Strontium sorption of fluorinated clinoptilolite, chabazite, and mordenite

Clinoptilolite, chabazite, and mordenite are zeolites commonly used for the removal of radioactive strontium from contaminated water in the environment. However, their strontium removal performances are relatively poor for the treatment of acidic processing wastes, e.g., those generated from the small-scale processing of irradiated fuel sand isotopes. Zeolite fluorination, which improves the acidity of the functional groups, has potential for improving strontium sorption onto zeolites under acidic conditions. Fluorinated low-cost zeolites can be more economical for the treatment of small volumes of acidic waste, as compared to multiple-step processes involving pH adjustment, precipitation, and ion exchange on organic resins. This study prepared fluorinated clinoptilolite, chabazite, and mordenite at fluorine loadings of 4 to 25 mg-F/g-zeolite. The fluorine loading depended on the quantities of Ca²⁺, Mg²⁺, and H⁺ in the zeolites, as fluoride replaced the OH^- groups associated with these ions. The fluorinated materials were tested for strontium sorption at pHs of 6.7 and 1.5, the known pHs of contaminated environmental water and hot-cell wastes, respectively. After fluorination, the H-form mordenite and chabazite, which were known to have no strontium sorption in their unmodified form, showed significant strontium sorption capacities at near-neutral pH, but lost the strontium sorption capacities at acidic pH values, relative to their capacity at near-neutral pH. Improvements in strontium removal at acidic but not at neutral pH were observed for the fluorinated Na-form clinoptilolite and chabazite. The improved strontium sorption onto the fluorinated Na-form clinoptilolite and chabazite was caused by the exchange between strontium and aluminium after the fluorination. (author)