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Nanoscale uniformity of desalination membranes

By Scott Jenkins |

Reverse-osmosis (RO) desalination membranes use a polyamide selective layer formed on a microporous support to separate water molecules from salts. Past efforts to characterize the polyamide membrane structure have been limited by the internal variability of the selective layer. Now, leveraging new capabilities in microscopy and modeling, a team of academic and industrial researchers has published a study describing key insights for the structure-property relationships of RO membranes. The study, published in a recent issue of the journal Science, resulted from a collaboration of researchers from the Pennsylvania State University (State College; www.psu.edu), University of Texas at Austin (www.utexas.edu), Iowa State University (Ames; www.iastate.edu), Dow Inc. (Midland, Mich.; www.dow.com) and Dupont Water Solutions (Edina, Minn.; www.dupont.com).

Using a specialized type of scanning transmission electron microscopy along with detailed computational modeling, the researchers were able to generate three-dimensional spatial information that reveals where the polymer is (and is not) concentrated within the membrane at the nanoscale. By constructing these nanoscale polyamide density maps of RO membranes, the researchers gained valuable insight into the relationship between density and membrane thickness in how water is transported.

It its analysis, the research team compared RO membranes manufactured under varying process conditions and concluded that “systematic control over nanoscale polyamide inhomogeneity” in desalination membranes is key to maximizing water permeability without sacrificing salt selectivity. While conventional modeling suggests that water passage should decrease with thicker membranes, that would be the case only if the membrane had a uniform thickness and density. A thicker membrane could result in improved water transport if the membrane was less dense, but more homogeneous, according to the study findings.

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