Membranes for Oil/Water Separation: A Review

Abstract

Recent advancements in separation and membrane technologies have shown a great potential in removing oil from wastewaters effectively. In addition, the capabilities have improved to fabricate membranes with tunable properties in terms of their wettability, permeability, antifouling, and mechanical properties that govern the treatment of oily wastewaters. Herein, authors have critically reviewed the literature on membrane technology for oil/water separation with a specific focus on: 1) membrane properties and characterization, 2) development of various materials (e.g., organic, inorganic, and hybrid membranes, and innovative materials), 3) membranes design (e.g., mixed matrix nanocomposite and multilayers), and 4) membrane fabrication techniques and surface modification techniques. The current challenges and future research directions in materials and fabrication techniques for membrane technology applications in oil/water separation are also highlighted. Thus, this review provides helpful guidance toward finding more effective, practical, and scalable solutions to tackle environmental pollution by oils.

Keywords: antifouling; membrane design; membrane materials; membrane technologies; oil/water separation.

Figure 1.

Schematic representation of membrane exposed to a liquid droplet. a) Young’s, b) Wenzel’s, and c) Cassie–Baxter’s. Adapted with permission.^[16] Copyright 2018, Wiley-VCH.

Figure 2.

a) The upper images show coated cotton fabric self-cleaning performance and Ag-TiO₂@PDMS photocatalytic coated cotton fabric efficiency in the bottom. b) Photocatalysis mechanism of the same materials in (a) for methylene blue and oil. Reproduced with permission.^[39] Copyright 2022, Springer Nature B.V. c) photocatalytic and superwetting of Ag₂O/TiO₂@CuC₂O₄ membranes for the application of dye removal and oil/water separation, Reproduced with permission.^[40] Copyright 2021, Elsevier Ltd. d) Figure illustrates rotating reactor and preparation of integrated photocatalysis-adsorption-membrane separation from Ag@BiOBr/AC/GO in rotating reactor for RhB removal. Reproduced with permission.^[27] Copyright 2020, Elsevier Ltd.

Figure 3.

Schematic illustration of pressure-driven membrane filtration processes. Reproduced with permission.^[65] Copyright 2017, Elsevier B.V.

Figure 4.

A chart illustrates the most common membrane materials.

Figure 5.

Examples of metallic membranes: (i) schematic diagram of superhydrophobic 3D ZIFI preparation and associated digital photos of water droplets and immersed membrane in water under external force (a–d). Adapted with permission.^[69] Copyright 2020, Elsevier B.V. (ii) membrane surface morphology via FESEM images shows superhydrophobic copper boat floating top view on a water surface and a side view of the weight-loaded superhydrophobic boat immersed in water (a–k). Adapted with permission.^[67] Copyright 2017, Elsevier B.V.

Figure 6.

i-a) schematic illustration of PAN membrane with a hydroxylamine-induced phase-inversion technique and corresponding water droplet on the membrane. Adapted with permission.^[92] Copyright 2017, Elsevier B.V. ii-a) Electrospinning setup of CA nanofiber membrane, b) deacetylating of CA nanofiber d-CA nanofiber membrane. c) Schematic diagram of oil/water mixture based on selective separation, d) schematic diagram illustrating emulsified oil/water separation. Adapted with permission.^[96] Copyright 2019, Elsevier B.V.

Figure 7.

i) Scheme illustrates co-axial electrospinning setup for FPB/SNP nanofiber membrane modified with PI/CA: membrane designed as cellulose-acetate (CA-shell), polyamide acid (PAA-core), polyimide (PI-core). Adapted with permission.^[57] Copyright 2016, Elsevier B.V. ii) SEM images of PES-G-PANCMI membranes and cross-section view and membranes optical image. Adapted with permission.^[75] Copyright 2016, Elsevier Ltd.

Figure 8.

The inorganic-based membranes; i) membrane structure characterization with photographic images and SEM images beside crude oil used to demonstrate the anti-oil-adhesion and self-cleaning performance and crossflow oil/water separation. Adapted with permission.^[124] Copyright 2018, American Chemical Society. ii) Schematic description of inorganic ZnO-Co₃O₄ overlapped membrane structure and switchable wettability when immersed in different media and the corresponding separation capacities of oil/water emulsions, as well as the SEM images of the inorganic membrane and the results of the span80-stabilized water-in-diesel emulsion. Adapted under the terms of the CC-BY 4.0 license.^[125] Copyright 2015, The Authors, published by Springer Nature.

Figure 9.

i) Metal-organic framework (MOF)@GO nanocomposite. Adapted with permission.^[132] Copyright 2020, American Chemical Society. ii) separation process of diesel oil with Z8/PC-2 monolith MOF membrane, Adapted with permission.^[47] Copyright 2020, Elsevier Ltd. iii) MOF-based film procedure and color change of ESSM@PDA@MOF after the heat and cold treatments and antifouling tests with self-cleaning ability against crude oil of the MOF-based film. Adapted with permission.^[135] Copyright 2018, Royal Society of Chemistry.

Figure 10.

i) Schematic diagram of the synthesis procedure of zwitterionic nanogels dispersion and PAN nanofibrous modification. ii) TEM and b) SEM images of the zwitterionic nanogels iii) underwater anti-oil-adhesive performance of the ZPAN membrane. Adapted with permission.^[169] Copyright 2020, Elsevier B.V.

Figure 11.

(i) fabrication of polymer/PDA-coated membrane and illustration of the polymer/PDA-coated membrane. Adapted with permission.^[170] Copyright 2019, Elsevier B.V. ii) Bio-inspired hollow PDMS sponge, cactus water absorption and storage space schematic, and optical image of oil absorption in hollow PDMS sponge and schematics showing PDMS sponge fabrication. Adapted with permission.^[176] Copyright 2018, Elsevier B.V.

Figure 12.

i-a) Schematic of the fabrication of superhydrophobic cotton fabrics through vapor phase deposition process. b) Water droplets on the superhydrophobic cotton fabric and hexadecane droplets spread and permeate through the fabric. c) Water droplets on the coated textile, d) a jet of water bouncing off the surface, e) the textile immersed in water by an external force, f) water droplets on the oil-contaminated textile, Adapted with permission.^[182] Copyright 2013, American Chemical Society. ii) Synthesis of carbon foam using LPF resin. Adapted with permission.^[183] Copyright 2021, Elsevier.

Figure 13.

Different types of external stimuli on membrane materials.

Figure 14.

a) Schematic of an environmentally responsive cotton fabric prepared by coating a μ-PDMS-b-PDMAEMA-b-PIPSMA ABC miktoarm terpolymer and its permeability after contact with oil or water, and photographs of continuous separation of DCE water–hexane triple mixtures. Adapted with permission.^[195] Copyright 2019, American Chemical Society. b) Schematic illustration of fabrication process of photothermal responsive Au nanorods/pNIPAm-co-AAm hybrid SWCNT ultrathin membranes. Adapted with permission.^[196] Copyright 2015, American Chemical Society. c) Characterization of morphology and wettability of water and oil on the PANI mesh, and graphic mechanism diagrams of the electric field induced oil/water separation process based on the PANI mesh. A small amount of water in the device proved that the mesh film could not permeate the oil/water mixture, and electric field-induced oil/water separation. Adapted with permission.^[197] Copyright 2016, Wiley-VCH.

Figure 15.

Schematic illustration i-A) manufacture of PDA modified cotton fabric and kapok fabric. B) Modified fabrics were applied to selectively separate oil and water. Adapted with permission.^[222] Copyright 2020, Wiley-VCH. ii) The fabrication and design of TFC pervaporation membranes via LbL interfacial polymerization. Adapted with permission.^[223] Copyright 2019, Elsevier B.V.

Figure 16.

i) Schematic of preparation Janus fabric membrane and ii) water droplets on surfaces of Janus fabric at the oil/water interface. Adapted with permission.^[227] Copyright 2018, Elsevier B.V. iii) Schematic representation of the a) PFOTS/CNT functionalization, b) the c-PVA/f-CNT nanocomposite fabrication process, and d) the crosslinking details of the c-PVA. iv) SEM Images of Janus c-PVA nanofiber membrane. Adapted with permission.^[231] Copyright 2019, Elsevier B.V. v) Janus fabric membrane fabrication using liquid/liquid interface-confined surface engineering strategy and possible mechanism. Adapted with permission.^[232] Copyright 2019, Elsevier B.V.

Figure 17.

i) 3D printing process via FDM. a) 3D printing orthogonal mesh considering diameter, spacing, and layers. b) Floating oil removal by 3D-printed spherical oil skimmer and (c) diesel dyed in green with barrel skimmer. Adapted under the terms of the CC-BY 4.0 license.^[251] Copyright 2019, The Authors, published by MDPI. ii-a) 3D printing of a porous membrane using nanosilica filled PDMS ink and images of b) pristine steel mesh and the printed membrane under bending and stretching, c) image of membrane pores and costed with superhydrophobic PDMS/silica coating. Adapted with permission.^[252] Copyright 2017, Royal Society of Chemistry.

Figure 18.

i) This figure illustrates oil/water separation device using two antagonistic polymer. Adapted with permission.^[299] Copyright 2015, American Chemical Society. ii) Spilled oil collection via SSM@ZnO@PFOTS membrane prepared with inspiration design to mimic water skipper float on water. Adapted with permission.^[300] Copyright 2019, Elsevier B.V. iii) Figure shows device through the targeted treatment of silica meshes. The inset panel (A) shows device components of (i) glass syringe plunger, (ii) shortened glass syringe barrel, (iii) silicone adhesive, and (iv) superhydrophobic silica membrane. Panel (B) shows the device collects a range of oils from water and pannel (C) dispersed oil droplets. Adapted under the terms of the CC-BY 3.0 license.^[301] Copyright 2015, The Authors, published by Taylor & Francis. iv) Shows a functionally integrated device for oil/water separation, (a) a drop of water placed on the outer surface of the device. b) Upon tilting the device, the drop of water slid off quickly. c) A drop of water is placed inside the device. d) Water drop placed on the surface of the untreated copper foam. Adapted with permission.^[302] Copyright 2015, Royal Society of Chemistry.