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The microscopic reversibility of the sorption of Sm and Yb onto kaolinite and smectite is investigated by introducing an isotopic disequilibrium between the clay and the solution. The experiments are performed at 25°C, in 0.025 or 0.5 M NaClO4 and from pH 4 up to pH 7. The isotopic exchange is monitored as a function of time over a duration of 355 hours. The first stage of the experiment consists of equilibrating the clays with a natural or spiked lanthanide solution. The second stage consists of interchanging the solutions between twin phials (same clay, pH and ionic strength, but with different lanthanide isotopic compositions). The isotopic composition and concentration of aqueous lanthanides are analysed by ICP-MS. The results are as follows: (1) the lanthanide isotopic composition of the solution is rapidly modified and stabilised within 24 h; (2) the isotopic exchange between the solid and the solution is not always complete; (3) the degree of microscopic reversibility (isotopic exchange) decreases with increasing pH. These results are explained by the fact that exchange is easier for lanthanides linked to the surface as outer-sphere complexes (physical sorption), which predominate at low pH, than for atoms sorbed as inner-sphere complexes (chemical sorption) which predominate at high pH. The contrasted kinetics observed for the different kind of sites provide additional constraints for the modeling of migration processes in natural systems.

Experiments were carried out to investigate the sorption of the complete lanthanide series (Ln or rare earth elements, REE) on a kaolinite and an a Na-montmorillonite at 22°C over a wide range of pH (3–9). Experiments were conducted at two ionic strengths, 0.025 and 0.5 M, using two different background electrolytes (NaNO 3 or NaClO 4 ) under atmospheric conditions or N 2 flow (glove box). The REE sorption does not depend on the background electrolyte or the presence of dissolved CO 2 , but is controlled by the nature of the clay minerals, the pH and the ionic strength. At 0.5 M, both clay minerals exhibit the same pH dependence for the Ln sorption edge, with a large increase in the sorption coefficient ( K D ) above pH 5.5. At 0.025 M, the measured K D is influenced by the Cation Exchange Capacity (CEC) of the minerals. Two different behaviours are observed for smectite: between pH 3 and 6, the K D is weakly pH-dependent, while above pH 6, there is a slight decrease in log K D . This can be explained by a particular arrangement of the particles. For kaolinite, the sorption coefficient exhibits a linear increase with increasing pH over the studied pH range. A fractionation is observed that due to the selective sorption between the HREEs and the LREEs at high ionic strength, the heavy REE is being more sorbed than the light REE. These results can be interpreted in terms of the surface chemistry of clay minerals, where two types of surface charge are able to coexist: the permanent structural charge and the variable pH-dependent charge. The fractionation due to sorption observed at high ionic strength can be interpreted either because of a competition with sodium or because of the formation of inner-sphere complexes. Both processes could favour the sorption of HREEs according to the lanthanide contraction.

The stability of aqueous complexes formed by gadolinium (Gd 3+ ) and ytterbium (Yb 3+ ) aqueous species with the acetate ion (AcO − ) was investigated at 25°C, 40°C, 60°C and 80°C, and saturated vapor pressure, using potentiometric measurements. In the large range of [AcO] T /[REE] T molal ratio investigated (1 to 120), the results are interpreted in terms of the formation of two complexes, REEAcO 2- and REE(AcO) 2 - (where REE denotes Gd or Yb). The corresponding association constants β 11 and β 12 that we proposed are successfully used to predict high Gd(OH) 3 (cr) 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 [Deberdt, S., Castet, S., Dandurand, J.-L., Harrichoury, J.-C., Louiset, I., 1998. Experimental study of La(OH) 3 and Gd(OH) 3 solubilities (25 to 150°C), and La–acetate complexing (25 to 80°C). Chem. Geol. 151, 349–372.] demonstrates that the stability of the REE–AcO − complexes increases from La to Gd, then decreases to Yb.

The stoichiometry and stability constants of Na-aluminate (NaAl(OH) 4 0 ) ion pair were determined from both boehmite solubility and potentiometric measurements. Boehmite solubility was measured at temperatures from 125 to 350 °C at pressures corresponding to equilibrium between liquid and vapor ( P SAT ) in the system Na-NH 4 -Cl-OH. Potentiometric measurements were performed at temperatures from 75 to 200 °C in Na-Al-Cl-OH solutions, using a Na-selective glass electrode. NaAl(OH) 4 0 stability constants derived independently from these two methods are in excellent agreement and increase markedly with increasing temperature from 0.8 at 25 °C to 209 at 350 °C. Experimental results obtained in this study were combined with corresponding data reported by Castet et al. (1993) for boehmite solubility to generate, within the framework of the revised HKF model, the standard partial molal properties and equations of state parameters for Al(OH) 4 − , Al(OH) 3 0 , and NaAl(OH) 4 0 . The solubilities of gibbsite, boehmite, and corundum calculated in Na-rich solutions using the thermodynamic data for Al aqueous species generated in this study are in close agreement with their experimental counterparts. Thermodynamic calculations carried out at temperatures up to 800 °C and pressures up to 5 kbar indicate that the formation of NaAl(OH) 4 0 ion pairs can markedly increase the solubility of Al-bearing minerals and thus, Al mobility, both in sedimentary basin and metamorphic fluids at pHs > 4.

The solubilities of bayerite and boehmite were measured as a function of pH and acetate concentration at temperatures from 60 to 2OO"C, to determine the stoichiometries and stability constants of aluminum-acetate complexes. Analysis of these solubility data indicate that the three aluminum- acetate complexes, AlAcO *+ Al(OH)AcO+, and Al(Ac0); are significant. The logarithm ofthe stability constants of these complexei are found to be linearly related to the reciprocal of temperature over the range 60-200°C. The slopes of these relationships yield association enthalpies of -75 kJ/mol. Ther- modynamic calculations indicate that acetate can markedly enhance the solubility of Al-bearing minerals in sedimentary basin fluids over the temperature range from 100 to 2OO"C, at pH < 5, but have little effect in solutions having higher pHs.

A critical review of the literature concerning the solubility of aluminium hydroxides and oxyhydroxides has revealed some problems concerning the measurements in alkaline solutions; in addition, it has led us to regard the solubility measurements of gibbsite in acid solutions as being very questionable. As a result of this review, solubility measurements have been made in weakly concentrated alkaline solutions (which allow for easy thermodynamic modelling) for: - synthetic bayerite: dissolution and precipitation between 21 and 65°C; - synthetic gibbsite: dissolution between 21 and 70°C; - synthetic boehmite: dissolution and precipitation between 70 and 300°C; - natural diaspore: dissolution between 130 and 300°C. Our measurements permit the determination of the solubility products of these minerals with good accuracy and allow for the calculation of their thermodynamic properties as well as of the Gibbs free energy of formation of the aluminate ion, Al(OH) 4 #75 . The following values are proposed for the Gibbs free energy of formation at 298.15 K: Al(OH) 4 #75 : -1305.42 kJ/mol; gibbsite: -154.889 kJ/mol; bayerite: -1149.80 kJ/mol; boehmite: -917.82 kJ/mol; diaspore: -924.59 kJ/mol. Therefore, we can propose that the order of increasing stability of the aluminium hydroxides and oxyhydroxides at 25°C is: diaspore > boehmite ∼ gibbsite > bayerite

The effect of temperature on the sorption of cations onto a dioctahedral smectite was investigated by running batch experiments at 25, 40, 80 and 150°C. We measured the distribution coefficient (Kd) of Cs + , Ni 2+ and 14 lanthanides (Ln 3+ ) between solutions and the montmorillonite fraction of the MX80 bentonite at various pH and ionic strengths. Up to 80°C we used a conventional experimental protocol derived from Coppin et al. (2002). At 150°C, the experiments were conducted in a PTFE reactor equipped with an internal filter allowing the sampling of clear aliquots of solution. The results show a weak but measurable influence of the temperature on the elements sorption. Kd’s for Ni 2+ and Ln 3+ increase by a factor 2 to 5 whereas temperature raises from 25 to 150°C. This effect seems higher at high ionic strength. The estimated apparent endothermic sorption enthalpies are 33 ± 10 kJ.mol −1 and 39 ± 15 kJ.mol −1 for Ni 2+ and Eu 3+ , respectively. On the other hand, the temperature effect on Cs + sorption is only evidenced at low ionic strength and under neutral conditions where the Kd decreases by a factor 3 between 25 and 150°C. Apparent exothermic sorption enthalpy for Cs + on the montmorillonite is −19 ± 5 kJ.mol −1 . Experiments conducted at the four temperatures with the coexistence of all of the cations in the reacting solution (100 ppb of each element in the starting solution) or only one of them, produced similar values of Kd. This suggests the absence of competition between the sorbed cations, and consequently a low degree of saturation of the available sites. A fractionation of the lanthanides spectrum is also observed at high pH and high ionic strength whatever the temperature. The conclusion of this study is that the temperature dependence on sorption reflects, as the fractionation of REE or the pH and ionic strength effects, the chemical process which controls the overall reaction. In the case of an exchange dominated reaction (low pH and low ionic strength), the temperature effect is negligible. In the case of surface complexation (high pH and high ionic strength), the observed increase of Kd with temperature reflects either an increase of the sorption equilibrium constant with temperature or an endothermic property for reactions describing the montmorillonite surface chemistry.