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From Popular Mechanics

Scientists have found using metal organic frameworks (MOF) could improve water desalination speed by at least one order of magnitude.

The Carnegie Mellon team used 3D solid modeling to calculate water flux and ion rejection.

Membranes of advanced nanomaterials are improving all kinds of water filtration.

A mechanical engineer at Carnegie Mellon University has developed a new, micro-thin material to make membrane water desalination even better. Amir Barati Farimani, with fellow researchers Zhonglin Cao and Vincent Liu, has calculated how much better his metal organic framework (MOF) works than the traditional membrane method.

In this application, membrane filtration is a blanket term for a form of filtration where a very thin, physical barrier allows some particles through, but holds others back. Think about a cell membrane, where water and other things pass through freely, but all the cell’s guts are kept inside. It’s a pasta colander writ micro. In seawater desalination, membranes allow fresh water to pass through while trapping larger salt particles.

Farimani seeks to replace existing membranes with a more efficient version. Membrane filtration can also be called reverse osmosis filtration and is often combined with other filtration methods: the membrane removes particles larger than water and a second method purifies the water-sized particles left behind. Research into hybrid membrane solutions is also ongoing because of how useful membrane filtration is when water is dirty or polluted, not just salty.

But Farimani’s research is specifically about desalinating water, which is a different use case altogether. His team ran simulations using virtual mockups of different MOF membranes. These MOF materials are a combination of metal and organic material that bonds together into strong but very thin materials—just a few atoms thick— with repeating patterns of open pores. The research team tested five materials: graphene, which is a simple hexagonal array format of carbon; molybdenum sulfide in monolayer form, and three MOF membranes.

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The researchers modeled everything in the experiment using a drafting program called SolidWorks, which makes even micromaterials easy to look at and shape. From those models, they were able to calculate the overall number of pores and the surface area of each pore. They ran simulations of different sizes of salt ions and found that the MOF membranes had higher flux of water, or permeation rate, while keeping comparable ion rejection—meaning a lot more water pumped through and the filtration was just as effective.

Think of it like your showerhead before and after you run it through a CLR bath: suddenly the same amount of surface area is pushing through a lot more water, because more pores are open.

It’s exciting that the MOF membranes allow more flowthrough to begin with, but the research team also notes that this is without having to specially open pores like they do in a graphene or molybdenum sulfide membrane. The MOF naturally forms into pores. With a material that opens its own pours and allows anywhere from one to six orders of magnitude more water permeation (depending on layers and arrangement), Farimani says his hope is that massively more efficient desalination will help some of the four billion people in the world who don’t have steady access to fresh, clean water—many of whom live in places with abundant seawater all around.

If tiny pieces of membrane in one simulation were this much more effective, Farimani has high hopes for membrane filtration at scale in desalination plants.

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