Hydrophilic Fe2O3 dynamic membrane mitigating fouling of support ceramic membrane in ultrafiltration of oil/water emulsion
Graphical abstract
Introduction
Oil-in-water (O/W) emulsion is stable dispersion of microscopic oil droplets (size < 10 μm) in an aqueous solution and daily produced by various industrial activities [1], [2], [3]. The ever-increasing O/W emulsion with high pollution loads has become a severe environmental issue. Ceramic ultrafiltration membrane is an effective technology for O/W emulsion separation. It has been widely applied in the treatment of O/W emulsion with its advantages of no addition of chemicals, relatively low operation cost and stable quality of purified water [4], [5], [6]. However, the performance of ceramic UF membrane is compromised by severe oil fouling during the treatment of O/W emulsion, which remains a major challenge for ceramic membrane to be successfully applied in O/W emulsion treatment [7], [8], [9].
Extensive efforts have been dedicated toward membrane modification through introducing hydrophilic material to reduce membrane fouling, e.g., the antifouling ability of PVDF membrane was significantly enhanced through post-fabrication tethering of surface-tailored superhydrophilic silica nanoparticles. [10], [11], [12], [13]. However, the process of membrane modification is relatively complicated, and thus leading to an increase cost in practical industrial application. Dynamic membrane has advantages of simple preparation, ease of removal and regeneration [14]. It can be formed in situ by deposition of a suspension on a porous support, which was used as filtration membrane to enhance the rejection rate of protein or salt, etc. [15]. In the treatment of O/W emulsion, membrane fouling is mainly caused by direct contact and interaction between oil droplets and membrane surface [16]. Therefore, hydrophilic dynamic membrane formed on the surface of support membrane can be used to mitigate fouling of support membrane through enhancing the hydrophilicity of membrane surface and reducing exposures of support membrane to oil foulants.
Dynamic membranes can be classified into two basic types: pre-coated membrane and self-forming membrane [17]. Pre-coated membrane is formed before the filtration of target solution by deposition of dynamic membrane material on the surface of a porous support. Self-forming membrane is formed during the filtration of target solution that contains dynamic membrane material. To our best knowledge, antifouling performance of these two types of dynamic membranes has never been compared, which will be helpful for the selection of appropriate dynamic membrane for O/W emulsion treatment. In addition, since membrane fouling is inevitable, an efficient membrane cleaning method for dynamic membrane under mild conditions is essential for its sustainable O/W emulsion treatment.
In this study, highly hydrophilic Fe2O3 dynamic membrane prepared by two methods was used as a protective membrane to reduce the fouling of support ceramic membrane in O/W emulsion treatment. The antifouling performance of Fe2O3 composite membranes (i.e., Fe2O3 dynamic membrane and ceramic memebrane) was compared, and possible fouling mechanism was proposed. In addition, correspondingly cleaning method for Fe2O3 composite membranes was developed. This study provides a facile strategy to effectively reduce the fouling of ceramic membrane and enhance membrane cleaning efficiency in oily wastewater treatment.
Section snippets
Preparation and characterization of O/W emulsion
Crude oil (Saudi Aramco) and sodium dodecyl benzene sulfonate (SDBS) (Sigma–Aldrich) were used as base oil and surfactant, respectively, to prepare oil/water (O/W) emulsion. Crude oil (100 mg L−1) and SDBS (10 mg L−1) were mixed in ultra-pure water (Milli-Q Academic, Millipore) and sonicated (Q700, Qsonica) for 50 min. The prepared O/W emulsion was at pH 6 (i.e., pH of the ultra-pure water) and stable in terms of average oil-droplet size and zeta potential for 5 days. All the experiments were
Oil/water emulsion properties
Table 1 presents major characteristics of crude oil/water emulsion. O/W emulsion was negatively charged at pH 6.0 and pH 8.0. The zeta potential of oil droplets slightly increased from −45.5 ± 3.0 to −36.52 ± 3.0 mV with pH increase. The average size of oil droplets at pH 6.0 was almost the same as that at pH 8.0, which was independent to the pH. This phenomenon is consistent with that reported by Lobo et al. [24]. The images of oil droplets show that the emulsion was composed of fine oil droplets
Conclusion
- 1.
Pre-coated Fe2O3 dynamic membrane had better performance for O/W emulsion treatment than self-forming dynamic membrane in terms of reducing ceramic membrane fouling. It is a simple and efficient strategy to reduce the fouling of support membrane in oily wastewater treatment.
- 2.
Fouling mechanism for two types of Fe2O3 dynamic membranes was proposed and confirmed, which helps to select appropriate dynamic membrane for oily wastewater treatment.
- 3.
High cleaning efficiency for Fe2O3 composite membranes
Acknowledgements
This work was supported by National Key Technology Support Program of China (2012BAC05B02). The authors want to thank Daqing Oilfield Company of China for providing the crude oil for the experiment.
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