Tuning Ion Transport in Phyllosilicate Membranes for Selective Lithium Extraction from Water

Water resources including seawater, salt lake brines and waste streams represent the largest reservoirs of critical materials yet remain largely underutilized. Compared with conventional evaporation-based recovery processes, membrane-based extraction offers a more sustainable alternative due to low energy consumption, continuous operation, and high separation efficiency. However, traditional polymeric membranes suffer from fouling, limited long term stability, and an inherent permeability-selectivity tradeoff. Two-dimensional (2D) materials offer a promising platform to overcome these limitations, as their laminar structures enable tunable transport channels with molecular level precision. Whereas their practical implementation is hindered by high material and processing costs as well as limited structural integrity. Phyllosilicates are abundant and low cost natural 2D materials that allow for cost-effective and scalable membrane solutions. However, their hydrophilic nature and limited control over surface chemistry result in poor water stability and insufficient selectivity. Moreover, although phyllosilicates provide the most accessible laminar architectures among 2D materials, their potential as membrane platforms remains underexplored, and systematic studies linking interlayer structure, surface chemistry, and ion transport properties are still lacking. In this thesis, we develop laminar phyllosilicate membranes for resource recovery applications, with a focus on lithium extraction, and establish molecular level strategies to simultaneously enhance stability and separation performance. Beyond tuning ion transport through membrane structure and chemistry, we identify transport modulated by coexisting ions as a new mechanistic handle for achieving exceptional separation performance. Chapter 1 introduces the diverse family of phyllosilicate materials and provides the research background of clay science. In the context of lithium extraction from water, this chapter discusses the key challenges facing phyllosilicate membranes and outlines the proposed strategies that form the basis of this thesis. In Chapter 2, a crosslinking strategy is developed to overcome the water instability of phyllosilicate membranes by strengthening interlamellar interactions through electrostatic binding. Specifically, organic diamine crosslinkers are incorporated into montmorillonite membranes, resulting in improved structural stability. By varying the molecular size of the diamine crosslinkers, the interlayer spacing is precisely controlled, enabling tunable ion transport. Chapter 3 extends this crosslinking strategy to inorganic crosslinkers and establishes a doping strategy for regulating ion transport. Specifically, alumina pillared vermiculite membranes are fabricated by forming inorganic pillars that covalently bridge adjacent layers. In addition, sodium ions are introduced as ubiquitous coexisting species to modulate membrane charge and pore structure, enabling enhanced lithium selectivity over both monovalent and multivalent competing cations. Building on the pillared vermiculite membrane platform, polymeric species are further intercalated into pillared clay membranes as interlayer fillers to precisely control pore size, enabling the separation of monovalent cations that is difficult to achieve using conventional polymeric membranes with broad pore size distributions. Inspired by the discovery of dopant modulated transport in pillared systems, Chapter 4 establishes transport modulated by coexisting ions as a mechanistic handle for controlling ion separation, using phosphoryl functionalized vermiculite membranes as a model platform. In contrast to the conventional view of membranes as static structures, phyllosilicate membranes are demonstrated to dynamically respond to alkali metal cations, thereby regulating interlayer organization and suppressing the partitioning of multivalent cations to enable exceptional sorption dominated lithium separation. Further analysis under electrodialysis reveals emergent monovalent ion selectivity. Isothermal titration calorimetry (ITC) demonstrates that this selectivity originates from preferential binding between phosphoryl groups and competing monovalent cations, suggesting broader relevance to ion-selective membrane systems