Development and Characterization of Ultrafiltration TiO2 Magnéli Phase Reactive Electrochemical Membranes

Mechanistic Study of the Validity of Using Hydroxyl Radical Probes To Characterize Electrochemical Advanced Oxidation Processes

The detection of hydroxyl radicals (OH^•) is typically accomplished by using reactive probe molecules, but prior studies have not thoroughly investigated the suitability of these probes for use in electrochemical advanced oxidation processes (EAOPs), due to the neglect of alternative reaction mechanisms. In this study, we investigated the suitability of four OH^• probes (coumarin, p-chlorobenzoic acid, terephthalic acid, and p-benzoquinone) for use in EAOPs. Experimental results indicated that both coumarin and p-chlorobenzoic acid are oxidized via direct electron transfer reactions, while p-benzoquinone and terephthalic acid are not. Coumarin oxidation to form the OH^• adduct product 7-hydroxycoumarin was found at anodic potentials lower than that necessary for OH^• formation. Density functional theory (DFT) simulations found a thermodynamically favorable and non-OH^• mediated pathway for 7-hydroxycoumarin formation, which is activationless at anodic potentials > 2.10 V/SHE. DFT simulations also provided estimates of E° values for a series of OH^• probe compounds, which agreed with voltammetry results. Results from this study indicated that terephthalic acid is the most appropriate OH^• probe compound for the characterization of electrochemical and catalytic systems.

Bacteria inactivation at a sub-stoichiometric titanium dioxide reactive electrochemical membrane

This study investigated the use of a sub-stoichiometric TiO2 reactive electrochemical membrane (REM) for the inactivation of a model Escherichia coli (E. coli) pathogen in chloride-free solutions. The filtration system was operated in dead-end, outside-in filtration model, using the REM as anode and a stainless steel mesh as cathode. A 1-log removal of E. coli was achieved when the electrochemical cell was operated at the open circuit potential, due to a simple bacteria-sieving mechanism. At applied cell potentials of 1.3 and 3.5V neither live nor dead E. coli cells were detected in the permeate stream (detection limit of 1.0 cell mL(-1)), which was attributed to enhanced electrostatic bacteria adsorption at the REM anode. Bacteria inactivation in the retentate solution increased as a function of the applied cell potential, which was attributed to transport of E. coli to the REM and stainless steel cathode surfaces, and direct contact with the local acidic and alkaline environment produced by water oxidation at the anode and cathode, respectively. Clear evidence for an E. coli inactivation mechanism mediated by either direct or indirect oxidation was not found. The low energy requirement of the process (2.0-88Whm(-3)) makes the REM an attractive method for potable water disinfection.

Electroconductive and electroresponsive membranes for water treatment

In populated, water-scarce regions, seawater and wastewater are considered as potable water resources that require extensive treatment before being suitable for consumption. The separation of water from salt, organic, and inorganic matter is most commonly done through membrane separation processes. Because of permeate flux and concentration polarization, membranes are prone to fouling, resulting in a decline in membrane performance and increased energy demands. As the physical and chemical properties of commercially available membranes (polymeric and ceramic) are relatively static and insensitive to changes in the environment, there is a need for stimuli-reactive membranes with controlled, tunable surface and transport properties to decrease fouling and control membrane properties such as hydrophilicity and permselectivity. In this review, we first describe the application of electricity-conducting and electricity-responsive membranes (ERMs) for fouling mitigation. We discuss their ability to reduce organic, inorganic, and biological fouling by several mechanisms, including control over the membrane’s surface morphology, electrostatic rejection, piezoelectric vibrations, electrochemical reactions, and local pH changes. Next, we examine the use of ERMs for permselectivity modification, which allows for the optimization of rejection and control over ion transport through the application of electrical potentials and the use of electrostatically charged membrane surfaces. In addition, electrochemical reactions coupled with membrane filtration are examined, including electro-oxidation and electro-Fenton reactions, demonstrating the capability of ERMs to electro-oxidize organic contaminates with high efficiency due to high surface area and reduced mass diffusion limitations. When applicable, ERM applications are compared with commercial membranes in terms of energy consumptions. We conclude with a brief discussion regarding the future directions of ERMs and provide examples of several applications such as pore size and selectivity control, electrowettability, and capacitive deionization. To provide the reader with the current state of knowledge, the review focuses on research published in the last 5 years.