Filtering and treating water, both for human consumption and to clean industrial and municipal wastewater, accounts for about 13 percent of all electricity consumed in the U.S. and releases about 290 million metric tons of CO2 into the atmosphere every year — roughly equivalent to the combined weight of every human on earth.
One of the most common methods of processing water is passing it through a membrane with pores that are sized to filter out particles that are larger than water molecules. However, these membranes are susceptible to “fouling” — clogging by the very materials they are designed to filter out — necessitating more electricity to force the water through a partially clogged membrane and frequent membrane replacement, both of which increase water-treatment costs.
New research from the Wyss Institute for Biologically Inspired Engineering at Harvard University and collaborators at Northeastern University and the University of Waterloo demonstrates that the Wyss’ liquid-gated membranes (LGMs) filter nanoclay particles out of water with twofold higher efficiency and nearly threefold longer time to foul, and reduce the pressure required for filtration over conventional membranes. It’s a solution that could reduce the cost and electricity consumption of high-impact industrial processes such as oil and gas drilling. The study is reported in APL Materials.
“This is the first study to demonstrate that LGMs can achieve sustained filtration in settings similar to those found in heavy industry, and it provides insight into how LGMs resist different types of fouling, which could lead to their use in a variety of water-processing settings,” said first author Jack Alvarenga, a research scientist at the Wyss Institute.
LGMs mimic nature’s use of liquid-filled pores to control the movement of liquids, gases, and particles through biological filters using the least energy possible, much like the small stomata openings in plants’ leaves allow gases to pass through. Each LGM is coated with a liquid that acts as a reversible gate, filling and sealing its pores in the “closed” state. When pressure is applied to the membrane, the liquid inside the pores is pulled to the sides, creating open, liquid-lined pores that can be tuned to allow the passage of specific liquids or gases, and that resist fouling due to the liquid layer’s slippery surface. The use of fluid-lined pores also enables the separation of a target compound from a mixture of different substances, which is common in industrial liquid processing.
The research team decided to test the LGMs on a suspension of bentonite clay in water, as such “nanoclay” solutions mimic the wastewater produced by drilling activities in the oil and gas industry. They infused 25 mm discs of a standard filter membrane with perfluoropolyether, a type of liquid lubricant that has been used in the aerospace industry for more than 30 years, to convert them into LGMs. They then placed the membranes under pressure to draw water through the pores but leave the nanoclay particles behind, and compared the performance of untreated membranes to LGMs.
