The concrete answer to pollution

The development comes down to the introduction of photocatalysts – substances that promote chemical reactions using only the sun’s rays as input. When the photocatalysts are incorporated into a material such as concrete, they trap pollutants and convert them into clean products.

For Dr Andrea Folli – a scientist at the Danish Technological Institute – the technology offers an attractive way to improve the functionality of one of the most widely used building materials. ‘Our built-up infrastructure offers an incredibly big surface area that we could use in order to promote reactions that improve air quality,’ said Dr Folli, who is project manager of the Light2Cat project to develop pollution–degrading concrete.

Pollution-degrading concrete is not a new idea, but it has not been wholly successful. For years manufacturers have sold cement infused with the photocatalyst titanium dioxide.

When titanium dioxide is exposed to light, it converts water vapour in the air into hydroxyl and peroxyl radicals. These radicals react eagerly with airborne pollutants, such as nitrogen oxides in exhaust fumes, producing totally benign products.

The trouble is twofold, according to Dr Folli. First, titanium dioxide is expensive relative to cement, so that currently available pollution-degrading concrete costs about four times as much as normal concrete. Second, titanium dioxide is only sensitive to ultraviolet light, which is plentiful in hot regions such as the Mediterranean, but harder to come by in the north.

Concrete solutions

The EU-funded Light2CAT consortium – which includes various academic institutions and industries in Europe – already has a way to make it cheaper. They can make concrete ‘active’ either by spraying previously set blocks with titanium dioxide-infused cement, or by forming a block in a mould lined with the photocatalyst. In both instances, the concrete block ends up containing the reactive substance only on the outside, where the cost–benefit is greatest.

To solve the problem of it only being sensitive to ultraviolet light, Dr Folli and colleagues needed to get more scientific. They found that by doping the titanium dioxide with other elements and compounds, it became more sensitive to visible light – and hence more suitable for colder latitudes.

Recently the consortium had an entire street in Copenhagen paved with their special concrete. Although the first results of their tests mixing titanium dioxide with other compounds will not be available until spring 2015, a previous trial with ordinary titanium dioxide showed that, during the middle of summer, nitrogen-oxide pollution could be significantly reduced. ‘The effect is best on very narrow streets with very tall buildings, where the circulation of the air is not very good,’ said Dr Folli.

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‘Our built-up infrastructure offers an incredibly big surface area that we could use in order to promote reactions that improve air quality.’

Of course, there is another way to give building surfaces an overhaul – paint them. That is the approach of another EU-backed project, 4G-PHOTOCAT, which is trying to incorporate photocatalysts like titanium dioxide into paint.

Professor Radim Beránek, a chemist at the Ruhr University Bochum in Germany and project coordinator of 4G-PHOTOCAT, said that his group has been looking at two major issues. The first was improving the rate at which titanium dioxide degrades pollution, and for this they had to develop other catalysts – known as co-catalysts – to work in tandem and assist in the reaction.

The other was finding a suitable binder that could immobilize the photocatalysts in the paint, without blocking their effect. Fortunately one of the consortium’s industrial partners, Advanced Materials-JTJ in the Czech Republic, invented a binder that automatically pushes the photocatalysts to the surface upon drying, so that their effect is not diminished.

Cheap makeover

Prof. Beránek and his colleagues are hoping that their paint will allow the creation of cheap water-treatment centres. If the paint is applied to a flat surface over which water runs, then sunlight alone will kill many of the passing waterborne pollutants. They are currently running field tests in rural Vietnam, where clean water is scarce. ‘I’m quite optimistic,’ said Prof. Beránek.

Dr Polycarpos Falaras, a physical chemist at the National Center for Scientific Research ‘Demokritos’ in Athens, Greece, is also interested in applying photocatalysts to water treatment. He has coordinated the EU-backed project CleanWater, which aims to combine photocatalysts with conventional ceramic and nano-composite membranes.

CleanWater is a more comprehensive approach to water treatment, which takes the best of both old and new technologies. The membranes remove relatively large pathogenic microorganisms, while the photocatalysts remove the smaller pollutants that would otherwise slip through the net.

Dr Falaras said they are currently constructing a module that will provide several thousand litres of clean water every day – one that he hopes will prove useful on remote Greek islands. ‘It is very important to see that we’re able to combine different technologies within such a difficult European project, and propose practical solutions,’ he added.