The feasibility of in situ phosphate- and metal- (calcium, aluminum, and iron) solution treatment for 90Sr immobilization was investigated. Batch and column experiments were performed to find optimum conditions for coprecipitation of 90Sr with Ca-, Al-, and Fe-phosphate compounds in contaminated soils. Separate columns were packed with artificially 85Sr-contaminated acid soil as well as 90Sr-contaminated soil from the Oak Ridge Reservation. After metal-phosphate treatment, the columns were then leached successively with either tapwater or 0.001 M CaCl2 solution. Most of the 85Sr coprecipitated with the metal phosphate compounds. Immobilization of 85Sr and 90Sr was affected by such factors as solution pH, metal and phosphate concentration, metal-to-phosphate ratio, and soil characteristics. Equilibration time after treatments also affected 85Sr immobilization. Many technology aspects still need to be investigated before field applications are feasible, but these experiments indicate that phosphate-based in situ immobilization should prevent groundwater contamination and will be useful as a treatment technology for 90Sr-contaminated sites.

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