## 3 Results and Discussion
### Fixed-Bed Adsorption  
Under dynamic operation, S914 continued to exhibit exclusive Cu selectivity and extraction from the PLS as evidenced by the almost immediate breakthrough of all other ions from the column, which reached complete breakthrough within the first five BV throughput (Figure 5). Numerical modelling for these metals indicated very low loading capacities for these ions (2.02-2.15 mg/g, MDR, Table 2), but considering the speed at which these metals broke through and considering the lack of displacement following complete breakthrough, this is likely an overestimation of loading capacity. Cu breakthrough began to occur at around 5 BV throughput and gradually increased, following a slightly sigmoidal pattern, until reaching a concentration ration of 0.92 at BV 80 (Figure 5). While the breakthrough curve was not sharp, Cu extraction appeared unaffected by other metal ions in solution and was best-defined by the MDR model, which indicated a loading capacity of 19.84 mg/g. Figure 4: XPS Cu \(2p\) scan for Cu-loaded MTS9140 (CPS = counts per second). Changing solution flow rate to 5 BV/h had little effect on the uptake of Co, Fe, Mn, Ni, and Zn, which passed through the column with little-to-no interaction with the thiourea-functionalised resin (Figure 6). For Cu, which was exclusively extracted by S914, the breakthrough profile appeared sharper than it did when loaded at 10 BV/h, which was confirmed by an associated increase in the MDR model constant 'a' (Table 3). Complete Cu breakthrough was also achieved earlier at the 5 BV/h flow rate, where Cu concentration ratio reached 1 at 68 BV throughput and corresponded to a \(Q_{o}\) value of 20.68-22.77 mg/g according to the breakthrough modelling (MDR and Thomas model had equal goodness-of-fit, R\({}^{2}\) = 0.998). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{} & \multicolumn{3}{c}{**Modified Dose Response**} & \multicolumn{3}{c}{**[PERSON]**} & \multicolumn{3}{c}{**[PERSON]**} & \multicolumn{3}{c}{**[PERSON]**} \\ \hline  & \(a\) & \(b\) & \(Q_{o}\) & \(R^{2}\) & \(K_{a}\) & \(W\) & \(R^{2}\) & \(K_{t}\) & \(Q_{o}\) & \(R^{2}\) & \(K_{ym}\) & \(t_{50}\) & \(R^{2}\) \\ \hline Al & 9.15 & 15.45 & 2.10 & _0.996_ & 0.14 & 3.27 & _0.996_ & 0.14 & 2.12 & _0.996_ & 0.49 & 18.22 & _0.998_ \\ Co & 9.64 & 15.44 & 2.15 & _0.999_ & 0.14 & 3.34 & _0.999_ & 0.14 & 2.17 & _0.999_ & 0.52 & 18.20 & _0.996_ \\ Cu & 3.01 & 160.26 & 19.84 & _0.998_ & 0.005 & 33.07 & _0.989_ & 0.005 & 21.20 & _0.989_ & 0.02 & 199.98 & _0.989_ \\ Fe & 9.97 & 16.48 & 2.13 & _0.980_ & 0.16 & 3.31 & _0.979_ & 0.16 & 2.15 & _0.979_ & 0.53 & 19.44 & _0.995_ \\ Mn & 9.06 & 15.57 & 2.02 & _0.986_ & 0.15 & 3.14 & _0.985_ & 0.15 & 2.04 & _0.985_ & 0.49 & 18.36 & _0.997_ \\ Ni & 10.07 & 15.37 & 2.10 & _0.999_ & 0.15 & 3.26 & 0.999 & 0.15 & 2.11 & _0.999_ & 0.54 & 18.11 & _0.997_ \\ Zn & 10.07 & 15.37 & 2.01 & _0.999_ & 0.14 & 3.27 & _0.997_ & 0.14 & 2.12 & _0.997_ & 0.48 & 18.96 & _0.999_ \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Breakthrough model parameters for MTS9140 at 10 BV/hour flow rate. When reduced to 2 BV/h, a further sharpening of the breakthrough of ions was observed. For all metals other than Cu, almost immediate complete breakthrough occurred, after which concentration ratios plateaued at values between 1 and 1.1 (Figure 7). It is important to note that a decision was made to increase sampling resolution during the 2 BV/h flow rate study to increase the number of data points during the frontal portion of the breakthrough curve. Given the limited space for sample collection in fraction collectors, this resulted in only the first 40 BV being sampled. Nevertheless, sufficient Cu breakthrough was captured to allow effective modelling and comparison to other flow rates. Cu was first detected in effluent solutions beyond 25 BV throughput (Figure 7), much later than observed for 10 and 5 BV/h (BV 5 and 10, respectively).