Clays and Clay Minerals, Vol. 47, No. 3, 3 7 5 - 3 8 5 , 1999.
P R E P A R A T I O N A N D PROPERTIES OF H E M A T I T E W I T H
STRUCTURAL PHOSPHORUS
NATIVIDAD GALVEZ, VIDAL BARRON, AND JOSE TORRENT
DepartaInento de Ciencias y Recursos Agricolas y Forestales, Universidad de Cdrdoba, Apdo. 3048, 14080 Cdrdoba, Spain
Abstract--Synthetic hematites prepared in the presence of phosphate can incorporate phosphorus (P) in
forms other than phosphate adsorbed by ligand-exchange on the crystal surface. To investigate the nature
of such occluded R which is also found in some natural specimens, we prepared 13 hematites by aging
ferrihydrite precipitated from Fe(NO3)3-KH2PO4 solutions. The P/Fe atomic ratio of the resulting hematites
ranged from 0 to 3% and all incorporated significant amounts of OH. As P content is raised, particle
morphology changes from rhombohedral to spindle or ellipsoid-shaped. Despite the grainy appearance in
transmission electron microscope images, X-ray diffraction data indicate that the particles are single
crystals. Specific surface area ranged from 66 to 91 m 2 g 1, partly in micropores. The intensity of the
absorption bands due to Fe 3+ ligand field transition in the visible region, as measured by the second
derivative of the Kubelka-Munk function, suggests that both OH and P contribute to an Fe deficiency in
the structure. Such a deficiency is also apparent from the 104/113 peak intensity ratio in the X-ray
diffraction patterns. The c unit-cell length increases with increasing P content. The infrared spectra exhibit
four bands in the P-OH stretching region (viz., at 936, 971, 1005, and 1037 cm -1) which suggest that
o c c l u d e d P O 4 possesses a low symmetry. Congruent dissolution of P and Fe was observed on acid
treatment of the hematites, the dissolution rate being negatively correlated with the P content. All observations are consistent with the occluded P in the hematites being structural. A model is proposed where
P occupies tetrahedral sites in the hematite structure, thus resulting in an Fe deficiency and facilitating
proton incorporation.
Key Words--c~-Fe203, Hematite, Color, Crystal Chemistry, Dissolution, IR Spectra, Structural R Structure.
INTRODUCTION
1981; Torrent e t al., 1990), m o s t o f the p h o s p h a t e app e a r s to b e in an o c c l u d e d form, a n d the n a t u r e o f this
f o r m h a s not b e e n studied carefully.
I n this p a p e r w e i n v e s t i g a t e the f o r m in w h i c h P is
o c c l u d e d in h e m a t i t e s p r e p a r e d f r o m f e r r i h y d r i t e s w i t h
c o p r e c i p i t a t e d p h o s p h a t e . In particular, w e test the h y p o t h e s i s that the P is structural. T h i s i n v e s t i g a t i o n h a s
g e o c h e m i c a l r e l e v a n c e since m a n y n a t u r a l h e m a t i t e s
a n d o t h e r F e o x i d e s a p p e a r to c o n t a i n significant
a m o u n t s o f n o n - a l k a l i - e x t r a c t a b l e P (Ruiz et al., 1997,
a n d u n p u b l , results). T h u s , it is likely that the a m o u n t
o f P o c c l u d e d in h e m a t i t e reflects the P c o n c e n t r a t i o n
in the m e d i u m in w h i c h this m i n e r a l w a s f o r m e d .
T h e f o r m a t i o n o f F e oxides a n d o x y h y d r o x i d e s in
a q u e o u s e n v i r o n m e n t s is affected b y the type a n d conc e n t r a t i o n o f cations, anions, a n d n e u t r a l m o l e c u l e s
p r e s e n t in s o l u t i o n ( C o r n e l l a n d S c h w e r t m a n n , 1996).
O n e s u c h solute is ( o r t h o ) p h o s p h a t e , a u b i q u i t o u s ani o n that interacts strongly w i t h Fe o x i d e s via specific
a d s o r p t i o n ( l i g a n d e x c h a n g e ) w i t h surface O H groups.
L a b o r a t o r y e x p e r i m e n t s s h o w e d that p h o s p h a t e s l o w s
the rate o f t r a n s f o r m a t i o n o f f e r r i h y d r i t e to c r y s t a l l i n e
F e oxides a n d influences the t y p e o f o x i d e p r o d u c t a n d
its characteristics. G~ilvez et al. ( 1 9 9 9 ) r e p o r t e d that
the h e m a t i t e / g o e t h i t e ratio in the p r o d u c t s o b t a i n e d
f r o m the t r a n s f o r m a t i o n o f f e r r i h y d r i t e w i t h c o p r e c i p itated p h o s p h a t e at t e m p e r a t u r e s b e t w e e n 2 9 8 - 3 7 3 K
d e p e n d s o n the P / F e ratio, a n d the r e s u l t i n g h e m a t i t e s
are s p i n d l e - s h a p e d or ellipsoidal. T h e s e s h a p e s are obs e r v e d also in h e m a t i t e s f o r m e d in FeC13 s y s t e m s w i t h
a k a g a n 6 i t e as p r e c u r s o r i f p h o s p h a t e is p r e s e n t ( R e e v e s
a n d M a n n , 1991; K a n d o r i et al., 1992; M o r a l e s et al.,
1992; M a t i j e v i c , 1993; S u g i m o t o e t al., 1993; Ocafia
et al., 1995; S u g i m o t o a n d M u r a m a t s u , 1996).
H e m a t i t e s p r e p a r e d f r o m Fe3+-rich solutions c o n taining p h o s p h a t e i n c o r p o r a t e significant a m o u n t s o f P
(G~ilvez e t al., 1999; Serna, p e r s o n a l c o m m u n i c a t i o n ) .
Alkali t r e a t m e n t results in r e m o v a l o f o n l y a s m a l l
f r a c t i o n o f the p h o s p h a t e in the solid phase. S i n c e alkali t r e a t m e n t is c o n s i d e r e d to b e e f f e c t i v e in r e m o v ing p h o s p h a t e in surface c o m p l e x e s ( C a b r e r a et al.,
Copyright 9 1999, The Clay Minerals Society
MATERIALS AND METHODS
Preparation of hematite with occluded P
T h i r t e e n ferrihydrites w i t h c o p r e c i p i t a t e d P w e r e
p r e p a r e d b y a d d i n g 1 M K O H ( - - 1 0 c m 3 m i n -1) to 2 0 0
c m 3 o f a c o n t i n u o s l y stirred 0.135 M Fe(NO3) 3 s o l u t i o n
c o n t a i n i n g KH2PO 4 until the p H w a s 4. T h e P / F e a t o m ic ratio i n the solution, e x p r e s s e d as p e r c e n t a g e ,
r a n g e d f r o m 0 to 3%; g r e a t e r ratios w e r e n o t u s e d
b e c a u s e t h e y did not result in c r y s t a l l i z a t i o n o f hem a t i t e ( G N v e z e t al., 1999). T h e r e s u l t i n g f e r r i h y d r i t e
s u s p e n s i o n was c e n t r i f u g e d a n d - - 1 9 0 c m 3 o f the sup e r n a t a n t w e r e discarded. T h e n , solid M g ( N O 3 ) z . 6 H 2 0
a n d w a t e r w e r e a d d e d to m a k e the M g c o n c e n t r a t i o n
2 M ; this w a s d o n e to p r e v e n t f o r m a t i o n o f g o e t h i t e
in the f o l l o w i n g s y n t h e s i s steps ( C o l o m b o , 1993). Af375
376
G~_lvez, Barr6n, and Torrent
ter r e a d j u s t i n g p H to 4, the s u s p e n s i o n was p o u r e d into
a p y r e x bottle w h i c h w a s p l a c e d in a n o v e n at 373 1 K. C r y s t a l l i z a t i o n o f h e m a t i t e was c o n s i d e r e d to b e
c o m p l e t e w h e n the v o l u m e o c c u p i e d b y the initial gel a t i n o u s precipitate was r e d u c e d to less t h a n o n e t e n t h
o f the original, a n d the color o f the s e d i m e n t w a s
b r i g h t red. C r y s t a l l i z a t i o n o c c u r r e d f r o m < 1 d in the
P-free s a m p l e to --5 d in the s a m p l e w i t h P / F e = 3%.
A f t e r cooling, c e n t r i f u g a t i o n , a n d d e c a n t a t i o n , the sedi m e n t w a s w a s h e d o n c e (twice in the P - r i c h s a m p l e s )
w i t h 0.5 M H N O 3 to d i s s o l v e a n y residual ferrihydrite,
t w i c e w i t h 0.1 M KNO3 at p H 3, a n d t w i c e w i t h 0.1
M K O H - 0 . 5 M KNO3 to r e m o v e p h o s p h a t e a d s o r b e d
o n the surface o f h e m a t i t e . T h e s u s p e n s i o n w a s finally
b r o u g h t to the p H o f m a x i m u m flocculation (7.2 to
7.4), w a s h e d four t i m e s w i t h water, a n d d i a l y z e d in
d e i o n i z e d water. A b o u t o n e third o f the r e s u l t i n g susp e n s i o n was freeze-dried. T h e rest was stored as stock
s u s p e n s i o n a n d the h e m a t i t e c o n c e n t r a t i o n was m e a s u r e d b y e v a p o r a t i n g 2 c m 3 at 383 K. A p o r t i o n o f the
f r e e z e - d r i e d s a m p l e was h e a t e d at 1073 K for 2 h.
Physical, chemical, and mineralogical analyses
T h e specific surface area a n d the m i c r o p o r e surface
area w e r e d e t e r m i n e d b y N 2 a d s o r p t i o n ( B E T a n d tplot methods, respectively) with a Micromeritics
A S A P 2 0 1 0 surface area analyzer. C o l o r was determ i n e d w i t h a V a r i a n C a r y 1E s p e c t r o p h o t o m e t e r
e q u i p p e d w i t h a diffuse reflectance a t t a c h m e n t . To prev e n t particle orientation, c o l o r was m e a s u r e d in m i x tures o f 3 wt. % h e m a t i t e - 9 7 wt. % w h i t e s t a n d a r d
B a S O 4. T h e r e f l e c t a n c e (R) values were t a k e n at int e r v a l s o f 0.5 n m in the 3 8 0 - 7 1 0 n m r a n g e a n d converted to the C I E L*a*b* color p a r a m e t e r s , as des c r i b e d b y B a r r 6 n a n d Torrent (1986). T h e s e c o n d der i v a t i v e c u r v e o f the K u b e l k a - M u n k f u n c t i o n [(1 R)2/2R] for this r a n g e was o b t a i n e d b y u s i n g a c u b i c
spline fitting p r o c e d u r e applied to a series o f 22 consecutive R values (Press et al., 1992; S c h e i n o s t et al.,
1998).
To d e t e r m i n e total F e a n d P contents, the h e m a t i t e
was d i s s o l v e d b y t r e a t i n g 0.1 c m 3 o f the stock s u s p e n sion w i t h 2 c m 3 o f 11 M HC1. In the r e s u l t i n g solution,
Fe was d e t e r m i n e d b y the o - p h e n a n t h r o l i n e m e t h o d
( O l s o n a n d Ellis, 1982) a n d P b y the m o l y b d e n u m b l u e
m e t h o d ( M u r p h y a n d Riley, 1962). T h e loss o f w a t e r
in the 2 9 3 - 1 0 7 3 K r a n g e w a s e x a m i n e d w i t h a Cam e c a t h e r m o b a l a n c e set at a h e a t i n g rate o f 5 K rain 1.
T h e loss of w a t e r m e a s u r e d in this w a y w a s n o t significantly different f r o m the loss m e a s u r e d in s a m p l e s
h e a t e d at 473 K for 24 h a n d t h e n at 1073 for 70 h.
B e c a u s e o f g r e a t e r precision, the w a t e r loss in the
4 2 3 - 1 0 7 3 K r a n g e m e a s u r e d w i t h the t h e r m o b a l a n c e
w a s used, t o g e t h e r w i t h the P/Fe ratio, to c a l c u l a t e the
structural f o r m u l a Fe~Py(OH)wO3_ w, w h e r e 3x + 5y =
6 - w.
Clays and Clay Minerals
The phosphate adsorption capacity was determined
in s u s p e n s i o n s c o n t a i n i n g 1 0 - 2 5 m g o f h e m a t i t e a n d
0 . 1 0 0 m g o f P as KH2PO 4 i n 8 c m 3. A f t e r a d d i n g the
p h o s p h a t e a n d stirring the s u s p e n s i o n for 1 rain, the
p H w a s a d j u s t e d to 6. T h e n , the s u s p e n s i o n w a s stirred
o c c a s i o n a l l y d u r i n g the n e x t 3 h, the p H r e a d j u s t e d to
6 and, finally, s h a k e n in a p o l y e t h y l e n e b o t t l e at 3 H z
for 3 d. A f t e r c e n t r i f u g i n g a n d d e t e r m i n i n g t h e c o n c e n t r a t i o n o f P in the s u p e r n a t a n t , the p h o s p h a t e ads o r p t i o n c a p a c i t y was c a l c u l a t e d f r o m the d i f f e r e n c e
b e t w e e n total P a d d e d a n d P r e m a i n i n g in solution.
F o r s o m e d i s s o l u t i o n e x p e r i m e n t s , h e m a t i t e w i t h the
surface saturated w i t h p h o s p h a t e w a s p r e p a r e d b y adding a n a m o u n t o f p h o s p h a t e in e x c e s s o f the p h o s p h a t e
a d s o r p t i o n c a p a c i t y to the h e m a t i t e s u s p e n s i o n . A f t e r
2 h, the s u s p e n s i o n w a s c e n t r i f u g e d , the s u p e r n a t a n t
d e c a n t e d , a n d the s e d i m e n t w a s h e d w i t h w a t e r a n d
freeze-dried.
T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s ( T E M ) w e r e obt a i n e d w i t h a Jeol J E M - 2 0 0 C X e l e c t r o n m i c r o s c o p e
after d i l u t i n g s m a l l p o r t i o n s o f the stock s u s p e n s i o n in
w a t e r a n d d r y i n g a d r o p o f the r e s u l t i n g t h i n s u s p e n sion o n a thin film o f c a r b o n o n a c o p p e r grid. I n f r a r e d
(IR) spectra o f K B r pellets (0.5 wt. % s a m p l e ) w e r e
r e c o r d e d f r o m 380 to 4 0 0 0 c m 1 w i t h a P e r k i n - E l m e r
2 0 0 0 F T I R s p e c t r o m e t e r u s i n g 4 c m -~ r e s o l u t i o n a n d
an a v e r a g e o f 100 scans. X - r a y d i f f r a c t i o n ( X R D ) traces were o b t a i n e d w i t h a S i e m e n s D 5 0 0 0 a p p a r a t u s
e q u i p p e d w i t h CoKct r a d i a t i o n a n d a g r a p h i t e m o n o chromator. T h e h e m a t i t e s a m p l e s , m i x e d w i t h 10% silicon as i n t e r n a l standard, a n d p r e p a r e d as self-supp o r t i n g p o w d e r m o u n t s , w e r e s c a n n e d at a p p r o p r i a t e
a n g u l a r r a n g e s to c o n t a i n the 012, 104, 110, 113, 024,
116, 018, 214, a n d 300 reflections at 0.02 ~
steps
a n d 10 s c o u n t i n g time. T h e s c a n s w e r e fitted to a
p s e u d o - V o i g t f u n c t i o n (FIT p r o g r a m for S i e m e n s diff r a c t o m e t e r s ) , f r o m w h i c h the C a u c h y a n d the G a u s s
i n t e g r a l b r e a d t h s w e r e d e r i v e d (de K e i j s e r et al.,
1983). M e a n c o h e r e n c e l e n g t h ( M C L ) p e r p e n d i c u l a r
to a g i v e n p l a n e (MCLhk/) w a s c a l c u l a t e d f r o m the
C a u c h y i n t e g r a l b r e a d t h ( c o r r e c t e d for the C a u c h y
b r e a d t h o f well c r y s t a l l i z e d quartz) a c c o r d i n g to the
S c h e r r e r f o r m u l a . In this f o r m u l a , the v a l u e s o f k w e r e
t a k e n f r o m S t a n j e k (1991). T h e h e m a t i t e a a n d c unitcell l e n g t h s w e r e d e r i v e d f r o m the p o s i t i o n s o f the n i n e
reflections ( a b o v e ) a n d refined w i t h a least-squares
procedure. Oriented mineral aggregate mounts on
glass slides w e r e p r e p a r e d b y m i x i n g a p o r t i o n o f the
stock s u s p e n s i o n c o n t a i n i n g 0.2 m g o f h e m a t i t e w i t h
1 c m 3 o f water, s p r e a d i n g the s u s p e n s i o n o n a n area
o f - 1 . 5 c m 2, a n d letting it d r y at r o o m t e m p e r a t u r e .
T h e d i f f r a c t o g r a m s o f t h e s e m o u n t s w e r e o b t a i n e d for
the a p p r o p r i a t e a n g u l a r r a n g e s to c o n t a i n the 012, 104,
a n d 110 reflections at 0.01 ~ steps a n d 15 s c o u n t i n g
time.
Vol. 47, No. 3, 1999
Hematites with structural phosphorus
Table 1. Chemical composition of the hematites.
P/Fe a t o m i c
P a r a m e t e r s o f the Fe~P~,(OH),,.O3.,,. f o r m u l a
Sample I
ratio x 100
x
y
w
3-w
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
0.00
0.23
0.48
0.71
0.95
1.16
1.38
1.69
1.80
2.11
2.30
2.82
2.97
1.81
1.80
1.80
1.80
1.78
1.76
1.73
1.75
1.72
1.71
1.70
1.68
1.68
0.000
0.004
0.009
0.013
0.017
0.021
0.024
0.030
0.031
0.036
0.039
0.048
0.050
0.57
0.57
0.57
0.55
0.57
0.62
0.68
0.59
0.68
0.69
0.70
0.71
0.72
2.43
2.43
2.43
2.45
2.43
2.38
2.32
2.41
2.32
2.31
2.30
2.29
2.28
t Sample code is the atomic P/Fe ratio (in percentage) in
the initial Fe(NO3)3-KH2PO4 solution.
Acid dissolution experiments
Two c m 3 o f the stock suspension (equivalent to 1 0 33 m g of hematite) were added to 8 c m 3 of a magnetically stirred 8.75 M HC1-0.875 M H2SO4 solution
at 298 K. Portions of 0.4 c m 3 w e r e taken f r o m the
resulting suspension at times of 5 to 360 min and immediately centrifuged for 2 min at an acceleration o f
1.2 • 105 m s-L Then, the concentrations o f F e and
P in the supernatant were determined by the above
methods.
RESULTS AND DISCUSSION
Chemical composition
M o s t o f the Fe in the iniIial solutions was precipitated as ferrihydrite, which incorporated m u c h of the
P This P was largely retained by the hematite f o r m e d
f r o m the ferrihydrite, as suggested by Table 1, which
shows that the P/Fe ratio of the hematites is only
slightly smaller than the P/Fe ratio of the initial solutions. These small differences in the P / F e ratio may be
due to phosphate retained by the surface o f the hematite resulting f r o m the crystallization ferrihydrite.
Such phosphate was r e m o v e d when, in the course of
the preparation of the stock suspension, the hematite
was washed with 0.1 M K O H . Hereafter, the term " o c cluded P " will be used to refer to the non-hydroxylextractable P that is incorporated in the hematite.
All the hematites incorporate hydroxyl, as indicated
by the significant value o f w in the hypothetical structural formula FexPy(OH)wO3 w (Table 1). The O H content is positively and significantly correlated with the
P content (r 2 = 0.78).
Morphological and surface properties
The T E M images show a gradual change in particle
m o r p h o l o g y as P content increases (Figure 1; Table 2),
f r o m nearly equidimensional (apparently cube-like)
crystals of 6 0 - 7 0 n m for P/Fe = 0 to spindles or el-
377
lipsoids with a length of 1 7 0 - 2 3 0 n m and a m a x i m u m
width o f 4 0 - 6 0 n m for P/Fe > 2 . 5 % . The face index
of the surface planes of the cube-like hematites is
(012), as suggested by the high oriented m o u n t / p o w d e r
intensity ratio for the 012 reflection (Figure 2). This
suggests that these particles are imperfect r h o m b o h e dra with {012} faces, similar to s o m e synthetic hematites ( S u g i m o t o et al., 1993). T h e long axis o f the
spindle-shaped hematites corresponds to the [001] direction because the value of such ratio is large for the
110 reflection and small for the 104 reflection (which
corresponds to a plane at a high angle to 110) (Figure
2). A similar orientation was o b s e r v e d by R e e v e s and
M a n n (1991).
All hematites exhibit a " g r a i n y " appearance, with
the apparent grains of about one tenth o f the particle
size. These grains are elongated in the [001] direction
for spindle-shaped particles. The m e a n c o h e r e n c e
lengths (MCLs) perpendicular to the (012), (104), and
(110) planes (Table 2) are generally similar to the dim e n s i o n s of the particles as o b s e r v e d in T E M images,
suggesting that the apparent grains diffract coherently.
Therefore, the particles are essentially monocrystalline, as is c o m m o n with hematites prepared in the
presence of phosphate by the sol-gel m e t h o d (Sugim o t o et al., 1998). The separation b e t w e e n the grains
suggested by the T E M images m a y be limited to the
grains on the particle surface.
The specific surface area (Table 2) ranges f r o m 66
to 91 m 2 g L and bears little relation with P/Fe for the
samples with P/Fe < 2 . 5 % . For the two samples with
P/Fe > 2 . 7 5 % , the values are smaller (45 and 58 m z
g 1). In the latter samples, the proportion of the total
surface area in micropores is < 3 0 % , which contrasts
with the greater proportion ( > 4 5 % ) o b s e r v e d in other
samples. The contribution of microporosity and surface rugosity to the surface area (as also suggested by
the T E M images) is substantial, g i v e n that the specific
surface area calculated on the basis of the external
particle shape ranges f r o m 15 to 20 m 2 g-t. The phosphate adsorption capacity on a surface area basis (Table 2) ranges f r o m 1.5 to 2.5 txmol P m -2 and shows
little relationship with P/Fe. These values are consistent with the T E M images, w h i c h suggest the presence
o f a significant proportion of the m o s t active P-adsorbing faces, such as the (012) and the (110) ( C o l o m bo et al., 1994; Barr6n and Torrent, 1996).
U p o n heating at 1073 K, the granular appearance o f
the particles was lost, the surface b e c a m e smooth, and
vesicles, probably caused by the escape o f water, appeared (Figure 3). The specific surface area of the
heated samples (not shown) ranged f r o m 15 to 20 m 2
g-a, consistent with the o b s e r v e d sizes based on T E M
data and with the absence of surface rugosity and micropores.
378
Clays and Clay Minerals
G41vez, Barr6n, and Torrent
Figure 1.
TEM i m a g e s of hematite samples (A) 0.00, (B) 1.00, (C) 2.00, and (D) 3.00. Bar represents 100 nm.
Table 2.
M o r p h o l o g i c a l and surface properties of hematite.
Mean coherence lengths •
Original samples
Sample
Size and shape (TEM)
(012)
(104)
the given planes
Samples heated at 1073 K
(110)
(012)
(104)
(110)
Specific
surface
area
Micropore
surface
area
Phosphate
adsorption
capacity
p~mol
.............
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
6 0 - 7 0 nm cube-like
120 • 70 n m spindles
120 X 65 nm spindles
180 • 70 nm spindles
150 • 65 n m spindles
200 • 50 nm spindles
200 X 55 nm spindles ~
Partly ellipsoids.
88
116
119
108
115
106
91
107
100
82
90
90
84
81
104
107
105
107
97
91
104
95
88
86
93
82
nm . . . . . . . . . . . . . .
73
82
85
74
74
72
66
71
68
61
59
67
62
133
94
111
149
127
86
134
98
97
100
87
153
86
123
85
74
70
57
68
102
102
92
105
84
89
83
101
78
59
58
46
52
70
65
64
59
57
60
59
m2g ~ -70
87
77
81
71
84
91
66
90
89
90
58
45
38
52
43
45
34
42
44
31
40
39
40
18
11
in 2
2.2
1.8
2.0
1.9
1.8
1.7
1.8
1.7
1.7
1.7
1.5
1.5
2.5
Vol. 47, No. 3, 1999
Hematites with structural phosphorus
379
O
'~
1.5
cc(D
2;3
O
Q_
0.5
03
O
0
0
0.5
'
i
1.'5
2'
2.5
'
3
P/Fe x 1 O0
Figure 2. The ratio between the intensities of the XRD
peaks of the oriented mineral aggregates and those of the
corresponding powder samples as a function of P/Fe for the
012, 110, and 104 reflections.
Color
The color of the hematites was bright red, w h i c h is
characteristic of hematites in the 5 0 - 2 0 0 n m size
range (Torrent and Schwertmann, 1987). U p o n heating
to 1073 K, there was a change (Figure 4) to less saturated, m o r e purple hues (lower a* and b* values).
This effect increased markedly with decreasing P/Fe,
and thus the pure hematite showed a dark purple color
and the hematites with P/Fe > 2 . 5 % were bright red.
Purple hues in hematite are associated with large particle sizes (Morris et al., 1985). So, it can be hypothesized that particles sintered at 1073 K, the degree of
sintering decreased with increasing P content. It is possible that P makes hematite more refractory or, by
means o f its influence on the initial particle shape,
makes the particle contacts less favorable to sintering.
In fact, w h e n sintering was prevented by thoroughly
mixing a portion of the hematite suspension with
B a S O 4 before heating at 1073 K (to decrease the n u m ber of hematite-hematite contacts), color c h a n g e d little
regardless of P/Fe (results not shown).
M i n i m a of the second derivative c u r v e s o f the (1 R)Z/2R function, which indicate the position of the absorption bands due to Fe 3+ ligand field transitions, are
o b s e r v e d at --405, 4 2 5 - 4 3 5 , 445, 488, and 538 n m
(Figure 5). The bands at 405, 445, and 538 n m can be
assigned to hematite and correspond respectively to
the 6A 1 ---ff 4T 2 (4D), 6A 1 ---) 4 E 4 A 1 (4G), and 2(6A1)
2(4T1) electron transitions (Sherman and Waite, 1985).
The 4 2 5 - 4 3 5 and 488 n m bands h a v e a smaller intensity than the former ones and are typical of the goet h i t e 6A 1 --~ 4E4A1 (4G) a n d 2(6A1) ----) 2(4T1(4G)) tran-
Figure 3. TEM images of two hematites heated at 1073 K:
(A) 1.00, and (B) 2.00. Bar represents 100 nm.
sitions. Therefore, the hematites b e h a v e optically as if
they contain a goethite impurity. B e c a u s e no goethite
was detected by X R D in any of the samples, this " o p tical goethitic c o m p o n e n t " is probably due to structural and surface hydroxyls.
The position o f the absorption bands is not related
to the P/Fe ratio. To measure band intensity, w e took
the band amplitude, defined as the difference in ordinate b e t w e e n the corresponding m i n i m u m and the next
m a x i m u m at longer w a v e l e n g t h (e.g., the difference in
ordinate b e t w e e n the hematite m i n i m u m at 538 n m
and the next m a x i m u m at 574 nm). The relative band
intensity changes with the P/Fe ratio. In particular, the
2(6A1) ~ 2(4T1) (538 nm) hematite band, which is assigned to Fe3+-Fe 3§ pair excitations, decreases with P/
Fe, w h i c h indicates decreasing degree of Fe3+-Fe 3+
coupling. The m a g n i t u d e of the optical goethitic c o m ponent relative to hematite can be quantified as the
4 8 8 - 5 0 4 n m (goethite) to the 5 3 8 - 5 7 4 n m (hematite)
band amplitude ratio. This ratio increases with increasing P/Fe in the unheated hematites (Figure 6). Since
O H content increases with increasing P/Fe, this result
380
Clays and Clay Minerals
G~ilvez, Barr6n, and Torrent
40
Original samples
3.00
yellow
0,00
~
,.@~
o
30
3.00
,,
C'4
2O
O,d
cE
I
o~.S'~'~mmples heated at 1073 K
10
/
q---
O
(D
11--0
9
~
0.00
J- red
o3
k_
-10
03
2:3
Z3
cc-M
9 blue
6
lo
a*
15
Figure 4. The CIE a* and b* parameters of mixtures of
hematite (3 wt. %) and white standard, BaSO 4 (97 wt. %),
for the untreated samples and after heating at 1073 K.
is consistent with the hypothesis that O H creates the
aforementioned optical goethitic component.
The position of the absorption bands changed little
w h e n the samples were heated at 1073 K. As with the
unheated samples, the ( 4 8 8 - 5 0 4 n m ) / ( 5 3 8 - 5 7 4 nm)
band amplitude ratio increases significantly with increasing P/Fe (Figure 6). This increase cannot be due
to differences in structural OH, which is eliminated by
heating, nor to differences in surface adsorbed hydroxyl, because the samples have similar surface area.
Therefore, in analogy with OH, the P occluded in the
hematite particles produces an optical goethitic effect,
We did not observe any significant change in the band
amplitude ratio w h e n hematite was surface-saturated
with phosphate, i.e., adsorbed phosphate does not affect the second derivative spectrum. The slope o f the
regression line of the amplitude ratio against P/Fe is
steeper relative to that of the heated samples, consistent with both O H and occluded P contributing to the
optical goethitic effect.
Structural properties
The c unit-cell length is significantly and positively
correlated with P/Fe (Table 3; Figure 7); this effect
was noted also by Ocafia et al. (1995). In contrast, the
a length varies little (Table 3) and is unrelated to P/
Fe. The value of c in the hematites heated at 1073 K
is also correlated with P/Fe, but the regression line has
a l o w e r slope (Figure 7). This difference in slope can
be explained by the effect structural hydroxyl has in
increasing c (Stanjek and Schwertmann, 1992). H o w ever, g i v e n the high covariance b e t w e e n P/Fe and hydroxyl content, the relative contribution o f P and hy-
40O
500
600
700
Wavelength (nm)
Figure 5. Second-derivative curves of the Kubelka-Munk
function of hematite-BaSO4 (3 97 wt. %) mixtures for hematite samples 0.00, 1.00, and 3.00. The curves for the mixtures heated at 1073 K are indicated by 1.00 (h), 2.00 (h),
and 3.00 (h).
o
~ 0.8
xz~
~ 0.7
cz
E
Y=0.39+ O.
P = o.g3
~ 0.6
o
~ 0.5
~b-
[]
[]
c+
0.4
CO
u
Y = 0.25 + 0.051X
r2 = 0.74
,~
~'_
0.3
L
~_
o
~ 0.2
-I
0
I
J
1
1
2
3
P/Fe x 1 O0
Figure 6. The (488-504 nm)/(538-574 nm) band amplitude
ratio in the second derivative spectra of the Kubelka-Munk
function of hematite-BaSO4 mixtures as a function of P/Fe.
Open symbols: untreated hematite; solid symbols: hematite
heated at 1073 K.
Vol. 47, No. 3, 1999
Hematites with structural phosphorus
1.384
Table 3. Unit cell parameters of hematite.
Original samples
Sample
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
381
Samples heated at 1073 K
a (nm)
c (nm)
a (nm)
c (nm)
0.5032
0.5031
0.5035
0.5031
0.5033
0.5032
0.5028
0.5031
0.5031
0.5033
0.5032
0.5029
0.5028
1.3776
1.3785
1.3793
1.3788
1.3801
1.3800
1.3800
1.3805
1.3809
1.3814
1.3811
1.3828
1.3824
0.5034
0.5033
0.5037
0.5036
0.5035
0.5034
0.5035
0.5034
0.5034
0.5034
0.5033
0.5030
0.5031
1.3744
1.3746
1.3748
1.3755
1.3756
1.3754
1.3758
1.3759
1.3759
1.3764
1.3762
1.3762
1.3767
[]
1.382
[]
[]
1.38
[]
E
C
v 1.378
:~./[]
re = o.g4
f
(o
[]
1.376
Y = 1.3747 + 0.00069X
7 = 0.89
1.374
I
0
d r o x y l to the i n c r e a s e in c c a n n o t b e d e t e r m i n e d f r o m
the r e g r e s s i o n analysis.
Wolska (1981) and Stanjek and Schwertmann
(1992) s h o w e d that structural O H in h e m a t i t e ( a n d the
r e s u l t i n g F e deficiency in the c a t i o n i c s u b s t r u c t u r e )
c a n b e d e t e c t e d b y a r e d u c t i o n o f the ratio b e t w e e n
the intensities o f the h k l a n d the 113 X - r a y reflections
(Ihk/1113) , since the 113 reflection is i n d e p e n d e n t o f the
cations. In o u r case, IhJlll3 is n e g a t i v e l y c o r r e l a t e d
w i t h P/Fe, as s e e n for the 104 reflection (Figure 8).
T h i s is c o n s i s t e n t w i t h the p o s i t i v e c o r r e l a t i o n b e t w e e n
O H c o n t e n t a n d P/Fe. Interestingly, the n e g a t i v e corr e l a t i o n persists after d e h y d r o x y l a t i n g the s a m p l e s at
1073 K (Figure 8), s u g g e s t i n g t h a t P also p r o d u c e s
structural F e deficiency.
F i g u r e 8 s h o w s the I104/1113 ratio c a l c u l a t e d b y Rietveld s i m u l a t i o n o f the X - r a y p a t t e r n ( R I E T A N ; I z u m i ,
1993) as a f u n c t i o n o f P / F e for: (1) h e m a t i t e s c o n t a i n ing P in t e t r a h e d r a l sites (but n o OH), a n d (2) h e m a tites w i t h o u t P b u t w i t h structural O H in a m o u n t s
e q u i v a l e n t to t h o s e o f the u n h e a t e d samples. T h e s u m
o f the slope o f t h e s e t w o lines is s i m i l a r to the slope
o f the line for the u n h e a t e d hematites. T h i s f u r t h e r
supports the h y p o t h e s i s that b o t h O H a n d P c o n t r i b u t e
to structural F e deficiency.
S e l e c t e d I R spectra o f the h e m a t i t e s are s h o w n in
F i g u r e 9. T h e spectra h e m a t i t e s c o n t a i n i n g P s h o w
four d i s t i n c t i v e a n d relatively n a r r o w b a n d s in the P O ( H ) s t r e t c h i n g region, at --936, 971, 1005, a n d 1037
c m -l. T h e i n t e n s i t i e s o f these b a n d s relative to t h o s e
o f the h e m a t i t e b a n d s i n c r e a s e w i t h i n c r e a s i n g P c o n tent. S a t u r a t i o n o f the surface o f the h e m a t i t e s w i t h
p h o s p h a t e p r o d u c e s relatively b r o a d b a n d s at 1015 a n d
1105 c m -1 (but n o t at the f o r m e r frequencies), as illustrated in F i g u r e 9 b y the P-free sample; t h e s e t w o
b a n d s are close to t h o s e r e p o r t e d for diffuse reflect a n c e IR spectra o f p h o s p h a t e d h e m a t i t e s ( P e r s s o n et
aL, 1996). T h e s e o b s e r v a t i o n s are i n t e r p r e t e d as evid e n c e for the l o w s y m m e t r y o f the o c c l u d e d F O 4 units
relative to PO4 ions a d s o r b e d o n the surface. Alter-
I
I
1
2
PfFe x 1 00
3
Figure 7. The c-cell length as a function of P/Fe. Open symbols: untreated hematite; solid symbols: hematite heated at
1073 K.
natively, o c c l u d e d P m a y e x h i b i t h i g h e r s y m m e t r y b u t
b e p r e s e n t in m o r e t h a n o n e e n v i r o n m e n t .
T h e h e m a t i t e s h e a t e d at 1073 K s h o w four b a n d s at
9 2 6 c m 1 (weak), 9 6 0 c m 1 ( v e r y w e a k ) , 1000 c m -I
( v e r y weak), a n d 1032 c m i. T h e s e b a n d s are slightly
d i s p l a c e d to l o w e r f r e q u e n c i e s relative to the b a n d s o f
the u n h e a t e d h e m a t i t e s a n d are w e a k e r t h a n the latter.
Tetrahedral P
3.5
o
J
,~ =
~>~ 3
9
0.s0
9
--
......
-
9
tO
C-
(1)
.+=..
C-
[]
2.5
//
- ~'~'~-~mm
Structural OH
[]
o
Y=2.64-0.22x
:
0
o
0.sg
[]
-
~
[]
I
I
I
1
2
3
P/Fe x 1 00
Figure 8. The ratio of the 104 to the 113 reflection from
XRD data as a function of P/Fe. Open symbols: untreated
hematite; solid symbols: hematite heated at 1073 K. The figure also shows the ratios obtained from Rietveld calculations
based on hematite without OH and with P in tetrahedral position (upper dash line) and for hematite without P but with
OH in amounts equal to that of the studied samples (lower
dash line).
382
G~ilvez, Barr6n, and Torrent
-
~
Clays and Clay Minerals
a
100
'
'
I
I
I
I
I
I
i
i
i
i
(~
i
i
i
i
~
I
I
I
I
"~
10371005
971
.
.
.
.
80
6o
936
CO
r
20
0
0
100
200
300
400
Time (min)
i
I
I
1200
I
[
I
11 O0
I
I
I
I
I
1000
I
~
~
I
i
900
i
i
i
I
800
Figure 10. Acid dissolution curves of hematite samples
0.00, 1.00, 2.25, and 3.00.
c m -1
Figure 9. Infrared spectra of samples (a) 0.00, (b) phosphated 0.00, (c) 1.00, (d) 2.00, (e) 3.00, (f) 3.00 heated at
1073 K, (g) phosphated ferrihydrite heated at 1073 K, and
(h) strengite heated at 1073 K.
This decrease in intensity in the P - ( O H ) stretching region is likely associated with an increase of s y m m e t r y
caused by loss o f OH. The spectra of the heated hematites differ clearly f r o m the spectra o f two products
that contain a crystalline FePO4 phase and are dehydroxylated, viz., P-saturated ferrihydrite and strengite
(FePO4.2H20) heated at 1073 K (Figure 9).
the P-free hematite with phosphate and d i s s o l v e d it in
acid. The corresponding dissolution c u r v e was markedly c o n v e x towards the y-axis (Figure 12). Therefore,
the surface phosphate desorbed in the initial hematite
dissolution stages r e m a i n e d largely in solution. This
lack of readsorption m a y be due to the strong c o m petition b e t w e e n phosphate and sulfate (which is present in the acid mixture used in the experiments) for
the adsorption sites on the hematite surface. Consequently, dissolution is congruent.
A c i d dissolution
The dissolution curves are generally sigmoidal (Figure 10), i.e., they indicate an acceleration o f dissolution f o l l o w e d by deceleration of dissolution. This
shape is characteristic o f m a n y hematite studies (Cornell and Giovanoli, 1993) and is explained by the increase in surface area as the result of enlargement of
pores in the hematite after initial dissolution (Weidler
et al., 1998). The presence of P increased markedly
the time needed for dissolution (Figure 10). To c o m pare samples, we used the initial dissolution rate,
which was computed as the slope of the regression line
fitted to 3 - 5 aligned points in the range 2 - 2 0 % total
Fe dissolved, This rate (calculated on a surface area
basis) decreases with increasing PriZe (Figure 11).
The plot of P dissolved against Fe dissolved is atways close to the 1:1 line, as illustrated in Figure 12.
This plot indicates either true congruent dissolution or
P dissolves preferentially to Fe during the initial period, but the phosphate that is released is mostly readsorbed on the surface of the hematite. To dismiss the
possibility that phosphate is readsorbed, we saturated
@
ox--x
=m
5
4
E
o9 3
__L-L
O
E
v
o9
2
co
_~
9
co
0~
5
q
0
P/Fe x 1 O0
Figure 11.
P/Fe.
Initial hematite dissolution rate as a function of
Vol. 47, No. 3, 1999
Hematites with structural phosphorus
383
100
-(3
03
>
60
o9
~_
-(:3
El_ 40
2O
0
I
I
I
I
I
0
20
40
60
80
100
% Fe dissolved
Figure 12. Percent P dissolved against percent Fe dissolved
in HC1-H2SO 4 for samples 0.00, 1.00, 2.25, 3.00, and phosphated 0.00.
DISCUSSION
Congruent dissolution o f P and Fe indicates that the
occluded P is evenly distributed in the hematite crystals. One possibility is that P occurs in the f o r m of
clusters of phosphate ions adsorbed on walls of micropores. These micropores are not accessible from the
outer solution and are evenly distributed within the
particle. However, T E M images and X R D data appear
to indicate that micropores separating subcrystals are
confined to the surface of the particles, and one w o u l d
expect the P O 4 ions adsorbed on them to be released
in the first dissolution stages, which is not the case.
The IR data suggest (see above) that the occluded
PO4 units have a lower s y m m e t r y relative to that of
the adsorbed P O 4 units. The latter usually possess high
s y m m e t r y (e.g., C3v), as suggested by Persson et al.
(1996) for protonated monodentate c o m p l e x e s on goethite surfaces. Therefore, to be consistent with the I R
data, if the occluded PO4 units are adsorbed on micropore walls, they will have a molecular configuration
m u c h different f r o m that of the P O 4 adsorbed on the
external crystal surface. This, however, is an ad-hoc
explanation not supported by other data.
Another hypothesis consistent with P-Fe congruent
dissolution is the presence o f a FePO4 nanophase (e.g.,
strengite) e v e n l y distributed in the hematite particles.
However, h o w such a phase could be intimately m i x e d
with a hematite m o n o c r y s t a l is difficult to visualize.
Neither of the above hypotheses accounts for the
change that P causes in the c unit-cell length, since
the structure of a hematite crystallite is unlikely to be
influenced by adsorbed phosphate or an adjacent
FePO4 nanophase. Moreover, these forms o f PO4 are
Figure 13. Model of a unit cell of hematite with P in one
tetrahedral site. The distances of the P atom (black circle) to
the Fe atoms of the neighboring octahedra are shown. P-Fe
distances for the labelled octahedra are as follows: octahedron
1, 0.16 nm; 2, 0.19 nm; 3, 0.22 nm; 4, 0.26 nm; 5, 0.31 nm;
6, 0.34 nm; 7, 0.35 nm; 8, 0.36 nm; 9, 0.36 nm; 10, 0.39 nm.
unlikely to cause the Fe deficiency indicated by the
L,k]I113 ratio (Figure 8).
The most simple and reasonable hypothesis is that
the occluded P is mostly structural. This hypothesis is
consistent not only with the relationship b e t w e e n the
c dimension and P/Fe, but also with the low degree of
m o l e c u l a r s y m m e t r y of the occluded P 0 4 , a fact c o m m o n l y o b s e r v e d for PO4 confined in crystal lattices
(Persson et al., 1996). If P is in the hematite structure,
the regressions of y on x and w in the hypothetical
structural formulae of Table 1 indicate that, per each
additional P atom incorporated in the hematite, --2.9
atoms of Fe are lost and 3.6 oxygens are replaced by
hydroxyls. The existence of a Fe deficiency due to
structural P, already suggested by the decrease in the
lhkl[lll 3 ratio, is further substantiated by the increase in
the ratio between the amplitudes of the goethitic 2(6A1)
---4 2(4T1(4G)) (488 nrn) and the hematite 2(6Al) ----)
2(4T1) (538 nm) bands. These bands are assigned to a
double exciton process, which is m o r e intense in hematite than in goethite because the strength of the
magnetic coupling b e t w e e n face-sharing FeO6 octahedra in hematite is greater than that of the edge-sharing FeO6 octahedra in goethite. Thus, Fe deficiency
384
Gfilvez, Barrdn, and Torrent
h a s a greater effect o n the h e m a t i t e b a n d t h a n o n the
b a n d a s s i g n e d to the optical goethitic c o m p o n e n t bec a u s e e a c h F e O 6 o c t a h e d r o n in h e m a t i t e shares o n e
face with a (first) neighbor. Finally, the d e c r e a s e o f the
F e d i s s o l u t i o n rate w i t h i n c r e a s i n g P c o n t e n t is consistent w i t h F e deficiency a n d the structural c h a n g e s
that P m a y create, b u t it is u n l i k e l y i f the s a m p l e s
consist of a s i m i l a r p u r e h e m a t i t e a d m i x e d w i t h a
FePO4 n a n o p h a s e .
A s i m p l e structural m o d e l a c c o u n t i n g for the a b o v e
o b s e r v a t i o n s is s h o w n in F i g u r e 13. T h e P a t o m s occupy the t e t r a h e d r a l sites o f the hematite. T h e O, ( O H )
ions at the c o r n e r s o f the t e t r a h e d r o n are s h a r e d w i t h
a total o f ten o c t a h e d r a ( n u m b e r e d 1 to 10 in F i g u r e
13). O c c u p a n c y of the t e t r a h e d r a l site b y P results
p r o b a b l y in the loss o f s o m e or all of the t h r e e closest
Fe ions, w h i c h c o r r e s p o n d to o c t a h e d r a 1, 2, a n d 3 in
the case o f a structure not distorted b y the i n c o r p o r a tion o f R T h e r e s u l t i n g c h a r g e deficit is c o m p e n s a t e d
b y a d d i t i o n a l p r o t o n s a s s o c i a t e d w i t h the o x y g e n ions
o f the closest octahedra.
ACKNOWLEDGMENTS
This research was supported by the Spanish
C.I.C.Y.T., P r o j e c t A M B 9 4 - 0 3 2 2 . T h e s e n i o r a u t h o r
a c k n o w l e d g e s the a w a r d o f a r e s e a r c h g r a n t b y the
S p a n i s h P l a n N a c i o n a l I + D . U. S c h w e r t m a n n , Techn i s c h e Universit~it M t i n c h e n , a n d S. M y n e n i , L a w r e n c e
B e r k e l e y N a t i o n a l L a b o r a t o r y , B e r k e l e y , m a d e useful
c o m m e n t s o n a n earlier v e r s i o n o f this paper. T h e authors are i n d e b t e d to H. Stanjek, T e c h n i s c h e U n i v e r sit,it M t i n c h e n , for his s u g g e s t i o n s a n d c a l c u l a t i o n s
w i t h the R I E T A N p r o g r a m .
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(Received 22 September 1998; accepted 1 February 1999;
Ms. 98-117)