Solar alchemy: Photocatalysts to clean water and recover chemicals

In many places of the world water is highly polluted by organic chemicals from industrial wastes. The experiments carried out at the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw prove that the biomass can be successfully transformed into useful chemicals and fuel. Due to appropriately selected photocatalysts, the transformation of polluted water into clean one and chemicals does not require specialized plants and takes place under conditions that are commonly met in nature.

Catalyst is a substance that participates in the chemical reaction, speeds its course and fully recovers after the reaction is completed. In typical catalytic processes the catalysts are activated at high temperatures, typically of several hundreds degrees centigrade, often at a significantly increased pressure.

"Photocatalysts studied by us differ in many respects from traditional catalysts. They are activated by light, and the temperature has no significant effect here," says Dr Juan Carlos Colmenares from the IPC PAS. The reactions with participation of photocatalysts occur at good exposure to sun rays, at temperature about 30 degrees centigrade and at normal atmospheric pressure -- so at conditions occurring naturally all year round in many equatorial countries.

The photocatalysts studied at the IPC PAS are solids based on titanium dioxide, TiO₂. The catalysed reaction occurs in a liquid containing organic pollutants. After the reaction is completed, the catalyst can be isolated almost without losses and used again.

"My work resembles somewhat alchemy," jokes Colmenares. "I take a 'magic' powder, pour it into polluted water, stir and expose to the sun. After several hours, I get clean water plus chemicals that can be used to make useful things, for instance drugs."

The research on photochemical degradation of pollutants has been carried out in the world already in the late 1960's. By intensive UV irradiation chemical compounds with simple structures have been obtained at that time.

The research pursued at the IPC PAS aims at such a selection of photocatalysts and reaction conditions that the reaction can occur without using specialized equipment, and the degradation of biomass stops at a precisely defined stage. With titania-based photocatalysis the researchers managed to produce carboxylic acids used, e.g., in pharmaceutical and food industries. It is also possible to prepare a photocatalyst so as to have the reaction completed and yielding substances with the simplest structure, such as hydrogen or carbon dioxide. The latter compound is undesirable and would require disposal, hydrogen, however, has excellent prospects as the fuel of the future.

"In laboratory conditions, the reactions of the biomass with participation of photocatalysts are promising already now. In this year we are going to attempt the first tests in the pilot biochemical photoreactors at the University of Cordoba, Spain. The reactions will occur there in liquids with volumes measured in tens of litres," says Colmenares while making clear that still many tests and studies are to be carried out before the new technology gets disseminated.

The co-authors of the paper published in the Bioresource Technology journal, describing application of photocatalysts to glucose degradation and to production of valuable chemicals are Agnieszka Magdziarz and Dr Anna Bielejewska, who passed away late last year. The research has been financed from an international Marie Skłodowska-Curie reintegration grant under the 7th Framework Programme of the European Union.