Bart Van der Bruggen, Yan Zhao
Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, Leuven, Belgium
Currently, the main resources of lithium compounds are from limited rock reserves. The production of lithium from these rocks could cause environmental problems. On the other hand, the abundance of saline lakes as lithium resources (about 62% of the total worldwide lithium resources) has attracted people’s attention to reinforce the exploitation of lithium. The application of membrane technology to separate and extract lithium has recently seen considerable growth as a research topic, driven by the growing demand for energy and the status of this technology as environmentally-friendly. In particular, ion exchange membranes (IEMs), which are the core elements of electrodialysis (ED), have unique advantages compared to membranes applied in pressure driven separations, in their ability to separate or extract ions in highly concentrated brines and avoid the scaling.
IEMs with selective separation properties for monovalent ions have been explored. In classical theories, the electrostatic repulsion effect between solution ions and the charged membrane surface and the size sieving effect between the hydrated ionic diameter and the membrane structure are the most important rules for preparation of the AEMs/CEMs with selective monovalent ion separation properties. For example, we controlled the tunable interlayer spacing and the functionalization of graphene oxide membranes by polyelectrolytes or by monomers allows for an efficient and selective separation of larger hydrated cations in ED.
However, based on the electrostatic repulsion effect, the coexistence of a large number of monovalent monovalent cations, such as Li+ and K+, with chemically similar properties is difficult to be selectively separated from each other. Furthermore, the size sieving effect may cause an inevitable low desalination efficiency, which leads to a high energy consumption. According to the classical mechanics of ion motion, the focus is usually only on the electric field effect in which ions move across IEMs in a perfect manner.
The quantum state of lithium in electric field can be used to describe the quantum mechanics of lithium in three dimensional states. Under the electric field, the lithium ion has its specific coherent superposition of waves and the wave function, so that the lithium is able to control the architecture of the multilayer. This electric field-based ionic control of selective separation layers shows great promise when applied to the fabrication of various other functional ion exchange membranes for extracting clean ion resources. Based on this mechanism, the potential for lithium purification from aqueous sources is presented.