Cadmium and copper adsorption by δ-MnO2 was investigated by potentiometric titration over a range of pH. Adsorption increased with increasing pH because of hydrolysis of metal cations and/or variable charge sites on δ-MnO2 δ-MnO2. The surface acidity constant for δ-MnO2 was determined using the Triple Layer Model to be pKint,a2 = 5.34. The data from titrations with pH as master variable were analyzed to determine the Triple Layer Model intrinsic stability constants, p*Kint,cd2+ = 0.81, p*Kint.cdoH+ = 6.89, p*Kint,cd2+ = 1.66, and p*K+int,CuOH+ = 3.79 for cadmium and copper adsorption onto δ-MnO2. Titrations with metal ion as the master variable were performed at constant pH to estimate the adsorption capacities. The data conformed to a Langmuir isotherm and could be modeled with the Triple Layer Model constants. For cadmium, at pH 5.5, 7.0, and 8.0, the adsorption capacity is 0.434, 1.08, and 1.92 mmol/g, respectively. The affinity of δ-MnO2 for Cu, 1.54 mmol/g at pH 5.5, is greater than that for cadmium. The results show δ-MnO2 has high adsorption capacities and high adsorption affinities for cadmium and copper even in acidic conditions.
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Fu et al. (1991) studied this question.