Geochemical Controls on Uranium Release from Neutral-pH Rock Drainage Produced by Weathering of Granite, Gneiss, and Schist

We investigated geochemical processes controlling uranium release in neutral-pH (pH ≥ 6) rock drainage (NRD) at a prospective gold deposit hosted in granite, schist, and gneiss. Although uranium is not an economic target at this deposit, it is present in the host rock at a median abundance of 3.7 µg/g, i.e., above the average uranium content of the Earth’s crust. Field bin and column waste-rock weathering experiments using gneiss and schist mine waste rock produced circumneutral-pH (7.6 to 8.4) and high-alkalinity (41 to 499 mg/L as CaCO3) drainage, while granite produced drainage with lower pH (pH 4.7 to >8) and lower alkalinity (<10 to 210 mg/L as CaCO3). In all instances, U release was associated with calcium release and formation of weakly sorbing calcium-carbonato-uranyl aqueous complexes. This process accounted for the higher release of uranium from carbonate-bearing gneiss and schist than from granite despite the latter’s higher solid-phase uranium content. In addition, unweathered carbonate-bearing rocks having a higher sulfide-mineral content released more uranium than their oxidized counterparts because sulfuric acid produced during sulfide-mineral oxidation promoted dissolution of carbonate minerals, release of calcium, and formation of calcium-carbonato-uranyl aqueous complexes. Substantial uranium attenuation occurred during a sequencing experiment involving application of uranium-rich gneiss drainage into columns containing Fe-oxide rich schist. Geochemical modeling indicated that uranium attenuation in the sequencing experiment could be explained through surface complexation and that this process is highly sensitive to dissolved calcium concentrations and pCO2 under NRD conditions.

A Theoretical and Experimental Study on Hydrogen-bonding Interactions between 4H-1,2,4-triazole-3,5-diamine and DMSO/water

In this paper, 4TZDA-DMSO/water complexes formed by hydrogen bonding interactions were investigated by a combined experimental and computational approach. Two conformations of 4TZDA molecule were considered. Seven hydrogen-bonded 4TZDA-DMSO/H2O complexes were characterized in terms of geometries, energies and vibrational frequencies. The optimizations and calculations were performed for the complexes by Density Functional Theory. In the experimental part, the DMSO/H2O solutions of 4TZDA were prepared and infrared spectra of the solutions were recorded. After the solvation process, significant shifts in the existing bands and new band rising were observed in the experimental spectra of 4TZDA. Following results are found from this study: 1) 4TZDA (I) is more stable than 4TZDA (II). 2) Seven 4TZDA-DMSO and 4TZDA-H2O complexes are investigated and it is seen that all nitrogen atoms of 4TZDA are hydrogen bond acceptor and all hydrogen atoms are hydrogen bond donors. 3) Aqueous complexes of 4TZDA are found to form stronger hydrogen bonds compared to DMSO complexes. 4) It is determined that the most stable structures are intermolecular interactions of lpO⋯H-N and lpN⋯H-O type for the complexes. For these interactions, h-bond lengths are calculated as 1.78 and 1.90 Å and interaction energies are -7.10 kJ/mol for 4TZDA-DMSO and -50.5 kJ/mol for 4TZDA-H2O. Because of this energy difference in the complexes, it can be said 4TZDA forms more stable complexes with water molecules compared to DMSO molecules and with this property, it is an ideal molecule for pharmacological purposes.

Thermodynamic parameters for the complexation of Tc(IV) with bromide under aqueous conditions

Technetium-99 is a long-lived fission product present in nuclear wastes, found mainly as Tc(VII) and Tc(IV) in the environment. The quantification of the equilibrium constants for the formation of Tc(IV) aqueous complexes has been limited to carboxylate ligands and interactions with the halides is mostly unknown. This work reports equilibrium constants of the formation of the TcO(OH)+ complexes with Br−, in a 3 M NaClO4 solution of pcH 2 and varied temperature, using a liquid-liquid extraction system. Neutron activation confirmed the suitability of the extraction technique for this work. Under the working conditions, Br− forms a weak exothermic TcO(OH)Br complex, with a Gibbs free energy (ΔGr) of 3 ± 3 kJ · mol−1 at a temperature of 273.15 K. The values for ΔHr (−32 ± 3 kJ · mol−1) and ΔSr (106 ± 9 J · mol−1 · K−1) of the complexation reaction were quantified using a van’t Hoff analysis. This work also showed that bromide addition does not displace the hydroxide from TcO(OH)+, as the equilibrium constant of bromide addition is much weaker than the first hydrolysis constant of the metal.

Unraveling Zr/Hf fractionation: Hydrothermal zirconium and hafnium fluoride complexation up to 400°C

&lt;p&gt;High field strength elements (HFSE) such as Zr and Hf are relatively insoluble in most natural hydrothermal solutions and consequently immobile in most geological systems. However, fluoride forms stable aqueous complexes with many HFSE ions, including Zr&lt;sup&gt;4+&lt;/sup&gt; and Hf&lt;sup&gt;4+&lt;/sup&gt;, and is thus a potent mobilizer of these elements. Due to their identical charge and similar ionic radius (590 pm and 580 pm, respectively), Zr and Hf behave almost identically in geological system and are therefore referred to as geochemical twins. Fluoride complexation in hydrothermal environments is one of few processes in the Earth's crust that can effectively fractionate them from one another. This fact can be used to trace past fluoride activity in fossil hydrothermal systems by investigating Zr/Hf ratios, if fluoride complexation of Zr and Hf is sufficiently well understood. Mobility of metals as complexes is controlled by two distinct but related mechanisms: Formation of the complex itself and solvation of that complex in the solvent. Poly(hydrogen-fluoride) bridging of fluoride complexes to the surrounding aqueous solvent is crucial to the understanding of the solvation and therefore the mobility of fluoride complexes.&lt;/p&gt;&lt;p&gt;We report geometries of Zr and Hf fluoride complexes up to 400&amp;#176;C, determined by extended X-Ray absorption fine structure (EXAFS) in a hydrothermal autoclave. Existing data sets on the stability of those complexes at lower temperatures are extended to 400&amp;#176;C. Our data show strong temperature dependence of the complex stability for both metals. However, the effect of temperature is not equally strong for Zr and Hf. Fractionation of the twin pair is thus a function of temperature as well as fluoride activity.&lt;/p&gt;

On the colorimetric measurement of aqueous Si in the presence of organic ligands and common pH buffering agents

The effects of the presence of ten organic ligands and common pH buffering agents – acetate, oxalate, tartrate, citrate, phthalate, EDTA, carbonate, TRIS, phosphate and borate – on aqueous Si concentration measurements made using the common molybdate blue method were quantified. The concentrations of these additives ranged from 0.01 to 0.1 mol/kg in the measured aqueous solutions. Whereas measured Si concentrations were not affected by the presence of up to 0.1 mol/kg acetate, hydrogen phthalate or TRIS, the presence of 0.1 mol/kg of all other selected additives altered substantially the measured Si concentrations using the molybdate blue method. For example the presence of 0.1 mol/kg of citrate, EDTA, and hydrogen phosphate decreased the ratio of measured to true Si concentrations to ~0.05. These variations are interpreted to stem from the formation of competing aqueous complexes in the aqueous phase limiting the formation of the characteristically blue SiMo12O404– complex.