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Abstract: 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 0.01wt% 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 24h 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. [Copyright &y& Elsevier]

Increasing demand for oil and gas leads to the generation of substantial amount of produced water, bringing about deleterious impacts on the environment. Direct-contact membrane distillation (DCMD) could be a possible option for dewatering oil-in-water (O/W) emulsions because of many benefits brought by the DCMD process. However, these low surface tension solutions pose some difficult issues such as membrane fouling and pore wetting. The mechanisms involved are not fully understood due to the lack of study of the interaction between the emulsions and the membrane surface in the DCMD domain. To address the challenges, this study aims at developing a fundamental understanding of the relationship between surfactant-stabilized O/W emulsions and polyvinylidene fluoride (PVDF) membrane surface in DCMD operations. Effects of surfactant types (Span 20, Tween 20, and sodium dodecyl sulfate), oil concentration, and oil types (petroleum and vacuum pump oil) were systematically studied to better understand the fouling and wetting mechanisms involved. The results reveal that surfactant concentration and hydrophobicity had an influence on the membrane fouling and wetting behaviors. Surfactants with a lower hydrophilic-lipophilic balance (HLB) value could make the PVDF membrane surface less hydrophobic and cause less severe fouling by restraining the adsorption of oil droplets on the membrane surface. These findings suggest that membrane surface modification is required to achieve anti-fouling and anti-wetting properties to make DCMD an energy-efficient and effective technology for treating produced water. [ABSTRACT FROM AUTHOR]