Neutron transmission studies with GdCl3 for tracing concentration polarization in reverse osmosis

Concentration polarization (CP) is determined by neutron transmission in a reverse osmosis (RO) system consisting of two cells, one with and the other without an RO membrane, to get the net neutron response for the feed solution in front and far from the membrane separately. The aqueous feed was a dilute solution of NaCl and GdCl 3 . The special feature of these experiments is that GdCl 3 can be selectively determined with neutrons, since the neutron absorption cross-section of Gd is larger orders of magnitude compared to other elements such as Na, Cl, and water, and therefore has a strong influence on the overall transparency of the sample for neutrons. It is therefore the transmission of neutrons through both cells (one with and the other without membrane), by which the concentration and CP of GdCl 3 can be determined. Transmission and small-angle scattering (SANS) were measured in parallel, demonstrating the performance of this method as an in-situ method for determining CP as a function of time under near-realistic conditions. The neutron experiments are complemented by measurements of permeate flux and electrical conductivity of the feed, which were used to determine the salt concentrations, the osmotic pressure and the concentration polarization of the two salts individually and of a mixture of both. The CP results of this study are in good agreement with the values presented in the literature, determined mainly from flux data under a wide variety of conditions. • Operando reverse osmosis. • Neutron transmission to determine GdCl 3 concentration. • Permeate flux. • Concentration polarization of GdCl 3 and NaCl.