Thursday, October 18, 2012

'Smog-eating' cement to curb air pollution in Chicago's industrial corridor

Source: Drew Kann
Medill Reports - Chicago, Northwestern University

Dubbed the “greenest street in America” by city officials, the technology employed in the new Pilsen Sustainable Streetscape on the West Side of Chicago is impressive to read about, and even more so to see in person.

Solar panels and wind turbines power overhanging high-efficiency streetlights.

Ninety-five species of native grasses, shrubs and trees line the sidewalks and roadway.

However, the most notable feature of the streets are the roads themselves.

The bike lanes and parking lanes that flank the main roadway are made of semi-permeable photocatalytic cement, also known as “smog-eating” cement, a compound that reacts with ultra-violet light to remove a variety of pollutants from the surrounding air.

While this type of cement has been utilized in building materials and other projects for some time, the Pilsen Streetscape represents the first-ever use on a commercial roadway. When completed, the project will span two miles of Cermak Road and Blue Island Avenue.

The active ingredient in the cement is a chemical called titanium dioxide, a compound commonly included in toothpaste and house paint for its cleaning properties.

According to professor SonBinh T. Nguyen, an organic chemistry professor at Northwestern University, 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.”

According to Nguyen, the titanium dioxide on the cement surface absorbs UV light and uses this energy to react with water vapor in the surrounding air.

The result of this reaction is a highly reactive particle known as a hydroxyl radical.

It is these unstable hydroxyl radicals that in turn decompose a host of other compounds in the surrounding air, including nitrous oxide, a harmful greenhouse gas released in car exhaust.

The end products of this reaction are much more innocuous: water and nitrogen, which accounts for 78 percent of Earth’s atmosphere.

Exactly how effective this cement will be at cleaning the air on Blue Island Avenue, an area characterized by heavy traffic and industrial activity, remains to be seen.

David Leopold, project manager for the Chicago Department of Transportation, did say the photocatalytic cement is more expensive than regular pavement, but the city expects to see considerable improvement in street-level air quality as a result.

The cement manufacturer, an Italian-based company called Italcementi Group, claims that in closed-chamber tests, the cement reduced nitrous oxide concentrations by 91 percent.

Based on pre-installation estimates, “on a windless day up to about eight feet from the pavement’s surface, you can see demonstrated improvements in air quality,” said Leopold. “Coincidentally, that’s about the height of a person on a bike.”

Leopold said that while the city doesn’t have exact air-quality measurements to gauge the pavement’s impact since installation, the entire project has the potential to serve as a blueprint for future transportation projects in the city.

“The purpose of this entire streetscape is to look at sustainable design practices across a pretty wide spectrum -- to kick the tires, monitor it, and then get some good, solid data.”

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