Development and Characterization of Ultrafiltration TiO 2 Magné li Phase Reactive Electrochemical Membranes Lun Guo, Yin Jing, and Brian P. Chaplin* Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton Street, Chicago, Illinois 60607 * S Supporting Information ABSTRACT: This research focused on the synthesis, charac- terization, and performance testing of a novel Magné li phase (Ti n O 2n-1 ), n = 4 to 6, reactive electrochemical membrane (REM) for water treatment. The REMs were synthesized from tubular asymmetric TiO 2 ultrafiltration membranes, and optimal reactivity was achieved for REMs composed of high purity Ti 4 O 7 . 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 ms -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 Ti 4 O 7 REM and not on less reduced phases (e.g., Ti 6 O 11 ). REMs were also tested for oxyanion separation. Approximately 67% removal of a 1 mM NO 3 - 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. ■ INTRODUCTION Reactive electrochemical membranes (REMs) are a promising technology that combine an electrochemical advanced oxidation process (EAOP) and physical separation into a single water treatment device. EAOP is the process by which water is oxidized on an anode surface to form hydroxyl radicals (OH • ), which react with a wide range of recalcitrant organic and inorganic compounds often at diffusion-controlled rates. 1 Magne ́ li phase titanium oxides (Ti n O 2n-1 ), n = 4 to 10, have been utilized for REM fabrication because they can be synthesized into porous monolithic structures at low cost and are reported to produce OH • via water oxidation. 2,3 The unique chemical, electrical, and magnetic properties of Ti n O 2n-1 have motivated their use as battery electrodes, 4-6 fuel cell supports, 6,7 memristor devices, 8,9 photocatalysts, 10 and electro- des for electrochemical oxidation, 11,12 and reduction 6,13,14 of water contaminants. The work by Zaky and Chaplin demonstrated that commercial, tubular Ti n O 2n-1 monolithic electrodes (Ebonex) could be utilized as REMs for the oxidation of several organic compounds at high current efficiencies. 3 Results showed that reaction rates were limited by convection to the REM, due to the fast radial diffusion of compounds in the micron-sized REM pores. 3 These promising results suggest that reaction rates can be increased by simply increasing the permeate flux and that intrinsic reaction rates of the electrode should be obtained at sufficiently high fluxes. However, the Ebonex REM pore structure was not tailored for water treatment, which resulted in a high-pressure drop across the membrane and thus low pressure-normalized permeate fluxes (e.g., 50-70 L m -2 h -1 bar -1 (LMH bar -1 )). 2,3 Additionally, Ebonex electrodes often contain a range of Magné li phases (n = 4 to 10), 15 which can affect electrode conductivity and presumably EAOP perform- ance. While it is well-known that Ti 4 O 7 is the most conductive Magne ́ li phase suboxide (e.g., 20 000-100 000 S m -1 ), 6,15 studies focused on providing a link between Magne ́ li phase composition and EAOP performance are lacking. Another unexplored area of research for REMs is their use as electrostatic barriers for ion separation. Related technologies, such as carbon nanotube-polymer composite membranes, 16,17 have shown the ability to reduce membrane fouling due to electrostatic repulsion of negatively charged organics at cathodically polarized membrane surfaces. However, to our knowledge, work focused on ion separation by an electrically conductive membrane has not been reported. Although electrodialysis (ED) is similar in function, the technology is fundamentally different from REMs. ED membranes possess high ionic conductivity but are electrically insulating. Polymeric ED membranes suffer from high production costs, are Received: September 8, 2015 Revised: December 25, 2015 Accepted: January 5, 2016 Article pubs.acs.org/est © XXXX American Chemical Society A DOI: 10.1021/acs.est.5b04366 Environ. Sci. Technol. XXXX, XXX, XXX-XXX