Preparation And Properties Of PVC Ultrafiltration Membrane Blended With Functionalized Multi-walled Carbon Nanotubes

Membrane separation technology,with a simple operation,energy saving,no by-products and other features,is one of the priorities of drinking water treatment technology of the 21 st century.According tooperating pressure and membrane pore sizemembranescan be divided into microfiltration?MF?,ultrafiltration?UF?,nanofiltration?NF?and reverse osmosis?RO?.UF as the core technology of the third generation has been widely used in urban drinking water treatment processes.With the economic and technological development and the increasing environmental protection standard,the requirements for membrane properties such as membrane structure,anti-pollution ability,the ability to remove small molecule substances are also rising.Carbon nanotubes?CNTs?,because of its excellent mechanical,thermal and adsorption properties,have attracted much attention since it was discovered.Therefore,it has great theoretical value and profound practical applications to blending carbon nanotubes with polymers to prepare membranes for the purpose of improving anti-pollution,increasing the capacity of entrapping small molecule contaminants,improving the structure and properties of membranes.For the above targets,treat carbon nanotubes with functionalization and then blend it with polyvinyl chloride?PVC?to prepare UF membrane.The main experimental procedure and results are as follows:?1?Preparation and characterization of the additive.Multi-walled carbon nanotubes?MWCNTs?pre-treatmented by mixed acids were loaded by Fe₃O₄?MWCNTs/Fe₃O₄?through chemical co-precipitation method.Infrared spectroscopy?IR?and X-ray diffraction?XRD?analysis showed that pre-treatmented MWCNTs were purified and were introduced hydroxyl and carboxyl groups on its surfaces,and that structural integrity of Fe₃O₄ were loaded on the acid treated MWCNTs surfaces.?2?Three different PVC UF membranes were prepared by blending with different additives,namely: pure PVC membrane,PVC membrane with pre-treatmented MWCNTs as additive,PVC membrane with MWCNTs/Fe₃O₄ as additive.The three different UF membranes all have a typical UF membrane structure with membrane skin layer of dense tiny pores and the support layer of finger-like pore structures.With the addition of the additives,the properties of the membranes improved in surface roughness,porosity and average pore diameter,meanwhile the membranes hydrophilicity were also improved.Correspondingly,the water fluxes of membranes increased and retentions for BSA and BPA also increased.MWCNTs/Fe₃O₄ as additive played a better supporting role in the membrane forming process than pre-treatmented MWCNTs,resulting in forming a more dense,a more narrow pore structure,moreover,the membrane have better hydrophilicity and a higher water flux.?3?The factors such as initial concentration,ionic strength,pH,membrane fouling and trans-membrane pressure?TMP?influencing the retention efficiencies of BPA by three different UF membranes were investigated.The results showed that the effect of initial concentrationon retentions is relatively minor.The retentions were promoted because of the existence of Ca²⁺,which played a "bridge" role and promoted the adsorption of negatively charged molecules and negatively charged membrane surfaces.PH affected not only the existing state of the molecules,but also the membrane surface Zeta potentials.When the pH of solution approached to its pKa value,BPA exsited mainly in anion,at the same time the membrane surfaces were negatively charged.Electrostatic repulsion hindered the adsorption between molecules and the active sites on membrane surfaces,so retentions significantly reduced.The retentions were all improved after HA fouling because of the HA block in the membrane pores,but this caused serious membrane fouling and affected the filtration of membranes.Because of the BPA concentration gradient increased across both sides of the membrane with increasing pressure and the adsorption forces here were weak,the retentions decreased with increasing trans-membrane pressure.?4?The membrane with MWCNTs/Fe₃O₄ as additive was used about cleaning research.The cleaning effects of ultrasonic cleaning,NaOH cleaning,citric acid cleaning and NaClO cleaning on membrane were studied.The results showed that ultrasonic cleaning,NaOH and NaClO cleaning all had good flux recoveries,but meanwhile they damaged the membrane structure and properties.Especially NaClO had the maximum damage because of its strong oxidizing.Citric acid cleaning had theminimal damage to the membrane,but because of the hydrophobic groups of protein and other organic substances exposed in the acidic environment,which made the contaminants adhere the membrane more closely,resulting in flux recovery is minimal.So for the high concentration organic wastewater,citric acid should not be used as cleaning agents.?5?The preparation and properties of magnetic induced ordered-modified UF membrane were discussed.It found that the ordered-modified UF membrane had not too much difference compared with disordered-modified UF membrane in membrane porosity,average pore diameter,hydrophilicity,water flux and retention of BPA.It need further study.