EXAFS studies on adsorption irreversibility of Zn(II) on TiO2: 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 ( 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 ( H 0 ) and entropy ( 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 cornersharing 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.

XAFS study of starch-stabilized magnetite nanoparticles and surface speciation of arsenate

“…The spectra were processed by removing the background absorbance, normalizing the atomic absorption, and extracting the EXAFS signals from the spectra. The coordination numbers (CN) and interatomic distances (R) were then determined by fitting the model-simulated values to the experimental spectra (Li et al, , 2008Pan et al, 2004).…”

Coordination Structure of Adsorbed Zn(II) at Water−TiO2Interfaces

“…The body amplitude reduction factor (S 0 2 ) of 0.87 was used for data fitting. 4,12 The ZnO reference was fitted by use of the Zn-O and Zn-Zn scattering paths calculated from the crystalline structure of ZnO to estimate the accuracies for the fit parameters. All fits were performed in a sequential manner, by adding backscattering paths stepwise and recording reductions of the residual of the fit.…”

Advances in Interfacial Adsorption Thermodynamics: Metastable-Equilibrium Adsorption (MEA) Theory

“…The adsorption irreversibility of Zn(II) on anatase at various temperatures was studied using a combination of macroscopic thermodynamic methods and microscopic spectral measurement. Adsorption isotherm results 29 showed that, when the temperature increased from 5 to 40 °C, the Zn(II) adsorption capacity increased by 130% ( Figure 13). The desorption isotherms significantly deviate from the corresponding adsorption isotherms, indicating that the adsorption of zinc onto anatase was not fully reversible.…”

“…The thermodynamic index of irreversibility (TII) proposed by Sander et al 30 was used to quantify the adsorption irreversibility. The TII was defined as the ratio of the observed free energy loss to the maximum possible free energy loss due to adsorption hysteresis, which was given by eq eq eq eq ln ln TII ln ln 29 EXAFS spectra results showed that the hydrated Zn(II) was adsorbed on anatase through edge-sharing linkage mode (strong adsorption) and corner-sharing linkage mode (weak adsorption), which corresponded to two average Zn-Ti atomic distances of 3.25±0.02 and 3.69±0.03 Å, respectively. 29 According to the DFT results (Figure 14), 13 EXAFS measured the corner-sharing linkage mode at the Zn-Ti distance of 3.69 Å may be a mixture of 4-coordinated bidentate binuclear (BB, 3.48 Å) and 6-coordinated monodentate mononuclear (MM, 4.01 Å) MEA states.…”