Environmental Engineering Reference
In-Depth Information
However, cost-effective production of suficiently high photocatalyst surface area in con-
tact with water, and delivery of enough energetic photons to the semiconductor to activate
it, has proven dificult. Systems employing UV-activated TiO
2
slurries have been demon-
strated to be effective in breaking down most organic contaminants [11,12], but require
complicated, expensive systems for management of the slurry material. Puralytics uses
an order-of-magnitude increase in surface area in a ixed-bed reactor with a signiicant
improvement in mass transport over these slurry systems.
Optimized illumination sources are also needed for cost-effective water puriication
systems. At low UV intensities, less than ~3 mW/cm
2
at wavelengths below 400 nm, pro-
duction of hydroxyl radicals by UV-illuminated anatase TiO
2
photocatalyst is known to
be linearly proportional to the UVA intensity, while the production of hydroxyl radicals
has been reported [7,8] to increase sublinearly at higher UVA intensities. Most research
to date has been done with lamps illuminating a slurry. These lamps have typically been
low-pressure mercury lamps emitting at 254 nm or mercury “black light” lamps emitting
in the UVA band near 365-370 nm with limited optical lux and eficiency. LEDs are now
able to more eficiently emit a band or bands of light that can more optimally excite photo-
catalytic processes, with important advantages:
• The UVA intensity can be signiicantly increased without exceeding the range of
linear proportionality between intensity and hydroxyl radical production.
• The photocatalyst can be applied to a transparent, ixed substrate, increasing both
surface area and mass transport compared with slurry systems.
• LED illumination avoids the issues associated with using lamps.
24.6.2 Photocatalytic Reduction
Free electrons produced on the illuminated photocatalyst instantly react with many posi-
tive valence compounds, including heavy metals and inorganics, reducing them to a less
toxic, more elemental state. These reduced compounds demonstrate an enhance afinity
for adsorption to the TiO
2
surface, where further oxidation or deposition can occur. Many
inorganic compounds and heavy metals have been reported to photoreduce [8].
24.6.3 Photoadsorption
The light-activated photocatalyst strongly and irreversibly adsorbs heavy metals includ-
ing mercury, lead, selenium, arsenic, permanganate, and other toxic compounds. Previous
reduction reactions enhance this process. Heavy metals are permanently retained in the sys-
tem, and properly managed when the catalyst is replaced. While TiO
2
is already an excellent
medium for contaminant adsorption, anatase TiO
2
under exposure to UV light becomes an
even more aggressive adsorber, and can also irreversibly photodeposit certain contaminants
on the TiO
2
surface. Compounds involving noble metals and non-noble heavy metals with
favorable redox potentials have been shown to photodegrade [13] into molecular compo-
nents, photoreduce into less toxic forms, and then photodeposit onto the catalyst.
24.6.4 Photolysis
High-energy photons directly disassociate many chemical compounds, complementing
and enhancing the effectiveness of the other processes. The multiple wavelengths of
