Microchim. Acta 147, 31–37 (2004) DOI 10.1007/s00604-004-0224-4 Original Paper Solid Phase Extraction of Cesium from Aqueous Solution Using Sol-Gel Encapsulated Cobalt Hexacyanoferrate Diep Vu Ca and James A. Cox  Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056, USA Received November 13, 2003; accepted February 26, 2004; published online April 26, 2004 # Springer-Verlag 2004 Abstract. The synthesis and evaluation of silica sol- gels doped with cobalt hexacyanoferrate (CoHCF) for solid phase extraction of Cs þ from aqueous solution is described. The CoHCF is formed by first introducing K 4 Fe(CN) 6 (HCF) into the sol-gel and subsequently contacting the porous solid to a solution containing Co 2þ . Generally, sols contain alcohol as a co-solvent, which limits the solubility of HCF. Inclusion of gen- eration-4 polyamidoamine (G4-PAMAM) dendrimer in the sol-gel increases the level of HCF and, in turn, the CoHCF. The uptake capacity of this composite was 0.43  0.01 mmol Cs þ g 1 . A second approach to increasing the level of HCF silica is to exclude alcohol from the sol and use ultrasound to obtain a homoge- neous suspension; HCF concentrations up to 0.17 M in the sol were thereby achieved. After gelation and reac- tion with Co 2þ , the resulting composite had a capacity of 0.61  0.01 mmol Cs þ g 1 . With 0.5 mM Cs þ as the sample, the presence of either 0.5–100 mM Na þ or 10 mM Ca 2þ did not change that value at the 95% confidence level. Key words: Solid phase extraction; sol-gel; cesium; cobalt hexa- cyanoferrate; pore size distribution. The selective preconcentration of cesium from aqueous solutions containing high concentrations of alkali metals is an important problem in the treatment of radioactive wastes. Among the materials being studied are precipitates of hexacyanoferrate, HCF, with divalent cations of metals such as copper, cobalt, nickel and zinc. These precipitates are formed as bulk solids or as films on a variety of supports [1]. Cobalt hexacyanoferrate (CoHCF), which in some cases is charge-balance with other cations such as K þ and H þ , has perhaps been the compound most used. The mechanism of the uptake of Cs þ by metal hexacya- noferrates has been reported as either ion exchange or sorption into the cage structure of these compounds. For example, Lehto et al. [2] investigated the distribu- tion coefficients for alkali and alkali earth cations on K 2 Co II Fe II (CN) 6  4H 2 O; a high selectivity toward Cs þ was reported. They speculated that the uptake mechanism was probably ion exchange. Ramaswamy [3] compared the distribution of Cs þ on HCFs of Cu II , Co II , Ni II , and Zn II . The metal HCFs were in the form of composites on anion exchange resins. The efficacy of these metal HCFs for the extraction of Cs þ was in the sequence Co > Ni > Cu  Zn. The composite materials were reported as advantageous compared to the unsupported solids in terms of total capacity [3]. In acid solution, a low concentration of hydrazine was added to obtain high distribution coefficients. An analogous study by Ramaswamy but with silica as the support reported that the exchange capacity for Cs þ from neutral solu- tion was greatest with CuHCF [4].  Author for correspondence. E-mail: coxja@muohio.edu