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Effective separations of oil/water mixtures and emulsions are challenges worldwide because of the expanding production of industrial oily wastewaters and the frequent oil spills that arise from industrial accidents and the sinking of oil tankers and other ships. In 2010, the explosion of BP’s Deepwater Horizon oil rig resulted in 210 million gallons of oil being released into the Gulf of Mexico.
Traditional techniques for oil/water separation such as air floatation, gravity separation combined with skimming, oil-absorbing materials, coagulation, and flocculation are limited and are not effective for separating emulsions, making further treatment necessary. Hence, a facile synthesis of superoleophobic or superhydrophobic materials is gaining a lot of attention from the industries.
As a result, there is a need to develop new materials that would allow oil/water separations to be performed efficiently, at low cost, with high selectivity. Recently, materials possessing both superhydrophobic and superoleophilic properties have attracted broad attention because of their capacity to mediate the efficient separation of oils, organic pollutants, and other hydrophobic organic solvents from the water. Although such previously developed materials can be effective agents for oil/water separation, they are readily fouled, or even blocked up, by oils because of their intrinsic oleophilicity. Furthermore, because water is generally denser than oil, it tends to settle below an oil phase, forming a barrier layer above the separation material and inhibiting oil permeation into it. Accordingly, materials possessing superhydrophobicity and superoleophilicity are not suitable for the separation of water-rich oil/water mixtures or oil-in-water emulsions.
Inspired by the wetting behavior of fish scales, it has been possible to construct underwater superoleophobic surfaces in oil/ water/solid three-phase systems. The development of simple, inexpensive, eco-friendly, and readily scalable fabrication process superhydrophilic and underwater-oleophobic materials may lead to a more practical, alternative, and feasible approach for oil/water separations.
Oleophobicity is a phenomenon where a material does not allow oil to spread on it, i.e. the contact angle of oil on the material is >90°. However, oleophobic materials do not ensure the complete non-wetting behavior in a material.
In order to ensure a complete non-wetting behavior, the contact angle between the material and oil must be >150°. This is known as a superoleophobic material.
RELATION BETWEEN AREA OF CONTACT, SURFACE ROUGHNESS ON OLEOPHOBIC BEHAVIOR
This is Cassie- Baxter equation for the contact angle of oil in an oil/water/solid system with a rough surface where, the area fraction of the solid the contact angle of the oil droplet on a smooth surface in water the contact angle of the oil droplet on a rough surface in water
A smaller area fraction indicates a lower opportunity of the oil droplet contacting the solid surface, and the larger the contact angle of oil in water. The melamine sponges possess a very rough surface, which intimates a rather small area fraction of solid and a large oil contact angle.
This is Young’s equation for the contact angle of oil in water on a flat surface where
?OA: the oil/air interface tension
?OA: the contact angle of oil in the air
?WA: the water/air interface tension
?WA: the contact angle of water in the air
?OW: the oil/water interface tension
?OW: the contact angle of oil in water
Since the surface tension of oil and organic liquids is much lower than that of water, we can see that hydrophilic surfaces in the air can become oleophobic in water. The melamine sponge became superoleophobic when immersed in water. Underwater oil droplets were nearly spherical on the melamine sponge surface and exhibited high contact angles (> 150°). The rough surface structure and superhydrophilicity of the melamine sponge combined to result in a particular wettability characterizing an oil/water/solid three-phase system.
Melamine sponge is a commercially available 3-dimensional porous material consisting of a formaldehyde-melamine-sodium bisulfite copolymer. These sponges exhibit superhydrophilicity and superoleophilicity. Melamine sponges pre-wetted with water display superhydrophilicity and superoleophobicity and could be used in effective oil/water separation.
Wang et al. presented a simple and inexpensive dipping method for the fabrication of a superhydrophilic and underwater super oleophobic polyvinylpyrrolidone (PVP)-modified melamine sponge.
Melamine sponges are commercially available three-dimensional porous materials exhibiting superhydrophilicity and underwater superoleophobicity.
When dipped in a PVP solution to enhance its oleophobicity, the as-prepared modified melamine sponge exhibited high separation capacity, allowing separation of oil/water mixtures continuously for up to 12 h without any increase in the oil content in the filtrate.
The excellent performance of the PVP modified melamine sponge in oil/water separations and its preparation through an industrially feasible process suggest that it has potential applicability in both academic and industrial settings.
Behavior OF RAW SPONGE
When a water droplet was placed on the surface of the raw melamine sponge, it spread out and permeated into it instantly, resulting in a contact angle of approximately 0°. The same situation occurred when using a droplet of oil. Both processes were complete within 1 s, suggesting both hydrophilicity and oleophilicity of the melamine sponge in the air.
The melamine sponge became superoleophobic when immersed in water. When we immersed the melamine sponge in water, water became trapped within its rough microstructure (a water layer is formed on the skeleton of the sponge) that then formed an oil/water/ solid composite interface in the presence of oil. The trapped water molecules significantly decreased the contact area between the oil and the surface of the sponge, leading a large oil contact angle.
The pre-wetted sponge showed similar behavior for other organic solvents like n-hexane, diesel, and isooctane.
Oil/water separation experiment, driven by gravity alone, was performed where a sponge was fixed between two glass tubes and then an oil/water mixture (1:2 v/v) was poured into the upper tube. The water quickly passed through the pre-wetted melamine sponge and entered the beaker below. Meanwhile, all of the oil was retained above the sponge, due to the underwater superoleophobicity of the pre-wetted melamine sponge. The flux was quite high.
PROBLEM WITH RAW SPONGE
In continuous oil/water separation experiments of diesel/water mixtures, however, the diesel would permeate through the melamine sponge within 3 minutes.
The raw sponge was unable to separate emulsions.
NECESSARY MODIFICATION TO RAW SPONGE
The melamine sponge was modified with polyvinylpyrrolidone (PVP) to enhance its oleophobicity by the following process.
Polyvinylpyrrolidone (PVP) (C6H9NO)n is a water-soluble polymer. It is soluble in water and other polar solvents. It exhibits excellent wetting properties in solution form and forms films. Hence, it can be used as a coating agent or as an additive to ensure a good coating.
A piece of raw sponge was soaked in 1.0 wt% aqueous PVP for 30 min. The treated sample was dried at 85 °C. It was then cured at 150 °C for 5 min. The obtained sample was washed multiple times with hot (50 °C) water. In the compression procedure, for emulsion separation experiments the PVP-modified sponge was compressed into a compact form.
CHARACTERIZATION OF THE MODIFIED SPONGE
The modified melamine sponge retained the microstructure of the pristine melamine sponge.
FESEM images of the pristine sponge showed that the surface of the fibers in the pristine melamine sponge was quite smooth and plain.
Upon treating with PVP, the FESEM images showed that some sort of deposition had taken place on these fibers. This deposition made the fibers quite rough in nature which highly enhances the oleophobicity, considering the Cassie-Baxter equation and Young’s equation as mentioned above.
Since the aqueous PVP solution used was very dilute (1.0 wt% strength), so any effect of the deposition is only visible in the microstructure level. Despite the deposition, the morphology was mostly retained in the modified sponge from the original condition.
For further inspection of the coating on the fibers of the modified sponge, Fourier-transform infrared spectroscopy (FTIR) was performed on the raw sponge and the modified sponge for comparison. In the spectrum of the modified sponge, a peak appeared at 1654 cm-1 which is assigned to the C=O groups of PVP. This peak is not present in the spectrum of the raw sponge.
Hence, we can say that the rough coating on the modified sponge, as seen in the FESEM images, is PVP. It has successfully adhered to the fiber surface of the sponge.
PERFORMANCE ENHANCEMENT UPON
Using the PVP-modified melamine sponge, we could perform continuous separations of diesel/water and n-hexadecane/water mixtures for up to 12 hours, with no oil in the collected water during the entire process, indicating the effectiveness of the separation of the oil/water mixture using the modified sponge.
Successful separation of several oil/water and organic solvent/water mixtures, including ones containing n-hexadecane, isooctane, and diesel was observed. We performed a continuous oil/ water separation test by adding water into the upper glass tube continuously while maintaining the height of the oil/water mixture at 7 cm. The absence of oil content in the filtrate further demonstrated the robustness and antifouling properties of the melamine sponge. Oil/water mixtures, including those containing n-hexane, n-hexadecane, and isooctane, could separate in these continuous separation tests over a period of at least 1 h while also maintaining a high flux value (L m-2 h-1).
This only further proves that the PVP, that had been deposited on the sponge, has fixed very strongly to the cotton fiber and is serving its purpose by improving the roughness to enhance oleophobicity.
Emulsified oil in wastewater is also a major environmental issue affecting a range of industries. Direct discharge of such wastewater harms both the environment and human health. Because of its large pores (>50 µm), the raw melamine sponge could separate only free oil/water mixtures; it could not separate oil-in-water emulsions, where the typical droplet size is less than 20 µm. The modified melamine sponge could be used successfully for the separation of emulsified oil/water after applying a simple compression process.
Compared with the original sponge, the surface morphology changed such that the pores had smaller diameters and that the skeleton of the melamine sponge was packed more compactly. The increase in surface roughness also helped in more efficient oleophobic nature. This compressed modified melamine sponge also possessed high efficiency when separating surfactant-stabilized oil-in-water emulsions. The water permeated through the sponge continuously, causing the emulsion droplets to demulsify, leaving behind the oil.
A simplistic, inexpensive method for the fabrication of a PVP-modified melamine sponge that displays superhydrophilicity and underwater superoleophobicity was developed. This material should have practical application in the effective separation of water-rich immiscible oil/water mixtures with extremely high separation efficiency and separation capacity. The PVP-modified melamine sponge exhibited excellent antifouling properties during long-term use. Moreover, after performing a simple compression process, this modified sponge allowed the effective separation of surfactant-free and -stabilized oil-in-water emulsions with high separation efficiency. This kind of sponge is a promising candidate material for use in the treatment of wastewater produced industrially and in daily life.
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