Novel membrane surface modification to enhance anti-oil fouling property for membrane distillation application

• A novel approach was explored to obtain a hydrophilic surface for PVDF membrane.
• The modified membrane was used for membrane distillation of feed containing oils.
• The mechanism of oil affinity onto a membrane surface was studied by the XDLVO theory.
• The modified membrane presents a stable water flux over 24 h of operation without oil fouling.
• The pH value of the feed solution plays an important role in oil fouling.

This study aims at developing an effective method to modify hydrophobic polyvinylidene fluoride (PVDF) membranes for membrane distillation (MD) which is tolerant to salty feed solution containing mineral oil. A novel approach was explored to obtain a highly hydrophilic membrane surface via plasma induced grafting of polyethylene glycol (PEG) and subsequent TiO2 particles deposition onto the membrane surface. The mechanism of oil affinity onto a membrane surface was quantitatively investigated by the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. A series of characterizations were conducted for the virgin and modified membranes. Membrane distillation tests for synthetic seawater containing 0.01 wt% of mineral oil were also performed to investigate the influences of membrane hydrophobicity, membrane surface structure, pH value of the feed solution and hydrodynamic conditions on oil fouling behavior.The SEM and FTIR measurements confirmed the presence of PEG and TiO2 particles on the PVDF membrane substrate. The hydrophilically modified PVDF-PT membrane exhibits a positive interfacial free energy with oil in the feed, indicating the TiO2 deposition effectively improve the anti-fouling property of PVDF-P membrane and PVDF membrane. It was also found that the PVDF-PT membrane presents a stable water flux over 24 h of operation without fouling and wetting due to combined effects of enhanced surface hydrophilicity and reduced membrane pore sizes. In addition, the pH value of the feed solution plays an important role in oil fouling. More severe fouling can be observed at the pH which made the membrane surface and oil droplets carry opposite charges. Moreover, a higher flow velocity enhances the shear force exerted on the oil droplets, thus leading to less oil present on the membrane surface and less oil fouling of the membrane.