Superhydrophobic membranes for membrane distillation and crystallization

Abstract

Membrane distillation is a promising alternative to conventional desalination processes, offering lower operating temperature and pressures, high salt rejection and the ability to process high concentration feeds. However, inefficiencies exist in the form of low permeate flux and membrane wetting. Superhydrophobic modifications to the membrane surface could potentially overcome this issue. While superhydrophobic surfaces have been known to display anti-fouling properties, how this affects membrane distillation performance is still not well understood. In this study, membranes were coated with TiO2 and fluoro-silane to create superhydrophobic surfaces for membrane distillation. The use of templating agents was found to affect the surface structure and wettability of superhydrophobic membranes, but once superhydrophobicity was achieved, there was negligible impact on membrane distillation performance. However, the presence of a superhydrophobic surface significantly improved the desalination performance of the membranes, with higher salt rejection and longer operating times compared to commercial hydrophobic membranes. Despite this, eventual pore wetting was still observed. The surface structure and wettability of superhydrophobic membranes also played an active role in solute interactions, leading to a significant difference in foulant rejection and crystal yield compared to hydrophobic membranes. Crystallization in membrane distillation was further examined using direct observation. Crystal growth occurred at isolated nucleation sites on hydrophobic membranes, while crystal deposition and growth remained largely uniform on superhydrophobic membranes. Static salt crystallization tests were used to verify the effects of the membrane properties on heterogeneous crystallization. The surface energy, roughness and membrane pore size played an active role in determining the crystal structure and yield. Finally, to improve the overall mass transfer, nanofiber membranes with high porosity and low thickness were fabricated for membrane distillation via forcespinning. Optimum salt rejection was achieved by using a composite membrane consisting of a microporous support with a thin nanofiber layer at the surface, which trapped the deposited salts within the nanofiber mesh and reduced the amount of solutes passing through to the microporous support. This indicates good potential for handling more aggressive feeds in membrane distillation.