K. Abels, A. Barbosa Botelho Junior, W. Kau, V. Yang, W. Tarpeh
Stanford University,
United States
Keywords: lithium separation, ligand-functionalized polymer membrane, partitioning, diffusion, Donnan dialysis, lifecycle analysis, technoeconomic analysis, process modelling
Summary:
Lithium is classified as a critical mineral due its high importance in the energy sector and high supply risk. Given the economic and environmental implications of a strained lithium supply, selective lithium recovery from unconventional water sources such as brines and spent battery waste is critical in the expansion of low-carbon and renewable energy technologies. Membranes are an ideal candidate separation and concentration technology because of their energy efficiency and low chemical input; however, further innovation is required at multiple scales to render membrane technologies feasible for ion-selective separation and concentration. To this end, our research efforts span investigation of 1) ion transport theory fundamentals to guide the development of highly selective membranes for ion separations and 2) process-level innovation in the design of a Donnan dialysis system for more efficient lithium brine concentration. Ion Transport in Ligand-Functionalized Membranes: Commercial ion exchange membranes are size- and charge-selective but are not yet ion-selective. A promising strategy to achieve ion-specific selectivity involves the incorporation of ligands into membranes that have ion-specific interactions beyond general size-sieving and electrostatic interaction. Studies have probed the effect of ion-ligand affinity on ion permeabilities and selectivities, but the effect of membrane ligand content remains understudied. Moreover, while ion transport has been studied at both extremes of dry and highly-swollen polymers, a gap remains in the study across a broad water content regime. Our fundamental studies of ion transport across libraries of membranes with variable ligand species, ligand content, and water content serve to provide elucidate membrane structure-function relationships pertinent to ion-selective separation applications. We have demonstrated that not only ligand identity but also ligand content – a previously understudied parameter – are critical factors for ion permeability, specifically for ions capable of multidentate ligand coordination (Figure 1) [1]. As we gradually lower membrane water content, our preliminary results show an inversion in permeability trends that could be explained by a shift in dominant transport mechanisms from diffusion through water volumes to ligand-mediated hopping. These results will support the development of ion transport models based on macroscopic polymer properties and will help inform design rules for ligand-functionalized membranes for selective ion separations. Donnan dialysis for lithium brine concentration: Beyond ion-selective separations, membranes could also prove useful for lithium brine concentration. One of the most energy-intensive steps of direct lithium extraction is the concentration of the dilute lithium eluate following lithium extraction. Currently, membrane-based reverse osmosis (requiring high pressure pumping) and thermal evaporation techniques (requiring significant heat input) contribute to the high energy intensity. We have performed bench-scale validation of Donnan dialysis (DD) as an alternative, low-energy process can be used in place of RO for the concentration of lithium brines [2]. Our pilot-scale modelling identifies priorities for further membrane material innovation (Figure 2) and quantifies the trade-off in life cycle impacts and techno-economics compared to incumbent concentration processes. Together, our material design and process innovation studies serve to advance membrane-based ion-ion separation and concentration technologies for sustainable lithium recovery.