Effects of coating technique on the physicochemical properties and structure of MoS₂-TiO₂/PVDF-PES for membrane distillation application

BACKGROUND: Membrane distillation (MD) for desalination encounters significant hurdles as a result of pore wetting, leading to reduced permeate flux or deterioration in permeate quality over time. Therefore, membranes used in MD must be highly hydrophobic to prevent liquid from entering the pores and contaminating the permeate stream. In this study, a thin layer of MoS₂-TiO₂ composite was coated using poly (DOPA) as an adhesive to enhance attachment. Two coating methods, interlinking and layering, were compared to determine the most effective method for forming a thin hydrophobic coating layer onto PVDF-PES hollow fiber membrane. RESULTS: Interestingly, this study revealed that increasing the number of dipping cycles led to higher flux, defying conventional expectations. This unexpected outcome is likely to have resulted from the thicker coatings creating a more hydrophobic surface, reducing water molecule adhesion and preventing wetting. Consequently, the membrane demonstrated enhanced flux despite its increased thickness. Among the membranes tested, the MoS₂-TiO₂/PVDF-PES membrane coated using the layering method exhibited the best performance, leading to a flux of 2.06 kg m⁻² h⁻¹ and 99.99% salt rejection. This superior performance can also be attributed to the membrane's hydrophobic properties, evident in its water contact angle (WCA) of 99°. By contrast, the interlinking method resulted in lower performance, with a permeate flux of 1.70 kg m⁻² h⁻¹ and 99.96% salt rejection, which is likely to have resulted from its lower WCA of 86°. CONCLUSION: The findings from this study suggest that the coating method has the potential to greatly improve membrane distillation performance through wettability modification rather than altering membrane porosity.