WO1999054258A1

WO1999054258A1 - Soil and/or groundwater remediation process

Info

Publication number: WO1999054258A1
Application number: PCT/US1999/008579
Authority: WO
Inventors: Richard S. Greenberg
Original assignee: Greenberg Richard S
Priority date: 1998-04-20
Filing date: 1999-04-19
Publication date: 1999-10-28
Also published as: AU3653899A

Abstract

A method of treating contaminants in soil and/or groundwater including adding a source of a peroxide and ozone to the in situ environment in amounts capable of producing reactive species sufficient to oxidize at least one of the contaminants.

Description

SOIL AND/OR GROUNDWATER REMEDIATION PROCESS

FIELD OF THE INVENTION

The present invention is directed to methods and systems for converting

contaminants contained in soil and/or groundwater to non-contaminating or harmless

compounds. The methods and systems include treatment of the contaminants with a

peroxide (e.g. hydrogen peroxide) and ozone which produce the hydroxyl radical and

other reactive species to thereby promote and control the conversion of the

contaminants to harmless by-products.

BACKGROUND OF THE INVENTION

The treatment of contaminated soils and groundwater has gained increased

attention over the past few years because of the increasing number of uncontrolled

hazardous waste disposal sites. It is well documented that the most common means

of site remediation has been excavation and landfill disposal. While these procedures

remove contaminants, they are extremely costly and in some cases difficult if not

impossible to perform.

More recently, research has focused on the conversion of contaminants

contained in soil and groundwater based on the development of on-site and in situ

treatment technologies. One such treatment has been the incineration of contaminated

soils. The disadvantage of this system is in the possible formation of harmful by- products including polychiorinated dibenzo-p-dioxins (PCDD) and polychlohnated

dibenzofurans (PCDF).

In situ biological soil treatment and groundwater treatment is another such

system that has been reviewed in recent years. So-called bioremediation systems,

however, have limited utility for treating waste components that are biorefractory or toxic

to microorganisms.

Such bioremediation systems were the first to investigate the practical and

efficient injection of hydrogen peroxide into groundwater and/or soils. These

investigations revealed that the overriding issue affecting the use of hydrogen peroxide

in situ was the instability of the hydrogen peroxide downgradient from the injection

point. The presence of minerals and the enzyme catalase in the subsurface catalyzed

the disproportionation of hydrogen peroxide near the injection point, with rapid evolution

and loss of molecular oxygen, leading to the investigation of stabilizers as well as

biological nutrients.

During the early biological studies from the 1980s, some investigators

recognized the potential for competing reactions, such as the direct oxidation of the

substrate by hydrogen peroxide. Certain researchers also hypothesized that an

unwanted in-situ Fenton's-like reaction under native conditions in the soil was reducing

yields of oxygen through the production of hydroxyl radicals. Such a mechanism of

2 - contaminant reduction in situ was not unexpected, since Fenton's-type systems have

been used in ex situ systems to treat soil and groundwater contamination.

Other investigators concomitantly extended the use of Fenton's-type systems to

the remediation of in situ soil systems. These studies attempted to correlate variable

parameters such as hydrogen peroxide, iron, phosphate, pH, and temperature with the

efficiency of remediation.

As with the bioremedial systems, in situ Fenton's systems were often limited by

instability of the hydrogen peroxide in situ and by the lack of spatial and temporal

control in the formation of the oxidizing agent (hydroxyl radical) from the hydrogen

peroxide. In particular, aggressive/violent reactions often occurred at or near the point

where the source of the oxidizing agent (the hydrogen peroxide) and the metal catalyst

were injected. As a consequence, a significant amount of reagents including the source

of the oxidizing agent (hydrogen peroxide) was wasted because activity was confined

to a very limited area around the injection point. In addition, these in situ Fenton's

systems often required the aggressive adjustment of groundwater pH with acid, which

is not desirable in a minimally invasive treatment system. Finally, such systems also

resulted in the mineralization of the subsurface, resulting in impermeable soil and

groundwater phases due to the deleterious effects of the reagents on the subsurface

soils.

- 3 Other researchers have investigated the use of ozone, either alone or in

combination with hydrogen peroxide, in ex situ advanced oxidation processes (AOPs).

These systems suffered from a similar limitation as the ex situ Fenton's systems;

namely, the necessity to pump contaminants from the in situ media to an external

reaction vessel, a requirement which was both expensive and inefficient. Ozonation

processes also suffered from low selectivity of contaminant destruction and high

instability of the ozone and reactive species generated.

It would be of significant advantage in the art of removing contaminants from soil

and/or groundwater to provide a system by which the source of the oxidizing agent can

travel from the injection point throughout the aerial extent of the contamination in order

to promote efficient destruction of the contaminant plume without the acidification of the

subsurface or the resultant mineralization of the soils. It would also be a significant

advantage in the art to generate a variety of reactive species in sufficient quantity to

allow the efficient degradation of a number of contaminants including traditionally

recalcitrant chlorinated solvents such as polychloroethylenes and trichloroethylenes.

It would be a further benefit in the art to provide a system which efficiently

generates the hydroxyl radical to provide a cost efficient and effective method of

oxidizing contaminants in soil and/or ground water.

4 - SUMMARY OF THE INVENTION

The present invention is directed to methods and systems for treating

contaminants in an in situ environment in which a reactive agent (hydroxyl radical

and/or other reactive species) obtained from the reaction of a peroxide, such as

hydrogen peroxide, with ozone, in an in situ environment reduces or eliminates

contaminants present therein in a cost efficient and effective manner.

In accordance with one aspect of the invention, there is provided a method and

system of treating contaminants in an in situ environment comprising adding an

effective amount of a peroxide to the in situ environment together or separately with an

effective amount of ozone, which by their reaction and generation of reactive species,

are capable of oxidizing at least one of the contaminants to reduce the concentration

thereof in the in situ environment. The present system enables temporal and spatial

control of the oxidation process so that the reactive species (e.g. hydroxyl radical) is

able to be generated into areas where contaminants are present. As a result,

aggressive/violent reactions at the point of injection are minimized and less reactive

species is wasted. In addition, due to the generation of hydroxyl radicals throughout

the plume and the presence of other reactive species, contaminants normally

recalcitrant to ex situ Fenton's or advance oxidation processes are now able to be

converted to harmless by-products.

5 - In accordance with preferred aspects of the invention, the peroxide component

is first injected into the in situ environment, either the groundwater or the soil above the

groundwater. The gaseous ozone is preferably injected into the groundwater through

multiple sparge points dispersed throughout the aerial extent of the plume where it

combines with the peroxide to generate the reactive species (e.g. hydroxyl radical).

Alternatively, the ozone can be injected into the subsurface as an aqueous solution into

the groundwater or as a gas into the vadose zone.

In accordance with another aspect of the invention, the methods and systems

herein can be applied to oxidizing contaminants in formations which are difficult to

access such as fractured bedrock. In particular, the peroxide and ozone are injected

at elevated pressures into the fractured bedrock to treat contaminants whose density

is greater than water and are often trapped in bedrock fractures.

In a further aspect of the invention, the peroxide and ozone are injected into the

in situ environment to enhance the operation and efficiency of traditional remediation

technologies such as pump and treat and solvent vapor extraction systems. The

present invention enhances these conventional systems that are based on mechanical

removal of the contaminants. This is because the oxidation reactions which convert the

contaminants to harmless compounds also enhance desorption of the contaminants

from organic carbon in soil and/or groundwater and generally result in enhanced

volatilization. The breakdown of contaminants into smaller compounds and the

- 6 increased production of carbon dioxide in the method of the present invention also

enhances volatilization and reduces adsorption of organic carbon in the soil.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to methods and systems for removing

contaminants from soil and/or groundwater by converting the same to harmless by¬

products. Such contaminants typically arise from petroleum storage tank spills or from

intentional or accidental discharge of liquid hydrocarbons or compositions containing

the same. Typical examples of contaminants are hydrocarbons including, but not

limited to: gasoline, fuel oils, benzene, toluene, ethylbenzene, xylenes, naphthalene,

pesticides, herbicides and other organic compounds; lubricants; chlorinated solvents,

including polychiorinated ethylenes, trichlorinated ethylenes, vinyl chlorides, dichloro

ethylenes, polychiorinated biphenyls (PCBs), pentachlorophenol (PCP); and metals,

cyanides and the like. The list of contaminants provided herein is exemplary. It should

be understood, however, that other contaminants capable of being oxidized into

harmless compounds, such as carbon dioxide and water, is within the purview of the

present invention.

In accordance with the present invention, the methods and systems for

remediation of a contaminated environment in situ is performed by providing a stabilized

source of a peroxide, for example hydrogen peroxide, and a source of ozone so that

- 7 they may react in situ to form a reactive species (e.g. hydroxyl radical), as hereinafter

described. It has been found that the reactive species generated in this way are found

throughout the extent of the plume with a resultant higher efficiency of contaminant

destruction.

In one embodiment of the invention, the peroxide and the ozone are alternately

injected (i.e. pulsed) into the soil and/or groundwater. In another embodiment of the

invention, the stabilized peroxide is allowed to disperse or migrate throughout the plume

and subsequently the ozone is bubbled into the groundwater through, preferably,

multiple sparge points.

The sources of the reactive species employed in the present invention is the

combination of a peroxide and ozone. Peroxides include, for example, hydrogen

peroxide, calcium peroxide, and sodium peroxide. Calcium peroxide generates

hydroxyl radicals under acidic conditions in the presence of iron (II) salts. Calcium

peroxide is very slightly soluble in water and is generally more expensive than hydrogen

peroxide. However, calcium peroxide can be used as an effective source of oxidizing

agent for hydrocarbon-contaminated sites. Sodium peroxide has been found to behave

in a manner similar to calcium peroxide and can be used as well. Hydrogen peroxide

is the preferred peroxide for use in the present invention.

8 - The peroxide (e.g. hydrogen peroxide) reacts with ozone in situ to produce

reactive species, including but not limited to hydroxyl radicals, hydroperoxide ion, and

ozonide ion. Ozone has previously been used as a disinfectant and in more recent

applications to oxidize refractory organic contaminants. The concentration of ozone

suitable for employment in the present invention can be supplied from known

commercial ozone generators (e.g. from about 3 to 15% ozone in air).

What is essential is that the system be capable of generating reactive species

in sufficient quantity and for a sufficient length of time to convert existing contaminants

(e.g. hydrocarbons) to harmless compounds (e.g. carbon dioxide and water vapor).

Prior to injection, the peroxide is preferably stabilized. Stabilization prevents the

immediate conversion of the peroxide via native iron or catalase into hydroxide radicals

or oxygen at positions only immediately adjacent to the injection points. Once stabilized

the peroxide is introduced into the in situ environment, typically in water at a

concentration of up to about 35% by weight. It will be understood that the concentration

of peroxide in the in situ environment will significantly decrease as the peroxide spreads

out through the soil and/or groundwater. Suitable stabilizers include acids and salts

thereof. The most preferred acid is phosphoric acid and the most preferred salt is

monopotassium phosphate.

- 9 The in situ environment for most soil and/or groundwater sites includes a water

table which is the uppermost level of the below-ground, geological formation that is

saturated with water. Water pressure in the pores of the soil or rock is equal to

atmospheric pressure. Above the water table is the unsaturated zone or vadose region

comprising the upper layers of soil in which pore spaces or rock are filled with air or

water at less than atmospheric pressure. The capillary fringe is that portion of the

vadose region which lies just above the water table.

The capillary fringe is formed by contact between the water table and the dry

porous material constituting the vadose region. The water from the water table rises

into the dry porous material due to surface tension because of an unbalanced molecular

attraction of the water at the boundary.

The source of the reactive species (peroxide and ozone) can be administered

in any zone in the in situ environment by any method considered conventional in the art.

For example, administration can be directly into the groundwater through a horizontal

or vertical well or into subterranean soil through a well or infiltration trenches at or near

the site of contamination. In a preferred form of the invention, the peroxide is placed

into the subsoil where it leaches therethrough into the groundwater. Ozone can be

introduced directly to the groundwater as a gas or as an aqueous solution (preferably

through multiple sparge points) or may be injected into the vadose zone.

10 As previously indicated, peroxide and ozone can be administered under elevated

pressures into hard to reach places such as fractures within underlying bedrock. These

fractures are collecting places for contaminants which are typically more dense than

water. When administered the peroxide and ozone are able to penetrate the fractures,

contact the contaminants and convert the same to harmless compounds.

Injection of the peroxide and ozone can be accomplished by installing steel or

polyvinylchloride lined wells or open hole type wells into the bedrock. Packers and

bladders conventionally employed in downhole drilling can be employed to assist in

isolating discrete fractures and accessing the contaminants with the reagents. The

reagents are then injected into the fractures at applied elevated pressures, typically in

the range of from about 40 to 100 psi.

The administration of the peroxide and ozone into the in situ environment

including bed rock fractures under elevated pressures can be accomplished either alone

or in conjunction with conventional treatment systems. Such systems include pump and

treat systems which pump the contaminated groundwater out of the in situ environment

and solvent vapor extraction systems in which a vacuum is applied to the site of

contamination to physically enhance volatilization and desorption of the contaminants

from soil and/or groundwater.

- 11 The employment of ozone as a reactant for the formation of reactive species is

advantageous because ozone can be continuously generated by readily available

ozone generators which do not require excessive labor to operate. In addition, unlike

conventional Fenton's systems which are highly dependent on pH and require the

aggressive adjustment of site pH, it has been found that the present system functions

efficiently at neutral to alkaline pH, consistent with the native pH found in many

subsurface environments.

As indicated above, the peroxide and ozone can be administered directly into the

in situ environment. In a preferred form of the invention, the amount of the peroxide

and ozone and the number of treatment cycles are predetermined. For example,

samples of the contaminated soil and/or groundwater are taken and the concentrations

of the peroxide and ozone required for in situ treatment are then determined based on

the amount of each reagent needed to at least substantially rid the samples of the

contaminants contained therein.

More specifically, a sample of the soil and/or groundwater is analyzed to

determine the concentration of the contaminants of interest (e.g. hydrocarbons).

Analysis of volatile hydrocarbons can be made by gas chromatographic/mass

spectrometric systems which follow, for example, EPA Method 624. Semi-volatiles are

analyzed in a similar manner according to, for example, EPA Method 625.

12 - Results from these analyses are used to determine the combinations of peroxide

and ozone for treatment of the sample based on the type and concentration of the

contaminants. A specific weight ratio of the peroxide and ozone is used for the sample

based on prior research, comparative samples and the like. Typical sample volumes

can be in the range of from about 120 to 150 ml. It has been found that a weight ratio

of from about 0.2 to 1.5 w/w peroxide/ozone in the in situ environment is preferred

because operation within this ratio provides the highest efficiency of contaminant

destruction for most contaminants.

Sample analysis is also employed to determine the number of treatment cycles

which may be necessary to achieve the desired reduction in the level of contaminants.

While one treatment cycle may be used, it is often desirable to employ a plurality of

treatment cycles depending on the type and concentration of contaminants. The

number of treatment cycles is determined in part by monitoring the performance of the

peroxide and ozone once injected into the soil and/or groundwater.

In operation, the peroxide and ozone are injected into sealed vials with a syringe.

The doses of peroxide and ozone are given as hourly treatment cycles with the

expectation that the samples will typically require as few as one treatment cycle and as

many as five treatment cycles in order to substantially or completely convert the

contaminants to harmless by-products.

13 A control sample is set up for each type of sample undergoing the study to

correct for any volatization loss. All experimental vials are allowed to sit overnight at

room temperature. On the following day the samples are analyzed to determine the

concentration of contaminants by the above-mentioned EPA procedures. Once the

results are obtained, they may be extrapolated to provide a suitable amount of the

peroxide and ozone necessary to treat the contaminants in situ.

Injection of the stabilized source of the peroxide may be performed under both

applied and hydrostatic pressure into the in situ environment. Flow rates will vary

depending on the subsurface soil characteristics with faster rates associated with more

highly permeable soils (e.g. gravel and/or sand). Slower rates as low as 0.1 gallons per

minute may be used for less permeable soils (e.g. clays and/or silts). The stabilized

source of the peroxide may be injected into the subsurface and allowed to disperse

over a specific period, typically about 24 hours. The length of time may be varied

depending on the soil type.

In less permeable soils, injection procedures are preferably associated with a

pressurized system. A typical system involves injection wells installed with screens set

at specific levels to allow for higher pressures and countered by pumping into less

permeable soils. The pumping system can include a low horsepower pump at

pressures ranging from between about 10 and 40 pounds per square inch. The

14 stabilized peroxide may be pumped in short pulse injections or in a long steady flow as

desired.

In a preferred form of the invention, the stabilized peroxide is injected directly into

the capillary fringe, located just above the water table. This can be accomplished in a

conventional manner by installing a well screened in the capillary fringe and injecting

the reagents into the well screen.

After the peroxide has been injected into the in situ environment and allowed to

disperse throughout the aerial extent of the plume in this preferred embodiment, ozone

is generated on site and pumped into the subsurface as described previously.

Preferably the ozone is injected as a gas into the groundwater through multiple injection

points arranged throughout the aerial extent of the plume. In another embodiment, the

ozone and peroxide can be alternatively pulsed into the in situ environment.

In particular, the effects of naturally occurring minerals including their reactivity

with the peroxide and ozone can have a dramatic effect on the extent of the formation

of the reactive species. It has been found that injection of the stabilized source of the

peroxide followed by subsequent repeated ozone injections allows for improved

efficiency of conversion to reactive species throughout the plume in the subsurface.

15 - In particular, a monitoring system employs a free radical trap to directly measure

the concentration of the reactive species contained within the in situ environment. More

specifically, a sample of the soil and/or groundwater is combined with a specified

amount of a free radical trap such as methylene blue dye. The mixture is stabilized and

precipitated and/or colloidal matter removed. The absorbance of the color remaining

in the sample is measured using a spectrophotometer at a wavelength capable of

measuring the absorbance of the blue dye (e.g. 662 nm). The absorbance value is then

compared to the standard curve of absorbance vs. reagent value determined for the

particular site.

The free radical concentration of the sample is expressed as a reagent value (R)

which is proportional to the concentration of the radical and is representative of the

amount of the peroxide and ozone initially added that are remaining at that point as

shown in Table #1. This amount is expressed as a fraction proportional to the total

number of treatment cycles X, wherein X is the number of treatment cycles originally

recommended for the sample.

16 - TABLE 1

Reagent Explanation Value (R)

2 100% of the peroxide and ozone initially added are still present. The amount of free radicals produced in 10 minutes is highest for this sample.

1 50% of the peroxide and ozone initially added are still remaining in the sample. The amount of free radicals produced in 10 minutes for R=1 sample is one half the amount produced for R=2 sample.

0.5 25% of the peroxide and ozone initially added are still remaining in the sample. The amount of free radicals produced in 10 minutes for R=0.5 sample is one quarter the amount produced for R=2 sample.

EXAMPLE 1

The following experiments were conducted in two (2) - 1500 cm³ air-tight glass

reactors with inlets provided for ozone, hydrogen peroxide, sample collection and

venting. A representative contaminated sample as shown in Table 2 was prepared by

spiking appropriate concentrations of the contaminants into tap water. Exactly 1000

cm³ of the substrate solution was decanted into each of two reactors designated for

control and treatment purposes and placed on magnetic stirrers for a mixing operation.

The Ozonator used in the experiments had a maximum ozone generating capacity of

6 g/h. Hydrogen peroxide was introduced into the treatment reactor immediately prior

to ozonation. Ozonated oxygen (approx. 4% v/v O₃) was introduced and allowed to

bubble at the bottom of the reactor.

- 17 The control reactor was simultaneously treated by introducing distilled water in

place of hydrogen peroxide and bubbled with pure oxygen instead of ozone at similar

flow rates. Water samples were withdrawn periodically for residual hydrogen peroxide

analysis. Water and air samples for volatile organic analysis were collected after 60

minutes of ozonation at which time hydrogen peroxide was completely consumed.

Air samples were collected in tedlar bags and water samples in 40 ml VOA vials

preserved in HCI. Both air and water samples were analyzed for targeted volatile

organics by an EPA certified laboratory. Hydrogen peroxide concentration was

determined by complexing with Ti(IV) and measuring the absorbance of the yellow color

developed spectrophotometrically (FMC 1989). Ozone concentration in both air and

water was measured by iodometric titration (Standard Methods, 1989). The results are

shown in Table 2.

TABLE 2

Contaminant Before Treatment After Treatment Percent

(ppb)^* (ppb)^* Reduction (%)

Trichloroethene 518 7.1 98.6

Perchloroethene 472 11.3 97.6

Vinyl Chloride 610 9.0 98.5 cis-1 ,2-Dichloroethene 450 5.0 98.9

Benzene 87 3.5 95.9

Toluene 112 ND** 100.0

^* parts per billion ** Not Detected

18 - As shown in Table 2, the reduction in the level of contaminants after treatment

with the reactive species formed by the reaction solution containing the peroxide and

ozone was between about 95.9 and 100%.

19

Claims

IN THE CLAIMS:

1. A method of treating contaminants in an in situ environment comprising:

adding a source of a peroxide and ozone in amounts capable of producing reactive

species sufficient to oxidize at least one of the contaminants.

2. The method of claim 1 wherein the source of the peroxide is selected

from the group consisting hydrogen peroxide, sodium peroxide and calcium peroxide.

3. The method of claim 2 wherein the source of the peroxide is hydrogen

peroxide.

4. The method of claim 1 further comprising stabilizing the source of the

peroxide.

5. The method of claim 4 comprising stabilizing the source of the peroxide

with a stabilizer selected from the group consisting of acids, salts and mixtures thereof.

6. The method of claim 5 wherein the stabilizer is selected from the group

consisting of phosphoric acid, monopotassium phosphate and combinations thereof.

20 -

7. The method of claim 1 comprising first adding the source of the peroxide

to the in situ environment and then adding the ozone to the in situ environment.

8. The method of claim 1 wherein at least a portion of the reactive species

comprises hydroxide radicals.

9. The method of claim 1 further comprising monitoring the concentration of

the reactive species in the in situ environment.

10. The method of claim 1 wherein the in situ environment is selected from

the group consisting of soil, groundwater and fractured bedrock.

11. The method of claim 1 adding the source of the peroxide and ozone at an

elevated pressure.

12. The method of claim 11 wherein the elevated pressure is from about 40

to 100 psi.

13. The method of claim 11 wherein the in situ environment is fractured

bedrock.

- 21

14. The method of claim 1 wherein the source of peroxide is peroxide in water

at a concentration of up to 35% by weight.

15. The method of claim 1 wherein the weight ratio of peroxide to ozone is in

the range of from about 0.2 to 1.5 w/w.

16. The method of claim 1 comprising supplying ozone to the in situ

environment mixed with air at a concentration of from about 3 to 15%.

17. The method of claim 1 comprising injecting the ozone through a plurality

of injection points in the in situ environment.

22