MXPA98005166A - Method to reduce hexavalent chrome in soils, sediments, industrial waste and other contaminated materials, with use of ascorb acid - Google Patents

Abstract

The present invention relates to a process for reducing the concentration of potentially toxic hexavalent chromium, (Cr (VI)), in existing soils / chromium-bearing materials in the form of soils, sludges, sediments, fillings, industrial wastes or other materials, the concentration is reduced by applying and mixing a single reducing agent, ascorbic acid, to effect the chemical reduction of Cr (VI) to a less toxic valence state. The ascorbic acid is added at room temperature in the form of an aqueous solution or suspension and mixed with the soils / materials bearing Cr (VI) in amounts based on the test results of representative samples of the material to be treated. The ascorbic acid can also be added in an anhydrous form if sufficient moisture is present in the soils / materials to allow the dissolution of the ascorbic acid and the reaction with the Cr (VI) in the material. The percentage of reduction in the concentration of Cr (VI) is greater and is obtained more quickly than what is previously reported when using other organic chemical reducing agents. The process, which does not require the modification of the pH of the material bearing Cr (VI), can be applied in situ to the materials bearing Cr (VI), in which unsaturated and / or saturated soils are included in the same column of the soil, using appropriate mixing equipment for the depth of the soil or material to be treated, and can be applied to materials that are stored in a container or that have been excavated from the soil or other deposit.

Description

METHOD TO REDUCE HEXAVALENTE CHROME IN SOILS, SEDIMENTS, INDUSTRIAL WASTE AND OTHER CONTAMINATED MATERIALS, WITH USE OF ASCORBIC ACID Background of the invention Field of the invention The present invention consists of a treatment process for soils, sediments, industrial wastes, fillers or other materials that carry hexavalent chromium. { Cr (VI)} , or other materials that have been contaminated, that have been released to the environment or have been generated in the production process of chromium compounds. Specifically, a process is described which reduces the toxic hexavalent form of chromium, Cr (VI), to trivalent chromium. { Cr (III)} by adding ascorbic acid (CAS: 5081-7; CQRQOQ) 1 to materials / soils bearing Cr (VI). The ascorbic acid is mixed with the material that carries Cr (VI) in if you (in place) without the need to separate the soils / materials from the place where they rest or can be applied ex situ (above the surface of the soil) to a material bearing Cr (VI) that has been excavated or otherwise separated from the ground or other deposit. The process can be carried out at room temperature and atmospheric pressure. The reduction of Cr (VI) to Cr (III) is rapid and does not require the addition of other agents or previous alteration of the pH of the soils / materials that carry Cr (VI). The process is particularly applicable to soils exposed to Cr (VI) as a result of either: 1) release of chemicals from chromate or 2) mixed with residue carrying Cr (VI) that was generated during the production of chromium, such as those derived from the processing of the chromite ore, as described by Austin ^ and Westbrook- ^, or to a processing residue of ore or ore of chromite containing highly desirable levels of Cr (VI). 10 Regulatory and Factual Background According to the efforts to submit to hazardous waste sites throughout the United States, chromium, chromate and / or Cr (VI) have been identified as contaminants of concern in a significant number of sites of the "National Priority List (Superfund)" under the "Comprehensive Environmental Responsibility and Liability Compensation Act" 4 as well as many hundreds of other sites where chemical compounds have been released from chromium into the environment. Cr (VI) is classified in the category of a human inhalation carcinogen5, and many forms of Cr (VI) are highly soluble and mobile in the environment. The inhalation of particles transported by air carrying Cr (VI) is the most significant trajectory of potential exposure to humans. The high solubility and high mobility characteristics of many chromate compounds are additional concerns for the soils / materials bearing Cr (VI) since the contamination of Cr (VI) can be spread to significant distances from a strong site via spillage or runoff surface water or groundwater migration. There is also concern about the ingestion of water containing high concentrations of total chromium, which is reflected in the standard of chromium concentration of the federal drinking water of 0.1 mg / l. In contrast to Cr (VI), Cr (III) is not classified as a human carcinogen and is still considered an essential trace nutrient for mammals. Although some soluble forms of Cr (III) have been shown to be toxic to certain aquatic species, soluble Cr (III) is rarely found in aquatic systems. The trivalent forms of chromium are found more frequently in insoluble forms in the environment6. Thus, the soils / materials bearing Cr (III) are considered a health concern significantly lower than the materials bearing Cr (VI). Cr (VI) is more frequently the focus of decision making of correction operations at sites where high levels of chromium have been identified in soils. Under the Resource Conservation and Recovery Act (RCRA), the Environmental Protection Agency of the United States of America (USEPA) has established test criteria to determine when a waste or soil containing chromium is considered a hazardous waste. When the USEPA Standard Toxicity Leaching Procedure (TCLP) is applied to a material and the total concentration of chromium present in the leachate is greater than 5 mg / l, the material is designated as a "characteristic hazardous waste", subject to the treatment, storage and disposal regulations of the RCRA. Cr (VI) is not found naturally in most soil / water environments. Thus, essentially all the Cr (VI) contamination found in soils, sediments, waste and other materials is the result of human activities7. Much of the contamination by Cr (VI) is the result of materials carrying Cr (VI) spilled or discarded, which include such widely used chromate chemical compounds as K2Cr2? 7, a2Cr? 4, and CrO 8. Such Cr (VI) contamination can cause physical, chemical and / or biological changes that alter the properties of contaminated soils / materials, which include pH, permeability, porosity, salinity, oxidation-reduction potential (redox) and microbial population. Another type of contamination by Cr (VI) of the soils is associated with the processing residue of ores or chromite minerals (COPR). This residue has unique physical / chemical properties (for example, it fluctuates in particle size from spherical particles of mud size and granular sand to cement-like monoliths, agglomerates greater than baseballs) as described by James 9'10 , compared to most soils in their 'natural state. This residual material is produced by roasting the chromite ore in a furnace under alkaline oxidation conditions to commercially extract and produce various chromium3'11 compounds. Because this extraction process is not complete, COPR contains Cr (VI) soluble and insoluble residual12. COPR is also quite alkaline, normally exhibiting a pH greater than 11, due to the use of quick lime (CaO) and sodium carbonate (Na2C? 3) in the roasting process3. Its color may vary based on the source of the mineral and the materials used in the roasting process, although the grayish-black and reddish-brown materials are typical.

During many decades of the twentieth century until the 1970s, COPR was used with other filler material to reclaim wetlands near the chemical manufacturing facilities that produce it. The Cr (VI) and total chromium content of these soils / materials bearing COPR varies widely based on many factors, which include the source and characteristics of the processed chromite ore, the aggregate materials during ore processing , the actual processing conditions at the time the waste leaves the process, the nature of the soils or the filling material with which the COPR was mixed during its deposition and the degree of exposure to the weather that has occurred since its placement in the environment. For soils that are highly enriched with COPR, the total chromium concentration may exceed 30,000 mg / Kg, with approximately 33% to 66% existing as Cr (VI). However, such high concentrations of total chromium and Cr (VI) in soils bearing COPR are rarely found, except in sites that were predominantly filled with COPR. Common COPR-bearing soils contain concentrations of total chromium from several hundred to 4,000 mg / Kg with Cr (VI) concentrations that represent approximately 1% to 8% of total chromium. The other main cations in soils enriched with COPR are usually iron, aluminum, calcium and magnesium, the concentrations of which can also vary widely according to the source of the chromite ore, the "processing conditions and the mixture of COPR with other filler material.13 Compared to COPR-bearing soils, natural soils in the United States of North America contain from 1 mg / kg to 2,000 mg / kg of total chromium, with an average of 54 mg / kg14 and negligible concentrations of Existing chrome such as Cr (VI) Also, in soils that carry COPR that have been mixed with sediments or soils rich in organic compounds, Cr (VI) concentrations can be approximated to undetectable levels (<5 mg / Kg ) due to the reducing conditions of the soil matrix12'15 although the total chromium concentration can be higher than 10,000 mg / kg.

Description of the Related Art Physical separation and / or isolation, (eg, solidification / stabilization, encapsulation, suspension walls, containment caps, etc.) are conventional practices that have been used to treat soil / material contamination by Cr (VI). In U.S. Patent No. 4,504,321, Kapland and Robinson teach the combination of chrome ore waste with certain mud or dredged mud and the addition of 5-30% finely ground blast furnace slag stabilizes the mixture to a hardened state after curing In the process described in US Patent No. 3,937,785, Gancy and Wamser teach that the reduction of the COPR particle size by grinding, such that at least 20% by weight passes through a 200 mesh screen, can decrease the filtration of Cr (VI) from soils that carry COPR after its contact with water at the waste site. Another methodology of physical treatment for corrective operation of sites with Cr (VI) bearing soils has involved either excavation and washing of the soils or washing them in their place16. However, this method commonly exhibits limited effectiveness, except in soils containing easily leachable Cr (VI) compounds. A problem with the prior art is that the techniques of physical separation, relocation or isolation do not usually change the Cr (VI) to a less dangerous valence state. When using these techniques, the materials contaminated with Cr (VI) are: 1) only transferred to another deposit; 2) captured in an aqueous stream that must then be treated to reduce Cr (VI) and separate it from water; or 3) physically contained in the place of the site in which they exist where they remain a long-term concern for the release of Cr (VI) uncontrolled potential to the environment. The reduction of Cr (VI) to Cr (III) represents a way to decrease the toxicity and mobility of Cr (VI) present in soils / materials without having to extract or change the total chromium content of the materials. Chemical reduction processes that reduce Cr (VI) in aqueous or solid materials to Cr (III) have been described in the prior art, sometimes combine solidification / stabilization processes with the reduction of Cr (VI) to obtain a reduction of the greater danger of the treated material. Pal and Yost describe a process for the reduction of Cr (VI), stabilization and fixation of chromium in contaminated materials, in U.S. Patent No. 5,397,478. Waste soils or waters containing Cr (VI) are first treated with a reducing agent such as sodium dithionite, sodium hydrosulfite, ferrous sulfate, sulfur dioxide or one of several forms of sodium sulfite, then treated by addition of lime to stabilize the reduced chromium and finally treated by addition of phosphate to complete the fixation process. The chemical processes taught in the prior art have cited a variety of reducing and treatment agents for Cr (VI), which include the use of the following: 1) reducing sugars such as sucrose, glucose and maltose (Elges et al. al., North American Patent No. 3,784,669; 2) water-insoluble lead compounds (eg, lead oxide, lead carbonate and lead hydroxide) to precipitate lead chromate from wastewater (Nieuwenhuls, US Patent No. 3,791,520), 3) barium carbonate for the direct precipitation and recovery of chromium from waste water containing chromic acid and / or metal chromate salts in an aqueous medium acidified with glacial acetic acid (Feltz and Cunningham, US Patent No. 3,969,246); 4) non-pulverized elemental iron to reduce Cr (VI) to Cr (III), by treating acidified industrial wastewater in a gravity flow system (Roy, US Patent No. 4,108,770); 5) hydrogen peroxide and oxalic, malic or maleic acid with polyvinyl alcohol and application of lime to precipitate Cr (0H) 2 from waste water carrying Cr (VI) (Hawxhurst and Slobbe, US Patent No. 4,321,149); and 6) alkali metal dithionite to reduce the Cr (VI) to Cr (III) in the waste material and subsequently to separate the co-precipitated material from the waste (Pilznienski, US Patent No. 5,200,088).

Another chemical method described by Ladd and Miller in U.S. Patent No. 4,798,708, is a complex process involving the recovery of chromium from chromium-bearing material comprising the use of one or more strategic metals or other metals (e.g. cobalt, nickel, molybdenum, tungsten, aluminum, iron and tin) by atomizing the chromium-bearing material into a powder that can flow and heating it in an alkaline oxidizing environment, followed by the formation of a water suspension and adjusting pH to approximately 9.6, to capture insoluble materials (this is cobalt, nickel, iron and aluminum), followed by pH adjustment to less than 2 and addition of methyl alcohol, in an amount sufficient to reduce Cr (VI) to Cr (III). Then, adsorption by activated carbon is applied to separate the tungsten and molybdenum from the suspension, followed by adjusting the pH to a value of about 5.0 to 8.5 with base to form a precipitate, followed by separation of essentially all the chromium from the spent liquor. resulting. Another method taught by Schwitzgebel in the US Patent No. 5,285,000 is a process in which the soil contaminated by the metal is treated with ferrous iron to reduce Cr (VI) to Cr (III), followed by the addition of sodium silicate to form a waterproof gel that decreases the permeability of the treated soil matrix. Perrone et al, in US Patents No. 4,401,573 and 4,560,546, teaches that the treatment of waste water containing chromium, with the use of acetic acid or alkali acetates and alkali hydroxides at a temperature of 40-100 ° C and values of pH greater than 6.5, produces chromium hydroxide that is quickly filtered for chromium recovery. Chemical treatment by addition of iron particles and mechanical stirring to reduce Cr (VI), followed by pH adjustment and separation of insoluble precipitates from waste water have been addressed in the prior art taught by Thornton (US Patent No. 5,380,441). In addition to chemical reduction methods, biological corrective operation processes have been developed to reduce Cr (VI) to Cr (III). To treat water contaminated by Cr (VI) in the soil, Lupton et al, in US Patent No. 5,062,956, describe a treatment process using anaerobic sulfate-reducing bacteria and an alkaline additive to reduce Cr (VI) to Cr (III) and immobilize Cr (III) as an insoluble hydroxide. Lupton et al, in US Patent No. 5,155,042, subsequently describe another biological corrective operation process for treating solid waste contaminated by Cr (VI), by first treating it with acid to separate Cr (VI) from the solids and produce an aqueous solution with a pH in the range of 6.5 to 9.5, followed by the addition of anaerobic sulfate-reducing bacteria and nutrients to reduce Cr (VI) to Cr (III). A variation of the process is described by Higgins in US Patent No. 5,562,588 for the in-situ mixing of an acid or mineral base to adjust the pH of the Cr (VI) carrying solids to the range of 6.5 to 9.5, followed by the mixed organic matter, such as animal or peat fertilizer, to provide bacteria and nutrients to reduce Cr (VI) to Cr (III) without having to separate the soil from the soil. Higgins teaches that the treatment materials must be sufficiently mixed with the soil carrying Cr (VI), in place, a hollow shaft auger or other appropriate mechanical mixer is used and also teaches that ferrous sulfate can be added to increase the speed of reduction of Cr (VI). Finally, a hybrid process of chemical reduction and in situ mechanical mixing described by Stanforth in US Patent No. 5,202,031, treats solid waste containing arsenic, cadmium, chromium, and / or copper by first mixing the waste with a phosphate agent. or carbon or ferrous sulfate and possibly an agent for pH control, followed either by dispersion of the material above the ground and mixed with one or more mechanical devices or by the use of chemical injection and mixing equipment such as nozzles or cavities, infiltration galleries or a hollow shaft auger to effect mechanical mixing in the place. For one or more of the following reasons, most of the patented processes mentioned above are not used in the present to treat soils, sediments, fillings or wastes containing Cr (VI) or corrective operation sites contaminated with Cr (VI): 1) the complexity of the process, 2) the transport distance and cost of the materials needed in the process, 3) the difficulty of managing waste produced through the process, 4) poor performance effectiveness to reduce Cr (VI) to Cr (III), 5) complications associated with shallow groundwater at one site, and / or 6) the high cost of applying the process. Additionally, most of the methods of the prior art are limited in the range of materials which can be treated at the same time, in particular, Cr (VI) contamination that exists in soils not saturated with water and saturated with water in the soil. Same floor column of a site.

Brief description of the invention The present invention is based on a method for reducing the concentration. { [Cr (VI)]} of Cr (VI) in si tu or ex situ, different from that described in the prior art. This invention consists of a process in which a single organic compound, ascorbic acid, is added and mixed in situ or ex situ with soils, sediments, sludge, filler, waste or other materials bearing Cr (VI). In the preferred embodiment of this invention, the materials do not have to be separated from the ground or other deposit in which they rest. This significantly reduces the cost and effort required to clean a contaminated site. Alternatively, the process can be applied to materials contaminated with Cr (VI) as they are generated or to materials contaminated with Cr (VI) that have been excavated from the soil or another deposit (ex si tu). Ascorbic acid is a non-hazardous substance which, when properly handled in accordance with manufacturers' safety precautions, can be used over a wide range of concentrations without concern for human health or environmental damage. To carry out the present invention, ascorbic acid can be added to soils / materials as an aqueous solution or supersaturated suspension. Alternatively, it can be applied in a dry form if the soil or other material contaminated by Cr (VI) has adequate moisture to allow sufficient mixing and dissolution of the ascorbic acid.

To carry out the process, first determine the [Cr (VI)] (concentration of Cr (VI)) in the material, together with such auxiliary parameters as pH and En (redox potential) that help to characterize the state of oxidation-reduction of the soils / materials to be treated. In the process, an amount of ascorbic acid in excess of the theoretical stoichiometric amount required for the determined Cr (VI) concentration is added. An excess is used to provide an amount of ascorbic acid that sufficiently reduces the bound and unlinked forms of Cr (VI) in the soils / materials. Multiple samples are treated with different amounts of ascorbic acid and analyzed to determine the amount of ascorbic acid needed (in excess of the stoichiometrically theoretical amount) to obtain the desired reduction of [Cr (VI)]. The treated material is normally analyzed for [Cr (VI)], pH, and E ^. Based on these data, the appropriate amount of ascorbic acid can be selected for its addition and mixing in itself or ex if you with the soils / materials to be treated, either in a single stage or in a multi-stage process for reduce the Cr (VI) to Cr (III) to obtain the desired treatment goal. It has been found that ascorbic acid is substantially more effective in reducing Cr (VI) to Cr (III) in soils bearing Cr (VI), compared to other common organic acids, such as acetic acid and citric acid and many others organic compounds that are considered potential reducing agents. It was found surprising that the reduction of the concentration of Cr (VI) to Cr (III) in soils bearing Cr (VI), by ascorbic acid was significantly faster than what had been reported for other potential organic reducing agents and in comparison with the biological corrective operation processes described in the prior art. After the addition of ascorbic acid, a significant [Cr (VI)] reduction was obtained in hours, compared to the days needed for other organic reducing agents and the many months that are commonly required for the biological corrective operation processes get similar results. In laboratory tests, it was also found that when ascorbic acid or a mineral acid were each applied to soils that carry highly alkaline COPRs, minimal amounts of gas were generated by ascorbic acid compared to the significant amounts of gas and heat generated by the mineral acid. This finding is significant with respect to field applications where the formation of excess gas could: 1) limit the amount of additive (s) that can be mixed in if you and 2) inhibit the effectiveness of the reduction of [Cr (VI)] described in the prior art. For sites where the soils / materials bearing Cr (VI) exist in the unsaturated zone, above the mantle or water table, and in the saturated zone, below the water table, the process of the present invention has the ability to treat Cr (VI) in both soil layers and the associated groundwater, if the There are, at the same time. Mechanical and mixed injection equipment that can penetrate more than 15.24 m (50 ft.) To the surface of the subsoil and provide a supply of concurrent ascorbic acid and in-situ mixing within the column of the Cr (VI) in a single stage it is well known to those skilled in the art. The process of this invention can also be used to treat surface soils that carry Cr (VI) up to a few meters below the soil surface, where the reduction of [Cr (VI)] is • necessary or planned only for these floors of the surface. The ascorbic acid can be added as an aqueous suspension or solution or in dry form and mixed in itself in a single step or in multiple applications. Depending on the moisture content of the soil, it may be necessary to add water to the soils / materials to allow sufficient mixing to effect solubilization of the ascorbic acid with the material bearing Cr (VI). The process of this invention can also be used to treat soils / materials that have been stored in terrestrial or separate (e.g. excavated) vessels or structures from the earth or other deposit. The use of such conventional equipment as excavators, conveying systems, chemical feeding systems, kneader mills, front-loading loaders and transport vehicles to apply this ascorbic acid treatment process is considered well known to those skilled in the art for treat soils / contaminated materials. The process of this invention does not usually require adjustment of the pH of the soils / materials bearing Cr (VI), nor does it require the addition of other organic matter and nutrients, materials or other organisms and the subsequent maintenance of appropriate pH conditions for its propagation. Unlike some prior art processes, which require the addition of heat, the process of this invention can be carried out in soils / materials carrying Cr (VI) at ambient temperatures and pressures. The collective characteristics of this invention provide a faster, more complete and more economical treatment of the soils / materials bearing Cr (VI) in comparison with the processes taught in the prior art. A first object of this invention is to provide a method for reducing the potentially toxic forms of the chromium existing in the environment, in particular hexavalent chromium, to less toxic forms, mainly trivalent chromium, by reduction with ascorbic acid. A further object of the invention is to provide a method for the reduction of hexavalent chromium by using ascorbic acid, which can be applied in situ to soils / materials contaminated with chromium, without the need for the separation of soils / materials from their resting place. . A further object of this invention is to provide a method for the treatment of waste contaminated with hexavalent chromium in all forms, whether in-situ or ex-if with ascorbic acid to reduce potentially toxic hexavalent chromium to less dangerous trivalent chromium. A further object of this invention is to provide a method for analyzing soils / materials contaminated with chromium to determine the appropriate amount of ascorbic acid to be used to obtain a desired level of hexavalent chromium reduction. Still a further object of this invention is to provide a method for treating soils / materials contaminated with chromium which reside mainly in the near surface environment. A further object of this invention is to provide a method that obtains the rapid reduction of hexavalent chromium in a single process stage that uses ascorbic acid. A further object of this invention is to provide a method for measuring and evaluating the effectiveness of a method for the rapid reduction of hexavalent chromium to trivalent chromium by using ascorbic acid. A further object of this invention is to provide a method that uses ascorbic acid to reduce hexavalent chromium to trivalent chromium which keeps the treated material in a reduced valence state. A further object of this invention is to provide a treatment process for materials contaminated with hexavalent chromium that reduces hexavalent chromium to trivalent chromium, "which then exists in a state of stable reduced valence.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. A to EE show concentrations of Cr (VI) against the time after treatment with ascorbic acid for samples A to E at different stages.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT The present invention consists of a process in which ascorbic acid is added to and mixed sufficiently with soils, sediments, sludge, waste, filler or other materials bearing Cr (VI) (with or without separation of soil materials or other resting place) to chemically reduce Cr (VI) to Cr (III). By using a relatively safe organic acid, the hazards associated with some additives used in the prior art are avoided (e.g., hydrogen peroxide, sulfuric acid, ferrous sulfate, lime, hydrazine). The only additive used with this process, ascorbic acid, when used in accordance with the manufacturer's safety handling instructions, does not present a hazard or environmental concern to corrective operation workers, nearby residents and / or other nearby workers or the environment, because ascorbic acid has essentially no potential city to humans. Indeed, ascorbic acid is important for human health in the prevention of scurvy and is used as a beneficial food supplement. It is believed that the reduction reactions involved in the process can be described by the following chemical equation in which the ascorbic acid is consumed and dehydroascorbate is formed at the same time as the chromate ion is reduced to the trivalent state in the form of chromium hydroxide, an insoluble compound that is not prone to re-oxidation.

Cr042"+ 2H + + 1.5C6H806 -> Cr (OH) 3 + 1.5C6H606 + H20 (Eq. 1) Although this relationship is believed to be a principal and representative mechanism for the reduction of Cr (VI) to Cr (III) by ascorbic acid, it is possible that other chemical reactions occur simultaneously and potentially at different reaction rates than this reaction. Indeed, as will be seen below, the fact that quantities in excess of the stoichiometric quantities defined above are required in real-world systems suggests that other reactions and perhaps competent reactions may be involved. However, based on this equation as a reference, the stoichiometric amount of ascorbic acid to reduce 1 g of Cr (VI) to Cr (III) in the form of insoluble chromium hydroxide is 5.1 g (see Equation 1). The first step in the application of the process of this invention is to measure the concentration of Cr (VI) of representative samples of the soils / materials to be treated. This can be done by using the standard alkaline extraction procedure and analysis described by Vítale et al12. It is also important to measure such auxiliary parameters as the concentration of total chromium, pH and E ^ as described by James et al ^ to help confirm the changed redox condition (condition of increased reduction) of the treated material compared to the material without try. Based on the concentration of [Cr (VI)] determined, the stoichiometric amount of ascorbic acid can be calculated, according to Equation 1, required to reduce the concentration of [Cr (VI)]. In addition, several multiples of the stoichiometric amount are calculated, usually in the range of 2X to 9X. The purpose of determining the stoichiometric amount and related multiples is to provide a range of ascorbic acid dosages with which to test the materials contaminated with Cr (VI). As will be seen in the examples below, the effect of different dosages varies with the sample. This is expected for several reasons: 1) the materials from which the samples are taken are not homogeneous even in the same place; 2) Samples are complex organic and inorganic mixtures of materials contaminated with Cr (VI); and 3) the nature and source of the materials contaminated with Cr (V) varies widely. For example, as discussed above, chromite processing produces COPR with particle sizes, alkalinity, and physical composition as well as variable chemistry. Normally, it has been found that stoichiometric multiples of 2X to 7.5X are useful and sufficient to cover the range of materials tested. However, clearly, those skilled in the art which practice this invention will recognize those circumstances when a greater range of stoichiometric multiples must be employed. Then, the stoichiometric and multiples are added to - and sufficiently mixed with - the untreated samples representative of the material bearing Cr (VI). A control or reference sample (without ascorbic acid) is also included for a reference measurement. Three main measurements are carried out on the materials treated as a function of the time after mixing, the concentration of Cr (VI), E ^ and pH. These three measurements can be used to determine: 1) whether a sufficient reduction of [Cr (VI)] has been obtained with a particular material and 2) whether the concentration of [CR (III)] is likely to remain reduced with step time. In a chemical reduction process of this invention, based on the observed results, the following should be presented. The ascorbic acid in solution penetrates and disperses in the pores and between the soil particles to reduce Cr (VI) to Cr (III). When the ascorbic acid is provided in a dose sufficiently in excess of the theoretical amount, the reduction of the desired Cr (VI) is obtained. The desired level of [Cr (VI)] reduction depends on a variety of factors that include, but are not limited to, the location of contaminated materials, probable human exposure, and regulations to currently applicable halves and the criteria for cleaning. One such reduction criteria are the soil filtration references established by the USEPA in 1996 for use in "Superfund" sites for the corrective operation of Cr (VI) contaminated soils in residential locations. The established principle is a maximum soil concentration of 270 mg Cr (VI) / Kg based on an exposure or inhalation trajectory17. Based on the results of the treatability tests of the representative samples described above, a dosage amount (stoichiometric multiple) is chosen which will produce the desired reduction of [Cr (VI)]. The ascorbic acid is applied in if you or your ex as an aqueous solution or a supersaturated suspension with sufficient chemical and mechanical mixing to test the reduction of Cr (VI) to Cr (III) in the soils / materials. Depending on the object of concentration reduction for a particular contaminated site and the response of the soils / materials to the treatment by ascorbic acid, the appropriate stoichiometric multiple can be selected based on the test results to reduce [Cr (VI)] on a short or long time interval. The larger multiples of ascorbic acid normally reduce Cr (VI) to Cr (III) more rapidly than the smaller multiples, but at a higher cost because of the amount of ascorbic acid required. If a lower multiple can obtain the desired reduction of [Cr (VI)] over a longer and more acceptable time interval, it may be economically advantageous to select the lower multiple. Such selection should also consider the other cost elements of supplying and mixing the ascorbic acid with the soils / materials to be treated. The solution or suspension of ascorbic acid can be added and mixed simultaneously with the soils / materials or added first and then mixed. The mixing method and duration depends mainly on the moisture content, viscosity, cohesiveness and other properties of the soils / materials that affect the ability of the selected equipment to sufficiently mix the ascorbic acid with the material bearing Cr (VI). As indicated above, the blended methods are well known to those experienced in the corrective operation technique and include, but are not limited to, soil depth mixing devices or agricultural implements that are used to mix soils relatively close to the surface. Samples of the treated soils can be collected and analyzed to determine if the treatment objectives have been achieved. The process of this invention can also be carried out by using an anhydrous form of ascorbic acid, if there is sufficient moisture that occurs naturally in the soil, atmospheric precipitation is frequent and sufficient or if moisture can be applied separately to the soils to help dissolve the ascorbic acid and distribute it in the pores within the matrix of the soils / materials. When anhydrous (eg powder) is used, ascorbic acid can be used to treat the surface and soils / shallow materials (eg, a few meters below the surface of the soil) in one or more applications, using the treatability results described above to assist in the selection of the dosage. The ascorbic acid is mixed in the soils / materials by using one or more appropriate shallow soil mixing devices, well known in the art, such as cultivators, discs, mixing paddles, maneuvering handles or other such implements that are used commonly in the preparation of agricultural soils. The soils / materials to be treated must be mixed during or immediately after the addition of the ascorbic acid to effect the maximum benefit for the field application. The use of a hollow rod auger with mixing blades allows the injection of ascorbic acid in an aqueous solution or suspension directly into the soils / materials at the same time that the soils / materials are mixed. This is an effective method to treat soils / materials that are located more than one tier from 0.61 m (2 feet) to 0.91 m (3 feet) and can be used to treat soils, sludge, sediments and the like that carry Cr ( VI) at depths of 15.24 m (50 feet) or more below the surface as long as there are no obstructions, rocks or other conditions that limit the mixing capacity of the selected device. It is advantageous from a logistical or technical reason or another reason to separate the soils / materials bearing Cr (VI) from the soil or soil deposit or other storage site for the treatment with ascorbic acid, the use of conventional equipment (for example, excavators, conveyors, storage / feeding systems for chemical products, mixing vessels and vehicles to move the earth) to apply and mix the ascorbic acid, are considered in the experience of those experienced in the technique of soil treatment / contaminated materials. Another distinctive advantage of the process of this invention is that it can be practiced at ambient temperatures, normally found in soils bearing Cr (VI). Since soil temperatures generally fluctuate from somewhat below the freezing point (0 ° C or 32 ° F) to more than 30 ° C (8 ° F) near the soil surface and are relatively close to approximately 13 ° C (55 ° F) below the freezing line, the rate of reduction of [Cr (VI)] exhibited in laboratory tests at standard temperature and pressure will differ somewhat under field conditions. For each 10 ° C (18 ° F) decrease in soil temperature, the reaction rate is expected to decrease by approximately half, the inverse is expected for the increased soil temperature. This means, for example, that the amount of reduction of [Cr (VI)] obtained within a particular time interval in the laboratory may require more time under field conditions at temperatures lower than the freezing line. If you are going to treat soils / frozen materials, for example, mixing of additional ascorbic acid and / or heat may be necessary to obtain the desired reduction in [Cr (VI)] within an acceptable period of time. Such modifications to the process of this invention are considered within the level of experience of those familiar with the corrective treatment / operation technique. Under such circumstances, the appropriate preliminary tests in line with the present teaching would be appropriate to determine the appropriate conditions for the application of the process. It has been discovered that ascorbic acid has superior reducing properties (with respect to its effectiveness and rate of reduction of [Cr (VI)] in soils / materials bearing Cr (VI) compared to more than a dozen other organic compounds with Several functional groups, including acetic acid, citric acid, lactic acid, oxalic acid, mandelic acid, salicylic acid, and substituted phenols, can be expected from their molecular structure and their common uses to function as ascorbic acid. reducing agent However, the exceptional rapidity and the greater effectiveness with which it reduces the Cr (VI) to Cr (III) in the soils / materials that carry Cr (VI) in comparison with other organic substances, were unexpected before the discovery OF THIS INVENTION The results of comparative laboratory tests of several soils bearing different Cr (VI) containing widely varying variant Cr (VI) With different site characteristics, they show that ascorbic acid produces a much higher percentage of reductions in Cr (VI) concentration than either acetic acid or citric acid. Acetic acid and citric acid effect less than 55% reduction in Cr (VI) concentration compared to approximately 88 to 99.9% when using ascorbic acid at similar treatment doses. In addition, these results were obtained in only hours when using ascorbic acid. This is also quite fast compared to the many months required by the biological corrective operation processes described in the prior art to obtain a comparable reduction of [Cr (VI)]. It was also unexpectedly found that the rate of reduction of [Cr (VI)] by ascorbic acid was much higher than the rates reported by Deng and Stone18 when using numerous organic compounds to treat the reduction of [Cr (VI)] in aqueous solutions that contain approximately 1 mg / l Cr (VI) in the presence and absence of oxide surfaces (IO2) • Deng and Stone's research focused on 16 low molecular weight organic compounds (in which ascorbic acid is not included) in six functional groups (a-hydroxycarboxylic acids, their esters, a-keto acids, oxalic acid, benzaldehyde and phenolic compounds), none of which reduced significant amounts of Cr (VI) in the absence of oxide surfaces. In the presence of oxide surfaces, Deng and Stone found that the reduction of [Cr (VI)] normally fluctuated from 15 to 80 percent in periods of approximately 5.5 to 8 days. Only two of the compounds, 4-methoxyphenol and mandelic acid, obtained more than 80% of the reduction of [Cr (VI)] (97 and 94% for approximately 35 and 54 hours, respectively). Thus, nothing in the prior art remotely suggests that ascorbic acid would produce such spectacular results with the soil carrying Cr (VI) and waste materials.

Methods and materials Experimental Protocol Laboratory tests were carried out to determine the effectiveness of the reduction of ascorbic acid from Cr (VI) to Cr (III) in a variety of soils bearing Cr (VI). Two other common organic acids, acetic and citric acid, were also tested by comparison. The tests included the use of 50 g samples (aliquots) of different soils bearing COPR containing moderate to high concentrations of Cr (VI). To the 50 g aliquots taken from each sample site, different amounts of ascorbic acid (0.55 g / ml in deionized water) are added and vigorously mixed manually for approximately 30 seconds at room temperature. Less acid levels and sampling intervals were used, however, for the tests of acetic acid or citric acid than with the ascorbic acid, although all the aliquots were mixed in a similar manner. After the initial mixing and for the rest of the test, the samples were exposed to ambient air at 25 ° C, but they were not mixed again. Sufficient quantities of control or reference samples (untreated) and treated were prepared to allow the collection and analysis of individual aliquots at various time intervals of up to at least 28 days for each sample source. In each time interval, the E ^ and pH were measured using calibrated measuring devices and the total chromium was measured using methods 3060A / 7196A of SW-84619'20. The data of E ^ are related to the redox state of a sample, so that the values of E ^ decreased in a sample treated with ascorbic acid, compared to the untreated sample, indicate that the conditions in the treated sample are more conducive to the reduction of Cr (VI) than in the untreated sample. Quality control samples were also included, including duplicates and laboratory preforms in the protocol to demonstrate that the test results met the essential requirements regarding the acceptance of the quality of the data. The following examples illustrate the fields of application of the process of this invention in soils bearing COPR containing a wide range of Cr (VI) concentrations. Each of the samples was collected from a different location in identified sites because they have Cr (VI) contamination. It was known or assumed that soils bearing COPR have been deposited in each of the locations along with other unidentified filling materials of unknown origin. There were no other common characteristics among the five samples discussed below. Based on prior sampling and testing experience with the type of material contaminated with inhomogeneous Cr (VI) used to test the process of this invention, multiple control or reference aliquots were collected and analyzed (no acid aggregate ascorbic) in terms of the concentration of Cr (VI), [Cr (VI)] in the same time intervals as the samples treated with ascorbic acid were collected for analysis. Since, as expected, the samples were not homogeneous and produced variable Cr (VI) concentrations for the different aliquots, the arithmetic mean of the multiple control samples was used to calculate the percentage of reduction of [Cr (VI)] in each stoichiometric multiple, although only the reduction percentages of [Cr (VI)] of the 7.5X stoichiometric multiples at different time intervals are shown in the tables below.

Example 1 The protocol described above was applied to a soil bearing COPR (designated "Sample A") containing an average during the test period of 4,060 mg of Cr (VI) / Kg. The untreated (control) sample exhibited an initial pH of 12.1 and an initial redox potential (Eh_) of 533 mV. Table 1 summarizes the results of the tests.

Table 1 Treatment with Ascorbic Acid of Soils bearing Cr (VI) Sample A Tierpc Multiple E_stequic_rétrico Time C 2.0 3.5 5.0 7.5 0.0 2.0 3.5 7.5 0.0 2.0 3.5 7.5 Cr.VI, _g / Kg% Cr.VI) Units of pH Eh, mV Reduced from Schedule 12.1 533 1 3, 420 2,240 1, 420 1,380 42 99.0 10.6 9.3 8.7 149 181 191 3 3, 150 1, 490 1,090 837 185 95.4 10.6 9.2 8.7 151 168 182 . 5,230 1, 130 307 795 45 98.9 10.4 9.3 9.0 124 145 151 24 5, 400 1,390 826 589 25 99.4 11.1 10.2 9.7 115 118 144 168 3,? 40 435 229 293 35 99.1 11.2 10.6 10.1 156 149 168"CO 3, 260 99 138 36 12 99.7 11.2 10.7 10.5 182 136 128 ? RCM 4, € C The table summarizes the stoichiometric multiples of ascorbic acid (0, 2.0, 3.5, 5.0 and 7.5) applied to the different aliquots of sample A and the results of the Cr (VI), pH and En concentrations of the control samples. or reference (multiple 0) and samples treated with ascorbic acid at the increased time interval after initial mixing (time 0). The arithmetic mean (average) of the control or reference samples is presented in the lower left part of the table and is used to calculate the indicated percentages of reduction of [Cr (VI)] obtained. It can be seen that for Sample A the stoichiometric multiple 7.5X reduces the concentration of Cr (VI) in the soil matrix by approximately 99% within the first hour and exhibits more than 99% reduction of [Cr (VI)] after exposure to environmental conditions for 720 hours (30 days). For the stoichiometric multiple of 7.5 X, the pH of the sample decreased markedly from 12.1 at a level as low as 8.7 after the ascorbic acid was added. After 24 hours, the pH would have increased to 9.7 and continues to increase to 10.5 after 720 hours. This behavior is attributed to the highly alka condition of the sample. ' For the stoichiometric multiple of 7.5X, the highly oxidized condition of the control sample (untreated), represented by an E ^ of 533 mV, was substantially decreased by the addition of ascorbic acid to an E ^ of 195 mV in the first hour after treatment. The E ^ continues with its gradual decrease to 120 mV during the test period of 720 hours. The data indicate that the decrease in E ^ coincide with the reduction of Cr (VI) to Cr (III) immediately after the addition and mixing of the ascorbic acid. Furthermore, by uniformly decreasing the E ^ it is shown that the treated material remains in a reduced state, that is, in a condition not favorable for the oxidation reactions that could occur in the soil and generate Cr (VI).

At stoichiometric multiples less than 7.5X, the reduction reaction of [Cr (VI)] was not as rapid (see Figures ÍA - ÍE for 0 to 24 hours) or as effective as the stoichiometric multiple 7.5X (see Table 1, columns 2.0, 3.5 and 5.0). By illustration, the 2X stoichiometric multiple produced only approximately 66%, 89% and 98% reduction in [Cr (VI)] in 24 hours, 168 hours (7 days), and 720 hours (30 days), respectively. However, concentrations of Cr (VI) for all stoichiometric multiples measured at 720 hours contained 138 mg Cr (VI) / mg or less, equivalent to more than 96.6% reduction. The results of the treatment of Sample A when using acetic acid and citric acid are presented in Table 2. Table 2 Treatment of Soils Bearing Cr (VI) by Acetic Acid or Citric Acid Sample At Time Stoichiometric Multiple Time 0 3.5 7.5 0 7.5 0 7.5 Addition of Cr.VI), mg / Kg% Cr (VI) Units of Eh, mV Treatment Reduced pH 0 4,060 12.1 533 Acetic acid 4 2,750 2,290 43.6 9.9 287 Acetic acid 144 2,370 Citric acid 4 2,400 2,120 47.8 7.9 375 Citric acid 144 2,220 1,980 51.2 9.7 300 These data show that these two organic acids were significantly less effective in reducing [Cr (VI)] than ascorbic acid at doses of similar treatments. A reduction of less than 52% was obtained 6 days after the treatment, and the treated samples continued to exhibit oxidizing conditions (high E ^) consistently after the addition of acetic acid or citric acid.

Example 2 The protocol described above was applied to a soil bearing COPR (designated "Sample B") containing an average during the test period of 314 mg of CR (VI) / Kg. Table 3 summarizes the results of the tests.

Table 3 Treatment by Ascorbic Acid of Soils Carrying Cr (VI) Sample B Tisrpo Multiple Estequi.trétricc r 'tou. 2.0 3.5 5. 0 7.5 0.0 2.0 3.5 7.5 0.0 2.0 3.5 7.5 r VI). mg / Kg% Cr.VI) pH Units Eh, mV Reduced Pr of redio 13.5 409 - 452 111 58 34 89.2 9.0 8.1 6.7 171 179 190 3 265 1Ü3 57 47 22 93.0 3.6 8.6 7.2 139 143 181 3 ^ 6 136 65 65 38 87.9 8.5 8.1 7.9 134 144 146 28 290 31 40 35 15 95.2 8.3 8.2 8.0 127 156 117 3 12 _r 38 - > 7 16 94.9 9.2 9.2 8.2 140 128 105 168 234 -a 29 14 12 96.2 9.6 9.3 8.8 200 181 180 336 319 55 - 1. 99.4 9.5 9.7 8.1 248 263 146 or "2 357 50 10 2 or 99.4 9.5- 9.5 * 8.8 * 388 * 584 * 164 * * The sample had dried and required the addition of moisture to obtain the measurement. The untreated sample (control or reference) exhibited an initial pH of 10.5 and an E ^. of 409 V. Which indicates that it was highly alkaline and was in an oxidized condition. Within one hour after the treatment, the stoichiometric multiple of 7.5X of ascorbic acid reduced the concentration of Cr (VI) in sample B by approximately 89%, the pH had decreased to 6.7, and the En had decreased to 190 mV. The pH was gradually increased to a slightly lower level of pH 9 during the 672 hours (4 weeks), while the E ^ remained in the range of approximately 110 to 180 mV. The concentration of Cr (VI) also continues its decrease to approximately 2 mg of Cr (VI) / Kg, which represents a reduction of [Cr (VI)] of more than 99%. At lower stoichiometric multiples, the reduction of [Cr (VI)] was not as fast or effective as at the maximum dosage applied. For illustration, the 2X stoichiometric multiple produces approximately 74%, 81% and 84% reduction of [Cr (VI)] in 28 hours, 168 hours and 672 hours respectively. All stoichiometric multiples collected at 672 hours, however, contained 50 mg of Cr (VI) / Kg, or less, representing at least 84% reduction of [Cr (VI)]. The results of the treatment of Sample B with the use of acetic acid and citric acid (Table 4) demonstrate that these two organic acids did not essentially obtain [Cr (VI)] reduction 144 hours after treatment using similar treatment dosages as for the ascorbic acid and the treated sample continued to exhibit highly oxidizing conditions consistently after the addition of acetic acid or citric acid.

Table 4 Treatment of soils that carry Cr (VI) with acetic acid or citric acid Sample B Stoichiometric Multiple Time Time 0 3.5 7.5 0 7.5 0 7.5 Additive of Cr (VI), mg / Kg% Cr (VI) Units Eh, mV Reduced Treatment of pH 0 314 10.5 409 Acetic acid 4 360 435 -38.4 8.3 433 Acetic acid 144 350 350 -11.3 9.6 362 Citric acid 4 440 470 -49.5 8.2 425 Citric acid 144 250 300 4.6 9.6 371 The negative values (increased concentrations) in the table probably reflect variations associated with the sampling of inhomogeneous materials.

Example 3 The protocol described above is applied to a soil bearing COPR (designated as "Sample C") which contains an average over the test period of 3,980 mg of CR (VI) / kg. The untreated sample (control or reference) exhibits an initial pH of 11.8 and an E ^ of 516 mV, which indicates that it was highly alkaline and was in an oxidized condition. Table 5 summarizes the results of the tests. Table 5 Treatment of Ascorbic Acid Soils Bearing Cr (VI) Sample C Tieirpo Estequicmetric Multiple Hour D • 2.0 3.5 5.0 7.5 0.0 2.0 3.5 .5 oO 2.0 3.5 7.5 r (VI), mg / Kg Cr (VI) pH Units Eh, mV Reduced from Average 0 11.8 516 4, 640 1, 490 1, -iCC 952 i 99.7 8.6 8.3 7.1 179 179 175 3 5,340 1, 680 1,220 466 1 99.97 8.9 8.3 7.3 173 155 134 9 4, 320 1, 580 1, 990 1,200 £ 6 98.6 9.2 8.6 7.6 140 138 93 24 4, 330 1, 600 455 487 43 98.9 9.8 8.8 7.8 139 100 83 144 4, 610 518 6 99.8 432 2,200 803 453 481 25 99.4 10.9 10.3 8.5 164 142 51 .16 2, 250 687 726 192 54 98.6 11.7 10.6 9.2 179 174 80 PSCM 3, 980 Over the course of one hour of ascorbic acid addition and mixing, the stoichiometric multiple of 7.5X showed a reduction of [Cr (VI)] greater than 99.9%. The pH had decreased to almost neutral at 7.1 and the Eh_ had decreased to 175 V. During the course of 816 hours (34 days), the treated sample exhibited an increase of more than two pH units, which was attributable to the condition highly alkaline of the initial sample. During the same period the concentration of Cr (VI) was measured within the range of 1 to 56 mg / kg, which represents more than 98% of the reduction of [Cr (VI)]. These data, coupled with the increase in pH, suggest that a gradual leaching of Cr (VI) from the internal porous matrix of the soil had been prolonged. However, E ^ continues its decrease to 80 mV in 816 hours, which confirms that the sample continues to exhibit an environmental reduction more than a month after treatment. At lower stoichiometric multiples, the reduction of [Cr (VI)] was not as fast or effective as at the maximum applied dosage. For illustration, the stoichiometric multiple of 2X produces approximately 60%, 80% and 82% reduction of [Cr (VI)] in 24 hours, 432 hours and 816 hours, respectively. The results of treating Sample C, using acetic acid and citric acid, as shown in Table 6, demonstrate that these two organic acids were significantly less effective in reducing [Cr (VI)] than ascorbic acid at treatment doses Similar .

Table 6 Treatment of Soils bearing Cr (VI) by Acetic Acid or Citric Acid Sample C Stoichiometric Multiple Time Time 0 3.5 7.5 0 7.5 0 7.5 Additive Cr (VI), mg / Kg% Cr (VI) Units of Eh, mV Treatment Reduced pH 0 3,980 11.8 516 Acetic acid 4 2,620 2,640 33.7 7.7 376 Acetic acid 144 2,300 2,410 39.4 9.3 322 Citric acid 4 1,860 1,780 55.3 7.4 428 Citric acid 144 2,180 2,120 46.7 8.6 362 The reduction in the range in only 33 to 55% of [Cr (VI) is obtained at 144 hours after stoichiometric multiple treatment of 7.5X and samples treated with acetic acid or citric acid continue to exhibit oxidative conditions (high E ^) consistently after treatment.

Example 4 The protocol described above was applied to a soil bearing COPR (designated "Sample D") which contains an average during the test period of 336 mg Cr (VI) / kg. The untreated (control) sample exhibits an initial pH of 9.8 and an E ^ of 573 V, indicating that it was highly alkaline and in an oxidized condition. Table 7 summarizes the results of the tests. Table 7 Treatment of Ascorbic Acid of Soils bearing Cr (VI) Sample D - Tierpo Multiple E cequicmeric Hour 0 2.0 3.5 5.0 7.5 0.0 2.0 3.5 7.5 0.0 2.0 3.5 7.5 Cr (VI), rrg / Kg% Cr. VI) - pH units Eh, mV Reduced from the average 9.8 573 - 336 2C4 124 88 66 90.3 9.3 8.5 7.8 226 225 240 377 200 107 84 61 81.8 9.1 8.8 8.1 220 215 217 323 131 113 es 69 79.4 9.2 8.7 3.3 216 218 204 23 313 92 79 55 42 37.5 9.3 9.0 8.5 220 215 196 43 286 ._ 34 56 39 38.4 9.3 9.0 3.4 227 210 191 168 302 95 79 64 42 87.5 9.3 9.2 8.5 309 294 191 336 323 9D '_ 64 42 .z 9.0 8.3 8.0 309 154 165 6"2 369 126 66 61 56 33.3 8.2 * 8.3 * 3.4 * 653 * 687 * 644 * _5CM.

* The sample had dried and requires the addition of moisture to obtain the measurement. Over the course of one hour of ascorbic acid addition and mixing, the stoichiometric multiple of 7.5X showed a reduction of [Cr (VI)] of about 80%. The pH had dropped to 7.8, and the Eh. it had decreased to 240 mV. Over the course of 1.032 hours (43 days), the sample treated with the highest dose of ascorbic acid exhibits a gradual increase in pH of about 8.5. During the same period, the concentration of Cr (VI) was consistently in the range of 39 to 69 mg / kg, representing between 79 and 88% reduction of [Cr (VI)]. The E ^ remains in the range of approximately 160 to 260 mV for 408 hours (17 days). At smaller stoichiometric multiples, the reduction reaction of [Cr (VI)] was not as fast or as effective as the maximum dosage applied. For illustration, the stoichiometric multiple of 2X produces approximately 73%, 72% and 62% reduction of [Cr (VI)] in 24 hours, 192 hours (8 days) and 1,032 hours, respectively. The results of treating sample D with the use of acetic acid and citric acid, as shown in Table 8, demonstrates that these two organic acids did not essentially obtain [Cr (VI)] reduction at 144 hours after treatment. dosages of treatment similar to treatment with ascorbic acid.

Table 8 Treatment of Soils bearing Cr (VI) by Acetic Acid or Citric Acid Sample D Stoichiometric Multiple Time Time 0 3.5 7.5 0 7.5 0 7.5 Additive Cr (VI), mg / Kg% Cr.VI) Units of Eh, mV Reduced Treatment pH 0 336 9.8 12.1 Acetic acid 4 315 330 1.6 7. 9 452 Acetic acid 144 360 390 -16.2 8. 8 419 Citric acid 4 530 380 -13.3 8. 0 386 Citric acid 144 270 280 16.5 8. 9 387 The negative values (increased concentrations) in Table 8 probably reflect variations associated with the sampling of inhomogeneous materials. The En values of the samples treated with acetic acid or citric acid were substantially higher than the samples treated with ascorbic acid and remained in a substantially oxidizing range.

Example 5 The protocol described above was applied to a soil bearing COPR (designated as "Sample E") containing an average, during the test period of 940 mg Cr (VI) / kg. The untreated sample (control) exhibits an initial pH of 10.6 and an En of 340 V. Table 9 summarizes the results of the tests. Table 9 Treatment with Ascorbic Acid of the Soil carrying Cr (VI) Sample E Time Multiple Stoichiometric Hour 0 2.0 3.5 5.0 7 5 0.0 2.0 3.5 7.5 0.0 2.0 3.5 7.5 Cr.VI), mg / Kg of pH units Eh. mV Cr.V_) reduce. of prom 10.6 340 1 1.020 3 9 173 138 88 90.6 9.5 8.7 8.3 133 111 112 3 875 336 155 104 85 91.0 9.2 8.8 8.6 140 123 113 8 871 557 131 92 63 93.3 10.2 9.3 8.6 136 144 111 2 869 189 101 80 65 93.1 11.6 11.0 10.1 153 123 121 168 738 119 78 57 40 95.7 11.5 11.1 10.2 199 156 147 336 1.070 125 89 80 51 94.6 11.3 11.0 10.1 672 1.140 78 63 65 53 94.4 11.2 * 10.7 * 10.1 * 352 * 242 * 191 * * The sample had dried and requires addition of moisture to obtain the measurement. In the course of one hour of addition of ascorbic acid and mixed, the stoichiometric multiple of 7.5X shows a reduction of [Cr (VI)] greater than 90%. The pH had decreased to 8.3 and E ^ had decreased to 112 mV. During the course of 672 hours (28 days), the treated sample exhibits an increase of almost two pH units, which are attributed to the highly alkaline condition of the initial sample. During the same period, the concentration of Cr (VI) was measured at 40 to 88 mg / Kg, which represents a reduction in [Cr (VI)] of up to 95%. The E ^ remains in the reducing range of approximately 110-190 mV during a period of 672 hours. At lower stoichiometric multiples, the reduction reaction of [Cr (VI)] was not as fast or effective as the maximum dosage applied. By illustration, the 2X stoichiometric multiple produces approximately 80%, 87% and 92% reduction of [Cr (VI)] in 24 hours, 168 hours and 672 hours, respectively. However, all treated samples contained less than < 78 mg of Cr (VI) / Kg (91% reduction) at 672 hours after treatment with ascorbic acid. The results of the treatment of Sample E with the use of acetic acid and citric acid (Table 10) shows that these two organic acids were significantly less effective in reducing [Cr (VI)] than ascorbic acid at similar treatment dosages.

Table 10 Treatment of Soils bearing Cr (VI) by Acetic Acid or Citric Acid Sample E Stoichiometric Multiple Time Time 0 3.5 7.5 0 7.5 0 7.5 Cr.VI additive), mg / Kg% Cr.VI) Units of Eh, mV Reduced Treatment pH 0 940 10.6 340 Acetic acid 4 930 900 4.3 10.6 258 Acetic acid 144 880 850 9.6 10.9 277 Citric acid 4 870 840 10.7 9.6 295 Citric acid 144 765 685 27.2 10.5 249 The reduction in the range of approximately 9 to 27% of [Cr (VI)] is obtained 144 hours after the treatment, and the treated samples continue to exhibit significantly higher E ^ values compared to those for the samples treated with ascorbic acid.

Summary of the exemplary data Figures A to EE show that the test data from the previous tables, plotted for each of the five samples bearing Cr (VI) (A-E). Only the first 24 hours of the data are shown, since the percentage of the greatest reduction occurs within this period. Clearly, the higher amounts of ascorbic acid added to the sample (stoichiometric multiples / higher dosages) result in lower Cr (VI) concentrations in the treated sample. However, the actual differences in the resulting concentrations of Cr (VI) for the different dosages depend on the nature / composition of the sample. Thus, a reduction range in concentration of more than 80% reduction of [Cr (VI)] for sample D to more than 99.9% reduction of [Cr (VI)] for sample C is seen at the dose of higher applied ascorbic acid (stoichiometric multiple of 7.5X. Figures 1A-1E and Tables 1, 3, 5, 7 and 9 also show that, over longer periods of time, percentages of reductions similar to those obtained with the 7.5X multiple (>90%) are obtained at smaller treatment doses for almost Sample D (83%). Additionally, "Figures 1A-1E show that the speed during the first hour of the reduction of [Cr (VI)] was not only rapid and substantially similar for each of the different samples, but was also independent of the dose, as indicated by the similar initial slopes of the curves of the first hour for the different stoichiometric multiples For all samples, the majority of the reduction of [Cr (VI) 3 (for example, 65-85% and 90-99.9 % at stoichiometric multiples of 3.5 and 7.5 respectively) and decrease in Eh (more than 200 mV) occurs essentially immediately (within the first hour) after the addition of ascorbic acid and mixing. 3, 5, 7 and 9 also show that the pH initially decreases sharply after mixing with decreases greater in magnitude for larger doses of ascorbic acid than for the smaller doses, then the pH increases uniformly. Memente for days and weeks. The final pH was generally less than or equal to the initial pH although, here again, some dependence on the stoichiometric multiple can be seen. The gradual increase in pH after the abrupt initial decrease, immediately after the ascorbic acid treatment is probably due to the leaching of the intrinsic alkalinity of the internal pores of the source materials at a decreased rate. During the same period of time, the E ^ remains normally in the range at which it had decreased simultaneously with the reduction of [Cr (VI) _ initial. These data indicate that enough ascorbic acid had been incorporated into the matrix of the material of the samples in such a way that the reducing conditions persisted long after the treatment.

In order to carry out the process of this invention to reduce the concentration of Cr (VI) in soils / complex materials to acceptable levels, the ascorbic acid must be mixed appropriately with the soils / materials to provide contact with the dissolved ascorbic acid with the Cr (V) found: 1) on the surfaces of the floors / materials; 2) within a short distance to the floors / materials; 3) within the interstices and pores of the soils / materials that carry Cr (VI) and 4) in the associated aqueous environment. However, as can be seen from the previous examples, no fracturing, sieving, scraping or meshing, or other special mechanical procedure is required. The liquid medium (water) in which the ascorbic acid dissolves or suspends properly carries the ascorbic acid to the materials. Certainly, this transport is so effective that the actual reduction of the majority of Cr (VI) is obtained in the course of one hour of mixing, since the physical composition (for example, density / compactibility) of contaminated soils / materials with Cr (VI) varies over a wide range, no individual mixing protocol will work in all situations, however, mixing methods are well known to those experienced in the environmental corrective operation technique and appropriate mixing protocols can be determined for the various soil / material compositions without undue experimentation for given soils / materials One of the spectacular benefits of the process of this invention is that no restrictions are placed on mixing methods by the process itself, unlike other processes In the art, the reduction of Cr (VI) by ascorbic acid is as effective as the process of this invention can be. er used in si tu for the treatment of soil / contaminated materials without the need to separate the floors / materials from their resting place. Based on the test results and knowledge of soil chemistry, it is expected that the resulting Cr (III) formed by the reduction of Cr (VI) will be thermodynamically stable in typical environmental soils and not oxidized back to Cr (VI over time under most environmental conditions where soils / materials bearing Cr (VI) require a corrective operation Since ascorbic acid is soluble in water, the process can be applied in itself soils / materials that are: 1) unsaturated and located above the water table; 2) constantly saturated below the upper part of the water table; or 3) unsaturated and saturated in the soil column at the same time. In the case of saturated soils, sufficient ascorbic acid must be added to reduce [Cr (VI)] in the soils and associated groundwater. The process of this invention also eliminates the need to excavate the soil for the ex-treatment, which would necessitate the replacement of the treated soil to the soil or deposit from which it originated or its disposal elsewhere. However, ex situ treatment is also possible with the method of this invention by mixing the soils / materials with ascorbic acid after excavation and returning the treated materials to the landscape from which they were separated. The present invention is also based on the recognition that, unlike the methods taught in the prior art, the chemical reduction of soils bearing Cr (VI) can be obtained without the need to: 1) leach or extract the Cr (VI) of the soils / materials and treat the Cr (VI) contained in the leachate; 2) modifying the pH of soils bearing Cr (VI) or associated pore water by treatment with acids or bases to provide appropriate conditions for the reducing agents to react or to provide appropriate conditions for the propagation of bacteria; or 3) mix the bacteria or material that contains bacteria, nutrients or organic material complementary to the soils.

One of the important observations of this invention is that minimal gas is generated when ascorbic acid is added to the alkaline soils bearing Cr (VI) discussed above. However, when a mineral acid is added to samples of the same materials, substantial amounts of gas are generated and noticeable increases in soil temperature are observed. This gas is most likely released by lowering the pH of the soils sufficiently to favor the release of the carbon dioxide gas with respect to the bicarbonate solution. If the prior art mineral acid processes were applied to such highly alkaline Cr (VI) soils / materials under field conditions, the generation of significant amounts of gas would likely limit the amount of reducing agent which can be mixed in. if you with the soils / materials and consequently limit the degree of reduction of Cr (VI) obtained. If the gas eventually dissipates, then it would be necessary to repeat the treatment in a subsequent time. The problem of excess gas is minimized by using ascorbic acid, thereby significantly decreasing the effort and time required for treatment compared to the use of mineral acid processes to reduce Cr (VI) to Cr (III) in soils / materials that carry highly alkaline Cr (VI).

The comparison of the results of ascorbic acid with the results of acetic acid and citric acid (Tables 2, 4, 6, 8 and 10) for each of the five tested samples clearly demonstrates that the reduction of [Cr (V)] It was much more effective when using ascorbic acid than comparable treatment doses of acetic acid or citric acid. Similarly, the collective treatment results show that ascorbic acid is more effective (a greater percentage of reduction) and reacts much more quickly to reduce [Cr (VI)] than all organic compounds studied by Deng and Stone.1 ^ shown by the data in Figures 1A-1E, for different soils / source materials, the same multiples of doses result in different reductions in [Cr (VI)], although in all cases the percentage reduction was very significant. Clearly, differences in soil com- positions, as described above, slightly affect their response to treatment. These data indicate the importance of carrying out preliminary tests, as taught by this invention, on representative soil / material samples of the site in order to determine the appropriate dosage to reach a desired degree of reduction of [Cr (VI) ] and reaction rate for each source soil. t I * REFERENCES CITED 1. Sax, N. I and Lewis, R.J., Mr. eds. 1993. Hawley's Condensed Chemical Dictionary, 13th edition, Van Nostrand Reinhold, New York, p.101. 5 2. Austin, G.T. 1984. Shreve 's Chemical Process Industries. 5a. edition, McGraw-Hill, New York. 3. estbrook, J.H. 1991. Chromium and Chromium Alloys. 10 In: Kirk-Othmer Encyclopedia of Chemical Technology, 4a. edition, Vol. 6, Kroschwitz, J.l. and Howe-Grant, M., Wiley-Interscience, New York. 4. The Hazardous Waste Consultant, 1993. EPA Updates 15 CERCLA Priority and Li st of Hazardous Substances, 11 (3), 2.26-2.30. McCoy and Associates, Inc., La ewood, CO.

. USEPA, 1995a. IRIS (Integrated Risk Information System). An updated electronic database of continuously by the Environmental Protection Agency of the United States of America. Bethesda, MD. 6. Bartlett. R.J. and James. B.R., 1988. Mobility and Bioavailability of Chromium in Soils. In: Chromium in 25 Natural and Human Environments. Nriagu, J.O. and Nieboer, E., Wiley-Interscience editions: New York, 267-304.

Y 7. Nriagu, J.O. and Nieboer, E., Edicioness. 1988. Chromium in the Natural and Human Environments. Wiley-Interscience, New York. 8. Page, B.J. and Loar, G.W., 1991. Chromium Compounds. In: Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, Vol. 6, Kroschwitz, J.l. and Howe-Grant, M., ed., Wiley-Interscience, New York. 9. James, B.R., 1994. Hexavalent Chromium Solubility and Reduction in Alkaline Soils Enriched with Chroirtite Ore Processing Residue; J Environ. Qual. 23, 227-233.

. James, B.R., 1996. The Challenge of Remediating 15 Chromium-Contaminated Soil. Environ Sci. Technol. 30 (6), 248A-251A. 11. Copson, R.L., 1956. Production of Chromium Chemicals. In: Chromium. I. Chemistry of Chromium and Its Compounds, Udy, M.J., ed., Reinhold Publishing Corp., New York. 12. Vítale, R.J., Mussoline, G.R., Petura, J.C., James, B.R. 1994. Hexavalent Chromium Extraction from Soils: Evaluation of an Alkaline Digestion Method, J Environ.

Qual. , 23, 1249-1256. 13. Environmental Science and Engineering (ES & E), 1989. Remedial Investigation for Chromium Sites in Hudson County, New Jersey. Prepared by the New Jersey Department of Environmental Protection. Environmental Science and Engineering, Inc. 14. Dragun, J. and A. Chiasson, 199 1. Elements in North American Soils. Hazardous Materials Control Resources Institute, Greenbelt, MD.

. James, B.R., Petura, J.C., Vitale, R.J. and Mussoline, G.R. 1995. Hexavalent Chromium Extraction from Soils: A Comparison of Five Methods, Environ. Sci. Technol. , 29: 2377-2381. 16. The Hazardous Waste Consultant, 1996. Remediation of Soils and Sediments Contaminated with Heavy Metals, 14 (6), 4.1-4.57 Elsevier Science, Inc., New York. 17. USEPA 1996. Soil Screening Guidance. Technical Background Document, EPA / 540 / R95 / 128, PB96-963502. U.S. EPA Office of Solid Waste and Emergency Response, Washington, DC. 18. Deng, B. and Stone, A. 1996. Surface-Catalyzed Chromium (VI) Reduction: Reactivity Comparisons of Different Organic Reductants and Different Oxide Surfaces, Environ. Sci. Technol. 30 (8), 2484-2494. 19. USEPA, 1995 Test methods for evaluating solid wastes, physical / chemical methods, SW846, 3a. update, 3rd. edition. Method 3060A - Alkaline Digestion for Hexavalent Chromi um, Washington, DC: U.S. EPA Office of Solid Waste and Emergency Response.

. USEPA, 1994. Test methods for evaluating solid wastes, physical / chemical methods, SW846, 2a. update, 3rd edition. Method 7196A - Hexavalent Chromi um (Colorimetric), U.S. EPA Office of Solid Waste and Emergency Response, Washington, DC.

It is noted that, in relation to this date, the best known by 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 (11)

1. Rei indications 1. A method for decreasing the concentration of hexavalent chromium in soils / materials contaminated with chromium to a desired level, characterized in that it comprises treating the soils / materials with a sufficient amount of ascorbic acid to chemically reduce the hexavalent chromium to a state of lower valence.
2. 2. A method for decreasing the concentration of hexavalent chromium in soils / materials contaminated with chromium to a desired level, characterized in that it comprises the following steps: a. determining the amount of ascorbic acid necessary to chemically reduce sufficient hexavalent chromium to a lower valence state; and b. mix the determined amount of ascorbic acid with contaminated soils / materials.
3. 3. The method according to claim 2, characterized in that the determination of the amount of ascorbic acid necessary to chemically reduce sufficient hexavalent chromium to a lower valence state further comprises the following steps: a. obtain representative samples of contaminated soils / materials; b. mix the samples with several doses of ascorbic acid; c. measuring the concentration of hexavalent chromium remaining in the samples as a function of time after mixing; and d. select the dose of ascorbic acid which reduces the concentration of hexavalent chromium to the desired level.
4. 4. The method of compliance with the claim 3, characterized in that the determined amount of ascorbic acid is mixed with the contaminated soils / materials in itself.
5. 5. The method of compliance with the claim 4, characterized in that the mixing in itself is carried out by means of a rotary hollow shaft auger-like device, which allows the injection of ascorbic acid during mixing.
6. 6. The method according to claim 2, characterized in that the determined amount of ascorbic acid is mixed with the contaminated soils / materials ex si tu.
7. 7. The method of compliance with the claim 2, characterized in that the determination of the amount of ascorbic acid necessary to chemically reduce sufficient hexavalent chromium to a lower valence state further comprises the following steps: a. obtain representative samples of contaminated soils / materials; b. mix the samples with assorted doses of ascorbic acid; c. measuring the concentration of hexavalent chromium remaining in the samples as a function of time after mixing; d. measure the redox potential of the samples as a function of time after mixing; and e. select the doses of ascorbic acid which reduces the concentration of hexavalent chromium to a desired level and maintains a decreased redox potential, which corresponds to the desired reduction of hexavalent chromium.
8. 8. The method of compliance with the claim 7, characterized in that the determined amount of ascorbic acid is mixed with the contaminated soils / materials in itself.
9. The method according to claim 8, characterized in that the mixing in itself is carried out by means of a rotary hollow shaft auger-like device which allows the injection of ascorbic acid during the mixing.
10. 10. The method according to claim 7, characterized in that the determined amount of ascorbic acid is mixed with the contaminated soils / materials ex si tu.
11. 11. A method for rapidly decreasing the concentration of hexavalent chromium in the soils / materials contaminated with chromium to a desired level, characterized in that it comprises treating the soils / materials with a sufficient amount of ascorbic acid to chemically reduce the hexavalent chromium to a state of lower valence in a few hours. SUMMARY OF THE INVENTION A process for reducing the concentration of potentially toxic hexavalent chromium, (Cr (VI).), In soils / materials carrying existing chromium in the form of soils is described., sludge, sediments, fillings, industrial waste or other materials, the concentration is reduced by applying and mixing a single reducing agent, ascorbic acid, to effect the chemical reduction of Cr (VI) to a less toxic valence state. The ascorbic acid is added at room temperature in the form of an aqueous solution or suspension and mixed with the soils / materials bearing Cr (VI) in amounts based on the test results of representative samples of the material to be treated. Ascorbic acid can also be added in an anhydrous form if sufficient moisture is present in the soils / materials to allow dissolution of ascorbic acid and reaction with Cr (VI) in the material. The percentage of reduction in the concentration of Cr (VI) is greater and is obtained more quickly than what is previously reported when using other organic chemical reducing agents. The process, which does not require the modification of the pH of the material bearing Cr (VI) can be applied in itself to the materials bearing Cr (VI), in which unsaturated and / or saturated soils are included in the same column of soil, using appropriate mixing equipment for the depth of soil or material to be treated, and can be applied to materials that are stored in a container or that have been excavated from the ground or other deposit.