Influences of microwave irradiation on performances of membrane filtration and catalytic degradation of perfluorooctanoic acid (PFOA).

Perfluorooctanoic Acid (PFOA), one of the common per- and poly fluorinated alkylated substances (PFASs), is increasingly detected in the environment due to the diverse industrial applications and high resistance to degradation processes. This study evaluated degradation of PFOA in microwave-assistant catalytic membrane filtration, a process that integrates microwave catalytic reactions into a ceramic membrane filtration. First, water permeation of the pristine and catalyst-coated membranes were examined under the influence of microwave irradiation to analyse the impacts of the coating layer and water temperature increase on permeate flux, which were well interpreted by the Carman-Kozeny and Hagen-Posieulle (non-slipping and slit-like) models. Then, the PFOA removal was first assessed in a continuous filtration mode with and without microwave irradiation. Our results show that PFOA first adsorbed on membrane and catalyst materials, and then fully penetrated the membrane filter after reaching adsorption equilibrium. Under microwave irradiation (7.2 W·cm-2), approximate 65.9% of PFOA (25 µg·L-1) in the feed solution was degraded within a hydraulic time of 2 min (at the permeate flow rate of 43 LMH) due to the microwave-Fenton like reactions. In addition, low flow rates and moderate catalyst coating densities are critical for optimizing PFOA removal. Finally, potential degradation mechanisms of PFOA were proposed through the analysis of degradation by-products (e.g., PFPeA). The findings may provide new insight into the development of reactive membrane-enabled systems for destruction of refractory PFAS.

Microalgae Filtration Using an Electrochemically Reactive Ceramic Membrane: Filtration Performances, Fouling Kinetics, and Foulant Layer Characteristics.

Electrochemical membrane filtration has proven to be successful for microbial removal and separation from water. In addition, membrane fouling could be mitigated by electrochemical reactions and electrostatic repulsion on a reactive membrane surface. This study assessed the filtration performances and fouling characteristics of electrochemically reactive ceramic membranes (a Magneli phase suboxide of TiO2) when filtering algal suspension under different dc currents to achieve anodic or cathodic polarization. The critical flux results indicate that when applying positive or negative dc currents (e.g., 1.25-2.5 mA·cm-2) to the membrane, both significantly mitigated membrane fouling and thus maintained higher critical fluxes (up to 14.6 × 10-5·m3·m-2·s-1 or 526 LMH) compared to the critical flux without dc currents. Moreover, applying dc currents also enhanced membrane defouling processes and recovered high permeate flux better than hydraulic and chemical backwash methods. Moreover, fouling kinetics and the cake layer formation were further analyzed with a resistance-in-series model that revealed many important but underexamined parameters (e.g., cake layer resistance and cake layer thickness). The cake layer structures (e.g., compressibility) were shown to vary with the electrochemical activity, which provide new insight into the biofouling mechanisms. Finally, the algogenic odor, geosmin, was shown to be effectively removed by this reactive membrane under positive dc currents (2.5 mA·cm-2), which highlights the multifunctional capabilities of electrochemically reactive membrane filtration in biomass separation, fouling prevention, and pollutant degradation.

Reactive Photo-Fenton ceramic membranes: Synthesis, characterization and antifouling performance.

To develop reactive and antifouling membrane filtration systems, a photo-Fenton ceramic membrane was prepared by coating goethite (α-FeOOH) catalysts on a zirconia/titania alumina membrane via a cross-linking method. Scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize α-FeOOH catalysts and the surface coating quality. The cross linker yielded stable covalent binding between catalyst and membrane under room temperature and produced a homogeneous and smooth coating of catalyst on ceramic membranes. Photo-Fenton reactions were initiated with addition of H2O2 under UV irradiation to improve the foulant degradation on membrane surface while filtration. Membrane fouling was simulated by bovine serum albumin (BSA) and humic acid (HA). Our results show that the photo-Fenton reactions on the coated membranes slowed down the fouling kinetics and even reversed the fouling, leading to a stable transmembrane pressure (TMP) over time of filtration, as opposed to a monotonous increase of TMP due to surface fouling. The batch experiments verified that the photo-Fenton reactions achieved the degradation rates of 76% and 86% for HA and BSA respectively within 60 min, with the mineralization rates of over 80% as indicated by the total organic carbon measurement. This study embarks on a novel antifouling membrane filtration process via incorporation of photo-Fenton reactions. The findings are also important for diverse applications of surface fouling mitigation and rationale design of fouling resistant surfaces or materials through photo-Fenton or other catalytic reactions.

Influences of Air, Oxygen, Nitrogen, and Carbon Dioxide Nanobubbles on Seed Germination and Plant Growth.

Nanobubbles (NBs) hold promise in green and sustainable engineering applications in diverse fields (e.g., water/wastewater treatment, food processing, medical applications, and agriculture). This study investigated the effects of four types of NBs on seed germination and plant growth. Air, oxygen, nitrogen, and carbon dioxide NBs were generated and dispersed in tap water. Different plants, including lettuce, carrot, fava bean, and tomato, were used in germination and growth tests. The seeds in water-containing NBs exhibited 6-25% higher germination rates. Especially, nitrogen NBs exhibited considerable effects in the seed germination, whereas air and carbon dioxide NBs did not significantly promote germination. The growth of stem length and diameter, leave number, and leave width were promoted by NBs (except air). Furthermore, the promotion effect was primarily ascribed to the generation of exogenous reactive oxygen species by NBs and higher efficiency of nutrient fixation or utilization.

Generation of nanobubbles by ceramic membrane filters: The dependence of bubble size and zeta potential on surface coating, pore size and injected gas pressure.

Generation of gaseous nanobubbles (NBs) by simple, efficient, and scalable methods is critical for industrialization and applications of nanobubbles. Traditional generation methods mainly rely on hydrodynamic, acoustic, particle, and optical cavitation. These generation processes render issues such as high energy consumption, non-flexibility, and complexity. This research investigated the use of tubular ceramic nanofiltration membranes to generate NBs in water with air, nitrogen and oxygen gases. This system injects pressurized gases through a tubular ceramic membrane with nanopores to create NBs. The effects of membrane pores size, surface energy, and the injected gas pressures on the bubble size and zeta potential were examined. The results show that the gas injection pressure had considerable effects on the bubble size, zeta potential, pH, and dissolved oxygen of the produced NBs. For example, increasing the injection air pressure from 69 kPa to 414 kPa, the air bubble size was reduced from 600 to 340 nm respectively. Membrane pores size and surface energy also had significant effects on sizes and zeta potentials of NBs. The results presented here aim to fill out the gaps of fundamental knowledge about NBs and development of efficient generation methods.

Visible-Light-Responsive Graphitic Carbon Nitride: Rational Design and Photocatalytic Applications for Water Treatment.

Graphitic carbon nitride (g-C3N4) has recently emerged as a promising visible-light-responsive polymeric photocatalyst; however, a molecular-level understanding of material properties and its application for water purification were underexplored. In this study, we rationally designed nonmetal doped, supramolecule-based g-C3N4 with improved surface area and charge separation. Density functional theory (DFT) simulations indicated that carbon-doped g-C3N4 showed a thermodynamically stable structure, promoted charge separation, and had suitable energy levels of conduction and valence bands for photocatalytic oxidation compared to phosphorus-doped g-C3N4. The optimized carbon-doped, supramolecule-based g-C3N4 showed a reaction rate enhancement of 2.3-10.5-fold for the degradation of phenol and persistent organic micropollutants compared to that of conventional, melamine-based g-C3N4 in a model buffer system under the irradiation of simulated visible sunlight. Carbon-doping but not phosphorus-doping improved reactivity for contaminant degradation in agreement with DFT simulation results. Selective contaminant degradation was observed on g-C3N4, likely due to differences in reactive oxygen species production and/or contaminant-photocatalyst interfacial interactions on different g-C3N4 samples. Moreover, g-C3N4 is a robust photocatalyst for contaminant degradation in raw natural water and (partially) treated water and wastewater. In summary, DFT simulations are a viable tool to predict photocatalyst properties and oxidation performance for contaminant removal, and they guide the rational design, fabrication, and implementation of visible-light-responsive g-C3N4 for efficient, robust, and sustainable water treatment.

Effects of anodic oxidation of a substoichiometric titanium dioxide reactive electrochemical membrane on algal cell destabilization and lipid extraction.

Efficient algal harvesting, cell pretreatment and lipid extraction are the major steps challenging the algal biofuel industrialization. To develop sustainable solutions for economically viable algal biofuels, our research aims at devising innovative reactive electrochemical membrane (REM) filtration systems for simultaneous algal harvesting and pretreatment for lipid extraction. The results in this work particularly demonstrated the use of the Ti4O7-based REM in algal pretreatment and the positive impacts on lipid extraction. After REM treatment, algal cells exhibited significant disruption in morphology and photosynthetic activity due to the anodic oxidation. Cell lysis was evidenced by the changes of fluorescent patterns of dissolved organic matter (DOM) in the treated algal suspension. The lipid extraction efficiency increased from 15.2 ± 0.6 g-lipidg-algae(-1) for untreated algae to 23.4 ± 0.7 g-lipidg-algae(-1) for treated algae (p<0.05), which highlights the potential to couple algal harvesting with cell pretreatment in an integrated REM filtration process.