The adsorption of Pb from pure Pb(NO 3 ) 2 solutions by Ca‐montmorillonite at pH 5.00 and 25°C was found to be sensitive to the weight of the clay sample. This apparent sensitivity of the adsorption of Pb to the weight of the sample was attributed to Ca competition for exchange sites as determined from Pb/Ca ratios found in the solutions. Adsorption of Pb from 0.1 M Ca(ClO 4 ) 2 solutions was substantially lower and was insensitive to the weight of the clay sample. This further supported our conclusion that adsorption of Pb was dependent on the Pb/Ca ratio. Application of the “simple” Langmuir equation to Pb adsorption by 0.05 and 0.10 g of clay resulted in a linear relation over the entire concentration range (0 to 1,200 ppm Pb). However, increasing non‐linearity was observed at the lower Pb concentrations when 0.50 and 1.00 g of clay were used, respectively. These Langmuir plots yielded lower adsorption maxima than maxima obtained from the two lower weights of clay and the shapes of the isotherms indicated two distinct regions of adsorption. The data were then plotted according to a “competitive” Langmuir equation, which successfully resolved the data for all four weights of clay into a single linear adsorption plot.
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Griffin et al. (1977) studied this question.