Surfactants can stabilize oil and water in an emulsion, a useful mechanism in many industrial processes and in cleaning up oil spills. But separating and recovering the oil and water, such as for eliminating waste discharge, can be difficult. A team of researchers from Zhejiang University (Hangzhou, China; www.zju.edu.cn) has developed a membrane system for separating oil and water from complex mixtures that the researchers say can recover up to 97% of water and 75% of oil, both at purities of up to 99.9%.
To achieve this, the team designed a membrane system with narrow, confined channels that can be adjusted between 4 and 125 mm. The channel of membranes — called a Janus channel of membranes, after the two-faced ancient Roman god of transitions and duality — is constructed with pair of hydrophilic and hydrophobic membranes. “The confined Janus channel can amplify the membrane pair through a feedback loop that involves enrichment and demulsification,” the researchers, led by Xin-Yu Guo, write in the November 7 issue of Science.
As the oil-water emulsion is forced to pass through the narrow channel, pressure drives water to move toward the hydrophilic membrane. This allows water to pass through and results in an elevated concentration of oil within the channel. The increase in oil density triggers hydrodynamic forces that encourage oil droplets to collide and merge, forming larger droplets with reduced surface energy. These larger droplets then pass through the hydrophobic membrane. “This continuous process creates a feedback loop of enrichment, coalescence and demulsification, allowing for the simultaneous, high-purity separation of oil and water without the concentration issues seen in traditional systems,” according to a research summary from the American Association for the Advancement of Science (AAAS), publisher of Science.
The versatility of the technique “may enable near-zero liquid discharge for a range of separations,” the researchers say, such as treatment of oily wastewater and sorting of biological materials. The Janus channel of membranes can process oil-in-water emulsions with oil content as high as 40%, still reaching over 50% water recovery and over 80% oil recovery, the study says.