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

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)(3)Al2O3SiCH3, 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)(3)Al(2-x)FexO(3)SiCH(3), where x = 0.025 or 0.050) were prepared by ion exchange of preformed methyl-imogolite with FeCl3 x 6H(2)O 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 4.9 eV to 2.4 eV. At higher Fe content (sample with nominal composition (OH)(3)Al1.950Fe0.050O3SiCH3), some Fe oxohydroxide 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 2 h, likely due to the formation of reactive AlOOH groups promptly generating HO center dot radicals. With sample having nominal composition (OH)(3)Al1.975Fe0.025O3SiCH3, Fe3+ species undergo efficient photo-Fenton reaction under UV light, leading to 90% TOC removal after 2 h. Conversely, the Fe oxo-hydroxide clusters at nanotubes outer surface likely worsen the photo-Fenton activity of the sample with (OH)(3)Al1.950Fe0.050O3SiCH3 nominal composition as far as the TOC removal is concerned.