Development of high flux PVDF/modified TNTs membrane with improved properties for desalination by vacuum membrane distillation

In this work, titanium dioxide nanotubes (TNTs) were first synthesized via hydrothermal method at 110 °C. To reduce hydrophilicity, the synthesized TNTs were annealed at 350 °C and then aged in air at ambient temperature. Subsequently, polyvinylidene fluoride (PVDF) mixed matrix membranes (MMMs) containing the modified TNTs were prepared by phase inversion method. Performance of the prepared PVDF/TNTs MMMs was evaluated in vacuum membrane distillation (VMD) process. Effects of polymer concentration and content of the modified TNTs on the fabricated membrane performance in the VMD process were investigated. Hydrophilicity reduction of the synthesized TNTs after annealing and aging was confirmed by FTIR analysis. It was found that the modified TNTs with partial hydrophilicity could improve the VMD performance of the fabricated MMMs. The results showed that the modified TNTs improve the membrane porosity, finger-like cavities, thermal and mechanical properties without having a significant negative effect on the hydrophobicity of the PVDF membrane. Compared to the neat PVDF membrane, water flux and salt rejection of the MMM containing 2 wt% of the modified TNTs were improved by 38.7% and 47.0%, respectively. Response surface methodology (RSM) was used to study the effects of operating parameters of VMD process such as feed flow rate, feed temperature and feed concentration on the membrane performance. The obtained optimum conditions are: flow rate of 36.99 LPH, temperature of 77.79 °C and concentration of 10.18 g/l. Under optimum conditions, water flux and salt rejection were maximized as 92.55 L/m2.h and 99.9%, respectively. The prepared PVDF/TNTs MMMs with improved properties can be used in MD applications successfully.