Date of this Version
Geochimica et Cosmochimica Acta, Vol. 67, No. 16, pp. 2893–2912, 2003
The desorption of 137Cs+ was investigated on sediments from the United States Hanford site. Pristine sediments and ones that were contaminated by the accidental release of alkaline 137Cs+ -containing high level nuclear wastes (HLW, 2 X 106 to 6 X 107 pCi 137Cs+/g) were studied. The desorption of 137Cs+ was measured in Na+, K+, Rb+, and NH4+ electrolytes of variable concentration and pH, and in presence of a strong Cs+-specific sorbent (self-assembled monolayer on a mesoporous support, SAMMS). 137Cs+ desorption from the HLW-contaminated Hanford sediments exhibited two distinct phases: an initial instantaneous release followed by a slow kinetic process. The extent of 137Cs+ desorption increased with increasing electrolyte concentration and followed a trend of Rb+ ≥ K+ > Na+ at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of 137Cs+ desorption was influenced by surface armoring, intraparticle diffusion, and the collapse of edge-interlayer sites in solutions containing K+, Rb+, or NH4+. Scanning electron microscopic analysis revealed HLW-induced precipitation of secondary aluminosilicates on the edges and basal planes of micaceous minerals that were primary Cs+ sorbents. The removal of these precipitates by acidified ammonium oxalate extraction significantly increased the long-term desorption rate and extent. X-ray microprobe analyses of Cs+-sorbed micas showed that the 137Cs+ distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of 137Cs+. Controlled desorption experiments using Cs-spiked pristine sediment indicated that the 137Cs+ diffusion rate was fast in Na+-electrolyte, but much slower in the presence of K+ or Rb+, suggesting an effect of edge-interlayer collapse. An intraparticle diffusion model coupled with a two-site cation exchange model was used to interpret the experimental results. Model simulations suggested that about 40% of total sorbed 137Cs+ was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60–80%. The remainder of the sorbed 137Cs+ was fixed or desorbed at much slower rate than our experiments could detect.