Sorption and desorption of radiocesium by muscovite separated from the Georgia kaolin

https://doi.org/10.1016/j.jenvrad.2019.106074Get rights and content

Highlights

  • Muscovite separated from kaolin is promising as a low-cost agent to sequester radiocesium.

  • Like illite, this moderately weathered muscovite has cation-exchange sites that are highly selective for Cs.

  • Unlike frayed-edge sites of illite, these sites are only slowly accessible for cation exchange.

  • At trace Cs levels, most added radiocesium reaches such sites in several months.

  • The radiocesium that reaches such sites appears to be effectively sequestered therein.

Abstract

Radiocesium (137Cs) sorption by moderately weathered, sand-sized muscovite, obtained as a byproduct of kaolin ore processing, was observed at various concentrations of added stable Cs (0–100 μmol/L) over a 130 d period. After 18 h of batch sorption with 1 mmol/L NaCl as background electrolyte, conditional 137Cs Kd values were near 2000 L/kg across the entire range of added stable Cs. Over four succeeding months, the Kd values increased by large factors for suspensions with little added Cs but increased only slightly for the suspensions with the most added Cs. The large grains of muscovite used in this study behaved distinctly differently than previously studied, much finer illite in that highly Cs-selective but low-abundance cation exchange sites appeared to be unavailable to the aqueous Cs during the first few days of the experiment. Diffusion pathways to highly Cs-selective sites were thought to be much longer in the muscovite than in frayed edges of illite, causing the highly Cs-selective sites to be isolated from the bulk solution. The longer diffusion pathways may be due to much greater stiffness of the material bounding interlayer wedges in the muscovite than in illite. This isolation from solution led to slow uptake at trace levels of Cs though the final Kd values (after 130 d) at those levels were comparable to those found for some illite. After 130 days, the original solutions were replaced by new electrolyte solutions containing no Cs, to observe 137Cs desorption over another 130 d period. There was no indication of desorption of 137Cs from the slowly accessible, highly Cs-selective sites apparently reached by most of the 137Cs during sorption at the low Cs levels. The byproduct mica from kaolin processing might serve effectively as a chemically stable sorbent to isolate accidently released radiocesium and to hold it until the 137Cs is virtually gone.

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

References (54)

  • M.S. Prasad et al.

    Kaolin: processing, properties and applications

    Appl. Clay Sci.

    (1991)
  • J.M. Zachara et al.

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

    Geochem. Cosmochim. Acta

    (2002)
  • P.W. Birkeland

    Soils and Geomorphology

    (1999)
  • E. Brouwer et al.

    Cesium and rubidium ion equilibria in illite clay

    J. Phys. Chem.

    (1983)
  • R.N.J. Comans et al.

    Sorption of cesium on illite: non-equilibrium behaviour and reversibility

    Geochem. Cosmochim. Acta

    (1991)
  • M. Chino et al.

    Preliminary estimation of release amounts of 131I and 137Cs accidently discharged from the Fukushima Daiichi nuclear power plant into the atmosphere

    J. Nucl. Sci. Technol.

    (2011)
  • T. Degen et al.

    The HighScore suite

    Powder Diffr.

    (2014)
  • H.M. Dion et al.

    Cesium-137 in floodplain sediments of the lower three runs creek on the DOE Savannah River site

    J. Radioanal. Nucl. Chem.

    (2005)
  • R.A. Eggleton et al.

    Illite from Muloorina, South Australia

    Clay Clay Miner.

    (2011)
  • W.C. Elliott et al.

    A paleoclimate interpretation derived from pedogenic clay minerals from the Piedmont Province, Virginia

    Chem. Geol.

    (1997)
  • A.M. Elser

    The Provenance and Weathering of Muscovite from the Georgia Kaolin Deposits

    (2004)
  • Environmental Protection Agency

    Acid Digestion of Sediments, Sludges, and Soils. Method 3050B, Revision 2

    (1996)
  • M. Findley

    Characterizing the Environmental Availability of Trace Metals in Soils at the Savannah River Site

    (1998)
  • C.W. Francis et al.

    Preferential adsorption of 137Cs to micaceous minerals in contaminated freshwater sediment

    Nature

    (1976)
  • M. Goto et al.

    Freundlich and dual Langmuir isotherm models for predicting 137Cs binding on Savannah River Site soils

    Health Phys.

    (2008)
  • M. Goto et al.

    Interactions of radioactive and stable cesium with hydroxy-interlayered vermiculite grains in soils of the Savannah River Site, South Carolina, USA

    Clay Clay Miner.

    (2014)
  • A. Grütter et al.

    Sorption, desorption, and isotope exchange of cesium (10−9–10−3 M) on chlorite

    Clay Clay Miner.

    (1986)
  • Cited by (5)

    • Comparative study of strontium adsorption on muscovite, biotite and phlogopite

      2020, Journal of Environmental Radioactivity
      Citation Excerpt :

      With the extension of contact time, the fixation of Sr(II) by dioctahedral muscovite was weakened under alkaline conditions, but it is relatively stable under acidic conditions. This might result from the relatively stable structure of muscovite that is resistant to chemical weathering to illite or kaolinite (Kwong-Moses et al., 2020; Nicolini et al., 2009; Bergaya et al., 2006). However, the adsorption of Sr(II) by biotite and phlogopite was enhanced in the alkaline environment as the contact time prolonged, and this enhancement even started at pH ~4.3 for phlogopite.

    • Synthesis and Sorption Properties of Lithium Aluminosilicate

      2023, Protection of Metals and Physical Chemistry of Surfaces
    • Thallium adsorption onto phyllosilicate minerals

      2022, Environmental Science: Processes and Impacts
    View full text