Radiolysis of simple quaternary ammonium salt components of Amberlite resin

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Abstract

The radiation chemical yields of gaseous products, H2 and CH4, in the radiolysis of dry methylammonium chloride, dimethylammonium chloride, trimethylammonium chloride, tetramethylammonium chloride and benzyl trimethylammonium chloride by γ-rays and 5 MeV helium ions have been investigated. Some of these amines are the different components of the quaternary ammonium resin Amberlite, which is a strongly basic anion exchange resin based on a polystyrene divinylbenzene copolymer. Molecular hydrogen yields with γ-radiolysis range from a high of 4.43 molecules per 100 eV for trimethylammonium chloride to 0.07 and 0.05 molecules per 100 eV for tetramethylammonium chloride and benzyl trimethylammonium chloride, respectively. Yields of methane gas are generally negligible except for trimethylammonium chloride and tetramethylammonium chloride, 0.26 and 0.02 molecules per 100 eV, respectively. Isotopic labeling studies suggest that the first step in H2 production is the breakage of the Nsingle bondH bond followed by abstraction of Hradical dot atom from the methyl groups. EPR analysis shows the formation of both N and C centered radicals. A comparison is made between the radiolysis of Amberlite and its various components.

Introduction

Anion exchange resins have received a large amount of attention because they are used widely in the nuclear industry as a part of fuel reprocessing, waste management, and purification of reactor coolants [1]. Earlier investigations generally examined physic-chemical properties such as total exchange capacity, water retention capacity, weight loss, and similar properties that were related to the functional performance of resins [2], [3], [4], [5], [6]. However, exposure of anion exchange resins to ionizing radiation leads to molecular degradation releasing gaseous and other products like amines into the systems [5], [6]. This radiolytic degradation might impede the separation process and can also cause serious engineering and maintenance problems. In this connection, several studies on the radiation induced decomposition of resins have reported on probable mechanisms involving radiolytic splitting of the functional groups and deterioration of the resin matrix to release basic products, amines and ammonia, on exposure to high doses [7], [8], [9], [10].

Radiation induced decomposition studies of the anion exchange resin Deacidite FF has revealed the destruction of the strong base functional group resulting in the release of water soluble tertiary, secondary and primary amines [9]. Gamma radiolysis of aqueous solutions of benzyl trimethylammonium ions, which mimic the functional group of quaternary ammonium based resins, also reveal the cleavage of the benzylsingle bondN bond resulting in the formation of trimethylamine as the major nitrogenous deamination product [11]. Reactions of various aryl alkyl amines have been studied in detail using pulse radiolysis techniques [12]. The results show that reductive deamination of arylalkylamines by eaq- is an important process having diffusion-controlled rate coefficients. Radiolysis of the strong basic anion exchanger resin Amberlite IRA400 in various anionic forms shows an excess production of H2 with about 85% water weight fraction samples [13]. This enhancement in H2 production was reported to be due to dimethylamine and trimethylamine that leached from the resin into the aqueous phase. These studies show that the presence of aliphatic amines either due to deamination or to leaching from the resin plays an important role in the radiolytic production of H2 and other gases.

In the present work, the radiation chemical yields of gaseous products in the γ-radiolysis of different components of Amberlite, a strongly basic anion exchange resin based on polystyrene divinylbenzene copolymer, have been evaluated. These compounds include: methylammonium chloride, dimethylammonium chloride, trimethylammonium chloride, tetramethylammonium chloride and benzyl trimethylammonium chloride in the solid state. Fourier transform infrared, FTIR, Raman, and electron paramagnetic resonance, EPR, spectroscopy of the parent compounds were also performed. The results are compared with that obtained with polyethylene, and polystyrene [14] to predict what is observed in the radiolysis of Amberlite. In realistic conditions, resins are irradiated in association with water or aqueous solutions. All compounds examined in this work are in the dry state in order to understand the direct radiation effects induced by the deposition of energy in the resin. Indirect radiation effects can occur when water decomposition products react with the resin and the overall outcome will depend on the amount of water present. Further studies on the radiolysis of Amberlite components in association with water will be combined with the results of this work in order to understand the heterogeneous radiolysis of the resin in a practical scenario.

Section snippets

Materials

Methylammonium chloride, dimethylammonium chloride, trimethylammonium chloride, tetramethylammonium chloride, and benzyl trimethylammonium chloride were obtained from Alfa Aesar and used without further purification. Special care was taken in sample preparation as these amines are hygroscopic in nature. The amines were ground into uniform size and dried under vacuum at 35 °C, weighed, and placed in a drying chamber with Drierite (calcium sulfate).

Irradiations

Samples for γ-radiolysis were placed in Pyrex

Gas production

Radiation chemical yields, G values, are presented here as molecules of H2 or CH4 formed per 100 eV of total energy absorbed by the sample (1 molecule/100 eV = 0.104 μmol/J). The G values for H2 in the γ-radiolysis of dry methylammonium chloride, dimethylammonium chloride, trimethylammonium chloride, tetramethylammonium chloride and benzyl trimethylammonium chloride were listed in Table 1. These yields vary from a low of 0.05 with the aromatic benzyl trimethylammonium chloride to 4.43 molecules/100 eV

Conclusion

Radiolysis of the various quaternary ammonium compounds that can be found in the Amberlite resin have been examined. These compounds are the various methylammonium chlorides and benzyl trimethylammonium chloride. The yield of molecular hydrogen increases with the addition of methyl groups and then decreases dramatically for tetramethylammonium chloride. Methane production is relatively small except for the trimethylammonium chloride. Isotopic and spectroscopy studies suggest that the formation

Acknowledgments

The authors thank Prof. Michael Wiescher for making the facilities of the Notre Dame Nuclear Structure Laboratory available to us. The Nuclear Structure Laboratory is supported by the U.S. National Science Foundation. Maria Vyushkova helped in obtaining the EPR spectra. The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. This contribution

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