Zeolite/ZnAl-layer double hydroxides with different Zn/Al ratios and intercalated anions as the substrate of constructed wetlands: synthesis, characterization and purification effect of Hexavalent chromium.

Abstract

The study aimed to synthesize novel zeolite substrates modified with four types of ZnAl-LDHs including Cl-LDHs(1:1), Cl-LDHs(3:1), CO3-LDHs(1:1), and CO3-LDHs(3:1); investigate Cr(VI) removal efficiencies in lab-scale constructed wetlands (CWs); and explore the effect of different Zn/Al ratios and intercalated anions on the removal efficiencies of Cr(VI) by modified zeolite. Different ZnAl-LDHs were prepared by co-precipitation method and coated onto the surface of original zeolite. Field emission scanning electron microscope and energy dispersive spectrometer were used to analyze physicochemical properties of zeolite/ZnAl-LDHs. Obtained results confirmed the successful LDHs-coating modification. The results of both X-ray diffraction and Fourier transform infrared suggested that the typical diffraction peak and functional groups of ZnAl-LDHs were detected in modified zeolites, and the peak of CO32- in CO3-LDHs at 1362cm-1 was stronger and sharper than Cl-LDHs. It could be demonstrated by above results that the synthesis crystallinity and coating effect of CO3-LDHs was better than Cl-LDHs. Furthermore, it could be found that under the condition of same intercalated anion, LDHs with metal molar ratio of 1:1 had better crystallinity than LDHs with metal molar ratio of 3: 1. Subsequent determination of the removal performance of Cr(VI) by purification experiments revealed that zeolite/Cl-LDHs(3:1) showed the best Cr(VI) removal performance, and the removal rate of Cr(VI) was improved by 32.81% compared with the original zeolite, which suggested that could be an efficient substrate of CWs for Cr(VI) removal. The high crystallinity indicated that the structure of LDHs was stable and it was difficult to remove Cr(VI) by ion exchange. The above explained why the Cr(VI) removal efficiency by zeolite/Cl-LDHs is superior to that by zeolite/CO3-LDHs under the condition of same metal molar ratio. With the increase of metal molar ratio, the charge density of LDHs layers and intercalated anion increased, thus enhancing the electrostatic attraction of LDHs layers to Cr(VI) and the interlayer anion exchange capacity. However, the effect of charge density on Cr(VI) removal efficiency may be greater than crystallinity on removal efficiency, which could be responsible for the fact that zeolite/ZnAl-LDHs(3:1) had better Cr(VI) removal efficiency than zeolite/ZnAl-LDHs(1:1) under the condition of same intercalated anion.