Adsorption of Strontium onto Synthetic Iron(III) Oxide up to High Ionic Strength Systems

In this work, the adsorption behavior of Sr onto a synthetic iron(III) oxide (hematite with traces of goethite) has been studied. This solid, which might be considered a representative of Fe3+ solid phases (iron corrosion products), was characterized by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS), and its specific surface area was determined. Both XRD and XPS data are consistent with a mixed solid containing more than 90% hematite and 10% goethite. The solid was further characterized by fast acid-base titrations at different NaCl concentrations (from 0.1 to 5 M). Subsequently, for each background NaCl concentration used for the acid-base titrations, Sr-uptake experiments were carried out involving two different levels of Sr concentration (1·10−5 and 5·10−5 M, respectively) at constant solid concentration (7.3 g/L) as a function of −log([H+]/M). A Surface Complexation Model (SCM) was fitted to the experimental data, following a coupled Pitzer/surface complexation approach. The Pitzer model was applied to aqueous species. A Basic Stern Model was used for interfacial electrostatics of the system, which includes ion-specific effects via ion-specific pair-formation constants, whereas the Pitzer-approach involves ion-interaction parameters that enter the model through activity coefficients for aqueous species. A simple 1-pK model was applied (generic surface species, denoted as >XOH−1/2). Parameter fitting was carried out using the general parameter estimation software UCODE, coupled to a modified version of FITEQL2. The combined approach describes the full set of data reasonably well and involves two Sr-surface complexes, one of them including chloride. Monodentate and bidentate models were tested and were found to perform equally well. The SCM is particularly able to account for the incomplete uptake of Sr at higher salt levels, supporting the idea that adsorption models conventionally used in salt concentrations below 1 M are applicable to high salt concentrations if the correct activity corrections for the aqueous species are applied. This generates a self-consistent model framework involving a practical approach for semi-mechanistic SCMs. The model framework of coupling conventional electrostatic double layer models for the surface with a Pitzer approach for the bulk solution earlier tested with strongly adsorbing solutes is here shown to be successful for more weakly adsorbing solutes.

An Experimental Study of the Solubility and Speciation of MoO3(s) in Hydrothermal Fluids at Temperatures up to 350°C

The solubility of molybdenum trioxide (MoO3(s)) in aqueous solutions has been investigated experimentally at 250°, 300°, and 350°C and saturated water vapor pressure, and total Na concentrations ranging from 0 to 3 molal (m). Results of these experiments show that the solubility of MoO3(s) increases with increasing temperature and at 350°C can reach several thousand parts per million at high salinity (>1 m NaCl). At low Na+ activity, MoO3(s) dissolves dominantly as HMoO4,− whereas at high Na+ activity, the dominant species is NaHMoO40. The two dissolution reactions are MoO3(s)+H2O=HMoO4−+H+(1) and MoO3(s)+H2O+Na+=NaHMoO40+H+.(2) The values of the logarithms of the equilibrium constants for reaction (1) are –5.20 ± 0.12, –5.31 ± 0.17, and –5.50 ± 0.09 at 250°, 300°, and 350°C, respectively, and for reaction (2) the values are –3.40 ± 0.11, –3.25 ± 0.19, and –2.97 ± 0.09 for the same temperatures. In combination, these equilibrium constants yield equilibrium constants for the reaction relating the two aqueous species: Na++HMoO4−=NaHMoO40.(3) The values of the logarithms of the equilibrium constants for reaction (3) are 1.80 ± 0.16, 2.06 ± 0.25, and 2.53 ± 0.13 at 250°, 300°, and 350°C, respectively. Calculations, based on the results of this study and thermodynamic data available for other species, suggest strongly that in ore-forming hydrothermal systems, molybdenum is transported mainly as NaHMoO40 and deposits as molybdenite in response to cooling and possibly a reduction in fO2.

Equilibrium and kinetic simulation of groundwater demanganation and deironing

Analysis of chemical equilibria among iron and manganese aqueous species at various Eh-pH conditions and aqueous CO2 concentration is done. Thermodynamic and equilibrium-kinetic simulation of iron and manganese aqueous species oxidations is developed for groundwater demanganation and deironing. Numerical simulation of chemical interactions in the system groundwater-aqueous oxygen-rock minerals-aqueous carbon dioxide is shown that deironing is effective enough but aqueous manganese(II) concentration is increased. It occurs because (Fe,Mn)CO3 solubility rate is too slow and (Fe,Mn)CO3 dissolution and removal of aqueous iron species results in secondary MnCO3 formation. Using published experimental data on carbonate dissolution kinetics, iron and manganese oxidation kinetics and the critical values of rate constants of iron and manganese homogenous oxidation, iron and manganese carbonates solubility, manganese homogenous catalytical oxidation on iron hydroxide suspension are chosen. The kinetics-thermodynamics model of underground oxidation of iron and manganese by dissolved oxygen have been developed. By numerical simulation of chemical interactions in the system groundwater saturated by oxygen-stratal water-intake rock minerals shows that deironing occurs effective enough but aqueous manganese concentration increased. It happens due to aqueous manganese slow oxidation and dissolution of (Fe,Mn)CO3. Also secondary MnCO3 formation is possible due to removal of aqueous iron specis. So underground demanganation is possible if there is no (Fe,Mn)CO3 among intake rock minerals or inconvenience of water contact with it.

Thermodynamic Properties of Aqueous Species Calculated Using the HKF Model: How Do Different Thermodynamic and Electrostatic Models for Solvent Water Affect Calculated Aqueous Properties?

Thermodynamic properties of aqueous species are essential for modeling of fluid-rock interaction processes. The Helgeson-Kirkham-Flowers (HKF) model is widely used for calculating standard state thermodynamic properties of ions and complexes over a wide range of temperatures and pressures. To do this, the HKF model requires thermodynamic and electrostatic models of water solvent. In this study, we investigate and quantify the impact of choosing different models for calculating water solvent volumetric and dielectric properties, on the properties of aqueous species calculated using the HKF model. We identify temperature and pressure conditions at which the choice of different models can have a considerable effect on the properties of aqueous species and on fluid mineral equilibrium calculations. The investigated temperature and pressure intervals are 25–1000°C and 1–5 kbar, representative of upper to middle crustal levels, and of interest for modeling ore-forming processes. The thermodynamic and electrostatic models for water solvent considered are: Haar, Gallagher and Kell (1984), Wagner and Pruß (2002), and Zhang and Duan (2005), to calculate water volumetric properties, and Johnson and Norton (1991), Fernandez and others (1997), and Sverjensky and others (2014), to calculate water dielectric properties. We observe only small discrepancies in the calculated standard partial molal properties of aqueous species resulting from using different water thermodynamic models. However, large differences in the properties of charged species can be observed at higher temperatures (above 500°C) as a result of using different electrostatic models. Depending on the aqueous speciation and the reactions that control the chemical composition, the observed differences can vary. The discrepancy between various electrostatic models is attributed to the scarcity of experimental data at high temperatures. These discrepancies restrict the reliability of the geochemical modeling of hydrothermal and ore formation processes, and the retrieval of thermodynamic parameters from experimental data at elevated temperatures and pressures.

COMPARATIVE STUDY OF GEOCHEMICAL SPECIATION MODELING USING GEODELING SOFTWARE

Geochemical modeling is used to understand water-rock interactions that occur in the sedimentary basin. Activities of aqueous species are usually calculated using the Davies equation, Debye-Hückel equation, B-Dot or Newton-Raphson. We perform a comparative study of geochemical speciation using three different software: PHREEQCTM, Geochemist's WorkbenchTM (GWB) and GEODELING (own code). Details of each software are carefully analyzed, taking into account the distribution, mobility, and availability of chemical species in geological environments. We can observe very similar results in speciation when working with low-temperature systems. GWBTM, GEODELING employ an integrated system to define when to use Davies, Debye-Huckel or B-Dot equation, according to the value of the solution ionic strength. The utilization of GEODELING allows comparing the results with software GWBTM and PHREEQCTM with a high degree of acceptance for low temperatures. Soon to high temperatures, we need to be cautious in their use.