The surface properties of electrically conductive membranes (ECMs) govern their advanced abilities. During operation, these properties may differ considerably from their initially measured properties. Depending on their operating conditions, ECMs may undergo various degrees of passivation. ECM passivation can detrimentally impact their real time performance, causing large deviations from expected behaviour based on their initially measured properties. Quantifying these changes will enable consistent performance comparisons across the active and electrically conductive membrane research field. As such, consistent methods must be established to quantify ECM membrane properties. In this work, we proposed three standardized methods to assess the electrochemical, chemical, and physical stability of such membrane coatings: 1) electrochemical oxidation, 2) surface scratch testing, and 3) pressurized leaching. ECMs were synthesized by the most common approach - coating support ultrafiltration (UF) and/or microfiltration (MF) polyethersulfone (PES) membranes with carbon nanotubes (CNT) cross-linked with polyvinyl alcohol (PVA) and two types of cross-linkers (either succinic acid (SA) or glutaraldehyde (GA)). We then evaluated these ECMs based on the three standardized methods: 1) We evaluated electrochemical stability as a function of electro-oxidation induced by applying anodic potentials. 2) We measured the scratch resistance to quantify the surface mechanical stability. 3) We measured physical stability by quantifying the leaching of PVA during separation of a model foulant (polyethylene oxide (PEO)). Our methods can be extended to all types of electrically conductive membranes including MF, UF, nanofiltration (NF), and reverse osmosis (RO) ECMs. We propose that these fundamental measurements are critical to assessing the viability of ECMs for industrial MF, UF, NF, and RO applications.•Anodic-oxidation was used to check the electrochemical stability of ECMs•Depth of penetration resulted from scratch test is an indicator of the electrically conductive membrane coating's mechanical stability•The leaching of the main components forming the nanolayer was quantified to assess the membranes' physical stability.
Biofouling detection enables the adoption of effective cleaning strategies for biofouling prevention. This work investigates the use of electrical impedance spectroscopy (EIS) to monitor the biofilm development and the use of electric fields to mitigate biofouling on the surface of gold-coated membranes. The multi-bacterial suspension was injected into a two-electrode crossflow filtration system where the permeate flux and impedance spectra were recorded to monitor the biofilm growth. Permeate flux declined over time while the impedance at low frequency regions (<10 Hz) rapidly decreased with fouling at the early stages of fouling, and then gradually decreased as biofilm matured. The normalized diffusion-related impedance (Rd), an EIS-derived parameter, was extracted to determine the sensitivity of EIS detection. We observed that impedance-based detection was more sensitive to changes as compared to the decline of permeate flux during the early stage of biofouling. With early detection of fouling, fouling mitigation strategies could be applied more effectively. Further, under the same conditions as fouling detection, either applying an intermittent cathodic potential (−1.5 V) or cross-flow flushing delayed the biofilm growth on the electrically conductive membranes (ECMs). EIS sensitivity was repeatably recovered across four cycles of mechanical fouling removal. Hence ECMs were demonstrated to play a dual function: EIS-enabled detection of biofouling evolution and surface biofouling mitigation.
Stability of electrically conductive membranes (ECM) is critical for expanding their application in separation-based technologies. In this work, ECMs were synthesized by coating polyethersulfone membranes with carbon nanotubes (CNT) crosslinked to polyvinyl alcohol (PVA) using two types of crosslinkers (succinic acid or glutaraldehyde). ECMs demonstrated a 21% reduction in flux over 4 h under cathodic potential (2 V) in comparison to a 69% reduction in flux for control experiments when filtering a realistic bacterial suspension. Subsequently, the electrochemical, physical, and mechanical stability of the ECMs were explored using chronoamperometry and cyclic voltammetry, an evaluation of polymer leaching from membranes, and micro mechanical scratch testing, respectively. ECMs were shown to be unstable under anodic potentials (2–4 V) with the glutaraldehyde crosslinking demonstrating the highest electrochemical stability. PVA was shown to be a physically unstable crosslinking agent for CNTs under concentration polarization conditions. Instability was moderated by extending CP layers through thicker and less dense nanolayers. ECMs showed higher mechanical stability and resistance to surface damage, in particular when coated with glutaraldehyde. We quantified the relationship between ECM surface instability and their physical and electrochemical properties. In so doing, we provide guidance for making practical and scalable electrically conductive membranes. • Applied potential impedes the development of membrane biofouling. • Electrically conductive membranes are unstable under operating conditions. • Physical, mechanical, and electrochemical stability of membranes were investigated. • PVA and GA protect CNT from anodic electro-oxidation.
Electrically conductive membranes have shown significant promise in combining conventional separations with in situ contaminant oxidation, but little has been done to consider chlorine removal. This study demonstrates the simultaneous chlorine removal and oxidation of organic compounds during filtration using an electrochemically assisted electrically conductive carbon nanotube (CNT) membrane. As much as 80% of chlorine was removed in the feed by CNT membranes at the initial phase of continuous filtration. The efficacy of these CNT membranes toward chlorine removal was dependent on the mass of CNTs within the membranes and the applied pressure to the membranes, indicating the central role of available CNT active sites and sufficient reaction time. Furthermore, the removal mechanism of chlorine by CNTs was revealed by studying the degradation of benzoic acid and cyclic voltammetry on the membrane surface. Reactive oxidants were generated by the reductive decomposition of chlorine through the catalytic interaction with CNTs. Subsequently, electrical potentials were applied to the CNT membrane surfaces during the filtration of chlorinated feed waters. The simultaneous decomposition of chlorine and oxidation of benzoic acid were significantly enhanced by applying a cathodic current to CNT membranes enabling continuous dechlorination. The cathodic current applied to CNT membranes is believed to regenerate CNT membranes by providing electrons for the reductive decomposition of chlorine. In situ chemical-free dechlorination coupled with membrane filtration offers great opportunity to reducing the environmental impact of desalination, while maximizing the lifetime of reverse osmosis membranes and demonstrating greener approaches available to industrial water treatment.
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 H2O2 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 H2O2 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 H2O2, 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 H2O2 concentration (188 μM). Electrical current-induced pH change at the membrane surface was shown to be more effective in reducing bacterial viability than H2O2 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.
Activated carbons have been widely used for water treatment due to their large surface area and structural stability. Their high cost has motivated the development of sustainable bio-based sorbents. However, their industrial acceptance within the water industry is limited by lower surface areas and poorer adsorptive capacities as compared with commercial sorbents. We herein report a green, high performance porous carbon produced from boreal peats for organic micropollutant removal. Boreal peatlands are increasingly damaged due to climate change-induced wildfires and droughts, which lead to increased run-off and impeded forest regrowth. Fire-impacted peatland soils therefore were excavated and converted into value-added porous carbons through ZnCl 2 activation at low temperature (400 – 600 °C). These products have significantly higher surface areas (> 1377 m 2 /g) than commercial activated carbon Norit GSX (965 m 2 /g). Adsorption of p -nitrophenol, a micropollutant, onto the porous carbons is efficient, and superior to that of Norit GSX and most sorbents reported in the literature. Adsorption mainly occurred through multi-layer chemisorption and was impacted by the electron donor-acceptor complexes mechanism, π-π interactions and steric effects. Because of the massive environmental and economic benefits, peat porous carbons are strong candidates for use in large-scale water treatment facilities. • Simple and rapid synthesis of highly porous carbons from damaged peatland soils. • Peat porous carbons exhibit extraordinary removal for p -nitrophenol (> 530 mg/g). • Maximum adsorption capacity substantially greater than literature values. • Boreal peat porous carbons are eco-friendly high-performance bio-based sorbents for market use.
Abstract The combination of Cu(II) with peroxymonosulfate (PMS) (i.e., the Cu(II)/PMS system) synergistically inactivated P. aeruginosa cells in the planktonic state, and in biofilms grown on RO membranes. The enhanced bacterial inactivation by the Cu(II)/PMS system appears to be due to the reactive oxidants produced by the catalytic reactions of the Cu(II)/Cu(I) redox couple with PMS. In the presence of chloride ion (Cl−), the Cu(II)/PMS system showed increased microbicidal effects on the planktonic P. aeruginosa cells, which was explained by the role of hypochlorous acid (HOCl) produced by the reaction of chloride with PMS. In addition, the combination of Cu(II) with HOCl showed synergistic microbicidal effects on the planktonic cells. Compared to planktonic cells, biofilm cells were more resistant to the Cu(II)/PMS treatment. Cl− did not significantly affect the inactivation of biofilm cells by the Cu(II)/PMS system. It is believed that the extracellular polymeric substances of biofilms play a role as oxidant sinks (particularly HOCl), protecting the cells inside the biofilm matrix. The HOCl-generating systems, such as PMS/Cl− and Cu(II)/PMS/Cl−, greatly degraded proteins and polysaccharides in biofilms. Experiments on the cross-flow filtration of NaCl solution showed that the Cu(II)/PMS treatment of fouled RO membranes resulted in partial recovery of permeate flux.
Boreal peatlands provide critical global and regional ecosystem functions including climate regulation and nutrient and water retention. Wildfire represents the largest disturbance to these ecosystems. Peatland resilience depends greatly on the extent of post-fire peat soil hydrophobicity. Climate change is altering wildfire intensity and severity and consequently impacting post-fire peat soil chemistry and structure. However, research on fire-impacted peatlands has rarely considered the influence of peat soil chemistry and structure on peatland resilience. Here we characterized the geochemical and physical properties of natural peat soils under laboratory heating conditions. The general trend observed is that hydrophilic peat soils become hydrophobic under moderate heating and then become hydrophilic again after heating for longer, or at higher, temperatures. The loss of peat soil hydrophilicity initially occurs due to evaporative water loss (250 °C and 300 °C for <5 min). Gently but thoroughly dried peat soils (105 °C for 24 h) also show mass losses after heating, indicating the loss of organic compounds through thermal degradation. Gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared (FTIR) spectroscopy were used to characterize the chemistry of unburned and 300 °C burned peat soils, and various fatty acids, polycyclic compounds, saccharides, aromatic acids, short-chain molecules, lignin and carbohydrates were identified. We determined that the heat-induced degradation of polycyclic compounds and aliphatic hydrocarbons, especially fatty acids, caused dried, hydrophobic peat soils to become hydrophilic after only 20 min of heating at 300 °C. Furthermore, peat soils became hydrophilic more quickly (20 min vs 6 h) with an increase in heat from 250 °C to 300 °C. Minimal structural changes occurred, as characterized by BET and SEM analyses, confirming that surface chemistry, in particular fatty acid content, rather than structure govern changes in peat soil hydrophobicity.
Abstract Detecting the onset of membrane fouling is critical for effectively removing membrane foulants during microfiltration (MF) separation. This work investigates the use of electrical impedance spectroscopy (EIS) on the surface of electrically conductive membranes (ECMs) to measure early development of membrane surface fouling. An electrochemical cell was developed in which an ECM acted as a working electrode and a graphite electrode acted as the counter electrode. Conductive membranes were fabricated by coating single-walled/double-walled carbon nanotubes (f-SW/DWCNT) on microfiltration polyethersulfone (PES) supporting membranes. Membrane fouling was simulated by pressure depositing different amounts of latex beads onto the surface of the membrane in a dead-end filtration cell. Changes in membrane water permeability were correlated to the degree of membrane fouling. Clean membranes had water permeability of 392 ± 28 LMH/bar. Reduction of membrane water permeability of 13.8 ± 3.3%, 15.8 ± 4.7%, 17.8 ± 0.5% and 27.1 ± 4.6% were observed for membranes covered with 0.028 mg/m2, 0.28 mg/m2, 1.40 mg/m2 and 2.80 mg/m2 on the membranes, respectively. These small differences in fouling degree were statistically resolvable in measured Nyquist plots. It was observed that the diameter of the higher frequency charge transfer region (104–106 Hz) of the Nyquist plot semicircles increased with greater fouling. These observations were hypothesized to correspond to decreasing surface conductivities of the membranes by the incorporation of insulating materials (latex beads) within the porous conductive coating. This proposed hypothesis was supported by measured EIS results modeled with a theoretical equivalent circuit. Fouled membrane surface conductivity, surface hydrophilicity, and pore size were measured by SEM, four-point probe conductivity, contact angle, and MWCO experiments, respectively, to compare conventional characterization techniques with non-destructive EIS measurements.
Use of crystal nanocellulose to stabilize nano-ZVI has tremendous potential to improve the capability and applicability of nano-ZVI based subsurface remediation systems in an environmentally sustainable way.