NOx-Sucking Sidewalks Could Save Lives (or at Least, Lungs)


One of Malmo's busiest streets will get a stretch of NOx-sucking sidewalk (via Wikimedia Commons).

Cement has not had an easy time trying to profile itself as a green building material - it's energy intensive and has high carbon dioxide emissions, and cement kilns are a source of mercury emissions. But it has continued to try to do so. That's why the story of cement product concrete's abilities to both suck nitrogen dioxide (NO2) from the air and be self-cleaning have been almost magic for the cement industry, or at least for Italcementi, the company that has patented this process. Now after lots of pre-press, the additive is making its way into building panels as well as stretches of sidewalk.
Rome's Jubilee church hopes to stay white with Italcementi's self-cleaning cement.

Malmo tries the NOx-sucking sidewalk

Italcementi's patented additive consists of nano-particles of titanium dioxide, a naturally-occurring mineral used in everything from paint to sunscreen to food coloring. When the nano-particles are embedded in the top layer of sidewalk pavers or building panels and get hit by sunlight, a photocatalytic process turns the NOx from air pollutant to a water soluble nitrate NO-3.

Breathing nitrogen dioxide or nitrogen oxides is associated with respiratory problems, such as more coughs, bronchitis, impaired pulmonary function, and even premature death. NOx pollution is acute in car-choked cities, and the EU has set limits for the amount of NOx and ozone in air. (The U.S. hasn't revised its standard since 1971.) Especially during the summer months, though, many cities fail to meet the standards, which will tighten in the EU at the start of 2010.

So Sweden's third-largest city Malmö is taking a pre-emptive action by installing a sidewalk embedded with the catalyst (called TioCem in some European countries and TioMix in Sweden).

As product manager Fredrik Jansson of Sweden's Cementa notes, adding a one centimeter layer of TioMix to paving tiles is a 'costful' undertaking, but the catalytic effect is, as far as the company can determine, lasting. As long as there is enough sunlight hitting the pavement, the catalytic effect works (so, yes it is effective on cloudy days, but not, for example, if the pavement is covered with snow).

Exposure to sunlight does help break down some airborne pollutants naturally, so TioCem basically speeds up a job nature is already doing. In controlled tests, between 40 to 80 % of NOx spewed onto a TioMix sidewalk by internal combustion engines was removed from surrounding air. Here's how Heidelberg Cement, a German company distributing TioCem, describes the additive's potential:

If a traffic surface the size of a football pitch (around 7,500 square meters)
is covered with photocatalytically active slabs exposed on average to 2000 hours of solar radiation annually, it will be capable of breaking down the pollutant emissions from more than 190,000 car kilometers.

That old precautionary principle
The catch with this cool and potentially pollution-mitigating process is the long term effects versus the long term benefits. Jansson says the nano-particles of titanium dioxide in TioCem and TioMix don't effect the concrete's ability to be 'recycled' or broken down and used for other types of pavement projects.

However, the use of nanoparticles in general is still being studied, and thus far there are continued questions on the health effects of these tiny structures. In fact, a recent study suggests that titanium dioxide nanoparticles may change the lung's structures.

But Åsa Nilsson at Heidelberg subsidiary Cementa said she is convinced that the titanium dioxide in TioMix and TioCem will not be of concern, for two reasons. The first, she said, is that the nanocrystalline particles of titanium dioxide in the additive are 'agglomerated' into larger particles.

"This titanium dioxide, by the time it is in the mix, it isn't nano-sized anymore," she said.

In addition, even if concrete containing the TioMix was broken or crushed in a recycling process, Nilsson didn't believe the titanium dioxide would be released. She also said that the resulting NO3- nitrate that is produced from the catalytic process will likely react with the calcium in the concrete to become calcium nitrate.

Italcementi also isn't the first to commercialize the NOx neutralizing powers of titanium dioxide. The technology is decades old - and Mitsubishi has had their NOx absorbing Noxer tiles on the market for years, though mostly used in Japan.

Outside of Sweden, the NOx-absorbing additive has shown up in English gardens and Italian buildings, including an IKEA in Milan.

Read more at TreeHugger about NOx
Italy's Trash Robot is a Real-Life Wall-E
Nanotech & Environment: A Match Made in Heaven
Photocatalytic Tiles: Cleaner House Without the Chemical

Tags: Buildings | Concrete | Green Building