Nanofiltration membrane and Sorbent-Based Technologies for Lithium Recovery from Salt Lake Brine

The recovery of lithium from high-salinity salt lake brines is increasingly critical for supporting global energy transition and battery supply chains. However, selective separation of lithium from competing divalent cations such as magnesium and calcium remains a major technical challenge, especially under complex brine chemistries. This dissertation investigates two complementary strategies for lithium recovery: nanofiltration (NF) membrane pretreatment and sorbent-based selective adsorption, with an emphasis on the Salton Sea geothermal brine as a representative U.S. resource. The separation performance of three commercial NF membranes (NF270, DL, and NF90) was systematically evaluated under varying feed salinities (20-200 g/L) and pH conditions. At high salinity levels, membranes exhibited low lithium-to-magnesium selectivity (1-2), reduced rejection of divalent ions, and diminished water flux (28-42 L·m-2·h-1 at 30 bar). Upon dilution of the feed brine to 20 g/L, both flux (135 L·m-2·h-1) and rejection performance (79.1%) improved significantly. Molecular dynamics simulations revealed that reduced hydrogen bond lifetimes at high salinity decreased water mobility without significantly altering salt diffusivity, which in turn explained the observed decline in separation efficiency. A preliminary process design and economic assessment indicated that a combination of upstream crystallization to remove calcium, followed by brine dilution and staged NF treatment, could achieve molar Li/Mg and Li/Ca ratios exceeding 99. Sorbent-based strategy employing lithium manganese oxide (LMO) composites was investigated. Three LMO phases, LiMn2O4, Li1.66Mn1.66O4, and Li4Mn5O12, were synthesized and embedded into granular matrices with hydroxyethyl cellulose (HEC), chitosan, and colloidal silica as binders. Structural characterizations via XRD confirmed high phase purity and crystallinity across all sorbents. Static batch adsorption experiments revealed that the Li1.66Mn1.66O4-HEC composite exhibited the highest lithium uptake (~39 mg·g⁻¹) and reached equilibrium within 4 hours. Kinetic and isotherm modeling indicated pseudo-second-order behavior and strong agreement with Langmuir monolayer adsorption, with favorable binding affinity supported by Freundlich analysis. Dynamic column experiments further confirmed the superior performance and regeneration stability of the 1.66-HEC composite under flow-through conditions.