【摘要】 用延展X射线吸收精细结构光谱(EXAFS)研究了不同温度对Zn(Ⅱ)-锐钛矿型TiO2吸附产物微观构型和吸附可逆性的影响机制.宏观的吸附-解吸实验表明,不同温度下的吸附等温线可以用Langmuir模型进行较好的描述(R2≥0.990).随温度升高,吸附等温线显著升高,Zn(Ⅱ)在TiO2表面的饱和吸附量由5℃时的0.125mmol·g-1增至40℃时的0.446mmol·g-1;而体系的不可逆性明显减弱,解吸滞后角θ由32.85°减至8.64°.求得体系反应的热力学参数ΔH、ΔS分别为24.55kJ·mol-1和159.13J·mol-1·K-1.EXAFS结果表明,Zn(Ⅱ)主要是通过共用水合Zn(II)离子及TiO2表面上的O原子结合到TiO2表面上,其平均Zn-O原子间距为RZn-O=(0.199±0.001)nm.第二配位层(Zn-Ti层)的EXAFS图谱分析结果表明,存在两个典型的Zn-Ti原子间距,即R1=(0.325±0.001)nm(边-边结合的强吸附)和R2=(0.369±0.001)nm(角-角结合的弱吸附).随温度升高,强吸附比例(CN1)基本不变而弱吸附比例(CN2)增加,两者比值(CN1/CN2)逐渐减小.该比值的变化从微观角度解释了宏观实验中温度升高,不可逆性减弱的吸附现象.更多还原

【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 ℃ to 40 ℃, the adsorption capacity increased from 0.125 mmol·g-1 to 0.446 mmol·g-1, while the desorption hysteresis angle (θ) decreased from 32.85° to 8.64°. The thermodynamic parameters ΔH and ΔS 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 reversibility 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.更多还原