Surface modification of glass fiber membranes by fluorographite coating for desalination of concentrated saline water with humic acid in direct-contact membrane distillation

Abstract

Amphiphobic materials are gaining popularity in many applications because they have self-cleaning properties. Self-cleaning substrates are important for applications in antifouling surfaces in seawater and antifouling membranes for desalination; membrane-based desalination and particularly distillation uses hydrophobic membranes. However, the lack of commercial amphiphobic membranes has prevented the exploration of customized membranes. In this research, we applied fluorographite (FGi) particles to glass fiber (GF) membranes with polydimethylsiloxane (PDMS) to render superhydrophilic filters into Amphiphobic membranes. The coating was applied via a dip/immersion method using hexane, a low-surface tension solvent, and PDMS paired with reaction assistants for the PDMS systems. The surface modifications introduced functionality to the membrane surfaces by the incorporation of FGi and PDMS, enhancing the evaluated amphiphobicity. Contact angles (CA) were observed using water and organic solutions to determine oleophobicity. We also evaluated the effects of the amount of FGi particles, PDMS, reaction assistants, and the drying and curing reaction times on the CA and liquid entry pressures (LEPs) of the coated membranes. The post-coating CA with water and the low-surfacetension liquid like dimethyl sulfoxide showed improved amphiphobicity. Furthermore, the surface characteristics were evaluated by Fourier-transform infrared spectroscopy and scanning electron microscopy to identify the chemical groups developed during coating and the morphological changes before and after membrane coating. Finally, the modified membrane was applied in direct-contact membrane distillation (DCMD) with a highly concentrated feed solution containing 1 M NaCl and 10 mg/L humic acid (HA) as an organic foulant. The DCMD results demonstrated the success of the coating in membrane distillation applications.