Electrocatalytic Perchlorate Reduction Using an Oxorhenium Complex Supported on a Ti4O7 Reactive Electrochemical Membrane.

An organometallic rhenium catalyst was deposited on a Ti4O7 reactive electrochemical membrane (Re/REM) for the electrocatalytic reduction of aqueous ClO4- to Cl-. Results showed increasing ClO4- reduction upon increasing cathodic potential (i.e., -0.4 to-1.7 V/SHE). A 5 mM ClO4- solution was reduced by ∼21% in a single pass (residence time ∼0.2 s) through the Re/REM at a pH of 7, with >99% Cl- selectivity and a current efficiency of ∼100%. Kinetic analysis indicated that the reaction rate constant increased from 3953 to 7128 L h-1 gRe-1 at pH values of 9 to 3, respectively, and was mass transport-limited at pH < 5. The rate constants were 2 orders of magnitude greater than reported values for an analogous catalytic system using hydrogen as an electron donor. A continuous flow Re/REM system reduced 1 ppm ClO4- in a groundwater sample by >99.9% for the first 93.5 h, and concentrations were lower than the EPA ClO4- guideline (56 ppb) for 374 h of treatment. The fast ClO4- reduction kinetics and high chloride selectivity without the need for acidic conditions and a continual hydrogen electron donor supply for catalyst regeneration indicate the promising ability of the Re/REM for aqueous electrocatalytic ClO4- treatment.

Mechanistic Investigation of Haloacetic Acid Reduction Using Carbon-Ti4O7 Composite Reactive Electrochemical Membranes.

Carbon-Ti4O7 composite reactive electrochemical membranes (REMs) were studied for adsorption and electrochemical reduction of haloacetic acids (HAAs). Powder activated carbon (PAC) or multiwalled carbon nanotubes (MWCNTs) were used in these composites. Results from flow-through adsorption experiments with dibromoacetic acid (DBAA) as a model HAA were interpreted with a transport model. It was estimated that ∼46% of C in the MWCNT-REM and ∼10% of C in the PAC-REM participated in adsorption reactions. Electrochemical reduction of 1 mg L-1 DBAA in 10 mM KH2PO4/K2HPO4 at -1.5 V/SHE (hydraulic residence time, ∼11 s) resulted in 73, 94, and 96% DBAA reduction for Ti4O7, PAC-Ti4O7, and MWCNT-Ti4O7 REMs, respectively. The reactive-transport model yielded kobs values between 9.16 and 33.3 min-1, which were 2 to 4 orders of magnitude higher than previously reported. PAC-Ti4O7 REM was tested with tap water spiked with 0.11 mg L-1 of nine different HAAs in a similar reduction experiment. The results indicated that all HAAs were reduced to <20 µg L-1. Moreover, the total combined concentration of five regulated HAAs was lower than the regulatory limit (60 µg L-1). Density functional theory simulations suggest that a direct electron transfer reaction was the probable rate-determining step for HAA reduction.

Charged Layered Boron Nitride-Nanoflake Membranes for Efficient Ion Separation and Water Purification.

2D layered nanomaterials have attracted considerable attention for their potential for highly efficient separations, among other applications. Here, a 2D lamellar membrane synthesized using hexagonal boron nitride nanoflakes (h-BNF) for highly efficient ion separation is reported. The ion-rejection performance and the water permeance of the membrane as a function of the ionic radius, ion valance, and solution pH are investigated. The nonfunctionalized h-BNF membranes show excellent ion rejection for small sized salt ions as well as for anionic dyes (>97%) while maintaining a high water permeability, ≈1.0 × 10-3 L m m-2 h-1 bar-1 ). Experiments show that the ion-rejection performance of the membrane can be tuned by changing the solution pH. The results also suggest that the rejection is influenced by the ionic size and the electrostatic repulsion between fixed negative charges on the BN surface and the mobile ions, and is consistent with the Donnan equilibrium model. These simple-to-fabricate h-BNF membranes show a unique combination of excellent ion selectivity and high permeability compared to other 2D membranes.

Electrochemical Production of Si without Generation of CO2 Based on the Use of a Dimensionally Stable Anode in Molten CaCl2.

The current Si production process is based on the high-temperature (1700 °C) reduction of SiO2 with carbon that produces large amounts of CO2 . We report an alternative low-temperature (850 °C) process based on the reduction of SiO2 in molten CaCl2 that does not produce CO2 . It utilizes an anode material (Ti4 O7 ) capable of sustained oxygen evolution. Two types of this anode material, dense Ti4 O7 and porous Ti4 O7 , were tested. The dense anode showed a better performance. The anode stability is attributed to the formation of a protective TiO2 layer on its surface. In situ periodic current reversal and ex situ H2 reduction could be used for extending the lifetime of the anodes. The findings show that this material can be applied as a recyclable anode in molten CaCl2 . Si wires, films, and particles were deposited with this anode under different cathodic current densities. The prepared Si film exhibited ≈30-40 % of the photocurrent response of a commercial p-type Si wafer, indicating potential use in photovoltaic cells.

Simultaneous Adsorption and Electrochemical Reduction of N-Nitrosodimethylamine Using Carbon-Ti4O7 Composite Reactive Electrochemical Membranes.

This study focused on synthesis and characterization of Ti4O7 reactive electrochemical membranes (REMs) amended with powder-activated carbon (PAC) or multiwalled carbon nanotubes (MWCNTs). These composite REMs were evaluated for simultaneous adsorption and electrochemical reduction of N-nitrosodimethylamine (NDMA). The carbon-Ti4O7 composite REMs had high electrical conductivities (1832 to 2991 S m-1), where carbon and Ti4O7 were in direct electrical contact. Addition of carbonaceous materials increased the residence times of NDMA in the REMs by a factor of 3.8 to 5.4 and therefore allowed for significant electrochemical NDMA reduction. The treatment of synthetic solutions containing 10 µM NDMA achieved >4-log NDMA removal in a single pass (liquid residence time of 11 to 22 s) through the PAC-REM and MWCNT-REM with the application of a -1.1 V/SHE cathodic potential, with permeate concentrations between 18 and 80 ng L-1. The treatment of a 6.7 nM NDMA-spiked surface water sample, under similar operating conditions (liquid residence time of 22 s), achieved 92 to 97% removal with permeate concentrations between 16 and 40 ng L-1. Density functional theory calculations determined a probable reaction mechanism for NDMA reduction, where the rate-limiting step was a direct electron transfer reaction.