Facilitated strontium transport by remobilization of strontium-containing secondary precipitates in Hanfod Site subsurface

Highlights: ► Remobilization of secondary precipitates increased with elevated flow rate and pH. ► Mobility of Sr-containing colloid decreased with decreased solution ionic strength. ► Sr transport through both desorption and colloid-facilitation in subsurface. -- Abstract: Significantly enhanced immobilization of radionuclides (such as 90Sr and 137Cs) due to adsorption and coprecipitation with neo-formed colloid-sized secondary precipitates has been reported at the U.S. Department of Energy's Hanford Site. However, the stability of these secondary precipitates containing radionuclides in the subsurface under changeable field conditions is not clear. Here, the authors tested the remobilization possibility of Sr-containing secondary precipitates (nitrate-cancrinite) in the subsurface using saturated column experiments under different geochemical and flow conditions. The columns were packed with quartz sand that contained secondary precipitates (nitrate-cancrinite containing Sr), and leached using colloid-free solutions under different flow rates, varying pH, and ionic strength conditions. The results indicate remobilization of the neo-formed secondary precipitates could be possible given a change of pH of ionic strength and flow rate conditions. The remobility of the neo-formed precipitates increased with the rise in the leaching solution flow rate and pH (in a range of pH 4–11), as well as with decreasing solution ionic strength. The increased mobility of Sr-containing secondary precipitates with changing background conditions can be a potential source for additional radionuclide transport in Hanford Site subsurface environments