Ion adsorption on oxides : surface charge formation and cadmium binding on rutile and hematite

The adsorption of charge-determining (H <sup><font size="-2">+</font></SUP>and OH <sup><font size="-2">-</font></SUP>) and cadmium ions on rutile (TiO _{<font size="-2">2</font>} ) and hematite (α-Fe _{<font size="-2">2</font>} O _{<font size="-2">3</font>} ) has been studied in different concentrations KNO _{<font size="-2">3</font>} as a function of temperature and pH.Rutile and hematite show identical surface charge - pH behavior, except for the position of the point of zero charge (pH <sup><font size="-1">o</font></SUP>): normalized with respect to pH <sup><font size="-1">o</font></SUP>(σ _{<font size="-1">o</font>} ,pH) curves of both oxides are experimentally indistinguishable.Measurement of the surface charge density on these oxides at temperatures other than room temperature is well possible with the potentiometric titration technique, provided certain precautions are taken. The surface charge - pH relations of rutile and hematite have been studied as a function of temperature in the range 5<T<70 °C. Temperature changes affect the relative position of these curves but do not significantly change their shapes. This phenomenon has been called 'temperature congruence of surface charge'. It is shown that temperature congruence Is to be expected for high capacitance double layers on Nernstian surfaces. This high capacitance value is related to the specific structure of oxide surfaces in aqueous solutions.A thermodynamic treatment has been put forward, that relates the observed shifting of (σ _{<font size="-1">o</font>} ,pH) curves with temperature to the enthalpy of proton adsorption/desorption In the interfacial region. We showed that pH <sup><font size="-1">o</font></SUP>is determined by the enthalpy of proton association with the surface rather than by the entropy. Entropy changes accompanying charge formation are aspecific in the sense that for hematite and rutile identical values for ΔS <sup><font size="-1">o</font></SUP>are obtained. The adsorbing proton loses about half of its hydration water on adsorption.Enthalpies obtained by the thermodynamic analysis could be verified by direct microcalorimetric titrations of rutile and hematite suspensions. Calorimetric heats of charge formation are independent of the surface charge density on the oxides. This behavior Is shown to be typical for high capacitance interfaces, because in this case electrical and countercharge contributions to the enthalpy of charge formation practically compensate each other.The adsorption of cadmium ions on rutile and hematite has been measured using a potentiometric pH-stat technique. The influence of temperature on the adsorption is relatively small. The surface charge density of the ox ide, however, is a major factor determining the adsorbability of Cd <sup><font size="-1">2+</font></SUP>ions.On adsorption of cadmium Ions, hydroxyl Ions are depleted from the solution. This OH <sup><font size="-1">-</font></SUP>co-adsorption could be explained in terms of 'surface charge adjustment'. Our surface charge adjustment theory is based on the constant potential principle: on specific adsorption of (cat)ions, the surface potential is assumed to maintain a constant (Nernst) value. Considering the specific characteristics of oxide - solution interfaces, our OH <sup><font size="-1">-</font></SUP>co-adsorption data for the adsorption of cadmium onto rutile and hematite at different temperatures, and those taken from literature for other heavy metal ions onto Inorganic oxides, could be explained satisfactorily within the framework of the theory.Experimental adsorption isotherms of Cd <sup><font size="-1">2+</font></SUP>were analysed according to the Frumkin - Fowler - Guggenheim model, where chemical, electristatic and lateral interactions are taken into account. The chemical contribution to the Gibbs energy of adsorption was shown to be insensitive towards changes in surface charge density and electrolyte concentration, which is an Indication of the correctness of the model.The enthalpy of cadmium adsorption in the rutile-Cd(NO _{<font size="-1">3</font>} ) _{<font size="-1">2</font>} -KNO _{<font size="-1">3</font>} system could be obtained from calorimetric acid/base titrations. The adsorption of Cd <sup><font size="-1">2+</font></SUP>ions on rutile was shown to be slightly endothermic, even though calorimetric measurements always showed exothermic overall heat effects. It was inferred that Cd <sup><font size="-1">2+</font></SUP>adsorption on rutile is driven by a gain in hydration entropy.A strong correlation between Cd <sup><font size="-1">2+</font></SUP>adsorption and the formation of Cd(OH) <sup><font size="-1">+</font></SUP>complexes in aqueous solutions is observed. Both reactions are mainly driven by a gain in hydration entropy. This finding supports the Idea that surface groups on oxides resemble bulk OH <sup><font size="-1">-</font></SUP>ions.The fact that the adsorpion of protons and cadmium Ions on oxides In this study were shown to be strongly related processes, has Important practical implications. With a minimum of experimentation It is now possible to predict the adsorption of heavy metal ions on oxides, using essentially easily obtainable and accurate potentiometric acid/base titration data.