vxEPA
United
Environmental Protection
Agency
Emerging Technology
Summary
Photoelectrocatalytic
Degradation and Removal of
Organic and Inorganic
Contaminants in Ground Waters
Photocatalytic oxidation offers a
means of remediating low concentra-
tions of organics in aqueous and air
streams. Commercial development of
this technology is limited by relatively
low rates of oxidation of organics in
aqueous systems and by fouling of the
catalyst by other components of the
waste stream. Results from this project
indicate that applying an appropriate
electric field across the photocatalyst
extends the range of applications for
this technology. The resulting "biased"
Photoelectrocatalytic reactor demon-
strates ca. 40-60% higher rates of deg-
radation of the test organic (25 ppm (as
C) formic acid) than are observed in
the corresponding non-biased reactor.
However, the overall rate of reaction is
still slow even when biased (a half-life
of ca. 1 hour). This biased photoreactor
successfully treated a waste contain-
ing both formic acid and dissolved cop-
per. In addition, the biased photoreactor
was not adversely affected by use in
either relatively saline media or in me-
dia containing no dissolved oxygen.
Non-biased photoreactors do not func-
tion under these conditions. Earlier stud-
ies of biased photoreactors employed
photocatalysts coated on conductive
glass. Because such photoelectrodes
may not be commercially viable,
photoelectrodes that were stable dur-
ing repeated use were prepared for this
project by coating the photocatalyst on
a metallic substrate.
Introduction
Many remediation technologies treat
relatively high concentrations of contami-
nants. Numerous laboratory studies sug-
gest that photocatalytic oxidation (PCO)
effectively treats low concentrations of or-
ganic contaminants (ca <100 ppm, either
by weight in water or by volume in air) [1-
3]. This emerging technology utilizes vis-
ible or near-UV light to activate a
photocatalyst that then reacts with chemi-
cal species at or near the catalyst sur-
face. Specifically, the photocatalyst must
absorb a photon with enough energy to
excite an electron from the occupied va-
lence band of the material to its unoccu-
pied conduction band (i.e., radiation with
energy greater than the band gap energy
of the material). Electron promotion also
produces a positive hole. If this
photogenerated electron-hole pair recom-
bines, heat is produced. However, if they
reach the surface of the material before
recombining, either with themselves or
other photogenerated electrons and holes,
they can then react with species in the
surrounding medium.
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In most PCO studies, the
photogenerated holes combine with wa-
ter to form highly reactive hydroxyl radi-
cals that then oxidize organic
contaminants. Under appropriate reaction
conditions, the organic contaminants are
completely oxidized to CO2, water, and
halide ions with minimal, if any, genera-
tion of undesired byproducts [2,3]. How-
ever, several constraints on this process
have limited commercialization of this tech-
nology for purifying water.
This project addresses three specific
constraints. 1) Low apparent efficiencies
of photon utilization (ca. 1-2% at best),
where efficiency refers to the number of
molecules degraded per incident photon.
Relatively low rates of oxidation result,
caused in part by electron-hole recombi-
nation within the photocatalyst. 2) Need
to remove photogenerated electrons from
the catalyst. Dissolved oxygen is gener-
ally employed for this purpose, but its
concentration in water is low (ca. 8 ppm
for room temperature water equilibrated
with air). The resulting low rate of elec-
tron removal by this small amount of dis-
solved oxygen may limit the rate of
oxidation that is observed in aqueous sys-
tems. 3) Catalyst deactivation. Many ma-
terials present in natural or waste waters
can deposit on photocatalysts, deactivat-
ing them. Dissolved metal ions are a ma-
jor concern because many metal ions
(e.g., copper, silver) react with
photogenerated electrons to form zero-
valent metals that deposit (or electroplate)
on the catalyst.
Applying an electric potential across a
thin film of the catalyst, to produce a "bi-
ased photoreactor", can minimize these
constraints. Such reactors employ sepa-
rate electrodes: a working electrode
coated with photocatalyst where holes
oxidize organic contaminants (the
photoanode) and a counter electrode
where electrons reduce other species (the
cathode). Applying a positive potential
across the photoanode pulls
photogenerated electrons to the cathode,
thus minimizing electron-hole recombina-
tion within the catalyst and increasing the
rate of oxidation of the organics. By sepa-
rating the electrodes at which oxidation
and reduction occur, concerns about de-
activation of the photocatalyst are allevi-
ated. Because the electron-accepting
(reduction) reactions occur on a separate
electrode, several species in the test so-
lution can act as electron acceptors, pos-
sibly increasing the overall rate of
reaction. One might even employ biased
photoreactors to treat mixed wastes con-
taining both organic contaminants and
dissolved heavy metals (and/or reducible
oxyanions such as nitrate or perchlorate)
and then reclaim the metals after they
deposit on the cathode.
Early studies of biased photoreactors
suggested two further advantages of this
approach as compared with conventional
PCO. 1) Solutions containing only a dis-
solved organic contaminant could be
treated in biased photoreactors even if
no oxygen was present, whereas con-
ventional PCO requires dissolved oxy-
gen. 2) Solutions containing relatively high
concentrations of dissolved salts (specifi-
cally NaCI) could be treated in biased
photoreactors, whereas saline solutions
inhibit conventional PCO.
Most of these early studies utilized elec-
trodes in which the photocatalyst was
deposited on conductive glass, a formu-
lation that may not be suitable for com-
mercial water purification systems. In this
research project we fabricated
photoelectrodes by coating the catalyst
on a thin piece of conductive metal. Initial
studies focused on selecting a substrate
for the photoanodes from copper, alumi-
num, stainless steel, and titanium. Then a
method was developed for fabricating ef-
fective and stable photoanodes on the
selected substrate. Finally, a bench-scale,
flow-through biased photoreactor contain-
ing an appropriate photoelectrode formu-
lation was constructed and tested to
elucidate its operating characteristics.
All studies were performed with titania-
based photocatalysts synthesized using
sol-gel processing techniques. Thin films
(<1 micron thick) of these materials were
coated on various substrates and then
heated to sinter the film to the substrate
and to obtain the desired crystal structure
in the film [4]. Although the small amount
of catalyst present in these thin films may
limit the rate of oxidation, titania strongly
absorbs the near-UV radiation required
to activate these photocatalysts. As a re-
sult, a film of titania only a few microns
thick will absorb all of the incident UV
radiation [5]. Therefore, the loss in photo-
catalytic activity associated with using ti-
tania films <1 micron thick is not as great
as initially expected.
At present, it appears that biased
photoreactors are best used with aque-
ous wastes that contain semivolatile or-
ganic contaminants (e.g., pesticides).
Aqueous wastes containing low levels of
organic compounds that are easily
stripped from water are probably better
treated by air stripping and passing the
resulting contaminated air through a gas-
phase PCO reactor.
Procedure
Synthesis of Photocatalysts
All photocatalysts were synthesized as
stable suspensions of nanoparticles (di-
ameters <10 nm) in either water or t-amyl
alcohol via the controlled hydrolysis of
the appropriate metal-alkoxide precursor
[4]. All alkoxides and alcohols were used
as obtained from Aldrich Chemical.
Preparation of Photoelectrodes
All tests conducted to select an effec-
tive metallic support and to determine
appropriate fabrication conditions em-
ployed 5x10 cm plates of the metals with
a thickness of 0.5 mm. All metals were
initially cleaned for 5 h in an ultrasonic
bath containing acetone with the bath
cycled on and off for 15-min periods. Stain-
less steel plates (but no other metals)
were then heated in air at 450°C to gen-
erate an oxide film that improved the ad-
herence of the photocatalyst.
After cleaning, all metal plates were
dip coated one time with the alcoholic
suspension of titania using a withdrawal
speed of 1.5 cm/min, air dried, and heated
in air at 450°C for 1 h. Following pretreat-
ment, the plates were dip coated with
aqueous suspensions of either titania or
mixed zirconia-titania. Other processing
variables tested were number of coat-
ings, firing temperature, and withdrawal
speed. For multiple coatings, the plates
were air dried and fired at the desired
temperature after each coating. X-ray dif-
fraction analysis of the coated titanium
plates indicated that the coatings con-
tained the anatase form of titania.
Preliminary experiments indicated that
titanium was the most appropriate sup-
port for use in the flow-through reactor,
as it was the only support material that
did not visibly degrade during testing, ei-
ther by pitting or causing the test solution
to change color [4]. Based on results of
these tests, a piece of pretreated titanium
foil (20x14 cm, 0.05 mm thick) was dip
coated two times with the aqueous sus-
pension of titania using a withdrawal
speed of 21.6 cm/min and fired in air at
300°C for 5 h after each coating to obtain
the photoelectrode used in the flow-
through photoreactor [6,7].
Testing of Photoelectrodes
Preliminary experiments to select a suit-
able metallic support and appropriate pro-
cessing conditions for preparing
photoelectrodes involved measuring the
catalytic activity of individual
photoelectrodes in a batch reactor. This
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reactor consisted of a rectangular boro-
silicate glass vessel placed on a mag-
netic stirrer and set 10 cm from two parallel
15-W fluorescent UV bulbs (F15T8-BLB
with a broad emission over 300-400 nm
centered at ca. 365 nm). The irradiance
at the photoelectrode was 1.35 mW/cm2
based on a single measurement with a
photometer. (Note, however, that the irra-
diance from the bulb decreases with in-
creased time of operation. In general, this
loss of irradiance is an inherent property
of fluorescent bulbs; however, for these
experiments this loss of UV output was
accompanied by a decrease in the rate of
the desired reaction over the same time
period.)
Electrical potentials were controlled by
a potentiostat operated in either a two- or
three-electrode configuration using the
photoelectrode (photoanode), a platinum
mesh (2x5 cm) counter electrode (cath-
ode), and a saturated calomel electrode
(SCE) as a reference when needed, as
shown in Figure 1. In this project, all volt-
ages are measured with respect to an
SCE (indicated as (vs SCE)), with the
exception of the test of the flow-through
photoreactor using a mixed waste.
Catalytic activities of photoelectrodes
were determined by monitoring changes
in the concentration of total organic car-
bon (TOC, but not including volatiles) in
70 ml of an aqueous solution of formic
acid (25 ppm (as C)) in 0.01 M NaCI
through which oxygen was bubbled con-
tinuously. Formic acid was selected be-
cause it is chemically unreactive in both
electrochemical (expect for applied volt-
ages > +2.75 V vs SCE) and direct photo-
chemical (except for incident radiation
with wavelengths < 260 nm) applications
Reference
Electrode
and is relatively nontoxic when diluted.
Initial tests confirmed that there was lim-
ited, if any, adsorption of formic acid on
these photoelectrodes and no loss of for-
mic acid through volatilization. TOC mea-
surements were employed because no
long-lived intermediates were expected
to form and our interest was in monitoring
carbon removal rather than in quantifying
any intermediates produced. Specifically,
catalytic activities were evaluated by moni-
toring the change in TOC concentration
in the test solution after exposure of the
photoelectrode to UV radiation for 3 h.
Fabrication of Flow-through
Photoreactor
The flow-through photoreactor was fab-
ricated concentrically around an 8-W fluo-
rescent UV bulb (F8T5BL). A 26-cm long
Pyrex glass tube (22-mm OD) placed
around the bulb served as the transpar-
ent inner wall of the reactor. The outside
wall was a 26-cm long Pyrex tube (45-
mm OD), providing a total reactor volume
of ca. 200 ml. The photoelectrode was
rolled into a cylindrical tube and fitted
inside the outer glass wall. The irradi-
ance at the photoelectrode was ca. 5 mW/
cm2. The cathodes were 15-cm long, 5-
mm diameter, reticulated vitreous carbon
(RVC) rods that contained 500 pores per
inch. Platinum wires attached to the rods
provided electrical contact. (Platinum wire
or mesh also act as effective cathodes
but were not used because of their ex-
pense. Smooth graphite sheets were
tested but released carbon into the test
solution. RVC rods are not the optimum
choice because they are relatively weak.)
Three cathodes were employed to obtain
Potentiostat
Cathode
Photoanode
Figure 1. Schematic diagram (not to scale) of batch reactor system employed to test activities of
photoelectrodes (photoanodes).
a relatively uniform electrical field through-
out the reactor. Teflon end caps held the
feed lines and cathodes. This reactor de-
sign was difficult to assemble without
leaks, so the reactor was disassembled
only after performing several tests once a
leak-tight seal was obtained [7].
Testing of Flow-through
Photoreactor
This photoreactor was evaluated by
recirculating a test solution from a 500-
mL reservoir. Initial tests and a study with
a surrogate mixed waste (25 ppm (as C)
formic acid and 50 ppm Cu(ll) (from cu-
pric nitrate) in 0.01 M NaCI) were con-
ducted at flow rates from 4 to 27 mL/min
even though mass transfer limitations
were present (i.e., formic acid degraded
faster at higher flow rates). In tests with
the mixed waste, it typically required ca.
7 h to degrade 80-90% of the TOC and to
remove ca. 50% of the dissolved copper
at 12 mL/min and potentials up to +2.75
V. Clearly, this system can treat this waste,
although these results cannot be used to
estimate treatment rates in scaled-up units.
Later tests employed only formic acid and
showed a constant rate of removal at flow
rates between 63 and 127 mL/min. Most
tests were performed at 90 mL/min, typi-
cally requiring 3 h to remove 80-90% of
the TOC.
The atmosphere in the test system was
controlled by continuously bubbling ei-
ther oxygen or nitrogen into the test solu-
tion after cleaning and humidifying the
gas stream by passing it through deion-
ized water. When necessary, pH and/or
reaction temperature were measured in
the solution in the reservoir. All samples
were obtained from the reservoir.
Several characterization tests were per-
formed with the solution used for the batch
tests, 25 ppm (as C) formic acid in 0.01 M
NaCI. To determine a kinetic expression
relating the rate of reaction to the con-
centration of formic acid, formic acid con-
centrations between 4 and 70 ppm (as C)
were utilized. For the mixed waste test,
we did not verify the assumption that for-
mic acid and cupric nitrate did not react.
For all tests of the flow-through reactor,
the rate of removal of the contaminant of
interest was observed by measuring the
amount of contaminant remaining in the
test solution at a minimum of seven differ-
ent times, ideally with at least two mea-
surements obtained after 50% removal of
the contaminant. Throughout this project,
measured concentrations of two samples
obtained at a given time were averaged
to obtain individual data points (i.e., mea-
surements).
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Results and Discussion
Initial tests were performed to develop
an appropriate method for fabricating ac-
tive photoelectrodes that provide reason-
able reproducible performance. Results
of these tests are reported in references
4 and 6. However, representatives of the
U.S. Environmental Protection Agency
(EPA) have not assessed the quality of
the data underlying these reported re-
sults. These papers should be read with
this caveat in mind. Results presented in
the following sections are based on data
that have been verified by the EPA. Some
of the results presented herein are dis-
cussed further in reference 7.
Ability to Treat a Surrogate
Mixed Waste
In initial tests of the flow-through
photoreactor at flow rates of 4-27 mL/min
(i.e., residence times of 8-50 min in the
reactor per pass), higher activities were
observed as the flow rate increased, indi-
cating the presence of mass transfer limi-
tations. Consequently, several tests were
repeated at a flow rate that was high
enough (ca. 90 mL/min with a residence
time of some 2 min) to avoid these limita-
tions. Results of tests conducted at 90
mL/min are discussed later.
One key test conducted at a low flow
rate (12 mL/min) evaluated the ability to
treat a mixed waste solution [formic acid
+ Cu(ll)] at different operating conditions.
Copper metal was observed to deposit
on the three cathodes while formic acid
was oxidized on the photoanode. For just
this specific test, the applied potentials
were measured directly between the
photoanode and the RVC cathodes (i.e.,
a two-electrode configuration) instead of
utilizing a saturated calomel reference
electrode (i.e., a three-electrode configu-
ration). This approach was taken in part
to mimic the expected operation of a bi-
ased photoreactor in field applications,
where a two-electrode configuration
would likely be employed. However, be-
cause of this difference in operation, the
values of applied potential reported in
this section (not vs SCE) may not corre-
spond directly to the values of applied
potential reported elsewhere (vs SCE) in
this project summary.
Given the difficulty in obtaining a leak-
tight seal with the reactor design em-
ployed in these tests, the reactor was not
disassembled after each test and copper
was not cleaned off of the cathodes. As a
result, recirculating the acidic test solu-
tion (pH > 3) through the reactor before
turning it on caused some deposited cop-
per to dissolve, leading to somewhat dif-
ferent initial concentrations of dissolved
copper in each test. In spite of this con-
founding factor, the rates of removal of
copper and carbon appeared to follow
first-order kinetics. Up to 60% of the dis-
solved copper and over 80% of the car-
bon were removed in an 8-h period at
applied potentials between +1.0 and
+2.75 V. Direct comparisons of rates of
removal of these contaminants are not
feasible because of changes in irradi-
ance in the reactor during the ca. 60 h of
operation required to perform this set of
experiments.
In addition, these rate constants are
affected by the mass transfer limitations
in the reactor. However, the presence of
such limitations should not change the
conclusion that a surrogate mixed waste
can be treated with this configuration of a
biased photoreactor.
The remaining tests were performed
with just the formic acid test solution at a
flow rate (ca. 80-90 mL/min) high enough
to minimize these concerns. Increasing
the flow rate to either 85 or 125 mL/min
resulted in oxidation of ca. 80% of the
formic acid in a 3-h period rather than 5
to 6 h as required at a 12 mL/min flow
rate. In addition, because a given set of
experiments could be completed in 10-
15 h, changes in irradiance from the bulb
were minimized. Tests with the mixed
waste were not repeated at 90 mL/min
because of time constraints.
Kinetic Expression for TOC
Removal (at Constant pH)
Eventual scale-up of this system re-
quires understanding the kinetics of
photodegradation in this reactor, so the
effect of varying the concentration of for-
mic acid on its rate of oxidation was de-
termined. These experiments were
performed in oxygen with a potential of
+ 1.0 V (vs SCE) applied across the
photoelectrode. Although the mixed waste
experiments were modeled assuming
pseudo-first-order kinetics at a constant
initial concentration of contaminant, these
experiments required a more complex ki-
netic expression for a reasonable fit. The
following Langmuir-Hinshelwood-
Hougen-Watson expression proved suit-
able:
dCF/dt = -(kKFCF)/{S(1+[KF/S]CF)}
where k is a rate constant, KF is a con-
stant representing the adsorption of for-
mic acid on the titania photocatalyst, S is
a constant incorporating oxygen adsorp-
tion on the photocatalyst and the dis-
solved oxygen concentration in the test
solution, and CF is the concentration of
formic acid (as represented by TOC) at
time t.
Integrating this equation gives an ex-
pression for the half-life (t1/2, the time re-
quired to oxidize half of the formic acid)
of the reaction as a function of initial con-
centration of formic acid, CFO:
t1/2 = S In 2 / (k KF) + CFO / (2 k).
A plot of t1/2 vs CFO is linear (r2 = 0.9957)
and yields the following values: k = 0.0206
± 0.0025 mmol min-1 L~1 (equivalent to
0.247 ± 0.030 ppm min-1) and KF/S = 1.284
± 0.488 L/mmol-1 (equivalent to 0.107 ±
0.041 ppm-1). However, this analysis is
only valid when the pH of the formic acid
test solution remains near 3.2 during
photoelectrooxidation. Changes in pH
during the reaction affect both the rate of
oxidation of formic acid and the adsorp-
tion constant KF. These values are also
expected to change if one treats a mixed
waste rather than pure formic acid.
Effect of Applied Potential on
the Photodegradation of Formic
Acid
An advantage of operating at a higher
flow rate was that it allowed comparisons
of the rates of removal of formic acid
(monitored by TOC) as a function of ap-
plied potential. Rates of oxidation of TOC
were compared with the reactor operated
as a purely photocatalytic reactor (i.e., no
applied potential, which is not the same
as an applied potential of 0.0 V) and at
applied potentials of 0.0, +1.0, and +2.0
V (vs. SCE). As shown in Figure 2, rates
of oxidation increased as the applied po-
tential increased.
Close inspection of Figure 2 indicates
that little reaction occurred during the first
few minutes of operation. Therefore, rate
constants for the comparisons discussed
below were calculated without including
the data points at the start of the tests. In
addition, the last two data points for the
NAP test were excluded, as the reaction
essentially ceased after 150 min of op-
eration with a corresponding increase in
the pH of the test solution to above 5.
Table 1 presents the rate constants,
error estimates for those rate constants to
within a 95% confidence interval, and the
coefficients of determination (i.e., r2 val-
ues) for the fit of each set of data to a first-
order rate expression for each of the
conditions shown in Figure 2. Based on
the results of a least significant difference
multiple comparison procedure, it appears
that, to within a 95% confidence interval,
the rates of degradation for the pure pho-
tocatalytic reaction and any
photoelectrocatalytic reaction with an ap-
plied potential of 0.0 V or higher are dif-
ferent. On the other hand, in comparing
photoelectrocatalytic reactions at differ-
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Figure 2. Effect of applied potential on the photodegradation of an oxygenated formic acid test solution
(25 ppm as C in 0.01 M NaCI). (NAP = No Applied Potential)
ent applied potentials, there is no statisti-
cal difference at a 95% confidence level
in the rates of degradation at +1.0 V and
+2.0 V. No other comparisons were made
at this time. (The value of the rate con-
stant for an applied potential of +2.0 V
corresponds to a half-life of about 1 h for
25 ppm as C formic acid. Because half-
life is directly proportional to the initial
concentration of contaminant in this sys-
tem, this reactor is more effective for treat-
ing wastes with low concentrations of
organics.)
Effect of Operating Conditions
on pH in the Biased
Photoreactor
Colloid chemists have observed that
the properties of oxides such as titania
are strongly affected by pH when in con-
tact with water. The variations in oxida-
tion rate noted above correlate with the
pH of the test solution, which reached 3.2
after recirculation through the non-oper-
ating reactor. In the biased photoreactor,
the pH of the test solution remained con-
stant (at 0.0 V) or decreased slightly (to
2.9 at +2.0 V) during the test. Thus, the
fastest degradation of formic acid was
observed at the lowest pH at applied po-
tentials >+1.0 V, which also indicates
that hydrogen ions were generated in the
biased photoreactor because formic acid
was continually degraded during the ex-
periment. However, when the reactor was
operated in a purely photocatalytic mode,
the pH of the test solution continually in-
creased, reaching 4.3 after 100 min and
5.4 after 150 min, when degradation es-
sentially ceased.
When only the 0.01 M NaCI background
electrolyte was recirculated through the
reactor with no illumination and no ap-
plied potential, the pH of this solution
increased from 6.4 to 9.0, most likely
caused by an ion exchange reaction in
which the photocatalyst adsorbs chloride
ions and releases hydroxide ions. When
the salt solution was illuminated without
applying a potential or when a potential
was applied without illumination, the pH
remained near 9.0. However, under both
UV illumination and an applied potential,
the pH dropped dramatically over 30 min,
to 4.2 at 0.0 V and to 3.5 at +2.0 V.
Photogenerated holes can reach the sur-
face of the titania catalyst readily in a
biased reactor. Apparently these holes
react with surface hydroxyl groups and/or
adsorbed water molecules to release hy-
drogen ions to the solution, as suggested
above.
The pH changes described above were
not observed when saline solutions of
formic acid were tested because the for-
Table 1. Effect of applied potential on the pseudo-first-order rate constants, errors in these rate constants at a 95%
confidence interval, and coefficients of determination (r2) for the kinetic analysis of each data set shown in Figure 1.
Applied Voltage (V)
None (NAP)
0.0
1.0
2.0
Rate Constant (min "')
6.92 x 10-3
8.70 x 10-3
10.03 x 10-3
11.18x 10-3
Error in Rate Constant (min "' J
1.07xlO-3
1.00 xlO'3
1.41 xlO-3
1.48x 10-3
Coefficient of Determination
0.9930
0.9902
0.9899
0.9910
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mic acid buffers the test solution, although
the processes discussed above should
still occur. These processes have two spe-
cific implications for useful operation of a
biased photoreactor. 1) Because there
appears to be ion exchange between an-
ions in the test solution and the catalyst
surface, the composition of the test solu-
tion may affect the performance of the
reactor. In particular, phosphate ions,
which bind strongly to most oxides, may
inhibit reactions. 2) This biased reactor
was tested with acidic waste streams.
Because acid is generated in the biased
reactor during operation, it may be pos-
sible to treat non-acidic wastes in such a
reactor. However, this possibility may only
apply to oxygenated solutions. A reason-
able potential cannot be applied to the
reactor when only the deoxygenated sup-
porting electrolyte is present. On the other
hand, the formic acid test solution can be
oxidized in the biased photoreactor
whether or not oxygen is present. Appar-
ently hydrogen ions are reduced at the
cathode when no oxygen is present, thus
allowing the biased reactor to operate
normally. If this assumption is correct, then
treatment of non-acidic organic species
in biased photoreactors under anaerobic
conditions may require the addition of
acid from a separate source.
Conclusions and
Recommendations
Several conclusions result from this
study.
1. Photoelectrocatalytic (biased) reac-
tors can be employed to treat aque-
ous mixed-waste solutions.
2. Stable photoelectrodes can be pre-
pared by coating titania on titanium
supports.
3. Such photoelectrodes can be used
in saline solutions. These
photoelectrodes were tested in 0.01
M NaCI for this project. Further test-
ing at higher salt concentrations is
needed.
4. Reticulated vitreous carbon rods
serve as effective cathodes in biased
photoreactors. Platinum can also be
employed but is more expensive.
5. Because biased photoreactors gen-
erate acid during operation, only a
small amount of additional acid may
have to be added to a non-acidic
waste to obtain reasonable treatment
rates.
6. Biased photoreactors are relatively
unaffected by operation under
anaerobic conditions, at least for the
systems tested in this project.
7. The kinetics of degradation in biased
reactors is modeled better by a
Langmuir-Hinshelwood-Hougen-
Watson expression than by a simpler
first-order rate expression.
8. Rates of reaction in this biased
photoreactor are relatively slow with
half-lives for reaction on the order of
1 hour. Applications for this technol-
ogy will remain limited unless reac-
tion rates can be increased by at
least an order of magnitude over the
rates observed in this project for
photoelectrocatalysis.
This investigation generated several
observations and questions concerning
the operation of biased photoreactors that
lead to the following recommendations.
1. Because of the relatively slow rates
of reaction in biased reactors, they
are probably best considered for
treating wastes that contain relatively
low levels of contaminants (<100 ppm
might be a reasonable cutoff).
2. It is not clear that simply scaling up
the reactor will provide the desired
improvement in performance for this
reactor. One option is to replace the
15-W UV light source used in this
reactor with a 40-W bulb. This change
would lengthen the reactor but would
also require using larger cathodes.
Another option is to test different cath-
odes and cathode geometries. A third
possibility is to employ a photocata-
lyst that contains a small amount of a
dopant (e.g., 1 wt% niobium) that in-
creases the photoconductivity of the
catalyst.
3. The underlying processes that con-
trol the behavior of biased
photoreactors are rather complex and
require further elucidation, preferably
by research groups with a back-
ground in both electrochemistry and
the colloid chemistry of the interac-
tions between metal oxides and aque-
ous solutions.
4. Bench-scale tests of biased
photoreactors with actual waste
streams would be appropriate. Given
the apparent interactions of chloride
ions with the surface of the titania
photocatalyst, the ability of biased
photoreactors to treat waste streams
containing other anions should be
evaluated. Of particular interest are
anions that bind to metal oxides more
strongly than chloride (e.g., phos-
phate).
5. Alternative designs for biased
photoreactors should be evaluated.
Such alternatives include designs that
increase the turbulence of the flow
through the reactor and that place an
externally illuminated photoanode
around a cathode sitting at the cen-
ter of the reactor.
6. One consideration that was not stud-
ied in this project is to replace the
simple DC bias that we employed
with a modulated AC bias in an at-
tempt to partially disrupt the water
layer around the electrodes.
References
1. R.W. Matthews, J. Catal., 111 (1988)
264-272.
2. D. Bahnemann, J. Cunningham, M.A.
Fox, E. Pelizzetti, P.Pichat, and N.
Serpone, in Aquatic and Surface Pho-
tochemistry (G.R. Helz, R.G. Zepp,
D.G. Crosby, Eds.), CRC Press, Inc.,
Boca Raton, FL, 1994, 261-316.
3. M.R. Hoffmann, ST. Martin, W. Choi,
and D.W. Bahnemann, Chem. Rev.,
95 (1995) 69-96.
4. R.J. Candal, W.A. Zeltner, and M.A.
Anderson, J. Adv. Oxidat. Technol.,
3[3] (1998) 270-276.
5. J. Peral and D.F. Ollis, J. Catal., 136
(1992) 554-565.
6. R.J. Candal, W.A. Zeltner, and M.A.
Anderson, J. Environ. Eng., 125[10]
(1999) 906-912.
7. R.J. Candal, W.A. Zeltner, and M.A.
Anderson, Environ. Sci. Technol.,
34[18] (2000) 3443-3451 and Sup-
porting Material on the Web.
This Emerging Technology Summary
was prepared by principal investigators,
Marc A. Anderson, Roberto Candal, and
Walter Zeltner of the University of Wis-
consin and by the U.S. Environmental
Protection Agency Project Manager,
Vicente Gallardo.
The principal investigators can be con-
tacted at:
Environmental Chemistry and
Technology Program
660 North Park Street
University of Wisconsin-Madison
Madison, Wl 53706
Telephone: 608 262-2470
The U.S. EPA Project Manager can be
contacted at:
U.S. EPA (Mail Stop 481)
26 W. Martin Luther King Drive
Cincinnati, OH 45268
Telephone: 513 569-7176
Email: gallardo.vincente@epa.gov
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