Removal of ⁸⁵Sr, ¹³⁴Cs, and ⁵⁷Co Radionuclides from Acidic and Neutral Waste Solutions by Metal Doped Antimony Silicates

Abstract

Antimony silicates doped with a metal, (M)_x,ySb_2-xSi_2-yO₇ (M=W, Ti, Nb), were studied for the removal of the key radionuclides ⁸⁵Sr, ¹³⁴Cs, and ⁵⁷Co from nuclear waste solutions. Emphasis was given to the removal of radionuclides from acidic effluents for which no efficient commercial exchangers are available. Initial screening tests showed that the undoped antimony silicates (Sb:Si molar ratio ∼1:1) are highly selective for ⁸⁵Sr. Distribution coefficients (K_D) of 35,500 mL g⁻¹ were obtained in 0.1-M HNO₃. The antimony silicates also have high or reasonable selectivity for ⁵⁷Co, ⁵⁹Fe, and ²⁴¹Am; but the selectivity for ¹³⁴Cs is low. An attempt to increase the selectivity for ¹³⁴Cs was conducted by doping the antimony silicate with Ti⁴⁺, Nb⁵⁺, or W⁶⁺. Best results were obtained with a material doped with tungsten, which resulted in an almost ten-fold increase in cesium selectivity in acid. The pyrochlore structure of the materials was also expected to have an effect in creating cesium selectivity. The granular antimony silicate doped with tungsten performed very well in column tests, and high-breakthrough capacities were observed for ⁸⁵Sr ( >17,000 BV with 1% breakthrough) and ¹³⁴Cs (5,000 BV) in 0.1-M HNO₃, and for ⁵⁷Co (5,200 BV) and ⁸⁵Sr (11,000 BV) in neutral simulated pond water. In general, the performance of the metal doped antimony silicates was considerably better than that of commercial materials such as a zeolite, a sodium titanate, and a silicotitanate, which function effectively only in neutral or alkaline conditions and were tested in parallel for reference.

Keywords:

• Ion exchange
• Pyrochlore
• Hydrous metal oxide
• Nuclear waste
• Strontium
• Cesium
• Cobalt

Acknowledgments

This work was funded by the European Commission within the Nuclear Fission Safety Program (Contract No. FI4W-CT95-0016). The authors thank all the partners for helpful discussions.