Smart urban planning: controlled pollution
The photocatlysis
Today, there is broad consensus among the sectors involved regarding the strong growth that the photocatalytic industry will experience in the coming years. In fact, photocatalysis, based on the use of semiconductor materials, has numerous applications in fields such as improving air and water quality, reducing building maintenance and conservation costs due to its self-cleaning capacity, health and general well-being due to its biocidal and organoleptic effect, and energy. However, this promising technology is not well known either by public and private infrastructure managers or by the general public. In the field of environmental pollution, a stronger legislative framework, with greater regulation to eliminate fraudulent products from the market, as well as greater knowledge of this technology among technical specifiers, would have a positive impact on the application of photocatalysis.[1]
“AirClean® reduces quickly and effectively harmful NO2 (nitrogen dioxide), thus helping to comply with established limits. The basis of the effectiveness of AirClean® pavements and façades is photocatalysis: they decontaminate the air, destroy dirt and prevent the growth of microorganisms.”
Gemma Pagès. Technical Architect. Breinco
Photocatalytic concrete
Air purification through photocatalysis consists of several steps: under UV light, the photoactive TiO₂ on the material surface becomes activated. Subsequently, pollutants are oxidised due to the presence of the photocatalyst and are deposited on the material surface. Finally, they can be removed by rainwater or washing.
Photocatalysis using titanium dioxide as a catalyst is a rapidly developing field in environmental engineering, as it has great potential to address increasing pollution. In addition to self-cleaning properties, it has been known for almost 100 years that titanium dioxide acts as a photocatalyst capable of decomposing pollutants under UV radiation. (4)
The use of TiO₂ in its photoactive anatase crystalline phase for air purification originated in Japan in 1996.(7) Since then, a wide range of products has appeared on the market for both indoor and outdoor applications.
Regarding traffic emissions, it is important that exhaust gases remain in contact with the active surface for a certain period of time. Street configuration, traffic speed, and wind speed and direction all influence the final in-situ pollutant reduction rate.
Applications
Photocatalysts, typified by TiO₂, are defined as substances that promote reactions through light absorption without being altered before or after the reaction. When titanium dioxide is exposed to a light source, such as sunlight or fluorescent light, it shows both oxidative decomposition and superhydrophilic properties, which can be effectively used for deodorisation, antibacterial action, pollution prevention (NOx and VOC removal), anti-fogging, dirt removal and sterilisation.
Photocatalysis in a concrete paver
In a concrete paver, anatase is added in the wear layer of the pavers, approximately 4–10 mm thick. The presence of TiO₂ throughout this layer means that even if surface wear occurs, for example due to traffic or weathering, fresh TiO₂ remains available to maintain photocatalytic activity (unlike the abrasion of a paint or coating layer).
The use of TiO₂ in combination with cement leads to the transformation of NOx into NO₃, which is absorbed on the surface due to the alkalinity of concrete. A synergistic effect is therefore created by the cement matrix, which helps effectively capture reactive gases (NO and NO₂) together with the formed nitrate salt. Subsequently, the deposited nitrate can be washed away by rainwater or cleaning. These nitrates do not represent a real threat to groundwater contamination, as the resulting concentrations are very low.
According to the Air Quality Report 2015 by Ecologistas en Acción:
- During 2015 there was a very significant increase in NO₂ and O₃ pollution levels in Madrid, influenced by prevailing weather conditions.
- The annual NO₂ limit value for human health protection is set at 40 µg/m³. In 2015, 13 of the 24 stations exceeded this limit, and two more matched it.
- EU legislation also sets an hourly NO₂ limit of 200 µg/m³, which should not be exceeded more than 18 hours per year. In 2015, 8 stations exceeded this limit.
- Madrid has therefore breached NO₂ legal limits for six consecutive years (2010–2015), leading to an EU infringement procedure.
Drastic measures to reduce pollution
The high concentration of NOx is highly harmful to human health, so affected cities are adopting increasingly strict measures to reduce pollution levels.
Thanks to companies grouped in the Iberian Photocatalysis Association (AIF) and the CSIC, photocatalytic technology has made significant progress in recent years.
More and more public administrations are interested in smart and sustainable urbanism. The city councils of Madrid and Barcelona, which both face high NOx levels, are among the most active in Spain in studying and applying photocatalytic materials.
In Barcelona, the City Council took an important step by publishing in 2013 the “Technical Instructions for the Application of Sustainability Criteria”, which include the use of photocatalytic construction materials to reduce NOx levels in the air.
The first large-scale application in Barcelona took place in 2010 with the paving of Can Rosés square using photocatalytic slabs in the Les Corts district. Other interventions include pavements in Diagonal Avenue, streets around the Sants market and the Sants High Line, the Fabra i Puig wall, and the façade of the Leitat building in the 22@ district.
Also noteworthy is the redevelopment of the L’Estartit Marina in Girona, with 2,000 m² of decontaminating urban surface that combines environmental and aesthetic value.
Standards. Laboratory and real-condition testing.
For NOx removal, JIS R 1701-1:2010 is the basis of the ISO 22197-1:2007 standard, widely used worldwide and published in Spain as UNE-ISO 22197-1:2012.
Thanks to research by PAVINOx and ANDECE, a specific standard for prefabricated concrete products was developed in Spain.
The UNE 127197-1:2013 standard defines the test method for evaluating air purification performance using photocatalytic materials embedded in precast concrete products.
Current EU-funded LIFE projects, such as LIFE PHOTOSCALING, are testing these materials under real conditions to assess efficiency, durability, and potential side effects, helping scale the technology to real urban environments.
Bibliography:
[1] AIF, Iberian Photocatalysis Association, White Leaflet. 2017. [2] Renz, C. Lichtreaktionen der Oxyde des Titans, Cers und der Erdsäuren. Helv. Chim. Acta 1921, 4, 961-968 [3] Fujishima, A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972. 238, 37-38 [4] Fujishima, A. rao, T.N. Tryk, D.A. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C 2000, 1, 1-21 [5] Sopyan, I.; Watabane, M.; Murasawa, S.; Hashimoto, K.;Fujishima. A. An efficient TiO2 thin-film photocatalyst:Photocatalytic properties in gas-phase acetaldehyde degradation. J. Photochem.Photobiol. A 1996, 98, 79-86.
