United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/079 Jan. 1988
Project Summary
Analysis of Modified Wet-Air
Oxidation for Soil Detoxification
Walter Unterberg, R. S. Willms, A. M. Balinsky, D. D. Reible,
D. M. Wetzel, and D. P. Harrison
This report presents the results of
research on wet-air oxidation as a
method for the destruction of hazard-
ous wastes. For organics in the pres-
ence of large amounts of water, the
water need not be vaporized during
wet-air oxidation, an attractive charac-
teristic for energy conservation. The
feasibility of using wet-air oxidation
was investigated in terms of the effects
of temperature, pressure, and the
presence or absence of soil on the
oxidation rate of three model
compounds.
Wet-air oxidation is a semi-
commercial process that has been used
to treat a variety of weakly toxic
chemical wastes and for the regener-
ation of activated carbon. In this study
wet-air oxidation research was carried
out in a 1 -liter batch reactor at temper-
atures from 130 to 275°C and pres-
sures from 703-1760x103 kg/m2 on
three substances: m-xylene, tetrachlo-
roethylene (TCE), and malathion. This
research was performed with and
without addition of soil.
Any attempt to balance the effect of
residence time and the cost of energy
requires an accurate description of the
oxidation kinetics for the compound or
waste stream in question. Because of
the sampling technique used during
this investigation and the inherent
nature of the wet-air oxidation process,
a variety of potential problems with the
interpretation and analysis of the raw
concentration-time data were encoun-
tered during the study. These included
vapor-liquid equilibrium effects, the
effects of sample withdrawal from the
batch reactor, and density variations
between the reactor and the sample
injector. Corrections for each of these
effects must be incorporated into the
analysis in order to extract the inherent
kinetic information. Considering these
factors, the results of the study showed
that malathion was destroyed quickly
even at low temperatures, with or
without soil. M-xylene required a
minimum critical temperature (200°C)
to react; adding soil slowed down the
reaction. TCE required a minimum
temperature of 250°C, then reacted at
a much slower rate than m-xylene. This
rate was unaffected by the presence or
absence of soil. Wastes that are dilute
to moderately dilute, e.g., 1-30%
oxidizable waste, can be economically
destroyed without a prior dewatering
step. The moderate temperatures of
wet-air oxidation, however, result in
long (minutes or hours) reaction times.
This Project Summary was devel-
oped by EPA's Hazardous Waste Engi-
neering Research Laboratory, Cincin-
nati. OH . to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The reauthorization of the Resource
Conservation and Recovery Act (RCRA)
in 1984 and the passage of the Com-
prehensive Environmental Response
Compensation and Liability Act of 1980
(CERCLA), commonly known as Super-
fund, have placed great emphasis on the
ultimate destruction of hazardous
wastes. Currently, the most commonly
used ultimate disposal method is incin-
eration. For wastes containing in excess
of 30% combustible organic matter,
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incineration is both effective and rela-
tively inexpensive. More dilute wastes,
however, typically require heating and
vaporization of large quantities of water.
The expense of providing the additional
energy requirements for dilute wastes
has provided the impetus to identify
alternative destruction methods.
Among the host of alternative destruc-
tion methods that have been proposed
is wet-air oxidation. Wet-air oxidation is
a semi-commercial process that has
been used to treat a variety of weakly
toxic chemical wastes and for the regen-
eration of activated carbon. As the name
implies, wet-air oxidation is the destruc-
tive oxidation of waste compounds by
dissolved oxygen in a moderate temper-
ature (130-400°C) aqueous phase. The
source of the dissolved oxygen is com-
pressed air. The process operates at
pressures of 703-2110x103 kg/m2 to
reduce vaporization of the aqueous
phase and raise the equilibrium
dissolved-oxygen content of the reaction
medium. Thus, wastes that are dilute to
moderately dilute, e.g., 1 -30% oxidizable
waste, can be economically destroyed
without a prior dewatering step. The
moderate temperatures of wet-air oxida-
tion, however, result in long reaction
times. Minutes or hours are required as
opposed to seconds required for
incineration.
Any attempt to balance the cost of the
residence time and the cost of energy
requires an accurate description of the
oxidation kinetics for the compound or
waste stream in question. Many previous
kinetics studies can be faulted in that
non-specific measures of the reaction
efficiency have been employed, such as
oxygen demand reduction and percent
destruction of the test compound after
a given time period. Transient kinetic
measurements are required to describe
the reaction rate and reaction mecha-
nism; both are requirements for devel-
oping an optimal process design.
Due to the sampling technique and the
inherent nature of the wet-air oxidation
process, a variety of potential problems
with the interpretation and analysis of
the raw concentration-time data became
apparent. These included vapor-liquid
equilibrium effects, the effects of sample
withdrawal from the batch reactor, and
seal failure in the sample injector valve.
Corrections for each of these effects
must be incorporated into the analysis
in order to extract the inherent kinetic
information.
The technical objective of the exper-
imental program was to measure the
time to 90% destruction of the three
selected hazardous waste compounds
with and without soils. The three hazard-
ous waste compounds selected for this
study included an aromatic compound
(m-xylene), a chlorinated organic
[tetrachloroethylene(TCE)], and a sulfur-
and phosphorus-containing pesticide
(malathion). The time required to achieve
90% destruction of these compounds
was measured as a function of temper-
ature, pressure, and the presence or
absence of soil. The goal of this exper-
imentation was a preliminary assess-
ment of the viability of wet-air oxidation
for the decontamination of soils. More
importantly, however, these experiments
were designed to identify potential
problems with the envisioned process
and to define possible process improve-
ments required to overcome these prob-
lems and limitations.
Observations
Modified Wet-Air Oxidation
Research
The following observations are based
on experiments conducted at tempera-
tures of 130 to 275°C and pressures of
703-1760x103 kg/m2 in an electrically
heated, stirred, 1-liter reactor with
analysis of unreacted contaminants by
gas chromatography (see Figure 1):
• Conventional wet-air oxidation can
destroy hazardous waste compounds
in water. Malathion was destroyed at
the lowest possible operating temper-
ature, 1 30QC, m-xylene required
about 50 minutes at 225°C and 1406-
1760x103 kg/m2, while TCE required
approximately 27 hours at 275°C and
1406x103 kg/m2. A "critical" temper-
ature was noted for both m-xylene
(200°C) and TCE (250°C), below
which the reaction was significantly
inhibited;
• Above the "critical" temperature,
reduced reaction times for m-xylene
and TCE were observed with
increases in temperature and
pressure;
• Adding soil (19.4 percent sand, 63.2
percent silt, and 17.4 percent clay) to
the reaction mixture resulted in
considerably longer reaction times for
m-xylene. At conditions identical to
the non-soil experiments, no reaction
of m-xylene could be detected in 48
hours. Increasing the reaction
temperature by 50°C to 275°C
resulted in a very long reaction time,
32 hours. In contrast, the soil had little
effect on reaction times for TCE.
Malathion reacted before the first
sample was withdrawn for analysis,
just as without soil. It is expected that
organophosphorus pesticides similar
to malathion will also be effectively
destroyed by wet-air oxidation even in
the presence of soil;
• For the three contaminants tested,
three different wet-air oxidation
mechanisms are postulated. Class I
(malathion) is probably a rapid hydrol-
ysis reaction whose products are
oxidized subsequent to this first step.
Wet-air oxidation is more than ade-
quate for dealing with this class of
compounds with or without soil. Class
II (m-xylene) is probably a free radical
chain reaction, apparently first-order,
with an induction period followed by
a rapid reaction phase ending in
complete oxidation. The addition of
soil which scavenges free radicals
results in very slow reaction rates.
Class III (TCE) applies to hydrogen-free
chlorocarbons, requires more severe
conditions to react, displays no induc-
tion period, and is not affected by soil.
This leads to speculation that, though
oxidized, these type of compounds
react according to some mechanism
other than a free radical based one.
Conclusions and
Recommendations
General
The wet-air oxidation process has
received promising technical validation
for three typical contaminants at the
research level and requires further work
in the areas of the reaction mechanisms
and pilot plant operation.
A feasibility and cost study should be
undertaken in which the wet-air oxida-
tion process is compared with existing
technologies and other novel treatment
processes in terms of further work
needed, speed of the treatment (which
indicates the severity of contaminants
that can be treated), and economics.
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Integrator
-—czzi
N/vN/V^/WS/^s/
/ -Liter
Stirred Reactor
with Furnace
00
M
M
a
Heat Traced Tubing
Rupture Disk
Ball Valve
Gate Valve
Check Valve
Needle Valve
Filter
Thermocouple
Diaphram Valve
Ftotameter
Figure 1. Experimental apparatus—wet-air oxidation.
Modified Wet-Air Oxidation
Research
• Conventional wet-air oxidation should
be considered for destruction of
organic wastes, present with or
without soil in fractions of 1 to 30
percent in water. The experiments
indicate that reasonably rapid des-
truction of the model compounds is
possible at low temperatures (130-
275°C) compared to incineration and
without vaporization of the water;
• A comparative cost study between
wet-air oxidation and incineration/
pyrolysis should be performed;
• Additional tests should be made to
determine the reaction mechanism of
TCE which does not appear to react
by a free radical chain mechanism;
Because malathion was effectively
destroyed with soil even under mild
conditions, evaluation of the wet-air
oxidation of soils contaminated with
other organophosphorus pesticides
should be conducted. The toxicity and
persistence of any potential partial
decomposition products of these
compounds should also be evaluated;
When conventional wet-air oxidation
yields slow reaction rates, it is recom-
mended that further studies be con-
ducted which evaluate the potential
for rate enhancement through: (a)
addition of free radical initiators such
as hydrogen peroxide, (b) addition of
metal ion or heterogeneous catalysts,
(c) generation of localized hot spots
within the reactor for free radical
formation, and (d) destruction in
combination with a more rapidly
oxidizing compound to take advantage
of possible synergistic effects;
When the presence of soils slows wet-
air oxidation reaction to unacceptable
treatment levels, further studies
should be performed to determine (a)
the relationship between soil concen-
tration and degree of reaction inhibi-
tion; (b) conditions or additives which
reduce the inhibition effect of the soil;
and (c) which soil component (sand,
silt, clay, etc.) impedes the reaction.
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Walter Unterberg is with Rockwell International (subsequently Environmental
Monitoring and Services, Inc.) Camarillo, CA 93010; R. S. Willms. A. M.
Balinsky, D. D. Reible. D. M. Wetzel, and D. P. Harrison are with Louisiana
State University, Baton, Rouge, LA 70803.
Hugh Masters is the EPA Project Officer (see below).
The complete report, entitled "Analysis of Modified Wet-Air Oxidation for Soil
Detoxification," (Order No. PB 88-102 397/AS; Cost: $11.95, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Releases Control Branch
Hazardous Waste Engineering Research Laboratory—Cincinnati
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
tm^us-OF£!£!£LM£
EPA
Official Business
Penalty for Private Use $300
EPA/600/S2-87/079
0000329
60604
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