## 3 Results and Discussion
### Automated Column Sorption System
#### 3.2.2. Acidity Levels  
Natural zeolites are known to raise the pH level in acidic aqueous solutions, which is due to: (1) the ion-exchange of H\({}^{+}\) ions, (2) the binding of H\({}^{+}\) ions to the Lewis basic sites linked to the oxygen atoms in the zeolite framework, and (3) the OH\({}^{-}\) ions in solution deriving from hydrolysis of some species present in the zeolite [11]. The pH level of the aqueous solution controls the overall sorption process; adsorption of the HMI at the solid-water interfaces as well as the ion-exchange of cations within the zeolite structure. [PERSON] et al. [10] points out that for all minerals, a decrease in the ion-exchange capacity of HMIs occurs for a pH range of 1 to 2. However, very low pH levels may positively influence the sorption process with the hydrolysis of the HMIs in solution [10]. Table 5 presents the pH levels of the effluent for both columns, of equally distributed selected time-step checkpoints of Cx-3, Cx-6, Cx-9, and Cx-13. When the acidified influent stock is combined to a 3-L volume, the average multi-component MM pH level has a value of 1.90. By maintaining a very low initial pH level and the use of highly soluble nitrate salts, the precipitation of the HMIs is avoided. Additional trials verified that the filtered and unfiltered HMI influent stock concentrations are the same, indicating both effective dilution practices and complete solubility. As the sample traverses through the first column C1, the H\({}^{+}\) ions are captured by the zeolite, resulting in an increase in the pH level to 6.34 from the first sample C1-A. There is an interesting \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**pH Level**} \\ \cline{2-3}  & **SCI** & **SC2** \\ \hline C1-A & 6.34 & - \\ Cx-3 & 4.79 & 6.84 \\ Cx-6 & 3.99 & 6.72 \\ \hline TW1 & \multicolumn{2}{c}{6.05} \\ \hline Cx-9 & 3.86 & 6.33 \\ Cx-13 & 3.60 & 5.76 \\ \hline TW2 & \multicolumn{2}{c}{5.44} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The pH levels of selected sorption column samples. observation between the columns' pH levels, which is a direct reflection of the zeolite's removal capacity for both the HMIs of interest and the competitive H\({}^{+}\) ions in solution. The pH level gradually decreases in both columns, with the levels of C2 being slightly greater than that of C1. The total waste (TW) collects in the effluent basin throughout the analysis period, and its pH level decreases from the half-way check point of 6.05 (115:45 min:s) to 5.44 at the final collection (195:00 min:s). This is a clear indication that the zeolite capacity is becoming exhausted for the competing H\({}^{+}\) ions, as well as hindering the sorption process of the HMIs. Research conducted by [PERSON] et al. [34] also demonstrated that the pH values changed during the uptake process, following the opposite shape of the typical breakthrough curves. At breakpoint, a drastic change in the pH value occurred, which corresponded with a rapid Pb\({}^{2+}\) concentration increase. The maximum pH level is reached at the breakthrough point, due to the absence of HMIs in the effluent. The minimum pH level is reached at the exhaustion point, due to the increase of the concentration of HMIs in the effluent and due to their hydrolysis in solution. After the exhaustion point, the pH level is constant [12]. These findings suggest that the continuous monitoring of pH levels is important and considerably contributes to the prediction of breakthrough and exhaustion points [11; 12; 34]; in order to monitor the progress of the service life and inevitably regeneration (adsorption/desorption cycles), both of which are very significant for practical industrial applications [34]. Also, the pH level may influence the ionization degree of the sorbate (HMI solution) and the surface property of the sorbent (zeolite mineral) [44].