| Polyamide composite nanofiltration membrane is the most common membrane prepared by interfacial polymerization method. This kind of membrane has advantages of high flux, good chemical stability, etc. So many researchers are conducting further experienments on this kind of nanofiltration membrane and making the products commercialized. In the actual production and operation process, the main problem of nanofiltration treatment is the membrane fouling.Serious membrane fouling can cause the decrease of water flux and the reduction of thelongevity of membrane, which aggravates the burden of maintenance.There are different types of membrane fouling,such as organic fouling, inorganic fouling, microbial fouling,colloidal fouling, etc.A large number of organic foulants and inorganic ions in natural waters have a significant impact on the performance of nanofiltration membrane. Currently, there are only a few researchesabout the combined fouling caused by organic matters and inorganic ions. In the engineering application of nanofiltration membrane, membrane fouling caused by natural organic matter(NOM) and Ca2+ which are rich in natural waters will significantly reduce the economic benefits for the application of nanofiltration membrane in the actual production processes.This paper aimed to reveal the effects of fouling layers formed by different organic matters on the behavior of calcium sulphate scaling on the polyamide composite nanofiltration membrane surface. This research is consisted of two parts. The first part is the preparation of polyamide composite nanofiltration membrane using interfacial polymerization method, and characterization of the performance of polyamide composite nanofiltration membrane. The second part is investigating the influence of organic fouling layers formed by three different typical organic matters on calciumsulphate scaling behavior.Bovine serum albumin(BSA), humic acid(HA) and alginate(SA) were chosen as typical organic foulantsto contaminate the composite polyamide nanofiltration membrane. After that, the calcium sulphate scaling experiment was conducted. The morphological features of calcium sulphate crystals formed on different membranes were observed by using scanning electron microscopy(SEM).The interaction forces of membrane-calcium sulphate and calcium sulphate-calcium sulphate for different membranes were investigated by usingatomic force microscope(AFM)combined with self-made calcium sulphate probe. Quartz Crystal Microbalance with Dissipation(QCM-D) was used to simulate the foulants adsorption processin microscale.The quantification of calcium sulphate adsorption in different organic fouling conditions was measured. The main conclusions are as follows:(1) Ultrafiltration membranes were prepared by using polysulfone(PSF) as polymer material.Polyamide composite nanofiltration membranes were prepared by interfacial polymerization using piperazine(PIP) as aqueous monomerandtrimesoyl chloride(TMC) as organic monomer.(1)The ultrafiltration membrane was an asymmetric membrane with typical finger-pore structure.(2)Under 0.6MPa at 20±1℃, the pure water flux of polyamide composite nanofiltration membrane was 40±5 L/(m2·h),the molecular weight cutoff was 400±50Da. The flux of the membrane for 2000 mg/L Mg SO4 solution was 28L/(m2·h), and the rejection was above 90%.The flux sequence for several kinds of inorganic salt solutions was: Na2SO4<Mg SO4<Mg Cl2<Na Cl, and the sequence of rejection rate was Na2SO4 > Mg SO4 > Mg Cl2 > Na Cl. At room temperature, the contact angle is 55±2° with deionized water as the test solution.(3)According to the surface topography figures, the support membrane surfacewas relatively flat and the surface of polyamide composite nanofiltration membrane was rough. Accordingly, the results indicated that there was a relative rough layer formed on the membrane surface during the interfacial polymerization process.(4)Contents of element C, N, O in the polyamide composite nanofiltration membrane surface were: 81.76%, 9.62%, 8.62%, respectively,which approximated thetheoretical values of polyamide polymer.(5)Attenuated total reflectionfourier transformed infrared spectroscopy(ATR-FTIR) showed that a polyamide function layer formedon the support membranesurface.These test results showed that a selective polyamide layerwasgenerated on the support membrane through interfacial polymerization reaction by using PIP and TMC as monomers.(2)The organic fouling degree of polyamide composite nanofiltration membrane increased in the order of SA>HA>BSA.The calcium sulphate scaling degree of four kinds of membrane surface conditions decreased in the order of SA > HA > BSA>virgin membrane. The flux recovery rate after calcium sulphate scaling decreased in the order of virgin membrane > BSA > HA > SA. The results demonstrated that the calcium sulphate formed in SA fouling condition had a strong combinationwith the membrane surface and was not easily remove. Besides, it is showed that differentorganic foulantshad different influence on calcium sulphate scaling of nanofiltration membranes.(3)Combining the results of SEM images, AFM adhesionforces and QCM-D, it is indicated that the membrane surface propertieswas changed by the adhered organic macromolecules, and further impacted the formation mechanisms of calcium sulphate crystal. The interaction between SA and Ca2+could shorten the nucleation time of calcium sulphate crystals, enlarge the crystals size and result in the fastest decrease of membrane flux. The complexation between HA and Ca2+ increased the thickness and compaction of the scaling layer, causing a relative serious membrane flux decline.Besides, the weak Ca2+ combination ability and particular heart-shape molecular structure of BSA led to a lower degree of gypsum scaling compared to other two kinds of organic conditions.( 4) The calcium sulphates scaling of nanofiltration membrane could be significantly reduced if targeted NOMs could be effectively removed in the feed water pretreatment process. However, the specific operationsstill need further exploration. |
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