THE ESTIMATION OF THE STABILITY OF IONITE COMPLEXES FORMED
2123
mass of dry ionite (g), and g is the sorption capacity of ratio between the concentrations of copper cations in
dry ionite (meq/g, eq/kg).
the ionite [M] and equilibrium solution [M] (mol/l) is
the distribution coefficient λ, which allows Eq. (2) to be
rewritten as
The concentration of copper ions in solution was
determined on an SF-56 spectrophotometer with lead
diethyldithiocarbamate [8].
The determination of the stability constant by the
destruction of ammonia copper complexes in contact
with the anionite was performed as follows. Ammonia
in concentration varying from 0.164 to 5.56 M was
[
M] 1
1
K = ----------------- -- = λ-------- -- .
(3)
st
n
n
[M]
[
L]
[L]
The concentration of free coordination-active
added to aqueous solutions of copper sulfate (c0(Cu2+)
.55 mM). Air-dry AM-7 anionite in the OH form
0.5 g) was introduced into 100 ml of the resulting
solution containing ammonia copper complexes
=
groups in the ionite [L] (mol/kg ionite) was calculated
on the basis of the following considerations. In sorption
by the ionite, each copper cation coordinates n ligands
2
(
(
coordination-active groups of the anionite). If the max-
imum sorption capacity of the ionite g and sorption
2
+
0
[
Cu(NH ) ] . Once the equilibrium was established
3 4
capacity g for the given equilibrium copper concentra-
tion in solution are known, the concentration of free
ionite coordination-active groups can be calculated by
multiplying the difference of the maximum and equilib-
rium sorption capacities by the coordination number,
(5 days), the equilibrium concentration of ammonium
2+
complexes [Cu(NH ) ] in solution was determined.
3
4
The difference between the initial c0(Cu2+) and equilib-
rium ceq concentrations was used to determine the
amount of sorbed copper per 1 kg anionite (in Eq. (1),
molecular weight of copper, 63.5 g/mol, was taken
instead of the equivalent mass E). This gave the sorp-
tion capacity of the anionite (mol/kg ionite).
[
L] = ng – ng = n(g – g).
(4)
0
0
Substituting this [L] value in Eq. (2), we obtain
1
K = λ----------------------------.
(5)
st
n
(
n(g – g))
0
RESULTS AND DISCUSSION
The dissociation of the ionite complex at equilib-
rium between the anionite and solution can be
described by the equation
Taking the logarithm of Eq. (5) yields
logλ = (logK + nlogn) + nlog(g – g).
(6)
st
0
The resulting equation in the coordinates logλ =
MLn
M + nL,
(I)
f(log(g – g)) describes a straight line with a slope
0
where ML is the ionite complex of copper in the
n
equal to the coordination number n, and its y-intercept
anionite, M denotes copper cations in solution, L is the is the sum (logK + nlogn ), from which K can be
st
st
free coordination-active group in the ionite, and n is the
number of coordination-active groups coordinated by
copper as a result of sorption. The charge of the copper
cation (M) in reaction (I) is omitted for simplicity.
determined if n is known.
The maximum sorption capacity was found by pro-
cessing the experimental data on the sorption of Cu
depending on the equilibrium concentration of cations
in the coordinates of the Langmuir equation [9],
2+
Coordination-active groups in the AM-7 anionite
˙˙
˙˙
˙˙
are ≡N , =NH , and –NH . The sorption of copper cat-
g = αc /(1 + βc ),
(7)
2
eq
eq
ions occurs via the formation of coordination bonds
between metal cation free d orbitals and the electron
pair of nitrogen in the functional groups of the ionite by
the donor–acceptor mechanism,
where g is the sorption capacity of the ionite (meq/g
ionite), ceq is the equilibrium concentration of metal
cations in solution (mg/l), and α and β are empirical
constants.
If sorption proceeds according to Eq. (7), the exper-
imental results must form a straight line in the coordi-
nates 1/g = f(1/ceq),
2
+
2+
˙
˙
≡
N + ꢀCu
[≡N : Cu] .
(II)
The equilibrium described by Eq. (I) can be quanti-
tatively characterized by the stability constant of the
ionite complex,
1
/g = α/β + 1/αceq.
(8)
n
It follows from the experimental data (Fig. 1) that
the Langmuir equation can be used to describe the sorp-
tion of copper cations to a satisfactory accuracy.
K = [ML ]/[M][L] .
(2)
st
n
The concentration of the complex in the anionite
The maximum sorption capacity g calculated by
0
[
ML ] (mol/kg ionite) is equal to the concentration of
n
approximation (8) at ceq
∞ is 3.5 ± 0.3 meq/g. This
sorbed metal [M] , which, in turn, corresponds to the value is in satisfactory agreement with the experimental
sorption capacity of the ionite g (mol/kg ionite). The data, 3.33 ± 0.25 meq/g. A comparison of the experi-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 12 2008