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The researchers achieved simultaneous oil and water recovery of up to 97 per cent and 75 per cent, respectively, with purity levels approaching 99.9 per cent for both, according to the study published on Friday by the peer-reviewed journal Science.
The record-breaking result was achieved using a Janus Channel of Membranes (JCM) – named after the ancient two-faced Roman god to reflect its dual-purpose design. Traditional separation technologies typically recover only oil or water.
According to co-authors Yang Haocheng, Zhang Chao and Xu Zhikang from Zhejiang University, the JCM consists of a water-attracting membrane and one that repels water, arranged to create a confined channel between them.
The oily-water mixture is pumped into the channel, where water is drawn through the hydrophilic membrane while oil is channelled to the hydrophobic membrane for collection, the paper said.
Industries such as petrochemicals, metallurgy, food production and pharmaceuticals generate vast quantities of oily waste water – particularly emulsified waste water that has been stabilised by surfactants and is among the hardest to treat.
Whether the emulsion consists of oil droplets dispersed in water, such as in mayonnaise, or water droplets dispersed in oil, as is the case in margarine, separating these mixtures is notoriously challenging.
Unlike traditional methods that typically recover only oil or water, the JCM can separate both, not only at the same time but also to a level that meets environmental discharge standards, according to the paper.
The researchers found that the width of the gap between the two membranes was crucial – too large and they operated independently, yielding low separation efficiency. However, by narrowing the distance they achieved a significant boost, the paper said.
By narrowing the gap from 125mm (5 inches) to just 4mm, the team observed a jump in oil recovery from 5 to 97 per cent on the hydrophobic side, while water recovery improved from 19 to 75 per cent on the hydrophilic side.
The team attributed the improvement to a “positive feedback mechanism” created at the smaller width by emulsified droplets colliding and accumulating within the channel.
As the hydrophilic membrane removes water, the emulsion’s concentration increases, encouraging droplets to merge and destabilise, facilitating oil separation, according to the paper.
Continual oil removal also lowers the concentration of emulsion on the membrane surface, further supporting water permeability and boosting the system’s overall separation efficiency, it said.
In an official press statement on the university’s website, co-author Yang said the JCM used simple materials and demonstrated “broad applicability to both oil-in-water and water-in-oil emulsions, encompassing a diverse range of oils and surfactants”.
“These membrane pairs hold promise for assembly into a multistage module to scale up,” the team said in the paper, indicating the technology’s potential to be used in practical industrial applications.
According to the paper, the JCM is also more robust than conventional hydrophilic membranes, which typically clog rapidly and perform well only with emulsions containing less than 10 per cent oil by volume.
In contrast, the JCM can handle emulsions with oil content of up to 40 per cent, achieving over 50 per cent water recovery and more than 80 per cent oil recovery, both with purities exceeding 99.9 per cent.
In a perspective article published in the same issue of Science, Professor Xing Yang from KU Leuven in Belgium said the approach “goes beyond simple oil-and-water separation and opens doors to other challenging separations on an industrial scale”.
“It has the potential to separate complex mixtures, such as removing water or glycerol from biofuels, recovering valuable metals from mining waste, and isolating proteins and vitamins from dairy products,” she said.