The rise in wastewaters has necessitated the need to develop robust and highly effective water treatment strategies to produce clean water for human consumption. To this end, various techniques such as filtration and biodegradation have been developed to purify oily wastewaters. In particular, three-dimensional (3D) porous materials such as foams, aerogel and sponge have proved efficient in separating oil/water mixtures. These materials are mostly made of polymers which have the advantages of ease of manufacturing, outstanding mechanical properties and tunable surface wettability. However, the main disadvantage of polymer sponges is their poor mechanical robustness which reduces their ability to squeeze out the absorbed liquid, thus limiting the cycling operations. Adding inorganic nanomaterials could enhance the mechanical stability of the polymers. Unfortunately, this method induces certain adverse effects that limit the effectiveness of the polymer sponges under certain pressure conditions.
Numerous studies have been conducted in an attempt to address the above challenges. Mechanically strong materials like graphene and carbon nanotubes have been used to develop robust polymer sponges for wastewater treatment purposes. Their application has been inspired by their ability to improve the mechanical properties of composite materials. Although the use of carbon nanomaterials improved the mechanical elasticity, the cyclic comprehension strain was insufficient. Similar results have been reported in other developed oil/water separation sponges. Thus, the need to develop oil/water separation methods capable of achieving high elasticity and improved mechanical robustness is highly desirable. The application of surface embedding as an alternative has been extensively studied. Previous research findings established its usefulness in improving the mechanical robustness and flexibility and reducing the adverse effect of nanomaterials.
Despite the numerous attempts to enhance the mechanical robustness and durability of sponges, the trade-off effects between mechanical robustness and elasticity have remained a challenge. Herein, Dr. Zihe Pan, Mr. Yihao Guan, Ms. Yanhong Liu and Professor Fangqin Cheng from Shanxi University proposed a new facile method for fabricating mechanical robust and hydrophobic and underwater superoleophilic graphene/polydimethylsiloxane (PDMS) sponge via mold-transfer surface embedding approach. The deposition of graphene on the PDMS surface was facilitated by using sugar cubic as a support matrix and as templates for generating pores. However, graphene was embedded on the sponge surface after the removal of the sugar cube template. SEM technique was used to investigate the resulting pore structure. The mechanical robustness and elasticity performance of the graphene/PDMS was tested and discussed. The work is currently published in the journal, Separation and Purification Technology.
The research team showed that embedding small amounts of graphene not only reduced the amount of graphene needed but also solved the trade-off effects by simultaneously improving the elasticity and mechanical durability. It increased the pore size distribution and the total surface area by 10% while at the same time reducing the porosity and medium pore diameter. At a maximum strain of 80%, the graphene/PDMS sponge returned to its original size after 100 cycles. This only required 0.4 MPa, which is half the stress needed to achieve the same strain for the PDMS sponge. Furthermore, the graphene/PDMS sponge exhibited an adsorption capacity of 400 – 1500% of its weight for different organic solvents and oils as it supported the continuous elimination of oil from oil/water mixture.
In summary, the study successfully demonstrated the fabrication of hydrophobic and underwater superoleophilic graphene/PDMS sponge through mold transfer-surface embedding. The resulting graphene/PDMS sponge exhibited improved elasticity, stability and mechanical robustness. Its advantages included a significant reduction of the underwater adsorption rate and a corresponding increase in the absorption capacity on high viscosity and density oils. Overall, the graphene/PDMS sponge outperformed other oil/water separation techniques. In a statement to Advances in Engineering, Dr. Zihe Pan, first author explained their study will contribute to developing high-performance oil/water separating sponges for producing clear and safe water.
Pan, Z., Guan, Y., Liu, Y., & Cheng, F. (2021). Facile fabrication of hydrophobic and underwater superoleophilic elastic and mechanical robust graphene/PDMS sponge for oil/water separation. Separation and Purification Technology, 261, 118273.