Innovative paving slabs that are coated in titanium dioxide nanoparticles can reduce the amount of nitrogen oxide in the air. Titanium dioxide is a photocatalyst; it uses sunlight to accelerate a naturally occurring chemical reaction, the speed of which changes with exposure to light.
The concentrations of toxic nitrogen oxide that are present in German cities regularly exceed the maximum permitted levels. In 2009, the amounts of toxic nitrogen oxide in the atmosphere exceeded the maximum permitted levels at no fewer than 55 percent of air monitoring stations in urban areas.
The paving slabs are to be laid the length of Petersberger Straße in the Baroque city of Fulda, where recorded pollution levels topped the annual mean limit of 40 micrograms per cubic meter (μg/m3) last year. The "Air Clean" nitrogen oxide-reducing paving slabs were developed by concrete manufacturer F. C. Nüdling Betonelemente. Proof of their effectiveness has been provided by the Fraunhofer Institute for Molecular Biology and Applied Ecology IME in Schmallenberg, where researchers also determined the risk to the environment posed by the resulting nitrates.
Dr. Monika Herrchen, a scientist at the IME, says: "Experiments in Italian cities had already shown that photocatalytic paving slabs can improve the air quality. We wanted to see if they would also be effective here in Germany, where we have lower levels of light intensity and fewer hours of sunshine. Of course, the more intense the sunshine, the quicker the degradation of harmful substances, so our aim was to identify the formula with the highest photocatalytic efficiency rating."
To begin with, F.C. Nüdling produced a range of sample slabs incorporating different surfaces, colors, types of cement, and TiO2 contents. Since the nitrogen oxide degradation rates achieved using standard commercial photocatalytic cement (cement that reacts to solar radiation) proved unsatisfactory, the company had to develop its own, more effective formula.
During an extended time field test, Herrchen and her team recorded nitrogen oxide degradation rates of 20 to 30 percent in specially-created street canyons. The measurements were taken at a height of three meters above the photocatalytic slabs, in variable wind and light conditions. When the wind was still, the experts recorded degradation rates as high as 70 percent for both nitrogen monoxide (NO) and nitrogen dioxide (NO2). Measurements likewise taken at a height of three meters above the Gothaer Platz in Erfurt, which is already paved with Air Clean paving slabs, revealed an average degradation rate of 20 percent for NO2 and 38 percent for NO.
The nitrate that is generated during the conversion process poses absolutely no risk to the environment. It runs off into the drainage system, then into a wastewater treatment plant, before finally ending up on a farmer’s field or in the groundwater. The maximum possible nitrate concentration traceable back to photocatalytic reactions is around five milligrams per liter (mg/l), while the maximum permitted concentration of nitrate in groundwater is 50 mg/l.