Study In Modification Of Polyvinylidene Fluoride(PVDF) Membrane For Enhanced Corrosion Resistance And Performance Optimization

Membrane separation technology has received considerable attention in various industrial fields due to its many advantages of such as low cost,high separation efficiency and minimization of secondary pollution,etc.With the rapid expansion in the scope of membrane separation technology for industrial effluent treatment,highly corrosive wastewater such as those from mining,metal plating and pharmaceutical manufacturing industries has posed new challenges to many of the conventional membrane.Commonly used polymer membrane materials such as cellulose,polysulfone,polyamide,polyolefin,etc.have a limited tolerance to corrosion by acid,alkali,not even mentioning organic solvents,and cannot work stably for a long time in a highly corrosive sewage environment.Ceramic membranes and polytetrafluoroethylene(PTFE)membranes,although they have good anti-corrosion performance,are limited in their applications by difficulties in their manufacturing and their high materials and operation costs,for common and large-scale industrial wastewater treatment.Polyvinylidene fluoride(PVDF)membrane is widely used and available in the market,and has good mechanical strength and moderate chemical stability for applications in the p H range of 2-12.The modification of PVDF membrane to enhance its corrosion resistance would be an effective way to expand its role in corrosive industrial wastewater treatment.In this study,two methods were investigated to modify PVDF membranes for their enhanced acid and alkali resistance,as well as organic solvent resistance.The effect of modification on the membrane structure,mechanical properties,permeability and corrosion resistance was systematically investigated.The reaction conditions though solvent modification were optimized,and the reaction mechanism and products were discussed.In the first part,PTFE nanoemulsion was blended with PVDF,and the PVDF/PTFE composite membrane was prepared by the common non-solvent induced phase separation method(NIPS).By adding surface active agent(Sodium dodecyl sulfate)to inhibit the agglomeration of PTFE nanoparticles in the casting solution,the agglomerated PTFE particle sizes were effectively reduced from 5-6 microns to about1 micron.The prepared membranes were immersed in 5 mol H⁺per liter of different types of acid solution and 0.1 mol OH^-per liter of alkali solution for up to 6 months in the experiments.The fracture strength,surface roughness,water flux,surface contact angle,etc.of the membranes were analyzed before and after the corrosion to characterize the corrosion resistance of the membranes.The results showed that the incorporation of PTFE nanoemulsion into PVDF significantly improved the corrosion resistance of the modified PVDF membranes(i.e.,PVDF/PTFE composite membrane).When the membranes were immersed in H₂SO₄,HCl,and HNO₃ solutions with an H⁺concentration of 5 mol/L,the performance of the membranes was generally weakened by corrosion with the immersion time increased.After soaking for up to 6months,the breaking strength of the pure PVDF membrane(M-0)decreased by44.23%,29.57%and 38.22%,while the breaking strength of the composite PVDF/PTFE membrane M-30(with a PTFE content of 30%)was much lower,at30.30%,25.93%and 28.62%,respectively.The membranes were corroded by alkaline solution to a larger extent.After immersed in Na OH solution with an OH^-concentration of 0.1 mol/L for 6 months,the fracture strength retention rates of M-0and M-30 became only 48.32%and 36.70%.The flux change rate of M-30 immersed in an acidic solution with an H⁺concentration of 5 mol/L for 6 months was equivalent to that of M-0 immersed in the same acidic solution for 3 months.The change rate of the roughness and surface contact angle of the PVDF/PTFE composite membranes were also significantly lower than that of the pure PVDF membrane.Soaked in H₂SO₄with an H⁺concentration of 5 mol/L and in Na OH with an OH^-concentration of 0.1mol/L for 6 months,the contact angle change rates of M-0 were 9.3%and 15.4%,while those of M-30 were only 8.1%and 12.9%.During the experiments,using the H₂SO₄ solution with an H⁺concentration of 0.01 mol/L in a dynamic filtration test for up to 336 hours(14 days).The PVDF/PTFE composite membranes showed more stable filtration performance than the unmodified PVDF membrane.The membrane flux change rate was 50%for M-0 but only about 30%for M-30 membrane.This indicates that the PVDF/PTFE modified membrane which can be prepared by a simple method has a greater anti-corrosion advantage than pure PVDF membrane in practical applications,especially for the treatment of acidic wastewater.In order to further improve the corrosion resistance of PVDF membrane,in the second part of the study,PEI network was wrapped on the surface of the PVDF membrane as the"protective layer",formed through cross-linking reactions to make the membrane with strong resistance to organic solvents.Through orthogonal experiments on the influencing factors such as co-heating temperature,co-heating time,oven thermal-treating humidity,and PEI concentration,etc.,the optimal wrapping reaction conditions were determined:PEI with molecular weights of 600,7W,and 75W,the mass fraction of each PEI is 1.3%of overall PEI aqueous solution.First in a solution at 90?for 17 hours,and then continued to react for 3 hours in an oven at a temperature of 90?and a humidity of 70%.The formed surface membrane under such condition has been found to have the best resistance to organic solvents.The modified M_600+7W+75W membrane had a mass retention rate of 90%after being immersed in a 50%NMP solvent for 7 days,which can be further used for the treatment of industrial organic solvent wastewater.Furthermore,the study carried out elemental analysis and chemical bond identification on the prepared membrane of the PEI wrapped PVDF.The modified membrane was found to contain five elements:H,C,N,O,and F,among which the O element was present in the cross-linked layer other than in PVDF and PEI.The reaction mechanism in PEI wrapped PVDF to form an organic solvent-resistant protective layer was preliminarily speculated and tested.Compared with the original PVDF membrane,the surface water contact angle of the modified membrane decreased from 78.6°to 51.8°,becoming more hydrophilic,and the pure water flux decreased from 7.8 LMH to 3.6 LMH,due to the decreased pore sizes.Since the wrapping reaction only occurred on the external and perhaps internal surfaces of the membrane,the macroscopic structure of the original membrane was almost retained,which improved the mechanical strength of the prepared membrane.The maximum tensile stress was found to be increased from 4.16 MPa before to 5.60 MPa after the modification.