Potentiometric determination of cadmium–acetate complexation in aqueous solutions to 250°C

Chemical Geology 167 Ž2000. 11–24 www.elsevier.comrlocaterchemgeo

Potentiometric determination of cadmium–acetate complexation in aqueous solutions to 2508C ) Pascale Benezeth , Donald A. Palmer ´ ´ Chemical and Analytical Sciences DiÕision, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6110, USA Received 15 December 1998

Abstract The molal formation quotients of cadmium–acetate complexes were measured potentiometrically Žhydrogen-electrode concentration cell. from 50 to 2508C at ionic strengths of 0.1, 0.3, and 1.0 molal in aqueous sodium trifluoromethanesulfonate ŽNaTr. media. Two cadmium–acetate species, namely CdŽAc.qand CdŽAc. 28, were identified by regressional analysis of the data. Their formation quotients were modeled by empirical equations to describe their temperature and ionic strength dependencies. The thermodynamic quantities, obtained by differentiating these equations with respect to temperature at 258C and at infinite dilution are for CdŽAc.q: log K 1 s 1.96 " 0.24, D H8 s y17 " 17 kJ P moly1 , D S8 s y21 " 53 J P Ky1 P moly1 and DCp8 s 660 " 370 J P Ky1 P moly1 ; and for CdŽAc. 28: log K 2 s 3.15 " 0.10, D H8 s 4 " 14 kJ P moly1 , D S8 s 75 " 47 J P Ky1 P moly1 and DCp8 s y400 " 260 J P Ky1 P moly1. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Potentiometry; Cadmium; Acetate; Formation quotients; Thermodynamic properties

1. Introduction Cadmium is a toxic metal that belongs to group IIb in the Periodic Table, together with zinc and mercury. It is a relatively rare element and is not found in the pure state in nature. Commercial production only became significant at the beginning of this century and over 65% of the cumulative world production has taken place in the last two decades ŽWilson, 1988.. Cadmium is used in a large variety of consumer and industrial materials Že.g., protective

) Corresponding author. Tel.: q1-865-574-4960; fax: q1-865574-4961. E-mail address: [email protected] ŽP. Benezeth ´ ´ ..

plating on steel; stabilizers for polyvinyl chloride; pigments in plastics and glasses; electrode material in nickel–cadmium batteries; and a component of various alloys.. Because of its many uses in industry, significant amounts of cadmium may be released to the environment. Cadmium is widely distributed in the Earth’s crust at an average concentration of about 0.1 mg P kgy1 ŽHeinrichs et al., 1980.. High levels may accumulate in sedimentary rocks; marine phosphates often contain about 15 mg P kgy1 ŽGESAMP, 1984.. Since cadmium is closely related to Zn, it is mainly found in Zn, Pb–Zn and Pb–Cu–Zn ores Že.g., Mississippi Valley Type ore deposits., where high cadmium soil concentrations are more commonly found. Weathering also results in the riverine transport of large quantities of cadmium to the

0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 9 7 - 7

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

12

world’s oceans and this represents a major flux of the global cadmium cycle. An annual gross input of 15,000 tons has been estimated ŽGESAMP, 1987.. The behavior of cadmium in surface water, soil and groundwater was studied extensively not only because of its high toxicity, but also because it has been recognized as being representative of other elements, in particular Zn, Co and Cu that are common for example in coal combustion residues. Therefore, knowledge of cadmium behavior in aqueous systems has important environmental and geological consequences. Its availability and mobility in natural aqueous systems is not simply controlled by the total dissolved concentrations, but rather related to the chemical species present in the solution, i.e., as a free or hydrolyzed ion, and inorganic and organic complexes. Acetate, which is the most abundant organic acid found in oil field brines Žup to 10,000 ppm, Willey et al., 1975; Carothers and Kharaka, 1978. and many geothermal systems, may be an effective agent in the migration of cadmium in natural fluids. Numerous experimental studies of metal– acetate complexes have been reported in the literature, demonstrating that acetate complexes could transport significant amounts of metals such as Al ŽFein, 1991; Benezeth et al., 1994; Palmer and Bell, ´ ´ 1994., Zn ŽHennet et al., 1988; Yang et al., 1989; Giordano and Drummond, 1991., Fe ŽPalmer and Drummond, 1988a; Palmer and Hyde, 1993., Mg ŽSemmler et al., 1990; Fein, 1991., Ca ŽFein, 1991; Seewald and Seyfried, 1991. and Pb ŽHennet et al., 1988; Giordano, 1989; Yang et al., 1989. and more recently rare earth elements, La, ŽDeberdt et al., 1998. and Nd ŽWood et al., 1999.. However, few attempts have been made in the past to measure the association constants of cadmium–acetate over a wide range of temperature and ionic strength. Shock and Koretsky Ž1993. have made

a theoretical prediction of stability constants for cadmium–acetate complexes up to 3508C and 2 kbar, which is considerably different from the only experimental study available over a range of temperature, that of Choudhary and Prasad Ž1975.. This paper presents the first experimental stability constants for cadmium–acetate complexes as a function of temperature and ionic strength.

2. Experimental 2.1. Materials Cadmium trifluoromethanesulfonate, CdŽCF3 SO 3 . 2 or ŽCdTr2 ., solutions were prepared by dissolving a known amount of cadmium oxide powder ŽEM Science, lot 6101 and Aldrich, lot 24,478-3. in 1.308 molal trifluoromethanesulfonic acid ŽHCF3 SO 3 or HTr.. This solution was diluted with deionized water to produce a stock solution of approximately 0.05 molal of cadmium. The total cation ŽCdŽII.. equivalence of the stock solution was measured by passing a know mass of solution through a cation-exchange column ŽDower 50 WX8-100 ion-exchange resin, 8% cross linking, 50–100 dry mesh. in the hydrogen form and titrating the effluent to neutral pH with standardized NaOH stock solution. The sodium trifluoromethanesulfonate, NaCF3 SO 3 , ŽNaTr., was prepared by neutralization of the acid with a concentrated NaOH solution as described earlier ŽPalmer and Drummond, 1988b. and recrystallized twice from hot ethanol. Stock solutions of both HTr and NaTr were titrated directly and after passing through a cation-exchange column, respectively. Glacial acetic acid ŽMallinckrodt, lot 3121. was used without purification. A stock solution of diluted acetic acid ŽHAc. was prepared and analyzed

Table 1 Summary of solution compositions for Cd–acetate runs Ionic strength

0.1 0.3 1.0

Reference

Test

3

10 m HT r

m NaTr

2.000 2.000 2.000

0.0980 0.2980 0.9982

3

Titrant

10 m CdTr2

3

10 m HTr

m NaTr

m HAc

m NaOH

m NaTr

5.000 5.000 5.001

2.000 1.989 1.991

0.0829 0.2849 0.9849

0.1100 0.2211 0.4999

0.0999 0.2009 0.4499

0.0499 0.1608 0.6499

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

13

Table 2 Summary of solution compositions for Cd–acetate runs without titration Žsee text. Ionic strength

0.1

Reference

Test

m HA c

m NaTr

m NaOH

m HAc

10 3 m CdTr2

10 3 m HTr

m NaTr

10 3 m NaOH

0.09816

0.09734

0.003035

0.1008

5.009

2.000

0 .0829

3.005

by direct titration. The total concentration of cadmium used in all the experiments was kept constant at 5 millimolal. The solution compositions for the test, reference and titrant are summarized in Tables 1 and 2.

in the HECC. The test and reference solution compositions for these runs are reported in Table 2. No titrant was added to the test solution so that only one acetate concentration was investigated at each temperature.

2.2. Equipment and procedure The hydrogen-electrode concentration cell ŽHECC. has been described in numerous publications Že.g., Mesmer et al., 1970; Kettler et al., 1991; Palmer and Hyde, 1993; Benezeth et al., 1997.. The ´ ´ temperature was controlled to within "0.028C using a waterrethylene glycol-filled bath for the experiments performed at 508C and an aluminum block tube furnace for higher temperatures. The cell configuration for the runs performed with the stoichiometric molal concentrations reported in Table 1 was as follows: H 2 , Pt



H 2 , Pt

whereas the titrant consisted of NaOH, HAc and NaTr. NaTr was used as a non-complexing supporting electrolyte to maintain constant ionic strength. It is a stable and strong electrolyte and due to the delocalized charge on this large anion, interactions with metal ions are weak even at high temperature ŽScott and Taube, 1971; Fabes and Swaddle, 1975.. Moreover, NaTr has been successfully used in previous metal–acetate complexation ŽPalmer and Drummond, 1988a,b; Giordano and Drummond, 1991.. Note that chloride forms strong complexes with cadmium Že.g., Hahne and Kroontje, 1973; Palmer et al., unpublished results. such that NaCl is a less desirable supporting electrolyte. In addition, four independent high temperature experiments were carried out at ca. 0.1 mol P kgy1 ionic strength Ž150–2508C.

3. Results and discussion The method applied in the present study relied on the competition between Hq and Cd 2q for association with acetate anions. Thus, measurement of pH m Žnote that in the present study, pH m s ylogw Hqx. provides an accurate measure of cadmium–acetate association. The hydrogen ion molality w Hqx in the test solution can be calculated from the Nernst expression: ylog w Hq x test s

2.303F RT

Ž E q Elj . y log w Hq x ref Ž 1.

where w Hqx test and w Hqx ref refer to the stoichiometric molalities of hydrogen ions in the test and reference compartments, respectively. The ideal gas and Faraday constants are designated by R and F, respectively; T denotes the temperature in Kelvin. E and E lj represent the measured potential and calculated liquid junction potential, respectively. The value of E lj was calculated according to the Henderson equation ŽBaes and Mesmer, 1976., which involves the limiting conductivities of the individual ions. The limiting molar conductances Ž l8. of Naq, Hq and OHy were taken from Quist and Marshall Ž1965., that for Try was taken from Ho and Palmer Ž1995., whereas the following assumptions were made for the other minor component ions involved: l8 Acy4

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

14

s l8 Cly. , l8 CdŽAc.q4 s l8 Naq4 , and l8 Cd 2q 4 s l8 Mg 2q 4 . The calculated liquid junction potentials were less than 1.8 mV, which gives an uncertainty in the logarithms of the association quotients of "0.005, assuming that the Henderson equation predicts the value of Elj to within 25% ŽMesmer, 1991.. Hydrogen ion and free acetate molalities in the test cell were calculated at each point in the titration using an iterative method involving Eq. Ž1. and the dissociation quotients of HAc and water, and a stepwise refinement for the liquid junction potential and the ionic strength. The values for the dissociation quotients of HAc and water in NaTr media were taken from Palmer and Bell Žunpubl. data. and Palmer and Drummond Ž1988b., respectively. The complexing equilibria investigated in this study are described by the following general equation: 2y n

Cd 2qq nAcy° Cd Ž Ac . n

Ž 2.

with: 2y n

Qn s

Cd Ž Ac . n

w Cd 2q xw Acy x

n

.

Ž 3.

The degree of association of cadmium is defined in terms of average ligand number, n, calculated from the following expressions: n

Ýn

y

ns

m HA c y w HAc x y w Ac x m CD 2q

s

2y n

Cd Ž Ac . n

1

m Cd 2q

Ž 4. where m HA c represents the stoichiometric concentration of acetic acid, wHAcx and wAcyx are the calculated acetic acid and free acetate concentrations,

Fig. 1. Average ligand number, n versus the logarithm of the acetate ion molality for ionic strengths of Ža. 0.1, Žb. 0.3, and Žc. 1.0 at each temperature investigated in this study. The symbols represent the experimental values Ž n observed., whereas the solid line represents the calculated values from the regression.

respectively, and m Cd 2q represents the total stoichiometric metal concentration.

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

The values of n determined in a given titration were regressed with the ORGLS non-linear leastsquares program ŽBusing and Levy, 1962. to determine cadmium speciation and formation constants according to: n

Ý nQn wCd 2q xwAcy x ns

1

m Cd 2q

n

.

Ž 5.

Two cadmium–acetate complexes were identified by the fitting procedure as CdŽAc.q and CdŽAc. 28. Examples of titration curves obtained are shown in Fig. 1 as n obs versus the logarithm of free acetate ion in solution at 0.1 Ža., 0.3 Žb. and 1.0 Žb. molal ionic strength. The association quotient values determined by the regression are listed in Table 3 together with their uncertainties, which represent three times the standard deviation Ž s ., and are reported in Figs. 2a–b and 3a–b as a function of the reciprocal of temperature ŽK. and the square root of ionic strength, respectively.

15

For some of the experiments performed with the stoichiometric concentrations reported in Table 1, small n values were obtained and therefore the values of Q1 and Q2 could not be determined accurately and unambiguously in a single regression. For these specific titrations, a fit with only CdŽAc.q overestimated values of Q1 , indicating that CdŽAc. 28 species cannot be neglected in the regression analyses. This is demonstrated by Fig. 4 where the values of n obs are reported for a run performed at 758C and 0.1 molal ionic strength. As can be seen in this figure, the values calculated with only CdŽAc.q Ždashed curve. overestimated the low n obs values and did not fit the highest ones. Consequently, the data analysis adopted involved first fitting the values of Q2 obtained from a regression of the data where both log Q1 and logQ 2 were accessible, then fixing the Q 2 values obtained from the fit Žbold values in Table 3. in the subsequent regression of the titration results for which only the values of log Q1 could be determined. The result of this fitting procedure is shown in Fig. 4 by the solid line, which confirmed that the presence of CdŽAc. 28 complex yield a substantially better description of the experimental data, especially at high n values.

Table 3 Measured stability quotients for Cd–acetate complexes as a function of temperature and ionic strength T Ž8C.

49.7 49.8 74.7 75.0 99.7 100.2 125.0 125.4 140.2 149.6 149.8 149.8 150.0 150.3 150.7 199.8 249.9 a

0.1 m

0.3 m

logQ1

log Q2

1.45 " 0.02 1.38 " 0.04 1.57 " 0.01 1.45 " 0.02 1.59 " 0.03 1.53 " 0.02 1.83 " 0.01

2.71 " 0.07 2.70 " 0.04 a 2.82 " 0.04 2.87 " 0.04 a 3.11 " 0.04 a 3.12 " 0.03 3.48 " 0.02

2.18 " 0.04 2.24 " 0.02 ) b 2.23 " 0.02 ) b

2.14 " 0.01 2.61 " 0.01) b 3.10 " 0.01) b

log Q1

1.0 m logQ2

1.44 " 0.01

2.62 " 0.04

1.52 " 0.02

2.83 " 0.05

1.73 " 0.01 2.03 " 0.02

2.98 " 0.03 3.31 " 0.08

2.03 " 0.02 2.04 " 0.02

3.39 " 0.04 a 3.39 " 0.04 a

log Q1

logQ2

1.33 " 0.03

2.35 " 0.06

1.34 " 0.03

2.36 " 0.04

1.48 " 0.02 1.76 " 0.02 1.71 " 0.02 1.92 " 0.02

2.41 " 0.05 2.50 " 0.02 2.55 " 0.02 2.64 " 0.20

3.78 " 0.04 a

3.80 " 0.04 a

Constants fixed from the smoothed values ŽEq. 6. in the fitting procedure. Asterisk represents the values extracted from the second set of experiments ŽTable 2. Žsee text..

b

16

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

where polynuclear species are formed in small amounts Ž n - 0.02, Baes and Mesmer, 1976. before the precipitation of cadmium hydroxide. No data on cadmium hydrolysis are available at temperatures higher than 258C, but hydrolysis and precipitation may have occurred at pH m - 7 as the temperature increases and may be the reason of the drift observed at 1408C or 1508C.

Fig. 2. Experimental log Q1 Ža. and log Q2 Žb. for the formation of CdŽAc.q and CdŽAc. 2 8, respectively, as a function of reciprocal temperature ŽK. at each ionic strength investigated. The curves were computed from Eqs. Ž6. and Ž7.. The symbols define the measured values taken from Table 3 except for the diamonds, which represent the values from Archer and Monk Ž1964. at 258C and zero ionic strength.

At 1408C and 1508C, titrations were terminated when the cell potential drifted rapidly to smaller values following small additions of titrant, generally for a pH m greater than 5. The drift may correspond to precipitation of cadmium hydroxide from solution. Biedermann and Ciavatta Ž1962. have shown that hydrolysis of cadmium according to the reaction: Cd 2q H 2 O ° CdŽOH.qq Hq, becomes significant above pH m s 7 at 258C in concentrated solution,

Fig. 3. Experimental logQ1 Ža. and log Q2 Žb. versus the square root of ionic strength where the curves result from Eqs. Ž6. and Ž7.. The symbols define the measured values taken from Table 3 except for the diamonds, which represent the values from Archer and Monk Ž1964. at 258C and zero ionic strength.

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

17

Shock and Koretsky Ž1993. for Cd–acetate or for Fe–acetate ŽPalmer and Drummond, 1988a. and Zn–acetate ŽGiordano and Drummond, 1991.. All of the equilibrium quotients were regressed as a function of temperature and ionic strength using the ORGLS general least squares program incorporating the literature values at infinite dilution and 258C from Archer and Monk Ž1964. Žlog K 1 s 1.93 and log K 2 s 3.15. to help constrain the fit at low temperature and the extrapolation to infinite dilution. The empirical equations that best described the data are: logQ1 s y Fig. 4. Average ligand number, n versus the logarithm of the acetate ion molality for 0.1 ionic strength at 758C. The symbols represent the experimental values Ž n observed., whereas the dashed line represents a fit performed with only the 1:1 complex and the solid line represents the calculated values obtained by considering both 1:1 and 1:2 complexes in the fitting procedure.

=

4 Aw ln Ž 10 .

½

'I 1 q b'I

a q

ž / b

ln Ž 1 q b'I .

5

y 444.138 q 16,313.13rT q 72.22ln Ž T . y 0.06747T q 0.00382 Ž FI . T

In the case of the four experiments conducted at 0.1 mol P kgy1 ionic strength from 1508C to 2508C ŽTable 2., only one acetate concentration could be investigated and hence it was impossible to extract more than one formation quotient from each experiment. Data treatment was based on solving the acetate balance equation, which yielded n values ranging from 0.3 Ž1508C. to 0.53 Ž2508C.. These values implied that the solutions contained predominantly Cd 2q and CdŽAc.q with a minor contribution from CdŽAc. 28. Note that the pH was approximately 3.2 so that hydrolysis could be effectively ignored, as supported by the fact that no precipitation occurred. However, in order to account for the minor contribution of CdŽAc. 28, the ratio of Q 2rQ1 observed for the corresponding CdŽCl. 28 and CdŽCl.q complexes ŽPalmer et al., unpublished results. was imposed on the acetate complexes in an iterative manner until final Q1 values for CdŽAc.q were obtained. Note that if the CdŽAc. 28 species were completely neglected in the data treatment, then the resulting Q1 values were 0.1 to 0.15 log units larger than those reported in Table 3 Žasterisk.. This correlation was also supported by the fact that same Q 2rQ1 ratio used above from 1508C to 2508C are in good agreement with the ratio that can be calculated from

logQ2 s y

=

Ž 6.

6 Aw ln Ž 10 .

½

'I 1 q b'I

a q

ž / b

ln Ž 1 q b'I .

5

y 17.058 q 2896.13rT q 0.03521T y Ž 2.4485.10y6 . IT 2 q 636.243 Ž FIrT .

Ž 7. with FI s 1.0 y expŽy2.0'I .Ž1.0 q 2.0'I .. The first term in Eqs. Ž6. and Ž7. represents the extended Debye–Huckel expression used in the ion¨ interaction model ŽPitzer, 1973. Žwith a and b assigned values of 2.0 and 1.2, respectively, and A w taken from Bradley and Pitzer, 1979.. The next four terms in Eq. Ž6. and three terms in Eq. Ž7. define the equilibrium constant K n at infinite dilution, whereas the remaining terms account for the deviation from the Debye–Huckel limiting slope. ¨ The goodness of fit to the data for the formation of the CdŽAc.q and CdŽAc. 28 species are illustrated in Figs. 2 and 3. As previously demonstrated for many metal carboxylate complexes, the stability of

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

18

the cadmium–acetate interaction increases with temperature Žexcept for logQ1 from 25–508C.. Conversely, as the ionic strength increases, complexation becomes weaker with the effect being greater for the second association quotient compared to the first one. This effect is due to charge neutralization, which is greater for the diacetato complex. Differentiation of Eqs. Ž6. and Ž7. with respect to temperature yields the thermodynamic quantities D H, D S and DCp for the formation of CdŽAc.q and CdŽAc. 28 species, respectively. These values are re-

ported in Tables 4 and 5 for selected temperatures and ionic strengths, together with the corresponding association quotients calculated from Eqs. Ž6. and Ž7.. These parameters show that the increased stability with temperature is due to the dominant effect of the entropy change to more positive values, which outweighs the positive change in enthalpy values. This behavior is typical for association reactions in which electrostricted water is released ŽMesmer et al., 1991.. Association also increases as the dielectric constant of water decreases and similarly, lowering

Table 4 Thermodynamic quantities for the formation of CdŽAc.q at saturation vapor pressure. The uncertainties listed represent three times the standard deviation Ž s . T Ž8C.

log Q1

25 50 75 100 125 150

1.94 " 0.12 1.83 " 0.08 1.90 " 0.09 2.08 " 0.09 2.34 " 0.09 2.63 " 0.09

25 50 75 100 125 150

1.57 " 0.12 1.45 " 0.07 1.50 " 0.07 1.66 " 0.07 1.88 " 0.06 2.14 " 0.07

25 50 75 100 125 150

1.48 " 0.14 1.37 " 0.08 1.42 " 0.07 1.57 " 0.05 1.79 " 0.05 2.03 " 0.06

25 50 75 100 125 150

1.45 " 0.15 1.34 " 0.09 1.39 " 0.07 1.55 " 0.06 1.76 " 0.06 2.00 " 0.08

25 50 75 100 125 150

1.41 " 0.17 1.31 " 0.12 1.36 " 0.10 1.52 " 0.10 1.73 " 0.10 1.96 " 0.12

D H1 ŽkJ P moly1 . I s 0.0 y15 " 11 y0.5 " 6 12 " 3 24 " 4 33 " 4 41 " 4 I s 0.1 y14 " 11 0.6 " 5 14 " 3 25 " 4 35 " 5 43 " 4 I s 0.3 y13 " 11 2"5 15 " 3 27 " 4 37 " 5 45 " 4 I s 0.5 y12 " 11 3"5 16 " 3 28 " 4 38 " 5 47 " 4 I s 1.0 y11 " 11 4"5 18 " 3 30 " 4 40 " 5 49 " 5

D S1 ŽJ P Ky1 P moly1 .

DCp1 ŽJ P Ky1 P moly1 .

y13 " 35 34 " 18 72 " 9 104 " 10 129 " 11 148 " 10

610 " 270 550 " 210 480 " 140 420 " 90 350 " 60 290 " 80

y17 " 35 30 " 17 68 " 9 99 " 10 124 " 11 143 " 10

620 " 270 550 " 210 490 " 140 430 " 90 360 " 60 300 " 80

y15 " 34 32 " 17 71 " 8 102 " 10 127 " 11 146 " 11

630 " 270 560 " 210 500 " 150 440 " 90 370 " 60 310 " 80

y13 " 34 34 " 16 73 " 8 105 " 10 130 " 11 149 " 11

630 " 270 570 " 210 500 " 150 440 " 90 380 " 60 320 " 90

y10 " 33 38 " 16 77 " 8 109 " 10 134 " 13 154 " 13

640 " 270 580 " 210 510 " 150 450 " 90 390 " 60 330 " 90

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

19

Table 5 Thermodynamic quantities for the formation of CdŽAc. 2 8 at saturate vapor pressure. The uncertainties listed represent three times the standard deviation Ž s . T Ž8C.

logQ 2

25 50 75 100 125 150

3.15 " 0.10 3.28 " 0.15 3.52 " 0.19 3.84 " 0.16 4.23 " 0.10 4.68 " 0.18

25 50 75 100 125 150

2.63 " 0.15 2.70 " 0.08 2.87 " 0.12 3.12 " 0.10 3.43 " 0.08 3.78 " 0.20

25 50 75 100 125 150

2.53 " 0.27 2.54 " 0.13 2.65 " 0.09 2.84 " 0.07 3.09 " 0.09 3.39 " 0.21

25 50 75 100 125 150

2.49 " 0.34 2.46 " 0.17 2.54 " 0.11 2.69 " 0.08 2.90 " 0.08 3.15 " 0.19

25 50 75 100 125 150

2.40 " 0.41 2.30 " 0.21 2.31 " 0.12 2.39 " 0.08 2.53 " 0.07 2.70 " 0.15

D H2 ŽkJ P moly1 . I s 0.00 5 " 14 15 " 8 26 " 3 38 " 8 51 " 17 65 " 25 I s 0.1 3 " 15 13 " 9 24 " 3 36 " 7 49 " 15 63 " 24 I s 0.3 0.08 " 16 10 " 10 21 " 5 33 " 6 46 " 14 59 " 22 I s 0.5 y2 " 18 8 " 12 19 " 6 31 " 6 43 " 13 57 " 21 I s 1.00 y5 " 20 5 " 14 15 " 9 26 " 7 38 " 11 51 " 18

the ionic strength favors increased ion association and complexation. The heat capacity changes shown in Tables 4 and 5 are slightly larger for the diacetato complex because of the greater charge neutralization associated with their formation.

4. Comparison with literature As can be seen in Table 6, some measurements of the association of cadmium–acetate have been made near room temperature at different ionic strengths,

D S2 ŽJ P Ky1 P moly1 .

DCp 2 ŽJ P Ky1 P moly1 .

75 " 48 109 " 26 143 " 9 177 " 21 210 " 41 244 " 63

400 " 260 440 " 280 470 " 300 500 " 320 540 " 350 570 " 370

59 " 49 92 " 28 125 " 10 157 " 19 189 " 39 221 " 60

400 " 260 430 " 280 470 " 300 500 " 320 530 " 340 570 " 360

42 " 55 81 " 30 112 " 13 144 " 17 175 " 36 205 " 56

390 " 250 430 " 270 460 " 290 490 " 310 520 " 330 560 " 360

42 " 53 73 " 33 104 " 17 135 " 15 165 " 33 194 " 53

390 " 250 420 " 270 450 " 290 480 " 310 520 " 330 550 " 350

28 " 60 58 " 41 88 " 25 116 " 18 145 " 28 172 " 45

380 " 240 410 " 260 440 " 270 460 " 290 490 " 310 520 " 330

but with poor agreement between the different studies. For the same temperature and ionic strength, these results are generally in disagreement with the values obtained in this study, as reported in parenthesis in Table 6, except for the values of Kolat and Powell Ž1962., at 0.1 molal ionic strength. Excellent agreement was obtained with their values at infinite dilution of Archer and Monk Ž1964.. The fit excluding their values yielded log K 1 s 2.07 and log K 2 s 3.27, which are only 0.1 log units larger than the values reported in Tables 4 and 5, but are within the combined estimated uncertainties. Shock and Koret-

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

20

Table 6 Experimental literature data for the formation of Cd–acetate complexes. Values from this study are in parenthesis log Q1

log Q2

I ŽMedium.

Method

T Ž8C.

Reference

1.70 1.75 1.70 1.43 Ž1.62. 1.20 Ž1.51. 1.30 Ž1.44. 1.50 Ž1.57. 1.61 Ž1.62. 1.93 Ž1.94. 0.7 " 0.2 Ž1.37. 1.27 " 0.02 1.19 " 0.01 Ž1.45. 1.18 " 0.02 Ž1.41. 1.23 " 0.02 1.32 " 0.04 1.26 " 0.04 Ž1.41. 1.62 1.54 1.49 1.45 1.41 1.40 1.39

– 2.75 – – – – – 2.68 Ž2.63. 3.15 Ž3.15. 1.4 " 0.2 Ž2.37. 2.00 " 0.08 1.90 " 0.03 Ž2.49. 1.82 " 0.03 Ž2.40. 1.98 " 0.03 2.32 " 0.06 – 3.93 3.83 3.76 3.69 3.64 3.59 3.55

- 0.55 ? 0 0.2 0.2 0.2 0.1 0.1 ŽNaClO4 . 0 1.0 ŽNaClO4 . 0.25 ŽNaClO4 . 0.50 ŽNaClO4 . 1.0 ŽNaClO4 . 2.0 ŽNaClO4 . 3.0 ŽNaClO4 . 1.0 ŽNaClO4 . 0 0 0 0 0 0 0

Glass electrode Glass electrode Solubility Polarography Polarography Polarography Glass electrode Glass electrode Glass electrode Extraction Potentiometry Potentiometry Potentiometry Potentiometry Calorimetry Kinetic Potentiometry Potentiometry Potentiometry Potentiometry Potentiometry Potentiometry Potentiometry

20 30 25 15 25 35 25 20 25 30 25 25 25 25 25 25 5 10 15 20 25 30 35

Ferrell et al., 1934 Aditya and Prasad, 1953 Bardhan and Aditya, 1955 Tanaka et al., 1960 Tanaka et al., 1960 Tanaka et al., 1960 Yasuda et al., 1960 Kolat and Powell, 1962 Archer and Monk, 1964 Hellwege and Schweitzer, 1965 Gerding, 1968 Gerding, 1968 Gerding, 1968 Gerding, 1968 Gerding, 1968 Hutchinson and Higginson, 1973 Choudhary and Prasad, 1975 Choudhary and Prasad, 1975 Choudhary and Prasad, 1975 Choudhary and Prasad, 1975 Choudhary and Prasad, 1975 Choudhary and Prasad, 1975 Choudhary and Prasad, 1975

sky Ž1993. also used the experimental data from Archer and Monk Ž1964. to estimate the stability constants of cadmium–acetate complexes up to 3508C and 2 kbar. A comparison of equilibrium constants for cadmium–acetate complexes obtained in this study with corresponding values previously reported in the literature Žat zero ionic strength, Table 6. is given as a function of the reciprocal of temperature in Fig. 5. For K 1 , the values from this study are in relatively good agreement with the predictions of Shock and Koretsky Ž1993., but differ by ; 0.5 log units for K 2 inside the experimental range represented by the solid line in Fig. 5. It should be noted that their predictions were based solely on one set of experimental values at 258C. The standard state properties for the 1:1 and 1:2 complexes were calculated from the thermodynamic quantities reported in Tables 4 and 5, together with the thermodynamic properties for Cd 2q and acetate ions from Shock and Helgeson Ž1988. and Shock and Helgeson Ž1990., respectively. The values obtained are listed in Table 7 and compared to the standard state properties of cadmium–acetate complexes reported by Shock and Koretsky Ž1993.. It

can be seen from this table that the entropy differs by 27 J P Ky1 P moly1 and up to 72 J P Ky1 P moly1

Fig. 5. Comparison with literature data of log K n values versus reciprocal temperature ŽK.. The solid curves were generated from Eqs. Ž6. and Ž7., whereas the dotted lines represent extrapolations outside the measured temperature range. The symbols represent the literature values from Table 6 and the dashed curves are the predicted values from Shock and Koretsky Ž1993..

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

21

Table 7 Comparison of the standard state properties of CdŽAc.q and CdŽAc. 2 8 species determined in this study with the values reported by Shock and Koretsky Ž1993. 2q

Cd Acy CdŽAc.q CdŽAc. 2 8

D f G 298.158 ŽkJ P moly1 .

D f H298.158 ŽkJ P moly1 .

S298.158 ŽJ P Ky1 P moly1 .

Reference

y78 y369 y458 y458 y834 y834

y76 y486 y569 y577 y1065 y1043

y73 86 28 0.4 103 175

Shock and Helgeson Ž1988. Shock and Helgeson Ž1990. Shock and Koretsky Ž1993. This study Shock and Koretsky Ž1993. This study

for the 1:1 and 1:2 complexes, respectively, compared to the corresponding values of Shock and Koretsky. However, the uncertainties assigned to the thermodynamic properties in Tables 4 and 5 are large Ž3 s . at 258C, because of the lack of precise experimental measurements at low temperatures. The values for different divalent metal–acetate formation constants are shown in Fig. 6 as a function of the reciprocal temperature. It is obvious from this figure that the stability constants for divalent metals–acetate complexes at 258C follow the order Hg ) Pb ) Zn ( Cd ) Fe ( Mn with similar temperature dependencies. The formation constants for the same divalent

metals with chloride are shown in Fig. 7. At 258C the following order in the stability constants is Hg ) Cd ) Pb ) Zn ) Fe ) Mn, while at higher temperatures the order is Hg ) Zn ) Cd ( Pb ) Mn ) Fe. The zinc–acetate complexes are only slightly more stable than the cadmium complexes ŽFig. 6., which reflects their similar periodic group behavior. However, at low temperature cadmium forms stronger complexes with chloride than does zinc ŽFig. 7.. Although zinc, cadmium and mercury belong to the same group of elements in the periodic classification, mercury, which is more toxic than cadmium, exhibits different properties. Mercury–acetate and chloride

Fig. 6. Log K n values versus reciprocal temperature ŽK. for different divalent metal acetate complexes. The values for Fe– acetate complexes are from Palmer and Drummond Ž1988a. and Giordano and Drummond Ž1991. for ZnAc, Shock and Koretsky Ž1993. for Pb–, Mn– and Hg–acetate, and from this study for CdAc.

Fig. 7. Log K n values versus reciprocal of temperature ŽK. for different divalent metal–chloride complexes. The values for Fe–, Hg–, Pb–, Zn–, and Cd–chloride complexes are from Sverjensky et al. Ž1997. and the values for Mn–chloride are taken from Gammons and Seward Ž1996..

22

P. Benezeth, D.A. Palmerr Chemical Geology 167 (2000) 11–24 ´ ´

complexes have the highest stability among the divalent metals displayed in Figs. 6 and 7 and have a different configuration. This tendency is also exhibited in the corresponding hydrolysis constants. At 258C, hydrolysis of HgŽII. becomes important at pH values above 2, whereas PbŽII., ZnŽII., and CdŽII. hydrolyze above pH 5, 7, and 8, respectively ŽHahne and Kroontje, 1973..

5. Conclusions The association quotients of 1:1 and 1:2 cadmium–acetate complexes were measured by potentiometry in NaTr media for the first time over a wide range of temperature and ionic strength. The empirical equations and thermodynamic properties generated from these results provide a model for the behavior and speciation of cadmium in acetatebearing solutions Žas a model of other organic ligands.. Complexation of Cd 2q by thermally stable organic ligands may influence its mobility, bioavailibility and hence its toxicity in natural systems. Some recent studies related to the treatment and removal of heavy metals from aqueous solutions and industrial wastewaters by adsorption show that the presence of organic ligands such as EDTA, citrate and acetate, decreased the adsorption of cadmium onto metal oxides due to the competition between the ligand and oxide surface for the complexation of metal ion Že.g., Boily and Fein, 1996; Namasivayam and Ranganathan, 1998. and thereby enhance cadmium mobility.

Acknowledgements This research was sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation. We are grateful to David Wesolowski ŽORNL. for his comments and review of this paper. The authors wish to thank Jeremy B. Fein and an anonymous reviewer for their helpful comments, and suggestions for improvement of the manuscript.

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