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Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H1.6Mn1.6O4) Derived from Li1.6Mn1.6O4 Ramesh Chitrakar, Hirofumi Kanoh, Yoshitaka Miyai, and Kenta Ooi* Separation Technology Group, National Institute of Advanced Industrial Science & Technology, 2217-14 Hayashi-cho, Takamatsu 761-0395, Japan
Manganese oxide adsorbent (H1.6Mn1.6O4) was synthesized from precursor Li1.6Mn1.6O4 that was obtained by heating LiMnO2 at 400 °C. LiMnO2 was prepared by two methods: hydrothermal and reflux. The crystallite size of Li1.6Mn1.6O4 and its delithiated product was slightly higher by the hydrothermal method as compared to the reflux method. The adsorbents prepared by the two methods were compared in terms of physical characteristics and lithium adsorption from seawater. The maximum uptake of lithium from seawater by the adsorbent was 40 mg/g, which is the maximum value among the adsorbents studied to date. Introduction Interest in lithium sources has been increasing because of wide applications of the metal in high-energy storage lithium batteries and its compounds in other fields.1 Lithium also has an important potential use in thermonuclear fusion.2 The lithium requirement for the blanket region of controlled thermonuclear fusion reactors is estimated to be between 4.3 × 107 and 109 kg beyond the year 2030.3 Present lithium reserves not including that in seawater are about equal to the estimated demand for lithium required for fusion reactors and battery applications. Lithium appears in lake brines and several minerals such as spodumene, petallite, lepidolite, and amblygonite. Seawater is also considered as a vast source of lithium (about 2.5 × 1014 kg), although the concentration of lithium is very low, i.e., 0.17 mg/L.3 Several methods such as adsorption (ion exchange),4-9 solvent extraction,3 and coprecipitation10 have been investigated for the extraction of lithium from seawater, brine, and geothermal water. The adsorption method is suitable for recovery of lithium from seawater because certain inorganic ion-exchange materials show extremely high selectivity for lithium ions only. In the past 20 years, several studies have been done on the recovery of lithium from seawater using different types of inorganic adsorbents. Among the inorganic adsorbents, spinel-type hydrous manganese oxides are interesting materials because of their extremely high affinity toward lithium ions only. Many lithium manganese oxides and their lithium extraction/insertion reactions have been studied for the development of selective adsorbents and cathode materials for rechargeable batteries.11-17 The spinel-type manganese oxide MnO2‚ 0.31H2O derived from Li1.33Mn1.67O4 is suitable as a lithium selective adsorbent, because it has a high chemical stability against lithium insertion-extraction in the aqueous phase as well as selective lithium uptake.6 We have developed a new type of lithium selective manganese oxide, MnO2‚0.5H2O (H1.6Mn1.6O4) derived from Li1.6Mn1.6O4.18 First, the ion-exchange capacity was markedly larger than that of the other manganese oxides, because the theoretical exchange capacity reaches 10.5 mmol/g on the basis of the chemical composition. * Corresponding author. E-mail:
[email protected].
Second, the chemical stability is sufficiently high, because it contains only tetravalent manganese. The purpose of this study is to evaluate the applicability of H1.6Mn1.6O4 for the recovery of lithium from seawater. Experimental Section Preparation of Material. Two methods are reported. LiMnO2 was prepared by the hydrothermal method as reported earlier.18 A total of 2 g of γ-MnOOH (Toyo Soda Co., Japan) were mixed with 40 mL of a 4 M LiOH solution (1 M ) 1 mol/L) in a Teflon-lined stainless steel vessel (50 mL) and autoclaved at 120 °C for 1 day. The precipitate was filtered, washed with deionized water, and dried at 60 °C. The obtained product (LiMnO2) was then heated at 400 °C for 4 h in air to obtain Li1.6Mn1.6O4. The Li+ extraction was carried out batchwise by stirring 1.5 g of material with 2 L of a 0.5 M HCl solution for 1 day. The acid-treated materials were filtered and washed with deionized water and airdried. The samples at each step were designated as LiMnO2(HT), Li1.6Mn1.6O4(HT), and H1.6Mn1.6O4(HT). In the reflux method, 10 g of γ-MnOOH was mixed with 200 mL of a 4 M LiOH solution and boiled for 8 h. The remaining procedure was the same as that described in the hydrothermal method; i.e., the product was filtered, washed, heated, etc. The samples at each step were designated as LiMnO2(RF), Li1.6Mn1.6O4(RF), and H1.6Mn1.6O4(RF). Physical Analysis. X-ray diffraction (XRD) analysis was carried out using a Rigaku type RINT 1200 X-ray diffractometer with a graphite monochromator. Differential thermal analysis-thermogravimetry (DTATG) curves of materials were performed on a MAC Science thermal analyzer (system 001 200 TG-DTA) at a heating rate of 10 °C/min. The water contents of the samples were calculated from the weight loss in the temperature range of 120-400 °C. Chemical Analysis. The adsorbent (50 mg) was dissolved in a 0.1 M HCl solution containing hydrogen peroxide, and the lithium and manganese contents were determined with a Shimadzu AA-760 atomic absorption spectrometry. The mean oxidation number (ZMn) of manganese was evaluated after determination of the available oxygen by the standard oxalic acid method.19
10.1021/ie000911h CCC: $20.00 © 2001 American Chemical Society Published on Web 04/06/2001
Ind. Eng. Chem. Res., Vol. 40, No. 9, 2001 2055 Table 1. Composition of LiMnO2 Samples Heated at 400 °C and Their Delithiated Materials sample
Li/Mn
LiMnO2(HT) Li1.6Mn1.6O4(HT) H1.6Mn1.6O4(HT) LiMnO2(RF) Li1.6Mn1.6O4(RF) H1.6Mn1.6O4(RF)
1.01 0.99 0.014 0.98 0.99 0.016
H2O/Mn
0.51 0.52
ZMn
formula
3.03 3.96 4.02 3.02 3.89 3.96
Li1.01MnO2.02 Li1.59Mn1.61O4 MnO2‚0.50H2O Li0.98MnO2 Li1.64Mn1.64O4 MnO2‚0.52H2O
Adsorption of Lithium from Seawater. (i) Rate of lithium adsorption: The rate of lithium adsorption from seawater was determined by stirring 15 mg of adsorbent in 5 L of seawater. (ii) Adsorption isotherm: Adsorption isotherms of lithium were obtained by stirring different amounts of adsorbent with 5 L of seawater for 3 days. (iii) Maximum metal ion uptakes: The metal ion uptakes were determined by stirring 100 mg of adsorbent with 8 L of seawater, which was replaced eight times until equilibrium was reached (1 month). The concentration factor (CF) was calculated as follows:
CF ) metal ion uptake (mg/g) by adsorbent/metal ion concentration in seawater (mg/L) (iv) Adsorption and desorption of metal ions: Repetition of adsorption/desorption was studied batchwise. The adsorption of metal ions was carried out by stirring 200 mg of adsorbent with 50 L of seawater, which were replaced four times to attain the equilibrium (1 month).
After the adsorption, desorption of metal ions was carried out by stirring the adsorbent (200 mg) with a 0.5 M HCl solution (250 mL). All of the adsorption experiments were carried out at room temperature. The metal ion uptakes were determined by atomic absorption spectrometry after dissolution of the sample with a mixed solution of HCl and H2O2. All of the calculations of metal ion uptakes were based on the manganese content in the original hydrogen form adsorbent. Results and Discussion Preparation of LiMnO2. We adopted two methods: hydrothermal and reflux. LiMnO2 prepared by the hydrothermal method appeared to be a light greenish color, while the intensity of light greenish color was much lower in LiMnO2 prepared by the reflux method. The formula of LiMnO2 was confirmed for both of the samples from the chemical analysis of lithium and manganese and the mean oxidation number (ZMn) of manganese (Table 1). The XRD patterns of the present materials showed that the crystal system was identical with that of orthorhombic LiMnO2 (JCPDS no. 35-0749; Figure 1.). LiMnO2 prepared by the reflux method showed broader and weaker peaks and was therefore less crystalline than LiMnO2 prepared by the hydrothermal method. The reaction of γ-MnOOH with the LiOH solution was an ion-exchange-type reaction replacing H+ with Li+ to form LiMnO2. The transition from monoclinic-type γ-MnOOH to orthorhombic-type
Figure 1. XRD patterns of manganese oxides prepared by the hydrothermal method (top) and the reflux method (bottom).
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Figure 2. DTA-TG curves of adsorbents. Bottom left: LiMnO2(HT). Bottom right: H1.6Mn1.6O4(HT). Top left: LiMnO2(RF). Top right: H1.6Mn1.6O4(RF).
LiMnO2 involves a certain amount of local rearrangement of the atomic structure of the MnO2 “host” lattice because the close-packed oxygen layers and Mn chains are common to both structures.15 Preparation of Li1.6Mn1.6O4. Li1.6Mn1.6O4 was prepared by thermal treatment of LiMnO2 at 400 °C, which brought about the oxidation of manganese from trivalent to tetravalent. The DTA-TG curve of LiMnO2 prepared by the hydrothermal method showed a sharp exothermic peak around 380 °C with abrupt weight gain, followed by a slow weight gain between 400 and 520 °C (Figure 2). The weight gain reached its maximum at 520 °C and then decreased slightly with a rise in temperature above 520 °C. LiMnO2 prepared by the reflux method showed a broad exothermic peak around 330 °C. The formula of Li1.6Mn1.6O4 was confirmed for both of the heat-treated samples by chemical analysis. The mean oxidation numbers of Mn were 3.96 and 3.89 for Li1.6Mn1.6O4(HT) and Li1.6Mn1.6O4(RF), respectively (Table 1). The low oxidation state of Mn in the sample prepared by the reflux method was due to incomplete oxidation of Mn3+ to Mn4+ in LiMnO2 as evidenced by the smaller weight gain from the TG-DTA curve (7% for LiMnO2(HT) and 4% for LiMnO2(RF)). The XRD patterns of the samples are shown in Figure 1. LiMnO2 was less crystalline by the reflux method as compared to the hydrothermal method; its heated material Li1.6-
Mn1.6O4 also showed a smaller crystallite size. Both of the Li1.6Mn1.6O4 samples could be indexed to a facecentered-cubic system (Fd3m) with a lattice constant of 8.15 Å. Preparation of H1.6Mn1.6O4. The extractability of lithium from the Li1.6Mn1.6O4 samples was investigated using a 0.5 M HCl solution; the Li+ extractability reached 99%. The percentage of the dissolution of manganese during the acid treatment was 2. The mean oxidation states of manganese in the acidtreated samples were nearly equal to 4 (Table 1). This shows that the extraction progresses mainly by the Li+/ H+ ion-exchange mechanism. The water content was determined by the weight loss between 100 and 400 °C, assuming the product to be β-MnO2. The H2O contents were nearly equal to the theoretical proton content calculated based on the Li+/H+ exchange reaction, suggesting that the H2O contents corresponded to the amount of lattice hydroxyl groups formed by the exchange reaction. The formulas of H1.6Mn1.6O4 were confirmed for both of the samples. The XRD patterns of the acid-treated samples showed the preservation of the cubic structure with a slight decrease of the lattice constant (Figure 1). The patterns could be indexed to a face-centered-cubic system (Fd3m) with a lattice constant of 8.05 Å. The relative intensities of the peaks were almost the same as those of the precursor Li1.6Mn1.6O4. This indicates that the lithium extraction progressed topotactically, preserving the cubic structure. DTA-TG curves of H1.6Mn1.6O4 were almost the same for both of the samples: two distinct endothermic peaks around 170 and 210 °C with weight loss (Figure 2). These peaks could be ascribed to the condensation of hydroxyl groups, which are responsible for the ionexchange reaction. The exothermic peak around 260 °C could be ascribed to the crystallization from spinel to β-MnO2. The large endothermic peak around 540 °C with weight loss was due to the transformation from β-MnO2 to the more stable R-Mn2O3 phase accompanied by a loss of oxygen. Lithium Adsorption from Seawater. The rate of lithium adsorption was studied by the batch method. The time dependence of lithium adsorption from seawater is shown in Figure. 3. The lithium uptake reached 28 mg/g in 1 day and increased to 35 mg/g in 2 days; equilibrium was reached within 2 days. The time
Figure 3. Rate of lithium adsorption from seawater. H1.6Mn1.6O4(HT) ) 15 mg. Seawater ) 5 L.
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has a strong potential for both lithium adsorption and lithium recovery. The slow adsorption of lithium might be due to the small amount of adsorbent versus the large volume of seawater because all of the adsorbent particles could not be distributed throughout the seawater. The seawater with adsorbent could not be adequately stirred constantly in the container. Because of the very small concentration of lithium in seawater (0.17 mg/g), the time required to reach equilibrium at maximum lithium uptake would be very long for the small amount of adsorbent with a large amount of seawater. Lithium uptakes were high (34-40 mg/g), while the uptakes of other metal ions were low (
