New insights into short
ordered Fe-Al oxyhydroxide
coprecipitates
X-ray absorption spectroscopies at the K
a
edges of Fe and Al were conducted at LUCIA
and SAMBA beamlines. They were decisive
in understanding how these two cations
interact during mineral phase formation
by forced hydrolysis. The classical view of
an oxyhydroxy-polymer with substitution
between Al and Fe is now refined.
Low crystallinity iron oxyhydroxides are
ubiquitous in environmental systems,
where they act as strong sorbents,
thus playing a dominating role for
the distribution of trace compounds
between solid and solution [1]. They
are rarely present as a pure iron phase;
the association of Fe(III) and Al in
oxyhydroxides is particularly common
because of the abundance of both ions
at the Earth surface and because of
similarities in aqueous ion speciation.
We investigated the products of hydrolysis
of a solution containing both metal
ions. The objective was to identify the
interactions occurring between the cations
during precipitation and to evaluate
the effect of changes in Al/Fe ratio.
The two solid phase end-members in the
Fe-Al system, hydrous ferric oxide, HFO
and hydrous aluminous oxide, HAO are
quite distinct chemically and structurally.
The solubility constants for HFO and HAO
are 6 orders of magnitude apart and the
two precipitates differ in the degree of
polymerisation, Al remaining significantly
more hydrated than Fe. Crystallinity of the
pure end members is also different. HFO is
a 2-line ferrihydrite characterised by two
diffuse diffraction lines on XRD patterns,
also well identified by SAED. Conversely,
HAO is amorphous towards X-rays and
electrons. Despite these differences in the
end-member phases, it has been claimed
that hydroxide formation in solutions with
both cations present, results in mixed
phases (HFAO) with substitution between
Fe and Al [2].
Fe-Al oxyhydroxy-coprecipitates were
synthesized by forced hydrolysis
of Fe(NO
3
)
3
.9H
2
O and Al(NO
3
)
3
.9H
2
O
solutions. Solids containing 0, 1, 10, 25,
50, 75, 90 mol% Al were characterised
for composition, texture, mineral structure
and local atomic environment.
EXAFS analyses at the Fe K-edge
conducted on the co-precipitates were
well fitted by assuming only Fe and O as
close and distant neighbours of the target
iron atom. Substituting Al for Fe in the
local environment of Fe could not provide
satisfactory EXAFS fits, suggesting that Al
was not present in the local environment
around iron atoms. Nonetheless Al was
generally associated with HFO. At less
than 50% Al, the Al/Fe ratio measured by
TEM-EDXS on individual particle spots was
homogeneous and representative of the
bulk ratio, suggesting that the Al-hydroxide
forms a coating around the HFO nuclei
[3]. Such a surface rim can be explained
by Al(H
2
O)
6
3+
binding to hydroxy sites at
the surface of the HFO nuclei, as soon
as these are formed during synthesis.
Al K-edge XANES spectra revealed the
presence of structurally well organised
Al-octahedra, best interpreted through
a mono/multilayer hypothesis (Fig.
➊
).
Below 50 % Al, planar gibbsite-like local
structures were observed while above 50
% Al, three-dimensionally linked octahedra
appeared. The planar Al linkages may
reflect monolayer formation at the HFO
surface. Multiple layer sorption should
favour the formation of 3D linkages as
they were indeed observed with the
occurrence of boehmite-like organisation
above 50 % Al.
Fe and Al hydroxypolymers
Location of Al in HFAO’s
CHEMISTRY AND PHYSICAL CHEMISTRY, NANOCHEMISTRY
38
SYNCHROTRON
HIGHLIGHTS
2013
