| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | October 28, 2003 |
| PATENT TITLE |
Methods of using polylactate release compounds |
| PATENT ABSTRACT | The preferred embodiments provide for a family of novel compositions to serve as substrates that release hydroxy acid slowly over time. Preferably the hydroxy acid is an .alpha.-hydroxy acid, more preferably it is lactic acid. The compositions are preferably made by reaction of poly(lactic acid) with multifunctional alcohols. Also disclosed are formulations based on the compounds and methods of use for both the compositions and the formulations. The preferred use of the compositions and formulations is for bioremediation purposes wherein they provide a time-release source of lactic acid to support the growth and reductive activity of microbes present in a system or medium, such as an aquifer, bioreactor, soil, industrial process, wastestream, body of water, river or well. The microbes destroy or inactivate compounds which are capable of being reduced, such as nitrogen-containing organic compounds, oxygen-containing organic compounds, polyaromatic hydrocarbons, and halogen-containing organic compounds |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | November 7, 2001 |
| PATENT REFERENCES CITED |
S. Karjomaa et al Polym. Degrad Stab (1998) 59 (1-3) 333-336.* Smatlak, et al., Comparative Kinetics of Hydrogen Utilization for Reductive Dechlorination of Tetrachloroethene and Methanogenesis in an Anaerobic Enrichment Culture. 1996, Environmental Science .sctn. Technology, 30, pp. 2850-2858. Fennell, et al., Comparison of Butyric Acid, Ethanol, Lactic Acid, and Propionic Acid as Hydrogen Donors for the Reductive Dechlorination of Tetrachloroethene, 1997, Environmental Science .sctn. Technology, 31, pp. 918-926. DiStefano, et al., Reductive Dechlorination of High Concentrations of Tetrachloroethene to Ethene by an Anaerobic Enrichment Culture in the Absence of Methanogenesis, 08/91, Applied and Environmental Microbiology, vol. 57, No. 8, pp. 2287-2292. Biodegradable Poly (Lactic Acid) Compositions with Improved Physical Properties, May 7, 1995, Chemical Abstracts, vol. 126, No. 11, p. 637, No 144978t. Shin, et al., Biodegradability of Degradable Plastics Exposed to Anaerobic Digested Sludge and Simulated Landfill Conditions, 1997, Chemical Abstracts, vol. 126, No. 22, p. 1009, No. 297116k. Kakizawa, Yasutoshi, Process for the Preparation of Lactic Acid-Based Polyester Compositions, Apr. 2, 1997, Chemical Abstracts, vol. 126, No. 23, p. 563, No. 306163c. Morse, et al., A Treatability Test for Evaluating the Potential Applicability of the Reductive Anaerobic Biological In Situ Treatment Technology (RABITT) to Remediate Chloroethenes, Aug. 25, 1997, Department of Defense-Technical Protocal (Draft). Stover, Michael Augustine, Abstract/Thesis, 1993, No. S8892, Cornell University. Smatlak, Concordia Ruth, Abstract/Thesis, 1995, No. S6362, Cornell University. DiStefano, et al., Hydrogen as an Electron Donor for Dechlorination of Tetrachloroethene by an Anaerobic Mixed Culture, 11/92, Applied and Environmental Microbiology. vol. 58, No. 11, pp. 3622-3629. Rafler, et al., Novel Biodegradable Polyesters, Abstract, American Chemical Society (1992). Fennell, et al., Comparative Studies of Hydrogen Donors for Stimulation of Tetrachloroethene Dechlorination, 1997, International In Situ and On-Site Bioremediation Symposium-New Orleans, LA, p. 11. Becvar, et al., In Situ Dechlorination of Solvents in Saturated Soils, 1997, International In Situ and On-Site Bioremediation Symposium-New Orleans, LA, pp. 39-44. Brennan, et al. Anaerobic Microbial Transformation of Trichloroethylene and Methylene Chloride in Pinellas Soil and Groundwater, 1997, International In Situ and On-Site Bioremediation Symposium-New Orleans, LA, p. 45. Dybas, et al., Slow-Release Substrates for Transformation of Carbon Tetrachloride by Pseudomonas Strain KC, 1997, International In Situ and On-Site Bioremediation Symposium-New Orleans, LA, p. 59. Acree, et al. Site Characterization Methods for the Design of In-Situ Electron Donor Delivery System, 1997, International In Situ and On-Site Bioremediation Symposium-New Orleans, LA, pp. 261-266. Gibson, et al., Stimulation of Reductive Dechlorination of Tetrachloroethene in Anerobic Aquifer Microcosms by Addtion of Short-Chain Organic Acids or Alcohols, 04/92, Applied and Environmental Microbiology, vol. 58, No. 4, pp. 1392 and 1393. Carney, Anna P., Abstract-Ethylene as Inhibitor of Methanogenesis/Thesis, 1995, No. C289, Cornell University. Freedman, et al., Biological Reductive Dechlorination of Tetrachloroethylene and Trichloroethylene to Ethylene under Methanogenic Conditions, 09/89. Applied and Environmental Microbiology, vol. 55, No. 9, pp. 2144-2151. Fathepure, et al., Complete Degradation of Polychlorinated Hydrocarbons by a Two-Stage Biofilm Reactor, 12/91, Applied and Environmental Microbiology, vol. 57, No. 12, pp. 3418-3422. Fathepure, et al., Anaerobic Bacteria that Dechlorinate Perchloroethene, 11/87, Applied and Environmental Microbiology, vol. 53, No. 11, pp. 2671-2674. Ballapragada, et al., Effect of Hydrogen on Reductive Dechlorination of Chlorinated Ethenes, 1997, Environmental Science & Technology, vol. 31, No. 6, pp. 1728-1734. Carr, et al., Enrichment of High-Rate PCE Dechlorination and Comparative Study of Lactate, Methanol, and Hydrogen as Electron Donors to Sustain Activity, 1998, Environmental Science & Technology, vol. 32, No. 12, pp. 1817-1824. Yang, et al., Competition for Hydrogen within a Chlorinated Solvent Dehalogenating Anaerobic Mixed Culture, 1998, Environmental Science & Technology, vol. 32, No. 22, pp. 3591-3597. |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A method of aiding bioremediation of contaminants remediated through microbial reduction in a medium, comprising applying a composition comprising an ester of poly(.alpha.-hydroxy acid). 2. The method of claim 1, wherein said composition has the formula: ##STR6## n=1 to 4; m=0 to 3; and x=1 to 9. 3. The method of claim 1, wherein said composition is glycerol tripolylactate. 4. The method of claim 1, wherein said composition is xylitol pentapolylactate. 5. The method of claim 1, wherein said composition is sorbitol hexapolylactate. 6. The method of claim 1, wherein said application is performed by injecting said composition into said medium using a high pressure pump. 7. The method of claim 1, wherein said contaminants are selected from the group consisting of nitrogen-containing organic compounds, oxygen containing organic compounds, polyaromatic hydrocarbons, and halogen containing organic compounds. 8. The method of claim 1, wherein said contaminants comprise at least one of the following: chlorinated aromatic hydrocarbons and chlorinated aliphatic hydrocarbons. 9. The method of claim 1, wherein the medium is an underground aquifer. 10. The method of claim 9, comprising the steps of: packing said composition into plastic tubes having holes or slits in the sides thereof; placing the tubes into holes drilled through the ground into said underground aquifer. 11. The method of claim 1, wherein the medium is selected from the group consisting of an aquifer, a bioreactor, soil, an industrial process, a wastestream, a body of water, a river, and a well. 12. The method of claim 1, wherein said composition is part of a formulation comprising: 65-99% by weight said composition; and 1-35% by weight inorganic salts. 13. The method of claim 12, wherein said formulation further comprises 30% or less by weight of one or more compounds selected from the group consisting of: nutrients, buffers and pH modifiers, ethylene, chelating agents, surfactants, vitamins, enzymes, compounds that inhibit competing microorganisms, and bacteria and other microbes. 14. The method of claim 13, wherein the nutrient is a compound selected from the group consisting of yeast extract, urea, potassium-containing compositions, nitrogen-containing compositions, phosphorus-containing compositions, sulfur-containing compositions, molydenum salts, iron salts, zinc salts, and copper salts. 15. The method of claim 13, wherein the vitamin is vitamin B.sub.12. 16. The method of claim 13, wherein the enzyme is selected from the group consisting of lipase and esterase. 17. The method of claim 1, wherein said composition is part of a formulation comprising; 14-98% by weight said composition; 1-15% by weight inorganic salts; and 1-85% by weight of at least one diluent which does not interfere with the hydrolysis of an ester. 18. The method of claim 17, wherein said formulation comprises 10-25% by weight of said diluent. 19. The method of claim 17, wherein said formulation comprises 50-85% by weight of said diluent. 20. The method of claim 17, wherein said diluent is selected from the group consisting of water, glycerin, esters, and alcohols. 21. The method of claim 17, wherein said diluent is selected from the group consisting of isopropyl alcohol, ethyl alcohol, ethyl acetate and ethyl lactate. 22. The method of claim 17, wherein said formulation further comprises 30% or less by weight of one or more compounds selected from the group consisting of: nutrients, buffers and pH modifiers, ethylene, chelating agents, surfactants, vitamins, enzymes, compounds that inhibit competing microorganisms, and bacteria and other microbes. 23. The method of claim 22, wherein the nutrient is a compound selected from the group consisting of yeast extract, urea, potassium-containing compositions, nitrogen-containing compositions, phosphorus-containing compositions, sulfur-containing compositions, molydenum salts, iron salts, zinc salts, and copper salts. 24. The method of claim 22, wherein the vitamin is vitamin B.sub.12. 25. The method of claim 22, wherein the enzyme is selected from the group consisting of lipase and esterase. 26. A method of aiding bioremediation of contaminants remediated through microbial reduction in a medium, comprising applying a polylactate ester to the medium. 27. The method of claim 26, wherein said polylactate ester is glycerol tripolylactate. 28. The method of claim 26, wherein said polylactate ester is xylitol pentapolylactate. 29. The method of claim 26, wherein said polylactate ester is sorbitol hexapolylactate. 30. The method of claim 26, wherein said contaminants are selected from the group consisting of nitrogen-containing organic compounds, oxygen-containing organic compounds, polyaromatic hydrocarbons, and halogen-containing organic compounds. 31. The method of claim 26, wherein said contaminants comprise at least one of the following: chlorinated aromatic hydrocarbons and chlorinated aliphatic hydrocarbons. 32. The method of claim 26, wherein said medium is selected from the group consisting of an aquifer, a bioreactor, soil, an industrial process, a wastestream, a body of water, a river and a well. 33. The method of claim 26, firther comprising the step of adding an oxygen-liberating compound downstream from said polylactate ester. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to methods of using polylactate release compounds. 2. Description of the Related Art Compounds which release hydroxy acids slowly over time preferably .alpha.-hydroxy acids can serve as a time-release source of lactic acid for biodegradation of chemical compounds in various media, including soils, aquifers, bioreactors, wastestreams, industrial processes, and other systems. The compounds may also be the basis of formulations which provide a time-release source of lactic acid and other materials and compounds which stimulate growth of microbes and facilitate bioremediation. The lactic acid, which is itself a nutrient for microbes, is broken down to form other compounds which provide both additional nutrients and a source of electrons to support the microbial biodegradation of chemical compounds, preferably halogenated hydrocarbons. Halogenated hydrocarbons are compounds composed of hydrogen and carbon with at least one hydrogen substituted by a halogen atom (e.g. Cl, Br, or F). Halogenated hydrocarbons are used for many purposes, such as solvents, pesticides, and degreasers. Degreasing products have widespread use in several industries, including dry cleaning, microelectronics, and equipment maintenance. Some of the most common halogenated hydrocarbons are methylene chloride, chloroform, carbon tetrachloride, tetrachloroethane (TCA), tetrachloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE), and vinyl chloride (VC). Such compounds are commonly known as "chlorinated hydrocarbons" or "chlorinated solvents." Chlorinated hydrocarbons have been widely used for several decades. This use, in addition to improper handling and storage, has led to extensive soil and groundwater contamination, and these solvents are among the most prevalent groundwater contaminants in the United States today. Contamination of groundwater by chlorinated hydrocarbons is an environmental concern because these compounds have known toxic and carcinogenic effects. One common technique for decontaminating aquifers that is in current use is the pump-and-treat method. As practiced, this method utilizes a series of extraction wells drilled into a contaminated aquifer. Contaminated water is drawn through an extraction well, treated to remove or degrade the contaminant, and then returned to the aquifer through one or more injection wells or discharged to sewers or other points of non-origin. This method can be time consuming and cost-prohibitive. Recently, attempts have been made to biodegrade chlorinated solvents in-situ using anaerobic bacteria. Some species of anaerobic bacteria used in bioremediation of chlorinated solvents degrade these solvents by reductive dechlorination. This reductive process requires a steady supply of an electron donor such as hydrogen. Some current research supports the proposition that delivery of hydrogen in a slow, steady manner is an effective way to stimulate and maintain organisms that perform reductive dechlorination and reduce competition for ambient hydrogen by other organisms. Several methods have been proposed to supply the hydrogen needed for reductive dechlorination: addition of short chain organic acids or alcohols; addition of sodium benzoate (as disclosed in U.S. Pat. No. 5,277,815); addition of fats and oils; sparging with hydrogen gas (as disclosed in U.S. Pat. No. 5,602,296); and generating hydrogen gas in-situ by electrochemical reactions or electrolysis (also disclosed in U.S. Pat. No. 5,602,296). All of the previously mentioned methods have serious shortcomings. Addition of short chain organic acids or alcohols as well as the addition of simple organic esters or organic salts such as sodium benzoate have the problem that essentially all of the chemical is released at once in the area and is free to flow away from the contaminated area. Thus, frequent addition of the chosen compound is needed to keep a sufficient concentration of the compound in the contaminated area over time. The constant injection of high volumes of solutions will increase the volume of the system or aquifer and thereby potentially cause further spread of the contamination. Furthermore, unless special measures are taken to deoxygenate the water and solutions which are injected, the level of oxygen in the system or aquifer will rise, thus harming the anaerobic atmosphere which fosters the microbes performing the reduction. Sparging with hydrogen requires the installation and use of pipes, manifolds, valves, and other equipment and the handling of large quantities of a highly flammable and explosive gas under pressure. Generation of hydrogen gas in-situ by chemical reaction or electrolysis as disclosed in U.S. Pat. No. 5,602,296 is, by those inventors' own admission, experimental in nature and like sparging suffers from the additional limitation in that hydrogen gas has very low solubility in water. Lastly, addition of fats and oils can provide for the slow release of hydrogen, but the method does not provide a mechanism for controlling the amount of hydrogen released. Furthermore, the amount of hydrogen released is very low compared to the weight of fat or oil that must be added. One of the most effective substrates to provide hydrogen to a biological system is lactic acid. During anaerobic processes the conversion of lactic acid (or lactate salt) to acetic acid (or acetate salt) liberates two moles of dihydrogen (four moles of elemental hydrogen) for each mole of lactic acid or lactate consumed. ##STR1## Thus the process produces both an electron source (hydrogen) and a nutrient source for bacteria. A convenient method of delivering lactic acid is in the form of an ester. Esters of lactic acid hydrolyze to produce free lactic acid, or lactate salt, depending on the pH of the solution. ##STR2## The hydrolysis reaction can be catalyzed by either acid or base, and the alcohol produced can also serve as a nutrient source for surrounding bacteria. The rate of hydrolysis is dependent upon both the pH and the alcohol with which the ester was formed. Although simple esters of lactic acid, such as ethyl lactate, delay the release of free lactic acid into solution, the lactic acid is still released and converted to hydrogen at a very high rate. This rate may be higher than the rate at which bacteria performing reductive dechlorination can consume it, and thus either be wasted or used by other bacteria which compete with the reductive dechlorinators. SUMMARY OF THE INVENTION The preferred embodiments relate to compounds, characterized by their ability to release hydroxy acids slowly over time. The preferred embodiments also relate to formulations comprising the compounds, as well as methods for their use in aiding bioremediation of media contaminated by contaminants capable of being remediated by microbial reduction. In one aspect, the preferred embodiments provide for a composition comprising a multifunctional alcohol ester of a poly(hydroxy acid), wherein the poly(.alpha.-hydroxy acid) is either a .alpha.-hydroxy acid or a .beta.-hydroxy acid, and each hydroxyl group on the multifunctional alcohol has reacted to form an ester bond with a molecule of poly(hydroxy acid). In the preferred embodiments, the poly(hydroxy acid) is an .alpha.-hydroxy acid. In especially preferred embodiments, the composition has the formula: ##STR3## wherein n=1 to 4, m=0 to 3, and x=1 to9. The preferred embodiments also provide a formulation comprising 65-99% by weight of a multifunctional alcohol ester of poly(hydroxy acid) and 1-35% by weight inorganic salts. Another formulation comprises 14-98% by weight of a multifunctional alcohol ester of poly(hydroxy acid), 1-15% by weight inorganic salts, and 1-85% by weight of a diluent which does not interfere with the hydrolysis of an ester. Preferably, the diluent is selected from the group consisting of water, glycerin, esters, and alcohols. In other embodiments, the formulations above further comprise 0-30% by weight of one or more compounds selected from the group consisting of nutrients such as yeast extract, urea, potassium-containing compositions, nitrogen-containing compositions, phosphorous-containing compositions, sulfur-containing compositions, molybdenum salts, iron salts, zinc salts, copper salts, buffers and pH modifiers such as sodium carbonate and potassium carbonate, ethylene, chelating agents, surfactants, vitamins such as B.sub.12, enzymes such as lipase and esterase, compounds that inhibit competing microorganisms, and bacteria and other microbes Especially preferred compounds include glycerol tripolylactate, xylitol pentapolylactate, and sorbitol hexapolylactate. There is also provided a process of making multifunctional alcohol esters of poly(.alpha.-hydroxy acids) comprising the steps of charging a reaction vessel with solution of .alpha.-hydroxy acid; adding a catalytic amount of a strong inorganic acid; heating the reaction vessel to drive off water and cause polymerization resulting in poly(.alpha.-hydroxy acid); adding a multifunctional alcohol to the reaction vessel; heating the reaction vessel to cause esterification of the poly(.alpha.-hydroxy acid); and adding an inorganic base to neutralize at least some of the inorganic acid in the reaction vessel. In embodiments wherein the reaction vessel has a large volume, the heating step to drive off water is preferably done under vacuum. The above process may further comprise steps wherein a solvent is added with the .alpha.-hydroxy acid and the solvent is removed following addition of the inorganic base. The preferred embodiments also provide for a method of aiding bioremediation of contaminants remediated through microbial reduction in a medium, comprising contacting the medium with applying a composition comprising an ester of an .alpha.-hydroxy acid. In preferred embodiments, the .alpha.-hydroxy acid is polymerized to form a poly(.alpha.-hydroxy acid). In other preferred embodiments, the composition comprises a multifunctional alcohol ester of poly(.alpha.-hydroxy acid) wherein each hydroxyl group on the multifunctional alcohol has reacted to form an ester bond with a molecule of poly (.alpha.-hydroxy acid). The method may also utilize formulations, as described above, which comprise the poly(.alpha.-hydroxy acid)esters. The medium is preferably selected from the group consisting of an aquifer, a bioreactor, soil, an industrial process, a wastestream, a body of water, a river, and a well. When the medium is underground, the preferred method of aiding bioremediation comprises injecting the composition or formulation into the medium with a high pressure pump. Another preferred method comprises the steps of packing the composition into tubes or canisters having holes or slits in the sides thereof, and placing the canisters into holes drilled into the ground. There is provided a method of aiding remediation of chemical compositions in a medium, comprising applying a polylactate ester to the medium. Preferably the contaminants are selected from the group consisting of nitrogen-containing organic compounds, oxygen-containing organic compounds, polyaromatic hydrocarbons, and halogen-containing organic compounds. More preferably, the contaminants comprise chlorinated aromatic or aliphatic hydrocarbons. In preferred embodiments, the polylactate ester is glycerol tripolylactate, xylitol pentapolylactate, and sorbitol hexapolylactate. The medium is preferably selected from the group consisting of an aquifer, a bioreactor, soil, an industrial process, a wastestream, a body of water, a river and a well. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a chart which shows the rate of release of lactic acid from ethyl lactate in water, in the absence of bacteria. FIG. 2 is a chart which shows the rate of release of lactic acid from glycerol tripolylactate in water, in the absence of bacteria. FIG. 3 is a chart which shows the rate of release of lactic acid from xylitol pentapolylactate in water, in the absence of bacteria. FIG. 4 is a chart which shows the rate of release of lactic acid from sorbitol hexapolylactate in water, in the absence of bacteria. FIG. 5 is a graph showing reduction in TCE in two separate test tube tests using a sterile sand substrate to which bacteria and sorbitol polylactate were added. FIG. 6 is a graph showing the decrease in TCE and the concomitant rise in lactic acid concentration for one of the test tube tests of FIG. 5. FIG. 7 is a graph showing the decreased rate of TCE metabolism in a test tube system of the type of FIG. 5 wherein the bacterial concentration is diminished by a factor of ten. FIG. 8 is a graph showing the rate of TCE metabolism in a test tube system of the type of FIG. 7 in which three times the sorbitol polylactate has been added. FIG. 9 is a graph showing the decrease in TCE concentration with time for two soil samples having different initial concentrations of TCE, to which sorbitol polylactate was added. FIG. 10 is a graph showing the increase in lactic acid concentration with time for the two soil samples of FIG. 9. FIG. 11 is a chart showing the reduction of TCE, DCE and VC in a well following the addition of sorbitol polylactate. FIG. 12 is a schematic of the recirculating well system used in the field experiment of Example 12. FIG. 13 is a chart showing the concentrations of chlorinated ethenes in a well used to monitor the recirculating well system of FIG. 12 over time in a system treated with sorbitol polylactate. FIG. 14 is a chart showing the concentrations of chlorinated ethenes in four wells, an injection well and three monitoring wells, before addition of sorbitol polylactate. FIG. 15 is a is a chart showing the concentrations of chlorinated ethenes in the same four wells of FIG. 14 one hundred eighty-nine days following the addition of sorbitol polylactate. FIG. 16 is a schematic showing the points at which glycerol polylactate was injected into a system and groundwater monitoring points in the experiment of Example 13. FIG. 17 is a series of drawings showing the change in concentration of PCE over time in the system of Example 13. FIG. 18 is a series of drawings showing the change in concentration of TCE over time in the system of Example 13. FIG. 19 is a series of drawings showing the change in concentration of cis-1,2-DCE over time in the system of Example 13. FIG. 20 is a graph showing the total mass change of chlorinated ethenes described in FIGS. 17-19 over time in the system of Example 13. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In view of the prior art, a need remains for a method to provide hydrogen that is cost-effective, safe, efficient, and requires a minimum of active management to perform. Furthermore, the method would preferably provide a known amount of hydrogen, the release of which is controlled over time, and a high quantity of hydrogen per unit weight or volume of substrate used. The preferred embodiments provide novel compounds, formulations, and methods that have some or all of these desirable qualities. A family of preferred compounds to serve as substrates releases hydroxy acid slowly over time. Preferably the hydroxy acid is an .alpha.-hydroxy acid, more preferably it is lactic acid. The compounds may be used to provide a hydroxy acid source for bioremediation of chemical compounds in aquifers, soils, wastestreams, industrial processes, or other systems, preferably by reductive dechlorination. The preferred embodiments also provides for formulations based on the family of novel compounds, as well as methods for their use in promoting bioremediation of contaminants. The preferred compounds are based upon polymers of .alpha.-hydroxy acids having the general formula CH.sub.3 (CH.sub.2).sub.m CHOHCOOH, preferably where m=0, 1, 2, or 3. The most preferred embodiment is where m=0, commonly known as lactic acid. Although .alpha.-hydroxy acids where m=0 to 3 are preferred, other hydroxy acids are within the scope of the preferred embodiments such as: .alpha.-hydroxy acids where m>3; .beta.-, .gamma.- or other such hydroxy acids; di- tri- or other multi-hydroxy acids; branched hydroxy acids; or substituted hydroxy acids. Although the preferred embodiments relate to a wide variety of hydroxy acids, as discussed above, for the sake of simplicity the preferred embodiments are disclosed in terms of the most preferred hydroxy acid, lactic acid. Therefore, when in this disclosure the acids, polymers, and esters are referred to as lactic acid, lactate, poly(lactic acid), polylactate, or polylactate ester, it should be understood that it relates to all hydroxy acids, including .alpha.-hydroxy acids, and the polymers, esters, and esterified polymers thereof. If lactic acid is first polymerized to make poly(lactic acid) and the poly(lactic acid) is reacted with a multifinctional alcohol such as glycerol, xylitol, or sorbitol, polylactate esters result that are semisolid, easy to handle, very insoluble in water, and release lactic acid at a controlled rate. The rate of release of lactic acid from these polylactate esters is comparable to the requirement for lactic acid of microbes, such as those involved in remediating halogenated solvents by reductive dechlorination. Modification of the polylactate esters, such as varying the degree of neutralization of the ester, or by adding other compounds to a formulation based upon polylactate esters, can further regulate the rate of lactic acid release. The preferred embodiments further provide for formulations based on the polylactate esters that serve as a source of lactic acid and other materials that may be desired in a particular application, as determined by one of skill in the art. The formulations are high in lactic acid content, and thus hydrogen releasing ability, for their weight. The formulations are preferably comprised of polylactate esters and inorganic salts. Formulations may also comprise one or more diluents, such as water, glycerin or alcohols. Additionally, formulations may contain other inorganic salts; nutrients such as yeast extract, urea, potassium-containing compositions, nitrogen-containing compositions, phosphorous-containing compositions, sulfur-containing compositions, molybdenum salts, iron salts, zinc salts, copper salts, buffers and pH modifiers such as sodium carbonate and potassium carbonate, ethylene, chelating agents, surfactants, vitamins such as B.sub.12, enzymes such as lipase and esterase, compounds that inhibit competing microorganisms, and bacteria and other microbes. The materials other than the polylactate esters are not required for remediation, but they can provide an improved or more consistent environment for the growth and sustenance of the bacteria responsible for bioremediation. The preferred embodiments further provide methods for the biodegradation of chemical compounds which are remediated by microbial reduction, preferably chlorinated solvents, in aquifers, soils, bioreactors, wastestreams, industrial processes, or other media and systems. The methods utilize the compounds and/or formulations, discussed above, to provide a source of lactic acid either alone or in combination with other compounds to provide a source of electron donors (hydrogen), nutrients, and, in some embodiments, other compounds which serve to support bacterial growth. For purposes of the disclosure herein, the term "poly(lactic acid)" is used to refer to the compound made from polymerizing lactic acid, and the term "polylactate ester" is used to refer to esterified poly(lactic acid). It is recognized that poly(lactic acid) itself is produced through an esterification process, and comprises ester groups. However, as used herein, the term "polylactate ester" is not intended to cover poly(lactic acid) before it has been esterified by reaction with a molecule other than lactic acid or homopolymers thereof. It should also be noted that for purposes of this disclosure, the words "system" and "medium" are used in a very broad sense to refer not only to sites, systems and media in nature such as soils, aquifers, lakes, rivers, and the like, but also to man-made systems including reservoirs, holding tanks, bioreactors, wastestreams, industrial processes, wells, and the like. The polylactate ester compounds serve as a time-release source of lactic acid and thus, hydrogen. These compounds, which may be incorporated into formulations, are preferably used to stimulate bacterial growth and facilitate bioremediative reduction of chemical compounds. The lactic acid released by these compounds and formulations is converted to hydrogen to serve as a source of electrons which aid in bioremediation, as well as other products which provide nutrients for the growth of the bacteria. The compounds and formulations have utility in aiding the destruction or inactivation of compounds which may be reduced, including metal compounds and metals such as chromium VI and organic compounds. Examples of some reducible organic compounds are: nitrogen-containing organic compounds such as quinoline; polynuclear aromatic hydrocarbons (PAHs) such as naphthalene; oxygen-containing organic compounds such as methyl tert-butyl ether (MTBE); and halogen-containing hydrocarbons such as trichloroethene (TCE), PCBs, and chlorofluorocarbons. The family of compounds can be used for purposes other than the preferred use, thus the applicants do not disclaim other unnamed uses for these compounds. The family of compounds is referred to generally herein as polylactate release compounds or polylactate esters. Esters of poly(lactic acid) are preferred, but one may use esters of polymers of hydroxy acids other than lactic acid, such as: .alpha.-hydroxy acids other than lactic acid; .beta.-, .gamma.- or other such hydroxy acids; di- tri- or other multi-hydroxy acids; branched hydroxy acids; or substituted hydroxy acids. The compounds are produced by first polymerizing lactic acid to form poly(lactic acid). Under the preferred conditions disclosed herein, the lactic acid appears to preferentially polymerize, on average, to the tetralactate. This is determined by the amount of water released during the polymerization reaction: ##STR4## The poly(lactic acid) is then combined with an alcohol, preferably a multifunctional alcohol, in the presence of an acid catalyst to produce the ester. For purposes of this disclosure, multifunctional alcohol is defined as an aliphatic hydrocarbon wherein two or more of the carbon atoms have one hydrogen substituted by a hydroxyl group. The carbon atoms of the multifunctional alcohol can contain carbonyl groups on some of the carbons or the end groups of the molecule can be carboxyl groups. Preferred multifunctional alcohols can be further characterized in that they would not cause further pollution or contamination of a system or medium in which they are placed, and would be easily biodegraded or more preferably be used as a nutrient source for the bacteria. The most preferred multifunctional alcohols are those of the type CH.sub.2 OH(CHOH).sub.n CH.sub.2 OH where n is preferably from 1 to 4, more preferably 1, 3, or 4, corresponding to glycerol, xylitol and sorbitol, respectively. Other preferred multifunctional alcohols are complex alcohols such as sugars, reduced sugars, and pentaerythritol. Examples of preferred polylactate esters made from reaction of poly(lactic acid), which has been polymerized to the tetralactate, with preferred multifunctional alcohols are: ##STR5## Esters of different multifunctional alcohols will hydrolyze at different rates under the same conditions, dependent upon such factors as size and structure. Using such knowledge and a minimal amount of experimentation, one of skill in the art will be able to get a desired rate of hydrolysis for a particular application by appropriate choice of multifunctional alcohol as well as by varying the surrounding pH and other factors. The preferred polylactate esters are characterized in part by their ability to hydrolyze to poly(lactic acid), which in turn breaks down and slowly releases lactic acid monomers. These lactic acid monomers are converted, preferably by anaerobic microbial metabolism, to form a variety of compounds including acetic acid, carbon dioxide, hydrogen, and methane. The process of releasing the lactic acid monomer into the aqueous phase from the polylactate ester takes place at a slow, controlled, and predictable rate which is dependent upon the multifunctional alcohol used in the esterification, the pH, the temperature, concentration of the polylactate ester, the surface area of the polylactate ester, and the presence of other hydrolysis catalysts such as added lipase or esterase enzymes. The rate of monomer release is also dependent upon and proportional to the microbial demand for lactic acid. The rate at which the released monomer forms hydrogen and other products is also dependent upon the microbial population in a system and the state of growth and nutrient availability. This rate can be regulated by additives in a formulation based on polylactate esters if not otherwise adequate. The three preferred polylactate esters were also tested to determine their lactic acid release rates in water over time and compared with the lactic acid release rate of a non-polymerized lactate ester, ethyl lactate. These results are shown in FIGS. 1 through 4. The lactic acid release rates for the polylactate esters are far lower than that of the simple ethyl lactate ester and the 24 hour lactic acid release rates for polylactate esters are comparable to that required to remediate TCE. It should be noted that these figures show results when there is no biological demand and the esters are simply placed in water. The release of lactic acid by the poly(lactic acid) is retarded by the presence of free lactic acid in solution. On the other hand, the release of lactic acid is enhanced by the presence of bacteria, with the rate of release being in concert with the demand for lactic acid by the bacteria. Thus, if lactic acid is used by microbes as it is produced, the release will continue at a rate to meet the demand of the microbes. In other words, if all of the lactic acid released in 24 hours was consumed by bacteria in those 24 hours, the next 24 hours would show a continuous higher rate of release, not the decreasing amount shown on the graphs in FIGS. 2-4. Synthesis of Polylactate Esters The synthesis of the polylactate esters is a process remarkable in that it produces no waste products other than water in a relatively simple one pot reaction. All materials formed, and all of their degradation products, are biologically compatible and fulfill some need of bacteria used in bioremediation. The first step in synthesizing the polylactate esters is to make poly(lactic acid). This is done by the polymerization of lactic acid. A quantity of a lactic acid solution, preferably 80% to 100% by weight, more preferably 85% to 88% by weight, is placed in a suitably sized container or vessel. Then preferably 0.1% to 5%, more preferably 1% to 3% by weight of a strong inorganic acid, preferably phosphoric acid, is added as a catalyst. This mixture is heated to a temperature preferably between 20.degree. C. and 180 .degree. C., more preferably to approximately 120.degree. C., to drive off the water and polymerize the lactic acid. The lower temperatures of the preferred range are used if the mixture is under negative pressure (vacuum), as is preferred when the volume of compound being produced is large. If longer chain hydroxyacids are used in place of lactic acid, a solvent such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO) can be added to the reaction vessel. The solvent can remain in the vessel throughout the synthesis of the ester and be removed by evaporation once the synthesis is complete. Slightly more than 3 moles of water should be driven off for every mole of lactic acid charged into the original reaction container. The reaction is complete when there is no visible sign of water being removed, or the proper amount of water has been removed. This generally takes from 2 to 8 hours, preferably from 3 to 4 hours, but is largely dependent on the heating rate, the stirring rate, and the method for condensing and removing the water. Application of a negative pressure (vacuum) can be used to facilitate water removal in larger vessels. This process results in poly(lactic acid) that is polymerized to about the tetralactate. The second step is the esterification of the poly(lactic acid). Any multifunctional alcohol can be used, as the term is defined above. Preferred multifunctional alcohols are of the type CH.sub.2 OH(CHOH).sub.n CH.sub.2 OH where n is preferably from 1 to 4, more preferably 1, 3, or 4, corresponding to glycerol, xylitol and sorbitol, respectively. While adding the alcohol, the temperature of the reaction container and its contents is preferably reduced to 60.degree. C. to 100.degree. C., more preferably approximately 80.degree. C. Alternatively, the temperature can be kept at that used for the polymerization, and in such a case the alcohol will preferably be added under pressure. Preferably, an amount of multifunctional alcohol is added to the vessel so that there is one poly(lactic acid) molecule therein for each hydroxyl group added. In other words, the most preferred molar ratio of total poly(lactic acid) in the reaction vessel to total molar hydroxyl groups on multifunctional alcohols added is approximately 1:1. Ratios from 2:1 to 1:2 are also preferred, but the most preferred ratio is 1:1 to 1:1.1. The temperature of the mixture is preferably set to 20.degree. C. to 180.degree. C., more preferably 120.degree. C. Lower temperatures are chosen if the mixture is under negative pressure (vacuum). When approximately one mole of water for each molar equivalent of hydroxyl group added has been removed, the heat is turned off. The removal of water will generally take from 1 to 3 hours, preferably about 2 hours, depending on the heating rate, stirring rate, and the method for condensing and removing the water. In larger vessels, negative pressure may facilitate water removal. After the heat is turned off, an amount of an inorganic base approximating the neutralization equivalent of the inorganic acid is added with stirring. The inorganic base is preferably any of a wide variety of metal oxides and hydroxides or other basic species, such as magnesium hydroxide, calcium hydroxide, magnesium oxide, magnesium carbonate, calcium carbonate, sodium carbonate, or potassium carbonate. The base can be added soon after the heat is turned off or it can be added at any time during cooling. If the base is added once the reaction mixture has cooled to a temperature at or near room temperature, the mixture is preferably reheated to allow good mixing. If a solvent such as DMF or DMSO was added to the reaction vessel, that solvent is removed by evaporation, preferably under reduced pressure, to give the final product. The degree of neutralization of the acid catalyst as determined by the amount of base added, can be chosen as to buffer the aqueous treatment system or effect a mechanism to create a microenvironment of reduced or elevated pH in the polylactate ester matrix. Reduced or elevated pH in the matrix facilitates hydrolysis of the polylactate ester when the matrix is exposed to water. This is because hydrolysis of the ester may be catalyzed by either acid or base, with the rate increasing the farther the pH gets from neutral in either direction. The polylactate esters are preferably semisolid, colorless to tan to dark amber, are not light sensitive, and can be stored in a manner to create long term stability. The preferred polylactate esters generally melt in the range of 60.degree. C. to 90.degree. C. and can be made to flow in containers of any shape or shaped into any configuration. They can be mixed with powders or other solids if a more granular product is desired |
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