Clay Minerals (1997)32, 21-28
Iodide, caesium and strontium adsorption
by organophilic vermiculite
J. B O R S , A. G O R N Y *
AND S. D U L T Z t
Centre of Radiation Protection and Radioecology, Hannover University, Herrenhiiuser Str. 2, D-30419 Hannover,
Germany, *Dahlemer Str. 4, D-30982 Pattensen, Germany, and t lnstitute of Soil Science, Hannover University,
Herrenhi~user Str. 2, D-30419 Hannover, Germany
(Received 7 November 1995; revised 22 August 1996)
A B S T R A C T: Distribution coefficients and isotherms for the adsorption of radioiodide, caesium
and strontium ions (125I-, 134Cs+ and 85Sr2+) were determined in untreated vermiculite and in
vermiculite treated with the chloride salt of hexadecylpyridinium (HDPy+). Experimental data reveal
that some of the alkylammonium ions were incorporated as HDPyCI ion pairs into the vermiculite.
The fraction of ion pairs adsorbed is reflected by an increase in distribution coefficients with
increasing saturation levels. Consequently, a considerable part of the iodide adsorption could be
attributed to an anion exchange process. At smaller amounts of Cs§ and Srz+ adsorbed, distribution
coefficients of the modified clay mineral are significantly reduced in comparison with the original
material. These differences are less pronounced when greater amounts of cations are adsorbed.
Generally, larger distribution coefficients were found for the Cs+ compared with the Srz§ ion in the
untreated and modified samples.
Soils represent an important filter and buffer system
for many cations and anions. Generally, their
sorption properties are determined by the contents
and compositions of clay minerals and humic
substances. A great part of the soil organic matter
is associated with clay minerals forming organomineral complexes (Greenland, 1965) which are of
pronounced importance with respect to element
transport and/or retention and to aggregate stability.
Clay minerals also interact with organic cations.
The replacement of inorganic cations by alkylammonium ions causes considerable modification in
the hydration and swelling properties of the clays.
Research in this area has significantly increased the
knowledge on the physico-chemical behaviour of
both clay minerals and organo clay minerals
(Weiss, 1963; Mortland, 1970; Theng, 1974;
Lagaly, 1981; Rausell-Colom & Serratosa, 1987).
Cation exchange capacities (CEC) of naturally
occurring organo-mineral complexes were determined by Leinweber et al. (1993). It was observed
in samples extracted from nine different soils that
the potential CEC decreased with increasing
equivalent diameters of the separated particles (i.e.
fine and medium clay: 489-813 mmolc kg-l;
coarse clay: 367-749 mmolc kg-1; fine silt:
202-587
mmolc kg-l;
medium
silt:
63-345 mmolc kg-l). The CEC varied with
factors associated with the soil genesis, the
mineralogical composition of the <6.3 p.m sizefraction and the C and N contents of the particle
size-fractions.
It has been shown in extensive investigations that
organophilic clay minerals can adsorb non-ionic
organic compounds (Mortland et al., 1986; Jaynes
& Boyd, 1991; Xu & Boyd, 1994) as well as
iodide, which exists predominantly in the anionic
form in terrestrial ecosystems (Lieser & Steinkopff,
1989). After modification of bentonite, vermiculite
and Cretaceous clay by hexadecylpyridinium
(HDPy +) and benzethonium (BE+), these organoclay minerals exhibited adsorption rates and
amounts for iodide ions which are several orders
of magnitude higher than those of untreated
samples. Moderate increases in the adsorption
parameters were found after cation exchange with
hexadecyltrimethylammonium (HDTMA+), while
the treatment with trimethylphenylammonium
9 1997 The Mineralogical Society
22
J. Bors et al.
(TMPA +) and tetramethylammonium (TMA § were
ineffective in this respect (Bors, 1990, 1992).
Studies with partially saturated HDPy-vermiculite
showed that the increased organic C content and the
e x p a n s i o n of the basal spacing ( ~ 2 . 7 nm
maximum) were related to the increased amounts
of radioiodide adsorbed. The log-log plot of the
adsorption and desorption isotherms exhibited
linearity at equilibrium concentrations up to
10 -4 mol 1-1 or 10 -1 mol kg -1 and became nonlinear at higher I - loadings (Bors & Gorny, 1992).
Adsorption experiments with different particle
size-fractions (clay and silt fractions) of Cretaceous
clay, the mineral fraction of a loess soil and
vermiculite, each treated with HDPy +, revealed
highest iodide adsorption in the medium clay
fractions (0.2-0.6 gm and 0.6-2.0 gin, respectively). For Cretaceous clay and clay from loess,
iodide adsorption was related to the amount of
HDPy + incorporated, but in vermiculite no clear
relationship between distribution coefficients and
HDPy content of the different size-fractions was
observed (Bors et al., 1995).
Alkylammonium cations with ten or more C
atoms in the aliphatic chain are adsorbed by
montmorillonite in amounts greatly exceeding the
CEC (Greenland & Quirk, 1962; Rausell-Colom &
Serratosa, 1987). Such sorption of HDPy + beyond
the CEC was observed in the experiments
mentioned above with Cretaceous clay and with a
clay from a loess soil also (Bors et al., 1995). These
organic cations exchange inorganic cations at the
external surface in a first step or exchange occurs
simultaneously with the interlamellar cations. A
portion of the organic cations is adsorbed, probably
together with their gegen ions (Patzko, 1991;
Lagaly, 1995).
A different adsorption process may operate in
organo-vermiculite. It has to be noted that in spite of
the application of HDPy + in amounts equivalent to
100% of the CEC, the maximum adsorption of
HDPy + represesented only ~ 5 0 % of the CEC (Bors
& Gorny, 1992). In this case, C1- ions are adsorbed
via HzO molecules around the alkyl chains (Holz &
SOrensen, 1992; Lagaly, 1995). In both examples, the
ion exchange of I - against C1- could be responsible,
at least partially, for the iodide adsorption.
In order to improve the understanding of iodide
adsorption mechanisms, the contribution of
HDPy-C1 ion pairs to the sorption behaviour was
investigated. The ability of organophilic vermiculite
to interact with inorganic cations was examined by
batch experiments using 134Cs+ and 85Sr2+ as tracers
with the corresponding carrier substances in
different concentrations.
MATERIALS
AND
METHODS
Preparation and characterization of
HDPy-vermiculite.
A thermally expanded vermiculite from Russia
(Thermax, A-3300 Greinsfurth) ground using an
Ultra-Centrifugal Mill was used for the experiments; according to XRD measurements, the
structure of the samples was not affected by this
procedure. To determine the CEC, the samples were
washed with a MgCI2 solution (three times with 0.5
tool 1- j , three times with 0.01 mol 1-1), and
Ba(NO3)2 was used (six times with 0.1 mol 1-1)
as an extracting agent. In each case, 100 ml of salt
solution per g vermiculite was employed. The
concentration of MgC12 in the solution was
determined by measuring the C1- concentration by
potentiometric titration, while that of the Mg 2+ ions
was determined by atomic absorption spectrometry
(AAS). The original vermiculite contained N85%
of the exchangeable Mg 2+ and 15% of the Ca 2+.
The charge per half unit-cell was determined by
chemical analysis to be 0.755 and the CEC was
1.62 mole kg -I (Pesci, 1994).
After dispersion of 20 g of the vermiculite in 1 1
of distilled water, the chloride salt of the quaternary
alkylammonium ion of HDPy* was added in
amounts equivalent to 100% CEC. Additionally,
HDPy-vermiculites of different HDPy + loadings
were prepared, adding the organic ion in amounts
corresponding to 10, 20, 30, 50 and 100% of the
CEC of the vermiculite. The suspensions were
stored for 18 h and filtered. The filter residues were
then washed at least eight times with water (1 1
total) or different mixtures of ethanol:water (10:90;
30:70; 50:50; 70:30; 90:10) to remove excess
organic salt. The samples were air dried, and the
uptake of HDPy + was determined by measuring the
C content with a LECO C instrument (IR 12). The
chloride concentration of the alkylammonium
solution before and after adsorption was measured
using a DIONEX DX 100 ion chromatograph.
Adsorption experiments
The adsorption of I - , Cs + and Sr 2+ was
investigated using the batch technique and was
Iodide, Cs and Sr adsorption by organophilic vermiculite
23
TABLE 1. Specifications of the HDPy-vermiculite and steady-state distribution coefficients.
HDPyC1 applied
[% CEC]
[mg g-l]
10
20
30
50
100
58
116
174
290
580
HDPy+ adsorbed
[mg g-l]
H D P y C 1adsorbed
[mg g--l]
Kd
[1 kg -1]
45
80
80
110
116
2
5
40
80
160
25
68
205
2630
3390
characterized by the distribution coefficient (Kd
value). The Ko value is defined as the ratio between
the concentration of solute sorbed on the solid
matrix (tool kg -1) and the concentration of the
solute in the equilibrium solution (mol l-l).
According to the given guidelines, distribution
coefficients are used frequently for presentation of
data in experiments investigating and attempting to
predict the long-term behaviour of radionuclides
(Kim & Lang, 1982; Baeyens & Bradbury, 1995).
For the anion adsorption experiments, ~0.5 g of
the organo-vermiculite was dispersed in 10 ml of
bi-distilled water containing 37 kBq of 1z5I( ~ 5 • 10 -12 tool 1-1) and KI in a concentration
range of 1 • 10 -8 to 1 tool 1-1. The samples were
incubated in 30 ml centrifuge tubes and shaken at
22~ for seven days, this time being sufficient to
establish equilibrium conditions. Sorption isotherms
were calculated using the formulae:
[I-]1 = [I-] ~ aj/a ~
(1)
0-Is = Ka [l-]~
(2)
where [I- h is the equilibrium concentration of
iodide in the solution in tool 1-1, [I-] ~ the initial
iodide concentration in the solution (tool l-l), AI is
the radioactivity of 1251- in the liquid at the end
of the adsorption e x p e r i m e n t measured in
cpm rnl- l , A~ is the total radioactivity of 125I- at
the beginning of the experiments in cpm ml - l and
[I-]s is the amount of iodide adsorbed in mol kg -1.
In the same manner, the adsorption of Cs + and
Sr 2+ was determined using 10 kBq 13aCs+
( ~ 1 x 10 -12 mol 1-1) and 10 kBq SSSr2+
( ~ l x 10 -12 mol 1-1) and CsCI or SrCl2 solutions
in initial concentrations of 1 x l0 -7 to 1 x l0 - l
tool 1-1. The solid and liquid phases were separated
by centrifugation (15 000 rpm, 20 000 g for 15 rain).
The experiments were carried out in duplicate.
In a double-labelling experiment with Cs + and
Sr2+, the competing adsorption of the two elements
by organo-vermiculite (with 276 mg HDPy + per g,
resulted from 100% CEC application) and by
untreated samples was tested. About 0.5 g of the
vermiculite was dispersed in 10 ml of the
equilibrium solution. The sum of the ion concentrations in equivalents was kept constant (1 x 10-3
mol 1-1), but the relation of Cs + to Sr2+ was varied
(equivalents Cs:Sr = 1:4, 2:3, 3:2 and 4:1). After
attaining equilibrium, the distribution coefficients
were determined.
RESULTS
AND
DISCUSSION
The distribution coefficients for the iodide adsorption, the adsorbed amounts of HDPy + ions and
HDPyCI ion pairs are listed in Table 1. The
application of HDPy + in increasing concentrations
results in increased organophilicity (total HDPy §
content) and consequently in increased distribution
coefficients of radioiodide. It is also obvious that a
portion of the alkylammonium ions is bound as
HDPyC1 ion pairs, the fraction of which increases
with increasing saturation levels. The data of sorbed
HDPyCI agree, at least qualitatively, with the
distribution coefficients.
In Fig. 1, distribution coefficients for the
differently saturated HDPy-vermiculite are related
to the concentrations of adsorbed iodide. They
increase with increasing organophilicity in a wide
range of iodide loadings and decrease very sharply
from 4.000-6.000 to ~ 1 0 1 kg - l at loadings
exceeding 10 -1 tool kg - l indicating saturation of
adsorption sites. Considering these data, the
adsorption capacity of the organo-vermiculite for
iodide is estimated to be ,-~5 x 10-1 tool kg - l .
As expected, the washing procedure with
different mixtures of ethanol:water reduced the C
24
J. Bors et al.
HDPY ~
~ 10000t
1000
IX
content
190 mg g-'j
120 mg gl
47mgg ~
1
10~
I0 ~
10 ~
lff z
10o
iodideadsorbed[rnol kg-1]
FIG. 1. Distribution coefficients for iodide ions on vermiculite treated with HDPy § in different concentrations as a
function of the amount of iodide adsorbed.
and C1 content of the HDPy-vermiculite (Fig. 2).
The minimum organic C content of the HDPyvermiculite and the maximum CI- concentration in
the washing solution were found for an ethanol:
water ratio of 70:30 vol%. The samples washed
with this ethanol:water mixture exhibited the lowest
iodide sorption (Fig. 3).
An adsorption maximum at an iodide concentration of ~ 1 0 -2 tool kg - t is observed for the
samples with the lowest HDPy-content (47 mg g - l ,
Fig. 1), and in those washed with the most effective
ethanol:water mixture (70:30 vol%, Fig. 3). Similar
unexpected effects were reported previously for the
HDPy + derivatives of vermiculite size-tractions of
0.6-2.0 and 2.0-6.0 Ixm (Bors et al., 1995). The
XRD patterns of these samples showed distinct
basal reflections with d-values of 3.0 nm and broad
shoulders to higher d-values. These reflections were
absent in the two smaller particle size-fractions
which showed no sorption maximum. Changes in
the interlayer arangement of the alkyl chains or/and
in the adsorption mechanism of iodide ions at the
24
18
7__
22
=m
~
14
20
E
g
r
10 .~
I
"5
m
16-
"5
S
(.~ 144
O
12-
2
10
20
40
Ethanol
60
contentof the washingsolution
00
8
0
100
[vol%]
FIG. 2. Carbon content of the HDPy-vermiculite and C1- concentration of the washing solution as a function of
the composition of the washing solution (ethanol:water).
Iodide, Cs and Sr adsorption by organophilic vermiculite
800
25
~
70% E t h a n o h
600 t
2O
.t
10 7
,
- -
,it"
10 ,5
,
,
10 ~
I0 -I
- : :
Q
I0 ~
Iodide a d $ o r ~ d [mol kg~:%
Fit. 3. Distribution coefficients for iodide ions as a funtion of the amount of iodide ions adsorbed in HDPyvermiculite (with 276 mg g-] HDPy § washed with different mixtures of ethanoL:water.
relatively high loading may be responsible tbr the
observed phenomenon.
Considering that some of the alkylammonium
ions are adsorbed together with their gegen ions
(Table 1) and that the iodide sorption is low in
samples washed with the ethanol:water ratio of
70:30 vol% (Fig. 2), ion exchange is assumed to be
one of the processes controlling iodide incorporation. On the other hand, no substantial effects on
the distribution coefficients were found for the
organo-vermiculite washed with the other ethanol:
water mixtures. These data and the strong decrease
of Ka values at high loading (Fig. 1) indicate that
besides ion exchange, another mechanism may be
involved in iodide adsorption. It is likely that the
organo-vermiculite contains different binding sites
for iodide anions. More work is needed to clarify
these hypotheses.
The distribution coefficients (Ka) for Cs ~ and
Sr2+ on original vermiculite and HDPy-vermiculite
(with 276 mg HDI~ "+ g-a) as a function of the
amount of Cs + and Sr2+ adsorbed are shown in
Figs. 4 and 5. The distribution coefficients are
distinctly different for the two materials. At higher
concentrations (1 x 10 -4 moi kg - / for Cs + and
1 • 10 -2
mo ~. kg -~ for St2+), the distribution
coefficients of vermiculite and HDPy-vermiculite
become very similar. The adsorption of the
untreated samples is reduced to the level of those
of HDPy-vermiculite. The observed sorption of
cations on the HDPy-vermiculite, especially evident
in the case of Cs +, can be explained by the mixedlayered structure (the existence of altered and
unaltered silicate layers) of this organophilic clay
mineral ( B o r s &
Gorny, 1992). The partial
e x c h a n g e of i n t e r l a y e r cations by H D P y +
(Table 1), support this assumption. One of the
reasons for the incomplete exchange in the highlycharged vermiculite may be due to the relatively
short reaction time of 18 h (Lagaly, 1982).
Considerably higher Kd values are obtained for
Cs + compared with the bivalent Sr 2+ for both
vermiculites. The preferential adsorption of Cs + can
be explained by its low hydration energy of Cs +.
Figure 6 illustrates the adsorption isotherms for
Cs + with unaltered and HDPy-vermiculite. The loglog plot of the isotherm of the original vermiculite
is linear up to ~ 1 x 10 - 4 tool 1-~ or 5 x 10 -~
mol kg -1. The corresponding isotherm for HDPyvermiculite is probably not linear. The HDPyvermiculite adsorbed less Cs + up to 1 x 10 -5
i
'~i
,oooi
\\
origin=l vl~mk~jte ~
\
FIG. 4. Distribution coefficients for caesium ions
related to the amount of caesium ions adsorbed,
original vermiculite and HDPy-vermiculite (with
276 mg g-1 HDPy+).
26
J. Bors et al.
2SO-
lO,]
10-1
50,
~l~ 100"
10 4 .
SO,
o
lO"
,
1o"
10"
in~"
104
tn~2
102
~
10 -s.
,
t~~
10
Sd" iclsorbedImol kg'~l
//
~
<
10 .7.
Fx6. 5. Distribution coefficients for strontium ions
related to the amount of strontium ions adsorbed,
original and HDPy+-vermiculite (with 276 mg g-~
HDPy+).
t
/
lO-O
10 "11
I-~
10 9
10 .5
rite
HDPy -vermiculite J
10-2
10;1
Cs* c o n c e n t r a t i o n of t h e equilibrium solution [mol 1-1]
m o l l -] or 1 •
-3 tool kg -]. At higher Cs §
concentrations the amount of Cs + sorbed becomes
similar for both vermiculites. As Figs. 4 and 5
demonstrate, the differences in the isotherms appear
more evident in the distribution coefficients.
The isotherms for Sr 2+ are linear over almost the
whole concentration range studied (Fig. 7). The
HDPY+-treatment influences the sorption of Sr z§
much more than that of Cs +.
Distribution coefficients obtained from a doublelabelling experiment confirm the preferential sorption of Cs in competition with Sr (Fig. 8). With
increasing Cs+:Sr 2+ ratios the adorption of Cs §
increases, whereas the value for Sr 2+ remains
constant. The especially high values for Cs § in
vermiculite with 120 mg g-1 HDPy+ content
(corresponding to 25% of the CEC) are remarkable.
This phenomenon is not well understood yet and
needs further attention, considering different
particle size-fractions. According to experimental
results (Graf v. Reichenbach, 1973), basal spacing
expansion and ion-exchange reactions are dependent on particle size.
FJc. 6. Adsorption isotherms for Cs § in original and
HDPY- vermiculite (with 276 mg g-1 HDPy*).
mechanism may operate in iodide sorption which
could not be identified on the basis of the
experimental data obtained. Additional work
101
10 -1 ,
~'
E
10 -~ .
"6
10 4 .
<
ite t
I " - ~ HDPy'vermiculite t
"
9
/
lO-'r.
CONCLUSIONS
The incorporation of alkylammonium cations
(HDPy § in vermiculite modifies the adsorption of
anions as well as of cations. A considerable fraction
of HDPy § is incorporated into the vermiculite as
HDPyC1 ion pairs. As a consequence, iodide
sorption is attributed, at least partially, to ion
exchange processes. Besides ion exchange, another
10 -9
10"'
10"
1()~
1()~
1~1
Sr z* c o n c e n t r a t i o n o f t h e e q u i l i b r i u m solution [mot 1-1]
FIG. 7. Adsorption isotherms for Sr2+ in original and
HDPY- vermiculite. (with 276 mg g-1 HDPy+).
lodide, Cs and Sr adsorption by organophilic vermiculite
27
t200 -
1000 9
HDPy* content
800 9
Cs*
9
276 m g g-~
9
120 m g g-~
9
0 mg g-1
-- ~-- 276 m g g-1
~
600
S~* ..i-
120 m g g-1
--*-
O m g 9-1
2OO
0
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
1
2
3
4
CII * I S r 2" r a t i o
F[(;. 8. Distribution coefficients for caesium and strontium as a function of the initial equivalent ratio of the two
cations, original vermiculite and HDPy-vermiculite. The composition of the equilibrium solution was held
constant (1 • 10 -3 eq 1-1). The solid and dashed lines represent best-fit curves.
including structural analysis is necessary to improve
the u n d e r s t a n d i n g of the i o d i d e s o r p t i o n
mechanism.
Cation adsorption is observed in original and
HDPy-vermiculite. However, in HDPy-vermiculite,
the cation adsorption is substantially reduced,
especially for Sr 2+. Generally, Cs § ions exhibited
a considerably higher affinity to the original and
HDPy+-exchanged vermiculite than Sr 2+ ions. The
adsorption of cations to the organophilic vermiculite can be attributed to the incomplete exchange of
interlayer cations by HDPy § under the experimental
conditions employed. As a consequence, a distinct
number of sorption sites remains available for the
sorption of inorganic cations.
ACKNOWLEDGMENTS
The skilled technical assistance of Mr. K.-H. Iwannek
is gratefully acknowledged.
REFERENCES
Baeyens B. & Bradbury M.H. (1995) A quantitative
mechanistic description of Ni, Zn and Ca sorption on
Na-montrnorillonite. PSI-Bericht Nr. 95-10,
Wtirenlingen and Villigen, Switzerland.
Bors J. (1990) Sorption of radioiodine in organo-clays
and soils. Radiochim. Acta, S1, 139-143.
Bors J. (1992) Sorption and desorption of radioiodine on
organo-clays. Radiochim. Acta, 58/59, 235-238.
Bors J, & Gorny A. (1992) Studies on the interactions of
HDPy-vermiculite with radioiodine. Appl. Clay Sci,
7, 245-250.
Bors J., Gorny A, & Dultz S. (1995) Sorption of
radioiodide on different particle size fractions of
hexadecylpyridinium-clays. Proc. lOth Int. Clay
Conf. Adelaide, 74-78.
Graf v. Reichenbach H. (1973) Exchange equilibria of
interlayer cations in different particle size fractions
of biotite and phlogopite. Proc. Int. Clay Conf.
Madrid, 457-466,
Greenland D.J. (1965) Interaction between clays and
organic compounds in soils. Pt. 2. Adsorption of soil
organic compounds and its effect on soil properties.
Soils Fert. 28, 521-532.
Greenland D.J. & Quirk J.P. (1962) Adsorption of 1-nalkyl-pyridinium bromides by montmorillonite. Clay
Miner. 9, 484-499.
Holz M. & S6rensen M. (1992) Direct detection of an
attractive interaction between anions and hydrophobically hydrated unpolar groups. Ber. Bunsenges.
Phys. Chem. 96, 1441-1447.
Jaynes W.F. & Boyd S.A. (1991) Clay mineral type and
organic compound sorption by hexadecyltrimethylammonium-exchanged clays. Soil Sci. Soc. Am. J.
55, 43-48.
Kim J.L. & Lang H. (1983) Standardization of the batch
experiment for the migration study of radionuclides
in the geologic media. In: Proc. of the U.S./FRG
Bilateral Workshop Berlin, Munich, (K.E. Maas &
A. Huf, editors) Hahn-Meitner Institut ftir
Kernforschung, Berlin.
Lagaly G. (1981) Characterization of clays by organic
compounds. Clay Miner. 16, 1-21.
28
J. Bors et al.
Lagaly G. (1982) Layer charge heterogeneity in
vermiculite. Clays Clay Miner. 30, 215-222.
Lagaly G. (1995) Surface and interlayer reactions:
bentonites as adsorbents. Proc. lOth Int. Clay Conf.
Adelaide, 137-144.
Leinweber P., Reuter G. & Brozio K. (1993) Cation
exchange capacities of organo-mineral particle size
fractions in soils from long term experiments. J. Soil
Sci. 44, 111-119.
Lieser K.H. & Steinkopff T.H. (1989) Chemistry of
radioactive iodine in the hydrosphere and in the
geosphere. Radiochim. Acta, 46, 49-55.
Mortland M.M. (1970) Clay-organic complexes and
interactions. Adv. Agron. 2 2 , 7 5 - 1 1 7 .
Mortland M. M., Shaobai S. & Boyd S.A. (1986) Clayorganic complexes as adsorbents for phenol and
chlorophenols. Clays Clay Miner. 34, 581-585.
Patzko A. (1991) Ion exchange and molecular adsorp-
tion of a cation active surfactant on montmoriUonite.
Proc. 7th Euroclay Conf. Dresden, 827-830.
Pesci N. (1994) Einflufl des Sr/K-Austausches auf die
Struktur aufweitbarer Schichtsilikate. PhD thesis,
Univ. Hannover, Germany.
Rausell-Colom J.A. & Serratosa J.M. (1987) Reactions
of clays with organic substances. Pp. 371-422 in:
Chemistry of Clays and Clay Minerals (A.C.D.
Newman, editor) Mineralogical Society, London.
Theng B.K.G. (1974) The Chemistry of Clay-Organic
Reactions. Hilger, London.
Weiss A. (1963) Mica-type layer silicates with alkylammonium ions. Clays Clay Miner., Monogr. No.
12, 10, 191-223.
Xu S. & Boyd S.T.A. (1994) Cation exchange chemistry
of hexadecyltrimethylammonium in a subsoil containing vermiculite. Soil Sci. Soc Am. J. 58,
1382-1391.