## 3 Results and Discussion
### Resin Elution  
The proposed reductive extraction mechanism of Puromet MTS9140 suggested an oxidative eluent to be applied to liberate Cu(I) as Cu(II) from the resin. While no literature exists on the elution of Cu from Puromet MTS9140 specifically, a limited number of articles do exist that explore the elution of Cu from non-commercial dual-functionalized resins containing, among other groups, thiourea functionality. One such paper reports effective batch Cu recovery from a thiourea/acyl bifunctional resin using concentrated nitric acid (equivalent to 3.0 M) as an eluent [30], and was initially explored (see Supplementary Material).  
Based on initial experimentation, it was deemed that the use of concentrated nitric acid as an eluent for MTS9140 would be unsuitable for two main factors; (1) the observed formation of nitrous gases could hinder process scale-up, and (2) the safety concerns of handling large volumes of highly concentrated nitric acid after process scale-up. However, given the presence of Cu(I) on the thiourea resin surface, an oxidative elution approach remained a possible avenue of exploration, and so sodium chlorate (NaClO\({}_{3}\)) was chosen for further elution studies, given the higher reduction potential of chlorate (1.152-1.451 V) over nitrate (0.803-0.934 V) in acidic media [31]. In addition to acting as a stronger oxidising agent, the use of NaClIO\({}_{3}\) over HNO\({}_{3}\) allowed elution to be performed under less-acidic conditions.  
The elution profile of Cu from S914 using a 0.5 M solution of NaClO\({}_{3}\) at pH 2 is presented in Figure 8. The concentration of Cu in effluent solutions increased sharply beyond 4 BV throughput, reaching a maximum concentration of 511 mg/L at 20 mL (-14 BV) throughput. Following peak maximum, Cu concentration exhibited a steep decline, which gradually levelled out to below 10 mg/L by the end of the experimental run; allowing the complete profile to be captured. Integration of the area below the curve (Table 5) revealed effective Cu elution by this eluent, with an overall Cu recovery percentage of 78.91%.  
Doubling the concentration of sodium chlorate had the effect of increasing the maximum Cu concentration to 612 mg/L (Figure 9). Despite the higher Cu concentration during peak maximum, the width of the elution peak (FWHM) was smaller than that in Figure 8, resulting in a recovery efficiency of 81.86%; fairly similar to the Cu recovery using 0.5 M NaClIO\({}_{3}\) (Table 5). Following elution using 1 M NaClIO\({}_{3}\), a slight but notable colour change was observed, with the 1M-contacted S914 taking on a grey hue when compared to the 0.5 M-contacted resin. It is theorised that this grey colouration was the result of copper oxide formation on the surface of the resin bead.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Cu** & **Bed** & **Total Cu** & **Cu** & **FWHM** & **Recovery** \\
**[NaClO\({}_{3}\)]** & **Loaded** & **Volume** & **on Bed** & **Recovered** & **(mL)** & **(\%)** \\  & **(mg/mL)** & **(mL)** & **(mg)** & **(mg)** & **(mg)** & **(\%)** \\ \hline
0.5 M & 8.64 & 1.4 & 12.10 & 9.55 & 16.2 & 78.91 \\
1.0 M & 8.36 & 1.4 & 11.70 & 9.58 & 13.2 & 81.86 \\ \hline \end{tabular}
\end{table}
Table 5: Details of Cu elution investigations using NaClO\({}_{3}\) (FWHM provided for comparison of peak widths).  
No gas formation was observed during chlorate elution, which was also evidenced by the more regular shape of elution profiles when compared to those obtained during HNO\({}_{3}\) elution (see Supplementary Information). Cu elution profiles were fairly symmetrical with a steep frontal curve and slightly longer tail-end. This curve profile is often observed where  
Figure 8: Elution of Cu from MTS9140 using 0.5 M NaClO3 at pH 2 (HCl matrix, 2 BV/h).  
Figure 9: Elution of Cu from MTS9140 using 1 M NaClO\({}_{3}\) at pH 2 (HCl matrix, 2 BV/h).  
effective elution is achieved [32; 33; 34], and implies a favourable exchange (or removal) of the target ion in favour of the eluent counter-ion.