Clay Minerals (1967) 7, 167.
SORPTION
OF a - A M I N O - A C I D S
BY C O P P E R
MONTMORILLONITE
W. B O D E N H E I M E R
AND L. H E L L E R *
Geological Survey of Israel, Jerusalem
(Received 26 June 1967)
ABSTRACT: Sorption of an acidic, amphoteric, sulphur containing and basic
7-amino-acid (glutamic acid, glycine, methionine and lysine) by copper montmorillonite was studied by chemical and X-ray methods. With glutamic acid
complex formation occurs only in solution but increasing basicity of the aminoacid favours complex formation in the clay interlayers.
The association of amino-acids with montmorillonite has been studied sporadically
over the last 12 years, most recently by Cloos et aI. (1966). Summarizing previous work Cloos et al. concluded that although there are considerable differences
between the results and interpretations of various investigators, it is generally agreed
that low pH values favour sorption of amino-acids by acid and neutral montmorillonite. Accordingly they carried out experiments under more or less acid conditions,
using a hydrochloride form of the amino-acid with Ca and Na montmorillonite
and neutral amino-acids with hydrogen montmoriUonite.
Under acid conditions amino-acids occur as cations and as neutrality is
approached they tend towards the zwitterion form:
K1
HsN + -- R -- C O O H ~
HsN + -- R -- C O 0 - .
(Cation)
(Zwitterion)
At low p H values the reaction of amino-acids with clays is an ion exchange
reaction (Sieskind, 1963). Under the experimental conditions employed by Cloos
et al. (1966) the amino-acids were sorbed partly in the cationic and partly in the
zwitterion form.
Greenland, Laby & Quirk (1958) found that with concentrated amino-acid
solutions some adsorption occurred also at pH values near neutral.
Studies of the sorption of amines and catechols by montmorillonite containing
exchangeable transition metal ions, particularly copper, showed that there is a
strong tendency to complex formation in the clay interlayers (Bodenheimer et al.,
1962; Bodenheimer, Kirson & Yariv, 1963; Bodenheimer, Heller & Yariv, 1963a, b,
1964, 1966a, b, 1967). Amines, particularly diamines, can be extracted by copper
*Present address: Department of Geology, Hebrew University, Jerusalem.
168
W. Bodenheimer and L. Heller
montmorillonite, even from very dilute solutions. Complexes which are known to
be stable in aqueous solution are sometimes formed preferentially in the clay interlayers and in addition some which are unstable in aqueous solution are readily
obtained in interlayer space.
By analogy it seemed possible that complexes may be formed between exchangeable metal cations in montmorillonite and amino-acids. However, whilst copper
complexed with amine retains its net charge, complexing with amino-acids renders
it neutral. Thus if complex formation with amino-acids occurs in interlayer space,
cations must be supplied to the clay, probably in the form of protons and the
mechanism of such a reaction must differ basically from that governing the formation
of amine complexes.
Naturally occurring amino-acids may be divided into basic, acidic, neutral and
sulphur containing ones. It is the object of the present research to establish whether
copper ions can affect the sorption of different types of amino-acids by clays from
relatively dilute solutions and at various pH values.
EXPERIMENTAL
Materials
Wyoming bentonite was supplied by Wards Natural Science Establishment. The
following amino-acids were used: lysine, supplied by Light & Co., further purified
with active carbon; glycine, glutamic acid, and methionine as supplied by N.B.C.,
Ohio.
METHOD
Copper montmorillonite was prepared by shaking 200 g of natural Wyoming bentonite with 600 ml N Cu(NO~)2 solution overnight, washing six times with distilled
water, centrifuging and drying at 100 ~ C. The product, which contained 2"8 To Cu 2+,
i.e. 90 meq/100 g clay, was used as the starting material for a systematic investigation of the reaction of copper montmorillonite with amino-acids.
Different amounts of amino-acids were dissolved in 80 ml of water and added
to 500 mg of copper montmorillonite. The mixtures were shaken on a mechanical
vibrator for various periods of time.
Chemical analysis
After reaction with the amino-acid the clay was separated by centrifuging from
the supernatant liquid, which was analysed for amino-acid and copper. Amino-acids
were determined by the method of Harding & MacLean (1916) modified by the
addition of EDTA to avoid interference by the metal ions. The amount of aminoacid sorbed by the clay was mostly determined as the difference between the amount
added at the beginning of each experiment and that determined in the supernatant
liquid. Some experiments were, however, carried out to evaluate the amount sorbed
Amino-acids in Cu-montmorillonite
169
directly, by re-exchanging the clay with NaC1. The agreement between the measurements obtained by the two methods was within the timits of experimental error.
Copper in the supernatant solution was determined with diethyldithiocarbonate,
by the method of Frear (1939), adapted to the conditions of the experiments.
X-ray diffraction
Samples of clay after reaction with amino-acid solutions were sedimented on
glass slides and brought to equilibrium for 2 days in desiccators maintained at
different relative humidities with solutions of magnesium perchlorate (R.H. < 2%),
sulphuric acid (R.H. 35 %) and magnesium nitrate hexahydrate (R.H. 56 %). X-ray
diffraction patterns were obtained using General Electric XRD6 equipment, Cu K~
radiation.
RESULTS
The results obtained are summarized in Tables 1--4 and Fig. 1. Experiments with
natural montmorillonite and amino-acids showed that even after prolonged treatment glutamic acid, glycine and methionine are not sorbed by the clay from
solutions at their natural pH values. After 20 hr 7 to 8 mMol lysine were sorbed
out of a total of 40 mMol added to 100 g clay. No change was observed after standing
for two days.
The reactions of the four amino-acids examined with copper montmorillonite
differ from each other but show some common features, which distinguish them
from similar reactions of, e.g. amines and polyamines. Whereas with amines complex formation occurs preferentially in interlayer space and copper is liberated to
the solution only when excess amine is added, with amino-acids some copper is
extracted from the clay immediately, even when no amino-acid is sorbed. With
lysine and methionine the amount of copper extracted is practically independent
of the concentration of amino-acid added in the range of 20-200 mMol/100 g
clay. With glutamic acid and glycine the amount of copper extracted from the
clay increases with increasing concentration of amino-acid, but there is no simple
stoichiometric correlation between the concentration of copper and either the total
amino-acid added or that present in solution.
It appears that extraction of copper is the first stage of the reactions which
occur when various amino-acids are brought in contact with copper montmorillonite. These are much more time-dependent than ion exchange reactions or complex
formation between amines and copper clay, which are usually completed within
1 hr. With amino-acids slight changes in the systems were observed even after
several days.
Except with glutamic acid the presence of copper increased the ability of the
clay to sorb amino-acids under the conditions studied (Tables 1--4). However, the
distribution of copper and amino-acid between the clay and solution differs for the
different amino-acids.
170
W. Bodenheimer and L. Heller ~
70
/
60
50
e,
Lysine
"9....
/
/
[r~tla[ concentration o~Cu
..........;--:=:-:-_/121~22222
.........................
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'~"
--
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Nethionine
~ __
......
40
~0
80
1130 1 0
140 160
rnMo[ Amino-acid added per 100g clay
180
200
Fx~. 1. Concentration of copper and amino-acid in clay against total amount of
amino-acid added to Cu montmorillonite-water system.
With glycine and methionine the ratio amino-acid sorbed/amino-acid in solution
increases with decreasing total concentration of amino-acid and concomitantly
greater concentration of copper in the clay interlayers, i.e. amino-acids are extracted
relatively more efficiently from dilute solutions. A similar trend was observed with
lysine, except at total amino-acid concentrations of 20 and 40 mMol/100 g clay,
where the ratios are reversed. Experiments were carried out at the natural p H
values of the amino-acid solutions and in addition, the pH values of some glycine
solutions were adjusted to various values with HCI and NaOH, and with glutamic
acid some experiments were carried out at pH values of 9"9. In the course of the
reactions the p H values of all the systems changed, in general trending towards
neutral values. With glycine solutions of equal concentration maximum sorption
of amino-acid occurred in the vicinity of p H 7.
On addition of glutamic acid to copper montmorillonite copper is extracted
from the clay but no acid is sorbed. With glycine, too, the tendency to extract
Amino-acids in Cu-montmorillonite
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W. Bodenheimer and L. Heller
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174
W. Bodenheimer and L. Heller
copper from the clay is greater than that of the clay to sorb amino-acid, but with
solutions containing less than 200 mMol/100 g clay some amino-acid is sorbed.
The amino-acids containing an additional basic functional group, methionine
and lysine, are sorbed more strongly by copper montmorillonite than glutamic acid
and glycine. Lysine is also sorbed by natural montmorillonite, though to a lesser
extent. With methionine a change in concentration of the amino-acid solution does
not appreciably affect the distribution of copper in the clay and the solution, but
the ratio amino-acid in clay-amino-acid in solution increases with decreasing concentration and associated increase in pH.
With lysine only relatively small amounts of copper are extracted from the clay
and amino-acid is strongly sorbed. The ratio Cu-lysine in the clay increases with
decreasing concentration, as with methionine. The distribution factor lysine in
clay-lysine in solution reaches a maximum at an amino-acid concentration of 40
mMol/100 g clay. Sorption is almost complete after 1 hr but on standing for 18 hr
part of the acid returns into solution, whilst the amount of copper extracted from
the clay remains virtually unchanged.
DISCUSSION
The r2 values for Cu-glutamic acid, Cu-glycine and Cu-methionine complexes are
very similar--14.8-15-1, 15-15.9 and 14"7, respectively (Martell, 1964). The different
reactions observed with the three acids, therefore, cannot be attributed to differences in the relative stabilities of their copper complexes in aqueous solution. The
stability of the Cu-lysine complex is somewhat lower (f12 = 13.6-13"7) but this is
not sufficient to explain the profound difference in its reaction with Cu-montmorillonite.
Whilst stability constants do not explain the reactions of the amino-acids
examined with copper montmorillonite, these can be readily understood in the light
of the chemical nature of the organic compounds.
Glutamic acid dissolved in water gives rise to acid solutions in which the aminoacid is in one of the anionic forms :
H H H
H H H
I I 1
I r I
HOOC--C--C---C---CO0or
-OOC---C---C--C---COOH .
I I I
E r I
NH2 H H
NH2 H H
In this form there is no tendency for complex formation in the clay interlayers.
A cation exchange reaction presumably occurs between protons in solution and
copper, which is extracted from the clay to form a copper-glutamic acid complex
in the aqueous solution. The clay is thereby probably partially converted into a
hydrogen clay.
Amphoteric amino-acids are sorbed by copper clay from dilute aqueous solution.
(Preliminary experiments with a alanine showed that this reacted similarly to glycine.)
There is a tendency to complex formation both in the clay interlayers and in aqueous
solution. The ratio amino-acid in the clay-amino-acid in solution is greatest at
Amino-acids in Cu-montmorillonite
175
pH values 6-7, i.e. near the isoelectric point, and at any particular concentration
the amount of copper extracted from the clay is least in this range of pH values.
It appears, therefore, that neutral pH values and dilute solutions favour complex
formation of copper clay with amphoteric amino-acids. Complex formation is
inferred, since no glycine or alanine are sorbed by natural Wyoming bentonite under
similar conditions.
The strong sorption of lysine by copper montmorillonite can no doubt be
attributed to its alkalinity and the ability of basic amino-groups to co-ordinate with
copper.
Lysine eccurs in two configurations (A) and (B):
H ~N--(CH2)4---CHNH3 +
I
and
+H3N--(CH2)4--'CHNH2--COO-
COO-
(B)
(A)
In aqueous solution (B) predominates in the ratio 5"6:1 rendering the copper
complex relatively unstable (Li & Doody, 1952). It cannot be predicted which form
predominates in the clay interlayers, but both could probably co-ordinate with copper
in interlamellar space. Sorption of lysine by copper clay may therefore be favoured
both by the tendency of the amino-acid to form complexes in the clay interlayers
and by the lower stability of the complex in aqueous solution.
Methionine is intermediate in behaviour between amphoteric amino-acids and
lysine.
Basal spacings of the clay-amino-acid compounds were determined for samples
dried at different relative humidities. Except with lysine the spacings obtained
correspond more or less to those of copper montmorillonite under equivalent drying
conditions (12-1-12"4 A) and are therefore not included in Tables 1-3. Only with
lysine was a larger basal spacing observed, suggesting that the amount of organic
material sorbed was sufficient to determine the interlayer separation (Table 4). It
must be noted that the X-ray data do not correspond exactly to the chemical data:
these were obtained after specified periods of treatment ranging from 1 to 144 hr,
whereas X-ray measurements were made on washed samples which had stood in
desiccators for two or more days.
Preliminary experiments with vermiculite showed that the presence of copper
affects the sorption of amino-acids. The effect may be negative or positive, depending
on various factors.
CONCLUSION
Sorption of amphoteric and basic amino-acids by clays from dilute solutions at
pH values above 5 is enhanced by the presence of copper. Since copper is preferentially sorbed by clay minerals, e.g. from water containing only 2 ppm of copper
(Heydemann, 1959) this process may be of considerable ~,ignificance in nature. The
stability of the amino-acid--copper--clay associations, the tendency to peptide bond
formation, the effect of other metal cations and of substitution of the amino-acids
by various active and inert groups remains to be investigated.
176
W. Bodenheimer and L. Heller
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