- Scientists examine reverse osmosis membranes on a molecular level.
- The findings may help improve the performance of membranes used in reverse osmosis systems.
- This could significantly decrease the cost of water purification.
Earth contains about 326 million trillion gallons of water, most of which is in the oceans and only 3% is fresh/drinkable water. This water never sits still: it evaporates from the sea, travels through the atmosphere, rains down on the ground and then flows back into the sea.
One of the leading techniques of converting seawater into drinking water is reverse osmosis. According to the International Water Association, more than 25,000 million gallons of fresh water is created using this technique.
In reverse osmosis (RO), a partially permeable membrane is used to remove molecules, ions and larger particles from water. It’s not a new technology, but the structure of membranes used in the purification process was not previously well-understood, especially on molecular levels.
The commercial RO systems pressurize salty water to force clean water through the membrane. Doing so, they consume a lot of energy: to make 100 gallons of clean water, an RO system consumes nearly 1 kilowatt-hour of energy, which is enough to light a 50-watt of bulb for 20 hours.
Improving the performance of membranes used in these systems could result in significant energy savings. Thinner and more permeable polyamide barrier layers, with an optimal level of wettability and cross-linking, could enhance the functional characteristics, elevate permeance, and decrease the cost of water purification.
Therefore, researchers at Brookhaven National Laboratory decided to study reverse osmosis membranes on a molecular level. They figured out what role does molecular structure play in making membranes highly efficient. The findings can be used to develop more advanced membranes.
Developing and Illuminating Membrane
The researchers used a technique known as interfacial polymerization — very much similar to commercial approach — to develop a thin polymer sheet at the water/oil interface. More specifically, they added one of the molecular substances to the oil while the other is added to the water.
Reference: ACS Macro Letters | doi:10.1021/acsmacrolett.9b00077 | Brookhaven National Laboratory
These two molecular substances react with each other at the point where oil and water come in contact. The reaction forms a thin sheet of polymer — thousand times thinner than a human hair — which is quite similar to the membranes used in commercial RO, but it’s much smoother.
To analyze such thin polymer sheets, the team used ultrabright X-rays (from the National Light Source II) along with advanced simulation tools. They observed the scattering patterns of X-rays through a grazing-incidence X-ray scattering method, in order to better understand the sheets’ molecular structure.
X-ray hitting the membrane and scattering off the surface | Courtesy of researchers
This scattering method involves hitting membrane with X-rays at a slightly tilted angle so that they scatter off the surface. A special device then captures the X-rays and records their scattering pattern, which is specific to the structure of the membrane.
The scattering pattern shows how neighbor molecules within the polymer are aligned with respect to each other. This is what researchers call molecular packing motifs. In this study, they were able to detect perpendicular motif and parallel motif.
As per findings, the perpendicular motif is associated with optimal filtration characteristics. Also, the molecular structure is usually oriented with the surface of the membrane. One can correlate this point with the orientation of water pathways in the membrane.
Read: New Material Can Filter Salt and Ions From Seawater
The team plans to build a comprehensive structure-function relationship by exploring different kinds of membranes and comparing their X-ray scattering patterns. They hope their finding will accelerate the development of ‘next-generation membrane’ for efficiently filtering water.
