The point of zero charge (pHpzc) indicates in which pH the adsorbent and adsorbate prefer to adsorb each other. At this pH, the number of positive charges are equal to the number of negative charges. To evaluate the pHpzc, 0.03 g of the adsorbent (PTA@MIL–53 (Fe)) was added to each 10 different 60 mL beakers containing 0.1 M KNO3 solution. HCl (1.0 N) as a strong acid and NaOH (1.0 N) as a strong base were applied to adjust the initial pH of each beaker (pHi) between 2–11. The samples were stirred for 24 hrs to get equilibrated and then the final pH values (pHf) correspond to the pH at which there is no net OH− or H+ adsorption, were measured. Then the diagram of pHi–pHf was plotted versus pHi. As depicted in Figure 5.8, the pHpzc was found to be 4.3 which was obtained from the intersection between the sketched curve and horizontal axis.
To determine the pHpzc, it is ideal that the amount of positive and negative charges in the solid-liquid interface, represented by cations and anions, are equal. For this it is important that the ions are symmetrical in size, or of approximate sizes. In addition to good solubility in water, availability, relative low toxicity, potassium nitrate results in ions K + and NO3- which both present an effective hydrated ionic radius of approximately 3 Å, which is also the effective radius of the Cl-.
Perhaps the methodology could be optimized by replacing NaOH with KOH, since Na + has a hydrated ionic radius of 4 Å.
this question has already been answered here:
https://www.researchgate.net/post/What_is_the_role_of_KNO3_or_NaCl_in_the_determination_of_the_point_of_zero_charge
KNO3 is preferred to other salts (e.g. KCl) because NO3- does not interact specifically with the surface (no surface complexation)
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