*3.5. Validation of the Electrochemical Accumulation*

The effect of electrochemical accumulation pertaining to the proposed adsorption mechanism was investigated by quantifying the amount of Pb(II) ions adsorbed by the modified device. Originally, the Pb(II) concentration of each test solution was 100 μg/L. The concentration was subsequently measured by the ICP-MS instrument after every 5 min of adsorption. The results obtained are presented in Figure 8. When the electrochemical accumulation was not activated, i.e., only the HAMA hydrogel was responsible for the adsorption of Pb(II) ions (blue dots in Figure 8), the adsorption rate slowly increased within the first 20 min. Thereafter, there was no significant increase in the adsorption rate. The adsorption behavior of the HAMA hydrogel could be attributed to the diffusion effect. Initially, metal ions moved into the vicinity of the hydrogel driven by the high-ion-concentration gradient established between the solution and the hydrogel. This corresponded to the increase in the adsorption rate observed at 5, 10, 15, and 20 min. Thereafter, the concentration gradient was significantly reduced, which could have resulted in a rapid saturation of adsorption for the HAMA hydrogel, corresponding to the data points observed at 25, 30, 35, and 40 min. It was calculated that only 43.05% of Pb(II) ions were adsorbed by the device if electrochemical accumulation was not triggered.

On the other hand, when the electrochemical accumulation was activated (red dots in Figure 8), the adsorption rate continuously increased up to 30 min. In addition, the slope of the increase was significantly higher than in the case where electrochemical accumulation was not involved. It was found that 94.08% of Pb(II) ions were adsorbed by the device within 30 min once the electrochemical accumulation had been triggered. These experimental results confirm that electrochemical accumulation is particularly beneficial in facilitating the adsorption rate for the device by maintaining a high concentration gradient between the solution and the hydrogel. Considering the above discussion, it is apparent that both intermolecular complexation and electrochemical accumulation make a significant contribution to the adsorption capability of the HAMA hydrogel-modified electrochemical devices. A comparison of the Pb(II) removal efficiency of the HAMA hydrogel-modified electrochemical device and other polymeric adsorbents is shown in Table 2. Based on the data presented in Table 2, it can be observed that the developed HAMA hydrogel-modified electrochemical device is capable of effectively removing Pb(II) ions with a short adsorption time of 30 min.

**Figure 8.** Comparison of adsorption efficiency for the HAMA hydrogel-modified devices without (blue dots) and with (red dots) electrochemical accumulation. Data obtained using three devices.


**Table 2.** Comparison of Pb(II) removal efficiency of different adsorbents.
