Interactions of W(VI) and Mo(VI) Oxyanions with Metal Cations in Natural Waters

Selective Crystallization of Four Tungstates (La2W3O12, La2W2O9, La14W8O45, and La6W2O15) via Hydrothermal Reaction and Comparative Study of Eu3+ Luminescence

“…Such results may be understood from the solution chemistry of La and tungstate ions. It is known that tungstate anions present themselves as monomeric [WO 4 ] 2– in a solution of sufficient alkalinity (pH ≥ 8) while the protonated and polymerized forms of [H 18 (WO 4 ) 12 ] 6– , [H 10 (WO 4 ) 6 ] 2– , [H 7 (WO 4 ) 6 ] 5– , and [HWO 4 ] − prevail in acidic and neutral solutions of pH 4–7 by the reaction ,,− On the other hand, La 3+ undergoes hydrolysis in aqueous solution, according to the reaction and a higher pH produces more hydroxyls in the complex ion. , The protonated tungstate ions are acidic species, which react more readily with the hydrolyzed La ions (a base) than [WO 4 ] 2– via neutralization reaction. The crystallization of pure La­(OH) 3 at (13, 0.5) conforms to the above discussion and suggests that [WO 4 ] 2– exhibits negligible reactivity toward La­(OH) 3 under highly alkaline conditions.…”

“…This could possibly be hindered by the fact that tungstate anions present themselves as complicated species by polymerization and protonation in solution, 29 and are readily influenced by tungstate concentration, the presence of other ionic species, and interaction of the tungstate anions with other anions. 30 Lanthanum (La) is the most inexpensive RE and La 3+ is optically inert, because of its vacant 4f shell; for thesse reasons, La compounds are frequently employed as host lattice for luminescence. 31 In this work, we thoroughly investigate the crystallization of La tungstates through hydrothermal reaction at the highest available temperature of 200 °C, without using any organic additives, and the effects of W/La molar ratio and solution pH on the characteristics of the products are unveiled.…”

“…It was concluded from previous studies that hydrothermal parameters, especially the RE/W molar ratio, solution pH, and reaction temperature, have remarkable influence on the direct and calcination product, but a systematic study has been rare for the factors and also their operating mechanism. This could possibly be hindered by the fact that tungstate anions present themselves as complicated species by polymerization and protonation in solution, and are readily influenced by tungstate concentration, the presence of other ionic species, and interaction of the tungstate anions with other anions . Lanthanum (La) is the most inexpensive RE and La 3+ is optically inert, because of its vacant 4f shell; for thesse reasons, La compounds are frequently employed as host lattice for luminescence .…”

Effects of tungsten source and tartrate additive on the microstructure and photoluminescence of hydrothermally crystallized ZnWO4

“…The morphology of S2 is almost identical to that of ZnWO 4 nanorods obtained with Na 2 WO 4 as the tungsten source under otherwise identical hydrothermal conditions, where the individual nanorods were analyzed via lattice imaging and electron diffraction to develop along the [001] crystallographic direction [19,25]. Tungstate anions exhibit rich solution chemistry and, critically depending on pH of solution, may exist in one or more of the ionic forms of WO 10− in an aqueous solution [32]. It is known that monomeric WO 4 2− can be stabilized by raising the pH value of solution to ~ 6.2, and then becomes the prevailing one (> 99%) when the pH value is over 7.8 [33].…”

NaLaW2O7(OH)2(H2O): Crystal Structure and RE3+ Luminescence in the Pristine and Annealed Double Tungstates (RE = Eu, Tb, Sm, and Dy)

“…As a result, subsequent annealing is needed to convert the precursor into the targeted ARE­(WO 4 ) 2 :Ln compound (A = Na and K; RE = La, Y, Gd, and Lu; Ln = Eu 3+ , Tb 3+ , Sm 3+ , Dy 3+ , and Sm 3+ /Nd 3+ ; etc. ). ,,, This is primarily due to the rich solution chemistry of tungstate anions, which present themselves as monomeric [WO 4 ] 2– in a solution of sufficiently high alkalinity (pH >8), while the various protonated and polymerized forms of [H 18 (WO 4 ) 12 ] 6– , [H 10 (WO 4 ) 6 ] 2– , [H 7 (WO 4 ) 6 ] 5– , and [HWO 4 ] − appear in an acidic or neutral solution (pH 4–7). − Furthermore, the type and concentration of tungstate ions are affected by not only the solution pH but also the temperature, the initial concentration of the tungstate source (such as Na 2 WO 4 ·2H 2 O in this work), and the attendance and concentration of alien anions. On the other hand, RE 3+ undergoes hydration and hydrolysis in an aqueous solution to form [RE­(OH) x (H 2 O) y ] 3– x complex ions, whose x value increases with an increase in the solution pH. , Numerous reaction pathways are, therefore, possible under hydrothermal/solvothermal conditions, which, in turn, make the product rather sensitive to the synthesis parameter. ,, It was recently shown that the reactivity of tungstate species toward [RE­(OH) x (H 2 O) y ] 3– x decreases with an increase in solution pH, and the hydrothermal product of a higher pH tends to have a lower W/RE molar ratio and vice versa. , The complicated solution chemistry and reaction pathway may account for why most of the studies to date left the hydrothermal/solvothermal precursors unanalyzed for chemical composition, phase purity, phase constituent, and phase structure.…”