Article pubs.acs.org/IECR
Synthesis and Adsorption Properties of Li1.6Mn1.6O4 Spinel Jia-Li Xiao, Shu-Ying Sun,* Jin Wang, Ping Li,* and Jian-Guo Yu National Engineering Research Center for Integrated Utilization of Salt Lake Resource, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Orthorhombic-LiMnO2, spinel-type Li1.6Mn1.6O4, and spinel MnO2·0.5H2O were synthesized via a combination of hydrothermal synthesis and solid-phase reaction. o-LiMnO2 with different stacking fault concentrations were synthesized by controlled hydrothermal reaction using a KMnO4, MnCl2, and LiOH mixed solution. Spinel-type Li1.6Mn1.6O4 precursor was prepared via heat treatment of o-LiMnO2. The effects of hydrothermal and solid-phase reaction on the structure and ionexchange properties were examined with powder X-ray diffraction, scanning electron microscopy, and Li+ selective adsorption measurements. The results showed that stacking fault concentration of o-LiMnO2 affects the adsorption properties of Li1.6Mn1.6O4. Li+ adsorption capacity reached 6.06 mmol·g−1 at equilibrium. Distribution coefficients of metal ions were in the order Li+ ≫ Na+ > Mg2+ > K+ > Ca2+ and were significant for Li+ extraction from low grade brine. The equilibrium uptake of Li+ from salt lake brine remained at 3.62 mmol·g−1 after six cycles.
1. INTRODUCTION Lithium manganese oxides have been widely used as cathode materials, adsorbents, and catalysts.1−4 The primary lithium resources are mainly found in salt lakes.5 However, it is difficult to separate Li+ ions from the brine of most of the Chinese lakes due to their low concentration as compared to that for Na+ and Mg2+ ions. A highly selective lithium ion-sieve (adsorbent) would be advantageous for Li+ ion recovery from low-grade salt lake brine/seawater, offering both cost-effective and environmentally friendly technology. The Li+ ion desorption/adsorption properties of LiMn2O4, Li1.33Mn1.67O4, and Li1.6Mn1.6O4 in aqueous phase have been reported.6−10 Li1.6Mn1.6O4 is an attractive adsorbent for Li+ ions because of its large ion-exchange capacity and high chemical stability. However, it can be only obtained by the calcination of orthorhombic LiMnO2 (o-LiMnO2) at an appropriate temperature.11 Many types of raw materials have been used to synthesize oLiMnO2 in different processes as follows:12−14 (1) Ramesh15 synthesized o-LiMnO2 by the hydrothermal reaction of LiOH and γ-MnOOH. Li1.6Mn1.6O4 was then synthesized by the calcination of LiMnO2. The maximum Li+ ion adsorption capacity (from Li+-enriched seawater) of the thus prepared lithium ion-sieve reaches 5.3 mmol·g−1 (36.8 mg·g−1). (2) Shi16 used LiOH and Mn2O3 as the starting materials to synthesize LiMnO2. Li1.6Mn1.6O4 was then synthesized by the calcination of LiMnO2. The maximum Li+ ion adsorption capacity (from brine) of the thus prepared lithium ion-sieve at 323 K reaches 3.9 mmol·g−1 (27.1 mg·g−1). The complex synthetic processes of γ-MnOOH (in the first process) or Mn2O3 (in the second process), and the unstable nature of γ-MnOOH limits the commercial use of both these process. Furthermore, in the second process, the effect of stacking fault concentration of LiMnO2 on the Li+ ion adsorption/desorption properties of Li1.6Mn1.6O4 has not been studied. In this study, a simple and effective method was developed to synthesize Li1.6Mn1.6O4. First, o-LiMnO2 with different stacking fault concentrations were synthesized by a controlled hydro© 2013 American Chemical Society
thermal reaction from commercially available KMnO4, MnCl2, and LiOH. Next, the thus prepared o-LiMnO2 materials were calcined to obtain Li1.6Mn1.6O4. Systematic studies on the effect of o-LiMnO 2 stacking fault concentration on the Li + adsorption/desorption properties of Li1.6Mn1.6O4 were carried out. The structural characteristics and ion-exchange properties of o-LiMnO2, and Li1.6Mn1.6O4 ion-sieve were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Li+ selective adsorption measurements.
2. EXPERIMENTAL SECTION 2.1. Preparation of Lithium Ion-Sieve. To synthesize pure-phase Li1.6Mn1.6O4 with high Li+ ion adsorption capacity, o-LiMnO2 products with different stacking fault concentrations were first synthesized by controlled low-temperature hydrothermal reactions, as shown in eq 1. MnO4 − + 4Mn 2 + + 12OH− + 5Li+ → 5LiMnO2 + 6H 2O
(1)
MnO4 − + 4Mn 2 + + 12OH− → 5MnOOH + H 2O
(2)
MnOOH + Li+ + OH− → LiMnO2 + H 2O
(3)
The aqueous solutions of KMnO4 and LiOH were successively pumped into the MnCl2 solution at a stirring speed of about 1000 rpm resulting in a 700 mL brown mixture. The mole ratio of MnCl2/KMnO4 was 4, according to eq 1. When the mole ratio of Li/Mn was
