1. Electrocatalysis with a TiO2 coated foil produces appreciable levels of the germicidal species triiodide (I3-) from iodide (I-) at applied voltages between 0.80-1.0 volts. 2. Higher voltages between 0.80-1.0 volts resulted in greater production of I3-, while voltages at or below 0.5 volts produced only background levels. 3. The production of I3- was decreased by approximately 71% when the system was purged with nitrogen gas, indicating oxygen plays a role in one of the pathways for I3- synthesis.

1. Gamma irradiation of aqueous I- yields triiodide (I3
-) as a product,
along with a family of intermediate radicals, oxidants and reductants
(ROR) that function as potent germicides. We hypothesized that
when 0.8V-1.0V was applied to a chemically treated (CT) titanium
oxide coated foil, the system would also synthesize I3
- due to
electron ejection from the surface of the TiO2 semiconductor, and/or
the presence of 'oxidizing holes', which also appear on a ‘voltage
activated’ TiO2 surface. A TIO2 coated-foil was wired into a
complete circuit; submerged in a buffered I- solution, and a range of
voltages was applied to the coated foil for 5 minutes. I3
- absorbance
at lambda max 350nm was read for four unique foils with a Tecan
microplate reader and are reported here as an average of all foils as
Mean +/- SD (N) # of trials/foil: 1.0V=0.389±0.042 (5);
0.80V=0.146±0.033 (5); and 0.50V=0.0625±0.063 (5). Nitrogen
purging before and during the experiment decreased I3
- production
by ~71%. We conclude that in order to generate iodine germicidal
species, a CT-TiO2 coated foil is required along with a voltage of at
least 0.8V. Our methodology has significant safety and cost
advantages, when compared to gamma ray generation of germicidal
iodine species.
Electrocatalysis with a TiO2 coated foil yields appreciable germicidal iodine species from I-, at
applied voltages between 0.80-1.0 volts.
Alejandro Morales1, Patricia Barreto1, Jordan Brown1, Jose Barreto1,2
Green Technology Research Group1,Dept of Chemistry and Mathematics2, Florida Gulf Coast University, Fort Myers, FL 33965
Abstract Data
.
Results
Acknowledgements
Discussion/Conclusion
Introduction
Materials and Methods
This work has been funded by the Office of Naval Research (ONR)
grant # N00014-10-1-0927.
Possible mechanism for the production of I3
-
on a titanium (IV) oxide electrocatalyst
An Increased voltage increased I3
- production
References
Voltage, a TiO2 electrocatalytic surface, and
iodide anion are all required for I3
- production
I3
- production in our system, depends partly
on the presence of oxygen in ambient air
Figure 4: Both reactive electrons and oxidizing holes
can potentially produce I3
- via the pathways
delineated above. If direct reduction by electrons is
an important pathway in the mechanism, the most
likely electron acceptor is molecular oxygen from
ambient air. All other species in the aqueous solution
are very difficult to reduce. Note that removal of
oxygen in figure 3 greatly diminished I3
- production.
1. We only observed I3
- production in our system when all the three
variables were present (1) voltage, (2) TiO2 electrocatalyst, and
(3) iodide.
2. Higher voltage (up to 1.0 volt) produced more I3
-. 0.5 volts and
below, produced background values of I3
- .
3. Nitrogen purging before and during the experiment decreased I3
-
by ~71%.
We were able to produce I3
- electrocatalytically on a TiO2
semiconductor surface. Increasing the applied voltage from
0.5V increased I3
- production (although we did not increase
the voltage past 1.0V to prevent the electrolysis of water). Removal
of oxygen by purging the solutions with nitrogen gas greatly
decreased I3
- production. We hypothesize that O2 is the only species
in our solutions which can be easily reduced by free electrons from
the TiO2 surface, and superoxide anion production creates a new
pathway for triiodide anion synthesis (see fig 4). It is also possible
that 'oxidizing' holes contribute to triiodide synthesis.
Significance: The production of I3
- by electrocatalysis is important
because useful, short lived, germicidal intermediates appear to
be generated, which will enable us to disinfect water.
It has been reported that gamma radiation of aqueous concentrated
bromide and iodide anions produces isopolyhalogen anions (X3
-)
(Balcerzyk, 2011, Rahn 2002). It has also been shown that these
useful germicidal isopolyhalogen anions are very stable and can be
easily measured spectrophotometrically, (Mirdamadi-Esfahani,
2009). We hypothesized that TiO2 electrocatalysis could more safely
and inexpensively replace gamma radiation in producing I3
-. We now
demonstrate that electrocatalytic I3
- production can be engendered
with TiO2. We have also shown the dependence of the reaction to
oxygen by purging the system with nitrogen gas.
Beaker Protocol: A CT-foil, and an untreated Ti mesh (the mesh
served as a counter-electrode) were placed in a 10mL beaker. A Ti
wire was micro-welded to the metallic edge of a circular coated foil
and led to the negative output of a DC power supply (the coated foil
face has an edge of uncoated Ti metal). A separate Ti wire
connected the mesh and the positive output. 5mL of 100mM KI in
10mM K3PO4 buffer at pH 7.20 was then added to the beaker. A
silicon stopper was put on the beaker, the Ti wires were passing
through it; two needles were put through the stopper for nitrogen
purging, one in the solution (nitrogen in), and one outside of the
solution (nitrogen out). The solution was purged with nitrogen for
10 minutes before the reaction was started, nitrogen purging
continue for the duration of the reaction. 1.0 volts was applied to a
coated foil for 10 minutes. I3
- absorbance at lambda max 350nm
was read with a Tecan microplate reader.
Figure 1: 0.8 volts were applied to a chemically
treated Ti foil (coated with TiO2) in a solution of
100mM KI, 10mM potassium phosphate buffer at pH
7.2. When treated for 5 minutes the foil creates I3
-
only when all three variables (+++) are present. All
the other controls are below the background of the
instrument (red line), indicating that no I3
- was
produced.
Figure 2: Do to variations in CT foil synthesis, the
foils vary in their ability to create I3
- four separate
foils are shown above. At 0.5V no I3
- is created (the
red line is background absorbance) however at 0.8V
an average of 0.1456 absorbance units (350nm) was
generated by I3
- production. Note that at 1.0V a much
higher average absorbance of 0.3891 was generated
by I3
- production. The ‘well plate electrocatalytic cell
protocol’ was used for these experiments.
Figure 3: Using the ‘beaker electrocatalytic protocol’
1.0V was applied for 10 minutes to the CT foil
exposed to ambient air, yielding an absorbance of
0.3590 (green bar, Mean +/- SD N=3). The
experiment was repeated with nitrogen purging
before and during electrocatalysis, yielding 0.1055
absorbance (blue bar).
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
(---) (++-) (+-+) (+--) (--+) (-+-) (-++) (+++)
Absorbance(350nm)
Treatment ( Voltage, Electrocatalyst, Iodide)
(---)
(++-)
(+-+)
(+--)
(--+)
(-+-)
(-++)
(+++)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Foil 1 Foil 2 Foil 3 Foil 4
Absorbance(350nm)
0.5V
0.8V
1.0V
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Before Nitrogen Purging After Nitrogen Purging
Absorbance(350nm)
Before Nitrogen Purging
After Nitrogen Purging
Wellplate Protocol: A CT-foil, and an untreated Ti mesh (the mesh
served as a counter-electrode) were separated by a plastic tube within
a plastic well; (in a standard 24 well-plate). A Ti wire was micro-
welded to the metallic edge of a circular coated foil and led to the
negative output of a DC power supply (the coated foil face has an
edge of uncoated Ti metal). A separate Ti wire connected the mesh
and the positive output. 2.5mL of 100mM KI in 10mM K3PO4 buffer
at pH 7.20 was then added to the well. A range of voltages was
applied to a coated foil for 5 minutes. I3
- absorbance at lambda max
350nm was read with a Tecan microplate reader.
Balcerzyk, A., et al., 2011. Direct and Indirect Radiolytic Effects in Highly
Concentrated Aqueous Solutions of Bromide. J. Phys. Chem, 115,
4326-4333
Mirdamadi-Esfhani, M, et al., 2009. Radiolytic formation of tribromide ion
Br3
- in aqueous solutions, a system for steady-state dosimetry.
Radiation Physics and Chem., 78, 106-111
Rahn, R.O., et al., 2002. Dosimetry of ionizing radiation using an
iodide/iodate aqueous solution. Applied Radiation and Isotopes, 56,
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Materials and Methods Cont.