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Roads that eat pollutants

Titanium dioxide has been turning up in a lot of applications recently: road surfaces, sound barriers, construction site hoardings, roof tiles, paving slabs – and probably other things too.

The reason is that titanium dioxide is a photocatalytic material which reacts in sunlight to absorb nitrogen oxides – NO and NO2, collectively referred to as NOx – and coverts them into nitrates. That could be good news for urban areas or highways that have high densities of traffic which produces pollution including NOx which is harmful to health.

This isn’t a new concept. Back in 2004, Italcementi – now part of HeidelbergCement - announced that it had developed a special concrete which could be used to coat streets or buildings in cities to reduce levels of NOx, as well as carbon monoxide and sulphur dioxide.

Then in 2010, researchers from the Eindhoven University of Technology reported that using concrete containing titanium dioxide on roads reduced the concentration of NOx by between 25 and 45%. More recently highway authorities in several US states have reported applications of the pollutant-eating concrete on roads and airport pavements.

Around eight years ago, Shell Bitumen was said to be developing an asphalt that could also remove such pollutants, although that hasn’t yet materialised on the market. However, a 2022 trial at the Port of Skagen in Denmark has seen AirClean concrete granules, containing titanium dioxide, scattered over the top of freshly laid asphalt to reduce NOx levels. AirClean also makes paving slabs that can be installed in footpaths, cycle ways or landscaping to do the same job.

Closer to home, National Highways conducted a trial of SmogStop barriers, designed by Envision SQ and Gramm Barrier Systems, on a section of the M1 between junctions 28 and 29. The barriers have a coating that contains – you guessed it – titanium dioxide. A report into the barrier trial published last year said that a 3m high, 100m-long carrier removed the equivalent of NOx emissions from 230 vehicles from 100,000 passing vehicles.

Roof tile manufacturers have also been promoting this technology for several years, perhaps with limited take up due to the added cost. Housing association New Charter Homes was a trail-blazer, re-roofing 1,000 properties in Greater Manchester with Marley Eternit’s EcoLogic tiles which have a titanium dioxide coating.

Sir Robert McAlpine has trialled a coating containing titanium dioxide on site hoardings. Following a trial of Guard Industry’s Dtox Guard at its Manchester office, it applied the coating to site hoardings at 21 Moorfields in London.

What happens to all these nitrates when they are formed on roads or barriers or hoardings? They are washed away by the rain and end up in our surface water.

Some researchers have suggested that this could be a potential problem, since an excess of nitrates can be damaging to the environment, contributing to algae blooms and harming aquatic life. Since much surface water runoff from roads is not filtered before entering rivers and other water courses, it’s certainly a risk that will have to be evaluated.


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