24 July 2013 — Among the groundbreaking inventions to deliver more sustainable buildings is smog eating buildings, which use titanium dioxide coatings to “eat” air pollution. The range of success stories reads like science fiction but the technology is new and needs further work. A study released only a few weeks ago casts doubt on the product in real-world conditions, given that high levels of ammonia could be converted by the coating into nitrous oxides, the very gases the product is meant to reduce. Leon Gettler reports.
Studies show that air pollution contributes around 2.5 million deaths a year with 470,000 people dying as a result of ozone and 2.1 million deaths linked to fine particulate matter. We have seen Beijing’s pollution spike to record levels, and serious pollution problems in the Iranian city of Ahvaz where the pollution is three times worse than Beijing.
Something has to be done and new developments suggest the answer might come from the way we build our cities. Think of smog-eating buildings, concrete and cement. It sounds like science fiction but apparently, it all comes down to the chemical titanium dioxide, the stuff that neutralises air pollutants.
Titanium dioxide, a component in some paints, is already used to coat surfaces that are hard to clean. It functions as a self-cleaning chemical, breaking down algae and dirt so the surface stays clean. One processed form of titanium dioxide is what is called superhydrophilic – literally, “water-loving”. It means that when water hits a dirty titanium dioxide surface, that surface will draw in a whisper-thin sheet of water across its whole surface, displacing grime that then washes neatly away.
But its second trick is fighting pollution. That happens when a bit of the ultraviolet light in sunlight frees up electrons from the material, creating “free radicals” that actively break down pollutants including so-called nitrogen oxide gases (molecules of varying proportions of nitrogen and oxygen) or VOCs (volatile organic compounds).
The Italians have been working on this since 2006. They have developed a patented pollution-reduction technology – which in Italy is becoming known as “cemento mangiasmog” or “smog-eating cement”. It hastens the decomposition of organic and inorganic pollutants and prevents their build-up on surfaces, helping to preserve a building’s pristine appearance over time. They have been producing blocks costing about one-third more than usual paving. Still, it’s far less than the long-term cost of doing nothing.
Dutch scientists have gone one step further. They have now made the big breakthrough, developing smog-eating concrete using titanium dioxide. Eindhoven University of Technology scientists have installed air-purifying cement onto a city block in Hengelo, Netherlands and found it reduced nitrogen oxide air pollution up to 45 per cent in ideal weather conditionsm and an average reduction of 19 per cent each day.
The concrete is made with run-of-the-mill cement. There’s nothing special about that. But it’s sprayed with titanium dioxide.
The problem at the moment is that titanium dioxide isn’t cheap. As Reuters reports, the stuff costs $175 per tonne in Australia and there are only a few places in the world where deposits are readily accessible. As a result, it’s the most expensive of the common pigments.
Still, there are signs that it could be taking it off and there are ways of processing it to make it cheaper. And the more it is used, the cheaper it will get.
Professor Jos Brouwers of the Department of Architecture, Building and Planning at Eindhoven University of Technology told CNN the cost isn’t that big a factor. True, the material is around 50 per cent more expensive than normal concrete. But Brouwers is adamant that when the total cost of fitting is included, the overall increase in cost is only 10 per cent.
“Sure, it is slightly more expensive, but if you look at the total pavement costs where the stone is one part – there is also labour, foundations etc. to calculate – then you are only looking at a slightly higher cost,” he says.
Certainly, it’s now being used around the world to cut back on pollution. For example, the Mexico City Hospital, the Torre de Especialidades, is shielded with a facade of Prosolve370e, a new type of tile whose special shape and chemical coating can help neutralize the chemicals that compose smog. Actually, it purportedly neutralises the equivalent produced by 1000 cars driving by each day. When UV light cuts through smoggy air and hits the titanium dioxide on the tiles, there’s a chemical reaction. The smog is then broken down into small amounts of less noxious chemicals, including calcium nitrate, carbon dioxide, and water. The titanium dioxide itself remains unaffected, so it can keep making reactions happen.
The really cool part is that that the tile is shaped to scatter more light and collect more pollutants. That means more chemical reactions.
Then there’s Chicago which has just opened the greenest street in America. It’s a street that purportedly has smog-eating pavement. The new roadway uses what the boffins call photocatalytic cement. It’s a new paving surface that contains nano-particles of titanium dioxide. It literally “eats” smog and removes nitrogen oxide gases from the surrounding air. Additionally, the sidewalks are paved with 30 per cent recycled content, and more than 60 per cent of all construction waste is recycled.
According to the newsletter from Chicago’s Northwestern University, the cement manufacturer, an Italian-based company called Italcementi Group, claims that in closed-chamber tests, the cement reduced nitrous oxide concentrations by 91 per cent. Professor SonBinh T Nguyen, an organic chemistry professor at Northwestern University says its smog-fighting power lies in the microscopic size of its crystals. “The [titanium dioxide] crystals in your toothpaste container are about 200 to 250 nanometers in diameter while the ones in the pollution fighting form are only about seven nanometers,” said Nguyen. “That’s what makes them capable of fighting pollution.”
Alcoa in America has developed smog-eating buildings that use technology to apply a titanium dioxide coating, called EcoClean, to a pre-painted aluminium surface, producing the world’s first coil-coated aluminium architectural panel that helps clean itself and the air around it. Free radicals generated from the titanium dioxide reacting with water and oxygen in the air attack the nitrogen oxide which is the primary component of smog. The free radicals oxidise the nitrogen oxide molecules, converting them to a harmless nitrate. All that’s needed for the process is oxygen and water vapour.
Alcoa claims that by adding 10,000 square feet of the EcoClean panels to a building is enough cleaning power to offset smog created by four cars every day, which is the approximate cleaning power of 80 trees.
Alcoa says: “When it rains, water doesn’t bead on the surface but instead collapses and runs evenly off the building, taking most of the broken-down organic matter and nitrates with it. Even the slightest amount of rain creates the effect, so the building is constantly washed clean of contaminants. That means lower maintenance costs for owners, and a consistently cleaner image for the building over time.”
Boral produces smog-eating tiles which can be installed on both new residential and commercial buildings, as well as re-roofing projects. The tile technology works via a catalyst embedded in the upper portion of the roof tile. When exposed to sunlight it speeds up oxidization reducing nitrogen oxide.
And this is where it becomes interesting. The fact that such large manufacturers like Alcoa and Boral have started working with it means that it can be finally assessed and used on a large scale, which would drive down the costs even further.
And as large building projects make use of titanium dioxide-coated products further large-scale experiments can be carried out. As reported here, the coating is now being mandated by many new construction regulations in Europe. In Britain and Belgium governments have launched projects to measure the real-world effectiveness of the coating in reducing pollution. The European project Photopaq has coated the interior of the Leopold II tunnel in Brussels, carrying out detailed measurements of the before-and-after effects.
However, it’s still a relatively new technology and some researchers have raised questions about whether the coatings are safe. Science Daily reports that research by Indiana University found that pollution removal technology might be doing more harm than good.
They said that in real-world conditions, high levels of ammonia could be converted by the coating into nitrous oxides, the very gases the product is meant to reduce. However, Science Daily reports this could lead to further research into developing more effective pollution-control equipment as well as improvements in industrial processes involving ammonia. Which in turn could potentially address the titanium dioxide issue, if there is a problem.
The Pharos team, which looks at the sustainability of building products, says that the use of titanium dioxide needs further study. Even if there is a net gain, titanium dioxide’s impact on human health and environmental safety is still unstudied with some evidence suggesting it could be carcinogenic.
This is all new territory for the building industry. With further research, smog-eating buildings and concrete could be very much the shape of the future for the construction industry around the world, changing our cities into more sustainable places.
Leon Gettler is a freelance business journalist, author and podcaster. He works for a range of publications and produces two podcasts for RMIT every week: Talking Business and Talking Technology. He has an acute interest in the environment, its impact on business and the response of businesses and governments.