Oil spill accidents not only cause huge loss of energy resources, but also bring long-term damage to marine ecosystems. The marine oil spills are usually in the state of large area, low thickness, high viscosity, and so on, which makes it very difficult to be cleanup.

Recently, PHOS has been demonstrated as promising oil sorbents for the remediation of oil spills due to its high oil/water separation efficiency, easy to manipulation, environmentally friendly, and low cost features.

However, with the increase of oil viscosity, traditional sorbents will gradually lose its effectiveness because of their decreased oil diffusion speed, which hinds the application of oil sorbent in the cleanup of real ocean crude oil spills.

To promote the wide application of oil sorbents, it is highly in demand to develop a new sorbent design in order to improve the oil uptake speed for viscous crude-oil spill.   

Prof. YU Shuhong's team from the University of Science and Technology of China (USTC) developed new sorbent design to clean-up viscous crude oil spill faster by using joule heated graphene wrapped sponge(GWS), decreasing the oil sorption time was decreased by 94.6% compared with normal GWS.

They for the first time introduced joule-heating effect to porous hydrophobic and oleophilic sorbents (PHOS). Reviewers from Nature Nanotechnology commented "This study takes advantage of the joule heating properties of the graphene in order to form functional sponges able to reduce in situ the viscosity of the crude oil and to remove it from water. The idea is pretty original and innovative". 

Figure 1: Schematic illustration of using joule heated graphene wrapped sponge to cleanup viscous crude oil spill. (Image by YU Shuhong)

To solve the above problem, they for the first time introduced joule-heating effect to PHOS. Though the centrifuge-assisted dip-coating method they developed previously, they prepared a graphene wrapped sponge which could be heated up by apply an electric current though the conductive graphene coating.

When the GWS is put on the viscous crude oil spill and applied with an electric current, the heat generated from the GWS will be transferred to the surrounding oil and warm it up. Due to its temperature-depended viscosity, the hot crude oil spill will pass through the pores into the GWS quickly (Figure 1).

According to their study, the increase of oil temperature dramatically decreased the oil viscosity, thus increased the oil sorption coefficient (Ks) of GWS. Finally, with the aid of joule heating effect, the GWS could rapidly absorb the high viscos oil from water surface (Figure 2).

To improve the heat-utilization efficiency, they confined the heat region to the bottom part of GWS, which not only saved 65.6% of electric energy but also decreased consumption of graphene oxide (GO) by 50% (Figure 3).

More importantly, the oil sorption time was decreased by 94.6% compared with normal GWS. To demonstrated the possibility of large scale application of joule-heated GWS. They also proposed a dense electrodes design, which could heat up large-area GWS to a high temperature under a low applied voltage. 

Figure 2: Effect of joule heating on the oil sorption kinetics. a. The influence of oil temperature to oil sorption speed of GWS. b. Oil sorption coefficient (Ks) of GWS and plot of square root of the ratio of oil surface tension to oil viscosity ( ) showed a similar trend. c. The change of oil density (dl) and θas the function of oil temperature. d. The plot of versus oil temperature. e. The change of oil viscosity and surface tension as the function of oil temperature. f. Different crude oil sorption speed of GWS under different power density. (Image by YU Shuhong) 

Figure 3: Effects of heat distribution on the utilization efficiency of heat energy.  a. Simulated temperature distribution on GWS-x (different electrode height) and GWS-MS-x (different RGO coating height) under same power density. b. Experiment results correspond to the simulations in (a). c. Schematic illustration of the setup for the measurement of the sinking speed of oil sorbents and temperature change of water below the oil/water interface. d. Oil sorption time for GWS-x, GWS-MS-x under the same electric power density (0.16 W/cm3). e. Maximum electric powers (Pmax) that could be applied on GWS-x and GWS-MS-x. f. Oil sorption time for GWS-x, GWS-MS-x under their maximum electric power densities. g. Electric energy and oil sorption time for GWS-x-Pmax and GWS-MS-x-Pmax. h. Simulated heat-transfer of GWS-MS-10-Pmax during the oil sorption process. (Image by YU Shuhong)

This research opens a new window for the high speed cleanup of viscous crude-oil spill by oil sorbent. With further optimization of material composition and heat transfer, we can envision that this joule-heated sorbent design will find widely application in the remediation of future marine oil spill accidents.  

This study entitled “Joule-heated graphene-wrapped sponge enables fast clean-up of viscous crude-oil spill” has been published in the journal of Nature Nanotechnology, and was highlighted at the same day by the journal Nature.

This work is supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China, the National Natural Science Foundation of China, Key Research Program of Frontier Sciences, CAS, the Chinese Academy of Sciences, National Basic Research Program of China, Collaborative Innovation Center of Suzhou Nano Science and Technology, CAS Center for Excellence in Nanoscience, the Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS and the Fundamental Research Funds for the Central Universities.