Strategies for Strontium Recovery/Elimination from Various Sources

Abstract

Not having the same grade of popularity as other metals like rare earth elements, gold, copper, etc., strontium is a chemical element with wide uses in daily life, which is why it appears in the EU 2023 list of Critical Raw Materials. Among the sources (with celestine serving as the raw material) used to recover the element, the recycling of some Sr-bearing secondary wastes is under consideration, and it is also worth mentioning the interest in the removal of strontium from radioactive effluents. To reach these goals, several technological alternatives are being proposed, with the most widely used being the adsorption of strontium or one of its isotopes on solid materials. The present work reviews the most recent advances (for 2024) in the utilization of diverse technologies, including leaching, adsorption, liquid–liquid extraction, etc., in the recovery/elimination of Sr(II) and common ⁹⁰Sr and ⁸⁵Sr radionuclides present in different solid or liquid wastes. While adsorption and membrane technologies are useful for treating Sr-diluted solutions (in the mg/L order), liquid–liquid extraction is more suitable for the treatment of Sr-concentrated solutions (in the g/L order).

1. Introduction

As one of the 34 raw materials included by the EU in its 2023 Critical Raw Materials (CRMs) list, strontium is used in a series of applications, with Iran (37%) and Spain (34%) being the major worldwide producers of this element [1].

While there are no negative effects of stable strontium in humans at the levels normally found everywhere, this element has positive effects in the treatment of several diseases. However, the current research on its role in human health is not exhaustive, so future research is crucial for unlocking the full potential of strontium in enhancing human health [2]. On the other hand, with ⁹⁰Sr considered an important mobile water migrant [3], the negative effect that the presence of this radionuclide has on life is well known. This is why, in addition to the production of this element from its main raw material (celestine, SrSO₄) or the recycling from other sources, the recovery of ⁹⁰Sr from contaminated radioactive effluents has captured the attention of many researchers. However, as will be seen later in this review, the majority of these investigations have been carried out on synthetic solutions containing non-radioactive strontium salts; thus, the behavior of the respective systems does not or may not correspond “exactly” to real situations, in terms of radioactive issues, as one would find in real environmental emergencies.

In addition to the usefulness of the element as well as ⁹⁰Sr in medicine, strontium and many of its compounds are currently used in several fields, e.g., ceramic magnets, alloys, and pyrotechnics. The utility of SrO for use as a heterogeneous catalyst in the production of biodiesel via a transesterification process has been described [4]. Hexaferrites are a type of magnetic material with applications in magnetic sensors and microwave and energy storage devices. In particular, M-type strontium nanohexaferrite is a promising material due to its remarkable properties, especially its high magnetic susceptibility [5,6,7].

With respect to the recovery of the element from non-conventional sources, the use of bitterns (residual brines) as potential Sr-bearing materials located in the Mediterranean region, including Southern Europe, North Africa, the Near East coasts, and parts of the Atlantic regions, has been mentioned in the literature [8]. Also, the recovery of strontium from zinc electrolytic anode slimes (ZEASs) is of interest [9]. While the price of celestine concentrates is known to be increasing in value, the recycling of strontium, at least in the EU, is very low, if not non-existent [1]. Despite the above, some efforts are being carried out to recover strontium from cathode-ray tubes or other waste materials.

Several technologies are currently being proposed for the recovery of radioactive and non-radioactive strontium from solutions; among them, adsorption on different materials is mostly used, i.e., alkali-activated materials [10], metal and covalent organic frameworks and their nanocomposites [11,12], and porous organic polymers [13]. Graphene-based materials are used in the removal of various types of contaminants including strontium [14]. Biotechnology, in the form of microbially induced carbonate precipitation, has also found its place in the recovery of this element [15,16].

The present work reviewed the significant contributions that appeared in the literature during the year 2024 in relation to the recovery/elimination of strontium or some of its isotopes from different sources. As mentioned above, adsorption is the most popular technology to achieve the recovery of this element (especially ⁹⁰Sr) from liquid sources/wastes; however, as also mentioned above, most of these investigations used mimic solutions, which, naturally, do not match the actual situations that may be encountered in the event of a radioactive emergency.

2. Recovery of Harmless Strontium(II)

2.1. Leaching Operation

Though not exactly the same but reported by the same authors, references [17,18] presented investigations about the use of biometabolites produced by Aspergillus niger in the bioleaching of waste LCDs. Prior to the bioleaching step, the e-wastes were dismantled, crushed-milled, and thermally treated in B₂O₃ medium at 1100 °C for seven hours. From this treatment or activation step, the resulting powder (2.33% Sr, 7.01% Al, 0.13% As, 0.02% In, 0.94% Ba, 3.64% Ca, and 23.84% Si) was bioleached. Under the optimum bioleaching conditions of 10 g/L pulp density, temperature of 70 °C, and leaching time of 24–29 h, similar but non-identical leaching efficiencies were found (

Table 1. Bioleaching efficiencies in the treatment of spent LCDs with Aspergillus niger.

%Sr	%Al	%As	%In	Ref.
33.3	81.4	69.1	60.0	[17]
36.2	82.6	70.8	64.5	[18]

Adapted from [17,18].

). No data were reported about the separation of the various elements from the leachate.

2.2. Adsorption Operation

Though several procedures are used to remove metals from aqueous media, among these methodologies, adsorption is one of the most popular due to its reliability, operational easiness, and somewhat low cost.

Among the various materials used to recover strontium, ladle slag, a by-product of steelmaking, was found. In reference [19], a ladle slag, provided by a Belgian company, was used to treat a Belgian-originated electroplating wastewater of pH 10 containing 0.63 mg/L Sr(II), 10.4 mg/L Al(III), 1.1 mg/L B(III), 0.13 mg/L Ba(II), 1.7 g/L Cr(III), 0.23 g/L Mg(II), 0.60 g/L Pb(II), and 1.5 g/L Zn(II). The treatment of the solution with 15 g/L slag during eight hours provided metal removal rates exceeding 90%. Neither data about the recovery/separation of the elements loaded onto the adsorbent nor the recyclability of the ladle slag.

An anionic metal–organic framework, DGIST-12, formed from In³⁺ and partially deprotonated 4,6-dihydroxy-1,3-benzenedicarboxylic acid, was used to investigate its performance in the removal of strontium from synthetic solutions [20]. While Sr²⁺ removal occurred in the pH range of 4–11, the element is loaded onto the adsorbent via a cation exchange mechanism between Sr²⁺ and [NH₂Me₂]⁻ groups present in the MOF. Strontium was adsorbed preferentially to Co²⁺, Ni²⁺, Mn²⁺, Cd²⁺, Zn²⁺, and Cs⁺. Desorption tests demonstrated that the adsorbed strontium remained in the adsorbent after several washes with water.

MnO₂@ZIF-8 nanocomposites were synthesized and used in the removal of Sr²⁺ from the aqueous solution [21]. The as-prepared nanocomposites showed a core–shell structure. High adsorption capacity was reached at pH 4, with strontium loaded onto the adsorbent due to ion exchange, electrostatic attraction, and complexation. While loaded Sr can be desorbed using 0.1 M nitric acid solutions, there was a systematic loss of adsorption–desorption capacity under continuous cycles: 100% in the first cycle against 68% adsorption and 62% desorption in the fifth cycle.

Another nanocomposite, in this case, an organic–inorganic hybrid material IIGK@MnO₂ (2-naphthalenemethyl-isoleucine-isoleucine-glycine-lysine@manganese dioxide), was designed and used for the removal of Sr²⁺ [22]. This material presented a large specific surface area and negative surface charges, with strontium being the best removed from the solutions at pH 6 and 25 °C. The interactions between N-H groups on IIGK and oxygen atoms on MnO₂ with Sr²⁺ were responsible for the strontium loading onto the adsorbent. Desorption was carried out in 1 M HCl medium, however, as, in the previous reference, there was a continuous loss of effectiveness after continuous cycles: about 75% in the first cycle and near 60% in the third cycle.

Among adsorption methodologies, bioadsorption presented a series of advantages like high performance, cost effectiveness, the ability to use live and dead biomass, etc. [23]. Thus, biadsorption investigations with live mycelia of Schizophyllum commune to remove strontium and heavy metals from aqueous solution were performed [24]. With respect to strontium(II), the maximum adsorption capacity was reached at pH 5 and 180 min, increasing this efficiency in the 20–70 °C range of temperatures. The removal of this element (and lead, zinc, and cadmium) involved both biosorption and intracellular accumulation with –CH, amino acid, fatty acid, carboxyl groups, and alkane and alkyl groups of S. commune. The adsorption capacity of the biomass for the removal of heavy metals under the optimized conditions was Sr(II) > Pb(II) > Zn(II) > Cd(II), whereas the adsorption of these elements resulted in cellular and morphological alterations in the fungal biomass.

Using titanium isopropoxide or anatase as precursors and citric acid as a complexing agent, Engelhard titanosililicate (ETS-4), to be used as adsorbent for Sr²⁺ and Cs⁺ from aqueous media, was formulated [25]. Cesium and strontium exchange capacities of the adsorbent were found to be 220 mg/g and 76.2 mg/g, respectively. The saturation exchange equilibrium was attained within 60 min. No desorption data were included in the work.

In order to investigate the removal of strontium from solutions, birnessite (mixture of manganese oxides) and dopped-birnessite were used to study their stability under continuous adsorption–desorption (0.3 M nitric acid) cycles [26]. The results showed that, with pure birnessite, there was a continuous decrease in the strontium adsorption efficiency (0.97 mmol Sr/g in the first cycle and 0.52 mmol Sr/g in the fifth cycle). The presence of dopants (5 mol %) Fe, Co, and Ni produced and increased strontium removal efficiency with successive cycles. These results were attributed to the introduction of these doping ions, which are not disproportionate under ion exchange conditions and did not manifest the Jahn–Teller effect.

Based on previous investigations about water purification by the gas hydrate process for removing various ions from seawater and hypersaline water [27], another study investigated the removal characteristics of Sr²⁺, Co²⁺, Cs⁺, and I⁻ in the context of the hydrate-based water purification process [28]. In the study, 1,1,2-Tetrafluoroethane (HFC-134a) was selected as the hydrate-forming gas, and the investigations were carried out both in pure water and sea water. The results (

Table 2. Removal percentages using the hydration process.

	Sr	Co	Cs	I	TDS, mg/L
Water	85.8	86.3	87.1	87.3	820
Sea water	80.9	84.8	85.8	89.2	726 a

^a Total TDS: 32,294 mg/L considering the presence of Na, Mg, Ca, K, chloride, sulfate, and bromide. Adapted from [28].

) showed that the removal efficiency is practically the same when both water systems are used in the tests.

It is worth noting that, when the sea water sample is used, together with Sr, Co, Cs, and iodide, the other elements present in the solution are also removed from this, including the following: Na (84.7%), Mg (84.2%), Ca (83.9%9), K (84.3%), chloride (85.2%), sulfate (85.5%), and bromide (84.2%).

Ceramic matrices can be useful in forming adsorbent materials, and thus, Ti–Ca–Mg composite phosphates with different ratios of Ti/(Ca+Mg) were used to remove Sr(II) from the solutions [29]. This removal was attributed to an ion exchange mechanism between Sr²⁺ ions and all components of the Ti–Ca–Mg composite phosphates. The efficiency in the removal increases with an increase in the titanium content in the adsorbent material. After the calcination of the Sr-saturated adsorbents, the products of heat treatment are compounds with the structure of Sr-substituted hydroxyapatite, whitlockite, and pyrophosphates. The presence of these species is responsible for the immobilization strength of Sr(II) ions, resulting in desorption rates of 0.03–0.12% or 0.31–0.54% for samples derived from the processing of Sr-bearing water or seawater, respectively.

Geopolymers based on the metakaolin were used to remove Sr- and Cs-containing aqueous solutions [30]. The forms in which the geopolymers were synthesized influenced the removal of these elements, with spherical rods favoring the adsorption of strontium and pyramidal rods that of cesium. In any case, the use of these geopolymers increased the maximum strontium (and cesium) loading with respect to that yielded with kaolin, 57 mg/g geopolymer versus 15 mg/g metakaolin. No data about the desorption step were included in this work.

Commercial polymeric (IRC747, S940, MTX8010) and inorganic (SbTreat, SrTreat) adsorbents, along with another synthesized inorganic one (CuHCF), were used for recovering strontium (as well as cobalt, gallium, germanium, rubidium, and cesium) from synthetic solutions resembling saltwork brines (bitterns) [31]. The results demonstrated that the polymeric material containing aminophosphonic groups (Na-form) (IRC747) is effective for the removal of strontium (about 80%) at pH 7 (as well as cobalt and gallium). The commercial inorganic adsorbent SrTreat (sodium titanium oxide hydrate) is useful for removing this element (about 85% at pH 8) (as well as cobalt, gallium, and germanium), whereas CuHCF (copper(II) hexacyanoferrate) removes strontium with an efficiency of near 30%. In dynamic column experiments, SrTreat also exhibited an adsorption capacity of 7 mg/g for strontium. Acidic (1 M HCl) desorption recovered (efficiencies greater than 70%) most of the elements from the adsorbents.

Zeolite derivatives are widely used in the removal of metals from aqueous solutions, and dodecahedral zeolitic imidazolate framework-67 (ZIF-67) was modified by potassium hexacyanoferrate (25 nm nanoparticles) and used as an adsorbent for Sr²⁺ removal from model wastewater (pH 5–13) [32]. Desorption was performed using 1 M KOH in ethanol medium. There is a continuous decrease in the Sr-adsorption efficiency after continuous use (up to three adsorption–desorption cycles). In comparison, as described in the title of the work, the investigation was not carried out on a nuclear wastewater but on solutions derived from the dissolution of harmless strontium salt in deionized water or tap water.

The silico antimonate (SiSb) nanocomposite (particle size ranged from 2.04 to 3.07 nm and was mainly composed of SiO₂ and Sb₂O₅) was investigated, both batches, through continuous experimentation, as adsorbents for removing strontium and rubidium from synthetic aqueous solutions [33]. The maximum strontium adsorption occurred at pH 6–8, and thus, the best Sr/Rb separation occurred at these same pH values. Column experiments confirmed the results derived from batch studies. In these column experiments, HNO₃ was used to desorb the loaded metals onto the adsorbent material, whereas, with 0.1 M HNO₃, only Rb was released to the aqueous phase; from 0.25 M acidic solutions, strontium was desorbed from the metal-loaded nanocomposite.

Tin molybdate talc adsorbent, synthesized by the co-precipitation technique using SnCl₂·2H₂O and Na₂MoO₄·2H₂O solutions as starting materials, was utilized to investigate its performance in the removal of Sr (and Eu) from aqueous media [34]. The maximum strontium uptake was achieved at pH 6 with equilibrium attained after 3.5 h. The metal-loaded sample was desorbed from the adsorbent with 0.1 M HCl solutions, with an operational efficiency of near 97%.

In the next reference [35], nanoporous aluminum borosilicate (AlBS) was used to remove strontium ions from solutions of different pH values. With a maximum strontium uptake at a pH around 6 (one hour and 25 °C), the Sr-loaded AlBSs were treated at various temperatures to investigate the immobilization of the element on the adsorbent. After heat treatment (60–1100 °C), the samples were leached with 1 M NaCl or aqueous solutions of pH 4 or 7. In the case of using 1 M NaCl solutions, the materials heated at 60 and 400 °C demonstrated poor stabilization, with the release of 100% and 63.81% of the initially adsorbed Sr²⁺, respectively. This value decreased to 3.43% when the material was heated at 1100 °C, and this decrease was attributed to the formation of an amorphous glassy-like phase that entrapped strontium ions. When solutions of pH 4 or 7 were used, an adequate immobilization of strontium onto the heat-treated material was found for specimens heated at 800 °C.

With yttrium, not strontium, the target of this work, another biotechnological study used Aspergillus terreus, isolated from liquid radioactive waste (Hot Laboratory Center of the Egyptian Atomic Energy Authority), as a dead biosorbent material to separate Y(III) from a synthetic solution containing both Sr(II) and Y(III) [36]. Using monoelemental solutions and batch experiments, Y(III) was demonstrated to be preferentially removed over strontium(II) in the 1–6 pH range. Both elements can be desorbed from the loaded material using the 0.1 M nitric acid medium. The column experiments showed that the breakthrough capacities were 63 and 1.43 mg/g for yttrium and strontium, respectively.

This work [37], as many others, compared with that mentioned in each respective title and abstract, used experimental solutions prepared from ultrapure water and laboratory salts. In this context, several chitosan/chabazite-K/epicholorohydrin composite beads (

Table 3. Formulations used in the removal of strontium(II) using chabazite-K derivatives.

Material	Abbreviation	a Sr(II) Maximum Uptake, mmol/g
Chitosan/chabazite-K	Cts-CHA-NCl	0.613
Chitosan/chabazite-K/epichlorohydrin	Cts-CHA2	0.551
Chitosan/chabazite-K/epichlohydrin	CTS-CHA1	0.656

Cts-CHA₁ and Cts-CHA₂ have different synthesis routes. ^a Based on the Langmuir model. Adapted from [37].

) were utilized to investigate their performance in the removal of strontium (and cesium) from these mimic aqueous solutions. Table 3 presented the maximum strontium capacities (pH equilibrium values in the 6.5–7 range) obtained with these formulations, which are surprisingly (or not) lower than that derived with the use of pristine chabazite-K (1290 mmol/g). Desorption was carried out in the 1 M KCl medium. The main advantage of using these composites is represented by their formulation as beads, which is somewhat an improvement from the pristine materials because the beads settled immediately at the end of the experimentation and chabazite-K did not.

The solid-state reaction was used to formulate nanostructured CaTiO₃, which was utilized for the removal and recovery of Sr²⁺ from the various water sources (deionized water, tap water, well water, lake water, and seawater) [38]. Experiments carried out on deionized water showed that the maximum strontium removal was achieved at pH 9, though it was not clear that this removal was not only from the use of the adsorbent or that strontium hydrolysis but also contributed to it [33]. Though desorption can be performed using 0.5 M HCl, the Sr-free material necessitated a further conditioning step, with the 0.5 M NaOH solution and 600 min, prior to its reutilization in another loading cycle. The water source influenced the efficiency of this adsorbent on the removal of strontium, and the results derived from this experimentation attest to this as follows: deionized water (97%), tap water (65.6%), well water (76.5%), lake water (73.9%), and seawater (17.8%).

Though this investigation was based in the SIR (solvent impregnated resin) methodology, and thus, the extractant (in the present case Cyanex 572, a phosphorous-based chelating reagent) was only responsible for removing the metals from the solution, the authors of this work treated the data as derived from an adsorption process, and this is why the reference was considered in this subsection. As in a previous reference [36], strontium was not the primary target of the investigation but zirconium(IV). This investigation uses Amberlite XAD–4 resin impregnated with 0.2 M Cyanex 572 in n-dodecane to separate Zr(IV) from Y(III) and Sr(II) [39]. The separation of these elements was studied using both batch and continuous flow procedures. With a removal order of Zr > Y > Sr, the highest separation factors Zr/Y and Zr/Sr were obtained at 0.05 M HCl. Double-distilled water can be used to desorb the residual strontium adsorbed onto the impregnated resin, while zirconium is effectively recovered by elution with 0.5 M oxalic acid and yttrium with the 3 M HCl + 0.25 M NaCl mixed solution. The column experiments showed the same tendency of one being derived from batch experimentation.

Also, under the SIR concept, in the next investigation and using vacuum impregnation, a porous resin (DtBuCH₁₈C₆@CG-71) was synthesized by the introduction of 1 M 4′,4″(5″)-di-tert-butyldicyclohexano-18-crown-6 in 50% iso-octanol and n-dodecane solution onto the Amberlite CG-71 resin [40]. The strontium-removal effectiveness of this resin increased with the increase (0.1–8 M) in the nitric acid concentration in the feed aqueous solution and then levels off at higher (8–12 M) acidic concentrations. With water being a suitable eluant, its continuous use showed that there is no loss in loading capacity after fifteen cycles, but the efficiency in the removal step continuously decreases after the ninth cycle. The column experiments showed the expected tendency with respect to the initial strontium concentration in the feed phase, that is, complete metal loading in the column is achieved at higher bed volume values as the strontium concentration in the initial solution decreases.

A potassium fluoroaluminate, K₂[(AlF₅)H₂O], which was synthesized with a low-temperature one-step method, was used to remove strontium from aqueous solutions [41]. The material adopted a 1D AlF6-chain structure, consisting of exchangeable potassium ions in between the infinite chains of octahedral aluminum centers. The maximum adsorption capacity is near 120 mg/g. There were no data about the desorption step.

The next investigation is related to the distinction in the binding configurations of Sr²⁺ (and Cs⁺) on various 2:1 phyllosilicate (illite, vermiculite, and montmorillonite) [42]. Similarly to many systems, the removal of this element is influenced by the pH of the aqueous phase, reaching the maximum at pH values above 6 in the three cases. The ionic strength (NaCl) also influenced the removal of strontium at every pH value, and this removal decreased with the increase in the ionic strength from 0.001 M to 0.01 M. Again, the investigation does not include data about the desorption step.

Returning to the nanoworld, nanocrystalline low-silica X with Al-rich framework (Si/Al of near 1.00) was synthesized and investigated in the removal of strontium from aqueous phases [43]. All the three synthesized formulations (with Na/Al in the 0.72–0.77 values range and K/Al in the 0.20–0.23), presented almost the same strontium loading capacity of 230–235 mg/g, which is not better than the value of 237 mg/g presented by conventional Na/Al zeolite (Si/Al = 1, Na/Al = 1, and K/Al = 0). However, continuous (fixed bed column) experiments showed that the nanocrystalline compound (Si/Al = 1, Na/Al = 0.76 and K/Al = 0.21) show a 5.5-fold larger breakthrough volume than conventional NaAl, which is attributed to the faster strontium-exchange kinetics presented by the former adsorbent. There were no data about the removal of strontium from the loaded adsorbent.

Some limitations (low adsorption rate, slight selectivity, and long operational time) shown by different adsorbents have led to a continuous search for new materials to improve these performances. Thus, a pH-responsive and dual-purpose adsorbent Mg_0.17V₂O₅·0.83H₂O (MgVO) for removing strontium was formulated [44]. The maximum adsorption capacity (about 265 mg/g) was reached at pH 2, with a sharp decrease with increased pH values, i.e., 80 mg/g at pH 3. Strontium was removed from the solution by a cation exchange mechanism with interlayer Mg²⁺ ions from the adsorbent. Several solutions were tested to remove strontium from the loaded MgVO adsorbent (

Table 4. No-desorption efficiency of Sr(II)-loaded on MgVO adsorbent.

Desorbent	% No-Desorption
Water	100
0.5 M NaCl	94.7
2 M NaCl	93.1
0.05 M Na-EDTA	100
0.01 M HCl	98.7
0.015 M HCl	100

Time: 2 h. Adapted from [44].

), and the results showed that neither of the solutions removed the element from the solid material; alternatively, strontium was effectively retained or immobilized (under mild conditions) on this type of adsorbent.

Zirconium phosphate derivatives presented a considerable potential for use as adsorbents of metals due to their tunable interlayer spacing and ion exchange properties. Based on these premises, a calcium zirconium phosphate (Ca_0.55ZrH_0.9(PO₄)₂) with ultra-wide spacing was obtained by means of layer-spacing modulation and used in the removal of strontium from aqueous solutions [45]. With a maximum adsorption capacity at a pH around 7, the adsorbent performed well after irradiation, with 100 kGy (β and γ) of the solid adsorbent, as well as a decrease of 5% (100 kGy β) and 4% (100 kGy γ) from 85% of the pristine simple. There were no desorption data available in the published manuscript.

Nanostructure titanium carbide MXene (Ti₃C₂T_x) was modified with KH₂PO₄ and chitosan to remove strontium from aqueous solutions [46]. At pH 7, the best strontium loading (about 365 mg/g) was derived from the use of the 8% KH₂PO₄-modified pristine material, with results that compared well with those of Ti₃C₂(OH)_x-chitosan (about 285 mg/g) or pristine Ti₃C₂(OH)_x (near 250 mg/g). It was concluded that electrostatic reactions and intra-sphere complexation dominated the removal of strontium from the aqueous solution. Desorption was carried out using the 0.1 M HCl medium. It should be mentioned here that this is another reference that mismatched what it is mentioned in the title, abstract, and introduction—“nuclear waste”—and what is actually presented in the manuscript—non-radioactive aqueous solutions.

The fundamentals of the aqueous adsorption behavior of strontium, with respect to a commercially available ethyl/butyl phosphonate-functionalized silica (Na-form) were investigated [47]. However, prior to its use, the material was converted to its H-form by treatment with the 1 M HCl solution. Maximum adsorption was obtained at an equilibrium pH value of 2.4–2.7, with negligible strontium removal at pH values below 1.2. Column experiments showed near 55% retention of capacity, and breakthrough could be well-described via the modified dose–response model. There were no desorption data included in this work.

A Cu-Zn-MOF, [NH₄]₃[(Cu₃Zn₂(FDA)₆(H₂O)₈Cl·3H₂O] (FDA = 2,5-furandicarboxylate), is formulated for the adsorption of Sr²⁺ by the cation exchange mechanism [48]. This MOF has an anionic 2D → 3D interpenetration framework-based trinuclear centers, which are charge-balanced by amino cations. Strontium was removed from the Sr-loaded MPF by the use (ten hours) of 1 g/L ammonium chloride solution with a slight decrease in the adsorption capacity after three adsorption–desorption cycles.

Another framework (K-Zn-MOF), with [Me₂NH₂][KZn(pydc)₂₍H₂O)₂]∙H₂O (pydc = 2,6-pyridinedicarboxylic acid) was used in the removal of strontium from synthetic aqueous solutions [49]. With experiments carried out at “neutral conditions” as mentioned in the text, equilibrium was reached after one hour of reaction between the solid and the strontium-bearing solution. This is another example of a work that does not include desorption data.

In this reference [50], Na_0.03Natroxalate_2.47Si_1.44Nb_0.08V_1.92O₅·1.2H₂O (Nb4-NxSiVO), with a layer spacing of 14.9 Å, was formulated and used in the removal of strontium from inert solutions [50]. This material effectively removed strontium at pH values in the 4–10 range (while bearing in mind the question: is there any strontium precipitation at alkaline pH values?), with this removal being more effective as the temperature increases from 25 °C (254 mg/g) to 45 °C (289 mg/g). This investigation did not include data about the desorption step.

Another MOF, in this case, with formulation [Zr₆(µ₃-O)₄(µ₃-OH)₄(OBA)₄(OH)₃(H₂O)₃(Me₂NH₂)]_n (OBA = 4,4-oxy bis(benzoic acid), including cationic dimethyl ammonium groups and anionic Zr(OBA)_n^n– species, was used to remove strontium rom aqueous solutions [51]. The pH of the aqueous phase influenced the strontium loading capacity, increasing it with the increase in pH from 1 to 8–10 (bearing in mind the question: is this another case of strontium hydrolysis?). It is claimed that the adsorbent resisted high acidic conditions; however, the loading sharply decreased with the increase in the nitric acid concentration in the feed solution (240 mg/g at 0.5 M nitric acid versus 180 mg/g at 5 M acid). The desorption step consisted of various steps, namely (i) contact with 1 M CaCl₂ solution, (ii) wash with 0.2 mM/L ammonium nitrate solution, (iii) wash with (deionized) water, and (iv) dry at 80 °C during six hours. The results, based on several adsorption–desorption cycles, showed that, up to eight cycles, there is a continuous decrease in the loading–desorption capacity.

The adsorption behavior of strontium(II) on weathered micaceous minerals (biotite and vermiculite) with different weathering degrees was investigated [52]. The type of mineral clearly influenced strontium loading onto the different materials (

Table 5. Sr loadings onto micaceous minerals.

Mineral	[Sr(II)], mg/g
Na-Biotite	5.4
Ca-Biotite	6.6
SCa-Biotite	19.0
Na-Vermiculite	16.0

pH = 6.5. Temperature: 25 °C. Adapted from [52].

), with a sharp increase in the loading capacity, of the four adsorbents, from pH 7 to alkaline pH values (possibly strontium hydrolysis). There were no data about the desorption step.

A one-pot hydrothermal procedure was used for the fabrication of Na₃FePO₄CO₃ (NFPC) to be used for the removal of strontium from the simulated solutions [53]. With respect to this removal, the adsorbent becomes effective from pH 4, and thus, acidic conditions were used to desorb the metal, i.e., 97% efficiency at pH 1 and four hours of contact time. The adsorption efficiency decreased after five adsorption–desorption cycles from 99.1% in the first cycle to 71.2% in the fifth cycle.

Fly ash-based zeolites aimed at the removal of strontium were the adsorbents used in the next investigation [54]. The utilization of NaNO₃-based zeolite increased the adsorption capacity of the material at NaOH concentrations lower than 2 M while reducing it at high concentrations of NaOH (exceeding 4 M). In the case of fly ash-based zeolite solely influenced by NaOH, the adsorption followed the Langmuir isotherm model, while, if the adsorption process is influenced by both NaOH and NaNO₃, the removal of strontium followed the Freundlich isotherm model. This is another investigation with no data about the desorption step.

A functional adsorbent CEPA@SBA-15-APTES was formulated through the phosphorylation of amino-modified mesoporous silica with an organic content of near 20 wt% and used in the removal of strontium from (again) synthetic non-radioactive solutions [55]. The adsorbent lost efficiency when the acidity of the feed phase increases from 10⁻⁵ M to 10⁻² M nitric acid. The Sr uptake onto the adsorbent increases when the temperature increases from 25 °C to 45 °C. Column experiments showed that the breakthrough point decreased as the acid concentration in the feed phase increased (195 mL, loading capacity 40.7 mg/g at 3 M nitric acid versus 367 mL, 64.5 mg/g at 10⁻⁶ M acid concentration). There were no desorption data included in the work.

2.3. Liquid–Liquid Extraction Operation

From early studies published on the purification of uranyl nitrate with ether extraction [56,57], the true liquid–liquid extraction operation was developed in the course of World War II to purify uranium; further, the operation was introduced as a scientific–technological and industrial tool for recovering a series of metals like copper, gold and other precious metals, zinc, cobalt, etc. The operation needs an extractant (the organic agent responsible for the extraction of the metal) and an organic diluent, both to conform an organic phase suitable for each liquid–liquid extraction operation.

Among these diluents, 1,1,7-trihydrododecafluoroheptanol is a potential diluent for the development of strontium-selective extractants based on individual stereoisomers of dicyclohexano-18-crown-6 (DCH₁₈C₆) [58,59]. Through the use of this mixture of extractant/diluent, another work studied the extraction of nitric acid using this extractant [60]. The results revealed that the extraction of nitric acid depended on the acidity of the feed aqueous phase (increasing it with the increase in the nitric acid concentration in the 1–8 M concentrations range) and did not depend on DCH₁₈C₆ stereoisomerism. Moreover, the use of the mixture crown-ether/fluorinated alcohol is the key point to developing a system that inhibits the transfer of the macrocyclic ligand from the organic to the aqueous phase. An important result derived from this investigation is that there is a decrease of about 17% in the value of the distribution ratio of strontium after irradiation to the adsorbed dose of 285 KGy. The distribution ratio is defined as follows:

$$\mathrm{D}={\displaystyle \frac{{\left[\mathrm{S}\mathrm{r}\right]}_{\mathrm{o}\mathrm{r}\mathrm{g}}}{{\left[\mathrm{S}\mathrm{r}\right]}_{\mathrm{a}\mathrm{q}}}}$$

(1)

where [Sr]_org and [Sr]_aq represent the strontium concentrations in the respective organic and aqueous phases at the equilibrium. The chemical radiation transformations of the DCH₁₈C₆ solutions in fluorinated alcohol at the early stages of radiolysis follow a scenario in which both the solvent and the macrocyclic ligand undergo destruction. At the later stages of radiolysis, the DCH₁₈C₆/1,1,7-trihydrododecafuoroheptanol extractant behaved like a solution of DtBCH₁₈C₆ in 1-octanol [61], where intermediates from the diluent interacted with the macrocycle to promote its destruction.

An organic solution formed by N,N,N′,N′-tetracyclohexyl diglycolamide and o-nitrophenyl hexyl ether as extractant and diluent, respectively, was used to extract strontium from nitric acid solutions [62]. The extraction of this element increased with the increase in the nitric acid concentration from 10⁻⁴ M to 1 M and then levels off and decreased from 2 M acidic concentration in the feed aqueous phase. This tendency is typical of the formation of a neutral complex in the organic phase as follows:

$${\mathrm{S}\mathrm{r}}_{\mathrm{a}\mathrm{q}}^{2+}+{2\mathrm{N}\mathrm{O}}_{3\mathrm{a}\mathrm{q}}^{-}+{3\mathrm{L}}_{\mathrm{o}\mathrm{r}\mathrm{g}}\leftrightarrow \mathrm{S}\mathrm{r}{(\mathrm{N}\mathrm{O}}_3{{)}_2·3\mathrm{L}}_{\mathrm{o}\mathrm{r}\mathrm{g}}$$

(2)

where L represents the extractant. Strontium can be stripped from the organic phase by the use of low (deionized water or pH 4 solutions) or high acidic (6 M) concentrations. Though strontium is completely stripped after four consecutive stages using any of the three media, in the first stage, efficiency is much higher in the case of 6 M nitric acid (89%) than when water (0.48%) or pH 4 solution (0.48%) is used.

Also utilizing simulated solutions, the extraction of strontium (and other metals) using the same extractant as in the previous reference but with the inclusion of cyclohexanediamine tetra-acetic acid (CDTA) as a complexing agent to inhibit the extraction of Zr(IV) and precious metals was investigated [63]. A 0.5 M oxalic acid solution was used to strip loaded strontium from the organic phase.

With yttrium as the element of interest, the complete separation of Y³⁺ from Sr²⁺ in the HCl medium was investigated using tributyl phosphate (TBP), as the extractant, diluted in a diluent formed with a hydrophobic deep eutectic solvent (DES) consisting of oleic acid (OA) and 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) in an 8:2 molar ratio [64].

2.4. Membrane Operation

Membrane operations, in their most simple form, are characterized because the solution containing the element(s) to separate or concentrate passed across a solid support of variable composition, pore size, pore tortuosity, etc. As can be seen below, recent investigations included the use of graphene oxide (GO)-based membranes.

A zirconium metal–organic framework/graphene oxide (UiO-66-NH₂/GO) nanofiltration membrane was prepared using branched polyethyleneimine (PEI) in order to improve the uniform dispersion of UiO-66-NH₂ within the graphene oxide membrane, thus increasing the structural stability of the membrane during the operation [65]. The experimental results indicated that the PEI-UiO-66-NH₂/GO presented the maximum strontium rejection with near-maximum water permeation in relation to the use of graphene oxide, UiO-66-NH₂/graphene oxide, or PEI–graphene oxide membranes. The results also indicated that the membrane presented adequate strontium separation performance with the change in the strontium feed concentration (10–100 mg/L), operational pressure (0.1–0.5 MPa), pH (2–10), in the presence of accompanying cations (K⁺, Ca²⁺, and Mg²⁺), and during continuous testing over 7 days.

A graphene oxide membrane was modified through the incorporation of polydopamone, followed by a thermal reduction treatment to reduce the transport of strontium across it [66]. The key to reaching this objective is to obtain appropriate interlayer spacing and a high surface charge density. Optimized conditions favoring water permeation and strontium rejection are established as follows: 0.042 mg/cm² graphene oxide dosage, three hours for the reaction of polydopamine and graphene oxide, and the polydopamine–graphene oxide mass ratio of 0.5:1. Moreover, it has been found that the best temperature for the thermal treatment, and thus reducing the membrane swelling, is 80 °C. At pH 11, the membrane presented reasonable (90 h) stability with respect to water permeation (near 0.5 L/m²· h·bar) and strontium rejection (about 90%).

The transport of strontium across a reduced graphene oxide membrane, prepared by the amino-hydrothermal method from graphene oxide suspensions, in the presence of other divalent cations (Co, Ni, and zinc in the form of chloride salts) was investigated [67]. Using single-cation solutions, the rejection rates were 28.7% for strontium and about 99.9% for the other cations, with water permeances of 53.6 (Sr), 63.5 (Co), 67.4 (Ni), and 74.0 (Zn) L/m²·h·bar. In the case of cation-binary solutions, the rejection was of 99.9% for all the cations but strontium (0.4–0.6%), with water permeances in the 45–50 L/m²·h·bar range. Another permeation experiment using a binary solution of SrCl₂ and CoCl₂ (50 mg/L each) spiked with 250 Bq/L ⁸⁵Sr and 250 Bq/L ⁶⁰Co was carried out. The results showed that the rejection rates were 3.5% (⁸⁵Sr) and 99.8% (⁶⁰Co) with a water permeance of 47.8 L/m²·h·bar.

2.5. Miscellaneous Operations

Ion exchange resins are useful materials to recover/remove valuable/hazardous metals especially from dilute solutions; thus, the NaSO₃-based Purolite C100 cation exchange resin was used to remove Sr(II) (and Co(II)) from solutions of various pH values (1–6) [68]. As in many systems, the variation in the aqueous pH is a key point to achieving the maximum strontium removal. In the present case, this maximum is reached at pH 6, one hour of reaction time, and increased temperature from 25 to 55 °C. Different media (0.1 M each) were used to investigate the elution step, and the results indicate that the efficiency followed the sequence as follows: KCl > EDTA > NaOH > NaCl > CaCl₂. Loading-elution cycles showed that there is a continuous decrease in efficiency after repeated use (98.8% loading-93.7% elution in the 1st cycle, 67% loading-85.5% elution in the 7th cycle, 16.2% loading-53.4% elution in the 14th cycle).

The joint use of fine clinoptilolite (zeolite) and co-precipitated barite produced a composite core–shell coagulant used in the simultaneous removal of strontium(II) (and cesium(I)) [69]. While strontium is preferentially removed to Cs⁺, the Na-activation of natural clinoptilolite did not produce an enhancement in strontium-removal efficiency; with respect to the natural product, it showed a relative increase in this removal in the case of Cs⁺. The real advantage about the use of these core–shell coagulants is their faster settling and high-degree dewatering.

Spark plasma sintering was utilized for the fabrication of ceramic matrices designed for the immobilization of strontium [70]. The ceramic matrices (cenospheres) were formed from the use of two aluminosilicate mineral-like phases: pollucite (Cs,Na)AlSi₂O₆ and gehlenite Sr₂Al₂SiO₇. Within these microspheres or cenospheres, the removal of strontium from the solutions reached more than 98% efficiency. Under GOST R 52126–2003, similar to the American National Standards Institute/American Nuclear Society 2019 (ANSI/ANS 16.1) methodology, leaching tests (25 °C, distilled water of pH 6.8, 30 days, and static experimentation) on Sr-bearing samples, showed leaching rates in the range of 10⁻⁵–10⁻⁶ g/cm²·day.

The coal industry is also a source of effective adsorbents, and one of these materials, coal gangue (rich in silica-alumina oxides) was utilized as a starting material, after calcination at 900 °C for five hours. The as-calcined material was used to immobilize strontium in the form of Sr-cancrinite (Sr_xNa_8−2x(AlSiO₄)₆(NO₃)₂, where x ranges from 0.2 to 1) through a hydrothermal process (180 °C and one day) [71]. Under the PCT test [72], the resulting material showed adequate stability when x < 1.

The ceramic products, derived from cesium–strontium-co-doped geopolymers ((Cs, Sr)-GP), were synthesized by microwave sintering to achieve the co-immobilization of Cs and Sr [73]. The results show that (Cs, Sr)-GP with various Cs/Sr ratios (2–4) form Sr-feldspar–pollucite composite ceramics, the formation temperatures of monoclinic Sr-feldspar decrease from 1000 °C (for Sr-GP) to 800 °C (for (Cs, Sr)-GP) with Cs addition. The ceramic product heat-treated from the (4Cs, Sr)-GP at 1000 °C has a high crystalline and optimal leaching resistance and is considered an ideal host for immobilizing the real fission products containing Cs and Sr (no data included in the work).

A hydrate-based desalination (HBD) technology using methane, as a hydrate former, for freshwater recovery and the removal of Sr (and Cs) from simulated wastewater, was used [74]. The complete exclusion of strontium (and Cs⁺) ions from solid methane hydrates was confirmed by phase equilibria, spectroscopic investigations, thermal analyses, and theoretical calculations, resulting in the recovery of freshwater and the removal of these ions from wastewater.

Sr_0·5Zr₂(PO₄)₃-SmPO₄ dual-phase ceramics were prepared via in situ synthesis process and use in the removal of strontium [75]. In the synthesis process, the optimum microwave-sintering temperature and heat preservation time were 1050 °C and 1.5 h, respectively. Under the PCT rule, the leaching rate for strontium reached the value of 10⁻⁴ g/m²·day.

Phosphoric acid-activated metakaolin-based geopolymer (PAMG) was used in the solidification of strontium [76]. With the results indicating that the compressive strength of PAMG increased and then decreased as the P/A ratio increased, the highest compressive strength of 98.1 MPa after 28 days of curing resulted from the use of a P/A molar ratio of 1.8. The addition of Sr resulted in a decrease (86.7 MPa) in the compressive strength of PAMG; however, after 42 days, the leaching rate was of 5.0 × 10⁻⁶ cm/day.

3. Elimination of Radioactive ⁹⁰Sr from Liquid Solutions

3.1. Adsorption

A lithium titanate-decorated Ti₃C₂T_x MXene (LTO-MX) composite was synthesized using etching and alkali procedures and then immobilized using polyacrylonitrile (PAN) polymer via a phase inversion method. The resulting material (LTO-MX-PAN beads) was used in the removal of ⁹⁰Sr (5 Bq/mL) solution at different dosages and water conditions [77]. The results are summarized in

Table 6. ⁹⁰Sr adsorption using various water sources and adsorbent dosages.

Water Source	2 g/L	10 g/L	50 g/L	100 g/L
Distilled water	>90	>90	>90	>90
Ground water	70	>90	>90	>90
Sea water	20	45	70	80

Temperature: 25 °C. pH: 6.5. Time: one day. Adapted from [77].

.

These results showed that, when distilled water is the liquid source, the removal of ⁹⁰Sr was not affected by the adsorbent dosage. In the case of ground water, this effect was only noted when the lowest dosage was used to remove ⁹⁰Sr; in any case, this influence was critical in the case of sea water, with a sharp increase in the removal efficiency as the adsorbent dosage increases. This influence was attributed to the presence of competing cations in the respective waters: none in the case of distilled water (none); sodium (18.0 mg/L), potassium (1.7 mg/L), calcium (27.5 mg/L), and magnesium (9.5 mg/L) ions in the case of ground water; and sodium (10 g/L), potassium (0.39 g/L), calcium (0.41 g/L), and magnesium (1.3 g/L) ions when simulated sea water was used in the experimentation. Under the experimental conditions of 2 g/L adsorbent dosage, the ⁹⁰Sr removal reached the same efficiency as when the 1 mg/L Sr²⁺ non-radioactive solution was used.

The reaction of MnSO₄ and KMnO₄ solutions in the alkaline medium produced a pelletized adsorbent based on mixed Mn(III, IV) oxides. The best conditions for this synthesis were determined as follows: Mn²⁺/MnO₄^– molar ratio of 1.70–1.80, reaction mixture pH of 11.0–12.5, and calcination temperature of 220 °C. As a source for ⁹⁰Sr, a 0.01 M calcium chloride solution of pH 6.0, containing ⁹⁰Sr (105 Bq/mL), was utilized as the liquid phase. The adsorption efficiency remained constant with the variation (0.01–1.3 M) in Na⁺ ions in the feed solution but decreased with the increase (0.01–0.08 M) in calcium(II) in the liquid phase [78].

In order to improve the adsorption characteristics of zeolites, the next investigation used the partial interzeolite transformation as a direct and tunable procedure to reach better adsorptive properties [79]. Under column experiments (10 mL/min), a mixed solution of ⁹⁰Sr (37 Bq/mL) and ¹³⁷Cs (1000 Bq/mL) flowed through the beds of different adsorbent materials: Mud Hills (zeolite from California), mordenite (MOR topology from Java), and composites 85 MOR:9 GIS and 57 MOR:34 GIS (GIS topology: aluminous zeolite P). The results showed that, after having passed 0.25 L of the solution through the column, the ⁹⁰Sr activity in the outlet solution decreased to 40% (57:34 composite), 50% (85:9 composite), and about 55% for the other two tested adsorbents. This decrease is near 10% for the four adsorbents after 1.7 L of the solution flowed through the column. In the case of ¹³⁷Cs, the same efficiency was demonstrated.

Natural (NB) and sodium-modified (SMB-20) bentonites were proposed in the treatment of the Cherkasy deposit to remove ⁹⁰Sr [80]. From a model solution containing ⁹⁰Sr (6.57 Bq/mL), the addition of CaCl₂ and the variation in the solution pH was used to investigate the performance of the adsorbents in the removal of the radionuclide. The addition of calcium decreased the removal efficiency of both adsorbents, as did the variation in the pH from 7.4 to 11.8. Under all performance conditions, with respect to ⁹⁰Sr removal, the Ca-bentonite was better than that of the Na-modified material.

A molten salt synthesis strategy was used to investigate the reaction of different types of MXenes with nitrates. Among all the formulations, K⁺-intercalated hierarchical titanate nanostructures (K-HTNs) was used to investigate the removal of ⁹⁰Sr from solutions [81]. The initial ⁹⁰Sr activity (50–500 Bq/mL, pH 8) had little effect on the adsorption efficiency of K-HTNs; however, the type of water greatly influenced this removal: Milli-Q water (100%) versus simulated seawater (32.5%). The influence of the pH on ⁹⁰Sr removal in simulated seawater and real seawater (Zhejiang province, China) was also investigated; for both types of waters, the removal rate of the radionuclide increased slowly as the pH value increased from 5.0 to 10.0 and reached a maximum of 42% at pH 10.0.

Tetraselmis chui was used in the elimination of radioactive ⁹⁰Sr from a culture medium mainly composed of seawater, and this removal capacity was attributed to the intracellular formation of micropearls [82]. The removal of this radionuclide depends on T. chui cell density in the cultures, as well as on the initial ⁹⁰Sr concentration in the medium. It is shown that culture growth is not affected at ⁹⁰Sr concentrations of 3.1 × 10⁻¹² M (1.4 Bq/mL), but a heterogeneous effect on T. chui cultures containing higher ⁹⁰Sr concentrations (i.e., 2.7 × 10⁻¹⁰ M or 124 Bq/mL) that may be related to high levels of ionizing radiation.

3.2. Liquid–Liquid Extraction Operation

In order to yield ⁹⁰Y, for its application in various fields of cancer therapy as a radiopharmaceutical or as a selective internal radiation therapy (SIRT) brachytherapy source, ^235,238U-, ²³⁹Pu(nfast,f)-, ⁹⁰Sr(β−)-, ⁹⁰- irradiated fuel from Fast Breeder Test Reactor (Kalpakkam) was used as a basis for the separation of ⁹⁰Sr and ⁹⁰Y [83]. After the dissolution of the fuel in 11.5 M nitric acid, the leachate was treated in a liquid–liquid extraction operation using 0.1 M DtBuCH₁₈C₆ in an octanol medium. In the extraction step, Sr, Ru, Sb Cs, and Eu were loaded into the organic phase, which was subjected to four scrubbing steps using 12 M nitric acid to separate impurities, and an additional stripping step with dilute nitric acid as a strippant to yield a solution containing Sr, Ru, and Sb. After the separation of ⁹⁰Sr, along with trace-level impurities, the simultaneous purification of ⁹⁰Sr and ⁹⁰Y using single-stage ion chromatography was carried out.

4. Elimination of ⁸⁵Sr Radionuclide from Liquid Solutions

4.1. Adsorption

A magnesium molybdenum titanate (MgMoTi) composite was used to remove ⁸⁵Sr from aqueous solutions under various experimental conditions [84]. From the different formulations, the one containing MgCl₂·6H₂O (0.3 M)/Na₂MoO₄·2H₂O (0.3 M)/TiCl₄ (10% v/v) in 2/1/2 volumetric ratios presented the best ⁸⁵Sr removal efficiency (96.2%) at an initial pH value of 12 (pH_eq = 10, pH_eq is the pH value of the aqueous solution after the equilibrium in the adsorption process was reached). Several solutions were used (

Table 7. ⁸⁵Sr desorption using different solutions.

[Desorbent]	EDTA	ZnCl2	HCl
0.03 M	18.6	64.4	90.1
0.05 M	32.6	74.8	93.7
0.1 M	45.8	81.7	95.9

Adapted from [84].

) to investigate the best desorbing agent for the system. Experimental data concluded that the 0.1 M HCl solution was suitable for the removal of ⁸⁵Sr loaded into the adsorbent. Under continuous adsorption–desorption cycles, it was found that there is an important loss of efficiency: first cycle—96.6% loading and 95.6% desorption; fourth cycle—56.8% loading and 68.9% desorption; and seventh cycle—21.5% loading and 57.1% desorption.

As in previous works, a granular manganese oxide-based adsorbent was fabricated from MnSO₄ and KMnO₄ using a [Mn²⁺]/[MnO₄⁻] relationship of 1.8, aging the precipitate at pH 11, and subjected it to a final calcination step at 160 °C [85]. Experiments were carried out on a simulated water resembling one of Mayak-Russian origin, to which ⁸⁵Sr (100 Bq/mL) was added. Batch experiments showed that the ⁸⁵Sr/Ca separation factor was in the 65–67 order. Column experiments showed that, after three adsorption–desorption (0.5 M nitric acid) cycles, the adsorbent maintained its removal characteristics; moreover, variations in the granule size and in the hydraulic resistance of the bed were not observed.

Tin molibdenum (SnMo) adsorbent was used in the removal of ⁸⁵Sr (and ⁶⁰Co) radionuclides from aqueous solutions [86]. The variation in the pH of the solution showed that both radionuclides were better removed from solutions at pH values in the 5–8 range, with a removal order of ⁸⁵Sr > ⁶⁰Co. Additionally, 0.1 M HCl solutions were selected to desorb ⁸⁵Sr from the loaded adsorbent. After up to 15 adsorption–desorption cycles a continuous loss of the ⁸⁵Sr removal efficiency from the feed solution was observed: 1st cycle—98%, 7th cycle—47%, and 15th cycle—3%.

Continuing with the use of Ti-based adsorbents, sodium titanates were employed to remove ⁸⁵Sr from solutions [87]. Similarly to previous systems, the maximum ⁸⁵Sr removal was attained at pH values in the 6–10 range. There were no desorption data included in the investigation.

The spent alum sludge was employed for the removal of specific fission products, including ⁸⁵Sr (and ¹³⁷Cs, ^152+154Eu, ⁹⁹Mo, and ⁹⁹Tc) radionuclides from radioactive waste [88]. The second Egyptian Research Reactor provided the radioisotopes of ⁸⁵Sr (and ^152+154Eu). With a maximum uptake (about 90%) of ⁸⁵Sr at pH 12, there were no further data about what to do with the radionuclides-loaded adsorbent material.

4.2. Liquid–Liquid Extraction Operation

The well-known DCH18C6 crown ether, in this investigation, dissolved in various ionic liquids of formulation 1-alkyl-3-methylimidazolium bis[(trifuoromethyl)sulfonyl]imides ([C_nmim⁺][NTf₂⁻], n = 2,4, 6, 7, 10), was used in the removal of ⁸⁵Sr from nitric acid solutions [89]. Generally, ⁸⁵Sr was best-extracted from diluted (0.001 M) nitric acid solutions than from the concentrated (10 M) ones. With respect to the ionic liquid formulation, the extraction order followed the sequence n = 2 > 4 > 6 > 7 > 10 at every (0.001–10 M) nitric acid concentration. ⁸⁵Sr extraction responded to the formation of two species in the organic phase: SrCE(NO₃)₂ and SrCE²⁺ (CE = crown ether). In this last case, C_nmim⁺ species from the ionic liquid was released to the aqueous phase or raffinate (bearing in mind the question: is this likely to be the cause of potential raffinate poisoning?). No information about the stripping stage is available in the published material. For those less familiarized with ionic liquid conception, it is worth noting that these are a type of chemical composed entirely of ions and characterized, among other properties, as liquids at temperatures below 100 °C. From various compositions, the most used are those formed by couples of organic cation–organic anion and organic cation–inorganic anion.

5. Conclusions

This work reviewed the latest contributions (from the year 2024) on the recovery/removal of Sr²⁺, and the most common ⁹⁰Sr and ⁸⁵Sr radionuclides present in different media. In our opinion, unfortunately, many authors have confused the objectives of their work, since they first intend to use different methodologies to remove strontium radionuclides, but they use non-radioactive media in their research, which as understood, are situations very far from reality. Thus, in our opinion, the results of these investigations should be quarantined with respect to their usefulness in the elimination of these radionuclides since the effect that radioactivity can have on the stability of the materials (solids or liquids) used in the experimentation is not contemplated. As mentioned in [59], the presence of a certain dose of radioactivity degrades the extraction reagent, so the results are worse than when inert solutions are used in experiments; moreover, the extractant cannot be recovered for further use. Also, the authors of [45,67,82] discussed the loss of efficiency of the material when radioactive solutions of limited activity are used in their experimentation.

Also, the reviewers found that, in many works, especially in relation to adsorption processes, the respective authors do not consider the desorption stage, so the adsorption–desorption cycle was not complete, and the usefulness of the adsorbent used is not completely known.

This review showed that adsorption is the most widely used technology to remove strontium from different wastes. The direct comparison, in relation to the efficiency of strontium loading between all these adsorbents, is not straightforward since various investigations used very different experimental conditions (temperature, adsorbent dosage, strontium concentrations, etc.); however,

Table 8. Maximum strontium loadings onto different adsorbents.

Adsorbent	a Maximum Sr loading, mg/g	Reference
Ti3C2(OH)x-KH2PO4	1066	[46]
K-Zn-MOF	878	[49]
IIGK@MnO2	748	[22]
Zr-based-MOF	353	[51]
MgVO	316	[44]
Ca/Zr phosphate	227	[45]
Cu-Zn-MOF	221	[48]
K-HTNs	204	[81]
Sh. Comm.	182	[24]
NaNO3-based zeolite	122	[54]
AlBS	110	[35]
FCK(4)/ZIF-67	106	[32]
CaTiO3	102	[38]
CEPA@SBA-15-APTES	81.9	[55]
ETS-4	76.2	[25]
MnO2@ZIF-8	65.7	[21]
NFPC	63.5	[53]
GP-2	57.0	[30]
Functionalized silica	43.7	[47]
DGIST-12	37.8	[20]
Tin molybdate	33.5	[34]
Kaolin	15.0	[30]

^a Based on the Langmuir model.

summarized, just from a quantitative point of view, the maximum strontium loadings onto selected adsorbents. It can be from these the wide range of strontium loadings.

With respect to the various technologies used to adsorb this element of any of its radionuclides, the direct comparison between them is also not straightforward. The selection of one or another may be dependent on several circumstances, including strontium concentration (liquid–liquid extraction is recommended to treat higher strontium concentrations, whereas adsorption and possibly membranes are adequate for the treatment of diluted ones). The treatment of the waste is urgently needed (it is worth noting that liquid–liquid extraction operation required minutes, whereas adsorption probably required extensive period times) but depends on the availability of the given technology in the site, i.e., dependent on environmental urgency, etc.

However, it is possible to establish some special features of each one of these operational units (

Table 9. Differences between the reviewed hydrometallurgical separation technologies applied on strontium removal.

Technology	Operational Time	Useful Orders to Treat Sr Concentrations	Selectivity (Co-Existing Ions)	Type of Feed Solution	Phases Separation
Adsorption	min/h	10−3–103 mg/L	average-good	clarified/non-clarified	good
Liquid–liquid extraction	min	101–105 mg/L	good	clarified	sometimes problematic
Membranes	min/h	10−3–103 mg/L	average-good	clarified	good

Other experimental conditions influencing Sr removal, including pH of the feed solution, temperature, adsorbent or extractant concentration, membrane composition, etc. All the above technologies presented a reasonable scaling-up grade. The performance of all these technologies may be sensitive to the radioactivity of the feed solution.

). In the case of Sr removal, none of these have been scaled-up beyond laboratory experimentation.

Along with the above comments, some future works and challenges may be considered as follows (neither in specific order not excluding others):

-

The use of biomineralization as a sustainable approach for the elimination of strontium (and other contaminants) within the natural ecosystem.

-

The integration of bioreactors into strontium and other radionuclide-bearing systems; thus, hybrid technology development may increase robustness and implementation into nuclear waste disposal flowsheets.

-

The synthesis of various and stable MOFs, including UiOs, MILs, and ZIFs, and their nanocomposites developed for use in various wastewater treatment procedures like adsorption, membrane separation, and ion exchange.

-

The exploration of porous organic polymers (POPs), that, due to some of their properties (tunable porosity and functionality), are potential adsorbent materials for the effective separation of radionuclides or non-radioactive pollutants from aqueous systems.

-

The development of microbially induced carbonate precipitation (MICP) as a methodology to remove all types of pollutants from solid or liquid effluents.

-

The development of graphene-based materials can be a challenge for their potential use in the recovery of harmful (but also valuable) elements.

-

Post-remediation monitoring and surveillance are key points of the nuclear remediation process. Their usefulness is to ensure that the remediation measures taken are effective and that unexpected complications do not appear over time.

-

The scaling up of the different methodologies used in the removal/elimination process and within the most realistic situations (the usage of radioactive samples to have a real control of what can be used and what cannot be used).

As cleverly mentioned in the recent literature [90], the recycling of strontium from waste materials does not actually compete with the mining industry compared to raw ores. Besides environmental concerns, the industrial scaling up of Sr recycling (also extendable to other metals) must consider several factors, including the location of the industry, labor costs, transportation, consumption, generation of wastes, etc. At the laboratory scale of the current investigations, these items are far from being known or even imagined.