Last month, we told you about an experiment with air-purifying concrete that was recently conducted in the Netherlands. Researchers resurfaced 1,000 square meters of a busy road with concrete paving stones that contained titanium dioxide (TiO2), a photocatalytic material that removes automobile-produced nitrogen oxides (NOx) from the air and converts them into nitrate with the aid of sunlight. When the air was tested up to one-and-a-half meters above those stones, NOx levels were found to be 25 to 45 percent lower than above regular concrete on the same road. Now, a similar study is underway in Germany, and is already showing promising results.

Last year, at 55 percent of urban air monitoring stations in Germany, NOx levels over the maximum permissible limit were recorded. One particularly bad street was Petersberger Straße in the city of Fulda. For the study, the length of that street is being covered with paving slabs coated with TiO2. The Air Clean slabs were developed by F. C. Nüdling Betonelemente, and their effectiveness was subsequently proven by researchers at the Fraunhofer Institute for Molecular Biology and Applied Ecology IME.

Finding the right formulation for the slabs was a tricky process. The team at F. C. Nüdling tried various surfaces, colors, and TiO2 contents, and ultimately ended up having to create their own cement formula, as traditional mixes proved unsatisfactory. The slabs were then laid down in a specially-created street canyon testing ground, and left for an extended period of time. Over the course of the test, NOx degradation rates of 20 to 30 percent were recorded at a height of three meters above the slabs, in moderate to light wind conditions. When there was no wind, degradation rates were as high as 70 percent for both nitrogen monoxide(NO) and nitrogen dioxide(NO2).

The Air Clean slabs are already in use at the Gothaer Platz in Erfurt, where rates of 20 percent for NO2 and 38 percent for NO were recorded. After 14 to 23 months of use, the slabs appear to be just as effective at neutralizing pollutants as they were when they were first implemented.

The researchers from Fraunhofer also wanted to know what happened to the nitrate that was the end result of the NOx conversion process. As was the case in the Netherlands, rain washed it off the roads, down the storm sewers, into water treatment plants, and ultimately into the rivers and groundwater. In local water sources, the highest amount that the German team could trace back to the paving stones was about five milligrams per liter, which is well below the maximum permissible 50 milligrams per liter.

The study is being funded by the German Environment Foundation.