Abstract
Nuclear waste storage tanks at the Hanford site in southeastern Washington have released highly alkaline solutions, containing radioactive and other contaminants, into subsurface sediments. When this waste reacts with subsurface sediments feldspathoid minerals (sodalite, cancrinite) are formed, which can sequester pertechnetate (99TcO4−5 ) into their structure, thereby affecting the fate and transport of 99TcO4− in the vadose zone. This study investigates the potential for incorporation of perrhenate (ReO4−), a chemical surrogate for 99TcO4−, into mixed perrhenate/nitrate (ReO4−/NO3−) sodalite. Mixed-anion sodalites were hydrothermally synthesized in the laboratory from zeolite A in sodium hydroxide, nitrate, and perrhenate solutions at 90 °C for 24 to 168 hours. The resulting solids were characterized by bulk chemical analysis, X-ray diffraction, scanning electron microscopy and X-ray absorption near edge structure spectroscopy (XANES) to determine the products’ chemical composition, structure, morphology, and Re oxidation state. XANES data indicate that rhenium (Re) was incorporated as Re(VII)O4−. The non-linear increase of the unit cell parameter with increasing ReO4−/NO3− ratios suggests formation of two separate sodalite phases in lieu of a mixed anion sodalite. The results reveal that the sodalite cage is highly selective towards the NO3− over ReO4−, suggesting that incorporation of NO3− into the cage is energetically more favorable than incorporation of the larger ReO4−. Based on these results, it is expected that NO3−, which is present at significantly higher concentrations in alkaline waste solutions than 99TcO4−, will be strongly preferred for incorporation into the sodalite cage.
