Ph.D. Thesis Defense Announcement

Advanced Separation Technologies for Lithium-Ion Battery Recycling Using Cyanex 272–Blended PVDF Composites from Membranes to Fixed-Bed Columns

By Chengchao Xiao

Advisor:

Dr. Yongsheng Chen

Committee Members: Dr. Hailong Chen (MSE), Dr. Xing Xie (CEE),

Dr. Shane. A. Snyder (CEE), Dr. Joe F. Bozeman (CEE)

Date and Time: March, 06, 2026. 10am EST

Location: Daniel Lab 303

Selective separation of Co(II) from Ni(II) in sulfate-based lithium-ion battery leachates remains a major bottleneck in hydrometallurgical recycling because these adjacent transition metals exhibit closely similar aqueous chemistries. This dissertation develops a scalable solid-phase extraction platform by physically immobilizing the organophosphorus extractant Cyanex 272 within a polyvinylidene fluoride (PVDF) polymer matrix and using morphological engineering to translate molecular coordination selectivity into process-relevant separation performance. The study progresses from planar PVDF–Cyanex 272 adsorptive membranes to hierarchically porous beads fabricated via controlled non-solvent induced phase separation, and finally to continuous fixed-bed column operation to quantify how material structures govern dynamic selectivity and mass-transfer limitations. Planar membranes confirm
that Cyanex 272 retains its intrinsic Co(II) preference upon immobilization, achieving Co(II)/Ni(II) separation factors up to 209.5 at pH 6.8 and 75 °C, while also revealing a capacity ceiling (1.42 mg·g⁻¹) imposed by two-dimensional geometry and restricted extractant accessibility. Morphology-engineered beads overcome this limitation by creating interconnected transport pathways that improve internal site utilization, increasing cobalt capacity to 14.1 mg·g⁻¹. Under continuous flow, fixed-bed experiments exhibit pronounced Ni(II) roll-up, which indicates competitive chromatographic displacement by the higher-affinity Co(II) front. Breakthrough analysis indicates diffusion-limited saturation behavior, for which our Modified Yan model provides superior predictive performance across operating conditions (𝑹𝑹² > 0.99) compared with idealized models such as Thomas. Process optimization identifies pH 6.15 as a practical set point balancing uptake kinetics and thermodynamic selectivity, and the system demonstrates robust reusability with regeneration efficiencies exceeding 97.8% over ten cycles using 0.5 M H₂SO₄. Overall, the dissertation establishes a mechanistically grounded, performance-intensified solid-phase route for Co(II)/Ni(II) separation that bridges coordination chemistry, morphology, and fixed-bed process design for advanced battery recycling.