Isothermal titration calorimetry of selenium(IV) sorption processes onto iron oxides

As a consequence of nuclear waste disintegration heat, elevated temperatures in the near field of geological repositories may influence radionuclide retention at interfaces significantly. However, experimental data on free Gibbs energy (ΔRG), enthalpy (ΔRH) and entropy (ΔRS) of reactions of most radionuclides including fission products such as 79Se are sparse. Using the Se(IV)/maghemite system, we intended to show that microcalorimetry can provide these thermodynamic parameters with high accuracy and in a manner that allows studying various radionuclides.
The detection of the heat of the sorption reaction of Se(IV) onto maghemite was accomplished by isothermal titration calorimetry. Experiments were carried out at temperatures ranging from 20 to 40 °C and at pH 5. The heat flow was recorded as a function of time during the titrations (Figure 1).
The adsorption process was found to be exothermic, in agreement with findings of batch experiments. As the number of injections increases, the signal continuously decreases. Indeed, during the course of injections, the binding sites of maghemite are being gradually saturated by Se(IV), and the exothermic effect gets consequently reduced until only the heat of dilution is detected.
The heat (in J) related to each injection can be derived from single peak areas. The sum of all injections represents the corresponding enthalpy of the overall reaction (ΔRH). Additionally, the molar enthalpy of adsorption (J mol−1) can be determined and, thus, the number of adsorbed Se(IV) molecules as well. By applying the Langmuir isotherm, and assuming the proportionality of the maximum adsorption capacitiy qm to the mass to volume ratio, the Langmuir constant (log KL) can be derived.
In the future, spectroscopic techniques evidencing the nature of the adsorption process and the number of relevant species at the surface will be combined with microcalorimetry. Thus, a thermodynamic description of the selenium mobility in natural systems will be assessed with much more confidence and lower uncertainties.