Sorption of Cs+ to micaceous subsurface sediments from the Hanford site, USA

Abstract

The sorption of Cs ϩ was investigated over a large concentration range (10 Ϫ9 -10 Ϫ2 mol/L) on subsurface sediments from a United States nuclear materials site (Hanford) where high-level nuclear wastes (HLW) have been accidentally released to the vadose zone. The sediment sorbs large amounts of radiocesium, but expedited migration has been observed when HLW (a NaNO 3 brine) is the carrier. Cs ϩ sorption was measured on homoionic sediments (Na ϩ , K ϩ , Ca 2ϩ ) with electrolyte concentrations ranging from 0.01 to 1.0 mol/L. In Na ϩ electrolyte, concentrations were extended to near saturation with NaNO 3(s) (7.0 mol/L). The sediment contained nonexpansible (biotite, muscovite) and expansible (vermiculite, smectite) phyllosilicates. The sorption data were interpreted according to the frayed edge-planar site conceptual model. A fourparameter, two-site (high-and low-affinity) numeric ion exchange model was effective in describing the sorption data. The high-affinity sites were ascribed to wedge zones on the micas where particle edges have partially expanded due to the removal of interlayer cations during weathering, and the low-affinity ones to planar sites on the expansible clays. The electrolyte cations competed with Cs ϩ for both high-and low-affinity sites according to the trend K ϩ ϾϾ Na ϩ Ն Ca 2ϩ . At high salt concentration, Cs ϩ adsorption occurred only on high-affinity sites. Na ϩ was an effective competitor for the high-affinity sites at high salt concentrations.In select experiments, silver-thiourea (AgTU) was used as a blocking agent to further isolate and characterize the high-affinity sites, but the method was found to be problematic. Mica particles were handpicked from the sediment, contacted with Cs ϩ (aq) , and analyzed by electron microprobe to identify phases and features important to Cs ϩ sorption. The microprobe study implied that biotite was the primary contributor of high-affinity sites because of its weathered periphery. The poly-phase sediment exhibited close similarity in ion selectivity to illite, which has been well studied, although its proportion of high-affinity sites relative to the cation exchange capacity (CEC) was lower than that of illite. Important insights are provided on how Na ϩ in HLW and indigenous K ϩ displaced from the sediments may act to expedite the migration of strongly sorbing Cs ϩ in subsurface environments.

Reversibility of radiocaesium sorption on illite

“…In particular, our mechanistic interpretation of the competitive ion-exchange of radiocaesium, ammonium, and potassium on illite is consistent with (in situ) observations of reversible sorption behaviour of radiocaesium in aquatic sediments (Evans et al, 1983;Smith and Comans, 1996;Comans, 1999), as well as the observed persistence of this radionuclide in water and food products (fish and vegetation) for Table 1. many years following the Chernobyl accident (Smith et al (2000) discuss the relationship between radiocaesium sorption reversibility on illite and uptake in food products in more detail). It also explains the recent observations by Zachara et al (2002) of incomplete exchange of caesium against 0.1 mol/L KNO 3 on Na-saturated micaceous subsurface sediments from the Hanford site after only short caesium adsorption times.…”

“…These results indicate that the desorption reaction is still far from equilibrium after 8 weeks of reaction in 1 mol/L NH 4 Ac. Incomplete exchange of caesium against 0.1 mol/L KNO 3 after only short caesium adsorption times (0.5 d) has also been observed by Zachara et al (2002) for Na-saturated micaceous subsurface sediments from the Hanford site.…”

“…Cs sorption on Fithian illite (Comans et al, 1991), which is used in the present study, was also analysed and well described by their model and parameter set. Finally, Zachara et al (2002) have recently modelled Cs sorption to micaceous subsurface sediments with a single site for the FES and have found a similar selectivity coefficient (log K Cs K C ) of 4.7 Ϯ 0.1 as Bradbury and Baeyens (2000). Below, we use the model and generic parameter set of Bradbury and Baeyens (2000) to calculate equilibrium Cs sorption to Fithian illite at the different experimental conditions that we have applied to investigate the reversibility of the sorption process.…”

Effect of waste mica on transfer factors of134Cs to spinach and lettuce

“…This was considered to be due to a slow and continued release of K from mica through the weathering process. Another reason might be due to high fixing capacity of waste mica for cesium (Zachara et al, 2002;Thiry et al, 2005;Sreenivasa Chari, 2010). The data revealed that soil type significantly influenced the 134 Cs transfer factors.…”

Modelling the radionuclide transfer from bedrock to surface systems at Forsmark site (Sweden)

“…In the case of caesium, the presence of frayed edge sites (FES) in illite is a powerful sink for this element [4,5]. Co-precipitation of strontium with calcite is also a relevant retention mechanism in the till environment.…”