Characterization of the Adsorption Behavior of Copper (II) Ions on the Surface of Manganese-Dioxide Particles

The adsorption of Cu2+ ions on MnO2 was examined as a function of pH and concentrations of Cu2+ and MnO2. The adsorption isotherm shows that the surface concentration of adsorbed Cu2+ increases with pH and concentration of Cu2+ in solution, and that, at higher coverages, the surface concentration is lower than predicted by the Langmuir isotherm The behavior was characterized by considering the stoichiometry and equilibrium and concentration conditions of the adsorption. The surface acid hydroxyl groups, ≡MnOH(a), react with Cu2+ ions to form the following two surface complexes.≡MnOH(a)+Cu2++NO-3_??_≡MnOCu+·NO-3+H+, 2≡MnOH(a)+Cu2+_??_(≡MnO)2Cu+2H+. The equilibrium conditlons of these surface complexations are given by. K1int=Q1eq exp (B1[≡MnOCu+·NO-3]), β2int=Q2eq exp(B2[(≡MnO)2Cu]), where K1int and β2int are“intrinsic”stability constants of[1:1]and[1:2] surface complexes, Q1eq and Q2eq are concentration quotients at equilibrium, and B1 and B2 are constants expressing the extent of suppression of surface complexation by the respective complexes. The equations of equilibrium conditions and of concentration conditions for both solution and surface species were fitted to the adsorption isotherm, and the values of the constants were determined by the nonlinear leastsquares method. The values are: K1int=8.15×10-3m3mol-1, β2int=2.41×103m-1, B1=3.81×106m2mol-1, and B2=1.62×106m2mol-1 at an ionic strength of 0.1 (NaNO3) and 25°C. These values well describe the observed adsorption behavior.