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

Selective removal of radioactive strontium (⁹⁰Sr) from the environment is important, and selective adsorption/ion exchange is appropriate for removal of trace amounts of ⁹⁰Sr from large volumes of ⁹⁰Sr-contaminated water. Although various inorganic ion-exchange materials, including zeolites, have been investigated intensively for removal of Sr²⁺ due to their excellent resistance to radiation and high ion-exchange capacity, their ion-exchange selectivity for Sr²⁺ is poor in the presence of competing ions such as Ca²⁺ and Mg²⁺. Here, sulfur-modified NaA zeolite (S-NaA) was prepared for low-cost, selective ⁹⁰Sr removal because the elemental sulfur encapsulated in micropores provides additional Lewis acid-base interactions with Sr²⁺ during the Sr²⁺ ion-exchange. Our ion-exchange experiments revealed that S-NaA with 3 wt% sulfur (3 S-NaA) showed the highest Sr²⁺ selectivity among various S-NaAs containing up to 10 wt% sulfur because ion exchange involving bulky hydrated Sr²⁺ depends on the reduced micropore volume of S-NaA after sulfur loading. Most importantly, 3 S-NaA effectively and efficiently (>99.4%) removed ⁹⁰Sr from groundwater containing 8.4 ppt ⁹⁰Sr, demonstrating its excellent potential for practical application in the treatment of ⁹⁰Sr-contaminated water.