Synergistic Solvent Impregnated Resin for Adsorptive Separation of

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Synergistic Solvent Impregnated Resin for Adsorptive Separation of Lithium Ion Kenta Onishi, Takahide Nakamura, Syouhei Nishihama, and Kazuharu Yoshizuka* Department of Chemical Engineering, The UniVersity of Kitakyushu, Hibikino 1-1, Kitakyushu 808-0135, Japan

A novel synergistic solvent impregnated resin (SIR), containing both 1-phenyl-1,3-tetradecanedione (C11phβDK) and tri-n-octylphosphine oxide (TOPO), was used for the selective adsorption of Li+ in aqueous chloride media. The extractants have a synergistic effect on the ability of the SIR system to adsorb Li+ ion with high selectivity relative to Na+ and K+. The extent of adsorption of Li+ is increased with an increase in the amount of TOPO impregnated in the SIR. Li+ can be effectively separated from a solution containing a high concentration of Na+ by column chromatography using a column packed with SIR. 1. Introduction Rare metals are important materials in high-technology industries, and the demand for such metals has been increasing in recent years. A sustainable supply of rare metals is important for supporting the development of high technology.1 Lithium, one of the rare metals, is widely used for lithium ion batteries. Although lithium is obtained at present from mines and the brine of salt lakes, the supply of lithium will diminish with increase in demand, mainly due to the increase in the large-scale lithium ion battery industry.2 Seawater, which contains 230 billion tons of lithium in total, is expected to be an attractive source of lithium, although large amounts of sodium, potassium, and magnesium coexist with lithium in seawater. Development of an effective separation and recovery process for lithium from seawater has been investigated.3 In a previous work, a spineltype manganese dioxide (λ-MnO2) adsorbent was used to recover Li+ as a chloride salt from seawater, using a benchmark plant.4 However, the purity of Li+ in the salt obtained was too low (ca. 33%), due to contamination by Na+ (the major component in seawater), and a procedure for purification of Li+ against Na+ is still needed to obtain high-purity lithium. Solvent extraction has been widely applied as a process for separation, purification, and recovery of metals, due to its simplicity of equipment and operation. Over the past few decades, there have been several studies on the synergistic extraction of Li+ with mixtures of β-diketones and neutral organophosphorus compounds as extractants.5-13 In these synergistic extraction systems, the β-diketone and neutral organophosphorus compounds act as chelating ligand and dehydration reagents, respectively. Miyai et al. reported recovery of Li+ from the residual solution of Li2CO3 precipitation, with a mixture of commercial β-diketones (LIX-51, R-perfluoroalkanoyl-m-dodecylacetophenone) and TBP (tri-n-butyl phosphate).14 They established that Li+ could be quantitatively extracted in the presence of the other alkali metal ions. Zushi et al. also investigated the extraction behavior of Li+ using a mixture of β-diketones with several alkyl chains and neutral organophosphorus compounds.15,16 The synergistic effect for the extraction of Li+ was obtained with the combinations of β-diketones, such as 1-phenyl-1,3-tetradecanedione (abbreviated hereafter as C11phβDK; the structure shown in Figure 1), and neutral organophosphorus compounds, such as tri-n-octylphosphine oxide (abbreviated as TOPO), though almost no extraction progressed with each single-extractant system. They estimated * To whom correspondence should be addressed. Tel.: +81-93-6953281. Fax: +81-93-695-3374. E-mail: [email protected].

that the β-diketone acts as the cation exchanger for Li+, while the neutral organophosphorus compounds are the solvating reagent. The disadvantages of the solvent extraction method, such as the need for a large amount of organic solvent, have been pointed out recently, especially from the environmental point of view. Combination of solvent extraction with adsorption and/ or ion exchange, such as with a solvent-impregnated resin (abbreviated as SIR)17-21 and microcapsules encapsulating extractant,22-25 has been investigated as a second generation extraction system. Recently, as a Li+ separation system a molecular imprinted polymeric adsorbent containing a mixture of C11phβDK and TOPO has been developed to combine a synergistic extraction system with the ion exchange method.26,27 Although the adsorbent possesses high selectivity for Li+, the loading capacity is quite poor (of the order of several micromoles per gram). The SIR is easily prepared by simply treating the polymeric resin with an organic solution of extractant, and the loading capacity of the SIR for the metal ion is sufficiently high (of the order sub-millimoles per gram). SIRs containing two kinds of extractants for synergistic extraction have scarcely been reported. An SIR impregnated with a combination of bis(2ethylhexyl) phosphoric acid (abbreviated as D2EHPA) and TOPO was reported for mutual separation of Zn2+, Cu2+ and Cd2+.28 An antisynergistic effect for Cu2+ and Cd2+ together with no effect for Zn2+ were, however, observed for the D2EHPA/TOPO SIR system, even though the separation of Zn2+ against Cu2+ and Cd2+ can be improved by increasing the TOPO content of the SIR. An SIR system impregnated with a combination of crown ethers and bis(2,4,4′-trimethylpentyl) dithiophosphinic acid (Cyanex 301) was recently reported for the selective uptake of thorium.29 They concluded the sufficient adsorption capacity of thorium could be obtained in a 15crown-5 and Cyanex 301 system. In the present work, a novel SIR synergistic extraction system, containing C11phβDK and TOPO, was prepared to investigate the adsorption behavior of Li+ in aqueous chloride media. The SIR was first applied to conventional batchwise adsorption, and the synergistic adsorption of Li+ was investigated. The ability of the SIR to separate Li+ from several alkali and alkaline earth metal ions was also investigated in a batchwise procedure. In

Figure 1. Chemical structure of C11phβDK.

10.1021/ie100145d  2010 American Chemical Society Published on Web 06/24/2010

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Figure 4. Effect of coexisting metal ions on the adsorption of Li+. [Li+]initial ) 1.0 mmol/L; pH ) 12 ( 0.1. Figure 2. Effect of equilibrium pH on the adsorption of Li+ with several SIRs. [Li+]initial ) 1.0 mmol/L.

addition, the SIR was applied to column operation for selective adsorption of Li+ in the presence of a high concentration of Na+. 2. Experiment 2.1. Reagents and Procedures. Tri-n-octylphosphine oxide (TOPO), supplied by Tokyo Chemical Industry, was used without further purification. DIAION HP2MG, a methacrylic ester copolymer without any ligand, was kindly supplied by Nippon Rensui. All other organic and inorganic reagents were supplied by Wako Pure Chemical Industries, as analytical-grade reagents. 2.2. Preparation of 1-Phenyl-1,3-tetradecanedione. C11phβDK was synthesized according to the procedure reported by Zushi et al.15 The product was identified by 1H NMR spectrometry (JEOL, JMM-ECP500) and FT-IR spectroscopy (Shimadzu, FTIR-8400S). Elem. anal. (Yanaco, MT-6 Corder). Found: C, 78.61; H, 10.02%. Calcd for C20H30O2: C, 79.42; H, 10.00%. 2.3. Impregnation Procedure. The SIR containing C11phβDK and TOPO was prepared by the following method. HP2MG was washed with methanol and dried in vacuo and then immersed overnight in a toluene solution of C11phβDK and/or TOPO. The toluene was then removed by evaporation and the resin dried in vacuo for 24 h. The amount of C11phβDK impregnated in the resin was fixed at 0.66 mmol/g, while the amount of TOPO impregnated was varied from 0 to 0.66 mmol/

g. The preliminary batchwise experiments for the leakage of extractants from the SIR indicate that the SIR has high affinity for the extractants, and the extractants in SIR leaked at a quite low extent into the aqueous solution over the whole pH range. 2.4. Batchwise Adsorption of Metal Ions. Aqueous feed solutions of Li+, other alkali metals (Na+ and K+), and alkaline earth metals (Mg2+, Sr2+ and Ca2+) were prepared by dissolving each chloride salt in deionized water. The concentration of each metal ion, in the case of single metal systems for pH dependency experiments, was set at 1.0 mmol/L, while the concentration of Li+ was varied from 1 to 40 mmol/L for adsorption isotherm experiments. In mixed metal systems, the concentration of Li+ was set at 1.0 mmol/L and the concentrations of other metal ions were varied from 10 to 100 mmol/L. The pH was adjusted by adding the appropriate amount of sodium hydroxide solution to the aqueous solution. A 20 mg amount of SIR was added to 10 mL of aqueous feed solution, and the mixture shaken at 298 K for 12 h. After filtration, the equilibrium pH was measured. The concentrations of metal ions were determined by an atomic adsorption spectrophotometer (AAS; Shimadzu AAS-6800). The amount of metal ion adsorbed, qM, is defined by qM )

([M]initial - [M])L w

(1)

where [M]initial and [M] represent initial and equilibrium concentrations of metal ion in the aqueous phase. L and w are the volume of the aqueous solution and the weight of adsorbent, respectively.

Figure 3. Effect of TOPO content on the Li+ adsorption isotherm, for C11phβDK contents (a) 0.66 and (b) 0.33 mmol/g, at pH ) 12 ( 0.1.

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remaining feed solution. The loaded metal ions were then eluted with 1.0 mol/L hydrochloric acid solution. The effluents were collected with a fraction collector (EYELA DC-1500). The pH and metal ion concentration were measured by pH meter and AAS, respectively. The bed volume (BV) of the effluent is defined as BV )

Vt V

(2)

where V, t, and V are the flow rate of solution, the time for which the feed solution was supplied, and the wet volume of adsorbent, respectively. Figure 5. Effect of pH on the adsorption of Li+ and divalent ions. [Mn+]initial ) 1.0 mmol/L.

2.5. Chromatographic Adsorption of Li+. SIR was treated with 0.1 wt % sodium lauryl sulfate solution to make the surface of the resin hydrophilic, prior to column operation. The SIR used for the column experiments was impregnated with 0.66 mmol/g of both C11phβDK and TOPO. A 4.0 g amount of SIR (wet volume ) 7.0 mL) after hydrophilic treatment was packed into a column with 10 mm diameter and washed with deionized water. An aqueous feed solution with pH ) 12.3 containing 10 mmol/L of Li+ and 435 mmol/L of Na+ was fed upward to the column at flow rate 0.4 mL/min (SV ) 3.4 h-1, which is defined as the ratio of flow rate to wet volume of SIR), using a dual plunger pump (Flow KP-11). After breakthrough of Li+, deionized water was fed into the column to wash out the

3. Results and Discussion 3.1. Adsorption of Li+ on SIR. Figure 2 shows the effect of equilibrium pH on the amount of Li+ adsorbed by SIRs with varying TOPO content and constant C11phβDK content (0.66 mmol/g). The adsorption capability was improved with an increase in TOPO content. This trend indicates that a synergistic effect on the adsorption of Li+ is operative in this SIR system, and TOPO is capable of dehydrating Li+ even in the polymer matrix, as in the conventional solvent extraction system. The amount of Li+ adsorbed increased with increasing pH for a large TOPO content, and effective Li+ adsorption was achieved at about pH 12. The adsorption of Li+ with this SIR occurs via a cation exchange mechanism between Li+ and H+ of the enol tautomer of the β-diketone. Figure 3 shows the effect of TOPO content on adsorption isotherms at pH ) 12. The mole ratio C11phβDK:TOPO was

Figure 6. (a) Breakthrough and (b) elution profiles of Li+ and Na+. [Li+]initial ) 10 mmol/L; [Na+]initial ) 435 mmol/L; pHinitial ) 12.3.

Figure 7. (a) Breakthrough and (b) elution profiles of Li+ and Na+ for reusability of the SIR. [Li+]initial ) 10 mmol/L; [Na+]initial ) 435 mmol/L; pHinitial ) 12.3.

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varied from 1:0.1 to 1:1 (Figure 3a) and from 1:1 to 1:2 (Figure 3b). The maximum amounts of Li+ adsorbed increased with an increase in the TOPO content up to the ratio 1:1, and no increase in the amount of Li+ adsorbed was observed when TOPO was present in excess of C11phβDK. Thus, the maximum synergistic effect was obtained with equal proportions of C11phβDK and TOPO. In the case of SIR containing 0.66 mmol/g of each extractant, the maximum amount of Li+ adsorbed was 0.60 mmol/g, almost equal to the extractant contents. On the basis of these observations the stoichiometry of the metal complex in the SIR is suggested to be Li+:C11phβDK:TOPO ) 1:1:1. The present impregnated resin possesses much higher adsorption capacity for Li+ than the molecular imprinted polymeric adsorbent containing C11phβDK and TOPO, though there were no data of the maximum adsorption amount of the molecular imprinted polymeric adsorbent based on the adsorption isotherm.26,27 3.2. Separation Ability for Li+ against Alkali and Alkaline Earth Metals. Figure 4 shows the effect of coexisting metal ions on the adsorption of Li+. The amount of Li+ adsorbed was reduced to a negligible extent by coexisting alkali metal ions (Na+ and K+), whereas adsorption of Li+ was strongly suppressed by the alkaline earth metal ions (Ca2+ and Sr2+). Figure 5 shows the effect of pH on the adsorption of Li+ and alkaline earth metals in single-metal systems. The adsorption selectivity of metal ions was in the order Mg2+ > Ca2+ > Sr2+ . Li+, which agrees with the selectivity order for the conventional solvent extraction system.7 Since suppression of Li+ adsorption in the presence of alkaline earth metals is due to the selective extraction of the alkaline earths, the alkaline earth metal ions must be removed before recovery of Li+. The separation of Li+ from Na+ and K+ is, however, easily achieved with the present SIR. 3.3. Chromatographic Adsorption of Li+. Figure 6a shows breakthrough profiles of Li+ and Na+. As expected from the batchwise experiments, Li+ was selectively adsorbed, while Na+ was adsorbed to an insignificant extent. Figure 6b shows the elution profiles of Li+ and Na+ with 1.0 mol/L hydrochloric acid solution as eluent. Li+ was quantitatively eluted with high purity (99.8%) at BV ) 2.7, while Na+ was scarcely eluted. The maximum concentration of Li+ was reached at 360 mmol/ L, corresponding to 36 times the concentration of Li+ in the feed solution. The reusability of the SIR was also investigated. Figure 7 shows the breakthrough and elution profiles of Li+ and Na+ for reuse of the SIR. The adsorption-elution processing of Li+/ Na+ was conducted 10 times with the same column. The elution profiles were hardly changed from those in Figure 6b, and the average elution yield was 96.9 ( 6.3%. This indicates that there is little solvent loss during column operation and the SIR has sufficient loading capacity for use in repeated processing. 4. Conclusions The adsorptive separation of Li+ with a SIR impregnated with a mixture of the extractants C11phβDK and TOPO was investigated with the following results. (1) A synergistic effect of the two extractants can be successfully obtained in the SIR system, and the adsorption abilities were improved with increased TOPO content in the SIR. (2) The SIR has high selectivity for Li+ against Na+ and + K . Li+ can be effectively separated from aqueous solution containing a large excess of Na+ concentration by column operation.

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(3) The present SIR can be reused for repeated processing and thus has high potential as an adsorbent for recovery of Li+ from seawater. Acknowledgment We are grateful for the financial support through a Grantin-Aid for Scientific Research (B) (No. 20360353) from the Japan Society for the Promotion of Science (JSPS). Nomenclature L ) volume of liquid phase, L M ) metal ion q ) adsorption amount in the impregnated resin, mmol/g t ) time for the feed solution supplied, min V ) flow rate of solution, mL/min V ) wet volume of the impregnated resin, mL w ) weight of the impregnated resin, g [ ] ) concentration of metal ions in the brackets, mmol/L

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ReceiVed for reView January 21, 2010 ReVised manuscript receiVed March 29, 2010 Accepted June 9, 2010 IE100145D