comparison of adsorption and post-adsorption behavior of
TRANSCRIPT
1
Comparison of adsorption and post-adsorption behavior of oxyanions between ferrihydrite and schwertamnnite
Khandala Khamphila
Candidate for the Degree of doctoral Supervisor: Prof.Tsutomu Sato
Division of sustainable resources engineering
Introduction
Water contamination is a serious problem around the
world. Many toxic elements such as arsenic, chromium
and selenium are seriously problem in the surface and
groundwater, because most of them are oxyanions in the
natural water and they are highly mobiles over a wide
range of redox conditions.
The migration of dissolved trace species in surface
condition are initially retarded by the adsorption process
at mineral water interface. Especially, low crystalline
ferric oxides are known to be the most effective
scavengers for these species. The interaction of low
crystalline ferric oxide with cationic metal species have
been widely investigated by laboratory experiments and
field observations. However, these with anionic species
have been lacking information investigated due to their
complex behavior in the natural system. Ferrihydrite
and schwertmannite are iron oxide minerals and low
crystalline minerals had high specific surface area [1],
[2]. Ferrihydrite and schwertmannite occur in natural
with different pH conditions, ferrihydrite precipitated in
soil and sediment with pH in neutral conditions,
schwertmannite occurrences in pH acidic condition in
mining tailing [3], there are different with surface
properties, ferrihydrite surface are hydroxyl group with
had ligands exchange with oxyanions. Meanwhile,
schwertmannite has hydroxyl group and SO4 group and
there is some information adsorption arsenate [4], [5].
The low crystalline Fe(III) oxide are metastable phase
and eventually transform to more a stable phase with
time. Therefore, it is important to understand the
stability of the host mineral and sorbed species for the
prediction and assessments of long term behavior of
anionic species. Ferrihydrite and schwertmannite both
are metastable phase and transform to stable phase as
goethite by dissolution and reprecipitation. Ferrihydrite
is known as a ferric iron oxide mineral, which is highly
effective for waste water treatment and has application
to predict the adsorption capacity such as surface
complexation modeling, meanwhile, information is
lacking in which surface complexation modeling of
schwertmannite. The comparison of both ferrihydrite
and schwertmannite is important to better understand
the adsorption and post-adsorption properties.
Particularly, this study aims to
To understand the differences and similarities
in adsorption behavior of oxyanions by
ferrihydrite and schwertmannite
To understand the different and similarities in
post-adsorption behaviors of oxyanions
between ferrihydrite and schwertmannite
To apply surface complexation modeling for
different kinds of oxyanions on ferrihydrite and
schwertmannite to check applicability of
existing double layer model.
Method and material
Preparation of adsorbents
Ferrihydrite was prepare as adsorbent by using digital
titration machine TOADKK-AUT-701, 0.1M of
Fe(NO3)3.9H2O, 500 ml was prepared and set on the
stirrer, 0.1 of KOH solution was also prepared about
500 ml set into the machine, Auto titration was added
0.04ml/min (KOH) to the solution, until reach to pH7
[6], sample was centrifuge 3000rpm, 40 minutes’ wash
in 6 times, then filtered through 0.2µm cellulose
membrane. The resulting of solids was freeze dried and
identify the synthesized phases solids was examined by
Rigaku X-Ray diffractometer with CuK radiation
(40kV and 40 mA), the result of XRD pattern was
identical to that of the previously reported of
ferrihydrite [7]. Schwertmannite was prepared by the
method previously reported by Bigham et al [8]. Mixing
solution prepared by 0.04 M Na2SO4 solution and 0.04
M Fe(NO3)3.9H2O solution was held at 60C for 12
minutes, then cooled and dialyzed for 30 days,
deionized water used for the dialyzing was changed
every day. To remove the salt in the surface of mineral
the product was clean by using deionized water and was
filtered through a 0.2 µm cellulose membrane then
immediately freeze-dried to prevent transformation to
another phase. Further, X-ray diffraction (XRD)
analyses were conducted to identify the synthesized
phases by Rigaku X-Ray diffractometer with CuKα
radiation (40kV and 40mA). From the XRD analyses,
the synthesized products were identified as a
schwertmannite because the XRD pattern was identical
to that of the previously reported schwertmannite by
Bigham et al [2].
Adsorption experiment
All adsorption experiments were conducted at a
constant ionic strength (I=0.01 M, NaNO3) by using a
50 ml centrifuge tube adding 40 ml solution of
Na2HPO4, Na2HAsO4, Na2CrO4, and Na2SeO4 with
concentrations from 0 to 2 mM and 40 mg of the
synthetic schwertmannite and ferrihydrite. The pH of
the adsorption media was adjusted to 7.00 ± 0.15 by
0.1M NaOH solution. The samples were placed in a
2
reciprocal shaker at 25C, 100 rpm for 24 hours. The
adsorption experiments for surface complexation
modeling were performed as function of pH from pH 3
to pH 12. The filtered solids were freeze-dried for
measurements of Zeta potential (Malvern Zetasizer
Nano series Nano-ZS90 instrument) and liquid samples
were used for the inductively coupled plasma atomic
emission spectroscopy, (ICPE-9000, ICP-AES) and ion-
chromatography (Metrohm 861 Advanced Compact IC
instrument) to determine the concentration of elements
after the adsorption. The released SO42-
in the
equilibrium solutions were also determined for
schwertmannite.
Alteration experiment
The dried powders of schwertmannite and ferrihydrite
were added to the solutions of the anions, for the
adsorption process, the pH was adjusted by 0.1 M
NaOH and 0.1 HNO3 to 7.00±0.12 and 30-35 mg/g of
the solid phase of anions of arsenate, phosphate,
chromate or selenate were adsorbed on the
schwertmannite, separately. The solids after the
adsorption were mounted and dried on silica glass. The
glasses with the mounted solid samples were kept in
boxes with wet cotton at 60C to accelerate alterations
in a moisture condition. At the different aging times, the
samples were analyzed by XRD to determine the extent
of the phase transformation.
Result and discussion
Adsorption behavior of oxyanions onto ferrihydrite and schwertmannite
The result of adsorption capacities between ferrihydrite
and schwertmannite, under these conditions showed that
the schwertmannite’s adsorption capacity is higher than
the ferrihydrite’s adsorption capacity. However, the
adsorption selectivity of oxyanion adsorption on both
schwertmannite and ferrihydrite decreases in the
following order: arsenate phosphate > chromate
>>selenate (Figure 1). The change of zeta potential for
ferrihydrite and schwertmannite before and after
adsorption shown the changing of zeta potential, when
decreasing of zeta potential consistent with previous
work which explain the mechanism of oxyanion
adsorption. Arsenate, phosphate might form inner-
sphere complexes with the surface of ferrihydrite and
schwertmannite. The different behavior is the selenate
and sulfate ions form outer-sphere complexes with the
surface of schwertmannite. Chromate in between
arsenate and selentate with surface of schwertmannite
assumed that chromate might make intermediate
complexes. For ferrihydrite chromate and selenate
might be made similar behavior as intermediate on
ferrihydrite. Strong base anions such as arsenate and
phosphate can form inner-sphere complexes, which
induces a strong adsorption with ferrihydrite and
schwertmannite as well as provides a high adsorption
capacity (Error! Reference source not found.)
________________________________________________________________________________________________
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
0 0.5 1 1.5
Ad
sorp
tio
n (
mo
l/m
2)
Initial concentration (mmol)
Ferrihydrite
a
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
0 0.5 1 1.5
Ad
sorp
tio
n (
mo
l/m
2)
Initial concentration (mmol)
Schwertmannite
b
0
20
40
60
80
100
0 0.5 1 1.5
Figure 1. Oxyanions adsorption on ferrihydrite and schwertmannite as function of the initial concentration of oxyanion
in solution, with pH adjustment (: Arsenate, : Phosphate, : Chromate and : Selenate)
3
-60
-40
-20
0
20
40
3 4 5 6 7 8 9 10 11
Zet
a p
ote
nti
al
(mV
)
pH
-60
-40
-20
0
20
40
3 4 5 6 7 8 9 10 11
Zet
a P
ote
nti
al
(mV
)
pH
(a) (b)
Figure 2. The zeta potential with function of pH, (a) display Zeta potential for ferrihydrite, (b) display zeta potential
for schwertmannite (: Original, : Arsenate, : Phosphate, : Chromate and : Selenate)
________________________________________________________________________________________________
Post-adsorption behavior of oxyanions onto ferrihydrite and schwertmannite
The post-adsorption behavior of oxyanion onto
ferrihydrite and schwertmannite was investigated. To
better understand the stabilization of minerals. The
solubility of schwertmannite with different oxyanions
was calculated by the solid solution theory. A
comparison of post-adsorption behavior between
schwertmannite and ferrihydrite showed that solubility
of ferrihydrite is lower than schwertmannite’s solubility
that is why ferrihydrite is more stable than
schwertmannite shown in XRD pattern (Error!
Reference source not found. and Error! Reference
source not found.). In case of comparison of other
oxyanions adsorption on schwertmannite showed that
the degree of retardation in transformation to goethite
decreased as
arsenate=phosphate>chromate>selentate>sulfate
(Error! Reference source not found.). The solubility
of mineral after adsorption increase in the following
order: arsenate<phosphate<chromate
<selenatesulfate.
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Fh-Chromate
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Fh-Selenate
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Ferrihydrite
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Fh-Phosphate
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Fh-Arsenate
º2CuK º2CuKº2CuKº2CuKº2CuK
Figure 3. X-ray diffractogram of synthetic ferrihydrite and ferrihydrite after adsorption of each oxyanion with different
aging times
4
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Chromate
G
G G
G=Goethite
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Selenate
G
GGG
G=Goethite
G
10 20 30 40 50 60 70
7 days
14 days
21 days
30 days
GG G
GG
G
Sulfate
G
G
0 day
G=Goethite
G
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Phosphate
10 20 30 40 50 60 70
0 day
7 days
14 days
21 days
30 days
Arsenate
º2CuK º2CuKº2CuKº2CuKº2CuK
Figure 4. X-ray diffractogram of the synthetic schwertmannite and schwertmannite after adsorption of each oxyanion
with different aging times.
________________________________________________________________________________________________
Therefore, oxyanions with a high selectivity can
stabilize schwertmannite by lowering the solubility of
schwertmannite after adsorption of oxyanions. The
similar characteristic of post-adsorption is the oxyanions
effected to stabilization of both minerals. The different
shown ferrihydrite’s solubility is lower than
schwertmannite’s solubility. But in pH acidic condition
to pH neutral condition schwertmannite’s solubility
decreased to similar with ferrihydrite’s solubility.
-10
-9
-8
-7
-6
-5
-4
-3
-2
3 4 5 6 7 8 9 10 11
log
tota
l F
e(II
I) a
ctiv
ity
pH
Solubility of Fe
Ferrihydrite Anion free, I=0.01 (Fukushi et al, 2005)
Ferrihydrite containing SO4 (Fukushi et al, 2005)
Ferrihydrite containing PO4 (Fukushi et al, 2005)
Ferrihydrite containing As(V) (Fukushi et al, 2005)
Ferrihydrite containing Cr(VI) (This study)
SCH anion free, I=0.01 (This study)
SCH containing Cr(VI) (This study)
SCH containing PO4 (This study)
SCH containing As(V) (This study)
Figure 5. Solubility of diagram of Fe(III) for
ferrihydrite and schwertmannite
Surface complexation modeling for many kinds of oxyanions onto ferrihydrite and schwertmannite
The surface complexation modeling which is known as
a theoretical method and a tool for prediction of
adsorption in the natural system was applied. However,
ferric oxide has already been established in many of the
adsorption conditions, such as ferrihydrite adsorbing
arsenate and phosphate as inner-sphere complexes [9],
they was modeled by extended triple layer modeling
(ETLM).
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tion
fra
ctio
n (
%)
pH
Ferrihydrite, 1 g L-1
1 mM As(V), 0.1 M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11A
dso
rpti
on f
ract
ion
(%
)pH
Ferrihydrite, 1g L-1
Se(VI) 1mM, 0.1 M NaNO3
Figure 6. DLM of arsenate adsorption onto ferrihydrite
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tion
fra
ctio
n (
%)
pH
Ferrihydrite, 1 g L-1
1 mM As(V), 0.1 M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tion
fra
ctio
n (
%)
pH
Ferrihydrite, 1g L-1
Se(VI) 1mM, 0.1 M NaNO3
Figure 7. DLM of selenate adsorption onto ferrihydrite
5
Meanwhile, adsorption information is lacking for
schwertmannite. The double layer modeling (DLM) was
performed following from previously study [10], by
using REACT in the Geochemist’s workbench (GWB)
[11], the result of arsenate adsorption onto ferrihydrite
with DLM, shown in Figure 6 the point is experimental
data and solid line is model, the experiment data was
fitting well with the model. The DLM was also applied
for selenate base on the data base of previously study
[10] by using REACT in GWB program. As shown in
the Error! Reference source not found., the
experiment data was not fitted well with this model
which just fit some of the data base, because selenate
was involved both surface species equation which are
inner-sphere and outer-sphere complexes. To better
fitted model, in this present study, extended triple layer
modeling (ETLM) was applied following previously
study [12], [13] for arsenate, chromate and selenate
adsorption onto ferrihydrite (Error! Reference source
not found., Error! Reference source not found. and
Error! Reference source not found.). The speciation
reaction equation for arsenate are following here:
O2HHAsOFeO)(AsOHFeOH2 222
0
43 (1)
O2HAsOOHFeO)(AsOHFeOH2 22
0
43 (2)
OHH2FeOAsOAsOHFeOH 2
2
3
0
43 (3)
The speciation reaction equation for selenate are
following here:
OHFeOSeOSeOHFeOH 23
-2
4 (4)
2
422
-2
4 _SeO)FeOH(SeOH2FeOH2 (5)
2
42
-2
4 _HSeOFeOHSeOH2FeOH (6)
The speciation reaction equation for chromate are
following here:
OHOOHFeOCrHCrOHFeOH 224 (7)
4224 _HCrO)FeOH(HCrO2HFeOH2 (8)
424 _HCrOFeOHHCrOHFeOH (9)
In case of applied ETLM onto schwertmannite for
oxyanions, in this present study surface protonation and
electrolytes adsorption equilibrium constants and
capacitances was calculated following previous study of
ferrihydrite. Because from the comparison adsorption
capacities, oxyanions selectivity and surface speciation
of ferrihydrite and schwertmannite. As known that
schwertmannite and ferrihydrite are precipitate in
mining site whereas rich iron; therefore, the different is
schwertmannite had SO4 sorbs in the tunnel structure. Although, the between schwertmannite and ferrihydrite
had similar, but additional to apply schwertmannite
ETLM, some reaction should be included
OHFeOSOSOHFeOH 23
2
4 (10)
42
2
4 _HSOFeOHSOH2FeOH (11)
As shown in Error! Reference source not found.
arsenate adsorption onto schwertmannite, the
experiment data was fitted to the model. In Error!
Reference source not found., dash line is the
estimating modeling, in this model involve only two
species surface reaction of chromate inner-sphere
complexes and bidentate outer-sphere complexes. From
the result of estimated modeling was fitted to the
experiment data.
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ra
cti
on
%
pH
Ferrihydrite, 1g L-1
0.2 mM As(V), 0.01 M NaNO3
0.6 mM As(V), 0.01M NaNO3
1 mM As(V), 0.01 M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ra
cti
on
%
pH
Ferrihydrite, 1g L-1
0.2 mM As(V), 0.1 NaNO3
0.6 mM As(V), 0.1M NaNO3
1 mM As(V), 0.1M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ra
cti
on
%
pH
Ferrihydrite, 1g L-1
1 mM As(V), 0.01 M NaNO3
1 mM As(V), 0.1M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10
% A
rse
na
te s
pecie
s
pH
Ferrihydrite, 1 g L-1
1 mM As(V), 0.1 M NaNO3
(>FeO)2AsO2-
HAsO4--
(>FeO)2AsOOH
>FeOAsO32-
H2AsO4-
a
dc
b
Figure 8. ETLM of arsenate adsorption onto
ferrihydrite
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tion
fra
ctio
n %
pH
Ferrihydrite, 1g L-1
0.2 mM Se(VI), 0.01M NaNO3
0.6 mM Se (VI), 0.01M NaNO3
1 mM Se(VI), 0.01M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ract
ion
%pH
Ferrihydrite,1g L-1
0.2 mM Se(VI), 0.1M NaNO3
0.6 mM Se(VI), 0.1M NaNO3
1 mM Se(VI), 0.1M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ract
ion
%
pH
Ferrihydrite, 1g L-1
1 mM Se(VI), 0.01M NaNO3
1 mM Se(VI), 0.1M NaNO3
0
20
40
60
80
100
4 5 6 7 8 9 10
% S
elen
ate
sp
ecie
s
pH
Ferrihydrite, 1g L-1
Se(VI) 1mM, 0.1 M NaNO3
(>FeOH2+)_SeO4
-
NaSeO4-
SeO4--
>FeOH2+_HSeO4
-
>FeOSeO3-
a
dc
b
Figure 9. ETLM of selenate adsorption onto ferrihydrite
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tion
fra
cti
on
%
pH
Ferrihydrite, 1 g L-1
0.2 mM Cr(VI), 0.01 M NaNO3
0.6 mM Cr(VI), 0.01M NaNO3
1 mM Cr(VI), 0.01M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tion
fracti
on
%
pH
Ferrihydrite, 1 g L-1
0.2 mM Cr(VI), 0.1 M NaNO3
0.6 mM Cr(VI), 0.1M NaNO3
1 mM Cr(VI), 0.1M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tion
fracti
on
%
pH
Ferrihydrite, 1 g L-1
1 mM Cr(VI), 0.01M NaNO3
1 mM Cr(VI), 0.1M NaNO3
0
20
40
60
80
100
4 5 6 7 8 9 10
% C
hro
ma
te s
pecie
s
pH
>FeOHCrO4-
>FeOH2+_HCrO4
-
HCrO4-
CrO4--
Ferrihydrite, 1 g L-1
1 mM Cr (VI), 0.1 M NaNO3
a
dc
b
Figure 10. ETLM of chromate adsorption onto
ferrihydrite
6
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ract
ion
%
pH
Schwertmannite, 1 g L -1
0.6 mM As(V), 0.01M NaNO3
1 mM As(V), 0.01 M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tion
fra
ctio
n %
pH
Schwertmannite, 1 g L -1
0.6 mM As(V), 0.1M NaNO3
1 mM As(V), 0.1M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ract
ion
%
pH
Schwertmannite, 1 g L -1
1 mM As(V), 0.01 M NaNO3
1 mM As(V), 0.1M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11 12
% A
rsen
ate
sp
ecie
s
pH
Schwertmannite, 1 g L-1
1 mM As(V), 0.1 M NaNO3
1.46 mM SO4-- sorbing on
H2AsO4-
HAsO4--
(>FeO)2AsO2-
>FeOAsO3-
>FeOSeO3-
SO4--
(>FeOH2+)2_SO4
-
a
dc
b
Figure 11. ETLM of arsenate adsorption onto
schwertmannite
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ra
cti
on
%
pH
Schwertmannite, 1 g L-1
0.6 mM Cr(VI), 0.01M NaNO3
1 mM Cr(VI), 0.01M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ra
cti
on
%
pH
Schwertmannite, 1 g L-1
0.6 mM Cr(VI), 0.1M NaNO3
1 mM Cr(VI), 0.1M NaNO3
0
20
40
60
80
100
3 4 5 6 7 8 9 10 11
Ad
sorp
tio
n f
ract
ion
%
pH
Schwertmannite, 1 g L-1
1 mM Cr(VI), 0.01M NaNO3
1 mM Cr(VI), 0.1M NaNO3
0
20
40
60
80
100
4 5 6 7 8 9 10 11
% C
hro
ma
te s
pecie
s
pH
Schwertmannite, 1 g L-1
>FeOCr(OH)O2
>FeOSO3-
>FeOH2+_SO4
2-
>FeOH2+_HCrO4
-
CrO42-
SO42-
>(FeOH2+)2_HCrO4
-
1 mM Cr(VI), 0.1 M NaNO3
a
d
c
b
Figure 12. ETLM of chromate adsorption onto
schwertmannite
Conclusion
Oxyanions were classified that arsenate and phosphate
are inner-sphere complexes, chromate, selenate and
sulfate are intermediate complexes. The ferrihydrite’s
solubility is lower than schwertmannite’s solubility
indicate that ferrihydrite is more stable than
schwertmannite. Oxyanions were effected to
stabilization of both minerals. In the natural is complex
system, surface complexation model as ETLM is useful
to predicted the adsorption capacities for oxyanions
adsorption on ferrihydrite and schwertmannite. In the
natural water treatment systems for both acid mine
drainage and ground water system, schwertmannite is
the appropriate material use for water treatment system,
because schwertmannite had high adsorption capacities.
To choose the materials are depending on exciting of
the materials near in the water treatment system site and
the concentration of the toxic elements. In case of
disposal site for materials, if used schwertmannite
adsorbs arsenate and phosphate, the disposal of
adsorbent will be safe. Base on the elementary
properties of cation and base on this studies result, other
oxyanions which had similar properties such as arsenate
and vanadate may have made inner-sphere complexes;
selenate, manganate and chromate may have made
intermediate complexes, but to better understand we
need experiments to support this prediction. For
oxyanions, which made intermediate and outer-sphere
complex we can consider to choose ferrihydrite and use
surface complexation modeling to predict the
concentration of adsorbents.
Reference
[1] R. M. Cornell and U. Schwertmann, “Influence of
Organic Anions on the Crystallization of
Ferrihydrite,” Clays and Clay Minerals, vol. 27, no.
6, pp. 402–410, 1979.
[2] J. M. Bigham, U. Schwertmann, L. Carlson, and E.
Murad, “A poorly crystallized oxyhydroxysulfate of
iron formed by bacterial oxidation of Fe(II) in acid
mine waters,” Geochimica et Cosmochimica Acta,
vol. 54, no. 10, pp. 2743–2758, 1990.
[3] J. M. Bigham, U. Schwertmann, S. J. Traina, R. L.
Winland, and M. Wolf, “Schwertmannite and the
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