Stabilization Effect of Alkaline Materials on U and Th in Monazite Waste Residue
Several materials such as CaO,Mg(OH)2,Ca3(PO4)2,sepiolite,CaCO3 were used to stabi-lize radioactive monazite waste residue.Different dosing ratios and long-term leaching experiments were carried out,the changes in U,Th,and pH were monitored,and the stabilization mechanism of U and Th was revealed by means of XRD,SEM-EDS,and variation of the distribution and forms.The results show that under the condition of 5.0％and 7.5％dosage ratio,the leaching solution of all experimental groups meet the standard requirements of pH= 6～9.In subsequent long-term leaching experiments,only Ca3(PO4)2 has the long-term stability ability for U and Th nuclides.In the short term,the increase in pH of waste residue has a stabilizing effect on U,and the activation and redisso-lution of U after stabilization is a long-term process unrelated to acid reflux.The stability of Th is mainly affected by pH,pH＞6 can achieve long-term stability of Th.During the stabilization process,the surface of the waste residue become rough due to the formation of precipitation,and the precipita-ted U and Th secondary phases are mostly amorphous.The stabilization of U and Th nuclides is main-ly related to promoting the transition of exchangeable state and carbonate complex state to iron-man-ganese oxidation state and residual state.
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