Yiming He

Membrane fouling in a membrane bioreactor: High filtration resistance of gel layer and its underlying mechanism

A membrane bioreactor (MBR) was continuously operated to investigate mechanisms of fouling caused by the gel layer in this study. Agar was used as a model foulant for gel layer formation, and filtration resistance of gel layers was systematically assessed. The results showed that gel layer possessed unusually high specific filtration resistance (SFR) and high measured porosity as compared with cake layer. Current knowledge cannot explain the contradiction between high filtration resistance and high porosity of gel layer. A new fouling mechanism based on Flory-Huggins theory was then proposed. Filtration resistance of agar gel layer was found to be independent of pH and ionic strength, but linearly increase with gel thickness. The results are accordant with the mechanism deductions. Simulation of the mechanism model showed that the filtration resistance induced by mixing chemical potential variation was comparable to the experimental data of filtration resistance of agar gel layer, indicating that the proposed mechanism is the predominant mechanism responsible for the high filtration resistance of gel layer. The proposed mechanism was further verified from the bound water viewpoint.

Effects of hydrophilicity/hydrophobicity of membrane on membrane fouling in a submerged membrane bioreactor.

The interfacial interactions between sludge foulants and four different types of membranes were assessed based on a new combined calculation method. Effects of membrane surface hydrophilicity/hydrophobicity on the interfacial interactions were investigated. It was found that, membrane surface hydrophilicity/hydrophobicity was not directly relevant to the interfacial interactions with sludge particles. Increasing membrane surface zeta potential could significantly increase the strength of the electrostatic double layer (EL) interaction and the energy barrier. For membrane with a surface roughness of 300nm, the total interaction was continuously repulsive in the separation distance coverage of 0-4nm in this study. The results suggest that, under conditions in this study, designing membranes with a high zeta potential and certain roughness can significantly mitigate membrane fouling, whereas, the strategy of improving membrane surface hydrophilicity cannot alleviate sludge adhesion in the membrane bioreactor.

Fabrication and characterization of hollow CdMoO4 coupled g-C3N4 heterojunction with enhanced photocatalytic activity.

This research was designed for the first time to investigate the activities of CdMoO4/g-C3N4 heterojunction in photocatalytic degradation of rhodamine B (RhB) and converting CO2 to fuels. The composite was synthesized via a simple mixing-calcination method and characterized by various techniques including Brunauer-Emmett-Teller method (BET), X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and electrochemical method. The results showed that the introduction of CdMoO4 to g-C3N4 exerted little effect on the property of light absorption, but resulted in an increase in the BET surface area, which was beneficial for the adsorption of RhB. More importantly, formation of a hetero-junction structure between CdMoO4 and g-C3N4 significantly promoted the separation of electron-hole pairs and ultimately enhanced the photocatalytic activity. The optimal CdMoO4/g-C3N4 composite could degrade RhB 6.5 times faster than pure g-C3N4 under visible light irradiation. Meanwhile, the composite showed a CO2 conversion rate of 25.8 μmol h(-1) gcat(-1), which was 4.8 and 8.1 times higher than those of g-C3N4 and P25, respectively, under simulated sunlight irradiation. This work might represent an important step in simultaneous environmental protection and energy production by g-C3N4 based materials.

A new method for modeling rough membrane surface and calculation of interfacial interactions.

Membrane fouling control necessitates the establishment of an effective method to assess interfacial interactions between foulants and rough surface membrane. This study proposed a new method which includes a rigorous mathematical equation for modeling membrane surface morphology, and combination of surface element integration (SEI) method and the composite Simpsons approach for assessment of interfacial interactions. The new method provides a complete solution to quantitatively calculate interfacial interactions between foulants and rough surface membrane. Application of this method in a membrane bioreactor (MBR) showed that, high calculation accuracy could be achieved by setting high segment number, and moreover, the strength of three energy components and energy barrier was remarkably impaired by the existence of roughness on the membrane surface, indicating that membrane surface morphology exerted profound effects on membrane fouling in the MBR. Good agreement between calculation prediction and fouling phenomena was found, suggesting the feasibility of this method.

Synthesis and characterization of a ZrO2/g-C3N4 composite with enhanced visible-light photoactivity for rhodamine degradation

The visible-light-driven ZrO2/g-C3N4 hybrid photocatalysts were prepared by direct heating of ZrO2 and melamine. Compared to pure g-C3N4 or ZrO2, the synthesized ZrO2/g-C3N4 exhibited much higher photocatalytic activity for rhodamine (RhB) degradation under visible light irradiation. In order to reveal the origin of the high photoactivity, the ZrO2/g-C3N4 composites were characterized by various techniques including N2 adsorption, thermogravimetric analysis (TG), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), and electrochemical methods. The characterization results demonstrated that ZrO2 nanoparticles were well distributed on the surface of g-C3N4. Although the anchoring of ZrO2 on g-C3N4 increased the surface area and light absorption ability, the hetero-junctions formed between the two semiconductors which retarded the recombination of electrons and holes were believed to result in the enhanced photoactivity of the ZrO2/g-C3N4 composite. In addition, it was found that holes and ˙O2− generated in the photocatalytic process played a key role in RhB degradation over the ZrO2/g-C3N4 hybrids.

Quantification of interfacial interactions between a rough sludge floc and membrane surface in a membrane bioreactor

Interfacial interactions between foulants and membrane directly determine foulant adhesion and membrane fouling. In this study, surface of sludge foulant particles (flocs) was found to be rough, and could be modeled by a sinusoidal sphere function. A novel method, which combined surface element integration (SEI) method, differential geometry and composite Simpsons rule, was developed to quantify the interfacial interactions between the modeled rough floc surface and membrane surface. Application of the novel method in a membrane bioreactor (MBR) provides broad profiles of quantitative interactions with rough floc surface with separation distance. It is also found that increase in the scaled amplitude of floc surface significantly reduced the interaction strength. Derjaguins approximation (DA) can be regarded as a special case of the novel method, indicating the extensive application prospect of the novel method. The novel method for interaction calculation was verified to be correct and feasible. Finally, roles of the novel method in membrane fouling research were discussed.

Realization of quantifying interfacial interactions between a randomly rough membrane surface and a foulant particle

Quantification of interfacial interaction with randomly rough surface is the prerequisite to quantitatively understand and control the interface behaviors such as adhesion, flocculation and membrane fouling. In this study, it was found that membrane surface was randomly rough with obvious fractal characteristics. The randomly rough surface of membrane could be well reconstructed by the fractal geometry represented by a modified Weierstrass-Mandelbrot function. A novel method, which combined composite Simpsons approach, surface element integration method and approximation by computer programming, was developed. By using this method, this study provided the first realization of quantifying interfacial energy between randomly rough surface of membrane and a foulant particle. The calculated interactions with randomly rough surface of membrane were significantly different from those with smooth surface of membrane, indicating the significant effect of surface topography on interactions. This proposed method could be also potentially used to investigate various natural interface environmental phenomena.

One-step degradation of cellulose to 5-hydroxymethylfurfural in ionic liquid under mild conditions.

One-step conversion of cellulose to HMF (5-hydroxymethylfurfural) has been achieved by using metal chlorides (CrCl3, CuCl2, SnCl4, WCl6) in [BMIM]Cl. The effects of temperature, reaction time, amount of catalysts, and the purity of [BMIM]Cl on the performance have been studied and discussed in detail. More than 63% yield of HMF and 80% yield of TRS (total reducing sugar) were obtained in [BMIM]Cl with CrCl3 at 120 °C under atmospheric pressure. Filter paper and cotton were also used as a source for cellulose degradation to HMF, but only a moderate yield of HMF was obtained (40% for filter paper and 12% for cotton). The reutilization of this system was examined and the reaction mechanism was also discussed.

Photodegradation of RhB over YVO4/g-C3N4 composites under visible light irradiation

A series of novel YVO4/g-C3N4 photocatalysts were prepared by a facile mixing and calcination method. The obtained composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, ultraviolet visible diffuse reflection spectroscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and a photocurrent–time experiment. The rhodamine B dye was selected as a model pollutant to evaluate the photocatalytic activity of the as-prepared YVO4/g-C3N4 composite. It shows that the photocatalytic activity of g-C3N4 can be largely improved by the doping of YVO4. The optimal YVO4 content is determined to be 25.8 wt%; and the corresponding degradation rate is 2.34 h−1, about 2.75 folds that of pure g-C3N4. A possible mechanism of YVO4 on the enhancement of visible light performance is proposed. It suggests that YVO4 plays a key role, which may lead to efficiently suppressing the recombination of photogenerated charge carriers, consequently, improving the visible light photoactivity.

Fractal reconstruction of rough membrane surface related with membrane fouling in a membrane bioreactor.

In this paper, fractal reconstruction of rough membrane surface with a modified Weierstrass-Mandelbrot (WM) function was conducted. The topography of rough membrane surface was measured by an atomic force microscopy (AFM), and the results showed that the membrane surface was isotropous. Accordingly, the fractal dimension and roughness of membrane surface were calculated by the power spectrum method. The rough membrane surface was reconstructed on the MATLAB platform with the parameter values acquired from raw AFM data. The reconstructed membrane was much similar to the real membrane morphology measured by AFM. The parameters (including average roughness and root mean square (RMS) roughness) associated with membrane morphology for the model and real membrane were calculated, and a good match of roughness parameters between the reconstructed surface and real membrane was found, indicating the feasibility of the new developed method. The reconstructed membrane surface can be potentially used for interaction energy evaluation.