## 3 Results and Discussion
### Pd(II) Sorption in Complex Systems
#### 3.3.1 Effect of Anions and Cations
Interestingly, the effect of SO\({}_{4}\)\({}^{2-}\) on Pd(II) binding is slightly less marked compared to the other two anions. Similarly, [PERSON] et al. [61] studied the effect of a series of anions on As removal and concluded that sulfate anions have a lower impact on As removal compared to other anions such as nitrate. It is probably due to the lower affinity of the functional groups (i.e., amine groups in this study) for sulfate than for other anions. The influence of different cations on Pd(II) sorption was investigated by adding Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\), CuSO\({}_{4}\), and Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) salt to palladium solution. The results in Figure 7 show that the trends of those salts are similar but Cu(II) exhibits a slightly higher inhibition on Pd(II) sorption. As discussed above, Pd(II) sorption occurs mainly through electrostatic attraction. Thus, the influence of metal cations should be limited. However, those cations may influence the sorption by forming metal sulfate complexes. The free sulfate anions in the solution are thus reduced and this leads to less competition between these anions and PdCl\({}_{3}\)\({}^{-}\) or PdCl\({}_{4}\)\({}^{2-}\) for the sorption sites. Indeed, as shown in Supplementary Materials Table S3, the addition of the salts does not significantly change palladium speciation in the solution. While a certain amount of SO\({}_{4}\)\({}^{2-}\) are captured by Fe\({}^{3+}\) and Al\({}^{3+}\) to form FeSO\({}_{4}\)\({}^{+}\) and AlSO\({}_{4}\)\({}^{+}\), respectively, most of the SO\({}_{4}\)\({}^{2-}\) in the solution added with CuSO\({}_{4}\) exists as SO\({}_{4}\)\({}^{2-}\) (19.6%) and HNO\({}_{4}\)\({}^{-}\) (74.6%); These anions are likely to compete with chloro-palladium complexes for the sorption sites. Figure 6: Influence of anions on Pd(II) sorption using (a) LD/PEI and (b) CC/PEI beads (C\({}_{0}\): 0.8 mmol L\({}^{-1}\); SD: 0.25 g L\({}^{-1}\); pH: 1, adjusted by H\({}_{2}\)SO\({}_{4}\) or NaOH; Contact time: 72 h; T: 20 \({}^{\circ}\)C). Figure 7: Influence of metal ions on Pd(II) sorption using LD/PEI and CC/PEI beads (C\({}_{0}\): 0.8 mmol L\({}^{-1}\); SD: 0.25 g L\({}^{-1}\); pH: 1, adjusted by H\({}_{2}\)SO\({}_{4}\) or NaOH; Contact time: 72 h; T: 20 \({}^{\circ}\)C). The influence of platinum on palladium sorption has been carried out at pH 1 with different concentrations of platinum, while the concentration of palladium was fixed at 0.5 mmol L\({}^{-1}\). The results in Figure 8 show that with the increase of Pt(IV) concentration from 0 to 0.5 mmol L\({}^{-1}\), the sorption capacity of LD/PEI for Pd(II) decreases from 0.80 to 0.57 mmol g\({}^{-1}\), while in the case of CC/PEI, it decreases from 1.35 to 1.09 mmol g\({}^{-1}\). However, the sorption capacity for Pt(IV) rises from 0 to 0.22 mmol g\({}^{-1}\) for LD/PEI and to 0.35 mmol g\({}^{-1}\) for CC/PEI beads. The molar ratio of Pd(II) to Pt(IV) loaded on LD/PEI and CC/PEI beads is 2.59 and 3.11, respectively, when the initial solution contains an equal amount (0.5 mmol L\({}^{-1}\)) of Pd(II) and Pt(IV). Therefore, although the presence of Pt(IV) affected the sorption of Pd(II), the sorbents maintain a preference for Pd(II) over Pt(IV).