Desorption kinetics of radiocesium from subsurface sediments at Hanford Site, USA

The desorption of Cs-137(+) was investigated on sediments from the United States Hanford site. Pristine sediments and ones that were contaminated by the accidental release of alkaline Cs-137(+)-containing high level nuclear wastes (HLW, 2 x 10(6) to 6 x 10(6) pCi Cs-137(+)/g) were studied. The desorption of Cs-137(+), 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). Cs-137(+) desorption from the HLW-contaminated Hanford sediments exhibited two distinct phases: an initial instantaneous release followed by a slow kinetic process. The extent of Cs-137(+) desorption increased with increasing electrolyte concentration and followed a trend of Rb+ greater than or equal to K+ > Na+ at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of Cs-137(+) 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 Cs-137(+) distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of Cs-137(+). Controlled desorption experiments using Cs+-spiked pristine sediment indicated that the Cs-137(+) 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 Cs-137(+) was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60-80%. The remainder of the sorbed Cs-137(+) was fixed or desorbed at much slower rate than our experiments could detect. Copyright (C) 2003 Elsevier Ltd.