Abstract
Robust hydrophobic and superhydrophobic mixed matrix electrospun nanofibrous membranes (MM-ENMs) have been prepared from low- and high- molecular weight polyvinylidene fluoride with either multi-walled carbon nanotubes or graphene oxide nanofillers (0.05–0.5 wt%). The polymer solutions' properties, including their electrical conductivity, viscosity, and surface tension, were determined and used to guide the design of single-, dual-, and triple-layered MM-ENMs combining layers with different hydrophobic character. All MM-ENMs were subsequently prepared and characterized in terms of their morphology, hydrophobicity, mechanical properties, and direct contact membrane distillation (DCMD) performance. A thinner hydrophobic layer with a thicker hydrophilic support layer in dual-layered MM-ENMs reduced water vapor transport resistance and improved DCMD performance relative to single-layer MM-ENMs. Conversely, placing an intermediate hydrophilic layer between two hydrophobic layers in triple-layered MM-ENMs promoted water condensation (water pocket formation) and thus reduced DCMD performance. Over 10 h DCMD, the best-performing dual-layered MM-ENM allowed ultra-high permeate fluxes of up to 74.7 kg/m2 h while maintaining a stable permeate electrical conductivity of around 7.63 μS/cm and a salt (NaCl) rejection factor of up to 99.995% when operated with a feed temperature of 80°C, a permeate temperature of 20°C, and a feed solution containing NaCl at a concentration of 30 g/L.
Highlights
Membrane Distillation (MD) is a thermal separation process in which porous hydrophobic membranes are used as the separation media
A higher viscosity promotes the formation of thick fibers; a higher electrical conductivity favors the formation of thinner fibers, while a high surface tension promotes bead formation
MM-electrospun nanofibrous membranes (ENMs) with favorable mechanical and hydrophobic properties for direct contact membrane distillation (DCMD) were electrospun by mixing nanofillers (MWCNs or graphene oxide (GO)) with polymeric solutions of different poly vinylidene fluoride (PVDF) molecular weights, Low molecular weight PVDF (LMPVDF) and High molecular weight PVDF (HMPVDF)
Summary
Membrane Distillation (MD) is a thermal separation process in which porous hydrophobic membranes are used as the separation media. Various strategies have been proposed to improve the properties and efficiency of direct contact membrane distillation (DCMD) membranes These include coating with hydrophobic nanoparticles [9,10,11,12,13], the use of Surface Modifying Macromolecules (SMMs) [8,14,15], blending [16,17,18], incorporating nanofillers [19,20,21,22], and using dual and triplelayered with hydrophobic/hydrophilic structures [23,24,25,26,27,28,29]. Different triple-layered MM-ENMs were prepared by changing the nature of the middle layer while keeping the electrospinning time (i.e. thickness) of the top and bottom layers the same
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