Reasearch On Modification Of PVDF Ultrafiltration Blend Membrane And Its Anti-fouling Behavior Of Effluent Organic Matter

Membrane fouling has been always the key factor for the wider application of PVDF ultrafiltration membrane in wastewater reclamation.It is important significance to identify the intrinsic cause of membrane fouling for its anti-fouling modification.In this paper,effluent organic matter?EfOM?in municipal wastewater was chosen as the main foulants,modified colloid probes of membrane and foulant for atomic force microscopy?AFM?and quartz crystal microbalance?QCM-D?chips were independently developed,and the adsorptive fouling behavior of EfOM on specific membrane interface was measured by AFM and QCM-D.In addition,the mechanism and process of membrane fouling of EfOM were analyzed in microcosmic level to reveal the essential reason of anti-pollution modification of ultrafiltration membrane.Based on the results of microscopic observation of membrane fouling mechanism,hydrophilic copolymer polyvinyl alcohol?PVA?and silicon dioxide?Si O₂?were used as additives to control the composition of casting solution,the phase inversion condition and the hollow fiber spinning process,thus the excellent performance and controllable structure of highly anti-fouling PVDF ultrafiltration membranes were prepared.The main conclusions were as follows:?1?The AFM blend membrane material colloidal probe and EfOM coated colloid probe were developed,both of which were used in the technology to measure the micro-force between membrane-EfOM and EfOM-EfOM,the determination of pollution absorption on the membrane was achieved via integrating QCM-D characterization of absorption and measure of AFM colloid probe,a new method to reveal the performance of pollution on the membrane with micro-mechanism was gained and supports the anti-fouling evaluation of modified PVDF ultrafiltration membrane and the select of additive technically.?2?Three representative PVDF blending modified ultrafiltration membranes were prepared with PVA,PEG and PVP which were used as macromolecular blending additives.The adsorption behavior of three kinds of macromolecule modified PVDF ultrafiltration membranes at different contact angles was investigated by using self-developed AFM colloid probes modified with membrane and EfOM.AFM and QCM-D technique were used to unravel dynamic adsorption behavior and adhesion force of EfOM with the PVDF ultrafiltration membranes modified with three different additives.Results showed that the adhesion force between EfOM and EfOM was much larger than that between EfOM and EfOM.However,with the decrease of hydrophilic water contact angle,the adhesion tendency of EfOM-EfOM on the three kinds of membrane surface was significantly different.The order of influence of three hydrophilic additives on the adsorption layer of EfOM was PVA> PVP> PEG.The adhesion between EfOM and EfOM decreased from 0.46mN/m to 0.19mN/m on PVA-modified PVDF membrane,indicating that the hydrophilicity modification of membrane surface could not only reduce the interaction force between pollutants and membrane,but also can reduce the adhesion between pollutants and pollutants,the adsorption layer became loose.The results further reveal the essential reason for the improvement of the anti-fouling property of PVDF ultrafiltration membrane,and it is clear that the high hydrophilicity additive,which was rich in hydroxyl-OH,can effectively inhibit the adsorption behavior of pollutants.?3?In order to solve the problem of loss of PVA in blending modified PVDF ultrafiltration membranes,anhydrous FeCl3 was used as a coordination metal ion source to immobilize the highly hydrophilic modifier PVA and to prepare PVDF/PVA with permanent hydrophilic modification Fe interpenetrating network crosslinked ultrafiltration membrane.The ratio of PVDF/PVA/Fe3+ was investigated and optimized by taking the physical and chemical properties of the membrane,the interface structure parameters and the anti-pollution performance as the index.The results showed that water flux was 277 L/?m2·h?,the tensile strength was 2.53 MPa and the BSA rejection was 83% for PVDF/PVA at 4/1.After the crosslinking of FeCl3,the degree of immobilization of PVA in PVDF/PVA blended film was improved and the degree of membrane swelling decreased.However,the loss attenuation of flux during the operation of ultrafiltration membrane was reduced,and the membrane permeable performance was stable,compression resistance increased.Membrane filtration rate and membrane fouling resistance generally decreased,and membrane steady-state flux and flux recovery rate increased when filtering the secondary effluent.The membrane fouling analysis showed that the membrane fouling caused by the polar hydrophilic component in the secondary effluent was the biggest,and the neutral hydrophilic component caused the least pollution.The structure of PVDF/PVA/Fe3+?4?The effect of SiO₂ content on the structure and properties of the membrane was investigated by using the stable nano-SiO₂ as the inorganic hydrophilic additive.Combined with QCM-D and AFM techniques,the interaction mechanism between EfOM and SiO₂ modified PVDF UF membrane was studied and analyzed.The results show that the membrane water flux is 987 L /?m hollow fiber membrane was dense,the porosity reduced,the average pore diameter and the pure water flux were low,but the strength of membrane was improved greatly.2h?,the BSA rejection is 92.7%,and the contact angle decreases to 55° when the addition of SiO₂ is 0.5%.The results of QCM-D show that the adsorption process of EfOM has gone through two stages:?a?the frequency and dissipation factor of EfOM during the initial adsorption phase change obviously,which indicates that EfOM accumulates on the membrane surface rapidly;?b?after the adsorption process reached equilibrium,the frequency change tends to be stable,but dissipation continues to change,indicating that the conformational changes of adsorption layer continue to occur.The degree of conformational change in the contaminant layer is mainly determined by the interaction of EfOM with the membrane surface.The better the hydrophilicity of the membrane surface,the better the viscoelasticity of the adsorption layer.AFM analysis showed that repulsive force was only observed on the surface of SiO₂ added PVDF membranes?M2 and M3?,indicating that the introduction of SiO₂ could effectively prevent the contact of EfOM with the membrane surface,and the irreversible pollution was significantly decreased.The results of membrane flux decay and pollution index analysis of SiO₂ modified PVDF membrane on the secondary effluent ultrafiltration process confirmed the above results.The effect laws of membrane preparation conditions such as air gap,inner bore fluid and coagulation bath composition on the shape,strength and water permeability of the hollow fiber membrane were investigated and the PVDF/SiO₂ blends hollow fiber membrane spinning process was optimized.The irregular defects of membrane profile were solved.?5?In order to realize the controllability of flux and pore size of hollow fiber ultrafiltration membrane,the hollow fiber spinning technology was innovated.PVDF/CaCO₃ blending hollow fiber ultrafiltration membrane was prepared by submerged precipitation phase separation?NIPS?with micron-sized CaCO₃ as additive.Using the chemical reaction between the chemical agent and the additive CaCO₃ introduced in the core liquid and the gel bath,the dynamic chemical reaction of the spinning process can be used to strengthen and control the pore so as to improve the water flux of membrane without changing the spinning process and the production equipment.The effects of the acid species,the acid concentration in the core liquid and the external gel bath on the properties and structural parameters of the membrane were investigated.It was confirmed that the dynamic chemical porosity of imperfectly cured nascent membrane was effective in enhancing the water permeability of the membrane by promoting the formation of microporous and interconnected pores.The optimum spinning conditions are as follows: the core liquid is 10% HAc solution at 60 ?,the second gel bath is 1mol/ L hydrochloric acid at 40 ?,and the dwell time is 20 min.