Efficiency and mechanisms of cadmium removal via core-shell zeolite/Zn-layer double hydroxides

To investigate the removal mechanisms of cadmium (Cd) by Zn-layer double hydroxides-modified zeolites substrates in constructed rapid infiltration systems (CRIS), the ZnAl-LDHs and ZnFe-LDHs were synthesized and in-situ coated on the original zeolites through co-precipitation method. The prepared Zn-LDHs-modified and original zeolites were characterized by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) methods, whose results provided the evidences that the Zn-LDHs were successfully coated on the original zeolites. From the results of purification experiments, the average Cd removal rates of ZnAl-LDHs-modified, ZnFe-LDHs-modified and original zeolites were 88.40, 86.00 and 32.52%, respectively; demonstrating that the removal rates of zeolites could significantly improve. Additionally, the modification of Zn-LDHS could enhance the theoretical adsorption ability. According to the results of isothermal adsorption and desorption tests, the desorption rates of Zn-LDHs-modified zeolites were higher than that of original zeolites. Cd adsorption capacity of ZnFe-LDHs-modified zeolites was 1428.57 mg kg^-1 and original zeolites was 434.783 mg kg^-1. In the adsorption kinetic studies, the pseudo-second-order models were used to well describe the experimental results of Zn-LDHs-modified zeolites, indicating that their adsorption types were attributed to be more stable chemisorption. Besides, the relevant microbial tests also confirmed that microbial enzymatic activity and extracellular polymeric substances (EPS) were significantly promoted on surface of Zn-LDHs-modified zeolites. The contents of EPS on the surface of zeolites were as following: ZnAl-LDHs-modified zeolites (78.58128 μg/g) > ZnFe-LDHs-modified zeolites (71.85445 μg/g) > original zeolites (68.69904 μg/g). Meanwhile, the results of high-throughput sequencing showed that modification by Zn-LDHs improved microbial diversity and relative abundance. The Proteobacteria was the dominant phylum and the Acidobacteria was conducive to Cd removal. Overall, it could be concluded that ZnAl-LDHs-modified zeolites might be applied as an efficient substrate for Cd removal in CRIS.