Leihong Zhao

Enhanced photodegradation activity of methyl orange over Z-scheme type MoO3–g-C3N4 composite under visible light irradiation

Novel Z-scheme type MoO3–g-C3N4 composites photocatalysts were prepared with a simple mixing–calcination method, and evaluated for their photodegradation activities of methyl orange (MO). The optimized MoO3–g-C3N4 photocatalyst shows a good activity with a kinetic constant of 0.0177 min−1, 10.4 times higher than that of g-C3N4. Controlling various factors (MoO3–g-C3N4 amount, initial MO concentration, and pH value of MO solution) can lead to the enhancement of the photocatalytic activity of the composite. Only MoO3 and g-C3N4 are detected with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) spectra. N2 adsorption and UV-vis diffuse reflectance spectroscopy (DRS) results suggest that the addition of MoO3 slightly affects the specific surface area and the photoabsorption performance. The transmission electron microscopy (TEM) image of MoO3–g-C3N4 indicates a close contact between MoO3 and g-C3N4, which is beneficial to interparticle electron transfer. The high photocatalytic activity of MoO3–g-C3N4 is mainly attributed to the synergetic effect of MoO3 and g-C3N4 in electron–hole pair separation via the charge migration between the two semiconductors. The charge transfer follows direct Z-scheme mechanism, which is proven by the reactive species trapping experiment and the ˙OH-trapping photoluminescence spectra.

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.

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.

Novel indicators for thermodynamic prediction of interfacial interactions related with adhesive fouling in a membrane bioreactor.

This study focused on developing indicators to predict adhesive membrane fouling in a membrane bioreactor (MBR). Thermodynamic interactions between membrane surface and foulants in various interaction scenes were comprehensively evaluated. It was revealed that, the total interaction energy in contact could be considered as a critical value affecting adhesion of foulants. Surface hydrophilicity cannot be simply represented by water contact angle. Statistical analysis showed that membrane acid-based (AB) surface tension, Lifshitz-Van der waals (LW) surface tension, total tension, zeta potential and water contact angle had no apparent correlation with adhesive fouling, suggesting the infeasibility of these parameters as fouling predictors. It was found that, interaction between two identical membrane surface in water (ΔGsws) and membrane surface electron donor tension (γ-) strongly correlated with adhesive fouling, and could be reliable indicators to predict adhesive fouling. This study identified the relationships of series membrane surface properties with adhesive fouling in MBRs.

Effects of surface charge on interfacial interactions related to membrane fouling in a submerged membrane bioreactor based on thermodynamic analysis

Effects of both membrane and sludge foulant surface zeta potentials on interfacial interactions between membrane and sludge foulant in different interaction scenarios were systematically investigated based on thermodynamic methods. Under conditions in this study, it was found that zeta potential had marginal effects on total interfacial interaction between two infinite planar surfaces, and the total interfacial interaction between foulant particles and membrane would be more repulsive with increase of absolute value of zeta potential. Adhesion of foulant particles on membrane surface should overcome an energy barrier. There exists a critical zeta potential below which energy barrier would disappear. Results also showed that rough surface membrane corresponded to significantly low strength of interfacial interactions. This study not only provided a series of methods to quantitatively assess the interfacial interactions between membrane and sludge foulants, but also reconciled the contradictory conclusions regarding effects of zeta potential in literature, giving important implications for membrane fouling mitigation.

Fabrication, characterization and photocatalytic activity of g-C3N4 coupled with FeVO4 nanorods

A novel FeVO4/g-C3N4 composite photocatalyst was synthesized via a simple mixing-calcination method and characterized by various techniques including the Brunauer–Emmett–Teller method, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and an electrochemical method. The photocatalytic activity was evaluated by degradation of rhodamine B (RhB). Results indicated that the FeVO4/g-C3N4 composite exhibited a much higher photocatalytic activity than pure g-C3N4 under visible light illumination. The rate constant of RhB degradation for the optimal FeVO4/g-C3N4 composite (5.0% FeVO4/g-C3N4) is approximately 2.2 times that of pure g-C3N4. The formation of a heterojunction structure between FeVO4 and g-C3N4 which efficiently promoted the separation of electron–hole pairs was believed to be the origin of the enhanced photoactivity.

Membrane fouling in a submerged membrane bioreactor with focus on surface properties and interactions of cake sludge and bulk sludge

In this study, the fouling behaviors and surface properties of cake sludge and bulk sludge in a submerged membrane bioreactor (MBR) were investigated and compared. It was found that the specific filtration resistance (SFR) of cake sludge was about 5 times higher than that of bulk sludge. Two types of sludge possessed similar extracellular polymeric substances (EPS) content, particle size distribution (PSD) and zeta potential. However, their surface properties in terms of surface tensions were significantly different. Further analysis showed that cake sludge was more hydrophilic and had worse aggregation ability. Moreover, cake sludge surface possessed more hydrocarbon, less oxygen and nitrogen moieties than bulk sludge surface. It was suggested that, rather than EPS and PSD differences, the differences in the surface composition were the main cause of the great differences in SFR and adhesion ability between cake sludge and bulk sludge.

Modeling three-dimensional surface morphology of biocake layer in a membrane bioreactor based on fractal geometry

While the adsorptive fouling in membrane bioreactors (MBRs) is highly dependent of the surface morphology, little progress has been made on modeling biocake layer surface morphology. In this study, a novel method, which combined static light scattering method for fractal dimension (Df) measurement with fractal method represented by the modified two-variable Weierstrass-Mandelbrot function, was proposed to model biocake layer surface in a MBR. Characterization by atomic force microscopy showed that the biocake surface was stochastic, disorder, self-similarity, and with non-integer dimension, illustrating obvious fractal features. Fractal dimension (Df) of sludge suspension experienced a significant change with operation of the MBR. The constructed biocake layer surface by the proposed method was quite close to the real surface, showing the feasibility of the proposed method. It was found that Df was the critical factor affecting surface morphology, while other factors exerted moderate or minor effects on the roughness of biocake layer.

Influences of acid-base property of membrane on interfacial interactions related with membrane fouling in a membrane bioreactor based on thermodynamic assessment.

Failure of membrane hydrophobicity in predicting membrane fouling requires a more reliable indicator. In this study, influences of membrane acid base (AB) property on interfacial interactions in two different interaction scenarios in a submerged membrane bioreactor (MBR) were studied according to thermodynamic approaches. It was found that both the polyvinylidene fluoride (PVDF) membrane and foulant samples in the MBR had relatively high electron donor (γ(-)) component and low electron acceptor (γ(+)) component. For both of interaction scenarios, AB interaction was the major component of the total interaction. The results showed that, the total interaction monotonically decreased with membrane γ(-), while was marginally affected by membrane γ(+), suggesting that γ(-) could act as a reliable indicator for membrane fouling prediction. This study suggested that membrane modification for fouling mitigation should orient to improving membrane surface γ(-) component rather than hydrophilicity.