Environ. Sci. Techno/. 1995, 29, 2150-2153
Stability of Mixed-Ligand Complexes of Metal Ions with Humic !hbstmees and Low Molecular Weight Ligands MARTIN A. GLAUS,* W O L F G A N G HUMMEL, A N D LUC R. VAN LOON Laboratory for Waste Management, Paul Scherrer Institute, Wiirenlingen and Villigen, CH-5232 Villigen-PSI,Switzerland
Introduction General. Humic substances (HS) are some of the most powerful metal-binding agents among natural organic substances (I,,?). Depending on the concentration of HS and the environmental conditions (e.g., pH), the concentration of metal ions in natural waters and consequently their mobility in the geosphere may be altered due to complexation by HS. These are the reasons for the continuous interest in data and models describing metal complexation by HS. Most of the current binding models (for an overview, cf. ref 3) assume a 1:l stoichiometry of the metal ion (M) with the ligand sites (S) of HS. It is however possible that a metal ion bound to a ligand site exchanges coordinated water molecules or hydroxide ions against one or several low molecular weight ligands (L), thereby forming mixed-ligand S-M-Li (i = 1, 2, ..., n) complexes (hereafter denoted as MSLi complexes). Neglecting such mixed-ligand complexes in speciation calculations for systems containing low molecular weight ligands, e.g., carbonate-rich groundwaters, can thus lead to an underestimation of the degree of complexation of M by HS. Information about the formation of MSLi complexes in the literature is scarce. Experimental evidence for the existence of such complexes was first provided by acidbase titration combined with ion-selective measurement of Cu2+(4). The metal concentrations used in t h i s study were of the order of millimolar and thus not relevant for trace metal studies in environmental systems. Powell and Town (5) postulated from data obtained by using spectroscopic and voltammetric techniques that MSLi complexes were formed in significantamounts. Unfortunately, no stability data were reported in this study. Buffle (6) finally corrected literature data for the complexation of Cu(I1) by HS in the presence of L by introducing mixedligand complexes. He tentatively assumed that the change in free energy for the addition of L to a metal-humate complex and for the addition of L to the metal aqua ion is the same. As a result, the corrected data fit better to the binding models used in that study. The lack of thermodynamic data for the formation of MSLi complexes prompted us to determine stability constants of such complexes for a few metal-ligand pairs in combination with a fulvic acid using the equilibrium dialysis-ligand exchange (EDLE) technique (7, 8) and to * Telepone: +41 56-99 22 93: Fax: +41 56-99 22 05: E-mail address:
[email protected].
2150 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8 , 1995
test whether the data obtained are compatible with the concepts of mixed-ligand complex formation in homogeneous aqueous solution (9). Role of Mixed-LigandComplexesin EDLE Experiments. In the EDLE technique, the unequal distribution of M across the inner dialysis compartment (containing HS) and the outer dialysis compartment is used to calculate ‘KHs, the conditional stability constant for the formation of MHS, which is defined as
where all the terms in brackets are given as moles per liter. Both compartments additionally contain a low molecular weight ligand (L), in the following also denoted as the reference ligand, which is freely diffusable across the membrane and competes with HS for M. ‘KHSis calculated by formulating mass balance equations for M in the two dialysis compartments. If these mass balances include only M, MLi complexes, and MHS complexes, the calculated ‘KHS(for a detailed derivation, cf. ref 8) is denoted in the following as K‘. If MSLi complexes are also considered in these mass balance equations,the calculated ‘KHSis denoted in the following as K (the derivation is similar to that described in ref 8). It can be shown that Kis related to K‘ by the following equation: n
x“ = K(1 + D?[L]’) i= 1 ,BlmiX is the stability constant for the formation of MSLi complexes (MS + z Z = MSLi): mix
pi
- [MSL,] - [MSl[L]’
(3)
Equation 2 shows on the one hand that K‘ asymptotically approximates Kif Cy=l/31mk[L1i-K 1, which means that, in such cases, mixed-ligand complexes can be neglected. On the other hand, if Cy=41mix[L]i>> 1, mixed-ligand complexes are predominant and must be considered in speciation calculations.
Experimental Section The elemental composition and the characterization by synchronous excitation fluorescence spectroscopy of Laurentian soil fulvic acid (LFA),which is used in this study, as well as the experimental procedure for the determination of ‘KHS of LFA by the EDLE technique are described elsewhere (8). Experiments were carried out at room temperature at pH 7.00 i 0.05 and at LFA concentrations between 50 and 100 ppm, depending on the complexation strength of L. For the determination of ‘KHSfor the binding of Eu(III),the isotope 1 5 2 Ewas ~ used at a concentration of 250 kBq L-’ (approximately0.2nM)in the presence of stable Eu(1II) at 3 nM.
Results and Discussion If K is measured at varying total concentration of L, Kand be obtained from the best fit of the data to eq 2. In the case of i = 1, this can be done by linear regression.
,BzmiXcan
0013-936W95/0929-2150509.00/0
0 1995 American Chemical Society
5*51
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An example is given by Figure 1, which shows data for the complexation of Co(I1) by LFA in the presence of oxalate as a reference ligand. Linear regression gives a log K of 4.25 and a log ,61mix of 2.2. The dashed lines in Figure 1 indicate the asymptotic behavior of the model described by eq 2. At very low oxalate concentrations, mixed-ligand complexes are not formed, and K' approximates K (slope ofzero);whereas at very high oxalate concentrations, mixedligand complexes are predominant (slope of 1). The possible existence of mixed-ligand complexes in which i > 1 cannot be excluded from the data presented in Figure 1. Note that it is not possible to investigate a larger range of ligand concentrations because of limitations of the EDLE technique. At lower concentrations of oxalate, the radionuclide is overly enriched in the dialysis compartment containing LFA, and at higher concentrations of oxalate, the radionuclide is distributed equally between the two dialysis compartments, both of which cases lead to large statistical errors in the determination of the equilibrium constant (8). Another example is shown in Figure 24 in which data for the binding of U O P by LFA in the presence of oxalate as a reference ligand are plotted. These data clearly demonstrate that only MSLl is formed, since most of the data lie in a quasi-linear region with slope of 1. Linear regression gives a log K of 9.4 and a log PImixof 2.3. It is an inherent drawback of the linear regression procedure that the accuracy of the resulting K value is strongly dependent on the accuracy of the highest values on the abscissa. As an example, it was calculated that an uncertainty of f 0 . 2 log unit in the data at 0.1 M oxalate results in an uncertainty of log K of approximately 2 orders of magnitude. Therefore, additional experiments with EDTA as a reference ligand were conducted in order to independently determine K. EDTA was suggested to occupy four sites in the equatorial plane of the coordination sphere of U0z2+ (IO). The apical sites are occupied by the two oxyanions. A similar coordination geometry was observed in a crystal structure of VOZ+-EDTA complexes ( 1 1 ) . The additional binding of a bulky ligand such as HS to the U0z2+-EDTA complexis thus not possible without serious
distortion of the UOZ2+-EDTAbindings, and therefore, the formation of mixed-ligand complexes ofthe type S-UOzZ+EDTA is not expected. In other words, K can be approximated by K', since Z$4imix[L]i e 1 (cf. eq 2). In fact, over the whole range of EDTA concentrations tested, log K' is found to be 10.0 zk 0.2 (cf. Figure 2B), which is in fair agreement with the value obtained with oxalate. Note that differences of this order of magnitude between stability constants determined by using different reference ligands are not unusual (7). The average of log Kvalues from the experiments performed in the presence of EDTA and of those performed in the presence of oxalate gives a log Kof 9.7. Using this value, a log /31mix of 2.1 is calculated. This shows that the calculation of PImixis quite robust within the uncertainty range of K. Estimates for the formation of mixed-ligand complexes in systems containing LFA and the metallligand pairs UOZ2+/carbonate,Eu(II1)lnitrilotriacetate and Eu(III)/oxalate were obtained from similar experiments (data not shown). Again, no evidence for the formation of MSLi complexes in which i =. 1 was found. The Blmixvalues of all the metallligand pairs assessed in this study are summarized in Table 1. Mixed-ligandcomplex formation in homogeneous aqueous solution containing low molecular weight ligands was found, in many cases, to be stronger than one would predict on the base of statistical arguments (9).Statistical considerations (17) predict that the equilibrium constant of the following equation (cf. eq 3) is approximately twice the
Mx+Y;=r.MXY
(4)
value of KZL,the stability constant for the addition of Y to MY. Table 1 shows that the measured /?lmix values are smaller than KzLthroughout. This indicates that the formation of mixed-ligand complexes with HS as a ligand is rather weak compared to the formation of mixed-ligand complexes with low molecular weight ligands. Tentative explanations for this finding might be, for example, steric hindrance effects between L and HS in the mixed-ligand complex or electrostaticrepulsion between L and HS, which is negatively charged at pH 7 (18). Due to the polyelectrolyte character of HS, this repulsion is stronger than between low molecular weight ligands. In this context, it is interesting to mention that Schindler (19)reported the formation of mixed-ligand complexes with Si02 and TiO2 surfaces being generally not preferred. Both surfaces are negatively charged at the pH of these experiments. Despite the relatively low stability of MSLi complexes, these can become important, e.g., in groundwaters with high carbonate contents, as is shown in Figure 3 by the example of complexation of U0z2+. However, the figure shows that the error committed by ignoring mixed-ligand complexes is restricted to a rather narrow range of carbonate concentrations around a total carbonate concentration of 0.01 M. Note that, in the case in which mixed-ligand complexes are considered, the fraction of UOz2+associated with HS also decreases with increasing carbonate concentration. The reason for this decrease is found in the stoichiometry of the complexes formed. Under the conditions given,U022+(C03z-)2 is the dominatingspecies among the binary carbonato complexes. As can be seen from Table 1, the formation of U022+(C032-)2is preferred over the formation of S-UOZ~*-CO~~-. VOL. 29, NO. 8, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY 12151
,
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11.5
11
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-
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9
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1
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8 88
I
lei
log[EDTA]
log [Oxalate]
FIGURE 2. Plot of log K as a function of total reference ligand concentration for the complexation of U0z2+by LFA in the presence of oxalate (A) and of EDTA ( 8 )as a reference ligand at pH 7 and an ionic stren@h of 0.3 molR. The solid line shows the best fit of the linear data to eq 2; the dashed lines indicate the asymptotic behavior of the model. TABLE 1
Experimentally Obtained and KzL Values from literature
B1IRix Values
metal ion
ref ligand
Co2+ UOz2+ U0z2+ Eu3+ Eu3+
oxalate oxalate carbonate oxalate nitrilotriacetate
ionic strength (molR)
log /?Pix log hL *
0.2 0.3 0.3
2.0
0.1 0.1
2.3
2.1
4.3
5.0
7.3
3.4 8.0
3.7 9.3
a Conditional constants valid for the ionic strength given: values taken f r o m (refs 12-16).
too
Acknowledgments
v)
I
2
This work was supported financiallyby the Swiss National Cooperative for the Disposal of RadioactiveWaste (Nagra). The authors would like to thank A. Laube for his assistance with the experimental work. I. Grenthe, F. N e d , and P. Schindler provided critical and helpful comments during preparation of the manuscript.
80-
U
C
a
2
other metallligand combinations, it would be possible to estimate, even for cases in which ,BlmiX is not known, the critical concentration limit of L ([L],,J, above which mixedligand complexes become important. To a first approximation, it would be sufficient to assume that BlmK equalsKZL.[L],fit further depends on what fraction of ternary complexes (withrespect to the total concentration of metalhumate complexes1is regarded as significant. If we assume this fraction being lo%, [LI,,, equals 0.1/KzL (cf.eq 2). Note that this estimate results in considerablyhigher [Ll,,.it values than the approach of Buftle,who assumed that,Blmkequals the stability constant of the binding of the first L to the metal aqua ion (6').
60
literature Cited
c
0
8
FIGURE 3. Fraction of U0z2+ bound to HS as a function of total carbonate concentration at pH 8, ionic strength of 0.016 M, and of 4 x l W 4 W (approximately 100 ppm of HS). The solid line was calculated by considering mixed-ligand complex in the speciation (log /3pix= 5, cf. Table 1); the dotted line represents the result obtained by ignoring these species.
[a
Conclusions
It has been shown here that the fraction of metal bound to HS in a system containing HS, a metal ion and an additional ligand can be modeled better if mixed-ligand complexes of the type MSLi are also included. However, these complexes are rather weak in comparison to mixedligand complexes with low molecular weight ligands, since it was found that j31mix< KzL. If this relation is valid for 2152 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8,1995
(1) BuMe, J. Complexation Reactions in Aquatic Systems: An Analytical Approach; Ellis Horwood Limited: Chichester, 1988. (2) Boggs, S., Jr.; Livermore, D.; Seitz, M. G. Rev. Mucromol. Chem. Phys. 1985,25, 599-657. (3) Tipping, E. Radiochim. Acta 1993, 62, 141-152. (4) Manning, P. G.; Ramamoorthy, S.J. Inorg. Nucl. Chem. 1973,35, 1577-1581. (5) Powell, H. K. J.; Town, R. M.Anal. Chim.Acta 1991,248,95- 102. (6) Buffle, J. Anal. Chim. Acta 1980, 118,29-44. (7) Van Loon, L. R.; Granacher, S.; Harduf, H.Anal. Chim.Acta 1992, 268, 235-246. (8) Glaus, M. A.; Hummel, W.; Van Loon, L. R. Anal. Chim. Acta 1995, 303, 321-331. (9) Sigel, H. In Metal Ions in Biological Systems, Volume 2: Mixed-
Ligand Complexes;Sigel,H., Ed.; Marcel Dekker, Inc.: New York, 1973. (10) Brighli, M.; Lagrange,1.; Lagrange, P. Polyhedron 1984,3,469-
474. (11) Scheidt, W. R.; Countryman, R.; Hoard, J. L. 1.Am. Chem. SOC. 1971, 93, 3878-3882. (12) Sillen, L. G.; Martell, A. E. Stability Constants of Metal-Ion
Complexes;Special Publication 25; Chemical Society: London, 1971.
(13) Martell, A. E.; Smith, R. M. Critical Stability Constants, Volume 5,Second Supplement; Plenum Press: New York, 1982. (14) Smith, R. M.; Martell, A. E. Critical Stability Constants, Volume 6, Second Supplement; Plenum Press: New York, 1989. (15) Grenthe, I.; et al. In Chemical Thermodynamics of Uranium; Wanner, H., Forest, I., Eds.;NEA-OECD: Elsevier: Amsterdam, 1992. (16) Van Loon, L. R.; Kopajtic, Z. Rudiochim. Acta 1991,54,193-199. (17) DeWitt, R.; Watters, J. 1.1. Am. Chem. SOC.1954, 76, 3810-3814.
(18) Stevenson, F. J, Humus Chemistry;Wiley-Interscience: New York, 1982. (19) Schindler, P. W. InReuiews in Mineralogy,Volume23: MineralWater Interface Geochemistry Hochella, M. F., Jr.; White, A. F., Eds.; Mineralogical Society of America: Washington, DC, 1990.
Received for review January 17, 1995. Revised manuscript received May I, 1995. Accepted May 25, 1995. ES950024J
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