A review on solid phase extraction of actinides and lanthanides with amide based extractants

Journal of Chromatography A, 1499 (2017) 1–20

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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma

Review article

A review on solid phase extraction of actinides and lanthanides with amide based extractants Seraj A. Ansari, Prasanta K. Mohapatra ∗ Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e

i n f o

Article history: Received 10 November 2016 Received in revised form 14 March 2017 Accepted 17 March 2017 Available online 19 March 2017 Keywords: Solid phase extraction Amides Separation Actinides Radioactive wastes

a b s t r a c t Solid phase extraction is gaining attention from separation scientists due to its high chromatographic utility. Though both grafted and impregnated forms of solid phase extraction resins are popular, the later is easy to make by impregnating a given organic extractant on to an inert solid support. Solid phase extraction on an impregnated support, also known as extraction chromatography, combines the advantages of liquid-liquid extraction and the ion exchange chromatography methods. On the flip side, the impregnated extraction chromatographic resins are less stable against leaching out of the organic extractant from the pores of the support material. Grafted resins, on the other hand, have a higher stability, which allows their prolong use. The goal of this article is a brief literature review on reported actinide and lanthanide separation methods based on solid phase extractants of both the types, i.e., (i) ligand impregnation on the solid support or (ii) ligand functionalized polymers (chemically bonded resins). Though the literature survey reveals an enormous volume of studies on the extraction chromatographic separation of actinides and lanthanides using several extractants, the focus of the present article is limited to the work carried out with amide based ligands, viz. monoamides, diamides and diglycolamides. The emphasis will be on reported applied experimental results rather than on data pertaining fundamental metal complexation. © 2017 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

5.

6.

7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Novel extractants for actinide separations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Effective ‘green’ separation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Extraction chromatography with impregnated resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Monoamide impregnated resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.2. Malonamide impregnated resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.3. Diglycolamide impregnated resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.4. Multi-podant DGA ligands in extraction chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Extraction chromatography with chemically bonded resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1. Monoamide grafted resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.2. Malonamide grafted resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.3. Diglycolamide grafted resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 RTIL based extraction chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.1. TODGA/RTIL resin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.2. C4DGA/RTIL and T-DGA/RTIL resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Composite beads for extraction chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

∗ Corresponding author. E-mail address: [email protected] (P.K. Mohapatra). http://dx.doi.org/10.1016/j.chroma.2017.03.035 0021-9673/© 2017 Elsevier B.V. All rights reserved.

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S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

Nomenclatures ADHA ␣-HIBA An C4DGA CMPO DB2EHM DEHAA DEHBA DEHiBA DGA DIAMEX DIDPA DMDBTDMA DMDOHEMA Dw EC EDHBA EDTA ERIX Process HAN HDEHP HEDTA HLW HN Kd Ln MCF MAREC PUREX RTIL SHLW TBP T2EHDGA T-DGA THOREX TODGA TRPO

P-amino-N,N-dihexyl acetamide Alpha hydroxyl isobutyric acid Actinide Diglycolamide-functionalized calix[4]arene Octyl-(phenyl)-N,N-diisobutyl carbamoyl methyl phosphine oxide Di-bis-(2-ethylhexyl)malonamide Di-2-ethyhexyl acetylamide Di-2-ethylhexyl butyramide Di-2-ethyhexyl isobutryamide Diglycolamide Diamide extraction Di-isodecylphosphoric acid N,N’-dimethyl-N,N’-dibutyl tetradecyl malonamide N,N’-dimethyl-N,N’-dioctyl-2-(2hexyloxy-ethyl)-malonamide Weight distribution coefficient Extraction chromatography 4-Ethoxy-N,N-dihexyl-butanamide Ethylenediamine tetraacetic acid Electrolytic reduction and ion exchange process Hydroxylamine nitrate Bis-2-ethylhexyl phosphoric acid N-(2-Hydroxyethyl)-ethylenediamine triacetic acid High level waste Hydrazine Distribution coefficient Lanthanide Mesostructured cellular foams Minor actinide recovery by extraction chromatography Plutonium uranium reduction extraction Room temperature ionic liquid Simulated high level waste Tri-n-butyl phosphate N,N,N’,N’-tetra-2-ethylhexyl diglycolamide Tripodal diglycolamide Thorium extraction N,N,N’,N’-tetraoctyl diglycolamide Trialkyl phosphate oxide

1. Introduction In view of the fast dwindling natural resources of fossil fuels, nuclear energy is slowly emerging as one of the major alternative energy resources [1]. However, due to the limited availability of the naturally occurring fissile material (235 U), the future of the nuclear energy program largely depends upon the availability of the manmade fissile materials such as 239 Pu and 233 U. To sustain the nuclear power programme beyond the availability of the naturally occurring 235 U, it is imperative, therefore, to follow the closed nuclear fuel cycle option [1–3]. The closed nuclear fuel cycle emphasizes on the reprocessing of the spent nuclear fuel using a suitable methodology. During reprocessing of the spent fuel, valuable activation products such as 239 Pu (formed in uranium based fuel) and 233 U (formed in thorium based fuel) are recovered in the well-known hydrometallurgical processes, PUREX (Plutonium Uranium Reduc-

tion EXtraction) and THOREX (THORium EXtraction), respectively [4]. Though the TBP (tri-n-butyl phosphate) based PUREX process has been the workhorse of the nuclear fuel reprocessing industry for the past several decades, continued R&D efforts have identified a few drawbacks associated with the use of TBP which have raised concerns [5–10]. The main problems of TBP are (i) its vulnerability to high radiation field and deleterious nature of its degradation products affecting the product recovery, (ii) its solubility towards aqueous phase, and (iii) non-incinerable nature of the spent solvent, thus, yielding large volumes of secondary wastes. To overcome these draw-backs, amide based extractants have been studied for possible application in the nuclear reprocessing since the work of Siddall [11,12]. These extractants offer several advantages over TBP, especially with respect to the (i) innocuous nature of their degradation products, viz. carboxylic acids/amines, and (ii) the possibility to incinerate the used solvent leading to reduced volume of secondary wastes [11–15]. Additionally, the physico-chemical properties of this class of ligands can be tuned by the judicious choice of alkyl groups. Systematic studies have been undertaken to investigate (i) linear N,N-dialkyl amides as alternatives to TBP in the PUREX process [16–18], and (ii) branched chain N,N-dialkylamides as alternatives to TBP in the THOREX process [18–22]. Though the reprocessing of spent nuclear fuel gives fissile nuclear materials such as 239 Pu, it also leads to the generation of large volumes of highly radioactive liquid waste, known as High Level Waste (HLW) which contains >95% of the total radioactivity of all radioactive wastes. The HLW generated during the reprocessing of the spent nuclear fuel contains the un-extracted U, Pu (from the PUREX losses), bulk of the minor actinides (Am, Np, Cm), a host of fission product elements like lanthanides (Lns), Tc, Pd, Zr, I, Cs, Sr and activation products such as Ni, Sb, Zr [23–27]. Fission products are highly radioactive (beta/gamma emitters) and pose serious threat to the human life as well as to the ecology. The challenge for the final disposal of HLW is largely due to the radiotoxicity associated with the minor actinides (Ans), which are alpha emitters and have half-lives in the range of few hundreds to millions of years. At present, the most accepted strategy for the management of HLW is to vitrify the waste oxides in borosilicate glass matrices followed by disposal in deep geological repositories [28–30]. Due to the long half-lives of the minor Ans, the surveillance of HLW containing vitrified blocks for such a long period is a daunting task. An alternative/complimentary concept is the P&T (Partitioning and Transmutation), which is being explored by several countries worldwide for the safe disposal of HLW. The P&T process envisages the complete removal of minor Ans from HLW and their subsequent burning in high flux reactors/accelerators in suitable chemical forms [31–36]. After removing the minor Ans along with the long-lived fission products, the residual waste can be vitrified and buried in sub-surface repositories at a much reduced risk and cost. The successful application of this approach is expected to reduce the stringent requirements for geological repository capacity substantially.

2. Novel extractants for actinide separations Separation scientists have been engaged in the development of new extractants for actinide partitioning from HLW for the past few decades [37–39]. In this context, several organophosphorus as well as amide based extractants have been developed and evaluated systematically (Fig. 1). Amongst the organophosphorus compounds, octyl-(phenyl)–N,N-diisobutyl carbamoyl methyl phosphine oxide (CMPO) has been investigated extensively [38–43]. The bifunctional nature of CMPO facilitates the extraction of trivalent Ans (Am and Cm) at a moderate acidity of 3–4 M HNO3 . However, the stripping of the extracted An ions from the loaded

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

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Fig. 1. Structural formulae of the ligands used for actinide extraction from acidic feeds.

organic phase is rather cumbersome and requires multiple contacts with complexing solutions [44]. Other phosphorus based extractants, such as trialkyl phosphine oxide (TRPO) [45,46] and diisodecylphosphoric acid (DIDPA) [47,48] have limitations as they extract trivalent Ans at a much lower acidity (≤1 M HNO3 ). Though these extractants (TRPO and DIDPA) are useful for ‘actinide partitioning’ from the HLW containing a much lower acidity (0.5–1.0 M HNO3 ), they are not effective for more common type of HLW which contains 3–4 M HNO3 [23–25]. Substituted malonamide extractants, on the other hand, have been found to be more promising for minor An extraction under HLW conditions [49–52]. The fact that these extractants are not as efficient as CMPO at lower acidities (≤1 M HNO3 ) makes them versatile during their subsequent back extraction. In addition, malonamides are completely incinerable, which implies that the amount of secondary waste generated could be reduced significantly. Amongst several malonamide extractants, N,N’-dimethyl-N,N’-dibutyl tetradecyl malonamide (DMDBTDMA) and N,N’-dimethyl-N,N’-dioctyl-2-(2hexyloxy-ethyl)-malonamide (DMDOHEMA) have been extensively studied in the proposed DIAMEX process [49–52]. The distribution ratio values with malonamides for trivalent Ans, however, are significantly lower as compared to those obtained with CMPO, which necessitates the use of relatively higher concentrations of these ligands. Efforts to design more efficient extractants with similar amide skeleton led to the subsequent development of diglycolamide (DGA) extractants, which display far superior extraction properties as compared to the malonamides as well

as CMPO based extractants [53–57]. The distribution ratio (D) of the minor An ions such as Am3+ and Cm3+ with DGA is one order magnitude higher at a much lower extractant concentration. Subsequent counter-current extraction studies using DGA extractants have underlined their importance for actinide partitioning [58–63]. Recently, the affinity of DGA ligands towards the trivalent Ans have been further enhanced several folds after functionalization of multiple DGA moieties either onto a tripodal ligand [64–66] or a calix[4]arene scaffold [67–71]. Increased extraction efficiency of such ligands is ascribed to the cooperative complexation of 3–4 DGA moieties available in a pre-organized manner in these multiple DGA ligands. 3. Effective ‘green’ separation methods In view of their continuous nature, solvent extraction based methods have been employed extensively in the industrial scale operations. However, the major disadvantages associated with this technique includes the generation of large volumes of secondary waste and handling of large volumes of volatile organic compounds. In this context, techniques such as extraction chromatography (EC) have been explored for the treatment of wastes containing low concentrations (sub-millimolar) of recoverable constituents [72–75]. EC is often suggested as a technique that combines the selectivity of solvent extraction with the ease of operation of chromatography. Horwitz et al. [76] found a similar correlation for the extraction of Lns by HD2EHP (di-2-ethylhexyl phosphoric acid) with the two

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S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

techniques. There is, however, a difference between solvent extraction and EC with respect to the activities of the extractant and the extracted complex due to the presence of solid support. EC, part of the broader concept of solid phase extraction, has been increasingly used for the pre-concentration of trace as well as ultratrace amounts of inorganic species from complex matrices [72–75]. There are two methodologies which are frequently adopted for designing the EC resins. The first involves the physical sorption of ligands onto an inert solid support, and the other is based on covalent coupling of the ligands with the polymer backbone through certain functional group such as N N or CH2 groups. The latter approach renders a resin that is more stable to leaching of the loaded ligands which can be recycled over a long period of time. In this article, a brief summary of the recent literature on the separation of Ans and Lns from aqueous streams with EC will be presented. Efforts will be made to present the EC work carried out with (i) ligand impregnated resins, and (ii) ligand functionalized polymers (chemically bonded resins). Though there are enormous studies in the literature on the EC separation of Ans and Lns using several extractants, the focus of the present article will be limited to the work carried out with amide based ligands like monoamides, diamides (including malonamides) and diglycolamides. Emphasis will be made to summarize only the results reported on separations of Ans and Lns. The fundamental complexations of ligands with the metal ions are out of scope of this article.

4. Extraction chromatography with impregnated resins As the name indicates, these EC materials are prepared by physical impregnation of the ligands on the commonly employed inert solid supports such as Chromosorb-W, Chromosorb-102, XAD-4, XAD-7, Amberchrom CG-71, etc. Impregnation is done by dissolving the ligand in a volatile solvent (such as acetone, methanol, toluene, etc.) and mixing the solution with a solid support in a suitable composition followed by solvent removal by a suitable method. The resultant material is vacuum dried and the weight percentage of the extractant loaded on the resins is calculated from the weight gain of the solid support after the loading. The loading of the ligand onto the solid support is mostly confirmed by elemental (C, H, N) analysis. Generally, the resins having ligand composition of 10–50% (w/w) on the support is prepared. Higher loading of the ligand is not advisable due to the loss of free flowing nature of the resins as the materials may become sticky. Studies have shown that the extraction behaviour of the resin is independent of the nature of the diluent used for the impregnation of the ligand [77]. However, the performance of the EC resin is strongly dependent on the nature of the solid support [78–83]. While XAD-7 (a polyacrylic ester, CH2 CH(COOR) ]n , with a hydrophobic surface of moderate polarity) based resins have been found to be very good for metal ion uptake in certain cases [78], XAD-4 (styrene divinylbenzene) has been effectively used in other cases [79]. On the other hand, Chromosorb-W (dimethyl dichlorosilane treated acid washed celite diatomaceous silica) was found to be a better support for DGA ligands as compared to Chromosorb-102 (styrene divinylbenzene), ® XAD-4 and XAD-7 [83]. Amberchrom CG71, a macroporous polyacrylic ester, has been used as the support material for EC resins offered by Eichrom Technologies, USA [82]. Batch distribution coefficient (Kd ; unit: mL/g) measurements are performed by equilibrating the XC resin with solutions containing the metal ions at a given aqueous phase acidity for a given time. After attaining the equilibrium Kd values, the resins are separated and the aqueous phases are analyzed for the metal ion content. The Kd value is calculated as the ratio of the amount of metal ion sorbed onto one gram of the resin to that remained in one mL of the aqueous solution. Since, the weight of the resins is involved in

the calculation of Kd , it is also referred to as “weight distribution coefficient” (Dw or Kd,w ). From the distribution coefficient data, one comes to know about the affinity of the resin towards the metal ion of interest, and the amount of metal ions that can be sorbed per g of the resin. From these fundamental data, the actual application of the resin is tested in column mode by packing it in a suitable column of required dimensions. In this section, the work on EC resins prepared by impregnating extractants such as monoamides, diamides and DGA have been briefly discussed. It is important to mention that monoamides show higher affinity for tetra- and hexavalent actinides over the trivalent actinides [38]. Diamides (such as malonamides) and DGA based extractants, on the other hand, are well studied for trivalent actinide separations [38]. Table 1 lists a series of monoamide, diamide and DGA based EC resins that have been studied for the separation of actinides and lanthanides. 4.1. Monoamide impregnated resins The potential of monoamide-based EC resins for An recovery was assessed by monitoring uranium separation with three different monoamides impregnated on XAD-4 and XAD7 [80]. The amides used were, di-2-ethylhexyl butyramide (D2EHBA), di-2-ethyhexyl-isobutryamide (D2EHiBA) and di-2ethyhexyl acetylamide (D2EHAA). The extraction of UO2 2+ by these ligands followed the order: D2EHAA > D2EHBA > D2EHiBA (Fig. 2). XAD-7 consistently resulted in the highest extraction of uranium. This was correlated to the average pore size of the resin material, where the extractant molecules could penetrate the material and sorb the metal into the interior of the resin. However, this proposed mechanism was not supported by experimental evidence. The XAD-4 has a styrene divinyl benzene backbone with an average pore size of 100 Å and a surface area of 750 m2 /g. Whereas, the XAD-7 resin has a polyacrylic ester backbone with an average pore size of 300–400 Å and a surface area of 380 m2 /g. Since the amount of extractant loaded on both the supports was identical (40% w/w), it was difficult to understand the higher extraction behaviour of the XAD-7 resins. For any given amide, the maximum Kd for the XAD-7 system was approximately 3 times higher than the corresponding Kd values with the XAD-4 based EC resin. There was a clear indication that either the resin pore size or other parameter might have affected the sorption behaviour, which showed such a vast difference in the Kd values. However, one of the most interesting conclusions from this work is that D2EHAA outperforms D2EHBA and D2EHiBA, two of the most widely studied monoamides for selective recovery of uranium in solvent extraction processes [18–22]. 4.2. Malonamide impregnated resins DMDBTDMA is probably the first amide based ligand proposed for the recovery of trivalent Ans and Lns from HLW [39]. The ligand has been extensively studied for actinide partitioning in DIAMEX process proposed by French researchers [49–52]. Though, the major studies reported with this ligand were based on solvent extraction method including large scale demonstrations (10–50 L scale) utilizing mixer-settlers or centrifugal contactors, reports on EC separation of Ans with this ligand are limited [77]. The DMDBTDMA resin was prepared by impregnating 50% w/w ligand on Chromosorb-W. The distribution behaviour of several Ans and fission product elements such as UO2 2+ , Pu4+ , Am3+ , Eu3+ , Cs+ , and Sr2+ from nitric acid solutions were investigated. As shown in Fig. 3, the resin showed selective and efficient sorption of Am3+ , UO2 2+ and Pu4+ over fission products elements such as Cs+ and Sr2+ from moderate acidities (3–5 M HNO3 ). At 3 M HNO3 , the uptake of metal ions followed the order: Pu4+ > UO2 2+ » Am3+ ∼ Eu3+ » Sr2+ ∼ Cs+ . The possibility of using DMDBTDMA resin for sorbing trace concentra-

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Table 1 Summary of the amide based extraction chromatography resins studied for the separation of actinides and lanthanides. Ligand

Support

Studies performed

Ref

D2EHAA D2EHBA D2EHiBA DMDBTDMA DMDOHEMA TODGA

XAD-7 XAD-7 XAD-7 Chromosorb-W ® Amberchrom GC71C Chromosorb-W

TODGA TODGA TODGA

Magnetic particles Silica/Polymer ® Amberchrom

TODGA/TBP T2EHDGA

Amberchrom ® Amberchrom

C4DGA T-DGA TODGA/RTIL C4DGA/RTIL T-DGA/RTIL

Chromosorb-W Chromosorb-W Chromosorb-W Chromosorb-W Chromosorb-W

Separation of U(VI) from acidic feed solution. Better than D2EHBA and D2EHiBA resins. Separation of U(VI) from acidic feed solution. Better than D2EHiBA. Separation of U(VI) from acidic feed solution. Inferior to D2EHAA and D2EHiBA resins. Separation of actinides and lanthanides under HLW conditions. Sorption of Am(III), Cm(III), Th(IV) and U(VI). The resin was found to be inferior in its performance than TODGA resin. (i) Separation of actinides and lanthanides under HLW conditions. (ii) Sr-90/Y-90 separation for medical application. (iii) Mutual lanthanide separation. Minor actinides recovery under HLW conditions. Minor actinides recovery by MAREC process. (i) Ac-225/Th-225 separation for medical application. (ii) Reprocessing of FBR-MOX fuel by ERIX process. (iii) Distribution behaviour of 60 elements. Separation of actinides and lanthanides under HLW conditions. (i) Separation of Ra/Ac for medical application. (ii) Separation of minor actinides, and Ra/Ac. Separation of minor actinides from moderate acidic feed conditions. Separation of minor actinides from moderate acidic feed conditions. Separation of actinides and lanthanides. Separation of actinides and lanthanides. Separation of actinides and lanthanides.

[80] [80] [80] [77] [81] [83] [98] [100] [85] [90] [95–97] [87] [99] [86] [101,102] [82] [103] [103] [134] [135] [135]

®

Fig. 2. Distribution behaviour of UO2 2+ with increasing concentration of amides on (A) XAD-7, and (B) XAD-4. Extractant loading: 40% w/w/Legends: : D2EHAA, 䊉: D2EHBA, 䊏: D2EHiBA. Reproduced with permission from Ref. [80]

tions of Am3+ in the presence of relatively large amounts of Nd3+ and UO2 2+ was also studied [77]. The results indicated that the presence of macro concentrations of Nd3+ , UO2 2+ and Fe3+ in the feed affected the Kd values of Am3+ significantly. The presence of 2 M NaNO3 , on the other hand, enhanced the Kd values in a lower acidity range (1–4 M HNO3 ), while an opposite effect was observed at higher acidities (5–6 M HNO3 ). Stripping of the metal ions (Am, U and Pu) from the loaded column was investigated employing 0.01 M HNO3 , 0.05 M oxalic acid and 0.1 M sodium carbonate. As shown in Fig. 4, stripping of Am3+ was most efficient with oxalic acid as quantitative stripping was accomplished in ∼3.0 bed volumes. With 0.01 M HNO3 , on the other hand, near quantitative recovery was possible with ∼4.5 bed volumes, but a tailing effect was clearly noticed. The stripping of UO2 2+ was equally efficient with 0.05 M oxalic acid and 0.1 M sodium carbonate where >99.9% recovery was possible in ∼4.0 bed volumes. On the other hand, quantitative stripping of Pu4+ was obtained with 0.05 M oxalic acid in ∼4.5 bed volumes of the eluent.

Hecke and Modolo [81] investigated N,N-dimethyl-N,N-dioctyl2-(2-hexylethoxyethyl) malonamide (DMDOHEMA) impregnated resin for the uptake of UO2 2+ , Th4+ , Am3+ , Cm3+ and fission products from nitric acid solutions. The sorption of metal ions from simulated Low Level Liquid Waste (LLLW) solutions was evaluated. The Kd values for UO2 2+ , Th4+ , Am3+ , Cm3+ and Eu3+ at different nitric acid concentrations are shown in Fig. 5. The sorption of tetravalent and hexavalent Ans was much better than that of trivalent Ans and Lns on DMDOHEMA resin due to higher ionic potential of the former Ans. The general sorption behaviour of Ans on DMDOHEMA resin was similar to that observed with DMDBTDMA resin described earlier. For the Ans as well as the Lns, a gradual increase in the Kd values were observed with increasing nitric acid concentration. For the Ln series, there was a gradual decrease in the Kd values with increasing atomic number at fixed HNO3 concentration. The sorption behaviour of Ans from LLLW by the DMDOHEMA resin was comparable to those observed with tracer concentrations of U, Th, Am and Cm. The results of these experiments revealed that the Ans

6

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

Fig. 3. Variation of distribution coefficient of Am3+ with nitric acid concentration by DMDBTDMA impregnated Chromosorb-W resin. Reproduced with permission from Ref. [77].

and Lns could be separated from the bulk of other fission products present in simulated LLLW solutions with DMDOHEMA resin. 4.3. Diglycolamide impregnated resins In the recent year, diglycolamide (DGA) based extractants, viz. N,N,N’,N’-tetraoctyl diglycolamide (TODGA) and N,N,N’,N’-tetra-2ethylhexyl diglycolamide (T2EHDGA), have been studied as one of the most promising ligands for extraction of trivalent Ans from acidic feed solution [55]. Horwitz et al. made a comparative EC separation of Ans using TODGA and T2EHDGA impregnated resins and found that the performance of the two resins was compara-

Fig. 5. Distribution behaviour of Am3+ , Cm3+ , Eu3+ , UO2 2+ and Th4+ from nitric acid solution by DMDOHEMA resin. Reproduced with permission from Ref. [81].

ble [82]. A thorough comparison of TODGA, TRPO (Cyanex-923), CMPO and DMDBTDMA impregnated resins (50% loading in each case) was made for the extraction of Am3+ from varying concentration of nitric acid solution [83]. As shown in Fig. 6, the Kd of Am3+ by TODGA resin increased sharply up to 1 M HNO3 and remained steady thereafter up to 6 M HNO3 . This behaviour was similar to that of the CMPO resin [84], but was in contrast to that of DMDBTDMA resin where sorption of Am3+ increased only gradually with nitric acid concentration and reached moderate values above 3 M HNO3 . On the other hand, TRPO resin exhibited reasonably good

Fig. 4. Elution profiles of Am3+ , UO2 2+ and Pu4+ from the DMDBTDMA resin column. Reproduced with permission from Ref. [77].

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

4

10

3

Kd, Am(III)

10

TODGA CMPO Cyanex-923 DMDBTDMA

2

10

1

10

0

10

0

1

2

3

4

5

6

[HNO3], M Fig. 6. Distribution coefficient (Kd ) of Am3+ as a function of HNO3 concentration by different extractant sorbed on Chromosorb-W. Reproduced with permission from Ref. [83].

Kd values only at a lower acidity (Kd = 140 at 0.5 M HNO3 ). The Kd values of Am3+ by different resins at 3 M HNO3 followed the order: 7200 (TODGA) > 2000 (CMPO) > 35 (DMDBTDMA) > 3 (TRPO). The performance of the TODGA resin was also compared with that of DMDOHEMA resin prepared by impregnating the two ligands ® on Amberchrom CG-71C [81]. The study revealed 3–4 orders of magnitude higher uptake of trivalent Ans by the TODGA resin as compared to the DMDOHEMA resin. On the other hand, the uptake of Mo(VI), Pd2+ and Zr4+ was relatively higher for the DMDOHEMA resin. Nevertheless, the sorption of Zr4+ and Mo(VI) were successfully masked by the addition of oxalic acid in the feed solution. Similarly, the extraction of Pd2+ was suppressed by its selective complexation with HEDTA in the feed solution without affecting the extraction of Ans/Lns. The sorption profiles of several metal ions such as Am3+ , Eu3+ , 4+ Pu , UO2 2+ , Fe3+ , Sr2+ and Cs+ by TODGA impregnated resin were obtained from a series of nitric acid solution [83]. The order of Kd values for An ions was similar to their DM values observed in solvent extraction studies, i.e. An3+ ∼ An4+ > AnO2 2+ . The Kd values for Cs+ and Fe3+ were less than 0.5 mL/g in the entire range of acidity investigated, suggesting insignificant sorption of these metal ions. An EC resin prepared by impregnating TODGA onto magnetic particles (as the inert support) indicated an identical result that was obtained by impregnating TODGA on Chromosorb-W [85], suggesting that the role played by the extractant is far more dominating than that by the support material. The advantages of such resins, however, includes the fact that these resins can be added directly to the bulk of the solution and can be separated by magnetic field after sorption of the metal ions, leaving behind the lean aqueous stream. To evaluate the process applicability of TODGA resin, the effect of some of the selected metal ions, viz. UO2 2+ and Nd3+ on the sorption of Am3+ was studied [83]. The sorption of Am3+ was not affected even in the presence of 20 g/L of uranium. However, the Kd value decreased sharply in the presence of Nd3+ suggesting a strong com-

7

petition between Nd3+ and Am3+ . In the column studies, 10 mg of europium could be loaded without any breakthrough of 241 Am. On the other hand, when a solution containing 20 g/L of U, spiked with 241 Am tracer (at 3 M HNO ), was passed through the column, no 3 breakthrough for 241 Am was observed even after passing 100 mL of the feed solution. This result indicated that UO2 2+ did not compete with Am3+ . In a similar work, an EC resin prepared by impregnating ® 30% TODGA + 10% TBP on Amberchrom CG-71C suggested that all the Ans and Lns could be separated from the other constituents of SHLW in the column operation [86]. Hoshi et al. [87] used TODGA impregnated resin for the development of flow sheet for ERIX (Electrolytic Reduction and Ion Exchange) process for the reprocessing of FBR-MOX fuel dissolved in 3 M HNO3 . The flow sheet for the ERIX process is given in Fig. 7. The process consists of three stages, viz. pretreatment, main process and finally, minor An separation process with a combination of TODGA/BTP column for the recovery of Ans and Lns separately. Zhang et al. [88–93] proposed TODGA impregnated resin in MAREC (Minor Actinide Recovery by Extraction Chromatography) process. They have extensively studied the radiolytic and hydrolytic stability of the resin under HLW conditions. Horwitz et al. [94] demonstrated the synergistic enhancement for the uptake of trivalent Ans/Lns by TODGA resin in the presence of carrier trivalent metal ions from HCl medium (viz. Fe, Ga, In, Tl which tend to form anionic species in HCl medium). They observed that the synergistically extracted species contained a trivalent An or a Ln complexed by three TODGA molecules with three anionic metal chloride complex ions balancing the charge. They succeeded in the separation of Am3+ and Pu4+ in tracer quantity from the bulk of soil sample. After leaching the Am3+ /Pu4+ from the soil sample by 3 M HCl, the leached solution was passed through a TODGA column where all the impurities were washed out except Am and Pu. Significant amount of Fe was also held onto the column which was washed out with 3 M HNO3 . Finally, Am3+ /Pu4+ were recovered with a mixture of 0.03 M oxalic acid and 0.25 M HCl (20 mL). More than 95% recovery of Am3+ /Pu4+ was possible with concentration factor of 25 from 450 mL of leached solution. Horwitz et al. [95,96] also patented the procedure for the separation of nuclear medicine grade 225 Ac from 229 Th target using TODGA coated resin column. Radchenco et al. [97] demonstrated the separation of Ac from an irradiated Th target by the TODGA-resin column. The irradiated Th target was dissolved in acid, and all Th was converted into anionic form and separated from Ac3+ through an ion exchange column. Finally, the Ac3+ , contaminated from the other cations such as Ra3+ , Ba2+ and Ln3+ , was separated through the TODGA column. Dutta et al. [98] reported an efficient EC based separation method using TODGA loaded Chromosorb-W as the resin for the separation of carrier free 90 Y from 90 Sr. This separation method used EDTA as the eluent and was found to be superior to that reported for similar separations by Horwitz et al. [82]. Pourmand et al. [99] determined the Kd values of sixty elements by TODGA resin. (Fig. 8). They also reported the elution behavior of 32 elements with the TODGAresin column. TODGA resin was found to be highly versatile with immense potential for matrix-column. TODGA resin was found to be highly versatile with immense potential for matrix-analyte separation for high-precision elemental and isotope analysis of terrestrial and extra-terrestrial materials. TODGA resins and columns are now available commercially from Eichrom Technologies, Inc. USA. The mutual separation of Lns was attempted by EC technique using TODGA as the stationary phase [100]. Lns reflected strong sorption onto the TODGA resin at acidity >2 M HNO3 . However, the Kd values of Lns differed significantly at lower acidity (0.1 M HNO3 ). The Kd values increased with the increasing ionic potential of the Ln ions, and the separation factor between La3+ and Lu3+ was found

8

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

Fig. 7. Advanced reprocessing process (ERIX process). Reproduced with permission from Ref. [87].

Fig. 8. Distribution coefficients (Kd ) of 58 elements on TODGA resin in logarithmic scale as a function of HCl concentration. Reproduced with permission from Ref. [99].

to be in the excess of 170 (Fig. 9). In the column studies, the breakthrough for La3+ and Lu3+ was observed after passing 1.5 mL and 4.5 mL of the effluent solutions, respectively. However, the mutual separation of all the Lns was not possible with TODGA resin col-

umn. Nevertheless, lower Lns like La3+ could be separated from the higher ones like Lu3+ in good purity. In contrast to the linear alkyl chain derivatives of DGA (for example, TODGA), the EC studies with branched alkyl chain DGA, viz. T2EHDGA are limited and only few studies have been reported in

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

4

10

Yb Ho

Table 2 Distribution coefficient (Kd ) values of metal ions by T-DGA and C4-DGA resins; Aqueous phase: 3 M HNO3 [103].

Lu

Er

Metal Ions

Tb 3+

Am Eu3+ Pu4+ UO2 2+ Sr2+ Cs+

Kd-Metals

Eu 3

10

Pm

a

Pr 2

10

La

2.8

2.9

3.0

3.1

3.2

3.3

3.4

3.5

9

3.6

Ionic potential of Ln(III) Fig. 9. Distribution coefficient (Kd ) of Ln3+ ions as a function of ionic potential by TODGA/Chromosorb-W resin; Aqueous phase: 0.1 M HNO3 . Reproduced with permission from Ref. [100].

Kd Values, mL/g at 3 M HNO3 T-DGA

C4-DGA

TODGAa

9939 ± 340 8335 ± 410 4757 ± 138 9.8 ± 0.50 2.6 ± 0.21 0.93 ± 0.52

6985 ± 253 8330 ± 380 5305 ± 168 0.50 ± 0.31 1.13± 0.11 0.87 ± 0.15

7212 9018 5130 110 76 0.25

Distribution coefficient values by 47% w/w TODGA on Chromosorb-W; Ref. [83].

order of magnitude as compared to the resin containing TODGA under comparable extraction conditions. EC resin materials were prepared by coating these ligands on Chromosorb-W after dissolving them in n-dodecane [103]. These resins were used for the removal of hazardous An ions like Am3+ , Pu4+ and UO2 2+ from acidic feed solutions and the results are listed in Table 2. At any acidity, the Kd values of Am3+ were higher with T-DGA resin as compared to C4-DGA resin. This behaviour was explained on the basis of steric effects where C4-DGA with four DGA pendants requires more energy to be spent for bringing the co-ordination sites favourable for complexation. Whereas, the extraction of trivalent and tetravalent Ans were very high, the hexavalent uranyl ion was poorly extracted. 5. Extraction chromatography with chemically bonded resins As the name indicates, these EC materials are prepared by chemical bonding of the ligating moieties onto a suitable solid support and are commonly known as grafted resins. The major advantages of these resins, in comparison to the impregnated resins, are their excellent stability against leakage of the ligands. Since, the ligands are covalently bonded with the solid support, the support can be washed and cleaned with organic solvents such as acetone and methanol. A list of amide grafted resins and their proposed applications for the separation of Ans and Lns are given in Table 3.

Fig. 10. Typical chromatogram for the separation of Ra-225/Ac-225 on T2EHDGA resin. After loading Ra-225/Ac-225 at 4 M HNO3 , the column is washed with 4 M HNO3 to remove Ra2+ and other impurities. Ac3+ is stripped with 0.05 M HNO3 . Reproduced with permission from Ref. [101]

the literature [82,101,102]. The performance of T2EHDGA resin was shown to be similar to that of TODGA resin for separation of Ans and Lns under identical conditions [82]. T2EHDGA resin has been shown to offer a new possibility for rapid, robust, and effective separation of 225 Ra from 225 Ac for application in targeted cancer therapy [101,102]. The resin showed a strong retention of Ac3+ from 6 to 7 M HNO3 and efficient stripping of Ac3+ in dilute nitric acid (Fig. 10), which can be loaded on a cation exchange column for production of 225 Ac/213 Bi generator [102]. Consequently, the use of T2EHDGA was implemented into the routine procedure used at ITU, Karlsruhe, for the production of 225 Ac/213 Bi radionuclide generators. 4.4. Multi-podant DGA ligands in extraction chromatography Multiple-DGA functionalized ligands are reported recently to be significantly superior to normal DGA ligands such as TODGA [64–71]. Two such ligands, viz. diglycolamide-functionalized calix[4]arene (C4DGA) and tripodal diglycolamide (T-DGA) (Fig. 1) resulted in enhancement in metal ion extraction by more than one

5.1. Monoamide grafted resins The EC studies with monoamide grafted resins for separation of Ans and Lns are limited [104,105]. Most of the studies with grafted resins have been reported with acidic functional groups for the separation of metal ions for environmental analytical purposes, thus are not the scope of this article. An EC material was synthesized by grafting 4-ethoxy-N,N-dihexyl-butanamide (EDHBA) on a chloromethylated polymer support for selective separation of UO2 2+ , Th4+ , La3+ and Nd3+ from highly acidic matrices [104]. The synthesis scheme of this resin is shown in Table 3. The grafted resin was found to be robust under various acidic conditions whose extractability was constant up to 20 cycles of repeated use. The developed amide grafted resin showed good uptake of Ans and Lns even at high acidities (6 M HNO3 ), with the maximum loading capacity being 0.36, 0.69, 0.32 and 0.42 mmol/g for UO2 2+ , Th4+ , La3+ and Nd3+ , respectively. The practical utility of this grafted resin was tested for the selective extraction and sequential separation of Ans and Lns using a synthetic nuclear waste mixture of fission products and common metal ions in >103 fold concentrations to that of the analyte ions [104]. The sample solution was loaded onto the resin packed column and the metal ions were separated by a sequential elution method. In this process, the An ions were first eluted using 100 mL of water followed by the recovery of the Ln ions with 20 mL of 0.01 M EDTA. Analysis of the results indicated 99.8% recovery of

10

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

Table 3 Summary of the amide grafted resins for the separation of actinides and lanthanides. Ligand [Ref.]

Support

Proposed application

EDHBA [104]

Polystyrene/DVB (Merrifield resin)

(i) Sequential separation of lanthanides and actinides with different solvent. (ii) Analytical separation of uranium over thorium.

ADHA [105]

Polystyrene/DVB (XAD-16)

Select separation of uranium over thorium for analytical applications.

DB2EHM [106]

Polystyrene/DVB (Merrifield resin)

Select separation of uranium over thorium for analytical applications.

DMDBMA [107,108]

Polystyrene/DVB (Merrifield resin)

Separation of Am/Pu/U from their mixture or separation of Am/Pu/Th from their mixture.

BenzoDODA [109]

Polystyrene/DVB (Merrifield resin)

Selective recovery of plutonium over other metal ions from acidic feed solution.

DGA [110,111]

Silica

Recovery of minor actinides over fission product elements under HLW conditions.

DGA [112,113]

Silica

Separation of Lanthanides

Synthesis scheme

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

Fig. 11. Acid dependency on metal ion extraction by ADHA functionalized resin. Reproduced with permission from Ref. [105]

Ans. A column loaded with the grafted resin was used for the separation of Th4+ from monazite sand sample. The amount of thorium recovered from the monazite ore was estimated to be 79.8 mg/g of ore, which was in close agreement with the certified values of 81.0 mg/g of ore. A similar grafted resin was prepared by chemical anchoring of p-amino-N,N-dihexyl acetamide (ADHA) on Amberlite XAD-16 support [105]. This resin showed greater selectivity for UO2 2+ over Th4+ and other An ions from highly acidic matrices and the material was proposed for an efficient separation of U/Th (Fig. 11). At 3 M HNO3 , the separation factor of UO2 2+ over Th4+ was 8.03. Moreover, the resin showed non-extractive behaviour for all transition metal ions along various diverse ions that are commonly present in nuclear spent fuel solutions. The sorption capacity for uranium was 0.69 mmol/g. The practical utility for the selective separation of UO2 2+ from Th4+ and rare earth ions was tested using a synthetic low-level nuclear reprocessing mixture. One litre of the waste solution was passed through the resin loaded column and UO2 2+ was recovered from the column using 15 mL of 0.1 M (NH4 )2 CO3 leading to >99% recovery. 5.2. Malonamide grafted resins A chelating polymeric sorbent was prepared by chemical functionalization of di-bis(2-ethylhexyl)malonamide (DB2EHM) ligand on Merrifield chloromethylated resin [106]. The synthesis scheme is shown in Table 3. The grafted sorbent showed superior binding affinity for UO2 2+ over Th4+ and other metal ions, even under high acidic conditions. Various physico-chemical parameters, like solution acidity, extraction kinetics, metal sorption capacity, electrolyte tolerance studies, etc., influencing the resin’s metal ion extractive behavior were studied by both the static and the dynamic methods. The influence of solution acidity on metal ion extraction was studied using varying concentrations of HNO3 and HCl and the results are shown in Fig. 12. It is evident from the figure that both UO2 2+ and Th4+ uptake showed a positive dependence with increasing acidity. However, the resin matrix showed a greater affinity towards UO2 2+ over Th4+ which was best achieved at ∼3 M HNO3 . The extraction kinetic studies revealed that a time duration of <15 min was sufficient for >99.5% metal ion extraction. On the other hand, similar studies preformed for Ln ions showed rather low uptake values (Kd < 50) with about 60 min required to get equilibrium Kd values. The metal ion loading capacities for UO2 2+ and Th4+

11

Fig. 12. Acid dependence metal ion uptake by DB2EHM functionalized resin. Reproduced with permission from Ref. [106]

at 5 M HNO3 were found to be 62.5 and 38.2 mg/g, respectively. The results also suggested the affinity of the diamide anchored polymer towards UO2 2+ and Th4+ over other metal ions, thereby proving its application in nuclear reprocessing plants. Metal ion desorption was effective with 1 M (NH4 )2 CO3 or 0.5 M 2-hydroxy-isobutyric acid (␣-HIBA). The column studies performed at 5 M acidity was found to give enrichment factor values of 310 and 250 for UO2 2+ and Th4+ ions, respectively. The application of the resin was demonstrated for the recovery of Th from monazite ore. A chelating polymeric material containing N,N -dimethyl-N,N dibutyl malonamide (DMDBMA) anchored with chloromethylated polystyrene-divinyl benzene polymer was successfully synthesized for the solid phase extraction of Ans [107,108]. The synthesis scheme is shown in Table 3. Sorption kinetics studies showed that <20 min was sufficient for >99.99% adsorption of Th4+ and UO2 2+ . The metal ion sorption kinetics followed the Lagergren pseudo-first order rate kinetics with sorption rate constant of 0.421 ± 0.015 and 0.348 ± 0.011 per min for Th4+ and UO2 2+ , respectively. The metal ion sorption capacity at 3 M HNO3 was found to be 18.78 ± 1.53 mg/g and 15.74 ± 1.59 mg/g for U and Th, respectively. The distribution behaviour of Am3+ , Pu3+ , Th4+ , Pu4+ and UO2 2+ onto DMDBMA grafted resin was investigated from varying concentrations of HNO3 [108]. As shown in Fig. 13, the Kd value of all the metal ions increased gradually with nitric acid concentration up to 4 M HNO3 . However, at higher acidities, there was a decrease in the free ligand concentration due to the formation of malonamideHNO3 adduct, thereby giving a near constant Kd values at higher acidities. Higher Kd values for tetravalent metal ions, viz. Th4+ and Pu4+ was attributed to their stronger complexation with the bidentate malonamide molecule due to higher charge density on the metal ions. On the other hand, trivalent Am and Pu shows significant distribution coefficient only above 3 M HNO3 . At 1 M HNO3 , the Kd values for Am3+ and Pu3+ ions were 4.9 and 7.4 mL/g, respectively, suggesting insignificant sorption of these metal ions at low acidities. The critical analysis of Fig. 13 suggested the possible analytical application of this resin for the separation of Am, Pu and U or Am, Th and Pu from each other at lower acidic region. The Kd values and the selectivity factors of these metal ions at 1 M HNO3 are listed in Table 4. Relatively good selectivity factor for Am3+ /Pu4+ , Am3+ /UO2 2+ and Pu3+ /UO2 2+ indicated the analytical separation of these metal ions. Desorption studies on U, Pu and Am from the loaded resin were conducted employing various stripping solutions such as 1 M HNO3 ,

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

d

K (mL/g)

12

10

3

10

2

10

1

10

0

10

Am(III) U(VI) Pu(IV) Pu(III) Th(IV)

-1

0

1

2

3

4

5

6

[HNO3], M Fig. 13. Distribution coefficient of actinide ions by malonamide grafted resin from varying concentration HNO3 . Reproduced with permission from Ref. [108]. Table 4 Selectivity factor for actinide ions by DMDBMA grafted resin; Aqueous phase: 1 M HNO3 [108]. Meta1 ions 3+

Am Pu3+ Pu4+ Th4+ UO2 2+

Kd (mL/g) 4.9 7.4 158 66 75

Metal ion pair 4+

3+

Pu /Am UO2 2+ /Am3+ UO2 2+ /Pu3+ Th4+ /Am3+ Th4+ /Pu3+

Fig. 14. Elution profile of Am, Pu and U from the malonamide grafted resin column. Reproduced with permission from Ref. [108].

the resin was tested in the column mode. The loading and elution curves for Pu4+ on the resin are given in Fig. 15. The feed solution for this experiment was 50 mg/L Pu4+ in 1 M HNO3 . The breakthrough for Pu4+ was observed after passing ∼680 mL of feed solution suggesting the capacity of 34 mg of Pu/g of resin. Finally, the loaded Pu4+ could be quantitatively eluted using 300 mL of 0.3 M HAN in 0.3 M HNO3 .

Selectivity factor 32.2 15.3 10.1 13.5 8.9

0.01 M EDTA, 0.25 M Na2 CO3 , 0.25 M oxalic acid, 0.1 M ␣-HIBA and a reducing mixture, viz. 0.2 M HAN + 0.2 M HN (used for the back extraction of Pu4+ ion) [108]. The maximum desorption of uranium was observed with Na2 CO3 due to strong complexation of carbonate ions with uranyl ions. However, comparable desorption yields were obtained using ␣-HIBA as well. Similarly, Pu was effectively desorbed with 0.25 M oxalic acid. Comparable desorption data were obtained when Pu4+ was reduced to the +3 state using the reducing mixture of hydroxyl amine nitrate (HAN) and hydrazine (HN) at 1 M HNO3 . On the other hand, though Am was only partially sorbed on the grafted resin, it could be quantitatively desorbed with 0.01 M EDTA solution. A synthetic mixture of known quantity of 241 Am, 233 U and 239 Pu (as Pu4+ ) at 1 M HNO was loaded onto a column 3 containing the grafted resin. While Am was eluted with 1 M HNO3 , Pu was quantitatively eluted with a reducing mixture of 0.2 M HAN and 0.2 M HN at 1 M HNO3 as Pu3+ and U was quantitatively eluted with 0.1 M ␣-HIBA (Fig. 14). The purity of the eluted fraction was determined by alpha spectrometry and found that the Am fraction was fairly pure with <0.01% contamination of U and Pu. However, it was noticed that the Pu fraction was contaminated with 6% of Am and <0.1% of U. Similarly, U fraction was contaminated with 4% of Am as well as 2% of Pu. Very recently, a new grafted resin (Fig. 15) was prepared by functionalizing benzodioxodiamide (BenzoDODA) on styrene divinyl benzene polymer matrix for selective recovery of Pu4+ from nitric acid medium over other metal ions of HLW [109]. Sorption of Pu4+ was found to decrease with the increase in nitric acid concentration, with very small sorption above 7 M HNO3 . After obtaining various sorption parameters of the resin in the batch mode, performance of

5.3. Diglycolamide grafted resins Two DGA grafted silica beads were prepared by chemically anchoring either one (DGAS-1) or two DGA moieties (DGAS-2) (Table 3) on the silica substrate [110,111]. The materials were used for the sorption of trivalent Ans and Lns from nitric acid solutions. As shown in Fig. 16, at any given acidity, the Kd values of Am3+ by DGAS-2 was higher as compared to DGAS-1. This feature was ascribed to the higher functional groups available on DGAS2. The Kd values of Am3+ between 1 and 6 M HNO3 were >100. This feature is an indication that these grafted silica substrates can be utilized for the removal of Am3+ from a wide range of acidic solutions. Similarly, very low Kd values at lower acidities (<0.1 M HNO3 ), indicated that the sorbed metal ions could be easily desorbed with dilute acid solution. The maximum sorption capacities of Eu3+ were 10.44 ± 1.13 mg/g and 13.92 ± 1.30 mg/g of DGAS-1 and DGAS-2, respectively. The sorption of Eu3+ on the grafted silica followed Langmuir monolayer sorption phenomenon with sorption energy of 16.51 ± 2.05 and 17.79 ± 1.63 kJ/mol for DGAS-1 and DGAS-2, respectively. After demonstrating the possible sorption of Am3+ on the DGA grafted silica substrate, an attempt was made to investigate the metal ion loading capacity in the column mode with a feed solution of 1 g/L Eu(NO3 )3 in 3 M HNO3 spiked with 152,154 Eu tracer [111]. For DGAS-2, the breakthrough for Eu3+ was observed after passing 10 mL of the feed solution, suggesting the column capacity of 10 mg of Eu (Fig. 17). On the other hand, only 6 mg of Eu3+ could be loaded on DGAS-1 column before any breakthrough. Elution of the loaded Eu3+ was efficiently achieved with 6 mL of 0.01 M EDTA from both the columns. On the other hand, elution of the metal ions with distilled water was poor and a long tailing was observed. Florek et al. [112,113] carried out controlled grafting of DGA on mesoporous silica support for separation of Lns. They found that these highly porous DGA grafted silica were excellent for the sorption of Lns, and the material could be recycled in five successive

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

13

Fig. 15. (a) BenzoDODA grafted resin, and (b) Loading and elution profile for Pu4+ on the BenzoDODA grafted resin column. Column dimension: 15 cm × 1 cm; Bed volume: 2 mL containing 1 g resin; feed: 50 mg/L Pu4+ at 1 M HNO3 ; Eluent: 0.3 M HAN at 0.3 M HNO3 . Reproduced with permission from Ref. [109].

4

4

5x10

10

DGAS-1 DGAS-2 4

4x10 3

10

4

cpm / mL

Kd, mL.g

-1

3x10

DGAS-1 DGAS-2

4

2x10

2

10

4

1x10

0

1

10

0

1

2

3

4

5

6

0

5

[H NO 3 ], M Fig. 16. Distribution coefficient of Am3+ as a function of HNO3 concentration by grafted silica substrate. Reproduced with permission from Ref. [111].

cycles without any loss in its sorption properties. Authors also synthesized a series of ligand grafted materials by tuning the bite angle of the ligand coordination sites [113]. It has been shown that by tuning the ligand bite angle and its environment, it was possible to improve the selectivity towards specific rare earth elements. 6. RTIL based extraction chromatography Room temperature ionic liquids (RTIL) are being studied as alternate diluents to the conventional molecular diluents, and the metal ion extractions in RTILs have been shown to improve spectacularly [114–124]. Recent solvent extraction studies employing TODGA, C4DGA, and T-DGA for the extraction of Am3+ were highly encouraging as the metal ion extraction increased manifold in RTIL medium [125–130]. Furthermore, EC resins containing RTIL have shown to enhance metal ion extraction to a very large extent, though there were stripping issues [131–135]. In this section, results of the studies with EC resins containing TODGA, T-DGA and C4DGA in a RTIL, namely 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C4 mim·Tf2 N), have been summarized.

10

15

20

25

30

Volume passed, mL Fig. 17. Loading and elution curve for Eu3+ on grafted silica columns; For DGAS-1 column, passing solutions are: 15 mL feed, 3 mL washings (3 M HNO3 ) and 15 mL eluent (0.01 M EDTA); For DGAS-2 column: 15 mL feed, 3 mL washings (3 M HNO3 ) and 15 mL eluent (0.01 M EDTA). Column dimension: 5.5 cm × 0.4 cm; Bed volume: 0.82 mL containing 0.3 g resins; feed: 1 g/L Eu(NO3 )3 at 3 M HNO3 . Reproduced with permission from Ref. [111]

6.1. TODGA/RTIL resin A series of EC resins were prepared by varying compositions of TODGA and RTIL (C4 mim·NTF2 ) impregnated on Chromosorb-W [134]. In the first series, TODGA composition varied between 10 and 30% w/w at fixed (10% w/w) RTIL, while in the second series, the RTIL composition varied between 10 and 30% w/w at a fixed (10% w/w) TODGA concentration (Table 5). In another series, TODGA and RTIL compositions varied together from 10 to 30% w/w. This exercise was done to investigate the effect of ligand and RTIL on the extraction of Ans. Interestingly, no change in the extraction behaviour of resin was observed with all these resins in a wide range of acidity (0.01 M–6 M HNO3 ). The Kd values of Am3+ were in the range of 1 × 104 to 2.5 × 104 mL/g for all the resins at all acidities. The Kd values were distinctly different from those obtained with the resin prepared by pure TODGA impregnated on ChromosorbW [83], where the Kd value of Am3+ increased from 0.01 M HNO3 (Kd < 1) to 6 M HNO3 (Kd = 7500). Additionally, the Kd values of Am3+

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28

26 TODGA % with 10% RTIL RTIL % with 10% TODGA TODGA % + RTIL %

22

26

20 18 16 14 12

22 20 18 16 14 12

10

10

8

8 10

15

20

25

Experimental Theoretical

24

Eu(III) loading (mg/g)

Eu(III) loading (mg / g)

24

10

30

15

20

25

30

TODGA (%)

TODGA / RTIL (%)

Fig. 18. (Left) Loading of Eu3+ on TODGA-RTIL resins; (Right) Theoretical and experimental (actual) loading of Eu3+ on TODGA-RTIL resins; Aqueous phase: 3 M HNO3 . Reproduced with permission from Ref. [134]

Table 5 Composition of TODGA-RTIL resins prepared in various composition [134]. Resin

Weight ratio (L: RTIL: Solid)

TODGA (% w/w)

RTIL (% w/w)

Kd −Am at 0.01 M HNO3

Kd −Am at 3 M HNO3

R1 R2 R3 R4 R5 R6 R7

0.2 g: 0.2 g: 1.6 g 0.4 g: 0.4 g: 1.2 g 0.75 g: 0.75 g: 1.0 g 0.2 g: 0.4 g: 1.4 g 0.2 g: 0.6 g: 1.2 g 0.4 g: 0.2 g: 1.4 g 0.6 g: 0.2 g: 1.2 g

10 20 30 10 10 20 30

10 20 30 20 30 10 10

20770 14310 15870 17315 15930 12285 12895

15785 17170 19780 11540 17465 11085 17850

were lower with pure TODGA resin even with 47% w/w of ligand loading on the solid support. High Kd values of Am3+ at lower acidity (0.01 M HNO3 ) with TODGA/RTIL resins was an indication of ion exchange mechanism, and indicated difficulty in the stripping of the metal ions from the loaded resin. However, the sorbed metal ion was efficiently eluted with a complexing solution (EDTA), where the Kd values of Am3+ were <1 for all the resins. The loading capacity of Eu3+ by all the TODGA-RTIL resins (R1–R7, vide Table 5) were determined at 3 M HNO3 . As shown in Fig. 18, the loading of Eu3+ increased linearly with increasing TODGA concentration (at a fixed RTIL content) from 8.74 ± 0.40 mg/g at 10% TODGA to 23.66 ± 0.17 mg/g at 30% TODGA. The loading of Eu3+ also increased linearly with simultaneous increase in the concentrations of TODGA and RTIL. However, an increase in the RTIL content at a fixed TODGA amount in the resin did not affect the loading of Eu3+ . Solvent extraction studies have indicated the stoichiometry of the extracted species of Eu3+ with TODGA as 1:3 (metal/ligand) [128]. By knowing the amount of TODGA present in the solid support, one can calculate the theoretical loading capacity of the resin. The loading of Eu3+ on 10% TODGA resin reached to saturation values (stoichiometric ratio of 1:3 metal to ligand) as calculated. However, with increased TODGA concentration of the solid support, the actual experimental loading capacity of the resin deviated from the theoretical values and the metal to ligand stoichiometry became 1:3.4 (Fig. 18). This behaviour may be ascribed to the aggregation of ligand molecules at higher concentrations. The column studies with R1 resin were performed by packing 0.3 g of resin in a glass column having a bed volume of 1.02 cm3 . Fig. 19 shows the loading and elution curves of Eu3+ by the TODGA-

RTIL resin column. The breakthrough for Eu3+ was observed after passing 6 mL of the feed solution, suggesting the column capacity of 2.0 mg of Eu, which corresponds to 6.67 mg/g of resin. The elution of the loaded Eu3+ was efficiently achieved with 5 mL of 0.01 M EDTA solution.

6.2. C4DGA/RTIL and T-DGA/RTIL resins EC resins, prepared by impregnating C4DGA and T-DGA (Fig. 1) dissolved in RTIL (C4 mim·Tf2 N) on Chromosorb-W, gave excellent results for the removal of trivalent Ans from acidic waste solutions [135]. Fig. 20 shows the Kd values of Am3+ by the two resins with varying concentrations of HNO3 . The Kd values with the T-DGA resin were very high (>3 × 104 ) and remained constant in the acid range of 0.01–4 M HNO3 . On the other hand, the Kd value with C4DGA resin decreased linearly with increased aqueous phase acidity. The distribution pattern of Am3+ on the two resins indicated a different mechanism of extraction. Whereas, an ion-exchange mechanism was followed in the case of the C4DGA resin, a solvation mechanism could be the reason for constant Kd values at acidities between 0.1 M–4 M HNO3 in case of the T-DGA resin. This behaviour is in contrast to the observation made with these two resins prepared in n-dodecane, where the Kd values of Am3+ increased gradually between 0.1 M–3 M HNO3 and remained constant thereafter [103]. As evident from Fig. 20, the Kd value of Am3+ at any acidity was higher with the T-DGA as compared to the C4DGA resin, an identical pattern in the EC studies with these ligands in n-dodecane [103]. However, the Kd values of Am3+ with both the resins in n-dodecane were several folds lower than those of the resins prepared in RTIL (Table 6). A similar effect has been shown previously in solvent

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Table 6 Distribution coefficient of metal ions (Kd ) by T-DGA and C4DGA resins. Aqueous phase: 3 M HNO3 [135]. Metal ions

Kd at 3 M HNO3

Eu3+ Am3+ Cm3+ Pu4+ UO2 2+ Sr2+ Cs+ a b

T-DGA resin

C4DGA resin

TODGA/RTILa

TODGAb

50240 ± 3510 30910 ± 1975 47590 ± 2880 37210 ± 1960 80 ± 3.8 5.5 ± 0.3 <0.01

37550 ± 2070 8940 ± 1840 9710 ± 585 34680 ± 1935 10 ± 0.4 <0.01 <0.01

18415 ± 810 1575 ± 735 – 17840 ± 815 70.5 ± 5.4 4.3 ± 2.2 0.73 ± 0.01

9018 7212 – 5130 110 76 0.25

10% TODGA + 10% RTIL on Chromosorb-W [134]. 47% w/w TODGA on Chromosorb-W [83].

4

6x10

4

5x10

Loading curve

Elution curve

Washing

4

Dead volume

CPM

4x10

4

3x10

4

2x10

4

1x10

0 0

2

4

6

8

10

12

14

16

18

20

22

Volume (mL) Fig. 19. Loading, washing and elution curve for Eu3+ on TODGA-RTIL resin (R1) column; Passing solutions: 0–10 mL feed, 10–15 mL washings (3 M HNO3 ) and 15–22 mL eluent (0.01 M EDTA). Column dimension: 8.1 cm x 0.4 cm; Bed volume: 1.02 mL containing 0.3 g resin; feed: 0.5 g/L Eu(NO3 )3 at 3 M HNO3 (spiked with 152,154 Eu tracer). Reproduced with permission from Ref. [134].

4

4x10

4

3x10

T-DGA C4DGA

4

Kd, Am(III)

2x10

stripping with dilute acid solution would not be possible. However, the sorbed metal ions could be efficiently desorbed with a complexing solution containing guanidine carbonate and EDTA buffer which was used successfully for other ionic liquid based solvent systems [136]. Reusability of the resin was evaluated by successive sorption and desorption studies with Am3+ at 1 M HNO3 . The Kd values with the T-DGA resin remained constant, within the experimental error limits. On the other hand, the C4DGA resin showed a poor reusability, and the Kd value decreased to about 40% of the original value in the second cycle itself. The decrease in the Kd value was gradual up to 95% in 5th cycle. The results indicate that the T-DGA ligand has a strong hydrophobic-hydrophobic interaction with the solid support and that it remained inside the pores of the support. It was speculated that the C4DGA ligand may be showing a relatively weaker interaction with the solid support and comes out of the pores. Fig. 21 represents the loading and elution profiles of Eu3+ by the T-DGA and C4DGA resin columns. The breakthrough for Eu3+ was observed after passing 6.5 mL of the solution with the C4DGA column, ten times less than that seen with the T-DGA column under identical conditions. The column capacity for the two resins was found to be 0.23 and 2.2 mg for the C4DGA and T-DGA resins, respectively. Since, the column was packed with 0.5 g each of the resins; the capacity in the column is worked out to be 0.46 and 4.4 mg per g of C4DGA and T-DGA resins, respectively. The maximum loading capacities, determined at equilibrium in batch studies, were 4.91 and 0.52 mg/g of T-DGA and C4-DGA resins, respectively. This indicates that only 88.5% of the C4DGA and 89.8% of the T-DGA resins could be loaded with Eu3+ in the column. After loading the column with Eu3+ , its elution was performed with a buffer solution containing 1 M guanidine carbonate + 0.05 M EDTA. As evident from Fig. 21, sharp elution peaks could be obtained. 7. Composite beads for extraction chromatography

4

1x10

3

8x10

3

6x10

0.01

0.1

1

10

[HNO3], M Fig. 20. Influence of nitric acid concentration on the distribution coefficient of Am3+ on T-DGA and C4DGA resins. Reproduced with permission from Ref. [135].

extraction where the distribution values of metal ions are many folds higher in RTIL than in a paraffinic solvent such as n-dodecane [125]. As the Kd values of Am3+ at lower acidities were very high,

EC resins prepared by physical impregnation of the ligands on the solid support, sometimes, shows poor stability due to possibility of the leaching out of the ligand from the pores of the support. In this context, resin beads prepared by encapsulating the ligand molecules in the polymeric frame gives better stability of the resin. Recently, a polysulphone (PS) based composite polymeric bead containing TODGA was prepared by phase inversion technique [137]. The beads were tested for the uptake of An ions such as Am3+ , UO2 2+ , Pu4+ , Np4+ and fission product ions such as Eu3+ and Sr2+ . The distribution behaviour of Am3+ was not affected significantly by varying the TODGA content in the bead between 2.5–10% w/w. However, kinetics of extraction reduced with increasing TODGA content in the beads, and time required to reach the equilibrium condition increased from 90 min for 2.5% TODGA to 120 min for 10% TODGA. The uptake of Am3+ in the range of 0.01–6 M HNO3 was investigated by the composite beads and the results are shown in

16

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5

1.8x10

3.0x10

T-DGA C4DGA

4

1.5x10

(b) Elution curve

5

2.0x10

CPM / mL

CPM / mL

5

2.5x10

(a) Loading curve

4

1.2x10

T-DGA C4DGA

3

9.0x10

5

1.5x10

5

3

1.0x10

3

5.0x10

6.0x10

4

3.0x10

0.0

0.0 0

10 20 30 40 50 60 70

Volume of feed (mL)

0

4

8

12

16

Volume of eluent (mL)

Fig. 21. (a) Loading curve, and (b) elution curve for Eu3+ on T-DGA and C4DGA resin columns; Loading feed solution: 50 mg/L Eu3+ in 1 M HNO3 (spiked with 152–154 Eu tracer); Eluent: 1 M guanidine carbonate + 0.05 M EDTA; Column dead volume: 2 mL for both the columns. Reproduced with permission from Ref. [135].

Kd,Am

25000

10000

10 wt% TODGA 5 wt% TODGA 2.5 wt% TODGA

5000 0

1

2

3

4

5

6

[HNO3], M Fig. 22. Plots of Am3+ ion uptake onto the composite PS beads with varying TODGA content as a function of aqueous phase HNO3 concentration. Reproduced with permission from Ref. [137].

Kd values of Am3+ could be obtained with the same beads in five successive cycle. The column studies were performed in a glass column of ∼6 mL bed volume for each type of polymer beads. With a feed solution containing 0.95 g/L Eu3+ at 3 M HNO3 , the breakthrough of Eu was seen after passing only 0.75 bed volumes. After the breakthrough, a large tailing was recorded saturation levels (100% breakthrough) were obtained at 13 mL, 15 mL and 19 mL for the beads containing 2.5%, 5% and 10% TODGA, respectively. The elution of the loaded Eu was carried out using 0.01 M EDTA at pH 3. As shown in Fig. 23, relatively sharper elution profiles were obtained with the beads containing lower fraction of TODGA due to smaller size of the beads as compared to 5% and 10% TODGA loaded beads. Yun et al. [138] prepared a new solid material, siliceous mesostructured cellular foams (MCF), containing DGA-bonded RTIL for the sorption of Lns from aqueous solution. MCF and DGAbonded RTIL were soaked in ethanol in a certain weight proportion and stirred for 2 h at room temperature, followed by air drying to get the solid product. Experiments on solid-phase extraction with DGA-RTIL–MCF were carried out for La3+ , Eu3+ and Lu3+ , representing light, middle, and heavy Lns, respectively. The results revealed that the efficiency of Ln3+ sorption by the composite MCF increased with increasing the chain length of alkyl group of imidazolium cation of the RTIL. The nature of anionic component of RTIL, on the other hand, did not affect the sorption properties. Whereas, no sorption was observed with the MCF without DGA-bonded RTIL, the sorption increased linearly with increasing the ligand content. 8. Conclusions and perspectives

Fig. 22. The Kd values were usually significantly higher (>104 ) than the TODGA impregnated resin described above. The Kd values decreased slightly in the acid concentration range of 0.01–0.5 M, the reason for which could not be explained. Beyond 0.5 M HNO3 , an increase in the Kd values was seen, similar to the behaviour seen in the case of the solvent extraction studies [53]. Finally, a decrease in the Kd value was seen beyond 4 M HNO3 which could be attributed to the competitive extraction of HNO3 by the resin beads thereby affecting the Am3+ uptake. It was interesting to note that the composite beads were reasonably stable to HNO3 and within ±3% of the

EC based separation methods are gaining popularity as viable alternatives to the commonly employed solvent extraction or ion exchange based separation methods. The key to the improved selectivity shown by the EC based solid phase extraction methods vis-à-vis those reported by the well known ion-exchange methods using commercial resins is the selective extractants used in the former for the metal ion of interest. For this reason, EC is often described as a technique that combines the selectivity of solvent extraction and the ease of operation of ion-exchange when per-

6. 2. 4. 5. 7. 1. 3. 0 0 0 0x 0 0 0 10 - x10 -3 x10 - x10 -3 x10 -3 x10 -3 x10 3 3

17

thoroughly assess the stability of the extraction chromatographic resins which should include individually the radiation stability of the extractants (impregnated or grafted) and that of the solid support. On the other hand, the EC based separations performed for analytical applications will not be susceptible to radiation dose as the amount of analytes evolved will be very low as has been the case with the DGA-resins for separation of Th/Ra/Ac/Bi.

2.5 wt% TODGA 5.0 wt% TODGA 10 wt% TODGA

Acknowledgements The authors thank Dr. P.K. Pujari, Head, Radiochemistry Division for his keen interest and constant encouragement.

References

0. 0

Eu concentration (M)

-3

S.A. Ansari, P.K. Mohapatra / J. Chromatogr. A 1499 (2017) 1–20

0

1

2

3

4

5

6

Bed volume Fig. 23. Elution profile of Eu3+ from the column filled with the composite polymeric beads containing varying TODGA content. Feed: 1 g/L Eu carrier in 3 M HNO3 . Reproduced with permission from Ref. [137]

formed in the column mode. Additionally, the quantity of ligand and organic solvent used in an EC based method is extremely low as compared to a corresponding solvent extraction based method thereby terming the EC based method as a “green” alternative. In the case of EC based solid phase extraction, though the carrier extractant plays an important role in the separation of the metal ions, the nature of the solid support also has significance by controlling the ligand hold up inside its pores. Other factors which decide the efficiency of EC based resins include particle size, surface area and the pore size of the solid support material. Long term reusability of the EC resin is limited by the slow leaching of the impregnated extractant molecules from the pores of the solid support. Grafted resins can alleviate this problem and hence, are considered more efficient compared to the impregnated EC resins. Alternatively, composite polymeric resin beads obtained by phase inversion can be used and some reports showed exceptionally good uptake values for the metal ions as compared to both impregnated as well as grafted resins. Composite polymeric beads containing extractants are good option for extraction chromatography for those ligands which are otherwise difficult to impregnate onto the solid support or difficult to graft. For example, a calix-mono-crown-6 ligand, which could not be impregnated onto several supports such as Chromosorb-W, Chromosorb-102, Chromosorb-G, XAD-4 or XAD-7 [139], could be effectively used for the preparation of composite polysulfone beads and demonstrated for the pre-concentration of radio-cesium from environmental sample in a column operation. This method can be extended to other possible ligands where preparation of impregnated resins is difficult. The future of extraction chromatographic separation methods appears to be very bright in view of the very good separation possibilities though the uptake kinetics and low throughput are two of the major concerns. The low extractant inventory makes these methods very economical and ‘green’. Though it is preferred to process lean feeds by EC based separation methods, processing of feeds containing high concentrations of metal ions is also possible though it may be slow. The MAREC process aims at carrying out direct processing of high level waste solutions [90]. However, in such cases, radiation stability of the EC based resins needs to be considered as one of the deciding factors. Therefore, while considering application of solid phase extractants to radioactive feeds, it is required to

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