X-ray switch for rare earth element adsorption to a liquid interface

Ions at liquid surfaces and interfaces influence many scientific and technological areas, including molecular and nanoparticle assembly for energy and separations processes. Controlled transport of ions between interfacial and bulk liquids can lead to triggering ion-induced interfacial phenomena. Here, we show that X-ray exposure alters the competitive equilibrium of rare earth elements bound to chelating ligands in bulk water and to insoluble monolayers at the water surface. Controlling the X-ray exposure leads to reversible adsorption of rare earth trivalent ions to the liquid surface. Evidence for the exposure-induced temporal variations in the ion surface density is provided by synchrotron X-ray fluorescence near total reflection (XFNTR) measurements. Varying the X-ray penetration depth from 10 nm to 2.8 µm leads to a controlled exposure of either the surface region alone or the surface monolayer plus dissolved chelating ligands and bulk water. This separation of surface and bulk processes helped identify the role of aqueous radiolysis in the adsorption cycle. Comparison of different chelates identified amine binding sites as a contributor to the cycling mechanism. The primary molecules utilized for these studies – chelating ligand DTPA and organophosphoric acid extractant DHDP – are like those used in the separation of rare earth elements from ores and in the reprocessing of nuclear fuel. The observed reversible cycling of ion adsorption may provide an opportunity for further control over these processes and enhanced separation.