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Polyoxometalates in solution: speciation under spotlight[Review]

Polyoxometalates (POMs) are a large group of anionic polynuclear metal-oxo clusters with discrete and chemically modifiable structures. In most aqueous POM solutions, numerous, and often highly negatively charged, species of different nuclearities are formed. It is rather difficult to determine the dominant POM species or their combination, which is responsible for the specific POM activity, during a particular application. Thus, the identification of all individual speciation profiles is essential for the successful implementation of POMs in solution applications. This review article summarizes species that are present in isopoly- and heteropolyvanadates, -niobates, -molybdates and -tungstates aqueous solutions and covers their stability and transformations. The ion-distribution diagrams over a wide pH range are presented in a comprehensive manner. These diagrams are intended for the targeted use of POMs, and in a clear form shows species that are in equilibrium at the given pH value. Thus, the data accumulated in this review can serve as both a starting point and a complete reference material for determining the composition of POM solutions. Some examples are highlighted where the POM speciation studies led to a detailed understanding of their role in applications. In doing so, we aim to motivate the POM community for more speciation studies and to make the subject more comprehensible, both for synthetic POM chemists and for scientists with different backgrounds interested in applying POMs in biological, medical, electrochemical, supramolecular and nanochemistry fields, or as homogeneous catalysts and other water-soluble materials.

N. I. Gumerova, and A. Rompel,Polyoxometalates in solution: speciation under spotlight, Chemical Society reviews, 2020. DOI: 10.1039/D0CS00392A 

Hydration of Oxometallate Ions in Aqueous Solution

The strength of hydrogen bonding to and structure of hydrated oxometallate ions in aqueous solution have been studied by double difference infrared (DDIR) spectroscopy and large-angle X-ray scattering (LAXS), respectively. Anions are hydrated by accepting hydrogen bonds from the hydrating water molecules. The oxygen atom of the permanganate and perrhenate ions form weaker and longer hydrogen bonds to water than the hydrogen bonds in bulk water (i.e., they act as structure breakers), while the oxygen atoms of the chromate, dichromate, molybdate, tungstate, and hydrogenvanadate ions form hydrogen bonds stronger than those in bulk water (i.e., they act as structure makers). The oxometallate ions form one hydration shell distinguishable from bulk water as determined by DDIR spectroscopy and LAXS. The hydration of oxoanions results in X-O bond distances ca. 0.02 Å longer than those in unsolvated ions in the solid state not involved in strong bonding to counterions. The oxygens of oxoanions with a central atom from the second and third series in the periodic table and the hydrogenvanadate ion hydrogen bind three hydrating water molecules, while oxygens of oxoanions with a heavier central atom only form hydrogen bonds to two water molecules.

M. Śmiechowski, and I. Persson,Hydration of Oxometallate Ions in Aqueous Solution, Inorganic chemistry, 2020. 59, 8231–8239.


Aqueous speciation nitric acid

Chemical forms of molybdenum ion in nitric acid solution studied using liquid-phase X-ray absorption fine structure, Ultraviolet-Visible absorption spectroscopy and first-principles calculations

We have investigated chemical forms of molybdenum ion in nitric acid solution, using liquid-phase X-ray absorption fine structure, ultraviolet-visible absorption spectroscopy and first-principles calculations, from a viewpoint of disposal of high-level radioactive nuclear liquid wastes. The experimental and theoretical results indicated that Mo is a hexavalent ion and forms a hexa-coordination binuclear-structured complex in the 2 M nitic acid solution. The predominant chemical species of Mo complexes in the 2 M nitric acid solution (which is used for HLLW) are assigned to be [Mo2O5(H2O)(6)]2+ and [Mo2O4(OH)(H2O)(6)]3+. These species may play a crucial role of forming so-called yellow-phase in vitrified objects.

S. Watanabe, T. Sato, M. Nakaya, T. Yoshida, and J. Onoe,Chemical forms of molybdenum ion in nitric acid solution studied using liquid-phase X-ray absorption fine structure, Ultraviolet-Visible absorption spectroscopy and first-principles calculations, Chemical Physics Letters, 2019, 723, 76-81.


Nature of Monomeric Molybdenum(VI) Cations in Acid Solutions using Theoretical Calculations and Raman Spectroscopy

The composition and structures of the two protonated species formed from uncharged molybdic acid, MoO2(OH)2(OH2)20, in strongly acidic solutions have been investigated using a combination of density functional theory calculations, first-principle molecular dynamics simulations, and Raman spectroscopy. The calculations show that both protonated species maintain the original octahedral structure of molybdic acid. Computed pKa values indicated that the =O moieties are the proton acceptor sites and therefore that MoO(OH)3(OH2)2+ and Mo(OH)4(OH2)22+ are the probable protonated forms of Mo(VI) in strong acid solutions, rather than the generally accepted MoO2(OH)2-x(OH2)2+xx+ (x = 1, 2) species. This finding is shown to be broadly consistent with the observed Raman spectra. Structural details of MoO(OH)3(OH2)2+ and Mo(OH)4(OH2)22+, are reported.

N. Zhang, E. Koenigsberger, S. Duan, K. Lin, H. Yi, D. Zeng, Z. Zhao, and G. Hefter,Nature of Monomeric Molybdenum(VI) Cations in Acid Solutions using Theoretical Calculations and Raman Spectroscopy, The journal of physical chemistry B, 2019, 123, 3304−3311

Aqueous solutions complex formation and speciation

Speciation model of the Mo(VI)-Ni(II)-citrate-S(VI)-N(III) aqueous system for the study of the electrodeposition of molybdenum and nickel oxides films

A speciation model is proposed for the determination of the concentration of different species formed in an aqueous solution containing Mo(VI), Ni(II), citrate, ammonia and sulfate at 298 K, 105 Pa and variable pH and Mo(VI)/Ni(II) activity ratios. This solution is to be used as electrolyte in the electrodeposition process of thin films of Mo and Ni oxides, which appear to be a promising material for the fabrication of photo-anodes for water splitting in photo-electrochemical cells. The speciation model comprises 53 species and their stability constants, which are related through molar and charge balances to estimate the composition of the solution given a pH value and formal concentrations. As a result, predominance and distribution diagrams are produced, based on which electrolyte conditions (pH and central species concentrations) are recommended to maximize the availability of desired species for the electrodeposition process. Eh-pH diagrams for molybdenum and nickel species at the recommended activities (10-2 for Mo and Ni species, and 10-1 for citrate, ammonia and sulfate species) are also produced, to determine the adequate potential range to be applied for the electrodeposition of Mo and Ni oxides films. (C) 2018 The Electrochemical Society.

J. Morales-Santelices, M. Colet-Lagrille, and M. Garcia-Garcia, Speciation model of the Mo(VI)-Ni(II)-citrate-S(VI)-N(III)  aqueous system for the study of the electrodeposition of molybdenum and nickel oxides films, Journal of the Electrochemical Society, 2018, 165, D344-D353.


Species in aqueous solutions

Equilibria in aqueous solutions of molybdenum( VI) have been studied in detail . At molybdenum concentrations greater than 10-3 mol l-1 at pH >6 the predominant species is the tetrahedral [MoO4]2- ion. As the pH is lowered polymerisation condensation occurs giving at pH 5-6 the heptamolybdate ion [Mo7O24]6- and at pH 3-5 the octamolybdate ion [Mo8O26]4-. Both ions are built up from linked MoO6 octahedra. At pH 0.9 MoO3 precipitates and in more acidic solutions the [MoO2]2+ ion is formed.

The usual source of molybdenum in the physiological work described later is a molybdate; but it is not always clearly stated in the literature precisely which compound has been used. For example,"ammonium molybdate" may be any of the compounds (NH4)2MoO4, (NH4)2Mo2O7 (dimolybdate), (NH4)6Mo7O24.4H2O (heptamolybdate). The chemical similarities of the various molybdate species and the fact that they are in equilibria in aqueous media mean that major differences are unlikely in their physiological effects. The Table below summarises the aqueous chemistry of molybdenum(VI).

The molybdate species in aqueous solutions depend on the molybdenum concentration and the pH as shown in the Table. Since equilibria are established quickly (i.e., within the time of dissolution or mixing) these are the species whatever the starting compound. In alkaline and neutral solutions molybdates are present as the monomeric [MoO4]2- ion. As the pH is lowered the anion becomes protonated. Whether it polymerises to hepta- or octa- molybdate depends on the pH and the Mo concentration. Polymerisation occurs at higher Mo concentrations. At Mo concentrations above 10-3 mol Mo/l and pH 5 – 6 the heptamolybdate ion forms and at pH 3 – 5 octamolybdate. Note that these are the only polymeric species. The compounds which crystallise from solution under various conditions consist of linked molybdate ions. That we can crystallise a ‘dimolybdate’ does not mean that a [Mo2O7]2- ion is present in solution. At pH 0.9 MoO3 precipitates.

Species in aqueous molybdate solutions at ca 20°C
[Mo(VI)]/mol l-1 pHMain species
All   > 6  [MoO4]2-
10-5 > 5  [MoO4]2-(ca 100%)
10-5 [MoO4]2- (30%), [HMoO4]- or [MoO(OH)5]- (10%)
H2MoO4 or Mo(OH)6 (60%)
10-5  2 - 3  H2MoO4 or Mo(OH)6 (ca 100%)
10-5 1 H2MoO4 or Mo(OH)6 (80%) [H3MoO4]+ or [Mo(OH)5(H2O)]+ (20%)
< 10-3  > 1 Monomeric species only
> 10-3 5-6  [Mo7O24]6-, [HMo7O24]5-, [H2Mo7O24]4-
> 10-3 4 – 5  [Mo8O26]4-

Mitchell, P.C.H., in Ullmann’s Encyclopedia of Industrial Chemistry, 5th Ed., 1990, A16, Chap. 7, pp 675 - 682 and references therein.
Aveston, J., Anacker, E.W. and Johnson, J.S., Inorg. Chem., 1964, 3, 735.
Busey, R.H., and Keller, O.L., J. Chem. Phys., 1964, 41, 215.

In protonation of MoO42- at concentrations below 10(-3) M the dominant species is monomeric molybdic acid, H2MoO4. This is likely to be the species adsorbed on manganese oxide, a process thought to control MoO42- levels in the ocean, because of the strong proton dependence of MoO42- adsorption. A 919 cm-1 Raman band assigned to v(s)Mo= O of H2MoO4 was observed with 244 nm laser excitation. In DFT calculations on possible H2MoO4 structures the best fit for the 919 cm-1 band was obtained for MoO3( H2O)3.

Oyerinde, O.F., Weeks, C. L., Anbar, A. D., and Spiro, T. G., Solution structure of molybdic acid from Raman spectroscopy and DFT analysis, Inorganica Chimica Acta, 2008, 361, 1000-1007.

In oxidation state V molybdenum is less acidic than in oxidation state VI. Molybdenum(V) oxide, Mo2O5, and hydroxide, MoO(OH)3, are insoluble in neutral and alkaline solutions. Species of molybdenum(V) in hydrogen halide acids have been studied. In concentrated hydrochloric acid the main species is the mononuclear [MoOCl5]2- ion. At lower acid concentrations binuclear [Mo2O3]4+ and [Mo2O4]2+ ions are formed.

Contrary to earlier reports it is now believed that molybdenum(IV) is stable in aqueous solutions and is not subject to disproportionation.

Lamache, M., in Proceedings of the First International Conference on the Chemistry and Uses of Molybdenum, 1973,
Mitchell, P. C. H. (ed), Climax Molybdenum Co. Ltd, p. 278.

For molybdenum(III) the existence of the ion [Mo(H2O)6]3+and related dimeric ions has been demonstrated.

In ligand exchange reactions in aqueous solution molybdenum(VI) and (V) are kinetically labile and molybdenum(IV) and (III) are inert [Bowen and Taube, 1971; Sasaki and Sykes, 1973; Sasaki et al., 1975].

Bowen, A.R. and Taube, H., J. Amer. Chem. Soc., 1971, 93, 3287.
Sasaki, Y.and Sykes, A. G., in Proceedings of the first Climax International Conference on the Chemistry and Uses of Molybdeum, 1973, Mitchell, P. C. H., (ed), Climax Molybdenum Co. Ltd, London and Ann Arbor, p. 64.
Sasaki, Y., Taylor , R.S. and Sykes, A.G., J. Chem. Soc .( Dalton ), 1975, 396.

Species in aqueous solutions

Tetrahedral molybdate species in basic to near neutral solution phase of natural hydrothermal fluid; chloride complexes only significant in highly acidic (HCl) fluid

Molybdenum(VI) is expected to be the main form of molybdenum in hydrothermal fluids up to magmatic hydrothermal conditions. We conducted an in situ X-ray absorption spectroscopic study of molybdenum(VI) speciation in H2O-HCl-NaCl brines up to 385°C at a pressure of 600 bar. The EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near-Edge Structure) data supported by ab initio XANES calculations confirm that tetrahedral complexes containing the molybdate anion (MoO42-) predominate in basic to near neutral solutions with chloride concentration up to 5.5 M. The geometry of this species changes little over the investigated P-T range, and there is no complexation between MoO42- and chloride. In highly acidic solutions, molybdenum(VI) speciation is dominated by distorted octahedral oxo-chloro complexes MoOmCln6-2m-n; the number of chloride ligands increases with increasing temperature, to a maximum of ca 5 (e. g., MoOCl5-) at 340°C in 6.21 M HCl. These data suggest that molybdenum speciation is dominated by tetrahedral molybdate species (e. g., HMoO4- or MoO42-) in the basic to near neutral solution phase of natural hydrothermal fluid, and chloride complexes will only play a significant role in highly acidic fluid under our experimental P-T conditions. (C) 2012 Elsevier Ltd. All rights reserved

Borg, Stacey, Liu, Weihua, Etschmann, Barbara, Tian, Yuan, and Brugger, Joel, An XAS study of molybdenum speciation in hydrothermal chloride solutions from 25-385°C and 600 bar, Geochimica et Cosmochimica Acta, 2012, 92, 292-307.

Speciation molybdate aqueous solution

Frequency response analysis at 1000 Hz to 35 mHz and cyclic voltammetry were used to characterize the admittance, impedance, double layer capacitance, and semiconduction behavior of molybdate species in solution. The dominant species was MoO42- in the pH range 7-12 and protonated Mo7O246- pH range 3-5 and Mo8O264- below pH 2 consistent with potentiometric titrations.

Krishnan, C. V., Garnett, M., Hsiao, B., and Chu, B., Electrochemical measurements of isopolyoxomolybdates: 1. pH dependent behavior of sodium molybdate, International Journal of Electrochemical Science, 2007, 2, 29-51.

Dissolution and solubility of molybdates and molybdenum compounds
Solubility of molybdate at high temperatures and ore formation

Mo solubilities of 1.6 wt% in the case of KCl-bearing aqueous solutions and up to 0.8 wt% in pure H2O were found. Mo-oxo-chloride complexes are present at high salinity (> 20 wt% KCl) and ion pairs at moderate to low salinity (< 11 wt% KCl) in KCl - H2O aqueous solutions. In water molybdic acid is the dominant species in aqueous solution. High Mo concentrations can be transported in aqueous solutions. Mo concentration in aqueous fluids is not limiting factor for ore formation; precipitation and the availability of sulfur control the formation of molybdenite (MoS2).

Ulrich, T. and Mavrogenes, J., An experimental study of the solubility of molybdenum in H2O and KCl-H(2)Osolutions from 500 degrees C to 800 degrees C, and 150 to 300 MPa, Geochimica Et Cosmochimica Acta, 2008, 72, 2316-2330.

Electrochemical Behaviour of Isopoly- and Heteropolyoxomolybdates Formed During Anodic Oxidation of Molybdenum in Seawater

The electrochemical behaviour of isopoly- and heteropolyoxomolybdates formed during anodic oxidation of molybdenum in seawater at constant current intensity was investigated in this work.

The results supported by spectrophotometry clearly indicate the formation of mixed valence molybdates (V/VI) during this process.

The electrochemical behaviour of isopolyoxomolybdates shows a typical quasi-reversible mass-transport limited system coupled with an adsorption of reduced species and under some kinetic limitations.

For heteropolyoxomolybdates a reversible mass-transport limited system coupled with an homogeneous chemical reaction was found. The coupled reaction, probably protonation, prevents a rapid heterogeneous electron transfer for heteropolyoxomolybdate complex. The phenomenon is more noticeable for the phosphomolybdate complex since the protonation step has a greater influence on the registered voltammograms.

The presented results have a great importance in research areas where molybdate chemistry is used in detection of silicate and phosphate, namely in seawater

Jonca, J., Barus, C., Giraud, W., Thouron, D., Garcon, V., and Comtat, M., Electrochemical Behaviour of Isopoly- and Heteropolyoxomolybdates Formed During Anodic Oxidation of Molybdenum in Seawater, International Journal of Electrochemical Science, 2012, 7, 7325-7348.

In a study undertaken by IMOA for REACH the molybdenum species produced in water when molybdenum compounds are dissolved or stirred in suspension in water have been identified by ultraviolet spectroscopy of the water phase: sodium molybdate, Na2MoO4. 2H2O, ammonium dimolybdate, (NH4)2Mo2O7, ammonium heptamolybdate, (NH4)6Mo7O24.4H2O, ammonium octamolybdate, (NH4)4Mo8O26.5H2O, calcium molybdate, CaMoO4, molybdenum metal powder, ferromolybdenum, molybdenum dioxide, MoO2, molybdenum trioxide, MoO3, roasted molybdenum concentrate (MoO3), molybdenum disulfide, MoS2 and iron(III) molybdate, Fe2(MoO4)3. Species were identified by their characteristic uv spectra, peak positions and intensities. The solutions and supernatant liquids contain the molybdate ion and, in addition at lower pHs, protonated molybdate. At biological concentrations and pH the only molybdenum species produced from the molybdenum substances studied is the molybdate, [MoO4]2-, ion.

The Report  (Ultraviolet spectra and REACH read across, Report for the International Molybdenum Association and the REACH Molybdenum Consortium, P.C.H. Mitchell, March 2009) is available to download as a PDF file.

Download PDF (980 K)

Molybdate mine tailings speciation

The geochemical model for Mo mineralization in the JEB Tailings Management Facility (JEB TMF), operated by AREVA Resources Canada at McClean Lake, Saskatchewan, was investigated using X-ray Absorption Near-Edge Spectroscopy (XANES), an elemental-specific technique that is sensitive to low elemental concentrations. Twenty five samples collected during the 2013 sampling campaign from various locations and depths in the TMF were analyzed by XANES. Mo K-edge XANES analysis indicated that the tailings consisted primarily of Mo(6+) species: powellite (CaMoO4), ferrimolybdite (Fe2(MoO4)3.8H2O), and molybdate adsorbed on ferrihydrite (Fe(OH)3 - MoO4). A minor concentration of a Mo(4+) species in the form of molybdenite (MoS2) was also present. Changes in the Mo mineralization over time were inferred by comparing the relative amounts of the Mo species in the tailings to the independently measured aqueous Mo pore water concentration. It was found that ferrimolybdite and molybdate adsorbed on ferrihydrite initially dissolves in the TMF and precipitates as powellite.

Blanchard, P. E.,Hayes, J. R.,Grosvenor, A. P.,Rowson, J.,Hughes, K.,and Brown, C.,Investigating the Geochemical Model for Molybdenum Mineralization in the JEB Tailings Management Facility at McClean Lake, Saskatchewan: An X-ray Absorption Spectroscopy Study, Environ Sci Technol, 2015, 49, 6504.

Molybdate welding fumes analysis speciation

A novel analytical procedure was developed for the simultaneous speciation analysis of chromate, molybdate, tungstate and vanadate by anion-exchange high performance liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC-ICP-MS). Linear gradient elution from 100% water to 100% 0.7M NaCl was applied for chromatographic separation of metal species. In standard aqueous solution at neutral pH molybdate, tungstate and vanadate exist in several aqueous species, while chromate is present as a single CrO42- species. Consequently, only chromate can be separated from this solution in a sharp chromatographic peak. For obtaining sharp chromatographic peaks for molybdate, tungstate and vanadate, the pH of aqueous standard solutions was raised to 12. At highly alkaline conditions single CrO42-, MoO42- and WO42- are present and were eluted in sharp chromatographic peaks, while VO43- species, which predominates at pH 12 was eluted in slightly broaden peak. In a mixture of aqueous standard solutions (pH 12) chromate, molybdate, tungstate and vanadate were eluted at retention times from 380 to 420s, 320 to 370s, 300 to 350s and 240 to 360s, respectively. Eluted species were simultaneously detected on-line by ICP-MS recording m/z 52, 95, 182 and 51. The developed procedure was successfully applied to the analysis of leachable concentrations of chromate, molybdate, tungstate and vanadate in alkaline extracts (2% NaOH+3% Na2CO3) of manual metal arc (MMA) welding fumes loaded on filters. Good repeatability and reproducibility of measurement (RSD+/-3.0%) for the investigated species were obtained in both aqueous standard solutions (pH 12) and in alkaline extracts of welding fumes. Low limits of detection (LODs) were found for chromate (0.02ngCrmL(-1)), molybdate (0.1ngMomL(-1)), tungstate (0.1ngWmL(-1)) and vanadate (0.2ngVmL(-1)). The accuracy of analytical procedure for the determination of chromate was checked by analysis of CRM 545, Cr(VI) in welding dust loaded on a filter. Good agreement between determined and reported certified values was obtained. For molybdate, tungstate and vanadate the assessment of accuracy was performed by spiking welding fume filters. Good recoveries for all investigated species (98-101%) confirmed the accuracy of the analytical procedure.

Scancar, J.,Berlinger, B.,Thomassen, Y.,and Milacic, R.,Simultaneous speciation analysis of chromate, molybdate, tungstate and vanadate in welding fume alkaline extracts by HPLC-ICP-MS, Talanta, 2015, 142, 164.

Thiomolybdate sulfidic water speciation

The distinct reactivities of Mo and Re in oxic versus anoxic waters make these elements ideally suited for use as redox proxies. However, their full exploitation as geochemical tracers requires that their chemical transformations in sulfidic water be well understood. While thermodynamic data have been used to predict Mo and Re speciation within sulfidic waters, these predictions remain unsubstantiated because effective methodologies for separating the thiomolybdate ((MoOxS42-)-O-VI-x)) and thioperrhenate ((ReOxS42-)-O-VII-x) anions within natural settings have yet to be developed. Anion exchange chromatography (AEC) is often employed to provide separation of environmentally important anions. For example, thiometalates of As and Sb have been quantified in sulfidic geothermal waters using AEC coupled with inductively coupled plasma mass spectrometry (ICP-MS). Unfortunately, AEC methods are incapable of separating the thiomolybdate or thioperrhenate anions due to the extreme retention these thiometalates experience on AEC columns. Because reverse phase ion pair chromatography (RP-IPC) offers greatly diminished retention times, we have developed RP-IPC-methods that are capable of separating all stable thiomolybdates and thioperrhenates within 40 min. An isocratic method provides effective separation of the thiomolybdates. Thioperrhenates and mixtures of the thiomolybdates and thioperrhenates require gradient methods. Addition of p-cyanophenol (p-CP) to the eluent markedly reduces thiometalate retention and facilitates their separation. Our efforts could lead in the near future to coupling RP-IPC with ICP-MS or multi-collector ICP-MS for characterizing Mo and Re speciation in natural sulfidic waters as well as potential fractionation among Mo and Re isotopes during speciation changes. (C) 2015 Elsevier B.V. All rights reserved.

Vorlicek, T. P., Chappaz, A., Groskreutz, L. M., Young, N., and Lyons, T. W.,A new analytical approach to determining Mo and Re speciation in sulfidic waters, Chemical Geology, 2015, 403, 52.

Users of the Database should be aware that inclusion of an abstract in the Database does not imply any IMOA endorsement of the accuracy or reliability of the reported data or the quality of a publication.