Using Nanotechnology to Improve Photocatalytic Efficiencies for Water Treatment

Heterogeneous photocatalysis involves the use of a semi-conducting material which can be excited by the absorption of light. The applications of photocatalysis include water treatment and purification, air treatment and purification, and 'self-cleaning' surfaces. Photosynthetic applications are also widely reported including photoelectrolytic water splitting, CO₂ reduction and organic synthesis. There are a wide range of materials employed in photocatalytic research and applications. The important properties of these materials include the band gap energy and hence the wavelength of light required for excitation, the chemical and photochemical stability, particle size and surface area.

The use of nano-structured materials may lead to improved photocatalytic efficiencies where the reduction in particle size results in a greater surface area and possibly size quantisation effects. The former provides more active sites for reaction and the latter gives an increase in the absorption coefficient at specific wavelengths. The most commonly employed photocatalyst material for research and industrial applications is titanium dioxide (TiO₂). This is because it is photostable, chemically stable, photoactive, relatively inexpensive and non-toxic.

There are various routes to producing nanostructured titania including sol gel, hydrothermal, electrochemical oxidation of titanium, chemical vapour deposition and plasma sputtering deposition. Titania has a band gap energy of around 3.2 eV and therefore is a UV absorber. When excited by UV irradiation the electron-hole pairs in the titania can react with water and dissolved oxygen to form reactive oxygen species which can attack organic (and inorganic) pollutants in the water. Effectively, each excited particle becomes a nano-electrochemical cell driving redox reactions at the interface. Dispersed nano-structured (but most likely aggregated) titania can be utilised for water treatment and purification.

Suspensions have been used in lab based research and even on large scale treatment systems, however, the catalyst must be recovered from the water prior to discharge. Alternatively, the catalyst may be immobilised as thick or thin films on a solid supporting substrate to negate the catalyst recovery stage¹.

For water treatment applications, the use of immobilised films presents problems for reactor design due to mass transfer limitations². If the catalyst is immobilised onto an electrically conducting supporting substrate, one can employ this substrate as a photoanode in a photoelectrochemical cell (PEC), either in photolytic or photogalvanic mode). For example, one application may be the solar driven photocatalytic oxidation of organics and the simultaneous reduction of dissolved metal ions in a two compartment PEC³.

Photocatalysis has been reported to be effective against a wide range of chemical pollutants including persistent organic pollutants (POPs). One interesting application investigated at Ulster was the destruction of the female hormone 17-β-oestradiol and it's analogues. Hormones and hormone mimics are termed endocrine disrupting chemicals EDCs and pose a significant threat to the environment.

Photocatalysis is a degradative process where attack by reactive oxygen species results in the overall oxidation of an organic pollutant via intermediate products. These intermediates may be just as harmful as the parent compound. In relation to the oestrogen compounds, it is important to determine the destruction of the oestrogenic properties. This was achieved using a yeast screen bioassay which responds to the oestrogenic effect of pollutants. It was shown that photocatalysis was more effective than UVA photolysis in destroying the oestrogenic effect of 17-B-oestradiol, esterone and estriol^4,5.

More recent research has demonstrated the photocatalytic destruction of pharmaceuticals in water. Given that photocatalysis generates reactive oxygen species including hydroxyl radical, superoxide radical anion and hydrogen peroxide, it is a logical step to apply the treatment towards the disinfection of water containing pathogenic microogranisms. Indeed, photocatalysis has been reported to be effective against a wide range of microoganisms including bacteria, viruses and protozoa.

Our work at Ulster has investigated the inactivation of E.coli as a model organism⁶ using photocatalysis and electrochemically assisted photocatalysis. In the latter, the process is assisted by the application of an external electrical bias. While E.coli is a pathogen in its own right and is used as an indicator for faecal contamination, it is relatively easy to kill. Therefore, it is more interesting to study the inactivation of disinfection resistant species.

We have shown that photocatalysis and electrochemically assisted photocatalysis are effective against the spores of Clostridium perfringens⁷. Furthermore, we have also demonstrated that photocatalysis is effective against the protozoan oocysts of Cryptosporidium parvum. This organism is a big problem for the water industry because it is resistant to conventional disinfection and causes severe diarrhoea in humans.

Ulster is a partner in the EC FP6 Sodiswater project which aims to investigate the solar disinfection of water for use in developing countries. By simply filling a transparent bottle with water (preferably glass or PET) and placing in direct sunlight, one can inactivate most pathogens in the water, therefore rendering the water safer to drink. Given that around one sixth of the World's population do not have access to safe water, it makes sense to utilise the power of the sun in such a simple process.

While SODIS (solar disinfection) is used throughout the world by around 2 million people, the uptake for SODIS could be improved. Additionally, there is a need for improvements in SODIS efficiency and quality assurance for the end user.

To this end we have been investigating the use of photocatalysis to enhance the rate of kill of pathogens at pilot scale under real sun conditions at the Plataforma Solar de Almeria in collaboration with Pilar Fernandez, CIEMAT, Spain. Also, we have been developing sensor technologies to provide automated control and quality assurance for the end user. Our collaborators include partners in Kenya, Zimbabwe and South Africa. Nanotechnology could help save lives in the developing world.

References

1. Byrne J.A., Eggins B.R., Brown N.M.D., McKinney B., and Rouse M., Immobilisation of TiO2 powder for the treatment of polluted water. Applied Catalysis B: Environmental, 1998, 17, pp 25-36.
2. McMurray, T.A., Byrne, J.A., Dunlop,P.S.M., Winkelman, J.G.M., Eggins, B.R., and McAdams, E.T., "Intrinsic kinetics of photocatalytic oxidation of formic and oxalic acid on immobilised TiO2 films." Applied Catalysis A: General, 2004, 262, 1, 105-110.
3. Byrne J.A., Eggins B.R., Byers W., and Brown N.M.D., Photoelectrochemical cell for the combined photocatalytic oxidation of organic pollutants and the recovery of metals from waste waters. Applied Catalysis B: Environmental, 1999, 20, L85.
4. Coleman H.M., Eggins B.R., Byrne J.A., Palmer F.L., and King E., Photocatalytic degradation of 17-ß-oestradiol, Appl. Catal. B Environmental, 2000, 24, L1 - L5.
5. Coleman, H.M., Routledge, E.J., Sumpter, J.P., Eggins, B.R., and Byrne, J.A., "Rapid loss of estrogenicity of steroid estrogens by UVA photolysis and photocatalysis over an immobilised titanium dioxide catalyst," Water Research, 2004, 38, 3233-3240
6. Dunlop, P.S.M., Byrne, J.A., Manga, N., and Eggins, B.R., The photocatalytic removal of bacterial pollutants from drinking water, Journal of Photochemistry and Photobiology A: Chemistry, 2002, 148, pp 355-363.
7. Dunlop P S M, McMurray T A, Hamilton J W J, Byrne J A, "Photocatalytic inactivation of Clostridium perfringens spores on TiO2 electrodes", Journal of Photochemistry and Photobiology A: Chemistry, 2008, 196, 113-119