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 × 10
6 to 6 × 10
7 pCi
137Cs
+/g) were studied. The desorption of
137Cs
+ was measured in Na
+, K
+, Rb
+, and NH
4+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 NH
4+. 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.
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