Keywords

1 Introduction

Strontium (Sr) is a highly reactive alkaline earth metal. The occurrence of natural strontium is stable, but the mobility of its synthetic isotope 90Sr obtained from the nuclear fallout is one of the most harmful fission products [1, 2]. Therefore, the extraction of strontium can also be divided mainly into two parts: (1) minerals, commonly the celestite and strontianite (as shown in Fig. 1), and (2) spent fuels of nuclear fission containing 90Sr [3, 4], which has been focused primarily in this chapter.

Fig. 1
figure 1

A pictorial representation of the two major natural minerals of strontium

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2 Extraction from the Primary Minerals

Celestite or say celestine (SrSO4) is the main mineral source of strontium that commonly exists in highly gypsiferous soils. The preferential occurrence in decalcified parts of the groundmass suggests its resultant formation of calcite dissolution without getting affected by the conditions resulting in the dehydration and rehydration of gypsum minerals [3]. The carbonate mineral, strontianite (SrCO3), is another principal mineral of strontium albeit limited and sometimes mixed with celestite. Strontium predominantly as its carbonate salt is largely produced from Sr concentrates by following either black ash or direct leach process.

2.1 Extraction by a Black Ash Process

In the first process, the water-soluble species of strontium sulfide is formed via calcinating the celestite with coal fines in a rotary kiln at a higher temperature ~1100°C to expel out oxygen by forming the CO2. Thus obtained soluble sulfide of strontium is subjected to dissolve in water and filtered, where the residue contains most of the metal impurities as their insoluble sulfides. The presence of silicon and iron compounds has found to be problematic in this process as they can form water-insoluble silicates and ferrites, respectively, during the calcination process [5,6,– 7]. The filtrate is treated with soda ash or carbon dioxide to yield the precipitates of SrCO3 product. Nevertheless, this is the most widely used process, treating approximately 3 lakh tonnes annually [8]; the formation of SrCO3 during calcination causing the insoluble loss of strontium during water leaching is the major disadvantage of this process. Moreover, an energy-intensive process generating the undesirable pollution by-products such as CO2 and H2S is also not favorable from operational and environmental aspects [9].

The occurrence of calcination reactions in the black ash process can be given as:

$$ {\mathrm{SrSO}}_4+2\mathrm{C}\overset{\varDelta }{\to}\mathrm{SrS}+2{\mathrm{CO}}_2 $$
(1)
$$ {\mathrm{SrSO}}_4+3\mathrm{C}\overset{\varDelta }{\to}\mathrm{SrS}+2\mathrm{CO}+{\mathrm{CO}}_2 $$
(2)

However, the Gibbs free energy versus temperature data as shown in Fig. 2 for Eqs. (1) and (2) is thermodynamically possible at approximately 500°C; but if corroborated by the formation of CO and CO2 from carbon, it can be depicted that the occurrence of Eq. (2) is relatively more favorable than Eq. (1) at a temperature slightly below the 1200°C. Thus the formation of the calcined product according to Eq. (1) can be mainly due to often an increased temperature zone than the maintained value while dealing with carbon at high temperature.

Fig. 2
figure 2

Gibbs free energy versus temperature plot for celestite calcination with carbon

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The dissolution reaction of calcined product in water can be written as follows:

$$ \mathrm{SrS}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{SrCO}}_3+{\mathrm{H}}_2\mathrm{S} $$
(3)

Actually Eq. (3) occurs in two steps:

$$ \mathrm{Sr}\mathrm{S}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Sr}{\left(\mathrm{OH}\right)}_2+{\mathrm{H}}_2\mathrm{S} $$
(3a)
$$ \mathrm{Sr}{\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{SrCO}}_3+{\mathrm{H}}_2\mathrm{O} $$
(3b)

Therefore, it has been found that the progress of Eq. (3) mainly depends on the solubility of Sr(OH)2 by easier decomposition of SrS into hot water (as Eq. 3). The use of CO2 instead of Na2CO3 and the introduction of oxygen in carbonation column can increase the yield and purity of the product within a shorter carbonation time [10].

2.2 Extraction by Direct Leach Process

Using the direct leach process in the commercial production of SrCO3, the beneficiation of celestite simply involves crushing, sizing, acid washing (for dissolving impurities of CaCO3, BaCO3, and iron oxide), and sometimes flotation to obtain the concentrates that contain more than 90% SrSO4 [5]. A typical example of celestite beneficiation is given in Table 1 [11]. Thereafter the concentrates are directly leached in carbonate solution using a mechanically stirred tank at moderate temperatures (90–95°C) to yield the SrCO3.

Table 1 A typical composition of celestite mineral and concentrate after beneficiation works
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Depending upon the celestite grade and the application, SrCO3 can be used directly or subjected to further purification [6, 11, 12]. Dissolution in HCl followed by pH neutralization of the solution is carried out for removal of iron impurity as its precipitate along with silica. After filtration, the SrCl2 solution is subjected to another precipitation with soda ash to yield SrCO3 of high purity [7]. Alternatively, the impure SrCO3 is calcined at 1400°C to obtain SrO, which is then dissolved in water to separate SiO2, Fe2O3, Ca(OH)2, and Al2O3. The filtered solution of Sr(OH)2 is treated with carbon dioxide to yield the SrCO3 product. In the latter process of purification, the barium carbonate with a little tendency to decompose during calcination can also be eliminated. However, the direct leach process yields a relatively impure product (~95% SrCO3) as compared to the black ash process; it has been considered the simpler process of low cost with by-products of commercial interest [6, 11]. The reactions involved in a direct conversion of celestite to carbonate species followed by purification steps can be given as below:

For leaching:

$$ {\mathrm{SrSO}}_4+{\mathrm{Na}}_2{\mathrm{CO}}_3\to {\mathrm{SrCO}}_3+{\mathrm{Na}}_2{\mathrm{SO}}_4 $$
(4)

For purification via acid treatment:

$$ {\mathrm{SrCO}}_3+2\mathrm{HCl}\to {\mathrm{SrCl}}_2+{\mathrm{H}}_2{\mathrm{CO}}_3 $$
(5a)
$$ {\mathrm{SrCl}}_2+{\mathrm{Na}}_2{\mathrm{CO}}_3\to {\mathrm{SrCO}}_3+2\mathrm{NaCl} $$
(5b)

For purification via calcination:

$$ {\mathrm{SrCO}}_3\overset{\varDelta }{\to}\mathrm{SrO}+{\mathrm{CO}}_2 $$
(6a)
$$ \mathrm{Sr}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Sr}{\left(\mathrm{OH}\right)}_2 $$
(6b)
$$ \mathrm{Sr}{\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{SrCO}}_3+{\mathrm{H}}_2\mathrm{O} $$
(6c)

Further, the strontium metal can be produced either by the thermal reduction of strontium oxide and aluminum metal and distillation of elemental strontium on a cooled plate or by the electrolysis of a fused bath of SrCl2 and ammonium/potassium chloride following the reactions [8, 13]:

$$ {\mathrm{Sr}}^{2+}+2{e}^{-}\to \mathrm{Sr} $$
(7a)
$$ 2{\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_2+2{e}^{-} $$
(7b)

It needs to be mentioned here that low recovery and purity are the main reasons for comparatively being the less industrial use of the direct leach process; hence, this process is widely being studied to understand the direct conversion phenomena of celestite to its high-yield product. Below some of the most influential factors are briefly discussed.

2.2.1 Aqueous Chemistry for Leaching the Strontium

A difference in solubility products between SrSO4 (3.2 × 10−7) and SrCO3 (1.1 × 10−10) is the main driving force for the leaching reaction of Eq. (4) [14]; hence, their apparent solubility in terms of Sr ion concentration can be expressed as:

$$ \left[{\mathrm{Sr}}^{2+}\right]=\frac{3.2\times {10}^{-7}}{\left[{\mathrm{SO}}_4^{2-}\right]} $$
(8a)
$$ \left[{\mathrm{Sr}}^{2+}\right]=\frac{1.1\times {10}^{-10}}{\left[{\mathrm{CO}}_3^{2-}\right]} $$
(8b)

The solubility calculated for a reaction system of Na2CO3: SrSO4 from 1.0 to 1.2 (mole ratio) given in Fig. 3 indicates that regardless of the value of mole ratio, the solubility of SrSO4 is maximum in absence of SO4 2− ions and decreases with an increase in sulfate with the progress of leaching [11]. In contrast, the solubility of SrCO3 increases with consuming the CO3 2− ions. Leaching proceeds to the end of the reaction; the solubility of CO3 2− eventually becomes similar to the SO4 2− ions, which substantially decreases the driving force of leaching reaction. It can be handled by an excess of Na2CO3 in solution to keep leaching in progress.

Fig. 3
figure 3

Apparent solubility of celestite and SrCO3 as a function of sulfate species by reaction with Na2CO3 and SrSO4 at different molar ratios (Adopted with permission from Carrillo et al. [11])

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The studies by Iwai and Toguri [15] and Sutarno et al. [7] have clearly established that the stoichiometric conversion of celestite to SrCO3 (at Na2CO3: SrSO4 = 1:1) occurs only above solution pH 9.0 (as shown in Fig. 4). The influential pH for carbonate leaching has been corroborated by the dissociation constant values of H2CO3 [7, 11, 15, 16]:

Fig. 4
figure 4

Relationship between the apparent solubility of SrCO3 and SrSO4 with pH (Adopted with permission from Iwai and Toguri [15])

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$$ {\mathrm{H}}_2{\mathrm{CO}}_3\leftrightarrow {\mathrm{H}}^{+}+{\mathrm{H}\mathrm{CO}}_3^{-}\kern1em {K}_1={10}^{-6.35} $$
(9a)
$$ {\mathrm{H}\mathrm{CO}}_3^{-}\leftrightarrow {\mathrm{CO}}_3^{2-}+{\mathrm{H}}^{+}\kern1em {K}_2={10}^{-10.33} $$
(9b)

2.2.2 Parametric Influences on Direct Leach Process of Celestite

Castillejos and co-workers [16] have shown that the rate of reaction significantly increases with increasing carbonate concentration in solution. The fitted parabolic rate constants indicated the leaching is controlled by the diffusion of NaCO3 − into the pores of the thickening product layer. Notably, leaching celestite in >0.1 M Na2CO3 solution at 10 g L−1 pulp density caused a decrease in reaction rate due to a lower diffusivity with an increased dense product layer [15, 16]. It has been observed that under a stoichiometric Na2CO3 dosage and mild temperature, large conversion fractions are achievable, but not all. An entire conversion has found to be possible with a risen temperature (from 25 to 75°C) under the similar condition (as shown in Fig. 5). The calculated activation energy of 64.1 and 70.05 kJ mole−1 in different studies has revealed the transport through chemically controlled conversion reaction [17] and the effective diffusivity of carbonate ions fell in the range of 1.2 × 10−9 to 6.7 × 10−8 cm2 s−1. Studies on the effect of stirring speed and particle size have favorably demonstrated a high speed (>550 rpm) and small particle size (58 μm) follows shrinking-core model leaching is controlled by product layer reaction [16].

Fig. 5
figure 5

Rate curve for the conversion of celestite to strontium carbonate as a function of time with different temperatures (Adopted with permission from Castillejos et al. [16])

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2.2.3 Other studies

Besides the direct soda ash leach process of celestite, several alternatives have been explored either to increase the yield or purity of SrCO3 product. HCl leaching of celestite showed the activation energy of 68.8 kJ mole−1 for the process in 8.25 × 10−3 M BaCl2 solution equilibrated with 0.5 M HCl, and the reaction rate has found to be inversely proportional to the particle size which increases as 0.19, 0.70, and 0.73 powers of the Ba2+, Cl−, and H+, respectively [18]. A high purity of 98% celestite has been achieved by Dogan et al. [19], while a Turkish concentrate (~95% SrSO4) cleaning was performed with 50% H2SO4 and 1–3% HNO3 at 40–50°C. Using dry mechanochemical conversion of celestite with Na2CO3 in a planetary-type ball mill indicated the degree SrCO3 formation increased (>90%) with milling time above 30 min [20]. The milled powders leached in water at room temperature followed by washing with 1 M HCl give a desirable product of SrCO3, whereas Bingol et al. [21] used the activated SrSO4–(NH4)2CO3 mixtures to yield the conversion product of 98.1% SrCO3 [21].

3 Extraction of Radionuclide 90Sr from the HLW of Fission Products

In the processing of spent fuel of nuclear fission, the majority of heat load and radiation in the repository is mainly due to the high-level waste (HLW) of fission products. Approximately 27 tonnes of spent fuel is taken annually from the core of 1000 MWe nuclear reactor [22] that can be regarded entirely as waste (in the USA and Canada) or can be reprocessed (up to 97% in Europe and Japan). 137Cs and 90Sr with their relatively short half-lives of 30 and 28.8 years, respectively, contribute a larger heat load in HLW [23,24,25]. Therefore, the separation of 137Cs and 90Sr is potentially required to allow their management as a single product with reduced waste volume, saving repository capacity, and shortened the time for subsurface storage until they have decayed sufficiently to be disposed of as low-level waste (LLW). Additionally, the recovered 137Cs and 90Sr can be used as a radiation source in radiotherapy and micropower generator [26, 27]. Various methods (like solvent extraction, ion exchange, adsorption, precipitation, and membrane technology) have been studied and upscaled for the separation of 137Cs and 90Sr from HLW [24, 26,27,28,29,30]. Each of them has own limitations in their application, as given in Table 2 [31, 32]. In terms of stability and applicability for handling a large amount of radionuclide waste, below the prominent one, solvent extraction is being discussed in this chapter (for biosorption studies on strontium, please see Chap. 4).

Table 2 Features and limitations of each process applicable to radioactive waste treatments (After [31, 32])
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3.1 Solvent Extraction

The flexibility in stage design and regeneration of organic, convenience in combining with other processes, and easiness to be scaled up are the inherent properties of solvent extraction (SX) to gain more attention in fuel reprocessing, which in fact is invariably being used at present [33]. Selection of appropriate extractant is the vital factor of SX; hence, below the SX of 90Sr is divided on the basis of extractant used in the separation and recovery process. The compounds developed and widely used in strontium extraction have been given in Fig. 6 [30].

Fig. 6
figure 6

Various compounds used for solvent extraction of strontium extraction (Adopted with permission from Xu et al. [30])

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3.1.1 Crown Ethers

Crown ethers are highly selective compounds to interact with metal ions, which arise due to the interactions between the charged metal ion and dipole from the donor atom of crown ether [33]. The selectivity is primarily dependent on the suitability of the size of metal ions with the cavity of particular crown ether. The effect of electrostatic induction as the additional electron withdrawing or donating groups, the presence of altering heteroatoms (P, S, N, and O) within the crown ether, and types of solvents have the major role in the extraction process. Various electron donating and withdrawing substituents, including the alkyl groups (like halogen, methyl, nitro, and amine groups), have been introduced to modify the properties (like solubility and electronic property) of crown ether and are used in the extraction of 90Sr [25, 34]. For instance, 15-crown-5 (15C5) or 18-crown-6 (18C6) are strongly hydrophilic; hence, aryl or alkyl group must be added to enhance their lipophilicity for making them suitable to be used in SX, e.g., dicyclohexano-18-crown-6 (DC18C6) or dibenzo-21-crown-7 (DB21C7) [35]. The extraction of strontium from a nitrate solution by crown ethers can commonly be presented as below (where m and n are an integer value):

$$ \mathrm{Sr}{\left({\mathrm{NO}}_3\right)}_2+\overline{m\mathrm{C}n}\to \overline{\mathrm{Sr}{\left({\mathrm{NO}}_3\right)}_2\cdotp m\mathrm{C}n} $$
(10)

The rigid benzocrown ethers are effective in the extraction of monovalent cations, whereas the cyclohexano crowns are more suitable for divalent cations. The cis-syn-cis, cis-anti-cis, and trans-syn-trans isomers of DC18C6 exhibited the most efficient extractant (D Sr = 24.8) from nitric acid solution. The trans-syn-trans of DC 18C6 yields maximum extraction at a higher acidity, however, with only a 14.3 value of distribution coefficient. Using DC18C6 (0.1 M in chloroform) and DB21C7 (0.1 M in nitrobenzene with the addition of phosphomolybdic acid), the extraction of strontium and cesium could be quantitatively achieved, respectively [36]. The value of distribution coefficient decreases more with an increase in nitric acid concentration in the aqueous solution, as the D Cs and D Sr with bis-(4,4′(5′)-[1-hydroxy-2-ethylhexyl]-benzo)-18-crown-6 and bis-(4,4′(5′)-[1-hydroxyheptyl]cyclohexano)-18-crown-6 (0.02 M concentration of both crown ether in the mixture of 0.1 M nonyl-naphthalenesulfonic acid or TBP–kerosene) were only 1.6 and 1.98 from a 3 M HNO3 solution. The back-extraction of both metals required multiple stages of stripping with a dilute HNO3 solution [37,38,39]. In a similar type of studies by Dietz et al. [40, 41] with a higher acidic solution (4 M HNO3), the most efficient result (D Cs = 4) was with 0.1 M di-tert-butyl-benzo-21C7 diluted in methylpentanone and, as for the strontium, evidenced the role of dissolved water therein the diluents. The highest selectivity for cesium over sodium was with 21C7 derivatives; however, the efficient extractant was 4,4′-(5′)-di-[(1-hydroxy-ethylhexyl)-benzo]-18C6 associated with 0.2 M TBP, yielding D Cs = 30. It showed that none of the investigated compounds gave the desired combination of selectivity, efficiency, solubility, and stability. The examined dependency on nitrate ion for strontium extraction with various crown ethers revealed the contribution of two moles NO3 − and the relation with associated cations found in the order of Mg2+ > Al3+ > Na+ [42]. The extraction behavior of crown ethers from a lower 1.0 HNO3 solution to the higher 10.0 M HNO3 solution is given in Table 3.

Table 3 The behavior of Sr2+ extraction by different crown ethers from the feed aqueous solution of 1.0 and 10.0 M HNO3
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Horwitz et al. [43,44,45] used DC18C6 and its dimethyl or di-tert-butyl derivatives in various oxygenated and aliphatic diluents, for the SX of strontium from acidic solutions. The highest D Sr obtained with n-pentanol and n-hexanol could be corroborated to the metal cation extraction with the water contents combined in the diluent. Due to this, the diluent does not require the complete removal of the hydration shell of the nitrate associated with strontium for its mass transfer in the organic phase. Subsequently, the process, namely, “SREX” (stands for strontium extraction) was proposed using the di-tert-butyl-cyclohexano-18C6 diluted in 0.2 M 1-octanol from >1 M HNO3 solution. A good chemical and radiochemical stability showed by the organic constituents required three extraction stages for quantitative extraction (99.7%) of strontium in the SREX process. A trial on Na-bearing waste solution revealed that the interferences caused by the alkali and alkaline earth cations (like Na+, K+, Ca2+) did not affect the extraction efficiency of Sr2+. Nevertheless taking the di-tert-butyl-cyclohexano-18C6 (0.15 M) in 1.2 M TBP/Isopar L as (diluents), the co-extraction of strontium and lead followed by their selective stripping (using 16 centrifugal contactors) did not give the desired results. A poor selectivity and low extraction were observed due to the building up of metal cations into the organic phase as their insufficient stripping from the loaded organic after the extraction stages.

Combining the Cs extraction (CSEX) with SREX process using the bis-4,4′(5′)[(2-hydroxy-alkyl)-benzo]-18-crown-6 and bis-4,4′(5′)[(tert-butyl)cyclohexano]-18-crown-6 diluted in the mixture of 1.2 M TBP, lauryl nitrile, and Isopar-L, the process enables 99.99% cesium and strontium recovery from the waste solution containing 3.78 M HNO3, 0.486 M Al, 0.778 M Ca, 0.225 M Zr, and 0.015 M Na [46]. Lumetta et al. [47, 48] used di-tert-butyl-cyclohexano-18C6 in octanol for selective extraction of strontium after the extraction of lanthanides and actinides by CMPO (Octyl-phenyl-N,N-diisobutyl carbamoyl methyl phosphine oxide) yielding the decontamination factors that exceeded 7800. A relation between D Sr and water contents into the various organic phases (constituted of DC18C6 in alcohols and DB18C6 derivatives in nitrobenzene 4,4′(5′)-dinonaoyl-dibenzo-18C6, 4,4′(5′)-dihexanoyl-benzo-18C6, 4,4′(5′)-di-acetyl-benzo-18C6, di-tert-butylbenzo-18C6, dibenzo-18C6) revealed the highest D Sr with the mixture of butanol and octanol in a ratio of 80:20 [49, 50], whereas the highest D Cs could be achieved with ditert-benzo-18C6 in addition with phosphotungstic acid albeit the low solubility of it in nitrobenzene is disadvantageous. Strontium extraction carried out with 15C5/dicarbollide diluted in nitrobenzene from HClO4 with dibenzo-18C6/dicarbollide also diluted in C6H5NO2 enabled to get the extraction constants of metal-organic complexes [51, 52]. Also, the highest D sr and selectivity on the extraction of strontium over calcium was obtained with 15-crown-5 and with 15-benzo- and dibenzocrown-5; 12C4 and 18C6 have found to be less effective.

3.1.1.1 Extraction Influenced by the Diluents and Irradiation Effect

In most of the cases, the diluents comprise the larger part of a solvent. Therefore, proper selection of diluents is almost as crucial as selecting a suitable extractant because of the physical and chemical effects that the diluents can exhibit and also meet the environmental and safety requirements in an SX process [53]. Using the DtBuCH18C6, Mohapatra et al. [54] have investigated the effects of several diluents (including toluene, n-octanol, benzene, t-butyl benzene, o-dichlorobenzene, n-dodecane, chloroform, n-hexane, and the binary mixtures) on strontium extraction from a nitrate solution. A correlation between the Sr extraction and Schmidt’s diluent parameter has been determined, which is formulated by the various physical properties of the diluents (polarizability, H-bonding, viscosity, polarity, Hildebrand’s solubility parameter, etc.). Using DCH18C6 with several diluents (chlorobenzene, chloroform, dichloroethane, carbon tetrachloride, n-octanol, nitrobenzene, and its mixtures with benzene), Gupta et al. [55] found higher D Sr values for aromatic and chlorinated diluents with their higher dielectric constants. But due to safety concern in the nuclear process, these diluents are not recommended to be used, and, alternatively, a diluent mixture of TBP + dodecane + n-octanol (in the proportionate ratio of 30:20:50 vol.%) can be used. Use of dinitriles (glutaronitrile, succinonitrile, malononitrile, and adiponitrile) and diluents with DCH18C6 leads to form the solvated cationic complexes into the organic phase, resulting in an increased efficiency of strontium extraction [56].

The strong radiation by the 90Sr, 137Cs, and other radionuclide elements of the nuclear waste solution, exhibiting the irradiation effect on SX system during the operation, must be addressed [57]. The extractant and diluents in contact with the radioactive waste adversely affect the performance of organic that includes the efficiency, distribution, and separation factors and the recyclability of the organic solvents. The investigated irradiation effect on DCH18C6 with different diluents (chloroform, n-octanol, toluene, and cyclohexane) has been given in Fig. 7 [58]. The radiolytic degradation of DCH18C6 in all of the diluents, certainly with different degree, was observed, in which it was least in toluene and highest in chloroform. Exposure to γ-radiation could cause a darkening of solvent (0.12 M HCCD and 0.027 M PEG-400 in phenyltrifluoromethylsulfone) due to the formation of a water-soluble radiolytic product [59]. The value of D Sr gets decreased with an increasing absorbed dosage; however, stripping was unaffected. In a study with the organic phase (0.025 M DtBuCH18C6 in n-octanol or in the mixture of 20:80 vol% n-octanol/toluene), the irradiated organic phase with diluent mixture showed an improvement in strontium extraction up to a dose of 40 mrad, due to the increased uptake of acid into the organic phase [60]. A reduction in partitioning of Sr2+ to crown ether ionic liquids, [C(4)mim][PF6] and [C(4)mim][NTf2] phase, has been observed due to the formation of proton ion during γ-radiation of the ionic liquids, which can be handled by additional water washing step [61,62,63]. Interestingly, all the removal of Sr2+ ions from nuclear waste solution occurs via the irradiated [C4mim][NTf2] and via precipitation not by extraction with the organic phase. The precipitates as irradiation product identified as SrSO3 and/or SrSO4, depending on the contact time [63].

Fig. 7
figure 7

DCH18C6 concentrations remaining in selected solvents after irradiation (Adopted with permission from Takagi et al. [58])

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3.1.1.2 Complexed Structure into the Organic Phase

For a better understanding of extraction mechanism, the study on structure formation of metal ion complexed with the organic extractant can be helpful. Adopting the small-angle neutron scattering (SANS) methodology, Chiarizia et al. [64] investigated the extraction behavior of strontium from a LiNO3 solution by using the di-n-octylphosphoric acid (HDOP) and compared with HDOP/DCH18C6 mixture in toluene. The addition of DCH18C6 showed a synergistic effect on extraction with increased efficiency than that of with HDOP alone. The SANS analysis revealed that the extraction with HDOP alone in large stoichiometric excess predominantly forms the Sr(H(DOP)2)2 complex solvated into the organic phase (as shown in Fig. 8a). It is similar to the structure that was reported for the aggregates formation with HDEHP [65]. When Sr2+ is wrapped by the DCH18C6 ligand, its interaction with the next two dimmers of HDOP becomes more difficult and prefers to reorient itself as the mononuclear complex schematically is shown in Fig. 8b for the cis-syn-cis isomer of DCH18C6. Hence, the addition of crown ether takes part in the complex formation in the ratio of 1:2 for DCH18C6: HDOP while extracting one molecule of Sr2+. The large aggregates by HDOP alone could not be observed in both the cis-syn-cis and cis-anti-cis isomers of DCH18C6. The structure shown in Fig. 8b is quite supporting to that of the structural study carried out by Burns and Kessler [66] for the bis(dibutylphosphato)aquastrontuim-18crown-6, in which, the 9-coordinated Sr2+ ion is revealed to be complexed with six oxygen atoms of the crown ether, two monodentate dibutyl phosphato ions, and one molecule of H2O. Although a 10-coordinated Sr2+ can be found in [Sr(cis-syn-cis-DCHI8C6)(NO3)2]·CCI4 and [Sr(cis-syn-cis-DCHI8C6)(TMA)2]·H2O [67], the 9-coordinated Sr2+ ion appears to be more suitable for accounting the orientational flexibility of solution complexes [64].

Fig. 8
figure 8

The structure of Sr complexes (Adopted with permission from Thiyagarajan et al. [65])

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3.1.2 Dicarbollides

Dicarbollide is a boron cluster with a π-bonded cobaltic ion. It was originally prepared by Hawthorne in 1965 and studied for cesium extraction, later used in the extraction of strontium by adding the polyethylene glycols (PEGs) to yield the synergistic effect [23, 29, 30]. The chemical and radiation stability possess the uses of dicarbollides in radionuclide’s extraction even from the waste solution of very high acid concentration [68]. The poorly hydrated anions of dicarbollide associated with cations forming the ion-pair neutral compounds have greater solubility in polar-dissociating diluents like nitrobenzene than the water [30]. Its lipophilic anion is difficult to protonate and allows metal cations to be discriminated by their Gibbs energies of transfer. The extraction of strontium from a highly acidic solution can commonly be presented as below (where, m and n are an integer value):

$$ {\mathrm{Sr}}^{2+}+\overline{{n\mathrm{H}}^{+}}+\overline{{n\mathrm{CoB}}_2^{-}}\to {n\mathrm{H}}^{+}+\overline{{\mathrm{Sr}}^{2+}}+\overline{{n\mathrm{CoB}}_2^{-}} $$
(11)

The extraction of 90Sr from the nitric acid medium is carried out by dicarbollide anion {[π-(3)-1,2-B9C2H11]2Co} diluted in polar nitrobenzene. The addition of polyethylene glycol (PEG, HO–(C2H4O)n–H mainly the lipophilic p-nonylphenyl-nonaethylene glycol, Slovafol 909 HO–(C2H4O)9–C6H4–C9H19) to the dicarbollide can remove the water molecules surrounding the metal cations to improve its transfer to the organic phase. Dicarbollides display good stability toward irradiation; however, only effective in diluents of toxic nature like nitrobenzene and releasing halides during the reprocessing can raise the corrosion problem for the system. Using a diluent without nitro group, diethylene glycol ditetrafluoropropyl is proposed; Slovafol addition can lead to the extraction of 90Sr and 137Cs from 2 M HNO3 solution. Tetrahexyl-dicarbollide in aromatic hydrocarbon diluents exhibits comparable distribution coefficients to that of the dicarbollide in nitrobenzene. With a negligible loss into the aqueous phase, the equilibrium constant value of 1.7 was obtained for strontium, which was 800 and 0.5 for cesium and sodium ions, respectively [69,70,71]. Using tetrahexyl-dicarbollide in diethylbenzene for extracting the cesium and strontium from alkaline media yielded a diminished distribution coefficient with respect to increasing alkalinity of the solution. The value of D Sr could be changed from 30 to 7 when NaOH concentration varied from 0.01 to 1.0 M. Using the chlorinated cobalt dicarbollide in nitrobenzene and carbon tetrachloride mixture, with the addition of 1 vol.% Slovafol 909, a 99.8% recovery for 90Sr yielded the decontamination factor of above 500 [72].

Notably, the extracted 90Sr and 137Cs from the HLW are usually taken together for the final geological disposal; hence, their co-extraction can be advantageous [30]. The Universal Extraction (UNEX) process is a relatively well studied co-extraction process using dicarbollide and phosphorylated polyethylene glycols (PPEGs) diluted in m-nitrobenzotrifluoride (MNBTF) [73,74,75]. The organic phase constituted of the mixture of dicarbollide, PEG, and carbamoylmethyl phosphine oxide (CMPO) diluted in phenyltrifluoromethyl sulfone can be adopted for the simultaneous extraction of 90Sr and 137Cs from highly acidic media, along with the lanthanides and actinides. Diethylenetriaminepentaacetic acid (DTPA) or the guanidine carbonate solution can be used for the effective stripping of extracted metals [76]. Law et al. [77] demonstrated the improved UNEX process with simulated and genuine acidic liquid tank waste, as shown in Fig. 9, in which the suppression iron and zirconium in co-extraction with the strontium and cesium was achieved by adding a certain amount of HF, yielding efficiencies of 99.4% 90Sr and 99.99% 137Cs along with >99% actinides. But looking on the requirement of actinide separation for transmutation in the process of advanced nuclear fuel cycling, the co-extraction of actinides is not preferable; hence, the later use of FS-13 with 0.08 M dicarbollide +0.6 vol.% PEG-400 could significantly suppress the actinide over co-extraction of 99.9% strontium and 97.4% cesium [78].

Fig. 9
figure 9

Flowsheet for the demonstration of the UNEX process. (Adopted with permission from Law et al. [77])

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4 Conclusions

Celestite (SrSO4) and strontianite (SrCO3) are the main mineral source of strontium and predominantly being recovered as its carbonate salt by following the black ash or direct leach process. Nevertheless, the black ash process is widely used; the formation of SrCO3 during calcination leads to the strontium loss, while leaching in water is found to be disadvantageous for the energy-intensive process. On contrary, the direct leach process yields a relatively impure product (~95% SrCO3) as compared to the black ash process, but it is a low-cost simple process with by-products of commercial interests. There has been a plenty of researches devoted to strontium extraction from minerals; however, a high yield with the increased purity has still many scopes for new research to extract strontium from geo-environment.

At the same time, removal of radioactive and heat-emitting 90Sr and 137Cs from nuclear wastes and spent-fuel solutions is of great importance not only for the spent-fuel reprocessing and sustainable management of nuclear but also for the recovery of these two metals. As the requirement of radionuclide removal is still under discussion with respect to its economic aspects, the R&D on their recovery as value-added products will surely be beneficial for a further technical road map of the spent-fuel reprocessing. The solvent extraction has been potentially demonstrated for the simultaneous separation of cesium and strontium, mainly the CSEX/SREX process, and a combined crown ether/calixarene process. The stability of CSEX crown ether in HNO3 solution, however, limits its use in an industrial process, and without any further development, the CSEX/SREX process is not a feasible option for advanced fuel cycle initiatives. An improved stability of the extractant in presence of HNO3 must be addressed by the new research on these processes. In summary, the solvent extraction is a reasonable choice for strontium separation and recovery in advance fuel cycling, and more developments in this area can be well-expected in the near future.