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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。
Using urea as precipitating agent, Mg / Al layered double hydroxides (MgAl-LDH) were synthesized by hydrothermal method. The effects of calcination temperature, adsorption time, adsorption temperature, initial mass concentration and medium pH on the adsorption of hexavalent chromium (Ⅵ) on MgAl-LDH were systematically investigated by static adsorption experiments. On this basis, adsorption isotherm, adsorption kinetics and adsorption thermodynamics were studied. The results showed that the adsorption and removal of Cr (Ⅵ) by MgAl-LDH by high temperature calcination were significantly reduced. The optimum pH and adsorption temperature for the removal of Cr (Ⅵ) by MgAl-LDH were pH = 2.5 and 40 ℃, respectively. Adsorption isothermal thermodynamics studies show that MgAl-LDH is an exothermic process for adsorption and removal of Cr (Ⅵ), accords with Langmuir isothermal model and the maximum adsorption capacity is 105.15 mg / g. The adsorption kinetics of Cr (Ⅵ) by MgAl-LDH conforms to the quasi-second order kinetics, the Bangham and Elovich models, indicating that the adsorption process is dominated by chemisorption. The main mechanism of removal of Cr (Ⅵ) by MgAl-LDH was found to be the surface position of MgAl-LDH in the initial stage of adsorption of Cr (Ⅵ) based on the adsorption properties and kinetics, the pH value of the medium before and after adsorption, and the existence and amount of Cr Electrostatic interaction and ion exchange, the latter stage of ion exchange between layers, the former adsorption rate faster, the latter rate is slower.