镁铝水滑石的制备及其吸附Cr(Ⅵ)的性能

On the preparation of MgAl-LDH and its adsorption properties for chromium(Ⅵ)

以尿素为沉淀剂,采用水热法合成Mg/Al型层状双氢氧化物(MgAl-LDH)。通过静态吸附试验,系统考察了焙烧温度、吸附时间、吸附温度、初始质量浓度、介质pH值等因素对MgAl-LDH吸附6价铬Cr(Ⅵ)性能的影响。在此基础上,进行吸附等温、吸附动力学和吸附热力学研究。结果表明,高温焙烧导致MgAl-LDH对Cr(Ⅵ)吸附去除性能显著降低;MgAl-LDH吸附去除Cr(Ⅵ)的最佳pH值和吸附温度分别为pH=2.5和40℃。吸附等温和热力学研究表明,MgAl-LDH对Cr(Ⅵ)吸附去除过程为放热过程,符合Langmuir等温模型且最大吸附容量为105.15 mg/g。MgAl-LDH对Cr(Ⅵ)的吸附动力学符合准二级动力学、Bangham和Elovich模型,表明吸附过程为化学吸附主导。结合吸附性能和动力学、吸附前后介质的pH值变化及不同pH值下Cr(Ⅵ)的存在形式和数量,发现MgAl-LDH对Cr(Ⅵ)吸附初始阶段主要去除机理为MgAl-LDH表面位置的静电作用和离子交换,后阶段为层间离子交换,前者吸附速率较快,后者速率较慢。

The paper intends to introduce its method for preparing MgAl-layered double .hydroxide （ MgAl- LDH ） through hydro- thermal synthesis in autoclave, with urea taken as the precipitant and pH value for synthesis media being self-adjusted by way of continuous urea hydrolysis. In our work, we have clone static experiments by using MgAl - LDH as the adsorbent to remove Cr（VI） from the aqueous solution. Through the above said experiments, we have done detailed investigations for the effects of such factors, as the initial solution pH, the reaction time, the initial Cr（VI） concentration, the adsorption temperature and the calcined temperature on the removal efficiency of Cr（VI） by MgAl - LDH. As a resuh, we have discovered that the adsorption capacity of Cr（VI） by the calcined MgA1 - LDH had all been no- ticeably decreased, which can be attributed to the damage of the layered structure of MgA1 - LDH and the loss of interlayer anions on the basis of the characterization results of XRD and FT - IR. In such experiments, the optimal pH value and the adsorption temperature for Cr （VI） adsorption can be set up at 2. 5 and 40 ~C （313 K）, respectively. What is more, we have also investiga- ted the adsorption isotherm mode, the thermodynamics and kinet- ics of the adsorption in hoping to identify the adsorption behaviors of Cr（VI） by the said MgA1- LDH. What is more, the experi- mental data of Cr （VI） adsorption by MgA1 -LDH prove to be well in accord with Langmuir model with its maximum adsorption capacity taken as 105.15 mg/g. Thus, the results of the aforementioned experiments help to display the excellent capability of the adsorbing feature for Cr（VI） removal with the adsorption rate increasing with the initial concentration of Cr（VI） in accordance with the breaking factor RL of Langmuir model. In addition, the adsorption process has also been proved going on spontaneously when the adsorption temperature remains below 313 K, whereas the process tends to be non-spontaneous on the condition when the adsorption temperature is kept above 313 K. And, consequently, the kinetics data for the adsorption of Cr（VI） turn to be well in correspondence with the pseudo-second order kinetic model, Bangham model and Elovich model, demonstrating that the ad- sorption should be involved with much more complicated mecha- nism with the chemistry adsorption being dominant. Besides, when we investigated the adsorption features in light of adsorption behaviors, we have carefully studied the kinetic, pH values before and after Cr（VI） adsorption, as well as the forms and quantities of Cr （VI） anions under the effects of the various pH values. Thus, it can be found that pH value here can help to deduce the adsorption of Cr （VI） by MgAl - LDH mainly in the acidic media. Thus, the above adsorption features may also involve the electrostatic adsorption in the initial stage of the adsorption and anion exchange in that of the later stage. Hence, the above experimental facts can help to infer that the＂ corresponding adsorption mechanisms should be attributed to the following facts： at the initial stage of adsorption, MgAl - LDH should be considered as protonated with a positive charge, and then, Cr （VI） anions should be regarded as being removed through the electrostatic adsorption between MgAl- LDH and Cr（VI） anions, as well as removed via the anion exchange between OH- release to the outer surface of MgAl- LDI4 and Cr（VI） anions in the aqueous solution. However, in the later stage of adsorption, Cr （VI） anions should be removed by way of the anion exchange between the interlayer anions （ CO32- and NO3- ） in the interlayer of MgAl-LDH and those of Cr（ VI）, and the adsorption rate of the initial stage turns to be faster than that of the later stage. And, pH value of the media solution has been found increasing significantly after Cr（VI） adsorption by MgAl- LDH as a result of the consumption of H＋ in the aqueous solution and release of OH- and interlayer CO32- from MgAl- LDH.