Isomorphic substitution of aluminium by iron into single-walled alumino-silicate nanotubes: A physico-chemical insight into the structural and adsorption properties of Fe-doped imogolite

Abstract

Morphological, textural and surface properties are reported of Fe-doped aluminosilicate nanotubes (NTs) of the imogolite type (IMO, (OH)3Al2O3SiOH). Two samples with 1.4 wt% Fe were obtained by either direct synthesis (Fe-1.4-IMO) or post synthesis loading (Fe-L-IMO). Since formation of Fe2O3 clusters was observed in both cases, a third sample with a lower Fe content corresponding to 0.70 wt% Fe was prepared by direct synthesis, with the aim of preventing clusters formation (Fe-0.70-IMO). The samples were characterized by: High Resolution Transmission Electron Microscopy (HRTEM); X-ray Diffraction (XRD); N2 sorption isotherms; Thermo-Gravimetric Analysis (TGA); Diffuse Reflectance (DR) UV–vis and IR spectroscopies; electrophoretic mobility in water (ζ-potential) and adsorption of the azo-dye Acid Orange 7 (NaAO7) from water solutions. With Fe-0.70-IMO, Al3+ isomorphic substitution by Fe3+ was the main process occurring, leading to a material with formula (OH)3Al1.975Fe0.025O3SiOH, in which formation of Fe(OH)Al groups occurred at NTs outer surface. With Fe-1.4-IMO, some of the formed Fe(OH)Al groups probably acted as nucleation seeds, due to the natural tendency of iron to form Fe–O–Fe bridges, being responsible of the (undesired) formation of some Fe2O3 clusters. Comparison with Fe-L-IMO, in which only the formation of Fe2O3 clusters was, in principle, expected, showed on the contrary that some isomorphic substitution occurred also by post-synthesis loading.The intrinsic acidity of Fe(OH)Al groups was marginally different from that of Al(OH)Al groups, as shown by both IR spectroscopy and ζ-potential measurements, but the presence of iron modified the adsorption properties of IMO. Interaction with AO7− anions in water, indeed, occurred in different ways: i) with proper IMO, besides electrostatic interaction with the positively charged external surface of nanotubes, AO7− anions preferentially adsorb via H-bonding on inner SiOH groups; ii) with both Fe-0.70-IMO and Fe-1.4-IMO, Fe3+ cations of Fe(OH)Al groups act as coordination centres for N atoms in the AO7− moiety; iii) with Fe-L-IMO, larger Fe2O3 clusters likely hinder AO7− adsorption.