The nature of iron in Akaganéite (β-FeOOH)

Abstract

The Mossbauer spectra of a size range of synthetic akaganeite (β-FeOOH) samples containing chloride, and one sample containing fluoride, in the structural tunnels, have been obtained over 4-295 K. At 295 K the spectra consist of overlapping paramagnetic doublets which indicate the presence of at least two sites for iron. Typical parameters for a spectrum of chloride-containing β-FeOOH fitted to two doublets are: (i) δ (relative to iron metal) 0.372 mm s-1, Δ 0.55 mm s-1, Г 0.27mm s-1; (ii) δ 0.375 mm s-1, Δ 0.94mm s-1, Г 0.33 mm s-1, with peak area ratios (i) : (II) of 60 : 40 for lath-shaped particles and 53 : 47 for spindle-shaped particles. At 77 K the spectra consist of overlapping magnetic hyperfine components which indicate the presence of at least three sites for iron, a three-component model giving internal magnetic fields of about 470 kOe, 460 kOe, and 435 kOe. The spectra of fluoride-containing β-FeOOH are very similar to those for chloride-containing β- FeOOH. The results are explained in terms of two types of octahedra within the lattice, FeO3(OH)3 and FeO2(OH)4, the extra proton in the second type balancing the charge on halide ions within the tunnels. Secondary effects may arise from the interaction of the halide ions with adjacent octahedra. The effects of modifying the surfaces of β-FeOOH particles (heating, phosphate sorption) are consistent with modification of the outer surfaces of the particles only, and it is not necessary to invoke the presence of large internal pores, of the order of 30Ǻ by 30Ǻ, as has been suggested by some earlier workers.

Phosphorus binding by poorly crystalline iron oxides in North Sea sediments

“…Akageneite, in contrast, is very rare in soils. The conditions for formation of akageneite in the marine environment are favourable, however, as high Cl-(or F-) concentrations are a prerequisite for its formation (Childs et al, 1980;Schwertmann and Cornell, 1991). Murray (1978) found it to be the form of Fe that precipitates from Fe3+ in seawater and suggested (Murray, 1979) that hydrolysis of Fe*+ in seawater may also produce akageneite.…”

The preparation and characterization of iron trihydroxide, Fe(OH)3

“…This material was shown to be quite different from any other hydrated iron(II1) oxide or iron(II1) hydroxides that had been reported previously (2)(3)(4)(5)(6)(7)(8)(9)(10)(11). It was concluded that this sample of Fe(OH)3 had an average particle size of -80 P\ in diameter and that the two types of iron environment, visible in the low temperature 57Fe Mossbauer spectrum, were due to surface and bulk iron; the former having a hyperfine field of 437 kOe while the latter, presumably more magnetically ordered, has a field of 486 kOe.…”

Chlorine Occupancy Dependence of Crystal Structure of Pure β-Phase of Iron-Oxyhydroxide

“…Among candidates of such atoms or molecules, H 2 O will be most reasonable. Some previous investigations also suggested that H 2 O molecules would occupy some of tunnel sites 8,20) whereas Stahl et al suggested that tunnel sites does not contain water molecules. 13) In their analysis, they assumed that occupation factor, g ¼ 2=3 (they regarded as g ¼ u).…”