Cs Resins

Eichrom offers two cesium selective resins, AMP-PAN (Ammonium MolybdoPhosphate) and KNiF-PAN (Potassium Nickel HexacyanoFerrate(II)). Both resins were developed by Dr. Šebesta at the Czech Technical University in Prague and are based on very fine and selective inorganic materials embedded in polyacrylnitrile (PAN) resin beads. Embedding the inorganic materials in the polymer matrix improves their mechanical characteristics, improving flow through columns or cartridges packed with the Cs selective resins.

AMP-PAN resin is based on ammonium molybdophosphate, an inorganic ion exchanger known for its high selectivity for Cs (even at elevated acid concentrations), quick kinetics and radiation stability [1].

One of the main restraints to the use of AMP is its unfavorable microcrystalline structure. Accordingly, considerable work has been performed to improve its granulometry. Embedding the AMP in an organic matrix improves control of particle size, topography, porosity, hydrophilicity and cross-linking of the resin matrix as well as the amount of AMP embedded in the resin.

Šebesta and Štefula showed that embedding the AMP in a PAN matrix has only limited impact on its Cs uptake kinetics and on the Cs capacity of the embedded AMP [1]. The AMP-PAN resin is chemically stable even under relatively harsh conditions such as 1M HNO3 / 1M NaNO3 or 1M NaOH / 1M NaNO3. Even after storing the resin under these conditions for 1 month, no visible mechanical damage could be observed, while Kd values, sorption kinetics and capacity also remained unchanged [2]. Radiolytic stability of the resin was evaluated in acidic solution by exposing it to doses up to 106 Gy. No changes in Kd or sorption capacity were found.

Desorption of the cesium from the AMP-PAN is only possible using high concentrations of ammonium salts, such as 10 bed volumes of 5M NH4Cl [1]. Alternatively, NH4NO3 might be used [3]. The Cs may also be recovered by destroying the AMP using strong alkaline solutions, such as 5M NaOH.

High selectivity for Cs under harsh chemical conditions and high levels of radioactivity make the AMP-PAN resin a candidate for the treatment of radioactive waste solutions. Brewer et al. [3] tested the resin for the removal of Cs-137 from real and simulated acidic high-active liquid radioactive waste containing high amounts of potassium and sodium. Small scale tests were performed using 1.5 mL columns and two feed solutions, one simulated tank waste (spiked with 100 Bq.mL-1 Cs-137) and one actual tank waste. Both solutions were filtered, and pumped through the column at a flow rate of 26 – 27 bed volumes per hour.  Aliquots were taken at regular intervals and analyzed for Cs-137 activity. After the experiment, the AMP-PAN columns were eluted using 30 bed volumes of 5M NH4NO3, reconditioned and the effluents were passed over the column a second time. For the real waste samples a Cs breakthrough of 0.15% was observed after a sample loading volume of 1000 bed volumes during the first loading cycle (corresponding to a Cs decontamination factor greater than 3000) and 0.53% after 830 bed volumes during the 2nd loading cycle. Cs recoveries in the respective eluates were 87%.

The robustness of AMP-PAN against high salt concentrations also makes it interesting for use in environmental analysis, especially the analysis of Cs-134/7 in sea water. Pike et al. [4] used AMP-PAN for concentrating and purifying Cs from 20 liter seawater samples (acidified to pH 1 – 2, stable Cs was added for yield determination by ICP-MS). The authors employed 5 mL columns and worked at a flow rate of 35 mL/min. After extraction, the resin was rinsed from the column using 0.1M HNO3 and analysed by gamma spectrometry. Yields were found to be 93.5% +/- 5.0% (n=55). The authors further analysed an internal lab standard (WHOI) in triplicate and IAEA sea water reference material, results are summarized in Table 1.

Table 1: Comparison obtained and reference values, sea water samples [4]

Larger seawater samples were analysed by Kamenik et al. [5]. The authors evaluated, in addition to the AMP-PAN resin, also the use of KNiFC-PAN resin, which is based on potassium-nickel hexacyanoferrate(II) embedded in a PAN matrix.

The authors passed 100 liters of acidified seawater samples (and for KNiFC-PAN, unacidified seawater samples were tested as well) through 25 mL beds of AMP-PAN or KNiFC-PAN resin at flow rates up to 300 mL/min. The 100 liter samples were processed in less than 6 hours. As described previously, stable Cs was added to the seawater samples to allow the determination of the chemical yield via ICP-MS. After loading the samples, the resins were rinsed from the columns, dried and measured by gamma spectrometry using a coaxial HPGe detector with 43% relative efficiency in a Petri dish geometry. The chemical yields obtained are summarized in Table 2. Yields are generally high, with KNiFC-PAN showing slightly higher yields for the acidified seawater samples than AMP-PAN and comparable chemical yields for acidified and non-acidified seawater samples.

Table2: Comparison of obtained chemical yields, 100 L sea water samples, AMP-PAN and KNiFC-PAN [5]

Higher flow rates were tested for the processing of non-acidified sea water samples on KNiFC-PAN resin. At a flow rate of 470 mL/min, Cs yield is still greater than 85%. The authors calculated the minimum detectable activity (MDA) for 100 liter samples with 50 – 70 hour counting times and average chemical yields. For Cs-137 they calculated an MDA of 0.15 Bq/m3 and 0.18 Bq/m3 for Cs-134. KNiFC-PAN resin was further used for the determination of Cs isotopes in milk [6] and urine [7].

AMP based ion exchangers have also been used to separate Rb from other alkali metal ions in acidic media [8, 9].

Bibliography
[1] Sebesta F, Stefula V (1990) Composite ion exchanger with ammonium molybdophosphate and its properties. J Radioanal Nucl Chem 140(1):15 – 21
[2] John et al. (1999) Application of new inorganic-organic composite absorbers with polyacrilonitril binding matrix for separation of radionuclides from liquid radioactive wastes. Choppin and Khankhasayev (eds.) Chemical separation technologies and related methods of nuclear waste management, 155 – 168
[3] Brewer et al. (1999) AMP-PAN column tests for the removal of Cs-137 from actual and simulated INEEL high-activity wastes. Czechoslov J Phys 49(S1):959-964
[4] Pike et al. (2012) Extraction of cesium in seawater off Japan using AMP-PAN resin and quantification via gamma spectroscopy and inductively coupled mass spectrometry. Radioanal Nucl Chem. DOI 10.1007/s10967-012-2014-5
[5] Kamenik et al. (2012) Fast concentration of dissolved forms of cesium radioisotopes from large seawater samples. J Radioanal Nucl Chem. DOI 10.1007/s10967-012-2007-4
[6] Kamenik J et al. (2009) Long term monitoring of Cs-137 in foodstuffs in the Czech Republic. Appl Radiat Isot 67(5):974-977
[7] Bartuskova et al. (2007) Ingestion doses for a group with higher intake of Cs-137. In: IRPA regional congress for Central and Eastern Europe, Brasov, Romania
[8] Coetze CJ : The separation of a sodium-rubidium mixture on an ion exchanger. (1972) J Chem Edu 49(1): 33
[9] Smit, J van R, Robb W, Jacobs JJ: Cation exchange on ammonium molybdophosphate—I: The alkali metals (1959) J Inorg Nucl Chem, 12(1-2): 104-112