The adsorption of metals by clay minerals is a complex process involving different mechanisms, and is controlled by different variables which can interact. The aim of this work was to study the retention mechanisms of Cd on illite. We obtained Cd adsorption isotherms at constant pH, adsorption edges as a function of pH, adsorption isotherms at 5, 25 and 45°C (pH = 7), and response surfaces of the simultaneous effect of pH, initial concentration, ionic strength, and temperature on the retention of Cd on illite. Below pH 6, adsorption of Cd on illite is via ion exchange with H3O+ and Na+ ions which saturate the exchange sites, the exchange with Na+ being the main mechanism between pH 4.5 and 6.0. For pH values >6, the effect of ionic strength on the amount of Cd2+ retained decreased with pH, being negligible at pH 8; the proton stoichiometry was greater than for pH values <6 and an increase in the temperature favored the retention of Cd. These facts are compatible with a more specific process involving hydrolyzed species, in which Cd can associate with illite as an inner sphere complex.

The energetics of structural changes in the holo and apo forms of α-lactalbumin and the transition between their native and denatured states induced by binding Ca2+ and Na+ have been studied by differential scanning and isothermal titration microcalorimetry and circular dichroism spectroscopy under various solvent conditions. Removal of Ca2+ from the protein enhances its sensitivity to pH and ionic conditions due to noncompensated negative charge–charge interactions at the cation binding site, which significantly reduces its overall stability. At neutral pH and low ionic strength, the native structure of apo-α-lactalbumin is stable below 14 °C and undergoes a conformational change to a native-like molten globule intermediate at temperatures above 25 °C. The denaturation of either holo- or apo-α-lactalbumin is a highly cooperative process that is characterized by an enthalpy of similar magnitude when calculated at the same temperature. Measured by direct calorimetric titration, the enthalpy of Ca2+-binding to apo-LA at pH 7.5 is −7.1 kJ mol−1 at 5.0 °C, which is essentially invariant to protonation effects. This small enthalpy effect infers that stabilization of α-lactalbumin by Ca2+ is primarily an entropy driven process, presumably arising from electrostatic interactions and the hydration effect. In contrast to the binding of calcium, the interaction of sodium with apo-LA does not produce a noticeable heat effect and is characterized by its ionic nature rather than specific binding to the metal-binding site. Characterization of the conformational stability and ligand binding energetics of α-lactalbumin as a function of solvent conditions furnishes significant insight regarding the molecular flexibility and regulatory mechanism mediated by this protein.