Recovery of Lithium from Seawater with Lithium Titanium Oxide Ion Sieves and Photocatalytic Generation of H2

• Main Authors: Jia-WeiChen, 陳家偉
• Other Authors: Hong-Paul Wang
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/mjt759

碩士 === 國立成功大學 === 環境工程學系 === 103 === Lithium compounds have many applications (i.e., rechargeable batteries, nuclear fusion, and alloy of aircraft), leading to a highly increase of demand, however, the source of lithium is limited in the earth crusts. In spite of the low Li+ concentration in the seawater (0.17 mg/L), the potential resource can be as high as 2.5×〖10〗^14 kg. Therefore, developing an effective method for recovery of lithium from seawater is of increasing importance. Protonated titanate ion-sieves (H2TiO3) was used to capture Li+ from seawater due to a similar ion density between lithium ions (Li+) and protons (H+). The hydrothermal (HT) and solid-state reaction (SS) methods were used to prepare Li2TiO3 and LiTi2O4. By UV/Vis spectroscopy, the lithium titanates have the aborption zones at 200-410 nm, and a red-shift for protonated titanates was observed. The main objective of this study was, therefore, to investigate the feasibility for the recovery of lithium from seawater using the protonated lithium titanate ion-sieves (H2TiO3 and H0.23Li0.77Ti2O4). In addition, it is also of interest to study whether the lithium titanates have a capability for photocatalytic splitting of H2O in seawater for H2 fuels as well as photocatalytic degradation of organic pollutants during the lithium recovery. It is worth noting that the particle sizes of the LiTi2O4 (SS) and H0.23Li0.77Ti2O4 (SS) ion-sieves are in the range of 3-5 μm while the Li2TiO3 (SS) and H2TiO3 (SS) are much smaller (0.2-0.4 μm) which were determined by SEM. The Li2TiO3 (HT) and H2TiO3 (HT) ion-sieves have the average particle sizes of 0.1-0.3 μm. Since the surface exchange may play the primary role in the capture of Li+ from seawater, the larger ion-sieves turn out to possess a relatively low capability. Note that chemical structure of the lithium titanates was not perturbed during the consecutive exchanges of H+ and Li+, observed by XRD, Li7-nmr, and EXAFS. The Li2TiO3/H2TiO3 ion-sieve has negligible titanium dissolution during exchanges. After 3 cycles, the Li2TiO3/H2TiO3 (SS and HT) ion-sieve can sustain 82% of its best performance, while the LiTi2O4/H0.23Li0.77Ti2O4 (SS) one possesses 50-75%. After a 24-h UV-Vis light irradiation, the unconverted MB (1-XMB) for the Li2TiO3 is 8% (or 92% conversion) approximately. The Li2TiO3 prepared by the hydrothermal method have better photcatalysis performances with the unconverted MB in the range of 4-21%, while the lithium titianate prepared by the SS method has the performance in the range of 7-51%. However, in seawater, relatively low conversions are found, which may be associated the fact that the mean diameter and polydispersity index of the lithium/protonated titanates in water are greater than those in seawater. The lithium titanate ion-sieve catalysts that can also performance photocatalytic splitting of H2O to H2 during the lithium recovery processes has been preliminarily studied, suggesting that it is chemically feasible. It is worth to note that the regenerated Li2TiO3 prepared by the hydrothermal method has better yields of H2. Thus, in the cycles of the Li+ capture and regeneration, the lithium titanate ion-sieves can sustain desirable yields of H2 from photocatalytic splitting of H2O. Proceeding in this fashion, energy required for the lithium recovery processes may be self-supported. In addition, the concept developed in this work may be applied in the photocatalytic degradation of organic pollutants in waste or contaminated underground water. Selected metal ions therein may be captured by ion exchanges with the size-comparable ion-sieves.