Zn-LDHs覆膜改性麦饭石对Cd(Ⅱ)吸附性能及其作用机理研究

Adsorption behavior and mechanisms for Cd(Ⅱ) by modified maifanite coated with Zn-LDHs

采用水浴共沉淀法在碱性条件下制备两种Zn系层状双金属氢氧化物(Zn-LDHs),并对天然麦饭石进行覆膜改性得到Zn-LDHs覆膜改性麦饭石.运用能谱分析仪(EDS)配合场发射扫描电子显微镜(FE-SEM)检测改性前后麦饭石,及吸附试验后ZnFe-LDHs覆膜改性麦饭石的形貌和元素变化;采用X射线衍射仪(XRD)对天然麦饭石和ZnFe-LDHs覆膜改性麦饭石的结构进行表征;通过等温吸附试验、解吸附试验、吸附动力学试验、吸附热力学试验、不同pH值条件下的吸附试验和竞争离子吸附试验,探究Zn-LDHs覆膜改性麦饭石对Cd(Ⅱ)的吸附效果及其作用机理.结果表明,Zn-LDHs覆膜改性麦饭石对Cd(Ⅱ)的吸附性能显著提升,其中ZnFe-LDHs覆膜改性麦饭石饱和吸附容量(2500 mg·kg^-1)较天然麦饭石(714.29 mg·kg^-1)提高了3.5倍;吸附时间、pH值和温度条件均对吸附容量产生影响;Zn-LDHs覆膜改性麦饭石吸附过程更符合Langmuir等温模型、准二级动力学模型、Elovich模型,为均匀的单分子层化学吸附,而天然麦饭石吸附过程符合Freundlich等温模型和准一级动力学模型,吸附类型为非均匀多分子层物理吸附;Zn-LDHs覆膜改性麦饭石热力学参数ΔG^θ<0,ΔH^θ<0,ΔS^θ>0,说明Zn-LDHs覆膜改性麦饭石对Cd(Ⅱ)的吸附是自发放热过程,天然麦饭石吸附则为自发的吸热过程.研究结果可为ZnFe-LDHs覆膜改性麦饭石有效应用于生态工程除Cd(Ⅱ)提供理论依据和技术参考.

Two kinds of Zn-layered double hydroxides(Zn-LDHs) were synthesized by co-precipitation method under alkaline conditions and in situ coated on natural maifanite to obtain Zn-LDHs modified maifanite. The morphology structure and chemical composition of the maifanite before and after modification, and adsorbed ZnFe-LDHs modified maifanite were characterized with Energy Dispersive Spectroscopy(EDS) and Field Emission Scanning Electron Microscopy(FE-SEM). In addition, the crystal structure of the natural and ZnFe-LDHs modified maifanite were analyzed by using X-ray diffraction(XRD). The adsorption, desorption, adsorption kinetics, adsorption thermodynamic tests, adsorption tests under different pH conditions and competitive adsorption tests were carried out to investigate the adsorption effects and mechanisms of Cd(Ⅱ) adsorption by natural maifanite and Zn-LDHs modified maifanite. The results showed that the Cd(Ⅱ) adsorption performance of Zn-LDHs modified maifanite was significantly improved, and the saturated adsorption capacity of ZnFe-LDHs modified maifanite(2500 mg·kg^-1) was 3.5 times higher than that of natural maifanite(714.29 mg·kg^-1). The adsorption time, pH values and temperature had certain influence on the adsorption capacity of maifanite. The adsorption process of Zn-LDHs modified maifanite was well fitted with Langmuir model, pseudo-second-order kinetic model and Elovich model, which was homogeneous monolayer chemical adsorption. While the adsorption process of natural maifanite conformed to Freundlich model and pseudo-first-order kinetic model, and the adsorption type was attributed to be heterogeneous multilayer physical adsorption. The thermodynamic parameters of Zn-LDHs modified maifanite(ΔG^θ<0, ΔH^θ<0, ΔS^θ>0) indicated that the adsorption of Cd(Ⅱ) by Zn-LDHs modified maifanite was a spontaneous exothermic process, while the natural maifanite adsorption was a spontaneous endothermic process. The research results could provide theoretical basis and technical support for the effective application of ZnFe-LDHs modified maifanite for the removal of Cd(Ⅱ) in ecological engineering.