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Professor Sun Shipeng's group at Nanjing University of Technology has made new progress in the research direction of photocatalytic self-cleaning nanofiltration membranes
2019-12-26 Source: Polymer Technology

Nanofiltration membranes can effectively separate and recover small molecules due to their special pore size range, which makes them have good application prospects in high value-added industries such as medicine and petrochemicals. However, there are two problems in the application of nanofiltration membranes: the "trade-off" effect between the permeability and selectivity of the membrane and the flux attenuation caused by membrane fouling, including reversible concentration polarization And irreversible membrane fouling. Unlike reverse osmosis, the separation mechanism of nanofiltration membranes is more complicated, including the Donnan effect and pore size screening. Therefore, research on how to effectively reduce the flux reduction caused by concentration polarization of nanofiltration membranes is relatively slow. At present, Is still in its infancy. Due to the affinity between the organic nanofiltration membrane structure itself and organic pollutants, the surface of the nanofiltration membrane is easily contaminated when organic wastewater is treated. At present, researchers mainly focus on improving the hydrophilicity of the membrane to improve the antifouling performance of the nanofiltration membrane, but the improvement of the hydrophilicity can only alleviate the membrane pollution to a certain extent and cannot fundamentally solve the problem.

Aiming at the key problems in the operation of nanofiltration membranes, Professor Sun Shipeng's team successfully combined the membrane structure design and functional modification to successfully prepare a dual-function nanofiltration membrane with both concentration reduction and photocatalytic self-cleaning performance. . In order to avoid the heavy adsorption of pollutants due to the high specific surface area of the catalyst, the authors use carbon quantum dots with photocatalytic activity as an intermediate layer to make the nanofiltration membrane self-cleaning under visible light. In order to eliminate the adverse effects of the addition of sodium hydroxide to the interfacial polymerization process during the activation of carbon quantum dots, the authors used self-polymerized dopamine under alkaline conditions as an absorbent of sodium hydroxide, so as to achieve the dopamine and carbon quantum dot The synergistic regulation of the nanofiltration membrane makes the nanofiltration membrane have self-cleaning ability under visible light, and has a certain performance of alleviating concentration polarization.

Fig. 1 Schematic diagram of carbon quantum dots and dopamine synergistically regulated self-cleaning nanofiltration membranes

Figure 2 Surface morphology of different films. (A) TFCs; (b) CQDs-TFCs; (c) PDA-CQDs-TFCs; (d) Performance comparison chart of different films.

The author investigated the interfacial polymerization process and analyzed the causes of different morphologies from the reaction mechanism. By adjusting different experimental conditions and theoretical calculations, analyzing the surface morphology and corresponding performance of different membranes, it was confirmed that the rough membrane surface structure will increase the concentration polarization phenomenon of the nanofiltration membrane. Through the synergistic control of dopamine and carbon quantum dots to control the interfacial polymerization process, the formation of a nanofiltration membrane with a smoother surface can alleviate the flux attenuation caused by concentration polarization to a certain extent.

Figure 3 Structural analysis of different membranes. (a) potential; (b) pore size; (c) crystallinity; (d) surface wettability.

By analyzing a series of surface properties and internal structure changes of the nanofiltration membrane before and after modification, and combining the performance changes of the nanofiltration membrane before and after modification, the team proposed a possible mechanism for the influence of the structure of the nanofiltration membrane on the concentration polarization phenomenon. . Nanofiltration membranes rely on mutual repulsion between charged ions and common ions on the membrane surface to achieve salt separation. When the membrane surface is relatively smooth, the charged ions near the membrane surface can quickly diffuse into the bulk solution. When the membrane surface is relatively rough, due to the intricate structure and the Donnan effect, charged ions will gather in the rough structure, which will increase the osmotic pressure near the membrane surface, reduce the effective transmembrane pressure difference, and then increase the concentration difference.化 phenomenology.

Fig. 4.Effects of different structured film surfaces on the concentration polarization phenomenon.

The team further investigated the effect of carbon quantum dots as an intermediate layer on the photocatalytic performance of the film. It was found that for two different dyes with similar molecular weights and opposite electrical properties, methylene blue and golden orange II, due to the influence of the Donan effect, the film showed better photocatalytic degradation ability for methylene blue with opposite electrical properties. In addition, the film showed stable self-cleaning performance after two light exposures.

Figure 5 Comparison of the stability of different films and the surface pollution of the films before and after light exposure.

Related results were published in the well-known journal ACS Applied Materials & Interfaces under the title of "Self-cleaning Nanofiltration Membranes by Coordinated Regulation of Carbon Quantum Dots and Polydopamine". The first author of the thesis is Shao Dandan , a doctoral student in the School of Chemical Engineering and Technology of Nanjing University of Technology, and the corresponding author is Professor Sun Shipeng .

The above work was supported by the National Youth Thousand Talents Program, Jiangsu Outstanding Youth Fund, and the National Natural Science Foundation of China.

Article link: http://pubs.acs.org/doi/abs/10.1021/acsami.9b16704

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