The road to the future: air-cleaning asphalt

91% of the world’s population experiences substandard air quality on a daily basis, according to the World Health Organization. Dangerous substances such as soot, carbon monoxide and carbon dioxide, nitric oxide … can lead to serious health problems such as heart and lung diseases. In addition, they contribute to global warming. In order to achieve different climate goals and improve general health, it is obvious to break down these substances in both urban and rural areas. One possible solution to this is to treat titanium dioxide (TiO2) in asphalt mixtures.

Titanium dioxide or TiO2, a wonderful substance

Titanium dioxide is a well-known and relatively inexpensive photocatalytic semiconductor. This means that it is a nanomaterial that can initiate a chemical process through UV radiation. In this case, UV radiation causes this substance to use chemical energy to decompose various harmful substances into harmless liquids. The use of TiO2 has been known for a long time and is therefore very often used in research and industry. Now Arne Chantrain has investigated whether it can be used as an “air purifier” in our asphalt and optimized the technology.

Asphalt, the solution?

TiO2 is already used for various purposes. However, there is no commercial use. A new and very interesting option, which has not yet been thoroughly investigated, is the use of TiO2 on asphalt roads. In Belgium alone, there is more than 150,000 km of road surface, an incredibly large surface with a lot of light. In other words, this seems like the perfect combination for optimal air purification. In addition, most of our roads have a perfectly functioning sewer system, with which the end product, the harmless liquid substance, can be easily removed and treated.

To add TiO2 to an asphalt mixture, two choices can be made: either spray the substance on the asphalt surface, or mix the TiO2 particles with the asphalt mixture. Both methods have their advantages and disadvantages. The spraying method thus ensures a higher efficiency. However, due to the contact with car tires, it is less durable. When you mix the substance in the asphalt, it is exactly the opposite: it lasts longer, but it is less effective.

Soot particles, a great sinner!

During this research, Chantrain focused on the decomposition of soot, a seriously harmful substance that occurs from incomplete combustion of carbonaceous fuels (eg wood or coal, but also petrol and diesel). Soot is also known as elemental carbon and is characterized by its carbon black color. In order to detect the degradation of soot over time, he made various samples: small asphalt blocks, on which the researcher first applied a mixture of TiO2 and later a mixture of soot particles. These cubes were then placed in a special chamber where UV radiation shone on the surface. Then Chantrain looked at how much soot was being broken down by the asphalt. He saw that the demolition was successful and that after 50 days there was up to 90% less soot on the asphalt.

And what about the quality of the asphalt?

In addition to TiO2’s ability to degrade harmful substances, Chantrain has performed several tests of the quality of the asphalt in the presence of TiO2. All these tests confirm that there are no significant differences in the chemical and physical aspects of the asphalt. This means that the use of TiO2 in asphalt can only have positive effects on our environment without affecting the quality of our roads.

Air-purifying asphalt, a beautiful adventure?

Although some more research is still needed, it has clearly been shown that TiO2 asphalt can easily decompose large amounts of soot without affecting the quality of the asphalt. In the Netherlands, they have already tested this concept in a number of ways to filter nitric oxide from the air. There, they found that it is actually harder than in the laboratory, mainly due to the cold weather conditions and too little sunlight. The next step in this research is therefore to find out if this soot filtration technique also works as well on our roads as it does in the laboratory.

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