Augmenting the Interfacial Electrostatic Potential with Donnan-Enhanced Nanofiltration for Lithium Extraction from Salt-Lakes

The exponential growth in the demand for lithium in the renewable energy and electric vehicle industries necessitates significant improvement in the efficiency of lithium production from primary sources. While lithium is abundant in salt lakes within South America and Asia, existing extraction methods based on brine evaporation are relatively costly and suffer protracted production cycles. By avoiding brine evaporation altogether, membrane processes such as nanofiltration can selectively concentrate monovalent ions, optimizing the extraction process directly from salt lakes while eliminating the environmental impacts of evaporation ponds.<br/><br/>The selectivity of conventional nanofiltration membranes can be attributed to their cross-linked polyamide active layer. The polyamide layer is typically fabricated by a condensation reaction between trimesoyl chloride and piperazine, along the interface of a polysulfone support layer and the organic monomer mixture. As a result of the charged moieties in the polyamide, the electrostatic potentials that form along the solution-membrane interface inhibit ions of the same charge from partitioning into the membrane, a phenomenon known as Donnan exclusion. However, the high concentration of the residual carboxyl moieties of the polyamide layer imparts a negative volumetric charge density to the active layer and attenuates the rejection of multivalent cations by Donnan exclusion.<br/><br/>To impart monovalent cation selectivity, emerging variants of nanofiltration employ a positively charged surface layer on the membrane that displays high water permeability and a positive volumetric charge density. Here, a Donnan-enhanced nanofiltration membrane employing a polyethyleneimine (PEI) surface layer is fabricated through a condensation reaction between perfluorosulfonic acid and the amine moieties in the polyamide layer, to attain enhanced Li⁺/Mg²⁺selectivity. In this study, we present empirical evidence for the success of this method, based on experiments with high-salinity multi-cation brines that represent those encountered in salt-lake lithium extraction. Our conclusions are derived from 1000 original concentration measurements spanning three feed salinities and three pH levels.<br/><br/>The water permeability of the Donnan-enhanced NF membrane decreases from 17.2 % on average from the added hydraulic resistance. Our streaming potential measurements indicate that the zeta potential of the NF membrane increases from − 17.2 mV to 19.8 mV with the addition of the PEI surface layer. Our experiments with binary cation solutions (i.e., Li⁺, Mg²⁺) reveal that the Li⁺/Mg²⁺separation factor increases from 39.1 with the unmodified membrane to 129.2 with the Donnan-enhanced membrane. Using multicomponent salt-lake brines from Salar de Atacama, Chile, the recorded Li⁺/Mg²⁺separation factors are 13.2, 18.1, and 87.3, for solution pH of 7, 4 and 2, respectively. In a single pass, the Donnan-enhanced NF membranes register an Mg²⁺rejection of 96.8 % while raising the concentration of Li⁺by 17.7 %. The enhanced Li⁺/Mg²⁺selectivity at low solution pH arises from the protonation of the residual carboxyl moieties in acidic conditions (pk_a ~ 4). Lastly, we conduct a module-scale assessment of the process's membrane area requirement and energy efficiency.