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.