Cesium Adsorption on Clay Minerals: An EXAFS Spectroscopic Investigation BENJAMIN C. BOSTICK, † MURTHY A. VAIRAVAMURTHY,* ,‡ K. G. KARTHIKEYAN, § AND JON CHOROVER ⊥ Stanford University, Department of Geological and Environm ental Sciences, Stanford, California 94305-2115, Brookhaven National Laboratory, Energy Sciences and Technology Department, Upton, New York 11973, University of Wisconsin, Biological System s Engineering Departm ent, Madison, Wisconsin 53705, and University of Arizona, Department of Soil, Water and Environmental Sciences, Tucson, Arizona 85721-0038 Cesium adsorption on the clay minerals vermiculite and montmorillonite is described as a function of surface coverage using extended X-ray adsorption fine structure spectroscopy (EXAFS). Cesium (Cs) possessed a variable coordination environment consisting of Cs-O distances between 3.2 and 4.3 Å; however, disorder typical of the Cs coordination environments prevented the resolution of all oxygen shells. On the basis of the influence of Cs loading and exchange- ability on this structural arrangement, we could recognize both inner-sphere and outer-sphere adsorption complexes. The shorter Cs-O bond distance belongs to outer-sphere complexes typical of hydrated ions. In inner-sphere complexes, partially or fully dehydrated Cs coordinates directly to siloxane groups of the clay minerals forming longer Cs-O bonds. The inner-sphere adsorption complexes may have occurred within the interlayer or at frayed edge sites and were less extractable than the outer- sphere complexed Cs. Both coordination number ratios and linear combination fitting of EXAFS spectra were useful in estimating the fractions of inner-sphere and outer- sphere adsorption complexes. Our results show that X-ray absorption spectroscopy (XAS), and particularly EXAFS, is a valuable technique for exploring the type of Cs binding in environmental samples. Introduction Soilsand sedimentscontaminated byradionuclidesand toxic metals pose serious environmental threats and challenge remediation.Contamination byhighlysolubleradionuclides, such as 137 Cs,isparticularlydangerousbecause oftheirability to move with aqueous media in the subsurface. In fact, large volumes of contaminated wastes containing these radio- nuclides have leaked from storage tanks or other sources at DOEsites,such as the Hanford and the Savannah River sites, and thence into nearby groundwater and canals (1-3). Th e mobilityofthese soluble radionuclides is controlled bytheir sorption onto soil particles (4-6). A variety of factors, including the solution pH, ionic strength, moisture content, competitive sorption, and complexation with inorganic and organic ligands, influence the type and extent of sorption, thereby affecting the environmental transport of these cations. Th e Cs + ion doesnot form strongcomplexeswith dissolved inorganic or organic ligands (e.g.,EDTA),so that the sorption ofthe free ion on minerals is the dominant factor controlling its speciation and environmental fate. The major sorbents in soils are generally thought to be the layer-type silicates that bind Cs either through weak electrostatic interactions or through stronger bonds formed by partial sharing of electrons between Cs and the ligand sites ofthe claymineral. Electrostaticassociationsofhydrated Cswith anionicsurfaces within the basal plane or interlayer and dissociated edge hydroxyl groups form outer-sphere (OS) complexes. On the other hand,electronic bondingat the frayed edge sites(FES), externalbasalsites,or within the interlayer (4, 7-9)generates inner-sphere (IS) complexes. The FESrepresent particularly high-affinity sorption sites in which Cs may coordinate directlyto basaloxygenson adjacent siloxane groups.Because the radius of the dehydrated Cs + ion is similar to that of the ditrigonalsiloxane cavityoflayer silicates,adsorption ofCs + to negativelycharged siloxane sites can result in inner-sphere complexation (4, 5). The reactivity of the inner-sphere complexation sites may differ widely. The most reactive of these sites probably constitute only a small portion (∼10 -8 mol/kg)ofthetotal(3, 7, 10).However,other relativelystrong complexes may form at other surface sites at higher concentrations. Isomorphic substitution ofAl 3+ for Si 4+ in the tetrahedral sheet comprising the siloxane site (e.g., as with illite and vermiculite) enhances the stability of the Cs + -siloxane surface complex,presumablybypromotingthe dehydration ofthe sorbed Cs cation (4, 11).In contrast,when isomorphic substitution is restricted dominantly to the more distant octahedral sheet (e.g., as with montmorillonite), a weaker OS complex may result (12). Therefore, cesium adsorption and itssubsequent transport are influenced bymineralogical differences and the associated crystal chemistry of clays. In Hanford-site soils, the dominant 2:1 layer silicate at shallow depthsismontmorillonite(13),whereas illite and vermiculite are more prevalent in the surface layers (2, 3, 14); thus, changes in sorption with depth maypotentiallyinfluence Cs mobilityalongtheflowpath.Using 133 Csmagicanglespinning nuclear magnetic resonance (MASNMR) spectroscopy, Kim and co-workers (5, 11, 15) and Brouwer et al. (10) found that much of the Cs sorbed to montmorillonite was present at interlayer sites. For illite, both the FES and crystallite basal surfaces participated in the sorption reaction (13, 16). In fact, Brouwer et al. (10) identified multiple FES sites. X-rayabsorption spectroscopy(XAS),includingboth X-ray absorption near-edge structure (XANES)and extended X-ray absorption fin e structure (EXAFS), is a nondestructive, element-specific technique to probe adsorbate structures at the mineral-water interface. Information from EXAFS, especiallythe number and distance ofthe nearest neighbors in the first and second shells, can be used to discern the predominantsorption mechanismsforconcentrationsaslow as 50-100mg/kg.Forexample,O’dayetal.(17, 18)identified different Co 2+ sorption complexes (including mononuclear and multinuclear complexes)on kaolinite and Al2O3 surfaces as a function of adsorbate-to-adsorbent ratios. Such a knowledge isimportant because the type ofsorption complex *Corresponding author phone: (631) 344-5337; fax: (631) 344- 7905; e-mail: vmurthy@bnl.gov. † Stanford University. ‡ Brookhaven National Laboratory. § University of Wisconsin. ⊥ University of Arizona. Environ. Sci. Technol. 2002, 36, 2670-2676 2670 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 12, 2002 10.1021/es0156892 CCC: $22.00  2002 American Chemical Society Published on Web 05/17/2002