# Efficient Capture of Sr\({}^{2+}\) Ions by a Layered Potassium Neodymium Phosphate
## 3 Results and Discussion
### Adsorption Mechanism Study
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**adsorbent** & \(q_{\text{m}}\)**\({}^{\text{Sr}}\) (mg/g) & \(t_{\text{c}}\)**\({}^{\text{Sr}}\) (min)** & **Removal** & **PH \({}^{1}\)** & **Solution Composition \({}^{2}\)** & **Distribution** & **Ref** \\ \hline
2D \({}^{3}\)-K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) & 42.60 & 1440 & IE \({}^{4}\) & 8.03 & Sr & 1.46 \(\times\) 10\({}^{6}\) & This work \\
2D-K\({}_{2}\)Zr(PO\({}_{4}\))\({}_{2}\) & 52.83 & 90 & IE & 7.00 & Sr & \(\sim\)30,000 & [32] \\
2D-U\({}_{3}\)OH\({}_{2}\)OH\({}_{4}\)\({}_{2}\)OH\({}_{2}\) & 98.957 & UM \({}^{5}\) & OTH \({}^{6}\) & 2.00 & Sr & 6.90 \(\times\) 10\({}^{3}\) & [61] \\
2D-K\({}_{3}\)F\({}_{4}\)Ga(PO\({}_{4}\))\({}_{2}\) & 384.00 & 120 & IE & 7.00 & Sr & 2.30 \(\times\) 10\({}^{4}\) & [62] \\
2D-H\({}_{3}\)Sb\({}_{2}\)O\({}_{4}\) & 314.73 & \(<\)15 & IE & 2.1-2.5 & Sr & \(>\)10\({}^{6}\) & [36] \\
2D-Ce(PO\({}_{4}\))(HPO\({}_{4}\))\({}_{0.5}\)(H\({}_{2}\)O\({}_{0.5}\) & UM & UM & AD \({}^{7}\) & \(>\)9.00 & Np, Am, U, Th, Sr & UM & [40] \\
3D-K\({}_{2}\)Ce(PO\({}_{4}\))\({}_{2}\) & 45.65 & 60 & IE & 14.00 & Sr & \(\sim\)8000 & [32] \\
3D-H\({}_{5}\)Sb\({}_{2}\)O\({}_{2}\)(PO\({}_{4}\))\({}_{2}\)\({}^{7}\):3H\({}_{2}\)O & 123.19 & NC \({}^{8}\) & IE & \(<\)1.00 & Sr & \(\sim\)10\({}^{5}\) & [35] \\ Ca\({}_{2}\)\({}_{0.7}\)Mg\({}_{3}\)(NH\({}_{4}\))\({}_{2}\)OH\({}_{2}\) & UM & UM & NC & 5.00 & Cs, Sr, Co & 0.14 \(\times\) 10\({}^{3}\) & [63] \\ Ca\({}_{2.6}\)Mg\({}_{3}\)(NH\({}_{4}\))\({}_{3}\)(PO\({}_{4}\))\({}_{4}\)(CO\({}_{3}\))\({}_{0.3}\):6H\({}_{2}\)O & UM & UM & NC & 5.00 & Cs, Sr, Co & 7.91 \(\times\) 10\({}^{3}\) & [63] \\ phosphated montmorillonite & 12.5 & UM & IE/SC \({}^{9}\) & 5.00 & Sr, Cs & UM & [64] \\
2P-TiO\({}_{2}\) & 94.1 & 60 & AD & 8.00 & Sr & UM & [65] \\
8P-TiO\({}_{2}\) & 128.9 & 60 & AD & 9.00 & Sr & UM & [65] \\
4P-TiO\({}_{2}\) & 172.5 & 60 & AD & 9.00 & Sr & UM & [65] \\ \hline \hline \end{tabular}  
\({}^{1}\) The pH of solutions corresponding to the distribution coefficient values; \({}^{2}\) the composition of solutions corresponding to the distribution coefficient values; \({}^{3}\) the dimension is displayed in front of the adsorbent, the mixture is not displayed; \({}^{4}\) IE: ion exchange; \({}^{5}\) UM: not mentioned; \({}^{6}\) OTH(other): the formation of the Sr\({}^{2+}\) formation of UO2 HPO4-4H2O; \({}^{7}\) AD: adsorbent; \({}^{8}\) NC: not clear; \({}^{9}\) IE/SC: ion-exchange (pH \(<\) 8) and surface complexation (pH \(>\) 8).  
\end{table} 
Table 2: The comparison of the adsorption abilities of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) and other reported adsorbents for Sr\({}^{2+}\). In order to better clarify the adsorption mechanism, we tested the XPS of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) and K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr samples. In the XPS survey spectra (Figure 5a), the characteristic peak at \(\sim\)133 eV in pristine K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) was ascribed to P2p (Figure 5b). The characteristic peaks of Sr3d and Sr3p appeared in the XPS survey spectra of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr at \(\sim\)134 eV and \(\sim\)296.66 eV, respectively (Figure 5a). Two obvious peaks (133.6 eV and 135.5 eV) can be clearly observed in the high-resolution XPS spectra of Sr3d for K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr, corresponding to Sr3d\({}_{5/2}\) and Sr3d\({}_{3/2}\), respectively (Figure 5c). Meanwhile, the characteristic peaks of K2s and K2p of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr were lower than the pristine K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) at \(\sim\)377.7 eV and \(\sim\)292.4 eV, but they did not disappear completely in the XPS survey spectra of K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr (Figure 5a). This phenomenon can be seen more clearly in the high-resolution XPS spectra of K2p for K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\) and K\({}_{3}\)Nd(PO\({}_{4}\))\({}_{2}\)-Sr (Figure 5d).