Oil/Water Mixtures and Emulsions Separation Methods-An Overview

Abstract

Catastrophic oil spill accidents, oily industrial wastewater, and other types of uncontrolled release of oils into the environment are major global issues since they threaten marine ecosystems and lead to a big economic impact. It can also affect the public health of communities near the polluted area. This review addresses the different types of oil collecting methods. The focus of this work will be on the different approaches to materials and technologies for oil/water separation, with a special focus on water/oil emulsion separation. Emulsified oil/water mixtures are extremely stable dispersions being, therefore, more difficult to separate as the size of the droplets in the emulsion decreases. Oil-absorbent materials, such as sponges, foams, nanoparticles, and aerogels, can be adjusted to have both hydrophobic and oleophilic wettability while displaying a porous structure. This can be advantageous for targeting oil spills in large-scale environmental and catastrophic sets since these materials can easily absorb oil. Oil adsorbent materials, for example, meshes, textiles, membranes, and clays, involve the capture of the oily material to the surface of the adsorbent material, additionally attracting more attention than other technologies by being low-cost and easy to manufacture.

Keywords: oil microdroplets; oil removing; oil-absorption materials; oil-adsorption materials; oil/water emulsions; oil/water separation; water removing.

Figure 1

(a) Photo of an oil/water emulsion; (b) Optical microscopy image of an oil/water emulsion before filtration. (c) Optical microscopy image of an oil/water emulsion after filtration. Adapted by permission from Springer Nature: Springer, Cellulose, All-cellulose composite membranes for oil microdroplet collection, Ana P. C. Almeida, João Oliveira, Susete N. Fernandes, Maria H. Godinho, João P. Canejo [Copyright] (2020) [3].

Figure 2

Fabrication of a superhydrophobic surface using copper foam as a substrate. Reprinted from Applied Surface Science, Vol 413, Wei Zhou, Guangji Li, Liying Wang, Zhifeng Chen, Yinlei Lin, “A facile method for the fabrication of a superhydrophobic polydopamine-coated copper foam for oil/water separation”, Pages No.140, Copyright (2017), with permission from Elsevier [42].

Figure 3

Demonstration images for the removal of hexadecane oil, dyed with red, from the water surface using a piece of foam controlled by a magnet (a–c); Reprinted from Colloids and Surfaces A: Physicochemical and Engineering Aspects, Bo Ge, Xiaotao Zhu, Yong Li, Xuehu Men, Peilong Li, Zhaozhu Zhanga, Versatile fabrication of magnetic superhydrophobic foams and application for oil/water separation, Pages No.4 Copyright (2015), with permission from Elsevier [50].

Figure 4

SEM images of (a) the original stainless-steel mesh and (b–d) the TiO₂-coated mesh surface at low and high magnifications, respectively. Reprinted from Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 489, Jian Li, Long Yan, Wenfang Hu, Dianming Li, Fei Zha, Ziqiang Lei, Facile fabrication of underwater superoleophobic TiO₂ coated mesh for highly efficient oil/water separation, Pages No. 3, Copyright (2016), with permission from Elsevier [52].

Figure 5

(a) A mixture of water (dyed with blue for easy observation) and petroleum ether was on the fabric surface, petroleum ether passed through the surface, and the water was held on the surface. (b) Spreading of petroleum ether on the coating surface. (c) After 3 min, the petroleum ether volatilized completely, and the coating recovered to its superhydrophobicity. Reprinted from Chemical Engineering Journal, Volume 231, Xia Zhang, Tie Geng, Yonggang Guo, Zhijun Zhang, Pingyu Zhang, Facile fabrication of stable superhydrophobic SiO₂/polystyrene coating and separation of liquids with different surface tension, Pages No. 414, Copyright (2013), with permission from Elsevier [64].

Figure 6

Schematic of a fouled membrane. Reproduced from [76].

Figure 7

Optical microscope image of electrospun cellulose acetate nanofibers.

Figure 8

Schematic of a typical electrospinning setup.

Figure 9

Manufacturing procedure of the PTFE nanofibrous membrane. Used from [92].

Figure 10

Different steps involved in the amine functionalisation of xGnP. Reprinted from Water Research, Volume 103, J.A. Prince, S. Bhuvana, V. Anbharasi, N. Ayyanar, K.V.K. Boodhoo, G. Singh, Ultra-wetting graphene-based PES ultrafiltration membrane—A novel approach for successful oil-water separation, Pages No. 311, Copyright (2016), with permission from Elsevier. [102].

Figure 11

Optical image of oil/water emulsion observed in optical microscope.

Figure 12

(a) Schematic showing the synthetic aerogel steps; (b) Optical image of fiber aerogel on a large scale of 2.5 L; (c–e) Microscopic architecture of fiber aerogels at various magnifications; Y. Si et al., “Superelastic and Superhydrophobic Nanofiber-Assembled Cellular Aerogels for Effective Separation of Oil/Water Emulsions”, ACS Nano, vol. 9, no. 4, pp. 3791–3799, 2015, https://doi.org/10.1021/nn506633b. Copyright (2015) American Chemical Society [111].

Figure 13

SEM images of (a,b) a simple PVDF membrane, (c,d) a PVDF membrane with pDA coating, and (e,f) the final membrane with SiO₂ nanoparticles added to the final membrane surface. Reprinted from Separation and Purification Technology, Volume 209, Jiuyun Cui, Zhiping Zhou, Atian Xie, Minjia Meng, Yanhua Cui, Siwei Liu, Jian Lu, Shi Zhou, Yongsheng Yan, Hongjun Dong, Bio-inspired fabrication of a superhydrophilic nanocomposite membrane based on surface modification of SiO₂ anchored by polydopamine towards effective oil-water emulsions separation, Pages No. 4, Copyright (2018), with permission from Elsevier [116].

Figure 14

Schematic illustration of the preparation process for AT-based NFMs. (a) Unsintered AT-based NFMs, (b) Sintered AT-based NFMs. Reprinted from Ceramics International, Volume 43, Yekai Zhu, Dajun Chen, Novel clay-based nanofibrous membranes for effective oil/water emulsion separation, Pages No. 9465, Copyright (2017), with permission from Elsevier [119].

Figure 15

Schematic showing the preparation and reaction mechanism of the superhydrophobic coated polyurethane foam Reprinted with permission from F. Wu, K. Pickett, A. Panchal, M. Liu, and Y. Lvov, “Superhydrophobic Polyurethane Foam Coated with Polysiloxane-Modified Clay Nanotubes for Efficient and Recyclable Oil Absorption”, ACS Appl. Mater. Interfaces, vol. 11, no. 28, pp. 25445–25456, July 2019, https://doi.org/10.1021/acsami.9b08023. Copyright (2019) American Chemical Society [124].