Immobilization and exchange of perrhenate in sodalite and cancrinite

Highly alkaline nuclear tank wastes containing key anionic contaminants have leaked into the subsurface at the U.S. Department of Energy's Hanford Site. Laboratory studies showed that when simulated caustic tank wastes contact subsurface sediments, mineral dissolution and precipitation result in feldspathoid formation. Feldspathoids are environmentally important for waste management and disposal purposes because of their purported potential to sequester contaminants of interest (e.g., 99 TcO 4 − , 137 Cs + , 90 Sr 2+ ) into their structure. We investigated the incorporation of perrhenate (ReO 4 − ), a surrogate for TcO 4 − , in the presence of competing anions (X) including OH − , NO 2 − , NO 3 − and Cl − on feldspathoid formation and under conditions mimicking tank waste solution compositions. The resulting solids were characterized by their chemical composition, structure and morphology. Regardless of solution pH, sodalite formed in the presence of Cl − and NO 2 − whereas NO 3 − promoted either cancrinite formation in 16-mol OH − /kg (16 m ) or mixed sodalite/cancrinite phases in 1-mol OH − /kg (1 m ) solutions. In the presence of Cl − , NO 2 − , and NO 3 − less than 0.02 mol fraction of ReO 4 − was incorporated into the feldspathoid phase(s). Although, the NO 2 -sodalite and mixed NO 3 -cancrinite/sodalite phases incorporated significantly more ReO 4 − than NO 3 -cancrinite or Cl-sodalite phases, the total ReO 4 − fraction was ∼1% or less of the total sites. The ReO 4 − immobilized in ReO 4 -sodalite, NO 2 -sodalite, mixed NO 3 -cancrinite/sodalite and NO 3 -cancrinite was resistant to ion exchange with either NO 2 − or NO 3 − . The results imply that ReO 4 − , and thus by analogy 99 TcO 4 − , does not compete well with smaller ions for incorporation into feldspathoids, but, once sequestered, is difficult to exchange.