and nitrogen. The biomass may be native to the environment, or may be "seeded" through the introduction of exogenous organisms. Nutrients such as carbon, oxygen, and nitrogen are also typically found in the environment, or can be added. Through practice of this invention, a selected phosphorous source having the appropriate characteristics as described herein, shown in Fig. 1 as TEP, is provided through bioventing or biosparging. The biomass, through normal metabolic processes, produces phosphate, shown as PO . Because the phosphorous source provides an amount of phosphorous in excess of the stoichiometric amount needed for metabolism, the primary source of phosphate from the biomass is believed to be through metabolic conversion of the source nutrient. Some phosphate will also be provided by turnover in the biomass. As shown, it is anticipated that a portion of the produced phosphate will be recycled by the biomass as a nutrient source. A significant quantity of phosphate, however, will be available for participation in inorganic reactions that will immobilize, in one way or another, metal contaminants. Known remediation methods, however, are often limited to use in contaminant zones having organic contaminants to serve as carbon sources. Where the contaminants consist only of inorganic contaminants such as metals, these methods cannot be used. Alternatively, these methods must include in the amendments a volatile source of organic material for use by the microbial population as a food source. Adding additional amendments requires determination of a suitable amendment, and addition of the amendment. These requirements increase the costs and complications of these methods. It has been discovered, as an aspect of this invention, that by selecting a suitable organophosphate as an amendment, the organophosphate can serve both a carbon source and a phosphorous source for the biomass. In a preferred mode of the invention, TEP is used as an amendment. The TEP is volatile, and can be added along with the macronutrients oxygen and nitrogen in a vapor phase. The ethyl moieties of the molecule provide a good source of carbon, while the phosphorous moiety provides (1) the phosphate needed for cell growth and (2) through metabolism, the inorganic phosphate needed to treat the metal contaminants within the contaminant zone. By this means, two required macronutrients, carbon and phosphorous, are supplied to the biomass to enhance growth and provide phosphates as a byproduct.

TEP has several advantages as an amendment in bioremediation. It is volatile under environmental conditions, making it possible to add TEP as a vapor phase amendment. Also, TEP provides a means to carefully control the desired growth of the biomass. As shown in Equations I and II above, cell growth requires a carbon:phosphorous (C:P) ratio of about 60:1. TEP has a C:P ratio of 6:1. Thus, a representative cell mass having a C:N:P ratio of 60:12:1 could be derived solely from TEP in the presence of surplus oxygen and a nitrogen source. Atmospheric air supplies the oxygen and nitrogen, and the total amendment is then complete with the addition of TEP.

Because the C:P ratio of TEP is 6:1 , there will be an abundant production of inorganic phosphate. Theoretically, as much as 90% of the supplied phosphorous would be converted to phosphate for use in metal contaminant treatment. As a practical matter, some of the surplus phosphate will be consumed by the biomass in the degradation of other organics or by secondary biomass oxidation. The majority of the phosphorous, however, will be released as inorganic phosphate.

The method according to the current invention can therefore be used in a wide variety of contaminant zones. Where the contaminant zone contains an abundance of carbon sources (either as contaminants, e.g., hydrocarbons, or as natural organic matter), TEP can be used to control biomass growth by controlling the stoichiometric amount of phosphorous added. Where the contaminant zone is lacking a carbon source, the stoichiometric carbon (ethyl) content of TEP can be used to control biomass growth and produce an abundance of phosphate. Oxygen and nitrogen can also be used as the controlling macronutrient where appropriate. As noted above, however, additional carbon sources in the form of volatizable carbon compounds, can be added to the nutrient mix. The method using the preferred organophosphate TEP thus provides a very simple amendment, which can be simply air and TEP, that can be added to the contaminant zone with high efficiency and low cost. At the same time, the mixture can be used in almost any environment, and provides a variety of controls for the growth or other manipulation of the biomass.

In a most preferred application of the method, the method is utilized at sites contaminated only with metal contaminants. Such sites include, for example, those sites contaminated by run-off from mine tailings and industrial sites. The contaminant zones at such sites are often "carbon poor," that is, there are typically no organic sources of carbon, either in the form of contaminants or naturally occurring carbon. The site can be assayed via known techniques to determine the characteristics of the contaminant zone. This assay will provide information regarding (i) the types and amounts of metal contaminants; (ii) the location of the contaminants; and (iii) the movement, if any, of the contaminant zone through, e.g., an aquifer. The site will also be assayed to determine the type and amount (population) of microorganisms, and the nutrient requirements thereof in the environment at the contaminant zone. The microorganism population may require alteration or augmentation, that is, it may be necessary or desirable to inject additional numbers and/or types of microorganisms. The nutrient requirements of the microorganisms may be known, once the microorganisms are identified, or may be determined by known techniques.

Once the metal contaminants have been located and characterized, and the microorganism population and nutrients needs determined, a determination is then made with respect to the nutrients to be provided to the biomass. According to the current invention, one of the nutrients will be an organophosphorous. The preferred organophosphorous is TEP, with TPP, TBP, or mixtures of two or more of these alkyl phosphate esters also being preferred. Other suitable organophosphate compounds and mixtures thereof may be utilized. While it is preferred that the organophosphorous added via biosparging or bioventing be a nontoxic compound, it may be that in certain sites it will be useful to add at least some amount of an organophosphorous compound that is, under ordinary circumstances, toxic. This may be desirable, for example, where a different carbon:phosphorous ratio is useful. Other nutrients may be injected along with the vapor-phase organophosphorous compound. Oxygen and nitrogen can be added as discreet components, or as air. At some sites, it may be desirable or necessary to add additional carbon sources to the injected nutrient mix. For this purpose, it is anticipated that compounds such as methane and ethane can be used.

In a preferred method according to the current invention, the organophosphorous will comprise the only or at least the main source of carbon to the biomass. Under these conditions, the carbon:phosphorous ratio, e.g., about 6:1 for TEP, of the nutrient mix will provide an excess of phosphorous when compared to the bulk C:N:P ratio (carbon:phosphorous::60:1). The biomass will therefore produce "waste" phosphorous in the form of inorganic phosphates. These phosphates, in turn, will be available for participation in precipitation and co-precipitation reactions with contaminant metals, and will provide other mechanisms by which the contaminant metals are immobilized or otherwise be made unavailable to processes such as uptake by plants or animals.

The nutrient mix comprising the organophosphorous compound and other needed nutrients is provided to the contaminant zone via bioventing or biosparging. The biomass is fed with the nutrients, producing inorganic phosphates. The bioventing or biosparging is continued for a period of time sufficient to produce enough inorganic phosphate to permit the various precipitation, co-precipitation, and other mechanisms relating to the metal contaminants to proceed to completion, or at least to a point where the soil or water is considered sufficiently remediated.

The method of the invention can be implemented through use of systems exemplified by those described in U.S. Patent Nos. 5,480,549 and 5,753,109. Bioventing or biosparging injection wells are established, as are extraction wells. Depending on the hydrogeologic conditions presented, the latter may not be necessary to effect treatment. Typically, sampling wells or other sampling equipment will be used to monitor the process.

As described above, nutrient amendments are injected through the injection wells to create, enhance, and/or control the microbial population within the contaminant zone. Oxygen can be added in the form of atmospheric oxygen or as oxygen-rich compounds where aerobic communities are desired. Nitrogen, either in atmospheric form or in the form of compounds such as ammonia, is also contained in the nutrient mix. The oxygen amendment can be omitted where anaerobic microbial activity is desired. These nutrients are injected in vapor form through the vertical and/or horizontal injection wells and either naturally or through the action of extraction wells are drawn through the contaminant zone. Other nutrients such as trace minerals can also be added. In hydrocarbon contaminant zones, carbon is generally present in excess of what is needed, but where necessary carbon can be added, e.g., as carbon dioxide. The biomass may consist of indigenous or exogenous microbes, which may be naturally occurring and/or genetically engineered. It has been found in most bioventing and biosparging processes that the nutrients carbon, oxygen, and nitrogen are generally present in excess of what is needed. The key nutrient thus becomes phosphorous. As described in the referenced patents, phosphorous can be added to bioventing and biosparging processes in the form of vapor phase alkyl phosphate esters. The most preferred compounds for this purpose are TEP, TPP, and TBP. Other phosphate compounds may be used, but they must exhibit the necessary characteristics. As is now known in the art, these include an adequate vapor pressure at environmental temperatures and safety. Trimethyl phospate, for example, exhibits a higher vapor pressure than TEP at environmental temperatures (1 mm (Hg) at 26.0°C. compared with 1mm (Hg) at 39.6°C for TEP). Trimethyl phosphate, however, is both toxic and flammable. Volatile organo-dichlorophosphates such as phenyl dichlorophosphate are toxic. Data necessary for the selection of appropriate phosphates can be found in common reference sources such as the CRC Handbook of Chemistry and Physics, 77^th Ed. (1996-97) and Material Safety

Data Sheets provided by manufacturers.

The addition of phosphorous in vapor form as the controlling nutrient for the biomass provides both process control and a significant increase in biodegradation rates. Using Equations I and II above in general, and using specific data from the contamination zone, the stoichiometric requirements of the desired biomass can be calculated. The microbial population size can then be controlled to a desired level through controlling rates of injection of the various nutrients, and the nutrient mix can also be adjusted based on the types of microbes constituting the biomass.

Delivery of the nutrient mix to and through the contaminant zone results in increased size of the biomass, and an overall increase in metabolic activity. Where biodegradable contaminants exist within the contaminant zone, these contaminants are metabolized. The biomass effectively permeates the entirety of the contaminant zone, thus effectively treating the entire zone.

Importantly, the phosphate esters introduced into the contaminant zone are biologically converted to inorganic phosphates. The inorganic phosphates are utilized by the biomass, resulting in increased biomass through reproduction and resulting in increased biodegradation of the organic contaminants.

The phosphates are also available for other reactions, including the precipitation and stabilization of metal contaminants present in the contaminant zone. Because the nutrient mix enhances microbial growth throughout the contaminant zone, the zone is effectively permeated with the biomass. As a consequence, the phosphate produced also permeates the entire contamination zone. Even though the metal contaminants are widely dispersed throughout the contaminant zone at low concentrations, these contaminants will be exposed to the resulting inorganic phosphates.

It has been shown that the application of phosphates in solid form is effective in stabilizing contaminant metals. Adding a solid such as hydroxyapatite results in interactions with metals to form relatively insoluble compounds. As noted, addition of solids to a contaminant zone is of severely limited applicability. Addition of phosphate through metabolic conversion of organic phosphate esters is not subject to these limitations. For example, whereas addition of solid amendments is limited to shallow surface soils, bioventing and biosparging can be accomplished at much greater depths. Moreover, the distribution of the phosphate is not only more widespread throughout the contaminant zone but, because the biomass itself permeates the zone, distribution of the phosphate also permeates the zone, thus contacting all of the contaminant metal. Although the distribution of inorganic phosphate through biomass production is very different from that achieved by solid amendments, the reaction mechanisms for stabilization of the metals is very similar to that occurring with solid phase amendments. Solid amendments act to increase the concentration of dissolved phosphates, resulting in the precipitation of metals. With phosphate produced biologically, the phosphates are already in dissolved form, and function similarly. Depending on the nature and kind of the metal contaminants, some metals will precipitate as relatively insoluble phosphate compounds. The phosphate will also interact to precipitate iron, manganese, calcium, and aluminum, and certain other metal contaminants will co-precipitate with these compounds.

Another mechanism for immobilizing contaminant metals involves ion exchange. It has been demonstrated that cadmium will be removed from solution in the presence of hydroxyapatite. The cadmium exchanges with the calcium in the mineral and is effectively removed as a contaminant. This mechanism will also operate in a phosphorous amended bioventing system in the presence of phases such as crandallite (CaAI₃(PO )₂(OH)₅ H₂O) and strengite (FePO₄ ^■ H₂O). The mechanism of stabilization will depend in each case on the contaminant metal, the existing conditions affecting chemical reactions, and the geochemical makeup of the soil and/or water in the contaminant zone. Stabilization mechanisms for lead, cadmium, and chromium in soils subjected to bioventing with a phosphorous amendment are shown in Table 1. The mechanisms are dictated by the solubility of pertinent phosphate phases which are also shown in Table 1. Solubilities are shown as the concentration of the contaminant metal [M] in equilibrium with the pertinent phase at a dissolved phosphate concentration of 0.1 mg/l and a pH of 5.5.

Table 1: Solubilities ([M]) of phosphate minerals that are important to stabilization of lead, cadmium, and chromium.

Mineral/Phase Formula Ml mq/1

Comments

Chloropyromorphite Pb₅(P0₄)₃CI 0.012 (Cl)=1 mg/1

Hydroxypyromorphilla Pb₅(P0₄)₃OH 2.7

Plumbogummite PbAI₃(P0₄)₂(OH)₅ H₂0 0.002 Cd orthophosphate Cd₃(P0₄)₂ >30000 equilibrium with

Gibbsite(AI(OH)₃) Strengite FeP0₄ H₂0 0.02 (Fe(lll))

Crandalite CaAI₃(P0₄)₂(OH)₅.H₂0 No data available

Lead will precipitate as lead phosphates in view of the very low solubility of chloropyromorphite. It has been found that the water associated with most natural soil systems contains at least about 1 mg/l of chloride, which is sufficient to limit lead solubility to the value of 0.012 mg/l noted in Table 1. In acidic clay-rich soil, it is more likely that lead phosphate as plumbogummite will control lead solubility.

Cadmium will be stabilized by a variety of mechanisms. As shown in Table 1, cadmium solubility will not be controlled by the formation of cadmium orthophosphate. It has been found, however, that the activity of bacteria utilizing glycerol 2-phosphate will result in cell bound precipitation of cadmium in the form CdHPO₄. Thus, the method of the current invention may be used to precipitate cadmium in this form where such bacteria occur naturally. Where such bacteria are not naturally present, they may be injected along with the nutrient mix and supported by the bioventing process for remediation by this mechanism. Another useful mechanism for the immobilization of cadmium is through phosphate enhancement of the mineral goethite. It has been shown that adsorption of phosphate onto goethite significantly enhances the subsequent adsorption of cadmium by this mineral. Goethite is a common soil mineral and enhanced cadmium adsorption is thus expected in most soils following the introduction of phosphates according to the current method.

Cadmium may also be immobilized through exchanges with calcium in a variety of minerals. Crandallite, for example, is a common weathering mineral in many soils. Addition of phosphates will cause the precipitation of crandallite, and cadmium will co-precipitate through such exchange.

Chromium as Cr(lll) is similar in chemistry to iron as Fe(lll) and will be treated by the same type of mechanisms. Where the presence and sufficient concentration of phosphates cause the precipitation of iron phosphates, chromium will co-precipitate. Studies have shown, for example, that Cr(lll) co-precipitates with ferric hydroxides and ferric oxyhydroxides. It has also been observed that Cr(lll) will co-precipitate with ferric oxyhydroxides associated with a mine tailings pile. As shown in Table 1, at a dissolved phosphate concentration of about 0.1 mg/l, a solution will be saturated with respect to strengite at a ferric iron concentration of about 0.02 mg/l. Co- precipitation of chromium with the precipitation of strengite will occur in most soils.

The process according to the current invention enables additional mechanisms by which certain contaminant metals may be precipitated and/or immobilized. As described, the presence of phosphate under appropriate conditions will result in the formation of minerals or solids such as strengite and crandallite. The formation of these will result in the co-precipitation of some contaminant metals such as chromium as Cr(lll). Additionally, the formation of such solids will act to immobilize other chemicals by a different mechanism. Arsenic as As(V) has a chemistry (including physical characteristics such as effective size) very similar to phosphorous. During and after the formation of a compound such as strengite, As(V) if present in the environment will substitute for phosphorous and thereby be effectively immobilized. This same mechanism will apply to both antimony and bismuth should these metals be present also in the contaminant zone. This stabilization mechanism is more general than that for metals other than arsenic, antimony, and bismuth because the former does not require precipitation of a specific phosphate phase.

For purposes of this invention, the target concentration of inorganic phosphate sufficient to immobilize contaminant metals is determined using equilibrium chemistry with modifications as appropriate to implement the various immobilization mechanisms described above. By example, the reference design method for direct precipitation is based on the empirical formula of the desired (e.g., immobilized) solid and the relevant solubility product K_sp from the chemical literature. The K_sp of a solid containing a contaminant metal M^?+ (where "?+" represents the cationic valence of the contaminant metal M) and with the empirical formula M_x(PO )y is described by the equation: Eq.lll K_sp= {M^?+}^x{PO₄ ³l^y where {M^?+} and {PO₄°^} are the activities of the metal and phosphate in solution, respectively. Because the activity of an ion is approximately equal to its concentration under most environmental conditions, the target concentration of inorganic phosphate ([PO ^3"]) can be shown as follows: Eq.IV [P0₄*] = (K_sp/(target [M^?+]^x))^{1 y} where "target [M^?+]*" is the desired contaminant metal concentration after treatment. It will be understood, and is defined herein, that the term "empirical formula" expressed as M_x(PO )y means a solid formed by the interaction of the phosphate and the contaminant metal M. The solid may include other moieties including: anions, such as chloride found in chloropyromorphite and hydroxide found in plumbogummite; cations, such as calcium in crandallite; and other moieties such as water of hydration found in strengite. As is described herein, those of skill in the art will know how to account for these additional moieties in performing the calculations according to Equations III and IV. The following modifications to this simple calculations are typical: (1) for sites with high levels of dissolved solids having relatively high ionic strength, activity coefficient corrections may be performed; (2) for direct precipitation of solids containing other ions, these ions can be included in the K_sp calculation; (3) for co-precipitation, the precipitating metal is used in the primary calculation together with a co- precipitation ratio (the ratio of contaminant metal to primary precipitating metal) from the chemical literature; and (4) for enhanced sorption, the precipitating metal is used in the primary calculation together with a partition coefficient of the contaminant metal to that phase. For the described mechanism whereby arsenic as As(V) (and similar metals) substitutes for phosphorous in solids, coprecipitation calculations can be performed as described above. For this mechamism, the performance is calculated as for coprecipitation, except that the coprecipitation ratio is based on phosphorous. The foregoing modifications are well known to those ordinary skilled in the art, and the need for such modifications can be determined through assays of the contaminant zone and the surrounding area. These modifications, while considered the most typical, are not necessarily inclusive: other modifications based on conditions existing in or near the contaminant zone may be necessary, as will also be recognized by those of skill in the art.

Fig. 2 depicts diagrammatically the implementation of one embodiment of the current invention. Shown is a cross-section of a subsurface region in which a contaminant zone 1 is located. The contaminant zone 1 may be the result of a surface spill or other source, or may be a plume of contaminants moving through the subsurface, as in an aquifer. At a location selected with regard to the location and anticipated movement of the contaminant zone 1 , a biosparging or bioventing apparatus such as an injection well 3 is placed. Sparge air carrying a selected mixture, including an organophosphorous compound, is injected through well 3. At the lower end of well 3, the sparge mixture is discharged into the subsurface area through, e.g., an opening 5 in well 3. The opening 5 shown in Fig. 2 is by way of illustration only. The sparge mixture may be discharged at more than one point along the walls of well 3, or well 3 may be connected to one or more horizontal conduits (not shown) to more widely disperse the discharged mixture. The discharge in Fig. 2 is shown as occurring in a groundwater zone 7 (shown extending to the water table 9), which can be advantageous in helping to further disperse the discharged mixture. The discharge, however, may be made at any point selected with respect to the location of the contaminant zone, the hydrogeological characteristics of the subsurface area, and other considerations, as are known in the art. The nutrients, especially including the organophophorous nutrient, are dispersed through the subsurface, providing nutrients to the biomass. The metabolic action of the biomass then results in a widespread, highly dispersed area permeated by phosphate, forming a treatment zone 11. Contaminant metals will be precipitated or otherwise treated throughout the zone through the formation of stable phosphates and other mechanisms described herein. In one aspect of the invention, specialized bacteria, such as those using glycerol 2- phosphate, can be added to bio-precipitate contaminant metals. These specialized bacteria may be injected through well 3 with the nutrient mix and sparge air to also permeate the treatment zone 11. Biosparging with phosphate amendment can be used in the contaminant zone to increase the ability of certain natural minerals to adsorb contaminant metals such as cadmium, and will result in co-precipitation reactions. Where, for example, a contaminant plume is known to be moving through the soil, as in an aquifer, phosphate may be introduced into the aquifer ahead of the plume. The increased phosphate concentrations will produce an area of minerals having a high phosphate-enhanced adsorption rate for cadmium and other metals. When the contaminant plume flows through this area, these metals will be adsorbed at high rates. Also, this area of enhanced adsorption can be used to form a selective barrier or shield through which cadmium and other contaminants will not migrate. This will protect other, less accessible areas such as groundwater in another under- or overlying aquifer. Fig. 3 is an illustration of a second embodiment through which the current invention may be practiced, with elements numbered as in Fig.2. In this illustration, an injection well 3 has been provided with an opening 5 to discharge the sparge air and nutrient mix into the subsurface region. As in Fig. 2, this illustration shows the discharge occurring in a groundwater zone 5; the discharge point, however, will be selected as described above with due regard for factors such as the location and direction of movement of the contaminant zone 1. In the method illustrated in Fig. 3, an extraction well 13 has been added. Extraction wells such as extraction well 13 are known in the art. Operation of extraction well 13 will serve to shape treatment zone 11 by drawing the sparge air and nutrient mix from well 3 in the general direction of extraction well 13. Thus, a treatment zone 11 can be shaped and drawn such that it encompasses contaminant zone 1. Fig. 3 is intended as illustrative only. The method can be practiced using multiple injection wells and/or multiple extraction wells. By calculated placement of such wells, a treatment zone can be created in a wide variety of shapes and/or locations. It may be advantageous, for example, to establish one or more treatment zones in a location "downstream" from a contaminant plume moving through an aquifer, creating a phosphate-rich area through which the plume will travel. Appropriate placement of injection and extraction wells can provide lateral and vertical control of the desired treatment zone.

The process of treating contaminants through phosphate amended bioventing or biosparging is capable of use in a wide variety of forms and locations. The process may be used where organic (hydrocarbon) contaminants are present in addition to metal contaminants. In these cases, the organic contaminants may form part of the energy source for the biomass. The process can also be used where the contaminant is in the form of metals only. In the latter case, it may be necessary to include a carbon source as part of the nutrient mix.

The process may be used to carefully control the particular biodegradation process. Where aerobic reactions are desired, oxygen is included as a nutrient, enhancing the aerobic population. Where anaerobic mechanisms are more useful, oxygen may be omitted from the nutrient mix. The nutrient mix may be formulated, or changed, as desired to support a particular type of microbe. The injection wells used for venting or sparging the mixture can be used easily to also inject one or more desired varieties of exogenous microbes. The nutrient mix can also be used to exercise control over the biomass for purposes other than growth, where desired. As noted, atmospheric air provides an abundant source of two of the primary nutrients oxygen and nitrogen. Where necessary, it can function as a carbon source through the inclusion of carbon dioxide. Under these circumstances, the growth and overall metabolic level of the biomass can be very accurately controlled by the concentration of phosphate compounds in the nutrient mix. In other circumstances, the nutrient mix can be manipulated such that one of the other nutrients, such as nitrogen, becomes the controlling nutrient. This may be desirable where it is preferred, for example, to inject an excess of vapor phase phosphorous. This may be advantageous where the hydrogeochemical conditions favor the chemical conversion of organophosphates to inorganic phosphates, providing a mechanism for enhancing the phosphate concentration in the contaminant zone through processes other than bioconversion.

Once it has been determined, through assays of the contaminant zone, what nutrient mix, delivered with or without exogenous microorganisms, is to be used, a delivery point for the nutrient mix is determined. The delivery point, preferably using bioventing or biosparging, may be within the contaminant zone or outside the contaminant zone. It may be desirable, for example, to deliver a nutrient mix effectively downstream from a plume of contaminants in an aquifer, whereby the plume will thereafter enter an area of enhanced biomass and enhanced phosphate. The distribution of the nutrients can be accomplished by simple permeation of the nutrient mixture throughout an area, or the distribution may be controlled. A means for controlling the distribution of the mix can take the form of selectively choosing a delivery point, and selectively choosing a location for an extraction point, such as via installation of an extraction well. By operation of the extraction well, the flow of the nutrient mix can be controlled because the mix will be drawn in the direction of the extraction well. Selected placement of delivery points and extraction points can provide three-dimensional control of the distribution of the nutrient mix, and hence both the biomass and the resulting phosphate, in such a manner as to ensure contact between substantially all of the contaminants and the treating materials, that is, the biomass and the produced phosphate.

Phosphate amended bioventing or biosparging provides a cost- effective, controllable alternative to other methods of contaminant remediation. In addition to its value as a method of treatment for organic contaminants, it can now also be used to treat inorganic contaminants. It is capable of use in soil or groundwater, or both. It is not limited to surface or shallow level contamination, but may be used to reach contamination at great depths, both above and below the existing water table. For example, where the contaminant zone is in an aquifer that is beneath another, and separated from the upper aquifer by an impermeable zone, the lower aquifer can be efficiently treated without affecting the upper aquifer. The treatment of contaminants by phosphate amended bioventing is also environmentally acceptable. Once the contaminants have been treated, the nutrient addition is terminated, and the enhanced biomass in the treated area will quickly return to its native state. Also, while phosphate can be harmful in very high concentrations, treatment by the method of this invention does not create harmfully high concentrations. Moreover, even if the phosphate concentration is increased, phosphate is a natural compound that is much less toxic than the materials it is used to remediate and is not toxic perse. It can therefore be used in the method of this invention with minimal adverse effect on the environment.

As stated above, a preferred mode for carrying out the method of the current invention is to provide a nutrient mix in which the organophosphorous compound provides the sole source of carbon for the biomass. Where necessary or desirable, an additional carbon compound can be added to the nutrient mix. The method can also be used in conjunction with other bioremediation methods using bioventing or biosparging.

The method disclosed herein is capable of use in a wide variety of situations, and can be used with a wide variety of injection and extraction techniques. The particular phosphate amendment can be selectively tailored to match the needs of a particular site, as can the overall nutrient mix. There are thus many variations that can be introduced in the process without departing from the spirit and scope of the invention as disclosed herein.

Claims

ClaimsWhat is claimed is:

1. A method for treating metal contaminants in a contaminant zone, said method comprising: (a) creating a vapor phase nutrient mix comprising nutrients sufficient to enhance the growth of microorganisms within said contaminant zone, said mix comprising at least one vapor phase organophosphorous nutrient;

(b) delivering said nutrient mix to said contaminant zone in an amount at least equal to the nutrient requirements of said microorganisms; and

(c) continuing delivering said nutrient mix to said contaminant zone for a period of time sufficient to cause the production by said microorganisms of inorganic phosphates in a concentration sufficient to treat said metal contaminants by immobilizing said metal contaminants.

2. The method according to claim 1 , wherein said step of delivering said nutrient mix comprises delivering said nutrient mix by bioventing or biosparging.

3. The method according to claim 1 , wherein said at least one organophosphorous nutrient is an alkylphosphate ester.

4. The method according to claim 3, wherein said alkylphosphate ester is triethylphosphate, tripropylphosphate, tributylphosphate, or a mixture comprising two or more thereof.

5. The method according to claim 1 , said method also comprising delivering to said contaminant zone a population of at least one microorganism capable of metabolizing said organophosphorous nutrient to thereby produce inorganic phosphates.

6. The method according to claim 5, wherein said microorganism metabolically utilizes glycerol 2-phosphate.

7. The method according to claim 1, wherein said organophosphorous nutrient is delivered in an amount in excess of the amount required for cell mass formation.

8. The method according to claim 1, also comprising: providing an extraction well, whereby extraction from said extraction well enhances permeation of said nutrient mix through at least a portion of said contaminant zone.

9. A method for treating metal contaminants in a contaminant zone, said method comprising: (a) assaying said contaminant zone to determine the type and amount of metal contaminants therein;

(b) assaying said contaminant zone to determine the nutrient requirements of microorganisms therein;

(c) creating a vapor phase nutrient mix, said mix comprising nutrients sufficient to permit the growth of said microorganisms according to said nutrient requirements, said nutrient mix comprising at least one vapor phase organophosphorous nutrient;

(d) delivering said nutrient mixture at a delivery point to said contaminant zone in an amount at least equal to said nutrient requirements; and

(e) continuing said delivering for a period of time sufficient for said microorganisms to metabolically produce inorganic phosphates in an amount calculated to create a concentration of said inorganic phosphates in said contaminant zone sufficient to immobilize said metal contaminants within said contaminant zone.

10. The method according to claim 9, wherein said amount of inorganic phosphates is calculated to satisfy the condition: [P0₄*1 = (K_sp/(target[M^?+]^x))^{1 y} where x and y are defined by the empirical formula M_x(PO₄)y for a solid formed by the interaction of a contaminant metal M having a valence of ?+ and phosphate; [PO ^3"] is the concentration of inorganic phosphate produced; K_sp is the solubility product for said solid; and target[M^?+] is the desired final concentration of said contaminant metal.

11. The method according to claim 9, wherein said step of continuing said delivering is continued for a period of time sufficient for said microorganisms to metabolically produce inorganic phosphates in an amount sufficient to create a concentration of said inorganic phosphates in said contaminant zone of about 0.1 mg/l.

12. The method according to claim 9, wherein said method also comprises the step of delivering to said contaminant zone a population of at least one microorganism capable of metabolizing said vapor phase organophosphorous nutrient to inorganic phosphates.

13. The method according to claim 9, further comprising: providing an extraction well having an extraction point located away from said delivery point, whereby extraction through said extraction well distributes said nutrient mix between said delivery point and said extraction point.

14. The method according to claim 9, wherein said step of delivering said nutrient mix comprises delivering said nutrient mix by bioventing or biosparging.