Solubility Of MoO3 Research Articles

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

Abstract 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.

Compositional Dependence of Solubility/Retention of Molybdenum Oxides in Aluminoborosilicate-Based Model Nuclear Waste Glasses.

Molybdenum oxides are an integral component of the high-level waste streams being generated from the nuclear reactors in several countries. Although borosilicate glass has been chosen as the baseline waste form by most of the countries to immobilize these waste streams, molybdate oxyanions (MoO42-) exhibit very low solubility (∼1 mol %) in these glass matrices. In the past three to four decades, several studies describing the compositional and structural dependence of molybdate anions in borosilicate and aluminoborosilicate glasses have been reported in the literature, providing a basis for our understanding of fundamental science that governs the solubility and retention of these species in the nuclear waste glasses. However, there are still several open questions that need to be answered to gain an in-depth understanding of the mechanisms that control the solubility and retention of these oxyanions in glassy waste forms. This article is focused on finding answers to two such questions: (1) What are the solubility and retention limits of MoO3 in aluminoborosilicate glasses as a function of chemical composition? (2) Why is there a considerable increase in the solubility of MoO3 with incorporation of rare-earth oxides (for example, Nd2O3) in aluminoborosilicate glasses? Accordingly, three different series of aluminoborosilicate glasses (compositional complexity being added in a tiered approach) with varying MoO3 concentrations have been synthesized and characterized for their ability to accommodate molybdate ions in their structure (solubility) and as a glass-ceramic (retention). The contradictory viewpoints (between different research groups) pertaining to the impact of rare-earth cations on the structure of aluminoborosilicate glasses are discussed, and their implications on the solubility of MoO3 in these glasses are evaluated. A novel hypothesis explaining the mechanism governing the solubility of MoO3 in rare-earth containing aluminoborosilicate glasses has been proposed.

Solubility of MoO3 in acid solutions and vapor-liquid distribution of molybdic acid

This study reports the measurements of the solubility of crystalline molybdenum trioxide, MoO3, in acid (HClO4, HCl) aqueous solutions at 563, 573, 593, and 623 K, mostly at pressures close to the saturated water vapor pressure. The solubility data can be explained by the two dissolution reactions: a). MoO3(cr) + OH− = HMoO4−, with log10Ko values equal to (7.08 ± 0.20), (7.09 ± 0.20), (7.43 ± 0.20) at 563, 573, and 593 K, and b). MoO3(cr) + H2O(l) = H2MoO4(aq), with log10Ko equal to –(2.40 ± 0.20), –(2.22 ± 0.20), –(2.41 ± 0.20), –(1.95 ± 0.20) at 563, 573, 593, and 623 K, respectively. Combining these results with the literature data on the thermodynamics of H2MoO4 in the ideal gas state, we evaluated Henry's constants of this form at investigated temperatures, which turned out to be close to those of Si(OH)4, just as one would expect on the basis of chemical similarity. Reasonable estimates of the fugacity coefficients of H2MoO4 in steam allowed the calculation of the vapor-liquid distribution constant, KD=limx→0y/x, for this species, where y and x stand for the mole fractions of a solute in coexisting vapor and liquid phases, respectively. The value of the Krichevskii parameter, which governs the variation of KD in the investigated range of temperatures, is equal to –(228 ± 33) MPa for H2MoO4, being close to that of Si(OH)4, –(190 ± 10) MPa.

The solubility of MoO3 in aqueous solutions of HClO4 at T = 300°C and P = 100 bar by experimental data

The solubility of crystalline MoO3 in aqueous solutions of HClO4 at T = 300°C and P = 100 bar was determined in experiments. It was shown that the acidity of the solution is the determining factor of the solubility of molybdenum trioxide under hydrothermal parameters. The formation constant was calculated for the HMoO4− neutral complex (pK0 = 3.50 ± 0.20).

Multi‐Phase Glass‐Ceramics as a Waste Form for Combined Fission Products: Alkalis, Alkaline Earths, Lanthanides, and Transition Metals

In this study, multi‐phase borosilicate‐based glass‐ceramics were investigated as an alternative waste form for immobilizing non‐fissionable products from used nuclear fuel. Currently, borosilicate glass is the waste form selected for immobilization of this waste stream, however, the low thermal stability and solubility of MoO3 in borosilicate glass translates into a maximum waste loading in the range 15–20 mass%. Glass‐ceramics provide the opportunity to target chemically durable crystalline phases, e.g., powellite, oxyapatite, celsian, and pollucite that will incorporate MoO3 as well as other waste components such as lanthanides, alkalis, and alkaline earths at levels twice the solubility limits of a single‐phase glass. In addition a glass‐ceramic could provide higher thermal stability, depending upon the properties of the crystalline and amorphous phases. Here, glass‐ceramics were synthesized at waste loadings of 42, 45, and 50 mass% with the following glass additives: B2O3, Al2O3, CaO, and SiO2 by slow‐cooling from a glass melt. Glass‐ceramics were characterized in terms of phase assemblage, morphology, and thermal stability. Only two of the targeted phases, powellite and oxyapatite, were observed, along with lanthanide‐borosilicate and cerianite. Results of this initial investigation show promise of glass‐ceramics as a potential waste form to replace single‐phase borosilicate glass.

Electrical resistivity dip in Sb x V y Mo z O t phases

Electrical resistivity dips have been discovered in the temperature range 100–500 K both in the SbVO4.96 matrix and the Sb x V y Mo z O t phases for 10 mol% solubility of MoO3 in SbVO5. As the Sb content increases and simultaneously the V content decreases, the value of the resistivity at the dip, ρ d, decreases and shifts the dip to higher temperatures. The magnetic measurements showed a spontaneous magnetization and parasitic magnetism of the solid solutions under study. Characteristic for parasitic magnetism is a small value of the magnetic moment, here 0.014 μ B/f.u. at 4.2 K and at a magnetic field of 14 T as well as a small value of the mass susceptibility, here 10−5 cm3/g. The value of the Néel temperature, T N ≤ 8 K, and the Curie–Weiss temperature, θ CW ≤ −208 K, indicate a collinear antiferromagnetic (AFM) order. We suggest that neither the magnetism nor the Mo-content can be correlated with the resistivity anomalies. Therefore, these effects may rather be interpreted in terms of a small-polaron gas in the resistivity dip area. Alternatively, they could mark a lattice/electronic entropy-driven incomplete metal–insulator transition.

Comparison between the synthesis in molybdenum and antimony oxides system by high-temperature treatment and high-energy ball milling

The interactions between MoO3 and Sb2O3 or α-Sb2O4 taking place in the solid state in air during high-temperature as well as mechanochemical treatments have been investigated. The high-energy ball milling of MoO3 with Sb2O3 converts α-Sb2O3 to β-Sb2O3 and leads to formation of Sb2MoO6 and Sb4Mo10O31 phases. They are the final products of thermal synthesis in an inert atmosphere but not in air. The solid solution of MoO3 in β-Sb2O4 was obtained in high-temperature reaction of MoO3 with Sb2O3 or α-Sb2O4 as well as by milling of mixture MoO3/α-Sb2O4 for 14 h. The milling resulted in higher than 3 mol% solubility of MoO3 in β-Sb2O4. The constructed phase diagram of MoO3–α-Sb2O4 system is presented.

Phase relations in the system SbVO5-Sb3V2Mo3O21in solid-state and in the atmosphere of air

It has been established by XRD, DTA and TG methods that phases of solid solution type of MoO3 in SbVO5 are formed in the system V2O5-MoO3-a-Sb2O4. The Mo6+ ions are incorporated into the crystal lattice of SbVO5 instead of both Sb5+ and V5+, while the charge compensation occurs by a formation of cation defects (□) at Sb5+ and V5+. The phases Sb1-6x□xV1-6x□xMo10xO5 are stable in the solid-state up to 690±10°C and the limit of solubility of MoO3 in SbVO5 does not exceed 20.00 mol%.

Changes in the transport properties of silicon dioxide due to doping by foreign ions

Based on the model that in the intrinsic disorder of SiO2 oxygen-ion vacancies dominate at low oxygen pressure and oxygen-ion interstitials at high oxygen pressure, a model is presented which explains the formation of mobile interstitial silicon ions preferentially at high oxygen pressure by doping of SiO2 with Ti3+ and/or Ti2+ ions. Analogously an attempt has been made to understand an increase in the solubility of MoO3 in SiO2 by Al2O3 dissolved in SiO2. Doping and co-dissolution are in particular to be expected in the oxidation of SiO2 forming ceramics because SiO2 has very probable dissolved impurities of the ceramic or/and sinter additives.

Preparation of alumina supported molybdena catalysts by new slurry impregnation method: MoO3/Al2O3 sulfide hydrodesulfurization catalyst

A new slurry impregnation method (SIM) of the preparation of a catalyst or catalyst precursor MoO3/Al2O3 is described. Aqueous slurry of powdered MoO3 is mixed with alumina extradates and the mixture is refluxed. A low solubility of MoO3 is sufficient for transport of MoO3 from powder form via solution to the surface of the support. The catalysts were tested by hydrodesulfurization of thiophene at 1.6 MPa and 280–400°C. Their activity was similar to the activity of industrial and laboratory MoO3/Al2O3 samples prepared by conventional impregnation with solution of ammonium heptamolybdate. The advantage of the SIM method is that calcination, producing nitrogeneous waste gases, is not required and that all deposited molybdenum species are adsorbed and not precipitated.