Photocatalysis of Metals
Hazardous Waste Management
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.  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.
Titanium dioxide is found in many mineral forms. It is the fourth most abundant metallic mineral in the earth’s crust.  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 
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.  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 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 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.  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.
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.
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 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.
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 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