Zn系LDHs负载改性石英砂和沸石对Cr(VI)吸附效果对比及其作用机理研究

Study on adsorption performance and mechanism of Cr(VI) by Zn-LDHs coating on zeolites and quartz sands

为提升人工湿地常用填料对受污染水体中Cr(VI)的吸附性能,采用水热-共沉淀法在碱性条件下制备了Zn系LDHs涂层负载改性沸石和石英砂填料,并运用场发射扫描电子显微镜(SEM)、能谱分析仪(EDS)和BET等手段对改性前后填料的表面形态和化学成分进行了表征.最后,通过等温吸附试验、动力学吸附试验、吸附热力学试验对Zn系LDHs改性填料和天然填料的Cr(VI)吸附效果及其作用机理进行了对比研究.结果表明,通过Zn系LDHs涂层负载,沸石和石英砂对Cr(VI)的吸附类型由以物理吸附为主的多分子层吸附向以化学吸附为主的单分子层吸附转换,有利于吸附水中的Cr(VI);热力学相关参数ΔS^0<0,ΔG^0<0,ΔH^0<0,表明对Cr(VI)的吸附是自发放热过程.相比于天然沸石和石英砂,Zn系LDHs负载改性沸石和石英砂的表面物化性状发生明显变化,最大饱和吸附容量明显提高,尤其是石英砂的最大饱和吸附容量从21 mg·kg^(-1)提升到125 mg·kg^(-1).改性同种天然填料时,ZnAl-LDHs负载改性的效果更佳;而相对于天然沸石的改性,Zn系LDHs负载改性能更有效地提高石英砂对Cr(VI)的吸附能力.因此,本实验中ZnAl-LDHs负载改性石英砂可推荐用于实际人工湿地中Cr(VI)的去除.

In order to improve the adsorption performance of Cr(VI) by zeolites and quartz sand(QS), which are commonly used in constructed wetlands(CWs), Zn-LDHs-modified zeolites and QS were prepared by co-precipitation method under alkaline condition. The scanning electron microscope, energy dispersive spectrometer and BET were used to characterize the surface morphology and chemical composition of the substrates before and after modification. adsorption kinetics, isotherme and thermodynamic tests were conducted to explore the Cr(VI) adsorption mechanisms of Zn-LDHs-modified and natural substrates. The results revealed that Cr(VI) adsorption behavior of natural zeolites and QS were mainly due to the multilayer adsorption, which was mainly physical process. After Zn-LDHs coating, the Cr(VI) adsorption by Zn-LDHs-modified zeolites and QS were then transformed to monolayer and chemical adsorption, which was beneficial to adsorption of Cr(VI) in water body. Thermodynamic relevant parameters(ΔS^0<0,ΔG^0<0,ΔH^0<0) indicated that the adsorption process was a spontaneous exothermic process. After Zn-LDHs modification, the physical and chemical properties of the adsorbant were changed significantly, including the surface and the maximum adsorption capacity. In particular, the maximum adsorption capacity of QS was enhanced from 21 to 125 mg·kg^-1. The performance of ZnAl-LDHs coating modification was better than that of ZnFe-LDHs coating, and Zn-LDHs coating modification could more significantly improve Cr(VI) adsorption capacity of QS compared with natural zeolite. Therefore, ZnAl-LDHs coating QS can be recommended for Cr(VI) removal in actual CWs.