## 3 Results and Discussion
### Automated Column Sorption System
#### 3.2.5 Breakthrough Curve, Capacity and Usage Rate Analysis  
The breakthrough curve is displayed in Figure 3, as a plot of the solute outlet concentration (C\({}_{t}\)) from Table 6 normalized to the inlet concentration (C\({}_{o}\)) [34]. This normalized ratio trend over the service time of analysis [12; 25] at which the sampling chambers (SC1, SC2) are extracted is presented for each of the five (5) HMIs combined in the multi-component solution; for both the first sorption column (C1) and second sorption column (C2). The breakthrough point (BP) and exhaustion point (EP) of each HMI in each column are indicated. The first observation to be had is that the breakthrough curves of the first column C1 do not have a defined S-shape. However, in the second sorption column C2, the curves take on this typical shape. [PERSON] et al. [34] points out that the shape-change may be attributed to an improved solid-solution phase contact for sorption to take place. The influent stock concentration of the Ni\({}^{2+}\) ion is exceeded in the effluent solution, where the normalized C\({}_{t}\)/C\({}_{o}\) ratio surpasses 1 to reach an approximate maximum of 1.16 at 125 min and 1.10 at 190 min of service time, in sorption columns C1 and C2, respectively. The final ratio readings plateau at the end of service to approximately 5%-10% of the value of 1, given the nature of this experimental investigation. The effluent concentration that overshoots the influent concentration translates to concentration wave extremes inside the column [50]. [PERSON] et al. [12] investigates the breakthrough curves of Pb\({}^{2+}\) and Zn\({}^{2+}\) ions in dual-component solutions by natural zeolite; a similar trend is observed compared to this present study, where the C\({}_{t}\)/C\({}_{o}\) ratio even reaches a value of 2 for one set of operation conditions. This is attributed to the displacement of the bound Zn\({}^{2+}\) by the Pb\({}^{2+}\) from the influent, which is supported by a lower breakthrough capacity and higher exhaustion capacity in favour of the Pb\({}^{2+}\) ion, specifically [12]. It is important to note that the ion-exchange mechanism that attributes to the sorption process of HMIs transpires through the zeolite's framework of pores and channels. The presence of stronger binding HMIs, such as Pb\({}^{2+}\), weaken the chemical bonds between the functional group on the surface of zeolite and the weaker HMIs, such as Ni\({}^{2+}\) ions [9]. Given that zeolite demonstrates its highest preference towards Pb\({}^{2+}\), sorption site availability has reached its threshold, which may cause the leaching out of ions that zeolite holds a lower preference  
Figure 3: The multi-component system breakthrough curve. towards during this process. Therefore, careful screening on the selectivity of HMIs by zeolite should be conducted prior to adaptation. In summary, the major trends observed from the breakthrough curves are as follows:  
1. The zeolite holds the greatest preference towards to Pb\({}^{2+}\) ion, based on its complete removal throughout the analysis period;
2. The zeolite demonstrates the least preference towards the Ni\({}^{2+}\) ion; 1. A more sudden breakpoint occurring after just 25 min and 90 min of service time in columns C1 and C2, respectively; 2. An approximate exhaustion point after just 65 min and 165 min of service time in columns C1 and C2, respectively;
3. The Fe\({}^{3+}\) ion is removed entirely and sustained throughout the analysis period in C2, and;
4. The removal of both the Cu\({}^{2+}\) and Zn\({}^{2+}\) ions begin to plateau at 120 min of service time in C1, acting in parallel and do not reach the lower threshold of the exhaustion point in both columns throughout the analysis period. The breakthrough curves provide significant information from a perspective of sorption process performance, feasibility and optimization, which are vital for scaling-up the sorption system for industrial applications [28; 50].