Iodine Sorption onto Mixed Solid Alumina Cement and Calcium Compounds

Abstract

A substance for solidifying waste containing 129I is sought that effectively sorbs iodine to inhibit its release from repository into the environment. Three candidate media—commercial alumina cement mixed with calcium sulfate and/or calcium hydroxide—were investigated. The criterion applied for evaluating iodine sorption performance was the distribution coefficient Kd of iodine between solid and solution in an emulsion of sodium iodide solution and powder prepared from the alumina cement/calcium compound mixture, which was molded, cured and ground. Batch sorption experiments were performed on different combinations of the above-mentioned calcium compounds added to alumina cement. The solidified substance was also examined for mechanical strength. The highest iodine sorption performance was obtained with calcium sulfate added to alumina cement to a SC4/Ca mole ratio of 0.16, which ensured a Kd value raised to a level of not lower than 0.2 m3/kg from the (1.8–3.2)x10-3 m3/kg obtainable with alumina cement before calcium compound addition. The enhancement of iodine sorption with addition of calcium sulfate is attributed to formation in the substance of monosulfate (3CaO·Al2O3·CaSO412H2O) or tetracalcium aluminate hydrate (4CaO·Al2O3·XH2O (X=13 to 19)).