Preparation Of Positively Charged Nanofiltration Membrane And Its Application In The Treatment Of Dyeing Effluent

In consideration of the facts that the nanofiltration membranes with high separationperformance and low cost are enormous needed in the fields of water treatment fields, the hollowfiber membrane has the characteristics of higher surface to volume ratio and easiermanufacturing technique, and the submerged filtration technique has the characteristics of lowerenergetic consumption and higher energy efficiency, this study mainly focused on the preparationof positively charged hollow fiber composite nanofiltration membrane and its use in thetreatment of dyeing effluent.In the experiments, positively charged hollow fiber composite nanofiltration membraneswere developed by the dip-coatingmethodusing polypropylene hollow fibermicrofiltrationmembrane as support, polyvinyl alcohol (PVA) andpolyquaternium-10(the reaction product ofhydroxyethyl cellulose with a trimethylammonium substitutedepoxide, PQ-10) as coatingmaterials, and glutaraldehyde as cross-linking agent. Preparation parameters such as polymerconcentration, cross-linking reagent concentration, dip-coating time, cross-linking time andadditive concentration were systematically discussed to get the optimum membrane preparationconditions.Surface properties of the obtained PQ-10/PP composite membrane such as surface chemicalstructure, morphology, surface charge and hydrophilicity were measured through FE-SEM,ATR-FTIR, contact angle measurement and Zeta potential analyzer, respectively. The permeationproperties including molecular weight cut-off (MWCO), pure water permeability and rejectionsto different inorganic salts were estimated by pressurized filtration tests. Eventually, theremovalperformance to organic dyes, the performance in the treatment of real dyeing wastewater,membrane fouling mechanism as well as the cleaning process were evaluated through submergedfiltration tests. The conclusions obtained were showed below:(1) The results of ATR-FTIR measurement showed that an active skin separation layer hadbeen successfully formed through the reaction of PQ-10, PVA and GA on the surface of thesupport PP porous fiber. The SEM results indicated that the surface of PQ-10/PP compositemembrane was compact and smooth, and the thickness of the skin layer was around0.3μm. Theresults of zeta potential measurement revealed that the PQ-10/PP composite membrane waspositively charged at pH ranging from3to9. (2) The separation performance of the obtained membrane was largely affected by thepolymer concentration, cross-linking reagent concentration, dip-coating time, cross-linking timeand additive concentration. The increase of PQ-10concentration in aqueous solution and/or theprolongation of the cross linking time resulted in an increase in the rejection to CaCl2of theresultant membrane at the expense of flux. With increasing the additive concentration and/orprolonging the dip-coating time, the CaCl2rejection of the resultant membrane increased firstlyand then decreased. Theoptimum membrane preparation parameters forthe obtained compositenanofiltration membranes were showed below: PQ-10=2.5w/v%, glutaraldehyde=1.7w/v%,dip-coating time=10min, cross linking time=13.5h, additive concentration=1.0w/v%. Thedesired membrane had a pure water flux of31.1l·m-2·h-1and rejections of92.5%and41.2%toCaCl2and NaCl, respectively, when tested with aqueous solution containing500mg/l electrolyteat0.3MPa and25℃.(3) The salts rejections of the PQ-10/PP membrane to different inorganic salts were asfollows: CaCl2(91.3%)>MgCl2(86.9%)> KCl (41.8%)>NaCl (38.8%)>MgSO4(25.1%)>CaSO4(24.2%) at neutral pH, which was the typical characteristic of the positively chargedpolymeric nanofiltration membrane. With increasing feed salt concentration, both salt rejectionand flux decreased. The desired PQ-10/PP membrane had a MWCO of around685Da andexhibited removals of higher than95.0%to dyes victoria blue B, brilliant green and congo red.The dye removal ratio was affected by both of the dye content and the concentration of theco-solute salt presented in the aqueous solution.(4) The desired composite hollow fiber membranes could be used to treat the dyeingwastewater through submerged filtration for color removal and COD reduction. Under thetrans-membrane pressure of0.07MPa and the temperature of25℃, the permeation flux, CODreduction and color removal to raw dyeing waste water were4.2l·m-2·h-1,73.6%and63.3%,respectively, while to the secondary dyeing wastewater, the permeation flux, COD reduction andcolor removal were4.9l·m-2·h-1,36.9%and18.5%, respectively. During the submerged filtrationprocess, membrane fouling was composed of hydraulicallyreversible fouling resulted from thedeposition of contaminants on the membrane surface and hydraulically irreversible foulingresulted from the adsorption of contaminations into the membrane pores. The ratio of thepermeation resistance of the reversible fouling layer to the total permeation resistance of the fouling layer was usually higher than60%. Higher trans-membrane pressureresulted in moresever membrane fouling, while lower trans-membrane pressure resulted in higher COD reductionand flux recovery rate. The addition of CTAB in cleaning solution could improve the cleaningefficiency.