THE INFLUENCE OF TEMPERATURE ON THE ADSORPTION OF CADMIUM(II) AND COBALT(II) ON KAOLINITE

The adsorption of Cd(II) and Co(II) onto kaolinite was investigated at five temperatures between 10 and 70 degrees C. Adsorption edges showe d that both Cd(II) and Co(II) adsorbed onto kaolinite in two stages, s eparated by a plateau between pH 4 and 7. Initial adsorption commenced at about the same pH for both cations, but at each temperature the se cond-stage adsorption occurred at a slightly lower pH for Co(LI) than for Cd(II). At higher temperatures adsorption was generally shifted to lower pH. Adsorption isotherms at pH 5.50 for both cations could be f itted closely by a simple Langmuir model at all temperatures. A two-si te Langmuir model provided a substantially better fit for isotherms at pH 7.50 for Cd(II) and pH 7.00 for Co(II). At pH 5.50 the maximum ads orption density estimated from Langmuir modelling was approximately th e same (1 mu mol m(-2)) for both cations and at all temperatures. A si milar value was found for one of the model sites at pH 7.50 for Cd(II) and at pH 7.00 for Co(II). Potentiometric titrations of kaolinite sus pensions, in the presence and absence of added Cd(II) or Co(II), could be modeled accurately by a constant-capacitance surface complexation model. The data for adsorption of both cations could be fitted at all temperatures using a model that assumed ion exchange at permanent char ge sites on silanol faces and complexation to hydroxyl edge groups. Th ermodynamic parameters estimated from both the Langmuir and surface co mplexation models showed that adsorption of Cd(II) and Co(II) were end othermic. For the surface complexation model, enthalpies of adsorption on exchange sites were about 10 kJ mol(-1) but at the variable-charge sites the enthalpy changes were about 70 kJ mol(-1). For all these re actions the entropy changes were positive, with values of the order of 100 J K-1 mol(-1). Trends for the Langmuir model were qualitatively s imilar. (C) 1998 Academic Press.