Selective lithium extraction from natural salt lake brines via flow-electrode capacitive deionization: Separation performance, fouling behavior and transport paradigms

Salt lake brines have emerged as a strategic resource due to their abundance of high-value metal ions. However, the efficient separation of coexisting species, particularly Li⁺ and Mg²⁺, remains a critical challenge. This study systematically evaluated the separation performance and fouling behavior of a flow-electrode capacitive deionization (FCDI) system for treating five distinct real brines from Qinghai, China. The results demonstrated that the selective FCDI system serves as a highly effective pretreatment for lithium extraction, achieving mono/divalent separation factors up to 87.5 during a two-stage continuous operation. While natural brines introduced higher fouling risks compared to artificial solutions, membrane fouling exhibited a unique dual effect. Although inorganic scaling and non-mineral fouling led to flux decline, the fouling layer preferentially hindered divalent ion transport, inadvertently enhancing selectivity. Microbial analysis indicated that biofouling remains a noteworthy risk during long-term operation even in high-salinity environments. Moreover, this work also identified a kinetic trade-off among selectivity, yield, and membrane fouling, which was driven by a transition from a membrane-controlled to a diffusion-controlled regime. Despite the advantages in ion separation, successful industrial application of FCDI requires synergistic optimization of influent preconcentration, membrane materials, module configurations, and operational strategies.