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Designing Microstructures From Desert Beetles to Treat Oily Wastewater

A new magnetic demulsifier inspired by the Stenocara desert beetle can separate oil from water with higher efficiency and lower energy consumption.

In this age of environmental decline, superwetting materials have gathered much attention in the treatment of oily wastewater and oil spills. Such oil pollution in our waters destroys the insulating ability of sea otters, water repellency of birds’ feathers, and impairing the reproductive capacity of fishes, among others. In oily wastewater treatment, superhydrophilic filtration materials have demonstrated unmatched ability over other materials in separating oil from water due to its self-cleaning properties and high reusability. Yet, these materials lack separation efficiency and its high recovery energy consumption limit their practical application in oil-water emulsion separation.

Published in Chemical Engineering Journal, Professor Zeng Zhixiang and colleagues at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences developed a new magnetic demulsifier with microstructures inspired by the desert beetle to achieve high-efficiency oil-in-water emulsion separation with low recovery energy consumption.

The Stenocara beetle is native to the Namib Desert in southern Africa. To survive in the desert, the backs of the beetles have evolved microstructures consisting of hydrophilic bumps and superhydrophobic shell surface, which will enable these beetles to collect water. Numerous tiny water droplets in the air will gather and move towards the hydrophilic region, and the large water droplets will, under gravity, roll along the surface of the superhydrophobic surface and into the mouth of the beetle. To prevent water loss, the beetles have an efficient epidermal wax layer that reduces the transpiration of the cuticles, allowing the beetles to overcome the challenge of light-induced water evaporation.

Inspired by these water-collecting and water-retention strategies, Prof. Zeng and colleagues wondered if the hydrophilic-superhydrophobic combination structure can promote the aggregation of oil droplets in emulsion and improve the separation efficiency of crude oil-in-water emulsion. The team constructed a magnetic graphene oxide-based composite (Fe3O4/GO@C-SR) with desert beetle-like microstructures, which consist of underwater superoleophobic underlayer and superhydrophobic/superoleophilic (or oil-loving) bumps.

In a one-step solvothermal method, the magnetically active graphene oxide Fe3O4/GO particles were synthesised. Then the superhydrophobic/superoleophilic carbon black particles (C-SR) were assembled on the surface of the magnetic graphene oxide via mechanical ball milling to construct a beetle-like structure.

The team’s developed Fe3O4/GO@C-SR demulsifier demonstrated high hydrophobicity/oleophilicity in air and high oleophobicity underwater. When an aqueous solution of Fe3O4/GO@C-SR was added to the crude oil-water emulsion, the dark brown emulsion became a clear aqueous solution. However, when just C-SR particles or Fe3O4/GO particles were added to the crude oil-water emulsion, no significant changes were observed for C-SR particles, while Fe3O4/GO particles cannot completely clean oil from the water. This suggests that it is only with the combination of the two particles to form the novel Fe3O4/GO@C-SR demulsifier can a higher emulsion separation ability be attained.

This improved separation performance of the novel Fe3O4/GO@C-SR particles can be attributed to the improvement of adsorption capacity of Fe3O4/GO for micro- and nano-sized oil droplets due to the superhydrophobic C-SR particles, and the favourable dispersity in water and increased effective surface area due to the superhydrophobic/hydrophilic microstructures.

Furthermore, when compared to traditional magnetic demulsifiers, the team’s synthesised magnetic Fe3O4/GO@C-SR demulsifier displayed a higher photothermal conversion efficiency and better recyclability, thereby reducing the demulsifier’s energy consumption in the recovery process.

This beetle-inspired demulsifier may open up a new avenue for demulsifying particles in the design and construction of advanced, energy-saving materials in wastewater treatment. [APBN]

Source: Xu et al. (2022). Desert beetle-like microstructures bridged by magnetic Fe3O4 grains for enhancing oil-in-water emulsion separation performance and solar-assisted recyclability of graphene oxide. Chemical Engineering Journal, 427, 130904.