Reversible cycling of Nd adsorption Controlled and reversible adsorption of rare earth neodymium ions to the water-vapor interface was implemented by altering the competitive equilibrium between Nd 3+ bound to a chelating ligand DTPA (Fig. ) dissolved in water and to an insoluble monolayer of DHDP (Fig. ) spread on the water surface. Exposure of the water surface and nearby bulk solution to 10 keV X-rays alters the equilibrium between these two bound complexes, leading to the reversible transport of ions between surface and bulk liquid as illustrated schematically in Figs. and. X-ray exposure releases Nd 3+ ions bound to DTPA, allowing them to bind to the DHDP monolayer (Fig. ). Removing the continuous exposure of X-rays allows the DTPA to dominate the binding equilibrium once again and Nd 3+ ions leave the surface to bind to DTPA in the bulk aqueous phase (Fig. ).