Development and Characterization of Ultrafiltration TiO₂ Magnéli Phase Reactive Electrochemical Membranes

Abstract

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 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.

Mineralization of organic pollutants by anodic oxidation using reactive electrochemical membrane synthesized from carbothermal reduction of TiO2

“…Similar flux (3208 L m À2 h À1 bar À1 ) was obtained by Guo et al (2016) with TiO 2 membranes reduced to Ti 4 O 7 using a thermal method at 1050 C under 1 atm H 2 during 50 h . The median pore size diameter of these membranes was 2.99 mm, however, the 70 mm thick active layer with smaller pores could not be characterized by Hg porosimetry.…”

“…High permeate fluxes increased O 2 gas bubbles sweeping from the REM surface, thus resulting in the promotion of the competitive reaction of oxygen evolution. Guo et al (2016) reported the same behavior in a crossflow filtration configuration using TiO x REM from thermal reduction of TiO 2 under H 2 . Lower TOC removal rate (9.6 g m À2 h À1 ) but higher MCE (84%) was achieved for an optimal value of OA flux of 13.5 g m…”

“…This kinetic limitation depends on the nature of the substrate to oxidize, intrinsic kinetic reactivity of the REM and current supplied. From the comparison to values reported by Guo et al (2016) with a similar configuration, it is anticipated that the high permeability of the REM used during this study avoid diffusion limitations during electro-oxidation of organic compounds, even at the lowest flux used (TMP ¼ 40 mbar; J z 120 L h À1 m À2 ). The electrochemical performance of REM also tightly depends on the nature of the Magn eli phase.…”

“…Besides, research works from Guo et al (2016) showed that a convectionenhanced rate constant for Fe(CN) 6 4À oxidation of 1.4 10 À4 m s À1 e the highest reported in the literature, close to the kinetic limit e was reached by using porous sub-stoichiometric titanium oxide (TiO x ) reactive electrochemical membrane (REM) . The high membrane permeability (3200 L m À2 h À1 bar À1 ) resulted in a strong increase of the convective mass transport of pollutants from the bulk solution to electron-transfer sites at the surface and in the porosity of the material.…”

“…The median pore size diameter of these membranes was 2.99 mm, however, the 70 mm thick active layer with smaller pores could not be characterized by Hg porosimetry. Detailed study on limiting mechanisms for electrooxidation showed three different behaviors according to permeate flux through the membrane (Zaky and Chaplin, 2013;Guo et al, 2016). Mass transport rate is the key parameter since electro-oxidation reactions occur at the membrane/pore surface.…”