Xilin Wu

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.

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.

Bamboo-like carbon nanotubes derived from colloidal polymer nanoplates for efficient removal of bisphenol A

Bamboo-like carbon nanotubes (b-CNTs) were synthesized via a low-cost one-pot approach by using FeCl3 and self-assembled colloidal polymer nanoplates as precursors. The resulting b-CNTs were applied as both the adsorbent and the catalyst for the removal of bisphenol A (BPA) from aqueous solutions. The maximum adsorption capacity of BPA on the b-CNTs calculated by using the Langmuir model is 328.6 mg g−1 at 303 K. Thermodynamics studies revealed that the adsorption was spontaneous and exothermic. With the synergistic effect of adsorption and catalysis by the b-CNTs, BPA (70 mg L−1) was almost completely removed (over 97%) at pH 7.0 within 80 min after the adsorption–oxidative degradation process. The b-CNTs also displayed an excellent reusability for the catalytic oxidation of BPA (initial concentration of 70 mg L−1) with the degradation ratio remaining at 84% after 5 cycles. Moreover, the b-CNT catalysts can be easily recycled by using an external magnet and regenerated by simply washing. This study not only presents a cost-effective method of using magnetic nanocarbons for the cleanup of pollutants, but also demonstrates the potential application of the b-CNTs as both an adsorbent and a catalyst.

Enhanced visible-light-driven photocatalysis from WS2 quantum dots coupled to BiOCl nanosheets: synergistic effect and mechanism insight

Quantum dots (QDs) derived from two-dimensional (2D) nanosheets (NSs), especially ultrathin transition metal dichalcogenide (TMD, e.g. WS2, WSe2, MoS2, MoSe2) NSs, have attracted great attention due to their broad band absorption and high charge mobility. Herein, WS2 QDs, one of the emerging extraordinary zero-dimensional (0D) TMD materials, were applied to the preparation of novel 0D/2D heterojunctions of WS2 QDs/BiOCl nanosheets. The obtained WS2/BiOCl composites exhibited significantly enhanced visible-light-driven photocatalytic activity as compared with pure BiOCl. The results indicated that the holes (h+) and O2˙− are the main active species generated by the catalysts under visible light irradiation. The enhanced photocatalytic performance could be due to the broad band absorption and up-conversion properties of the WS2 QDs as well as the band alignment and the strong coupling between the WS2 QDs and BiOCl NSs, leading to a broadened light absorption range and enhanced efficiency for electron–hole pair production and separation. These findings offer exciting opportunities using the extraordinary 2D TMD material-derived quantum dots for the fabrication of novel 0D/2D composites and may provide new insights into the application of the novel 0D/2D composites in catalysis.

Magnetic ZnFe2O4@chitosan encapsulated in graphene oxide for adsorptive removal of organic dye

Graphene-based multifunctional composites were prepared by encapsulating of magnetic ZnFe2O4@chitosan (ZnFe2O4@CS) particles into graphene oxide (GO) layers. The obtained ZnFe2O4@CS/GO were characterized using scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Adsorption of basic fuchsin (BF) onto the ZnFe2O4@CS/GO was investigated using pH, adsorption time, initial BF concentration and temperature. Kinetic data and adsorption isotherms were better fitted by a pseudo-second-order kinetic model and Langmuir isotherm model, respectively. The values of activation parameters such as free energy (ΔG, −14.02 to −16.56 kJ mol−1), enthalpy (ΔH, 23.27 kJ mol−1) and entropy (ΔS, 127.75 J mol−1 K−1) were calculated, suggesting that the adsorption was a spontaneous, favorable and endothermic process in nature. The results demonstrated that the ZnFe2O4@CS/GO composites are potentially suitable materials for the removal of organic dyes from large volumes of wastewater.

Quantitative evaluation of the interfacial interactions between a randomly rough sludge floc and membrane surface in a membrane bioreactor based on fractal geometry

In this paper, a new method for quantification of interfacial interactions between a randomly rough particle and membrane surface was proposed. It was found that sludge flocs in a membrane bioreactor were of apparent fractal characteristics, and could be modeled by the modified two-variable Weierstrass-Mandelbrot (WM) function. By combining the surface element integration (SEI) method, differential geometry and composite Simpsons rule, the quantitation method for calculating such interfacial interactions was further developed. The correctness and feasibility of the new method were verified. This method was then applied to evaluate the interfacial interactions between a randomly rough particle and membrane surface. It was found that, randomly rough particle possesses stronger interaction strength than regularly rough particle but weaker strength than smooth particle with membrane surface, indicating significant effects of surface morphology and roughness. The proposed method in this study has broad application prospect in membrane fouling study.

New insights into bisphenols removal by nitrogen-rich nanocarbons: Synergistic effect between adsorption and oxidative degradation

In this work, nitrogen-rich graphene-like carbon sheets (N-GLCS) with high specific surface area (488.4m2/g), narrow pore distribution and high N-doping (18.4 at%) were prepared and applied as both adsorbent and catalyst for the removal of bisphenols. Adsorption experiments demonstrated the high adsorption capacities of the N-GLCS toward bisphenol F (BPF) (222.9mg/g), bisphenol A (BPA) (317.8mg/g), and bisphenol C (BPC) (540.4mg/g). Results showed that about 98.6% of BPA (70mg/L) was removed at pH 7.0 within 80min after the adsorption-catalytic degradation process. The N-GLCS also showed a superb reusability for the catalytic oxidative degradation of BPA (70mg/L) with the removal percentage maintains over 83% after 5 cycles. With the synergistic combination of the excellent adsorption and catalytic properties of the N-GLCS, trace amount of pollutants can be preconcentrated and immobilized at the surface of N-GLCs, at the same time, active radicals were also produced at the surface of the N-GLCS by the activation of peroxydisulfate (PS), and finally the pollutants can be degraded in-situ by the active radicals. These findings provide a new avenue towards the efficient removal of trace-level EDCs from water solution by using the coupled adsorption-advanced oxidation processes.

Nitrogen Doped Nanoporous Carbon Derived from Zizania Latifolia for Adsorptive Removal of Bisphenol A

Nitrogen doped nanoporous activated carbon (N-NPAC) was prepared via the facile and effective KOH activation method using Zizania latifolia (ZL), a common Chinese aquatic vegetable, as the raw material. The biomass derived N-NPAC exhibited high content of nitrogen (18.4 at%), large surface area (1493.4 m²/g) and abundant nanopores. The unique physical-chemical structure endows the N-NPAC with great application potential in adsorbents. The performance of the N-NPAC for the adsorptive removal of bisphenol A (BPA) was studied. The results showed the adsorption processes were barely affected by solution pH. The adsorption kinetics are well-fitted by the pseudo-second-order kinetic model and the adsorption isotherms followed the Langmuir isotherm model. The maximum adsorption capacity calculated by the Langmuir isotherm model is 555.5 mg/g at 313 K, demonstrating the promise of the N-NPAC for the application in water cleanup. This study provides an example using the inexpensive and abundantly available biomass as the raw materials for the large scale production of nanocarbons and paves an avenue for the development of bio-derived nanomaterials.

Characterization of molybdenum disulfide nanomaterial and its excellent sorption abilities for two heavy metals in aqueous media

ABSTRACT Here, molybdenum sulfide (MoS2) was characterized and its performance as an adsorbent for Co(II)/Ni(II) removal from water was estimated in batch experiments. The nanomaterial exhibits thin-layered structure and contains numerous hydroxyl groups. High pH enhanced Co(II)/Ni(II) sorption, while ionic strength had no effect. The chemical sorption between the binding sites of MoS2 and Co(II)/Ni(II) limited the rate. The Freundlich isotherm correlated better than Langmuir isotherm. The maximum sorption capacity for Co(II)/Ni(II) was 370.10/375.94 mg· g−1, obviously exceeding other adsorbents. Both adsorptions were endothermic and spontaneous processes. This study indicates the intrinsic value of MoS2 in removing heavy metals from water.

Application of biochar derived from rice straw for the removal of Th(IV) from aqueous solution

ABSTRACT Biochar is increasingly used as a low-cost and effective adsorbent for heavy metals in wastewater. Herein, biochar pyrolyzed from rice straw was employed as an adsorbent for the removal of Th(IV) from aqueous solutions. The sorption of Th(IV) on biochar was strongly dependent on pH, but independent on ionic strength at pH < 6.4. The inner-sphere complexation dominated the sorption mechanism of Th(IV) on biochar. The competition for Th(IV) between aqueous or surface-adsorbed cations/anions and functional groups of biochar was pivotal for Th(IV) sorption. The thermodynamic data suggested that Th(IV) sorption was a spontaneous and endothermic process.