- Email: [email protected]
Adsorption of lithium ions on novel nanocrystal MnO2
Chemical Engineering Science 62 (2007) 4869 – 4874 www.elsevier.com/locate/ces
Adsorption of lithium ions on novel nanocrystal MnO2 Qin-Hui Zhang, Shuying Sun, Shaopeng Li, Hao Jiang, Jian-Guo Yu ∗ State-Key Lab of Chemical Engineering, College of Chemical Engineering, East China University of Science and Technology, 200237 Shanghai, PR China Received 30 April 2006; received in revised form 8 January 2007; accepted 10 January 2007 Available online 25 January 2007
Abstract Various polymorphs of MnO2 were synthesized under a controlled hydrothermal method. The structure characteristics and ion exchangeability were studied by XRD, TEM, Li+ adsorption isotherm, kinetics and selectivity measurement. The MnO2 nanowires, mainly about 5 × 400 nm in diameter and length, are found to have a remarkable lithium ion-sieve property with equilibrium distribution coefficients (Kd ) in the order of Na+ < K+ < Mg2+ < Ca2+ Li+ , Li+ monolayer saturation amount of 2.43 mmol g−1 , and the first-order adsorption rate constant of 2.16 × 10−6 s−1 , which is promising in the lithium extraction from brine or seawater. 䉷 2007 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Ion exchange; Lithium; MnO2 ; Nanostructure; Separations
1. Instruction Various polymorphic forms of MnO2 , including -, -, -, - and -type, are widely used as catalysts, electrode materials in Li/MnO2 batteries (Armstrong and Bruce, 1996; Annundsen and Paulsen, 2001; Thackeray, 1997) and ion sieves applied in the lithium extraction from brine or seawater for analysis (Tsuji and Abe, 1984; Kannugo and Paride, 1984), geochemistry (Gray and Malati, 1979; Balistrieri and Murray, 1982) and water-pollution control (Hasany and Chaudhary, 1981). MnO2 ion sieve was first synthesized from manganese hydroxide saturated with Li+ (Ooi et al., 1986). Great efforts have been made to prepare bulky or nanocrystalline MnO2 with different structure (Hunter, 1981; Bach et al., 1995; Chitrahar et al., 2001a,b) followed by the extraction of Li+ with acid solution (Berg et al., 2001; Li et al., 2002; Kucza, 2002; Koyanaka et al., 2003; Ooi et al., 1987). However, there have been no systematic studies on the effect of different polymorphs of MnO2 precursor and the size effect of related crystal nanostructures on the Li+ extraction process. In this paper, a controlled hydrothermal method has been developed in the
∗ Corresponding author. Tel./fax:+86 21 64252826.
E-mail address: [email protected] (J.G. Yu). 0009-2509/$ - see front matter 䉷 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2007.01.016
synthesis of MnO2 ion sieve by the acid treatment of Li–Mn–O precursor. The structure characteristics and ion-exchange properties are studied by XRD, TEM, Li+ adsorption isotherm, kinetics and selectivity measurement. 2. Experimental 2.1. Ion-sieve preparation To prepare the Li/MnO2 ion-sieve precursor, different polymorphic forms of MnO2 were first obtained by controlled low-temperature hydrothermal synthesis via the oxidation of Mn2+ by S2 O2− 8 . Analytical grade MnSO4 · H2 O (0.25 mol) and a stoichiometric ratio of (NH4 )2 S2 O8 (0.25 mol) were put into 750 ml deionized water to form a homogeneous solution at room temperature, which was then transferred into a stainless steel autoclave, sealed, maintained for 12 h at 393 K (named MO-a) and 423 K (named MO-b), respectively. The resulted black solid products were filtered, washed completely with deionized water to remove ions possibly remaining in the final products, and finally dried at 393 K for 8 h in static air. The whole process was adjusted to prepare MO-c and -d by adding 0.47 mol of analytical grade (NH4 )2 SO4 to the reaction system, respectively; Li/MnO2 precursor counterpart, named LMO-a, -b, -c, -d, were prepared by wet impregnation of an
4870
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
Fig. 1. Diagram of MnO2 ion-sieve preparation process.
aqueous solution of LiOH · H2 O (0.35 mol l−1 , molar ratio of Li/Mn = 0.55) into MO-a, -b, -c, -d, accordingly, then the mixtures were heated to remove water by a rotary evaporator, dried at 393 K for 12 h and calcined at 923 K for 6 h in static air; the Li+ extraction from Li/MnO2 precursor was carried out by stirring 4 g of LMO-a, -b, -c, -d, respectively, in hydrochloric acid solution (200 ml, 0.5 mol l−1 ) for 120 h until the Li+ were extracted completely (in situ measured by Metrohm 861 IC with Metrosep C2 100/4.0 column). Then the acid-treated materials were filtered, washed with deionized water and dried at 393 K for 8 h to obtain the final MnO2 ion-sieve counterpart, named SMO-a, -b, -c, -d, accordingly. The material preparation is shown in Fig. 1 and further detail can be found in reports (Zhang et al., 2004, 2006; Wang and Li, 2002). The chemical reaction is Eq. (1), composed of two half reactions (2) and (3): MnSO4 + (NH4 )2 S2 O8 + 2H2 O → MnO2 + (NH4 )2 SO4 + 2H2 SO4 , (1) Mn2+ + 2H2 O → MnO2 + 4H+ + 2e 2− S2 O2− 8 + 2e → 2SO4
(E 0 = 1.23 V),
(E 0 = 2.01 V).
(2) (3)
On the basis of E 0 values, the G0 of reaction (1) is −151 kJ mol−1 , implying a very strong tendency to progress toward the right-hand side. 2.2. Characterization Phase purity of the samples was examined by X-ray diffraction (XRD) analysis using a Rigaku D/max 2550 X-ray diffrac˚ operating at tometer with Cu K radiation ( = 1.54056 A), 40 kV, 100 mA and scanning rate of 10◦ / min. The micro/nanostructure of the products was examined by transmission electron microscopy using a JEM-1200EX TEM (JEOL, Japan) with AV = 60 kV after ultrasonic dispersion of the powders for about 10 min in ethanol absolute. 2.3. Adsorption isotherm and kinetics measurement The Li+ adsorption isotherm was carried out by stirring (130 r/min) 100.0 mg of SMO-b ion sieve in 100 ml LiCl solution (pH = 9.19, adjusted by buffer solution composed of 0.1 mol l−1 NH4 Cl and 0.1 mol l−1 NH3 · H2 O, the molar ratio
Fig. 2. (a) XRD patterns of MnO2 polymorphs. (h k l): -MnO2 crystal face; r(h k l): ramsdellite MnO2 crystal face. (b) XRD patterns of Li–Mn–O precursor. c(h k l): cubic LiMn2 O4 crystal face; m(h k l): monoclinic Li2 MnO3 crystal face. (c) XRD patterns of MnO2 ion-sieve. (h k l): cubic MnO2 crystal face; m(h k l): monoclinic-Li2 MnO3 crystal face; c(h k l): cubic LiMn2 O4 crystal face.
equal to 2.0) with different initial Li+ concentration (1.80, 3.83, 5.82, 7.63, 9.91, 11.50, 15.80, 19.60, 23.80, 26.70 mmol l−1 , respectively) for about 102 h at 293 K. After attainment of equilibrium, the Li+ concentration of supernatant solution was determined in situ by IC. The exchange capacity or the amount of lithium ion adsorbed per gram of MnO2 ion sieve was calculated according to Q = (C0 − C) · V /W .
(4)
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
The lithium ion adsorption kinetics was carried out by stirring (130 r/min) 100.0 mg of SMO-b ion sieve in 100 ml LiCl solution (pH = 9.19) with uniform initial Li+ concentration of 9.91 mmol l−1 at 293 K, and the Li+ concentration of supernatant solution was determined in situ by IC at different reaction time until the attainment of equilibrium. The amount of Li+ adsorbed per gram of MnO2 ion sieve was also calculated according to Eq. (4). 2.4. Distribution coefficient (Kd ) measurement The uptake behavior of Li+ compared with other ions in brine was carried out by stirring 100 mg of SMO-b ion sieve in 10 ml solution (pH = 10.1, adjusted by buffer solution composed of 0.1 mol l−1 NH4 Cl and 0.1 mol l−1 NH3 · H2 O, the molar ratio equal to 0.25) containing Li+ , Na+ , K + , Ca2+ and Mg2+ , about 10 mmol l−1 , respectively, for 144 h at 303 K. After attainment of equilibrium, the metal ions in supernatant solution were determined in situ by IC. Distribution coefficient Kd , sepLi and concentration factor CF were calculated aration factor aMe according to Kd = Li aMe =
(C0 − C) V × , C W Kd (Li) , Kd (Me)
(5)
Me: Li+ , Na+ , K+ , Ca2+ and Mg2+ , (6)
CF =
Qe (Me) . C0 (Me)
(7)
3. Results and discussion 3.1. Material characterization The XRD patterns of MnO2 , Li–Mn–O precursor and MnO2 ion sieve are given in Fig. 2-a, b, c. The reflections of MO-a and -b in Fig. 2-a can be indexed to orthorhombic phase [S.G.: Pnma (62)] of ramsdellite-MnO2 with lattice ˚ b = 2.866 A, ˚ c = 4.5330 A ˚ (JCPDS 39constants a = 9.27 A, 0375), and tetragonal phase [S.G.: I 4/m(87)] of -MnO2 with
4871
˚ c = 2.8630 A ˚ (JCPDS 44-0141); while those in a = 9.7847 A, Fig. 2-a, MO-c and -d, can be readily indexed to a pure tetragonal phase of -MnO2 (JCPDS 44-0141) with a faint diffraction peak at 22.36◦ , indicating trace amount of orthorhombic MnO2 . Thus pure -MnO2 can be obtained by the control of different hydrothermal conditions. Tetragonal MnO2 is constructed from double chains of [MnO6 ] octahedral unit forming 2 × 2 2− tunnels (Thackeray, 1997). It is apparent that NH+ 4 and SO4 , 2− coexisting with the corresponding ions Mn2+ and S2 O8 in both reaction systems, play an important role in determining the crystal structure and morphology of the products, and the increase of SO2− 4 will result in a decrease in the formation rate 4+ of Mn and thus a decrease in the formation rate of various crystal faces of tetragonal MnO2 . It might be believed that reactant concentration has different effects on the formation rate of different crystal faces and applied to the preparation of the various crystal MnO2 structures. However, with the reaction temperature ascended from 393 to 423 K, we have not detected transformation of crystal structure as showed in MO-a/-b and MO-c/-d. Further research is still in process although it is possible that the effect of temperature is less than that of the reactant composition during the above synthesis of MnO2 polymorphs. Almost all the reflections in Fig. 2-b can be readily indexed to cubic phase [S.G.: F d3m (227)] of LiMn2 O4 with lat˚ (JCPDS 35-0782). The several faint tice constant of 8.2476 A diffraction peaks, at 33.04◦ , 38.36◦ , 55.54◦ and 66.22◦ , indicate trace amounts of monoclinic Li2 MnO3 (JCPDS 27-1252, S.G.: ˚ b=8.533 A, ˚ c=9.604 A), ˚ C2c(15), lattice constant a=4.928 A, probably resulted from relatively low calcination temperature (923 K) as typical LiMn2 O4 crystal phase will form completely above 1073 K (JCPDS 35-0782). The reflections in Fig. 2-c can be readily indexed to pure cubic phase [S.G.: F d3m (227)] ˚ (JCPDS 44-0992). of -MnO2 with lattice constant of 8.03 A Several faint diffraction peaks at 32.96◦ , 55.32◦ , 77.92◦ and 78.92◦ indicate trace amounts of monoclinic Li2 MnO3 and cubic LiMn2 O4 resulted from small amounts of undelivered Li+ on these sites. Theoretic calculations on the chemical bonding nature and lithium sites in Li–Mn–O system (Kim et al., 2002) also indicate that octahedral sites have lower Li+ /H+ exchange ability than tetrahedral sites.
Fig. 3. TEM images of the MO-b, LMO-b and SMO-b samples.
4872
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
It is interesting that the XRD patterns in Fig. 2-b and c are quite similar. The Li–Mn–O precursor and MnO2 ion sieve assemble the same cubic phase with lattice constants of ˚ respectively. Although the Li+ inserted 8.2476 and 8.03 A, in the Mn–O lattice results in larger lattice dimension, the Li–Mn–O precursor is very stable during the Li+ extraction process and the locations of manganese in the crystal structure are well maintained. The specific characteristic arises partly from their rigid structure with little swelling or shrinking in aqueous phases and brings about a strong steric effect or an ion-sieve effect for various ions depending on the hydrated or dehydrated size of the adsorbed ions. Koyanaka et al. (2003) reported a similar phenomenon.
Typical TEM images are shown in Fig. 3. The quality of these synthesized MnO2 nanowires is significant for the determination of the ion-exchange ability and thermal stability. In related reports, the wonderful TiO2 nanotube ion sieves were obtained (Sun and Li, 2003) and the SEM observations (Chitrahar et al., 2000) indicated the existence, not so clearly, of MnO2 nanowires. In our present work, freshly prepared MnO2 nanowires with about 5 × 400 nm in diameter and length are shown clearly (image MO-b in Fig. 3). However, during later process, the nanowire structure disappears and large polygons of about 200 × 500 nm particles are the primary structure (images LMO-b and SMO-b in Fig. 3). 3.2. Adsorption isotherm and kinetics
Fig. 4. Lithium adsorption isotherm of SMO-b ion-sieve and simulation according to Langmuir equation.
Fig. 4 shows the Li+ adsorption isotherm of the SMO-b ion sieve. It is apparent that the Li+ exchange process is well accorded with the Langmuir isotherm (1/Qe =KL /Qm ·1/Ce + 1/Qm ), and the monolayer amount of Li+ adsorbed per gram of MnO2 and Langmuir constant are calculated to be Qm = 2.43 mmol g−1 , KL = 3.88 × 10−3 mol l−1 , respectively. Fig. 5 shows the lithium adsorption kinetics of SMO-b ion sieve. It shows that the adsorption rate can be well described by firstorder kinetics Lagergren equation (ln(Qe −Qt )=ln Qe −kads ·t) (Trivedi and Patel, 1973) and the value of the adsorption rate constant is calculated to be 2.16 × 10−6 s−1 , indicating a rather slow adsorption rate. 3.3. Distribution coefficient (Kd )
Fig. 5. Lithium adsorption kinetics of the SMO-b ion-sieve and simulation according to Lagergren equation.
Table 1 shows the selectivity of Li+ compared with uptake behaviors for other ions in brine including Na+ , K+ , Ca2+ and Mg2+ . The equilibrium distribution coefficients (Kd ) of these metal ions are in the order of Na+ < K+ < Mg2+ < Ca2+ Li+ , indicating high selectivity for Li+ , but much less for Na+ , K+ , Ca2+ and Mg2+ . The relatively high selectivity for Li+ can well be explained by the ion-sieve effect of the spinel lattice with a three-dimensional (1 × 3) tunnel suitable in size for fixing lithium ions in cubic phase MnO2 ion sieve obtained from LiMn2 O4 precursor. Namely, Li+ can enter the (1 × 3) tunnel during the sorption process, while other metal ions can adsorb only on the surface sites because of their too large ionic radii, similar to the cases of the manganese
Table 1 Adsorption selectivity of Li+ on SMO-b ion sievea Metal ion
C0 (mmol l−1 )
Ce (mmol l−1 )
Qe (mmol g−1 )
CF (×10−3 l g−1 )
Kd (ml g−1 )
Li aMe
Li+ Na+ K+ Mg2+ Ca2+
9.35 9.25 8.84 8.35 13.43
0.11 8.90 8.49 7.97 10.86
0.913 0.017 0.034 0.124 0.307
97.6 1.84 3.85 14.8 22.8
7917.49 3.86 3.99 4.07 23.31
1.00 2051.16 1984.33 1945.33 339.66
aT
= 303 K, pH = 10.1, V = 10 ml, W = 0.10 g.
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
oxides prepared by solid-state reaction (Shen and Clearfield, 1986; Chitrahar et al., 2001a,b). The results also indicate that Na+ , K+ , Ca2+ and Mg2+ in solution do not interfere with Li+ during the adsorption process since high concentration factor (CF) values of Li+ are observed as compared to CF values of other metal ions. 4. Conclusion Various MnO2 polymorphs, including -, - and ramsdellitetype, have been synthesized with a new controlled hydrothermal method. The different MnO2 polymorphs transform to uniform ˚ and further cubic LiMn2 O4 with lattice constant of 8.2476 A + ˚ cubic MnO2 with smaller one of 8.03 A after Li are extracted from LiMn2 O4 . The typical TEM images of freshly prepared MnO2 mainly resemble nanowires of about 5 × 400 nm in diameter and length; however, the final MnO2 ion sieves mainly show polygonal particles of about 200×500 nm. The MnO2 ion sieve has high capacity for Li+ selective adsorption with Qm = 2.43 mmol g−1 , equilibrium distribution coefficients (Kd ) in the order of Na+ < K+ < Mg2+ < Ca2+ Li+ and first-order rate constant of 2.16 × 10−6 s−1 . This novel MnO2 nanocrystal is promising in the lithium extraction from brine or seawater. Notation Li aMe C C0 Ce Ct CF E0 G0 kads Kd KL Q
Qe Qm Qt T V W
separation factor concentration of metal ions, mol l−1 initial concentration of metal ions, mol l−1 equilibrium concentration of metal ions, mol l−1 concentration of metal ions at time t, mol l−1 concentration factor, l g−1 standard electrode potential, V standard Gibbs free energy change, kJ mol−1 adsorption rate constant, s−1 distribution coefficient, ml g−1 Langmuir constant, mol l−1 amount of metal ion adsorbed per gram adsorbent, mmol g−1 amount of metal ion adsorbed per gram adsorbent at equilibrium, mmol g−1 Li+ monolayer saturation amount adsorbed per gram adsorbent, mmol g−1 amount of metal ion adsorbed per gram adsorbent at time t, mmol g−1 absolute temperature, K solution volume, ml weight of adsorbent, g
Acknowledgments This work was supported by NSFC (No. 20576031), Risingstar Project (No. 05QMX1414) and Special Nano Scientific and Technical Project (No. 0552nm021) of STCSM.
4873
References Annundsen, B., Paulsen, J., 2001. Novel lithium-ion cathode materials based on layered manganese oxides. Advanced Materials 13, 943–956. Armstrong, A.R., Bruce, P.G., 1996. Synthesis of layered LiMnO2 as electrode for rechargeable lithium batteries. Nature 381, 499–500. Bach, S., Pereira-Ramos, J.P., Baffier, N., 1995. Synthesis and characterization of lamellar MnO2 obtained from thermal decomposition of NaMnO4 for rechargeable lithium cells. Journal of Solid State Chemistry 120, 70–73. Balistrieri, L.S., Murray, J.W., 1982. The surface chemistry of delta-MnO2 in major ion seawater. Geochimica et Cosmochimica Acta 46, 1041–1052. Berg, H., Rundlov, H., Thomas, J.O., 2001. The LiMn2 O4 to -MnO2 phase transition studied by in situ neutron diffraction. Solid State Ionics 144, 65–69. Chitrahar, R., Kanoh, H., Kim, Y., Miyai, Y., Ooi, K., 2001a. Synthesis of layered-type hydrous manganese oxides from monoclinic-type LiMnO2 . Journal of Solid State Chemistry 160, 69–76. Chitrahar, R., Kanoh, H., Miyai, Y., Ooi, K., 2001b. Recovery of lithium from seawater using manganese oxide adsorbent (H1.6 Mn1.6 O4 ) derived from Li1.6 Mn1.6 O4 . Industrial and Engineering Chemistry Research 40, 2054–2058. Chitrahar, R., Kanoh, H., Miyai, Y., Ooi, K., 2000. A new type of manganese oxide (MnO2 · 0.5H2 O) derived from Li1.6 Mn1.6 O4 and its lithium ionsieve properties. Chemistry of Material 12, 3151–3157. Gray, M.J., Malati, M.A., 1979. Adsorption from aqueous solution by dmanganese dioxide. II. Adsorption of some heavy metal cations. Journal of Chemical Technology & Biotechnology 29, 135–144. Hasany, S.M., Chaudhary, M.H., 1981. Adsorption studies of strontium on manganese dioxide from aqueous solutions. International Journal of Applied Radiation and Isotopes 32, 899–904. Hunter, J.C., 1981. Preparation of a new crystal form of manganese dioxide: -MnO2 . Journal of Solid State Chemistry 39, 142–147. Kannugo, S.B., Paride, K.M., 1984. Interfacial behavior of some synthetic MnO2 samples during their adsorption of Cu2+ and Ba2+ from aqueous solution at 300 K. Journal of Colloid Interface Science 98, 252–260. Kim, Y.S., Kanoh, H., Hirotsu, T., Ooi, K., 2002. Chemical bonding of ion-exchange type sites in spinel-type manganese oxides Li1.33 Mn1.67 O4 . Materials Research Bulletin 37, 391–396. Koyanaka, H., Matsubaya, O., Hatta, N., 2003. Quantitative correlation between Li absorption and H content in manganese oxide spinel -MnO2 . Journal of Electroanalytical Chemistry 559, 77–81. Kucza, W., 2002. Structural stability of MnO2 polymorphs and their reactivity vs. lithium. Electrochemistry Communications 4, 669–673. Li, W.J., Shi, E.W., Chen, Z.Z., Yin, Z.W., 2002. Solvothermal synthesis of superfine Li1−x Mn2 O4− powders. Journal of Solid State Chemistry 163, 132–136. Ooi, K., Miyai, Y., Katoh, S., 1986. Recovery of lithium from seawater by manganese oxide adsorbent. Separation Science and Technology 21, 755–766. Ooi, K., Miyai, Y., Katoh, S., 1987. Ion-exchange properties of ion-sievetype manganese oxides prepared by using different kinds of introducing ions. Separation Science and Technology 22, 1779–1789. Shen, X.M., Clearfield, A., 1986. Phase transitions and ion exchange behavior of electrolytically prepared manganese dioxide. Journal of Solid State Chemistry 64, 270–282. Sun, X.M., Li, Y.D., 2003. Synthesis and characterization of ion-exchangeable titanate nanotubes. Chemistry—A European Journal 9, 2229–2238. Thackeray, M.M., 1997. Manganese oxides for lithium batteries. Progress in Solid State Chemistry 25, 1–71. Trivedi, H.C., Patel, V.M., 1973. Adsorption of cellulose triacetate on calcium silicate. European Polymer Journal 9, 525–531 (It cites Lagergren (1972) Bil. K. Svenska Ventenskapasakad Handl. 24 (1898)). Tsuji, M., Abe, M., 1984. Synthetic inorganic ion-exchange materials. Synthesis of cryptomelane-type hydrous manganese dioxide as an ionexchange material and their ion-exchange selectivities towards alkali
4874
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
and alkaline earth metal ions. Solvent Extraction and Ion Exchange 2, 253–274. Wang, X., Li, Y.D., 2002. Selected-control hydrothermal synthesis of - and -MnO2 single crystal nanowires. Journal of American Chemistry Society 124, 2880–2881.
Zhang, Q.H., Li, Y., Xu, B.Q., 2004. Reforming of methane and coalbed methane over nanocomposite Ni/ZrO2 catalyst. Catalysis Today 98, 601–605. Zhang, Q.H., Sun, S.Y., Yu, J.G., 2006. Controlled synthesis of manganese dioxide ions sieves. 4th PBAST, Lecture, May 22–26, Tianjin, China.
Adsorption of lithium ions on novel nanocrystal MnO2 Qin-Hui Zhang, Shuying Sun, Shaopeng Li, Hao Jiang, Jian-Guo Yu ∗ State-Key Lab of Chemical Engineering, College of Chemical Engineering, East China University of Science and Technology, 200237 Shanghai, PR China Received 30 April 2006; received in revised form 8 January 2007; accepted 10 January 2007 Available online 25 January 2007
Abstract Various polymorphs of MnO2 were synthesized under a controlled hydrothermal method. The structure characteristics and ion exchangeability were studied by XRD, TEM, Li+ adsorption isotherm, kinetics and selectivity measurement. The MnO2 nanowires, mainly about 5 × 400 nm in diameter and length, are found to have a remarkable lithium ion-sieve property with equilibrium distribution coefficients (Kd ) in the order of Na+ < K+ < Mg2+ < Ca2+ Li+ , Li+ monolayer saturation amount of 2.43 mmol g−1 , and the first-order adsorption rate constant of 2.16 × 10−6 s−1 , which is promising in the lithium extraction from brine or seawater. 䉷 2007 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Ion exchange; Lithium; MnO2 ; Nanostructure; Separations
1. Instruction Various polymorphic forms of MnO2 , including -, -, -, - and -type, are widely used as catalysts, electrode materials in Li/MnO2 batteries (Armstrong and Bruce, 1996; Annundsen and Paulsen, 2001; Thackeray, 1997) and ion sieves applied in the lithium extraction from brine or seawater for analysis (Tsuji and Abe, 1984; Kannugo and Paride, 1984), geochemistry (Gray and Malati, 1979; Balistrieri and Murray, 1982) and water-pollution control (Hasany and Chaudhary, 1981). MnO2 ion sieve was first synthesized from manganese hydroxide saturated with Li+ (Ooi et al., 1986). Great efforts have been made to prepare bulky or nanocrystalline MnO2 with different structure (Hunter, 1981; Bach et al., 1995; Chitrahar et al., 2001a,b) followed by the extraction of Li+ with acid solution (Berg et al., 2001; Li et al., 2002; Kucza, 2002; Koyanaka et al., 2003; Ooi et al., 1987). However, there have been no systematic studies on the effect of different polymorphs of MnO2 precursor and the size effect of related crystal nanostructures on the Li+ extraction process. In this paper, a controlled hydrothermal method has been developed in the
∗ Corresponding author. Tel./fax:+86 21 64252826.
E-mail address: [email protected] (J.G. Yu). 0009-2509/$ - see front matter 䉷 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2007.01.016
synthesis of MnO2 ion sieve by the acid treatment of Li–Mn–O precursor. The structure characteristics and ion-exchange properties are studied by XRD, TEM, Li+ adsorption isotherm, kinetics and selectivity measurement. 2. Experimental 2.1. Ion-sieve preparation To prepare the Li/MnO2 ion-sieve precursor, different polymorphic forms of MnO2 were first obtained by controlled low-temperature hydrothermal synthesis via the oxidation of Mn2+ by S2 O2− 8 . Analytical grade MnSO4 · H2 O (0.25 mol) and a stoichiometric ratio of (NH4 )2 S2 O8 (0.25 mol) were put into 750 ml deionized water to form a homogeneous solution at room temperature, which was then transferred into a stainless steel autoclave, sealed, maintained for 12 h at 393 K (named MO-a) and 423 K (named MO-b), respectively. The resulted black solid products were filtered, washed completely with deionized water to remove ions possibly remaining in the final products, and finally dried at 393 K for 8 h in static air. The whole process was adjusted to prepare MO-c and -d by adding 0.47 mol of analytical grade (NH4 )2 SO4 to the reaction system, respectively; Li/MnO2 precursor counterpart, named LMO-a, -b, -c, -d, were prepared by wet impregnation of an
4870
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
Fig. 1. Diagram of MnO2 ion-sieve preparation process.
aqueous solution of LiOH · H2 O (0.35 mol l−1 , molar ratio of Li/Mn = 0.55) into MO-a, -b, -c, -d, accordingly, then the mixtures were heated to remove water by a rotary evaporator, dried at 393 K for 12 h and calcined at 923 K for 6 h in static air; the Li+ extraction from Li/MnO2 precursor was carried out by stirring 4 g of LMO-a, -b, -c, -d, respectively, in hydrochloric acid solution (200 ml, 0.5 mol l−1 ) for 120 h until the Li+ were extracted completely (in situ measured by Metrohm 861 IC with Metrosep C2 100/4.0 column). Then the acid-treated materials were filtered, washed with deionized water and dried at 393 K for 8 h to obtain the final MnO2 ion-sieve counterpart, named SMO-a, -b, -c, -d, accordingly. The material preparation is shown in Fig. 1 and further detail can be found in reports (Zhang et al., 2004, 2006; Wang and Li, 2002). The chemical reaction is Eq. (1), composed of two half reactions (2) and (3): MnSO4 + (NH4 )2 S2 O8 + 2H2 O → MnO2 + (NH4 )2 SO4 + 2H2 SO4 , (1) Mn2+ + 2H2 O → MnO2 + 4H+ + 2e 2− S2 O2− 8 + 2e → 2SO4
(E 0 = 1.23 V),
(E 0 = 2.01 V).
(2) (3)
On the basis of E 0 values, the G0 of reaction (1) is −151 kJ mol−1 , implying a very strong tendency to progress toward the right-hand side. 2.2. Characterization Phase purity of the samples was examined by X-ray diffraction (XRD) analysis using a Rigaku D/max 2550 X-ray diffrac˚ operating at tometer with Cu K radiation ( = 1.54056 A), 40 kV, 100 mA and scanning rate of 10◦ / min. The micro/nanostructure of the products was examined by transmission electron microscopy using a JEM-1200EX TEM (JEOL, Japan) with AV = 60 kV after ultrasonic dispersion of the powders for about 10 min in ethanol absolute. 2.3. Adsorption isotherm and kinetics measurement The Li+ adsorption isotherm was carried out by stirring (130 r/min) 100.0 mg of SMO-b ion sieve in 100 ml LiCl solution (pH = 9.19, adjusted by buffer solution composed of 0.1 mol l−1 NH4 Cl and 0.1 mol l−1 NH3 · H2 O, the molar ratio
Fig. 2. (a) XRD patterns of MnO2 polymorphs. (h k l): -MnO2 crystal face; r(h k l): ramsdellite MnO2 crystal face. (b) XRD patterns of Li–Mn–O precursor. c(h k l): cubic LiMn2 O4 crystal face; m(h k l): monoclinic Li2 MnO3 crystal face. (c) XRD patterns of MnO2 ion-sieve. (h k l): cubic MnO2 crystal face; m(h k l): monoclinic-Li2 MnO3 crystal face; c(h k l): cubic LiMn2 O4 crystal face.
equal to 2.0) with different initial Li+ concentration (1.80, 3.83, 5.82, 7.63, 9.91, 11.50, 15.80, 19.60, 23.80, 26.70 mmol l−1 , respectively) for about 102 h at 293 K. After attainment of equilibrium, the Li+ concentration of supernatant solution was determined in situ by IC. The exchange capacity or the amount of lithium ion adsorbed per gram of MnO2 ion sieve was calculated according to Q = (C0 − C) · V /W .
(4)
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
The lithium ion adsorption kinetics was carried out by stirring (130 r/min) 100.0 mg of SMO-b ion sieve in 100 ml LiCl solution (pH = 9.19) with uniform initial Li+ concentration of 9.91 mmol l−1 at 293 K, and the Li+ concentration of supernatant solution was determined in situ by IC at different reaction time until the attainment of equilibrium. The amount of Li+ adsorbed per gram of MnO2 ion sieve was also calculated according to Eq. (4). 2.4. Distribution coefficient (Kd ) measurement The uptake behavior of Li+ compared with other ions in brine was carried out by stirring 100 mg of SMO-b ion sieve in 10 ml solution (pH = 10.1, adjusted by buffer solution composed of 0.1 mol l−1 NH4 Cl and 0.1 mol l−1 NH3 · H2 O, the molar ratio equal to 0.25) containing Li+ , Na+ , K + , Ca2+ and Mg2+ , about 10 mmol l−1 , respectively, for 144 h at 303 K. After attainment of equilibrium, the metal ions in supernatant solution were determined in situ by IC. Distribution coefficient Kd , sepLi and concentration factor CF were calculated aration factor aMe according to Kd = Li aMe =
(C0 − C) V × , C W Kd (Li) , Kd (Me)
(5)
Me: Li+ , Na+ , K+ , Ca2+ and Mg2+ , (6)
CF =
Qe (Me) . C0 (Me)
(7)
3. Results and discussion 3.1. Material characterization The XRD patterns of MnO2 , Li–Mn–O precursor and MnO2 ion sieve are given in Fig. 2-a, b, c. The reflections of MO-a and -b in Fig. 2-a can be indexed to orthorhombic phase [S.G.: Pnma (62)] of ramsdellite-MnO2 with lattice ˚ b = 2.866 A, ˚ c = 4.5330 A ˚ (JCPDS 39constants a = 9.27 A, 0375), and tetragonal phase [S.G.: I 4/m(87)] of -MnO2 with
4871
˚ c = 2.8630 A ˚ (JCPDS 44-0141); while those in a = 9.7847 A, Fig. 2-a, MO-c and -d, can be readily indexed to a pure tetragonal phase of -MnO2 (JCPDS 44-0141) with a faint diffraction peak at 22.36◦ , indicating trace amount of orthorhombic MnO2 . Thus pure -MnO2 can be obtained by the control of different hydrothermal conditions. Tetragonal MnO2 is constructed from double chains of [MnO6 ] octahedral unit forming 2 × 2 2− tunnels (Thackeray, 1997). It is apparent that NH+ 4 and SO4 , 2− coexisting with the corresponding ions Mn2+ and S2 O8 in both reaction systems, play an important role in determining the crystal structure and morphology of the products, and the increase of SO2− 4 will result in a decrease in the formation rate 4+ of Mn and thus a decrease in the formation rate of various crystal faces of tetragonal MnO2 . It might be believed that reactant concentration has different effects on the formation rate of different crystal faces and applied to the preparation of the various crystal MnO2 structures. However, with the reaction temperature ascended from 393 to 423 K, we have not detected transformation of crystal structure as showed in MO-a/-b and MO-c/-d. Further research is still in process although it is possible that the effect of temperature is less than that of the reactant composition during the above synthesis of MnO2 polymorphs. Almost all the reflections in Fig. 2-b can be readily indexed to cubic phase [S.G.: F d3m (227)] of LiMn2 O4 with lat˚ (JCPDS 35-0782). The several faint tice constant of 8.2476 A diffraction peaks, at 33.04◦ , 38.36◦ , 55.54◦ and 66.22◦ , indicate trace amounts of monoclinic Li2 MnO3 (JCPDS 27-1252, S.G.: ˚ b=8.533 A, ˚ c=9.604 A), ˚ C2c(15), lattice constant a=4.928 A, probably resulted from relatively low calcination temperature (923 K) as typical LiMn2 O4 crystal phase will form completely above 1073 K (JCPDS 35-0782). The reflections in Fig. 2-c can be readily indexed to pure cubic phase [S.G.: F d3m (227)] ˚ (JCPDS 44-0992). of -MnO2 with lattice constant of 8.03 A Several faint diffraction peaks at 32.96◦ , 55.32◦ , 77.92◦ and 78.92◦ indicate trace amounts of monoclinic Li2 MnO3 and cubic LiMn2 O4 resulted from small amounts of undelivered Li+ on these sites. Theoretic calculations on the chemical bonding nature and lithium sites in Li–Mn–O system (Kim et al., 2002) also indicate that octahedral sites have lower Li+ /H+ exchange ability than tetrahedral sites.
Fig. 3. TEM images of the MO-b, LMO-b and SMO-b samples.
4872
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
It is interesting that the XRD patterns in Fig. 2-b and c are quite similar. The Li–Mn–O precursor and MnO2 ion sieve assemble the same cubic phase with lattice constants of ˚ respectively. Although the Li+ inserted 8.2476 and 8.03 A, in the Mn–O lattice results in larger lattice dimension, the Li–Mn–O precursor is very stable during the Li+ extraction process and the locations of manganese in the crystal structure are well maintained. The specific characteristic arises partly from their rigid structure with little swelling or shrinking in aqueous phases and brings about a strong steric effect or an ion-sieve effect for various ions depending on the hydrated or dehydrated size of the adsorbed ions. Koyanaka et al. (2003) reported a similar phenomenon.
Typical TEM images are shown in Fig. 3. The quality of these synthesized MnO2 nanowires is significant for the determination of the ion-exchange ability and thermal stability. In related reports, the wonderful TiO2 nanotube ion sieves were obtained (Sun and Li, 2003) and the SEM observations (Chitrahar et al., 2000) indicated the existence, not so clearly, of MnO2 nanowires. In our present work, freshly prepared MnO2 nanowires with about 5 × 400 nm in diameter and length are shown clearly (image MO-b in Fig. 3). However, during later process, the nanowire structure disappears and large polygons of about 200 × 500 nm particles are the primary structure (images LMO-b and SMO-b in Fig. 3). 3.2. Adsorption isotherm and kinetics
Fig. 4. Lithium adsorption isotherm of SMO-b ion-sieve and simulation according to Langmuir equation.
Fig. 4 shows the Li+ adsorption isotherm of the SMO-b ion sieve. It is apparent that the Li+ exchange process is well accorded with the Langmuir isotherm (1/Qe =KL /Qm ·1/Ce + 1/Qm ), and the monolayer amount of Li+ adsorbed per gram of MnO2 and Langmuir constant are calculated to be Qm = 2.43 mmol g−1 , KL = 3.88 × 10−3 mol l−1 , respectively. Fig. 5 shows the lithium adsorption kinetics of SMO-b ion sieve. It shows that the adsorption rate can be well described by firstorder kinetics Lagergren equation (ln(Qe −Qt )=ln Qe −kads ·t) (Trivedi and Patel, 1973) and the value of the adsorption rate constant is calculated to be 2.16 × 10−6 s−1 , indicating a rather slow adsorption rate. 3.3. Distribution coefficient (Kd )
Fig. 5. Lithium adsorption kinetics of the SMO-b ion-sieve and simulation according to Lagergren equation.
Table 1 shows the selectivity of Li+ compared with uptake behaviors for other ions in brine including Na+ , K+ , Ca2+ and Mg2+ . The equilibrium distribution coefficients (Kd ) of these metal ions are in the order of Na+ < K+ < Mg2+ < Ca2+ Li+ , indicating high selectivity for Li+ , but much less for Na+ , K+ , Ca2+ and Mg2+ . The relatively high selectivity for Li+ can well be explained by the ion-sieve effect of the spinel lattice with a three-dimensional (1 × 3) tunnel suitable in size for fixing lithium ions in cubic phase MnO2 ion sieve obtained from LiMn2 O4 precursor. Namely, Li+ can enter the (1 × 3) tunnel during the sorption process, while other metal ions can adsorb only on the surface sites because of their too large ionic radii, similar to the cases of the manganese
Table 1 Adsorption selectivity of Li+ on SMO-b ion sievea Metal ion
C0 (mmol l−1 )
Ce (mmol l−1 )
Qe (mmol g−1 )
CF (×10−3 l g−1 )
Kd (ml g−1 )
Li aMe
Li+ Na+ K+ Mg2+ Ca2+
9.35 9.25 8.84 8.35 13.43
0.11 8.90 8.49 7.97 10.86
0.913 0.017 0.034 0.124 0.307
97.6 1.84 3.85 14.8 22.8
7917.49 3.86 3.99 4.07 23.31
1.00 2051.16 1984.33 1945.33 339.66
aT
= 303 K, pH = 10.1, V = 10 ml, W = 0.10 g.
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
oxides prepared by solid-state reaction (Shen and Clearfield, 1986; Chitrahar et al., 2001a,b). The results also indicate that Na+ , K+ , Ca2+ and Mg2+ in solution do not interfere with Li+ during the adsorption process since high concentration factor (CF) values of Li+ are observed as compared to CF values of other metal ions. 4. Conclusion Various MnO2 polymorphs, including -, - and ramsdellitetype, have been synthesized with a new controlled hydrothermal method. The different MnO2 polymorphs transform to uniform ˚ and further cubic LiMn2 O4 with lattice constant of 8.2476 A + ˚ cubic MnO2 with smaller one of 8.03 A after Li are extracted from LiMn2 O4 . The typical TEM images of freshly prepared MnO2 mainly resemble nanowires of about 5 × 400 nm in diameter and length; however, the final MnO2 ion sieves mainly show polygonal particles of about 200×500 nm. The MnO2 ion sieve has high capacity for Li+ selective adsorption with Qm = 2.43 mmol g−1 , equilibrium distribution coefficients (Kd ) in the order of Na+ < K+ < Mg2+ < Ca2+ Li+ and first-order rate constant of 2.16 × 10−6 s−1 . This novel MnO2 nanocrystal is promising in the lithium extraction from brine or seawater. Notation Li aMe C C0 Ce Ct CF E0 G0 kads Kd KL Q
Qe Qm Qt T V W
separation factor concentration of metal ions, mol l−1 initial concentration of metal ions, mol l−1 equilibrium concentration of metal ions, mol l−1 concentration of metal ions at time t, mol l−1 concentration factor, l g−1 standard electrode potential, V standard Gibbs free energy change, kJ mol−1 adsorption rate constant, s−1 distribution coefficient, ml g−1 Langmuir constant, mol l−1 amount of metal ion adsorbed per gram adsorbent, mmol g−1 amount of metal ion adsorbed per gram adsorbent at equilibrium, mmol g−1 Li+ monolayer saturation amount adsorbed per gram adsorbent, mmol g−1 amount of metal ion adsorbed per gram adsorbent at time t, mmol g−1 absolute temperature, K solution volume, ml weight of adsorbent, g
Acknowledgments This work was supported by NSFC (No. 20576031), Risingstar Project (No. 05QMX1414) and Special Nano Scientific and Technical Project (No. 0552nm021) of STCSM.
4873
References Annundsen, B., Paulsen, J., 2001. Novel lithium-ion cathode materials based on layered manganese oxides. Advanced Materials 13, 943–956. Armstrong, A.R., Bruce, P.G., 1996. Synthesis of layered LiMnO2 as electrode for rechargeable lithium batteries. Nature 381, 499–500. Bach, S., Pereira-Ramos, J.P., Baffier, N., 1995. Synthesis and characterization of lamellar MnO2 obtained from thermal decomposition of NaMnO4 for rechargeable lithium cells. Journal of Solid State Chemistry 120, 70–73. Balistrieri, L.S., Murray, J.W., 1982. The surface chemistry of delta-MnO2 in major ion seawater. Geochimica et Cosmochimica Acta 46, 1041–1052. Berg, H., Rundlov, H., Thomas, J.O., 2001. The LiMn2 O4 to -MnO2 phase transition studied by in situ neutron diffraction. Solid State Ionics 144, 65–69. Chitrahar, R., Kanoh, H., Kim, Y., Miyai, Y., Ooi, K., 2001a. Synthesis of layered-type hydrous manganese oxides from monoclinic-type LiMnO2 . Journal of Solid State Chemistry 160, 69–76. Chitrahar, R., Kanoh, H., Miyai, Y., Ooi, K., 2001b. Recovery of lithium from seawater using manganese oxide adsorbent (H1.6 Mn1.6 O4 ) derived from Li1.6 Mn1.6 O4 . Industrial and Engineering Chemistry Research 40, 2054–2058. Chitrahar, R., Kanoh, H., Miyai, Y., Ooi, K., 2000. A new type of manganese oxide (MnO2 · 0.5H2 O) derived from Li1.6 Mn1.6 O4 and its lithium ionsieve properties. Chemistry of Material 12, 3151–3157. Gray, M.J., Malati, M.A., 1979. Adsorption from aqueous solution by dmanganese dioxide. II. Adsorption of some heavy metal cations. Journal of Chemical Technology & Biotechnology 29, 135–144. Hasany, S.M., Chaudhary, M.H., 1981. Adsorption studies of strontium on manganese dioxide from aqueous solutions. International Journal of Applied Radiation and Isotopes 32, 899–904. Hunter, J.C., 1981. Preparation of a new crystal form of manganese dioxide: -MnO2 . Journal of Solid State Chemistry 39, 142–147. Kannugo, S.B., Paride, K.M., 1984. Interfacial behavior of some synthetic MnO2 samples during their adsorption of Cu2+ and Ba2+ from aqueous solution at 300 K. Journal of Colloid Interface Science 98, 252–260. Kim, Y.S., Kanoh, H., Hirotsu, T., Ooi, K., 2002. Chemical bonding of ion-exchange type sites in spinel-type manganese oxides Li1.33 Mn1.67 O4 . Materials Research Bulletin 37, 391–396. Koyanaka, H., Matsubaya, O., Hatta, N., 2003. Quantitative correlation between Li absorption and H content in manganese oxide spinel -MnO2 . Journal of Electroanalytical Chemistry 559, 77–81. Kucza, W., 2002. Structural stability of MnO2 polymorphs and their reactivity vs. lithium. Electrochemistry Communications 4, 669–673. Li, W.J., Shi, E.W., Chen, Z.Z., Yin, Z.W., 2002. Solvothermal synthesis of superfine Li1−x Mn2 O4− powders. Journal of Solid State Chemistry 163, 132–136. Ooi, K., Miyai, Y., Katoh, S., 1986. Recovery of lithium from seawater by manganese oxide adsorbent. Separation Science and Technology 21, 755–766. Ooi, K., Miyai, Y., Katoh, S., 1987. Ion-exchange properties of ion-sievetype manganese oxides prepared by using different kinds of introducing ions. Separation Science and Technology 22, 1779–1789. Shen, X.M., Clearfield, A., 1986. Phase transitions and ion exchange behavior of electrolytically prepared manganese dioxide. Journal of Solid State Chemistry 64, 270–282. Sun, X.M., Li, Y.D., 2003. Synthesis and characterization of ion-exchangeable titanate nanotubes. Chemistry—A European Journal 9, 2229–2238. Thackeray, M.M., 1997. Manganese oxides for lithium batteries. Progress in Solid State Chemistry 25, 1–71. Trivedi, H.C., Patel, V.M., 1973. Adsorption of cellulose triacetate on calcium silicate. European Polymer Journal 9, 525–531 (It cites Lagergren (1972) Bil. K. Svenska Ventenskapasakad Handl. 24 (1898)). Tsuji, M., Abe, M., 1984. Synthetic inorganic ion-exchange materials. Synthesis of cryptomelane-type hydrous manganese dioxide as an ionexchange material and their ion-exchange selectivities towards alkali
4874
Q. Zhang et al. / Chemical Engineering Science 62 (2007) 4869 – 4874
and alkaline earth metal ions. Solvent Extraction and Ion Exchange 2, 253–274. Wang, X., Li, Y.D., 2002. Selected-control hydrothermal synthesis of - and -MnO2 single crystal nanowires. Journal of American Chemistry Society 124, 2880–2881.
Zhang, Q.H., Li, Y., Xu, B.Q., 2004. Reforming of methane and coalbed methane over nanocomposite Ni/ZrO2 catalyst. Catalysis Today 98, 601–605. Zhang, Q.H., Sun, S.Y., Yu, J.G., 2006. Controlled synthesis of manganese dioxide ions sieves. 4th PBAST, Lecture, May 22–26, Tianjin, China.