Fabrication Of Nanofibrous Composite Filtration Membrane For Water Softening

In recent decades, membrane softening is a new kind of water softening technology, belonging to nanofiltration(NF) separation process. It has been widely used for no pollution, no regeneration and simple operation. Since average pore size of the nanofiltration membrane is around 1nm and the film surface can obtain special electric charge, which can effectively remove divalent ions. The nanofiltration membrane used in water softening can efficiently removal of Ca2 +, Mg2 + and other divalent ions in water.Thin film composite(TFC) membranes have been widely researched for nanofiltration, which consist of a porous substrate providing mechanical support and a thin top barrier layer. It is considered as a key advantage that each layer in TFC membrane can be optimized. In this study, a series of high performance thin film nanofibrous composite(TFNC) nanofiltration membranes consisting of a polyelectrolyte(PEC) barrier layer, an ultrathin poly(acrylonitrile-co-acrylic acid)(PAN-AA) transitional mid-layer and a polyacrylonitrile(PAN) nanofibrous supporting layer, was successfully fabricated used as water softening composite membranes via layer-by-layer self-assemble technique(LBL) onto the double-layer nanofibrous substrates..The double-layer nanofibrous substrates were prepared by electrospinning a thicker layer of PAN nanofibrous mats firstly and then an ultrathin PAN-AA nanofibrous layer. The ultrathin PAN-AA hydrophilic transitional film layer was obtained by direct solution immersion method. The hydrophilic PAN-AA nanofibrous top layer was swollen to merge imperceptibly into an ultrathin transitional film layer on the PAN supporting layer by immersing PAN-AA/PAN double-layer mats into suitable buffer solution, by adjusting the nanofiber deposition time, buffer pH and immersion time to achieve the PAN-AA ultrathin transitional film layer, which could provide negative charge surface for attracting positive PEC and avoid the penetration of PEC solution during LBL.Compared with traditional coating method, the layer-by-layer self-assembly(LBL) is a controllable technique for fabricating composite membranes onto the nanofibrous substrates. And the filtration performance of membrane was evaluated with the obtained PEC barrier layer. Two kinds of PEC complex polydimethyldiallyl ammonium chloride(PDADMAC)/polyacrylic acid(PAA) and polyallylamine hydrochloride(PAH)/PAA were respectively covered multi-layers on PAN-AA/PAN nanofibrous substrate via LBL technique. An integrity hydrophilic barrier layer was established on the surface of substrate due to effect of static electric with polycation and polyanion, which is stable in structure and performance.The morphology and chemical structure of the as-prepared TFNC membranes were examined by scanning electron microscopy(SEM) and Fourier transform infrared spectroscopy(FT-IR), and the filtration performance of the composite membranes was investigated by cross-flow filtration test under 0.5MPa operating pressure. Water permeability and water contact angle of these composite membranes were also evaluated. The results indicated that, the permeate flux of the composite membranes decreased with the increase of the number of the PEC layers, while the rejection of salt solution increased sharply. It was found that the resulting PEC membrane can possess better filtration characteristic. Filtration performance of the resulting PDADMAC/PAA TFNC membranes was evaluated by different salt aqueous solution(CaCl2, MgCl2, MgSO4, Na2SO4) at 1000 ppm. It was showed that the composite membrane with 2 layers of PDADMAC/PAA could possess high flux(223.1 L/m2h) under the operating pressure(0.5 MPa). When concentration of PDADMAC is 0.3 wt %, the rejection of different salt aqueous solution with 11 layers of PDADMAC/PAA achieved the optimal effect as 89.6%, 83.3%, 73.5 % and 63.1%, respectively.The permeate flux and various salts rejection of PAH/PAA TFNC membranes were also measured. It was founded that the number of the polyelectrolyte multilayer determined the filtration performance of the composite membrane. When concentration of PAH is 0.4 wt %, the rejection of different salt aqueous solution with 11 layers of PAH/PAA achieved the optimal effect as 91.2%, 85.2%, 75.3% and 65.1%, respectively. The rejection of series salts for both PDADMAC/PAA TFNC membranes and PAH/PAA TFNC membranes were studied as follows: Ca Cl2>MgCl2>MgSO4>Na2SO4. Compared with PDADMAC/PAA polyelectrolyte composite membrane, the filtration performance of PAH/PAA composite nanofiltration membrane was better and more suitable for water softening applications.