Cleaning up pollutants with sunlight
23 Oct 2002
UK researchers are developing a new type of reactor to destroy persistent contaminants such as pesticides and pharmaceutical residues.
The technology, which breaks down polluting molecules into carbon dioxide and water, could provide a breakthrough for a sustainable way of cleaning up fresh water supplies and industrial wastewater.
The work is being led by Dr Gianluca Li Puma at the University of Nottingham. The project is funded by the Swindon based Engineering and Physical Sciences Research Council.
'Water treatment plants usually use granular activated carbon to 'soak up' (polluting) molecules. The carbon is then regenerated or disposed of by burning off the offending chemicals. However, this can result in noxious chemicals entering the atmosphere, so the problem changes from one of water pollution to one of atmospheric pollution,' says Dr Li Puma.
The Nottingham team is investigating a novel type of reactor, the 'fountain photo-catalytic reactor', to treat contaminated water using titanium dioxide. Titanium dioxide is an inexpensive white powder, which is used as pigment in paints, and also in health products such as toothpastes and sunscreens.
The idea is that contaminated water is pumped through a specially designed nozzle. Titanium dioxide, a photo-catalyst, is then added to the water. The nozzle produces an umbrella-shaped fountain of water, with the sunlight - or artificial ultraviolet light - falling on the 'canopy' of the umbrella.
'Titanium dioxide can absorb the ultraviolet component of sunlight, causing a change in its internal electron configuration. In this form it can split water into highly reactive components called free radicals,' says Dr Li Puma.
One of these, the OH radical, readily reacts with large carbon-based molecules, such as pesticides, converting them into carbon dioxide and water.
The fountain photo-catalytic reactor can be easily installed in current water treatment works by the simple distribution of these nozzles in an open-air, sunlight-activated lagoon treatment plant. In addition, the fountain photo-reactor has the potential to combine water disinfection and water detoxification in a single process.
'Once the pollutants have been removed the water can be passed to a settling tank where the titanium dioxide can be recovered and re-used for the same process,' says Dr Li Puma.
The team has already successfully demonstrated the feasibility of the concept using a 400 litre pilot plant and ultraviolet lamps to simulate sunlight.
'We see this as being a potentially sustainable technology which could have particular use in countries with plenty of sunshine, such as southern Europe, Central and South America, Africa and the Asia/Pacific region,' says Dr Li Puma.
In the UK, the present technology can use low-cost, low-power sun-tanning lamps.
