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Binding of calcium by humic acid
J. inorg nucL Chem Vol 43, pp 921-922, 19Sl Dinted in Great Brilain
0022-1902/81J050921-925020[I]0 Pergamon Press Ltd.
BINDING OF CALCIUM BY HUMIC ACID G. R. CHOPP1N and P. M. SHANBHAG Department of Chemistry, Florida State University, Tallahas~,ee. F[. 32306, U.S.A.
(Received 16 June 1980: received for publication 16 ,hdv 1980) ~bstraet--The binding of radiotracer 4SCa to humic acid was studied by a solvent extraction technique. The dependence of binding on pH and temperature was determined. For an ionic medium of 0. IM (NaC104), the binding constant varied from 2.25_+0.04 at pH 3.9 to 3.32+-0.04 at pH 5.0. Thermodynamic parameters of binding calculated from the temperature coefficient indicated that a large, positive entropy change accounts for the favorable free energy of complexation. INTRODUCTION ~s a part of a program to investigate the interaction of actinide ions with humic acid[I-3], we have also studied calcium binding. Calcium is a common cation in nature and would compete with the actinides for binding sites on humic acid. The binding for Ca(II) would be expected to be much weaker than for the actinides in III, IV or VI (mo2 :+ ) states but the latter would be likely to be only present at much lower concentrations. Consequently, the equilibrium competition could favor calcium, thereby allowing freer diffusion of actinides in humate rich environments. Humic acid is a natural polyelectrolyte. Study of the binding of Ca(IlL a hard acid of comparable radius to the hard acids Am(liD[I] and Th(IV)[2] that have already been studied allows further understanding of cation binding to such polyelectrolytes.
hydroxide solution. The capacity of ionizable acidic groups was determined to be 4.2 meq/g. The average of three titrations gave pKo = 4,19-+ 0.06 at 50% ionization [a = 0.5] for the carboxylate binding sites[6]. The variation of the distribution coefficient, D, between organic and aqueous phase as a function of humate concentration (in meq/1) at pH 4.14 (oe = 0.49) for 5 temperatures is shown in Fig. 1. The first power dependence of 1/D vs [Hu] indicates the binding reaction can be written:
EXPERIMENTAL Reagents. Di(2-ethylhexyl) phosphoric acid, HDEHP, from Pfaltz and Bauer, Inc., was purified by a modification[4] of the method of Peppard, et a/.[5]. Technical grade humic acid from Aldrich Chemical Co.. was purified earlier[l, 2]. Other chemicals were reagent grade and used with no further purification. a~Ca of initial specific actixity of 23 Ci/g was obtained from New England Nuclear Co. as a solution of CaCI2. Counting was done on a Packard Model 3320 liquid scintillation counter using an extractant scintillation cocktail which consisted of 6g/I HDEHP and 6g PPO in toluene which had been purified previously by passage through a column of alumina. ProcedUre: Both the aqueous and organic solutions were preequilibrated by contacting equal volumes of the two prior to use. The aqueous phase had a constant ionic strength of 0.1 M adjusted with (Na + H')C10~. Humic acid solutions were prepared fresh for each experiment by dissolving the proper amount in the aqueous solutions of 0.10 M ionic strength. The aqueous phase pH was adjusted by addition of 0.10 M HC104 or 0.10 M NaOH. In the distribution experiments, equal 10.0ml volumes of the two phases were placed in clean, dry pyrex glass vials, approximately 0.01 U Ci of the 4~Ca tracer added and the vials sealed. The vials were shaken in a constant temperature bath at approximately 7rpm for at least 20hr to ensure that the solutions reached distribution equilibrium. The two phases were allowed to separate, a portion of each phase was removed, centrifuged and 0.50ml duplicate aliquots taken for counting. They were added to the extractant scintillation cocktail, shaken and counted to an error of • ~ 1%.
where Do is the distribution coefficient in the absence of humate. Table 1 lists the values of /3, determined in acetate buffer. In the preliminary experiments of uranyl humate binding study[4], there was evidence of a dependence of ~ on the acetate buffer concentration. We studied the variation of 13, in acetate, isobutyrate and glycolale buffer media. With the results shown in Table 2, assuming
Ca ~2 + Hu ~ m CaHu ~n ~'
(1}
with a binding constant/3,. The value of/3, was obtained from least squares analysis of the relation
1/D = I/D,, +/3,/Do(Hu)
(2)
08
O6
r..--e
c~
0o
\_
/./o
00 ----
t 15
O0
RESULTS The cation exchange capacity of the humic acid was determined by titrating a solution of bumic acid in the presence of sodium acetate with standard sodium
c~---- d
(Humate)
I 30 meg
_ ~ _ 45
/ I
Fig. 1. Variation of the reciprocal of the distribution coefficient as a function of humate concentration: a = 298°: h 282~: c 307°; d - 290°; e - 275°K. 921
922
G.R. CHOPPIN and P. M. SHANBHAG
Table
1. Calcium(II) humate binding constants 0.10 M(NaCIO4); 0.01 M acetate buffer
pH
a}
log
~i
3.88
0.44
2.25
-+ 0 . 0 4 a
3.95
0.45
2.82
± 0.05 b
4.47
0.55
2.72
± 0.03 b
5.01
0.68
5.32
± 0.04 a
a. b.
I=
22.5°C 25°C. ~ = degree of ionization
of humic
acid
the same variation of/3, with a as in Table 1, we obtain corrections to a = 0.45:0.05 glycolate, log fl, = 2.52. In all cases, any effect of the buffer is small and probably reflects the formation of a small degree of mixed complex, Ca(Hu)(B). The thermodynamic parameters for calcium binding to humate, calculated from the temperature dependence of /3,, are given in Table 3. The temperature dependence of D was the same for Do, within error limits, so the enthalpy is zero and -AG-~ TAS. DISCUSSION
Schnitzer and Hansen studied the interaction of Ca(II) with soil fulvic acid [7]. Only a 1:1 complex was observed with stability constants in 0.10M ionic strength solution of 2.6+0.1 (pH 3) and 3,4-+0.1. (pH 5). Unfortunately, they do not report the degree of ionization of their fulvic acid at these pH values. A commercial sample of fulvic acid from Aldrich Chemical Co. has been found to have a = 0.05 at pH 3 and a = 0.84 at pH 5[2]. If these numbers are valid for the fulvic acid used in Ref.
[7], we can compare their log #, (CaHu)= 2.6--+ 0.1 (a = 0.55) with our log/3l (CaHu) = 2.72-+ 9.03. For Th(IV),/3z for ThFu was several orders of magnitude lower than that for ThHu. This was attributed to the greater effective charge per binding site of humate at the same degree of ionization due to the greater number of sites per unit weight[2]. Apparently, such an effect is not as significant in the calcium binding as the humic and fulvic acids have similar binding constants. The data in Table 3 indicates that the primary factor in the favorable free energy for calcium binding is the positive entropy change. For the actinide cations, large positive entropy changes were the source of favorable free energies of binding. As with these latter cations, we can attribute such entropy changes as reflecting dehydration of cation and anion to form inner sphere (contact) complexes. The binding of actinides to humate show both 1:1 and 1:2 stoichiometry with the latter accounting for the major sorption when [Hu] > 10-5 meq. I '. Based on the /3, (CaHu), and the /3, and /32 values of the actinide humate complexation, the presence of calcium in the ground water would not affect significantly the almost complete sorption of the actinides by soil humic acid. Acknowledgement--This research was supported by a contract
with the U.S.D.O.E. REFERENCES 1. E. k Bertha and G. R. Choppin, J. lnorg. NucL Chem. 40, 651 (1978). 2. K. L. Nash and G. R. Cboppin, J. lnorg. NucL Chem. 42, 1045 (1980). 3. G. R. Choppin, Trans. Am. Nucl. Soc. 32, 166 (1979). 4. P. M. Shanbhag, Ph.D. Dissertation, Florida State University, 1979. 5. D. F. Peppard, G. W. Mason, J. L. Maier and W. J. Driscoll, J. Inorg. Nucl. Chem. 4, 334 (1957). 6. G. R. Choppin and L Kullberg, J. Inorg. NucL Chem. 40, 651 (1978). 7. M. Schnitzer and E. H. Hansen. Soil Sci. 109, 333 (1970).
Table 2. Calcium(II) humate binding constants T=25°C, I= 0.10M (NaCIO4) pH
a*
buffer
log 81
3.88
0.44
0.I0 M acetate
2.25 ± 0,04
3.88
0.44
0.050 M acetate
2.47 ± 0.03
3.92
0.45
0.005 M glycolate
2.45 ± 0.04
4.10
0.48
0.050 M glycolate
2.65 ± 0.03
3.92
0.45
0.005 M i-butyrate
2.76
4.24
0.51
0.50 M i-butyrate
3.03 ± 0.03
degree
of ionization
± 0.04
of humic acid
Table 3. Thermodynamic parameters of Ca-humate binding T = 25°C' I = 0.I M (NaCIO4+ ~ycolate buffer) Species
CaHu
gG(kJ/eq)
-12.61
± 0.46
AH(kJ/eq)
0.0
AS(J/eq/K)
42.3
± .2
0.49
0022-1902/81J050921-925020[I]0 Pergamon Press Ltd.
BINDING OF CALCIUM BY HUMIC ACID G. R. CHOPP1N and P. M. SHANBHAG Department of Chemistry, Florida State University, Tallahas~,ee. F[. 32306, U.S.A.
(Received 16 June 1980: received for publication 16 ,hdv 1980) ~bstraet--The binding of radiotracer 4SCa to humic acid was studied by a solvent extraction technique. The dependence of binding on pH and temperature was determined. For an ionic medium of 0. IM (NaC104), the binding constant varied from 2.25_+0.04 at pH 3.9 to 3.32+-0.04 at pH 5.0. Thermodynamic parameters of binding calculated from the temperature coefficient indicated that a large, positive entropy change accounts for the favorable free energy of complexation. INTRODUCTION ~s a part of a program to investigate the interaction of actinide ions with humic acid[I-3], we have also studied calcium binding. Calcium is a common cation in nature and would compete with the actinides for binding sites on humic acid. The binding for Ca(II) would be expected to be much weaker than for the actinides in III, IV or VI (mo2 :+ ) states but the latter would be likely to be only present at much lower concentrations. Consequently, the equilibrium competition could favor calcium, thereby allowing freer diffusion of actinides in humate rich environments. Humic acid is a natural polyelectrolyte. Study of the binding of Ca(IlL a hard acid of comparable radius to the hard acids Am(liD[I] and Th(IV)[2] that have already been studied allows further understanding of cation binding to such polyelectrolytes.
hydroxide solution. The capacity of ionizable acidic groups was determined to be 4.2 meq/g. The average of three titrations gave pKo = 4,19-+ 0.06 at 50% ionization [a = 0.5] for the carboxylate binding sites[6]. The variation of the distribution coefficient, D, between organic and aqueous phase as a function of humate concentration (in meq/1) at pH 4.14 (oe = 0.49) for 5 temperatures is shown in Fig. 1. The first power dependence of 1/D vs [Hu] indicates the binding reaction can be written:
EXPERIMENTAL Reagents. Di(2-ethylhexyl) phosphoric acid, HDEHP, from Pfaltz and Bauer, Inc., was purified by a modification[4] of the method of Peppard, et a/.[5]. Technical grade humic acid from Aldrich Chemical Co.. was purified earlier[l, 2]. Other chemicals were reagent grade and used with no further purification. a~Ca of initial specific actixity of 23 Ci/g was obtained from New England Nuclear Co. as a solution of CaCI2. Counting was done on a Packard Model 3320 liquid scintillation counter using an extractant scintillation cocktail which consisted of 6g/I HDEHP and 6g PPO in toluene which had been purified previously by passage through a column of alumina. ProcedUre: Both the aqueous and organic solutions were preequilibrated by contacting equal volumes of the two prior to use. The aqueous phase had a constant ionic strength of 0.1 M adjusted with (Na + H')C10~. Humic acid solutions were prepared fresh for each experiment by dissolving the proper amount in the aqueous solutions of 0.10 M ionic strength. The aqueous phase pH was adjusted by addition of 0.10 M HC104 or 0.10 M NaOH. In the distribution experiments, equal 10.0ml volumes of the two phases were placed in clean, dry pyrex glass vials, approximately 0.01 U Ci of the 4~Ca tracer added and the vials sealed. The vials were shaken in a constant temperature bath at approximately 7rpm for at least 20hr to ensure that the solutions reached distribution equilibrium. The two phases were allowed to separate, a portion of each phase was removed, centrifuged and 0.50ml duplicate aliquots taken for counting. They were added to the extractant scintillation cocktail, shaken and counted to an error of • ~ 1%.
where Do is the distribution coefficient in the absence of humate. Table 1 lists the values of /3, determined in acetate buffer. In the preliminary experiments of uranyl humate binding study[4], there was evidence of a dependence of ~ on the acetate buffer concentration. We studied the variation of 13, in acetate, isobutyrate and glycolale buffer media. With the results shown in Table 2, assuming
Ca ~2 + Hu ~ m CaHu ~n ~'
(1}
with a binding constant/3,. The value of/3, was obtained from least squares analysis of the relation
1/D = I/D,, +/3,/Do(Hu)
(2)
08
O6
r..--e
c~
0o
\_
/./o
00 ----
t 15
O0
RESULTS The cation exchange capacity of the humic acid was determined by titrating a solution of bumic acid in the presence of sodium acetate with standard sodium
c~---- d
(Humate)
I 30 meg
_ ~ _ 45
/ I
Fig. 1. Variation of the reciprocal of the distribution coefficient as a function of humate concentration: a = 298°: h 282~: c 307°; d - 290°; e - 275°K. 921
922
G.R. CHOPPIN and P. M. SHANBHAG
Table
1. Calcium(II) humate binding constants 0.10 M(NaCIO4); 0.01 M acetate buffer
pH
a}
log
~i
3.88
0.44
2.25
-+ 0 . 0 4 a
3.95
0.45
2.82
± 0.05 b
4.47
0.55
2.72
± 0.03 b
5.01
0.68
5.32
± 0.04 a
a. b.
I=
22.5°C 25°C. ~ = degree of ionization
of humic
acid
the same variation of/3, with a as in Table 1, we obtain corrections to a = 0.45:0.05 glycolate, log fl, = 2.52. In all cases, any effect of the buffer is small and probably reflects the formation of a small degree of mixed complex, Ca(Hu)(B). The thermodynamic parameters for calcium binding to humate, calculated from the temperature dependence of /3,, are given in Table 3. The temperature dependence of D was the same for Do, within error limits, so the enthalpy is zero and -AG-~ TAS. DISCUSSION
Schnitzer and Hansen studied the interaction of Ca(II) with soil fulvic acid [7]. Only a 1:1 complex was observed with stability constants in 0.10M ionic strength solution of 2.6+0.1 (pH 3) and 3,4-+0.1. (pH 5). Unfortunately, they do not report the degree of ionization of their fulvic acid at these pH values. A commercial sample of fulvic acid from Aldrich Chemical Co. has been found to have a = 0.05 at pH 3 and a = 0.84 at pH 5[2]. If these numbers are valid for the fulvic acid used in Ref.
[7], we can compare their log #, (CaHu)= 2.6--+ 0.1 (a = 0.55) with our log/3l (CaHu) = 2.72-+ 9.03. For Th(IV),/3z for ThFu was several orders of magnitude lower than that for ThHu. This was attributed to the greater effective charge per binding site of humate at the same degree of ionization due to the greater number of sites per unit weight[2]. Apparently, such an effect is not as significant in the calcium binding as the humic and fulvic acids have similar binding constants. The data in Table 3 indicates that the primary factor in the favorable free energy for calcium binding is the positive entropy change. For the actinide cations, large positive entropy changes were the source of favorable free energies of binding. As with these latter cations, we can attribute such entropy changes as reflecting dehydration of cation and anion to form inner sphere (contact) complexes. The binding of actinides to humate show both 1:1 and 1:2 stoichiometry with the latter accounting for the major sorption when [Hu] > 10-5 meq. I '. Based on the /3, (CaHu), and the /3, and /32 values of the actinide humate complexation, the presence of calcium in the ground water would not affect significantly the almost complete sorption of the actinides by soil humic acid. Acknowledgement--This research was supported by a contract
with the U.S.D.O.E. REFERENCES 1. E. k Bertha and G. R. Choppin, J. lnorg. NucL Chem. 40, 651 (1978). 2. K. L. Nash and G. R. Cboppin, J. lnorg. NucL Chem. 42, 1045 (1980). 3. G. R. Choppin, Trans. Am. Nucl. Soc. 32, 166 (1979). 4. P. M. Shanbhag, Ph.D. Dissertation, Florida State University, 1979. 5. D. F. Peppard, G. W. Mason, J. L. Maier and W. J. Driscoll, J. Inorg. Nucl. Chem. 4, 334 (1957). 6. G. R. Choppin and L Kullberg, J. Inorg. NucL Chem. 40, 651 (1978). 7. M. Schnitzer and E. H. Hansen. Soil Sci. 109, 333 (1970).
Table 2. Calcium(II) humate binding constants T=25°C, I= 0.10M (NaCIO4) pH
a*
buffer
log 81
3.88
0.44
0.I0 M acetate
2.25 ± 0,04
3.88
0.44
0.050 M acetate
2.47 ± 0.03
3.92
0.45
0.005 M glycolate
2.45 ± 0.04
4.10
0.48
0.050 M glycolate
2.65 ± 0.03
3.92
0.45
0.005 M i-butyrate
2.76
4.24
0.51
0.50 M i-butyrate
3.03 ± 0.03
degree
of ionization
± 0.04
of humic acid
Table 3. Thermodynamic parameters of Ca-humate binding T = 25°C' I = 0.I M (NaCIO4+ ~ycolate buffer) Species
CaHu
gG(kJ/eq)
-12.61
± 0.46
AH(kJ/eq)
0.0
AS(J/eq/K)
42.3
± .2
0.49