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PHYSICAL SCIENCES / CHEMISTRY
Layered metal sulfides: Exceptionally selective agents for radioactive strontium removal
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*Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208; and Materials Science Division, Argonne National Laboratory, Argonne, IL 60439
Edited by James L. Dye, Michigan State University, East Lansing, MI, and approved January 4, 2008 (received for review December 6, 2007)
In this article, we report the family of robust layered sulfides K2xMnxSn3-xS6 (x = 0.5–0.95) (KMS-1). These materials feature hexagonal [MnxSn3-xS6]2x– slabs of the CdI2 type and contain highly mobile K+ ions in their interlayer space that are easily exchangeable with other cations and particularly strontium. KMS-1 display outstanding preference for strontium ions in highly alkaline solutions containing extremely large excess of sodium cations as well as in acidic environment where most alternative adsorbents with oxygen ligands are nearly inactive. The implication of these results is that simple layered sulfides should be considered for the efficient remediation of certain nuclear wastes.
chalcogenide | environmental remediation | ion exchange | layered materials | nuclear waste
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Crystals of K1.9Mn0.95Sn2.05S6 belong to the space group R-3m (no. 166) with a = 3.6969(5) Å, c = 25.403(5) Å, and V = 300.67(8) Å3. Other crystal data: Z = 1, Dc = 3.105 g/cm3, µ = 6.845 mm–1; total reflections, 1,007; independent reflections, 118 (Rint = 0.0483); R1 = 0.0271; wR2 = 0.0663; GOF = 1.176.
This article contains supporting information online at www.pnas.org/cgi/content/full/0711528105/DC1.
¶ The strontium adsorption rate at pH 
14 and 5 M Na+ is remarkably fast at the beginning and decelerates as the reaction proceeds. The kinetics of ion exchange is typically controlled by film diffusion or particle diffusion (see ref. 26). In our system, in the first few minutes the sorption is probably controlled by film diffusion, where the slow step is the diffusion of Sr through the liquid diffusion layer surrounding the sorbent particles. Other factors affecting the kinetics of strontium adsorption by KMS-1 may be the structure stability and crystallinity of KMS-1. The layered structure of KMS-1 was largely retained after at least 2 h of contact of the solid with the highly alkaline solution (pH 14). Within this reaction time, the strontium adsorption was very fast with KMS-1 absorbing 
50% and 64% of their 24-h Sr2+ uptake within 5 min and 2 h, respectively. However, the loss of the crystallinity of KMS-1 and their partial decomposition, which were observed with further increasing the reaction time (SI Fig. 12), presumably result in lowering the strontium adsorption rate. The final form of the material after 4 days of ion exchange contains almost no Sn and S, and Na-Mn in a ratio of 1:2. No Mn ions leached out from KMS-1 during the reaction. This decomposition form of KMS-1 also contains 1% wt of Sr, which is strongly bound because it could not be removed with copious washing with water.
|| According to the ion-exchange selectivity rules (see ref. 26, pp 158–159), the ion exchanger prefers the cation with the smallest hydrated sphere; i.e., that causing the least swelling. TGA (SI Fig. 11) and XRD (SI Fig. 10) analyses on strontium- and calcium-exchanged KMS-1 materials ions indicated similar hydration (5 H2O) of the intercalated cations and larger c-axes for the Sr2+-exchanged samples, respectively. Therefore, the preference of the material for softer ions explains its selectivity for Sr2+ over Ca2+ and not hydration-swelling effects.
To whom correspondence should be addressed. E-mail: m-kanatzidis{at}northwestern.edu
© 2008 by The National Academy of Sciences of the USA
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