Long-term immobilization of strontium ions using zeolite A/calcium phosphate nanocomposites

概要

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Zeolite A whose surface was evenly covered with tiny scaly calcium deficient hydroxyapatite (Ca-def HA) nanocrystals was prepared through the ion exchange reaction of ammonium and calcium ions as a novel encapsulation material for the long-term immobilization of radioactive Sr-90. The maximum capacity for Sr ion uptake in the zeolite A/Ca-def HA nanocomposite was determined to be 0.40 mmol g−1, which was comparable to the value for zeolite A alone; this was because of the loose arrangement of Ca-def HA nanocrystals on the surfaces. The nanocomposite obtained was heated at 800 to 1200°C to improve the solution stability of adsorbed Sr ions. The low crystalline Ca-def HA became tricalcium phosphate (TCP) after heating at over 850°C. A phase change from zeolite A to an amorphous phase was observed at 850°C and to feldspar at 1000°C. The elution of Sr ions from the nanocomposites heated was compared to those from the heated zeolite A, under alkaline conditions (pH 10.2) similar to those found in geological layers deeper than 300 m. The elution amounts of Sr ions from the nanocomposites decreased with heating temperatures over 950°C compared with those of zeolite A, due to the complete coverage of TCP crystals. These results indicate that calcium phosphate nano-layers on the zeolite A surfaces play an important role for the long-term encapsulation of Sr ions in the nanocomposites.