Journal of Alloys and Compounds 416 (2006) 284–290
Influence of ruthenium ions on the precipitation of -FeOOH,
-Fe
2
O
3
and Fe
3
O
4
in highly alkaline media
Stjepko Krehula, Svetozar Musi´ c
∗
Division of Materials Chemistry, Ru
–
der Boˇ skovi´ c Institute, P.O. Box 180, HR-10002 Zagreb, Croatia
Received 13 August 2005; accepted 1 September 2005
Available online 27 October 2005
Abstract
The influence of ruthenium ions on the precipitation of goethite (-FeOOH), -Fe
2
O
3
and Fe
3
O
4
in highly alkaline media was investigated
by
57
Fe M¨ ossbauer and FT-IR spectroscopies, thermal field emission scanning electron microscope (FE SEM) and EDS. The presence of Ru-
dopant strongly affected the precipitation of -FeOOH at highly alkaline pH, i.e. the formation of -Fe
2
O
3
was also noticed. A decrease of
hyperfine magnetic field (HMF) at RT from 35.1 T (undoped -FeOOH) to 31.3 T for sample with [Ru]/([Ru] + [Fe]) = 0.0196 was assigned to
the incorporation of ruthenium ions into the -FeOOH structure. M¨ ossbauer spectroscopy showed the formation of stoichiometric Fe
3
O
4
for
[Ru]/([Ru] + [Fe]) = 0.0291–0.0909. -Fe
2
O
3
and Fe
3
O
4
did not show a tendency to the formation of solid solutions with ruthenium ions. FE SEM
observations of the samples showed that reference -FeOOH sample contained acicular particles of good uniformity, which increased the length
up to ∼5 times with increase of concentration of ruthenium ions. On the other hand, large octahedral Fe
3
O
4
crystals (particles) were associated
with small particles of ruthenium (hydrous) oxide with a size in the range ∼100 nm or less. A possible catalytic action of ruthenium that created
reduction conditions for Fe
3+
ions and formation of Fe
2+
ions for precipitation of Fe
3
O
4
was discussed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Ru-dopant; Tetramethylammonium hydroxide; -FeOOH; -Fe
2
O
3
; Fe
3
O
4
;
57
Fe M ¨ ossbauer; FT-IR; FE SEM; EDS
1. Introduction
Goethite (-FeOOH) is naturally present in various soils,
marine sediments and ore deposits. As a rule, natural -FeOOH
is not pure, and it may contain metal cations substituted in
various concentrations. -FeOOH is a dominant fraction in
limonite ore (FeOOH·nH
2
O) which is used in the production
of iron. Also, -FeOOH is the constituent of the rust formed
by atmospheric or “wet” corrosion of iron (steel). The fraction
of -FeOOH phase in the rust depends on the conditions of the
rusting of iron (steel). The color of natural -FeOOH can vary
from lemon yellow to dark brown, and the changes in the color
are not only due to the particle size differences, but are also a
consequence of the metal cation substitutions.
Synthetic -FeOOH can be prepared in a chemical labora-
tory by: (a) hydrolysis of Fe(NO
3
)
3
aqueous solutions at low
pH values, (b) precipitation from Fe(III)–salt solutions at a very
∗
Corresponding author.
E-mail address: music@irb.hr (S. Musi´ c).
high pH and (c) oxidation of Fe(OH)
2
suspensions with air
or O
2
. Synthetic -FeOOH in the form of acicular -FeOOH
particles is used as a starting material in the production of aci-
cular maghemite (-Fe
2
O
3
) particles via magnetite (Fe
3
O
4
) as
a transition phase. The acicular shape of the particles could be
preserved during all stages of this synthesis. The researchers
made a significant effort in the investigation of the metal cation
substitutions in iron(III)-oxyhydroxides and -oxides due to their
importance in the industry and environment.
Cadmium ions in an aqueous environment are very toxic for
humans, and they come from mines or various industrial wastes.
Cd
2+
ions can be removed from contaminated aqueous solutions
by -FeOOH using an adsorption/coprecipitation mechanism.
Cd
2+
ions can be incorporated up to ∼9.5% of the Fe
3+
ions in
the octahedra of -FeOOH [1]. A progressive increase in the size
of the unit-cell parameters and unit-cell volume of -FeOOH
due to the much larger Cd
2+
ion (0.95
˚
A) compared with Fe
3+
(0.645
˚
A) was noticed, as well as a decrease of crystallinity. Sileo
et al. [2] monitored incorporation of Cd
2+
ions in -FeOOH
in alkaline media. The authors noticed a drastic decrease of
incorporation of Cd
2+
into -FeOOH structure for μ
cd
= 7.03
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.09.016