Hydraulic and earth engineering – Soil remediation – In situ contaminant removal or stabilization
Reexamination Certificate
1999-05-28
2001-09-04
Langel, Wayne (Department: 1754)
Hydraulic and earth engineering
Soil remediation
In situ contaminant removal or stabilization
C588S249000
Reexamination Certificate
active
06283675
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of remediating soil contaminated with hazardous substances. More particularly, the invention relates to a method of reducing the amount of carbon disulfide (CS
2
) in the soil through chemical oxidation.
In recent years, an increased awareness of preserving the environment has caused the Environmental Protection Agency (EPA) to take measures to counter the release or threat of release of hazardous substances from soil. The EPA investigated certain soils and determined that in some areas, there were hazardous concentrations of certain chemicals including CS
2
. The EPA has actually issued orders to remove CS
2
contaminated soil.
CS
2
can be converted to sulfate, a nonhazardous substance, by a chemical process known as alkaline oxidation. Alkaline oxidation of CS
2
can be achieved with the use of a reagent containing a peroxide radical. However, oxidation of CS
2
is a highly exothermic reaction which can release undesirable gaseous by-products such as H
2
S. Furthermore, the addition of oxidant to CS
2
can produce potentially explosive and flammable consequences.
Prior methods of remediating CS
2
-contaminated soil included excavation of the contaminated soil. However, CS
2
is highly volatile and can ignite or explode when exposed to air. Excavation of soil to remove CS
2
creates the hazards associated with exposing CS
2
to air. A process to remediate CS
2
-contaminated soil without exposing CS
2
to air is therefore desirable.
To date, there have been a number of methods developed to remediate contaminated soil. Examples include excavation and soil washing, electromigration, volatilization, incineration and land farming.
In addition, U.S. Pat. No. 5,868,523 describes an injection system to introduce biologicals, nutrients, water and air for bioremediation of contaminated soil and groundwater. The penetration can be both vertical and horizontal and does not disturb the soil in place. However, the '523 reference does not address rernediation of CS
2
-contaminated soil or the unique difficulties and hazards associated with handling CS
2
.
U.S. Pat. No. 5,755,285 describes concentrating contaminants in one area of the soil and excavating the area. The technology disclosed in the '285 reference involves the use of bore holes made in the soil and injection of a medium to cause harmful substances to migrate and concentrate to a smaller soil area. Excavation work is thus reduced to a great extent. The '285 reference does not address remediation of CS
2
-contaminated soil by treatment of the soil, nor does it address the difficulties and hazards encountered while treating CS
2
.
U.S. Pat. No. 5,536,898 describes immobilizing organic chemical waste in soil by absorption with biogenetic amorphous silica. The '898 reference discloses the mixing of untreated particulate rubber and biogenetic amorphous silica with soils contaminated with organic compounds in order to immobilize such compounds (i.e. they are no longer reactive) sufficiently to pass EPA requirements. The '898 patent does not address remediation of CS
2
-contaminated soil by treatment of the soil, nor does it address the unique difficulties and hazards encountered while treating CS
2
.
Other disclosures related to degrading contaminants within the soil include U.S. Pat. No. 5,840,191, which describes microbial degradation of soil contaminants. Soil contaminated with one or more organic species and one or more metal species is first treated with microbial agents to break down the organic compounds followed by treating the same soil with microbially produced sulfuric acid to solubilize and leach the metal species as a metal sulfate, and finally bioprecipitating the metal sulfate.
Similarly, U.S. Pat No. 5,667,690 describes breaking down organic compounds (generally hydrocarbons) in solids, water or gas by using a complex of a transition metal and a polyamine in the presence of a peroxide. These complexes are injected into the medium by a pump.
None of the technologies mentioned hereinabove address remediation of CS
2
-contaminated soil by treatment of the soil, nor do they address the unique difficulties and hazards encountered while treating CS
2
.
Thus, there is a need for a method to remediate CS
2
-contaminated soil which avoids the potential hazards associated with CS
2
oxidation.
SUMMARY OF THE INVENTION
According to the present invention, it has now been found that CS
2
-contaminated soil can be treated while avoiding the above-mentioned problems. Specifically, the present invention is a method of remediating CS
2
in soil by controlledly combining CS
2
-contaminated soil with an oxidizing agent such that CS
2
contained in the soil is disposed sufficiently near the oxidizing agent to react chemically therewith at a rate and under conditions which prevent volatilization of the CS
2
. Preferably, the controlled combining includes a stage-wise addition of the oxidizing agent while agitating the soil to ensure thorough mixing of the oxidizing agent with the soil.
In one embodiment, the combination step is preceded by treating the CS
2
-contaminated soil to reduce its acidity, such as by combining the soil with caustic before introducing an oxidizing agent. Sodium hydroxide (NaOH) is one preferred caustic for the pre-treatment. When a caustic is used, it should be combined with the soil such that the caustic to CS
2
molar ratio is from about 2:1 to about 4:1, preferably about 3:1. The pre-treatment can continue preferably not less than 24 hours, and most preferably not less than about 72 hours.
In one embodiment, the CS
2
-contaminated soil and the oxidizing agent is combined over a period from about 10 minutes to about 60 minutes, preferably from about 15 minutes to about 45 minutes, and most preferably for a time period of about 30 minutes.
The method can also include pre-churning of the CS
2
-contaminated soil to reduce unwanted concentrations of the CS
2
, and preferably to provide a substantially even distribution of the CS
2
throughout the soil.
The oxidizing agent useful for remediating the CS
2
can preferably be selected from a group consisting of perborate, percarbonate, hydrogen peroxide, and combinations thereof. Preferably, the oxidizing agent to CS
2
molar ratio is from about 2:1 to about 12:1, more preferably from about 6:1 to about 10:1, and most preferably is about 8:1. A preferred oxidizing agent is percarbonate, and in one embodiment, the percarbonate is suspended in a slurry.
To facilitate the suspension of percarbonate, it is preferable to use an inorganic suspension agent that would not be oxidized by the oxidizing agent. The suspension agent is preferably bentonite. When the oxidizing agent is included in a slurry, the oxidizing agent content (preferably percarbonate) is preferably from about 25% to about 75%, more preferably from about 40% to about 60%, and most preferably is about 50% by weight. In a most preferred embodiment of the invention, the suspension includes percarbonate and bentonite. In the case of a percarbonate:bentonite:water slurry, the preferred ratio is about 1.00:0.05:1.00 by weight.
A further refinement of the present invention includes the possibility of a pH buffer added to the combined oxidizing agent/CS
2
-contaminated soil. Preferably the buffer comprises substantially equal amounts of sodium carbonate and sodium bicarbonate, e.g., at a concentration of about 0.1N. Ideally the CS
2
-contaminated soil is maintained at a pH greater than about 7.0.
The method of the present invention can be implemented by controlledly combining the components by mixing with an auger. Moreover, even pre-churning can be implemented by use of an auger.
Yet in another embodiment of the present invention, mixing can be accomplished by trenching the CS
2
-contaminated soil while adding an oxidizing agent. This can be implemented by use of the trenching tool. In either case, the method can include adding a liquid containing the oxidizing agent to the soil during the mixing. When the liquid is a slur
Bybee Greg
Dulsey Rich
Fahrenthold Paul
Hazard John
Mausner Marvin
Crompton Corporation
Hoffmann & Baron , LLP
Langel Wayne
Nave Eileen E.
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