Studies Photocatalytic Algae Reduction to Help Homeowners
|Chemist Clovis Linkous compares different
photocatalyst coating formulations.
Research over the years at FSEC has shown that a white roof
helps to make a building more energy efficient since lighter
colors are more reflective of the sun’s rays. In
particular, white elastomeric roofing material with its 70
percent reflectivity can be an important contributing factor
to more energy-efficient buildings and energy and cost savings
for their owners.
However, in Florida as well as in many other hot-climate states,
algae growth on the surface of the roofing material causes
the color to darken. Within a short time, the darker
color causes the reflectivity to drop to approximately half
of what it was, significantly impacting the capability of the
roof to keep the building cool and greatly increasing the amount
of energy needed to cool the building.
Now, there may be a new strategy using the sun to get rid
of that algae problem. Since 1993, FSEC researcher Clovis
Linkous has been studying the effects of using photocatalytic
chemistry to destroy the algae that grow on a number of surfaces
in Florida’s hot and humid climate. The goal of this
work has been to develop photocatalytic paint formulations
that can be applied to a surface to protect it from biofouling
due to algae growth. “The idea first hit me one
day when I was cleaning my swimming pool,” he explained. “I
thought about this and realized that any solid surface upon
which algae collect might benefit from this strategy.”
In addition to materials such as roofing tile, shingles and
membranes, applications for the photocatalytic paint formulations
include stucco walls, swimming pools and outdoor fountains
Linkous adds that a significant side effect is that disinfection
(bacterial inhibition) is also possible with photocatalytic
paint, pointing out that counter tops in places like home kitchens
or hospital operating rooms could especially benefit from this
His research has included tests of the photocatalytic power
of the sun's rays to effectively inhibit the growth of algae
on cement surfaces. The following photos show the growth
of algae after one week, the effect of photocatalytic paint
in inhibiting the growth of algae, and the even more successful
effect of using a co-catalyst for algae inhibition.
|Unprotected cement surface after a one-week
exposure to algea attack.
||Using phocatalytic paint on cement surfaces
provides some protection.
|Co-catalyst effect on algae
inhibition: virtually no algae
have grown on the surface.
He explained that the extent of algae growth on the substrates
after one week was determined “via quantification of
chlorophyll content of the cellular growth on the surface.
The results showed that an unprotected substrate would become
completely covered with algae within a week. By coating
the substrate with a photocatalyst such as the P25 brand of
TiO2 made by Degussa Corporation, the rate of algae growth
was inhibited by about two-thirds.”
As this photocatalytic chemistry continued, he noted that “it
became apparent that it could be further improved by adding
small amounts of other catalytic substances, or co-catalysts,
to the light-absorbing particle surface. The results showed
that a Pt-TiO2 formulation could inhibit algae growth by 86
percent, nearly an order of magnitude less than an unprotected
Tungsten tri-oxide (WO3) was also tested as an algae inhibitor.
It was found that while WO3 by itself tended to facilitate
algae growth, inclusion of almost any co-catalyst showed
an improvement since the baseline for the unmodified photocatalyst
is actually a negative effect.
While Pt-TiO2 was found to be effective, researchers wanted
to find cheaper co-catalysts that represent a better value
in terms of oxidizing power per dollar. Studies showed
that for TiO2, the carbon, cobalt phosphide, and oxidized nickel
co-catalysts also improve baseline performance. It has been
demonstrated that certain formulations, particularly graphitic
carbon-TiO2, can reduce algae growth by nearly 90 percent.
Linkous has recently been conducting research in collaboration
with Ross Robertson of Firestone Building Products Company
on a method to destroy algae on elastomeric roofing material
and retain the high reflectivity to the material. Two types
of problems arise from the collection of algae on roofs. One
is aesthetic -- the roof looks as if it needs to be cleaned – and
the other is energy related. Traditional cleaning methods such
as soap, detergent or chemical solvents have proven to be ineffective
in ridding roofs of algae growth. Although they may be temporarily
effective in the short term, the algae return. “Algae
may be very simple organisms,” Linkous explained, “but
they are extremely difficult to destroy. The cleaning
first appears to get rid of the algae but their strong cellulosic
cell walls make them very difficult to kill and they soon begin
to grow again.”
Using one-foot squares of new roofing material from Firestone,
testing of these paint formulations has been conducted at a
test stand in Homestead, Florida. The Linkous and Robertson
study indicated that by modifying elastomeric roofing material
such as TPO, Thermoplastic Polyolefin, or PVC, Polyvinyl Chloride,
through a bonding process with the photocatalyst TiO2 and its
co-catalysts, a new material was created — one that is
both algae-resistant and highly reflective.
“We have now demonstrated a genuine solar photocatalytic
effect,” Linkous said. “The next step is
to determine whether our formulations can withstand the rigors
of the Florida climate, particularly the ultraviolet rays that
are absorbed during a hot summer afternoon, as well as the
action of wind and water. The good news is that so far,
so good, and I look forward to more research into this area.”
For more information on this research, see http://www.fsec.ucf.edu/en/research/environment/microorganism/index.htm.
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