Thermodynamic Stability Areas of Polyvanadates of Alkaline Earth Metals

Hindawi Journal of Chemistry Volume 2019, Article ID 7091781, 6 pages https://doi.org/10.1155/2019/7091781

Research Article Thermodynamic Stability Areas of Polyvanadates of Alkaline Earth Metals

Igor Povar ,1,2 Inga Zinicovscaia,1,2,3 Oxana Spinu,1 and Boris Pintilie1

1Institute of Chemistry, 3 Academiei Str., MD 2028, Chisinau, Moldova 2Joint Institute for Nuclear Research, Joliot-Curie Str., 6, 141980 Dubna, Russia 3Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, 30 Reactorului, Str. MG-6, Bucharest-Magurele, Romania

Correspondence should be addressed to Igor Povar; [email protected]

Received 28 September 2018; Revised 15 November 2018; Accepted 3 December 2018; Published 2 January 2019

Academic Editor: Ana Moldes

Copyright © 2019 Igor Povar et al. *is 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. 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 MeO-V2O5-H2O, 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. *e existing experimental data conﬁrm the results on the thermodynamic stability of polyvanadates obtained in this paper.

1. Introduction aluminum-vanadium alloys for alloying structural materials based on titanium, applied in space technology. Inorganic Polyvanadates obtained by solid-phase synthesis and sub- materials containing polyvanadates are used for production sequent study of their chemical properties in water-salt of optical quantum generators, ionic conductors, ferro- systems have a huge practical significance, because re- electrics, dielectrics, ceramics, etc. [1]. *e demand for gardless of the type of source of raw materials, their pro- polyvanadates is projected to grow by seven percent a year cessing includes stages of the synthesis of final products with till 2025 [2], thanks to the usage of vanadium-containing desired properties (high or low solubility, etc.), with the steels in both traditional areas and the implementation of transfer of vanadium in the solution and its subsequent new technologies for the manufacture of batteries [2]. *e precipitation. *e abovementioned processes are associated concentrations of vanadium in surface waters caused by with extremely complex chemical equilibria, the study of industrial wastes are small and, in general, are within the which would allow the development of technological basis natural content (up to 65 μg/L) [3]. Literature sources of a number of combined productions: obtaining techni- provide information on vanadium concentrations in surface cal and pure vanadium pentoxide, vanadium catalysts, dyes, waters of industrial wastes up to 2 mg/L [3]. *e chemistry of etc. [1, 2]. Up to 85–87% of the total amount of poly- aqueous solutions of vanadium is complicated by the ex- vanadates is used in ferrous metallurgy as highly efficient istence in solutions of numerous ions and polyions, the and habitually an indispensable alloying additive in the transition of which into an equilibrium state depends on the production of diverse steels. About 10–12% of polyvanadates pH value and oxidation-reduction potentials in the systems, is used in nonferrous metallurgy, mainly in the form of as well as its concentration and the content of other 2 Journal of Chemistry elements. *us, the composition and stability of solid phases Table 1: Standard Gibbs energy of formation of the soluble and formed in the MeO–V2O5–H2O system, where Me is the insoluble species at 295.15 K [4, 5]. alkaline earth element, exhibit complex functions of the 0 0 Species ΔGf (i) Species ΔGf (i) chemical composition of solution, pH, etc. Experimentally, it 2+ Ca −552.75 Ca3V10O28 −9376.52 has been demonstrated [4–6] that depending on the con- 2+ Sr −563.90 Sr3V10O28 −9428.23 ditions in the MeO–V2O5–H2O system, different poly- 2+ Ba −546.83 Ba3V10O28 −9403.50 vanadates MeV12O31, MeV6O16, Me3V10O28, Me(VO4)2, + VO2 −588.30 CaV6O16 −5031.30 Me2V2O7, and Me3(VO4)2 can precipitate. For the com- H2O −237.24 SrV6O16 −5043.05 2+ 2+ 2+ − pounds of Ca , Sr , and Ba the same authors determined VO3 −785.70 BaV6O16 −5029.60 3− the solubility products [4, 5], taking into account the mutual VO4 −897.60 CaV12O31 −9295.90 2− transformation reactions of the vanadium (V) ionic species. HVO4 −979 SrV12O31 −9307.60 − *e coexistence areas of these polyvanadates were also cal- H2VO4 −1021.12 BaV12O31 −9293.97 culated in the cited papers as a function of the solution pH H3VO4 −1043.95 Ca(VO3)2 −2165.56 4− V2O7 −1723.94 Sr(VO3)2 −2190.70 and alkaline earth metal concentrations. At the same time, the 3− HV2O7 −1767.73 Ba(VO3)2 −2181.97 reciprocal relationship between the solution composition and 3− V3O9 −2400.70 Ca2V2O7 −2870.75 solid phases was established in the form of diagrams log CV � 4− + + V4O12 −3202.23 Sr2V2O7 −2910.45 f(log[H ]) and log CM � f(log[H ]). *is procedure is not 6− V10O�28 −7688.78 Ba2V2O7 −2891.35 always efficient. In particular, the solubility diagrams provide 5− HV10O�28 −7721.88 Ca3(VO4)2 −3553.23 valuable information only if the solubility of solid phase is 4− H2V10O�28 −7742.43 Sr3(VO4)2 −3604.49 low. Ba3(VO4)2 −3574.97 In this paper, based on calculations of the global Gibbs energy variation of the dissolution-precipitation process of the polyvanadates of alkaline earth metals as a function of pH C 12C and total concentrations of vanadium and alkaline earth G ( ) � G0( ) + RT V Ca. (3) Δ S 1 Δ S 1 ln + 14 metals, their thermodynamic stability areas are determined. It []H has been proved that the value of global Gibbs energy vari- Note that the Gibbs energy variation in Reaction (1) ation under real conditions constitutes a more objective strongly depends on the pH value. At the same time, criterion for estimation of the stability areas of polyvanadates Equation (3) cannot also serve as a characteristic of the of alkaline earth metals than the solubility diagrams. thermodynamic stability of calcium dodecavanadate, because the vanadyl ion, in function of pH and CV, is subjected 2. Theoretical Part to complex chemical transformations in the solution (Table 2). In a series of papers [6–8], it has been shown that as a strict Obviously, in calculating ΔGS, all these equilibria must criterion of solid-phase stability serves the value of the Gibbs be taken into account. A rigorous thermodynamic analysis, energy variation of the solid-phase formation-dissolution developed in a series of papers [7, 8], shows the global Gibbs process. *is criterion will be applied for the determination energy variation of Reaction (1) under real conditions, of the thermodynamic stability areas of the alkaline earth considering the equilibria (Table 2), as described by the metal polyvanadates. following equation: *e essence of the calculation method will be explained C 12C ΔG (1) ��ΔG0(1) � −�RT ln α12 � +�RT ln V Ca�, by a concrete example of equilibrium of calcium dodeca- S S V []H+ 14 vanadate with the saturated aqueous solution: ( ) + + 2+ 4 CaV12O31(S) + 14H � 12VO2 + Ca + 7H2O. (1) where αV is the coefficient that takes into account the *e Gibbs energy variation under standard conditions is contribution of the equilibria (Table 2), which is determined equal: in the following way: G0( ) � G0 � + G0�2+ � + G0+ � + −2 + −4 + −3 + −2 Δ S 1 7Δ f H2O Δ f Ca 12Δ f VO2 αV � 1 + k1�H � + k2�H � + k3�H � + k4�H � G0� �, + −1 + + −6 + + −5 − Δ f CaV12O31(S) + k5�H � + 2� VO2 �k6�H � + 2� VO2 �k7�H � (2) + 2 + −6 + 3 + −8 + 3� VO2 � k8�H � + 4� VO2 � k9�H � 0 + 9 + −16 + 9 + −15 where ΔGf (i) is the standard Gibbs energy of formation of 0 + 10� VO2 � k10�H � + 10� VO2 � k11�H � the i species. *e ΔGf (i) values used (kJ/mol) are recal- + 9 + −14 culated from the selected equilibrium constants [4, 5] and + 10� VO2 � k12�H � , G0( ) shown in Table 1. *e Δ S 1 value cannot, however, serve (5) as a characteristic of the thermodynamic stability of calcium + dodecavanadate under real conditions. In the latter case, the where [VO2 ] is the equilibrium concentration of vanadyl + equilibrium is described by the equation of reaction iso- ion VO2 , computed for certain values of pH and CV from therm, which for Reaction (1) takes the following form: the mass balance conditions: Journal of Chemistry 3

Table 11 2: Complex chemical transformations of the vanadyl ion in 1 the solution at 295 ± 1 K [4, 5]. 1.0 Nr Equation of reaction log K + − + 0.8 2 1 VO2 + H2O � VO3 + 2H log K1 � −6.98 + 3− + 3 2 VO2 + 2H2O � VO4 + 4H log K2 � −29.94 + + � 2− + + � 3 VO2 2H2O HVO4 3H log K3 −14.68 0.6 4 + − + i 4 VO2 + 2H2O � H2VO4 + 2H log K4 � −7.30 f + + 5 VO2 + 2H2O � H3VO4 + H log K5 � −3.30 + 4− + 0.4 6 VO2 + 3H2O � V2O7 + 6H log K6 � −28.80 5 + 3− + 7 2VO2 + 3H2O � HV2O7 + 5H log K7 � −21.13 + 3− + 13 8 3VO2 + 3H2O � V3O9 + 6H log K8 � −13.30 0.2 + 4− + 7 9 4VO2 + 4H2O � V4O12 + 8H log K9 � −17.51 + 6− + 10 10 10VO2 + 8H2O � V10O28 + 16H log K10 � −16.15 6 8 9 12 + 5− + 0.0 11 10VO2 + 8H2O � HV10O28 + 15H log K11 � −10.35 + 4− + 2468101214 12 10VO + 8H O � H V O + 14H log K12 � −6.75 2 2 2 10 28 pH

+ 6 - H V O 3– 10 - HV O 3– 1 - VO2 3 10 28 2 7 + − 3− 2− C ��VO � +�VO � +�VO � +�HVO � 4– 2– 2– V 2 3 4 4 2 - H2V10O28 7 - H2V2O7 11 - HVO4 − 4− 3− 5– 6– 12 - V O 4– +�H2VO4 � +�H3VO4 � + 2� V2O7 � + 2� HV2O7 � 3 - HV10O28 8 - V10O28 2 7 4– 5– 3– 3− 4− 6− 4 - V4O12 9 - V5O15 13 - VO4 + 3� V3O9 � + 4� V4O12 � + 10� V10O28 � 5 - H VO – 5− 4− 2 4 + 10� HV10O28 � + 10� H2V10O28 � Figure 1: Partial molar fractions of the vanadium V(V) species for + + −2 + −4 + −3 −3 −1 ��VO2 � 1 + k1�H � + k2�H � + k3�H � CV � 1·10 mol·L . + k �H+ �−2 + k �H+ �−1 + 2� VO + �k �H+ �−6 4 5 2 6 4− 5− + + −5 + 2 + −6 H2V10O28 and HV10O28 are dominant. In alkaline so- + 2� VO �k �H � + 3� VO � k �H � 2− 2 7 2 8 lutions, after pH > 8, the HVO4 species prevail. + 4� VO + �3k �H+ �−8 + 10� VO + �9k �H+ �−16 *e obtained data (Figure 1) correlate well with existing 2 9 2 10 experimental evidences. It is well known that high-field + 9 + −15 + 9 + −14 + 10� VO2 � k11�H � + 10� VO2 � k12�H � �. NMR gives high-quality structural data and quantitative (6) information about speciation and concentration of a large variety of species in aqueous solvents [9]. Consequently, Analogous relations are valid for other vanadates. Under NMR 51V spectroscopy measurements [10] confirm the this approach, for −ΔGS > 0, the solid phase is thermody- speciation of vanadium (V) in aqueous vanadate solutions as namically unstable to dissolution according to Reaction (1) a function of pH and vanadate concentrations. As seen in and vice versa; if −ΔGS < 0, the dissolution of the solid phase Figure 1, in alkaline solutions, tetrahedrally coordinated takes place. vanadates, metavanadate, and pyrovanadate are abundant. Decavanadate only forms when metavanadate is added to solutions of pH 3 or less. Acidification of metavanadate 3. Results and Discussion solutions to pH 4 or lower polymerizes all the oligomers to Within the pH range where the solid phase (oxide) is form V10. Readjusting the pH near to 8 produces a de- thermodynamically unstable (dissolves), it is necessary to polymerization of V10 to form V2,V4, and V5, as it has been determine the areas of predominance of the soluble species stated in [11–13]. As the pH of metavanadate solutions in solution. In the presence of polynuclear species, it is increases, V1 and V2 become the predominant species [14]. As reported Ralston et al. [15], in the basic pH (between 8 necessary to calculate the partial molar fractions of the 4− 2− 2− and 9) solution, V4O12 and HVO4 (or VO3(OH) ) species as a function of pH and CV. *e last ones for va- 3− nadium compounds (V), for example, are determined in the coexist. Orthovanadates VO4 dominate at a solution pH following way [7]: greater than 13 [16]. As highly basic solutions are acidified to pH values between 9 and 12, orthovanadate tetrahedral units j� H V O � j� H V O � 4− 3− i j n i j n can combine to form pyrovanadates V2O7 , HV2O7 , and fijn � � . (7) 2− CV �i�j�nj� HiVjOn � HVO4 , which are colorless [16]. Continued acidification to pH values between 6 and 9 leads to the formation of colorless 4− 5− *e results of calculating these functions for vanadium or yellow metavanadates V4O12 ,V5O15 ,H3VO4, and −3 −1 − (V) compounds for CV � 1 × 10 mol·L as a function of H2VO4 [16, 17]. Additional acidification to pH between 2 pH are shown in Figure 1. Based on these results, it is easy to and 6 causes the coordination of vanadium to change from determine the thermodynamic stability of the species in the tetrahedral to octahedral. A combination of ten octahedral 6− 5− solution as a function of pH and CV. As one can see, in acidic units forms the decavanadate ions V10O�28 , HV10O�28 , 5− media, within the pH range between 2.4 and 5.5, the species and HV10O�28 , leading to orange or red solutions [16, 17]. 4 Journal of Chemistry

+ Pervanadyl VO2 is yellow and dominates at dilute con- 12 CaV12O31 centrations in strongly acidic solutions, at pH 1.0–2.5 10 CaV6O16 (Figure 1). 8 *e calculated dependence of ΔG on pH for C � C � Ca3V10O28 S V Ca 6 1 mol·L−1 for diﬀerent calcium polyvanadates are shown in 4 Figure 2. Ca(VO3)2 *e intersection points of the curves correspond to 2

the boundaries of the stability areas of the respective salts. –ΔG/2.3RT 0 For two salts, e.g., 1 and 2, the Gibbs energy values in the –2 Ca2V2O7 given conditions are ΔGS(1) and ΔGS(2); in the case –4 −ΔGS(1) < −ΔGS(2), salt 1 is thermodynamically more –6 Ca3(VO4)2 stable, and vice versa; for −ΔGS(1) > −ΔGS(2), salt 1 must transform into salt 2. In the case ΔGS(1) � ΔGS(2), equi- 02468101214 librium is established. In Figures 3–5, the ΔGS(pH) de- pH pendence for different calcium, strontium, and barium Figure 2: ΔGS versus pH for different CaO–V2 O5–H2O systems at polyvanadates are displayed for several CV and CMe values. −1 CV � CCa � 1 mol·L . For simplicity, only the lower boundaries of the ΔGS values are shown. As it is evident from Figures 2–5, the thermodynamic 7 2 28 ) O O 3 2 stability of polyvanadates towards each other strongly de- V 10 2 V pends on pH and total alkaline earth metal and vanadium 16 Ca Ca (VO ) 3 3 4 2 2 O Ca(VO concentrations. *e most insoluble compounds are barium CaV12O31 6 Ca 3 polyvanadates in comparison with calcium and strontium CaV ones. With decrease in the CV and CMe concentrations, the 0 polyvanadates become thermodynamically unstable to dissolution. From Figures 3–5, one can see that alkaline earth metal polyvanadates are thermodynamically instable when –2 −3 −1 CV � CMe ≤ 1 × 10 mol·L for all the examined pH values, –ΔG/2.3RT excepting barium polyvanadates in very acidic solutions (pH –4 2 < 3.3). *e last fact is not reflected in the + + log CV � f(log[H ]) and log CM � f(log[H ]) diagrams, –6 that is, the main priority of the criterion proposed in this 1 paper for assessing the thermodynamic stability of alkaline earth metal polyvanadates. *e existing experimental data 0 2 4 6 8 10 12 14 [4, 5, 18–20] confirm the results on the thermodynamic pH stability of polyvanadates obtained in this paper. Figure 3: ΔGS versus pH for different CaO-V2O5-H2O systems: (1) *e effect of temperature on the thermodynamics sta- −1 −1 −1 CV � CCa � 1 mol·L ; (2) CV � CCa � 1 × 10 mol·L ; (3) CV � CCa bility of hydrocomplexes HiVjOn and solubility of poly- � 1 × 10−3 mol·L−1. vanadates for different temperatures are estimated on the basis of van’t Hoff equation [7]: − Consequently, the VO2SO4 complexes may form in log K1 +1/T1 − 1/T2 �ΔH the presence of a large excess of ligand, C − >> C . *e log K � . (8) SO4 V 2 2.303R contribution of anions of aqueous electrolytes on the thermodynamics stability of soluble and insoluble species *e appropriate values of the standard enthalpies ΔH can be taken into account through the alpha coefficient, in can be found in handbooks available everywhere [21–27]. the form described in Equation (6) [7, 8]. Usually, it is supposed that T1 � 298.15 K and the temperature insignificantly influences the ΔH values within the investigated temperature interval. 4. Conclusions Vanadium in inorganic media (a mixture of an inorganic (i) A thermodynamic method for calculating the acids and their salts) may form complexes with such anions 2− − global Gibbs energy variation for describing het- such as SO4 , Cl etc. Note that the sulphate medium is erogeneous equilibria of transformations of poly- important due to its industrial applications [28–32]. Sche- vanadates of calcium, strontium, and barium, that matically, if L- denotes the ligand, the reaction of complex occur in systems MeO-V2O5-H2O, where Me is the formation may be written as follows: alkaline earth element, has been developed and + − (1−p)+ applied. VO2 + pA � VO2Ap . (9) (ii) Its quintessence consists in the thermodynamic *e thermodynamic equilibrium formation constant analysis of the real conditions of various pro- − ° of VO2SO4 at 25 C is equal to log K � 1.30 ± 0.02 [28]. cesses on the basis of their total thermodynamic Journal of Chemistry 5

Data Availability 2 O 28 16 O 10 6 V SrV O Sr 3 12 31 SrV Sr(VO3)2 Sr2V2O7 *e standard Gibbs energies of formation of chemical

0 Sr3(VO4)2 species used to support the ﬁndings of this study are in- 3 cluded within the article. –2 Conflicts of Interest

–ΔG/2.3RT –4 *e authors declare that they have no conﬂicts of interest.

–6 2 Acknowledgments *is work was partially supported by the Academy of –8 1 Sciences of Moldova (grant number 15.817.02.15A). 02468101214 pH References Figure 4: ΔG versus pH for diﬀerent SrO-V O -H O systems: (1) S 2 5 2 [1] R. R. Moskalyk and A. M. Alfantazi, “Processing of vanadium: a C � C � 1 mol·L−1; (2) C � C � 1 × 10−1 mol·L−1; (3) C � C � V Sr V Sr V Sr review,” Minerals Engineering, vol. 16, no. 9, pp. 793–805, 2003. 1 × 10−3 mol·L−1. [2] http://nextsourcematerials.com/posts/energizer-resources- provides-update-on-the-development-of-its-green-giant- vanadium-project/.

28 [3] M. Costigan, R. Cary, and S. Dobson, Vanadium Pentoxide 16 O O 2 10 6 and Other Inorganic Vanadium Compounds, World Health BaV12O31 V 3 BaV Ba Organization & International Programme on Chemical Ba(VO3)2 Ba V O 0 2 2 7 Safety, Geneva, Switzerland, 2001, http://www.who.int/iris/

Ba3(VO4)2 handle/10665/42365. –2 [4] A. A. Ivakin, I. G. Chufarova, N. I. Petunina et al., “Conditions 3 of the formation of meta-, pyro- and orthovanadates of –4 calcium, strontium, and barium,” Journal of Inorganic –ΔG/2.3RT –6 Chemistry, vol. 21, no. 7, pp. 177–774, 1976. [5] A. A. Ivakin, I. G. Chufarova, and N. I. Petunina, “Conditions of the formation of deca-, hexa- and dodecavanadates of –8 2 calcium, strontium and barium,” Journal of Inorganic –10 1 Chemistry, vol. 22, no. 6, pp. 1470–1474, 1977. [6] N. V. Podvalnaya and V. L. Volkov, “Hydrolytic precipitation 02468101214 of calcium polyvanadate in solutions of vanadium (IV), (V),” pH Journal of Inorganic Chemistry, vol. 52, no. 9, pp. 1566–1571, 2007. Figure 5: ΔG versus pH for different BaO-V O -H O systems: (1) S 2 5 2 [7] I. Povar and V. Rusu, “Aluminium heterogeneous speciation C � C � 1 mol·L−1; (2) C � C � 1 × 10−1 mol·L−1; (3) C � C V Ba V Ba V Ba in natural waters,” Canadian Journal of Chemistry, vol. 290, � 1 × 10−3 mol·L−1. no. 4, pp. 326–332, 2012. [8] I. Povar and O. Spinu, “*e role of hydroxy aluminium sulfate minerals in controlling Al3+ concentration and speciation in characteristics. On the basis of the selected acidic soils,” Central European Journal of Chemistry, vol. 12, thermodynamic data for involved soluble and no. 8, pp. 877–885, 2014. insoluble species, the thermodynamic stability [9] J. B. Lambert and E. P. Mazzola, Nuclear Magnetic Resonance areas of solid polyvanadates towards the solution Spectroscopy: An Introduction to Principles, Applications, and pH and vanadium and alkaline earth metal ion Experimental Methods, Chap. 1, Pearson Prentice Hall, concentrations in heterogeneous mixtures have Englewood Cliffs, NJ, USA, 1999. been established, taking into account the complex [10] M. Iannuzzi, T. Young, and G. S. Frankel, “Aluminum alloy formation reactions in multicomponent hetero- corrosion inhibition by vanadates,” Journal of the Electro- geneous systems. chemical Society, vol. 153, no. 12, pp. B533–541, 2006. [11] E. Heath and O. W. Howarth, “Vanadium-51 and oxygen-17 (iii) *e calcium, strontium, and barium polyvanadates −3 −1 nuclear magnetic resonance study of vanadate (V) equilibria and are dissolved for CV � CMe ≤ 1 × 10 mol·L for all kinetics,” J. Chem. Soc. Dalton Trans., no. 5, pp. 1105–1110, 1981. the examined pH values. [12] A. S. Tracey, J. S. Jaswal, and S. J. Angus-Dunne, “Influences of (iv) *e thermodynamic stability of soluble species as pH and ionic strength on aqueous vanadate equilibria,” In- organic Chemistry, vol. 34, no. 22, pp. 5680–5685, 1995. function of pH and CV has been determined. [13] J. W. Larson, “*ermochemistry of vanadium (5+) in aqueous (v) *e obtained results, based on the thermodynamic solutions,” Journal of Chemical and Engineering Data, vol. 40, analysis and graphical design of the calculated data, no. 6, pp. 1276–80, 1995. are in good agreement with the available experi- [14] G. Yoganandan and J. N. Balaraju, “Synergistic effect of V and mental data. Mn oxyanions for the corrosion protection of anodized 6 Journal of Chemistry

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