ADSORPTION OF CADMIUM AND COPPER BY MANGANESE OXIDE

Cadmium and copper adsorption by delta-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 delta-MnO2. The surface acidity constant for delta-MnO2 was determined using the Triple Layer Model to be pK(int,a2) = 5.34. The data from titrations with pH as master variable were analyzed to determine the Triple Layer Model intrinsic stability constants, p*K(int,Cd2+) = 0.81, p*K(int,CdOH)+ = 6.89, p*K(int,Cu2+) = 1.66, and p*K(int,CuOH+) = 3.79 for cadmium and copper adsorption onto delta-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 delta-MnO2 for Cu, 1.54 mmol/g at pH 5.5, is greater than that for cadmium. The results show delta-MnO2 has high adsorption capacities and high adsorption affinities for cadmium and copper even in acidic conditions.