## 3 Results and Discussion
### Isotherms Study  
The isotherm studies using the PAN-oxime fiber developed with a 15 min reaction time were conducted in single and binary metal systems of Pd(II) and Pt(IV). The adsorption capacities were compared with those of commercial ion-exchange resin (Amberjet(tm) 4200) which is predominantly used for metal adsorption. In order to evaluate the maximum adsorption capacity from the experimental data, the Langmuir isotherm model [21] was applied. The nonlinear Langmuir equation is shown below:  
\[\mathrm{Langmuir\ model:\ }q_{\mathrm{e}}=q_{\mathrm{m}}bC_{\mathrm{e}}/\left(1+bC _{\mathrm{e}}\right) \tag{2}\]  
where \(q_{\mathrm{e}}\) is the uptake of metal at equilibrium (mg/g) and \(C_{\mathrm{e}}\) is the concentration of the metal solution at equilibrium (mg/L). \(q_{\mathrm{m}}\) and \(b\) are related to the Langmuir constant, and they represent the adsorption capacity (mg/g) and affinity (L/mg), respectively. Figure 4: Effect of amidoximation time on the Pd(II) uptake by PAN–oxime fiber. As shown in Figure 5a, adsorption capacities for Pd(II) and Pt(IV) of both PAN-oxime fiber and Amberjet(tm 4200) increase in the single-metal systems as the equilibrium concentrations of respective metals increase. The estimated parameters are given in Table 1. In single-metal systems, the maximum uptakes of Pd(II) and Pt(IV) by PAN-oxime fiber were estimated to be 244.06 \(\pm\) 10.68 and 175.65 \(\pm\) 6.62 mg/g, respectively. Those of Pd(II) and Pt(IV) by Amberjet(tm 4200 were estimated to be 310.05 \(\pm\) 13.25 and 422.65 \(\pm\) 21.33 mg/g, respectively. In general, the sorption capacities of Amberjet(tm 4200 for Pd(II) and Pt(IV) were higher than those of the PAN-oxime fiber. In the binary metal system, the adsorption behaviors of the PAN-oxime fiber were quite interesting compared to those of Amberjet(tm 4200. In case of Amberjet(tm 4200, its metal uptakes in the binary metal mixture were lower than those in the single-metal systems, and it did not show any metal selectivity (Figure 5). However, in the same condition, Pd(II) uptakes of PAN-oxime fiber were similar in both cases of single (244.06 \(\pm\) 10.68 mg/g) and binary metal systems (234.54 \(\pm\) 11.53 mg/g). Note that the maximum Pd(II) uptake of PAN-oxime in the binary metal system may be higher than the calculated value, since the isotherm of Pt(IV) could not be fitted by the Langmuir model. Surprisingly, Pt(IV) uptake of PAN-oxime fiber, in the case of the multi-metal system, was significantly lowered (Figure 5b) from 62.99 mg/g in a single-metal system to 39.62 mg/L in a binary metal system. This indicates that PAN-oxime fiber has selective binding ability for Pd(II) in a mixture of Pd(II) and Pt(IV). For better comparison, the selectivity coefficients (\(\alpha_{Pd(II)/Pt(IV)}\)) of PAN-oxime and Amberjet(tm 4200 for Pd(II) in the mixture of Pd(II) and Pt(IV) were calculated using Equation (3). \[\alpha_{Pd(II)/Pt(IV)}=\frac{q_{t,Pd(II)}/C_{f,Pd(II)}}{q_{t,Pt(IV)}/C_{f,Pt(IV)}} \tag{3}\]  
where \(q_{t,Pd(II)}\) and \(q_{t,Pt(IV)}\) are the uptakes of Pd(II) and Pt(IV), respectively, and \(C_{f,Pd(II)}\) and \(C_{f,Pt(IV)}\) are the final concentrations of Pd(II) and Pt(IV), respectively. Based on Equation (3), the selectivity coefficients of PAN-oxime and Amberjet(tm 4200 at the initial metal concentrations (200-1000 mg/L) could be calculated. As can be seen in Figure 5c, the selectivity coefficients toward Pd(II) of PAN-oxime ranged from 15 to 33, which was approximately 19-89 times higher than those of Amberjet(tm 4200. The selectivity of PAN-oxime decreased when initial metal concentration increased.