Effect of Molybdenum on the Structure and on the Crystallization of SiO₂–Na₂O–CaO–B₂O₃ Glasses

Abstract

O-CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B 2 O 3 and MoO 3 were added to a SiO 2 -Na 2 O-CaO glass, and crystallization tendency was studied. Na 2 MoO 4 crystallization tendency decreased with the increase of B 2 O 3 concentration whereas the tendency of CaMoO 4 to crystallize increased due to preferential charge compensation of BO 4 À entities by Na 1 ions. 29 Si MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd 31 ) were shown to enter into CaMoO 4 crystals formed in glasses.

MoO3 incorporation in magnesium aluminosilicate glasses

“…Mo predominantly occurs as a hexavalent state Mo 6+ in glasses prepared under oxidising and neutral atmospheres, regardless of glass composition [9][10] with each Mo 6+ cation being coordinated with four oxygens to form a MoO4 2-tetrahedron. The average Mo-O distance range is 1.76-1.78 Å, indicating that Mo 6+ has a high field strength range of 1.89-1.94 Å -2 in glass [6,[10][11][12][13] and thus Mo 6+ cations have a strong ordering effect on surrounding oxygens [14]. Consequently, the MoO4 2-tetrahedra can be easily separated from the silicate glass network.…”

“…GCMs wasteform can be a useful compromise between glasses and ceramics, being easier and less expensive to prepare than conventional ceramics, but offering higher durability than glasses [40][41]. This is the approach adopted to date in France for waste with high MoO3 content [2,14]. Nevertheless, the glasses developed here represent an alternative approach in that promote high MoO3 solubility in a fully vitreous wasteform.…”

“…MoO4 2-tends to form alkali or alkaline earth molybdate crystals on separating out from the glass network and therefore the molybdate solubility can be related to the cations with which the MoO4 2-tetrahedra are associated. Tailoring glass composition so as to improve molybdate solubility in borosilicate glasses has been undertaken by a number of workers [2,[14][15][16][17]; however, the solubility is still low. For example, Caurant and co-workers [14,17] have found that increasing the B2O3 level in a Ca-Na borosilicate glass, which is supposed to consume more Na + cations thereby enabling more MoO4 2-units to be connected with Ca 2+ , does not enhance molybdate capacity in glass in reality.…”

Summary Report: Glass-Ceramic Waste Forms for Combined Fission Products

“…During FY 2010, the molar ratios of B 2 O 3 /alkali and CaO/MoO 3 were fixed at 1.75 (Caurant et al 2007) to force a Ca-rich molybdate with the powellite/scheelite structure, while waste loading was varied at 42, 45, and 50 mass%. The glasses were formulated to melt at ~ 1250-1300°C to avoid volatility of alkali (mainly Cs 2 O) and MoO 3 .…”

Effect of the Bromo-Group in 5-Octyloxymethyl-8-quinolinol on the Solvent Extraction of Molybdenum(VI) from Strongly Acidic Solutions

“…Molybdenum is one of the major fission products. It is known that the solubility of molybdenum oxides, such as MoO 3 and MoO 4 , is relatively low [3]. Molybdenum prevents the effective and economical vitrification of the high-level radioactive liquid waste.…”