Fabrication Of Micro Particles Doping Silicate Microfiltration Membrane And Its Catalytic Ozonation Decontamination Performance

Membrane separation technology is a physical process with no phase change and easily to operate.Membrane water treatment process is considered to be the core technology of the third generation of urban drinking water purification process.Among them,inorganic microfiltration membrane,as the basic first level filtration procedure of membrane separation technology,has the advantages of large water flux,low operating pressure and corrosion resistance.However,its pore size is too large to reject small pollutions.In addition,the high-temperature sintering process of membrane fabrication and expensive raw materials have become the key issues to limit the development and application of inorganic membrane.Non-sinter fabrication process and the low-cost raw materials of silicate microfiltration membrane（SMM）which can well solve the limited development of inorganic membranes.SMM also can catalytic ozonation.However,existing SMMs suffer from large average pore size,uneven pore size distribution,and low mechanical strength,resulting in poor retention capacity and short service life.In this study,to solve the existing problems of SMM,a novel silica microparticle doping silicate microfiltration membrane（Si-SMM）was fabricated by selected the type of silica,determining the ratio of Si O₂ micro-particle to silicate cement（s/c）,and adding alkaline additives.The pore size distribution,mechanical strength,porosity and pure water flux（PWF）of the membranes were investigated to determine an optimal membrane fabricated parameters and ensure the stability of Si-SMM.The XRD and FT-IR characterization of the raw materials were revealed that the low crystallinity Si O₂ can react with Ca（OH）₂ in silicate cement to form hydrated calcium silicate（C-S-H）,which promoted the hydration of silicate and thus improved the mechanical strength of the membrane.The effects of s/c on pore size distribution and mechanical strength of Si-SMM were investigated,and the results showed that the membrane pore size showed a bimodal distribution around 0.2μm and 1.2μm,respectively.The higher the s/c,the second pore size peak was closer to the theoretical silica particle accumulation peak.The bimodal peaks overlapped at s/c=0.5.The increase of s/c led to the decrease of membrane mechanical strength.The characterization by SEM of membrane cross-section confirmed that high s/c membranes had a loose structure,weak hydration and low bending strength.Since the hydration of silicate materials took place in alkaline environment,the addition of Na₂Si O₃ solution during the membrane preparation process can improve the bending strength of the membrane by about 27.6%.Combining the performance indexed of the membrane,the optimal Si-SMM preparation parameters were as followed:the ratio of C-10μm Si O₂ micro-particle to silicate cement s/c was 0.5,replacing ultrapure water with 1 mol/L Na₂Si O₃solution,90%relative humidity（RH）,and 20℃curing for 14 d.The average pore size of the fabricated Si-SMM was 0.633μm,bending strength was 5.40MPa,porosity was 37.9%,and PWF was 2617 L/m²/h/bar.The retention rate of BSA solution was 84.5%at transmembrane pressure（TMP）=40 k Pa.The surface functional groups and water retaining pore channels of powdered activated carbon（PAC）microparticles can promote the degree of silicate hydration.And the adsorption of PAC can remove small molecules organic compounds.Therefore,a new PAC microparticles doping silicate microfiltration membrane（C-SMM）was fabricated by replacing the silica powder to ensure the lifetime and the limitation of decontamination of SMM.Ensuring sufficient PWF and porosity,the mechanical strength of the C-SMM was greatly improved to ensure the stability of the membrane and prolong its service life.The effect of PAC doping amount on pore size distribution of the membrane was investigated,and it was determined that C-SMM with PAC doping amount of 10 wt%had the smallest critical pore size and the best permeability.XRD and FT-IR results showed that the addition of PAC did not change the mineral composition of cement hydration products.The appropriate amount of PAC retained water and provided nucleation for silica hydration reaction,due to its surface hydrophilic functional groups and rich pore structure.This performance of PAC strengthened the hydration of silicate,thus improved the mechanical strength of C-SMM.The size effect on the bending strength became more obvious as the thickness of the membrane decreased.The average pore size of the optimized C-SMM was 0.129μm and the bending strength was 12.69 MPa.Ozone coupling SMM process was designed for the small molecule organic pollutants degradation.Due to avoid the chance of ozone coupling SMM,6 kinds of small molecule organic poluutants,nitrobenzene（NB）,p-chloroaniline（p-CA）,benzophenone-4（BP-4）,p-chloronitrobenzene（p-CNB）,p-chlorophenol（p-CP）and p-chlorobenzoic acid（p-CBA）were chosen as model compounds for ozone coupling SMM degradation experiments.C-SMM had adsorption properties on organic compounds.Compared with ozone molecular oxidation alone,ozone coupling Si-SMM and ozone coupling C-SMM have synergistic effects,both of them can improve the degradation efficiency of organic removal by about 1.6～4.0 times.Moreover,both Si-SMM and C-SMM had strong alkaline buffering ability,which can level out the effect of p H solution on ozonation system.The alkaline environment inside the membrane pores and the pore-limiting effect could effectively accelerate the ozone decomposition to produce hydroxyl radicals with super oxidation capacity,thus accelerating the reaction rate and improving the ozone utilization.Six replicate experiments on ozone coupling Si-SMM and C-SMM for the removal of various organic pollutants demonstrated the fabricated membranes has a stable catalytic activity.Examination of ion leaching of the membranes revealed that Si-SMM was more suitable for alkaline environments and C-SMM was more applied for acidic conditions.By simulating actual water quality,continuous flow experiments of ozone coupling Si-SMM and C-SMM was designed.The results revealed the stability of the fabricated SMMs,which broadened the application range of SMMs in water treatment process,and provided theoretical support for the practical application of C-SMM and Si-SMM.