Potentiometric study of Gd– and Yb–acetate complexing in the temperature range 25–80°C

Chemical Geology 167 Ž2000. 75–88 www.elsevier.comrlocaterchemgeo

Potentiometric study of Gd– and Yb–acetate complexing in the temperature range 25–808C Samuel Deberdt ) , Sylvie Castet, Jean-Louis Dandurand, Jean-Claude Harrichoury L.M.T.G., Equipe de Geochimie, CNRSr UMR 5563 r OMP-UniÕersite´ Paul-Sabatier, 38 rue des Trente-Six Ponts, 31400 Toulouse, France ´ Received 17 November 1998

Abstract The stability of aqueous complexes formed by gadolinium ŽGd 3q . and ytterbium ŽYb 3q . aqueous species with the acetate ion ŽAcOy. was investigated at 258C, 408C, 608C and 808C, and saturated vapor pressure, using potentiometric measurements. In the large range of wAcOx TrwREEx T molal ratio investigated Ž1 to 120., the results are interpreted in terms of the formation of two complexes, REEAcO 2- and REEŽAcO. 2- Žwhere REE denotes Gd or Yb.. The corresponding association constants b 11 and b 12 that we proposed are successfully used to predict high GdŽOH. 3 Žcr. solubilities measured in acetate containing solutions in this study. The data show an increasing stability of the REE–acetate complexes with increasing temperature. The regression of the obtained association constant values using either a linear equation or a simplified formulation of the revised HKF thermodynamic model allows the retrieval of thermodynamic properties for the aqueous complexes. Combining these results with those obtained with lanthanum in a previous study wDeberdt, S., Castet, S., Dandurand, J.-L., Harrichoury, J.-C., Louiset, I., 1998. Experimental study of LaŽOH. 3 and GdŽOH. 3 solubilities Ž25 to 1508C., and La–acetate complexing Ž25 to 808C.. Chem. Geol. 151, 349–372.x demonstrates that the stability of the REE–AcOy complexes increases from La to Gd, then decreases to Yb. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Rare earth elements; Gadolinium; Ytterbium; Acetate complexes; Potentiometry; Thermodynamic properties

1. Introduction Most of the recent studies on REE aqueous behavior are focused on the marine environment ŽGoldberg et al., 1963; Elderfield and Greaves, 1982; Elderfield et al., 1990; Klinkhammer et al., 1983; De Baar et al., 1985; Sholkovitz, 1988; Piepgras and Jacobsen, 1992; Bertram and Elderfield, 1993; Shimizu et al., ) Corresponding author. Tel.: q33-5-61-55-65-02; fax: q33-561-55-81-38. E-mail address: [email protected] ŽS. Deberdt..

1994, and references therein.. The shale-normalized REE patterns reported in these works always exhibit the same shape, indicating an enrichment in heavy REE ŽHREE. relative to light REE ŽLREE., which is usually explained by a preferential HREE–carbonate complexation. Studies on dissolved REE loads in rivers and groundwater ŽKeasler and Loveland, 1982; Goldstein and Jacobsen, 1988; Elderfield et al. 1990; Sholkovitz, 1995; Gaillardet et al., 1997; Viers et al., 1997; Braun et al., 1998; Zhang et al., 1998. are still scarce and incomplete. Nevertheless, it emerges from these studies that, in these waters, the REE have a

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 2 0 1 - 6

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S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

much more variable behavior than in seawater. Indeed, continental waters drain various geological terrains, and present larger ranges of suspended and dissolved load compositions than oceanic waters. Very different shapes of shale-normalized REE patterns result from these variations, which cannot be quantitatively interpreted without reliable thermodynamic data for the aqueous REE complexes. Among the various ligands encountered in surface waters, organic anions are always present, and it is now recognized that they form strong complexes with REE, which could control their behavior in organic rich waters ŽSholkovitz, 1995; Viers et al., 1997.. However, organic substances in surface and groundwaters are such complex compounds that it is necessary to use simpler anions, representative of the dominant natural functional groups, such as carboxylic acids, in order to study their interactions with metals. In a recent review on REE complexes with simple carboxylic acids, Wood Ž1993. points out the difficulty in extrapolating association constants to zero ionic strength reference state from literature data, which are generally obtained in concentrated media. He also emphasized the lack of data at temperatures above 258C, and concludes that there is need for experimental work at elevated temperatures. Our REE complexing study is motivated by all these observations. The present work follows a previous study ŽDeberdt et al., 1998b. in which LaŽOH. 3 Žcr. and GdŽOH. 3 Žcr. solubility experiments and La–acetate potentiometric measurements have allowed the determination of the thermodynamic properties for La3q, Gd 3q, and the La–acetate complexation constants from 258C to 808C. In the present work, we have adopted the same experimental procedures as in Deberdt et al. Ž1998b. to extend our acetate-complexing study to heavier REE: gadolinium and ytterbium.

2. Materials and methods 2.1. Gd(OH)3 (cr) solubility measurements in acetate solutions The solubility of GdŽOH. 3 Žcr. was measured at 408C and 608C in acetate solutions. The pH was

adjusted by NaOH and NH 4 OH addition. The solid phase used in these experiments is the same as that previously described in Deberdt et al. Ž1998b.. The experiments were carried out in polypropylene reactors immersed in water thermostated baths Ž"28C.. Magnetic stirrers were used to homogenize the experimental suspensions. The pHs were measured in situ by means of a glass combined electrode ŽSchott H-61. standardized on activity scale using DIN 19266rNBS standard solutions ŽpH s 4.006 and 6.865 at 258C.. Experimental solutions were kept under a vapor saturated nitrogen flow to prevent atmospheric CO 2 contamination. The sampled solutions were also kept under nitrogen flow until their acidification Ž2% HNO 3 .. Values of Gd concentration were measured by ICP-MS ŽPerkin-Elmer Elan 6000.. Further details on experimental and analytical procedures are given in Deberdt et al. Ž1998b..

2.2. Potentiometric experiments The experiments consist of titrating an AcOH– NaOH buffered solution Žmolal ratio of about 2. with an REECl 3 solution. The potential was measured at each step of the titration by means of a Hq selective electrode. The buffered solution was prepared using pure AcOH ŽMerck 100%, Proanalysis., NaOH ŽMerck titrisol 1N., and double deionized ŽMilli-Q w . and degassed water. Titrant solutions were prepared from reagent grade Aldrich Ž99.9%. GdCl 3 P 7H 2 O and YbCl 3 P 7H 2 O. The REE and chloride contents in titrant solutions were accurately measured by ICPMS ŽPerkin-Elmer Elan 6000. and ion chromatography ŽDIONEX 2000-I., respectively. Both determinations agree within "4%. Experiments were performed at 258C, 408C, 608C and 808C in a 60-ml, double-walled thermostated glass cell, regulated within "18C. The cover of the cell has three openings, which allowed Ž1. potential measurement by means of a Schott H-61 combined glass electrode connected to a Philips PW9422 digital millivoltmeter, Ž2. a continuous flow of vapor saturated nitrogen to prevent CO 2 contamination of the solution, and Ž3. injection of the titrant solution. The titrant solution was added using a Gilmont Ž65-1200A-2 ml. micrometer burette, and the solu-

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88 Table 1 Values of the dissociation constants of water Ž K w ., acids, bases and salts used in this study t Ž8C.

log K wa

log b K NaCl

log c K NaOH

log d K HCl

log e K AcONa

log f K AcOH

25 40 60 80

y13.995 y13.539 y13.028 y12.605

0.925 0.983 1.014 0.995

0.717 0.568 0.388 0.224

0.670 0.706 0.708 0.663

0.180 0.130 0.086 0.037

y4.756 y4.775 y4.820 y4.887

a

Johnson et al. Ž1992.. Shock et al. Ž1992.. c Ho and Palmer Ž1996.. d Ruaya and Seward Ž1987.. e Benezeth et al. Ž1994.. ´ ´ f Fisher and Barnes Ž1972.. b

tion was continuously homogenized with a magnetic Teflon w-coated stirring bar. The electrode was calibrated at each experimental temperature with DIN 19266rNBS ŽpH 258C s 4.006 and 6.865. and a 0.01m HCl ŽpH 258C s 2.04; Bates, 1973. ŽMerck titrisol. standard solutions. The electrode potentials were found to be linearly related Ž r ) 0.999. with the pH of the standard solutions at all temperatures. The slopes, S, are equal to y58.50, y61.24, y64.92 and y68.22 mVrpH at 258C, 408C, 608C and 808C, respectively, which agree with the corresponding theoretical Nernst slopes within 2.5%. The titrations were repeated twice and the measured potentials

77

agree within "0.2 mV Žabout 0.004 pH unit.. The emf stabilisation of the cell was obtained in less than 2 min after each injection of the titrant and remained constant for at least 1 h within "0.2 mV. The pH of the experimental solutions were calculated from the emf Ž E . according to: pH s Ž E o q Ej y E . rS where E o stands for the sum of the standard glass electrode potential and the total reference electrode potential, and Ej denotes the liquid junction potential. The value of E o depends only on temperature and was derived from electrode calibration; Ej was estimated from the Henderson equation ŽBates, 1973. and the limiting equivalent conductivities of ions ŽRobinson and Stokes, 1959.. The contributions of the REEAcO 2q and REEŽAcO.q 2 species to Ej were assumed to be equal to those of Mg 2q and Naq ŽOelkers and Helgeson, 1989., respectively. The Ej contribution to the total potential relative to the standard pH solutions was F "1.3 mV, and its absolute variation was less than 2.2 mV between the initial AcOH–NaOH and the final AcOH–NaOH– REECl 3 solutions in the temperature range 25–808C. Taking into account the uncertainties on the measured potential Ž"0.2 mV., on E o Ž"0.2 mV. and Ej Ž"20%., the mean uncertainty on the calculated pH is estimated to be "0.01 pH unit and the maximum uncertainty is less than "0.03 pH unit.

Table 2 GdŽOH. 3 Žcr. solubility measurements at 408C and 608C in AcO-bearing solutions Solution compositions and experimental results. wAcOxT Žmolrkg H 2 O.

wNax T Žmolrkg H 2 O.

wNH 4 xT Žmolrkg H 2 O.

logwGdxT Žmolrkg H 2 O. Ž"0.02.

wAcOxT r wGdx T

Measured pH a Ž"0.02.

Ib Žm.

408C Gd-A40 Gd-B40

5.14 = 10y2 2.62 = 10y2

5.03 = 10y2 2.50 = 10y2

– 1.43 = 10y3

y3.41 y6.81

132 169160

7.29 8.20

5.0 = 10y2 2.6 = 10y2

608C Gd-A60 Gd-B60 Gd-C60

2.43 = 10y2 4.95 = 10y2 2.61 = 10y2

2.36 = 10y2 4.87 = 10y2 2.50 = 10y2

– – 1.02 = 10y3

y3.66 y3.63 y5.03

111 211 2797

6.81 7.04 7.31

2.4 = 10y2 4.8 = 10y2 2.6 = 10y2

Run

a b

pH measured at the run temperature. I s ionic strength.

78

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

2.3. Calculation of REE–AcO association constants The species considered in calculating the chemical equilibrium were Hq, OHy, Naq, Cly, REE 3q, AcOy, HCl o , NaOH o , NaCl o , AcOH o , AcONao , REEAcO 2q and REEŽAcO.q 2 . As demonstrated in our previous article ŽDeberdt et al., 1998b., REE hydrolyzed species can be disregarded at our experimental pH conditions. Similarly, the REE chloride complexes were not considered on account of their low stability ŽWood, 1990.. Chemical equilibria were computed by solving a system of non-linear equations relative to chemical equilibrium, electroneutrality and mass balance constraints on total REE, acetate, sodium and chloride contents. An iterative method was used to calculate activity coefficients. The standard states adopted in this study were unit activity coefficients for the pure solid phases, H 2 O and neutral aqueous species, at each temperature. For charged aqueous species, the standard state corresponds to unit activity coefficient for a hypothetical 1 m solution, which exhibits ideal behavior. The potentiometric titrations were performed in a range of low values of ionic strength Ž0.009 F I F 0.085.. Thus, activity coefficients of charged species are calculated using the extended Debye–Huckel equa¨ tion. In this equation, the values for the A g and Bg parameters were taken from Helgeson and Kirkham Ž1974. and the parameter a˚ was fixed to 9 = 10y8 cm for REE 3q and Hq, and 4 = 10y8 cm for the other charged species ŽHelgeson, 1969.. The dissociation constants needed for the chemical equilibrium computations are listed in Table 1. In the first step of the calculation, we only consider one complex of 1:1 stoichiometry over the whole range of wAcOx TrwREEx T molal ratio investigated. At each titration step, the value of the corresponding association constant b 11 is adjusted until the calculated pH value equaled the measured one. The resulting b 11 values were found to vary well beyond the uncertainty over each titration. Consequently, and by analogy with La–AcOy complexation ŽDeberdt et al., 1998b., a second complex of 1:2 stoichiometry, REEŽAcO.q 2 was considered in addition to the previous species. The constants b 11 and b 12 for the formation of the REEAcO 2q and REEŽAcO.q complexes, respectively, were calcu2 lated using an iterative method. At a given tempera-

ture, initial guessed values of b 11 and b 12 allowed the calculation of equilibrium concentrations and activity coefficients for each aqueous species and for each step of the titration. These speciation results were inserted in a non-linear least squares regression

Fig. 1. Solubility of GdŽOH. 3 Žcr. in acetate-bearing solutions at 408C Ža. and 608C Žb.. The symbols correspond to the experimental data Žopen square: wAcOxT s 0.025"0.001 m; black circle: wAcOx T s 0.050"0.001 m.. The dashed lines represent the calculated solubilities in non complexing media Žsee text.: Ž1. I s 0.025 m; Ž2. I s 0.050 m. The solid lines were computed using associaX tion constants generated in this study: Ž1 . I s 0.025 m and X wAcOx T s 0.025 m; Ž2 . I s 0.050 m and wAcOx T s 0.050 m.

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

79

Table 3 Results of the potentiometric measurements: characteristics of the solutions and values of the measured and calculated pH during the titrations with GdCl 3 solutions Ž0.1086 M. at 258C, 408C, 608C and 808C Žsee text. Injection number

wAcOx T Žmolrkg H 2 O.

wNax T Žmolrkg H 2 O.

wGdCl 3 x T Žmolrkg H 2 O.

wAcOxT r wGdx T

Measured pH

pH a

pH b

I Žm.

258C 0 1 2 3 4 5 6 7 8 9 10

1.944 = 10y2 1.941 = 10y2 1.938 = 10y2 1.934 = 10y2 1.928 = 10y2 1.915 = 10y2 1.888 = 10y2 1.840 = 10y2 1.789 = 10y2 1.736 = 10y2 1.667 = 10y2

1.000 = 10y2 9.983 = 10y3 9.967 = 10y3 9.950 = 10y3 9.918 = 10y3 9.853 = 10y3 9.710 = 10y3 9.465 = 10y3 9.205 = 10y3 8.931 = 10y3 8.575 = 10y3

0 1.802 = 10y4 3.597 = 10y4 5.387 = 10y4 8.949 = 10y4 1.600 = 10y3 3.154 = 10y3 5.808 = 10y3 8.638 = 10y3 1.161 = 10y2 1.548 = 10y2

– 107.72 53.86 35.91 21.54 11.97 5.98 3.17 2.07 1.50 1.08

4.735 4.725 4.714 4.706 4.685 4.651 4.578 4.476 4.387 4.313 4.234

4.736 4.734 4.732 4.730 4.727 4.720 4.711 4.698 4.687 4.678 4.669

– 4.726 4.716 4.707 4.688 4.652 4.579 4.475 4.387 4.313 4.233

9.97 = 10y3 1.05 = 10y2 1.10 = 10y2 1.16 = 10y2 1.27 = 10y2 1.51 = 10y2 2.12 = 10y2 3.33 = 10y2 4.76 = 10y2 6.37 = 10y2 8.54 = 10y2

408C 0 1 2 3 4 5 6 7 8 9

1.968 = 10y2 1.965 = 10y2 1.962 = 10y2 1.959 = 10y2 1.952 = 10y2 1.940 = 10y2 1.907 = 10y2 1.838 = 10y2 1.765 = 10y2 1.696 = 10y2

1.000 = 10y2 9.984 = 10y3 9.968 = 10y3 9.952 = 10y3 9.921 = 10y3 9.858 = 10y3 9.690 = 10y3 9.342 = 10y3 8.966 = 10y3 8.620 = 10y3

0 1.736 = 10y4 3.466 = 10y4 5.191 = 10y4 8.624 = 10y4 1.543 = 10y3 3.369 = 10y3 7.146 = 10y3 1.122 = 10y2 1.499 = 10y2

– 113.19 56.60 37.73 22.64 12.58 5.66 2.57 1.57 1.13

4.741 4.729 4.718 4.708 4.690 4.651 4.554 4.391 4.267 4.179

4.741 4.739 4.737 4.735 4.732 4.726 4.714 4.696 4.682 4.672

– 4.731 4.721 4.710 4.690 4.651 4.552 4.391 4.267 4.179

9.97 = 10y3 1.04 = 10y2 1.09 = 10y2 1.13 = 10y2 1.23 = 10y2 1.44 = 10y2 2.11 = 10y2 3.86 = 10y2 6.02 = 10y2 8.13 = 10y2

608C 0 1 2 3 4 5 6 7 8 9

2.007 = 10y2 2.003 = 10y2 2.000 = 10y2 1.994 = 10y2 1.981 = 10y2 1.957 = 10y2 1.927 = 10y2 1.865 = 10y2 1.806 = 10y2 1.732 = 10y2

9.580 = 10y3 9.565 = 10y3 9.550 = 10y3 9.520 = 10y3 9.460 = 10y3 9.343 = 10y3 9.201 = 10y3 8.902 = 10y3 8.623 = 10y3 8.269 = 10y3

0 1.720 = 10y4 3.434 = 10y4 6.845 = 10y4 1.361 = 10y3 2.687 = 10y3 4.300 = 10y3 7.682 = 10y3 1.085 = 10y2 1.486 = 10y2

– 116.51 58.26 29.13 14.56 7.28 4.48 2.43 1.66 1.17

4.732 4.716 4.707 4.683 4.640 4.555 4.458 4.284 4.163 4.051

4.732 4.730 4.728 4.725 4.718 4.708 4.698 4.683 4.672 4.661

– 4.721 4.710 4.687 4.642 4.555 4.456 4.287 4.167 4.051

9.54 = 10y3 9.95 = 10y3 1.04 = 10y2 1.12 = 10y2 1.31 = 10y2 1.74 = 10y2 2.36 = 10y2 3.95 = 10y2 5.63 = 10y2 7.89 = 10y2

808C 0 1 2 3 4 5 6 7 8

2.007 = 10y2 2.003 = 10y2 2.000 = 10y2 1.994 = 10y2 1.981 = 10y2 1.963 = 10y2 1.926 = 10y2 1.863 = 10y2 1.820 = 10y2

9.940 = 10y3 9.924 = 10y3 9.908 = 10y3 9.877 = 10y3 9.814 = 10y3 9.722 = 10y3 9.543 = 10y3 9.230 = 10y3 9.016 = 10y3

0 1.737 = 10y4 3.469 = 10y4 6.915 = 10y4 1.374 = 10y3 2.382 = 10y3 4.343 = 10y3 7.755 = 10y3 1.010 = 10y2

– 115.33 57.66 28.83 14.42 8.24 4.44 2.40 1.80

4.826 4.816 4.797 4.772 4.721 4.646 4.501 4.296 4.192

4.827 4.825 4.823 4.819 4.813 4.804 4.792 4.776 4.768

– 4.815 4.802 4.777 4.726 4.648 4.501 4.296 4.191

9.89 = 10y3 1.02 = 10y2 1.06 = 10y2 1.13 = 10y2 1.29 = 10y2 1.56 = 10y2 2.24 = 10y2 3.81 = 10y2 5.04 = 10y2

a b

pH calculated assuming that no complexation occurs. pH calculated using the complexing constants generated in this study.

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

80

Table 4 Results of the potentiometric measurements: characteristics of the solutions and values of the measured and calculated pH during the titrations with YbCl 3 solutions Ž0.0985 M. at 258C, 408C, 608C and 808C Žsee text. Injection number number

wAcOx T Žmolrkg H 2 O.

wNax T Žmolrkg H 2 O.

wYbCl 3 x T Žmolrkg H 2 O.

wAcOx T r wYbx T

Measured pH

pH a

pH b

I Žm.

258C 0 1 2 3 4 5 6 7 8 9 10 11 12

1.910 = 10y2 1.907 = 10y2 1.903 = 10y2 1.901 = 10y2 1.898 = 10y2 1.892 = 10y2 1.885 = 10y2 1.861 = 10y2 1.850 = 10y2 1.793 = 10y2 1.740 = 10y2 1.690 = 10y2 1.642 = 10y2

1.000 = 10y2 9.984 = 10y3 9.968 = 10y3 9.951 = 10y3 9.935 = 10y3 9.903 = 10y3 9.871 = 10y3 9.746 = 10y3 9.684 = 10y3 9.389 = 10y3 9.109 = 10y3 8.847 = 10y3 8.599 = 10y3

0 1.602 = 10y4 3.199 = 10y4 4.790 = 10y4 6.376 = 10y4 9.534 = 10y4 1.267 = 10y3 2.189 = 10y3 2.797 = 10y3 5.423 = 10y3 8.185 = 10y3 1.079 = 10y2 1.311 = 10y2

– 119.04 59.52 39.68 29.76 19.84 14.88 8.50 6.61 3.31 2.13 1.57 1.25

4.752 4.744 4.737 4.728 4.722 4.704 4.691 4.639 4.613 4.521 4.449 4.394 4.351

4.752 4.750 4.748 4.747 4.745 4.742 4.740 4.733 4.729 4.716 4.705 4.697 4.690

– 4.743 4.734 4.726 4.718 4.701 4.686 4.642 4.616 4.522 4.448 4.393 4.351

9.96 = 10y3 1.04 = 10y2 1.09 = 10y2 1.14 = 10y2 1.19 = 10y2 1.30 = 10y2 1.41 = 10y2 1.76 = 10y2 2.01 = 10y2 3.22 = 10y2 4.66 = 10y2 6.09 = 10y2 7.38 = 10y2

408C 0 1 2 3 4 5 6 7 8 9

1.930 = 10y2 1.927 = 10y2 1.924 = 10y2 1.918 = 10y2 1.905 = 10y2 1.887 = 10y2 1.858 = 10y2 1.791 = 10y2 1.729 = 10y2 1.662 = 10y2

1.000 = 10y2 9.984 = 10y3 9.968 = 10y3 9.936 = 10y3 9.873 = 10y3 9.779 = 10y3 9.628 = 10y3 9.282 = 10y3 8.961 = 10y3 8.612 = 10y3

0 1.584 = 10y4 3.162 = 10y4 6.305 = 10y4 1.253 = 10y3 2.172 = 10y3 3.665 = 10y3 7.068 = 10y3 1.023 = 10y2 1.366 = 10y2

– 121.67 60.83 30.42 15.21 8.69 5.07 2.53 1.69 1.22

4.758 4.747 4.740 4.722 4.686 4.640 4.570 4.442 4.354 4.280

4.758 4.757 4.755 4.752 4.746 4.739 4.730 4.714 4.703 4.693

– 4.749 4.740 4.722 4.688 4.639 4.568 4.442 4.356 4.282

9.96 = 10y3 1.04 = 10y2 1.08 = 10y2 1.17 = 10y2 1.37 = 10y2 1.69 = 10y2 2.30 = 10y2 3.93 = 10y2 5.61 = 10y2 7.52 = 10y2

608C 0 1 2 3 4 5 6 7 8 9

2.007 = 10y2 2.003 = 10y2 1.994 = 10y2 1.982 = 10y2 1.964 = 10y2 1.934 = 10y2 1.866 = 10y2 1.803 = 10y2 1.735 = 10y2 1.732 = 10y2

9.930 = 10y3 9.914 = 10y3 9.899 = 10y3 9.868 = 10y3 9.807 = 10y3 9.717 = 10y3 9.570 = 10y3 9.236 = 10y3 8.924 = 10y3 8.585 = 10y3

0 1.540 = 10y4 3.075 = 10y4 6.130 = 10y4 1.218 = 10y3 2.113 = 10y3 3.567 = 10y3 6.884 = 10y3 9.978 = 10y3 1.333 = 10y2

– 130.12 65.06 32.53 16.27 9.29 5.42 2.71 1.81 1.30

4.761 4.752 4.741 4.719 4.679 4.625 4.538 4.370 4.256 4.152

4.762 4.760 4.758 4.755 4.749 4.742 4.732 4.716 4.705 4.695

– 4.752 4.742 4.723 4.684 4.627 4.539 4.371 4.253 4.153

9.89 = 10y3 1.02 = 10y2 1.06 = 10y2 1.14 = 10y2 1.31 = 10y2 1.58 = 10y2 2.10 = 10y2 3.59 = 10y2 5.20 = 10y2 7.06 = 10y2

808C 0 1 2 3 4 5 6 7 8 9 10 11

2.007 = 10y2 2.003 = 10y2 2.000 = 10y2 1.994 = 10y2 1.976 = 10y2 1.964 = 10y2 1.923 = 10y2 1.889 = 10y2 1.856 = 10y2 1.814 = 10y2 1.784 = 10y2 1.736 = 10y2

9.940 = 10y3 9.925 = 10y3 9.909 = 10y3 9.878 = 10y3 9.787 = 10y3 9.727 = 10y3 9.524 = 10y3 9.356 = 10y3 9.194 = 10y3 8.986 = 10y3 8.836 = 10y3 8.598 = 10y3

0 1.535 = 10y4 3.065 = 10y4 6.111 = 10y4 1.514 = 10y3 2.106 = 10y3 4.124 = 10y3 5.788 = 10y3 7.394 = 10y3 9.451 = 10y3 1.093 = 10y2 1.330 = 10y2

– 130.52 65.26 32.63 13.05 9.32 4.66 3.26 2.51 1.92 1.63 1.31

4.827 4.821 4.805 4.777 4.704 4.660 4.498 4.388 4.294 4.195 4.133 4.051

4.827 4.825 4.823 4.820 4.811 4.807 4.793 4.785 4.778 4.769 4.765 4.758

– 4.816 4.804 4.781 4.710 4.662 4.500 4.382 4.288 4.189 4.131 4.050

9.89 = 10y3 1.02 = 10y2 1.05 = 10y2 1.10 = 10y2 1.30 = 10y2 1.45 = 10y2 2.11 = 10y2 2.81 = 10y2 3.59 = 10y2 4.67 = 10y2 5.48 = 10y2 6.82 = 10y2

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

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Fig. 2. Variation of the pH as a function of wAcOx T rwGdx T molal ratio along the titration curves from 258C to 808C. The symbols represent experimental data. The dashed lines show calculated pH variation assuming that no Gd–AcOy complexation occurs. The solid lines indicate the pH variation calculated using the complexation constants generated in this study.

routine to fit wREEx T over the experimental pH values, using its mathematical formulation:

w REEx T s w REEx 3q 1 q

b 11 w AcOy x g REE 3q gAcO y g REE Ž AcO . 2q 2

q

2 y b 12 w AcOy x g REE 3y gAcO

3. Results and discussion

g REE Ž AcO . q2 3.1. Solubility measurements

with

w AcOy x s

cal equilibrium computation. The iterative process was repeated until a steady set of b 11 and b 12 values was obtained.

K AcOH w AcOH o x

gAcO y 10yp H

The fitting procedure results in a new set of b 11 and b 12 values, which were used to reiterate the chemi-

The compositions of the experimental solutions are given in Table 2 along with the values of the pH measured at the run temperature and the calculated ionic strengths. The results are plotted as the loga-

Notes to Table 4: a pH calculated assuming that no complexation occurs. b pH calculated using the complexing constants generated in this study.

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S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

rithm of the total Gd concentration versus pH in Fig. 1a and b, at 408C and 608C, respectively. In these figures, the solubilities measured in acetate-bearing solutions Žsymbols. are compared to those computed, at the same ionic strength and in non-complexing solutions Ždashed lines., from the data of Deberdt et al. Ž1998b.. The presence of the organic ligand leads to an increase of the solubility of up to 1.5 and 2 log units in the presence of 0.05 m wAcOx T , at 408C and 608C, respectively. These variations of the solubility give a direct evidence for the formation of aqueous complexes between Gd and the acetate ions. 3.2. Potentiometric measurements The results of the potentiometric experiments are reported, as a function of temperature, in Tables 3

and 4 for REE s Gd and REE s Yb, respectively. These tables present, for each step of the titrations, the total concentrations of the constituents Žacetate, sodium and REE. of the experimental solutions, along with the corresponding ionic strengths, the wAcOx TrwREEx T molal ratios and the measured pH. For comparison, also listed are the calculated pHs of the solutions, assuming that no complexation occurs, and the corresponding pH values calculated using the complexation constants generated in this study. These different pH values are compared as a function of wAcOx TrwREEx T and at each temperature on Figs. 2 and 3 for Gd and Yb, respectively. It can be seen from these plots that the pHs calculated without taking account of the complexes formation Ždashed lines. differ dramatically from the measured ones Žblack symbols., over the wide range of wAcOx Tr

Fig. 3. Variation of the pH as a function of wAcOx T rwYbx T molal ratio along the titration experiments carried on from 258C to 808C. The symbols represent experimental data. The dashed lines show calculated pH variation, assuming that no Yb–AcOy complexation occurs. The solid lines indicate the pH variation calculated using the complexation constants generated in this study.

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

wREEx T molal ratios investigated Ž1 to 120.. This disagreement increases as the wAcOx TrwREEx T molal ratio decreases, clearly indicating the formation of aqueous REE–acetate complexes. According to Wood Ž1993. and Deberdt et al. Ž1998b., this difference was only attributed to the presence of monomeric species of the general formula: REEŽAcO. 3yn . Following this hypothesis, two n species of 1:1 and 1:2 stoichiometries are required to fit the experimental data. The logarithms of the corresponding complexation constants b 11 and b 12 are reported as a function of temperature in Table 5. The quality of the regressions can be clearly observed in Figs. 2 and 3, where the pH calculated using the proposed association constants Žsolid lines. are compared to the measured ones Žfilled circles.. It can be seen that the calculated pH values are closely consistent with the experimental data over the investigated range of wAcOx TrwREEx T ratios. In the case of Gd, the association constants of the complexes were also used, along with the first dissociation constant of GdŽOH. 3 Žcr. proposed by Deberdt et al. Ž1998b., to model the Gd trihydrate solubility in the presence of acetate at 408C and 608C. In Fig. 1a and b, the computed solubilities Žsolid lines. are compared to the measured ones Žsymbols. at 408C and 608C, respectively. The very good agreement between the two sets of data confirms the reliability of the association constants deduced from our potentiometric study. 3.3. Discussion The logarithms of the formation constants b 11 and b 12 are plotted as a function of the reciprocal Table 5 Association constants of REE acetate complexes at 258C, 408C, 608C and 808C REE

t Ž8C.

log b 11 Ž"0.05.

log b 12 Ž"0.12.

Gd

25 40 60 80 25 40 60 80

2.52 2.65 2.88 3.04 2.32 2.51 2.82 3.06

4.61 4.84 5.10 5.46 4.58 4.74 5.05 5.56

Yb

83

Fig. 4. Logarithms of the b 11 and b 12 Gd acetate association constants as a function of reciprocal temperature. The solid lines give the temperature dependence trend of the association constants resulting from a linear regression Žsee text.. The dashed lines represent the prediction of Shock and Koretsky Ž1993..

temperature in Figs. 4 and 5 for gadolinium and ytterbium, respectively. For both REE, the results reflect the increasing complexing ability of the acetate ligand with increasing temperature. A comparison of the equilibrium constants for Gd– and Yb– acetate complexes obtained in this study with the corresponding values previously reported in the literature at zero ionic strength is given in Figs. 4 and 5. No experimental data have been previously reported at temperatures above 258C. The values proposed by Wood Ž1993. are based on the experimental studies of Kovar and Powell Ž1966. and Sonesson Ž1958, 1959., obtained in 0.1 and 2 m ionic strength solutions, respectively. For the ytterbium–acetate complexes, values from Archer and Monk Ž1964, 1966. are also available. Archer and Monk Ž1964. obtained potentiometric values at ionic strengths - 0.1 m and used the Davies equation to compute activity coefficients. The second set of data ŽArcher and Monk, 1966. results from cation-exchange resin studies carried out at I s 0.1 and 0.05 m. For each ionic strength, the authors deduced a set of association constants at I s 0, which were slightly different, depending on the ionic strength of the experiment

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

84

Gibbs free energy of the first acetate complex forming reaction and the Gibbs free energy of formation of the cations, and Žb. log b 11 and log b 12 . It is obvious from Figs. 4 and 5 that these predictions are not constrained enough to accurately describe our experimental data. Similar large departure from the Shock and Koretsky Ž1993. model was found by Wood et al. Ž1999. for Nd–acetate complexation to 2508C.

3.4. Prediction of the thermodynamic properties of the Gd– and Yb–acetate complexes

Fig. 5. Logarithms of the b 11 and b 12 Yb acetate association constants as a function of reciprocal temperature. The solid lines give the temperature dependence trend of the association constants resulting from a linear regression Ž b 11 . and the revised HKF thermodynamic model Ž b 12 . Žsee text.. The dashed lines represent the prediction of Shock and Koretsky Ž1993..

ŽFig. 5.. Our data for b 11 at 258C are in close agreement with the literature values, whereas previous values for b 12 are systematically lower Žby 0.5 to 1.5 log units. than ours. The temperature dependence of the REE–AcO association constants has never been investigated experimentally. Thus, we can only compare our data with the predictions of Shock and Koretsky Ž1993.. These predictions used correlations between Ža. the

In agreement with the results previously obtained for lanthanum, the logarithm of the Gd– and Yb– acetate complexation constants are linearly related to the reciprocal temperature, except the b 12 Yb–AcO constant ŽFigs. 4 and 5.. In the cases where linear correlations occur, the standard enthalpies and eno tropies of the complex formation reactions Ž D r H298.15 o and D r S298.15 , respectively. are deduced from the simple equation: log b s y

o D r H298.15

Rln10

1

ž / T

q

o D r S298.15

Rln10

The Gibbs free energies for the reactions at 258C were calculated from the corresponding b values we o o propose. The resulting D r G 298.15 , D r H298.15 and o D r S298.15 values are listed in Table 6. The temperature dependence for the b 12 Yb–AcO constant was modeled using the revised HKF ŽHelgeson et al., 1981. equations of state and corre-

Table 6 o o o o . or deduced Ž D r H298.15 . from the regressions Thermodynamic data of complexes forming reactions used Ž D r G 298.15 , D r S298.15 , D r Cp 298.15 Ž . of log b 11 and log b 12 vs. 1rT Gd o ŽJ. D r G 298.15 o ŽJ. D r H298.15 o ŽJ Ky1 . D r S298.15 o ŽJ Ky1 . D r Cp 298.15

Yb

b 11

b 12

b 11

b 12

y14384" 285 19528 " 833 113.5 " 2.6 0

y26313" 685 30570" 2000 190.4 " 6.2 0

y13242" 285 27587" 935 136.6 " 2.9 0

y26142" 685 8234 " 698 115.3 " 7.7 1185 " 115

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

85

Table 7 Thermodynamic properties of aqueous species used or generated in the present study Normal font: selected values from the literature. Bold font: values calculated in the present study. AcOy Gd 3q Yb 3q GdAcO 2q GdŽAcO.q 2 YbAcO 2q YbŽAcO.q 2

o ŽkJ moly1 . D f G 298.15

o ŽkJ moly1 . D f H298.15

o ŽJ Ky1 moly1 . S298.15

o ŽJ Ky1 moly1 . Cp 298.15

y369.32 a y665.72 b y639.25c I1049.42 I1430.67 I1021.81 I1404.03

y486.10 a y689.46 b y670.50 d I1156.03 I1631.09 I1129.01 I1634.47

86.19 a y207.80 b y241.00 e I8.1 155.0 I18.2 46.7

25.9 a – y152.3 f – – – 1084.5

a

Shock and Helgeson Ž1990.. Deberdt et al. Ž1998b.. c o o Calculated from the corresponding values of D f H298.15 and S298.15 in this table. d Bettonville et al. Ž1987.. e David Ž1986.. f Shock and Helgeson Ž1988.. b

lations among equations of state parameters ŽShock and Helgeson, 1988, 1990.. Due to the ranges of temperature and pressure considered Ž25–808C, Psat ., and according to Sverjensky et al. Ž1997., we have used a simplified equation Žsee Eq. Ž39. in Sverjensky et al., 1997. in which the conventional Born coefficient, v PrTr , was substituted in the regression equation for its linear correlation with the standard partial molal entropy ŽEq. Ž40. in Sverjensky et al., 1997.. Values of the dielectric constant of H 2 O Ž ´ . and its partial derivatives with respect to temperature and pressure needed in these calculations are given by Shock et al. Ž1992.. Again, the Gibbs free energy for the reaction at 258C was deduced from the b 12 value at the same temperature stat and the only parameters which remain to retrieve are the standard partial entropy and heat capacity. The data used o Ž D r G 298.15 . or deduced from this regression o o Ž D r H298.15 , D r S298.15 . are listed in Table , D r Cp 298.15 6. In Figs. 4 and 5, the log b values computed using these data Žsolid lines. are compared to the experimental values. It can be seen that the regression curves are in very good agreement with the experimental points. The thermodynamic properties for the o o REE–acetate complexes Ž D f G 298.15 , D f H 298.15 , q o o Ž . D f S298.15 , and for the Yb AcO 2 species, Cp 298.15 . were calculated by combining the data for the complex formation reactions with the thermodynamic properties for the aqueous species. We have used the data proposed by Shock and Helgeson Ž1990. for the AcOy ion, and those from Deberdt et al. Ž1998b. for

Gd 3-. Following Morss Ž1994., we borrowed entropy and formation enthalpy values for Yb 3y from David Ž1986. and Bettonville et al. Ž1987., respectively. These data lead to a Yb 3y Gibbs free energy of formation equal to y639.25 kJ moly1 . The heat capacity for this ion was taken from Shock and Helgeson Ž1988.. The values of thermodynamic properties for aqueous species are listed in Table 7 along with those computed for REE–acetate complexes.

4. Conclusion In the range of wAcOx TrwREExT molal ratios investigated Ž1 to 120., the results of potentiometric titrations for GdCl 3 and YbCl 3 in AcOHrAcOyr Naq solutions gave evidences of two complexes with 1:1 and 1:2 stoichiometries. The regression of the experimental data leads to the determination of the association constants for these species, from 258C to 808C. The reliability of the proposed Gd– acetate association constants is validated by the good agreement between GdŽOH. 3 Žcr. solubilities measured at 408C and 608C in acetate bearing solutions and calculated corresponding values. The thermodynamic properties of the complexes were determined by regression of our data using a linear equation or a simplified formulation of the revised HKF thermodynamic model ŽHelgeson et al. 1981; Sverjensky et

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S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

of this assumption depends strongly on the behavior of the intermediate REEs. Additional experimental work are currently in progress to assess this hypothesis.

Acknowledgements Financial support was provided by the French program PROSE ŽProgramme de Recherche Sol et Erosion. INSUrORSTOM and the ‘‘Groupe de Recherche’’ FORPRO ŽForages Profonds. CNRSr ANDRA. This study was supported by a MESR fellowship. The authors wish to thank V. Ragnarsdottir for helpful comments, which have greatly enhanced the presentation and clarity of the manuscript. We are grateful to M. Valladon, B. Reynier, M. Thibault and J. Escalier for their help in physical and chemical analysis.

Fig. 6. Comparison between Ža. the variation of the b 11 and b 12 REE–acetate association constants vs. atomic number ŽGd and Yb: this study; La: Deberdt et al., 1998b., and Žb. REE pattern Žnormalised to shale. in the Nyong river ŽSouth Cameroon. ŽDeberdt et al., 1998a..

al., 1997.. The results of the present study combined with those obtained with lanthanum in a previous work ŽDeberdt et al., 1998b. allow, for the first time, comparison between thermodynamic constants for the REE–acetate complexes from the lighter- ŽLa. to the heavier-REE ŽYb., and their temperature dependencies. We found that the constants show a similar temperature dependence, whatever the REE. For all the temperatures investigated, the variation of association constants with REE atomic number ŽFig. 6a. shows a slight increase from La to Gd, then a decrease to Yb. This behavior may explain the convex normalized REE pattern observed for organicrich river waters ŽFig. 6b. Žfor example, Amazon, Rio Negro and Nyong rivers. ŽGoldstein and Jacobsen, 1988; Elderfield et al., 1990; Gaillardet et al., 1997; Deberdt et al., 1998a.. However, even if it is tempting to point to such a correlation, the validity

References Archer, D.W., Monk, C.B., 1964. Ion-association constants of some acetates by pH Žglass electrode. measurements. J. Chem. Soc. A, 3117–3122. Archer, D.W., Monk, C.B., 1966. Dissociation constants of some ytterbium ion-pairs from cation-exchange resin studies. J. Chem. Soc. A, 1374–1376. Bates, R., 1973. Determination of pH. 2nd edn. Wiley, New York, 479 pp. Benezeth, P., Castet, S., Dandurand, J.L., Gout, R., Schott, J., ´ ´ 1994. Experimental study of aluminium–acetate complexing between 60 and 2008C. Geochim. Cosmochim. Acta 58, 4561–4571. Bertram, C.J., Elderfield, H., 1993. The geochemical balance of the rare earth elements and Nd isotopes in the oceans. Geochim. Cosmochim. Acta 57, 1957–1986. Bettonville, S., Goudiakas, J., Fuger, J., 1987. Thermodynamics of lanthanide elements: III. Molar enthalpies of formation of 3q 3q 3q TbŽaq. , Ho Žaq. , YbŽaq. , TbBr3 Žcr., HoBr3 Žcr., and YbBr3 Žcr. at 298.15 K. J. Chem. Thermodyn. 19, 595–604. Braun, J.J., Viers, J., Dupre, ´ B., Polve, M., Ndam, J., Muller, J.-P., 1998. Solidrliquid REE fractionation in the lateritic system of Goyoum, East Cameroon: the implication for the present dynamics of the soil covers of the humid tropical regions. Geochim. Cosmochim. Acta 62, 273–299. David, F., 1986. Thermodynamic properties of lanthanide and actinide ions in aqueous solution. J. Less-Common Met. 121, 27–42.

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88 De Baar, H.J.W., Bacon, M.P., Brewer, P.G., Bruland, K.W., 1985. Rare earth elements in the Pacific and Atlantic Oceans. Geochim. Cosmochim. Acta 49, 1943–1951. Deberdt, S., Castet, S., Dandurand, J.L., 1998a. Potentiometric studies of REE–organic complexing. V.M. Goldschmidt Conference Toulouse 1998. Mineral. Mag. 62A, 362–363. Deberdt, S., Castet, S., Dandurand, J.-L., Harrichoury, J.-C., Louiset, I., 1998b. Experimental study of LaŽOH. 3 and GdŽOH. 3 solubilities Ž25 to 1508C., and La–acetate complexing Ž25 to 808C.. Chem. Geol. 151, 349–372. Elderfield, H., Greaves, M.J., 1982. The rare earth elements in seawater. Nature 296, 214–219. Elderfield, H., Upstill-Goddard, R., Sholkovitz, E.R., 1990. The rare earth elements in rivers, estuaries, and coastal seas and their significance to the composition of ocean waters. Geochim. Cosmochim. Acta 54, 971–991. Fisher, J.R., Barnes, H.L., 1972. The ion-product constant of water to 3508C. J. Phys. Chem. 76, 90–99. Gaillardet, J., Dupre, C.J., Negrel, P., 1997. Chemical ´ B., Allegre, ` ´ and physical denudation in the Amazon River Basin. Chem. Geol. 142, 141–173. Goldberg, E.D., Koide, M., Schmitt, R.A., Smith, R.H., 1963. Rare earth distribution in the marine environment. J. Geophys. Res. 68, 4209. Goldstein, S.J., Jacobsen, S.B., 1988. Rare earth elements in river waters. Earth Planet. Sci. Lett. 89, 35–47. Helgeson, H.C., 1969. Thermodynamics of hydrothermal systems at elevated temperatures and pressures. Am. J. Sci. 267, 729–804. Helgeson, H.C., Kirkham, D.H., 1974. Theoretical prediction of the thermodynamic behaviour of aqueous electrolytes at high pressures and temperatures: II. Debye–Huckel parameters for ¨ activity coefficients and relative partial molal properties. Am. J. Sci. 274, 1199–1261. Helgeson, H.C., Kirkham, D.H., Flowers, G.C., 1981. Theoretical prediction of the thermodynamic behaviour of aqueous electrolytes at high pressures and temperatures: IV. Calculation of the activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 6008C and 5 kb. Am. J. Sci. 281, 1249–1516. Ho, P.C., Palmer, D.A., 1996. Ion association of dilute aqueous sodium hydroxide solutions to 6008C and 300 MPa by conductance measurements. J. Solution Chem. 25, 711–729. Johnson, J.W., Oelkers, H.E., Helgeson, H.C., 1992. SUPCRT92. A soft package for calculating the standard molal properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 10008C. Comput. Geosci. 18, 899–947. Keasler, K.M., Loveland, W.D., 1982. Rare earth elemental concentrations in some Pacific Northwest rivers. Earth Planet. Sci. Lett. 61, 68–72. Klinkhammer, G., Elderfield, H., Hudson, A., 1983. Rare earth elements in seawater near hydrothermal vents. Nature 305, 185–188. Kovar, L.E., Powell, J.E., 1966. Stability constants of rare earths with some weak carboxylic acids. Report TID-4500. Ames Laboratory, Iowa State Univ., Ames. 54 pp. Morss, L.R., 1994. Comparative thermochemical and oxidation–

87

reduction properties of lanthanides and actinides. In: Handbook on the Physics and Chemistry of Rare Earths Vol. 18, pp. 239–291, Chap. 122. Oelkers, E.H., Helgeson, H.C., 1989. Calculation of the transport properties of aqueous species at pressures to 5 kbar and temperatures to 10008C. J. Solution Chem. 18, 601–640. Piepgras, D.J., Jacobsen, S.B., 1992. The behavior of rare earth elements in the seawater: precise determination of variations in the North Pacific water column. Geochim. Cosmochim. Acta 56, 1851–1862. Robinson, R.A., Stokes, R.H., 1959. Electrolyte solutions. 2nd edn. Butterworth, London, 571 pp. Ruaya, J.R., Seward, T.M., 1987. The ion-pair constant and other thermodynamic properties of HCl up to 3508C. Geochim. Cosmochim. Acta 51, 121–130. Shimizu, H., Tachikawa, K., Masuda, A., Nozaki, Y., 1994. Cerium and neodymium ratios and REE patterns in seawater from North Pacific Ocean. Geochim. Cosmochim. Acta 58, 323–333. Shock, E.L., Helgeson, H.C., 1988. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: correlation algorithms for ionic species and equation of state predictions to 5 kb and 10008C. Geochim. Cosmochim. Acta 52, 2009–2036. Shock, E.L., Helgeson, H.C., 1990. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: standard partial molal properties of organic species. Geochim. Cosmochim. Acta 54, 915–945. Shock, E.L., Koretsky, C.M., 1993. Metal-organic complexes in geochemical processes: calculation of standard partial molal thermodynamic properties of aqueous acetate complexes at high pressures and temperatures. Geochim. Cosmochim. Acta 57, 4899–4922. Shock, E.L., Oelkers, E.H., Johnson, J.W., Sverjensky, D.A., Helgeson, H.C., 1992. Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures: effective electrostatic radii, dissociation constants and partial molal properties to 10008C and 5 kbar. J. Chem. Soc., Faraday Trans. 88, 803–826. Sholkovitz, E.R., 1988. Rare earth elements in the sediments of the North Atlantic Ocean, Amazon delta and east China Sea: reinterpretation of terrigenous input patterns to the oceans. Am. J. Sci. 288, 236–281. Sholkovitz, E.R., 1995. The aquatic chemistry of rare earth elements in rivers and estuaries. Aquat. Geochem. 1, 1–34. Sonesson, A., 1958. On the complex chemistry of the tervalent rare-earth ions: II. The acetate systems of praseodymium, samarium, dysprosium, holmium, erbium and ytterbium. Acta Chem. Scand. 12, 1937–1954. Sonesson, A., 1959. On the complex chemistry of the tervalent rare-earth ions: IV. Ion-exchange studies of the gadolinium acetate and glycolate. Acta Chem. Scand. 13, 1437–1452. Sverjensky, D.A., Shock, E.L., Helgeson, H.C., 1997. Prediction of the thermodynamic properties of aqueous metal complexes to 10008C and 5 kb. Geochim. Cosmochim. Acta 61, 1359– 1412. Viers, J., Dupre, ´ B., Polve, ´ M., Schott, J., Dandurand, J.-L.,

88

S. Deberdt et al.r Chemical Geology 167 (2000) 75–88

Braun, J.-J., 1997. Chemical weathering in the drainage basin of a tropical watershed ŽNsimi-Zoetele site, Cameroon.: comparison between organic-poor and organic-rich waters. Chem. Geol. 140, 181–206. Wood, S.A., 1990. The aqueous geochemistry of the rare-earth elements and yttrium: I. Review of available low-temperature data for inorganic complexes and the inorganic REE speciation of natural waters. Chem. Geol. 82, 159–186. Wood, S.A., 1993. The aqueous geochemistry of the rare-earth

elements: critical stability constants for complexes with simple carboxylic acids at 258C and 1 bar and their application to nuclear waste management. Eng. Geol. 34, 229–259. Wood, S.A., Wesolowski, D.J., Palmer, D.A., 2000. Potentiometric study of Gd and Yb acetate complexing in the temperature range 25–808C. Zhang, C., Wang, L., Zhang, S., 1998. Geochemistry of rare earth elements in the mainstream of the Yangtze river, China. Appl. Geochem. 13, 451–462.