Quantifying the Impact of Electrically Conductive Membrane-Generated Hydrogen Peroxide and Extreme pH on the Viability of Escherichia coli Biofilms

Abstract

Electrically conductive membranes (ECMs) self-induce antifouling mechanisms at their surface under certain applied electrical currents. Quantifying these mechanisms is critical to enhancing ECMs' self-cleaning performance. Local pH change and H 2 O 2 production are among the most important self-cleaning mechanisms previously hypothesized for ECMs. However, the impacts of these mechanisms have not previously been isolated and comprehensively studied. In this study, we quantified the individual impact of electrochemically induced acidic conditions, alkaline conditions, and H 2 O 2 concentration on model bacteria, Escherichia coli. To this end, we first quantified the electrochemical potential of carbon nanotube-based ECMs to generate stressors, such as protons, hydroxyl ions, and H 2 O 2 , under a range of applied electrical currents (±0−150 mA, 0−2.7 V). Next, these chemical stressors with similar magnitude to that generated at the ECM surfaces were imposed on E. coli cells and biofilms. In the flow-through ECM systems, biofilm viability using LIVE/DEAD staining indicated biofilm viabilities of 39 ± 9.9%, 38 ± 4.7%, 45 ± 5.0%, 34 ± 3.1%, and 75 ± 4.9% after separate exposure to pH 3.5, anodic potential (2 V), pH 11, cathodic potential (2 V), and H 2 O 2 concentration (188 μM). Electrical current-induced pH change at the membrane surface was shown to be more effective in reducing bacterial viability than H 2 O 2 generation and more efficient than bulk pH changes. This study identified antibiofouling mechanisms of ECMs and provides guidance for determining the current patterns that maximize their antifouling effects.

Facile Synthesis of Electrically Conductive Membranes

“…This demonstrates that the graphite-PVDF ECM is stable under applied voltage and can be potentially used in water/wastewater treatment applications for, e.g., electrochemical degradation of contaminants. The solution pH was stable during this experiment, which is consistent with results by Halali et al 14 The reason why the pH does not change is the simultaneous production of protons at the anode (2H 2 O → O 2 + 4H + + 4e − ) and hydroxyl ions at the cathode (2H 2 O + 2e − → 2OH − + H 2 ) that neutralize each other in the bulk solution.…”

“…9−12 By applying an electric potential to an ECM, the membrane exerts strong electrostatic repulsion against contaminants. 13,14 Under applied voltages, the membrane can also generate strong oxidizing species (e.g., reactive oxygen and chlorine species) that could inactivate attached microorganisms, 15 reduce/oxidize toxic heavy metal ions (e.g., arsenite-(III) and chromium(VI)), 12,16 and degrade contaminants on membrane surfaces/pores or in water. 17 To fabricate ECMs, the choice of conductive material is a critical consideration.…”

“…17 To fabricate ECMs, the choice of conductive material is a critical consideration. 18 Since most conductive metal particles/ nanoparticles (e.g., silver (Ag) and copper) are relatively costly and tend to leach into solution, 19 carbon-based materials including graphene, 20 reduced graphene oxide (rGO), 19 and carbon nanotubes (CNTs) 14,21 have been evaluated in recent years, with the latter being the most widely reported. In terms of the method used to render membranes electrically conductive, poly(vinyl alcohol) (PVA)-based cross-linking with CNTs or graphene as conductive materials is the most commonly used method for functionalization of ultrafiltration (UF) membranes, 12,22−24 whereas the only reported approach for functionalization of polyamide-based osmotic membranes is the physical blending of polymer and CNTs or other conductive materials (e.g., carbon particles) during polyamide layer formation.…”

Methods for stability assessment of electrically conductive membranes

“…These antifouling mechanisms include electrostatic repulsion of like-charged foulants [11] , electrochemical [12] and electrocatalytic [13] reactions, and gas generation [14] . ECMs have presented several advantages as compared to conventional membranes such as: (a) controllably target foulants at the membrane/water interface which makes them more effective than traditional bulk solution cleaning (biocide dosing, pH adjustment) [15] , [16] , [17] , (b) use electrons, “clean reagents”, as antifouling mediators, making the process less chemical intense and easy to operate, which reduces the handling and storage costs of chemicals [18] , [19] , [20] , and (c) can tailor their antifouling mechanisms by tuning the applied electrical properties (polarity, magnitude, and frequency). Therefore, the antifouling mechanisms can be tailored to exclusively match the application [ 7 , 17 , 21 ].…”

Carbon nanomaterial-based membranes for water and wastewater treatment under electrochemical assistance

“…The inhibition of biofouling from low surface-charged bacteria could be also achieved by H 2 O 2 production through the electroreduction of oxygen. 275,276 The electrochemically produced H 2 O 2 reduced the microbial cell viability and increased cellular permeability, preventing bacterial attachment and ensuring biofilm-free conditions. Additionally, finite element modeling model demonstrated that both electrophoresis and dielectrophoresis are antibiofouling mechanisms for electroconductive membranes.…”