Effects of inorganic salts in the casting solution on morphology of poly(vinyl chloride)/bentonite ultrafiltration membranes

Modified polyvinyl chloride (PVC) composite ultrafiltration (UF) membranes are prepared by blending bentonite and varying amounts of different salts such as KCl, NH4Cl, NaCl, CaCl2, and MgCl2 in the casting solution (CS). Thermodynamics of PVC- based systems during phase inversion are determined by locating the binodal curve using Flory–Huggins theory. The modelled binodal curves are validated with the experimental binodal curve obtained from the cloud points data and found in close agreement with each other for different PVC-based systems. The blending of salt in the CS has two contradictory effects on the phase inversion rate as well as membrane morphology. A polymer-solvent axis shift occurs in the PVC/bentonite system in the presence of salts, reducing the thermodynamic stability, and a porous membrane with a 'finger-like' structure is obtained. In contrast, the viscosity of CS increases when salt is present in the CS, resulting in an increase in kinetic hindrance, and a membrane with a spongy structure is obtained. Hence due to the trade-off relation of thermodynamic and kinetic properties, the optimum salt concentration in the CS was evaluated by tailoring these properties. All the membranes involving salt and bentonite in the CS possessed enhanced membrane performance compared to the membrane without salt in CS (i.e., P1-membrane), and the best performing membrane is obtained for 1.0 g KCl in the CS (i.e., PK1.0). An enhanced pure water flux (i.e., 415.55 L m−2 h−1), permeate flux (i.e., 251.08 L m−2 h−1), oil rejection (i.e., 97.25%), and fouling resistance ability (i.e., FRR: 74.85%) is obtained for PK1.0 membrane due to improved membrane hydrophilicity and surface roughness.