Cs Resin , AMP-PAN

AMP-PAN (Ammonium MolybdoPhosphate) resins 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).

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

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