The conversion of seawater or wastewater into potable water is essential, especially in arid regions, but also in closed biotopes, such as a spacecraft. Reverse osmosis membranes (ROMs) consist of a 0.2-micrometre-thick active polyamide layer and a further 300-micrometre-thick supporting structure. This corresponds to a total of about three sheets of printer paper. These membranes produce pure water in an extremely energy-efficient manner, e.g. through desalination or purification of industrial water. The process is based on the selective transport of water molecules only, which are driven through a membrane under pressure.

Assessing membrane pores with neutrons

The performance of ROM is largely determined by its pore structure and the nature of the ROM material. Using neutrons at the KWS-1 and KWS-3 instruments operated by Forschungszentrum Jülich at the MLZ, an Israeli German research team succeeded in obtaining unique and detailed information about the pores in a commercial composite ROM during operation. The trick is to enhance the contrast for visualizing the pores in each layer by using a suitable mixture of light (H₂O) and heavy (D₂O) water. Heavy water scatters neutrons more effectively than light water and simultaneously exhibits a significantly higher transmission for neutrons, resulting in a significant amplification of the measurement signal without affecting the chemical and physical processes.

The complex network of pores

The key findings relate to the pore parameters and their evolution during operation in the selective surface and support layers of reverse osmosis “thin composite membrane”. Desalination is mainly achieved by a network of nanometer-sized pores in the fraction of a micrometer thin, selective Polyamide surface layer. The support layers of Polysulfone and non-woven Polypropylene also exhibit a pore structure, but with up to micrometer-sized pores, enabling permeate water flux through the entire membrane. The network of open nanometer pores in the polyamide layer exhibits a self-similar structure at multiple length scales, whereas micrometer-sized and smaller pores were identified in the support layer. These are not selective, however, and enable the transport of cleaned water. At the beginning of operation, the volume fraction these pores decreases slightly until equilibrium is reached. This process can take several hours. With this knowledge at hand, researchers can help to further optimize membranes, increasing their efficiency, and thus reducing the cost of water desalination.