Derry McPhail

Thermodynamic properties of aqueous tellurium species between 25 and 350

Tellurium occurs in trace amounts in a wide range of geological environments, the most important of which are epithermal ore deposits that contain precious- and base-metal telluride minerals. The behaviour of Te in hydrothermal fluids is poorly known, making it difficult to understand the formation of these types of deposits. This paper reviews the existing information for aqueous Te species and reports the derivation of their thermodynamic properties from 25 to 350°C. Free energies of formation at 25°C were derived mainly from experimental data and estimated at higher temperature using new experimental data for the solubility of tellurite (TeO2) up to 80°C and isocoulombic techniques. The stabilities of aqueous Te species were then predicted as a function of temperature, redox, acidity, and total tellurium concentration. For conditions typical of many ore-forming liquids telluride and tellurite species are predicted to be the most important Te species. Under relatively reduced (e.g., magnetite-stable) and acid to neutral pH conditions, the most important species are predicted to be H2Te (aq) and HTe −. In more oxidized (hematitestable) environments, H3TeO3+, H2TeO3, and HTeO3− predominate. There is a polymeric species (Te22−) that may be important under some conditions, but its stability is limited by the solubility of native Te and dependent on total Te concentration. At temperatures between 100 and 300°C, it is predicted to be stable at pH greater than approximately 8 and oxygen activities near and below the Te−II/TeIV boundaries. At even higher pH (>10 to 12) Te2− and Te032− predominate. The most oxidized Te species, the tellurates (H6TeO6, H5Te06− , and H4TeO6−) may be important at low temperature ( 7 or 8) the solubility of native Te increases with increasing pH regardless of the oxygen activity. Measured Te concentrations in experiments on the solubility of hessite + native Te + chlorargyrite in HCl solutions at 300°C are much higher (1s to 1000s of ppm) than predicted (10s to 100s of ppb) and may be evidence for unknown Te species (e.g., metal-tellurium complexes or aqueous Te in other valence states). The low predicted concentrations of aqueous Te may also indicate that vapour-phase transport of Te is important in the formation of telluride-bearing epithermal ore deposits.

Complexation of metal ions in brines: application of electronic spectroscopy in the study of the Cu(II)-LiCl-H2O system between 25 and 90°c

Abstract The concentration and transport of metals in hydrothermal solutions depend on how metals ions combine with ligands to form complexes, and experimental methods are necessary to identify the important complexes. UV-Vis-NIR spectrophotometry was used to study the formation of Cu(II)-chloride complexes in LiCl brines up to very high chlorinities (18 m LiCl), at temperatures between 25°C and 90°C. The number of Cu(II)-chloride complexes necessary to account for the variation in spectra with varying chloride molality at each temperature was estimated using principal component analysis. The molar absorptivity coefficients and concentrations of each complex were then determined using a “model-free” analysis, which does not require any assumption about the chemistry of the system, other than the number of absorbing species present. Subsequently, the results from the “model-free” analysis were integrated with independent experimental evidence to develop a thermodynamic speciation model, where the logarithms of the equilibrium constants for Cu(II)-chloride formation reactions were fitted to the data using a non-linear least-squares approach. Maps of the residual function were used to estimate uncertainties in the fitted equilibrium constants. The results of this study are similar to published properties of distorted octahedral [CuCl(OH 2 ) 5 ] + and [CuCl 2 (OH 2 ) 4 ] 0 at all temperatures, but diverge for [CuCl 3 (OH 2 ) 3 ] − and distorted tetrahedral [CuCl 4 ] 2− . Moreover, the data suggest the presence of [CuCl 5 ] 3− , probably with D 3h point group, at very high salt concentration. This study demonstrates that it is possible to determine apparent thermodynamic equilibrium constants for the formation of complexes of trace amount of metals in highly concentrated brines, such as those associated with many ore deposits. The results are dependent on the choice of activity coefficients for charged and neutral aqueous complexes, but this influence is relatively small compared with the experimental uncertainty. This study shows that Cu 2+ chloro-complexes, predominantly [CuCl 2 (OH 2 ) 4 ] 0 and [CuCl 4 ] 2− , will play a dominant role in nature where free oxygen is available (near-surface), and where chloride activities are very high (evaporitic basins; hypersaline soils).

An experimental study of copper(I)-chloride and copper(I)-acetate complexing in hydrothermal solutions between 50°C and 250°C and vapor-saturated pressure

The transport of copper is affected by complexing with ligands such as chloride, bisulphide and acetate. Many copper deposits are formed from hydrothermal waters and brines and in order to understand their formation we need thermodynamic properties of aqueous copper species. The solubility of cuprite experiments in pH-buffered acetate and chloride solutions was determined at 50°C, 150°C and 250°C and water vapor saturated pressures. Chloride and acetate concentrations were varied using NaCl (0.001m to 2 m) and NaAc (0.1m to 2 m, where Ac refers to acetate). Measured copper concentrations varied between 0.0001 m and 0.2 m and show systematic increases with increasing temperature and acetate or chloride concentration. Logarithms of the formation constants (log K) of copper complexes were fitted by a simplex function minimization method, then speciation models and uncertainties of fitted log K values were examined by residual maps and speciation plots of experimental solutions. Interpretation of our experimental data shows that CuCl(aq), CuCl2−, CuAc(aq) and CuAc2− were present in our experiments at all temperatures. CuCl32− is an important complex at 50°C, but cannot be detected unequivocally at 150°C and 250°C. This study presents the first experimentally-derived log K values for copper(I) acetate complexes (CuAc(aq) and CuAc2−). The derived log K values of copper(I) chloride complexes (CuCl(aq) and CuCl2−) are similar to recently published experimentally derived values (Xiao et al., 1998). Based on new log K values generated from this study, our calculations indicate that copper transport as acetate complexes is important in acetate-bearing fluids lacking competition of other ligands such as Cl− and HS−.

A spectrophotometric study of aqueous copper(I)-chloride complexes in LiCl solutions between 100°C and 250°C.

Copper transport and deposition in highly saline hydrothermal fluids are controlled by the stability of copper(I) complexes with ligands such as chloride and hydrosulphide. However, our understanding of the behavior of copper(I)–chloride complexes at elevated temperatures and in highly saline brines is limited by the conditions of existing experimental studies where the maximum chloride concentration is 2 m. This paper presents the results of a study of copper(I)–chloride complexes at much higher chloride concentrations, 1.5 m to 9.1 m, using a UV spectrophotometric method. The UV spectra of copper(I)-bearing LiCl solutions were measured at temperatures between 100 °C and 250 °C at vapor-saturated pressures and quantitative interpretation of the spectra shows that CuCl2−, CuCl32−, and CuCl43− were present in the experimental solutions. The fitted logarithms of formation constants (log K) for CuCl2− are in good agreement with the previous results of solubility experiments reported by Xiao et al. (1998) and Liu et al. (2001). The log K values for CuCl32− also agree with those of Liu et al. (2001) and theoretical estimates of Sverjensky et al. (1997). This study presents the first experimentally determined formation constants for CuCl43−, at temperatures greater than 25 °C, and indicates that this complex predominates at chloride concentrations greater than 5 m. Based on the new log K values generated from this study, the calculated chalcopyrite solubility in NaCl solutions indicates that in addition to cooling, fluid mixing (dilution of saline fluids) may be an important factor controlling the deposition of copper minerals from hydrothermal solutions.

Measurement of pH at elevated temperatures using the optical indicator acridine

AbstractA new method of measuring the pH of high-temperature aqueous solutions using optical indicators was developed. The absorbance spectrum of acridine changes as a function of pH, allowing its use as an optical indicator for pH. The pKa of acridine were experimentally determined in KOH-AcOH pH-buffer solutions at temperatures from 5 to 250°C at vaporsaturated pressures:

Continental-scale variation in chloride/bromide ratios of wet deposition.

pKa = 5.62 + 1158{\text{ }}/{\text{ }}T{\text{ - }}0.7168{\text{ }}lnT

Potential utilisation of micro-organisms in gold processing: a review

where T is in Kelvin. The technique is demonstrated by experimentally determining the pKa of benzoic acid at temperatures up to 250°C where the result was