Mg/Al双金属氧化物吸附Cr(Ⅵ)的动力学和热力学机理

THE KINETICS AND THERMODYNAMICS FOR Cr(Ⅵ) ADSORPTION ON Mg/Al LAYERED DOUBLE OXIDES

通过静态吸附实验,研究了Cr(Ⅵ)在Mg/Al水滑石焙烧产物层状双金属氧化物(layered double oxides,LDO)的吸附性能。考察了温度对其吸附性能的影响,探讨了吸附动力学和热力学规律。研究表明:吸附动力学符合准二级动力学模型,LDO的吸附活化能为21.42kJ/mol,表现为扩散控制过程。在293～333K温度范围内,吸附自由能(ΔG°)变小是Cr(Ⅵ)在LDO上的吸附推动力;吸附热(吸附焓变ΔH0)为5.12kJ/mol,是一自发的物理吸附过程;LDO吸附Cr(Ⅵ)为熵增过程,其熵变为49.12J/(mol·K)。结合XRD表征结果探讨了吸附机理:LDO吸附Cr(Ⅵ)无配位基交换、化学键等强的作用力,Mg/Al金属氧化物在吸附Cr(Ⅵ)后恢复为LDH层状结构,因此,LDO对Cr(VI)的强吸附能力是因为其具有较强的结构"记忆效应"。

The thermodynamics and dynamics for the adsorption of Cr （VI） onto layered double oxides （LDO） prepared after calcination by Mg-Al hydrotalcite under static conditions and the influence of temperature on the adsorption have been studied. The results indicate that the adsorption of Cr （Ⅵ） is consistent with the pseudo-second order kinetic model. The active energy value of the LDO is 21.42 kJ/mol, which suggests that the process of adsorption of chromium （Ⅵ） is controlled by diffusion. The adsorption that occurred was due to the decreasing of adsorption free energy （AGO）. The heat of adsorption （adsorption enthalpy △H^0） value is 5.12 kJ/mol, indicating that the adsorption process of Cr （VI） onto the LDO is a spontaneous physical adsorption process within the temperatttre range of 293-333 K. The adsorption of Cr （Ⅵ） onto the LDO is an entropy-increasing process and the adsorption entropy change value is 49.12 J/（mol·K）. Its adsorption mechanism is discussed in combination with the results of X-ray diffraction： there is no strong acting force such as a concerted radical exchange or a chemical bond. The structure of LDO shifts to a layer structure of LDHs after the adsorption, indicating that the memory effect of LDO accounts for the effective removal of aqueous Cr （Ⅵ）.