Smog-eaters sound like something out of a Harry Potter story but it’s actually a form of concrete treated with titanium dioxide that absorbs pollutants from the air.
Photocatalytic concrete is a viable idea in the laboratory but it’s proving difficult to quantify in the field.
In a concrete structure, photocatalysts decompose organic materials such as dirt, biological organisms, mold, bacteria and airborne pollutants like nitrous oxides and sulfuric oxides, the major components of smog.
The Ontario Ministry of Transportation (MTO) has been experimenting with a system of acoustic panels coated with the smog-absorbing substance since 2011 along at stretch of Hwy. 401, just west of Bayview Avenue, but it hasn’t been able to conclusively measure if there’s any impact on air quality in the area.
That’s not because the coating — a mix of titanium dioxide with a proprietary compound that is activated by sunlight — isn’t viable. Its properties have been proven in lab tests all over the world.
The problem with the MTO field test involves the difficulty of measuring how long polluted air is in contact with the panel and whether there’s any impact given that there’s a mass of air swirling around.
Still, the ministry is persevering, says David Rhead, a senior concrete engineer in the MTO’s material engineering research office, because the benefits are so promising.
"We used concrete acoustic panels but really couldn’t measure the impact," he says. "We’ve redesigned the panels in acrylic with a sort of a scoop which will create a laminar flow so we can control how long the air is in contact with the coating."
The MTO hopes to see some tangible results that will answer the questions around the long-term value of treated concrete.
They’re after the same answer researchers and concrete makers around the world are looking for.
Professor Daman Panesar at the University of Toronto’s building engineering research group has also been investigating those questions.
She played a role in the 2011 testing of the treated concrete material to determine whether the titanium dioxide would have any detrimental effect on the viability of the concrete.
"We found that there were problems with the freeze-thaw cycle and with road salt issues," Panesar says. "But the bigger problem was in measuring the impact on the air in the field."
At this stage it looks like the issues around freeze-thaw and salting made the material unsuitable for Ontario’s cold climate, especially around roads where salt is being used, she says.
However, Panesar adds, Professor Chi Sun Poon at the Hong Kong Polytechnic University has had good success in that area.
"But of course Hong Kong has a hot, humid climate with no freeze-thaw cycle and no road salt so it may be that photocatalytic cement is better suited for that kind of environment," she says.
Photocatalytic cement is also popular in India, Arabic countries, Australia and other hot climates where it is used in building structures and in making concrete blocks.
At this stage in Ontario, it’s hard to justify the added cost, though not outrageously more expensive, without any metrics to back it up.
Poon, however, has argued in scientific papers that on a "bright and clear day the process (involved in photocatalytic cement) can eliminate up to 90 per cent of nitrogen oxides, aldehydes, benzenes and chlorinated aromatic compounds."
The research dates back to 1972 when a couple of Japanese scientists created photocatalytic concrete using a conventional portland cement with silica sand, crushed stone and water and adding up to five per cent titanium dioxide.
The process was discovered somewhat by accident but after being replicated many times, the resulting photocatalytic effect was dubbed the Honda-Fujishima effect after Professor Kenichi Honda and his student Akira Fujishima.
Titanium dioxide is a common compound, easily mined and used commonly in toothpaste, paint and as a stain remover. It’s also been used to coat ceramic tiles and prompted for use in kitchens and bathrooms as self-cleaning.
The MTO trials in Ontario were a partnership between the Ontario Ministry of the Environment, the University of Toronto, Essroc Italcementi Group (the North American division of an international supplier of photcatalytic portland cement) and noise barrier specialist Armtec Durisol of Hamilton, Ont.
The results were never published, says Rhead, because of the problem with the data.
He’s hoping for more positive results with the acrylic panels that were chosen because the original cement supplier Essroc Italcementi is focused on other product lines.
However, as Panesar points out, there’s still a lot of hope for photocatalytic cement as a smog-eater.
It’s been used as a cement-based paint to coat the walls of tunnels in Italy to not only abate pollutants from traffic but also because of its self cleaning properties to ensure the walls of the tunnel stay white to assist with visibility and to maintain lumens.
To simulate the sunlight necessary to activate the titanium dioxide’s photocatalytic properties special UV lighting was added to the tunnel.
Results from 2007 testing found a significant decrease in pollutants.
More recently, similar installation and monitoring was carried out in the Leopold II Tunnel in Brussels, Belgium and the concept continues to be vigorously researched and investigated in an effort to find a value curve.