In the recent years, the photo-catalytic self-cleaning and "de-polluting" materials have been suggested as a remediation technology, mainly for NOx and aromatic VOCs. The European research project PhotoPAQ (Photocatalytic Remediation Processes on Air Quality, http://photopaq.ircelyon.univ-lyon1.fr) wants to find out to what extent these surface coatings are really able to improve the urban air quality. Previous measurements haven’t given a sufficiently clear picture. Therefore, a 100 meter long section of a road tunnel in Brussels, Belgium has been prepared with a photo-catalytic cement plaster. About 25,000 cars daily pass the tunnel and cause extremely high nitrogen dioxide concentrations. In September 2011 researchers of the Leibniz Institute for Tropospheric Research (IfT, Leipzig, Germany, http://www.tropos.de/eng/index.html), together with colleagues from France, Belgium, Italy and Greece, have conducted a measurement campaign in the tunnel,  which is evaluated in the coming months.

The basis of the de-pollutant effect of the new cement plaster is the added photo-catalyst titanium dioxide (TiO2). In order to develop its effectiveness, the catalyst requires UV radiation, which can be found for example in sunlight. Since the sun doesn’t shine in the tunnel, UV lamps had to be installed. As a reaction product, small amounts of nontoxic, water-soluble nitrate evolve on the surface of the concrete,

The European research project shall also find out, whether it is possible to influence the particulate air pollution, by the adsorption of particles and subsequent oxidation or - indirectly - by oxidation of gaseous particle-forming substances.

For learning more about the decomposition products, the effectiveness of the materials is also investigated under controlled conditions in the laboratory.

In addition to Brussels, the researchers plan to test the new materials at a house in Leipzig, Germany, which is coated with titanium dioxide-containing special color.

German manufacturers of building materials have already offered photo-catalytic products for several years. For example the roof tile producer Nelskamp (http://www.nelskamp.de/en/menue-oben/home.html) sells concrete roof tiles, whose surface contains titanium dioxide, since 2007. According to the company 200 square meters of the innovative roofing take 962 grams of nitrogen oxide per year out of the air - if the sun shines for 2,000 hours. Up to now, some 100,000 square meters were installed on roofs in Germany, for example on hospitals and kindergardens.

The company FC Nüdling offers an environmentally active paving stone (http://www.nuedling.de/produkte/product-details/produkte/t0/aircleanR.html). The evidence on the effectiveness of the stones was given by the Fraunhofer Institute for Molecular Biology and Applied Ecology (http://www.ime.fraunhofer.de/en.html). "Studies in Italian cities have already shown that photo-catalytic paving stones can improve the air quality. We wanted to examine whether these effects can also be measured in Germany - at a lower light intensity and sunshine duration. The more intense the sunlight, the faster the de-pollution. Therefore the aim was to find the recipe with the highest photo-catalytic efficiency," explained Dr. Monika Herrchen, a scientist at the IME. In specially constructed street canyons, the researchers proved nitrogen oxide reduction rates of 20 to 30 percent. The measurements were made at a height of three meters above the pavement and with varying brightness and wind conditions. In doldrums, the experts noticed isolated cases of reduction rates for nitric oxide (NO) and nitrogen dioxide (NO2) of up to 70 percent each.

The IME researchers have also investigated the environmental risk of the resulting nitrate. From the surfaces it gets into the sewers, from there to the waste water treatment plant and to the water cycle. According to the calculations of the scientists the maximum nitrate concentration, which can be attributed to photo-catalytic reactions is 5 milligrams per liter. By way of comparison: The limit for nitrate in groundwater in Germany is 50 milligrams per liter.

 

Picture: Nelskamp