Insights on uranium uptake mechanisms by ion exchange resins with chelating functionalities: Chelation vs. anion exchange

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Highlights

  • Most N-containing functionalities on resins uptake uranyl as [UO2(SO4)3]4−.

  • Amidoxime containing resins form coordination complexes with uranyl.

  • Mixed sulfonate/phosphonate functionalities on resins chelate uranyl.

  • No difference in uranyl speciation on resins loaded from fresh and sea water.

Abstract

X-ray absorption fine structure analysis has been successfully used to determine the coordination environment and therefore uptake mechanism towards the uranyl cation for a selection of commercially available ion exchange resins in non-saline and saline conditions ([Cl] = 22.7 g L−1, 0.64 M) similar to those found in sea water. The resins tested were Purolite S985, S910 and S957, Dowex M4195, Ps-EDA, Ps-DETA and Ps-PEHA, which contain polyamine, amidoxime, mixed sulfonic/phosphonic acid, bispicolylamine, ethylenediamine, diethylenetriamine and pentaethylenehexamine functional groups, respectively. Purolite S910 and S957 were both found to extract the uranyl cation through a chelation mechanism. The uranium coordination environment on uranyl loaded Purolite S910 was found to be either tetra- or hexa-coordinate in the equatorial plane, with a 2:1 ratio of amidoxime:uranium in the fit suggesting either monodentate or η2 coordination by two amidoxime groups. The uranium environment for uranyl loaded Purolite S957 was found to be tetra-coordinate in the equatorial plane, with both sulfonic and phosphonic acid groups being involved in sorption. The presence of chloride in the loading solution had no effect on the uranyl coordination environment observed on any of the resins. In contrast, Dowex M4195, Purolite S985, Ps-EDA, Ps-DETA and Ps-PEHA exhibited an anion exchange mechanism for uranyl uptake as the corresponding extended X-ray absorption fine structure (EXAFS) data best fit a [UO2(SO4)3]4− structure.

Introduction

Nuclear power is becoming an increasingly attractive sustainable energy source to cope with population growth and climate change. With this in mind, many countries around the world are investing in new nuclear reactors. There are currently over 60 reactors being constructed and many more in planning, with a predicted rise of up to 200% in global electricity produced from nuclear fission by 2050 [1]. As the number of nuclear reactors increases, so will the demand for uranium. Uranium mining requires vast quantities of fresh water, which can be problematic. Many mines are in arid environments, and the use of fresh water in mine processing circuits can put a strain on drinking supplies for local populations. There are also large costs involved in purifying wastewater from these processing circuits for release back into the environment and the option to desalinate low quality waters for use in processing is expensive and energy intensive. In order to meet uranium demands for future nuclear energy requirements in an economically and environmentally sustainable way, new extraction technologies need to be developed to allow the use of low quality waters for uranium recovery, such as those containing high saline. An example of this would be seawater, which has an average chloride concentration of 22.7 g L−1 (0.64 M).

The uranium mining industry has become heavily reliant on the use of strong base anion (SBA) exchange resins for the extraction of uranium. However, these conventional SBA resins are not compatible with the use of low quality waters, due to the suppression of ion exchange (IX) at high ionic strengths [2], [3]. These streams are reported to be more compatible with weak base anion (WBA) exchange resins and chelating IX resins than traditional anion exchangers [3], [4], [5], [6]. A fundamental understanding of the behaviour of UO22+ towards these resin types in sulfate processing liquors with/without high chloride content is lacking, as is structural data for the exchanged U species on their surface. Understanding the uranium uptake mechanisms in these systems could result in more effective uranium milling flowsheets, which can tolerate the presence of chloride as well as other anionic species, and could also lead to their implementation in environmental waste management strategies. This enhanced understanding of uranium recovery could also be transferred to the extraction of other high value, critical metals, such as rare earths and platinum group metals, ultimately allowing for the continued use of advanced technologies, enjoyed by many, which rely heavily on these elements.

WBA resins differ from SBA resins due to their different functionalities. SBA resins always contain a quaternary ammonium group, and are therefore always positively charged [7]. Charge balance is typically maintained by the presence of readily exchangeable anions found in the solution environments the resins are exposed to, such as sulfate. However, WBA resins are generally functionalised with primary, secondary and/or tertiary amines, and therefore their ability to become charged is dependent upon solution chemistry and their pKa values. They function in the same way as SBA resins, as when they become protonated they can exchange associated anionic co-ions with aqueous anions. In the case of uranyl recovery from acidic sulfate conditions, the extracted species is generally believed to be [UO2(SO4)3]4−, as reported for uranyl sorption on to WBA resins [8]. However, extended X-ray absorption fine structure (EXAFS) studies by Moon et al. on uranyl loaded tertiary amine WBA resin, Dowex Monosphere 77, from solutions with [Cl] of 0–0.5 M, 1–2 M and 3–5.8 M ([(SO4)2−] = 0.25 M, [(UO2)2+] = 4 mM) have shown the extracted species to be [UO2(SO4)2(H2O)]2−, [UO2(SO4)Cl.(H2O)2] and [UO2Cl4]2−, respectively [6]. A difference in uranyl speciation was also observed between the aqueous and resin phases in both studies [6], [8].

Chelation resins generally have molecular functionalities, attached to the surface of the bulk resin matrix, consisting of multiple ligating atoms that can coordinate to a metal ion resulting in the formation of a chelate ring. These chelate rings are typically thermodynamically favoured over equivalent coordination complexes with ligands that have only one binding site due to entropic considerations described by the chelate effect.

There are many molecules capable of forming multidentate complexes with uranium in aqueous and organic phases, many of which could be grafted onto a solid to produce an IX resin [9]. However, there are only a relatively small number of commercially available resins with molecular functionalities that are capable of chelating to metal ions, which could be applied to uranium extraction. Despite these resins being marketed as “chelation resins”, there is little direct evidence to indicate these do actually chelate metal ions. We have recently shown, by the application of EXAFS spectroscopy, that a series of polyamine functionalised resins, capable of chelating metal ions, actually perform as anion exchangers for uranyl from sulfate solutions at pH 2 [8]. In these examples, uranyl in the form of an anionic complex, [UO2(SO4)3] [4], binds to these resins by an ionic interaction with protonated amines on the resin. The branched polyamine functionalised resin Purolite S985 has also been shown to act as an anion exchanger under similar conditions [8].

This work aims to establish uranyl speciation upon a selection of loaded WBA and chelation resins from solutions containing saline concentrations similar to those in sea water (i.e. [Cl] = 22.7 g L−1 = 0.64 M), and therefore the mechanisms by which these resins uptake uranyl from low quality waters. This identified species are compared against those from the same resins loaded with uranyl under fresh water conditions. The chosen WBA resins for this work are three in house synthesised resins (Ps-EDA, Ps-DETA, Ps-PEHA; Fig. 1) and Purolite S985 (Fig. 1), all with polyamine functionalities, and the chosen commercial chelation resins are Purolite S910, Purolite S957 and Dowex M4195 with amidoxime, mixed sulfonate/phosphonate and bispicolylamine functionalities, respectively (Fig. 2). The establishment of the mechanism by which these resins uptake uranyl can then be compared to process performance parameters in order to identify criteria that can be used for the selection of IX resin/s that give optimum extraction properties based upon likely solution composition.

Section snippets

Materials and sample preparation

Dowex M4195 IX resin was purchased from Sigma Aldrich, with all other commercial resins kindly being supplied by Purolite. Manufacturer resin specifications are shown in Supp. Info. In house produced WBA resins were synthesised via previously established methods [8]. Resins were preconditioned by contacting with H2SO4 (1 M) for 24 h, with a resin:acid ratio of 1:10 (v:v). Aqueous uranyl sulfate solutions were supplied by the University of Sheffield.

Uranium uptake

All resin was ground to a fine powder before

Results and discussion

U LIII-edge EXAFS spectra were collected of uranyl-containing aqueous phases for both non-saline ([U] = 1.0 g L−1 = 4.2 mM, [(SO4)2−] = 1.4 g L−1 = 15 mM, pH 2.0) and saline conditions ([U] = 1 g L−1, [(SO4)2−] = 1.4 g L−1, [Cl] = 22.7 g L−1 = 0.64 M, pH 2.0), and of numerous ion exchange resins consisting of various chelating functionalities upon which uranyl was loaded from both the aqueous phases studied. Experimental data in R- and K-space are shown in Supplementary Fig. S1, Supplementary

Conclusions

A set of ion exchange resins consisting of various functionalities that are capable of chelating to metal ions have been analysed using EXAFS to determine their binding mode towards uranyl loaded from non-saline and saline conditions analogous to sea water environments. It has been shown that the presence of chloride has little effect on the uranyl binding mode by all studied resins.

Dowex M4195 and WBA resins Ps-EDA, Ps-DETA, Ps-PEHA and Purolite S985 were fit with the extracted species being

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank the EPSRC for funding this work (EPSRC reference: EP/G037140/1), the Diamond Light Source for the opportunity to perform these experiments (SP12643-1), the B-18 principal beamline scientist Dr. Giannantonio Cibin for his help with data acquisition, Professor Katherine Morris at the Research Centre for Radwaste & Disposal for assistance with sample preparation, and the University of Sheffield for the provision of uranyl sulfate.

References (43)

  • A. Taylor

    Short Course in Uranium Ore Processing

    (2016)
  • N. Kabay et al.

    Recovery of uranium from phosphoric acid solutions using chelating ion-exchange resins

    Ind. Eng. Chem. Res. 37

    (1998)
  • J. Kim et al.

    Recovery of uranium from seawater: a review of current status and future research needs

    Sep. Sci. Technol.

    (2013)
  • E.J. Zaganiaris

    Ion Exchange Resins in Uranium Hydrometallurgy

    (2009)
  • G.R. Choppin, M.P. Jensen, Actinides in Solution: Complexation and Kinetics, In The Chemistry of the Actinide and...
  • B. Ravel et al.

    ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT

    J. Synchrotron Radiat.

    (2005)
  • L. Downward et al.

    A variation of the F-test for determining statistical relevance of particular parameters in EXAFS fits

    AIP Conf. Proc.

    (2007)
  • W.C. Hamilton

    Significance tests on the crystallographic R factor

    Acta Cryst.

    (1965)
  • C. Hennig et al.

    The structure of uranyl sulfate in aqueous solution – monodentate versus bidentate coordination

    AIP Conf. Proc.

    (2007)
  • M. Walter et al.

    Sorption of uranium(VI) onto ferric oxides in sulfate-rich acid waters

    Environ. Sci. Technol.

    (2003)
  • C.W. Abney et al.

    XAFS investigation of polyamidoxime-bound uranyl contests the paradigm from small molecule studies

    Energy Environ. Sci.

    (2016)
  • Cited by (0)

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