Development of Ultrafiltration Titanium Dioxide Magnéli Phase Reactive Electrochemical Membranes

This research focused on the synthesis, characterization, and performance testing of a novel Magnéli phase (TinO2n-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 comprised of high purity Ti4O7. Probe molecules were used to assess outer-sphere charge transfer (Fe(CN)64-) and organic compound oxidation through both direct oxidation (oxalic acid) and formation of OH• (coumarin, terephthalic acid). High membrane fluxes (3208 L m-2 hr-1 bar-1 (LMH bar-1)) were achieved and resulted in a convection-enhanced rate constant for Fe(CN)64- oxidation of 1.4 x 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 mmole 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.