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Ion-exchange processes of lead and cobalt ions on the surface of calcium-montmorillonite in the presence of complex-forming agents II. The effect of DTPA, tartaric acid and citric acid on the sorption of lead ions on calcium-montmorillonite
COLLOIDS AND ELSEVIER
Colloids and Surfaces A: Physicochemicaland Engineering Aspects 137 (1998) 243-252
A
SURFACES
Ion-exchange processes of lead and cobalt ions on the surface of calcium-montmorillonite in the presence of complex-forming agents 1 II. The effect of DTPA, tartaric acid and citric acid on the sorption of lead ions on calcium-montmorillonite No6mi M. Nagy, J6zsef K6nya, Ilona K6nya Isotope Laboratory, Kossuth University, Debrecen, H-4010, Hungary Received 30 June 1997; accepted 24 November 1997
Abstract The sorption of lead ion on the surface of calcium-montmoriUonite is studied in the presence of complex-forming agent. The ion exchange processes are greatly influenced by the composition of the solution, pH and the stability constant of the complex forming agent. The total amount of lead contamination cannot be washed out of montmorillonite with the applied complex-forming agents. © 1998 Elsevier Science B.V.
Keywords: Calcium-montmorillonite; Citric acid; Cobalt ions; Complex formation; DTPA; Ion exchange; Lead ions; Tartaric acid
1. Introduction In Part I [1 ] the possible reactions of lead or cobalt ions was studied with calcium-montmorillonite in the presence and absence of E D T A as complex-forming agent. It was found that E D T A decreases the sorption of lead and cobalt ions on calcium-montmorillonite because they form negative complexes which cannot be sorbed on the surface. The amount of sorbed cobalt ions decreases practically to zero, whereas some lead ions always remain on the surface of montmorillonite. This was interpreted by the different sorption mechanisms of lead and cobalt ions. 1This work was presented at the 7th Conference on Colloid Chemistry in memoriam Alad~ir Buz~igh, Eger (Hungary), September 23-26, 1996. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0927-7757 (97)00381-6
Here, the effect of other complex-forming agents on the sorption of lead ions on calcium-montmorillonite is studied. As in Part I, the distribution of the total amounts of calcium and lead ions is measured by radioisotopic labelling, then the concentration of the different species (see fig. 1 in Part I [1]) are computed using the P S E Q U A D computer program [2].
2. Reactions possible in metal ion-diethyl-triamine penta-acetic acid (DTPA)-calcium-montmorillonite systems The reactions in Eqs. (1), (2a), (2b), (3), (4a), (4b), (5), (6a) and ( 6 b ) - ( 8 ) are the same as those in the presence of E D T A (see Part I [1] for
244
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
explanations). C a - m o n t + 45Ca2 +,¢~45Ca-mOnt + C a 2 +
( 1)
P b - m o n t + 212pb2 ÷,~212pb_mon t + p b 2 +
(2a)
C o - m o n t + 6°Co2 + ¢~6°Co-mont + Co 2 +
(2b)
H - m o n t + H ÷~ , H - m o n t + H ÷
(3)
C a - m o n t + p b 2 + , * : - P b - m o n t + C a 2+
(4a)
C a - m o n t + Co 2 ÷ ~ C o - m o n t + C a 2 +
(4b)
Ca-mont +2H+~2H-mont
+ C a 2+
(5)
P b - m o n t + 2 H +,=,2H-mont + Pb 2 +
(6a)
C o - m o n t + 2H + o 2 H - m o n t + C o 2÷
(6b)
The stability constants o f the complexes and protonated ligands are summarized in Table 1 [3].
3. Reactions possible in metal ion--tartaric acid (Tart)--caleium-montmorillonite systems
The reactions in Eqs. (1), (2a), (2b), (3), (4a), (4b), (5), (6a) and ( 6 b ) - ( 8 ) are the same as in the presence o f E D T A (see Section 2 and Part I [ 1 ]). The reactions o f the metal ions with Tart and the p r o t o n a t i o n reactions are as follows. Complex-formation equilibria (Eqs. ( 2 4 ) - ( 2 8 ) : Pb 2 ÷ + Tart 2- ~ P b T a r t
(24)
Ca 2+ + T a r t Z - o C a T a r t
(25)
3 C a - m o n t +2Fe3+,~,2Fe-mont + 3Ca 2+
(7)
Pb 2+ + H ÷ + T a r t 2 - c > H P b T a r t +
(26)
F e - m o n t + 3H +,*:,3H-mont + Fe 3+
(8)
Ca ÷ + H + + T a r t 2 - c > H C a T a r t +
(27)
The reactions o f the metal ions with D T P A and the p r o t o n a t i o n reactions are as follows. Complex-formation equilibria (Eqs. ( 9 ) - ( 1 7 ) :
Fe 3 ÷ + Tart 2 - o F e T a r t ÷
(28)
Ca 2÷ + D T P A S - c ~ - C a D T P A 3-
(9)
Tart 2- + H + ~ H T a r t -
(29)
Pb 2+ + D T P A S - , c ~ P b D T P A 3-
(10)
Tart 2- + 2H+,c~H2Tart 2-
(30)
Fe 3 + + D T P A 5 - , , > F e D T P A 1-
(11)
The stability constants o f the complexes and protonated ligands are summarized in Table 1 [3].
Ca 2 + + H + + D T P A 5 - ~ C a H D T P A 2-
(12)
Pb 2 + + H + + D T P A 5 - ~ P b H D T P A
(13)
4. Reactions possible in metal i o n - - c i t r i c acid
(14)
(Cit)---caleimn-montmorillonite systems
(15)
The reactions in Eqs. (1), (2a), (2b), (3), (4a), (4b), (5), (6a) and ( 6 b ) - ( 8 ) are the same as in the presence o f E D T A (see Section 2 [2] and Part
2-
Fe 3 + + H + + D T P A 5 - ¢ > F e H D T P A Fe 3+ + D T P A 5- + O H - ~ F e D T P A O H
3-
2Pb 2+ + D T P A S - c > P b 2 D T P A -
(16)
2Ca 2÷ + D T P A S - c ~ C a 2 D T P A -
(17)
Protonation
equilibria o f D T P A
(Eqs. ( 1 8 ) -
(23):
P r o t o n a t i o n equilibria (Eqs. (29) and (30):
I [1]). The reactions o f the metal ions with Cit and the p r o t o n a t i o n reactions are as follows. C o m p l e x - f o r m a t i o n equilibria (Eqs. (31)-(40): Pb 2 + + Cit 3 - ¢>PbCit -
(31 )
D T P A 5- + H + c > H D T P A a-
(18)
Ca 2+ + C i t 3 - ~ C a C i t -
(32)
D T P A 5- + 2 H + , * : , H 2 D T P A 3-
(19)
Pb 2+ + H + + C i t 3 - ~ H P b C i t
(33)
D T P A s - + 3H+,*:-H3DTPA 2-
(20)
Ca 2+ + H + + C i t 3 - ~ H C a C i t
(34)
D T P A 5- + 4 H + , : ~ H 4 D T P A -
(21)
Pb z+ + 2 C i t 3 - , ~ P b C i t 4-
(35)
D T P A 5- + 5 H + ~ H s D T P A
(22)
Pb 2+ + H ÷ + 2 C i t 3 - c ~ P b C i t 3-
(36)
D T P A 5- + 6H+ ¢>H6DTPA ÷
(23)
Pb 2+ + 2 H ÷ + Cit3-~e-H2PbCit ÷
(37)
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
245
Table 1 Logarithm of stability constants of different complexes and protonated ligands in the presence of DTPA, Tart and Cit [3] DTPA
Cit
Tart
Species
log K
Species
PbDTPA3HPbDTPA2 Pb2DTPA-
18 23.32 22.21
PbCit HPbCit PbCit~HPbCit 32H2PbCit ÷
log K 4.48 8.64 5.92
Species
log K
PbTart HPbTart ÷
3.12 5.72
10.61 11.70
CaDTPA3HCaDTPA2Ca2DTPA-
10.75 16.86 12.35
CaCit HCaCit H2CaCit +
3.45 7.79 11.00
CaTart HCaTart ÷
1.94 5,06
FeDTPA2FeHDTPAFeDTPAOH3-
28.00 31.56 18.22
FeCit FeHCit +
11.20 17.90
FeTart ÷
6,49
HDTPA4HzDTPA3H3DTPA2H4DTPAHsDTPA H6DTPA÷
10.49 19.09 23.37 26.01 28.01 29.61
HCit 2H2Cit H3Cit
5.66 10.00 12.90
HTart H2Tart
3.96 6.78
C a 2+ + 2 H + + C i t 3 - c : - H 2 C a C i t ÷
(38)
Fe 3 - + Cit 3 - ~ F e C i t
(39)
The substances a n d experimental c o n d i t i o n s are
Fe 3 ÷ + H ÷ + Cit 3- ~ F e H C i t ÷
(40)
very similar to those in P a r t I [1 ], b u t in this case D T P A , Tart a n d Cit were used as complex-form-
P r o t o n a t i o n equilibria (Eqs. ( 4 1 ) - ( 4 3 ) :
5. Experimental
ing agents.
Cit 3- + H + ¢~HCit 2-
(41)
Cit 3 - + 2 H ÷ , ~ H 2 C i t -
(42)
Cit 3- + 3H+,c~HaCit
(43)
The stability c o n s t a n t s o f the complexes a n d prot o n a t e d ligands are s u m m a r i z e d in Table 1 [3]. T h e r e s u l t a n t e q u i l i b r i u m state o f all systems is affected by the equilibria written above. I n o u r work we d e t e r m i n e d the total a m o u n t o f different ions (calcium a n d lead) in the solid a n d in solution by radioisotopic labelling, m e a s u r e d the p H , then we c o m p u t e d the a m o u n t o f different species in solution o n the basis o f the total a m o u n t s a n d stability c o n s t a n t s (Table 1).
6. Results and discussions 6.1. The
effect o f D T P A
The e q u i l i b r i u m fractions o f lead, calcium a n d h y d r o g e n ions o n the surface o f m o n t m o r i l l o n i t e versus p H in the presence o f D T P A at 1:1 a n d 1:4 P b : D T P A ratios are s h o w n in Figs. 1 a n d 2. I n this system the c o n c e n t r a t i o n s o f the following species were computed: P b 2 +, P b D T P A 3 - , H P b D T P A 2 - , P b 2 D T P A - , C a 2÷, C a D T P A 3-, H C a D T P A 2-, C a 2 D T P A - , Fe 3+, FeDTPA 2-, FeHDTPA-, FeDTPAOH a-, D T P A 5-, H D T P A 4-, H2DTPA 3-, HaDTPA 2-,
N.M. Nagy et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 137 (1998,) 243-252
246 1
0,9 0,8 0,7 A
X
0,S
a
•
A 0,4 •
A
0,3
~
mxCal
A
•
l.xH F • A
0,2 0,1 I 0
1
-
2
i
t
3
4
.--
t
-
-
5
6
pM
Fig. 1. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:DTPA = 1:1, the total initial concentration of lead and DTPA is 5 × 10 -4 mol dm -3.
1
0,9 0,8 0,7 0,6 x
0,5 0,4 0,3 0,2 0,1
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
pM
Fig. 2. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:DTPA = 1:4, the total initial concentration of lead is 2.5 x 10 -4 mol dm -3 and of DTPA is 10 -3 mol dm -3.
H 4 D T P A - , H s D T P A , H 6 D T P A +. A m o n g them the concentrations o f P b 2 D T P A - , C a D T P A 3-, Ca2DTPA-, Fe 3 +, F e D T P A 2-, F e H D T P A - , F e D T P A O H 3-, D T P A 5-, H D T P A 4-, H 4 D T P A - , H s D T P A and H 6 D T P A ÷ can be neglected, The equivalent fractions o f lead ions on the surface o f montmorillonite and the ratios o f different P b - D T P A complexes in the solutions are shown in Fig. 3. The equivalent fractions o f calcium ions on the surface o f montmorillonite and the ratios o f
different C a - D T P A complexes in the solutions are shown in Fig. 4. Figs. 1 and 2, together with fig. 6 in Part I [1], show similar tendencies as in the case o f E D T A ; only the degree o f the decrease o f lead sorption is smaller. We can see in Table 1, and in Table 1 o f Part I [ 1 ], that the stability constants o f P b - E D T A and P b - D T P A complexes, and even C a - E D T A and C a - D T P A complexes are very similar. In this case, however, the stability constants o f the protonated D T P A ligands characteristic at the p H exam-
N.M. Nagy et aL / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
0,9
247
II
0,8 0,7-
•
[e XPb
0,6
ill Pb2 +/PbtOt
0,5 -
/x HPbDTPA/PDtOt /
/A PbDTPA/Pbtot
0,4
~X (PbDTPA+ HPbDTPA)/PbtOt
03 0,2 0,1
O
1
2
3
4
5
6
PH
Fig. 3. The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of different lead-DTPA complexes in the solutions.
0,9 0,8 0,7 0,6
0,s
~
!
I-c~2÷'c~t°*i exCa
~DTPA/Catot~
0,4 0,3 0,2 0,1
1
2
3
4
5
6
pH
Fig. 4. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different lead-DTPA complexes in the solutions.
ined (H2DTPA 3- and H3DTPA 2-) are greater. As a result the concentration of hydrated Pb 2÷ is greater, which leads to a greater sorption of lead. The decrease of the sorbed lead is connected with the decrease of the concentration of hydrated, positive lead(II) ions, which is similar to the case with manganese ions [4]. In the case of lead, however, there is not a such strict relation between the sorbed quantity and the hydrated ion concentration, the quantity of the sorbed lead never decreases to zero. This is probably caused by the formation of Pb-O on the edges of layers, that is
on the pH-dependent charges (see Ref. [5] in Ref. [6]). The sorption method is less influenced by complex-formation; its effect is expressed in the decrease of the slope of the Xpb-pH curve. The sorption of calcium ions is as expected: at low pH-values, where calcium ions are present as Ca 2÷, they sorb as in the absence of complexforming agent; the quantity of the sorbed calcium increases when pH increases. At pH-values where negative Ca-EDTA complexes are already formed and the concentration of hydrated Ca 2÷ ions decreases, the sorption of calcium on the surface
248
N.M. Nagy et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 137 (1998) 243-252 1
o,9 0,8 0,7 o,6
0,4
•
j &
-
0,3
•
•
0,2 0,1
1,5
2
2,5
3
3,S
4
4,5
pH
Fig. 5. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:Tart = 1:1, the total initial concentration of lead and Tart is 5 x 10-4 mol dm -3.
I0,9 0,8 ,
~
0,7
0,6 .°,3
0,4
~
0,3-
~
0,2 0,1 0 1,5
-
A
~ I
I
~
i
2
2,5
3
3,5
p 4
[ 4,5
DH
Fig. 6. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:Tart = 1:4, the total initial concentration of lead is 2.5 x 10-4 mol dm -3 and of Tart is 10 -3 mol dm -3.
of montmorillonite also decreases. This was also the case for both the manganese and lead ions in the EDTA-calcium-montmorillonite system [4].
6.2. The effect of Tart The equilibrium fractions of lead, calcium and hydrogen ions on the surface of montmorillonite versus pH in the presence of Tart at 1:1 and 1:4 Pb:Cit ratios are shown in Figs. 5 and 6. In this system the concentrations of the following species were computed: Pb 2+, PbTart, HPbTart +, Ca 2+, CaTart, HCaTart +, Fe 3+, FeTart +, Tart 2-, Htart-, HzTart. The concen-
trations of HPbTart ÷, HCaTart +, Fe 3÷ and FeTart + are negligible. The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of different lead-tartrate complexes in the solutions at Pb:Tart= 1:1 are shown in Fig. 7. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different calcium-tartrate complexes in the solutions at Pb:Tart-- 1:1 are shown in Fig. 8. At Pb:Tart = 1:4 similar results are obtained. Figs. 7 and 8, together with fig. 6 in Part I [1], show that Tart influences the amount of sorbed lead on the surface of montmorillonite to a small
Physicochem.Eng. Aspects 137 (1998) 243-252
N.M. Nagy et al. / Colloids Surfaces A: 1 T
:
249
-
0,917,8-
0,7 o., [ A PbTart/PbtotJ
0,5 0,4
0,2
~
0,1 , l 0
a, i
I t
2
2,5
1,5
3
3,5
I
I
4
4,5
pH
Fig. 7. The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of different lead-tartrate complexes in the solutions.
0,9 0,8 0,7
I
O,.o,, 0,4
• ca2 + I C a t o ~ A
CaTart/cato~
t 0.3
,~
"
0,2
0,
1,5
AI
AI
2
2,5
A
t
3
--
a
I
3,5
•
~
4
4,5
IIH
Fig. 8. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different lead-tartrate complexes in the solutions.
degree. It seems that Tart at l : l = P b : T a r t ratio increases the amount o f lead to a small degree, The small effect o f tartrate on the sorption of lead is in accordance with the concentration o f positive, hydrated Pb 2÷ ions. The stability constants o f tartrate complexes is much smaller than the stability constants o f E D T A and D T P A complexes, so the concentration o f Pb 2+ is higher resulting in a relatively high sorption, The small increase o f lead sorption at Pb:Tart = 1:1 ratio is probably caused by the binding o f O H
groups of tartrate complexes on the edges of octahedral layers. In the case of calcium ions similar results are obtained as in the presence of E D T A and DTPA.
6.3. The effect of Cit The equilibrium fractions of lead, calcium and hydrogen ions on the surface o f montmorillonite versus pH in the presence o f citric acid at 1:1 and 1:4 Pb:Cit ratios are shown in Figs. 9 and 10.
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
250
0,8 0,7 0,6 O,S
x 0,3°'4t,~
~
~/mxCaI
0,2
1,5
2
2,5
3
3,5
4
4,5
Fig. 9. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:Cit = 1:1, the total initial concentration of lead and Cit is 5 × 10 -4 tool dm -3. 1 o,g
~
0,8
A
0,7 0,6 X
0,5
~
0,4
0,3 g,2 13,1 ~
,S
2
2,5
3
3,5
4
4,S
pH
Fig. I0. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:Cit 1:4, the total initial concentration of lead is 2.5 x l 0 - 4 tool dm - 3 and of Cit is 10- 3 mol dm- ~.
In this system the concentrations of the following species were computed: Pb 2+, PbCit-, HPbCit, PbCit 4- , HPbCit~-, H2PbCit + , Ca 2+, CaCit-, HCaCit, H2CaCit +, Fe 3+, FeCit, FeHCit +, Cit ~-, HCit z-, H~Cit-, H3Cit. Among them the concentrations of PbCit24-, HPbCit~-, CaCit-, HCaCit, HzCaCit +, Fe 3+, FeCit, FeHCit + and Cit 3- were less than 1% of the other species and so they are neglected, The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of
=
different Pb-citrate complexes in the solutions at Pb:Cit=l:l are shown in Fig. 11. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different Ca-citrate complexes in the solutions at Pb:Cit= 1:1 are shown in Fig. 12. In the case of Pb:Cit = 1:4 similar results are obtained; the concentration of the protonated citrate is increased in this case. Figs. 9 and 10, together with fig. 6 in Part I [1], show that Cit, as with EDTA and DTPA, decreases
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
251
0,9 0,8 0,7 x.p b ~ N
0,6
~
,
i
I • Pb2 */Pbtot I
IAPbdt/Fotot [
0,5
l x HPbCWPbtOtJ
0,4 + i 0,3 + 0,2 0,1
jr
o
- - - - -
%5
i~
2
iI
l
11 2,5
.
II
i
3
3,5
- - t
4
4,5
Fig, 11. The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of different lead-citrate complexes in the solutions.
1 T
;
_-
_-
•
m
:
0,9
0,8I 0,7 O,S
Lmca2 +/CatotJ
0,4 ~ 0,3
!
(],1 1,5
2
2,5
3 pH
3,5
4
4,5
Fig. 12. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different lead-citrate complexes in the solutions.
the amount of sorbed lead on the surface of montmorillonite, but the degree of decrease is rather great; this is surprising because of the relatively low stability constants of citrate complexes (Table 1). Figs. 11 and 12 also show that the ratio of hydrated cations (both Pb 2÷ and Ca 2÷) is great. So the amount of the sorbed lead cannot be interpreted on the basis of the Pb 2÷ concentration, It is known that citrate forms other polynuclear
complexes [7,8], though their stability constants are not included in Table 1. The computations were repeated considering these complexes, but the results were practically the same: the major part of the lead ions is present as Pb 2÷ in our experimental conditions. The effect of citrate cannot be interpreted on the basis of the ratio of pb2+; additional investigations are needed. It is well-known from the literature, however, that citrate frequently shows surprising
252
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
properties in the case of other cations, as well (e.g. see Refs [8,9]). This behaviour of citrate is strange from a complex-chemical point of view and needs further experimental investigation.
cially by the Foundation for Hungarian Higher Education and Research and National Research Foundation ( O T K A T23905, F19520).
7. Conclusions The ion-exchange processes are greatly influenced by the composition of the solution, p H and the stability constant of the complex-forming agent. However, when the sorption can take place in different ways these also affect the results. For example, lead ions can sorb on the surface of montmorillonite by different routes. Some of the lead ions always remain on the surface even when complexes with great stability are formed. As a consequence, lead contamination cannot be washed out of the surface of montmorillonite with the complex-forming agent studied.
Acknowledgement The authors thank R. Kir~ly for computations and discussions. The work was supported finan-
References [1] N.M. Nagy, J. K6nya, Colloids Surf. in press. [2|D.J.
Leggett (Ed.), Computational Methods for the
Determination of Formation Constants, Section 8, Plenum Press, New York, London, 1985. [3]A.E. Martell, R.M. Switch (Eds), Critical Stability Constants, vol. 1, Plenum Press, New York, 1974. [4] J. K6nya, N.M. Nagy, R. Kir~ly, J. Gelencsrr, submitted to Colloids Surf. [51 K. Hachiya, M. Ashida, M. Sasaki, H. Kan, T. Inoue, T.
Yasunaga, J. Phys. Chem. 83 (1979) 1866. [6] D.L. Sparks, Kinetics of Soil Chemical Processes, Academic Press, 1988, pp. 87-91. [7] L.-G. Ekstrrm, A. Olin, Chem. Scr. 13 (1978-1979) 10 [8] I. Grenthe, P. Wikberg, E.R. Still, Inorg. Chim. Acta 91 (1984) 25.
[9] D.R. Svoronos,S. Boulhassa,R. Guillaumont, M. Quarton, J. Inorg. Nucl. Chem. 43 (1981) 1541.
Colloids and Surfaces A: Physicochemicaland Engineering Aspects 137 (1998) 243-252
A
SURFACES
Ion-exchange processes of lead and cobalt ions on the surface of calcium-montmorillonite in the presence of complex-forming agents 1 II. The effect of DTPA, tartaric acid and citric acid on the sorption of lead ions on calcium-montmorillonite No6mi M. Nagy, J6zsef K6nya, Ilona K6nya Isotope Laboratory, Kossuth University, Debrecen, H-4010, Hungary Received 30 June 1997; accepted 24 November 1997
Abstract The sorption of lead ion on the surface of calcium-montmoriUonite is studied in the presence of complex-forming agent. The ion exchange processes are greatly influenced by the composition of the solution, pH and the stability constant of the complex forming agent. The total amount of lead contamination cannot be washed out of montmorillonite with the applied complex-forming agents. © 1998 Elsevier Science B.V.
Keywords: Calcium-montmorillonite; Citric acid; Cobalt ions; Complex formation; DTPA; Ion exchange; Lead ions; Tartaric acid
1. Introduction In Part I [1 ] the possible reactions of lead or cobalt ions was studied with calcium-montmorillonite in the presence and absence of E D T A as complex-forming agent. It was found that E D T A decreases the sorption of lead and cobalt ions on calcium-montmorillonite because they form negative complexes which cannot be sorbed on the surface. The amount of sorbed cobalt ions decreases practically to zero, whereas some lead ions always remain on the surface of montmorillonite. This was interpreted by the different sorption mechanisms of lead and cobalt ions. 1This work was presented at the 7th Conference on Colloid Chemistry in memoriam Alad~ir Buz~igh, Eger (Hungary), September 23-26, 1996. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0927-7757 (97)00381-6
Here, the effect of other complex-forming agents on the sorption of lead ions on calcium-montmorillonite is studied. As in Part I, the distribution of the total amounts of calcium and lead ions is measured by radioisotopic labelling, then the concentration of the different species (see fig. 1 in Part I [1]) are computed using the P S E Q U A D computer program [2].
2. Reactions possible in metal ion-diethyl-triamine penta-acetic acid (DTPA)-calcium-montmorillonite systems The reactions in Eqs. (1), (2a), (2b), (3), (4a), (4b), (5), (6a) and ( 6 b ) - ( 8 ) are the same as those in the presence of E D T A (see Part I [1] for
244
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
explanations). C a - m o n t + 45Ca2 +,¢~45Ca-mOnt + C a 2 +
( 1)
P b - m o n t + 212pb2 ÷,~212pb_mon t + p b 2 +
(2a)
C o - m o n t + 6°Co2 + ¢~6°Co-mont + Co 2 +
(2b)
H - m o n t + H ÷~ , H - m o n t + H ÷
(3)
C a - m o n t + p b 2 + , * : - P b - m o n t + C a 2+
(4a)
C a - m o n t + Co 2 ÷ ~ C o - m o n t + C a 2 +
(4b)
Ca-mont +2H+~2H-mont
+ C a 2+
(5)
P b - m o n t + 2 H +,=,2H-mont + Pb 2 +
(6a)
C o - m o n t + 2H + o 2 H - m o n t + C o 2÷
(6b)
The stability constants o f the complexes and protonated ligands are summarized in Table 1 [3].
3. Reactions possible in metal ion--tartaric acid (Tart)--caleium-montmorillonite systems
The reactions in Eqs. (1), (2a), (2b), (3), (4a), (4b), (5), (6a) and ( 6 b ) - ( 8 ) are the same as in the presence o f E D T A (see Section 2 and Part I [ 1 ]). The reactions o f the metal ions with Tart and the p r o t o n a t i o n reactions are as follows. Complex-formation equilibria (Eqs. ( 2 4 ) - ( 2 8 ) : Pb 2 ÷ + Tart 2- ~ P b T a r t
(24)
Ca 2+ + T a r t Z - o C a T a r t
(25)
3 C a - m o n t +2Fe3+,~,2Fe-mont + 3Ca 2+
(7)
Pb 2+ + H ÷ + T a r t 2 - c > H P b T a r t +
(26)
F e - m o n t + 3H +,*:,3H-mont + Fe 3+
(8)
Ca ÷ + H + + T a r t 2 - c > H C a T a r t +
(27)
The reactions o f the metal ions with D T P A and the p r o t o n a t i o n reactions are as follows. Complex-formation equilibria (Eqs. ( 9 ) - ( 1 7 ) :
Fe 3 ÷ + Tart 2 - o F e T a r t ÷
(28)
Ca 2÷ + D T P A S - c ~ - C a D T P A 3-
(9)
Tart 2- + H + ~ H T a r t -
(29)
Pb 2+ + D T P A S - , c ~ P b D T P A 3-
(10)
Tart 2- + 2H+,c~H2Tart 2-
(30)
Fe 3 + + D T P A 5 - , , > F e D T P A 1-
(11)
The stability constants o f the complexes and protonated ligands are summarized in Table 1 [3].
Ca 2 + + H + + D T P A 5 - ~ C a H D T P A 2-
(12)
Pb 2 + + H + + D T P A 5 - ~ P b H D T P A
(13)
4. Reactions possible in metal i o n - - c i t r i c acid
(14)
(Cit)---caleimn-montmorillonite systems
(15)
The reactions in Eqs. (1), (2a), (2b), (3), (4a), (4b), (5), (6a) and ( 6 b ) - ( 8 ) are the same as in the presence o f E D T A (see Section 2 [2] and Part
2-
Fe 3 + + H + + D T P A 5 - ¢ > F e H D T P A Fe 3+ + D T P A 5- + O H - ~ F e D T P A O H
3-
2Pb 2+ + D T P A S - c > P b 2 D T P A -
(16)
2Ca 2÷ + D T P A S - c ~ C a 2 D T P A -
(17)
Protonation
equilibria o f D T P A
(Eqs. ( 1 8 ) -
(23):
P r o t o n a t i o n equilibria (Eqs. (29) and (30):
I [1]). The reactions o f the metal ions with Cit and the p r o t o n a t i o n reactions are as follows. C o m p l e x - f o r m a t i o n equilibria (Eqs. (31)-(40): Pb 2 + + Cit 3 - ¢>PbCit -
(31 )
D T P A 5- + H + c > H D T P A a-
(18)
Ca 2+ + C i t 3 - ~ C a C i t -
(32)
D T P A 5- + 2 H + , * : , H 2 D T P A 3-
(19)
Pb 2+ + H + + C i t 3 - ~ H P b C i t
(33)
D T P A s - + 3H+,*:-H3DTPA 2-
(20)
Ca 2+ + H + + C i t 3 - ~ H C a C i t
(34)
D T P A 5- + 4 H + , : ~ H 4 D T P A -
(21)
Pb z+ + 2 C i t 3 - , ~ P b C i t 4-
(35)
D T P A 5- + 5 H + ~ H s D T P A
(22)
Pb 2+ + H ÷ + 2 C i t 3 - c ~ P b C i t 3-
(36)
D T P A 5- + 6H+ ¢>H6DTPA ÷
(23)
Pb 2+ + 2 H ÷ + Cit3-~e-H2PbCit ÷
(37)
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
245
Table 1 Logarithm of stability constants of different complexes and protonated ligands in the presence of DTPA, Tart and Cit [3] DTPA
Cit
Tart
Species
log K
Species
PbDTPA3HPbDTPA2 Pb2DTPA-
18 23.32 22.21
PbCit HPbCit PbCit~HPbCit 32H2PbCit ÷
log K 4.48 8.64 5.92
Species
log K
PbTart HPbTart ÷
3.12 5.72
10.61 11.70
CaDTPA3HCaDTPA2Ca2DTPA-
10.75 16.86 12.35
CaCit HCaCit H2CaCit +
3.45 7.79 11.00
CaTart HCaTart ÷
1.94 5,06
FeDTPA2FeHDTPAFeDTPAOH3-
28.00 31.56 18.22
FeCit FeHCit +
11.20 17.90
FeTart ÷
6,49
HDTPA4HzDTPA3H3DTPA2H4DTPAHsDTPA H6DTPA÷
10.49 19.09 23.37 26.01 28.01 29.61
HCit 2H2Cit H3Cit
5.66 10.00 12.90
HTart H2Tart
3.96 6.78
C a 2+ + 2 H + + C i t 3 - c : - H 2 C a C i t ÷
(38)
Fe 3 - + Cit 3 - ~ F e C i t
(39)
The substances a n d experimental c o n d i t i o n s are
Fe 3 ÷ + H ÷ + Cit 3- ~ F e H C i t ÷
(40)
very similar to those in P a r t I [1 ], b u t in this case D T P A , Tart a n d Cit were used as complex-form-
P r o t o n a t i o n equilibria (Eqs. ( 4 1 ) - ( 4 3 ) :
5. Experimental
ing agents.
Cit 3- + H + ¢~HCit 2-
(41)
Cit 3 - + 2 H ÷ , ~ H 2 C i t -
(42)
Cit 3- + 3H+,c~HaCit
(43)
The stability c o n s t a n t s o f the complexes a n d prot o n a t e d ligands are s u m m a r i z e d in Table 1 [3]. T h e r e s u l t a n t e q u i l i b r i u m state o f all systems is affected by the equilibria written above. I n o u r work we d e t e r m i n e d the total a m o u n t o f different ions (calcium a n d lead) in the solid a n d in solution by radioisotopic labelling, m e a s u r e d the p H , then we c o m p u t e d the a m o u n t o f different species in solution o n the basis o f the total a m o u n t s a n d stability c o n s t a n t s (Table 1).
6. Results and discussions 6.1. The
effect o f D T P A
The e q u i l i b r i u m fractions o f lead, calcium a n d h y d r o g e n ions o n the surface o f m o n t m o r i l l o n i t e versus p H in the presence o f D T P A at 1:1 a n d 1:4 P b : D T P A ratios are s h o w n in Figs. 1 a n d 2. I n this system the c o n c e n t r a t i o n s o f the following species were computed: P b 2 +, P b D T P A 3 - , H P b D T P A 2 - , P b 2 D T P A - , C a 2÷, C a D T P A 3-, H C a D T P A 2-, C a 2 D T P A - , Fe 3+, FeDTPA 2-, FeHDTPA-, FeDTPAOH a-, D T P A 5-, H D T P A 4-, H2DTPA 3-, HaDTPA 2-,
N.M. Nagy et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 137 (1998,) 243-252
246 1
0,9 0,8 0,7 A
X
0,S
a
•
A 0,4 •
A
0,3
~
mxCal
A
•
l.xH F • A
0,2 0,1 I 0
1
-
2
i
t
3
4
.--
t
-
-
5
6
pM
Fig. 1. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:DTPA = 1:1, the total initial concentration of lead and DTPA is 5 × 10 -4 mol dm -3.
1
0,9 0,8 0,7 0,6 x
0,5 0,4 0,3 0,2 0,1
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
pM
Fig. 2. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:DTPA = 1:4, the total initial concentration of lead is 2.5 x 10 -4 mol dm -3 and of DTPA is 10 -3 mol dm -3.
H 4 D T P A - , H s D T P A , H 6 D T P A +. A m o n g them the concentrations o f P b 2 D T P A - , C a D T P A 3-, Ca2DTPA-, Fe 3 +, F e D T P A 2-, F e H D T P A - , F e D T P A O H 3-, D T P A 5-, H D T P A 4-, H 4 D T P A - , H s D T P A and H 6 D T P A ÷ can be neglected, The equivalent fractions o f lead ions on the surface o f montmorillonite and the ratios o f different P b - D T P A complexes in the solutions are shown in Fig. 3. The equivalent fractions o f calcium ions on the surface o f montmorillonite and the ratios o f
different C a - D T P A complexes in the solutions are shown in Fig. 4. Figs. 1 and 2, together with fig. 6 in Part I [1], show similar tendencies as in the case o f E D T A ; only the degree o f the decrease o f lead sorption is smaller. We can see in Table 1, and in Table 1 o f Part I [ 1 ], that the stability constants o f P b - E D T A and P b - D T P A complexes, and even C a - E D T A and C a - D T P A complexes are very similar. In this case, however, the stability constants o f the protonated D T P A ligands characteristic at the p H exam-
N.M. Nagy et aL / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
0,9
247
II
0,8 0,7-
•
[e XPb
0,6
ill Pb2 +/PbtOt
0,5 -
/x HPbDTPA/PDtOt /
/A PbDTPA/Pbtot
0,4
~X (PbDTPA+ HPbDTPA)/PbtOt
03 0,2 0,1
O
1
2
3
4
5
6
PH
Fig. 3. The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of different lead-DTPA complexes in the solutions.
0,9 0,8 0,7 0,6
0,s
~
!
I-c~2÷'c~t°*i exCa
~DTPA/Catot~
0,4 0,3 0,2 0,1
1
2
3
4
5
6
pH
Fig. 4. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different lead-DTPA complexes in the solutions.
ined (H2DTPA 3- and H3DTPA 2-) are greater. As a result the concentration of hydrated Pb 2÷ is greater, which leads to a greater sorption of lead. The decrease of the sorbed lead is connected with the decrease of the concentration of hydrated, positive lead(II) ions, which is similar to the case with manganese ions [4]. In the case of lead, however, there is not a such strict relation between the sorbed quantity and the hydrated ion concentration, the quantity of the sorbed lead never decreases to zero. This is probably caused by the formation of Pb-O on the edges of layers, that is
on the pH-dependent charges (see Ref. [5] in Ref. [6]). The sorption method is less influenced by complex-formation; its effect is expressed in the decrease of the slope of the Xpb-pH curve. The sorption of calcium ions is as expected: at low pH-values, where calcium ions are present as Ca 2÷, they sorb as in the absence of complexforming agent; the quantity of the sorbed calcium increases when pH increases. At pH-values where negative Ca-EDTA complexes are already formed and the concentration of hydrated Ca 2÷ ions decreases, the sorption of calcium on the surface
248
N.M. Nagy et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 137 (1998) 243-252 1
o,9 0,8 0,7 o,6
0,4
•
j &
-
0,3
•
•
0,2 0,1
1,5
2
2,5
3
3,S
4
4,5
pH
Fig. 5. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:Tart = 1:1, the total initial concentration of lead and Tart is 5 x 10-4 mol dm -3.
I0,9 0,8 ,
~
0,7
0,6 .°,3
0,4
~
0,3-
~
0,2 0,1 0 1,5
-
A
~ I
I
~
i
2
2,5
3
3,5
p 4
[ 4,5
DH
Fig. 6. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:Tart = 1:4, the total initial concentration of lead is 2.5 x 10-4 mol dm -3 and of Tart is 10 -3 mol dm -3.
of montmorillonite also decreases. This was also the case for both the manganese and lead ions in the EDTA-calcium-montmorillonite system [4].
6.2. The effect of Tart The equilibrium fractions of lead, calcium and hydrogen ions on the surface of montmorillonite versus pH in the presence of Tart at 1:1 and 1:4 Pb:Cit ratios are shown in Figs. 5 and 6. In this system the concentrations of the following species were computed: Pb 2+, PbTart, HPbTart +, Ca 2+, CaTart, HCaTart +, Fe 3+, FeTart +, Tart 2-, Htart-, HzTart. The concen-
trations of HPbTart ÷, HCaTart +, Fe 3÷ and FeTart + are negligible. The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of different lead-tartrate complexes in the solutions at Pb:Tart= 1:1 are shown in Fig. 7. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different calcium-tartrate complexes in the solutions at Pb:Tart-- 1:1 are shown in Fig. 8. At Pb:Tart = 1:4 similar results are obtained. Figs. 7 and 8, together with fig. 6 in Part I [1], show that Tart influences the amount of sorbed lead on the surface of montmorillonite to a small
Physicochem.Eng. Aspects 137 (1998) 243-252
N.M. Nagy et al. / Colloids Surfaces A: 1 T
:
249
-
0,917,8-
0,7 o., [ A PbTart/PbtotJ
0,5 0,4
0,2
~
0,1 , l 0
a, i
I t
2
2,5
1,5
3
3,5
I
I
4
4,5
pH
Fig. 7. The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of different lead-tartrate complexes in the solutions.
0,9 0,8 0,7
I
O,.o,, 0,4
• ca2 + I C a t o ~ A
CaTart/cato~
t 0.3
,~
"
0,2
0,
1,5
AI
AI
2
2,5
A
t
3
--
a
I
3,5
•
~
4
4,5
IIH
Fig. 8. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different lead-tartrate complexes in the solutions.
degree. It seems that Tart at l : l = P b : T a r t ratio increases the amount o f lead to a small degree, The small effect o f tartrate on the sorption of lead is in accordance with the concentration o f positive, hydrated Pb 2÷ ions. The stability constants o f tartrate complexes is much smaller than the stability constants o f E D T A and D T P A complexes, so the concentration o f Pb 2+ is higher resulting in a relatively high sorption, The small increase o f lead sorption at Pb:Tart = 1:1 ratio is probably caused by the binding o f O H
groups of tartrate complexes on the edges of octahedral layers. In the case of calcium ions similar results are obtained as in the presence of E D T A and DTPA.
6.3. The effect of Cit The equilibrium fractions of lead, calcium and hydrogen ions on the surface o f montmorillonite versus pH in the presence o f citric acid at 1:1 and 1:4 Pb:Cit ratios are shown in Figs. 9 and 10.
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
250
0,8 0,7 0,6 O,S
x 0,3°'4t,~
~
~/mxCaI
0,2
1,5
2
2,5
3
3,5
4
4,5
Fig. 9. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:Cit = 1:1, the total initial concentration of lead and Cit is 5 × 10 -4 tool dm -3. 1 o,g
~
0,8
A
0,7 0,6 X
0,5
~
0,4
0,3 g,2 13,1 ~
,S
2
2,5
3
3,5
4
4,S
pH
Fig. I0. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:Cit 1:4, the total initial concentration of lead is 2.5 x l 0 - 4 tool dm - 3 and of Cit is 10- 3 mol dm- ~.
In this system the concentrations of the following species were computed: Pb 2+, PbCit-, HPbCit, PbCit 4- , HPbCit~-, H2PbCit + , Ca 2+, CaCit-, HCaCit, H2CaCit +, Fe 3+, FeCit, FeHCit +, Cit ~-, HCit z-, H~Cit-, H3Cit. Among them the concentrations of PbCit24-, HPbCit~-, CaCit-, HCaCit, HzCaCit +, Fe 3+, FeCit, FeHCit + and Cit 3- were less than 1% of the other species and so they are neglected, The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of
=
different Pb-citrate complexes in the solutions at Pb:Cit=l:l are shown in Fig. 11. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different Ca-citrate complexes in the solutions at Pb:Cit= 1:1 are shown in Fig. 12. In the case of Pb:Cit = 1:4 similar results are obtained; the concentration of the protonated citrate is increased in this case. Figs. 9 and 10, together with fig. 6 in Part I [1], show that Cit, as with EDTA and DTPA, decreases
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
251
0,9 0,8 0,7 x.p b ~ N
0,6
~
,
i
I • Pb2 */Pbtot I
IAPbdt/Fotot [
0,5
l x HPbCWPbtOtJ
0,4 + i 0,3 + 0,2 0,1
jr
o
- - - - -
%5
i~
2
iI
l
11 2,5
.
II
i
3
3,5
- - t
4
4,5
Fig, 11. The equivalent fractions of lead ions on the surface of montmorillonite and the ratios of different lead-citrate complexes in the solutions.
1 T
;
_-
_-
•
m
:
0,9
0,8I 0,7 O,S
Lmca2 +/CatotJ
0,4 ~ 0,3
!
(],1 1,5
2
2,5
3 pH
3,5
4
4,5
Fig. 12. The equivalent fractions of calcium ions on the surface of montmorillonite and the ratios of different lead-citrate complexes in the solutions.
the amount of sorbed lead on the surface of montmorillonite, but the degree of decrease is rather great; this is surprising because of the relatively low stability constants of citrate complexes (Table 1). Figs. 11 and 12 also show that the ratio of hydrated cations (both Pb 2÷ and Ca 2÷) is great. So the amount of the sorbed lead cannot be interpreted on the basis of the Pb 2÷ concentration, It is known that citrate forms other polynuclear
complexes [7,8], though their stability constants are not included in Table 1. The computations were repeated considering these complexes, but the results were practically the same: the major part of the lead ions is present as Pb 2÷ in our experimental conditions. The effect of citrate cannot be interpreted on the basis of the ratio of pb2+; additional investigations are needed. It is well-known from the literature, however, that citrate frequently shows surprising
252
N.M. Nagy et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 243-252
properties in the case of other cations, as well (e.g. see Refs [8,9]). This behaviour of citrate is strange from a complex-chemical point of view and needs further experimental investigation.
cially by the Foundation for Hungarian Higher Education and Research and National Research Foundation ( O T K A T23905, F19520).
7. Conclusions The ion-exchange processes are greatly influenced by the composition of the solution, p H and the stability constant of the complex-forming agent. However, when the sorption can take place in different ways these also affect the results. For example, lead ions can sorb on the surface of montmorillonite by different routes. Some of the lead ions always remain on the surface even when complexes with great stability are formed. As a consequence, lead contamination cannot be washed out of the surface of montmorillonite with the complex-forming agent studied.
Acknowledgement The authors thank R. Kir~ly for computations and discussions. The work was supported finan-
References [1] N.M. Nagy, J. K6nya, Colloids Surf. in press. [2|D.J.
Leggett (Ed.), Computational Methods for the
Determination of Formation Constants, Section 8, Plenum Press, New York, London, 1985. [3]A.E. Martell, R.M. Switch (Eds), Critical Stability Constants, vol. 1, Plenum Press, New York, 1974. [4] J. K6nya, N.M. Nagy, R. Kir~ly, J. Gelencsrr, submitted to Colloids Surf. [51 K. Hachiya, M. Ashida, M. Sasaki, H. Kan, T. Inoue, T.
Yasunaga, J. Phys. Chem. 83 (1979) 1866. [6] D.L. Sparks, Kinetics of Soil Chemical Processes, Academic Press, 1988, pp. 87-91. [7] L.-G. Ekstrrm, A. Olin, Chem. Scr. 13 (1978-1979) 10 [8] I. Grenthe, P. Wikberg, E.R. Still, Inorg. Chim. Acta 91 (1984) 25.
[9] D.R. Svoronos,S. Boulhassa,R. Guillaumont, M. Quarton, J. Inorg. Nucl. Chem. 43 (1981) 1541.