Adsorption studies with environmental significance using an acoustic wave sensor

The adsorption of copper and lead on MnO₂ was followed by monitoring the frequency of piezoelectric crystal coated with MnO₂ in contact with solutions of the metal under study, at controlled temperature. Each isotherm was constructed for concentrations of each metal between 1 and 58 mg L^-1. When absorption data for copper or lead were fitted to the Langmuir isotherm, two linear zones were obtained, the first one corresponding to data from 1 to 32 mg L^-1 and the second one from 34 to 58 mg L^-1. Each one of the linear zones corresponded to different adsorption sites, the first one to easily binding sites and the second one to more energetic sites, which begin to be occupied after the saturation of the first ones. The number of more favorable adsorption sites was higher than the number of the less favorable ones