MXPA96000682A - Process for the treatment or biorreduction in situ of solids that carry cr ( - Google Patents

Abstract

The present invention provides a process for the treatment of solids bearing Cr (VI) or earth by in situ biological reduction of Cr (VI) to Cr (III) insoluble. The pH of the Cr (VI) -containing solids is adjusted with an acid or base to a pH of about 6.5 to 9.5, and completely mixed in situ with an organic material (such as manure) that contains bacteria and nutrients.

Description

"PROCESS FOR THE TREATMENT OR BIORREION IN SITU OF SOLIDS THAT CARRY CR (VI)" Inventor (s): THOMAS E. HIGGINS, North American, domiciled at 2112 Lirio Court, Reston, Virginia 22091, E.U.A ..

Causaire: MAXUS ENERGY CORPORATION, Delaware State Corporation, E.U.A., domiciled at 717 North Harwood Street, Dallas, Texas 75021, E.U.A.

FIELD OF THE INVENTION The present invention is a process for in-situ bioremediation or bioscience of chromium (VI) (Cr (VI)) -solid solids, including soils, sediments and waste In particular, the present invention relates to a process for in situ treatment of solids carrying Cr (VI), where Cr (VI) is hinrebraided or bioclosed to Cr (III) without the need to remove the solids from their resting place.

DESCRIPTION OF THE RELATED TECHNIQUE A lot of effort has been spent on the remediation of waste deposit sites, contaminated with Cr (VI) in many places throughout the country. These solids carrying Cr (VI) potentially present a threat to health as compared to the low toxicity of Cr (III) carrying solids. A type of waste that carries Cr (VI) is the residue prod by a process of calcining mineral d-ßj chromite, typical, in which a portion of Cr (III) in chromite ore is oxidized to Cr (VI) by the , calcination in a calcination furnace and then the Cr (VI) salts soluble in water are extracted from the calcined ore. The chromite material processing residue (COPR) contains Cr (VI), due to incomplete leaching, and is usually highly alkaline, due to the use of quicklime (CaO) in the process of calcination. It is typical for Cr (VI) to be present in the COPR at concentrations ranging from 10,000 to 20,000 mg / kg and the Cr (VI) fraction of the total Cr in general, is in the range of 1% to 13%. However, when COPR has been mixed with other materials, it is common for the concentration of Cr (VI) in the mixture to vary widely, as a fraction of the total chromium concentration. The salts of Cr (VI) are very soluble in water, in comparison to Cr (III), which is precipitated as a hydroxide in neutral or alkaline pH. Actually, Cr (VI) is present- as a negative ion (anion) in water opposite to Cr (III) which is a positive ion (cation). Generally, anions are more mobile highs in the earth than cations, which are exchanged with other cations in the earth. The result is that Cr (VI) tends to be completely soluble and mobile and Cr (III) tends to be relatively insoluble and immobile. The conversion of Cr (VI) to Cr (III) has the benefit of greatly reng the migration of chromium into the environment. The biological reion of Cr (VI) to Cr (III) has been demonstrated in the laboratory and in the field. In a research report for The Engineering Foundation & American Society of Civil Engineers in 1979, Higgins reported in the biologic reion of Cr (VI) to Cr (III) and the subsequent removal of a wastewater stream. The researcher uses laboratory soil columns to investigate the movement of heavy metals to groundwater when treating the wastewater containing Cr (VI) and the Cd was applied to agricultural land for irrigation. The researcher initially found that Cr (VI) leaked freely through the soil columns, but that over time, the concentration of Cr (VI) in the filtrate decreased. Significant bacterial growth was observed on the surface of the columns. It was postulated that the removal of chromium is due to the biologic reion of Cr (VI) to Cr (III) followed by either hydroxide precipitation or adsorption or both. Feed for bacterial growth was supplied by biological, oxygen (residual) demand (BOD) in the filtered water. Patent No. 5,155,042 issued to Lupton et al. Refers to the bicrremediation or biocuration of solids carrying Cr (VI), specifically COPR. In the process, Cr (VI) is leached from the solids by injecting an acidic solution into the solids in one position and removing the leachate from a second position for treatment in an external biological reactor to which anaerobic reducing bacteria are added. of sulfate, sulfates and other nutrients, as necessary, for the growth of bacgterias. In the reactor, Cr (VI) is biologically reduced to Cr (III) which is then precipitated as a hydroxide and removed from the solution using solid separation processes. Then acid is added to the solution loaded with anaerobic sulfate-reducing bacteria to maintain a pH of 6.5 to 9.5 and the solution is recirculated in the solids bearing Cr (VI) to promote the reduction in situ and leaching of the remaining Cr (VI). . It is observed that the COPR exhibited an "alkaline reperfusion" effect where after the addition of sufficient acid to reduce the pH to the range of 6.5 to 9.5, the pH slowly rose above 9.5, due to the slow release of alkalinity of the COPR. It should be noted that the concentration of soluble Cr (VI) in the COPR should be less than 200 mg / 1 and the stabilized pH before a self-feeding population of anaerobic sulfate-reactive bacteria can be maintained in situ. Therefore, a process was proposed in which multiple applications of acid and sulfate-reducing anaerobic bacteria are required, and in which the external treatment of leaching in a biological reactor is used to reduce Cr (VI) to Cr (III). ). U.S. Patent No. 5,285,000 issued to Schwitzgebel is directed to the in situ chemical treatment of soil contaminated with Cr (VI). The method first uses a ferrous iron containing solution to reduce Cr (VI) to Cr (III) and coprecipitate Fe (0H) 3 and Cr (0H) 3 resulting with other heavy metals. A solution that forms sodium silicate gel is added to reduce the leaching of the metals. U.S. Patent No. 5,304,710 issued to Kigel et al. Refers to an ex situ process for chemically treating lands contaminated with chrome ore waste by acidification, chemical reduction, neutralization and stabilization. The methods include the steps of ground grinding acidification at a pH less than or equal to 3, the reduction of Cr (VI) to Cr (III) using a ferrous iron salt, raising the pH with an alkaline agent such as lime. live, precipitating chromium and iron as hydroxides, and if necessary to improve physical resistances, stabilizing the mixture by adding cement, very fine dust of cement kiln, inclusions in the metals of finely divided products of the combustion, slag or other agents. U.S. Patent No. 5,202,033 issued to Stanforth et al. Is directed to the in situ chemical treatment of soil contaminated with Cr (VI). The method for treating solid waste in soil or solid waste containing unacceptable concentrations of chromium includes mixing the waste or soil in situ with ferrous sulfate. The method consists in adding ferrous sulfate and a pH controlling agent such as magnesium oxide, magnesium hydroxide, calcium oxide or calcium hydroxide, to soil or waste, and mixing under conditions that withstand reactions that will convert the chromium to a non-leachable form. Treatment additives can be introduced and contacted by the following techniques: contacting the additives in the upper part of the earth or waste and mixing with a mechanical device, such as a rotating device; add the chemical treatment through an infiltration corridor as a solution or thick suspension; injecting a soluble additive through injection nozzles or injection wells, and adding a treatment additive through a recessed arrow auger, and a mechanical mixer. The methods that involve the treatment of solids carrying Cr (VI) mediate the acid filtration through the media, suffer two deficiencies: therefore, the acids react with the solids to significantly reduce the hydraulic permeability of the media, that limit the ability to continue filtering the treatment materials; and, the acid reacts with the first solid material it comes in contact with, which produces one less than the desirable or desirable pH in the water in the pores near the injection point and one greater than the desirable or convenient pH away from the point of injection. The result would be a significant variation in the pH of the media, with a little of the media in the desirable or desirable pH range of 6.5 to 9.5. Similarly, filtration is an inefficient method of distributing bacteria in a medium containing Cr (VI) solids. The tendency will be for the bacteria that are going to be removed by the filtration in the media. Bacteria that do not transcend would tend to be individual cells (to avoid media filtering effects) and would be exposed to high concentrations of Cr (VI), reducing the number of viable bacteria that could be injected through filtration. The North American pathogen No. 5,155,042 reported to Lupton et al. Is limited to the use of sulfate reducing bacteria for the reduction of Cr (VI). One of the patent holders, Defilipi, in "Bioremediation of Hexavalent Chromium in Water, Soil and Slag Using Sulfate Reducing Bacteria", a preprint of the Handbook of Process Engineering for Pollution and Waste Minimization, ed. by D.L. Wise and D.J. Trat it, determine that the sulfate reductive bacteria produce H S, which then reacts with Cr (VI) to reduce it to Cr (III), which is then precipitated as chromium hydroxide. A potential problem with this process is the generation of H2S, a toxic gas. Other research has shown that bacteria other than those that are reducing sulfate are effective in reducing Cr (VI) to Cr (III). Hi or ff i IH > gtJkÜal? l. fltrfi that bacteria present in the effluent of a domestic wastewater treatment plant would reduce Cr (VI). Blake and collaborators in "Chemical Transformation of Toxic Metals by a Pseudomonas Strain from a Toxic Waste Site", Environmental Geochemistry and Health, Vol. 15, No. 2, 1993, studied bacteria Tseßtf &pronos martophiTía. Their tests showed that "the reduction of Cr (VI) was catalyzed by a reductase of chromate bound to the membrane". Tests of Ohtake and Hardyo of Enterobacter cloacae found that bacteria anaerobically reduce Cr (VI) on the surface of cells. The reduced chromium then precipitated as an insoluble metal hydroxide. Their tests also indicated that the most favorable pH for the reaction was 7. See, "New Biological Method for Deposition and Removal of Hexavalent Chromium," Water Science and Technology, Vol. 25, No. 15, 1992. Consequently, it is an object of the present invention to provide an effective and efficient method for the in situ biological reduction of Cr (VI) -containing solids by mechanical mixing in place with organic nutrients, naturally occurring bacteria, and mineral acid or bases without excavation of solids. ,. - These and other objects of the present invention will become apparent in the revision of the following specification and the appended claims thereto. - '•. '** * * BRIEF DESCRIPTION OF THE INVENTION According to one aspect of the present invention, a method is provided for the bioremediation or bio-curing of solids bearing Cr (VI) without the removal of soil soil material. According to another aspect of the present invention, a method is provided for in situ bioremediation of Cr (VI) -containing solids, wherein Cr (VI) is reduced to Cr (III) such that the concentration is reduced. of Cr (VI) in the solids below the limit of detection of the methods using a normal test method. More specifically, the method of the present invention comprises adding organic material, naturally occurring bacteria, and a sufficient amount of an acid or mineral base and water to the solids bearing Cr (VI) to maintain the pH of the mix between 6.5 and 9.5, and then mix thoroughly in situ to promote the biological reduction of ^ Cr (VI) to Cr (III). Among other factors, the present invention is based on the recognition that the bioremediation of the solids carrying Cr (VI) can be successfully and efficiently achieved in situ, without the need for leaching, to reduce the concentration of Cr (VI) in solids or the use of cultured sulfate-reducing bacteria as required in the bioremediation processes of the prior art. By completely mixing in the settlement material, the organic material containing bacteria and sufficient acid or mineral base to maintain the pH between 6.5 and 9.6, it has been found that Cr (VI) to Cr (III) is reduced within the volume of treatment. The solids carrying Cr (VI) such as COPR and mixtures containing COPR, in this way can be remediated or cured more easily and efficiently by means of a biological process that until now, has been possible using other processes.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The present invention relates to a process for in situ bioremediation of solids bearing Cr (VI) without the removal of the material from the soil or compromising its current use. In the process, materials of., Treatment are mixed in the material containing Cr (VI). The treatment material includes bacteria, nutrients, and organic acids and bases, if necessary to establish a conductive pH for bacterial growth (typically between 6.5 and 9.5). Suitable sources of bacteria and organic material include manure, peat, sludge from wastewater treatment and the like. These materials contain a large population of various bacteria (particularly Escherichia coli, which has been found to be effective in reducing Cr (VI)). The semi-solid nature of these organic materials provides some protection for the bacteria against the potentially toxic effects of Cr (VI) on the water in the pores of the solids carrying Cr (VI). further, the semi-solid nature of these materials provides a long-term deposit of organic materials so that the bacteria or the reduction of Cr (VI) to Cr (III) takes place over a prolonged period of time. The flow of groundwater in the treatment area can be controlled in a manner to provide adequate moisture to promote bacterial growth. In the bioreduction process of the present invention, the following occurs: The Cr (VI) in the solids dissolves and migrates to the pores filled with fluid between the particles. Enough organic material, nutrients and bacteria are provided in the pores to effect the bioreduction of Cr (VI) and other easily reduced compounds. The proper conditions are maintained in the solids that carry Cr (VI) for the biological activity 6.5 and 9.5, the provision of sufficient nutrients such as nitrogen or phosphorus. With respect to the bioreduction process of the present invention, Cr (VI) -containing solids are characterized to determine the amount of mineral acid or base necessary to achieve a long-term pH of between 6.5 to 9.5 in the material to provide suitable for bacterial propagation. In general, the method of the present invention involves the following procedure. The acid or mineral base, sufficient to adjust the long-term pH, of the treated mixture in the range of 6.5 to 9.5, water and organic material, such as manure, peat or sludge from wastewater treatment, is applied in situ to the solids carrying Cr (VI) to increase the biodegradation process and effect the reduction of Cr (VI) to Cr (III). To increase the process of bioreduction, ferrous sulfate can be added as **** "One option: Successful bioreduction requires + ... to maintain the pH of the material between 6.5 and 9.5 after equilibrium conditions have been established.

The treatment material with the solids bearing Cr (VI) in situ to establish a uniform consistency is completely removed. Mixing can be done by any suitable means, including, by a recessed arrow auger, cultivator or another suitable mechanical mixer.

In the method of the present invention, the solids carrying Cr (VI) or earth are physically mixed in reactive solvents and the soil serves as the reactor. This allows bioremediation and the reduction of Cr (VI) is applied to large volumes of the solids bearing Cr (VI) without the need for the excavation of the solids bearing Cr (VI) and subsequent physical grinding of the material to facilitate a Faster reaction time at the site or near the treatment plant. Effective bioremediation and reduction of Cr (VI) in a site can be verified by the following procedure. Water samples can be taken from the soil or material and can be analyzed for Cr (VI), pH and the total microbial population (by the plate count method) to verify the progress of bioremediation. Representative samples of the "mixed solids of the earth or material can be taken and analyzed for the total Cr and Cr (VI) and a mass balance performed, to verify that the conversion of Cr (VI) to Cr (III) has occurred. ) and the conservation of the total Cr The invention will be illustrated in more detail by the following samples: These examples are given by way of illustration and do not mean that they limit the description or the claims that follow: the percentages in the example, and = another part in the specification, are in percent dry weight of the material by dry weight of the solids bearing Cr (VI) unless otherwise specified.

EXAMPLE 1 The invention was tested on the chromite ore processing residue. The composition of the COPR and the treatment materials are shown in Table 1.

Table 1 Composition of COPR and Treatment Materials Material Cr Cr P TKN COD Solids Total pH (VI) Total Totals (mg / kg)% Manure 15 10 8,600 735,000 48 8.6 Fresh Manure Composite 25 < 30 9.78023.000 1,430,000 14.1 - Peat 27 8 250 10,700 1,150,000 62.8 4.4 COPR 7,9703,520 250 300 34,000 80.8 11.1 The following procedures were used: Sr thoroughly mixed the COPR material and sieved through a 1.27 cm (1/2 inch) sieve to homogenize the samples and remove the debris.

The COPR samples were mixed with amounts of tap water, sulfuric acid, ferrous sulfate, fresh manure, compound manure and peat.

The mixture was transferred to a 15.24 cm (6 inch) diameter column about 30.48 cm (one foot) deep.

Water is added to the column to saturate the mixture.

Liquid samples were taken from a bottom tap and analyzed for Cr (VI), pH and total microbial population (by the plate count method).

The samples of the mixed solid were mixed and analyzed for nutrients (nitrogen and phosphorus), COD and total, and Cr (VI). 7. The columns were sealed and stored at room temperature (approximately 20 ° C). The columns were opened for sampling and resealing.

The water samples were drained from the column every month and analyzed for Cr (VI), pH and microbial population. The height of the water in the columns was monitored and water was added to maintain saturation. The columns were opened and the solid samples were collected and analyzed for, the total and Cr (VI).

The test conditions for the column tests are listed in Table 2. A mixture of turva and compound manure was tested to determine the effectiveness of these stabilized materials, biologically as compared to fresh manure. Ferrous sulfate was added to Columns Cl and C2 to determine if an initial reduction of Cr (VI) would be necessary to reduce the toxicity of potential chromium to bacteria and increase the process of bioreduction.

Table 2 Initial Conditions for the Column Biorreduction Test *! ^^ _____ COPR Column Initial Sulfuric Acid Water Peat Manure Com- Manure Sulfat or Sulphate kg (pounds) (mi) (liters) (%) position1 (%) Fresh1 (%) Ferrous2 (%) Ferrous (g) CO 7.257 (16) 100 2.0 0.0 0.0 0.0 0 0 Cl 7.257 (16) 75 2.0 0.0 0.0 0.0 50 115 C2 7.257 (16) 75 3.0 3.0 3.0 3.0 50 115 1 C3 7.257 (16) 100 4.0 5.0 5.0 0.0 0 0 C4 7.257 (16) 100 3.0 3.0 3.0 3.0 0 0 1 C5 7.257 (16) 100 4.0 5.0 0.0 5.0 0 0"Mass used as a percentage of the mass of COPR Amount used as a percentage of the req ernment is eq ui metric When testing the process of the present invention in the COPR solids carrying Cr (VI) the pHs of the samples were adjusted with sulfuric acid to the initial pHs of 5.6 to 8.1. (Table 3) In a month, the pHs stabilized between 7.8 and 9.4. These tests showed that the long term pH of the solids in the. COPR carrying Cr (VI) can be adjusted to the optimal range for the Cr (VI) bioreduction by an individual addition of mineral acid.

Table 3 Water pHs in the Pores of the Study Column Column Home 1 month 2 months 3 months 4 months 5 months 7 months 9 months 11 months CO 6.8 7.7 7.7 7.7 7.8 8.0 7.9 NM 7.6 Cl '5.6 7.6 8.1 8.1 7.8 7.7 7.5 7.5 7.5 C2 7.6 8.7 9.1 9.2 9.4 9.6 NM NM 8.8 C3 7.0 7.5 8.7 8.9 9.0 8.8 8.0 8.1 8.7 | C4 7.6 8.6 9.2 9.3 9.4 9.7 9.5 9.3 8.5 C5 8.1 8.9 9.3 8.7 9.1 9.0 8.6 8.9 8.4 1 MN = No Med i d o Table 4 shows that bacterial cultures can be developed and maintained in the COPR when several mixtures of manure and peat are applied to the material and the pH is adjusted to the optimum range (pH 6.5 to 9.5). CO and Cl, which had no organic supplement, both started with low populations. All columns with manure (compound or fresh) started with healthy bacterial populations. These populations tend to decrease "during the first couple of months and then increase, possibly due to the need for bacteria to condition a high concentration of Cr (VI) in the COPR.The C4 tube a very late recovery.This column also tube the highest pH (was greater than 9 during most of the study period), which reinforces the conclusion that high pH is greater than a toxicity factor than Cr (VI) .The column with the largest dose of manure Fresh (C5) developed the best bacterial culture.C2 also developed a healthy bacterial culture, although the pH also pointed to 9.3. '.i'fc-'i.

Table 4 Water plate counts in the pores of the columns dß; study (CFU * / ml) Column Home 1 month 2 months 3 months 4 months 5 months 7 months 9 months 11 months CO 2 37,500 4,700 31,800 16,400 1 60,000 22,800 7,700 Cl -ilO 15,900 1,200 53,000 278,000 3,000 147,000 30,000 34,600 C2 5,600 28,200 2,230 207,000 131,000 22,500 168,000 78,400 850 C3 9,900 2,100 109 430 18,600 46 1,090 258,000 2,400 C4 31,000 12,100 1,090 3,600 450 430 400 251,000 24,400 C5 > 300,000 3,100 1,100 > 300,000 118,000 26,000 377,000 166,000 10,800 K3"CFU is an abbreviation for Colony Forming Unit Table 5 shows the concentrations of Cr (VI) in the water between the pores and Table 6 shows the total Cr (VI) in the solid phase of the columns. The data in the control column (CO) basically shows no change during the test period. This demonstrates that although a reduction in pH promotes moderate bacterial growth, this does not result in a significant reduction of Cr (VI) in COPR. All samples treated organically showed an instantaneous and significant reduction in water between the Cr (VI) pores. The water in the pores in columns C3 and C5 (Table 5) decreased less than the detection limit of 0.01 mg / 1 after four months and remained undetectable for the rest of the time. The water in the pores in column C2 decreased less than the detection limit after 7 months, but was slightly higher than the limit of detection after the columns (C4) lagged behind the other columns, due to its higher pH and the resulting low bacterial population. After one year, the water between the Cr (VI) pores remained near or below the detection limit. After one year, all the solid phase Cr (VI) concentrations (Table 6) of the supplemented columns .. were less than 100 mg / kg and only the C4 was less than the detection limit of 20 mg / kg. In the above samples, collected after nine months, the solid phase Cr (VI) concentrations in columns C2 and C5 were lower than the lower detection limits of 6 and 8 mg / kg, respectively.

Table 5 Concentration of Cr (VI) in the water in the pono of the study column (mg / 1) Column Home 1 month 3 months 4 months 5 months 7 months 11 months 12 months CO 1,390 1,210 1,460 1,360 1,390 1,360 1,270 1,300 Cl 216 462 689 630 660 640 560 650 C2 309 795 40 2.79 0.13 0.01 0.02 0.02 C3 505 647 18 0.10 0.01 0.01 0.01 0.01 C4 837 782 188 101 51 17.3 2.37 0.16 C5 629 508 5 < 0.01 < 0.01 0.01 0.01 < 0.01 Table 6 Concentrations of Cr (VI) solid phase in the study column (mg / kg) Column Home 1 month 3 months 4 months 5 months 7 months 9 months 11 months CO 2,750 2,620 2,670 2,400 2,100 2,4? 3 1,800 2,200 O. 982 1,770 1,700 1,700 1,500 1,700 900 1,800 C2 917 988 163 145 80 20 ¿6 < 20 Table 6 (continued) Column Home 1 month 3 months 4 months 5 months 7 months 9 months 11 months C3 1,490 936 429 58 15 10 46 20 C4 1,530 1,580 915 250 67 60 61 70 C5 1,770 885 419 28 < 8 < 10 < 8 < twenty This test demonstrated the technical feasibility of the proposed method of in situ bioreduction of solids bearing Cr (VI). The pH of the Cr (VI) -containing solids can be adjusted to the optimum range for bacterial activity (6.5 to 9.5) by the addition of an acid or mineral base once. The addition of organic materials (several mixtures of fresh manure and compound, and peat with nutrients) provided an adequate population of bacteria and suitable environment for its propagation. It has been demonstrated that these bacteria can be conditioned to Cr (VI) concentrations in excess of 2,000 mg / kg, and that these bacteria will reduce the concentration of Cr (VI) in the solids bearing Cr (VI). The addition of ferrous sulfate resulted in a faster Cr (VI) reduction compared to organic treatment materials, alone.

EXAMPLE 2 The solids that carry Cr (VI) to be treated first can be tested to determine the appropriate quantities of additives required for the reduction of Cr (VI) of solids bearing Cr (VI). The acid or base requirements can be determined by acid or base titration of representative samples of the Cr (VI) -containing solids. HE can determine the requirements of organic material, ie, fresh manure or compound manure and / or peat, using the labotary columns, or experimental field test. The typical application rates for The additives are expected to be less than 5% manure and less than 2% acid or mineral base to the solids bearing Cr (VI) on a dry weight basis. * m * If necessary, the treatment area can be divided into separate treatment areas to facilitate e? use of different amounts of treatment additives. Once the amount of additive per area was determined, the additives are applied and mixed with the solids carrying Cr (VI). The application of additives and the mixing of these with the solids they carry Cr (VI), can be made using a hollow arrow auger or an egg-beater-like device, such as is used for in situ mixing of wellbore materials for stabilization / solidification, to apply simultaneously the additives in different depths and thoroughly mix the additives with the solids that carry Cr (VI). For solids carrying Cr (VI) that are close to the surface, agricultural implements such as plows or cultivators can be used to add additives to the solids. carrying Cr (VI). The use of the present invention can achieve effective reduction of Cr (VI) from a site in a relatively convenient and uncomplicated manner. In situ bioremediation incorporated in the present process, achieves the reduction of Cr (VI), excavation and chemical treatment of the prior art, which can give many benefits to the potential user. Once a site has been treated according to the in situ process of the present invention, the material can be left in situ in the soil containing less toxic and less mobile Cr (III) valence state. While the invention has been described with preferred embodiments, it is understood that it can be done Use of the variations and modifications as will be apparent to those skilled in the art. Such variations and modifications will be considered within the substance and scope of the appended claims thereto.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, property is claimed as contained in the following:

Claims (8)

1. A process for the bio-reduction or in situ bioauration of the solids that carry Cr (VI), characterized that includes: adding to the solids that carry Cr (VI) an organic material, bacteria, nutrients, a sufficient amount of acid or base mineral to maintain the mixture at a pH between 6.5 and 9.5, and water, and thoroughly mix the Cr (VI) bearing solids and additives in place to effect a reduction of Cr (VI) to Cr (III) in if you.

2. The process according to claim 1, characterized in that the organic material comprises manure (fresh or composite), peat, sludge from the wastewater treatment, or mixture thereof.

3. The process of conformance with the claim-characterized because it also includes the control of water flow-in the soil in a treatment area to ensure adequate moisture content for biological growth.

4. The process according to claim 1, characterized in that the addition and mixing is performed by a hollow arrow auger, a device similar to an "egg beater", or an agricultural implement.

5. The process according to claim 4, characterized in that the implement? (Agricultural is a plow or a cultivator.

6. The process according to claim 4, characterized in that the mixing is achieved using a hollow arrow auger.

7. The process according to claim 1, characterized in that the bacteria comprise Escherichia coli.

8. A process for the in situ bioreduction of the solids bearing Cr (VI), characterized in that it comprises: analyzing the solids carrying Cr (VI) that go. to be treated to determine the appropriate amounts of additives required for the reduction of Cr (VI). add to the solids that carry Cr (VI), the determined amounts of organic material, bacteria, nutrients, acid or mineral base, and water to reduce the mixture to a pH between 6.5 and 9.5, and thoroughly mix the solids that carry Cr ( VI) and additives to effect a reduction of Cr (VI) to Cr (III) in situ. In testimony of which I sign the present in this City of Mexico, D.F., on February 21, 1996. Attorney & ** _ * t $ l &