Fouling and crystallisation behaviour of superhydrophobic nano-composite PVDF membranes in direct contact membrane distillation

• Greater inorganic solute rejection and anti-fouling properties for superhydrophobic membranes.
• Organic foulants migrate through all membranes in the absence of apparent partial pore wetting.
• Proposal of an adsorption–desorption mechanism for organic foulant transmission.

Porous PVDF membranes were coated with TiO2 and fluoro-silane compounds to create superhydrophobic surfaces for membrane distillation and crystallisation. The effects of organic and inorganic foulants on the membrane performance were investigated in direct contact membrane distillation. The surface structure and wettability of superhydrophobic membranes played an active role in solute interactions, leading to a significant difference in foulant rejection and crystal yield compared to virgin membranes. Organic foulants were found to penetrate both virgin and superhydrophobic membranes despite the absence of partial pore wetting, leading to the proposal of an adsorption–desorption foulant migration mechanism. In particular, organic foulant migration was dependent on the adsorption strength of the foulant to the membrane. An increase in hydrophobicity of the membrane surface led to a decrease in crystal yield. However, the crystal yield of superhydrophobic membranes improved under aggressive fouling conditions due to closer contact between the feed solution and the membrane surface.

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