Download EFFECT OF AMINO ACID (GLYCINE)

Clay Minerals (1988) 23, 45-54
E F F E C T OF A M I N O A C I D ( G L Y C I N E ) ON THE
D I S T R I B U T I O N OF T R A N S I T I O N M E T A L
(Co-NI-ZN-Cu) A N D M A G N E S I U M D I V A L E N T I O N S
B E T W E E N S I L I C A T E GELS A N D A Q U E O U S
SOLUTIONS: AN EXPERIMENTAL STUDY
M. D A B I R A * ,
F. D E L B O V E ,
A. P E R R U C H O T
AND J. T R I C H E T t
Centre de Recherches sur la Synthdse et la Chimie des Mindraux GIS/CNRS-BRGM, 1A rue de la Fdrollerie,
45071 Orldans Cedex 02, France
t Universitd d'Orldans, Laboratoire de Gdologie Appliqude, 45067 Orldans Cedex 02, France
(Received 26 September 1987; revised 4 January 1988)
ABSTRACT : The study of M and Mg ion exchange (M = Co, Ni, Cu, Zn), between silicate
gels SiO2.q(M, Mg)O.nH20 and amino acid (glycine) saline aqueous solutions (M, Mg)SO4,
shows that the introduction of the complexing agent completely upsets the original distribution
equilibria of M and Mg between the two types of phases. The values of the measured bulk
distribution coefficient
(M/Mg)s,,
Dr
(M/Mg)~ol
are lowered considerably relative to the inorganic reference distribution coefficients. The
lowering of D may be accounted for by calculating the contents of the different species, MA,
MA+, M2+, under which the metallic element M is present in the solutions. The values of
(M/Mg)~
DTrue (M2+/Mg2+)sol
resulting from the calculated M2+ contents are identical to the values of Dref, determined in
systems without a complexing agent.
Previous investigations (Perruchot, 1976; Perruchot et al., 1981; Perruchot & Delbove,
1982a, b) have shown that synthetic magnesium silicate gels, which are analogous to the early
weathering products of basic and ultrabasic rock, selectively concentrate the divalent ions of
the first transition series (Co, Ni, Cu, Zn). In contact with solutions, such gels operate like ion
exchangers for these elements. The values of the distribution coefficients, measured by the
ratio: DA_Mg= (A/Mg)gJ(A/Mg)~olution confirm the great affinity of transition metals for
silicate gels. For example, DNi_Mg= 102"77,DCu_Mg= 105--106 (Perruchot & Delbove, 1982a, b).
In a general way, natural sedimentary and pedological environments contain, in varying
amounts, poorly-organized mineral matter which can be compared with these synthetic gels,
and, in fairly close association, organic substances whose metal fixation capacity has also
been clearly demonstrated (Jackson et al., 1978; Manskaya & Drozdova, 1968; Szalay &
Szilagyi, 1969; Schnitzer & Khan, 1972, 1978).
* Present address: Universit6 de Ouagadougou. I.S.P. BP 7021 Ouagadougou, Burkina Faso.
9 1988 The Mineralogical Society
46
M . Dabira et al.
For those reasons it was decided to investigate experimentally the metal fixation
efficiencies of such inorganic and organic compounds. Simplified two-phase systems were
chosen composed of a silicate gel and an aqueous organic saline solution; the distribution
(resulting from the ion-exchange reaction) of Mg and, successively, one of the elements, Co,
Ni, Cu, Zn, was investigated.
The gels had the composition: SiO2.q(M, Mg)O.nH20 where M was one of the above
elements. The solutions were aqueous solutions of (M, Mg)SO4 in which a simple and soluble
amino acid, glycine, (H2NCH2COOH), was added as a supplementary compound.
Glycine is one of the most important amino acids in living organisms, which accounts for
its abundance in soils, sediments and water. Due to its neutral properties, high solubility and
good stability in aqueous solution, glycine has also been one of the most widely used
compounds in experimental studies concerned with the geochemical behaviour of amino
acids (Flaig et al., 1975; Andreux, 1979).
EXPERIMENTAL
PROCEDURE
To an original mixture of 10 cm 3 of 0.1 M MSO4 and 10 cm 3 of 0.1 M MgSO4 solution, 10 cm 3
of a 0.1 Msolution of sodium silicate was added. These quantities are equivalent to 10-3 moles
for each reagent, the unit with reference to which the different species in the experimental
media will be counted subsequently. A gel precipitated immediately. Following this, 10 cm 3
of a 0.1 Msolution ofglycine was added. After periods of 2 days to 3 months during which the
system was agitated regularly and the pH measured, the solution was separated from the gel
by filtration and analysed for M and Mg by atomic absorption spectroscopy. The absence of
silica and the presence of glycine, in an amount equal to that introduced initially, were
checked. Silica was analysed by atomic absorption spectroscopy and glycine by colorimetry
(Ghuysen et al., 1965).
Two series of experiments were conducted: (i) a distribution vs. time study, for the pair
Ni-Mg, and (ii) a constant time distribution study, for the three pairs Co--Mg, Cu-Mg and
Zn-Mg.
RESULTS
If M, and Mg t are the total quantities of M and Mg measured in solution after the
experiments, the bulk composition of the solution is given by:
x2=
M, + Mg,
(1)
and the composition of the gel is given by:
(
M )
,-M,
x = ~r + Mg gel 2 - M , - Mg~
~"
( M + Mg'~
(2)
(3)
The terms x and x2 lead to the global distribution coefficient:
(M/Mg)ge,
x/(1 - x)
Dglobat= (M/Mg)~olutio" x2/(l - x2)
(1 - Mr)/(1 - Mgt)
MJMgt
(4)
Silicate gels--amino acid ion exchange
47
If glycine had not been added, the procedure would have been identical to that developed
during earlier experiments (Perruchot, 1976; Perruchot & Delbove, 1982a, b). As shown
then, this would have resulted in 50~ of the sum of the exchangeable cations being in the
solution, that is: Mt+ Mg, = 1, q = 1, with very high distribution coefficients. These
coefficients, which are now called D~f~...... are given in Tables 1, 2 and 3. The earlier
experiments also showed that no adsorption mechanism operates in the selective
concentration of transition elements by the silicate gel.
First series of experiments
The data obtained for the Ni-Mg pair, with gel/solution contact times varying from 2 to 90
days are given in Table 1. In comparison with glycine-free systems, the following differences
were observed:
(i) Instead of 50~o of the sum of the exchangeable cations, 60 to 65~ are in the solution
after the experiment: 1.2 ~< (M, + Mg,) ~< 1.3, which gives 0.7 <~q ~<0.8. As may be seen, q is
virtually independent of time.
(ii) Although still effective, the concentration of Ni by the gel is considerably reduced
(Ds~obal~ D~f~,r
Fig. 1 (curve 1) shows the variation of Dglob~~as a function of time and it is
apparent that the distribution equilibrium of metallic elements is, in effect, reached after 60
days.
Second series of experiments
The data obtained for the pairs Co-Mg, Cu-Mg, and Z n - M g with a gel-solution contact
time of 60 days are given in Table 2. The same differences, relative to glycine-free systems,
are observed as with the Ni-Mg pair: q < 1 and Dgloba~~ O r e f e . . . . . "
Fig. 2 (curve 1) is a representation of the values obtained for log Dgloba~as a function of the
corresponding values of log D~f,..... . No correlation is directly visible between the two sets of
values.
IogD
--
I
. . . . . . . . . . . . . . .
-
-
-
9
-
-
Dtrue
-
o,,~.
F10. 1. Variation of log Dg]ob~(curve 1) and log D,r,e (curve 2) vs. time for the pair Ni-Mg.
Dashed line: log D~;~dc,= 2-77 (see text for significance of these three values).
48
et al.
M. Dabira
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Silicate gels--amino acid ion exchange
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49
50
M. Dabira et al.
Iog D
log Dtrue
2
global
:2
NilCoZn I
'3''4
, Cu
5'6
I
log Dref
FIG. 2. Variationof log DOobaI (curve 1) and log Dtrue(curve2) vs. log Drefereneefor the four pairs:
Ni-Mg, Co--Mg, Zn-Mg and Cu-Mg.
TREATMENT
OF DATA
In order to account for the observed variations, an analysis is necessary of the part played by
the complexing agent in modifying the equilibria. Chemical analysis has shown that the gels
were free of glycine, so it is evident that, for the simple systems studied, the only interactions
to be considered are those between cation and complexing agent in the solutions, and the
resulting formation of various complexed species.
For glycine-free systems (Perruchot, 1976; Perruchot & Delbove, 1982a, b), the overall
precipitation-exchange reaction, after the M-Mg distribution equilibrium has been reached,
can be written:
SiO2Na20 + 2(M0.5, Mgo.5)SO4 --+
SiO2(ix, Mgl - x)O + (Mx,, Mg I _ xl)SO, + Na2SO,,.
(5)
For systems with glycine, the reaction will be written:
SiO2Na20 + 2(Mo.5, Mgo.5)SO4 + HA --~
SiO2, q(Mx, Mgt - x)O + r(Mx,, Mgt- x,)SO, + sMA2
+ 2 (MA)2SO, + uHA + vH2SO4 + wH20 + Na2SO4
(6)
where HA stands for glycine, and MA2, MA c§ for the complexed forms of M with glycine.
The latter formulation relies on the simplifying assumption that the Mg in solution is present
essentially as the 'free' Mg c2+)form, and the metal M is partitioned into the three forms: the
bidentate complex, MA2, the monodentate complex, MA ~+), and the 'free' uncomplexed
form, MC2+k Such a formulation ignores complexed species such as MgA2 and MgA c+~.
Consideration of the values of formation constants for the different possible complexed
species, MA2, MA c+), MgA2, MgA c+) (Table 3) indicates that MA2 and MA c+) are
predominant relative to ~Ac2+)
.... f~e', and inversely, MgA2 and MgA c+) are negligible relative to
M,,C2+)
Due
to
the
pH
conditions
of the systems investigated, complexed species such as
8'free'"
MA(f ) can also be estimated to be negligible (Greenstein & Winitz, 1961, p. 615). Both 'free'
species, M(2+)r~e~and ,,,sr~A"~2+)appear in the form (Mx,, Mg~_ x,)SO, in equations 5 and 6.
51
Silicate gels--amino acid ion exchange
TABLE3. Values of f12and K2 relative to the elements Mg, Co, Ni, Cu
and Zn, and of Kd relative to glycine (after Sillen & Martell, 1971)*.
Values of D,efe.... relative to the pairs Co-Mg, Ni-Mg, Cu-Mg and
Zn-Mg (after Perruchot et al., 1981; Perruchot & Delbove, 1982a, b).
f12
g2
Dref
Mg
Co
104
103
109
104.
103.29
Ni
101~176
10,*.95
102"77
Kd~glycin,) =
Cu
Zn
101525
106.95
105-6
10945
104.30
10375
10-9"75
* A more recent compilation of constant values by Perrin (1979) has
been pointed out to the authors, but could not be checked in time.
It should be noted that bulk formulations (5) and (6) ignore complexed species such as
MSO ~and MgSO ~ These can be included in Mfree and Mgf.... due to the fact that the values of
the corresponding constants are low and, moreover, equal ( _ 102, Sillen & Martell, 1971),
with the result that their intervention may be considered ineffective in so far as only the
Mfree/Mgfree ratios are concerned in both cases.
It should also be noted that the sulphate ion is considered in four forms: (i) sulphate bonded
to sodium, N a 2 S O 4 ( 1 . 1 0 -3 mol), (ii) sulphate bonded to the free cations (Mx,, Mgl_ x,)SO4,
(r. 10-3 mol), (iii) sulphate bonded to the monodentate complex MA2SO, (t/2.10 -3 mol) and
(iv) sulphuric acid H2SO, (v. 10-3 mol). As q and x are given directly by the experimental
data, only seven coefficients in equation 6 remain unknown: Xl, r, s, t, u, v, and w.
These coefficients are connected by the five mass balance equations (expressed in 10-3
units) for the five entities: M, Mg, A, SO,, H.
M:
qx + rxl + s + t = l
(7)
Mg: q x + r ( 1 - x l )
=1
(8)
A:
= 1
(9)
SO,: r + v + t / 2
=2
(10)
H:
=1.
(11)
2s+ t + u
u+2v+2w
Also, the two equations for the complexing equilibria (conventionally expressed in mo1-1
units) may be considered:
MA + + A- --~ MA2,
with K2 -
(MA2)
(MA+)(A -)
(12)
M 2+ + 2A- ~ MA2,
(MA2)
with f12 = (M2+)(A_)2'
(13)
However, the solution of these two equations requires the knowledge of the concentration of
the glycine anion, (A-), and, therefore, that of the corresponding quantity, A-, which
introduces an additional unknown. The acid dissociation equilibrium ofglycine gives access
to the latter unknown:
HA~A-+H
§
Kd=
(A-)(H §
(HA)
(14)
52
M. Dabira et al.
Now, if u2 denotes the quantity of A- and U1 the quantity of undissociated AH (in 10-3 mol
units), the mass balance equations (9) and (11) must be corrected accordingly:
2s + t + u2 + u2 = 1
Ul + 2v + 2w
= 1.
(9')
(11')
Thus the equilibria 12, 13, 14 can be written:
t. V. 103
-
-
r.x
-
1(2
S. V 2 . 10 6
-
-
r.x
=
f12
I .u 2
u2. (H +)
-
(15)
I .u 2
-
=
Kd
Ul
(16)
(17)
with V, the volume of solution expressed in litres, being taken as 0-04 (neglecting the amount
of water fixed by the gel).
The different values calculated for the eight unknowns are given in Tables 1 and 2
(columns 9 to 16). As expected, the metal is seen to be essentially in the complexed forms
(s + t > rxt), with the bidentate form being generally more abundant than the monodentate
one, and the free uncomplexed form being comparatively minor. If this free form is taken into
account, a 'free-ion' distribution coefficient, defined by
Dt~e =
(M/Mg)gel
(M2+/Mg2+),o I
(18)
and corresponding to the exchange reaction: Mggel+ M ~ ,~ Mget+ M g ~ , can be calculated.
The values of such 'free-ion' distribution coefficients are given in Tables 1 and 2 (column 17).
DISCUSSION
Fig. 1 (curve 2) is a representation of the values calculated for log Dtrue as a function of time for
the Ni-Mg pair. It can be seen that log Dt~~ tends to a limit; this limit is very close to
log Drefe...... as shown by the horizontal line with the ordinate 2.77 (Perruchot & Delbove,
1982a, b), which relates Dtrue at equilibrium and Dreferenee.
The Ni-Mg pair is not the only pair concerned, as may be seen on Fig. 2 (curve 2), where
the equilibrium values of log Dt,~ are plotted as a function of Dr~re..... for all the pairs
investigated; the representative points are distributed on a line with a slope of unity.
The equality between D,~ and Dr~fer~,cr is due to the fact that in both cases it is the same
fundamental exchange equilibrium which intervenes. This is the additive equilibrium, due to
the presence of complexing agent, which results in Dg~oba~being considerably lowered relative
to Dt~~.
Given (4) and (18), the relationship between DglobaI and Dref~..... (-~ Dt~e) is given by:
Dglobal
Dglobal= ( M 2 + /
All the factors (high values of/(2, f12, Kd and pH) which tend to decrease the proportion of
M 2+ relative to M, in the solution, tend to decrease DglobaI relative to Dreferenre. The same factors
Silicate gels--amino acid ion exchange
53
Jo
1~
15
V- . . . .
I
.,V,//
,
I
Co
Ni
'V (~ref,
(~
I
u
ii
Zn
FIG. 3. Valuesof log/~2relative to the four elements: Co, Ni, Cu and Zn. Valuesof log Dgloba I and
q relative to the four pairs: Co-Mg, Ni-Mg, Cu-Mg and Zn-Mg. Dashed line: values of
log D,erere.~e and q for glyeine-free systems.
act also on q, and tend to make the gel more siliceous. Fig. 3 is an illustration of the influence
of 132.
CONCLUSIONS
Considering these mechanisms globally, the introduction of an organic complexing agent
considerably upsets the global distribution equilibria of the elements M and Mg between the
gel and the solution. The gel is depleted in these elements in favour of the solution (q < 1), and
the distribution coefficient:
Dgl~
=
(M/Mg)gel
(M/Mg)~ol
decreases more or less considerably (Dooba~~ Drefo..... ). Thus, in the series of the four elements
(Co, Ni, Cu and Zn), Cu, the element having the greatest affinity for complexing agents
(Irving & Williams, 1953), is the most strongly affected by the introduction of glycine in the
experimental systems.
Information (from Tables of constants) on the thermodynamic parameters governing
inter-species equilibria in the aqueous amino acid solution can be used to calculate the
amounts of complexed and free ions in solution. Taking into account the latter, which are the
only species intervening in the exchange reaction between gel and solution, distribution
constants, D,~,~, may be calculated, which compare very well with the distribution constants,
D,ec, measured in simple (without complexing agent) systems. This shows that the presence of
amino acid does not alter the fundamental ion distribution mechanism between gel and
solution.
The affinity of metallic cations for the hydroxyl groups (OH) of the gel remains a real factor
to be considered, but this affinity, in the presence of a given complexing agent, appears to be
relatively unimportant.
54
M. Dabira et al.
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