# Efficient Capture of Sr\({}^{2+}\) Ions by a Layered Potassium Neodymium Phosphate
## 4 Conclusions  
A two-dimensional layered neodymium phosphate K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) presented efficient adsorption for Sr\({}^{2+}\) ions. The adsorption properties of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) for Sr\({}^{2+}\) were studied by batch experiments. The adsorption capacity of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) was 42.35 mg/g. The material had good stability in the range of pH from 3.07 to 12.17, with a low leaching percentage of Nd (<3.76%). At a pH of 8.07, the highest value of \(K_{4}\)\({}^{\rm Sr}\) was observed (>10\({}^{6}\)), and the removal efficiency was as high as 99.93%. The mechanism of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) adsorbing Sr\({}^{2+}\) ions was the ion exchange between K\({}^{+}\) and Sr\({}^{2+}\), which was confirmed by XPS, EDS, PXRD, etc. K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) represents the first two-dimensional rare earth phosphate studied for the removal of Sr\({}^{2+}\) ions by ion exchange. This work highlights the subsequent research on the removal of radionuclides by rare earth phosphate ion exchange agents.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13010497/s1](https://www.mdpi.com/article/10.3390/app13010497/s1)]([https://www.mdpi.com/article/10.3390/app13010497/s1](https://www.mdpi.com/article/10.3390/app13010497/s1)), Figure S1: Leaching percentages of Nd (%) in Sr\({}^{2+}\) solutions with different pH; Figure S2: Solid state UV /Vis absorption spectra for the pristine K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) (black line) and K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr (magenta line) measured at room temperature; Figure S3: Energy dispersive spectroscopy (EDS) analysis of the adsorption product K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr; Figure S4: Scanning electron microscope images for the pristine K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) (a) and the adsorption product K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr (b); Table S1: Adsorption capacities for K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) at different initial Sr\({}^{2+}\) concentrations; Table S2: The data for the concentrations of Sr\({}^{2+}\) (C\({}_{t}^{\rm Sr}\)) and the relative amounts of Sr\({}^{2+}\) removed (\(R^{\rm Sr}\)) for K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) at different time in kinetics experiments under neutral conditions; Table S3: The fitting data with the pseudo-first-order kinetic model and pseudo-second-order kinetic model for the removal kinetics of Sr\({}^{2+}\) ions of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\); Table S4: The adsorption results of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) for Sr\({}^{2+}\) in different pH solutions.  
Figure 5: (**a**) XPS survey spectra of the pristine K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) and the adsorption product K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr; (**b**) high-resolution XPS spectra of P2p for the pristine K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\): (**c**) high-resolution XPS spectra of Sr3d for the adsorption product K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr; (**d**) high-resolution XPS spectra of K2p for the pristine K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) and the adsorption product K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr.  
**Author Contributions:** Conceptualization, Y.Y. and M.F.; formal analysis, Y.Y., H.S., Y.G., C.C. and T.Z.; investigation, Y.Y.; data curation, Y.Y.; writing--original draft preparation, Y.Y.; writing--review and editing, M.F., J.L. and X.H.; visualization, H.S. and X.H.; project administration, M.F.; funding acquisition, M.F. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (grant numbers U21A20296, 22076185 and 21771183) and the Natural Science Foundation of Fujian Province (grant number 2020)06033).  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available in this article or supplementary material.  
The authors declare no conflict of interest.