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.
Potentiometric titrations of iron(iii) phosphate have been carried out in aqueous electrolyte solutions of KCI over the temperature range 299-333 K. The data were explained by the law of mass action. Sorption of Cu2+, Zn2+, Co2+ and Ni2+ on iron(iii) phosphate was studied over the temperature range 298-323 K. The sorption data fitted the linear form of the Langmuir adsorption equation. Further, the extent of sorption was found to increase with increase in temperature and concentration. The order of selectivity was found to be Cuz+ > Zn2+ > C02+ > Ni2+. The values of the Langmuir constants were used to calculate the thermodynamic parameters: AGO, AH0 and ASo.
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