# Adsorption of Ag (I) Ions at the Zirconium Phosphate/KNO3 Aqueous Solution Interface
## 3 Results and Discussion
### Electrophoretic Study of the Zirconium Phosphate Samples  
The FTIR studies showed the presence of Zr-OH and P-O-H groups in the samples. These groups on the ZrP surface can undergo the following ionization reactions, resulting in a surface charge formation:  
\[\equiv\mathrm{ZrOH}_{2}^{+}\leftrightarrow \equiv\mathrm{ZrOH}+\mathrm{H}^{+} \tag{1}\] \[\equiv\mathrm{ZrOH}\leftrightarrow \equiv\mathrm{ZrO^{-}}+\mathrm{H}^{+}\] (2) \[\equiv\mathrm{POH}\leftrightarrow \equiv\mathrm{PO^{-}}+\mathrm{H}^{+} \tag{3}\]  
The adsorption of the proton on the hydroxyl group according to reaction (1) leads to the formation of a positive charge while the dissociation of the proton from the hydroxyl group, reaction (2), or dissociation of the proton from the hydrogen phosphate group, reaction (3), leads to the formation of a negative charge. The charge accumulated on the ZrP surface is the algebraic sum of charges originating from the above-mentioned groups. In the case of a low concentration of the base electrolyte, [PERSON] and [PERSON] assumed that the surface charge density is approximately equal to the charge density of the diffusion layer which can be calculated from the zeta potential measurements [24]. Assuming further that for pH < pHip the surface charge is determined by the groups with the positive charge and for pH > pHip by the negatively charged groups, they proposed a method of calculating the thermodynamic constant equilibria of the ionization reaction of groups with the positive charge K\({}_{\mathrm{a1}}\) and those with the negative charge K\({}_{\mathrm{a2}}\)[24]. Figure 3a,b show the dependence of the zeta potential as a function of the pH of the xerogel and gel ZrP samples, respectively, at 0.001 mol/dm\({}^{3}\) KNO\({}_{3}\). The relationships of the zeta potential as a function of the pH of the xerP samples presented in Figure 3a differed significantly from the dependence of the zeta potential versus pH for the Initial sample. Initially the xerogel sample showed a similar course up to pH \(\sim\) 3.5, but above this pH value the zeta potential was higher, which indicates a smaller concentration of the negatively charged groups. The ZrP MWT xerogel sample in the range from pH = 2 to pH = 6.5 was characterized by smaller zeta potential values, which indicates greater ionization of the surface groups towards the formation of negatively charged groups. Based on the FTIR spectrum of this sample, it can be assumed that the hydrogen-phosphate groups can be responsible for such a course. The dependence of the zeta potential as a function of pH for the gel-type ZrP samples (Figure 3b) shows that the gel-type sample showed, in the initial pH range, up to pH = 5, lower pH values and higher than the those of ZrP initial sample. On the other hand, the ZrP MWT and ZrP MChT samples up to pH = 7 exhibited smaller zeta potential values, and above pH = 7 the zeta potential vs. pH was not much different from that of the initial sample. The pH\({}_{\mathrm{iep}}\) values and the negative logarithm of the thermodynamic equilibrium constants pK\({}_{\mathrm{a1}}\) and pK\({}_{\mathrm{a2}}\) calculated from these relationships are presented in Table 2. The pH\({}_{\mathrm{iep}}\) value of the xerogel sample in relation to the pH\({}_{\mathrm{iep}}\) of the Initial sample was slightly higher due to the lower concentration of negatively charged groups; the surface groups were prone to ionization as evidenced by the value of pK\({}_{\mathrm{a2}}\) = 7.2. On the other hand, the surface groups of the Gel, MWT gel, and MChT gel samples were more acidic. The pK\({}_{\mathrm{a2}}\) of the surface groups of these samples was significantly smaller than the pK\({}_{\mathrm{a2}}\) of the Initial sample (Table 2), and the pH\({}_{\mathrm{iep}}\) of these samples was shifted towards lower pH values compared with those of the Initial sample.