Clay Minerals (1994) 29, 361-367
A C I D A C T I V A T I O N OF A S P A N I S H S E P I O L I T E "
PHYSICOCHEMICAL CHARACTERIZATION, FREE
S I L I C A C O N T E N T A N D S U R F A C E A R E A OF P R O D U C T S
OBTAINED
M. A. VICENTE
RODRIGUEZ*t,
J . DE D . L O P E Z
M. A. BA/qARES
MU/qOZt
GONZALEZ*
AND
*Departamento de Qu#mica lnorgdnica, Facultad de Ciencias, Universidad Nacional de Educaci6n a Distanc&, Senda
del Rey s/n, 28040 Madrid, Spain, and ?Departamento de Quimica lnorgtinica, Facultad de Ciencias Qu[micas,
Universidad de Salamanca, Plaza de los Caidos s/n, 37008 Salamanca, Spain
(Received 6 April 1993; revised 29 October 1993)
A B S T R A C T: A sepiolite from Vallecas (Spain) was treated with solutions of HCI (1.25, 2.5, 5.0, 10.0 and
20.0 wt%) at 25~ for 2, 6, 24 and 48 h, respectively. The resulting solids were characterized by XRD, FT-IR
and thermal analyses, along with SEM, TEM and nitrogen adsorption isotherms at 77 K. The free silica was
digested and determined in all samples. Several samples were seen to have specific surface areas up to
450 m2/g, with a maximum value of 549 m2/g in the sample treated with 1.25 wt% HCI for 48 h. A sudden
decrease in specific surface areas was observed when free silica was digested.
The physicochemical behaviour of clay minerals is
studied because of its relation to their adsorbing
and/or catalytic properties. This behaviour is
governed by the extent and nature of their
external surface which can be modified by suitable acid and thermal treatments; these treatments are called activation treatments if they
produce an increase in the specific surface area
and/or in the number of acid centres in the clay
mineral. Acid and thermal treatments increase
the catalytic and adsorbing activity of some clay
minerals, and further and stronger treatments
decrease this activity (Grim, 1962; Fern~indez
Alvarez, 1970; Jim6nez L6pez et al., 1978; L6pez
Gonz~lez et al., 1981 ; Bonilla et al., 1981; Cetisli
& Gedikbey, 1990; Pesquera et al., 1992).
Sepiolite is a hydrous magnesium silicate whose
ideal formula is [Si12Mg8030(OH)a](H20)a.8H20
according to the model of Brauner-Preisinger
(Brauner & Preisinger, 1956; Caillere etal.,
1982). It has a fibrous morphology with microcrystalline channels 3.7 x 10.6 .A wide, running
parallel to the fibres and affording the clay a
microporous structure through which molecules
of water or other adsorbates of suitable dimensions can gain access to the channels without
swelling the host. Different acid and thermal
treatments are able to modify the porosity and
structure of the silicate, producing changes in its
surface and activity (Rautureau & Mifsud, 1977;
Fermindez Alvarez, 1978; L6pez Gonz~ilez et al.,
1981; Bonilla et al., 1981; Gonz~lez et al., 1984;
Cornejo & Hermosin, 1986). Both the natural
silicates and the solids obtained after acid activation are used as adsorbents, decontaminants,
deodorants and as catalysts or catalyst supports
(Campelo et al., 1987; Sugiura et aL, 1991).
Although much research has been carried out
on the acid and thermal activations of the
different clay minerals, certain aspects of their
nature and mechanism remain unclear. Accordingly, we have studied the evolution of some
properties of sepiolite when it is treated with HC1
over a wide concentration range and with very
different treatment times. These studies have
been extended to some activated samples after
removal of the free silica formed during the acid
treatments.
EXPERIMENTAL
The clay mineral used was a natural sepiolite from
Vallecas (Spain), supplied by T O L S A , SA. The
elemental analysis was carried "out by plasma
emission spectroscopy, using a Perkin-Elmer
emission spectrometer, model Plasma II after the
9 1994 The Mineralogical Society
362
M. A. Vicente Rodriguez et al.
solid had b e e n digested u n d e r pressure with a
nitric-hydrofluoric acid mixture in a P T F E
autoclave.
A f t e r grinding, the natural sepiolite was sieved
to a particle size of 0.60--0.75 m m . Five grams of
the clay were t r e a t e d by mechanical stirring with
1 5 0 m l of 1.25, 2.5, 5.0, 10.0 and 2 0 . 0 w t %
solutions of HCI at 25~ for 2, 6, 24 a n d 48 h.
A f t e r these t r e a t m e n t s , the resulting samples
were washed until no chloride anions could b e
detected. They were t h e n dried at 50~ and kept
over 50 w t % H2504. A portion was t a k e n from
these solids and the free silica was digested
following the m e t h o d described by Ross &
H e n d r i c k s (1945). T h e free silica extracted was
d e t e r m i n e d quantitatively by the reduced silicomolybdic complex formed, at 810 n m and using
an U V - V I S V a r i a n - T e c h t r o n , m o d e l 635 spectrop h o t o m e t e r (Chariot, 1956; modified by R o b e r t
& V e n e a u , personal c o m m u n i c a t i o n ) . A silica
standard solution (SiCI4 in 5 mol/! N a O H ) from
Merck (1.000 + 0.002 ppm) was used for calibrating the apparatus.
T h e specific surface areas of the samples were
d e t e r m i n e d by the c o r r e s p o n d i n g nitrogen
adsorption isotherms at 77 K, o b t a i n e d from a
Micromeritics A S A P 2000 analyser, after outgassing the samples at l l0~ for 8 h , with a
residual pressure of 10 -5 m m Hg. T h e B E T
m e t h o d was used for the c o r r e s p o n d i n g
calculations.
X-ray diffraction ( X R D )
p a t t e r n s were
o b t a i n e d on a Siemens D-500 diffractometer
using the Cu-Kol line (Z = 1.54050 A ) , with a Ni
filter, a graphite m o n o c h r o m a t o r (efficiency for
Cu-Ko: line up to 9 0 % ) and a D A C O - M P data
station.
T h e F o u r i e r - T r a n s f o r m infrared (FT-IR) spectra were o b t a i n e d in the region 4000-400 cm -1 on
a P e r k i n - E l m e r F T - I R M-1700 2B spectrophot o m e t e r e q u i p p e d with a 3600 data station, using
the K B r pellet technique.
The thermal analyses were carried out on
P e r k i n - E l m e r analysers TGS-2 and 1700 for
t h e r m o g r a v i m e t r i c ( T G ) and differential t h e r m a l
analyses ( D T A ) , respectively; b o t h were connected to a 3600 data station.
T h e morphological analyses were carried out
using a Zeiss DSM 940 scanning electron microscope on samples coated with A u by Bio-Rad
E5100 SEM Coating System e q u i p m e n t and a
Zeiss EM 902 transmission electron microscope
o n samples dispersed in w a t e r and deposited on a
grid.
RESULTS
AND
DISCUSSION
T h e original sepiolite was a good quality natural
sample, with quartz, feldspar and calcite as
mineralogical impurities, altogether accounting
for < 2 0 % of the weight.
T h e chemical composition of this sample,
equilibrated over 5 0 w t % H 2 S O 4 was: SiO2:
55.31; A1203: 1.68; Fe203: 0.53; MgO: 22.92;
CaO: 0.53; N a 2 0 : 0 . 1 3 a n d loss o n ignition
19.0 w t % . Traces of P and Ti were detected. T h e
structural formula of the dry sample was:
[Si5.88A10.t2][A10.09Mg3.65Fe0.04]O16Ca0.06Na0.03K0.07.
T h e X-ray diffractograms of natural sepiolite
and of the series t r e a t e d with 1.25% HCI are
p r e s e n t e d in Fig. 1. W h e n the time of the
t r e a t m e n t was increased, the crystallinity of the
samples decreased, as can b e qualitatively seen in
the main peak of the silicate at 12.1 ~ , b e c o m i n g
b r o a d e r a n d less intense as the time of t r e a t m e n t
progressed. A t the same time, during the treatm e n t t h e r e was a p r o n o u n c e d increase in the
quantity of silica in the samples, as shown by the
a p p e a r a n c e a n d increase of the b r o a d b a n d
characteristic of this c o m p o u n d situated in the
middle of the diffractogram, with the quartz peak
situated at 3.33 ~ . In the series treated with m o r e
c o n c e n t r a t e d HC1, the loss of crystallinity and the
a p p e a r a n c e of a m o r p h o u s silica was similar to
that described above. With increasing acid concentrations, the time required to produce the
o b s e r v e d effects decreased. W h e n c o n c e n t r a t e d
acid solutions were used for long t r e a t m e n t times,
the diffractograms showed only the silica b a n d
(Fig. le,f).
In o r d e r to o b t a i n more i n f o r m a t i o n a b o u t the
acid activation process of sepiolite, an exhaustive
t h e r m a l study was carried out. T h e D T A and T G
curves, b o t h for natural sepiolite and for the
series treated with 1.25% HC1, are plotted in
Fig. 2. T h e b e h a v i o u r of the natural sepiolite
agrees with that r e p o r t e d by J o n e s & G a l e n
(1988) for this silicate. O n the D T A curve,
e n d o t h e r m i c peaks a r o u n d 120,350 and 550~ are
observed, the first due to the loss of hydration and
zeolitic water and the others to ttie loss, in two
steps, of c o o r d i n a t e d water. T h e r e was a final and
very weak e n d o t h e r m i c effect at 820~ due to the
Acid activation of sepiolite and its effects
363
12.1A
3.33A
h~
E~
H
O3
e
z
z
H
e
b
3 . O00
2O
70.000
Ft6.1. XRD of natural sepiolite (a) and samples treated with [ .25% HCI for (b) 2, (c) 6, (d) 24 and (e) 48 h and with 20.0%
HCI for 48 h (f). Cu radiation.
loss of structural hydroxyl groups. This was
m a s k e d by the strong exothermic peak at 830~
p r o d u c e d by the structural change in the silicate
giving clinoenstatite (MgSiO3). All these processes c o r r e s p o n d to a weight-loss p h e n o m e n a in
the T G curves c o r r e s p o n d i n g to a total loss of
19% of the initial weight of the sample.
In the activated samples, when the intensity of
the acid t r e a t m e n t s was increased, the effects due
to coordination water decreased and finally
disappeared; this is reflected in the T G analyses
by a simplification in the curves and a decrease in
the value of the weight loss, which decreases to c.
9 w t % of the original solid in the most activated
samples. T h e s e water molecules were m o r e
weakly b o n d e d in the activated samples than in
the natural sepiolite, leaving the activated solids
at a b o u t 100-105~
as o b s e r v e d on the D T A
curves. T h e Mg 2+ cations migrate to the solution
and the water molecules and O H groups, initially
b o n d e d to these cations, disappear from the solids
as the acid t r e a t m e n t progresses (Cetisli &
G e d i k b c y , 1990). The solution of Mg 2+ cations,
a c c o m p a n i e d by destruction of the crystal structure, impedes the final structural change observed
in the natural sepiolite in which, as indicated
above, clinoenstatite was finally formed. Only
some of the hydration water persisted in the solids
after strong acid t r e a t m e n t , accounting for c. 9%
of its weight.
T h e study of the acid-activated solids by F T - I R
spectroscopy confirms the presence of silica w h e n
the sepiolite structure is destroyed (Fern~indez
Alvarez, 1972). T h e spectra of natural sepiolite
and of the series treated with 1.25% HCI are
shown in Fig. 3. As the t r e a t m e n t proceeds the
b a n d s due to the hydroxyl from the M g - O H group
(3669 cm -~) and to the different types of w a t e r
existing in the original solid (3635, 3570, 3420 and
3 2 5 0 c m ~) decreased in intensity and finally
disappeared. In the most activated solids a single
b a n d at 3440 cm J was observed. T h e b a n d at
1660 cm -~ , due to the b e n d i n g vibration m o d e of
the water, u n d e r w e n t a similar simplification
process. As indicated in the t h e r m a l analyses, the
water is c o o r d i n a t e d with the Mg 2+ cations and
disappears when these cations migrate to the
solution, causing these changes in the spectra as
well as the disappearance of the b a n d due to the
stretching vibration m o d e of M g - ( O H ) at
178(1 c m - 1.
Likewise, the b a n d situated between 1200 and
1000 cm 1, characteristic of sepiolite and attrib u t e d to S i - O - S i vibrations, disappeared as the
acid t r e a t m e n t progressed. As can be seen, the
attack b e c o m e s i m p o r t a n t after 24 h acid treat-
364
M. A. Vicente Rodriguez et al.
................
/ ......
"".....,.~
/
d
~
........... - 7 "
~-----~
/ ................................
,,
"~L22
~ ................~,.........
..........................
~"-....
ii
l~
i
I
120
........
............
5% WEIGHTLOSS
260
440
600
760
TEMPERATURE(nC)
Flo. 2. DTA and TG curves (broken and solid lines,
respectively), of natural sepiolite (a) and samples treated
with 1.25% HCI for (b) 2, (c) 6, (d) 24, and (e) 48 h.
m e n t in the series considered. As the characteristic b a n d s of the original silicate disappeared, new
b a n d s situated at 1200-1000, 795 and 470 cm -~
a p p e a r e d . T h e s e t h r e e b a n d s are characteristic of
silica, a l t h o u g h the shape of that at 1200-1000 cm-1 is different from that characteristic of
the original sepiolite at the same wave n u m b e r .
T h e same trends are o b s e r v e d in the o t h e r series
and the f o r m a t i o n of silica is faster w h e n the
c o n c e n t r a t i o n of the acid used in the t r e a t m e n t s
increases, as has b e e n shown by o t h e r techniques.
Electron microscopy was used to observe the
morphological changes p r o d u c e d in the solids
during the acid t r e a t m e n t . T h e silica o b t a i n e d
during the t r e a t m e n t maintains the fibrous structure of the natural sepiolite. This aspect was first
discovered by Gonz~ilez e t a l . (1984), who
r e p o r t e d an exfoliation p h e n o m e n o n as the only
structural change, p r o b a b l y p r o d u c e d by fractures in the silica sheets, which may occur at the
points of inversion of the silica t e t r a h e d r a , the
weakest points in the struture. T h e persistence of
the original structure during the acid t r e a t m e n t of
fibrous silicates has recently b e e n o b s e r v e d in
palygorskite by Su~irez et al. (1992).
T h e free silica digested increased with the
c o n c e n t r a t i o n of the acid and with time of attack
(Table 1) reaching --40% in the most strongly
attacked samples. P e r h a p s higher values would be
expected, but it should be b o r n e in mind that in
the most strongly attacked samples almost all of
the metallic cations and soluble impurities were
r e m o v e d , lending more i m p o r t a n c e to the impurities insoluble in e i t h e r acid or alkaline solutions.
This is clearly shown by different techniques.
W h e n the sepiolite was only slightly atacked by
the acid t r e a t m e n t , after silica digestion, the
solids showed X R D p a t t e r n s very similar to that
of the natural sample. H o w e v e r , if the acid
t r e a t m e n t was m o r e intense, the diffractograms
o b t a i n e d after silica digestion showed only the
insoluble impurities, identified as mica, quartz
and feldspar. Transmission electron microscopy
and F L I R studies disclosed the same kind of
b e h a v i o u r ; in the micrographs of the samples
without silica, a fibrous structure was observed in
slightly activated sepiolite, b u t w h e n the acid
attack was more intense, the fibrous structure
disappeared a n d only impurities, such as almost
spherical particles of mica, were observed. In FTI R spectroscopy of solids without silica, the
characteristic b a n d s of sepiolite a p p e a r e d w h e n
the acid t r e a t m e n t was weak, although the
complex a n d unclear spectra were o b t a i n e d , with
b a n d s due to the different impurities, w h e n the
acid t r e a t m e n t was stronger.
As indicated, the percentage of free silica
would seem low, a l t h o u g h o t h e r authors have not
found very different values. Only Fern~indez
Alvarez (1972) has r e p o r t e d a sample with 69%
free silica, using the Ross & Hendricks (1945)
digestion m e t h o d , although there is no description of the m e t h o d used in its d e t e r m i n a t i o n . This
value was p r o b a b l y calculated by the difference in
weight before and after the digestion process; in
this case, the data would not b e very accurate
because o n e must take into accouat t h a t before
digestion, the solids have --10% water, and after
digestion their w a t e r a d s o r b e n t capacity is, as we
Acid activation of sepiolite and its effects
I
i
I
I
I
I
I
I
I
I
I
l
I
I
I
I
I
365
I
I
I
i
I
,
e
o
z
E-,
20% TRANSMITANCE
4000
3000
2000
1500
1200 I000 "
500
WAVE NUMBER (cm- I )
FtG. 3. FT-IR spectra of natural sepiolite (a) and samples treated with 1.25% HCI ~r (b) 2, (c) 6, (d)24 and (e) 24 h.
M. A. Vicente Rodriguez et al.
366
have observed, m u c h lower. T h e m e t h o d described would u n d o u b t e d l y be effective for identifying the insoluble impurities in a silicate w h e n it
is difficult to observe t h e m in the diffractogram of
the natural sample.
The different solids o b t a i n e d after acid activation a n d after free silica digestion were studied
from their N2 adsorption isotherms at 77 K in
o r d e r to d e t e r m i n e their surface area. All the
isotherms are similar to type II of the B D D T
classification (Gregg & Sing, 1969) and are very
similar in shape. T h e specific surface areas of the
solids were calculated from the N2 adsorption
isotherms. T h e results o b t a i n e d are given in
T a b l e 1. T h e first value corresponds to natural
sepiolite: 293 m2/g, similar to that found in
previous works in which this sepiolite was used
TABLE 1. Free silica removed by Na:CO3 digestion and
surface area (m2/g) of natural and treated samples.
Sample
Natural sepiolite
Free
silica*
(m2/g)
2.0
293
SBET
Sepiolite
Sepiolite
Sepiolite
Sepiolite
1.25% HCI 2 h
1.25% HCI 6 h
1.25% HCI 24 h
1.25% HCI 48 h
4.l
5.8
14.3
30.7
306
314
406
549
Sepiolite
Sepiolite
Sepiolite
Sepiolite
2.5%
2.5%
2.5%
2.5%
HCI 2 h
HCI 6 h
HCI 24 h
HCI 48 h
5.1
8.6
40.3
38.7
299
323
492
460
Sepiolite
Sepiolite
Sepiolite
Sepiolite
5.11% HCI 2 h
5.11% HCI 6 h
5.0% HCI 24 h
5.11% HCI 48 h
7.5
13.3
37.6
40.2
290
355
439
381
Sepiolite
Sepiolite
Sepiolite
Sepiolite
111.11%HCI 2 h
11/.0% HCI 6 h
10.0% HCI 24 h
10.0% HCI 48 h
13.7
21.8
36.4
39.8
343
378
406
3(17
Sepiolite
Sepiolite
Sepiolite
Sepiolite
20.0%
20.0%
20.0%
20.0%
21.6
37.9
39.0
40.9
4111
41(1
328
291
Natural sepiolite, silica-free
-
222
Sepiolite 1.25% HCI 2 h silica-free
Sepiolite 1.25% HCI 6 h silica-free
Sepiolite 1.25% HCI 24 h silica-free
---
263
264
215
Sepiolite 2.5% HCI 48 h silica-free
-
HCI 2 h
HCI 6 h
HCI 24 h
HCI 48 h
* % of each sample weight.
- -
270
(Bafiares Mufioz & del A r c o S~nchez, 1989a,b).
F o r the series treated with 1.25% HC1, the value
of S increases continuously with the time of
t r e a t m e n t , reaching a m a x i m u m value of 549 m2/g
at 48 h. In o t h e r series, a m a x i m u m value of S is
o b t a i n e d , decreasing r e m a r k a b l y with longer
t r e a t m e n t s . This m a x i m u m is r e a c h e d after 24 h
w h e n 2.5% HCl (492 m2/g), 5.0% HCI (439 m2/g)
a n d 10.0% HCi (406 m2/g) are used. F o r the
samples activated with 20.0% HCI, the m a x i m u m
S (410 m2/g) is m a i n t a i n e d during the attack
period b e t w e e n 2 and 6 h.
O n the o t h e r h a n d , w h e n the d e p e n d e n c e of S
on the HC1 c o n c e n t r a t i o n is studied for each
constant t r e a t m e n t time it is seen that for 2 h of
t r e a t m e n t , S remains practically constant with the
more dilute acids, its value increasing w h e n
c o n c e n t r a t e d acids are used (10.0 and 20.0%).
A f t e r 6 h, the increase in S is practically linear and
for longer periods of t r e a t m e n t a m a x i m u m value
of S is reached, decreasing with more intense acid
t r e a t m e n t s . T h e m a x i m u m is o b t a i n e d with 2.5%
HC1 for samples treated for 24 h (492 m2/g) and
with 1.25% HCl after 48 h (549 m2/g).
In silica-free samples, the value of S decreases
in comparison with samples before digestion, thus
confirming that silica has a very i m p o r t a n t contribution to the total a m o u n t of the surface area.
The decrease in S is greater in the samples which
had b e e n more strongly attacked during the acid
t r e a t m e n t and, consequently, had more free
silica.
This part of the work can be s u m m a r i z e d by
indicating that, as in previous papers, suitable
conditions exist for the activation of a silicate,
where a m a x i m u m in the value of surface area is
o b t a i n e d , the a m o u n t of this magnitude decreasing with more intense treatments. With the
variables considered (HCI c o n c e n t r a t i o n and time
of t r e a t m e n t ) and in the ranges used, the maxima
of this magnitude are o b t a i n e d for the t r e a t m e n t
with 1.25% and 2.5% HC1 for 24 and 48 h; 5.0%
and 10.0% for 24 h; and 20.0% for 2 or 6 h.
Additional work is being carried out for the
d e t e r m i n a t i o n of the acid centres of the original
and acid activated solids.
REFERENCES
BAglARESMU~OZM.A. & DEL ARCOSANCHEZM. (1989a)
Estudio de la interacci6n de sepiolita de Vallecas con
DDT. I. Caracterizaci6n de la sepiolita por difracci6n de
Acid activation of sepiolite and its effects
rayos X, an~ilisis tfrmico diferencial, espectroscopfa IR y
adsorci6n de nitr6geno a baja temperatura. Agrochimica, XXXIII, 15-23.
BAIqARESMU~OZ M.A. & DEL ARCOSANCHEZM. (1989b)
Estudio de la interacci6n de sepiolita de Vallecas con
DDT. II. 'Parfimetros termodinfimicos para la adsorci6n
de DDT sobre sepiolita.' Agrochimica, XXXIII,
267-274.
BONILLAJ.L., LOPEZ GONZAEEZJ. DE D., RAMIREZSAENZ
A., RODRIGUEZREINOSOF. & VALENZUELACALAHORRO
C. (1981) Activation of a sepiolite with dilute solutions of
NO3H and subsequent heat treatments: II. Determination of surface acid centres. Clay Miner. 16, 173-179.
BRAUNERK. & PREISINGERA. (1956) Struktur und Entstehungdes sepioliths. Tschhermarks Min. Pert. Mitt. 6,
120-140.
CAILLERE S., HENINS. & RAUTUREAUM. (1982) Mindralogie des argiles, Vol. 1 et 2. INRA et Masson, Paris.
CAMPELO J.M., GARCIA A., LUNA O, & MARINAS J.M.
(1987) Catalytic activity of natural sepiolites in cyclohexene skeletal isomerization. Clay Miner. 22, 233-236.
CErtsu H. & GEDJKaEVT. (1990) Dissolution kinetics of
sepiolite from Eskisehir (Turkey) in hydrochloric and
nitric acids. Clay Miner. 25,207-215.
CHARLOTG. (1966) Les M(thodes de la Chimie Analytique.
Analyse Quantitative Mindrale. (Masson and Cie,
editors). Paris.
CORNEJO J, & HERMOSIN M.C. (1986) Efecto de la
temperatura en la acidez superficial del producto obtenido por tratamiento ficido de sepiolita. Bol. Soc. Esp.
Miner. 9, 15-138.
FERNANDEZ ALVAREZ T. (1970) 8uperficie especffica y
estructura de poro de la sepiolita calentada a diferentes
temperaturas. Proc. Reunirn Hispano-Belga de Minerales de la Arcilla, Madrid, 202-209.
FERNANDEZAEVAREZT. (1972) Activaci6n de sepiolita con
~icido clorhidrico. Bol. Soc. Esp. Ceram. Vidr. 11,
365-374.
FERNANDEZALVAREZT. (1978) Efecto de la deshidrataci6n
367
sobre las propiedades adsorbentes de la palygorskita y
sepiolita. Clay Miner. 3, 325-335.
GONZALEZ L , ISARRAL.M., RODRIGUEZA., MOYAJ.J. &
VALLE F.J. (1984) Fibrous silica gel obtained from
sepiolite by HC1 attack. Clay Miner. 19, 93-98.
GREGGS.J. & SINGK.S.W. (1982)Adsorption, Surface Area
and Porosity. Second Printing. Academic Press, London
& New York.
GRIM R.E. (1962) Applied Clay Mineralogy, pp. 307-318.
McGraw-Hill Book Co. Inc., London.
JIMENEZ LOPEZ A., LOPEZ GONZALEZ J. DE D., RAMIREZ
SAENZ A., RODRIGUEZREINOSOF., VAEENZUELACALAHORRO C. & ZURITA HERRERA L. (1978) Evolution of
surface area in a sepiolite as a function of acid and heat
treatments. Clay Miner. 13, 375-385.
JONES B.F. & GALANE. (1988) Sepiolite and palygorskite.
Rev. Miner. 19, 631-674.
LoPEz GONZALEZJ. DE D., RAMIREZSAENZA., RODRIGUEZ
REINOSO F., VALENZUELA CALAHORRO C. & ZURITA
HERRERA L. (1981) Activacirn de una sepiolita con
disoluciones diluidas de NO3H y posteriores tratamientos
trrmicos: I. Estudio de la supefficie especffica. Clay
Miner. 16, 103-113.
PESQUERA C., GONZALEZ F., BENITO I., BEANCO C.,
MENDIOROZ S. & PAJARES J. (1992) Passivation of a
montmorillonite by the silica created in acid activation.
J. Mater. Chem. 2, 907-911.
RAUTUREAUM. & MIFSUDA. (1977) Etude par microscope
61etronique des differents 6tats d'hydratation de la
s6piolite. Clay Miner. 12, 309-318.
Ross C.S. & HENDRICKS S.B. (1945) Minerals of the
montmorillonite group; their origin and relation to soils
and clays. Prof. Pap. U.S. Geol. Surv. 205-B, 23-79.
SU~,REZ M., FLORES L.V. & MARTIN POZAS J.M. (1992)
Textural study of palygorskite by acid treatment.
Abstracts Mediterranean Clay Meeting M.C.M.'92;
Lipari (Italy). 132-133.
SUGIURAM., HAYASHIH. & SUZUKIT. (1991) Adsorption of
ammonia by sepiolite in ambient air. Clay Sci. 8, 87-100.