Studies on rhenium (Re) solubility and heat capacity measurements onCa10-xRex(PO4)6(OH)2±δ, x= 0 to 0.7for radwaste immobilization

Abstract

Rhenium substituted Ca-hydroxy apatite (Ca10-xRex(PO4)6(OH)2±δ, x = 0−0.7) was synthesized by solid state reaction and characterized by XRD, Raman and TG-DTA analysis.Results show that the solubility limit of Re is below 5 mol %; above whichCa3(PO4)2 phase was separated out. In this study, Re is used as surrogate for technetium (Tc) which does not occur in nature. The incorporation and distribution of Re in Ca-hydroxy apatite surface was investigated by scanning electron microscopy (SEM) EDX. The prevalence of constituent elements was further examined by energy dispersive X-ray analysis (EDX) and neutron activation analysis (NAA). The heat capacity (Cp) of the compositions was measured by differential scanning calorimetry (DSC) at 298−800 K for the first time. The Cpvalue of Re substituted compositions (0.84-1.07 JK-1 g−1) was found to be higher than the pristine Ca-hydroxyapatite (0.77-1.02 JK−1 g−1).Thermodynamic functions of the compositions were derived by using the measured Cp values.