Preparation Of Biocatalytic Nanofiltration Membrane And Its Application In Aqueous Micronollutants Removal

Micropollutants such as antibiotics,endocrine disrupters,pharmaceutical by-products,etc.,have detrimental effects on public health and ecological environment.By integrating biodegradation and separation functions,the biocatalytic nanofiltration membranes(BCNMs)can effectively remove micropollutants from water.However,there still exist several problems such as low catalytic efficiency and poor stability for BCNMs.To improve their performance,this work,on the one hand,attempts to make an enzyme-membrane interface intensification design of BCNMs,thereby optimizing the confinement strength and microenvironment for the immobilized enzyme in the membrane.On the other hand,by elucidating the mechanisms of catalysis,mass transfer,and membrane fouling mechanism during micropollutants treatment process,the synergistic effect of adsorption-catalysis and the coupling efficiency of reaction-separation are enhanced.Using the above strategies,we expect to develop superior BCNMs with high catalytic efficiency,good long-term stability,and fast regeneration ability.The detailed results are as follows:(1)The graphene oxide(GO)-modified BCNMs were developed by introducing two-dimensional nanomaterials GO and its derivates into nanofiltration(NF)membrane by reverse filtration and co-deposition methods respectively,and the role of GO on the overall performance of BCNMs was explored.It was found that the enrichment effect and catalytic functions of GO improved both the activity and stability of laccase,while it would decrease the affinity of the substrate to enzyme.Moreover,the introduction of GO enhanced the adsorption capacity and strength of the support layer of NF membrane,thereby retarding the enzyme leakage and improving the storage stability of the BCNMs.As the introduction of GO accelerated the products accumulation,aggravated the membrane fouling,and increased the mass transfer resistance of the substrate/product,the GO-modified BCNMs did not show significant improvements in bisphenol A(BPA)removal and reusability under flow through mode.Notably,such GO-based BCNMs could be used for BPA semi-quantitative detection with the advantages of simple,fast,and portable operations.(2)To reduce such performance degradation caused by products accumulation,inspired by the fluid mosaic model of the cell membrane structure,we prepared a polyelectrolyte-based three-dimensional(3D)modified BCNM by using polyethyleneimine(PEI)to modify the support layer of NF membrane(termed 3D modification)and subsequent laccase reverse filtration.By rationally regulating the steric and electrostatic effects of the support layer to enzyme,the confinement strength of NF membrane to enzyme was optimized,thus delaying enzyme leakage with little increment in mass transfer resistance for substrate/product,and endowing the immobilized enzyme with requisite mobility for efficient catalysis.The prepared BCNM has advantages of uniform enzyme distribution,high enzyme loading,and superior storage stability.Moreover,it exhibited a negligible decline in the BPA removal for 7 reuse cycles or 36 h continuous operation,exhibiting extraordinary operation stability.For the first time,a simple method was proposed to quantify the mobility of immobilized enzyme which could directly reflect the confinement strength of the membrane and the performance of the BCNM.(3)With respect to further increasing the enzyme loading and enzyme stability,as well as achieving rapid enzyme reloading,the thermal-responsive poly(N-isopropyl acrylamide)(PNIPAM)hydrogel was used to modify the support of NF membrane by in-situ growing and co-deposition methods respectively.The thermal-responsive hydrogel-based smart BCNMs were prepared by reversely filtrating laccase into the support layer at 38℃(T>low critical solution temperature,LCST)when PNIPAM was in the shrinkage state.It was found that the PNIPAM modification could increase the specific surface area of the support layer,as well as the steric effects to laccase,thereby largely improving the enzyme loading,preventing enzyme accumulation beneath the skin layer of NF membrane,and avoiding the flux decline caused by enzyme excessive migration and aggregation.By optimizing the concentration of monomer,modification time,and modification strategies,the prepared smart BCNM showed better catalytic efficiency and reusability compared to the unmodified BCNM at room temperature 25℃(T<LCST)when PNIPAM was in the swollen state.Meanwhile,the deactivated enzymes can be quickly eluted by simply washing at 38℃,thus realizing enzyme reloading and BCNM regeneration.