Synergism Of Biological And Chemical Catalysis For Enhancing The Removal Of Nanofiltration Membrane Fouling

Nanofiltration（NF）technology can achieve the precise separation among a variety of substances especially for small molecules,and it has wide application prospect in industrial and medical fields.However,membrane fouling has negative impacts on the NF membrane properties,which decreases the permeate flux and affects the separation selectivity,thus seriously limiting its operating efficiency.Membrane cleaning is an effective approach to remove the membrane fouling and restore the membrane separation performance.The commonly used membrane cleaning methods,such as acid cleaning,alkali cleaning and oxidant cleaning,have the limitations of large consumption of cleaning agent,lower cleaning efficiency and possible membrane damage.Therefore,the development of efficient membrane cleaning strategy is of great significance for the practical application of NF technology.In this study,a biological and chemical cascade catalytic system is proposed for effective cleaning of NF membrane fouling,which is consisted of glucose oxidase（GOD）catalysis coupled with the Fenton oxidation process.Compared with traditional chemical cleaning methods,this biological and chemical cascade catalysis system shows high membrane cleaning efficiency without the addition of acid/alkali/oxidant reagents.Specific contents of this study are as follows:（1）Firstly,the biological and chemical cascade catalytic system composed of Fe₃O₄ nanoparticles and GOD is used to study the catalytic process and mechanism,which provides a theoretical basis for the subsequent application in NF membrane cleaning.The results show that GOD in the cascade system can efficiently produce hydrogen peroxide and gluconic acid in the presence of glucose.H2O2 will further initiate the Fenton reaction of Fe₃O₄,while gluconic acid can regulate the catalytic reaction environment and maintain the acidic conditions required for the Fenton reaction.The results show that Fenton reaction is the rate-limiting step in the cascade system,and H2O2,as the initiator of Fenton reaction,acts as a key factor in the catalytic degradation process.The GOD-Fe₃O₄ cascade reaction shows enhanced degradation performance,compared with the traditional Fenton reaction,for both bisphenol A and methyl blue dyes.By exploring the effects of different reaction conditions on GOD-Fe₃O₄ cascade reaction,we find that the improved catalytic efficiency is due to two reasons.The first is the effect of GOD adsorption which can promote the contact between the substrate and the catalyst.The second is that enzyme catalysis in the cascade system can continuously produce gluconic acid,which can regulate the pH of the reaction solution and facilitate the Fenton reaction of Fe₃O₄.（2）Based on the above results,GOD-Fe₃O₄ cascade system is used as a cleaning agent to clean the polyamide NF membrane,and the change of membrane performance before and after cleaning is evaluated.The results show that the GOD-Fe₃O₄ cascade system has promising cleaning effect compared with the other oxidant cleaning.After three rounds of filtration-cleaning experiments,water permeability of the NF membrane can still be restored to more than 95.8%by GOD-Fe₃O₄ cascade catalytic cleaning.Two commercial polyamide NF membranes are selected to explore the effects of GOD-Fe₃O₄ cascade catalysis and Fenton catalytic cleaning on membrane performance,respectively.It is found that Fenton catalytic reaction can cause slight damage of the polyamide NF membrane,resulting in the increase in membrane permeability and average pore size.Moreover,the ferric hydroxide sludge produced by the Fenton reaction can block the membrane pore,resulting in a decrease in membrane permeability.However,as for GOD-Fe₃O₄ cascade systems,GOD can act as the free radical scavengers and catalytic stabilizers.Therefore,GOD-Fe₃O₄ catalysis reaction has a less effect on polyamide NF membranes than the traditional Fenton reaction.（3）Finally,in order to reduce the amount of catalyst and improve its the reusability,self-cleaning NF membrane is prepared based on the strategy of surface modification and in-situ mineralization,which realizes the in-situ membrane self-cleaning.The polyamide NF membrane is first modified by a co-precipitation coating of tannic acid（TA）and（3-aminopropyl）triethoxysilane（APTES）,and then FeOOH is grown on the coating surface through a strategy of in-situ mineralization.After membrane fouling occurs,H2O2 or GOD-glucose is added to the membrane surface to initiate the Fenton reaction of FeOOH and realize the in-situ removal of membrane fouling.Through increasing the TA/APTES coating time and FeOOH loading,the hydrophilicity of the membrane surface increases significantly,and the anti-fouling performance and self-cleaning ability of the membrane are also improved.As H2O2 or GOD-glucose is used as self-cleaning initiators,the membrane permeability can respectively be recovered to 83.3%and 82.2%after three rounds of filtration-fouling and self-cleaning.In addition,in order to improve the reusability of GOD molecules and the cleaning efficiency,the GOD molecules are immobilized on Fe₃O₄ magnetic nanoparticles.The GOD@Fe₃O₄ magnetic nanoparticles are also used as the initiators of the self-cleaning membrane,where membrane permeability can still be recovered to 93.7%after three fouling-cleaning rounds.