Frayed Edge Sites Research Articles

The distribution coefficient for cesium in different clay fractions in soils developed from granite and Paleozoic shales in Japan

The distribution coefficient of cesium (Cs) (KdCs) was investigated for clays which were collected from upland forest soils developed from granites and Paleozoic shales in Japan. They were divided into three size fractions, coarse (2.0–1.0 µm), medium (1.0–0.2 µm), and fine-clay fractions (<0.2 µm), abbreviated CC, MC, and FC, respectively. The KdCs value increased in the fractions as follows: FC < MC < CC in all the horizon samples. The amount of potassium (K) in the clay fractions also followed this same pattern. As the amount of K in the clay fraction can be a quantitative indicator of illitic minerals, the positive relationship between the KdCs and the amount of K indicated that the coarser clay fraction had a higher 137Cs adsorption potential due to containing higher amounts of illitic minerals. The soil clay fraction from the Paleozoic shales both contained a large amount of K and showed greater KdCs values than that of the granite when comparing the same particle sizes. Hence, the type of parent material can be a significant factor in determining the 137Cs adsorption potential of the soils. In all particle fractions, those collected from deeper in the profile had smaller KdCs values than those from closer to the soil surface in the same profile, although amount of K present was similar. The X-ray diffraction (XRD) analysis showed that the hydroxy-aluminum (Al) polymer was present in the 2:1 interlayer sites to a greater extent in the clay fractions from deeper in the soil profile than in those from upper in the same soil profile. This result suggests that the hydroxy-Al polymers may be fixed at the expanded weathering front of the illitic minerals, blocking Cs adsorption on the frayed edge sites of these minerals. Thus, the KdCs value of the soil clays is determined by the degree of illitic minerals and hydroxy-Al interlayering present, with the larger the amount of illitic minerals the greater the Cs adsorption potential (with any hydroxy-Al interlayering reducing this effect).

Characterization of Portuguese geomaterials, the clay component of rañas, as potential liners for low and intermediate radioactive disposal sites

Safety assessment of low and intermediate level waste (LILW) repositories requires the understanding of radionuclides sorption-desorption mechanisms, mainly the degree of interaction between radionuclides and min- eral surfaces. Among the different radionuclides that are part of the Portuguese radwastes' inventory 137 Cs is one of the most important from the radiological point of view due to its high radiotoxicity. The clay component of natural geomaterials termed ranas, originated from an area located in the NE Mainland of Portugal, was studied for their potential as effective barriers. These geomaterials (grains- size fractions \63 lm, \36 lm) were characterized in terms of cationic exchange capacity (CEC), frayed edge sites (FES) and radiocaesium interception potential (RIP). Organic matter content and pH were also determined. The fractions were mineralogically analyzed by X-ray diffrac- tion and by instrumental neutron activation analysis (INAA) for chemical elemental composition. Smectite (montmorilonite), illite, kaolinite were identified. The capacity of FES is related to the samples granulometry as opposed to what was observed for CEC and RIP. The behavior of ranas was not similar for all samples which may be related to differences in mineralogy. Future studies with regard to the behaviour of radiocaesium are expected to clarify the possible application of these geomaterials as liner/backfill/buffer for a LILW repository.

Hydroxy‐Al polymers block the frayed edge sites of illitic minerals in acid soils: studies in southwestern Japan at various weathering stages

SummaryThe transformation of hydroxy‐interlayered vermiculite (HIV) to vermiculite in humid temperate soils may involve the gradual alteration of illitic minerals. However, it is difficult to detect such minor and progressive changes using conventional methods. We measured the amount of the frayed edge site (i.e. the weathering front of illitic minerals) in soil clays using Radiocaesium Interception Potential (RIP) methodology, and elucidated the effect of hydroxy‐Al polymers on the frayed edge site that may be occluded within HIV structures in the acidic soils of southwestern Japan. X‐ray diffraction patterns showed the progressive transformation of HIV to vermiculite, or further, to smectite in soil clays located in upper horizons and therefore subjected to more intensive podzolization. In this process, the amount of hydroxy‐Al polymers (represented by hot‐citrate Al) gradually decreased, while the vermiculitic charges increased (represented by Cs‐fixing capacity). However, the amount of the frayed edge site (represented by the RIP) firstly increased but then reversed and decreased in the upper layers of podzolic soils. After hot‐citrate extraction to remove the hydroxy‐Al polymers from HIV, the amount of the frayed edge site increased in HIV‐dominated clays, whereas there was a negligible increase in vermiculitic‐ or smectitic‐clays, thus indicating the blockage effect of hydroxy‐Al polymers on the frayed edge site. The sequential increase followed by the decrease that we documented in the amount of the frayed edge site along with the HIV‐vermiculite‐smectite transformation, suggested that the weathering front of illitic minerals was exposed as the HIV layers lose hydroxy‐Al polymers. Thereafter, the charges in the exposed frayed edge site might decline under the very acidic conditions of the upper layers of podzolic soils.

Pedogenic alterations of illitic minerals represented by Radiocaesium Interception Potential in soils with different soil moisture regimes in humid Asia

SummaryA frayed edge site can be a significant indicator of alterations in illitic minerals in soils evolving under different climates, because illitic minerals are known to enlarge the frayed edge site through vermiculitization. We investigated the relative abundance of this site in soil clays (&lt;2 μm) using Radiocaesium Interception Potential (RIP) methodology, and determined the mineralogical factors controlling its amount (represented by RIP) for soils under ustic soil moisture regimes (northern Thailand, TH) and udic regimes (southwestern Japan, JP; East Kalimantan in Indonesia, ID) in humid Asia. We found three different correlations between RIP and mineralogical‐ or chemical‐indices: positive (amount of illitic mineral and Cs‐fixing capacity), negative (amount of kaolinite and hot‐citrate extractable Al‐Si) and no correlation (amount of DCB‐oxides). The variable that correlated best with the logarithm of RIP in each region was Alcit‐Sicit (JP), amount of illitic mineral (TH) and amount of kaolinite (ID). Under an ustic soil moisture regime in TH, there was little vermiculitization, as was evidenced by the direct correlation between the frayed edge site (or RIP) and the amount of illitic minerals and the very small Cs‐fixing capacity. A distinct dry season restricts water leaching, which may reduce alteration in illitic minerals. For ID soils under an udic soil moisture regime, illitic minerals weather to form vermiculite, which is reflected in the enlargement of the frayed edge site. However, for JP soils, hydroxy‐Al interlayering reduces the frayed edge site formed through vermiculitization. The release of hydroxy‐Al polymers from vermiculitic layers can induce the re‐exposure of the frayed edge site that can be progressive under a podzolization process.

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

Quaternary sediments and soils at the Forsmark site (Sweden) would constitute the last barrier for radionuclide migration interposed between a deep repository of high level nuclear wastes (HLNW) and surface ecological systems. The retention capacity of these sediments is evaluated by means of reactive transport simulations. Two different scenarios have been modelled: (1) Inflow of deep fluids carrying radionuclides into shallow aquifers hosted in the carbonate glacial till, and (2) Inflow of deep fluids carrying radionuclides into organic matter-bearing clays. The model results predict that caesium is very efficiently retained in both scenarios for periods longer than 3000 years due to the strong affinity of this ion with the frayed edge sites of the illite layers. Strontium is also retained via cation exchange in illite and, in a lesser extent, via co-precipitation in calcite. The retention efficiency for strontium, however, quickly decreases to 0 after 1000 years. Uranium is retained in both scenarios with retention efficiencies higher than 40% at long term (>3000 years) due to precipitation of amorphous uraninite and adsorption on Fe(III) oxyhydroxides. Radium is only retained in the till aquifer since co-precipitation with barium sulphate does not occur in the clay porewater.

Characterization of the frayed edge site of micaceous minerals in soil clays influenced by different pedogenetic conditions in Japan and northern Thailand

Radiocesium interception potential (RIP), a quantitative index of a frayed edge site of micaceous minerals, was investigated on test clay minerals (illite, kaolinite, montmorillonite and vermiculite) and soil clays formed under different pedogenetic conditions in the Kinki district of Japan and in northern Thailand to elucidate the effect of pedogenetic weathering on the amount of frayed edge site of micaceous minerals. A frayed edge site is defined as the wedge-shaped intermediate zone between non-expansible (1.0 nm) and hydrated (1.4 nm) interlayers, on which Cs is selectively adsorbed. The test clay minerals demonstrated unique RIP values closely associated with their charge characteristics. Very small RIP in non-micaceous test minerals (0.006 mol kg−1 in kaolinite; 0.1 mol kg−1 in montmorillonite) supported the fact that negative charges in these minerals do not show selective adsorption of 137Cs. Large RIP values in vermiculite (25.9 mol kg−1) compared with illitic minerals (4.3 mol kg−1 in silt size; 11.8 mol kg−1 in clay size) suggested that the frayed edge site increased as the expanded layer formed in the micaceous minerals. In northern Thailand soil clays, the absence of a vermiculitic expanded layer was indicated. Periodic wet–dry conditions may restrict K depletion from micaceous minerals in the region. It is concluded that micaceous minerals were weathered without layer expansion and, therefore, the amount of the frayed edge site was regulated by the content of micaceous minerals in northern Thailand. In Japanese soil clays, large RIP values were found in the EB and Bs horizons of a Podzodic profile (SW1), which had large vermiculitic expanded layers like the test vermiculite. However, the RIP values drastically decreased from the Bs to E horizon, maintaining a high vermiculitic layer charge. Thus, intensive weathering in highly acidic soils in Japan might reduce the frayed edge site.

The accumulation of radiocesium in coarse marine sediment: Effects of mineralogy and organic matter

The controlling factors affecting the accumulation of 137Cs in marine sediment have not been investigated in detail, especially in coarse grained sediment. Eighty eight coarse marine sediment samples near Wuljin, Korea, were characterized by quantitative X-ray-diffraction (XRD), gamma-ray, and total organic carbon (TOC) analysis. Those factors were then compared. The grain size was in the range of −0.48 to 3.6 Md ϕ corresponding to sand grains. TOC content was in the range of 0.06–1.75%, and the concentration of 137Cs was <MDA to 4.0 Bq/kg-dry. The main identified minerals were general rock-forming minerals such as quartz, feldspars with minor contents of pyroxene, calcite, hornblende, a 10 Å phase of phyllosilicate assigned to biotite, and chlorite. Other clay minerals were not identified due to the large grain sizes of the investigated samples. Biotite (1–7 wt%) was the only mineral showing a positive correlation with 137Cs activity, which was first reported here, probably due to the weathered frayed edge site of biotite produced by a release of K. The samples with low TOC contents showed even better correlation between biotite content and 137Cs activity. For the entire samples, however, the TOC content showed better correlation with 137Cs activity than other single factors, indicating that biotite and organic carbon are the most important factors controlling 137Cs fixation. The combined effect of biotite and TOC for 137Cs fixation was also confirmed by multiple regression analysis ( 137Cs activity = 1.712 · TOC (wt%) + 0.202 · biotite (wt%) − 0.097; R 2 = 0.819). The regressed slopes indicated that the 137Cs-adsorption capacity of TOC was about 8.5 times higher than that of biotite. However, the amount of 137Cs adsorbed onto biotite was 30% more than that adsorbed onto TOC due to much greater biotite content in the sediment. The role of biotite in fixing 137Cs becomes more important in sediment with coarser grains, containing little TOC.

Effect of desorption kinetics on colloid‐facilitated transport of contaminants: Cesium, strontium, and illite colloids

To examine the importance of desorption kinetics to colloid‐facilitated transport, we conducted column experiments comparing the transport of cesium and strontium through a saturated quartz sand porous medium in the absence and presence of illite colloids at two ionic strengths. Because cesium desorption from illite was anticipated to be slower than that of strontium, we expected to see a contrast in the colloid‐facilitated transport of the cations. A model of colloid‐facilitated transport accounting for second‐order cation adsorption to and desorption from the quartz, second‐order cation adsorption to and desorption from fast and slow sites on the illite colloids, and second‐order colloid deposition to and release from the quartz accurately simulated the cation transport in the absence and presence of the illite colloids. The column results and model simulations revealed that cesium desorption was indeed slower than strontium desorption and that this contrast in desorption kinetics resulted in greater colloid‐facilitated transport of the cesium. The desorption of both cations was slow relative to the rate of advection. The fast and slow sites on the illite colloids behaved like planar and frayed edge sites typically identified for cesium adsorption to illite. The amount of cesium adsorbed to the slow, or frayed edge, sites was similar to the frayed edge site density of illite estimated by other researchers.

Measuring the specific caesium sorption capacity of soils, sediments and clay minerals

Two methods to quantify the specific Cs sorption capacity of soils and sediments, which is generally believed to be associated with the Frayed Edge Sites (FES) of illitic clay minerals, are described in detail and are critically reviewed. The first method is a direct measurement of the FES capacity, while the second quantifies the combined parameter K D Cs × [ K + ] ( = K C ( K → Cs ) × [ FES ] ) , i.e. the product of the FES capacity and the affinity of these sites for Cs. Both methods use the bulky AgTU-complex to mask non-specific sorption sites for Cs and are applied to a number of different soils and pure minerals. Measurement of the FES capacity of pure illite is straightforward. It is shown that the measured capacity is independent of the saturating ion, but does depend on particle size. This method could not be successfully applied to a peat bog soil with 90% organic matter, because the necessary correction for non-specific Cs sorption by the large pool of organic exchange sites overpasses the capacity of the small FES fraction. Measurement of the combined parameter K D Cs × [ K + ] is shown to be more appropriate in such cases. Application of the FES capacity method to the hydrous aluminosilicate mineral allophane, an important soil constituent of Andisols, shows that the AgTU-complex is unable to block all non-specific sorption sites for Cs on this mineral. The K D Cs × [ K + ] measurements show evidence of a very small number of specific Cs sorption sites on allophane, much smaller than inferred from the FES capacity measurement. The FES capacity of the clay mineral vermiculite is difficult to quantify because the high Cs concentrations that are needed to measure the FES capacity probably cause a collapse of the vermiculite interlayers, thereby creating more high-affinity sites for Cs. The K D Cs × [ K + ] method, in which only trace concentrations of Cs are used, is shown to be more appropriate for soils containing substantial amounts of vermiculite. It is concluded that both the direct FES capacity measurement and the measurement of the combined parameter K D Cs × [ K + ] can be very useful methods to isolate and characterise Cs-selective sorption sites in soils and sediments, but that results should be interpreted with great care.

Radiocesium reaction with illite and organic matter in marine sediment

The mineralogical effect on the 137Cs reaction with marine sediment has not been systematically studied yet, even though illite has been known to adsorb Cs preferentially on its frayed edge sites in a low Cs concentration. Ninety-three marine sediment samples were collected near Yangnam, Korea for quantitative X-ray-diffraction (XRD), gamma-ray, and total organic carbon (TOC) analysis. Illite content was in the range of 0–23 wt.% and those of 137Cs and TOC were minimum detectable activity (MDA) ∼ 7.19 Bq/kg-dry and ∼3.32%, respectively. The illite content in the marine sediment showed a good relationship with the 137Cs content ( R 2 = 0.69), but with an increase in the illite content, the relationship became less linear. This trend can be clearly shown in two groups of samples with different size fractions (< and >5Md ϕ). For the samples of larger particle sizes (low contents of illite), the relationship is linear, but for the samples of the smaller particle sizes (high illite content) it is less linear with a decreased slope, indicating that increase in illite content does not significantly contribute to the fixation of 137Cs in marine sediment. Rather, the TOC has a more linear relationship with 137Cs content with no slope change in all particle size ranges. This may indicate that humic materials in marine sediment block the access of 137Cs to the frayed edge site and reduces the adsorption of 137Cs on illite and that the organic materials in marine sediment play more important roles in adsorbing Cs than illite.

Rhizospheric Mobilization and Plant Uptake of Radiocesium from Weathered Micas

Potassium depletion in the soil solution around plant roots promotes the root uptake of radiocesium. However, it can also induce the transformation of mica through the release of interlayer K. In bulk soil, the formation of frayed edge sites (FES) with a high selectivity for Cs adsorption is usually related with mica weathering. We studied the effect of K level in the nutrient solution on the root-induced weathering of phlogopite as well as on the root uptake of radiocesium by willow (Salix viminalis L. var. Orm). The willows were grown for 7 wk in column lysimeters filled with a quartz-phlogopite mixed substrate continuously irrigated with nutrient solutions differing in K concentration (0-2 mM). From a potassium supply of 0.4 mM downward, we observed a decrease in root uptake of potassium as well as an increase in (i) potassium release from phlogopite, (ii) degree of transformation of phlogopite into vermiculite, and (iii) root uptake of radiocesium. Increasing K depletion had thus two effects: a decrease of the root uptake of potassium and an increase of phlogopite weathering in the rhizosphere, both of which promoted the root uptake of radiocesium.

Rhizosphere Effects on Cesium Fixation Sites of Soil Containing Micaceous Clays

Physical and chemical weathering processes in the rhizosphere may lead to the generation of a greater density of Cs‐selective frayed edge sites (FES) on rhizosphere soil as compared with bulk soil. This study was undertaken to determine if there are significant differences between bulk and rhizosphere soils from the Idaho National Laboratory (INL) with respect to their ability to bind Cs. The capacity of FES on bulk and rhizosphere soil materials and conditional Cs/K selectivity of FES cKexFES were determined as a function of both soil type and initial exchanger composition. The FES capacity was significantly higher in untreated, Ca‐saturated, and K‐saturated rhizosphere soil materials as compared with bulk soil materials. Sorption–desorption isotherms were obtained at Cs concentrations between 5 × 10−9 and 5 × 10−6 M No difference in Cs sorption was observed between bulk and rhizosphere soil materials. The composition of the exchanging solution had the greatest effect on the magnitude of Cs desorption; significantly more Cs was desorbed in the presence of KCl than in either CaCl2 or a mixed‐cation soil solution. In addition, Cs desorption was greater from rhizosphere soil materials relative to bulk soil materials. Cesium selectivity with respect to both Ca and K was significantly suppressed by weathering in the rhizosphere. We conclude that enhanced weathering in the rhizosphere increased the Cs sorption capacity of FES, but also reduced Cs selectivity on these sites. Enhanced Cs desorption from rhizosphere INL soils is likely in the presence of actively growing plants and associated microorganisms.

Clay mineral weathering and contaminant dynamics in a caustic aqueous system: II. Mineral transformation and microscale partitioning

Microscopic and spectroscopic studies were conducted to assess mineral transformation processes in aqueous suspensions of illite (Il), vermiculite (Vm) and montmorillonite (Mt) that were subjected to weathering in a simulated high-level radioactive tank waste leachate (0.05 m Al T, 2 m Na +, 1 m NO 3 −, pH ∼14, Cs + and Sr 2+ present as co-contaminants). Time series (0 to 369 d) experiments were conducted at 298 K, with initial [Cs] 0 and [Sr] 0 concentrations from 10 −5 to 10 − mol kg −. Incongruent clay dissolution resulted in an accumulation of secondary aluminosilicate precipitates identified as nitrate-sodalite, nitrate-cancrinite and zeolite X, by molecular spectroscopy and electron microscopy (XRD, IR, NMR, SEM-EDS and TEM-EDS). Contaminant fate was dependent on competing uptake to parent clays and weathering products. TEM-EDS results indicated that high Il affinity for Cs was due to adsorption at frayed edge sites. The Il system also comprised Sr-rich aluminous precipitates after 369 d reaction time. In Mt systems, Cs and Sr were co-precipitated into increasingly recalcitrant spheroidal precipitates over the course of the experiment, whereas contaminant association with montmorillonite platelets was less prevalent. In contrast, Cs and Sr were found in association with weathered Vm particles despite the formation of spheroidal aluminosilicate precipitates that were comparable to those formed from Mt dissolution.

Cesium Desorption from Illite as Affected by Exudates from Rhizosphere Bacteria

Biogeochemical processes in the rhizosphere can significantly alter interactions between contaminants and soil minerals. In this study, several strains of bacteria that exude aluminum (Al)-chelating compounds were isolated from the rhizosphere of crested wheatgrass (Agropyron desertorum) collected from the Idaho National Laboratory (INL). We examined the effects of exudates from bacteria in the genera Bacillus, Ralstonia, and Enterobacter on cesium (Cs) desorption from illite. Exudates from these strains of bacteria significantly enhanced Cs desorption from illite. In addition, Cs desorption increased with increasing Bacillus exudate concentrations. Cesium desorption from illite as a function of both exudate type and concentration was positively correlated with Al dissolution, suggesting that the Al-complexing ability of the exudates played an important role in enhancing Cs desorption. The density of frayed edge sites (FES) on illite increased as a result of treatment with bacterial exudates, while the Cs/K selectivity of FES decreased. These results suggestthat exudates from bacteria isolated from the rhizosphere can enhance Cs desorption from frayed edges of illite and, therefore, can alter Cs availability in micaceous soils.

Cesium-137 in floodplain sediments of the Lower Three Runs Creek on the DOE Savannah River Site

The legacy of nuclear weapons production has resulted in vast tracks of land contaminated with fission products, mainly 137 Cs, and at the U.S. Department of Energy's Savannah River Site (SRS) alone there is over 120 km 2 of land contaminated with low-levels of 137 Cs. Soils on the SRS are highly weathered and dominated by sand-sized quartz grains with the clay fraction consisting primarily of kaolinite and crystalline and poorly crystalline iron oxides ( 137 Cs in the Lower Three Runs Creek floodplain were retained in the sand-sized fraction (>52 mm) of the soil. Frayed edge site measurements were performed in order to probe the interaction between 137 Cs and the sand fraction, with the results indicating that the vast majority of the 137 Cs was strongly retained and existed in the residual fraction. These results prompted examination into the mineralogy of the soils in a hope to elucidate the mechanisms of 137 Cs retention by the sand fraction. The results from this study provide new evidence for selective retention of 137 Cs in larger-grained particles than previously demonstrated.

Microscale distribution of cesium sorbed to biotite and muscovite.

Individual 1-3 mm biotite and muscovite clasts from Hanford sediment were contacted with 0.08 M CsNO3. They were examined using electron or X-ray microprobe methods, as intact specimens or sectioned perpendicular to their basal planes. Cs+ was observed to preferentially sorb to mica edges, steps on mica surfaces, or fractured regions. The localization of Cs conformed to hypothesized strong binding to frayed edge sites in preference to sites on basal planes. In section, Cs+ was found to penetrate the mica interior, forming discrete zones of concentration, particularly in muscovite. In biotite, Cs was more abundant, permeating the clasts, but also forming discrete zones of higher concentration. Concentrated Cs on both clast edges and within clast interiors corresponded to microscopic but relatively extensive zones where K was depleted. The localization of sorbed Cs in areas where K was depleted suggested that weathering reactions had caused the formation of frayed edge sites within the micas. Cs+ accessed crystal interiors by diffusion along channels following crystal defects, cracks, or partings where pore fluids had previously migrated to form the interior alteration zones. On the nanometer scale, areas with localized Cs were disrupted, confirming that frayed edge sites were developed in clast interiors.

Experimental and modeling studies of radiocesium retention in soils

A two-compartment model was used to describe the irreversible sorption behavior of 137Cs on frayed edge sites (FES) of finite capacity in soils in Hong Kong reservoir. The sorption rate was assumed to depend on factors like the fractional water activity concentration, the difference between the activity concentration of 137Cs in aqueous phase and that in particulate phase, and the number of available sorption sites. By varying the exponents of the different factors, very good fitting was obtained between experimental data and theory. A relationship between the number of available frayed edge sites and the mass of soil was proposed.

Mobility of radiocesium in three distinct forest floors

The degree of mixing of organic matter with minerals in organic and hemi-organic horizons of forest soils largely differs between humus types. As clay minerals might control the mobility of radiocesium in these forest floor horizons, plant contamination could greatly vary with the kind of humus. We measured the mobility of radiocesium in the upper O, OAh and Ah horizons of three acid forest soils with three distinct humus types: eumoder, dysmoder and fibrimor. We used two different approaches: a physico-chemical test quantifying the radiocesium interception potential (RIP) and a biological assay simulating an experimental rhizosphere. The results show that the 137Cs horizon-to-plant transfer is directly governed by RIP, and thus by frayed edge sites born by weathered micaceous minerals. The inverse relationship between RIP and organic matter content indicates that in the three sites investigated the mixing of organic residues with Cs-fixing minerals is a key process in 137Cs mobility. These Cs-fixing clay minerals indeed decrease in the sequence eumoder>dysmoder>fibrimor because they are more diluted in forest floor with less bioturbation. Our results suggest that humus type might be an important parameter in classifying forest soils with respect to their ability to transfer radiocesium to the above standing vegetation.

Cesium migration in Hanford sediment: a multisite cation exchange model based on laboratory transport experiments

Cs + transport experiments carried out in columns packed with uncontaminated Hanford formation sediment from the SX tank farm provide strong support for the use of a multisite, multicomponent cation exchange model to describe Cs + migration in the Hanford vadose zone. The experimental results indicate a strong dependence of the effective Cs + K d on the concentrations of other cations, including Na + that is present at high to extremely high concentrations in fluids leaking from the Hanford SX tanks. A strong dependence of the Cs + K d on the aqueous Cs + concentration is also apparent, with retardation of Cs + increasing from a value of 41 at a Cs + concentration of 10 −4 M in the feed solution to as much as 282 at a Cs + concentration of 5×10 −7 M, all in a background of 1 M NaNO 3. The total cation exchange capacity (CEC) of the Hanford sediment was determined using 22Na isotopic equilibrium exchange in a flow-through column experiment. The value for the CEC of 120 μeq/g determined with this method is compatible with a value of 121.9 μeq/g determined by multi-cation elution. While two distinct exchange sites were proposed by Zachara et al. [Geochim. Cosmochim. Acta 66 (2002) 193] based on binary batch exchange experiments, a third site is proposed in this study to improve the fit of the Cs +–Na + and Cs +–Ca + exchange data and to capture self-sharpened Cs + breakthrough curves at low concentrations of Cs +. Two of the proposed exchange sites represent frayed edge sites (FES) on weathered micas and constitute 0.02% and 0.22% of the total CEC. Both of the FES show a very strong selectivity for Cs + over Na + ( K Na–Cs=10 7.22 and 10 4.93, respectively). The third site, accounting for over 99% of the total CEC, is associated with planar sites on expansible clays and shows a smaller Na +–Cs + selectivity coefficient of 10 1.99. Parameters derived from a fit of binary batch experiments alone tend to under predict Cs + retardation in the column experiments. The transport experiments indicate 72–90% of the Cs + sorbed in experiments targeting exchange on FES was desorbed over a 10- and 24-day period, respectively. At high Cs + concentrations, where sorption is controlled primarily by exchange on planar sites, 95% of the Cs + desorption was desorbed. Most of the difficulty in desorbing Cs + from FES is a result of the extremely high selectivity of these sites for Cs +, although truly irreversible sorption as high as 23% was suggested in one experiment. The conclusion that Cs + exchange is largely reversible in a thermodynamic sense is supported by the ability to match Cs + desorption curves almost quantitatively with an equilibrium reactive transport simulation. The model for Cs + retardation developed here qualitatively explains the behavior of Cs + in the Hanford vadose zone underneath a variety of leaking tanks with differing salt concentrations. The high selectivity of FES for Cs + implies that future desorption and migration is very unlikely to occur under natural recharge conditions.

Relationship of quantitative X-ray diffraction measurements of geologic materials to cesium sorption

SummaryThe amounts of clay minerals in geologic samples from the Australian arid zone were measured using the SIROQUANT program, which quantifies mineral abundances from X-ray diffraction patterns using the Rietveld method. The sorption of trace cesium on the same samples was investigated in batch experiments. The statistical relationships between the Cs distribution coefficients (Kdvalues) and the smectite, illite and kaolinite contents were assessed. Other characterisation data such as the BET surface area and total clay content were included in the statistical analysis. The correlation of the illite content with the Kdfor trace Cs was stronger than any other correlation that was tested. This can be attributed to the presence of frayed edge sites on illite which are highly selective for the adsorption of trace Cs. Sorption experiments were also carried out with higher Cs concentrations, which were expected to saturate the selective Cs sorption sites on illite. Under these conditions, there was no significant correlation between the illite content and the cesium Kd. However, the Kdwas related to the smectite content of the materials. There was no correlation between the kaolinite content and the Kdfor Cs sorption under any of the experimental conditions. The sorption of Cs on the samples was influenced by the BET surface areas, but normalisation of the Kddata by surface area did not fully explain the sorption results. Only the quantitative XRD data were able to explain the shapes of Cs sorption isotherms. The isotherm was distinctly different for an illite-rich sample and did not conform to the Freundlich isotherm exhibited by other samples. This study demonstrates that quantitative XRD measurements are valuable in predicting Cs sorption properties of complex geologic samples, particularly sorption data obtained with trace Cs levels where bulk measurements such as the BET surface area are less useful.