Inorganic nanotubes with permanent wall polarization as dual photo-reactors for wastewater treatment with simultaneous fuel production

Abstract

Photocatalytic production of fuels, even in small quantities, from the mineralization of hazardous pollutants, is a promising and renewable way of recycling wastewater. In the present work, the potential of methyl functionalized inorganic aluminosilicate nanotubes (methyl imogolite/Imo-CH₃) as photocatalytic nanoreactors for this application is demonstrated. Using the phototoxic polycyclic aromatic hydrocarbon dibenzo(a,h)anthracene (DBAN) as a model pollutant, we show that DBAN molecules can be efficiently trapped inside Imo-CH₃ nanotubes in an aqueous medium to undergo subsequent oxidative photo-degradation under UV light. The kinetics of this photo-degradation were shown to depend strongly on both the initial DBAN concentration in the nanotubes and the presence/absence of dissolved dioxygen. The photo-degradation process followed a complex mechanistic pathway, consisting of combined photo-oxidation and photo-cycloaddition reactions, where detection of carbon dioxide (CO₂) as a photo-oxidation product confirmed the mineralization of encapsulated DBAN. CO and CH₄ molecules were also formed, however these could arise from the further photo-reduction of CO₂ on the external surface of the nanotubes. Moreover, dihydrogen (H₂) was produced upon UV illumination under anaerobic conditions due to water reduction reactions on the external surfaces of Imo-CH₃ nanotubes. The possible mechanistic pathways of these processes are proposed, and the dual capability of Imo-CH₃ nanotubes for simultaneous pollutant degradation and H₂ production is then demonstrated – a rare feat for a single photocatalyst material.