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Aqueous Geochemistry

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Scott A Wood – 1st expert on this subject based on the ideXlab platform

  • the Aqueous Geochemistry of gallium germanium indium and scandium
    Ore Geology Reviews, 2006
    Co-Authors: Scott A Wood, Iain M Samson

    Abstract:

    Relatively little information is available in the literature regarding the speciation and solubility of Ga, Ge, In and Sc in Aqueous solutions, especially at elevated temperatures and pressures. In this paper we critically review stability constants for relevant Aqueous complexes of these metals and solubility products for relevant solid phases. Most of the available data refer to standard conditions of temperature and pressure (25 8C and 1 bar), although for Ga and Ge, experimentally derived data are available for some geologically relevant species and phases up to 200–300 8C. The stable oxidation states of the four metals in Aqueous solution are Ga(III), Ge(IV), In(III) and Sc(III). The ions of each of these metals are relatively hard in the Pearson [Pearson, R.G. 1963. Hard and soft acids and bases. Journal of the American Chemical Society 85, 3533–3539] sense, forming the strongest complexes with hard ligands such as hydroxide, fluoride, sulfate and phosphate, and weaker complexes with soft ligands such as chloride and bisulfide. The main exception to this rule is In(III), which forms reasonably stable chloride and bisulfide complexes. The hydrated Ga 3+ ,I n 3+ and Sc 3+ ions are all octahedrally coordinated by water molecules, but there is some evidence, e.g., for GaCl4 � and InCl4 , that a conversion to tetrahedral coordination may occur upon replacement of water by a sufficient number of other ligands. In most hydrothermal solutions, we predict that hydroxide complexes will be the most important forms of transport of Ga and Sc, although fluoride complexes will be important in environments where fluoride activities are relatively high (e.g., during greisen formation). In analogy with Si, the most important species for Ge is germanic acid, H4GeO4 0 , but again fluoride complexes may be important at high fluoride activities. Chloride complexes are not expected to play a significant role in the transport of Ga, Ge or Sc at temperatures below approximately 300 8C. The behavior of In is expected to be the most variable, with, depending on conditions, hydroxide, chloride, fluoride or bisulfide complexes all contributing to its transport. Sulfate and phosphate complexes of Ga, In and Sc may play limited roles in the hydrothermal mass transfer of these elements, but only under special conditions; normally these complexes will be less important than hydroxide or fluoride complexes. Solubility calculations for 25 8C indicate that In-sulfide and Sc-phosphate are less soluble (i.e., more stable) than the corresponding oxyhydroxides, even when In-bisulfide and Sc–phosphate complexes are taken into account. However, the phases GaPO4(s) and InPO4(s) are generally more soluble than the corresponding oxyhydroxides. Solubilities of a-GaOOH(s) and GeO2(tetragonal) have been estimated up to 300 8C and both increase with increasing temperature. The solubility of pure aGaOOH(s) at 25 8 C( b10 � 6 m) is quite low from pH 3 to pH 8, consistent with the general immobility of Ga in the weathering environment and its concentration in bauxites. D 2005 Published by Elsevier B.V.

  • the Aqueous Geochemistry of the rare earth elements part xiv the solubility of rare earth element phosphates from 23 to 150 c
    Chemical Geology, 2005
    Co-Authors: Ziya S Cetiner, Scott A Wood, Christopher H. Gammons

    Abstract:

    Abstract Rare earth element (REE) phosphates such as monazite and xenotime are important as ore minerals, potential hosts of radioactive waste, and target phases for isotopic dating. However, there are still insufficient thermodynamic data with which to model dissolution and precipitation of these phases in crustal fluids quantitatively. Therefore, the solubilities of end-member La(III)-, Nd(III)-, Sm(III)-, and Y(III)- phosphates were determined at 23 and 50 °C in NaCl–HCl and NaClO 4 –HClO 4 solutions with pH m from 0 to 2 and ionic strengths of 0.1, 0.5, 1.0, and 5.0 m. The solubility of Nd(III)-phosphate was also determined in chloride solutions at 150 °C. The La(III)- and Nd(III)-phosphates had the monazite structure, and Sm(III)-phosphate and Y(III)-phosphate had the rhabdophane and xenotime structures, respectively. The dependence of solubility on pH and chloride concentration, together with data from the literature, indicated that H 3 PO 4 0 and Ln 3+ (where Ln 3+ represents any free, hydrated trivalent REE ion) were the predominant species in our experimental solutions. At each ionic strength and temperature investigated, conditional equilibrium constants ( Q s3 ) were determined for reactions of the following type: LnPO 4 +3H + ↔Ln 3+ +H 3 PO 4 0 The conditional equilibrium constants determined at various ionic strengths were extrapolated empirically to obtain the equilibrium constants at infinite dilution ( K s3 ). These constants were then converted to solubility products ( K s0 ) for the following reaction: LnPO 4 (s)↔Ln 3+ +PO 4 3− using acid dissociation constants for H 3 PO 4 0 available in the literature. The values of log K s0 so obtained are: 23°C 50 °C 150 °C La −25.7 −25.4 – Nd −25.8 −26.6 −30.8 Sm −24.6 −24.8 – Y −27.9 −27.8 – These values are in reasonable agreement with the majority of those in the literature, with the exception of the values for Y(III)-phosphate, which are substantially lower. Our results, combined with data in the literature, suggest that the solubility products of REE phosphates are retrograde (i.e., decrease with increasing temperature) up to at least 300 °C. Moreover, the solubility of REE phosphate is quite low up to 300 °C, even at low pH and high chloride concentrations, confirming the robustness of these phases as hosts for radionuclides.

  • the Aqueous Geochemistry of the rare earth elements
    Chemical Geology, 2000
    Co-Authors: Scott A Wood, David J Wesolowski, Donald A Palmer

    Abstract:

    Abstract The concentration quotients for the following reactions with acetate have been measured at temperatures from 25°C to 225°C, at saturated water vapor pressure, and in 0.1 molal NaCl medium, using a potentiometric method: Nd 3+ + Ac − = NdAc 2+ Nd 3+ +2 Ac − = NdAc 2 + The concentration quotients for both of these reactions increase strongly with increasing temperature. The values of these constants obtained at 25°C are in good agreement with those reported in the literature for both NaCl and NaClO4 media, indicating that complexation of Nd3+ by chloride is weak at room temperature. The concentration quotients were corrected for complexation by chloride using experimentally determined stability constants from the literature; these calculations confirmed the lack of a significant effect due to chloride complexation at 25°C, but showed that the effect becomes more important with increasing temperature. Extrapolation of the chloride-corrected concentration quotients to zero ionic strength using an extended Debye–Huckel expression yielded values of the stability constants for the first and second complexation steps that are significantly higher than theoretical estimates reported in the literature. Our data show that Nd-acetate complexes are considerably more stable than Nd-chloride complexes, but that the stability constants for both systems exhibit very similar temperature dependences. In sedimentary basinal brines REE (rare earth elements)-acetate complexes may predominate over REE-chloride complexes, even where chloride is present in greater concentrations than acetate. However, oxalate, fluoride, or carbonate complexes could be more important than either acetate or chloride if the concentrations of the former ligands are relatively high.

Anthony E Williamsjones – 2nd expert on this subject based on the ideXlab platform

  • the Aqueous Geochemistry of the rare earth elements and yttrium vi stability of neodymium chloride complexes from 25 to 300 c
    Geochimica et Cosmochimica Acta, 1996
    Co-Authors: Christopher H. Gammons, Scott A Wood, Anthony E Williamsjones

    Abstract:

    Abstract The stability and stoichiometry of Nd (III) chloride complexes have been experimentally determined in the temperature range 40 to 300°C, P = Psat. The solubility of AgCl (s) was measured in solutions of fixed HC1 + NaC1 concentration (0.01 to 5.0 m) and varying ΣNd/ΣCl molar ratio (0.0 to 0.5), following the method of Gammons (1995). The results of over 250 individual solubility experiments were regressed to obtain the following smoothed values for the first and second cumulative association constants for the Nd(III) chloride complexes: Nd3+ + Cl− = NdC12+ (β1); Nd3+ + 2Cl− = NdCl2+ (β2):25°C50°C100°C150°C200°C250°C300°log β10.060.210.661.312.173.224.48±.50±.30±.20±.20±.15±.30±.50log β2——0.131.082.524.456.87±.50±.30±.15±.50c]±.50These are the first experimentally determined equilibrium constants for chloride complexes of any rare earth element (REE) at elevated temperature. At 25°C, neodymium exists mainly as Nd3+ in the absence of high concentrations of Cl− and other ligands (F−, CO3−, SO4−). However, complexation with chloride is greatly enhanced by increase in temperature, such that NdC12+, NdC12+, and possibly NdCl30 become the dominant species for NaCl HCl H2O brines at 300°C. The experimental data indicate a higher degree of complexation than predicted from earlier theoretical studies (Wood, 1990b; Haas et al., 1995), particularly in the case of log β2. Calculations of monazite solubility in seafloor hydrothermal systems (Wood and Williams-Jones, 1994) are re-evaluated in light of our new experimental data. Chloride complexes are shown to dominate the Aqueous Nd socciation at 300°C, and lead to solubilities that are (1) much higher than previously estimated and (2) much closer to the maximum concentrations that have been reported from active black smokers. However, the large fluxes of REEs in altered rock beneath ancient massive sulfide deposits are still difficult to explain assuming that modern seafloor hydrothermal systems are direct analogs to ore-forming processes. Significant differences in fluid chemistry (e.g., lower pH, higher Cl− or F− concentrations) and/or duration and intensity of hydrothermal activity (higher water/rock ratio) are required to explain the REE systematics in ancient volcanogenic massive sulfide deposits.

  • the Aqueous Geochemistry of zr and the solubility of some zr bearing minerals
    Applied Geochemistry, 1995
    Co-Authors: Scott A Wood, Anthony E Williamsjones

    Abstract:

    Abstract Literature data on the thermodynamics of complexation of Zr with inorganic species, at 25°C, have been critically reviewed. The preponderance of published complexation constants deal with F− and OH− ions. Stability constants for the complexation reactions are relatively independent of ionic strength and thus recomended values for each ligand type are averages of the most reliable data. Complexation constants under elevated conditions (T ⪕ 250°C andPv = PH2O) have been predicted for various Zr complexes (F−, Cl−, SO42− and OH−) using Helgeson’s electrostatic approach. Predominance diagrams (calculated for simple systems with these constants) suggest that, over a wide range of pH conditions, Zr(OH)4(aq) will dominate the Aqueous Geochemistry of Zr except under very high activities of competing ligands (e.g., F−, SO42−). The solubilities of vlasovite [Na2ZrSi4O11] and weloganite [Sr3Na2Zr(CO3)6·3H2O have been measured in KCI solutions (0.5–1.0 M) at 50°C. Weloganite dissolution is complicated by the predictable precipitation of strontianite (SrCO3) whereas vlasovite dissolves incongruently. Solubility products for the dissolution of welonganite and vlasovite are determined to be −28.96±0.14 and −20.40±1.18, respectively. Concentrations of Zr up to 10−3 m were present in the experimental solutions; the presence of large amounts of Zr in Aqueous solutions support the possibility of extensive remobilization of Zr during hydrothermal mineralization.

  • the Aqueous Geochemistry of the rare earth elements and yttrium 4 monazite solubility and ree mobility in exhalative massive sulfide depositing environments
    Chemical Geology, 1994
    Co-Authors: Scott A Wood, Anthony E Williamsjones

    Abstract:

    Abstract Although there is considerable evidence that rare-earth elements (REE), and particularly the light REE (LREE), have been mobile in the alteration zones below many massive sulfide deposits, there is some disagreement over the cause of this mobility. Most researchers have argued or implied that the REE were mobilized by analogues of modern seafloor hydrothermal vent fluids. However, the few measurements that have been made of the REE concentrations of these modern fluids suggest that their ability to mobilize REE is negligible. In order to shed further light on this problem we have carried out calculations of monazite solubility (monazite is thought to be the principal host for the LREE) in a model vent fluid at temperatures from 200° to 300°C. These calculations show that at 300°C the model fluid will contain 0.08 ppb Ce, 0.07 ppb La and 0.03 ppb Nd. At 200°C the concentrations increase to 0.54 ppb, 0.37 ppb and 0.15 ppb, respectively. A decrease in pH of one unit at 300°C also increases REE solubility significantly (7.4 ppb Ce, 5.7 ppb La and 1.1 ppb Nd). These solubilities are similar to the concentrations of REE measured in seafloor hydrothermal vent fluids. However, they are considerably lower than required to account for the scale of REE mobility in alteration zones associated with massive sulfide deposits. If the mass ratio of typical hydrothermal vent fluid to rock reached 1000, concentrations of REE in the altered rock the predicted to be in the range

Yongliang Xiong – 3rd expert on this subject based on the ideXlab platform

  • the Aqueous Geochemistry of thallium speciation and solubility of thallium in low temperature systems
    Environmental Chemistry, 2009
    Co-Authors: Yongliang Xiong

    Abstract:

    Environmental context. The Aqueous Geochemistry of thallium is not well known in comparison with cadmium and lead, although it is more highly toxic, and at the same time has a wide range of industrial applications. A database allowing us to reliably predict the speciation and solubility of thallium in various environments in low temperature systems would be invaluable in providing some understanding of thallium’s mobilisation and mitigation. We propose here such a thermodynamic database based on critical reviews. Abstract. Thallium is a highly toxic element, and at the same time it has a wide range of applications in industry. Therefore, it is important to know its speciation and solubility under low temperature conditions. This study expands the thermodynamic database of the first paper of this series on the Aqueous Geochemistry of thallium by providing the formation constants of some important thallium complexes, including TlEDTA3–, TlOx– (Ox: oxalate), TlSuc– (Suc: succinate), TlMal– (Mal: malonate) and TlHPO4–. This study also recommends the solubility product constant of TlCl(s) as 10–3.65. The combined database allows us to model reliably the speciation and solubility of thallium in the Earth surface environments. As an example, the speciation and solubility of thallium in soil solutions are presented based on thermodynamic calculations.