Jean-Louis Dandurand

Major and trace elements associated with colloids in organic-rich river waters: ultrafiltration of natural and spiked solutions

Abstract This study presents the results of ultrafiltration experiments (0.20 μm–300,000 Da–5000 Da–1000 Da) performed on natural rich-organic waters (30–40 mg l−1 of dissolved organic carbon) sampled in wetland area of Cameroon (Nsimi-Zoetele site). A very strong decrease in all cation concentrations (major and trace elements) except Si was observed after filtration. Speciation calculations using data available in the literature for metal–humic substance complexation observations suggest that ultrafiltration performed on this water source at a pH of 4.74 induces a very strong retention of cations only weakly bound to humic substances in the aqueous solution [Viers, J., Dupre, B., Polve, M., Schott, J., Dandurand, J.L., Braun, J.J., 1997. Chemical weathering in the drainage basin of a tropical watershed (Nsimi-Zoetele site, Cameroon): comparison between organic-poor and organic-rich waters. Chem. Geol., 140, 181–206]. To minimize this artifact, ultrafiltration must be performed at low pH (=3) or with the addition of high concentrations of a complexing metal (e.g., Lanthanum). The goal of these ultrafiltration experiments is to anticipate the affinity of several major and trace elements to form organo-metallic complexes with humic substances. Experiments using Sr and Ba isotopes at fixed ratios were entered in order to determine the exchangeable fraction of base cation. Natural Sr isotopic ratio ( 87 Sr / 86 Sr ) in different filtrates and retentates as well as isotopic ratios of Ba and Sr ( 86 Sr / 84 Sr and 138 Ba / 135 Ba ) in spiked and filtered samples appear to remain constant. These results suggest that there is only one source of Sr in these natural waters and that it is present in an exchangeable form (i.e., free ion and/or complexed with organic matter). By analogy, we suppose that elements such as Ca or Mg are not present in organic or mineral colloids but in an exchangeable position. This is in agreement with the hypothesis of the filtration artifact. In the filtration experiment, performed at pH 3, more than 95% of Al, Ga, Fe, U, Th, Y and REEs, ≅50% of Cr and V, 25% of Cu, 10% of Co, and ≤5% of Ca, Mg, Na, K, Mn, Ba, Rb and Sr are complexed with organic material. According to the data gathered in this study and the results of speciation calculations, the order and the overall constants (log K) for the formation of metal–humate complexes are the following: Al, Ga, Fe, Th, U, Y, REEs (more than 7)≫Cr (5.5)≫Co (3)>Rb, Ba, Sr, Mn, Mg (≈2). These data are obtained for both low ionic strength and low metal concentrations. Using the filtration experiment performed with an addition of La, we observe that REEs appear to be complexed with humic substances via two types of site. The first site has a strong affinity for the REEs but is not abundant. So the complexation by this site will be important when the REEs concentration is low. The second type of site is much more abundant but has a much smaller affinity for the complexation of REEs. This site will dominate when the REEs concentration is high. The first site remains unassigned but the second should be related to the carboxylic functional groups.

Kinetically controlled variations of major components and carbon and oxygen isotopes in a calcite-precipitating spring

Abstract A calcite-depositing stream in the Pyrenees, south France, enabled us to study the equilibration of major components and carbon and oxygen isotopes during the formation of calcite and the degassing of CO 2 , and to examine the possible influence of aquatic flora on these processes. Calcite is not precipitated from the solution until a supersaturation of ∼ 10 is attained. In order to start the reaction the activity of CO 2− 3 has to be increased from ∼ 5 · 10 −6 to 22 · 10 −6 . This is caused by an increase of pH due to the loss of CO 2 to the atmosphere. The excess free energy provided by CO 2 degassing is ∼ −5 kJ mol −1 . If formed under equilibrium conditions, the calcite should differ isotopically by Δ 13 C = + 2.3‰ from the dissolved carbonate. However, the observed difference is virtually zero and demonstrates substantial disequilibrium with respect to the stable carbon isotopes. Isotopic disequilibrium is also indicated by oxygen. 13 O-temperatures differ on the average by ∼ + 5°C from the observed temperatures of calcite precipitation. In contrast to calcite formation carbon-isotope equilibrium is attained between the dissolved carbonate and the atmosphere on the degassing of CO 2 . The observed changes of δ 13 C are in agreement with theoretical expectations, unless the surface area of the water is large. The variations of the major components and the carbon and oxygen isotopes observed during diurnal cycles are small and demonstrate that metabolic effects are negligible in a system with a high supply rate of ∼ 25 g HCO − 3 s −1 .

An experimental study of calcite and limestone dissolution rates as a function of pH from −1 to 3 and temperature from 25 to 80°C

Abstract Dissolution rates of single calcite crystals, limestones, and compressed calcite powders were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed in aqueous HCl solutions over the bulk solution pH range −1 to 3, and at temperatures of 25°, 50°, and 80°C. Corresponding rates of the three different sample types are identical within experimental uncertainty. Interpretation of these data using equations reported by Gregory and Riddiford [Gregory, D.P., Riddiford, A.C., 1956. Transport to the surface of a rotating disc. J. Chem. Soc. London 3, 3756–3764] yields apparent rate constants and H + diffusion coefficients. The logarithms of overall calcite dissolution rates ( r ) obtained at constant disk rotation speed are inversely proportional to the bulk solution pH, consistent with r = k 2 ′ a H + ,b , where k 2 ′ stands for an apparent rate constant and a H + ,b designates the hydrogen ion activity in the bulk solution. This variation of dissolution rates with pH is consistent with corresponding rates reported in the literature and the calcite dissolution mechanism reported by Wollast [Wollast, R., 1990. Rate and mechanism of dissolution of carbonates in the system CaCO 3 –MgCO 3 . In: Stumm, W. (Ed.), Aquatic Chemical Kinetics. Wiley, pp. 431–445]. Apparent rate constants for a disk rotation speed of 340 rpm increase from 0.07±0.02 to 0.25±0.02 mol m −2 s −1 in response to increasing temperature from 25° to 80°C. H + diffusion coefficients increase from (2.9 to 9.2)×10 −9 m 2 s −1 over this temperature range with an apparent activation energy of 19 kJ mol −1 .

Boehmite solubility and aqueous aluminum speciation in hydrothermal solutions (90–350°C): Experimental study and modeling

Abstract The solubility of a pure synthetic boehmite was measured in noncomplexing solutions over a wide range of pH (2–9) and temperatures (90–350°C). A least-squares linear regression of these data was used to determine the dissociation constants ( K ∗ s 0 to K ∗ s 4 ) of the mineral. Values obtained in basic solutions ( K ∗ s 4 ) are in close agreement with published data. In weakly acidic and neutral media, however, the values are lower than those reported previously in the literature. These differences are the result of the use of improved analytical techniques with lower detection limits and a more thermodynamically pure and stable solid material. The hydrolysis constants deduced in the present study indicate that, as temperature increases, aluminum hydrolysis becomes stronger, and the stability fields of Al(OH) − 4 and Al(OH) 0 3 expand at the expense of the other species. A density model (Franck, 1956; Anderson et al., 1991) was applied to extrapolate the experimental data to 25°C. This extrapolation permitted generation of a consistent set of thermodynamic data ( Δ 0 f ,298.15 and ΔH 0 f,298.15 ) for the aqueous aluminum monomer species. These data have been used to calculate the solubility of corundum at high temperature and pressure. The close agreement obtained with available experimental data indicates that the aluminate ion is dominant in most crustal fluids, in the absence of large amounts of complexing species.

Experimental studies of halite dissolution kinetics, 1 the effect of saturation state and the presence of trace metals

Abstract The comprehensive understanding of halite dissolution facilitates assessment of long-term radiogenic waste storage strategies, quantitative description of evaporite deposit formation and optimization of industrial processes. To this end, the dissolution rate of compressed halite powders was measured using the rotating disk method as a function of temperature, solute saturation state and the concentration of aqueous trace metals. The results of these experiments were used to generate dissolution and transport rate constants, as well as activation energies for these processes at temperatures from 25° to 80°C. Results indicate that in the absence of trace metals, constant-temperature halite dissolution rates vary linearly with the overall mineral saturation state. The presence of trace quantities of Fe and Zn were found to negligibly affect halite dissolution, while trace quantities of Co, Cr, Cd and Pb significantly lowered halite dissolution rates. At constant trace-metal concentration, halite dissolution rates were found to vary in the order: Fe ≈ Zn > Co > Cr > Pb > Cd. A combined transition state theory/Langmuir adsorption model was developed and demonstrated to effectively described measured rates as a function of solution composition.

An experimental study of calcite dissolution rates at acidic conditions and 25 °C in the presence of NaPO3 and MgCl2

Dissolution rates of single calcite crystals were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed at 25 °C in aqueous HCl solutions over the bulk solution pH range −1 to 3 and in the presence of trace concentrations of aqueous NaPO3 and MgCl2. These salts were chosen for this study because aqueous magnesium and phosphate are known to strongly inhibit calcite dissolution at neutral to basic pH. Reactive solutions were undersaturated with respect to possible secondary phases. Neither an inhibition or enhancement of calcite dissolution rates was observed in the presence of aqueous MgCl2 at pH 1 and 3. The presence of trace quantities of NaPO3, which dissociates in solution to Na+ and H2PO4−, decreased the overall calcite dissolution rate at pH≤2. This contrasting behavior could be attributed to the different adsorption behavior of these dissolved species. As calcite surfaces are positively charged in acidic solutions, aqueous Mg2+ may not adsorb, whereas aqueous phosphate, present as either the anion H2PO4− or the neutral species H3PO40, readily adsorbs on calcite surfaces leading to significant dissolution inhibition.

Experimental study of La(OH)3 and Gd(OH)3 solubilities (25 to 150°C), and La–acetate complexing (25 to 80°C)

The solubilities of pure synthetic La and Gd hydroxides were measured in aqueous solutions over a wide range of pH (5–9.5) and temperature (40–150°C). The regression of these data is used to determine the first dissociation constant (KS0*) of the minerals as a function of temperature. The logarithm of these constants is found to be linearly related to the reciprocal temperature, which allows their reliable extrapolation to 25°C. In the range of temperature and pH investigated, only the REE3+ ion (where REE denotes La or Gd) is present in solution, indicating that the first hydrolysis constants proposed by Baes and Mesmer (1976), which are generally admitted, are overestimated. The complexation of La with acetate ion (AcO−) was studied by both solubility and potentiometric measurements. Potentiometric titrations are interpreted in terms of the formation of two complexes: LaAcO2+ and La(AcO)2+, and are used to generate the corresponding stability constants in the temperature range 25–80°C. These data allow accurate description of the La(OH)3 solubility increase, evidenced in acetate-bearing solutions.

Convection in a North Sea oil reservoir: inferences on diagenesis and hydrocarbon migration

Cores drilled in a North Sea oil field reveal the existence of a siliceous level with an nextremely low porosity and permeability interbedded in the chalk oil reservoir. It is proposed that this silicification documents the mass transfer induced by a porous convective flow within the microfractured chalk. Numerical modelling of such circulation, using precise field observations as well as thermodynamic properties of aqueous solutions, is shown to support such a hypothesis. The precipitation of silica and calcite within the microfractures induced by the convection decreases the effective permeability of the chalk and destroys the convective circulation after a few million years. Nevertheless, during that period of time, a huge volume of water circulated (10 6 m 3 /m 2 ). It is also proposed that such a circulation can be a driving mechanism for oil migration.

Gibbs free energy of formation of kaolinite from solubility measurement in basic solution between 60 and 170 °C

Abstract The solubility of a natural hydrothermal kaolinite was measured in buffered alkaline solutions at temperatures ranging from 60 to 170 °C. At temperatures below 110 °C equilibrium was obtained from undersaturation, while above 110 °C two different steady states were obtained, one from undersaturation and another from supersaturation. Raman spectroscopy studies of the solid phases recovered at the end of each run, demonstrated that the “undersaturated” experiments attained equilibrium with respect to kaolinite, while the “supersaturated” experiments attained equilibrium with an illite-like mineral. Regression of kaolinite solubility products yields a Gibbs free energy of formation of kaolinite (Al 2 Si 2 O 5 (OH) 4 ) at 25 °C of −907.68 ± 0.2 kcal/mol. This value is more negative than most literature values, but similar to that generated from calorimetric data by Barany and Kelley (1961) . Furthermore, calculated thermodynamic parameters generated from kaolinite solubility obtained in this study are in close agreement with those deduced from boehmite-kaolinite equilibrium data at temperatures higher than 200 °C.

Modeling of the transport and deposition of tungsten in the scheelite-bearing calc-silicate gneisses of the Montagne Noire, France

Scheelite-bearing calc-silicate gneisses (CSG) oceur in the Montagne Noire within a series of dominant micaschists. Detailed petrographical and mineralogical studies reveal three successive stages of metamorphism and hydrothermal alteration: (1) stage 1, a regional metamorphism at 550°C and 4.5 kb where no mineralization is formed; (2) stage 2a, a hydrothermal alteration at 500 to 450°C and 4 to 3 kb which is characterized by an intense sericitization of feldpars and deposition of Sn in Sn-bearing cale-silicates; and (3) stage 2b, a hydrothermal alteration characterized by the crystallization of idocrasegrossular in CSG with concomittant precipitation of scheelite. Tungsten was transported through the micaschist environment and deposited as scheelite only in the CSG of stage 2b at relatively low pressures. To characterize the mechanism of tungsten transport, tungsten speciation at high P-T and scheelite solubility in aqueous solations buffered by the CSG and by the micaschists assemblages were calculated. It was found that H2WO04, HWO-4and WO2-4are the dominant tungsten aqueous species in H2O−NaCl (one molal) solutions at 500°C and 2–4 kb. Calculations also indicate that scheelite deposition is controlled by decreasing pressure and increasing activity of aqueous calcium in this system. This is consistent with the petrographical and mineralogical observations. The consequences of the presence of volatiles (N2, CH4, CO2) in the regional fluids were examined by determining the effect of N2 on tungsten speciation and scheelite solubility. The addition of N2 (up to 10 mol%) to the mineralizing fluids results in a marked increase in H2WO04and HWO-4concentrations relative to WO2-4and in a large decrease of scheelite solubility. This mechanism favours scheelite precipitation and accounts for the commonly observed association of W (and Sn) deposits with graphitic series generating mixed volatiles fluids.