Sulfur-modified zeolite A as a low-cost strontium remover with improved selectivity for radioactive strontium.

Selective removal of radioactive strontium (Sr) from the environment is important, and selective adsorption/ion exchange is appropriate for removal of trace amounts of Sr from large volumes of Sr-contaminated water. Although various inorganic ion-exchange materials, including zeolites, have been investigated intensively for removal of Sr due to their excellent resistance to radiation and high ion-exchange capacity, their ion-exchange selectivity for Sr is poor in the presence of competing ions such as Ca and Mg. Here, sulfur-modified NaA zeolite (S-NaA) was prepared for low-cost, selective Sr removal because the elemental sulfur encapsulated in micropores provides additional Lewis acid-base interactions with Sr during the Sr ion-exchange. Our ion-exchange experiments revealed that S-NaA with 3 wt% sulfur (3 S-NaA) showed the highest Sr selectivity among various S-NaAs containing up to 10 wt% sulfur because ion exchange involving bulky hydrated Sr depends on the reduced micropore volume of S-NaA after sulfur loading. Most importantly, 3 S-NaA effectively and efficiently (>99.4%) removed Sr from groundwater containing 8.4 ppt Sr, demonstrating its excellent potential for practical application in the treatment of Sr-contaminated water.