ION EXCHANGE ADSORPTION OF MOLYBDENUM WITH ZEOLITIC ADSORBENT

Transcription

1 J. Environ. Eng. Manage., 19(6), (009) 365 ION EXCHANGE ADSORPTION OF MOLYBDENUM WITH ZEOLITIC ADSORBENT Syouhei Nishihama* and Kazuharu Yoshizuka Department of Chemical Engineering The University of Kitakyushu Kitakyushu , Japan Key Words: Ion exchange, zeolite, molybdenum ABSTRACT Zeolitic adsorbents have been developed for molybdenum recovery from various sources. The batchwise adsorption ability of molybdenum ion (Mo 6+ ) with zeolite A type adsorbent could be improved when the counter cation (Na + ) was exchanged by divalent ions (Sr + and Ca + ), while no significant difference was observed in the case of zeolite X type adsorbent. The adsorption ability for Mo 6+ was therefore based on the pore size and cation exchange capacity of the zeolitic adsorbents. The column operations were also conducted with the granulated zeolitic adsorbents, to achieve the quantitative adsorption-elution processing of Mo 6+. INTRODUCTION The demand for rare metals has been increasing in recent years, and thus the separation and recovery of the rare metals are still active issues. Recycle of rare metals from various wastes, such as electric appliances and automobile catalysts, are especially quite important issues for establishing a sound materialcycle society. Among the several recovery technologies of rare metals, ion exchange adsorption is one of the most effective methods and is quite green process. Most of ion exchangers are based on the polymer matrix immobilizing various functional groups for uptake of metal ions. Also, inorganic adsorbent has been paid more attention, due to both separation ability and physical properties such as resistance against heat and radioactive irradiation [1-3]. Molybdenum (Mo) is one of the rare metals used for additive of ferrous alloys based on the nature of melleabil and ductil. Mo is also included in high level liquid waste (HLLW) solution of spent nuclear fuel. The separation and recovery of Mo from HLLW is required for nuclear fuel cycle process, since the solubility of Mo for vitrification is quite low. There are two ways for the hydrometallurgical separation of molybdenum ion (Mo 6+ ) in the aqueous solution: solvent extraction [] and ion exchange [5-7]. In case of solvent extraction, acidic organophosphorus compounds, such as bis(-ethylhexyl)phosphoric acid and -ethylhexyl phosphonic acid mono--ethylhexyl ester, were occasionally employed as extractants [8-10], and effective extraction of Mo 6+ from acid solution could be achieved. In case of ion exchange, anion exchange method was mostly applied, since molybdenum mainly exists as anionic species in the aqueous solution [11]. The cation exchange adsorption is also expected to be available in the acidic region where Mo 6+ exists as cationic species, as well as solvent extraction system, though there are few reports. Zeolite is the crystalline alumino-silicate, mainly based on SiO and Al O 3, and possesses molecular sieve property due to its regulated pore size. The zeolite has thus been used for the separation of gas components. Zeolite A and zeolite X have been also used for the water softening, since these possesses high cation exchange abilities for Ca + and Mg +. Recently, the zeolite has been attempted to be applied as the cation exchange adsorbent for heavy metals, especially in the field of nuclear fuel cycle [,1], indicating the zeolitic adsorbents possess high adsorption ability for various heavy metals in HLLW. In the present work, cation exchange adsorption of Mo 6+ with zeolitic adsorbents has been investigated, as a fundamental investigation for the recovery of Mo 6+ from the solution of hydrometallurgical recycle process of catalysts and also HLLW in nuclear fuel cycle process. Zeolite A and zeolite X were prepared by hydrothzzermal synthesis, and were characterized with X-ray diffraction (XRD) and filed emission scanning electron microscope (FE-SEM). The zeolitic *Corresponding author nishihama@env.kitakyu-u.ac.jp

2 366 J. Environ. Eng. Manage., 19(6), (009) adsorbents were then applied for evaluating the adsorption of Mo 6+ using batchwise and column operation. EXPERIMENT 1. Synthesis of Zeolitic Adsorbents Fraction collector Quartz Wool Zeolitic Adsorbent All chemicals used were supplied by Wako Pure Chemical Industries, as analytical-grade reagents, and were used without further purification. Sodium silicate (7 g for zeolite A and 15 g for zeolite X) and sodium aluminate (10 g for zeolite A and 5.5 g for zeolite X) were dissolved in 100 ml of M NaOH aqueous solution. These two solutions were mixed to react in an autoclave at 100 o C for h. The autoclave was immediately cooled with tap water after hydrothermal synthesis. The precipitates obtained were filtrated, washed, and dried at 60 o C for overnight, to obtain Na-types of zeolite A () and zeolite X (X-Na). These Na-type zeolites ( g) were contacted and shaken with 00 ml of SrCl (0. M, ph = ) or 00 ml of CaCl solution (0. M, ph = ) for 1 h, to obtain the counter-cation exchanged zeolites (A-Sr, A- Ca, X-Sr, and X-Ca, respectively) [13]. The zeolitic adsorbents were characterized with a powder XRD meter (0 kv and 0 ma, CuKα, Rigaku XRD-DSC-X II) and a SEM (Hitachi S-500). For the chromatographic operation, the granulated adsorbent is preferred, due to its smaller pressure drop. The powder-type zeolites obtained were therefore granulated for the column operation. The powdertype zeolite of 6 g was mixed with g of alumina based binder (CATALOID AP-1, Catalysts & Chemicals Industries) together with small amount of deionized water. The mixture was pressed out with the extruder from 1 mm hole in diameter, and dried at room temperature for overnight. The mixture was then calcined at 550 o C for 3 h, and then was cut for -1 mm in length.. Ion Exchange Adsorption of Mo 6+ The aqueous feed solution was prepared by dissolving Na MoO H O into deionized water to prepare 1 M of Mo 6+ solution. The ph value of the aqueous solution was adjusted with appropriate HCl or NaOH. The zeolitic adsorbent (0 mg) and the aqueous solution (10 ml) were contacted and shaken vigorously at 98 K for more than h. The concentration of Mo 6+ was analyzed by using an inductively coupled plasma atomic emission spectrometer (ICP- AES, Shimadzu ICPS-7000), and the corresponding adsorption amounts, q, were determined by material balance, as follows: ([Mo] feed [Mo]) L q = w Feed Solution Pump Fig. 1. Schematic flowsheet for the column adsorption apparatus setup. where [Mo] feed and [Mo] are initial and equilibrium concentrations of Mo 6+ in the aqueous phase, L is volume of aqueous solution, and w is weight of adsorbent. The aqueous phase ph value was measured with a ph meter (Horiba F-3). The chromatographic operation was also conducted using a column apparatus setup, as shown in Fig. 1. The granulated zeolitic adsorbent (0. g) was packed into the column tube (5 cm in length and 5 mm in inner diameter) together with quartz wool to be sandwiched. The zeolitic adsorbent in the column was treated with 1 M HCl (ph = ) prior to use. The aqueous feed solution, containing ca. mm of Mo 6+ at ph =, was then fed upward to the column using a dual-plunger pump at 5 ml min -1 (Flom KP-11). The effluent was collected with a fraction collector (Advantec CHF1SA), to measure the concentrations of Mo 6+ with ICP-AES. After the breakthrough, the Mo 6+ loaded in the adsorption column was eluted with 1 M of NaOH (ph = 1). Bed volume of the effluent is calculated by: v t Bed volume = V where v is flow rate of solution, t is supplying time of feed solution, and V is the wet volume of adsorbent. RESULTS AND DISCUSSION 1. Preparation of Zeolitic Adsorbents Figure shows the XRD patterns of and X-Na. Comparing the XRD patterns with the database [1], the required zeolites could be obtained by the synthesized method used in the present study. Figure 3 shows the SEM images for the and X-Na. Cubic shape particles are obtained in the case of, while aggregated particles of octahedron are found in the case of X-Na.

3 Nishihama and Yoshizuka: Adsorption of Molybdenum by Zeolite (a) Zeolite 0 mg + Mo solution 10 ml [Mo(VI)] feed = 1 mol/l Intensity (a.u.) X-Na A-Sr A-Ca θ (deg) Fig.. X-Ray diffraction patterns for zeolite A () and zeolite X (X-Na). q (mmol/g) 1.0 (b) Zeolite 0 mg + Mo solution 10 ml [Mo(VI)] feed = 1 mol/l X-Na X-Sr X-Ca Fig. 3. Scanning electron microscopy pictures of (a) zeolite A () and (b) zeolite X (X-Na).. Batchwise Adsorption of Mo 6+ with Zeolitic Adsorbents Figure a shows the effects of ph value on the adsorption amounts of Mo 6+ with, A-Sr, and A- Ca. In all cases, the adsorption of Mo 6+ occurs in the acidic ph region and the adsorption amount of Mo 6+ decreases with increase in the ph. This indicates that the adsorption progresses via cation exchange and only cationic species Mo 6+ takes part in the adsorption. In addition, the adsorption amounts are strongly enhanced, when the counter-cation is exchanged by divalent cations (A-Sr and A-Ca). This is due to the increase in the pore size, since the pore size of zeolite A has been reported to increase by exchanging countercation to divalent ions [13]. Figure b shows the adsorption behaviors of Mo 6+ on X-Na, X-Sr, and X-Ca. The adsorption amounts of Mo 6+ on all zeolite X type adsorbents are much lower than those of A-Ca and A-Sr, though the pore size of zeolite X is well known to be larger than that of zeolite A. The possible reason of the low adsorption amount on zeolite X is that the cation exchange capacity of zeolite X is smaller than that of zeolite A. The adsorption of Mo 6+ on zeolitic adsorbent is thus based on both pore size and cation exchange capacity of zeolites. In these zeolitic adsorbents, A-Ca is obviously seen to possess the most excellent adsorption ability for Mo Fig.. Effect of aqueous ph value on the adsorption amount of Mo 6+ with (a) Zeolite A, (b) Zeolite X. 3. Column Adsorption of Mo 6+ with Zeolitic Adsorbents Based on the batchwise adsorption, the column operation was conducted by using and A-Ca as the adsorbents. The column packed with zeolitic adsorbent was firstly treated with 1 M HCl to prevent from the increase in ph due to the hydrolysis and then the aqueous solution of Mo 6+ was fed. Figure 5a shows the breakthorough profile of Mo 6+ with. The ph value of the effluent was around 3.5, though the ph value of the feed solution was set at. Mo 6+ was therefore broken through at the early point of the adsorption, due to the slower adsorption rate in low ph region around. The breakthrough profile with A-Ca is also shown in Fig. 5. The adsorption amount of Mo 6+ was slightly improved, which is in agreement with the results of batchwise ph

4 368 J. Environ. Eng. Manage., 19(6), (009) Mo (mm) ph (a) Adsorption Zeolite: g g [Mo]feed = = ca. ca. mmmmol/l phfeed = phfeed = A-Ca Bed volume (-) 8 6 (b) Elution [NaOH] [NaOH]feed = 1 M 1 mol/l phfeed = 1 ph = Fig. 5. (a) Break through and (b) elution profiles of Mo 6+ on and A-Ca. The actual concentration of [Mo] feed = 1 mm () and 67 mm (A-Ca). experiments. After breaking through, the elution of Mo 6+ from loaded column was conducted with 1 M NaOH, as shown in Fig. 5b. In both cases of adsorption columns of and A-Ca, the quantitative elution of Mo 6+ could be achieved. Especially in case of A-Ca, the maximum concentration of Mo 6+ in the eluent was reached to 7.8 mm, which means 1 times concentration compared with 67 mm of Mo 6+ in aqueous feed solution. CONCLUSIONS The zeolite A and X type adsorbents have been prepared and ion exchange adsorptions of Mo 6+ were investigated, with following results. All types of zeolitic adsorbents could be successfully prepared by hydrothermal synthesis. The cation exchange adsorption of Mo 6+ with zeolitic adsorbent was progressed in the acidic region, and A-Ca possessed the highest adsorption capacity. The column operation was conducted with the granulated zeolite adsorbents to achieve the quantitative adsorption elution processing of Mo 6+. The more effective adsorption process of Mo 6+ could be proposed by improving adsorption rate by granulated zeolitic adsorbent. These results could contribute for the recovery and recycle of Mo 6+ from the solution of hydrometallurgical process of catalysts as well as from HLLW in nuclear fuel cycle process. ACKNOWLEDGMENTS The authors are grateful to Professor Sachio Asaoka of The University of Kitakyushu for his suggestion in the granulation of zeolitic adsorbents. The XRD measurement was performed at the Instrumentation Center, the University of Kitakyushu. REFERENCES 1. Kitajou, A., T. Suzuki, S. Nishihama and K. Yoshizuka, Selective recovery of lithium from seawater using a novel MnO type adsorbent II Enhancement of lithium ion selectivity of the adsorbent. Ars Separatoria Acta,, (003).. Sonohara, E., S. Nishihama and K. Yoshizuka, The synthesis of zeolite A ion exchanger for adsorption of Sr + and Cs +. J. Ion Exchange, 18(), (007). 3. Mimura, H., F. Tachibana and K. Akiba, Ionexchange selectivity for cesium in ferrierites. J. Nucl. Sci. Technol., 9(), (199).. Moris, M.A., F.V. Diez and J. Cosa, Solvent extraction of molybdenum and tungsten by Alamine 336 and DEHPA in a rotating disc contactor. Sep. Purif. Technol., 17(3), (1999). 5. Uchida, A., H. Koyanaka, S. Okubo and T. Fujita, Removal of Mo(VI) ion from waste water with lead compounds. Shigen-to-Sozai, 118(), (00). (in Japanese) 6. Zhang, A.Y., Y.Z. Wei and M. Kumagai, Properties and mechanism of molybdenum and zirconium adsorption by a macroporous silicabased extraction resin in the MAREC process. Solvent Extr. Ion Exc., 1(), (003). 7. Faghihian, H., A. Malekpour and M.G. Maragheh, Adsorption of molybdate ion by natrolite and clinoptilolite-rich tuffs. Int. J. Environ. Pollut., 18(), (00). 8. Sano, M., J. Shibata, M. Harada and S. Nishimura, Extraction of molybdenum and tungsten with DEHPA and LIX63, Nippon Kogyo Kaishi, 10(105), (1988). (in Japanese). 9. Sato, Y., K. Kondo and F. Nakashio, Extraction kinetics of molybdenum with -ethylhexyl phosphonic acid mono--ethylhexyl ester in membrane extractor using a hollow fiber. J. Chem. Eng. Jpn., (), 00-0 (1989). 10. Hirai, T., T. Hashimoto, I. Tsuboi, A. Hino and I. Komasawa, Extraction and separation of molybdenum and vanadium using bis(- ethylhexyl)monothiophosphoric acid and bis(- ethylhexyl)phosphoric acid. J. Chem. Eng. Jpn., 8(1), (1995). 11. Nakamura, T., S. Nishihama and K. Yoshizuka, A

5 Nishihama and Yoshizuka: Adsorption of Molybdenum by Zeolite 369 novel extractant based on D-glucosamine for the extraction of molybdenum and tungsten. Solvent Extr. Res. Dev., 16, 7-56 (009). 1. Miura, H., M. Kimura, K. Akiba and Y. Onodera, Separation of cesium and strontium by potassium nickel hexacyanoferrate(ii) loaded zeolite A. J. Nucl. Sci. Technol., 36(3), (1999). 13. Rakoczy, R.A. and Y. Traa, Nanocrystalline zeolite A: Synthesis, ion exchange and dealumination. Micropor. Mesopor. Mat., 60(1-3), (003). 1. Database of Zeolite Structures: ch/iza-sc/ Introduction.htm. (Jan 009) Discussions of this paper may appear in the discussion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication. Manuscript Received: April 16, 009 Revision Received: May 3, 009 and Accepted: June 7, 009