The desorption process includes the unbinding of Nd ions from the DHDP monolayer due to X-ray exposure, as well as the diffusion of the ion out of the surface region. This process is modeled for pH 3.0 and pH 4.5 by fitting the data in Fig. , which was measured in the absence of DTPA, to an exponential decay, 𝑐𝑒 *+ " # + 𝑑. The amplitude c and constant k2 were determined separately for the pH 3.0 and pH 4.5 data in Fig. , then used to parameterize the surface density 𝑛 !"