Inorganic Sorbents for Radiostrontium Removal From Waste Solutions: Selectivity and Role of Calixarenes

Abstract

The challenge in the remediation of ⁹⁰Sr-contaminated waters arises from the need to achieve very high removal efficiencies to meet discharge targets from waste effluents containing relatively high concentrations of non-radioactive cations. Low-cost natural zeolites are not selective for strontium over other divalent cations, notably such ions as calcium; and produce low ⁹⁰Sr removal performance, and large volumes of spent sorbent waste. The synthesis and use of selective, synthetic inorganic sorbents could prove to be a feasible approach for high ⁹⁰Sr removal efficiencies, and much smaller volumes of secondary solid waste generation. The essential advantages of inorganic sorbents include their stability and resistance to radiation, and the potential for producing stable waste forms such as vitrified glass or ceramics for disposal. However, the cost of strontium-specific sorbents is prohibitive for large-scale applications at present.

This paper is a review of the reported information on removal mechanisms and performance of Sr-specific inorganic sorbents. The analysis has revealed promising performance, efficiency and selectivity for strontium removal from solutions containing low and high concentrations of salts. The leading sorbents are crystalline silicotitanate and oxides of metals such as titanium. An initial assessment has also been made of the performance of calixarene-based macrocyclic compounds. These are known for their selectivity for strontium in solvent extraction processes.

From the initial strontium removal results in bench-scale tests using different solid substrates, impregnated with calixarene derivatives, only sodium-mordenite impregnated with calyx[8]arene octamide gave an overall strontium removal efficiency in the range of 90 to 95% in the presence of 3.5 ppm calcium. There was no improvement observed for strontium-removal efficiency or selectivity over calcium in the calixarene-impregnated inorganic sorbent matrix. In several tests, the overall efficiency of the sorbent was reduced by up to 50% in the presence of high concentrations of Ca (e.g., 15 ppm). Presently, the reasons for this behavior are not evident.

This paper also reports high strontium removal performance measured in column tests using crystalline silicotitanate sorbent with actual ground waters containing low and high concentrations of calcium. The spent sorbent waste volume has been estimated to be approximately one order of magnitude less than that using a natural zeolite such as chabazite. Until costs can be lowered, the presently available selective sorbents are not attractive for large volume applications. Further development in the modification of relatively low-cost sorbents, including natural zeolites and certain metal oxides such as titanium and zirconium oxides, would be advantageous.