Variable Hydrogen Bond Strength in Akaganeite

Akaganeite (beta-FeOOH) is a chloride-bearing iron oxy-hydroxide with a hollandite-type structure. This high specific surface area mineral has been the object of numerous studies given its high reactivity and involvement in natural and industrial processes. The important ion exchange attributes of this mineral involve similar to 0.4 x 0.4 nm wide channels in which chloride ions are stabilized by hydrogen bonding from bulk OH groups. This work provides further details on the relationship between bulk chloride ion loadings and hydrogen bond strengths. Molecular dynamics calculations were first carried out on chloride-free and bearing lattices to build a conceptual model for possible interactions in the akaganeite bulk. Experimental work was thereafter carried out on synthetic acicular particles (7 x 80 to 11 x 110 nm) reacted to aqueous solutions of HCl, then dried under dry N-2(g). These samples were studied by Fourier transform infrared spectroscopy, temperature-programmed desorption, X-ray photoelectron spectroscopy, and X-ray powder diffraction as well as transmission electron spectroscopy. Results collectively show that Cl/Fe molar ratios increasing from 0.169 up to 0.442 induce important changes in the hydrogen bonding environment of bulk hydroxyls. This can specifically be seen through shifts in bulk O-H stretching frequencies from 3496/3395 to 3470/3350 cm(-1). These changes are associated with a substantial shortening of particle lengths (97 to 45 nm), expansion of crystallographic lattice size (up to 0.9%), and increases in median thermal dehydroxylation temperatures (260 to 305 degrees C). Our work thereby highlights important variations in physicochemical attributes of akaganeite particles reacted with HCl. Such variations should consequently be considered in settings involving submicrometer-sized akaganeite particles.