Pascale Bénézeth

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.

Aqueous high-temperature solubility studies. I. The solubility of boehmite as functions of ionic strength (to 5 molal, NaCl), temperature (100 -290°C), and pH as determined by in situ measurements

Abstract The solubility of boehmite, γ-AlOOH, was measured in a modified hydrogen-electrode concentration cell, which provided continuous in situ measurements of hydrogen ion molality over the range of ionic strengths from 0.1 to 5.0 mol · kg−1 (NaCl) at temperatures from 100 to 290°C. A series of conventional solubility measurements was also carried out in acidic solutions over the same temperature range (i.e., pH was not monitored, but rather calculated from mass balance). The combined results yielded the molal solubility quotients, Qs0 and Qs4 for the equilibria: AlOOH(cr) + 3H+ ⇄ Al3+ + 2H2O Qs0 = [Al3+]/[H+]3 AlOOH(cr) + 2H2O ⇄ Al(OH)4− + H+ Qs4 = [Al(OH)4−][H+]In the regression of each isothermal data set, the values for the first hydrolysis quotient, Al3+ + H2O ⇄ Al(OH)2+ + H+ Q1,1 = [Al(OH)2+][H+]/[Al3+]were fixed according to a previous potentiometric study (Palmer and Wesolowski, 1993) . Moreover, for one series of titrations at 0.1 mol · kg−1 ionic strength at 150°C, the remaining two solubility quotients, Qs2 and Qs3, were determined simultaneously from the regression. However, at all other conditions, the values of Qs2 and Qs3 were also fixed at values consistent with the corresponding 0.03 mol · kg−1 ionic strength results (Benezeth et al., 2001) by invoking the isocoulombic assumption (i.e., an assumption of minimal ionic strength dependence for reactions with no net change in charge). The stability field with respect to pH of the Al(OH)2+ and Al(OH)30 aqueous species were found to be very narrow, and hence assumptions concerning their stabilities had little effect on the predicted shape of the solubility profiles at high ionic strength. Global fits were made of the log Qs0 and log Qs4 values as functions of temperature and ionic strength after combining with corresponding values from an analogous study at 0.03 mol · kg−1 ionic strength (Benezeth et al., 2001) , as well as appropriate constants taken from the literature. The fits were further constrained by inclusion of gibbsite solubility data ( Wesolowski 1992 , Palmer and Wesolowski 1992 after adjustment for the relative free energies of formation of gibbsite and boehmite. This treatment ensured that a continuous empirical model exists for Al3+ and Al(OH)4− from ambient conditions to 300°C and infinite dilution to five mol · kg−1 ionic strength.

Aqueous high-temperature solubility studies. II. The solubility of boehmite at 0.03 m ionic strength as a function of temperature and pH as determined by in situ measurements

Abstract The solubility of pure synthetic boehmite (γ-AlOOH) has been measured over a wide range of pH (2–10, depending on temperature), and temperature (100–290°C) at 0.03 mol · kg −1 ionic strength (NaCl) in a hydrogen-electrode concentration cell (HECC), which provided continuous in situ measurement of hydrogen ion molality (Palmer et al. 2001) . A least-squares regression of the results obtained was used to determine the molal solubility quotients (Q s0 to Q s4 ) of boehmite. The solubility products (Q sn ) were extrapolated to infinite dilution (K sn ), permitting calculation of the thermodynamic properties of aqueous species of aluminum for comparison with previous works, such as Bourcier et al. (1993) and Castet et al. (1993) . These results are generally consistent with the latter study conducted near infinite dilution, although some significant differences are apparent, particularly near the solubility minimum.

New Measurements of the Solubility of Zinc Oxide from 150 to 350°C

The solubility of zincite (ZnO) has been re-evaluated in noncomplexing solutions over a wide range of pH and from 150 to 350°C at pressures ranging from slightly above saturation vapor pressure to significantly higher pressures in NaOH, NH3, F3CSO3H/F3CSO3 Na (HTr/NaTr), CH3COOH/NaOH, and HTr/NH3 solutions using a hydrogen-electrode concentration cell (HECC) and a flow-through cell with downstream acid injection. The new results, coupled with our earlier results at 75–100°C, indicate that the solubility of zinc oxide is higher than we recently reported,(1) but substantially lower than previous estimates available in the scientific literature. It is believed that deposition of ZnO or a zinc hydroxide phase must have occurred near the solubility minimum in the HECC sample lines in our earlier study; a problem that is overcome in the flow cell by injecting acid into the sample line before the solution is depressurized and cooled to room temperature.

ZnO Solubility and Zn2 Complexation by Chloride and Sulfate in Acidic Solutions to 290°C with In-Situ pH Measurement

Abstract The solubility of zincite in mildly to strongly acidic aqueous solutions, according to the reaction ZnO + 2H+ ⇔ Zn2+ + H2O, has been measured at ionic strengths of 0.03–1.0 (stoichiometric molal basis) from 50 to 290°C at saturation vapor pressure in sodium trifluoromethanesulfonate solutions (NaTriflate, a noncomplexing 1:1 electrolyte). The hydrogen-electrode concentration cells employed in this study permit continuous and highly accurate pH measurement at elevated temperatures, and periodic sampling to determine the dissolved metal content of the experimental solution. The solubility of zincite is shown to be reversible at 200°C by addition of acidic and basic titrants, at constant ionic strength. The equilibrium constant is precisely described (±0.05 log units) by the function log K = −4.0168 + 4527.66/T. One additional adjustable parameter, together with an extended Debye-Huckel function, is sufficient to model the ionic strength dependence of the reaction. The solubility product at infinite dilution obtained from this study is in quantitative agreement with the thermodynamic model of Ziemniak 1992 . This experimental approach is demonstrated to be advantageous in studying the complexation of Zn2+ with Cl− and SO42−, by titrations involving the appropriate anion into NaTriflate solutions pre-equilibrated with zincite at constant temperature and ionic strength. Formation constants in 0.1 molal NaTriflate for the reaction Zn2+ + yLz− ⇔ Zn(L)y2−yz are reported for ZnCl+, ZnCl2° and ZnSO4° at 200°C (log Q = 1.7 ± 0.1, 3.0 ± 0.1, and 2.6 ± 0.1, respectively). Estimates of the equilibrium constants for the chloride species at infinite dilution and 200°C are log K = 2.5 ± 0.1 (ZnCl+), and 4.2 ± 0.1 (ZnCl2°). This value for the dichlorozinc complex agrees quantitatively with values reported by Bourcier and Barnes 1987 and Ruaya and Seward 1986 . However, the latter authors give a value for the monochlorozinc complex (log K = 4.01 ± 0.02) that is markedly different from our result and that of Bourcier and Barnes 1987 (log K = 3.1 ± 0.3).

Gallium speciation in aqueous solution. Experimental study and modelling: Part 1. Thermodynamic properties of Ga (OH) 4 to 300°C

Abstract The solubility of pure synthetic α-GaOOH was measured in dilute aqueous solutions at pH values ranging from 5 to 12.5, temperatures from 25 to 250°C, and at saturated water vapor pressures. Experimentally determined solubility constants were combined with the thermodynamic properties of α-GaOOH (Pokrovski et al., 1996) to generate, within the framework of the revised Helgeson-Kirkham-Flowers model (Tanger and Helgeson, 1988; Shock and Helgeson, 1988; Shock et al., 1992), the standard partial molal thermodynamic properties at 25°C and 1 bar, and revised equations of state parameters for Ga(OH)4−. Thermodynamic calculations indicate that Ga(OH)4− exhibits a chemical behavior very similar to that of Al(OH)4−.

Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics

Wollastonite (CaSiO3) dissolution rates were measured at 25°C in 0.01 M NaCl using a mixed-flow reactor as a function of pH (5 to 12) and concentration of forty organic ligands. Mostly stoichiometric dissolution was observed at these conditions. For seven ligands (acetate, citrate, EDTA, catechol, glutamic acid, 2,4-dihydroxybenzoic acid, glucuronic acid), batch adsorption experiments and electrokinetic measurements performed as a function of pH and ligand concentration confirmed the interaction of ligands with >CaOH2+ sites and allowed quantification of their adsorption constants. The effect of investigated ligands on wollastonite dissolution rate was modeled within the framework of the surface coordination approach taking into account the adsorption of ligands on dissolution-active sites and the molecular structure of the surface complexes they form. A positive correlation between surface adsorption constant and the stability constant of the corresponding reaction in homogeneous solution was observed. At neutral and weakly alkaline pH, the following total dissolved concentrations of ligands are necessary to double the rate of wollastonite dissolution: EDTA (10−4 M), phosphate (1.5 · 10−4 M), catechol (3 · 10−4 M), 8-hydroxyquinoline, gallic acid or adipate (5 · 10−4 M), 3,4-DHBA (7 · 10−4 M), PO3− (7.5 · 10−4 M), glutamate (0.002 M), citrate (0.003 M), malate or 2,4-DHBA (0.004 M), phthalate or succinate (0.005 M), tartrate (0.006 M), thioglycolate (0.008 M), aspartame (0.01 M), gluconate, ascorbate (> 0.01 M), malonate, diglycolate or lactate at pH 8.4 (0.02 M), formate or fumarate (0.05 M), oxalate (>0.05 M), bicarbonate (0.075 M), lactate at pH 5.6 (0.1 M), acetate (> 0.1 M), salicylate (0.15 M), humic acids (> 54 mg/L of dissolved organic carbon, DOC), gum xanthan (1.5-2.0 g/L). Sorbitol, mannitol, glucose, glucosamine, saccharose, fulvic acids and silica at pH ∼ 7 exhibit weakly inhibiting or no effect up to concentration of 0.1 M. The presence of the following ligands leads to a decrease of dissolution rates by a factor of 2: silica at pH 10.7 (2 · 10−4 M), glucuronic acid (0.001 M), algae exudates (30 mg/L DOC), mannit (0.02 M), urea (>0.05 M), pectin (>15 g/L), alginic acid (> 2 g/L). Overall, results of this study demonstrate that high concentrations (0.001-0.01 M) of organic ligands, whether they are originated from organic matter, enzymatic degradation or bacterial metabolic activity, are necessary to appreciably enhance wollastonite dissolution. This is further corroborated by batch experiments on live and dead cultures of soil bacteria Pseudomonas aureofaciens interaction with wollastonite. The release rates of both Ca and Si are only weakly affected by the presence of live or dead bacterial cells in inert electrolyte solution and in nutrient media: there is only ∼20 percent-increase of dissolution rate in experiments with live cultures compared to dead cultures. However, the reproducibility of rate measurements in ligand-free solutions at 7 ≤ pH ≤ 8 achieves ± 30 percent. Therefore, the effect of extracellular organic products on the weathering rate of Ca-bearing minerals is expected to be weak and the acceleration of “basic” silicate rocks dissolution in natural settings in the presence of soil bacteria is likely solely due to the pH decrease.

The solubility of zinc oxide in 0.03 m NaTr as a function of temperature, with in situ pH measurement

Abstract The solubility of zincite (ZnO) has been measured in noncomplexing solutions over a wide range of pH m (4–11), and temperature (75–200°C) at 0.03 mol · kg −1 ionic strength in NaTr media (sodium trifluoromethanesulfonate, a noncomplexing 1:1 electrolyte), in a hydrogen electrode concentration cell (HECC), which provided continuous in situ measurement of hydrogen ion molality. Total zinc content was analyzed by atomic absorption using graphite furnace, flame, and inductively coupled plasma (ICP) spectrometers. The direction of approach to the equilibrium saturation state was varied to demonstrate that the system was reversible thermodynamically. Separate experiments were performed in alkaline solutions (0.03 mol · kg −1 NaOH) at 25 and 50°C in polypropylene syringes, and between 50 and 290°C in a Teflon-lined pressure vessel. The aim of these experiments was to reach higher pH m (>8 depending on the temperature) to determine the thermodynamic properties of the negatively charged species, Zn(OH) 3 − . A least-squares regression of the results obtained at this ionic strength was used to determine the molal solubility products (Q sn ) of zincite. The solubility products (Q sn ) were extrapolated to infinite dilution (K sn ), permitting calculation of the thermodynamic properties of aqueous species of zinc for comparison with previous work.

Solubility and surface adsorption characteristics of metal oxides

Publisher Summary This chapter provides an overview of the recent developments in the understanding of the interaction of metal oxide and hydroxide minerals with hydrothermal solutions. With a few exceptions, metals exposed to aqueous solutions generally form an oxide surface film, which may or may not passivate the surface toward further oxidation. This phenomenon is driven by the relative Gibbs energies of the solid phases and the redox state of the system. Metal oxides formed in the presence of water are often hydrated regardless of whether the hydrous phase is thermodynamically stable relative to the pure oxide or a less hydrated phase. The ideal method of defining a solubility product is to demonstrate that the reaction is “reversible,” i.e., to show that the solution achieves the same IAP = K by approaching the equilibrium condition from under and super-saturation. The solubilities of metal oxides cannot be described without a detailed understanding of the hydrolysis and complexation of the aqueous reactants and products at elevated temperatures. At this time, there are no systems for which both dissolution/precipitation rate data and detailed information on the pH-dependent charging and adsorptive characteristics of the solid phase are known at elevated temperatures. However, the HECC and other emerging experimental techniques make such studies feasible and important targets for future research in hydrothermal oxide–water interactions.

Measurement of bacterial surface protonation constants for two species at elevated temperatures

This study reports the first potentiometric titrations of bacterial surfaces at elevated temperatures. We used a hydrogen electrode concentration cell to examine the protonation behavior of Bacillus subtilis (an aerobic, gram-positive species present in a wide variety of low-temperature, near-surface environments) at 30, 50, and 75°C. We also investigate the protonation of TOR-39 (an anaerobic, Fe-reducing, thermophylic bacteria isolated from a deep sedimentary basin) at 50°C and compare its surface properties to those of B.subtilis. Using a surface complexation approach that has been previously applied only to room temperature experiments involving bacteria, we determine the number of functional group types present on each cell wall, their acidity constants, and their site concentrations. We also characterize the temperature dependence of the acidity constants for B.subtilis. Our results indicate that the surface acidity can be successfully described using site-specific reactions involving discrete surface functional groups. Our results indicate that the acidity constants for the surface functional groups on B. subtilis do not change significantly over the temperature interval studied, and that the surface protonation at each temperature can be estimated using a single set of acidity constants and site densities. The best fitting model for B. subtilis is a three- site model with pKa values of 4.1, 5.8, and 7.7. Average site abundances for the three dominant functional groups on B. subtilis were determined to be 9.5 · 10−5, 1.0 · 10−4, and 8.8 · 10−5 moles of sites/gm. For TOR-39, the best fitting model was also a 3-site model with pKa values of 4.5, 5.8 and 8.2, with site abundances of 6.0 · 10−5, 5.0 · 10−5 and 3.0 · 10−5. This study suggests that bacterial surface site protonation reactions are not strongly temperature dependent, at least to 75°C. The lack of significant temperature dependence could greatly simplify the task of modeling bacteria-water-rock interactions at elevated temperature.