Nan Zhang


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Integrating biofouling sensing with fouling mitigation in a two-electrode electrically conductive membrane filtration system
Nan Zhang, Hye-Jin Lee, Yichen Wu, Mohamed A. Ganzoury, Charles‐François de Lannoy
Separation and Purification Technology, Volume 288

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.


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Simultaneous Dechlorination and Advanced Oxidation Using Electrically Conductive Carbon Nanotube Membranes
Hye-Jin Lee, Nan Zhang, Mohamed A. Ganzoury, Yichen Wu, Charles‐François de Lannoy
ACS Applied Materials & Interfaces, Volume 13, Issue 29

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.

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Upcycling wildfire-impacted boreal peats into porous carbons that efficiently remove phenolic micropollutants
Yichen Wu, Nan Zhang, Charles‐François de Lannoy
Journal of Environmental Chemical Engineering, Volume 9, Issue 4

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.


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Hydrophobicity of peat soils: Characterization of organic compound changes associated with heat-induced water repellency
Yichen Wu, Nan Zhang, G. F. Slater, J. M. Waddington, Charles‐François de Lannoy
Science of The Total Environment, Volume 714

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.

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Detection of fouling on electrically conductive membranes by electrical impedance spectroscopy
Nan Zhang, Mohamad Amin Halali, Charles‐François de Lannoy
Separation and Purification Technology, Volume 242

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.