Abstract
Vitrification is the most effective method for the immobilization of hazardous waste by incorporating toxic elements into a glass structure. Iron phosphate glasses are presently being considered as matrices for the storage of radioactive waste, even of those which cannot be vitrified using conventional borosilicate waste glass. In this study, a structural model of 60P2O5-40Fe2O3 glass is proposed. The model is based on the crystal structure of FePO4 which is composed of [FeO4][PO4] tetrahedral rings. The rings are optimized using the DFT method and the obtained theoretical FTIR and Raman spectra are being compared with their experimental counterparts. Moreover, the proposed model is in very good agreement with X-ray absorption fine structure spectroscopy (XANES/EXAFS) and Mössbauer spectroscopy measurements. According to the calculations the Fe(3+) is in tetrahedral and five-fold coordination. The maximal predicted load of waste constituents into the glass without rebuilding of the structure is 30 mol%. Below this content, waste constituents balance the charge of [FeO4](-) tetrahedra which leads to their strong bonding to the glass resulting in an increase of the chemical durability, transformation and melting temperatures and density.
Highlights
Besides their excellent chemical durability, iron phosphate glasses have the melting temperature of about 100–200 K lower than borosilicate glass and due to lower viscosity of the melts their homogenization time is about 1.5–3 hours shorter.[5]
Iron phosphate glasses are considered for the vitrification of waste containing a relatively high concentration of actinide elements, a high concentration of sodium, caesium sulphate and chloride, chloride waste from pyrochemical reprocessing of Pu metal and waste containing various metals, radioactive ceramics, polymers or carbon.[1,6]
Crystallization of 60P2O5–40Fe2O3 glass leads to the formation of two phases, namely a-FePO4 and Fe2P2O7, whose proportions strongly depend on the ratio of Fe3+/Fe2+ ions.[16]
Summary
Besides their excellent chemical durability, iron phosphate glasses have the melting temperature of about 100–200 K lower than borosilicate glass and due to lower viscosity of the melts their homogenization time is about 1.5–3 hours shorter.[5]. Iron phosphate glasses are considered for the vitrification of waste containing a relatively high concentration of actinide elements, a high concentration of sodium, caesium sulphate and chloride, chloride waste from pyrochemical reprocessing of Pu metal and waste containing various metals, radioactive ceramics, polymers or carbon.[1,6]. The structure of phosphate glasses can be described in a convenient way using the Qi notation. The Qi represents the phosphate tetrahedra, where i represents the number of neighbouring P-tetrahedra linked by common bridging oxygens.[4,7] Single component P2O5 glass is built of only Q3 tetrahedra
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