EPA/600/A-94/020
	 CHIEN T. CHEN
Assessment of the Applicability of
Chemical Oxidation Technologies for
the Treatment of Contaminants at
Leaking Underground Storage Tank
(LUST) Sites
ABSTRACT
The total number of confirmed releases from underground storage tanks is increas-
ing rapidly. In addition, the treatment of contaminants in soil and groundwater at
leaking underground storage tank (LUST) sites presents complex technical chal-
lenges. Research efforts focused on developing LUST site remediation technol-
ogies have produced a variety of physical treatment methods including: 1) in-situ
treatment technologies such as soil vapor extraction (SVE), radio frequency (RF)
heating, steam stripping (SS), soil flushing, and air sparging (AS), and 2) ex-situ
treatment technologies such as soil washing, thermal desorption, and solvent ex-
traction. Most of these technologies involve the separation of contaminants from
soil or groundwater. The separated contaminants may be adsorbed on activated
carbon, condensed to liquid, or separated from the extracting solvents by distil-
lation. The destruction or disposal of these waste mixtures is a tedious and expen-
sive task. Furthermore, most of the in-situ remediation technologies are only ef-
fective for removing volatile organic compounds (VOCs) and only certain semi-
volatiles from the vadose zone. The combination of SVE with either SS or AS is
being studied and developed, and indications are that these integrated systems may
be effective for removing the same contaminants for the saturated zone. However,
the efficiencies of the combined techniques has not been established. The most
commonly applied methods for the treatment of contaminants of high molecular
weight is excavation of the soil or pumping and treating the groundwater.
Biodegradation has been used for both in-situ and ex-situ destruction of organic
contaminants. Because of the slow and complicated bioreactions, however, the
mechanisms and efficacy of such treatments are not easy to establish.
Recently, new treatment methodologies have been investigated. Processes involv-
ing chemical oxidation have the potential to treat all types of organic contaminants
(volatile, semivolatile, and nonvolatile) in both vadose and saturated zones either
jn-situ or following excavation or, under certain conditions, to detoxify the hazard-
Chien T. Chen Ph.D.. Releases Control Branch. Risk Reduction Engineering Laboratory, US Environmental Protection Agency,
Ed:son. New Jersey 08837. USA
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ous materials that may be present in the off-gases that result from the use of vapor
extraction or thermal desorption technologies. The oxidative processes can entail
complete mineralization, transformation of complex substances into simple com-
pounds, or conversion of hazardous materials to more water-soluble compounds
that are typically less toxic and amenable to biodegradation.
INTRODUCTION
OBJECTIVE
The objective of this paper is to provide an overview of the published reports re-
garding chemical oxidation processes for degrading organic pollutants and to dis-
cuss in detail some of these processes that appear to have the greatest potential for
detoxifying hazardous materials at LUST sites. These technologies are of interest
if they can more completely degrade the contaminants, provide a reduction in the
time necessary to remediate a LUST site, or provide a cost savings over the
existing remediation methods. The goal is to identify which process applications
might be most worthy for further development as LUST remediation technologies.
CHEMICAL OXIDATION TECHNOLOGY
Chemical oxidation technologies, as discussed in this paper, involve the conversion
of environmental contaminants to nonhazardous or less toxic compounds that are
ideally more stable, less mobile, and/cr inert. This paper will assess the potential
for treating chemicals that have leaked from underground storage tanks by using
chemical oxidation to degrade hydrocarbons, chlorinated hydrocarbons, or other
organic chemicals to simpler molecules with the ultimate goal of complete oxida-
tion to C02, H20, and HCI. Incomplete or partial oxidation of molecules may
occur that typically renders them less toxic and in many instances more biodegrad-
able; for some contaminated sites, however, the combination of chemical oxida-
tion/bioremediation may yield acceptable results.
Chemical oxidation technologies have the following general advantages [1]:
•	Simple, readily available equipment and reagents.
•	They typically have low capital, operating, and maintenance costs.
•	They are capable of a high level of treatment at low concentrations.
•	Oxidants may have a positive effect on microbial degradation.
Chemical oxidation technologies have the following general limitations [1, 2, 3]:
•	Systems must be designed for the specific contaminant and soil type and
thus initial laboratory or pilot testing is required.
•	Some oxidants have safety and environmental concerns.
•	Oxidants are not discriminating and may react with nontarget com-
pounds; they are most suited to media with low concentrations of con-
taminants.
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The major cost of oxidation treatment is due to the cost of the oxidants. The
economics vary based on the contaminant and the media being treated. Contami-
nants in water or soil are the primary waste form treatable by chemical oxidation.
Chemical oxidation is applicable to both concentrated and dilute waste streams,
but the competing processes are more numerous for the concentrated streams [4].
UNDERGROUND STORAGE TANK (UST) CHARACTERIZATION
The EPA estimates that as many as 15 to 20 percent of the approximately 1.8
million regulated UST systems nationwide either are leaking or are expected to
leak in the near future [5]. Underground storage tanks in the United States pri-
marily are used to store gasoline and other petroleum products. However, other
organic and inorganic chemical substances such as solvents and various hydrocar-
bons may also be stored in USTs [5, 6]. USTs typically fail due to corrosion of the
tank or associated piping system.
Corrective actions that have been used or are being developed include the
following:
•	In-situ soil treatment - soil vapor extraction, soil flushing, radio frequency
heating, bioventing, hydrogen peroxide bio-oxidation, etc.
•	Ex-situ soil treatment - incineration, thermal desorption, soil washing,
solvent extraction, biodegradation, recycle and reuse (asphalt, cement
and brick manufacturing), etc.
•	In-situ groundwater treatment - air sparging, biodegradation, steam strip-
ping, chemical oxidation, etc.
•	Free product recovery.
•	Groundwater extraction and treatment by air or steam stripping, carbon
adsoprtion, chemical oxidation, physical separation, biodegradation, etc.
EVALUATION CRITERIA FOR ASSESSING CHEMICAL OXIDATION
TECHNOLOGY
During the evaluation of published reports, descriptions of chemical oxidation
processes were reviewed to ascertain the state of the art of chemical oxidation
technology and the potential for its use in treating contaminants at LUST sites. A
literature search was conducted of published literature including journal articles,
conference papers, computer data bases, vendor reports, and EPA documents. The
following criteria were used to evaluate the technologies:
•	Type, number, and completeness of the studies found.
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•	Safety or environmental impacts including toxicity of oxidant or products
formed and releases to all media.
•	Complexity of the process including range of physical conditions under
which the reactions occur and labor required to implement the process.
•	Estimated cost of the technology versus existing alternatives.
•	Applicability of the technology to LUST sites, and the extent of adapta-
tion necessary.
OVERVIEW OF CHEMICAL OXIDATION TECHNOLOGIES DESCRIBED IN
LITERATURE
Literature was reviewed to gather data pertaining to the following oxidants: chlo-
rine, chlorine dioxide, hydrogen peroxide, hypochlorite, ozone, and
ozone/ultraviolet light (UV). Table I presents information found on the advantag-
es and limitations of the oxidants that could be used in the treatment of contami-
nants at LUST sites.
COMMON CHEMICAL OXIDANTS
After the turn of this century, chlorine gas was applied as part of the drinking
water treatment process. In areas centrally supplied with drinking water in the
USA and Europe, this process reduced the typhoid mortality rate to an insignifi-
cant level. With the recent advancements of analytical methods for performing
trace analysis, however, the presence of chloroform and other chlorinated com-
pounds were detected in chlorinated drinking water. These findings have prompt-
ed a search for other drinking water treatment options.
Chlorine dioxide is an effective alternative to chlorine because it chemically con-
verts contaminants to salts and nontoxic organic acids. Chlorine dioxide (an un-
stable gas requiring on-site generation) is generated by reacting sodium chlorite
solution with chlorine gas or by reacting sodium chlorite solution with sodium
hypochlorite and hydrochloric acid [7], Concentrated aqueous solutions of chlorine
dioxide (0.5 to 3 g/L) may be stored for a few hours before application. In soil
treatment applications, the chlorine dioxide may be applied in-situ through conven-
tional injection wells or surface flushing [7]. If chlorine dioxide is used in place of
chlorine, chlorinated hydrocarbons are not created during the oxidation of organic
compounds. However, chlorine dioxide does give rise to other, unwanted substanc-
es. Large amounts of chlorite are formed and in some cases 50 to 70 percent of
the chlorine dioxide consumed is converted to chlorite [8].
Hydrogen peroxide is a powerful oxidizing agent whose reaction by-products (water
and oxygen) are nontoxic [12]. Solutions of hydrogen peroxide are relatively safe,
effective, and easy to use (with a natural decomposition rate of just 1 percent per
year in commercial storage) [12]. Hydrogen peroxide is a source of hydroxyl radi-
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TABLE I. SUMMARY OF ADVANTAGES AND LIMITATIONS OF
COMMON CHEMICAL OXIDANTS FOR USE AT LUST SITES
Oxidant
Technology
Chlorine
Advantages
Limitations
-	Biological activity may be killed
by chlorine [9],
-	Chlorine can convert organics to
chlorinated compounds which
may be carcinogenic or difficult
to biodegrade [9].
-	Not previously used at LUST
sites.
-	Chlorine gas is highly corrosive.
-	CI02 is a relatively unstable gas
which is not to be compressed
and liquified without danger of
explosion and therefore must be
generated on site [10],
-	Chlorine dioxide generates some
undesirable side products, na-
mely chlorites.
-	Aliphatic hydrocarbons do not
react significantly with chlorine
dioxide in practical conditions of
water treatment [10].
-	Toxic to microorganisms [11].
-	Use in conjunction with in-situ
bioremediation is not well de-
veloped [11].
-	Its decomposition in the sub-
surface may be so rapid that
much of the resulting oxygen will
bubble out of solution, becom-
ing unavailable to
microorganisms [11].
Chlorine
dioxide
Hydrogen
peroxide
Hypochlorite
-	Low initial cost
-	Long history of use in water treat-
ment
Does not react with organics to
form organo-chlorine compounds
as does chlorine [10].
May be generated on site [7],
Economical and readily available
[11].
Oxygen from disproportionate is
available for use by microorganisms
[11].
Can be added to environment at
high concentration, providing an
oxygen supply several orders of
magnitude more concentrated than
possible and saturating water with
pure oxygen [11],
Does not persist in the environment
[11].
Hydrogen peroxide is infinitely solu-
ble in water [12],
Low cost.
Readily available, and the aqueous
solution is easily transported,
stored, and metered into the react-
ing system [13].
- Forms chlorinated by-products.
(continued)
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TABLE 1 (continued)
Oxidant
Technology
Ozone
Advantages
Ozonation in the presence of UV
irradiation, ultrasound and/UV h202
causes organic oxidations to pro-
ceed at significantly increased
rates.
Generated on site from air; used
immediately; no storage or handling
of strong oxidants: stop generating
by turning off power [14],
Very strong oxidant; reacts with a
large variety of organics; does not
form chlorinated organics; residuals
react with constituents or revert
back to oxygen; short reaction
time; lower dose rates than other
oxidants; makes some organics
more biodegradable [14].
Temperature and pH less critical
than with other oxidants; treated
effluents are normally oxygen rich
[14].
Ozonation in the presence of UV
irradiation causes organic oxida-
tions to proceed at significantly in-
creased rates, especially for com-
pounds normally refractory to
ozone alone [16].
Limitations
Cannot be stored due to short
half life.
Ozonation of organic com-
pounds rarely proceeds com-
pletely to CO? and water.
Instead, intermediate oxidation
products form [15].
Ozone is produced on site. The
total equipment (air preparer,
ozone generator, contact cham-
bers) represent substantial cap-
ital and operational costs [14].
Will not degrade low-molecular
weight chlorinated organics;
must treat off-gas if ozone pres-
ent [14].
Ozone/UV
Cost of both UV and ozone gen-
erators may be high.
UV is not amenable to in-situ ap-
plication.
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cals, one of the most potent oxidizers known. The hydroxyl radical is second only
to fluorine in oxidation potential among the common oxidants.
There are two main methods of producing hydroxyl radicals with hydrogen perox-
ide:
1) By Fenton's method which involves the use of ferrous ion to enhance
the production of hydroxyl radicals.
Fe2' * H202 —> Fer * OH*+.OH	m
2) By "Advanced Oxidation Processes" [18, 19]
• H202/ultraviolet light
H202 	——> 2 -OH
2 2 <400 nm
• H202/ozone
H202 - 203 - 2HO- * 3Oz	(3)
• H202/ozone/ultraviolet light (e.g., Ultrox system [20]).
(Molar ratio of H?02/03 is varied by waste with at
least 13 possible reactions [21])
Sodium hypochlorite is a widely used oxidant. It is therefore readily available in
the form of aqueous solutions that are easily transported, stored, and metered into
the reacting system. One of the major uses of hypochlorite solutions is for the
treatment of cyanide-containing wastes from ore extraction, synthetic organic-chem-
ical manufacture, and metal finishing. However, chlorinated by-products may be
formed.
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Ozone is a powerful oxidant that has the ability to oxidize a great number of or-
ganic and inorganic materials. Ozone oxidation reactions in aqueous media usually
are dependent upon pH [15]. At higher pH ranges (8 to 9), ozone decomposes to
form hydroxyl free radicals (*OH), which are stronger oxidizing agents than free
ozone. At low pH, ozone reacts as the free ozone molecule, which has a slower
reaction rate than *OH. Ozonation of organic compounds rarely proceeds com-
pletely to C03 and water. Instead, intermediate oxidation products form. These
will contain more oxygen than did the starting organic materials. Because of this,
the oxidized materials are more polar, have a greater solubility in water, and are
usually more readily biodegradable. Saturated aliphatic hydrocarbons are unreac-
tive toward ozone. Unsaturated aliphatic compounds generally are readily reactive
with ozone, unless they have been halogenated to high levels. Alcohols are slowly
oxidized to acids. Oxalic acid and C02 are the most stable reaction products of
organic oxidation. Benzene is slowly oxidized with ozone. Other aromatic
compounds generally are easily oxidized by ozone, except when electron-
withdrawing substituents are present on the aromatic ring.
Ozonation in the presence of UV irradiation, ultrasound, and/or H202 causes or-
ganic oxidations to proceed at significantly increased rates. Reaction times can be
100 to 1000 times faster in the presence of UV light [2].
POTENTIAL CHEMICAL OXIDATION TECHNOLOGIES FOR USE AT LUST
SITES
There are four types of treatment operations for which chemical oxidation
technologies may be implemented at LUST sites: ex-situ water treatment, in-situ
soil and water treatment, ex-situ soil treatment, and treatment of gaseous emissions
generated by soil vapor extraction (SVE), thermal desorption and radio frequency
heating systems. Table II summarizes the characteristics of chemical oxidation
technologies that, on the basis of this review, appear to have the greatest potential
for use at LUST sites. In the following sections each of these four types of
treatment operations and the corresponding chemical oxidation processes which
may be applicable are discussed.
The discussion below emphasizes primarily technologies involving the use of H202,
ozone, and UV. Other chemical oxidation technologies (CU, C102, hypochlorite)
are not included here due to the lack of information regarding their application to
most common LUST contaminants as well as potential safety and environmental
concerns arising from the possible formation of chlorinated by-products.
EX-SITU WATER TREATMENT
Ex-situ water treatment at LUST sites may entail treatment of contaminated
groundwater or treatment of water generated on the site by processes such as soil
flushing. Since chemical oxidation processes have been used in wastewater treat-
ment and drinking water treatment for many decades, knowledge in this area is
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taih.e ii. chemical oxida tion riiCHNOLOGits wrm potential for use at lust sites
Oxidation Technology
Type of Studies,
Extent of Use
Potential Safety or
Environmental
Impacts
Complexityof
Proeess
Estimated Cost
Versus Existing
Alternatives
Adaptationlo
LUST Sites
References
Ex-Situ Water Technology
UV/II.O,
(Perox-pure™)
Used at over 70 sites
Minimal
System is com-
plex, but is com-
meitially available
Cosi-effcnivc for
toxic contaminants
1 las been used at
LUST sites
21,22, 2.1
UV/IIA/O,
(ULTROX)
Used at over 20 sites
Ozone must be
treated before
release to ambient
air
System is com-
plex, but is com-
mercially available
Cost-effective for
toxic contaminants
lias been used at
LUST sites
20, 22
UV/O,. 0,/HjOj,
UV/lascr/H,0,
Bench-scale and
laboratory studies
Ozone must be
treated before
release to ambient
aii
Systems are com-
plex
Unknown
Must be scaled up
and field tested
18, 24,25, 26, 27.
28, 29
In-S'nu Soil and Water Technology
HjO, (Venture)
Pilot scale study
Mimimal
Medium
Complexity
l-ow
Considemble
work is necessaiy to
understand best
conditions for use
50
o,
laboratory and pilot
scale studies
Unknown
Medium
Complexity
Medium
Considerable
work is nec-
essary to
understand
best condi-
tions for use
49, SI
H,0,/Bioremediation
Laboratory and
bench-scale with
some inconclusive
use in the Held
High concentrations
of HjO, toxic to
microbes
Process is not
complex, but best
conditions of use
are not well
understood
Unknown. May have
cost advantage if
time is important or
very toxic chemicals
are present
Considerable work
is necessaiy to
understand best
conditions for use
17. 30, 31, 32. 33.
34. 35, 36. 37, 38,
40
(continued)

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TABLE 2 (continued)
Oxidation Technology
Type of Studies,
Extent of Use
Potential Safety or
Environmental
Impacts
Complexltyof
Process
Estimated Cost
Versus Existing
Alternatives
Adaptatlonto
LUST Sites
Keferences
RX-Situ Soil Technology
MA
Laboratory and
bench-scale studies.
Some field tests.
Minimal
Not developed
enough to
ascertain
complexity
Unknown. May have
cost advantage if
time is important or
very toxic chemicals
present
Considerable work
is necessary to
identify best
conditions for use
17, 39, 41 52
UV/O, (Excalibur
Enterprises)
Pilot scale
Ozone must be
treated before
release to ambient
air
System is complex
Unknown
Must be field tested
16, 42
Technology to Treat Air From SVE Systems
LTV (Purus)
laboratory and
bench-scale studies
Minimal
Medium
complexity
May have cost
savings for smaller
LUST sites where
the capital cost of
carbon adsorption is
high
May be problem in
treating surges in
containment release
45, 46
IJV/TiOj (Nutech
r.nvironmenial)
Bench and pilot scale
Minimal
Medium
complexity
Unknown
Must be scaled up
and field tested
47, 4#
Air sparging with O,
'Pieoretical
Potential release of
o,
Medium
complexity
Unknown
Considcrublr work
needed
49

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comparatively advanced, and commercial systems are in use to oxidize organics and
chlorinated organics.
Several commercially available chemical oxidation water treatment technologies
are briefly described below:
•	The Perox-pure™ system developed by Peroxidation Systems Inc. em-
ploys UV and H202 and has been used at over 70 sites [21, 22, 23].
The system produces the hydroxyl radical and has degraded numer-
ous water contaminants including chlorinated solvents, pesticides,
polychlorinated biphenyls, phenolics, and fuel hydrocarbons in con-
centrations ranging from a few thousand milligrams per liter to one
microgram per liter [21, 23]. The system can also be used as a pre-
treatment to detoxify contaminants prior to biological treatment.
•	The Ultraviolet Radiation/Oxidation system developed by Ultrox
International uses UV, H202, and 03 and has been used at over 20
sites [20, 22], The system decomposes the ozone prior to release to
ambient air [20].
•	Bench-scale investigations have shown that UV/laser/H202,
03/H202, and UV/03 can decompose waterborne contaminants such
as chlorinated solvents, benzene, phenol, and other organics [18, 24-
29].
IN-SITU SOIL AND WATER TREATMENT
In a recent pilot scale study, hydrogen peroxide solution (35% in water) was inject-
ed into the saturated zone of a gasoline contaminated site (2,000 cubic yards) with
a porosity of 0.45. The total BTEX (benzene, toluene, ethyl benzene and xylene)
decreased from 10820 ppm to 580 ppm within 1-1/2 months. Four more injections
were conducted and the final BTEX was reduced to 384 ppb (52). The characters-
tics of the site are not clear and the test results were not verified.
Hydrogen peroxide (in conjunction with bioremediation) was the chemical oxidant
most often used for in-situ treatment of contaminated soils. Numerous articles
were reviewed describing bench-scale and field applications of H202 in conjunction
with bioremediation to treat fuel and chlorinated hydrocarbon spills in soil [17, 31-
35]. While the results of the studies are inconclusive, it appears that under the
proper conditions, the combination of H202 and bioremediation can result in a
significant reduction in the time necessary for remediation.
In situ aerobic biotransformation of underground organic pollutants is well known
and complete mineralization of many hydrocarbons is achieved under oxidant-rich
conditions [11]. Dissolved oxygen availability frequently limits the biotransforma-
tion of organic compounds in the subsurface due to the limited aqueous solubility
of oxygen, the relatively slow rate of reaeration of groundwater in the saturated
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zone, and the significant biological oxygen demand caused by aerobic metabolism.
In biological systems, hydrogen peroxide can supply dissolved oxygen and therefore
can be used to augment the oxidant capacity of the aquifer. H202 is disproportion-
ated by the action of microbial catalase and several inorganic catalysts such as iron
oxide species to give 0.5 mole of oxygen per mole of H202 consumed.
2h2q2 catalyses> 2H^Q +
The resulting dissolved oxygen should then be available for microbial respiration.
Injections of hydrogen peroxide and bacterial nutrients, such as nitrogen and phos-
phorus, into contaminated soil and groundwater has been performed to provide
oxygen and nutrients for bacteria that metabolize hydrocarbon contaminants. This
technology has been used to clean up soil and groundwater near leaking diesel and
gasoline tanks [31, 33, 34, 35].
The process generally entails passing a water solution through the contaminated
soil, collecting the water downstream (downgradient), and possibly reinjecting upgr-
adient. Suitable biological nutrients or other amendments are added to the water
prior to injection. It is often necessary to treat the water prior to reinjection to
remove contaminants. The previous section on water treatment described the use
of chemical oxidation in such cases.
In-situ bioremediation with hydrogen peroxide involves optimization of several
controlling variables including pH, catalysts, temperature, contact time, application
rate, and reactivity of the contaminants [17]. The optimum value for these factors
varies with the compound being oxidized. While the process is not complex, the
optimum conditions seem difficult to predict and identify. Other problems en-
countered include decomposition of peroxide near the distribution point, major
growth of bacteria near the injection point, obstruction of regular groundwater flow
by biomass, and inadequate penetration and availability across the contaminated
area [8, 32, 35, 40]. Iron catalyzes the decomposition of peroxide. Since iron ap-
pears in many soils around LUST sites, this can be a recurring concern. However,
the addition of phosphates will passivate iron and limit the effect on peroxide [32,
35, 39],
Bacterial enzymes (catalase) can also cause premature decomposition of peroxide
[40]. These enzymes can collect in the water/solution injection galleries, resulting
in a rapid loss of peroxide effectiveness. One method which has been used to
inhibit this effect is to filter water through charcoal beds, or not to use recycled
solution [8, 40]. Because these may not be cost-effective solutions, they may be
difficult to resolve if catalase-enhanced decomposition occurs.
Whether inhibited or not, decomposition of peroxide still occurs. Even with phos-
phate addition and the lack of any catalase activity, peroxide, after being released
into the environment, decomposes by 80 percent over as little as four hours [11,
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40]. Since solution flow is generally quite slow across a contaminated zone (due
primarily to the relatively low fluid porosity of subsurface soils), it often is difficult
for active peroxide solution to reach much of the contaminated area. For that
reason, multiple injection points are often advisable to provide suitable oxidant
activity to the zones of interest.
In order for peroxide to be effective in the chemical oxidation mode only, it must
decompose to form the OH radical, which is a potent oxidizer. Several studies (3,
36, 37) achieved substantial degradation of soil contaminants with peroxide at a pH
of 2-3. In most cases it would be difficult and expensive to acidify large volumes of
soil. Based on the published data, a further expense would be the extremely high
dosage utilized in the experiments, over 10,000:1, peroxide to contaminant, in some
cases.
Other research studies involving peroxide for soil remediation (39, 41) were per-
formed at neutral or slightly basic pH's (i.e., 9), where the concentration of OH
radical would be much higher (over 4 orders of magnitude higher than in the pre-
viously described study). These tests were in the form of a pretreatment of refrac-
tory constituents prior to bioremediation. Such an approach would be simpler, and
thus more feasible, to arrange in an ex-situ system. As such, it will be described in
the following section.
Another type of reaction is direct oxidation of organic compounds by H202 in the
presence of enzymes (peroxidases) or metal oxide catalysts [11]. Molecular oxygen
is not evolved as a result of this latter type of H202-consuming reaction.
In any case, the aqueous solution containing the oxidant must come into intimate
contact with the contaminant before oxidation/bioremediation can occur. Non-
wettable solids or contaminants or inaccessible soil pores and clay will retard and
limit the effectiveness of this approach. Contaminants in the vadose zone would
be difficult to access and treat by this approach.
The information which was reviewed indicated that H202 does not persist in the
environment. No reports were found which indicated that H202 reacts with envi-
ronmental constituents to form toxic by-products. At high H202 concentrations
(between 0.05 and 0.2 percent) H202 is toxic to the microorganisms [35].
The primary limitation in the use of this technology at LUST sites is the lack of
specific information regarding the concentrations of H202 which are optimal for
site specific conditions. The technology would be more costly than use of biore-
mediation alone due to the cost of the hydrogen peroxide; however, the reduction
in cleanup time and the ability to degrade more complex contaminants may result
in a net cost savings. The method could also be cost prohibitive if large quantities
of H202 are required due to a large contaminant concentration or other organic
matter in the soil that would consume the H202.
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On the plus side, hydrogen peroxide is the preferred oxidant for in-situ treatment
of contaminated soil because it is a liquid and is miscible with water, is relatively
inexpensive, and does not persist in the environment. Although it can be added to
the environment at high concentration, it can be injurious to microorganisms.
Even in somewhat lower concentrations, it can inhibit the growth of microor-
ganisms [11].
The greatest difficulty in the use of hydrogen peroxide (or any other chemical oxi-
dants) in situ is that the oxidant will not selectively oxidize the desired contaminant
but rather will react with organic substances, toxic or benign, in the soil, thereby
consuming H202. This makes the quantity of oxidant required prohibitively expen-
sive in some cases.
The oxidation of some PAHs in dry soil by ozone has been observed in laboratory
tests. Phenanthrene was completely removed, while pyrene and chrysene were
partially removed (33% - 94%) (49). Experiments for the ozone treatments of
contaminated soils have been conducted in laboratory and pilot scales. In labora-
tory, two kilograms each of PAH (2,250 mg/kg) and mineral oil (12,000 mg/kg)
contaminated soils were tested. The PAHs were reduced to 2.9 mg/kg in 20 days
and mineral oil was reduced to 3.5 mg/kg in 60 days. The pilot scale test was
conducted on 3 tons of contaminated soil, no test result is available. An in-situ
treatment was just started at a former gasoline station (51).
EX-SITU SOIL TREATMENT
In the literature reviewed, it was found that both hydrogen peroxide and
ozone/UV have been used in integrated treatment systems for treating con-
taminated soil. One of the difficulties in evaluating the application of chemical
oxidation to ex-situ treatment of soil is that few of the literature references discuss
applications to LUST types of contaminants. The discussions below emphasize
what fuel hydrocarbons and similar materials have been tested, but also draws
upon work with other compounds which may be relevant in evaluating LUST rem-
ediations.
Experiments with hydrogen peroxide (Fenton's reagent) involved mainly per-
colating a peroxide solution through a bed of the contaminated soil. These were
conducted at neutral or slightly basic conditions, up to a pH of 9 [39, 41, 42].
After 15 to 20 days of treatment of the contaminants under consideration, concen-
trations had fallen to the level at which bioremediation or landfarming could begin.
Benefits included accelerated degradation, production of intermediate products
which were more amenable to biological attack (primarily due to reduced struc-
tures which could more readily be absorbed into the cell), and reduction of con-
taminant to levels below that toxic to the bioorganisms.
Another study [43] blended a peroxide solution in with the contaminated soil using
rototilling. This likely resulted in volatilization of substantial amounts of the con-
taminant (gasoline, in this case), and very little actual oxidation occurred. One of
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the difficulties in dealing with ex-situ treatment of soils from UST sites is volatil-
ization, which can occur even during the excavation process. Any further handling
and soil turning or cultivation during treatment processes can result in almost total
loss of volatile constituents. Since volatilization is not an oxidation process, such
experiments cannot be utilized to evaluate chemical oxidation.
A pilot scale test was conducted on VOC (volatile organic compound) contaminat-
ed soil with aqueoue H20:. Seven cubic yards of the soil was agitated with 1,000
pounds of 35% H:0? for 15 minutes and let the mixture stood for one hour. The
concentration of methylene chloride reduced from a maximum of 14,000 ug/kg to
under the detection limit (11 ug/kg), toluene from 25,000 ug/kg to less than 5
ug/kg ethyl benzene from 24,000 ug/kg to less than 5 ug/kg, xylene from 133,000
ug/kg to less than 5 ug/kg (52).
UV/ozone offers some unique advantages for chlorinated aromatic compounds,
such as pesticides and herbicides. For example, UV most readily attacks the chlo-
rine-carbon bond and ozone attacks the aromatic ring. Most work with UV/ozone
has been with water solutions. One ex-situ treatment involves washing soil with
water, followed by UV/03 treatment of the leachate. Thus, if complicated chlori-
nated organics or polynuclear aromatic hydrocarbons (PAHs) are present, UV/-
ozone may be a viable alternative [16, 39, 44],
Generally, LUST soils contain simpler hydrocarbon fuel materials, which are readi-
ly degraded by less expensive methods. However, final treatment of soil washing
solutions prior to disposal (for any solution not being recycled) may benefit from
chemical oxidation to treat to very low concentration levels, providing an alterna-
tive to carbon beds.
The information found indicated that H202 did not persist in the environment and
did not form toxic products. Ozone, if present in the off-gas, must be treated prior
to release to the ambient air.
The ex-situ oxidation technology appears to be promising and may have a cost
advantage in cases where soils contain chemicals that are difficult to degrade.
Considerable work is necessary to identify the best conditions for the use of this
technology.
TREATMENT OF AIR GENERATED FROM SOIL VAPOR EXTRACTION
(SVE) SYSTEMS
Finally, soil vapor extraction (SVE) systems generate air streams which contain
organic contaminants that require treatment prior to release to the atmosphere.
At high organic concentrations, combustion is normally utilized. At lower organic
concentrations, carbon adsorption is typically used to remove the contaminant prior
to disposal. UV technologies may also be used to treat air streams containing
contaminants in the lower concentration range. The reason for substitution of UV
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technologies for carbon adsorption is that there may be a cost savings in some cases.
The literature contains several reports which describe bench- and pilot-scale as-
sessments of chemical oxidation technologies. These systems include the Photolytic
Oxidation Process by Purus, Inc. [45, 46], and the TiO,/Photolytic Air Treatment
System by Nutech Environmental [47, 48J. These systems use air (oxygen) or UV
and TiO:/UV.
Another bench-scale experiment proposed pumping ozonated air or oxygen instead
of air into a conventional vapor extraction system [49]. Ozone may present an
environmental hazard if not treated prior to release. No toxic reaction products
were identified. The processes are fairly complex and it is not clear if they could
handle surges in contaminant release as a conventional carbon adsorption system
could. The rationale for the use of these s>stems is that the high capital cost of a
carbon adsorption unit could be avoided in certain circumstances. However, no
cost data was identified to confirm this. In addition, no adaptation is necessary to
develop this technology for use at LUST sites.
CONCLUSIONS AND RECOMMENDATIONS
The primary data gaps for three of the four treat operations (except ex-situ water
treatment) is the lack of cost data and the lack of field experience at LUST sites.
The lack of cost data and field experience are serous impediments for selecting
these potentially useful technologies.
CONCLUSIONS
The following conclusions can be made regarding the potential use of the oxidation
technologies in each of the four LUST treatment operations.
Ex-situ water treatment using UV/H-,0?/Q, and UV/H-.Q-, technologies.
•	Commercial s>stems using these technologies are in use.
•	The technology appears best for low concentrations of contaminants
that are not amenable to biological treatment.
•	Other systems using UV/03, 03/H202, and UV/laser/H202 are being
investigated.
•	Market forces will determine the most cost effective technology.
In-situ or ex-situ soil and water treatment using aqueous H,Q?.
•	The technology has been field or pilot scale tested, the characterstics of
the site, contaminated soils and water are not clear.
240
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•	The conditions for the reactions to occur are not understood.
•	Considerable work is needed to identify the best conditions and soil or
water types for use of this technology.
•	The possibility of the detoxification of high molecular weight com-
pounds is unknown.
In-situ soil and water treatment using H^Or/bioremediation.
•	The technology has been field tested with inconclusive results.
•	The mechanisms are complex and not well understood in site-specific
applications.
•	Considerable work is needed to identify the best conditions and soil
types for use of this technology.
•	Cost versus other alternatives is a major unresolved question.
•	Very low pH (2-3) and higher pHs (9) as studied for Fenton's solution
application would be difficult to attain for in situ applications.
*	In-situ or ex-situ soil treatment using O,.
•	The preliminary experiments showed that this technology is very effec-
tive for the removal of some semi-volatile organic compounds (SVOCs)
with inconclusive results.
•	The reaction mechanisms are not understood.
•	Considerable work is needed to the applicability of this technology for
various compounds contained in USTs.
•	Cost and effecxtiveness versus other technologies have to be evaluated.
*	Ex-situ soil treatment using H.O-, or t IV/Ov
•	With ex-situ treatment, losses of the contaminant due to volatilization
can distort the reported results.
•	Considerable work is needed to identify the best conditions for use of
this technology.
•	The technology appears best for low concentrations of contaminants
that are not amenable to biological treatment.
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•	Cost versus other alternatives is a major unresolved question.
Treatment of air from SVE systems using UV. UV/TiCX:
•	Technologies destroy contaminants completely.
•	The technology appears best for low concentrations of contaminants
that are not amenable to carbon adsorption/desorption or incineration.
•	Lower capital costs may give technology a cost advantage over alterna-
tives.
•	Work is needed to identify the best conditions for use.
RECOMMENDATIONS
Technologies using UV/H202 and UV/ozone to treat contaminated water streams
are well advanced. Sufficient cost information generally is available so that market
forces will determine which technologies are preferable and in which situations.
For in-situ soil and water treatment using H202/bioremediation, the technology
appears to have potential in certain circumstances. However, since the mechan-
isms, best conditions for use, and cost parameters are uncertain, further inves-
tigation is recommended.
For ex-situ soil treatment using H202 or UV/03, the technology appears to have
potential in certain circumstances. However, since the best conditions for use and
cost parameters are uncertain, further investigation is recommended. Ex-situ ap-
plications should minimize soil movement to reduce contaminant volatilization.
Soil washing applications, possibly in conjunction with a subsequent H?02 enhanced
bioremediation step, thus appear to be the most fruitful for additional work.
The use of H,02 and 03 for in-situ and ex-situ treatments of soil and water have
shown faster remediation possibility. However, the data were inconclusive. Fur-
ther investigation on the applicability of these technologies to all the constituents
contained in UST and the conditions necessary for their effectiveness is recom-
mended.
For treatment of air from SVE systems using UV or UV/Ti02, the technology ap-
pears to have potential for treating low concentrations of contaminants that are not
amenable to other conventional treatment methods. Since the best conditions for
use and cost parameters are uncertain, further investigation is recommended.
Stringent air emission regulations may provide further impetus to apply chemical
oxidation in these applications. The adaption of these technologies for the treat-
ment of the off-gas from steam stripping and radio frequency heating also need
further investigation.
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ACKNOWLEDGEMENT
A portion of the information contained in this paper was collected by IT Corpo-
ration, to whom acknowledgement is made. It is a pleasure to thank Dr. Michael
L. Taylor, Mr. Larry M. Southwick and Edwin A. Pfetzing of IT for their evalua-
tion of the collecting information and Dr. S. Krishnamurthy and Ms. Esperanza P.
Renard of EPA for their helpful discussions and critical review.
DISCLAIMER
The information in this paper has been funded wholly or in part by the United
States Environmental Protection Agency under Contract No. 68-C2-0108 to IT
Corporation. It has been subjected to the Agency's administrative review. Men-
tion of trade names or commercial products does not constitute endorsement or
recommendation for use.
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