## 3 Results and Discussion
### Resin Reusability  
To determine the reusability of MTS9140 for the selective extraction of Cu, repeated loading, elution, and preconditioning cycles were performed. The 0.5 M NaClO\({}_{3}\) eluent was used given its effectiveness for Cu recovery. While the 1 M eluent did have a marginally higher recovery efficiency, the lack of resin discoloration indicated that 0.5 M NaClO\({}_{3}\) was a better option for investigating reusability. The resulting breakthrough curves are presented in Figure 10.  
During the first loading cycle, typical Cu adsorption behaviour was observed, with complete breakthrough encountered at 40 BV throughput (Figure 10). Integration of the area above the breakthrough curve (with an upper boundary of influent PLS concentration) indicated the adsorption of 7.95 mg Cu during the first loading cycle. Following Cu elution, the resin bed was loaded again using the 400 mg/L Cu solution, but exhibited very low adsorption of Cu, with only 1.55 mg of Cu removed before complete breakthrough occurred during both the second and third loading cycles (Table 6).  
Comparison of elution curves using 0.5 M NaClO\({}_{3}\) (Figure 11) revealed that for the first cycle, where 7.95 mg Cu was loaded to the resin bed, 6.79 mg was recovered, equating to a recovery efficiency of 85.34% (Table 6). The elution profile generated increased sharply, reaching a maximum Cu concentration of 418 mg/L after 0.028 L, before decreasing in an almost-symmetrical fashion, indicating effective desorption.  
The considerably lower extent of Cu extraction during the second and third loading cycles (Figure 10) was reflected in the elution profiles for the respective cycles (Figure 11). However, for elution cycle two, the recovery efficiency was consistent with recovery during cycle one (88.46 and 85.34, respectively; Table 6). While the extent of Cu loading between  
Figure 10: Breakthrough curves of Cu from MTS9140 over repeated loading cycles (1.4 mL BV, 5 BV/h; 400 mg/L Cu, pH 1.55).  
cycle one and two was similar, the mass of Cu eluted differed greatly, with a decline in recovery efficiency from 88.46% to only 46.27% during elution cycle three (Table 6).  
The lower mass of Cu recovered from S914 during the third elution cycle could potentially be explained by the progressive loss of resin functionality during elution cycles. It could be assumed that as the eluent contacts the exterior of resin beads before diffusing through the resin pores, the functional groups at the surface of each bead would be the first to be degraded, explaining the loss of capacity between cycle one and two. Assuming this'shrinking-core' hypothesis for loss of functionality is true, it holds that during the later elution cycles, the remaining functionality is located deeper within the resin beads and therefore less accessible to the eluent. While the ionic radii of Cu(I) (0.46-0.77 A) is substantially smaller than that of ClO\({}_{3}\)\({}^{-}\) (1.71 A), it is unlikely that Cu residing in pores inaccessible to ClO\({}_{3}\)\({}^{-}\) was responsible for this observation given the flexibility of gelled polymers [18]. Instead, this is more likely a result of kinetic limitations inherent to column operation, i.e., the limited residence time within the column is hindering sufficient mass transfer between the bulk eluent and resin, and in doing so reduces eluent efficiency [35]. It is expected that such effects are amplified when coupled with functionality degradation on resin outer surfaces.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cycle** & **Loading (mg)** & **Elution (mg)** & **Recovery (\%)** \\ \hline
1 & 7.95 & 6.79 & 85.34 \\
2 & 1.55 & 1.37 & 88.46 \\
3 & 1.55 & 0.72 & 46.27 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Masses of Cu loaded and eluted from MTS9140 over multiple operational cycles.  
Figure 11: Elution of Cu from MTS9140 using 0.5 M NaClO\({}_{3}\) (pH 2, HCl matrix, 2 BV/h) over repeated elution cycles.