The temporal evolution towards a new equilibrium state is probed by X-ray fluorescence near total reflection (XFNTR), a technique that measures the element-specific surface density of ions at the interface. XFNTR measurements of 5 seconds duration allow us to vary the X-ray exposure while measuring the surface density of ions. Continuous XFNTR measurements maximize the total exposure, while infrequent measurements without X-ray exposure between them are used to minimize the total exposure of the sample. X-ray exposure effects are probed by experiments that vary the X-ray penetration depth and the aqueous pH, as well as utilize different complexants and rare earth ions. A simple model of the kinetics summarizes the approach to equilibrium of the transport of ions from bulk liquid to interface and back. These experiments demonstrate a method to reversibly control the availability of rare earth ions at liquid interfaces.