Photo-activated degradation of tartrazine by H2O2 as catalyzed by both bare and Fe-doped methyl-imogolite nanotubes

Abstract

Imogolite-like nanomaterials were tested as heterogeneous catalysts for the photo-catalytic degradation of tartrazine (a food dye recalcitrant to biodegradation and responsible of allergic and/or intolerance reactions) in the presence of H2O2. Methyl-imogolite, a hybrid organic/inorganic material with chemical composition (OH)3Al2O3SiCH3, occurs as single-walled nanotubes with an inner surface lined by Si-CH3 groups and an aluminum oxo-hydroxide outer surface, where octahedral Al3+ ions may be isomorphically substituted by Fe3+ ions. Besides bare methyl-imogolite, novel Fe doped nanotubes (with nominal composition (OH)3Al2-xFexO3SiCH3, where x=0.025 or 0.050) were prepared by ion exchange of preformed methyl-imogolite with FeCl3×6H2O in water. Physico-chemical characterization of the materials showed that Fe doping positively modifies nanotubes light absorption capacity by lowering the band gap of methyl-imogolite from 4.9eV to 2.4eV. At higher Fe content (sample with nominal composition (OH)3Al1.950Fe0.050O3SiCH3), some Fe oxo-hydroxide clusters form, due to the natural tendency of Fe to form aggregates.Photo-degradation tests of tartrazine show that both bare and Fe-doped methyl-imogolite efficiently remove the dye from aqueous mixtures through different mechanisms. With bare methyl-imogolite, under UV light almost 65% Total Organic Carbon (TOC) is removed within 2h, likely due to the formation of reactive AlOOH groups promptly generating HO radicals. With sample having nominal composition (OH)3Al1.975Fe0.025O3SiCH3, Fe3+ species undergo efficient photo-Fenton reaction under UV light, leading to 90% TOC removal after 2h. Conversely, the Fe oxo-hydroxide clusters at nanotubes outer surface likely worsen the photo-Fenton activity of the sample with (OH)3Al1.950Fe0.050O3SiCH3 nominal composition as far as the TOC removal is concerned.