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US EPA Office of Research and Development
United States
Environmental Protection
Agency
EPA/540/F-01/502
April 2001
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology Bulletin
Photoelectrocatalytic Degradation and Removal of Organic and Inorganic
Contaminants in Ground Waters
Study Performed at the University of Wisconsin - Madison Water Chemistry Program
Technology Description: The University of Wisconsin - Madi-
son (UW-Madison) is developing a photocataiytic technology
that uses titanium dioxide (TiO2) suspensions to coat various
supports used in aqueous treatment applications. Photoca-
talysis involves the use of UV light to illuminate the surface of
a catalyst such as TiO2. As the photocatalyst absorbs near-
UV light, electrons in the valence band are excited into the
conduction band and produce highly reactive electrons and
holes that promote oxidation of organic compounds. In this
project researchers sought to overcome some of the challenges
of photocatalysis by developing a "biased" photoreactor. A
metallic substrate was coated with TiO2 to prepare a
photoelectrode which was then combined with a cathode to
form an electrolytic cell. UV light illuminates the TiO2 coating
on the electrode and allows the electrolytic cell to serve as a
biased photoreactor, which was the subject of this study. A
design for a biased photoreactor in an annular configuration is
shown below. A similar system was employed for this project.
In this approach, coating a conductive substrate (metal foils in
this project) with a thin film of TiO2 (< 1 micron thick) allows a
positive potential to be applied across the catalyst, producing
a photoanode. Electrons produced when the catalyst is illumi-
nated flow preferentially to a separate cathode. This approach
provides several advantages over the conventional photocata-
iytic oxidation (PCO) process.
1) By forcing photogenerated electrons and holes to move to
separate electrodes, electron-hole recombination within the
catalyst is minimized, thus increasing the rate of oxida-
tion of organic contaminants.
2) In conventional PCO, photogenerated holes oxidize dis-
solved organics in the waste stream. However, another
species, typically dissolved oxygen, is required to remove
photogenerated electrons or else the process will shut
down. In a biased photoreactor, the electron-accepting (re-
duction) reactions occur on a separate electrode (the cath-
ode). Consequently, several species in the test solution
can act as electron acceptors, possibly increasing the
overall rate of reaction.
3) In conventional PCO, many materials present in natural or
wastewaters can deposit on photocatalysts, deactivating
them. Specifically, dissolved metal ions such as copper
and silver react with photogenerated electrons to form zero-
valent metals that deposit (or electroplate) on the cata-
lyst. In a biased photoreactor, these reduction reactions
occur on one electrode whereas organic oxidation occurs
on a separate electrode, thus minimizing catalyst deacti-
vation. One might employ biased photoreactors to treat
mixed wastes containing 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 suggest two further ad-
vantages of this approach as compared with conventional PCO.
1) Solutions containing only a dissolved organic contaminant
could be treated in biased photoreactors with no oxygen
present, whereas conventional PCO requires dissolved oxy-
gen.
2) Solutions containing relatively high concentrations of dis-
solved salts (specifically NaCI) could be treated in biased
photoreactors, whereas saline solutions inhibit conven-
tional PCO. This phenomenon might extend to ions such
Water inlet
Outer wall of reactor
Water outlet
TiO2 coated titanium photoanode
Figure 1 . Biased photoreactor design employing a TiO2-coated
photoelectrode in which a sheet of titanium is coated with
TiO2, rolled into a cylinder, and placed inside the outer wall of
the reactor.
Recycled/Recyclable
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processed chlorine free.
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United States Environmental Protection Agency
National Risk Management Research Laboratory
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
EPA/540/F-01/502
BULK RATE
POSTAGE & FEES PAID
PERMIT No. G-35
as bicarbonate that are known to be problematic in the
UV/TiO2 treatment of contaminated groundwater.
In addition, the use of thin-film TiO2 photocatalyst coatings on
supports allows one to distribute the activating UV radiation
relatively uniformly throughout the reactor.This technology does
not require the addition of oxidants such as ozone or hydrogen
peroxide to achieve complete oxidation of organic contami-
nants. Because the catalyst adheres to a support, there is no
need for an additional unit to separate and recover the catalyst
from the purified water after the reaction is complete.
Waste Applicability: This technology is designed to treat
groundwater and dilute aqueous waste streams contaminated
with semivolatile organics, some dissolved metal ions, and
possibly some oxyanions. Organics are completely oxidized
to carbon dioxide, water, and halide ions. Dissolved metals
that plate out on the cathode can be stripped and subsequently
recovered. Oxyanions may be reduced to lower oxidation states
(e.g., perchlorate to chloride).
Status: The UW-Madison photoelectrocatalytic technology was
accepted into the SITE Emerging Technology Program in'19957
The overall objectives of this study are to develop an effective
photoelectrode and to refine the reactor design to allow the
unit to treat dissolved metals as well as organic contaminants.
Material development difficulties limited this project to labora-
tory studies at a bench-scale level. Most studies were per-
formed with a surrogate waste consisting of a solution of for-
mic acid (usually 25 ppm as C) in 0.01 M NaCI.
Stable photoelectrodes with reproducible behavior were pre-
pared by coating TiO2 on titanium supports. Coated copper,
aluminum, and stainless steel supports could not withstand
the treatment conditions.Tests employing a bench-scale, flow-
through biased photoreactor with both electrodes placed in one
compartment indicated that the rate of oxidation of formic acid
increased 50% by applying potentials of +1 to +2 V across the
photoelectrode as compared with operating with no applied
potential. Under these applied potentials, 500 mL of waste were
treated by recirculation at 90 mL/min with a half-life of ca. 1
hour. Because the half-life of this reaction was directly propor-
tional to the initial concentration of the organic contaminant,
this technology appears to be best applied to aqueous solu-
tions containing low concentrations of organic contaminants.
Rates of reaction were similar when either oxygen or nitrogen
was bubbled through the solution. In one test in which copper
(II) nitrate was added to the surrogate waste, both copper and
formic acid were removed from the waste.
In early 2001 the EPA will publish an Emerging Technology
summary that describes the results in greater detail.
For Further Information:
EPA PROJECT MANAGER:
Vince Gallardo
U.S. EPA
Nat]qnaJ Risk Management Research Laboratory, , „„
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7176
Fax: 513-569-7620
Gallardo.vincente@epamail.epa.gov
TECHNOLOGY DEVELOPER CONTACT:
Marc Anderson
Water Chemistry Program
University of Wisconsin - Madison
660 North Park Street
Madison, Wl 53706
608-262-2674
Fax: 608-262-0454
nanopor@facstaff.wisc.edu
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