Cation exchange equilibria of cesium and strontium with K-depleted biotite and muscovite Yunchul Cho a , Sridhar Komarneni b, ⁎ a Peter A Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, CA 95616, USA b Department of Crop and Soil Sciences and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA abstract article info Article history: Received 2 October 2008 Received in revised form 14 December 2008 Accepted 16 December 2008 Available online 31 December 2008 Keywords: K-depleted biotite K-depleted muscovite Cation exchange Radioactive species Cation exchange selectivity for Cs + and Sr 2+ with K-depleted biotite (Na-biotite) and K-depleted muscovite (Na-muscovite) was determined with equilibration for 4 weeks at room temperature. The cation exchange isotherms and Kielland plots indicated that both K-depleted micas show high selectivity for Cs + at low equivalent fraction of Cs + on solid. The K-depleted micas took up Cs up to approximately 50% of their theoretical cation exchange capacities. The XRD patterns after Na + →Cs + exchange reactions with K-depleted biotite showed that the d(001)-spacings collapsed from ~12.2 to ~11.2 Å with high Cs + concentrations. This collapse suggests that K-depleted biotite is able to immobilize or fix Cs ions in the interlayers. In case of 2Na + →Sr 2+ exchange, K-depleted biotite showed high selectivity for Sr ions at low equivalent fraction of Sr 2+ on solid. The XRD patterns showed that the main d(001)-spacing of the K-depleted biotite slightly increased from 12.16 Å to ~12.3 Å after the exchange reactions with the high Sr 2+ concentrations. These results suggest that K-depleted biotite could be used as an ion exchanger to remove radioactive 137 Cs as well as 90 Sr from groundwater. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In order to remedy or treat soil and groundwater contaminated by radionuclides, several remedial technologies can be applied: ion exchange, precipitation, solidification/stabilization, and phytoreme- diation (Komarneni and Roy, 1988; Bagosi and Csetényi, 1998; Mollah et al., 1998; Soudek et al., 2006). Among these treatments, some researchers showed that ion exchange is a potential remediation technology to separate 137 Cs and 90 Sr from groundwater or aqueous nuclear waste (Dyer et al., 1993; Gualtieri et al., 1999). Important characteristics of the ion exchange material for separation of radio- nuclides are selectivity and radiation stability. Among various inorganic ion exchangers for separation of radionuclides (Adabbo et al., 1999; Kodama et al., 2001; Solbra et al., 2001; Shimizu et al., 2004), some micaceous minerals were found to show high selectivity for cesium and strontium radioisotopes (Komarneni and Roy, 1988; Stout and Komarneni, 2003), and these are expected to show radiation stability (Komarneni and Roy, 1988). Naturally occurring micas must be modified for use as commercial and cost-effective ion exchangers suitable to separate radionuclides from groundwater or aqueous nuclear wastes because micas have low cation exchange capacity (CEC). The low CEC of micas is due to interlayer potassium ions which are fixed. Some attempts have been made to improve CEC of the micas. Interlayer potassium ions can be removed using sodium tetraphenyloborate (NaTPB), resulting in K- depleted micas (Scott and Smith, 1966). For example, K-depleted phlogopite (ideal formula NaMg 3 Si 3 AlO 10 (OH) 2 ·H 2 O) produced from naturally occurring phlogopite by the K removal treatment has a CEC of 239 meq/100 g. However, phlogopite [KMg 3 Si 3 AlO 10 (OH) 2 ] without K-depletion is expected to have about 5–10 meq/ 100 g. The K-depleted phlogopite was found to show high selectivity for Cs and be able to immobilize Cs in the interlayers (Komarneni and Roy, 1988). Also, interlayer potassium ions can be removed with NaNO 3 (Chaussidon, 1970). In addition to the chemical alteration or modification from naturally occurring micas, some researchers directly synthesized some swelling micas which have higher negative charge and greater cation exchange capacity than the K- depleted micas. A novel Na-4-mica with theoretical CEC of 468 meq/ 100 g was synthesized (Gregorkiewitz and Rausell-Colom, 1987). This large CEC value is due to four exchangeable sodium ions in the interlayer per unit cell. Although the K-depleted mica has low CEC compared to Na-4-mica, exchange process with the K-depleted mica may be faster than that with Na-4-mica because of high charge density of the Na-4 mica (Shimizu et al., 2004). The purpose of this investigation was to study cation exchange properties of two K-depleted micas (K-depleted biotite, and K-depleted muscovite) using some alkali and alkaline earth metal ions (Cs + and Sr 2+ ). Thermodynamic approach was applied to investigate their cation exchange properties. For instance, cation exchange isotherms for Cs + and Sr 2+ were determined. Also, Kielland plots were constructed to estimate selectivity coefficients. Applied Clay Science 44 (2009) 15–20 ⁎ Corresponding author. 205 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA. Tel.: +1 814 865 1542; fax: +1 814 865 2326. E-mail address: komarneni@psu.edu (S. Komarneni). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.12.015 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay