Surface Engineering of Ceramic Nanomaterials for Separation of Oil/Water Mixtures

Oil/water mixtures are a potentially major source of environmental pollution if efficient separation technology is not employed during processing. A large volume of oil/water mixtures is produced via many manufacturing operations in food, petrochemical, mining, and metal industries and can be exposed to water sources on a regular basis. To date, several techniques are used in practice to deal with industrial oil/water mixtures and oil spills such as in situ burning of oil, bioremediation, and solidifiers, which change the physical shape of oil as a result of chemical interaction. Physical separation of oil/water mixtures is in industrial practice; however, the existing technologies to do so often require either dissipation of large amounts of energy (such as in cyclones and hydrocyclones) or large residence times or inventories of fluids (such as in decanters). Recently, materials with selective wettability have gained attention for application in separation of oil/water mixtures and surfactant stabilized emulsions. For example, a superhydrophobic material is selectively wettable toward oil while having a poor affinity for the aqueous phase; therefore, a superhydrophobic porous material can easily adsorb the oil while completely rejecting the water from an oil/water mixture, thus physically separating the two components. The ease of separation, low cost, and low-energy requirements are some of the other advantages offered by these materials over existing practices of oil/water separation. The present review aims to focus on the surface engineering aspects to achieve selectively wettability in materials and its their relationship with the separation of oil/water mixtures with particular focus on emulsions, on factors contributing to their stability, and on how wettability can be helpful in their separation. Finally, the challenges in application of superwettable materials will be highlighted, and potential solutions to improve the application of these materials will be put forward.

Synthesis of Dual-Functional and Robust Underwater Superoleophobic Interfaces

Fish-scale-mimicked super oil-repelling wettability that functions under water has emerged as an important avenue for developing various functional materials. Mainly, polymeric hydrogel, brittle metal oxides, and electrostatic multilayers have been utilized for synthesizing an artificial underwater superoleophobic interface, and most of these reported artificial bioinspired wettabilities are likely to be compromised under practically relevant severe settings. Moreover, a design of a dual-functional fish-scale-mimicked interface that would be capable of separating water-soluble organic pollutants, in addition to the eco-friendly removal of different forms of contaminated oils under severe settings, could be highly useful in addressing globally recognized severe water pollution problems. In this report, a dual-functional underwater superoleophobic membrane is introduced for simultaneous removal of  contaminated dyes and oil/oily phase, where graphene oxide (GO) nanosheets were strategically integrated with naturally abundant and environmentally friendly cotton fibers by adopting mussel-inspired chemistry. The synthesized membrane was found to exhibit fish-scale-mimicked nonadhesive underwater superoleophobicity, and this super oil repellency remained unaffected even after prolonged exposures to various practically relevant harsh chemical and physical conditions. Moreover, this material was capable of rapid (within 2 min) adsorption of water-soluble cationic organic dyes with high adsorption capacity (136 mg/g for methylene blue), following linear pseudo-second-order kinetics. The biomimicked extreme oil repellency was exploited for separating different forms (bulk and emulsion) of oil/oily (both sedimenting and floating) contaminants with high separation efficiency (above 98%), and the immobilized GO in the biomimicked membrane parallely allowed cationic organic dyes (methylene blue and crystal violet) to be removed from the aqueous phase through a single-step gravity-driven filtration process. The performance of simultaneous removal of cationic dyes and oil/oily contaminants remained unaffected even under various practically relevant severe settings including extremes of pH, sea water, river water, and so forth. Furthermore, the dual-functional biomimicked membrane was repetitively (10 times) used for successful separation of both the contaminated cationic dye and oil/oily phase from the aqueous phase, without affecting the separation efficiency. This simple approach is likely to provide a facile basis for addressing the problem of water pollution under practically relevant diverse and severe settings.