Gleb S. Pokrovski

Iron (III)-silica interactions in aqueous solution: Insights from X-ray absorption fine structure spectroscopy

The influence of aqueous silica on the hydrolysis of iron(III) nitrate and chloride salts in dilute aqueous solutions (m(Fe) similar to 0.01 mol/kg) was studied at ambient temperature using X-ray absorption fine structure (XAFS) spectroscopy at the Fe K-edge. Results show that in Si-free iron nitrate and chloride solutions at acid pH (pH 3. At pH > 2.5 in the presence of aqueous silica, important changes in Fe(III) hydrolysis are detected. In 0.05-in Si solutions (pH similar to 2.7-3.0), the corner linkages between Fe octahedra in the polymeric complexes disappear, and the Fe-Fe distances corresponding to the edge linkages slightly increase (Fe-Fe-edge similar to 3.12-3.14 Angstrom). The presence of 1 to 2 silicons at 3.18 +/- 0.03 Angstrom is detected in the second atomic shell around iron. At basic pH (similar to12.7), similar structural changes are observed for the iron second shell. The Fe-Si and Fe-Fe distances and coordination numbers derived in this study are consistent with (1) Fe-Si complex stoichiometries Fe2Si1-2 and Fe3Si2-3 at pH < 3; (2) structures composed of Fe-Fe dimers and trimers sharing one or two edges of FeO6-octahedra; and (3) silicon tetrahedra linked to two neighboring Fe octahedra via corners. At higher Si concentration (0.16 m, polymerized silica solution) and pH similar to 3, the signal of the Fe second shell vanishes indicating the destruction of the Fe-Fe bonds and the formation of different Fe-Si linkages. Moreover, similar to20 mol.% of Fe is found to be tetrahedrally coordinated with oxygens in the first coordination shell (RFe-O = 1.84 Angstrom). This new finding implies that Fe may partially substitute for Si in the tetrahedral network of the silica polymers in Si-rich solutions. The results of this study demonstrate that aqueous silica can significantly inhibit iron polymerization and solid-phase formation, and thus increase the stability and mobility of Fe(III) in natural waters. The silica poisoning of the free corner sites of iron-hydroxide colloids should reduce the adsorption and incorporation of trace elements by these colloids in Si-rich natural waters. Copyright (C) 2003 Elsevier Ltd.

Thermodynamic properties and stoichiometry of As (III) hydroxide complexes at hydrothermal conditions

The stoichiometry and thermodynamic properties of As (III) hydroxide complexes were determined from both solubility and Raman spectroscopic measurements. Arsenolite, claudetite, and orpiment solubilities were measured at temperatures to 250 and 300 °C, respectively, in acid solutions (pH < 6) at the saturated vapor pressure of the system. Raman spectroscopic measurements were performed on As2O3-H2O solutions (0.02 ≤ As ≤ 6 m; 0 ≤ pH ≤ 9) at temperatures from 20 to 275 °C. Results indicate that H3AsO30(aq) is the dominant As-bearing species at concentrations up to ~1 m over a wide range of pH (0–8) and temperature (20–300 °C). At higher As concentrations (≥1–2 m), a polymerization-dehydration of H3AsO30(aq) occurs via the formation of As-O-As bonds, leading to the formation of poly-As aqueous complexes. These experimental results were combined with corresponding properties for arsenolite, claudetite, and orpiment obtained in this study to generate H3AsO30(aq) thermodynamic properties within the framework of the revised HKF equation of state (Helgeson et al., 1981; Tanger and Helgeson, 1988). Calculations carried out using these properties indicate that orpiment, realgar, and native As can control As concentration in epithermal fluids at T ≤ 150–200 °C. At higher temperatures (≥250 °C), it is shown that arsenopyrite in association with pyrite and pyrrhotite or cassiterite can control As deposition in hydrothermal environments.

Experimental study of arsenic speciation in vapor phase to 500°C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases

Abstract The stoichiometry and stability of arsenic gaseous complexes were determined in the system As-H2O ± NaCl ± HCl ± H2S at temperatures up to 500°C and pressures up to 600 bar, from both measurements of As(III) and As(V) vapor–liquid and vapor–solid partitioning, and X-ray absorption fine structure (XAFS) spectroscopic study of As(III)-bearing aqueous fluids. Vapor–aqueous solution partitioning for As(III) was measured from 250 to 450°C at the saturated vapor pressure of the system (Psat) with a special titanium reactor that allows in situ sampling of the vapor phase. The values of partition coefficients for arsenious acid (H3AsO3) between an aqueous solution (pure H2O) and its saturated vapor (K = mAsvapor /mAsliquid) were found to be independent of As(III) solution concentrations (up to ∼1 to 2 mol As/kg) and equal to 0.012 ± 0.003, 0.063 ± 0.023, and 0.145 ± 0.020 at 250, 300, and 350°C, respectively. These results are interpreted by the formation, in the vapor phase, of As(OH)3(gas), similar to the aqueous As hydroxide complex dominant in the liquid phase. Arsenic chloride or sulfide gaseous complexes were found to be negligible in the presence of HCl or H2S (up to ∼0.5 mol/kg of vapor). XAFS spectroscopic measurements carried out on As(III)-H2O (±NaCl) solutions up to 500°C demonstrate that the As(OH)3 complex dominates As speciation both in dense H2O-NaCl fluids and low-density supercritical vapor. Vapor–liquid partition coefficients for As(III) measured in the H2O-NaCl system up to 450°C are consistent with the As speciation derived from these spectroscopic measurements and can be described by a simple relationship as a function of the vapor-to-liquid density ratio and temperature. Arsenic(III) partitioning between vapor and As-concentrated solutions (>2 mol As/kg) or As2O3 solid is consistent with the formation, in the vapor phase, of both As4O6 and As(OH)3. Arsenic(V) (arsenic acid, H3AsO4) vapor–liquid partitioning at 350°C for dilute aqueous solution was interpreted by the formation of AsO(OH)3 in the vapor phase. The results obtained were combined with the corresponding properties for the aqueous As(III) hydroxide species to generate As(OH)3(gas) thermodynamic parameters. Equilibrium calculations carried out by using these data indicate that As(OH)3(gas) is by far the most dominant As complex in both volcanic gases and boiling hydrothermal systems. This species is likely to be responsible for the preferential partition of arsenic into the vapor phase as observed in fluid inclusions from high-temperature (400 to 700°C) Au-Cu (-Sn, -W) magmatic-hydrothermal ore deposits. The results of this study imply that hydrolysis and hydration could be also important for other metals and metalloids in the H2O-vapor phase. These processes should be taken into account to accurately model element fractionation and chemical equilibria during magma degassing and fluid boiling.

Experimental study of the complexation of silicon and germanium with aqueous organic species: implications for germanium and silicon transport and Ge/Si ratio in natural waters

Abstract The stability of aqueous complexes formed by Si and Ge with carboxylic acids (acetic, salicylic, oxalic, citric, tartaric) and phenols (phenol and catechol) has been investigated from 25 to 90°C via solubility and potentiometric measurements. Results show that Ge forms stable complexes with the di- and tricarboxylic acids and catechol, but that Si forms much weaker complexes with these ligands. Analysis of our results and of available literature data on Ge complexes formed with other types of aqueous organic species demonstrates that Ge forms complexes of chelate type with the following functional groups: (1) carboxylic in acid solutions (1 ≤ pH ≤ 6), (2) di-phenolic hydroxyls in neutral and basic solutions (pH ≥ 6), and (3) alcoholic hydroxyls in very basic solutions (pH ≥ 10). Conversely, Si forms very weak complexes with these compounds. Stability constants generated in this study for Ge- and Si-organic species have been used to approximate Ge and Si complexing with humic acids which possess the same organic functional groups as those used in this study. Our calculations show that Si-humic acid complexes are negligible in most natural waters. In contrast, the presence of humic acids can considerably affect Ge speciation in aqueous solution. For example, at pH ≥ 6 in a solution containing and 0.1 μg/L of Ge and 20 mg/L of dissolved organic carbon (DOC), Ge-humic acid complexes account for more than 95% of total aqueous Ge. These results can explain the increase of the Ge/Si ratio in organic-rich surficial waters. Ge-organic matter complexation should be thus taken into account when using Ge/Si ratios measured in surface waters and biogenic opals to estimate chemical-weathering intensity and Ge and Si global fluxes.

Stability and solubility of arsenopyrite, FeAsS, in crustal fluids.

The stability and solubility of natural arsenopyrite (FeAsS) in pure water and moderately acid to slightly basic aqueous solutions buffered or not with H2 and/or H2S were studied at temperatures from 300 to 450°C and pressures from 100 to 1000 bar. The solubilities of FeAsS in pure water and dilute HCl/NaOH solutions without buffering are consistent with the formation of the As(OH)30(aq) species and precipitation of magnetite. At more acid pH (pH ≤2), arsenopyrite dissolves either stoichiometrically or with formation of the As-FeAsS assemblage. In H2S-rich and H2-rich aqueous solutions, arsenopyrite dissolution results in the formation of pyrrhotite (±pyrite) and iron arsenide(s), respectively, which form stable assemblages with arsenopyrite. Arsenic concentrations measured in equilibrium with FeAsS in slightly acid to neutral aqueous solutions with H2 and H2S fugacities buffered by the pyrite-pyrrhotite-magnetite assemblage are 0.0006 ± 0.0002, 0.0055 ± 0.0010, 0.07 ± 0.01, and 0.32 ± 0.03 mol/kg H2O at 300°C/400 bar, 350°C/500 bar, 400°C/500 bar, and 450°C/500 bar, respectively. These values were combined with the available thermodynamic data on As(OH)30(aq) (Pokrovski et al., 1996) to derive the Gibbs free energy of FeAsS at each corresponding temperature and pressure. Extrapolation of these values to 25°C and 1 bar, using the available heat capacity and entropy data for FeAsS (Pashinkin et al., 1989), yields a value of −141.6 ± 6.0 kJ/mol for the standard Gibbs free energy of formation of arsenopyrite. This value implies a higher stability of FeAsS in hydrothermal environments than was widely assumed. Calculations carried out using the new thermodynamic properties of FeAsS demonstrate that this mineral controls As transport and deposition by high-temperature (>not, vert, similar300°C) crustal fluids during the formation of magmatic-hydrothermal Sn-W-Cu-(Au) deposits. The equilibrium between As-bearing pyrite and the fluid is likely to account for the As concentrations measured in modern high- and moderate-temperature (150 ≤ T ≤ 350°C) hydrothermal systems. Calculations indicate that the local dissolution of arsenopyrite creates more reducing conditions than in the bulk fluid, which is likely to be an effective mechanism for precipitating gold from hydrothermal solutions. This could be a possible explanation for the gold-arsenopyrite association commonly observed in many hydrothermal gold deposits.

Fluid density control on vapor-liquid partitioning of metals in hydrothermal systems

Hot aqueous fluids, both vapor and saline liquid, are primary transporting media for metals in hydrothermal-magmatic systems. Despite the growing geological evidence that the vapor phase, formed through boiling of magmatic ore-bearing fluids, can selectively concentrate and transport metals, the physical-chemical mechanisms that control the metal vapor-liquid fractionation remain poorly understood. We performed systematic experiments to investigate the metal vapor-liquid partitioning in model water-salt-gas systems H2O-NaCl-KCl-HCl at hydrothermal conditions. Measurements show that equilibrium vapor-liquid fractionation patterns of many metals are directly related to the densities of the coexisting vapor and liquid phases. Despite differences in the vapor-phase chemistry of various metals that form hydroxide, chloride, or sulfide gaseous molecules of contrasting volatile properties, water-solute interaction is a key factor that controls the metal transfer by vapor-like fluids in Earths crust. These findings allow quantitative prediction of the vapor-liquid distribution patterns and vapor-phase metal transport in a wide range of conditions. Our density model accounts well for the vapor-brine distribution patterns of Na, Si, Fe, Zn, As, Sb, and Ag observed in fluid inclusions from magmatic-hydrothermal deposits. For Au and Cu, the partitioning in favor of the liquid phase, predicted in a sulfur-free system, contrasts with the copper and gold enrichment observed in natural vapor-like inclusions. The formation of stable complexes of Cu and Au with reduced sulfur may allow for their enhanced transport by sulfur-enriched magmatic-hydrothermal vapors.

Thermodynamic properties of aqueous Ge(IV) hydroxide complexes from 25 to 350°C: Implications for the behavior of germanium and the Ge/Si ratio in hydrothermal fluids

Abstract The stoichiometry and thermodynamic properties of Ge(IV) hydroxide complexes were generated from both solubility and potentiometric measurements. The solubility of the tetrahedral germanium oxide (GeO 2 (tetr)) was measured at temperatures from 25 to 350°C in acid to alkaline solutions at the saturated vapor pressure of the system (P sat ). Potentiometric measurements were performed on GeO 2 -KOH aqueous solutions at temperatures from 21 to 200°C and P sat using a pH solid-contact glass electrode. Results indicate that Ge(OH) 4 ° (aq) is the dominant Ge-bearing species at concentrations up to at least 0.05 m over a wide range of pH (0–8) and temperatures (20–350°C). GeO(OH) 3 − forms in significant amounts only in alkaline solutions (pH > 8–9). These results were combined with the available low-temperature solubility data on the hexagonal germanium oxide (GeO 2 (hex)) and the thermodynamic properties of GeO 2 (tetr) and GeO 2 (hex) to generate Ge(OH) 4 ° (aq) and GeO(OH) 3 − thermodynamic parameters within the framework of the revised HKF equation of state (Helgeson et al., 1981; Tanger and Helgeson, 1988) . Calculations carried out using these parameters indicate that the distribution of Ge hydroxide species as a function of pH and temperature is similar to that of silicon hydroxide complexes. However, the significant differences between Ge(OH) 4 ° (aq) and Si(OH) 4 ° (aq) enthalpies of formation and heat capacities can lead to large variations with temperature of Ge/Si ratios in solutions in equilibrium with Ge-bearing silicates. For example, calculations show that the Ge/Si ratio in a fluid in equilibrium with a Ge-bearing wollastonite (Ca(Si,Ge)O 3 ) increases by an order of magnitude when temperature is raised from 25 to 500°C. This can be responsible for the high values of Ge/Si ratios measured in high temperature crustal fluids.

The S3– Ion Is Stable in Geological Fluids at Elevated Temperatures and Pressures

Instead of sulfide or sulfate, the trisulfur ion S3– is stable at high temperatures. The chemical speciation of sulfur in geological fluids is a controlling factor in a number of processes on Earth. The two major chemical forms of sulfur in crustal fluids over a wide range of temperature and pressure are believed to be sulfate and sulfide; however, we use in situ Raman spectroscopy to show that the dominant stable form of sulfur in aqueous solution above 250°C and 0.5 gigapascal is the trisulfur ion S3–. The large stability range of S3– enables efficient transport and concentration of sulfur and gold by geological fluids in deep metamorphic and subduction-zone settings. Furthermore, the formation of S3– requires a revision of sulfur isotope–fractionation models between sulfides and sulfates in natural fluids.

AN EXPERIMENTAL AND COMPUTATIONAL STUDY OF SODIUM-ALUMINUM COMPLEXING IN CRUSTAL FLUIDS

Abstract 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.

Adsorption of copper on Pseudomonas aureofaciens: protective role of surface exopolysaccharides.

Adsorption of copper on exopolysaccharide (EPS)-rich and (EPS)-poor soil rhizospheric Pseudomonas aureofaciens cells was studied as a function of pH and copper concentration at different exposure time in order to assess the effect of cell exopolysaccharides on parameters of adsorption equilibria. The surface properties of bacteria were investigated as a function of pH and ionic strength using potentiometric acid-base titration and electrophoresis that permitted the assessment of the excess surface proton concentration and zeta-potential of the cells, respectively. For adsorption experiments, wide range of Cu concentration was investigated (0.1-375 microM) in order to probe both weak and strong binding sites at the surface. Experimental results were successively fitted using a Linear Programming Model approach. The groups with pK(a) of 4.2-4.8 and from 5.2 to 7.2, tentatively assigned as carboxylates and phosphoryl respectively, are the most abundant at the surface and thus essentially contribute to the metal binding. The presence of exopolysaccharides on the surface decreases the amount of copper adsorbed on the bacterial cell wall apparently via screening the underlining functional groups of the cell wall. At the same time, dissolved EPS substances do not contribute to Cu binding in aqueous solution. Results of this study allow quantification of the role played by the surface EPS matrix as a protective barrier for metal adsorption on bacterial cell walls.