Photocatalytic properties of nanocrystalline titanium dioxide films in the degradation of domoic acid in aqueous solution: potential for use in molluscan shellfish biotoxin depuration facilities

Abstract

Domoic acid (DA) is a water-soluble marine neurotoxin produced and released by certain species of the diatom genus Pseudo-nitzschia. Present in coastal waters, it can be a threat to public health and marine life, and can result in severe economic losses to the molluscan shellfish and crustacean harvesting industries. Here we report on the efficiency of nanocrystalline (NC) titania (TiO₂) thin films used as a photocatalyst in the ultraviolet light photodegradation of DA. Titanium dioxide thin films produced by a sol-gel dip-coating method in the presence of polyethylene glycol of different molecular weights (200, 400 and 600) were deposited on glass substrates and crystallised at 90°C. The films were characterised using spectroscopic ellipsometry, Fourier transform infrared spectrometer (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The photocatalytic activity measurements were carried out by immersing the NC TiO₂ films in a DA solution (2500 ng ml⁻¹) and then exposing them for various times at room temperature to UVA irradiation (λ = approximately 350 nm). The degradation of DA, quantified by HPLC analysis, was not significant when using daylight or ultraviolet light irradiation alone, whereas the NC TiO₂ films prepared at low temperature proved to be a very efficient photocatalyst when used in conjunction with UVA light. The effectiveness of the photodegradation was improved by increasing molecular weight of polyethylene glycol, which increased the thickness of the film. The presence, transformation and degradation of three DA isomers were observed. The approach may eventually be practical for destroying DA in seawater used by aquaculture industry depuration facilities.

Keywords:

• domoic acid
• low temperature crystallization
• nanocrystalline titanium dioxide
• thin film
• photocatalytic activity
• photodegradation mechanism

Acknowledgements

The financial support of the Research Assistantships Initiative (New Brunswick Innovation Fund), the Con-sortium national de formation en santé (CNFS) volet Université de Moncton, concours 2007–2008, and of the Atlantic Innovation Fund (AIF) Round I, is gratefully acknowledged.