Performance improvement of polymer membranes with nano-additives for water purification

Abstract

Membrane fouling, which is mainly caused by adsorption of non-polar solutes and hydrophobic particles, is directly related to hydrophobicity of the membrane, and thus membranes are often modified to improve their hydrophilicity. Membranes are modified via various techniques to improve their performance of which the incorporation of inorganic nanoparticles in polymeric membranes has shown great potential. The effort to improve the membrane performance is still an ongoing progress and current trend in the field of membrane research is to develop new membrane materials and structures specifically to reduce fouling effects and to improve their function. A new two-dimensional zeolitic imidazolate framework (ZIF) with leaf-shaped morphology (ZIF-L) was incorporated into polyethersulfone (PES) ultrafiltration membranes to investigate how the ZIF nanoflakes affect membrane properties. The modified UF membrane with 0.5% ZIF-L loading showed around 75% increase in water flux while retaining its solute rejection. Also, the same membrane showed almost twice the fouling resistance with more than 80% water flux recovery. The improvement was due to the combined effect of the more negative zeta potential of the modified membrane, increased hydrophilicity and reduced surface roughness. While studying the stability of ZIF-L in organic solvent for the preparation of PES membrane, a new phase transformation of a zinc-2-methylimidazole-based ZIF was discovered, from ZIF-L to ZIF-8. The potential of the ZIF phase transformation in various solvents and also in solid phase were studied. Results indicated that the phase transformation occurs in the solid phase via the geometric contraction model (R2), a kinetic model new to ZIF. This work demonstrates the first topotactic phase transformation in porous ZIFs, from a 2D layered structure to a 3D structure, and provides a new insight into metal–organic framework crystallization mechanisms. Additionally, nanoporous titania nanoparticles synthesized via hydrothermal reaction was doped into polyethersulfone (PES) ultrafiltration membranes at a low concentration to improve nanoparticles dispersion in dope solution. All modified membranes showed narrower pore size distribution as TiO2 loading was increased. The composite membrane with 0.5% TiO2 nanoparticles loading showed ~20% increase in water flux and improvement in solute rejection (rejection of 100 kDa PEG from ~90% to ~92%). In addition, the same membrane showed improved fouling resistance (fouling rate of 0.58 kPa/ min compared to 0.70 kPa/ min of control membrane) with about 79% water flux recovery due to increased hydrophilicity, reduced surface free energy and reduced pore size. Beyond optimum loading of TiO2, the improvement was less significant due to the effect of agglomeration. Modification of the substrates’ properties is one of the most effective methods to improve the performance in forward osmosis (FO) process. A new Zn2GeO4 nanowire-modified PES ultrafiltration substrate with increased surface porosity and high water flux was used as a substrate for the fabrication of thin film composite (TFC) FO membrane, by coating a thin layer of polyamide on top of the substrate. The composite TFC membrane showed ~45% increase in water permeability and NaCl salt rejection of 80% under RO mode. In FO mode, the ratio of water flux to reverse solute flux was improved.