Sorption and desorption of radiocesium by muscovite separated from the Georgia kaolin
Introduction
The sorption of radiocesium by micaceous minerals has been studied in an effort to undertand the migration of radiocesium in the near-surface environment. In turn, the knowledge of radiocesium migration has aided the characterization of environmental and anthropocentric risks associated with radioactive waste disposal. The micaceous minerals were found to preferentially sorb and hold trace quantities of radiocesium released into natural waters (Lomenick and Tamura, 1965; Francis and Brinkley, 1976). In the case of the 2011 Fukushima Daiichi power plant accident, micaceous soil minerals were important in minimizing the migration of the accidently released radiocesium (Koarashi et al., 2012; Tanaka et al., 2012; Yoshida and Takahashi, 2012; Matsunaga et al., 2013). It has been generally accepted that cation exchange sites in the frayed edges of grains of illite and other fine micaceous minerals are extremely selective for cesium ions but are relatively few in number (Okumura et al., 2018). These “frayed-edge sites” (FES), owe their sorptive characteristics to a unique, stereoselective environment (Zachara et al., 2002) within interlayer wedges created by splaying of 2:1 aluminosilicate layers. Such splaying of phyllosilicate layers has been typically attributed to chemical weathering at grain edges (Jackson, 1962).
Although the sorption of radiocesium within soils and sediments has most commonly been attributed to illite and other clay-sized materials, Mukai et al. (2014, 2016) found relatively large (≈50 μm) weathered biotite grains to be among the most highly radioactive particles in a fallout-contaminated forest litter soil sample from Fukushima Prefecture. Along with illite, weathered biotite was recognized as a likely host of radiocesium in Fukushima Prefecture streambed sediments (Tanaka et al., 2018). The biotite grains, derived from abundant granite and granodiorite in Fukushima Prefecture, were typically deeply altered composites of biotite and non-uniformly distributed weathering products (Mukai et al., 2014, 2018; Tanaka et al., 2018). Study of Cs sorption by illite in sedimentary soils provided complementary information useful for understanding Cs interactions in Fukushima and neighboring areas (Ogasawara et al., 2019).
Muscovite is more resistant to chemical weathering than biotite (e.g. Birkeland, 1999). Consequently, muscovite grains that have been weathered only slightly are commonly found in soils and sediments. Detrital muscovite is widespread in sedimentary formations of the Atlantic Coastal Plain of the southeastern United States, but in near-surface soil horizons formed by intense weathering of these formations muscovite typically is not detectable by X-ray diffractometry (XRD). In highly weathered Florida soils, pedogenic Al-hydroxy interlayered vermiculite (HIV) grains contain nanoscale remnants of mica, which was inferred to have been the precursor of the HIV (Harris et al., 1992). FES developed on such mica remnants were thought to have controlled 137Cs dynamics in Savannah River Site (SRS) stream sediments (Dion et al., 2005). K-Ar dating of SRS soil clay fractions containing the mica-HIV intergrade confirmed the presence of mica remnants about 300 million years old in those HIV grains (Naumann et al., 2012). Significant enrichments of the natural Cs (stable 133Cs) in SRS soils were attributed to selective sorption and effective fixation of Cs ions in interlayer wedges of HIV (Wampler et al., 2012; Zaunbrecher et al., 2015a). Molecular modeling and molecular dynamics simulations further supported the idea of selective Cs sorption in mica-HIV interlayer wedges (Zaunbrecher et al., 2015b). The concept of effective fixation of Cs in the narrower parts of interlayer wedges of HIV arose from observations that those soils held nearly all their Cs against sequential extractions that would have removed exchangeable Cs and Cs in non-silicate phases (Findley, 1998; Goto et al., 2014) but lost most of their Cs to extraction by hot, strong acid (Zaunbrecher et al., 2015a). A key observation was that little of the K was extracted by the acid, which showed that the acid did not affect the ions in the fully closed portions of the remnant mica interlayers. In combination with evidence that both the K and the Cs in those soils were mostly within HIV grains, these observations provided strong support for the idea that Cs had been effectively fixed by migration into the narrower parts of interlayer wedges (Goto et al., 2014).
The demonstrated ability of muscovite remnants within HIV grains to sorb trace amounts of Cs and to hold that Cs for many years against leaching by soil water opened questions regarding the degree to which radiocesium could be sorbed and fixed by the moderately weathered detrital muscovite that is common in sedimentary formations underlying highly weathered near-surface soils. Such muscovite is abundant in Atlantic Coastal Plain formations and is coarser grained than the HIV-mica complex phases in the soils.
Sand-sized muscovite is a visible gangue mineral in mined kaolin ore and in its host sediments in Georgia, USA (Prasad et al., 1991; Hurst and Pickering, 1997). If the moderate weathering of such muscovite has created sites that sorb and hold trace amounts of Cs as effectively as muscovite remnants in HIV grains do, then muscovite separated from waste material of kaolin ore processing may be useful as a sorbent for radiocesium, particularly in situations where 137Cs (half-life 30.2 y) must be contained for hundreds of years. Consequently, we examined radiocesium sorption and desorption by a muscovite-dominant test material separated from kaolin ore. We measured the extent of sorption of 137Cs for 130 days across a wide range of added stable Cs (133Cs), because radiocesium sorption by micaceous materials is known to be strongly affected by the amount of stable Cs present and because stable Cs is ubiquitous and may be relatively enriched in such materials. The desorption of the 137Cs from the muscovite-dominant test material was studied for another 130 days following the sorption experiments. This study included mineralogical, chemical, and morphological characterization of the muscovite-rich test material and measurement of alkali and alkaline-earth elements in acid extracts of the material.
Section snippets
Sample description and initial processing
The muscovite-dominant test material (~0.2 kg) used in the present study was provided by Southeastern Performance Minerals, LLC (Deepstep, Georgia, USA). This muscovite concentrate was prepared from the coarse mineral fraction of slaked raw kaolin ore mined from kaolin deposits near Sandersville, Georgia. The ~0.2 kg sample was split into smaller (~12 g) subsamples with a Humbolt sample splitter. Some subsamples were crushed in a ball mill with a tungsten carbide ball for 15 min for X-ray
X-ray diffractometry
Powder XRD analysis of randomly oriented, crushed test material showed muscovite, kaolin group minerals, and quartz as the major constituents (Fig. S1). Semi-quantitative abundances of these minerals were determined as 76% muscovite, 21% kaolinite, and 3% quartz. The slight weathering of the muscovite grains is not evident by XRD. The 1.0 nm peak is closely similar to that of unweathered muscovite (Fig. Sx), and neither chlorite nor expandable phyllosilicate minerals (vermiculite and smectite)
Chemical, mineralogical, and morphological characteristics of the test material
The predominantly sand-sized (>64 μm) test material separated from the waste grit of mined kaolin ore was mostly muscovite (76%). Earlier studies showed muscovite grains separated from Coastal Plain sediments and kaolin ores to have lower K2O contents than unweathered muscovite (Kogel et al., 2000, Table 5.1; Elser, 2004). The low K2O content and the physical characteristics of the studied muscovite are indicative of chemical and physical weathering of these muscovite grains. Weathering of the
Conclusions
In this work, a moderately weathered muscovite material separated from Georgia kaolin ore was examined for application as a sorbent for 137Cs. Similar to well-known interactions between 137Cs and illite, 137Cs sorption to this weathered muscovite demonstrated 1) high Kd values indicative of strong sorptive interactions, 2) a multi-step process characterized by initially rapid uptake in 18 h followed by slower uptake over 130 days, and 3) much larger Kd values at very low levels of added stable
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This material is based upon work supported by the U.S. Department of Energy Office of Science, Basic Energy Sciences and Biological and Environmental Research programs, under Award Number DE-SC-00012530. The X-ray diffractometer used in this study was purchased from funding awarded by the National Science Foundation to D.M. Deocampo and W.C. Elliott (Award Number 1029020). The authors thank Dr. Shanna Estes of Clemson University for performing the Cs-CEC measurements. The results of this study
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