MXPA97003787A - A process of aerobic / anaerobic decontamination of soil contaminated with ddt through aerobic repeated treatments / anaerobi - Google Patents

Abstract

The present invention provides a soil and / or sediment decontaminant process containing DDT type contaminants, converting them into innocuous materials and therefore decontaminating the soil as necessary, either partial decontamination or

Description

AN AEROBIC / ANAEROBIC DECONTAMINATION PROCESS OF DDT-CONTAMINATED SOIL THROUGH REPEATED AEROBIC / ANAEROBIC TREATMENTS Background of the Invention This invention relates to a controlled process for decontaminating soil or sediments containing DDT contaminants. There are numerous land plots contaminated with insecticide DDT (1,1,1-trichloro-2,2-bis (p-chlorophenyl) ethane). Various methods have been used to reduce soil contamination including incineration, low temperature thermal desorption and chemical treatments. All these methods are extremely expensive and may not be suitable for many contaminated properties. Previous art reports reveal laboratory experiments on DDT biodegradation in soil slurries. These reports reveal the process of mixing soil contaminated with DDT with anaerobic sludge from municipal waste treatment plants, a non-ionic surfactant and a reducing agent in an aqueous liquid system. In these experiments there was an important biodegradation of DDT, but the toxic metabolites of DDT remained.

SUMMARY OF THE INVENTION The present invention provides a process for decontaminating soil and / or sediments containing contaminants of the DDT type, converting these contaminants to harmless materials and thus decontaminating the soil whatever the degree of decontamination desired, or partial decontamination. or total repair. The process comprises treating solids and / or sediments containing populations of feasible aerobic and anaerobic microbes capable of transforming DDT-type contaminants into harmless materials that are feasible under both aerobic and anaerobic conditions. The treatment includes forming a degradation zone comprising said soil and / or sediment; maintain the degradation zone at a temperature in the range of between about 15 ° C and 37 ° C, and maintain a microbial population of at least 105 cfu / g in the degradation zone during the aerobic and anaerobic steps of the process; supersaturate the degradation zone with water and convert it into anaerobic; keep the zone of degradation supersaturated with water and at a potential level of reduction-oxidation below about 200 mV negative until a significant amount of DDT-type pollutants is degraded; subsequently oxygenate the degradation zone to convert it into aerobic; maintain the degradation zone at a potential reduction-oxidation level above about 100 mV positive until a significant amount of DDT-type contaminants is degraded; and repeat the steps of aerobic and anaerobic degradation until achieving the desired decontamination.

Description of the Invention The term "DDT type contaminants" means 1, 1, 1 -trichloro-2, -bis (p-chlorophenyl) ethane (DDT); 1,1-dichloro-2,2-bis (p.chlorophenyl) ethane (DDD); 2,2-bis (p-chlorofenyl) -1,1-dichloroethylene, (DDE); and products of metabolic transformation of DDT, DDD and DDE including 1-chloro-2,2-bis (p-chlorophenyl) ethylene (DDMU), 2,2-bis (p-chlorophenyl) ethylene (DDOH), dichlorodiphenylmethane (DPM) ), dichlorobenzophenone (DBP), dichlorobenzidrol (DBZ), and unsim-bis (p-chlorophenyl) ethylene dichlorofenacetate (DDA). Some DDT type contaminants are present in the soil before decontamination through the present process; some can be formed as transformation products during the present process. The term "harmless materials" are unobjectionable materials in the concentrations present in the soil or sediment for the use to which they are intended. The term "decontamination" means transforming DDT-type contaminants into harmless materials, including the biodegradation of such contaminants and binding said contaminants to soil or other materials. The term "restoration" means the decontamination at an unobjectionable level of DDT-type contaminants in the soil for the use to which said soil is intended. The term "soil" means land, that is, humus, sand and particulate rock, and includes sediment from below the surface of the water. "Supersaturated with water" when referring to the decontamination zone means that the soil in the decontamination zone has a water content greater than 100% of its water retention capacity (CRA); that is, the soil is submerged in water with stagnant water above the ground, and / or excess water greater than 100% of the CRA is removed from the soil, such as grout being drained from the bottom of the zone. of degradation. In the process of the present invention, during degradation the soil to be decontaminated should contain appropriate types of viable native microbes capable of degrading DDT-type contaminants. These microbes must be feasible both in the anaerobic and aerobic conditions to which they will be subjected during the present process. Microbes are usually bacteria, fungi, actinomycetes, and to a lesser extent, protozoa. Preferably, the microbes are native to the contaminated soil, that is, they are present in the soil to be decontaminated; or are recycled from or together with soil already subjected to the present process. In some cases it may be beneficial to add an inoculant containing such viable microbial degrading DDT. In the practice of the present invention, a degradation zone comprising the soil is formed. The degradation zone is a space that contains the soil to be decontaminated. The soil can be surface soil and underground soil, that is, soil contaminated in situ. or preferably it accumulates in a decontamination zone. Preferably the decontamination zone is circumscribed to a well to retain the water in which the soil is supersaturated. The decontamination zone may contain a piping system to aerate and oxygenate the soil during aerobic treatment and / or to supply oxygen-free gas during anaerobic treatment. Normally it is protected against rains. During the aerobic and anaerobic steps of the present process, the temperature of the degradation zone remains between about 15 ° C and 37 ° C. This is usually the temperature range that exists when biodegradation occurs at a rate sufficient to achieve decontamination in a reasonable period of time. To achieve degradation within a reasonable period of time, microbial populations of at least 105, and preferably at least 107, aerobic and anaerobic culture units per gram are maintained during the aerobic and anaerobic steps. according to the measurement made in accordance with the standard plate counting techniques (cfu / g), these microbial counts include, of course, microbes other than those that degrade DDT-type contaminants, it may be desirable to add nutrients and / or a Source of appropriate microbes During the process, the aerobic microbes in the soil mixture remain alive for the next step of aerobic degradation and the anaerobic microbes remain alive for any further anaerobic degradation step that is necessary. The present invention is essential that viable aerobic and anaerobic microbes are maintained. In the present process, it is generally preferred to add surfactants to the degradation zone. Ionic and non-ionic surfactants are preferred. Normally a mixture of ionic and non-ionic surfactants is used. Surfactants should be biodegradable, non-inhibitory to the microbial population, and have the ability to improve the biodegradation of DDT and DDT metabolites. Suitable surfactants include polysorbates, octoxins, anionic alkyl sulfates, aryl anionic alkyl sulfonates and ethoxylates. Examples of suitable surfactants include "Tween" nonionic surfactants commercially available from ICI Americas, Inc., "Triton" nonionic surfactants commercially available from Union Carbide, and the "DAWN" detergent surfactant mixture commercially available from Procter & Gamble. A suitable mixture of surfactants is "Triton" X-100 and "DAWN". In the present process, the degradation zone becomes anaerobic supersaturated with water to substantially eliminate oxygen. Sufficient water is placed in the decontamination zone which exceeds 100% of the CRA to create an oxygen barrier. The oxygen in the water is depleted by microbes. During the anaerobic step, a low reduction-oxidation potential is maintained in the degradation zone, below about 200 negative mV. It has been found that this level is optimal for the anaerobic degradation of DDT type contaminants in the present process. Without limiting to the following theory, at lower levels of reduction-oxidation potential, there is apparently too much oxygen present for rapid degradation of DDT. The level of reduction-oxidation potential can be maintained within this range by the addition of conventional nutrients and / or reducing agents such as sulphite and / or acetate compounds. The first anaerobic step and any subsequent anaerobic step continues until a significant amount of contaminant is degraded DDT. This can be determined by analysis. Typically, degradation of between about 20% and 50% of the initial content of DDT-type contaminants is desirable in the first anaerobic step. After the soil DDT contaminant content decreases to the desired level during the first anaerobic degradation step, the degradation zone is oxygenated to become aerobic. The aeration continues sufficiently to maintain the degradation zone at a potential reduction-oxidation level greater than about 100 mV positive during the aerobic degradation step. Oxygenation is carried out by any conventional technique.

Preferably this is done by having the water level low enough for the air to penetrate the soil. Desirably, the soil is cultivated and / or air is passed through the decontamination zone to maintain the desired potential reduction-oxidation level. Aerobic conditions activate the degradation of DDT-type contaminants, specifically DDT metabolites, resulting in harmless materials. The step of aerobic degradation continues until a sufficient amount of DDT-type contaminants is degraded. In the present process, the desired degree of biodegradation of DDT-type contaminants can not be achieved for an acceptable repair in a reasonable time in the first sequence of anaerobic / aerobic treatment steps of potential negative / positive reduction-oxidation levels. Therefore, the sequence must be repeated, usually more than once, as necessary to achieve the desired decontamination of the soil. The substantially complete decontamination of DDT-type contaminants is easily accomplished by the present decontamination process of various sequences of anaerobic / aerobic degradation steps. Without attempting to be limited by the following theory, it is believed that during anaerobic degradation the anaerobic microbes remove at least one or two aliphatic chlorines from the DDT type contaminants. The toxic metabolites, mainly DDD and to a lesser extent DDE, are the biodegradation products of the initial anaerobic passage of DDT. The subsequent aerobic degradation reduces them to less toxic metabolites, mainly DDMU and DDOH, DPM, DBP, DBH and DDA. Aerobic degradation then transforms these metabolites into less chlorinated compounds. As important amounts of DDT-type contaminants, specifically metabolites, may also be bound to soil and / or organic materials in the present process producing harmless materials, the term "degradation" as used herein includes not only biodegradation but also such binding. of contaminants. A desirable feature of this process is that the DDT-degrading microbes are kept alive during the anaerobic / aerobic treatment cycles, so that it is not essential to supplement the microbes before repeating the treatment cycle. However, it may be desirable to add repair material containing more nutrient materials or other conventional fermentation ingredients, mainly to supplement the organic food supply. The addition of between 0 and 5% (by weight of soil) of nutrient material is preferred, which is added either in the aerobic or anaerobic degradation step, to maintain sufficient nutrient material to support the metabolism of high anaerobic microbial populations and aerobic. As previously mentioned, maintaining the potential levels of adequate reduction-oxidation of the soil in the anaerobic and aerobic steps is necessary for the efficient practice of the present invention. Absolute anaerobic and aerobic conditions are needed (although localized short excursions can be expected). Thus, for the purpose of defining the present invention, a potential reduction-oxidation level of less than about 200 mV negative is considered anaerobic and is required for anaerobic steps.; and a potential level of reduction-oxidation greater than about 100 mV positive is considered aerobic and is required for aerobic steps. The potential level of reduction-oxidation between about 200 mV negative and 100 mV positive is considered anoxic. Thus, "converting" the soil or degradation zone into anaerobic or aerobic means that the potential level of reduction-oxidation of the soil or degradation zone is less than about 200 mV negative (anaerobic) or greater than about 100 mV positive (aerobic). The soil to be treated can be analyzed and treated in the laboratory by the present process to determine the optimal treatment conditions, the additives and the times of the aerobic and anaerobic steps and the number of sequences thereof. In a preferred technique when practicing the present invention called immersion technique, the upper surface of the soil in the decontamination zone is more or less horizontal. This technique is especially useful in in situ treatment where contaminated soil is not excavated and transported to a decontamination cell. Desirably, the soil will be cultivated to facilitate the uniformity and speed of the treatment. In this immersion technique, which can also be practical in a closed cell (or even in a laboratory test tube), the soil is supersaturated with water for the anaerobic treatment step by submerging the soil in water to form an oxygen barrier . Depending on the depth and area of the decontamination zone and its exposure to the elements, a depth of immersion of between a few inches to a foot or more is used. Microbial activity consumes available oxygen by converting the decontamination zone into a strongly anaerobic zone, at a level of reduction-oxidation potential below 200 mV negative, where it is maintained until a significant amount of DDT-type contaminants is degraded. The addition of nutrients and / or microbes may be desirable to maintain the strong anaerobic conditions and the anaerobic microbial population of 105 cfu / g desired. Desirably, a surfactant mixture of the type described above is also added. In this immersion technique, water is then removed from the treatment zone to less than 100% of the CRA, but preferably greater than 50% of the CRA, and the decontamination zone is oxygenated. Oxygenation is normally achieved by cultivation and / or passage of air through the soil in the decontamination zone by any suitable means. Oxygenation makes the decontamination zone highly aerobic, activating aerobic microbes. A level of oxidation-reduction potential greater than about 100 mV positive is maintained. The addition of nutrition material and / or microbes to maintain the strong aerobic conditions and the desired population of aerobic microbes of 105 cfu / g may be desirable. This step of aerobic degradation continues until a significant amount of DDT-type contaminants is degraded. Generally at each aerobic step an important percentage, which often reaches substantially the complete degradation, of available DDT metabolites is achieved. The decontamination zone is then again supersaturated in water by immersion, giving the soil another step of anaerobic treatment. This in turn is followed by another step of aerobic treatment. The sequence of anaerobic / aerobic treatment steps is repeated as many times as necessary to achieve the desired level of decontamination. In most cases the restoration is easily achieved.

In another preferred technique, called the biopile technique ("biopile"), the decontamination zone is not immersed in water. The soil accumulates in the decontamination zone, generally a treatment cell containing water, with aeration medium and grout and recirculation collection. The soil in the decontamination zone is supersaturated with water during the anaerobic and aerobic degradation steps. Supersaturation is achieved by continuously supplying water at the top of the pile, such as by spraying or dripping water from one or more nozzles, so as to moisten substantially all the soil of the decontamination zone beyond the 100% of the CRA. Excess water seeps out from the bottom of the pile, and is preferably recycled and fed back into the top of the pile. In the decontamination zone there may be several batteries of this type. During the anaerobic step normally no air or gas is added or extracted through the system. The microbial activity converts the decontamination zone into anaerobic, and it remains anaerobic at a level of reduction-oxidation potential lower than around 200 mV negative maintaining an anaerobic microbial population of at least 105 cfu through the anaerobic passages. During the anaerobic degradation step, nutrients, additional microbes and mixtures of surfactants can be added. After a significant amount of DDT-type contaminants is degraded in an anaerobic degradation step, the biopile decontamination zone comprising the soil being treated is oxygenated by feeding gaseous oxygen, such as preferably air, through the soil to convert it. in aerobic and activate aerobic microbes. Through the degradation steps, water (preferably recycled leachate) is fed to the upper part of the biopile to keep the degradation zone oversaturated; gaseous oxygen is supplied to the soil; and the leachate is drained from the decontamination zone. Nutrients, supplementary microbes and surfactants may be added as necessary to achieve sufficient microbial action to maintain the aerobic microbial population of the decontamination zone of at least 105 cfu / g, temperature and rapid degradation of contaminants. After the desired significant amount of DDT-type contaminants is degraded in the initial aerobic step, the biopile decontamination zone is deoxygenated and anaerobically treated again, followed by oxygenation and aerobic decontamination treatment. The sequence of anaerobic / aerobic steps is repeated as necessary to achieve the desired decontamination. In most cases the complete restoration is easily achieved. Although the preferred technique of biopiles is not easily adapted for the treatment of soil in situ. and therefore generally requires the excavation and transport of decontaminated soil, the biopile technique can be better controlled and generally decontaminates the soil more quickly. This technique is especially useful for treating highly contaminated soil. As described above, the present process includes a plurality of sequences of an anaerobic degradation step followed by an aerobic degradation step. These sequences are necessary to properly degrade DDT and DDT metabolites in a reasonable period of time. However, it may be desirable that the soil is initially treated aerobically to decrease the content of preexisting DDT metabolites prior to the initial anaerobic stage. The following Examples illustrate the practice of the present invention: EXAMPLE 1 This example compares the method of treating the anaerobic / aerobic repeated sequence of this invention with simple anaerobic treatment and simple aerobic treatment in the degradation of DDT in a soil system. The soil in this example contains populations of viable aerobic and anaerobic microbes from >; 105 cfu / g capable of converting DDT contaminants into harmless ones, which is feasible under anaerobic and aerobic conditions. This soil contains DDT (200 ppm), DDD (22 ppm) and DDE (18 ppm). Each of the six soil samples is mixed with 25 ml of water containing 500 ppm of sulfite and cysteine reducing agents and 500ppm of "Triton" X-100. To these slurries is added 1 ppm of 4,4'-DDT-UL C14 ring in hexane. Two samples of 20 g are anaerobically incubated (in polypropylene tubes with a capacity of 50 ml at 35 ° C, stationary); two samples of 20 g are incubated aerobically (in rotary vats with foam plugs at 35 ° C, 150 rpm); and two samples of 20 g are incubated in a cycle of 2 anaerobic weeks / 2 aerobic weeks for a total of 8 weeks. The reduction-oxidation potential is mopitorea. Then the reduction-oxidation of the aerobic phase is greater than +100 mV and then the reduction-oxidation potential of the anaerobic incubation is lower than -200 mV. The incubation samples are extracted, and the fate of radiolabeled DDT is determined by thin layer chromatography and autoradiography / densitometry. The experiments are repeated to obtain average results. These average results are shown in Table 1.

Table 1 Destination of Radiolabelled DDT in the Soil System (% remaining after testing) Example 2 This example shows the anaerobic / aerobic sequence method used to decrease the contamination of DDT in the soil within 8 weeks, and the utility of different surfactants to aid the process. The soil to be repaired contains populations of > 105 cfu / g of feasible aerobic and anaerobic microbes capable of converting DDT-type contaminants into harmless and that can live under anaerobic and aerobic conditions. This soil contains DDT (220 ppm), DDD (22 ppm) and DDE (18 ppm).

Six samples of 20 g of soil are added to polypropylene tubes of 50 ml capacity with 25 ml of water containing 500 ppm each of cysteine and sulfite reducing agents. This creates a zone of supersaturated anaerobic degradation immersed in water. To each slurry sample is added 500 ppm of different surfactants and 1 ppm of radiolabelled 4,4'-DDT UL C14 ring in hexane. The tubes are capped and incubated constantly for 2 weeks at 35 ° C. After 2 weeks the tubes are opened and each of the contents is transferred to a vat with a foam lid, aerated by rotary incubation (150 rpm), for another 2 weeks at 35 ° C. The anaerobic / aerobic cycle is repeated 2 times. The samples are then extracted and the fate of the radiolabeled DDT is determined by thin layer chromatography and autoradiography / densitometry. The experiments are repeated to obtain average results. The average results are shown below.

IfiJa Destination of DDT Radiolabelled in the Soil System (% remaining after testing) Example 3 This example shows the effect of the anaerobic / aerobic cycle method of the present invention in a soil contaminated with DDT in a pilot-scale experiment. In a 8-foot-by-8-by-18-inch degradation zone box, three-quarters of a ton of soil containing DDT (728 ppm), DDD (87 ppm) and DDE (50 ppm) are placed. Alternatively it is submerged in water and drained cycles of a month. Sodium acetate nutrient (500 ppm) is added as required to maintain COD above 300 mg / L, and a mixture of "Triton" X-100 and "DAWN" non-ionic surfactants is added as required to maintain the surface tension of the slurry below 70 dynes / cm. The material is manually cultivated each week to aerate the degradation zone and convert it to aerobic during the drained phase. During the treatment, the temperature ranges between 22 ° C and 28 ° C. The potential level of reduction-oxidation in the treatment zone remains below 200 mV negative while submerged in water and above 100 mV positive when drained. While draining the water content of the soil ranges between 40 and 100% CRA. After 16 weeks, 72% of DDT, 31% of DDD and 54% of DDE are degraded, according to the measurement by solvent extraction and GC ECD analysis.

Example 4 This example shows the anaerobic / aerobic repeated sequence method of the present invention, which uses the biopile technique. Eight tons of soil containing DDT (728 ppm), DDD (87 ppm) and DDE (50 ppm) are placed in a closed box of degradation zone of 8 feet by 8 feet by 8 feet. The soil is repaired with 5% straw nutrient. Water is dripped continuously on the surface of the soil to supersaturate it, and the leachate is collected at the bottom and recycled at a rate of between 20 and 10.0 L? Onelada / day. The oxygenation is achieved by passing air through a mechanical distribution system in the soil at a rate of 44 L / tons / day, cutting the air weekly alternatively. The temperatures of the floor during the treatment oscillate between 22 ° C and 28 ° C. The potential oxidation-reduction level in the treatment zone remains below 200 mV negative when there is no aeration and greater than 100 m V when there is aeration. Sodium acetate is added as required to increase COD above 300 mg / L, and maintain population counts above 105 cfu / g of anaerobic and aerobic bacteria; and surfactants (a mixture of "Triton" X-100 and "DAWN") are added as required to maintain the surface tension below 70 dynes / cm measured in the leachate, which is recycled. After 16 weeks of cycles, 53% of DDT, 55% of DDD and 54% of DDE are degraded.

Claims (12)

1. CLAIMS Having described and illustrated the nature and main purpose of the present invention, as well as the manner in which it can be put into practice, it is claimed to claim as property and exclusive rights: 1 A PROCESS OF AEROBIC / ANAEROBIC DECONTAMINATION OF THE SOIL CONTAMINATED WITH DDT THROUGH AEROBIC / ANAEROBIC REPEATED TREATMENTS which includes a population of anaerobic and aerobic feasible microbes capable of transforming the DDT type pollutants in innocuous materials being able to live in anaerobic and aerobic conditions, characterized in that it comprises: a) forming an area of degradation comprising said soil; b) maintaining said degradation zone at a temperature in the range of about 15 ° C and 37 ° C, and maintaining in said degradation zone microbial counts of at least 105 cfu / g of anaerobic and aerobic bacteria; c) supersaturate the degradation zone with water; d) converting said degradation zone into anaerobic; e) maintaining said degradation zone supersaturated with water at a potential reduction-oxidation level of less than 200 mV negative until a significant amount of DDT-type pollutants is degraded; f) to oxygenate then said zone -degradation to convert it into aerobic; g) maintaining said degradation zone at a potential reduction-oxidation level of greater than about 100 mV positive until a significant amount of DDT-like contaminants degrades; and h) repeating steps c) to g).
2. 2. The process of Claim 1 characterized in that nutrient material is added to the degradation zone in an amount ranging from 0% to 5% by weight of soil.
3. 3. The process of Claim 1 characterized in that the microbes contained in the soil to be decontaminated comprise anaerobic and aerobic indigenous bacteria.
4. 4. The process of Claim 1 characterized by adding surfactant to said degradation zone.
5. 5. The process of Claim 1 characterized in that the water content of the degradation zone while aerobic remains above 50% of CRA.
6. 6. The process of Claim 1 characterized in that said degradation zone is kept supersaturated with water while aerobic. The process of Claim 1 characterized in that the water content of the degradation zone while aerobic is maintained below 100% CRA. The process of Claim 1 characterized in that said steps c) to g) are repeated a sufficient number of times to achieve repair. 9. The process of Claim 1 characterized in that said steps c) to g) are repeated once. The process of Claim 1 characterized in that said steps c) to g) are repeated more than once. The process of Claim 1 characterized in that the degradation zone is submerged in water to make it anaerobic and kept submerged in water while it is anaerobic; maintaining a water content of less than 100% CRA while aerobic. The process of Claim 1 characterized in that water is continuously fed over the top of the degradation zone and the leachate is removed from the bottom of said degradation zone while it is anaerobic and aerobic.