Irradiation of azo dyes and gold nanoparticles: beware of pitfall!

Azo dyes are well known chemicals used by the textile industry and found in wastewater. Promising solutions to degrade dyes in polluted water are based on radiation techniques. In this work, the discolouration and degradation of the three azo dyes Blue Evans, Tropaeolin O, and Procion Red have been studied as a function of the gamma irradiation dose. We quantified the discolouration of the dyes by spectroscopy and characterized the associated dye degradation by chromatography. In addition, we performed a mechanistic investigation, that indicated that both the hydroxyl radical and the solvated electron were involved in the dye degradation. We then added gold nanoparticles to the dye solutions as they are known to increase the radical production during radiolysis. Unexpectedly, an overall apparent decrease of sensitization was observed. We pointed out that the gold nanoparticles potentially had an enhancement effect but that it is probably limited on molecules adsorbed on their surface, de facto inducing the protection of the dyes in the bulk. Our results thus underline the need for in-depth physicochemical characterization of the complex system made up of dyes and nanoparticles before and after irradiation to avoid misleading observations of the effects induced by nanomaterials on the dye degradation.