Preparation Of Superhydrophobic Membrane And Its Application In Membrane Distillation Of Desulfurization Wastewater

Desulfurization wastewater is a typical industrial wastewater regularly discharged by thermal power plants to ensure the quality of gypsum during the desulfurization process.It has complex wastewater composition,high salinity,high suspended solids concentration,high heavy metal content,and certain amounts of organic pollutants.With the implementation of the"zero discharge"policy in recent years,achieve efficient treatment and recycling of desulfurization wastewater have become an urgent issue.Membrane distillation represetnts an emerging membrane separation technology with the advantages of simplicity,easy operation,100%rejection rate in theory,and utilization of cheap renewable（solar energy,geothermal）as energy source,hence making it widely used in deep purification of various industrial high-salt wastewater treatment.However,due to the high risk and complexity of industrial wastewater composition,the serious membrane fouling phenomenon in membrane distillation process reduces the recovery efficiency.It is therefore necessary to select an appropriate pretreatment or to be coupled with other processes（microfiltration,reverse osmosis）to improve the recovery efficiency.At present,there is still a lack of efficient pretreatment technology for the treatment of desulfurization wastewater by membrane distillation,and systematic analysis and research on the migration and transformation laws and formation mechanisms of pollutants during the treatment process.Therefore,in this thesis,the characteristics of the organic and inorganic foulant of membrane distillation in treating desulfurization wastewater were studied.Pror to improve the recovery efficiency of desulfurization wastewater,several methods such as chemical sedimentation,physical filtration,flocculation and other pretreatment methods were systematically performed,while the strategies of fabricated high steady superhydrophobic and amphiphobic membrane by mussel bionic technology was also adopted to improve the anti-fouling and anti-wetting properties.The research presents the effect of pollutants on flux during the treatment of desulfurization wastewater by polyvinylidene fluoride（PVDF）membranes.Then,the pioneer pollutants and dominant pollutants that cause flux changes at different periods in the treatment process were determined.The flow rate plays a dominant role in the distribution of the membrane fouling layer.A three-dimensional single-phase computational fluid dynamics（CFD）method was used to evaluate the flow state in the membrane module system,while SEM,X-ray energy dispersive spectrometer（SEM-EDS）,and FTIR were used to analyze the distribution,composition and structure of pollutants on the membrane surface in the horizontal and vertical directions in different fouling periods.It was found that organic pollutants are the pioneer pollutants that lead to a slow decline in flux in the early stage,and mineral scaling is the dominant factor in the rapid decline of flux in the later stage.A small portion of volatile organic pollutants can pass through the membrane pores and enter the permeate side,and the distribution of membrane foulant in the horizontal and vertical directions is different.The main pollutants in the membrane fouling process can be effectively removed by adding sodium carbonate,physical filtration,adding sodium hydroxide and adding flocculants.The pretreatment method inhibits the influence of mineral foulant on membrane distillation,and achieves a recovery rate of 90%of desulfurization wastewater.Based on the research on the main pollutant components and the formation mechanism in the process of membrane distillation treatment of desulfurization wastewater,the superhydrophobic modification of the membrane was proposed to improve the anti-fouling property of the membrane.The construction of micro-nano structure roughness and the modification of low surface energy materials are the key factors for the preparation of superhydrophobic membranes.In order to improve the stability of the combination of nanomaterials with the membrane surface,a biomimetic glue layer was deposited on the surface of PVDF membrane by mussel biomimetic technology.Using the chelating ability of polydopamine layer to metal ions,β-Fe OOH nanorods were generated by hydrothermal in-situ mineralization,and further grafted low surface energy material fluorosilane（PDTS）to realize the fabrication mussel biomimetic superhydrophobic PDTS-Fe-PDA/PEI-PVDF membrane.The modified superhydrophobic membrane has a contact angle of 162^o and a slding angle of 5.7^o.At the same time,the nanorods constructed on the membrane surface endow the superhydrophobic membrane with outstanding chemical and thermal stability under the adhesion of dopamine.The superhydrophobic membrane exhibits excellent anti-fouling and anti-wetting properties in the treatment of simulated wastewater（high salt,sodium dodecyl sulfate,calcium sulfate）.However,there is still a risk of easy contamination in the process of treating wastewater containing organic pollutants.The superhydrophobic membrane was further applied to the treatment of actual desulfurization wastewater.At the end of the experiment,1000 m L of high-quality water was recovered,and the flux of the pristine membrane was reduced to 65%;However,the flux of the superhydrophobic membrane remained stable within 21 h before the actual desulfurization wastewater was treated.At the end of the experiment,1000 m L of high-quality recycled water(conductivity lower than 5μS cm^-1)was obtained,and the flux decreased by only 15%,hence showing excellent stability and pollution resistance.In order to further improve the performance of PVDF membrane against organic pollutants and inorganic pollutants in the process of desulfurization wastewater treatment,the biomimetic glue layer was deposited on the surface of the membrane by mussel bionic technology（PDA）to provide active sites for the next modification,and then the Si O₂ multi-level micro-nano re-entrant structure was constructed on the surface of the biomimetic layer by the sol-gel method.Then,the low surface energy 17-chain fluorosilane（17-FAS）was grafted to reduce the surface free energy of the membrane,and an amphiphobic（hydrophobic and oleophobic）FAS-Si O₂-PDA-PVDF membrane was successfully constructed,thereby further improving the anti-fouling property of the membrane.SEM-EDS,contact angle,FTIR and other analytical methods were used to study the changes in the morphology,structure and surface properties of the amphiphobic membrane.The water contact angle and oil contact angle of the mussel biomimetic amphiphobic membrane were found to be 164^o and 113^o,respectively,with a lower sliding angle（4.3^o）.Amphiphobic membranes showed stable flux and excellent salt rejection（>99%）in both membrane distillation treatment of simulated humic acid-containing wastewater and calcium sulfate wastewater.Subsequently,the amphiphobic membrane was used to treat the actual desulfurization wastewater,and it was found that compared with the pristine PVDF membrane and the superhydrophobic membrane,the flux(10.86 L m^-2h^-1)of the amphiphobic membrane remained stable within the first 41 h of treating the actual desulfurization wastewater.The whole process showed more excellent stability and anti-pollution.