A precise iron-57 Mössbauer spectroscopic study of iron(III) in the octahedral and channel sites of akaganéite (β-iron hydroxide oxide)

Abstract

The ⁵⁷Fe Mössbauer spectra of β-FeO(OH)(akaganéite), prepared by two different methods and subjected to different washing procedures, have been determined under conditions of higher resolution than those previously reported. The spectra recorded at room temperature have been resolved, using a computer-based curve-fitting procedure, into two closely overlapping quadrupole-split iron(III) doublets, rather than the single doublet reported previously. The inner doublet having an average value of the quadrupole interaction Δ= 0.545 mm s^–1 and isomer shift δ= 0.155 mm s^–1 arises from Fe³⁺ in the Fe(O,OH)₆ octahedra of the β-FeO(OH) lattice. The outer doublet having Δ= 0.927–0.959 mm s^–1 and an average value of δ= 0.157 mm s^–1 probably arises from Fe³⁺ in the channels within the unit cells, with the range of Δ values presumably being due to variation in the Cl^–, [OH]^–, and H₂O content in these channels.