Temperature Effects on Adsorption-Desorption Irreversibility of Zn(II) onto Anatase

Abstract

Microscopic structures and mechanism of Zn(II) adsorbed onto anatase at different temperatures were studied using extended X-ray absorption fine structure (EXAFS) spectroscopy. Macroscopic adsorption-desorption experiments indicated that adsorption isotherms and adsorption reversibility increased substantially with increasing temperature. When temperature increased from 5 degrees C to 40 degrees C, the adsorption capacity increased from 0.125 mmol . g(-1) to 0.446 mmol-g(-1), while the desorption hysteresis angle (theta) decreased from 32.85 degrees to 8.64 degrees. The thermodynamic parameters Delta H and Delta S of the reaction were evaluated as 24.55 kJ . mol(-1) and 159.13 J . mol(-1) . K-1, respectively. EXAFS spectra results showed that Zn(II) was adsorbed onto the solid surface in the form of octahedral hydrous Zn(II) ions, which were linked to TiO2 surface by sharing O atoms, with an average bond length RZn-O=(0.199 +/- 0.001) nm. EXAFS analysis of the second Zn-Ti coordination sphere resulted in two Zn-Ti atomic distances of (0.325 +/- 0.001) nm and (0.369 +/- 0.001) nm, corresponding to edge-sharing linkage (stronger adsorption site) and corner-sharing linkage (weaker adsorption site), respectively. The number of stronger adsorption sites (CN1) remained relatively stable while the number of weaker adsorption sites (CN2) increased remarkably as the temperature increased, making the proportion of two adsorption modes (CN1/CN2) drop from 0.690 to 0.543. These results revealed that the increased adsorption capacity and reversibitity at higher temperature were due to the increase in CN2 and the decrease in CN1/CN2. This result implies that, in a given environment (soils or rivers), the bioavailability of zinc is higher at high temperature than that at low temperature.