Magnetic Lyogels for Uranium Recovery from Wet Phosphoric Acid

Article Cite This: Ind. Eng. Chem. Res. 2017, 56, 12644-12654

pubs.acs.org/IECR

Magnetic Lyogels for Uranium Recovery from Wet Phosphoric Acid Lev Bromberg, Ran Chen, Paul Brown, and T. Alan Hatton* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: The work introduces composite magnetic materials designed to capture uranium from wet-process phosphoric acid (WPA) containing 6 M H3PO4, 2% H2SO4, sodium fluoride, and metal salts of Fe(III) and Al(III). The materials include poly(vinyl chloride) (PVC) covalently modified with N,N-diethyldithiocarbamate (DEDTC) or O,O-diethyldithiophosphate (DEDTP) moieties by nucleophilic substitution of the >C−Cl bonds of PVC. To maintain the polymer processability, the maximum substitution degree was kept below 42%. The modified PVC formed stable organic gel (lyogel) materials with liquid uranium extractants such as di(2ethylhexyl)phosphoric acid (DEHPA) or a liquid mixture of trialkylphosphine oxides, Cyanex 923. To impart the magnetic recoverability to the lyogels, iron nanoparticles (20−50 nm) coated by carbon for chemical stability were incorporated. The resulting magnetic lyogels contain variable contents of liquid extractants, maintain particle shape, exhibit very low leaching of the extractants, and are chemically stable in extremely corrosive acidic environments. Kinetics of uranium capture and equilibrium sorption capabilities of the magnetic lyogels have been evaluated. The lyogels are readily recovered by a magnet and recycled without any loss of the material. Efficient uranium stripping from the lyogels is enabled by 1 M aqueous ammonium carbonate. Lyogel recyclability and reuse were demonstrated in at least three cycles of the uranium loading and recovery.

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INTRODUCTION Incorporation of uranium recovery into wet phosphoric acid (WPA) processes can potentially supplement a significant fraction of the world’s uranium production and simultaneously benefit human health and environment by reducing radiological and toxic contamination of the phosphate fertilizer products.1−5 WPA production involves leaching phosphate mineral concentrate with hydrochloric, nitric, or sulfuric acid from the phosphate rock, which is the only economically viable source of uranium.1 Phosphate rocks that contain recoverable uranium in the parts per million range are found throughout the world.1,3 On the industrial scale, solvent extraction and precipitation processes have been implemented for uranium recovery from WPA, with solvent extraction (SX) dominating the field. The leaching stage of the WPA process always involves solubilization of various impurities including uranium, cadmium, arsenic, fluorine, etc. into the phosphoric acid, which must then be purified by either extracting the impurities from the WPA leachate or extraction of the phosphoric acid itself from the leachate. SX technology is primarily used to separate and purify uranium from aqueous leachates that have been separated from the gypsum (CaSO4) and other solids formed in the process of the acid leaching.3 The uranium found in the WPA leachate is present as a mixture of its hexavalent and tetravalent forms. Despite the widespread acceptance of SX for the uranium recovery from the WPA leachates,6 the solvent extraction-based recovery processes are quite complex and disadvantageous from the standpoint of generating large amounts of solvent waste with the accompanying high costs of capital equipment for © 2017 American Chemical Society

solvent recovery. Solid−liquid separation methods can offer advantages over liquid−liquid solvent extraction due to large interfacial area, high selectivity, relatively high stability and recyclability, fast phase separation, and elimination of the use of diluent. The solvent recovery problem is potentially addressed by the separation processes utilizing solid materials capable of binding uranium selectively. A number of materials capable of capturing uranium from acidic media (but not necessarily the WPA leachates) have been reported. These include solid supports such as poly(styrene-co-divinylbenzene) beads, polyurethane foam, porous glass beads, and bentonite materials loaded with a solid-state extractant.5−14 Typical solid-state extractants include trioctylphosphine oxide (TOPO) and octyl(phenyl)-N,N-diisobutylcarbamoyl-methylphosphine oxide (CMPO), or the like. Solid−liquid extraction of rare earths from phosphoric acid has been reported using ionexchange resins modified with bifunctional phosphinic acid, amino phosphonic acid resin, as well as solvent impregnated (SIR) resins.7,8 After sorption to recover uranium or other metals, for instance in an ion-exchange column, the loaded sorption material is removed from the column and is either incinerated or acid digested to recover the metals. Conventional solvents may also be used to strip extractant and actinides from the support. However, in such systems where the solid supports are physically loaded with and not chemically bound Received: Revised: Accepted: Published: 12644

August 22, 2017 September 27, 2017 October 12, 2017 October 12, 2017 DOI: 10.1021/acs.iecr.7b03462 Ind. Eng. Chem. Res. 2017, 56, 12644−12654

Article

Industrial & Engineering Chemistry Research

halopolymer such as acid-resistant poly(vinyl chloride) (PVC) or the like polymer is covalently modified with a molecule that closely resembles properties of the chosen extractant, the resulting modified polymer can form stable gels with such liquid organic extractants.15−18 In the present work, we discovered that PVC modified with dithioalkyl-functionalized molecules can form stable gels with DEHPA and Cyanex 923, a liquid TOPO analogue. In addition, our choice of dithio-containing molecules was motivated by the known ability of such moieties to chelate uranium.19,20 Complexation of dithiocarbamic (dtc), xanthic (xan), and dithiophosphoric (dtp) acids with various metal ions is widely used in extraction, separation, and determination of metals. Dialkyldithiophosphates are very effective in extraction of Zn, Cd, Sb(III), Bi(III), Cr(III), U(VI), Mn(II), Fe(II), Ni, Cu(II), Pb(II), Fe(II), and Pd.21 The high complex stability of dithiocontaining compounds, complex pH dependency, and the low aqueous solubility of alkyl-substituted dithiocarbamates and sulfur-containing organophosphorus reagents have prompted considerable research into their application in uranium separation.20,22 Alkyl dithiocarbamates form stable complexes with uranyl ions, often forming N,N-dialkyldithiocarbamato uranylate anions. In such complexes, the linear UO22+ unit is surrounded by six sulfur donor atoms of three bidentately coordinating dialkyldithiocarbamate ligands.19,23 Herein, we combined the complexation ability of the dialkyldithio moieties grafted on PVC with the binding of uranium by the specifically designed liquid extractants via formation of lyogels, the latter being recoverable by a magnet because of the embedded ferromagnetic particles. Such composite magnetic lyogels are sufficiently stable, can capture uranium from the WPA medium, and are recyclable, as described below.

to the selective extractants, the support/extractant materials are not reusable and thus are not cost-effective. Merrifield chloromethylated resin grafted with CMPO represents a solid polymeric support that is chemically bound to the selective uranium extractant.9 Such grafted resin can be reused by sequential sorption of metal values from acidic solutions and stripping the metal off the solid support by ammonium carbonate. However, CMPO is inefficient at binding to uranyl ions in the presence of concentrated phosphoric acid characteristic of the WPA milieu. In contrast, several materials have been reported aiming specifically at uranium recovery from phosphoric acid. Thus, poly(ether sulfone)-based composite beads encapsulating a synergistic mixture of di(2-ethylhexyl) phosphoric acid (DEHPA) and liquid extractant Cyanex 923 (at 4:1 mol ratio) provided for quantitative recovery of uranium from phosphoric acid medium.10 A microcapsule adsorbent for separation of uranium from phosphoric acid solutions immobilizing the DEHPA−TOPO solvent in the polymeric matrix of calcium alginate adsorbed uranyl ion via both ion exchange and solvent extraction mechanisms.11 Grafting phosphorus-based ligands within the pores of mesoporous silica or mesoporous carbon enabled high capacities of the resulting sorbents to selectively extract uranium ions from phosphoric acid media.12 However, the above materials are somewhat cumbersome in recovery and reuse. In the 1990s, a group at Argonne National Laboratory undertook the development of a magnetically assisted chemical separation (MACS) process, wherein ferromagnetic (iron) and superparamagnetic (magnetite) nanoparticles coated with charcoal with an adsorbed layer of a solid uranium extractant (CMPO) as well as adsorbed DEHPA, TOPO, and potentially other extractants such as tributyl phosphate (TBP), amines, phosphnic acid, crown ethers, cryptands, poly(ethylene glycol), and poly(ethylene imine) were evaluated for use in the separation and recovery of americium, plutonium, and uranium from acidic nuclear waste solutions.13,14 These particles are readily recoverable from the aqueous media by magnetocollection or high gradient magnetic separation, but the extractant molecules adsorbed on the surface of the magnetic nanomaterials can dissociate and leave the magnetic carrier, i.e., the MACS particles were not recyclable. In addition, while carbon-coated iron is sufficiently stable in nitric acid, the nanoparticles can dissolve in concentrated phosphoric acid in the presence of sulfuric acid. Herein, we aimed at materials with enhanced stability that are capable of capturing uranium from ≥6 M phosphoric acid, in the presence of sulfuric acid and other highly reactive components typically found in the WPA media. The enhancement of stability of our materials is due to the carbon-coated iron nanoparticles being embedded into a hydrophobic polymeric material that is highly acid-stable and yet contains a conventional organophosphorus extractant such as DEHPA or TOPO. In order to achieve stability in such extractant-swollen organic gel (termed lyogel herein), the polymer and extractant must form a homogeneous phase that does not exhibit syneresis (phase separation) for extended periods of time, even upon exposure to an aqueous acid solution. Such stability will prevent leaching of the extractant from the lyogel into the aqueous acidic medium. A unique polymeric system that exhibits no syneresis when polymer is blended with a specific extractant is obtained by modifying halopolymers by substitution with a moiety of a substance compatible with the specific extractant.15 That is, if a

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EXPERIMENTAL SECTION Materials. Poly(vinyl chloride) (PVC, MW ∼ 48000), di(2ethylhexyl) phosphate (DEHPA, 97%, MW 322.4), sodium diethyldithiocarbamate trihydrate (DEDTC, 97%), diethyl dithiophosphate ammonium salt (DEDTP, 95%), phosphoric acid (85% in H2O), sulfuric acid (95−98%), sodium fluoride (≥99%), iron(III) sulfate hydrate (97%), aluminum sulfate hydrate (98%), and uranyl nitrate hexahydrate (98.0−102.0%) were all obtained from Sigma-Aldrich Chemical Co. Analyticalgrade uranyl nitrate hexahydrate solution (1005 ± 4 μg/mL) in 5% HNO3 was obtained from Inorganic Ventures (Christiansburg, VA). Carbon-coated iron nanoparticles (C@Fe NP) were purchased from Sun Innovations, Inc. (Fremont, CA). The nanoparticles are sized 7−50 nm, ∼25 nm average. The thickness of the carbon shell is 2.5−3 nm. The nanoparticles are composed of Fe (ferromagnetic), Fe3C (paramagnetic), some CFe15.1-Austenite (ferromagnetic), and some Fe2O3/ FeO depending on exposure to air. The elemental iron content in C@Fe material is 77 wt %; saturation magnetization (Ms) varies in the 80−130 emu/g range. Our measurements yielded saturation magnetization of the C@Fe NP utilized in the present work to be 120 emu/g. Cyanex 923 extractant was a generous gift from Cytec Industries Inc. (Woodland Park, NJ). Cyanex 923 is a liquid mixture of trialkylphosphine oxides (∼92%). It contains 18 components, 17 being trialkylphosphine oxides, mainly with normal (92.4%) hexyl and octyl groups. The average MW is 348, and the density (20 °C) is 880 kg/m3; aqueous solubility is 10 mg/L.24 12645

DOI: 10.1021/acs.iecr.7b03462 Ind. Eng. Chem. Res. 2017, 56, 12644−12654

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utilizing a PerkinElmer Optima 8000 spectrometer. The supernatant was diluted 100-fold by 5% aqueous nitric acid, and uranium concentration-net peak intensity calibration curves (λ = 409.014 nm) developed in the 0−2000 ppb concentration range were applied (relative standard deviation, RSD = 0.7%, recovery > 0.97%, correlation coefficient R2 = 0.999). Uranium partition coefficient (Kd) between the sorbent material and the WPA fluid was found from the expression

All other chemicals, gases, and solvents were of the highest quality available from the commercial sources and were used as received. Polymer Synthesis. Solution of PVC, 5−10 wt %, and DEDTC or DEDTP in anhydrous N,N-dimethylformamide (DMF) were deaerated by nitrogen flow at ambient temperature for 0.5 h in a glass reactor. The initial molar ratio of the PVC (molecular weight of repeat unit, 62.5 g/mol) to the dialkyldithio compound was set at 1:1 throughout.25,26 The reactor was sealed and equilibrated at 75 or 90 °C for specified time with shaking, then allowed to cool to ambient temperature; the polymer was precipitated while stirring the solution by dropwise addition of acetone/water (8:2). The polymer was filtered on a glass filter, washed with deionized water repeatedly, snap-frozen in liquid nitrogen, and lyophilized. The polymer samples were characterized by 1H NMR; Fourier transform infrared spectroscopy (FTIR); thermogravimetric analysis (TGA); and elemental analysis for chlorine, sulfur, and phosphorus. The chlorine substitution degree was expressed as SD, mol % = 100(number of substituted units/total number of units). 1H NMR (400 MHz, THF-d8): DEDTC-PVC, δ (ppm): 1.21 (m, 3H), 1.29 (s, 3H), 2.2 (m, 2H), 2.36 (m, 1H, >CH− S−N), 3.62, 3.82 (m, >CH−S), 4.04 (m, −CH2−N), 4.53 (m, >CH−Cl). DEDTP-PVC, δ (ppm): 1.20 (m, 3H), 1.65 (m, 2H, >CH2−), 2.34, 2.52 (m, 1H, >CH−S−P), 4.1 (m, >CH2−O− P). Intensity of signals of the >CH-Cl protons of PVC in the 4.4−4.5 ppm range decreased with substitution, whereas signals of the >CH−S protons at 2.36, 3.62, 3.82, and 4.04 ppm appeared and grew in intensity as the degree of substitution measured by S content increased.27 DEDTC-PVC FTIR (KBr), cm−1: 1673 (dithiocarbamate bond stretch), 1477 (C−N), 1414 (aliphatic CH), 1266 (CS2 asymm), 1247 (CH(Cl)), 1091 (C−C in PVC backbone), 980 (CS2 symm), 608 (C−Cl gauche). Preparation of Magnetic Lyogels. Magnetic lyogel materials were synthesized at room temperature by blending weighed amounts of solutions of modified PVC and chosen extractant in THF, followed by addition of weighed amounts of magnetic C@Fe nanoparticles, sonicating of the resulting blend for 15− 25 min, casting of the resulting homogenized blends on Teflon sheets, and THF evaporation under air flow for 48−72 h until constant weight. The gel-like soft solids were shredded into irregularly shaped particles sized 1−3 mm and dried under vacuum. The lyogel particles were collected by a NdFeB magnet (grade N52, 3 in. diameter × 1 1/2 in. thick, nickel plated, axially magnetized, K&J Magnetics, Inc., Plumsteadville, PA). Uranium Uptake Measurements. The materials were suspended in uranyl nitrate solution in the synthetic wetprocess phosphoric acid (WPA) fluid at effective particle concentrations of 30 or 50 mg/mL, and the suspensions were rotated in sealed polypropylene tubes at ∼100 rpm in a vertical position at room temperature. The synthetic aqueous WPA fluid was composed of 6 M H3PO4, 0.86% H2SO4, 0.9% NaF, 0.7% Fe2(SO4)3, and 0.35% Al2(SO4)3. Density of the WPA fluid was measured using a specific gravity bottle to be 1292 g/ L. Uranium concentration range studied (0−200 mg/L) corresponded to that reported in the WPA processes.28,29 At specified time t, the supernatant was separated from the polymer particles using a membrane filter (effective dpore, 0.22 μm) or using magnetocollection for the magnetic gel particles, and uranium concentration was measured by inductively coupled plasma optical emission spectrometry (ICP-OES)

Kd =

C0 − Ceq Vs Ceq Vm

where C0 and Ceq (mg/L) are the initial and equilibrium uranium concentration, respectively, measured before and after material equilibration with the uranyl nitrate solution in the WPA fluid; Vs and Vm are the volume of the uranyl solution and the material, respectively. Density of each material was measured using specific gravity bottles. All measurements were performed in triplicate. General Methods. Bruker Avance 400 spectrometer was used for all NMR measurements. Fourier transform infrared spectroscopy was performed with a Nicolet 8700 FTIR spectrometer (Thermo Scientific Inc.) using KBr pellets. Thermogravimetric analysis was conducted using a Q600 thermogravimetric analyzer (TA Instruments, Inc.). Samples were subjected to heating scans (20 °C/min) under a nitrogen atmosphere and in a temperature ramp mode. Saturation magnetization of samples weighing 3−4 mg, which were placed in sealed polypropylene tubes and mounted inside a plastic straw, was measured at 300 K over a 0−50 kOe range using a magnetometer equipped with a superconducting quantum interference device (Magnetic Property Measurement System model XL-5, Quantum Design, San Diego, CA). Elemental analysis measurements were performed in a certified laboratory using an ICP-MS.

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RESULTS AND DISCUSSION Preparation of Substituted PVC and Magnetic Lyogels. The formation of lyogels by blending our dialkyldithio-functionalized PVC polymers with extractants such as DEHPA or Cyanex is analogous to the well-developed plasticization of PVC, wherein the functional effect of plasticizers is sought in altering the mechanical properties of the (otherwise brittle) PVC.30 Polar (carbonyl) functionalities of the common PVC plasticizers such as di(2-ethylhexyl) phthalate, which can reach up to 40% by weight in some materials, interact positively with the polar carbon−chloride bonds in the polymer chains of PVC, but the nonpolar 2ethylhexyl groups are incompatible with the PVC chains possessing significantly large electric dipoles along the C−Cl bonds.31 Overall, the presence of a significant fraction of the plasticizer shields the van der Waals chain−chain interactions of the long PVC chains, preventing the formation of a rigid network and lowering the PVC glass transition temperature, rendering the gel-like material more flexible and malleable. Drawing an analogy with di(2-ethylhexyl) phthalate, we observed that the extractants such as DEHPA or trialkylphosphine oxides comprising Cyanex 923 act as PVC plasticizers, resulting in formation of malleable lyogels. However, in order to prevent the phase separation between PVC and the extractants, the PVC had to be modified to be less polar and more resembling the extractants by chemical grafting of dialkyldithio compounds. 12646


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Figure 1. Mechanism of nucleophilic substitution reactions on PVC by dialkyldithiocarbamate (a) and overall stoichiometry of the PVC substitution resulting in DEDTC-PVC (b) and DEDTP-PVC (c) studied herein.

Synthesis of PVC functionalized by dithioalkyl moieties such as N,N-dialkyldithiocarbamates and O,O-dialkyldithiophosphates by nucleophilic substitution in dipolar aprotic solvents has been reported previously.15−18,32 In this work, we chose N,N-diethyldithiocarbamate (DEDTC) and diethyldithiophosphate (DEDTP) as representative examples of dithio-functional compounds that are well-known chelators of heavy metals.21,33−35 The nucleophilic substitution of PVC with DEDTC in dipolar aprotic solvents such as DMF proceeds at moderately elevated temperatures via neighboring group participation through the formation of an intermediary cyclic carbocation (Figure 1a).25,32 The overall substitution degree, i.e., SD, % = 100p/n or 100q/n in Figure 1b,c is readily found from the elemental analysis. Kinetics of the PVC modification were monitored, and the results are presented in Figure 2. It was observed that the substitution by both DEDTC and DEDTP was rather facile at elevated temperatures. At ∼44 mol % SD, the polymer became completely insoluble in THF because of the formation of interchain −S−S− cross-linking. Therefore, the reaction time and temperature were optimized to achieve maximum SD that occurs prior to the onset of the cross-linking resulting in insoluble gels. The gelation onset was readily seen by turning the glass reactors upside down and observing the polymer solution flow under gravitation. At 90 °C, insoluble gels formed within approximately 1 h, and the gels appeared to be brownyellow and were difficult to process any further after cooling to ambient temperature. At 75 °C, appropriately large substitution degrees were achieved within 3−4 h, and the formed substituted polymer fractions passed THF solubility tests. In further studies, DEDTC-PVC and DEDTP-PVC species with ca. 40−42% SD were utilized. To prepare magnetic lyogels, solutions of dialkyldithiosubstituted PVC and DEHPA or Cyanex 923 in THF were blended with carbon-coated iron nanoparticles, forming homogeneously black suspensions. The C@Fe particles settled and phase-separated from unperturbed suspensions containing 5−10% of modified PVC within 1−2 h. When the suspensions were vigorously shaken and cast into films by depositing

Figure 2. Kinetics of PVC substitution with diethyldithiocarbamate (DEDTC-PVC) and ammonium diethyldithiophosphate (DEDTPPVC) in DMF at 75 °C.

suspension droplets, allowing THF to evaporate on air, the resulting lyogel films were visually homogeneous. The consistency of the lyogels depended strongly on the C@Fe content. Lyogels with more than 10 wt % nanoparticles appeared to be pliable soft solids, whereas materials with the NP contents ≤5 wt % were tacky and paste-like. Composition and thermal stability of the prepared lyogels were tested by thermogravimetric analysis and by elemental analysis. Typical TGA thermograms of the initial materials and resulting lyogels are shown in the Supporting Information (Figures S-1 and S-2). Thermal decomposition of Cyanex 923 started at ∼230 °C, while DEHPA started decomposing at 195 °C. Modification of PVC by DEDTC and DEDTP resulted in polymers that started degrading at 250−320 °C. Gels 12647


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Industrial & Engineering Chemistry Research containing Cyanex 923 degraded at T > 350 °C, whereas gels with DEHPA degraded at T > 250 °C. Combination of TGA and elemental analysis yielded composition (Table 1) that

therefore the magnetic properties on the time of exposure and temperature. Magnetic lyogels, designed to further protect the C@Fe nanoparticles by incorporating them into hydrophobic materials composed of acid-resistant, stable modified PVC swollen with water-insoluble, hydrophobic extractant, were significantly more stable than the C@Fe nanoparticles alone. The magnetization-field measurements by SQUID of lyogels composed of DEDTC-PVC (SD, 41%), DEHPA as an extractant, and Fe@C NP (material F in Table 1) before and after the material’s exposure to the WPA fluid for 7 days at room temperature demonstrated that the Ms values of the lyogels normalized for the C@Fe NP content were identical to those of the unaltered C@Fe and did not change upon equilibration of the lyogels with the WPA fluid (Figure S-3). The lyogels maintained their magnetic properties after equilibration with the WPA fluid at 60 and 80 °C for 48 h. Testing of two additional batches of the DEDTC-PVC, DEHPA lyogels in the WPA fluid further confirmed the stability of the magnetic material in the lyogels (Table 2). No loss of Fe by dissolution and leaching and no loss of saturation magnetization was observed in these experiments. The lyogel particles dispersed in the WPA fluid rapidly moved in strong magnetic fields and were readily removed from the fluids by a simple magnetocollection. Extractant Leaching from Magnetic Lyogels. The PVC plasticizers are not chemically bound to the polymer and can diffuse along the polymer chains and leach out of the material causing well-known environmental concerns.37 Likewise, aqueous miscibility of extractants utilized in the SX processes is undesirable and results in the extractant loss and diminished process efficiency. As we employed gel materials in the heterogeneous uranium removal processes, we were concerned about the potential of the extractant leaching as well. We thus performed a leaching study with materials C, E, and F (see Table 1), wherein suspensions of these lyogel species in 10 mM aqueous sulfuric acid (effective particle concentrations, 100 mg/mL) were agitated at room temperature and at 60 °C for 48 h; the supernatant and the particles were separated by a magnet, and the supernatant was subjected to the phosphorus content measurement using ICP-MS. The results are shown in Table 3. The data in Table 3 show that the fractions of the extractants leached from the tested lyogel materials were extremely low. For comparison, solubility of DEHPA in analogous sulfuric acid solutions at saturation at these temperatures has been reported in the range of 140−180 mg/L.38 Aqueous solubility of Cyanex 923 is approximately 10 mg/L at ambient temperature.24 That is, incorporation into the lyogels limited DEHPA and Cyanex dissolution in aqueous acid by approximately 10-fold and 5fold, respectively. Uranium Uptake by Magnetic Lyogels. Having established that the newly obtained magnetic materials were sufficiently stable under acidic conditions, we investigated

Table 1. Composition of Magnetic Lyogels of the Present Study material designation A B C D E F G H

polymer

a

DEDTCPVC DEDTPPVC DEDTCPVC DEDTPPVC DEDTPPVC DEDTCPVC DEDTCPVC DEDTCPVC

polymer content (wt %)

extractant

extractant content (wt %)

C@Fe (wt %)

100

na

0

0

100

na

0

0

36.5

Cyanex

52.0

11.5

23.5

Cyanex

65.0

11.5

23.5

DEHPA

66.5

10.0

38.5

DEHPA

50.0

11.5

25.8

DEHPA

68.8

5.4

5.0

DEHPA

90.8

4.2

a

Substitution degree (SD) was measured to be 41 and 42% for DEDTC-PVC and DEDTP-PVC, respectively.

corresponded well to the initial ratio of components set in the reaction mixtures during the composite material synthesis. Two polymers based on DEDTC and DEDTP with substitution degree in the 40−42% range were prepared, with two species of their gels containing Cyanex and C@Fe nanoparticles at a fixed 11.5% concentration. The majority of the studies focused on materials based on DEDTC-PVC. Three species of DEHPAcontaining gels based on DEDTC-PVC were prepared, with the polymer content varying from 5 to 38.5% and, conversely, DEHPA content varying from 50 to 90%. Magnetic Properties and Stability of the Lyogel Materials. Production of the wet-process phosphoric acid results in a highly aggressive medium containing a complex solution of ∼35% P2O5 and sulfuric acid, fluorine, ferrous, and other salts36 that would dissolve most ferro- and paramagnetic materials unprotected by specifically designed coating or encapsulation. Herein, stability of materials was tested upon their contact with the synthetic aqueous WPA fluid. In the control experiments, Fe(0) and Fe3O4 (magnetite) particles completely dissolved in the WPA fluid within 1−2 h at room temperature. Carbon-coated C@Fe nanoparticles lost 15−20% of their initial magnetization after 1 week of exposure to the synthetic WPA fluid at room temperature and completely dissolved at 80 °C within 48 h. Although the carbon shell of the C@Fe NP is designed to protect the crystalline iron core from the oxidation and dissolution in acids, imperfections in the shell resulted in a strong dependence of the particle integrity and

Table 2. Effect of the Synthetic WPA Fluid Exposure on Magnetic Properties of DEDTC-PVC Material Compositionsa Ms (emu/g total)

Ms (emu/g NP)

DEDTC-PVC (wt %)

DEHPA (wt %)

C@Fe NP content (wt %)b

Fe (wt %)c

as-made

after exposure

as-made

after exposure

38.5 30.8

50 69.2

11.5 5.38

8.8 4.1

13.1 6.5

13.7 6.6

119 120

125 120

Magnetic properties are characterized by saturation magnetization (Ms) measured by SQUID. Exposure to the WPA fluid at ambient temperature for 7 days at constant agitation (200 rpm). bAs made. cAfter exposure to the WPA fluid, measured by elemental analysis. a

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the highly acidic WPA fluid. In the control experiments with unmodified PVC, we observed no discernible uptake of uranium from the WPA fluid. The dialkylthiophosphatemodified DEDTP-PVC possessed a slightly higher binding affinity toward uranium than the dialkylthiocarbamate-modified PVC. As we could not attain the sorbent saturation (plateau) in the chosen uranium concentration range (Figure 3), it was possible to treat the sorption isotherms using their initial slope, which is commonly described by the Freundlich equation developed to reflect upon the adsorption characteristics of a heterogeneous surface:39−42

Table 3. Leaching of Extractants from Magnetic Lyogels into 10 mM Aqueous H2SO4 Solution for 48 h aqueous extractant concentration (mg/L)

fraction of extractant leached (%)b

materiala

extractant

25 °C

60 °C

25 °C

60 °C

C D F

Cyanex 923 DEHPA DEHPA

2 18 11

7 22 13

0.0038 0.027 0.022

0.013 0.033 0.026

a For material composition, see Table 1. bRelative to the effective initial extractant concentration in the lyogel suspension.

Q eq = K f Ceq1/ n

sorption isotherms by the modified PVC polymers and magnetic lyogels of various compositions. Although we have observed that the uranium partition equilibria could be established within a few hours depending on the material investigated, some of the kinetics plateaued within longer time periods close to 48 h; therefore, all the isotherms were studied at equilibration time of 48 h. The uranium concentration range was chosen to be 0−200 mg/L, which is a typical range of concentrations found in the wet-process phosphoric acid. The maximum uranium concentration of 200 mg/L would not lead to the saturation of the binding sites on the polymer or extractant in the lyogels at the material concentrations chosen. Figure 3 depicts typical sorption isotherms for the materials based on DEDTP-PVC (a) and DEDTC-PVC (b) polymers. As is seen, the dialkylthio-functionalized polymers possessed a significant affinity toward uranium and were able to sorb it from

(1)

where Qeq (mg/g) is the amount of uranium sorbed per gram of material at equilibrium; Ceq (mg/L) is the uranium concentration in solution equilibrated with the material; Kf (mg/g) is the Freundlich isotherm constant, indicating the sorption capacity; and n is the so-called adsorption intensity. The linearized form of Freundlich isotherm (eq 1), log(Q) = log(Kf) + (1/n)log(Ceq), was well-applicable to our experimental data (Figure S-4). The results of the sorption experiments of the studied materials of various compositions are collected in Table 4. Table 4. Composition of Magnetic Lyogels and Equilibrium Uranium Sorption Parameters: Freundlich Isotherm Constant (Kf), Adsorption Intensity (n), and Partition Coefficient (Kd)

a

material designationa

Kf c (μg/g)

n

A B C D E F G H

29.7 40.8 30.2 62.6 62.0 62.9 72.4 108.6

1.45 1.45 0.92 1.15 1.04 0.97 0.91 0.85

Kd 7 10 13 52 71 71 105 178

± ± ± ± ± ± ± ±

1 1 3 3 4 4 5 4

For material composition, see Table 1.

Partition coefficient Kd was found at the initial uranium concentration of 200 mg/L. It can be seen that the liquid extractants and the dithio-functionalized PVC possessed synergistic effect, increasing the sorption capacity of the resulting gels. DEHPA was more efficient than Cyanex 923, as expected. The values of 1/n of 0.7 found for the dialkyldithio-modified PVC indicate normal adsorption; whereas 1/n of ∼1.16 for material H (5% modified polymer and 90% DEHPA) indicates some degree of cooperative adsorption.41 Increasing DEHPA content at the expense of the polymer and nanoparticle content did augment the uranium sorption capacity and partition coefficient of the composite materials (compare materials F, G, and H), but increasing DEHPA content to 90% yielded very soft, pliable gels that were tacky and tended to agglomerate when dispersed in the WPA fluid. Hence, a balance should be found between the uranium uptake characteristics and mechanical properties/handling of the lyogels. Overall, the lyogel (material F) containing significant fraction of the uranium-binding polymer (DEDTC-PVC, 38.5%) and magnetic nanoparticles (11.5%) had a soft solid appearance and maintained the shape and size of its particles in

Figure 3. Typical uranium sorption isotherms measured with DEDTPPVC (a) and DEDTC-PVC (b) materials at 25 °C. Solutions of uranyl nitrate in the synthetic WPA fluid (concentration range, 0−200 mg/L) were equilibrated with weighed amounts of polymers or gel materials for 48 h; the solids were separated, and uranium concentration (Ceq, mg/L) was measured in the supernatant. Uranium concentration in the material (Q, mg/g) was obtained from the difference between the initial concentration in the WPA fluid and that at equilibration. Solid straight lines illustrate the initial slopes of the isotherms and are shown to guide the eye only. 12649


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concentration measurements by the ICP-OES as described in Experimental Section. The results of the uranium recovery (stripping) from material F lyogels by 1 M ammonium carbonate solutions are shown in Figure 4. It appeared that 1

suspensions when agitated; in addition, its particles did not aggregate and/or fuse together in the WPA fluid, unlike material H that contained 90% of DEHPA. We believe the optimum (sufficiently robust for recycling, and yet an effective sorbent) material found in this work was material F. It is interesting to compare the performance of the lyogel materials in uranium sorption to other materials reported previously. The Kd values herein can be correlated with the distribution coefficient of uranium (D, ratio of uranium concentration in the organic phase to that in the aqueous phase) in the SX process involving the wet-process phosphoric acid. The D values of 3−10 are typical for the traditional TOPO−DEHPA solvent extraction processes with the WPA but can be significantly enhanced (up to 70−100 and higher) through the use of the recently developed extractants such as bifunctional amido phosphonates and the like.3,29,43 These values are comparable to the Kd values for our DEDTC-PVC/ DEHPA lyogels (Table 4). On the other hand, the Kd values reported for uranium sorption from dilute nitric acid solutions using CMPO-loaded magnetic nanoparticles in the magnetically assisted chemical separation process (MACS)13,14,44 were up to 10 times higher than in our work. However, these results cannot be directly compared to the performance of the magnetic lyogels in the WPA fluid, because the MACS particles would not be able to efficiently adsorb uranium in the WPA fluid because of the uranium complexation with the phosphoric acid and the magnetic core dissolution. Similarly, any comparison with the rare earth metals sorption by magnetic materials developed for aqueous nitric acid or alkaline solutions40,42 will be precluded because of the inability of these materials to function in the WPA fluids. To the best of our knowledge, the lyogels reported herein are the first magnetic materials capable of uranium uptake from the wetprocess phosphoric acid. Uranium Recovery from Lyogels. The premise of utilization of the magnetic lyogels in the extraction of uranium from the WPA fluid is in the recovery (stripping) of the extracted metal into a more concentrated solution. In addition, the lyogel materials must be recyclable via magnetocollection or other means of manipulation by magnet in order to be economically feasible. The issue of uranium stripping pertaining to the commercial processes involving DEHPA−TOPO liquid−liquid extraction has been extensively investigated, with aqueous ammonium carbonate emerging as a more efficient and frequently utilized stripping agent.3 In the present work, we investigated uranium recovery from magnetic lyogels [material F, DEDTC-PVC (38.5 wt %), DEHPA (50.0 wt %), and Fe@C nanoparticles (11.5 wt %)] using 1 M (NH4)2CO3 solution. No uranium carbonate precipitation was observed at this carbonate concentration. The recovery experiments comprised placing weighed amounts of lyogel particles that had been equilibrated with the uranyl nitrate-containing WPA fluid for 48 h (followed by the particle rinsing with deionized water and air-drying), in 1 M ammonium carbonate solution at 50 mg/mL effective lyogel concentration. The resulting suspensions of lyogels in ammonium carbonate were agitated by rotation at 100 rpm at room temperature for 48 h. The particles were then collected by a magnet, rinsed by deionized water, dried, and weighed. In a control series of measurements, the uranium recovery was conducted using 1.5 and 2.0 M ammonium carbonate solutions, but no discernible differences in the recovery efficiency versus 1 M ammonium carbonate were observed. The supernatant was subjected to uranium

Figure 4. Uranium recovery (stripping) isotherm. Particles of material F [DEDTC-PVC (38.5 wt %), DEHPA (50.0 wt %), and Fe@C (11.5 wt %)] that had sorbed Q (mg/g) uranium nitrate in the uranium uptake experiments were placed in 1 M (NH4)2CO3 aqueous solution at 50 mg/mL concentration. Following equilibration with agitation for 48 h, the particles were removed by magnet and uranium concentration in the supernatant was assayed, yielding Q recovered (mg/g). Percent recovery was calculated from the (Q recovered/Q) ratio and was measured in 3 independent experiments.

M ammonium carbonate solution was quite efficient in stripping uranium from the DEHPA-containing lyogels, with the recovery from 60−100% throughout the range of the gel loads (Q) studied. Importantly, the stripped and washed lyogels did not lose their ability to sorb (uptake) uranyl nitrate from the WPA fluid in at least three sorption−recovery cycles, each followed by the particle washing (Figure S-5). We were able to extract on average 78, 82, and 87% of uranium nitrate (initial concentration, 200 mg/L) from the synthetic WPA fluid in cycle 1, 2, and 3, respectively, of which 73, 77, and 82% were recovered back by using 1 M ammonium carbonate in cycle 1, 2, and 3, respectively. Importantly, on the basis of the gravimetric measurements, 97−100% of the lyogel particles were recovered in each cycle by magnetocollection. Kinetics of Uranium Uptake. Representative magnetic lyogels material F [DEDTC-PVC (38.5%), DEHPA (50%), Fe@C NP (11.5%)], material H [DEDTC-PVC (5%), DEHPA (90%), Fe@C NP (5%)], and material C [DEDTC-PVC (36.5%), Cyanex 923 (52.0%), Fe@C NP (11.5%)] were subjected to kinetic studies. Uranium uptake kinetics for the materials studied is shown in Figure 5. The equilibrium uptake was achieved within 24 to 48 h, after which time the Q = Qeq values plateaued and no further uptake of uranium was observed. Uranium uptake with the lyogel containing Cyanex 923 reached equilibrium within approximately 4 h, but the Qeq = 1.5 mg/g with this material was significantly lower than with lyogels containing DEHPA as an extractant, wherein Qeq reached 3.4 and 3.0 mg/g with material F and H, respectively. This observation reflects upon DEHPA being a more powerful extractant than tri-n-octyl phosphine oxide (TOPO).13 The latter is the most prominent component of the Cyanex 923 extractant.24 The kinetic profiles in Figure 5 were analyzed with pseudofirst- and pseudo-second-order rate equations, which, along with the intraparticle diffusion models, are typically applied in studies of the sorption of rare earth and heavy metals on 12650


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magnetite nanoparticle clusters (sized ∼100 nm) coated with chitosan40 as well as magnetite-silica core−shell nanoparticles with shells functionalized by ammonium and phosphonate groups.42 Given that our lyogel particles were of considerably larger size (1−5 mm), the comparison indicates that the intraparticle diffusion of uranium in the lyogel particles toward its binding sites on the liquid extractant was not impeding the overall kinetics of uranium uptake. Effect of Temperature on Uranium Uptake by Lyogels. The effect of temperature on the uptake of uranium by material 1 [(DEDTC-PVC (38.5%), DEHPA (50%), Fe@C NP (11.5%)] was investigated in the range of temperatures of 25−80 °C (Figure 6).

Figure 5. Kinetics of uranium uptake (mg uranium/g lyogel) onto material F [DEDTC-PVC (38.5%), DEHPA (50%), Fe@C NP (11.5%)], material H [(DEDTC-PVC (5%), DEHPA (90%), Fe@C NP (5%)], and material C [(DEDTC-PVC (36.5%), Cyanex 923 (52.0%), Fe@C NP (11.5%)] from the synthetic WPA fluid at room temperature.

polymeric resins, solvent-impregnated resins (SIR), and nanoparticles.40,42 We applied the following expressions for the kinetics data analysis. The pseudo-first-order rate equation (PFOR) ln(Q eq − Q ) = ln Q eq − k1t

(2)

was applied to obtain the kinetic rate constants k1 from the initial slopes of the ln(1 − Q/Qeq) versus time fits. Sorption reaction half-life was calculated as τ1/2 = ln(2)/k1. The pseudosecond-order rate (PSOR) equation t 1 t = + Q Q eq k 2Q eq 2 (3)

Figure 6. Temperature dependence of uranium sorption isotherms with magnetic lyogel composed of DEDTC-PVC (38.5 wt %), DEHPA (50.0 wt %), and Fe@C nanoparticles (11.5 wt %) (material F). Effective gel concentration in the WPA fluid is 50 mg/mL; time of equilibration at each temperature, 48 h. Ceq (mg/L) and Q (mg/g) is the uranium concentration in the fluid equilibrated with the lyogel and the total uranium concentration in the lyogel, respectively. Solid lines indicate the initial slope of the Q vs Ceq curve and are shown to guide the eye only.

was applied throughout the entire time intervals studied. Excellent linear fits in the t/Q versus time coordinates were found [R2 = 0.999, 0.986, and 0.990 for materials F, H, and C, respectively (Figure S-6)]. The measured kinetic parameters are collected in Table 5. As is seen, material H containing a higher DEHPA concentration, but lower concentration of both the DEDTC-PVC polymer and impermeable Fe@C particles, demonstrated approximately 2-fold faster PSOR k1 rate constant and half-life of the uranium uptake reaction than its more “solid” counterpart, material F. On the other hand, material C that contained Cyanex 923 reached equilibrium 2.6-fold faster than its DEHPA-containing lyogel, material F, probably because of the 2.2-lower binding capacity of the Cyanex-based material compared to that of the DEHPA-based material. The experimental data in Table 5 and excellent correlation of the data to the PSOR model (Figure S-6) indicate that the ratelimiting step for uranium uptake is probably the chemical binding/sorption rate that involves the complexation of uranium to DEHPA. Interestingly, all rate constants measured with lyogels were equal to or in the same order of magnitude as the kinetic parameters reported with the cysteine-modified

The total uranium concentration adsorbed by the lyogel material gradually increased with temperature as is expected in an endothermic process. To obtain the partition coefficients (Kd), the initial slopes of the linear plots in the log Q versus log Ceq coordinates (R2 > 0.95 in all cases) were normalized for the lyogel densities measured using specific gravity pycnometers at each temperature. The obtained Arrhenius plot is shown in Figure 7. The slope and intercept in Figure 7 enable estimation of the thermodynamic parameters such as standard Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°): ln Kd =

−ΔH o ΔS o + RT R

ΔGo = ΔH o − T ΔS o The average values of the ΔH°, ΔS°, and ΔG° thermodynamic constants were measured to be 10.6 kJ mol−1, 71.1 J

Table 5. PFOR and PSOR Kinetic Parameters of Uranium Uptake from Synthetic WPA Fluid by Magnetic Lyogel Materials at 25 °C material

composition

k1 (×103 min−1)

τ1/2 (min)

k2 (×103 g mg−1 min−1)

Qeq (mg g−1)

F H C

DEDTC-PVC (38.5%), DEHPA (50%), Fe@C NP (11.5%) DEDTC-PVC (5%), DEHPA (90%), Fe@C NP (5%) DEDTC-PVC (36.5%), Cyanex 923 (52.0%), Fe@C NP (11.5%)

6.80 13.5 18.0

102 51 38

1.68 1.44 7.05

3.36 2.95 1.52

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CONCLUSIONS Magnetic nanoparticles incorporated in organic gels (lyogels) containing specific extractants offer desired properties such as lyogel recoverability by means of magnetocollection, recyclability, high content of the chosen extractants, chemical stability in acidic media, minimized extractant loss, and dramatically reduced waste streams versus liquid extraction routes. We have introduced magnetic lyogels based on (i) polymers modified with dialkylthio-functional molecules compatibilizing the polymer and the extractant and (ii) iron nanoparticles (20− 50 nm) coated by carbon for chemical stability. The resulting magnetic lyogels contain up to 90 wt % of liquid extractants; maintain particle shape; and are chemically stable in extremely corrosive environments such as blends of 6 M H3PO4, 2% H2SO4, and metal salts represented in the WPA fluid. We have demonstrated that DEHPA- or Cyanex 923-containing magnetic lyogels capture uranium from the WPA fluid, and kinetics and equilibrium sorption capabilities of the magnetic lyogels have been evaluated. The lyogels are readily recovered by a magnet and recycled without perceptible loss of the material. Efficient recyclability and reuse of the lyogels in at least three cycles of the equilibrium uranium loading−recovery (stripping) have been demonstrated. The uranium stripping solution consisted of 1 M aqueous ammonium carbonate.

Figure 7. Arrhenius plot of uranium partition coefficient (Kd) between magnetic lyogel and WPA fluid. Magnetic lyogel is composed of DEDTC-PVC (38.5 wt %), DEHPA (50.0 wt %), and Fe@C nanoparticles (11.5 wt %). Effective gel and initial uranium concentrations in the WPA fluid are 50 mg/mL and 200 mg/L, respectively; time of equilibration at each temperature, 48 h.

mol−1 K−1, and −12.4 kJ mol−1, respectively. The positive value of ΔH° indicates the endothermic nature of the reaction of the lyogel−uranium binding, whereas the negative value ΔG° attests to the thermodynamically favorable binding, i.e., the enhancement of uranium binding at higher temperature is observed. The positive value of ΔS° can be related to the release of the water of hydration from uranyl ions during the uptake process causing the increase in the randomness of the system. The dissociation of the hydration water and its dispersion in the bulk aqueous phase increases the entropy of the system. Overall, the observed temperature dependencies, endothermic nature of metal uptake, and estimates of the thermodynamic parameter values are characteristic of the processes of sorption of rare earths from aqueous media on solid sorbents such as magnetic nanoparticles and ion-exchange resins.40,42,45,46 Conversely, an exothermic process, i.e., a decrease of the partition coefficients with temperature and negative enthalpy change on uranium extraction in the TOPO− DEHPA liquid−liquid extraction process from wet-process phosphoric acid has been reported.29,47 The decrease in partition coefficients between the organic phase containing extractant and aqueous phosphoric acid phase with temperature is related to the increase in affinity of the phosphate ions to uranium. It is well-known that phosphoric acid complexes both the IV- and VI-valent forms of uranium:48 UO2

U

4+

2+

−

2+

+ nH 2PO4 ⇌ UO2 (H 2PO4 −

+ nH 2PO4 ⇌ U(H 2PO4

4−n

2−n

Article

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03462. Typical TGA thermograms measuring the materials utilized in preparation of magnetic lyogels and the lyogel materials based on DEDTC-PVC polymer, extractants, and C@Fe nanoparticles; typical magnetization versus field curves of the original C@Fe nanoparticles and a species of magnetic lyogel; Freundlich sorption isotherm for uranium uptake by magnetic lyogels; percent of uranium nitrate adsorbed from the synthetic WPA fluid and recovered in the stripping experiment in three uptake−recovery cycles; kinetics of uranium uptake by magnetic lyogel from the WPA fluid (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

)n

Lev Bromberg: 0000-0003-2326-8803 Paul Brown: 0000-0002-5660-0377 T. Alan Hatton: 0000-0002-4558-245X

)n

Such complexation strongly affects partition equilibria of uranium between the extractant-containing organic and aqueous phosphoric acid phases,29,46 increases with temperature, and lowers the partition coefficient between the organic and aqueous phases in the DEHPA-TOPO processes. Therefore, it appears that we created a unique material capable of maintaining the efficiency of the liquid−liquid extraction of uranium from a 6 M phosphoric acid solution while reversing the thermodynamic nature of uranium binding, which has previously been found only in solid adsorbents. No prior reports on successful heterogeneous uptake of uranium from phosphoric acid solutions by magnetic materials could be found.

Notes

The authors declare no competing financial interest.

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REFERENCES

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