## 3 Results and Discussion
### Pd(II) Sorption in Complex Systems  
#### 3.3.1 Effect of Anions and Cations  
The effect of anions was studied by adding different concentrations of NaCl, NaNO\({}_{3}\), or Na\({}_{2}\)SO\({}_{4}\) into initial solution containing a Pd(II) concentration of 0.8 mmol L\({}^{-1}\) and chloride concentration of 0.09 M (diluted from mother solution). Figure 6 shows that with the increasing concentration of the salts, the sorption capacity decreases, regardless of the sorbent. The addition of NO\({}_{3}\)\({}^{-}\) and Cl\({}^{-}\) may influence Pd(II) sorption through different mechanisms [45], depending on the main functional groups on the sorbents and the speciation of Pd(II):  
(1) If Pd(II) sorption occurs through metal-ligand coordination, such as Pd(II) bound onto carboxyl group-based alginate beads [53], chloride or nitrate anions in the solution may compete with the ligands on the sorbents for coordinating Pd(II). Thus, increasing such anions will reduce the amount of palladium bound to the sorbents, such as Pd(II) sorption onto calcium alginate beads reported by [PERSON] et al. [58];  
(2) If Pd(II) sorption occurs via electrostatic attraction between positively charged amine groups and negatively charged anionic palladium complexes, at the beginning, the addition of NaCl or NaNO\({}_{3}\) can promote the formation of PdCl\({}_{3}\)\({}^{-}\), PdCl\({}_{4}\)\({}^{2-}\), or Pd(NO\({}_{3}\))\({}_{3}\)\({}^{-}\), and consequentially increase the sorption capacity. However, when the concentration of these anions increases, anions may compete with chloro-anionic palladium species for binding on amine groups and the sorption capacity decreases. For example, [PERSON] et al. [59] reported an increase of palladium sorption onto chitosan (amine group-based sorbent) when the concentration of chloride increased from 0 to 33 mg L\({}^{-1}\), followed by a significant decrease of metal binding when the chloride concentration continued growing up to 500 mg L\({}^{-1}\). Similarly, [PERSON] et al. [60] observed that a maximum Pd(II) sorption capacity could be achieved for amine-functionalized resin when the concentration of chloride was 0.1 M;  
(3) If the bond between Pd(II) and the sorbents is not ionic, Pd(II) sorption could be almost independent on salinity variation, such as Pd(II) sorption onto activated carbon [5]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Model** & **Parameter** & **LD** & **CC** & **LD/PEI** & **CC/PEI** \\ \hline \multirow{4}{*}{Langmuir} & q\({}_{\text{q,exp}}\) (mmol Pd g\({}^{-1}\)) & 0.11 (0.01) & 0.04 (0.00) & 0.85 (0.03) & 1.34 (0.05) \\  & b ( L mmol\({}^{-1}\)) & 112.67 (30.47) & 26.79 (7.69) & 262.71 (102.8) & 129.01 (032.3) \\  & R\({}^{2}\) & 0.95 & 0.91 & 0.88 & 0.96 \\ \hline \multirow{4}{*}{[PERSON]} & K\({}_{\text{F}}\) (mmol g\({}^{-1}\))/(mmol L\({}^{-1}\))\({}^{n}\) & 0.14 (0.03) & 0.05 (0.01) & 0.90 (0.04) & 1.47 (0.09) \\  & n & 0.16 (0.04) & 0.24 (0.03) & 0.16 (0.03) & 0.16 (0.03) \\  & R\({}^{2}\) & 0.77 & 0.97 & 0.90 & 0.89 \\ \hline \multirow{4}{*}{[PERSON]} & q\({}_{\text{equal}}\) (mmol Pd g\({}^{-1}\)) & 0.11 (0.01) & 0.08 (0.01) & 0.88 (0.08) & 1.44 (0.05) \\  & b ( L mmol\({}^{-1}\)) & 382.54 (93.44) & 1.18 (0.43) & 16.64 (5.72) & 15.66 (5.69) \\ \cline{1-1}  & n & 0.80 (0.02) & 2.66 (1.07) & 1.72 (0.07) & 1.71 (0.06) \\ \cline{1-1}  & R\({}^{2}\) & 0.96 & 0.96 & 0.95 & 0.99 \\ \hline \end{tabular}  
* into parenthesis: standard error.  
\end{table} 
Table 2: Sorption isotherms—Modeling parameters for the Langmuir, Freundlich, and Sips equations. It is noted that, in this study, due to the high chloride concentration diluted from the mother solution, the predominant palladium species in the experiments are PdCl\({}_{4}\)\({}^{2-}\) and PdCl\({}_{3}\)\({}^{-}\). Therefore, Cl\({}^{-}\), NO\({}_{3}\)\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\) will compete with the chloro-palladium complexes for the binding sites; the sorption capacity decreases gradually when the concentration of these salts increases.