Development and Characterization of Ultrafiltration TiO₂ Magnéli Phase Reactive Electrochemical Membranes

This research focused on the synthesis, characterization, and performance testing of a novel Magnéli phase (Ti_nO_2n–1), n = 4 to 6, reactive electrochemical membrane (REM) for water treatment. The REMs were synthesized from tubular asymmetric TiO₂ ultrafiltration membranes, and optimal reactivity was achieved for REMs composed of high purity Ti₄O₇. Probe molecules were used to assess outer-sphere charge transfer (Fe­(CN)₆^4–) and organic compound oxidation through both direct oxidation (oxalic acid) and formation of OH^• (coumarin, terephthalic acid). High membrane fluxes (3208 L m^–2 h^–1 bar^–1 (LMH bar^–1)) were achieved and resulted in a convection-enhanced rate constant for Fe­(CN)₆^4– oxidation of 1.4 × 10^–4 m s^–1, which is the highest reported in an electrochemical flow-through reactor and approached the kinetic limit. The optimal removal rate for oxalic acid was 401.5 ± 18.1 mmol h^–1 m^–2 at 793 LMH, with approximately 84% current efficiency. Experiments indicate OH^• were produced only on the Ti₄O₇ REM and not on less reduced phases (e.g., Ti₆O₁₁). REMs were also tested for oxyanion separation. Approximately 67% removal of a 1 mM NO₃^– solution was achieved at 58 LMH, with energy consumption of 0.22 kWh m^–3. These results demonstrate the extreme promise of REMs for water treatment applications.