## 3 Results and Discussion
### Comparability and Selectivity of the b-HdA in a Multi-Ionic System  
Several radionuclide ions, such as Ni(II), Sr(II), Cs(I), and U(VI), have frequently been detected in groundwater and wastewater, which might significantly affect the adsorption behavior of Co(II) [3]. We observed that the removal efficiencies of Co(II) decreased consid  
Figure 5: Adsorption kinetic data fitted to (**a**) pseudo-second-order and three-stage models and (**b**) an intra-particle diffusion model for Co(II) adsorption onto the b-HdA at 293 K.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Models** & \multicolumn{2}{c}{**Parameters**} & **Value** \\ \hline \multirow{3}{*}{Pseudo-first-order} & \(q_{e}\) & (mg/g) & 2.87 \\  & \(k_{1}\) & (1/h) & 0.13 \\  & \(R^{2}\) & & 0.98 \\ \hline \multirow{3}{*}{Pseudo-second-order} & \(q_{e}\) & (mg/g) & 3.69 \\  & \(k_{2}\) & (g/mg h) & 0.15 \\  & \(R^{2}\) & & 0.99 \\ \hline \multirow{3}{*}{Three-stage} & \(\xi_{1}\) & & 0.07 \\  & \(\xi_{2}\) & & 0.93 \\ \cline{1-1}  & \(\alpha\) & (1/h) & 0.59 \\ \cline{1-1}  & \(\beta\) & & 0.89 \\ \cline{1-1}  & \(\gamma\) & & 0.09 \\ \hline \multirow{3}{*}{Intra-particle diffusion} & \(k_{ul}\) & (mg/g h\({}^{0.5}\)) & 1.4 \\ \cline{1-1}  & \(R^{2}\) & & 0.99 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Summary of the kinetic model parameters for Co(II) adsorption onto the b-HdA at 293 K.  
erably in the multi-ionic systems compared to those in the single systems (data not shown), which could be attributed to competing effects. Furthermore, the selective order in the adsorption of these radionuclides on the b-HdA was U(VI) > Co(II) > Ni(II) > Sr(II) > Cs(I), as illustrated in Figure 6a,b. Research has shown that the absorption preference is affected by various characteristics of the adsorbates, such as ionic valence, ionic radius, hydration radius, electronegativity, hydration enthalpy, and charge density [6, 40, 44]. Generally, the adsorptive sites are preferentially occupied by ions with higher electronegativity, bigger ionic radius and valence, and smaller hydration radius [12]. Furthermore, ions with a higher hydration enthalpy prefer the solution phase, where it may satisfy its hydration requirements. In contrast, ions with a lower hydration enthalpy prefer the adsorbent phase [12]. In this respect, the characteristic parameters of the investigated radionuclides are presented in Table S4. It can be observed that even the trends could not provide a consistent rule for examined radionuclides' selectivities for the b-HdA. Nevertheless, it is noteworthy that the selective order of examined ions only coincided with the order trend of charge density, which was U(VI) > Co(II) > Ni(II) > Sr(II) > Cs(I), suggesting that the adsorption affinity might more strongly depend on the charge density of ions than other characteristics. It was reported that ions with a larger charge density are capable of substituting Ca(II) ions on the HdA surface more easily and quickly than others [3, 19, 47]. Considering that ion exchange takes place as the main adsorption mechanism, this postulation is a reasonable way to interpret the selectivity of the adsorption process in this work. Our results are consistent with previous studies using hydroxyapatite-based materials as adsorbents for the uptake of metal ions [3, 17, 48].