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Illuminating Water Filtration

BNL researchers

For the first time, a team of researchers from Stony Brook University and the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have revealed the molecular structure of membranes used in reverse osmosis. The research is reported in a recently published paper in ACS Macro Letters, a journal of the American Chemical Society (ACS).

Reverse osmosis is the leading method of converting brackish water or seawater into potable or drinking water, and it is used to make about 25,000 million gallons of fresh water a day globally according to the International Water Association.

BNL researchers
Qinyi Fu, Francisco J. Medellin-Rodriguez, Nisha Verma, and Benjamin Ocko (from left to right) prepare to mount the membrane samples that mimic the membranes used in reverse osmosis for the measurements in the Complex Materials Scattering (CMS) beamline at the National Synchrotron Light Source II (NSLS-II).

“Most of the earth’s water is in the oceans and only three percent is fresh water, so water purification is an essential tool to satisfy the increasing demand for drinking water,” said Brookhaven Lab senior scientist Benjamin Ocko. “Reverse osmosis is not a new technology; however, the molecular structure of many of the very thin polymer films that serve as the barrier layer in reverse osmosis membranes, despite its importance, was not previously known.”

The thin polymer barrier layer used in reverse osmosis membrane is semipermeable. Small molecules such as water can cross from one side of the membrane to the other, but other molecules, such as hydrated sodium or chloride ions, cannot cross the barrier layer. This characteristic is what allows these membranes to filter the salt out of saline in order to make drinking water. During a commercial reverse osmosis process, the salty water is pressurized to force the fresh water through the membrane.

Since the seawater has to be pressed through the membrane, the energy consumption of reverse osmosis facilities is high. To make 100 gallons of fresh water with reverse osmosis, the energy cost is about one kilowatt-hour, the equivalent of running a 100-watt light bulb for 10 hours.

“Even small improvements in the performance of filtration membranes would result in huge energy and cost savings globally,” said Benjamin S. Hsiao, distinguished professor in Stony Brook’s College of Arts and Sciences. “Therefore, we are looking at the membranes on a molecular level. We want to figure out how molecular structure contributes to highly efficient membranes and use this knowledge to design improved membranes.”

According to Qinyi Fu, a Stony Brook University graduate student and the lead author of the study, “To resolve the molecular structure of the membranes, the team studied the scattering patterns of x-rays using a technique called grazing-incidence wide-angle x-ray scattering at NSLS-II’s Complex Materials Scattering (CMS) and Soft Matter Interfaces (SMI) beamlines.”

In this technique, the x-rays hit the membrane at a slight angle and scatter off the surface. They are then captured by a detector that records the so-called scattering pattern of the x-rays that is specific to the membrane’s molecular structure.

“In the scattering pattern, we are able to identify molecular packing motifs: how the neighboring molecules in the polymer are arranged with respect to each other. One is the parallel motif and the second is the perpendicular motif,” said Ocko. “While both packing motifs are present, the perpendicular packing motif is better correlated with optimal filtration properties.”

Hsiao added, “Our findings also show that the molecular structure is preferentially oriented with respect to the membrane surface. This is rather intriguing and may be related to how the water pathways in the membrane are oriented.”

Brookhaven National Laboratory (BNL) is a multipurpose research laboratory housing large, state-of-the-art facilities such as RHIC, NSLS-II, and the Center for Functional Nanomaterials. Stony Brook is a partner in Brookhaven Science Associates LLC, managing the Laboratory for the Department of Energy. Located less than 20 miles from campus, Brookhaven provides many opportunities for collaborative research efforts.

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