Preparation,Performance,and Fouling Alleviation Of PVDF/TiO₂ Modified Membrane

Polyvinylidene fluoride（PVDF）ultrafiltration membranes have some problems during the application,like,poor hydrophilicity,fouling resistance,or low flux,which is not conducive to the development of ultrafiltration technology on small scale during drinking water treatment.Blending nano TiO₂ is one effective way to improve the filtration performance of the PVDF membrane.In the preparation of PVDF/TiO₂ hydrophilic modified-membrane,nano TiO₂aggregation easily occurred in the casting solution,leading to poor modification performance.Ultrasound benefits improve the nano-TiO₂ dispersion in casting solution,but the effect of ultrasound on the structure and properties of the PVDF modified-membrane is not clear so yet.In this study,the PVDF/TiO₂ modified membrane was prepared and optimized by ultrasonic pretreatment,and its filtration performance was compared with the base membrane during actual raw water treatment.On this basis,low-pressure operation strategies were put forward to mitigate the fouling of PVDF/TiO₂ modified-membrane.The PVDF/TiO₂ modified-membrane was prepared by optimizing the ultrasonic conditions.Optimized condition of the PVDF membrane was described as follows.The content of PVDF polymer,DMAc and PVP was 19 wt%,78.7 wt%,and 1.3 wt%.Optimized condition of the hydrophilic additive was described as follows.The particle size of nano TiO₂ was around 5～10 nm,and the content was1%.The mixture of nano TiO₂ and DMAc dispersed in ultrasonic pretreatment for1 h with the power of 20 k Hz and intensity of 300W.The unmodified PVDF membrane was denoted as the base membrane,the PVDF/TiO₂ modified-membrane prepared by ultrasonic-assisted mechanical stirring was denoted as UMS membrane,and the modified-membrane prepared by mechanical stirring only was denoted as MS membrane.When the protein rejection rate was nearly around 60～67%,the ratio of pure water flux of UMS membrane,MS membrane,and the base membrane was 2.3:1.6:1.Results of SEM and element surface scanning analysis showed that the agglomerated nano TiO₂ was observed in the cross-sectional structure of MS membrane.The nano TiO₂ agglomeration reduced by adding the ultrasonic pretreatment.As the result,the unevenness of UMS membrane surface pores was reduced,while the membrane pore connectivity between the skin layer and sublayer was improved.Compared to the base membrane,the average pore size of the MS membrane decreased from 48.2 nm to37.4 nm,while which UMS membrane increased to 65.6 nm.The FT-IR result presented that the proportion of hydrophilic groups of the base membrane,MS membrane,and UMS membrane was 0.11%,8.56%,and 30.78%,respectively.And their surface contact was 74.04°,69.55°,and 63.54°,respectively,indicating that the hydrophilicity of the modified-membrane was significantly improved by adding nano TiO₂ and ultrasonic pretreatment.Besides,the fast filtration of protein solution illustrated that the normalized flux（J/J₀）of UMS membrane,MS membrane,and the base membrane was 0.47,0.37,and 0.32,respectively.At the same time,their irreversible fouling resistance ratio(R_ir/R_t)was 13.1%,17.7%,and 55.6%,respectively.The ultrasound helped to reduce the structure with a large height difference of membrane surface,and moderately increased the membrane surface roughness,possibly beneficial to enhance the anti-fouling ability of UMS membrane.The coefficientτof Tansel’s model is inversely proportional to the fouling rate.And the order ofτsorted from smallest to largest was,base membrane<MS membrane<UMS membrane,illustrating that UMS membrane had the slowest protein fouling rate.The actual filtration performance of UMS membrane and the base membrane was evaluated by treating micro-polluted raw water in three ways,such as direct filtration,pre-oxidation assists membrane filtration,and combined pretreatment assists membrane filtration.Retention capacity of the UMS membrane was similar to the base membrane at three filtration modes,according to the experimental result of effluent quality described as follows.The removal amount of COD_Mn and UV₂₅₄ was 0.3～0.7 mg/L and 0.01～0.06 cm^-1,of turbidity was 0.8～4.4 NTU.At the same time,the removal rate of total number of colonies was higher than 99%.The flux of UMS membrane（53.6 L/（m²·h））was 2.6 times that of the base membrane.Meanwhile,the total fouling resistance（R_t）,reversible fouling resistance(R_re),and R_ir of UMS membrane was 39.2%,43.2%,and 27.2%of the base membrane.Using the Fe²⁺/dithionite（DTN）/O₂ system as the pre-oxidation process,organics like COD_Mn,UV_254,and atrazine in raw water could be effectively degraded,through the dual effect of oxidation by free radicals and flocculation by iron salt.The effluent quality of UMS membrane was improved by pre-oxidation,and the flux was promoted to 58 L/（m²·h）.Compared to direct filtration mode,the R_t,R_ir,and R_re of UMS membrane was reduced by 7.6%,35.1%,and 4.3%,through adding Fe²⁺/DTN/O₂ pre-oxidation.Compared to the pre-oxidation pretreatment,using the above pre-oxidation and activated carbon denoting as the combined pretreatment,which can reduce the R_ir of UMS membrane from 0.85×10¹² to 0.58×10¹² m^-1.Results of modified Hermia’s model exhibited that,the internal fouling and cake layer fouling of UMS membrane were lighter than those of the base membrane.And,the internal fouling of UMS membrane was alleviated by the Fe²⁺/DTN/O₂ pre-oxidation,keeping the cake layer fouling less polluted.Additionally,the internal fouling of UMS membrane was further mitigated by the combined pretreatment,but the cake layer fouling was not alleviated too much.Based on the concept of critical flux and threshold flux,using the protein solution and aforementioned combined pretreated effluent as the feed solution,to study the fouling mitigation strategies of UMS membrane.When filtering protein solution,results of the Alternating Constant Pressure-step method showed that the critical flux（J_C）of UMS membrane is 5.10～10.62 L/（m²·h）,the filtration pressure（critical pressure,TMP_C）is 10 k Pa;And,the threshold flux(J_TH)is 23.82～23.91L/（m²·h）,and the corresponding filtration pressure(threshold pressure,TMP_TH)is 60 k Pa.For the protein solution,the J_C and J_TH of UMS membrane were 3.2times and 3.5 times that of the base membrane.Compared to the conventional pressure operation at 100 k Pa,the R_re and R_ir of UMS membrane were reduced by93%and 96%,respectively,when operating at TMP_C.And the R_ir of UMS membrane was reduced by 72%when operating at TMP_TH.This indicated that two low-pressure operation modes were useful to alleviate protein fouling of UMS membranes.Taking the combined pretreated effluent as the feed solution,the TMP_C of UMS membrane was 5 k Pa,the J_C was 6.22 L/（m²·h）;the TMP_TH was45 k Pa,and the J_TH was 27.5 L/（m²·h）.At the same time,the J_C and J_TH of UMS membrane were 3.5 times and 5.6 times that of the base membrane,respectively.Compared to conventional pressure operation,the R_re and R_ir of the UMS membrane were reduced by 91%and 93%,respectively,when operating at TMP_C.And,the R_re and R_ir were reduced by 19%and 78%,respectively,when UMS membrane operating at TMP_TH.Results of modified Hermia’s model presented that,as compared with the conventional operation,the blockage fouling and cake layer of UMS membrane was effectively alleviated by operating at TMP_TH.Moreover,there was almost no cake layer fouling for UMS membrane,meanwhile,the surface deposition fouling could be mitigated,when operating at TMP_C.To achieve the same water production rate produced under conventional pressure,the UMS membrane required 10.9 times of module to operate at TMP_C,and it only quired 1.8 times of module to operate at TMP_TH.Besides,when UMS membrane operating at TMP_TH,the filtration energy consumption was reduced by 17%,the chemical cleaning period was extended to 4.5 times,and the backwash period was extended to 1.2 times.This illustrated that the TMP_TH operation was more advantageous for fouling alleviation than the TMP_C operation when the UMS membrane was used in the small-scale ultrafiltration technology.In this paper,the hydrophilic modification of nano TiO₂ was used to improve the filtration performance of the PVDF membrane.Besides,the Fe²⁺/DTN/O₂ pre-oxidation and low-pressure operation were both used to alleviate the actual fouling of the PVDF/TiO₂ modified-membrane.The obtained results provide guides for the application and fouling control of PVDF/TiO₂ modified-membranes,which are applied in the small-scale ultrafiltration technology in drinking water treatment.