Research Article  Open Access
Igor Povar, Inga Zinicovscaia, Oxana Spinu, Boris Pintilie, "Thermodynamic Stability Areas of Polyvanadates of Alkaline Earth Metals", Journal of Chemistry, vol. 2019, Article ID 7091781, 6 pages, 2019. https://doi.org/10.1155/2019/7091781
Thermodynamic Stability Areas of Polyvanadates of Alkaline Earth Metals
Abstract
A thermodynamic method of the global Gibbs energy variation calculation for describing heterogeneous equilibria of transformations of calcium, strontium, and barium polyvanadates, that occur in systems MeOV_{2}O_{5}H_{2}O, where Me is the alkaline earth element, has been developed and used. Its quintessence consists in the thermodynamic analysis of the real conditions of various processes on the basis of their total thermodynamic characteristics. On the basis of the selected thermodynamic data for involved species, the thermodynamic stability areas of solid polyvanadates towards the solution pH and vanadium and alkaline earth metal ion concentrations in heterogeneous mixtures have been established, taking into account the complex formation reactions in multicomponent heterogeneous systems. The existing experimental data confirm the results on the thermodynamic stability of polyvanadates obtained in this paper.
1. Introduction
Polyvanadates obtained by solidphase synthesis and subsequent study of their chemical properties in watersalt systems have a huge practical significance, because regardless of the type of source of raw materials, their processing includes stages of the synthesis of final products with desired properties (high or low solubility, etc.), with the transfer of vanadium in the solution and its subsequent precipitation. The abovementioned processes are associated with extremely complex chemical equilibria, the study of which would allow the development of technological basis of a number of combined productions: obtaining technical and pure vanadium pentoxide, vanadium catalysts, dyes, etc. [1, 2]. Up to 85–87% of the total amount of polyvanadates is used in ferrous metallurgy as highly efficient and habitually an indispensable alloying additive in the production of diverse steels. About 10–12% of polyvanadates is used in nonferrous metallurgy, mainly in the form of aluminumvanadium alloys for alloying structural materials based on titanium, applied in space technology. Inorganic materials containing polyvanadates are used for production of optical quantum generators, ionic conductors, ferroelectrics, dielectrics, ceramics, etc. [1]. The demand for polyvanadates is projected to grow by seven percent a year till 2025 [2], thanks to the usage of vanadiumcontaining steels in both traditional areas and the implementation of new technologies for the manufacture of batteries [2]. The concentrations of vanadium in surface waters caused by industrial wastes are small and, in general, are within the natural content (up to 65 μg/L) [3]. Literature sources provide information on vanadium concentrations in surface waters of industrial wastes up to 2 mg/L [3]. The chemistry of aqueous solutions of vanadium is complicated by the existence in solutions of numerous ions and polyions, the transition of which into an equilibrium state depends on the pH value and oxidationreduction potentials in the systems, as well as its concentration and the content of other elements. Thus, the composition and stability of solid phases formed in the MeO–V_{2}O_{5}–H_{2}O system, where Me is the alkaline earth element, exhibit complex functions of the chemical composition of solution, pH, etc. Experimentally, it has been demonstrated [4–6] that depending on the conditions in the MeO–V_{2}O_{5}–H_{2}O system, different polyvanadates MeV_{12}O_{31}, MeV_{6}O_{16}, Me_{3}V_{10}O_{28}, Me(VO_{4})_{2}, Me_{2}V_{2}O_{7}, and Me_{3}(VO_{4})_{2} can precipitate. For the compounds of Ca^{2+}, Sr^{2+}, and Ba^{2+} the same authors determined the solubility products [4, 5], taking into account the mutual transformation reactions of the vanadium (V) ionic species. The coexistence areas of these polyvanadates were also calculated in the cited papers as a function of the solution pH and alkaline earth metal concentrations. At the same time, the reciprocal relationship between the solution composition and solid phases was established in the form of diagrams and . This procedure is not always efficient. In particular, the solubility diagrams provide valuable information only if the solubility of solid phase is low.
In this paper, based on calculations of the global Gibbs energy variation of the dissolutionprecipitation process of the polyvanadates of alkaline earth metals as a function of pH and total concentrations of vanadium and alkaline earth metals, their thermodynamic stability areas are determined. It has been proved that the value of global Gibbs energy variation under real conditions constitutes a more objective criterion for estimation of the stability areas of polyvanadates of alkaline earth metals than the solubility diagrams.
2. Theoretical Part
In a series of papers [6–8], it has been shown that as a strict criterion of solidphase stability serves the value of the Gibbs energy variation of the solidphase formationdissolution process. This criterion will be applied for the determination of the thermodynamic stability areas of the alkaline earth metal polyvanadates.
The essence of the calculation method will be explained by a concrete example of equilibrium of calcium dodecavanadate with the saturated aqueous solution:
The Gibbs energy variation under standard conditions is equal:where is the standard Gibbs energy of formation of the i species. The values used (kJ/mol) are recalculated from the selected equilibrium constants [4, 5] and shown in Table 1. The value cannot, however, serve as a characteristic of the thermodynamic stability of calcium dodecavanadate under real conditions. In the latter case, the equilibrium is described by the equation of reaction isotherm, which for Reaction (1) takes the following form:
Note that the Gibbs energy variation in Reaction (1) strongly depends on the pH value. At the same time, Equation (3) cannot also serve as a characteristic of the thermodynamic stability of calcium dodecavanadate, because the vanadyl ion, in function of pH and C_{V}, is subjected to complex chemical transformations in the solution (Table 2).


Obviously, in calculating ΔG_{S}, all these equilibria must be taken into account. A rigorous thermodynamic analysis, developed in a series of papers [7, 8], shows the global Gibbs energy variation of Reaction (1) under real conditions, considering the equilibria (Table 2), as described by the following equation:where is the coefficient that takes into account the contribution of the equilibria (Table 2), which is determined in the following way:where is the equilibrium concentration of vanadyl ion , computed for certain values of pH and C_{V} from the mass balance conditions:
Analogous relations are valid for other vanadates. Under this approach, for −ΔG_{S}> 0, the solid phase is thermodynamically unstable to dissolution according to Reaction (1) and vice versa; if −ΔG_{S}< 0, the dissolution of the solid phase takes place.
3. Results and Discussion
Within the pH range where the solid phase (oxide) is thermodynamically unstable (dissolves), it is necessary to determine the areas of predominance of the soluble species in solution. In the presence of polynuclear species, it is necessary to calculate the partial molar fractions of the species as a function of pH and C_{V}. The last ones for vanadium compounds (V), for example, are determined in the following way [7]:
The results of calculating these functions for vanadium (V) compounds for C_{V} = 1 × 10^{−3} mol·L^{−1} as a function of pH are shown in Figure 1. Based on these results, it is easy to determine the thermodynamic stability of the species in the solution as a function of pH and C_{V}. As one can see, in acidic media, within the pH range between 2.4 and 5.5, the species H_{2}V_{10}O_{28}^{4−} and HV_{10}O_{28}^{5−} are dominant. In alkaline solutions, after pH > 8, the HVO_{4}^{2−} species prevail.
The obtained data (Figure 1) correlate well with existing experimental evidences. It is well known that highfield NMR gives highquality structural data and quantitative information about speciation and concentration of a large variety of species in aqueous solvents [9]. Consequently, NMR ^{51}V spectroscopy measurements [10] confirm the speciation of vanadium (V) in aqueous vanadate solutions as a function of pH and vanadate concentrations. As seen in Figure 1, in alkaline solutions, tetrahedrally coordinated vanadates, metavanadate, and pyrovanadate are abundant. Decavanadate only forms when metavanadate is added to solutions of pH 3 or less. Acidification of metavanadate solutions to pH 4 or lower polymerizes all the oligomers to form V_{10}. Readjusting the pH near to 8 produces a depolymerization of V_{10} to form V_{2}, V_{4}, and V_{5}, as it has been stated in [11–13]. As the pH of metavanadate solutions increases, V_{1} and V_{2} become the predominant species [14]. As reported Ralston et al. [15], in the basic pH (between 8 and 9) solution, and (or ) coexist. Orthovanadates dominate at a solution pH greater than 13 [16]. As highly basic solutions are acidified to pH values between 9 and 12, orthovanadate tetrahedral units can combine to form pyrovanadates , , and , which are colorless [16]. Continued acidification to pH values between 6 and 9 leads to the formation of colorless or yellow metavanadates , , , and [16, 17]. Additional acidification to pH between 2 and 6 causes the coordination of vanadium to change from tetrahedral to octahedral. A combination of ten octahedral units forms the decavanadate ions , , and , leading to orange or red solutions [16, 17]. Pervanadyl is yellow and dominates at dilute concentrations in strongly acidic solutions, at pH 1.0–2.5 (Figure 1).
The calculated dependence of ΔG_{S} on pH for C_{V} = C_{Ca} = 1 mol·L^{−1} for different calcium polyvanadates are shown in Figure 2.
The intersection points of the curves correspond to the boundaries of the stability areas of the respective salts. For two salts, e.g., 1 and 2, the Gibbs energy values in the given conditions are ΔG_{S}(1) and ΔG_{S}(2); in the case −ΔG_{S}(1) < −ΔG_{S}(2), salt 1 is thermodynamically more stable, and vice versa; for −ΔG_{S}(1) > −ΔG_{S}(2), salt 1 must transform into salt 2. In the case ΔG_{S}(1) = ΔG_{S}(2), equilibrium is established. In Figures 3–5, the ΔG_{S}(pH) dependence for different calcium, strontium, and barium polyvanadates are displayed for several C_{V} and C_{Me} values. For simplicity, only the lower boundaries of the ΔG_{S} values are shown.
As it is evident from Figures 2–5, the thermodynamic stability of polyvanadates towards each other strongly depends on pH and total alkaline earth metal and vanadium concentrations. The most insoluble compounds are barium polyvanadates in comparison with calcium and strontium ones. With decrease in the C_{V} and C_{Me} concentrations, the polyvanadates become thermodynamically unstable to dissolution. From Figures 3–5, one can see that alkaline earth metal polyvanadates are thermodynamically instable when C_{V} = C_{Me} ≤ 1 × 10^{−3} mol·L^{−1} for all the examined pH values, excepting barium polyvanadates in very acidic solutions (pH < 3.3). The last fact is not reflected in the and diagrams, that is, the main priority of the criterion proposed in this paper for assessing the thermodynamic stability of alkaline earth metal polyvanadates. The existing experimental data [4, 5, 18–20] confirm the results on the thermodynamic stability of polyvanadates obtained in this paper.
The effect of temperature on the thermodynamics stability of hydrocomplexes and solubility of polyvanadates for different temperatures are estimated on the basis of van’t Hoff equation [7]:
The appropriate values of the standard enthalpies ΔH can be found in handbooks available everywhere [21–27]. Usually, it is supposed that and the temperature insignificantly influences the ΔH values within the investigated temperature interval.
Vanadium in inorganic media (a mixture of an inorganic acids and their salts) may form complexes with such anions such as , etc. Note that the sulphate medium is important due to its industrial applications [28–32]. Schematically, if L^{} denotes the ligand, the reaction of complex formation may be written as follows:
The thermodynamic equilibrium formation constant of at 25°C is equal to [28]. Consequently, the complexes may form in the presence of a large excess of ligand, . The contribution of anions of aqueous electrolytes on the thermodynamics stability of soluble and insoluble species can be taken into account through the alpha coefficient, in the form described in Equation (6) [7, 8].
4. Conclusions
(i)A thermodynamic method for calculating the global Gibbs energy variation for describing heterogeneous equilibria of transformations of polyvanadates of calcium, strontium, and barium, that occur in systems MeOV_{2}O_{5}H_{2}O, where Me is the alkaline earth element, has been developed and applied.(ii)Its quintessence consists in the thermodynamic analysis of the real conditions of various processes on the basis of their total thermodynamic characteristics. On the basis of the selected thermodynamic data for involved soluble and insoluble species, the thermodynamic stability areas of solid polyvanadates towards the solution pH and vanadium and alkaline earth metal ion concentrations in heterogeneous mixtures have been established, taking into account the complex formation reactions in multicomponent heterogeneous systems.(iii)The calcium, strontium, and barium polyvanadates are dissolved for C_{V} = C_{Me} ≤ 1 × 10^{−3} mol·L^{−1} for all the examined pH values.(iv)The thermodynamic stability of soluble species as function of pH and C_{V} has been determined.(v)The obtained results, based on the thermodynamic analysis and graphical design of the calculated data, are in good agreement with the available experimental data.
Data Availability
The standard Gibbs energies of formation of chemical species used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was partially supported by the Academy of Sciences of Moldova (grant number 15.817.02.15A).
References
 R. R. Moskalyk and A. M. Alfantazi, “Processing of vanadium: a review,” Minerals Engineering, vol. 16, no. 9, pp. 793–805, 2003. View at: Publisher Site  Google Scholar
 http://nextsourcematerials.com/posts/energizerresourcesprovidesupdateonthedevelopmentofitsgreengiantvanadiumproject/.
 M. Costigan, R. Cary, and S. Dobson, Vanadium Pentoxide and Other Inorganic Vanadium Compounds, World Health Organization & International Programme on Chemical Safety, Geneva, Switzerland, 2001, http://www.who.int/iris/handle/10665/42365.
 A. A. Ivakin, I. G. Chufarova, N. I. Petunina et al., “Conditions of the formation of meta, pyro and orthovanadates of calcium, strontium, and barium,” Journal of Inorganic Chemistry, vol. 21, no. 7, pp. 177–774, 1976. View at: Google Scholar
 A. A. Ivakin, I. G. Chufarova, and N. I. Petunina, “Conditions of the formation of deca, hexa and dodecavanadates of calcium, strontium and barium,” Journal of Inorganic Chemistry, vol. 22, no. 6, pp. 1470–1474, 1977. View at: Google Scholar
 N. V. Podvalnaya and V. L. Volkov, “Hydrolytic precipitation of calcium polyvanadate in solutions of vanadium (IV), (V),” Journal of Inorganic Chemistry, vol. 52, no. 9, pp. 1566–1571, 2007. View at: Publisher Site  Google Scholar
 I. Povar and V. Rusu, “Aluminium heterogeneous speciation in natural waters,” Canadian Journal of Chemistry, vol. 290, no. 4, pp. 326–332, 2012. View at: Publisher Site  Google Scholar
 I. Povar and O. Spinu, “The role of hydroxy aluminium sulfate minerals in controlling Al^{3+} concentration and speciation in acidic soils,” Central European Journal of Chemistry, vol. 12, no. 8, pp. 877–885, 2014. View at: Publisher Site  Google Scholar
 J. B. Lambert and E. P. Mazzola, Nuclear Magnetic Resonance Spectroscopy: An Introduction to Principles, Applications, and Experimental Methods, Chap. 1, Pearson Prentice Hall, Englewood Cliffs, NJ, USA, 1999.
 M. Iannuzzi, T. Young, and G. S. Frankel, “Aluminum alloy corrosion inhibition by vanadates,” Journal of the Electrochemical Society, vol. 153, no. 12, pp. B533–541, 2006. View at: Publisher Site  Google Scholar
 E. Heath and O. W. Howarth, “Vanadium51 and oxygen17 nuclear magnetic resonance study of vanadate (V) equilibria and kinetics,” J. Chem. Soc. Dalton Trans, no. 5, pp. 1105–1110, 1981. View at: Publisher Site  Google Scholar
 A. S. Tracey, J. S. Jaswal, and S. J. AngusDunne, “Influences of pH and ionic strength on aqueous vanadate equilibria,” Inorganic Chemistry, vol. 34, no. 22, pp. 5680–5685, 1995. View at: Publisher Site  Google Scholar
 J. W. Larson, “Thermochemistry of vanadium (5+) in aqueous solutions,” Journal of Chemical and Engineering Data, vol. 40, no. 6, pp. 1276–80, 1995. View at: Publisher Site  Google Scholar
 G. Yoganandan and J. N. Balaraju, “Synergistic effect of V and Mn oxyanions for the corrosion protection of anodized aerospace aluminum alloy,” Surface and Coatings Technology, vol. 252, pp. 35–47, 2014. View at: Publisher Site  Google Scholar
 K. D. Ralston, S. Chrisanti, T. L. Young, and R. G. Buchheit, “Corrosion inhibition of aluminum alloy 2024T3 by aqueous vanadium species,” Journal of The Electrochemical Society, vol. 155, no. 7, pp. C350–C359. View at: Publisher Site  Google Scholar
 R. J. H. Clark, The Chemistry of Titanium and Vanadium, Elsevier Publishing Co., Amsterdam, Netherlands, 1968.
 M. T. Pope, Heteropoly and Isopoly Oxometalates, SpringerVerlag, Berlin, Germany, 1983.
 V. N. Muzgin, L. B. Khamzina, V. L. Zolotavin, and I. Ya. Bezrukov, Analytical Chemistry of Vanadium, Nauka, Moscow, Russia, 1981.
 S. S. Korovin, D. V. Drobot, and P. I. Fedorov, “Rare and scattered elements. Chemistry and technology,” in Book II: Textbook for high schools, S. S. Korovin, Ed., p. 464, MISIS, Moscow, Russia, 1999. View at: Google Scholar
 L. Chen, F. Q. Liu, and D. B. Li, “Precipitation of crystallized hydrated iron(III) vanadate from industrial vanadium leaching solution,” Hydrometallurgy, vol. 105, no. 34, pp. 229–233, 2011. View at: Publisher Site  Google Scholar
 A. J. Bard, R. Parsons, and J. Jordan, Standard Potentials in Aqueous Solution (Ebook), Routledge, NY, USA, 2017.
 J. D. Cox, D. D. Wagman, and V. A. Medvedev, CODATA Key Values for Thermodynamics, Hemisphere Publishing Corp., New York, NY, USA, 1989.
 D. D. Wagman, W. H. Evans, V. B. Parker et al., “The NBS tables of chemical thermodynamic properties,” Journal of Physical and Chemical Reference Data, vol. 11, no. 2, 1982. View at: Google Scholar
 M. W. Chase, C. A. Davies, J. R. Downey et al., “JANAF thermochemical tables, third edition,” Journal of Physical and Chemical Reference Data, vol. 14, no. 1, 1985. View at: Google Scholar
 M. W. Chase, NISTJANAF Thermochemical Tables, (Journal of Physical and Chemical Reference Data Monograph), American Institute of Physics, College Park, MD, USA, 4th edition, 1998.
 T. E. Daubert, R. P. Danner, H. M. Sibul, and C. C. Stebbins, Physical and Thermodynamic Properties of Pure Compounds: Data Compilation, Extant 1994 (core with 4 supplements), Taylor & Francis, Bristol, PA, USA, 1994.
 J. D. Allison, D. S. Brown, and K. J. NovoGradac, MINTEQA2/PRODEFA2, A Geochemical Assessment Model for Environmental Systems: User Manual Supplement for Version 4.0, HydroGeoLogic, Inc. Herndon, Virginia, Allison Geoscience Consultants, Inc, Flowery Branch, GA, USA, 1999.
 M. Rakib and G. Durand, “Study of complex formation of vanadium (V) with sulphate ions using a solvent extraction method,” Hydrometallurgy, vol. 43, no. 1–3, pp. 355–66, 1996. View at: Publisher Site  Google Scholar
 I. Zinicovscaia, L. Cepoi, I. Povar et al., “Metal uptake from complex industrial effluent by cyanobacteria Arthrospira Platensis,” Water, Air, & Soil Pollution, vol. 229, no. 7, pp. 220–229, 2018. View at: Publisher Site  Google Scholar
 X. Zhou, C. Wei, M. Li et al., “Thermodynamics of vanadium–sulfur–water systems at 298K,” Hydrometallurgy, vol. 106, no. 1, pp. 104–112, 2011. View at: Publisher Site  Google Scholar
 T. H. Nguyen and M. S. Lee, “Solvent extraction of vanadium (V) from sulfate solutions using LIX 63 and PC 88A,” Industrial and Engineering Chemistry Research, vol. 31, pp. 118–123, 2015. View at: Publisher Site  Google Scholar
 M. R. Tavakoli, S. Dornian, and D. B. Dreisinger, “The leaching of vanadium pentoxide using sulfuric acid and sulfite as a reducing agent,” Hydrometallurgy, vol. 141, pp. 59–66, 2014. View at: Publisher Site  Google Scholar
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Copyright © 2019 Igor Povar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.