EXAFS studies on adsorption irreversibility of Zn(II) on TiO2: temperature dependence

Adsorption irreversibility of Zn(II) on TiO2 at various temperatures was studied using a combination of classical macroscopic methods and extended X-ray absorption fine structure (EXAFS) spectroscopy. When the temperature was increased from 5 to 40 degrees C, the Zn(II) adsorption capacity increased by 130%, and adsorbed Zn(II) became more reversible. The standard Gibbs free energy change (DeltaG 0) of the adsorption reaction at 5, 20, and 40 degrees C was determined to be -19.58+/-0.30, -22.28+/-0.10, and -25.14+/-0.21 kJ mol(-1), respectively. And the standard enthalpy (DeltaH 0) and entropy (DeltaS 0) were 24.55+/-2.91 kJ mol(-1) and 159.13+/-0.53 J mol(-1)K(-1), respectively. EXAFS spectra results showed that the hydrated Zn(II) was adsorbed through fourfold coordination with an average ZnO bond distance of 1.98+/-0.01 A. Two ZnTi atomic distances of 3.25+/-0.02 and 3.69+/-0.03 A were observed, which corresponded to an edge-sharing linkage mode (strong adsorption) and a corner-sharing linkage mode (weak adsorption), respectively. As the temperature increased from 5 to 40 degrees C, the number of strong adsorption sites (N1) remained relatively constant while the number for the weak adsorption sites (N2) increased by 31%. These results indicate that the net gain in adsorption capacity and the decreased adsorption irreversibility at elevated temperatures were due to the increase in available weak adsorption sites (N2) or the decrease in the ratio of N1/N2. Both the macroscopic sorption/desorption equilibrium data and the molecular level evidence of this study suggest that in a given environmental system (e.g., soils or natural waters) zinc and other similar heavy metals are likely more mobile at higher temperatures.