Research On Surface Photocatalyitc Modification Of Microfiltration Membrane Based On Atomic Layer Deposition And Characteristic For Membrane Fouling

The membrane separation technology for wastewater treatment is widely used in various fields of water treatment for its superior performance. It’s the dominant technology for efficient water treatment in our country. However, membrane fouling has hindered the further development and application of membrane technology for water treatment. The membrane photocatalytic modification can combine membrane material and photocatalysts and achieve effective membrane fouling control through photocatalytic degradation of membrane foulants. In this research, the modified membrane material with superior photocatalytic and membrane anti-fouling properties was acquired by membrane photocatalytic modification based on atomic layer deposition(ALD), which could be controlled precisely and deposit materials uniformly and stably in ALD deposition process.In this research, PVDF microfiltration membrane and Zn O were used as base material and photocatalyst, respectively. The optimization control of membrane photocatalytic modification was achieved by adjustment and control of parameters. The performance characteristics of Zn O-ALD photocatalytic modified membrane were characterized and the key technical advantages of Zn O-ALD photocatalytic modification were determined. The influence of Zn O-ALD photocatalytic modified microfiltration membrane on static adsorption and dynamic retention of pollutants was studied, the control effectiveness and mechanism of delaying membrane fouling was confirmed.Four key parameters of Zn O-ALD modification process including alkaline solution pretreatment time, deposition chamber temperature, deposition cycles and impulse waiting time were discussed. Their influences on the deposition layer quality and bonding strength, hydrophilic and hydrophobic properties, flow properties, membrane anti-fouling absorption capacity, photocatalytic performance of the membrane were investigated. And the process of ALD photocatalytic membrane modification was optimized. It was concluded that the optimal Zn O-ALD modification control conditions were as follows: Alkaline solution pretreatment time was 120 min, deposition chamber temperature was 100℃, ALD deposition cycles was 250 and ALD impulse waiting time was 5s. As a basis for comprehensive characterization of the functional performance of photocatalytic modified membrane, the key technical advantages were determined. The results showed that, a thin wurtzite phase Zn O modified layer was deposited on the surface of PVDF microfiltration membrane after the modification process. And the Zn element was all in the form of Zn2+ and O element was mainly in the form of O2- of wurtzite phase lattice. Although the modified porosity decreased by 5.3%, the membrane retention capacity was improved and surface roughness was decreased. In conclusion, all aspects of stability performance of Zn O-ALD photocatalytic modified microfiltration membrane were improved significantly. Among them, the tensile strengths, breaking elongation and thermal decomposition temperature increased substantially, while the decay degree of membrane flux decreased significantly. Zn O-ALD photocatalytic modified microfiltration membrane showed superior photocatalytic performance and excellent response under the xenon lamp light. The photocatalytic degradation efficiencies of modified microfiltration membrane on different concentration of MB solution under UV condition were investigated. It was found that the degradation efficiencies were greater than 90% after 100 min irradiation. In addition, the photocatalytic degradation efficiencies of modified microfiltration membrane on 5mg/L of MB solution under three light sources of a mercury lamp, an incandescent lamp and a xenon lamp were investigated, respectively. The results indicated that the degradation efficiencies were 82.5%, 19.0% and 97.3% after 60 min irradiation, respectively.The influences of Zn O-ALD photocatalytic modified microfiltration membrane on static adsorption of pollutants and components and characteristics of membrane surface cake layer after the retention were studied. Furthermore, the mechanism of delaying membrane fouling was analyzed. It was indicated that Zn O-ALD photocatalytic modification reduced the adsorption capacity of microfiltration membrane on key pollutants as polysaccharides and proteins in EPS. And the decay degree of membrane flux after membrane fouling was decreased by improving the filtration resistance of microfiltration membrane before and after membrane fouling. In addition, Zn O-ALD photocatalytic modification reduced the accumulation of membrane surface cake layer. Consequently, the adsorption and retention of SMP and EPS on the surface of membrane were slowed down and the accumulation of macromolecular organic compounds was reduced in the cake layer. The results also showed that Zn O-ALD deposition layer with hydrophilic property could effectively reduce the adsorption and retention of macromolecular organic compounds with hydrophobic property on the surface of membrane cake layer. On the other hand, because of the photocatalysis, organic pollutants on the membrane surface by adsorption and retention could be oxidized and decomposed into small molecular substances and even be mineralized into inorganic components.