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

Morphological, textural and surface properties are reported of Fe-doped aluminosilicate nanotubes (NTs) of the imogolite type (IMO, (OH)(3)Al2O3SiOH). 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); N-2 sorption isotherms; Thermo-Gravimetric Analysis (TGA); Diffuse Reflectance (DR) UV-vis and IR spectroscopies; electrophoretic mobility in water (zeta-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)(3)Al1.975Fe0.025O3SiOH, 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 zeta-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.