EP1079944A1

EP1079944A1 - Decontamination of soil contaminated with hcb

Info

Publication number: EP1079944A1
Application number: EP99920501A
Authority: EP
Inventors: Guy P. Moser; Neil C. C. Gray; David J. Gannon
Original assignee: Stauffer Management Co
Current assignee: Stauffer Management Co
Priority date: 1998-05-18
Filing date: 1999-05-14
Publication date: 2001-03-07
Also published as: TW446585B; AU3806899A; WO1999059743A1

Abstract

A microbe degradation process for decontaminating soil contaminated with HCB.

Description

DECONTAMINATION OF SOIL CONTAMINATED WITH HCB

This invention relates to a controlled microorganism degradation process for decontaminating soil or sediments contaminated with hexachlorobenzene (HCB). Hexachlorobenzene is produced as a reaction side-product in the production of chlorinated solvents, mainly carbon tetrachloride, perchlorethylene, trichlorethylene and dichlorethylene. HCB is also used in the manufacture of seed treatment agents and a variety of industrial applications. Numerous land sites exist that are contaminated, and have been contaminated for decades, with the highly toxic HCB. Many of these sites are unusable and will remain unusable until some economical process becomes available for large scale decontamination of the soil of such sites.

Zeneca Corp. U.S. Patents Nos. 5,660,612 and 5,660,613 disclose the remediation of soil contaminated with DDT by anaerobic composting followed by aerobic composting under specific conditions of water content, temperature, redox potential and the presence of unacclimated soil-indigenous anaerobic and aerobic microbes capable of transforming DDT into harmless materials.

The above method has been tried to decontaminate soil containing various contaminants other than DDT. While this method has been found promising with a limited number of specific contaminants, it was unsuccessful for many contaminants including HCB. There appears to be no reliable way to predict which compounds will be effectively decomposed by this method, and particularly no reason to expect that it would be successful, as with DDT, in decontaminating soil containing the chemically different HBC. Several researchers have attempted to degrade HBC by anaerobic composting in aqueous slurries under various minor modifications of anaerobic composting. See P. L. Van Hoof and C. T. Jaf ert, 1996, Environmental Toxicology and Chemistry, 15(11), 1914-24; J. Nowak, W. Hegeman and H. Stan, 1996, 45,700-709; S Masunaga, S. Susarla and Y. Yonezawa, 1996, Wat. Sci. Tech., 33(6), 173-180; and P. Rosenbrock, R. Martens, F. Buscot and J C. Munch, 1997, Appl. Microbial. Biotechnol, 48, 115-120. However none of the techniques of these researchers have been adequately successful to meet any known commercial standard of acceptance. For large scale soil treatment, the slurry treatments of these researchers are not well suited. In general all attempts at anaerobic composting of HCB have failed to completely dechlorinate and cleave the aromatic ring, always leaving mono- or multi-chlorobenzene toxic metabolites. There has been only partial removal of the chlorine atoms, and monochlorobenzene apparently has never been shown to be degraded anaerobically SUMMARY OF THE INVENTION

The present invention provides a process of decontaminating soil and/or sediments containing HCB contaminant by converting the contaminant into harmless materials. This process is capable of decontaminating the soil to whatever extent desired, either partial decontamination or complete remediation. More particularly, the invention provides a process of decontaminating soil contaminated with HCB comprising:

(a) preparing an inoculant by making a mixture comprising a minor amount of the contaminant that is contaminating the soil and a highly nutritious heterotrophic medium, and storing said mixture at about 20 °C. to 30 °C. for sufficient time for acclimation of microbes to occur and to produce viable anaerobic and aerobic microbes capable of transforming the contaminant in the soil to harmless materials, populations of the viable anaerobic and aerobic microbes being viable through both the aerobic and anaerobic conditions of the decontamination process; (b) admixing said soil with amendment material and inoculant to form a solid compost mixture comprising organic nutrient materials and from 1 to 20% of said inoculant,

(c) initially composting said soil while maintaining the temperature of said compost mixture in the range of about 20-65 °C and the water content of the compost mixture in the range of about 40-100% water holding capacity; (d) during said initial composting maintaining tile redox potential level of tile compost mixture below about negative 200 mV until a significant amount of said contaminant is degraded; and

(e) thereafter oxygenating said compost mixture to raise its redox potential level to above about positive 100 mV, and maintaining the redox potential level above about positive 100 mV until a significant amount of the contaminant is degraded.

The process comprises preparing an inoculant that contains populations of viable anaerobic and aerobic microbes that are capable of degrading the contaminants into harmless materials, and are viable under both the anaerobic and aerobic conditions throughout the process. The soil is then treated with these microbes by solid state composting as hereinafter described.

The inoculant is prepared by mixing a small amount of contaminant, preferably from 10 to 100 by wt. of the final inoculant mixture, with a conventional highly nutritious aqueous formulation normally referred to as a heterotrophic medium. Desirably, some soil is added to this mixture, which remains liquid. The liquid mixture is then stored at a temperature from about 20°C. to 30°C. for about 6 to 10 weeks. During this time, acclimated indigenous anaerobic and aerobic microbes form that are capable of transforming the specific contaminants into harmless materials and are viable under both the anaerobic and aerobic conditions throughout the process.

The decontamination treatment comprises mixing the HCB contaminated soil with amendment material to form a solid compost mixture containing organic nutrient materials; adding to the compost mixture 1 to 20% v/v of the inoculant to the compost mixture in the range of about 20 to 65 °C. and the water holding content of the compost mixture in the range of about 40% to 100% water holding capacity (WHC); during this composting maintaining the redox potential of the compost mixture below about negative 200 mV until a significant amount of contaminant is degraded, mostly to toxic HCB metabolites; thereafter raising the redox potential of the compost mixture above about positive 100 mV until a further significant amount of contaminant is degraded. This sequence of stages of negative/positive redox potential can be repeated as often as necessary to yield soil or sediment containing little contaminant. DEFINITIONS The term "composting" as used herein means transforming by degradation contaminants in the soil and/or sediment to harmless materials utilizing biological activity, the process being carried out in the solid state in the presence of organic nutrient material.

"Amendment", "amendment material" and "soil amendment material" mean material comprising organic nutrient for the microbes that is added to the soil to be decontaminated.

"Contaminants" means HCB and toxic metabolites from the degradation of HBC.

"Degrade" when used with HCB and its metabolites, means the removal of one or more chlorine atoms and/or cleaving the benzene ring.

"Harmless materials" means materials that are unobjectionable in the concentrations present in soil or sediment for its intended use.

"Toxic metabolites of HCB" means degradation products of HCB from which one or more chlorine atoms have been removed from the HCB molecule, but the HCB has not been completely degraded to harmless materials.

"Decontamination" means ttansforming the contaminants to harmless materials, including biodegrading the contaminants and/or binding the contaminants to soil or other material.

"Remediation" means decontamination to an unobjectionable level of the contaminants in the soil/sediment for the intended use of the soil/sediment.

"Soil" means earth, i.e. humus, sand and paniculate rock, and includes sediment from above and beneath the surface of water. DETAILED DESCRIPTION OF THE INVENTION

In the degradation steps of the process of the present invention, the soil to be decontaminated must contain throughout the process appropriate types of viable microbes capable of degrading the contaminants. Both anaerobic and aerobic microbes must be present. These microbes must be viable under both the anaerobic and aerobic conditions to which they will be subjected during the present process. Furthermore, when the compost mixture goes through the anaerobic and aerobic conditions of tile process, the microbial populations must not be killed or adversely affected so that they will not adequately degrade HCB. The microbes normally are bacteria, fungi, actinomycetes, and to a lesser extent protozoa. As aforementioned, the first step in the present invention is to prepare an appropriate inoculant from microbes, normally microbes initially present in the soil to be treated, that have been adequately acclimated to the contaminant that is to be subject to decontamination. Additional active microbes can be recycled from decontaminated soil.

In the practice of the present invention a solid compost mixture is prepared by mixing appropriate soil amendment into the soil to be decontaminated to give a total amount of amendment material of at least 10%, and up to about 95% by weight of the mixture, and preferably from about 30% to 70% by weight of amendment material, some of which may already be present in the soil. The soil amendment material comprises a conventional source of organic nutrients for the microbes during composting. The preferred amendment nutrient materials are agricultural waste and municipal waste sludge, preferably a manure such as horse, cow, sheep, turkey, chicken or fish manure, or activated sludge. Alfalfa, hay, sawdust, peat, grass and other bulking materials may be present in the compost mixture, originate in manure or be specifically added. The inoculant is added to the compost mixture in sufficient quantities to significantly influence the microbial population. At least 1 % is needed, and 1 to 20% inoculant v/v added to the amendment is preferred. In some cases it may be desirable to include in the soil amendment a surfactant to render the contaminants more available to biological degradation. Suitable surfactants include polysorbates, octoxynols, anionic alkyl sulfates, anionic alkyl aryl sulfates 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 DAWN™ detergent commercially available from Procter & Gamble. A suitable mixture of surfactants is TRITON^® and DAWN™. The amendment material may also contain, or be supplemented with, liquid or solid organic or inorganic nutrients. Organic materials high in nitrates and phosphates are normally used.

The compost mixture is maintained in a moist but solid state. Throughout the process the moisture level is maintained at less than 100% of the mixture water holding capacity (WHC), preferably in the range of about 40 to 70% WHC.

After mixing, biological degradation of organic matter in the mixture starts, raising the temperature and depleting the oxygen to an anaerobic condition. The temperature of the mixture is thereafter maintained within the range of about 20-65 °C. This is easily done by controlled air movement through the compost mixture (e.g. through pipes) and/or by the addition of nutrient material. Below about 20 °C, the biodegradation proceeds uneconomically slowly; above about 65 °C excessive microbe kill may take place. The preferred temperature range is within about 20-50 °C. The anaerobic microbes in the compost mixture remain viable for the subsequent aerobic degradation steps and the anaerobic microbes remain viable for any needed subsequent anaerobic degradation steps. Thus, it is essential that viable aerobic and anaerobic degradation microbes be maintained during the process of the present invention.

During the anaerobic step a low redox potential level is maintained in the compost mixture, at least about negative 200 mV, and preferably in the range of negative 300 mV to 500 mV. This level has been found to be optimum for the anaerobic process. The redox potential level can be maintained within this range by moist air movement through the compost and/or by the addition of conventional reducing agents such as sulfite and acetate reducing compounds. The first anaerobic step and any subsequent anaerobic steps are continued until a significant amount of the contaminant is degraded in the step. This can be determined by analysis. Typically, in the first anaerobic step degradation of about 30% to 70% of tile initial content of the HCB contaminant is desirable. After the contaminants content of the soil/manure mixture is decreased significantly in the first anaerobic stage, the mixture is oxygenated by any suitable means, preferably by air fed through and/or mixed with the mixture to achieve aerobic conditions. Sufficient oxygenation is applied for the redox potential level during the aerobic treatment to be maintained above about positive 100 mV. The aerobic conditions activate further degradation, yielding harmless materials. The aerobic degradation step is continued until a significant amount of the contaminants is degraded by the aerobic treatment.

In most cases, the desired degree of biodegradation of the contaminants for acceptable remediation will not be achieved in the first anaerobic/aerobic treatment sequence. In the highly preferred process of anaerobic/aerobic composting, the sequence is repeated one or more times as needed for acceptable soil decontamination. Substantially complete remediation is achievable by this multi-sequence preferred process.

A desirable feature of this process is that the degrading microbes are maintained viable throughout the anaerobic/aerobic treatment cycles, so that it is not essential that microbes be supplemented before repeating the treatment cycle. However, it may be desirable to add more amendment materials, manure, or other conventional fermentation ingredients, primarily to supplement the organic feed supply and to also introduce more bulking agent.

As aforementioned, maintaining the proper redox potential levels of the compost mixture in the anaerobic and aerobic steps is necessary for effective practice of the present process. The appropriate redox potential levels can be maintained by the addition of conventional nutrient materials and/or reducing agents such as sulfite and/or acetate compounds. Absolute anaerobic and aerobic conditions are needed (although short localized excursions can be expected). For the purpose of the present invention, a redox potential level of less than about negative 200 mV is considered anaerobic, and is required for the anaerobic steps; and a redox potential level greater than about positive 100 mV is considered aerobic and is required for the aerobic steps. During the anaerobic steps, the preferred redox potential level is in the range of about negative 300 to 500 mV; and during the aerobic steps it is in the range of about positive 200 to 300 mV. The redox potential level from about negative 200 mV to about positive 100 mV is considered anoxic. In the present invention, when going from anaerobic to aerobic conditions, and visa versa, anoxic conditions are present in the compost. During this period, degradation of some amounts of the contaminants appears to take place, but at low rates. Thus, speedy transition from one state to the other expedites overall degradation.

In the practice of the present invention, it may be desirable to have anaerobic conditions in some parts of the compost mixture and at the same time aerobic conditions in other parts. This may be desirable because of different initial contaminant levels and/or degradation rates in different parts of the compost mixture. Thus, to approach uniform degradation, it may be necessary for some parts of the compost to remain longer in an anaerobic or an aerobic state.

During composting high microbe counts are present, preferably up to 10⁸ aerobic colony forming units (cfu) per gram, as measured by standard plate count techniques, and up to 10⁶ anaerobic cfu/g. These microbe counts of course include microbes other than those that degrade the contaminants.

In practice the present process is conducted in a compost container, normally in a container cell or windrow. The soil to be treated can be analyzed and composted in the laboratory to determine optimum conditions of amendment composition, and anaerobic/aerobic treatment times and number of sequences.

As described above, the present process involves an anaerobic composting stage followed by an anaerobic stage. However, it may be desirable to initially treat the soil aerobically to lower the content of pre-existing undesirable materials prior to the first anaerobic stage.

The following Examples are illustrative of the preferred practice of the present invention. Example I An inoculant is prepared for use in decontamination of soil containing HCB by mixing together 25 mL of dilute heterotrophic* medium; 625 micro liter of a 2 mg/mL methanol solution of HCB and 50% of an uncontaminated soil. The inoculant is then stored under anaerobic conditions for 6 weeks at 22 degrees C inside an anaerobic chamber. 1 kg (60ml) of soil containing HCB at approximately 25 ppm is then mixed with a compost mixture containing 4000 ml of cattle manure and 50 ml straw and the above described 50 ml of inoculant. Duplicate samples of the mixture are composted anaerobically in a chamber for 6 weeks. The water holding capacity, redox potential and temperature of the anaerobic compost mixture during this time averages about 70% , below 250 mV, and 22 degrees C, respectively. Significant degradation of the HCB takes place.

Thereafter, the compost mixture redox potential is raised by oxygenation to aerobic conditions of the compost mixture. This aerobic phase is continued for 2 weeks. During this time the WHC, redox potential, and temperature averages respectively about 70%, above 200 mV and 34 degrees C. Significant further degradation of the contaminants occurs during this aerobic phase. These 8 week stages of sequential anaerobic/aerobic treatment are repeated one time, bringing about substantially more contaminant degradation. The extent of degradation obtained in this experiment indicates that, in large scale decontamination following this technique, remediation adequate for most desired uses will readily be obtained.

*Dilute heterotrophic media consists of:

Glucose - 0.1 g/L CaCl₂.2H₂0 - 0.07 g/L

Yeast extract - 0.1 g/L MOPS 0.1 g/L

Peptone - 0.05 g/L Vitamins (BME 100X) - l.O mL/L

Tritone - 0.05 g/L Ho-Le's Minerals l.O mL/L

MgS0₄.7H₂0 - 0.6 g/L 2M phosphate sol' n - 1.0 mL/LpH7.53

Examples 2-8

The procedure of Example 1 is followed except that the amendment (compost mixture) ingredients, the composting times and the compost conditions are as indicated in the following Table. In each example the inoculant is prepared by the following procedure of Example 1.

Table I

Ex. No. Amendment (v/v) Anaerobic/aerobic soil content phase times sequences

2 horse manure-20% 6/4 weeks 1

3 chicken manure-35% 4/2 weeks 3

4 municipal sludge-80% 6/3 weeks 1

5 cow manure-60% 8/3 weeks 2

6 30% sludge, 20% straw 3/3 weeks 1

7 fish manure, 80% 4/5 weeks 2

8 horse manure, 40% 4/4 weeks 2

The invention may be varied in any number of ways as would be apparent to a person skilled in the art and all obvious equivalents and the like are meant to fall within the scope of this description and claims. The description is meant to serve as a guide to interpret the claims and not to limit them unnecessarily.

Claims

WHAT IS CLAIMED:

1. A process of decontaminating soil contaminated with HCB comprising:

(a) preparing an inoculant by making a mixture comprising a minor amount of the contaminant that is contaminating the soil and a highly nutritious heterotrophic medium, and storing said mixture at about 20┬░C. to 30┬░C. for sufficient time for acclimation of microbes to occur and to produce viable anaerobic and aerobic microbes capable of transforming the contaminant in the soil to harmless materials, populations of the viable anaerobic and aerobic microbes being viable through both the aerobic and anaerobic conditions of the decontamination process;

(b) admixing said soil with amendment material and inoculant to form a solid compost mixture comprising organic nutrient materials and from 1 to 20% of said inoculant,

(c) initially composting said soil while maintaining the temperature of said compost mixture in the range of about 20-65 ┬░C and the water content of the compost mixture in the range of about 40-100% water holding capacity;

(d) during said initial composting maintaining tile redox potential level of tile compost mixture below about negative 200 mV until a significant amount of said contaminant is degraded; and

2. The process of Claim 1 wherein the first anaerobic step is continued until 30 - 70% of the contaminant is partially dechlorinated.

3. The process of Claim 1 wherein the sequence of steps (b) through (e) are repeated.

4. The process of Claim 1 wherein said compost mixture initially contains at least about 10% by weight of amendment material.

5. The process of Claim 1 wherein some organic nutrient material is present in the contaminated soil.

6. The process of Claim 1 wherein said amendment material comprises one or more of agricultural waste and municipal waste.

7. The process of Claim 1 wherein surfactant is added to said compost mixture.

8. The process of Claim 1 wherein said temperature is maintained in the range of about 30 - 55 ┬░C.

9. The process of Claim 1 wherein said compost mixture initially contains in the range of 30 - 70% by weight of said amendment material.

10. The process of Claim 1 wherein when said redox potential level is below negative 200 mV it is maintained in the range of about negative 300 to 500 mV, and when said redox potential level is above about positive 100 mV it is maintained in the range of about positive 200 to 300 mV.