Abstract: Adsorption of Co2+ and Cd2+ on Wyoming montmorillonite was studied by the batch equilibration technique, as a function of salt concentration (0.01–4 M NaCl and NaNO3), pH (5.0–6.5), adsorbate concentration (trace-10−2 moles/liter), and presence of complexing ions. Comparison was made with the adsorbability of Sr2+, known to follow simple ion-exchange equations. The distribution coefficients for Co and Cd in noncomplexing media varied with salt concentration (from ∼500 liters/kg in 0.01 M Na+ to ∼10 liters/kg in 1 M Na+; pH = 5), but to a lesser extent than that of St. Adsorbability varied also with pH (∼1 order of magnitude/pH unit), especially at high ionic strength, compared to a negligible pH effect on Sr. The distribution coefficients of Cd and Co decreased with increasing loading on the clay at a very low percentage (0.2%) of the ion-exchange capacity compared to Sr (20%). These data suggest two classes of sites participating in the adsorption of Cd and Co.
The adsorbability of Cd in highly concentrated chloride solution (>1 M) was less than 1 liter/kg, presumably because of the chloride complex formation. This effect increased with increasing pH. The low adsorbability of Cd on montmorillonite from concentrated NaCl solution is promising with respect to its use as a tracer for monitoring flow through formations containing montmorillonite.