Selective removal of uranium from rare earth leachates via magnetic

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Selective removal of uranium from rare earth leachates via magnetic solid-phase extraction using Schiff base ligands Laurence Whitty-Leveille, Nicolas Reynier, and Dominic Larivière Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03318 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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Industrial & Engineering Chemistry Research

Selective removal of uranium from rare earth leachates via magnetic solid-phase extraction using Schiff base ligands

Laurence Whitty-Léveillé1, 2, Nicolas Reynier1, 2, Dominic Larivière2* 1

: CanmetMINING, Natural Resources Canada, Ottawa, ON Canada, K1V 1E1 2 : Département de chimie, Université Laval, Québec, QC Canada, G1V 0A6

Abstract This study highlights a new type of extraction procedure combining a magnetic solid-phase support and a Schiff base ligand for the removal of uranium from rare earth leachates. After synthesizing and characterizing 3 Schiff base ligands (CH3Salen, H2Salophen and MeOSalophen), they were combined with magnetic nanoparticles and assessed for the selective extraction of uranium. To optimize extraction, the effects of parameters such as the pH, the mass of the ligand, and extraction time, were explored. The maximum adsorption capacity for U(VI) using the magnetic MeOSalophen Schiff base was 63 ± 3 mg g-1 at pH = 6 with 25 mg of extracting agent for a contact period of 24h. An acceptable extraction of U(VI) was also observed in more acidic conditions with a shorter extraction period. The magnetic ligand has shown a high selectivity towards U(VI) over a number of metals in a real rare earth element leaching solution.

Keywords : Actinides, Selective separation, Magnetic support, Schiff base, Adsorption

*

Corresponding author: Dominic.lariviè[email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Rare earth elements (REEs) have unique physicochemical properties that make them essential in many high technology components1 While REEs can be extracted from a large number of rare earth minerals, only three are predominantly used in the production of rare earth oxides, namely bastnasite ((Ce, La)(CO3)F), monazite ((Ce, La, Nd, Th)PO4) and xenotime (YPO4).2 In addition to REEs, uranium and thorium are often found in rare earth minerals such as monazite via lattice substitution.2 Uranium can usually be found at concentrations up to 5% and 0.1% in xenotime and bastnasite, respectively, and at trace levels in monazite.3 The presence of naturally occurring radioactive materials (NORM), including U and Th, in these minerals is problematic from both a regulatory and health physics perspective for the mine operator.4,5 Thus, effective methods to separate NORM from valuable REE constituents are essential.6 While most of the short-lived decay products from Th- and U-series, such as Ra, Pb, Po and Bi, can be segregated from REEs as they exhibit different chemical behavior during acid leaching and separation steps,7 the separation of naturally-occurring actinides from lanthanides still need to be addressed. Numerous strategies have been published to segregate U/Th from REE leach liquor. As an example, Sadri et al. recently reported that precipitation through pH adjustment followed by selective redissolution could be applied to REE leach liquor to isolate U, Th and REE in distinct fractions.7 However, these steps require precise pH adjustment and multiple phase separation to obtain proper level of separation. Zhu et al. also reported that the combination of solvent extraction, selective dissolution and ion-exchange resin could be used to purify REE fractions from


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U/Th impurities.6 However, this separation strategy, in addition to be cumbersome, generates significant volumes of radioactive acidic liquid wastes. The use of selective solid sorbents could be an interesting alternative both in terms of applicability and reusability to current hydrometallurgical processes. Recent developments in solid-phase extraction (SPE), with respect to support design and surface functionalization have provided improvements in terms of enrichment factors, phase separation (i.e. absence of emulsion) and recoveries, has shown promising performances for retrieving actinides in environmental matrices but have yet to emerge in hydrometallurgy.8 Molecular recognition-based sorbents, like molecularly imprinted polymers (MIP) and metal-organic frameworks (MOF), are a promising class of sorbent materials showing encouraging performances for actinides sequestration.9 While MIP and MOF generally exhibit very large extraction capacities (> 200 mg of analyte per g of adsorbent) and are designed to extract specific ions of interest,10 the real degree of selectivity of these materials is seldomly reported. In addition, the stringent synthetical routes and prohibitive costs also make MIP and MOF less attractive for commercial uses in large hydrometallurgical operations.11,12 MIP and MOF also frequently exhibit poor elution of the targeted analyte, limiting their reusability.14 Magnetic solid-phase extraction (MSPE), employing Fe3O4 nanoparticles as a solid support, is an attractive alternative to other solid supports in terms of production cost and phase separation.13,14 Fan and coworkers recently reported on the use of magnetic Fe3O4@SiO2 composite particles to remove uranium ions from aqueous solution.15 Adsorption process demonstrated a maximum calculated uranium sorption capacity onto their magnetic nanoparticles (MNPs) composite particles of approximately 52.0 mg g-1 at 25°C. The composite MNPs showed a good selectivity for uranium in the presence of other interfering ions such as Mg(II), Ca(II), Zn(II) and Sr(II). More 3 ACS Paragon Plus Environment


recently, U(VI) ions were separated in water with MSPE by Calì and co-workers with functionalized MNPs using a phosphate-based complex coating.16 Adsorption tests at pH = 7 in the presence of competing ions, such as Sr(II), Ca(II) and Mg(II), showed a high degree of selectivity for U(VI). While the magnetic nanoparticles provide an interesting strategy for phase separation, they lack the selectivity required for hydrometallurgical applications involving actinides and lanthanides. Polydentate ligands with both soft nitrogen and hard oxygen donor centers, such as salicylaldehyde derivatives, commonly known as Schiff bases, are particularly promising for actinide and lanthanide separation.17 Their chemical structures provide metal-binding pockets with controllable denticity and electronic properties suitable for selective extraction.18 The resulting coordination site, a tetradentate configuration based on the N2O2 binding pocket, is able to bond metals effectively. Amongst the member of the Schiff bases family, the salophen derivatives provide a rigid structure and persistent framework which is suitable for U complexation.19 Several adsorbents, such as bifunctional graphene20 and sodium dodecyl sulfate coated alumina21 have been functionalized or used with different Schiff base ligands for the coordination of actinides ions and have demonstrated some degree of selectivity and outstanding extraction capacity. In 2013, a composite material of Fe3O4 and N,N’-bis(3-methoxylsalicylidene)-1,2-phenylenediamine (MeOSalophen) for the extraction of uranyl ions from dilute aqueous solution was reported by Zhang et al.27. This composite material showed excellent adsorption capacity (94.30 mg U g-1) and rapid phase separation. However, the degree of selectivity of this material compared to competing elements as well as its applicability towards mineral leachates were not reported. Herein, the performances of magnetic Schiff bases for the selective adsorption of UO22+ were evaluated in surrogate solutions and REE leach liquors obtained from a REE-oriented Canadian 4 ACS Paragon Plus Environment

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venture. The effect of pH, extractant mass, contact time, uranyl concentration, temperature and the presence of other ions on the extractive properties of the proposed materials were assessed and reported. Finally, the thermodynamic parameters for the extraction of the uranyl ion were also evaluated using the Langmuir and Freundlich isotherm models.

2. Experimental 2.1 Preparation of samples Nanopure water (18.2 MΩ·cm at 25 °C) obtained using a Milli-Q system (Millipore, Bedford, MA) was used to prepare diluted solutions. An appropriate amount of UO2(NO3)2·6H2O (IBI Labs, Boca Raton, FL) was dissolved in nanopure water in order to achieve a 1000 mg L-1 stock solution of uranyl ion. Na2SO4, NaCl, or NaNO3, all purchased from Sigma-Aldrich (Saint-Louis, MO), were added to achieve a final anionic concentration of 4000 mg L-1, to mimic the nature of the media found in the mining industry.11A standard solution of uranium and thorium at a nominal concentration of 1000 mg L-1 (SCP Science, Montreal, QC) and a multi-element standard of REEs at a concentration of 100 mg L-1 (Inorganic Ventures, Christiansburg, VA) were used to prepare synthetic solutions. Working solutions were prepared by diluting the standard solution with nanopure water. The mining leachate solution was provided by Search Minerals (Vancouver, BC) and used as received.22 The composition of the solution is shown in Table 1. Reagents and solvents used for the synthesis of the Schiff bases were obtained from Sigma-Aldrich (Millipore Sigma, St. Louis, MO) and used without further purification.



Table 1. Composition of the major anions and cations present in Search Mineral leachate Component ClNO3SO42PO43-

Concentration (mg L-1) 28 ± 1 5.6 ± 0.3 11,800 ± 600 < 0.90

U(VI) Th(IV) LREE HREE

2.2 ± 0.1 13 ± 1 810 ± 20 66 ± 4

Fe(III) 1,020 ± 10 Si(IV) 220 ± 10 Ca(II) 160 ± 60 Pb(II) 2.0 ± 0.1 Cu(II) 3,700 ± 230 Ni(II) 5.6 ± 0.9 Cd(II) 53 ± 4 pH = 1.82 ± 0.03. Light rare earth element (LREE): Sc, La, Ce, Pr, Nd, Sm, Eu. Heavy rare earth element (HREE): Gd, Tb, Dy, Ho Er, Tm, Yb, Lu, Y.

2.1.1 Synthesis of Fe3O4 nanoparticles Fe3O4 nanoparticles were synthesized by a co-precipitation process: FeCl2·4H2O (0.5 g, 2.5 mmol) and FeCl3 (0.8 g, 5 mmol) were mixed in a 100 mL round bottom flask with 50 mL of nanopure water. The mixture was purged with N2 and heated at 70°C for 30 min. Then, 30 mL of 1 M NH4OH was added, and the mixture was reheated at 70°C for another 30 min. The resulting nanoparticles were isolated using a magnetic bar, washed several times with nanopure water, and dispersed in 20 mL of nanopure water. The nanoparticles were characterized with a Transmission Electron Microscope (TEM).


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2.1.2 Synthesis of Bis(2-hydroxyacetophenone) ethylenediimine (CH3Salen) Ethylenediamine (1.2 g, 20 mmol) was added to a solution containing 2-hydroxyacetophenone (5.5 g, 10 mmol) in 25 mL of ethanol. The solution was heated at 50°C and stirred for approximately 1 h, yielding a yellow precipitate. The precipitate was filtered and washed with ethanol and airdried at 40°C for 12 h. The CH3Salen was recrystallized from ethanol to give a final yield of 90%. Spectral data were identical to those previously reported.23 1H NMR (500 MHz, DMSO): δ 12.31 (s, 2H), 6.7–7.7 (m, 8H), 3.79 (s, 4H), 2.23 (s, 6H).

2.1.3 Synthesis of Bis(salicyla1dehyde) o-phenylenediimine (H2Salophen) Salicylaldehyde (2.0 g, 16 mmol) was mixed with o-phenylenediamine (0.9 g, 8.3 mmol) and stirred for 10 min without any solvent. As the reaction is exothermic, the temperature of the mixture increased by itself and it became viscous. The mixture was allowed to stand for 15 min and was then poured with vigorous stirring into 40 mL of 95% ethanol and agitated for 1 h. The solid mass that formed was filtered and air-dried at 40°C for 12 h. The orange product was recrystallized with ethanol. The overall yield was 81%. Spectral data were identical to those previously reported.24 1H NMR (500 MHz, CDCl3): δ 13.05 (s, 2H), 8.64 (s, 2H), 7.39 – 7.34 (m, 6H), 7.26 –7.23 (m, 2H), 7.05 (d, 2H), 6.92 (t, 2H).

2.1.4 Synthesis of N,N’-bis(3-methoxylsalicylidene)-1,2-phenylenediamine (MeOSalophen) Following a modified procedure conceived by Chen and Martell,25 a solution of o-vanillin (3.0 g, 20 mmol) in 25 mL of ethanol was mixed with a solution of o-phenylenediamine (1.1 g, 10 mmol) in 25 mL of ethanol. The mixture was stirred, and, an orange product precipitated after about 10 7 ACS Paragon Plus Environment


min. The mixture was stirred for another 60 min to ensure the completion of the reaction. The solid was filtered, washed with ethanol, and then air-dried in the oven at 40°C for 12 h. The MeOSalophen was then recrystallized using ethanol. The overall yield was 85%. Spectral data were identical to those previously reported.26 1H NMR (500 MHz, CDCl3): 13.17 (s, 2H), 8.62 (s, 2H,), 7.35-6.84 (m, 10H), 3.90 (s, 6H).

2.1.5 Preparation of the magnetic Schiff base A ligand/sodium dodecyl sulfate (SDS) solution was prepared by mixing 150 mg of MeOSalophen and SDS (0.25 g, 0.9 mmol) in 100 mL nanopure water. After the complete dissolution of the SDS, 40 mL of a solution containing 0.67 g (7.2 mmol) of Fe3O4 nanoparticles were added to the above ligand/SDS solution. The pH of the solution was adjusted to 2 using 2 M HNO3 in order to form ligand-deposited micelles which will interact with ferroferric oxide nanoparticles following the Pickering emulsion principle.27,28 The magnetic Schiff base (MSB), after being stirred for 2 h, was isolated using a magnetic bar, washed with water and then dried at room temperature.

2.2 Adsorption experiments The adsorption of uranium from the dilute aqueous solution was performed using 0.025 g of MSB mixed with 20 mL of metal solution in a conical tube. The solution was agitated at 20 °C on an oscillator at 230 strokes min-1. At the end of the adsorption period, the MSB were isolated from the solution by magnetic separation using a NdFeB magnet with a surface magnetic field of 3 723 G. The pH of the solution was measured prior and after every extraction test to determine the impact of the MSB on this parameter, but measured pH remained unchanged. Method blanks were


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performed by replicating the experiment in absence of either ligand or analyte. The initial and the equilibrium concentration of ions of interest in the supernatant were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES series 725, Agilent, Santa Barbara, CA). The adsorption capacity Qe (mg·g-1) was calculated according to the following equation:

𝑄𝑒 =

(𝐶0 ― 𝐶𝑒) ∙ 𝑉

(1)

𝑚

where C0 and Ce (mg L-1) are the initial ion concentration and at equilibrium (in the supernatant), respectively, V (L) is the volume of the testing solution, and m is the mass of sorbent (g). Intrinsic material characteristics such as distribution coefficient (Kd) of Ln and An ions between the solid and aqueous phases and separation factors (SFAn/Ln = KdAn/KdLn) are also critical when comparing solid phase extraction techniques.29,30 Kd were calculated by the following equation:

𝐾𝑑 =

𝐶𝑜 ― 𝐶𝑒 𝐶𝑒

𝑉

(2)

∙𝑚

where C0 (mg L-1) is the ion concentration in the initial solution, Ce (mg L-1) is the equilibrium concentration of ion in the supernatant, V (L) is the volume of the testing solution, and m is the mass of sorbent (g). As reported in the literature, the higher the Kd value, higher the extraction capacity on the solid sorbent is. Indeed, Kd values above 500 are considered acceptable, those above 5000 are considered very good, and Kd values in excess of 50 000 are considered outstanding.31

2.3 Characterization and analysis 9 ACS Paragon Plus Environment


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Transmission Electron Microscope (TEM) images were obtained using a Technai G2 Spirit BioTwin (FEI, Hillsboro, OR) with an accelerating voltage of 120 kV. Using those TEM images, the mean diameter of 100 nanoparticles was calculated using ImageJ software (National Institute of Health, Bethesda, MD). Attenuated Total Reflectance (ATR) study were collected on a MB3000 (ABB, Richmond, BC) using a MIRacle single bounce ATR diamond kit. Schiff base ligands were also analyzed by powder X-ray diffraction on a Panalytical Aeris (Malvern Panalytical, Royston, UK) X-ray benchtop diffractometer (Bragg-Brentano geometry, 40 kV, 7.5 mA)

3. Results and discussion 3.1 Selection of ligand Three Schiff base ligands were synthesized by condensation of a diamine and an aromatic compound. In Schiff base-uranium complexes, such as the salen and salophen tested in this paper (Scheme 1), the U(VI) ion is coordinated by two oxygen atoms in axial positions and five in the equatorial plane (pentagonal bipyramid).32 A)

B) N

N

OH HO

C) N

N

N

N

OH HO

OH HO O

O

Scheme 1. Structures of the Schiff base ligands synthesize and tested. A) CH3Salen, B) H2Salophen, C) MeOSalophen


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Computational studies performed by Brynda et al. highlighted the geometry and the interactions of the Schiff base complexes and the uranyl ion.32 Deprotonation of the Schiff base ligands, generate a complex between the uranyl ion and the ligand that is electronically neutral. The radius of the semi-cavity of the salophen moiety (2.39 Å) is slightly smaller than the ionic radius of the uranyl ion (2.80 Å) as reported by Brynda et al.32 Despite this apparent discrepancy, the presence of the free electron pairs on two imine nitrogens and the negative charge of the two deprotonated phenolic groups force the ion into the semi-cavity (Scheme 2).32 The presence of aromatic rings mean that the Schiff base structure is relatively adaptable, as demonstrated by Crane and MacLachlan,33 since different moieties can be added on the aromatic rings.

N

R O

R 1'

N

R

R1

U O

O

R4

R2'

O R4'

R3'

N

R2

O R3

R4

O U O

R1'

N

R2'

O R4'

R3'

Scheme 2. General structure of the Schiff base family ligands and its coordination of the uranyl ion. Substituents on the phenyl rings (R1-R4, R1’-R4’) are freely modified and may be used for easy control over the ligand properties.

By using various salicylaldehyde and/or diamines derivatives, the influence of the electronic and steric properties of the active center can be fine-tuned to enhance complexation properties.34 The effects of different moieties on the extraction of uranium is presented in Figure 1, where 0.08 mmol of each crystalline Schiff base ligand were used to complex a solution of 100 mg L-1 of U. This extraction test was performed at the pH obtained when UO2(NO3)2·6H2O is dissolved in nanopure water.



60 50 40

Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30 20 10 0 CH₃Salen

H₂Salophen

MeOSalophen

Figure 1. Effect of structure on the adsorption capacity of U(VI) ions by 3 crystalline Schiff bases. Initial uranium concentration 100 mg L-1, pH 6, temperature 20°C, adsorbent mass 0.08 mmol, stirring time 24 h.

Using the conditions described above, the adsorption capacity was 1.6-fold higher for MeOSalophen than for either CH3Salen or H2Salophen. The observed differences of adsorption capacity between the Schiff bases can be explained by enhanced electronic effects on the MeOSalophen. As the methoxy groups from MeOSalophen are electron donors, the phenolic oxygens should interact more efficiently with the uranyl cation, resulting in an enhanced adsorption capacity, which is consistent with the results obtained. Considering the above observations, the MeOSalophen was chosen as Schiff base ligand for the rest of this investigation.

3.2 Characterization of the magnetic Schiff base


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Chemical co-precipitation between Fe(II) and Fe(III) is a simple but effective method for preparation of hydrophilic nanosized magnetic particles. Fig. 2A depicts the TEM image of the asmade magnetic nanoparticles. They are spherical in shape with a mean diameter of (14 ± 4) nm. As shown in the image, the magnetic particles tend to aggregate together, which is due to their nanometric size.35 The TEM image of the MSB (Fig, 2B) confirms the successful deposition of Fe3O4 nanoparticles on the surface of the Schiff base crystal, suggesting a strong interaction between the two constituents. It has to be noted that some nanoparticles did not adhere to the Schiff base crystal, as seen in the upper corner of Fig. 2B.

Figure 2. A) TEM image of the Fe3O4 nanoparticles. B) TEM image of the magnetic Schiff base. To identify the key components for the complexation of the uranyl ions onto the MSB, ATR spectra of MSB (Figure 3) were compared before and after a 24h contact time period at pH = 6. After extraction, an adsorption band appears at 877 cm-1 (Figure 3A), which is a characteristic of O-U-O elongation and confirms the complexation of the uranyl ion on MSB. Moreover, in the region of 1265 cm-1, where the phenolic groups in MeOSalophen are identified (Figure 3B), the peak shifted to 1258 cm-1 after adsorption of U(VI). These observations confirmed that the phenolic groups contribute to the uptake of the uranyl ions and are in agreement with the DFT calculation made by Brynda and co-workers on a similar Schiff base ligand.32 13 ACS Paragon Plus Environment


Figure 3. ATR spectra of MSB before extraction (dotted line) and after extraction (full line) of a solution containing 100 mg L-1 of U(VI).

3.3 Effect of solution pH in various media As with many adsorption processes, elemental speciation based on the pH of the extracting environment plays a predominant role in the interaction between the solid phase and the analyte. At pH ≤ 4, UO22+ is the main species present and the adsorption on solid phases is generally limited due to the presence of H+ ions that compete for the binding sites on the ligand.36,37 At pH 4–6, where UO22+, UO2OH+, (UO2)2(OH)22+, (UO2)3O(OH)5+, (UO2)4(OH)7+and UO2(CO3)(OH)3coexist,38 the adsorption typically reaches a maximum. Furthermore, the possible precipitation of uranium in the conditions tested (medium, pH and concentration of U(VI) as high as 100 mg L-1) might result in an overestimation of the Qe value.39,40 Inversely, as reported by Li et al. 36, the formation of soluble carbonated complex at pH ≥ 7 combined with the deprotonation of the ligands


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could reduce the adsorption of negatively charged U species. 33,41 It is therefore expected that U(VI) adsorption on MSB will be maximized between pH 4 and 6. Thus, the adsorption of U(VI) onto magnetic Schiff base was tested at various pH (1.0–6.0) in the presence of various counter anions (NO3-, Cl-, SO42-); representative of leaching strategies commonly used in hydrometallurgy (Fig. 4). The pH value was adjusted using H2SO4, HCl or HNO3, depending on the media, and NaOH. 90 80 70 60

Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

NO₃⁻

40

Cl⁻

30

SO₄²-

20 10 0 1

2

3

4

5

6

pH

Figure 4. Impact of various media on the removal of U(VI) ions by MSB. Initial uranium concentration 100 mg L-1, anion concentration 4,000 mg L-1, temperature 20°C, adsorbent mass 0.025 g, stirring time 24 h. As shown in Fig. 4, the pH of the solution plays a critical role on the absorption capacity as predicted by the speciation study while the chemical nature of the anion plays a limited role in the complexation of the uranyl ions and thus has a limited impact on the adsorption capacity with the magnetic extractant. At low pH values (< 5), the extraction is limited in nitric media (< 10 mg g1),

intermediate for chloric media (< 15 mg g-1), and higher for sulfates media (20–30 mg g-1).



This observation suggests that the sulfate ions enhance the complexation between MSB and the uranyl ion than compared to their chloride or nitrate counterpart. The stronger binding of sulfate ions, is attributed to the charge-transfer contribution to the binding energy.42 As for the chloride anions, a study of their association constant with different types of salens has shown that they bind more strongly than nitrate anions, which were also observed in this study.43 At pH < 5, the slight decrease of the uptake of U in the acidic media may be attributed to the protonation of the lone pair of nitrogen and oxygen atoms on the Schiff base that hinder the uranyl ion complex formation.44,45 Independently of the acidic medium tested, adsorption capacity unequivocally increases at pH > 5. For the rest of this study, sulfate was chosen as the counter ion at pH = 6 for the extraction processes.

3.4 Effect of sulfate concentration The influence of the amount of sulfate on the efficiency of the adsorption capacity of MSB was also investigated. This investigation is important as mining leachate solutions tend to contain high concentrations of dissolved anions and cations. The impact of the concentration of sulfate (1000 – 13,000 mg L-1) on the adsorption capacity at pH = 6 was evaluated (Fig. 5). The results showed that the amount of sulfate present enhances the extraction of U(VI) ions by the adsorbents at concentrations up to 4000 mg L-1, where a plateau is reached. This finding and the influence of the nature of media on the adsorption process shown in Section 3.3, could indicate that the adsorption mechanism for the removal of uranyl ions by the proposed magnetic ligand is an inner-sphere surface complexation.46 This would then suggest that the selective adsorption mechanism is physical, rather than a chemical sorption.47 The results obtained


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also demonstrate a certain degree of tolerance of the system to sulfate ions, which is a welcome feature for mining applications. The concentration range tested is coherent with data reported in Table 1. 70 65 60

Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


55 50 45 40 35 30 1000

4000

7000

SO42- concentration

10000

13000

(ppm)

Figure 5. Effect of the sulfate concentration on the removal of uranyl(VI) ion by magnetic Schiff base. Initial uranium concentration 100 mg L-1, pH 6, temperature 20 °C, adsorbent mass 0.025 g, stirring time 24 h.

3.5 Effect of contact time and adsorption mechanisms Using the magnetic Schiff base, the adsorption kinetics parameters of U(VI) ions were assessed. The results are presented in Figure 6. The adsorption rate of the magnetic adsorbent is relatively slow ((5.3 ± 0.4) mg g-1 h-1) for the first 24 h, until saturation of the binding sites is achieved. The slow adsorption kinetics can be explained in part by the fact that some active sites on the crystallized ligand are barely accessible to the analyte, as reported by Zhang and co-workers.28



70 60 50

Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 0

12

24

36

48

Time (h)

Figure 6. Adsorption kinetic of U (VI). Initial uranium concentration in sulfate media 100 mg L1, pH 6.0, temperature of 20 °C, amount of magnetic Schiff base 0.025 g.

To determine the controlling mechanism of the adsorption process, the adsorption data was compared to pseudo-first and pseudo-second-order kinetic models simulations.48,49 The pseudofirst-order kinetic equation is given as: 𝑑𝑄 𝑑𝑡

(2)

= 𝑘1(𝑄𝑒 ― 𝑄𝑡)

where k1 is the rate constant of pseudo-first-order adsorption, Qe and Qt (mg g-1) refer to the amount of uranium(VI) ions adsorbed at equilibrium and at time (t), respectively. Integrating Eq. (2), the equation becomes: (3)

log (𝑄𝑒 ― 𝑄𝑡) = log (𝑄𝑒) ― 𝑘1𝑡/2.303

The pseudo-second-order kinetic model developed by Blanchard and co-workers50 uses experimental data to decipher the sorption mechanism onto a solid. The model is mathematically represented using the following equation: 18 ACS Paragon Plus Environment

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𝑑𝑄𝑡 𝑑𝑡

= 𝑘2(𝑄𝑒 ― 𝑄𝑡)2

(4)

When Eq. (4) is integrated for a period of time, t: 𝑡

1

𝑄𝑡

𝑡

(5)

= 𝑘 𝑄𝑒2 + 𝑄𝑒 2

where k2 is the rate constant of pseudo-second-order adsorption. The values of Qe, k1 and k2 are calculated from the intercept and slope values of the plot (Fig. S1A and B, in the supporting information) corresponding to Eqs. (3) and (5), and are presented in Table 2. It was concluded that the calculated and experimental equilibrium adsorption capacities are better correlated by the pseudo-first-order adsorption model. Therefore, it can be postulated that the adsorption kinetics follows a pseudo-first-order model indicating that the adsorption is more inclined towards physisorption than chemisorption.51

Table 2. Pseudo-first and pseudo-second-order constants and values of R2 for magnetic Schiff base Kinetic model Pseudo-first order Pseudo-second order

C0 (mg L-1) 100

Qe exp (mg g-1) 63.20

Qe cal (mg g-1) 62.67

k1 (h-1) 0.17

k2 (g mg-1 h-1) —

R2 0.9972

100

63.20

78.71

—

0.0015

0.9559

3.6 Adsorption isotherms of uranium Based on the adsorption kinetics shown in Fig. 6, it is possible to calculate the adsorption isotherms for the proposed system. These isotherms are used to describe the distribution in the liquid and solid phases of the sorbate after equilibration. The most commonly used adsorption models are the 19 ACS Paragon Plus Environment


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Langmuir and Freundlich models. The Langmuir isotherm model is based on the following assumptions: (i) each site accommodates only one atom or molecule, (ii) adsorption sites are localized on the ligand, and (iii) adsorption energy is constant over all sites.52 A linear representation of this model is: 𝐶𝑒

1

𝐶𝑒

(6)

𝑄𝑒 = 𝑏𝑞𝑚𝑎𝑥 + 𝑞𝑚𝑎𝑥

where Qe (mg g-1) is the amount of metal ions adsorbed at equilibrium, qmax (mg g-1) the capacity of adsorbent, and b (L mg-1) is a constant related to energy of adsorption. The values of qmax and b can be obtained by the slope and intercept of the correlation curve (Figure S2A, in the supporting information). The results are presented in Table 3. The Freundlich equilibrium isotherm describes the multilayer adsorption with interactions between adsorbed species. The model assumes that the energy associated with the adsorption surface is heterogeneous, and that the adsorption processes is proportional with the concentration in solution.53 The corresponding linear equation is: 1

(7)

ln𝑄𝑒 = ln𝐾𝐹 + 𝑛ln𝐶𝑒

where kF (mg g-1)·(mg L-1)n and n (a dimensionless parameter) are the constants determining adsorption capacity and adsorption intensity, respectively. They can be evaluated from the intercept and slope of the linear plot of ln Qe versus ln Ce (Figure S2B, in the supporting information) and are reported in Table 3. If 1/n equals one, the adsorption is linear. If the value is below one, process based on a chemical adsorption is proposed, whereas a physical process is expected when 1/n > 1. The closer the 1/n value tends towards zero, the more heterogeneous the surface is expected to be.54 20 ACS Paragon Plus Environment

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Table 3 shows the parameters determined using Langmuir and Freundlich isotherms model with the experimental data. The R2 values are used to evaluate the best fit of the experimental data. Based on the data presented in Fig. S2, the Langmuir isotherm model better describes the adsorption process occurring between UO22+ and the magnetic Schiff base. According to this model, a calculated qmax for the adsorbent equivalent to 60.20 mg uranyl g-1 is achievable in the experimental conditions tested. The closeness of the calculated qmax value to the one experimental value (63.20 mg g-1) confirms the applicability of this model for interpretation of the adsorption processes, which implies that the uranyl ion extracted forms a monolayer coverage at isolated adsorption sites. Table 3. Parameters determined using Langmuir and Freundlich models Langmuir Isotherm Parameter Unit Value 2 R 1.000 — -1 qmax mg g 60.20 b L mg-1 6.68x10-15

Freundlich Isotherm Parameter Unit Value 2 R 0.8228 — KF 0.38 — n 0.88 — 1/n 1.13 —

3.7 Effect of the amount of magnetic Schiff base The mass of MSB used is one of the key factors in an industrial extractive process, as the cost of the extractant is a non-negligible parameter in hydrometallurgy. In Figure 7, uranyl ion extractions using different masses of MSB were performed with a solution containing 100 mg L-1 of U at pH = 6. The quantity of MSB used in this test ranged from 5 to 100 mg. Initially, U(VI) removal increased significantly with an increase in adsorbent (5 to 25 mg), and reached a maximum capacity of approximately 60 mg g-1 when larger quantities of MSB was used. The change in the maximum capacity at lower masses of MSB was unexpected since Qe is supposed to be constant. 21 ACS Paragon Plus Environment


This odd behaviour was also reported by Zhang et al.27 whom also observed a change in Qe for a concentration < 20 mg of Schiff base. The authors reported that the increase in adsorption from 5 to 20 mg of sorbent was the result from an increase in surface area and a greater availability of adsorption sites. However, these hypotheses were never investigated. It could also be postulated that such decrease in capacity at lower sorbent masses could be caused by the partial solubility of the Schiff base in the solution. Since the Qe value is determined by the difference in the uranium concentration (initial versus final), the measure of dissolved MSB - uranium complexes would be interpreted as uncomplexed uranium. This would then affect the calculated Qe value. This hypothesis was investigated by repeating the extraction experiment using a larger volume of solution (40 mL instead of 20 mL) while the other conditions were kept identical (i.e. amount of ligand, pH, [U], contact time, temperature). In those conditions, the values obtained at lower sorbent masses (5-10 mg) were about half of those reported in Figure 7 for similar masses, strongly supporting the hypothesis regarding the partial solubility of MSB. Based on those data, it was possible to calculate the solubility for the MSB in the conditions presented (130 ± 70 mg L-1). In order to minimize the amount of MSB used while ensuring a limited effect of the solubility on the Qe, an extractant mass of 25 mg was used throughout the rest of this experiment.


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70 60 50 Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30 20 10 0 0

20

40 60 Mass (mg)

80

100

Figure 7. Effect of magnetic Schiff base mass on the uptake of U(VI) in sulfate media. Initial uranium concentration 100 mg L-1, pH 6, temperature 20 °C, adsorbent mass 0.025 g, stirring time 24 h.

3.8 Selectivity of magnetic Schiff base for actinides in solution The selectivity of the adsorbent for actinides over lanthanides was then investigated (Figure 8). The adsorption performance for a specific cation was evaluated in terms of Kd values. In a surrogate solution composed of U, Th and REEs at pH = 6, a high selectivity toward actinides was observed, and the reported Kd value exceeded 5,000 mL g-1 for uranium. Much lower Kd values were obtained for REEs (< 50 mL g-1) in the conditions tested.



9,000 8,000

Distribution coefficient Kd (mL/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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7,000 6,000 5,000

Fe₃O₄

4,000

MSB

3,000 2,000 1,000 0 U (VI)

Th (IV)

La (III)

Eu (III)

Dy (III)

Figure 8. Selectivity results of the magnetic Schiff base in a sulfate surrogate solution. Initial concentrations of U and REEs were1 mg L-1, initial concentration of Th was 10 mg L-1, pH 6.0, temperature 20°C, mass of adsorbent 0.025 g, stirring time 24 h.

This translated into a separation factor (KdU/KdEu) of more than 90, which compares favourably to other ligands recently proposed in the literature for the segregation of actinides and lanthanides (Table 4).


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Table 4. Comparison of different SFU/Ln values for ligands in various aqueous solutions

Ligand 2,2’-dipyridyl-6,6’-dicarboxylic acid 2,9-bis (1,2,4-triazin-3-yl)-1,10phenanthroline (BTPhen) BTPhen Tri-n-butyl phosphate (TBP)

SFU/Eu 14 7.9

Parameters Solvent Phenyl(trifluoromethyl)sulfone cyclohexanone

Aqueous phase 1 – 4 M HNO3 < 1 M HNO3

Ref. 55 56

400 2250

octanol Xylene

4 M HNO3 4 M HNO3

57 58

3,4-Hydroxypyridinone Schiff base N,N’-dimethyl-N,N’dibutyl malonamide MeOSalophen

1.1 1.2 15.3

N.A. N.A. N.A.

pH = 8.1 pH = 5.5 1 M HNO3

59 60 61

94

N.A.

pH = 6

Present study

N.A. – Not Applicable To evaluate the potential of the proposed magnetic adsorbent for selective removal of uranyl ions, the competitive extraction of U in the presence of numerous ions was performed with a mining leaching solution provided by Search Minerals at pH = 1.82 ± 0.03 (Table 1).

As expected from the complex chemical nature of the leaching solution, the Kd value for U(VI) (756 mL g-1) decreased (Fig. 9). Nonetheless, the selectivity for U over other REEs was preserved, especially considering that the amount of REE is one to two orders of magnitude higher than U. A significant degree of selectivity (10-fold) for U(VI) over Th was also observed, though this was not observed with surrogate solutions (Figure 8). It should be noted that the pH of the leaching and the surrogate solutions are different, suggesting that the pH might play a role in the selectivity enhancement between the two actinides. This could be associated with differences in sorption mechanisms onto the MeOSalophen surface or speciation, but further studies would require to validate these hypotheses.



The results also demonstrate significant selectivity of the MSB for UO22+ compared to other constituents of the leaching solution, even if they are present in much larger concentrations (Table 1). This experiment also showed that while lower extraction capacities were reported in acidic conditions (Fig. 4), the proposed magnetic Schiff base ligand can still effectively extract U(VI) in acidic conditions and in the presence of competitive ions.

900 800

Distribution Coefficient Kd (mL/ g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

700 600 500 400 300 200 100 0 U (VI) Th (IV) La (III) Eu (III) Dy (III) Fe (III) Si (IV) Ca (II) Pb (II) Cu (II) Ni (II) Cd (II)

Figure 9. Competitive adsorption of concurrent ions on magnetic Schiff base, magnetic nanoparticles and MeOSalophen in real leaching solution. pH 1.82, temperature 20 °C, mass of adsorbent 0.025 g, stirring time 24 h.

The respective Kd values of the nanoparticles and MeOSalophen ligand separately were also calculated, using the leaching liquor to determine the impact of each component of the MSB on the degree of selectivity. The nanoparticles and the ligand appeared to have a combined effect on the SF obtained by the MSB, as seen in Table 5. While the hydroxyl groups at the surface of the magnetic nanoparticles could play a role in the adsorption of uranium,62 the MeOSalophen ligand is always the main contributor for the preferential extraction of U(VI) by MSB. These results 26 ACS Paragon Plus Environment

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suggest that the use of magnetic nanoparticles alone is not an effective strategy for the segregation of U(IV) from mining leachates. Table 5. Effect of the Schiff base and Fe3O4 nanoparticles component on the separation factors of magnetic Schiff base ligand in the mining leachate SFU/Element MSB Schiff base Fe3O4 nanoparticles

Th (IV) 7 7 4

La (III) 10 7 < 0.5

Element Eu (III) Dy (III) Fe (III) 10 10 16 7 8 11 4 < 0.5 5

Si (IV) 4 4 1

Ca (II) 7 6 2

4. Conclusions U(VI) was extracted using magnetic Schiff bases with MeOSalophen. Using this magnetic extractant, that is conveniently separated by an external magnetic field, facilitate phase separation problems commonly encountered with more traditional adsorbents used in heterogeneous separation. Optimal extraction of U(VI) was achieved at pH = 6.0 with a contact time of 24 h in the presence of 25 mg of MSB. An adsorption capacity (Qe) of (63 ± 3) mg g-1 for U and a Kd value of over 5,000 mg L-1 was found using those conditions. Using real REE leaching solutions, a lower Kd value of approximately 760 mg L-1 was determined, but the MSB maintained an acceptable degree of selectivity for uranium (VI) over a number of coexistent ions. The present study illustrates that MSB is usable as adsorbents for the effective removal of uranyl ions from complex mining solutions at various pH values. In the future, to improve the adsorption rate and increase the number of available complexation sites, appropriate Schiff base ligands could be chemically grafted onto magnetic nanoparticles instead of only deposited.

Acknowledgments 27 ACS Paragon Plus Environment


The authors would like to thank J.-D. Hamel for the complete characterization of the Schiff base ligand, M.-P. Côté and A. Picard-Lafond for the TEM analysis, and S. Groleau for the ATR study. We also acknowledge the Bioimaging platform of the Infectious Disease Research Centre, funded in part by an equipment and infrastructure grant from the Canadian Foundation for Innovation (CFI) in support of the Transmission Electron Microscope (TEM) equipment. The authors declare no competing financial interests.

© Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2018


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Table of Content



References

1 Massari, S.; Ruberti, M. Rare earth elements as critical raw materials: Focus on international markets and future strategies. Resour. Policy 2013, 38, 36. 2 Jordens, A.; Cheng, Y.P.; Waters, K.E. A review of the beneficiation of rare earth element bearing minerals. Miner. Eng. 2013, 41, 97. 3 Gupta, C.K.; Krishnamurthy, N. Extractive Metallurgy of Rare Earths. CRC Press: Boca Raton, 2005. 4 Barakos, G.; Mischo, H.; Gutzmer, J. Rare earth underground mining approaches with respect to radioactivity control and monitoring strategies. In Rare Earths Industry; De Lima, I.B., Filho, W.L., Eds.; Elsevier: Amsterdam, 2015. 5 Kanazawa, Y.; Kamitani, M. Rare earth minerals and resources in the world. J. Alloy. Compd. 2005, 408, 1339. 6 Zhu, Z.; Pranolo, Y.; Cheng, C.Y. Separation of uranium and thorium from rare earths for rare earth production–A review. Miner. Eng. 2015, 77, 185. 7 Sadri, F.; Nazari, A.M.; Ghahreman, A. A review on the cracking, baking and leaching processes of rare earth element concentrates. J. Rare Earth. 2017, 35, 739. 8 Das, D.; Gupta, U.; Das, A.K. Recent developments in solid phase extraction in elemental speciation of environmental samples with special reference to aqueous solutions. TrAC–Trend. Anal. Chem. 2012, 38, 163. 9 Xiao, C.; Silver, M.A.; Wang, S. Metal–organic frameworks for radionuclide sequestration from aqueous solution: a brief overview and outlook. Dalton T. 2017, 46, 16381. 10 Abney, C.W.; Mayes, R.T.; Saito, T.; Dai, S. Materials for the Recovery of Uranium from Seawater. Chem. Rev. 2017, 117, 13935. 11 Pan, J.; Yao, H.; Guan, W.; Ou, H.; Huo, P.; Wang, X.; Zou, X.; Li, C. Selective adsorption of 2, 6–dichlorophenol by surface imprinted polymers using polyaniline/silica gel composites as functional support: equilibrium, kinetics, thermodynamics modeling. Chem. Eng. J. 2011, 172, 847. 12 Gao, B.; Zhang, Y.; Xu, Y. Study on recognition and separation of rare earth ions at picometre scale by using efficient ion–surface imprinted polymer materials. Hydrometallurgy 2014, 150, 83. 13 Lemos, V.A.; Teixeira, L.S.G.; Bezerra,, M. de A.; Spínola Costa, A.C.; Castro, J.T.; Cardoso, L.A.M.; de Jesus, D.S.; Santos, E.S.; Baliza, P.X.; Santos, L.N. New materials for solid‐phase extraction of trace elements. Appl. Spectrosc. Rev. 2008, 43, 303. 14 Giakisikli, G.; Anthemidis, A.N. Magnetic materials as sorbents for metal/metalloid preconcentration and/or separation. A review. Anal. Chim. Acta 2013, 789, 1. 15 Fan, F.–L.; Qin, Z.; Bai, J.; Rong, W.–D.; Fan, F.–Y.; Tian, W.; Wu, X.–L.; Wang, Y.; Zhao, L. Rapid removal of uranium from aqueous solutions using magnetic Fe3O4@ SiO2 composite particles. J Environ Radioact 2011, 106, 40. 16 Calì, E.; Qi, J.; Preedy, O.; Chen, S.; Boldrin, D.; Branford, W.R.; Vandeperre, L.; Ryan, M.P. Functionalised magnetic nanoparticles for uranium adsorption with ultra–high capacity and selectivity. J. Mater. Chem. A 2018, 6, 3063. 17 Gorden, A. E.; DeVore, M. A.; Maynard, B. A. Coordination chemistry with f-element complexes for an improved understanding of factors that contribute to extraction selectivity. Inorg. Chem. 2012, 52, 3445. 30 ACS Paragon Plus Environment

Page 30 of 33

Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18 Sessler, J.L.; Melfi, P.J.; Pantos, G.D. Uranium complexes of multidentate N–donor ligands. Coordin. Chem. Rev. 2006, 250, 816. 19 Dalla Cort, A.; Pasquini, C.; Schiaffino, L. Nonsymmetrically substituted uranyl-salophen receptors: New opportunities for molecular recognition and catalysis. Supramol. Chem. 2007, 19, 79-87. 20 Pan, N.; Li, L.; Ding, J.; Wang, R.; Jin, Y.; Xia, C. A Schiff base/quaternary ammonium salt bifunctional graphene oxide as an efficient adsorbent for removal of Th (IV)/U (VI). J. Colloid Interf. Sci. 2017, 508, 303. 21 Tashkhourian, J.; Abdoluosofi, L. M.; Pakniat, M.; Montazerozohori, M. Sodium dodecyl sulfate coated alumina modified with a new Schiff's base as a uranyl ion selective adsorbent. J. Hazard. Mater. 2011, 187, 75. 22 Alam, S.; Kim, H.; Neelameggham, N. R.; Ouchi, T.; Oosterhof, H. Rare metal technology 2016. John Wiley & Sons: New Jersey, 2016. 23 Naeimi, H.; Salimi, F.; Rabiei, K. Convenient, mild and rapid synthesis and characterization of some Schiff–base ligands and their complexes with uranyl (II) ion. J. Coord. Chem. 2008, 61, 3659. 24 Chen, T.; Cai, C. Selective Oxidation of Benzyl Alcohols to Aldehydes with a Salophen Copper (II) Complex and tert–Butyl Hydroperoxide at Room Temperature. Synthetic Commun. 2015, 45, 1334. 25 Chen, D.; Martell, A.E. Dioxygen affinities of synthetic cobalt Schiff base complexes. Inorg. Chem. 1987, 26, 1026. 26 Khan, T.; Datta, A. Enhanced fluorescence with nanosecond dynamics in the solid state of metal ion complexes of alkoxy salophens. Phys. Chem. Chem. Phys. 2017, 19, 30120. 27 Zhang, X.; Jiao, C.; Wang, J.; Liu, Q.; Li, R.; Yang, P.; Zhang, M. Removal of uranium (VI) from aqueous solutions by magnetic Schiff base: kinetic and thermodynamic investigation. Chem. Eng. J. 2012, 198, 412. 28 Chevalier, Y.; Bolzinger, M.–A. Emulsions stabilized with solid nanoparticles: Pickering emulsions. Colloid. Surface A 2013, 439, 23. 29 Lavrov, H.V.; Ustynyuk, N.A.; Matveev, P.I.; Gloriozov, I.P.; Zhokhov, S.S.; Alyapyshev, M.Y.; Tkachenko, L.I.; Voronaev, I.G.; Babain, V.A.; Kalmykov, S.N.; et al. A novel highly selective ligand for separation of actinides and lanthanides in the nuclear fuel cycle. Experimental verification of the theoretical prediction. Dalton T. 2017, 46, 10926. 30 Mathur, J.N.; Murali, M.S.; Nash, K.L. (2001). Actinide partitioning-a review. Solvent Extr. Ion Exc. 2001, 19, 357. 31 Fryxell, G.E.; Lin, Y.; Fiskum, S.; Birnbaum, J.C.; Wu, H.; Kemner, K.; Kelly, S. Actinide sequestration using self-assembled monolayers on mesoporous supports. Environ. Sci. Techol. 2005, 39, 1324. 32 Brynda, M.; Wesolowski, T.A.; Wojciechowski, K. Theoretical investigation of the anion binding affinities of the uranyl salophene complexes. J. Phys. Chem. A 2004, 108, 5091. 33 Crane, A.K.; MacLachlan, M.J. Portraits of Porosity: Porous Structures Based on Metal Salen Complexes. Eur. J. Inorg. Chem. 2012, 1, 17. 34 Casellato, U.; Tamburini, S.; Tomasin, P.; Vigato, P.A. Uranyl (VI) complexes with [1+ 1] asymmetric compartmental ligands containing a Schiff base and a crown ether–like chamber. Inorg. Chim. Acta 2002, 341, 118.



35 Yang, D.; Hu, J.; Fu, S. Controlled synthesis of magnetite− silica nanocomposites via a Seeded Sol− gel approach. J. Phys. Chem. C 2009, 113, 7646. 36 Li, Z.; Chen, F.; Yuan, L.; Liu, Y.; Zhao, Y.; Chai, Z.; Shi, W. Uranium (VI) adsorption on graphene oxide nanosheets from aqueous solutions. Chem. Eng. J. 2012, 210, 539. 37 Zhang, A.; Asakura, T.; Uchiyama, G. The adsorption mechanism of uranium (VI) from seawater on a macroporous fibrous polymeric adsorbent containing amidoxime chelating functional group. React. Funct. Polym. 2003, 57, 67. 38 Fan, Q.H.; Hao, L.M.; Wang, C.L;, Zheng, Z.; Liu, C.L.; Wu, W.S. The adsorption behavior of U (VI) on granite. Environ. Sci.-Proc. Imp. 2014, 16, 534. 39 Kang, M.J.; Han, B.E.; Hahn, P.S. Precipitation and adsorption of uranium(VI) under various aqueous conditions. Environ. Eng. Res. 2002, 7, 149. 40 Kramer-Schnabel, U.; Bischoff, H.; Xi, R.H.; Marx, G. Solubility products and complex formation equilibria in the systems uranyl hydroxide and uranyl carbonate at 25 C and I = 0.1 M, Radiochim. Acta 1992, 56, 183. 41 Zhou, P.; Gu, B. Extraction of oxidized and reduced forms of uranium from contaminated soils: effects of carbonate concentration and pH. Environ. Sci. Technol. 2005, 39, 4435. 42 Craw, J.S.; Vincent, M.A.; Hillier, I.H.; Wallwork, A.L. Ab initio quantum chemical calculations on uranyl UO22+, plutonyl PuO22+, and Their nitrates and sulfates. J.Phys. Chem. 1995, 99, 10181. 43 Rudkevich, D.M.; Verboom, W.; Brzozka, Z.; Palys, M.J.; Stauthamer, W.P.R.V.; van Hummel, G.J.; Franken, S.M.; Harkema, S.; Engbersen, J.F.J.; Reinhoudt, D.N. Functionalized UO2 salenes: neutral receptors for anions. J. Am. Chem. Soc. 1994, 116, 4341. 44 Marahel, F.; Ghaedi, M.; Montazerozohori, M.; Biyareh, M.N.; Kokhdan, S.N.; Soylak, M. Solid–phase extraction and determination of trace amount of some metal ions on Duolite XAD 761 modified with a new Schiff base as chelating agent in some food samples. Food Chem. Toxicol. 2011, 49, 208. 45 Mashhadizadeh, M.H.; Pesteh, M.; Talakesh, M.; Sheikhshoaie, I.; Ardakani, M.M.; Karimi, M.A. Solid phase extraction of copper (II) by sorption on octadecyl silica membrane disk modified with a new Schiff base and determination with atomic absorption spectrometry. Spectrochim. Acta B 2008, 63, 885. 46 McBride, M.B. A critique of diffuse double layer models applied to colloid and surface chemistry. Clay Clay Miner. 1997, 45, 598. 47 McBride, M.B.; Baveye, P. Diffuse double–layer models, long–range forces, and ordering in clay colloids. Soil Sci. Soc. Am. J. 2002, 66, 1207. 48 Min, X.; Yang, W.; Hui, Y.F.; Gao, C.Y.; Dang, S.; Sun, Z.M. Fe3O4@ ZIF–8: a magnetic nanocomposite for highly efficient UO22+ adsorption and selective UO22+/Ln3+ separation. Chem. Commun. 2017, 53, 4199. 49 Simonin, J.P. On the comparison of pseudo–first order and pseudo–second order rate laws in the modeling of adsorption kinetics. Chem. Eng. J. 2016, 300, 254. 50 Blanchard, G.; Maunaye, M.; Martin, G. Removal of heavy metals from waters by means of natural zeolites. Water Res. 1984, 8, 1501. 51 Pignatello, J.J.; Xing, B. Mechanisms of slow sorption of organic chemicals to natural particles. Environ. Sci. Technol. 1995, 30, 1. 52 Aly, Z.; Luca, V. Uranium extraction from aqueous solution using dried and pyrolyzed tea and coffee wastes. J. Radioanal. Nucl. Chem. 2013, 295, 889. 32 ACS Paragon Plus Environment

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53 Dolatyari, L.; Yaftian, M.R.; Rostamnia, S. Removal of uranium (VI) ions from aqueous solutions using Schiff base functionalized SBA–15 mesoporous silica materials. J. Environ. Manage. 2012, 169, 8. 54 Belgacem, A.; Rebiai, R.; Hadoun, H.; Khemaissia, S.; Belmedani, M. The removal of uranium (VI) from aqueous solutions onto activated carbon developed from grinded used tire. Environ. Sci. Pollut. Res. 2014, 21, 684. 55 Alyapyshev, M.Y.; Babain, V.A.; Tkachenko, L.I.; Paulenova, A.; Popova, A.A.; Borisova, N.E. New diamides of 2, 2′–dipyridyl–6, 6′–dicarboxylic acid for actinide–lanthanide separation. Solvent Extr. Ion Exc. 2014, 32, 138. 56 Lewis, F.W.; Harwood, L.M.; Hudson, M.J.; Afsar, A.; Laventine, D.M.; Šťastná, K.; John, J.; Distler, P. Separation of the minor actinides Americium (III) and Curium (III) by hydrophobic and hydrophilic BTPhen ligands: exploiting differences in their rates of extraction and effective separations at equilibrium. Solvent Extr. Ion Exc. 2018, 36, 115. 57 Whittaker, D.M.; Griffiths, T.L.; Helliwell, M.; Swinburne, A.N., Natrajan, L.S.; Lewis, F.W.; Harwoods, L.M.; Parry, S.A.; Sharrad, C.A. Lanthanide speciation in potential SANEX and GANEX actinide/lanthanide separations using tetra–N–donor extractants. Inorg. Chem. 2013, 52, 3429. 58 Menzies, I.A.; Rigby, F., Separation of thorium from uranium and rare- earth elements by solvent extraction with tri-n-butylphosphate-xylene. J. Appl.Chem. 1961, 11, 104. 59 Johnson, B.E.; Santschi, P.H.; Chuang, C.Y.; Otosaka, S.; Addleman, R.S.; Douglas, M.; Rutledge, R.D., Chouyyok, W.; Davidson, J.D.; Fryxell, G.E.; et al. Collection of lanthanides and actinides from natural waters with conventional and nanoporous sorbents. Environ. Sci. Technol. 2012, 46, 11251. 60 Shiri-Yekta, Z.; Yaftian, M.R.; Nilchi, A. Silica nanoparticles modified with a Schiff base ligand: an efficient adsorbent for Th (IV), U (VI) and Eu (III) ions. Korean J. Chem. Eng. 2013, 30, 1644. 61 Ansari, S.A.; Mohapatra, P.K. A review on solid phase extraction of actinides and lanthanides with amide based extractants. J. Chromatogr. A 2017, 1499, 1. 62 Waite, T.D.; Davis, J.A.; Payne, T.E.; Waychunas, G.A.; Xu, N. Uranium (VI) adsorption to ferrihydrite: Application of a surface complexation model. Geochim. Cosmochim. Ac. 1994, 58, 5465.


Selective removal of uranium from rare earth leachates via magnetic

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