(Lecture, Jan 9) Membrane Technology for Clean Water
time: 2018-01-09

Title: Membrane Technology for Clean Water
1:Understanding Nanofiltration: A Molecular Separation with Nanometer Effects
2:Approaches to Prepare HF PVDF based MF, UF & NF Membranes for Clean Water

Speaker: Professor Wang Xiaolin (Tsinghua University)
Time: 15:00–17:00p.m., Jan. 9th, 2018
Venue: Room 214, Henghua Building (Building No. 16), Wushan Campus

Understanding Nanofiltration: A Molecular Separation with Nanometer Effects
Xiao-Lin WANG
Department of Chemical Engineering, Tsinghua University, Beijing 100084, P.R. China

Abstract: Nanofiltration (NF) membrane, firstly named as “loose” Reverse osmosis (RO) or “dense” Ultrafiltration (UF) membrane, has two remarkable features: one is the molecular weight cut-offs (MWCO) ranges from 200 to 2000Da, and the other is the salt rejection depends on the ion valence and concentration. Several models for NF processes have been proposed, such as the pore model based on the sieving effect, the charge model based on the electrostatic effect, the electrostatic steric-hindrance (ES) model, and the Donnan steric pore model (DSPM) have been proposed, which play an important role in understanding the separation mechanism and promoting the application of NF. Afterward, almost all of the RO membrane manufacturers have produced a series of NF membranes for the purification and advanced treatment of water. However, the performances of these NF membranes with features of “loose” RO membranes cannot be predicted by commercial RO simulation software. It leads to a long period of previous experiments and scale-up process, which severely restricts the large scale standardization applications of NF. In regard to these problems, we proposed a simple simulation model for the separation performance of mixed salts solution across NF membranes to promote the application of NF during the water treatment in the light of the competitive effect among co-ions and regulation effect among counter-ions. Both two effects can be determined by some specific experiments. And then based on the in-depth experimental studies on rejection performance and the attendant electrokinetic properties, some researchers have found that the performance of NF membranes cannot be predicted completely by merely considering the sieving and electrostatic effect, but some drawbacks still exist in the analysis of electrokinetic properties. The further studies have contributed to a deeper understanding on the particular effect caused by the nano-scale pore size and charge features caused by the complicated interaction in solution. Moreover, the dielectric effect in the transport process of ions through NF membranes has been addressed and quantitatively analyzed. Recent studies have been paid much attention on the new generation of NF membranes improved by various nanostructured materials. We also made some try to develop some novel thin-film nano-composite NF membranes derived from the dual layer (PES/PVDF) hollow fiber UF membranes.

Approaches to Prepare HF PVDF based MF, UF & NF Membranes for Clean Water
Xiao-Lin WANG
Beijing Key Laboratory of Membrane Materials and Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, P.R. China

Abstract: Polyvinylidene fluoride (PVDF) hollow fiber (HF) Microfiltration (MF) membranes were firstly prepared via thermally induced phase-separation (TIPS) method, where diphenyl carbonate (DPC) and diphenyl ketone (DPK) were used as primary diluents. The liquid - liquid phase-separation phenomena were found and the monotectic points of PVDF/DPC and PVDF/DPK systems appeared at PVDF concentration approximately 30 and 56 %(wt), respectively. The effects of polymer concentration and quenching temperature on the pore structure, porosity and tensile strength of the membranes were also investigated. Secondly a novel HF PVDF based ultrafiltration (UF) membrane was prepared by forming a thick poly(sulfobetaine) (PSB) layer on the hollow fiber PVDF MF membrane. The PVDF based polySB UF membrane has sieving effects with the MWCO of 5.2 µm and 85~105 kDa, respectively, which contributed to the greatly improved hydrophilicity, membrane strength and thermal property. The another way to prepare a HF PVDF based UF membrane was forming a thick polyethersulfone (PES) outer layer by using the non-solvent induced phase separation (NIPS) method. Thirdly Nanoparticles (NPs) reinforced thin-film composite (TFC) membranes containing a range of 50~200 nm nanoparticles [MWCNTs, GOs, LTA zeolites] in a polypiperazine-amide (PA) thin film layer were synthesized via sequential interfacial polymerization on PES/PVDF hollow fiber substrates. The hydrophilization process of the NPs was conducted to ensure the homogenous dispersion in the aqueous phase containing piperazine prior to the interfacial reaction, and their morphologies in the PA layer were confirmed by FT-IR spectroscopy, SEM, EDX, XPS, and TEM. For all the NPs reinforced TFC membranes, the water flux increased significantly. The separation performances of the monovalent and divalent ions of NaCl/Na2SO4 solutions were conducted. Finally a novel thin film nanocomposite (TFN) hollow fiber membrane was fabricated comprising the sulfobetaine polymer functionalized multiwalled carbon nanotubes (ZCNT). The TFN(ZCNT) hollow fiber membrane had much narrower pore sizes than TFN(CNT) hollow fiber membranes, which was due to the grafting PSB layer at the end of the open-mouth-ended CNTs. By increasing the chain length of PSB, the TFN(ZCNT) hollow fiber membrane showed simultaneously improved water permeability and separation capacity of dextrans and electrolytes.