United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-89/007 Aug. 1989
&EPA Project Summary
Novel and Simple Approach to
Elimination of Dilute Toxic
Wastes Based on
Photoelectrochemical Systems
G. Cooper and A. J. Nozik
This work investigated several
basic parameters that are important
for extending photocatalytic purifi-
cation of water contaminated with
trichloroethylene (TCE) from the
laboratory to the field. Some of these
variables strongly influence the
decomposition kinetics of the TCE.
Parameters investigated were the
effect of solution pH, initial TCE
concentration, presence of naturally
occurring ions, temperature, and
presence of "color bodies." It was
found that the photocatalytic decom-
position of aqueous TCE occurs more
than twice as fast in the pH range 6.4
to 9 than in the range 3.4 to 5. The
rate of TCE decomposition was
shown to be strongly dependent on
its initial concentration. Photoelectro-
chemically nonreactive ions such as
Ca + 2, Mg + 2, and SO42 did not play a
measurable role in the reaction.
Temperature vs TCE decomposition
rate constant data exhibited non-
classical dependence by yielding an
activation energy of about 2 kcal/
mole and a pre-exponential factor of
about 1 min-1. The decomposition
rate of TCE was not seriously
diminished by the presence of 0.059
weight percent of powdered iron
oxide color bodies. Additionally,
various photocatalyst materials were
also tested but titanium dioxide
exhibited superior activity in decom-
posing TCE. In this laboratory study
low intensity irradiation having the
equivalent intensity of 1/4 to 1/5 Solar
at AM1 was employed in order to
exemplify the feasibility of large scale
water purification utilizing natural
solar light
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fulty documented in a
separate report of the same title (see
Project Report ordering information at
back).
Identification of Problem
TCE is a ubiquitous soil and ground-
water contaminant. Its widespread
appearance in the United States
environment is a result of causal disposal
practices and TCE's popularity as an
industrial solvent. Once in the soil, its
degradation does not occur at an
appreciable rate. The action of rain, snow
melt, and underground waters percolating
or passing through the ground gradually
cause the dissolution and migration of
TCE from the original site of contam-
ination into the general environment. The
contaminant plume contributes to ground
and surface water contamination far
removed from the original dump site.
TCE and other halocarbons have been
shown to be potentially deleterious to
animal health and, by extension, human
health as a carcinogen and/or mutagen,
and it is associated with adverse effects
on the heart, liver, kidney, and immune
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and nervous systems at elevated con-
centrations. The adverse effects of low
level, long duration ingestion of TCE-
contaminated water has most recently
and dramatically been litigated. Although
the causal relationship between lengthy
exposure to extremely small concentra-
tions of TCE and adverse health effects is
still being debated by the technical
community, clearly it is a problem whose
time has come for solution.
Prior Cleanup Methods
Various methods, both chemical and
physical in nature, have been proposed
and tested for the cleanup of volatile
organics (VOCs) at disposal sites. The
cleanup problem can be divided into two
general areas of interest; (a) cleansing or
removal of soil laden with these organics,
and (b) cleansing contaminated waters
originally derived from these soils. For
example, in one method pressurized
steam injected beneath the soil was used
to volatilize the organic contaminants.
The resulting condensate containing the
VOCs was further treated with activated
charcoal. However, non-optimal adsorp-
tion isotherms for the charcoal-aqueous
interphase resulted in inefficient transfer
of VOCs from the aqueous phase to the
charcoal phase. Other methods of
extracting the VOC from the soil into an
aqueous phase exist, but the subsequent
problem of their destruction has not yet
been surmounted. In a method for
removing the aqueous VOC, the contam-
inated water is vigorously sparged with
air. The attempt of the system's highly
dispersed aqueous phase to maintain the
volatiles equilibrium vapor pressure with
the unsaturated air results m efficient
transfer of volatiles from the water to the
air. Photochemical reaction in the atmos-
phere supposedly results in the degrada-
tion of these volatiles. However, this is
tantamount to converting water pollution
into an air pollution problem. Clearly, the
mitigation of contaminants at the source
by chemical destruction is the best way
to guarantee diminution of pollution in the
general environment.
Photocatalytic Cleanup Method
A new technology, exhibiting dramatic
laboratory successes, has been shown to
effect the complete mineralization of
aqueous chlorinated hydrocarbons such
as TCE, trichloromethane, carbon tetra-
chloride, pentachlorophenol and others
into carbon dioxide and hydrochloric
acid. In this process, solar or artificially
UV irradiated semiconductor powders
suspended in the water photocatalytically
oxidize the organic contaminants. Water
purification based on this technology has
the potential of being uncomplicated and
inexpensive. This is because the semi-
conductor powders only have to be
suspended in the water and the sus-
pension provided with sufficient mixing to
ensure replenishment of the depleted
reactants at the particles' surfaces. Addi-
tionally, the powders are inexpensive,
recyclable and nontoxic.
The energetic considerations of the
process are the following: By absorbing
photons having energy equal to or
greater than the band gap, the semi-
conductor creates electron/hole pairs.
These carriers can then transfer out of
the semiconductor into the surrounding
solution to do redox chemistry; alter-
natively, they may recombine, with the
photon energy becoming lost to thermal-
ization. In order for chemistry to proceed,
the following thermodynamic conditions
must be met: (1) The energy levels of the
electron acceptor must be below the
conduction band edge. (2) The energy
levels of the hole acceptor must be
above the valence band edge. (3) Redox
couples satisfying the above conditions
must exist simultaneously. For titanium
dioxide (anatase), the valence band edge
is at +3.0 V vs NHE. Holes with this
energy have oxidation potentials large
enough to attack most organic bonds.
The electrons are injected at the
conduction band edge at -0.3 V vs NHE
where dissolved 02 is first reduced to 02~
and then undergoes reactions resulting in
the production of the powerful oxidizing
radical species HO- and HOO-. The
photoinduced production of the powerful
oxidizing holes and HO- and HOO-
radicals is the reason why titanium
dioxide is effective at photodecomposing
such a wide class of organic molecules.
The satisfaction of the above three
thermodynamic conditions is necessary
but not sufficient to ensure that a given
reaction will proceed at an appreciable
rate. The oxidation of organic molecules
in the presence of Oj is energetically a
"downhill" process and should occur
spontaneously. This reaction may not
occur if the kinetics are slow. However,
the reactions proceed rapidly in the
presence of irradiated, naked titanium
dioxide .
Objectives of This Study
The experiments in this study were
performed with the objective of
elucidating the effect of certain basic
parameters on the photocatalytic decom-
position of aqueous TCE expected under
field conditions. These vari;
(temperature, ions derived from diss
minerals, solution pH, and initial
concentration, and turbidity) had not
previously investigated. If these p
eters exhibited severe practical re
tions to water purification in the
situation existing in this laboratory i
this would indicate the use of caut
committing future resources to
scale purification of industrial or n
ipal water sources by this technolog
the other hand, if this investk
uncovered no serious limitations (
process with respect to the var
discussed above, further scale
studies employing contaminated r
waters or industrial effluents wou
appropriate.
Results
Effect of Dissolved Ions
There was no measurable effect
photocatalytic decomposition rate c
caused by the presence of Ca + 2,
and SO4-2 ions in solution hav
combined ionic strength of 0.01
presence of up to about 1.5 x 1
dissolved CO2 did not appear to
the decomposition of TCE.
EffectofpH
The photocatalytic decomposit
TCE occurs at twice the rate in
range of 6.4 to 9.2 than in the rar
to 5.0.
Effect of Temperature
The photocatalytic decompositii
of TCE was not strongly depenc
temperature. For example, increas
temperature by about 30 °C cai
factor of 1.4 increase in reaction
classical reaction having a 25 kc
activation energy would exhibit i
mately a 63 fold increase in reactic
Effect of Initial TCE
Concentration
The photocatalytic decompositi
of aqueous TCE was shown
strongly dependent on the initi
concentration. This means that, c
the decomposition rate can be de
by an apparent first-order expoi
decaying equation, the reaction is
not first-order according to the
definition. For example, the valu
for 100 and 20.8 ppm initii
concentrations are 0.0169 and
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miir1, respectively. This means that
water contaminated by an initial con-
centration of 20.8 ppm TCE is purified
almost three times as fast as water
containing 100 ppm TCE.
New Materials as Photocatalysts
The new materials tested for activity in
photocatalytically decomposing TCE
were Fe203, SrTiO3, WO3, and SrZr03.
All of these compounds, except for the
latter, have been shown to exhibit photo-
electrochemical activity. As photo-
catalysts for the decomposition of TCE,
they were all demonstrated to be less
active than Ti02 by several orders of
magnitude.
Effect of 'Color Bodies'
By introducing 0.059 wt% of powdered
Fe203 to serve as a potentially interfering
'color body1 into the 0.10 wt% Ti02
slurry, the photocatalytic decomposition
rate for TCE was decreased by approxi-
mately 32% of the rate for TCE decom-
position in the absence of Fe203. The
decrease in the decomposition rate is
attributable to light absorption and scat-
tering by the powdered Fe203.
Recommendations
The laboratory results discussed
above, obtained with TCE-spiked deion-
ized water, demonstrate that a potential
exists for employing photocatalysts for
the removal of TCE from contaminated
waters. However, the effects of variable
light intensity, reactor design, and matrix
constituents in naturally occurring waters
must be investigated prior to a pilot-scale
investigation.
Light Intensity Dependence
The light employed in these experi-
ments had UV intensities approximately
one fourth to one fifth of the solar
intensity at AM1. It would therefore
appear that a photocatalytic purification
system could potentially be operated in
most parts of the United States.
Assuming the absence of diffusion limi-
tation, the TCE decomposition rate
should be directly proportional to the light
intensity. However, commercial feasibility
may strongly depend on exactly how the
TCE decomposition rate depends on the
UV light intensity. Although this will
depend, to a degree, on reactor design
the proportionality assumed above should
be demonstrated.
Reactor Design
Diffusion limitation, which occurs when
reactants must diffuse through a signifi-
cant quiescent layer occurring micro-
scopically on the photocatalyst particle or
macroscopically within the reactor, must
be avoided by the generation of suitable
fluid turbulence. The generation of good
mixing is energy intensive, however, and
will constitute a principal operation ex-
pense. Therefore, in order to minimize
operating expenses over-mixing in the
reactor must be avoided. Rudimentary
reactor designs that optimize energy
usage for turbulence generation must be
investigated.
Effect of Matrix Constituents
An attempt to duplicate some of the
water constituents present in naturally-
occurring water was made in this project.
However, it is impossible to duplicate the
dissolved minerals, natural organic prod-
ucts resulting from decayed vegetation
and other sources, organic and inorganic
man-made pollutants, organic and inor-
ganic particulates, and other substances
which occur in natural waters in the
laboratory. Experiments should be per-
formed which ascertain the recyclability
of the photocatalyst in various naturally
occurring waters. This would be another
factor affecting this technology's com-
mercial feasibility. Another concern is the
presence of other aqueous organics
which could compete with the targeted
toxicant for the photogenerated oxidative
species that effect decomposition. There-
fore, the next logical step is to demon-
strate that the photocatalytic decomposi-
tion of TCE contained in natural waters
occurs at an appreciable rate. This
successful demonstration would be a
very important advance towards applying
this technology to the decomposition o?
aqueous toxicants that pollute enviror-
mental waters.
The full report was submitted in ful-
fillment of Cooperative Agreement
CR813055 by Solar Energy Research
Institute under the sponsorship of the
U S. Environmental Protection Agency.
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G. Cooper and A. J. Nozik are with Solar Energy Research Institute, Golden, CO
80401.
T. David Ferguson is the EPA Project Officer (see below).
The complete report, entitled "Novel and Simple Approach to Elimination of Dilute
Toxic Wastes Based on Photoelectrochemical Systems," (Order No. PB 89-161
855/AS; Cost: $13.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
~ A p
- U .L
Official Business
Penalty for Private Use $300
EPA/600/S2-89/007
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