EXAFS studies on adsorption irreversibility of Zn(II) on TiO 2: Temperature dependence

Abstract

Adsorption irreversibility of Zn(II) on TiO 2 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 °C, the Zn(II) adsorption capacity increased by 130%, and adsorbed Zn(II) became more reversible. The standard Gibbs free energy change ( Δ G 0 ) of the adsorption reaction at 5, 20, and 40 °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 ( Δ H 0 ) and entropy ( Δ S 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 Zn O bond distance of 1.98 ± 0.01 Å . Two Zn Ti atomic distances of 3.25 ± 0.02 and 3.69 ± 0.03 Å 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 °C, the number of strong adsorption sites ( N 1 ) remained relatively constant while the number for the weak adsorption sites ( N 2 ) 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 ( N 2 ) or the decrease in the ratio of N 1 / N 2 . 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.