Formulation and Characterization of Dextran Sulfate-Polyethyleneimine Complexes for Lithium Recovery from Oilfield and Enhanced Geothermal System Brines

The accelerating demand for lithium and projected shortages of lithium carbonate (Li₂CO₃) by 2030 have intensified interest in oilfield and geothermal brines as alternative resources. Conventional recovery methods, including solvent extraction and nanofiltration, are limited by fire-safety risks and frequent operational maintenance associated with membrane fouling. This study introduces a coagulation-based lithium recovery method using polyelectrolyte complexes (PECs) formed from polyethyleneimine (PEI) and dextran sulfate (DS). By optimizing the PEI: DS mass ratio, pH, and polymer concentration, PECs rapidly coagulated and phase-separated from brine matrices while efficiently capturing Li⁺. Under optimized conditions, two-stage extraction produced 73 – 76% lithium recovery within 12 minutes per stage for both single-ion solution and untreated oilfield brine. A Li⁺–Mg²⁺ separation factor of 11 was achieved without pretreatment, demonstrating selective uptake in multivalent environments. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis confirmed Li⁺ coordination through interactions between PEI amine and DS sulfate groups. Supernatant recycling experiments further showed that residual PEI supports DS-only regeneration, enabling multi-cycle PEC formation and cumulative Li⁺ recovery. Furthermore, integration with ultrafiltration membranes is proposed to enable scalable polymer recovery and continuous operation, offering a simplified, low chemical input alternative to conventional lithium extraction technologies. • First application of polyelectrolyte complexes (PECs) for lithium recovery from oilfield brine. • Achieved 73–76% lithium extraction within 12 minutes without pretreatment. • Li/Mg²⁺ separation factor of 11 demonstrates strong selectivity. • Future directions include functional group optimization of PECs, controlled Li release, and polymer recycling via ultrafiltration.