Mineral sorption/coprecipitation is thought to be a principal sequestration mechanism for radioactive 90Sr and 137Cs in sediments impacted by hyperalkaline, high-level radioactive waste (HLRW) at the DOE's Hanford site. However, the longterm persistence of neo-formed, contaminant bearing phases after removal of the HLRW source is unknown. We subjected pristine Hanford sediments to hyperalkaline Na-Al-NO3-OH solutions containing Sr, Cs, and I at 10-5,10-5, and 10-7 molal, respectively, for 182 days with either <10 ppmv or 385 ppmv pC0 2. This resulted in the formation of feldspathoid minerals. We leached these weathered sediments with dilute, neutral-pH solutions. After 500 pore volumes (PVs), effluent Sr, Cs, NO3, Al, Si, and pH reached a steady-state with concentrations elevated above those of feedwater. Reactive transport modeling suggests that even after 500 PV, Cs desorption can be explained by ion exchange reactions, whereas Sr desorption is best described by dissolution of Sr-substituted, neo-formed minerals. While, pC02 had no effect on Sr or Cs sorption, sediments weathered at <10 ppmv pC0 2 did desorb more Sr (66% vs 28%) and Cs (13% vs 8%) during leaching than those weathered at 385 ppmv pC02. Thus, the dissolution of neoformed aluminosilicates may represent a long-term, low-level supply of 90Sr at the Hanford site.
ASJC Scopus subject areas
- Environmental Chemistry