Abstract
Melamine (MA) and polyurethane (PU) foams, including both commercial sponges for daily use as well as newly synthesized foams are known for their high sorption ability of both polar and unipolar liquids. From this reason, commercial sponges are widely used for cleaning as they absorb a large amount of water, oil as well as their mixtures. These sponges do not preferentially absorb any of those components due to their balanced wettability. On the other hand, chemical and physical modifications of outer surfaces or in the bulk of the foams can significantly change their original wettability. These treatments ensure a suitable wettability of foams needed for an efficient water/oil or oil/water separation. MA and PU foams, dependently on the treatment, can be designed for both types of separations. The particular focus of this review is dealt with the separation of oil contaminants dispersed in water of various composition, however, an opposite case, namely a separation of water content from continuous oily phase is also discussed in some extent. In the former case, water is dominant, continuous phase and oil is dispersed within it at various concentrations, dependently on the source of polluted water. For example, waste waters associated with a crude oil, gas, shale gas extraction and oil refineries consist of oily impurities in the range from tens to thousands ppm [mg/L]. The efficient materials for preferential oil sorption should display significantly high hydrophobicity and oleophilicity and vice versa. This review is dealt with the various modifications of MA and PU foams for separating both oil in water and water in oil mixtures by identifying the chemical composition, porosity, morphology, and crosslinking parameters of the materials. Different functionalization strategies and modifications including the surface grafting with various functional species or by adding various nanomaterials in manipulating the surface properties and wettability are thoroughly reviewed. Despite the laboratory tests proved a multiply reuse of the foams, industrial applications are limited due to fouling problems, longer cleaning protocols and mechanical damages during performance cycles. Various strategies were proposed to resolve those bottlenecks, and they are also reviewed in this study.
Highlights
The Oil and grease are organic substances composed by different hydrocarbons, soaps, fatty acids, and waxes [1]
Petroleum wastewaters formed during a production and processing of crude oil, gas, shale gas extraction and oil refineries, food and metal processing waters mostly contribute to the formation of oily polluted waters [1]
Emulsification of oil often occurs in water treatment processes due to the intensive use of emulsifiers, surfactants, and polymeric additives in petrochemical industry, mainly in the processes of chemically enhanced oil recovery of bypass oil and oil trapped in porous reservoirs [3,4]
Summary
The Oil and grease are organic substances composed by different hydrocarbons, soaps, fatty acids, and waxes [1]. Emulsification of oil often occurs in water treatment processes due to the intensive use of emulsifiers, surfactants, and polymeric additives in petrochemical industry, mainly in the processes of chemically enhanced oil recovery of bypass oil and oil trapped in porous reservoirs [3,4] These processes result in the production of huge volumes of oil-emulsified wastewater what implies serious problems of efficient water cleaning associated with a relatively high stability of emulsified mixtures and small particles size (below 5 microns) of oily droplets. Structural strength, manufacturability and multifunctionality of polymers are preferable for manufacturing membranes for oil/water separation with some cases demanding surface modification for hydrophobic/hydrophilic properties [22] Both phase inversion methods involving complex physical-chemical processes and in situ elimination methods are applied for generating porous polymeric membranes, which is much helpful in selectively separating oil/water mixtures and emulsions [12,23]. Various modifications performed on the MA and PU foams in regulating the separation capabilities of the materials are discussed by emphasizing the following aspects: (i) characteristic properties of the foams and their dependence on the oil separating strategies, (ii) nature of oil/water emulsions and its influence on the material performance, (iii) commercial preferences and how the goals can be achieved by modification methods and (iv) different challenges during the development of advanced foam separators
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