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Four methods, fast titration, salt addition, mass titration, and ζ potentiometry, were employed to determine the point of zero charge (PZC) and isoelectric point (IEP) of NiO. However, this work mainly focuses on the fast titration method because it limits the release of Ni from the substrate. Furthermore, the fast titration is simple, rapid, and accurate and requires a small quantity of substrate as compared to the salt addition and mass titration techniques. The similarity between the PZC and IEP values rules out the specific adsorption of the bulk electrolyte onto the NiO surface. However, the PZC values shift downward with the increase in Cd ion concentration, which indicates the specific adsorption of Cd onto the NiO. In addition, temperature has a mild effect on the deprotonation constants and PZC of NiO. It was also observed that the thermodynamic parameters favor the deprotonation of NiO. ' MATERIALS AND METHODSMaterials. Analytical grade reagents were used in this study. All glassware and polyethylene bottles were rinsed with 10%

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The sorption of Zn2+, Ni2+, and Co2+ on ZnO was studied as a function of pH, temperature, and concentration of the metal cations. The extent of sorption was found to increase with the increase in pH and concentration and decrease with the increase in temperature. The potentiometric titrations data showed that the sorption mechanism of the transition metal cations changed from adsorption to precipitation with the increase in pH. The sorption and precipitation of the metal cations were explained with the help of a new equation derived from the law of mass action. FTIR studies revealed the formation of a double hydroxide of Zn and metal ions concerned on the surface of the solid. The dissolution studies of the ZnO were also employed to evaluate the sorption mechanism of the metal cations on the ZnO. The decrease in the extent of adsorption of the metal cations with temperature was due to an increase in dissolution of the ZnO.

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