Introduction:
Electroless plating processes were developed in the 1950’s for metals such as
nickel-phosphorous, copper, cobalt and nickel-boron. This process is widely used to
finish steel, aluminum and plastic. [1] The most common use of these materials is for
plating electronics. The spent plating baths contain large quantities of the various metals,
and may become health risks if the solutions are released into the environment. TiO2
photocatalysis can be effective for removing these heavy metals from water by
photocatalytic reduction or photocatalysis. This basically involves a dissolved metal in a
positive valence state being reduced to metallic form, thus causing it to plate onto
something that can be removed from the water, a photocatalyst. TiO2 is the photocatalyst
in TiO2 photocatalysis. During this process, the water is oxidized to molecular oxygen.
Organics are involved in the process. Light intensity greatly affects the rate of reaction.
TiO2 can also be used to treat organometallic complexes.
Background:
Titanium dioxide is found in many mineral forms. It is the fourth most abundant
metallic mineral in the earth’s crust. [3] It is readily available, and inexpensive to get
from the earth. Titanium is commonly thought of as a very strong metal that is alloyed
with other metals. Chemists however, recognize the powerful oxidizing properties of
titanium dioxide. In fact, paint manufacturers must coat titanium dioxide with silica to
prevent it from oxidizing the resin base of the paint. If the TiO2 were left uncoated, paint
would break down in about two days [4]
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The process of photocatalysis is relatively simple. Light energy from ultraviolet
radiation (light) in the form of photons, below 390nm, excites the electrons on the surface
of titanium atoms suspended in the contaminated water, moving them from the valence
band to the conductance band. [2] Photoexcitation always requires photons of below
390nm. The result of this energy change is the formation of holes in the surface of the
titanium atom, and free electrons which are now available to form hydroxide (~OH), or
other radicals, which can oxidize organic chemicals, or reduce metal species. There are
currently two methods that can be used to perform TiO2 photocatalysis: slurry and fixed
phase reactors. In the slurry phase, TiO2 powder is added to the waste stream and
exposed to ultraviolet light. The light can come from sunlight or lamps. In the fixed
phase, the TiO2 is annealed to the surface of a supporting plate such as microporous
titainia ceramics. These can actually be used for liquid and gas streams. The ceramic
titanium dioxide nanoparticles are deposited as a porous film on a glass tube or used as a
particle aggregate in a packed bed reactor. Ultraviolet light is then used to activate these
semiconducting metal oxide particles.
Both techniques offer different advantages. For example, the slurry phase reduces
contaminant concentrations more readily than a fixed phase system, but requires a
secondary treatment to remove titanium dioxide from the aqueous solution.
Experiments:
Experiments have been performed to test this method of metal removal; TiO2
photocatalysis still seems to be in a developing phase. All chemicals used in any
experiment are reagent grade. Deionized water is also used for the test solution. Amounts
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of test solution range from 300 to 500mL. Intensity of the lights range from 2.9 to 46
mW/cm2 . A 1-ml sample is usually removed just prior to starting the ultra violet
illumination. This provides a starting point for photocatalysis. The sample can also be
used to determine how much metal absorbed onto the titanium dioxide. Control
experiments must be also performed. This is to verify that the photocatalyst was
responsible for all of the behavior observed in the experiment. Treated samples are
filtered through Teflon or Nylon syringe filters to remove any catalyst. Total Organic
Concentrations are then taken using analytical total organic analyzers.
The Solar Thermal Technology Department, Sandia National Laboratories,
Albuquerque, NM performed TiO2 photocatalysis experiments on solutions of metals:
Cd, Cu, Ni, and Pt, and atomic absorption were used for Hg and Ag. [5] A single glass
jacketed reactor vessel was used. Each experiment used 300-ml solution. The
photocatalyst was titanium dioxide powder suspended in solution at 0.1wt%. pH
adjustments were made using dilute HNO3 and NaOH. A Sylvania Par 38 mercury bulb
was used, providing 100 W. Additionally, an Oriel 100 W ozone free Hg (Xe) lamp was
used. Control experiments were performed to verify that the photocatalyst was
responsible for all the behavior observed.
Experimental Results:
Fig 1 shows the results of the experiment. It displays the fraction of metals
remaining in solution after adsorption and after exposure to UV radiation for 40 min.
Silver, chromium, mercury, and platinum were significantly reduced. Copper, cadmium
and nickel were not.
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Fig 1:
This experiment seems to be very thorough in its preparation. As a result,
the data obtained supports theory and seems very reliable. It is important to note that the
waste streams treated in the workplace may not be as simple as the ones used in the
experiment. Organometallics may be present or impurities may alter the properties of the
solution. It does however, give a sound foundation which can be used for further
research. The next step would be to move the process from the laboratory batch reactor to
the field scale.
Experimental results in general show that photocatalytic oxidation of organics and
reductions of metals are synergistic. This means that photocatalysis is an ideal for
application to waste water that naturally contains metals and organics.
From experiments, it has been shown that there is a role for organics in the
reduction of Chromium. It is suspected that organics donate electron directly to the
valence band of TiO2, resulting in high reduction rates: electrons from the organics fill
valence band holes leaving more conduction-band electrons available for reducing
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Chromium. Hydroxyl radicals oxidize other organics: they lower the rate of reaction.
Holes are only filled by formation of the hydroxyl radicals. This is why the total organic
carbon analysis is used for a process that is removing metals.
The practicality of removing metals by photocatalysis is often questioned when
thinking about the practical applications of photocatalysis. This is something to be
concerned about especially when considering solar sources of UV light since solar
concentrators would be very expensive to build at a site needing cleanup. This is another
reason why the help supported by organics is so important. High organic concentrations
tend to optimize reduction reactions. It is also important to note that a N2 purge has been
shown to help increase the effectiveness of TiO2 photocatalysis.
Another problem arises when the scope of metals to be removed from the
wastewater is increased. If we had a very complex polluted source of water with many
complex compounds, it may be possible that the metal is bound to an organic material.
These compounds occur in industrial waste. For example, phenyl mercury is composed of
two compounds that can be treated by photocatalysis: Hg (II) and benzene. Experiments
show that this compound is readily absorbed onto the TiO2. It is not fully understood why
this compound breaks down so well. It is a surprising result of an experiment since it was
common knowledge that organometallics are very difficult to treat. It may, however, lead
to further research on how to break down the organometallic waste.
Conclusions:
The relationship between the reduction of metals and the oxidation of organics is
shown through the basic oxidation-reduction cycle. This can be used by applying
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photocatalysis to water contaminated with reducible, toxic metals. There seems to be no
difference between driving a photocatalytic process using solar ultra-violet or artificial
ultra-violet light. The only important design factor is the light intensity. Process
efficiency shows a very nonlinear dependence on UV light intensity. This is very
important when designing a solar detoxification system that can is dealing with very toxic
metals in water. The treatment of metal waste streams can be an expensive and energy
consuming process. It is the price paid for the safe removal of hazardous materials from
water used to produce the products we use every day.
References:
1. Reiche, H., Dunn, W.W., and Bard, A.J. J. Phys. Chem. 83,2248-2251 (1979)
2. Venkatadri and Peters, Chemical oxidation technologies: Hazardous Waste and
Hazardous Materials, Vol. 10(2), pp 107-149 (1993)
3. Titanium Industries, Inc., 1996. Titanium & Titanium Alloys, Fairfield, NJ
4. Wilson, 1996
5. MR. Prairie,B.M. Stange, and L.R. Evans, TiO2 Photocatalysis for the Destruction of
Organics and the Reduction of Heavy Metals
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Titanium Dioxide Photocatalysis of Metals
ENVE 436-01
Hazardous Waste Management
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