Progress of using zeolite materials in soil remediation engineering 沸石材料在土壤修复工程中的应用研究进展

Soil contamination has become a global environmental crisis due to the development of the industry. Exploring costeffective and long-term active soil remediation materials is the key to tackle the soil contamination problem. Based on excellent absorption and cation exchange capacity (CEC), zeolite molecular sieve materials have been widely used in soil remediation. Herein, we summarize the available works in the literature on conventional zeolite materials which have been applied in soil remediation. Especially, focusing on composition and structure features of zeolite materials, we elaborate on their structure-activity relationship down to details. Besides, the microscopic interaction between zeolites and soil contaminants have elucidated. The research process of zeolite used in soil remediation has been reviewed. As discussed in this review, for the conventional zeolite materials used for soil remediation can be classified into pure zeolite materials and zeolite-based composite materials. Pure zeolite materials can be divided into natural zeolites and synthetic zeolites. Natural zeolites are cheap but with multiple elements inside the structure and composition, which is of low purity, and small pore volumes. Ion exchange, acid/alkali washing, and heat treatment are standard methods that have been used to improve the sorption capacity of natural zeolites. Compared to natural zeolites, synthetic zeolites are with tunable compositions and frameworks with high purity. But the cost for synthetic zeolites is higher, which hinders their large-scale application. In recent years, zeolite-based composite materials have been frequently employed as a composite material mixed with other kinds of materials, including nanoscale zero-valent iron or biochar, or humic acid, etc., for soil remediation. The obtained composite materials can be nanoscale zero-valent iron/zeolite composite materials (Z-nZVI), humic acids/zeolite composite materials (HZ), and biochar/zeolite composite materials (BZ). Usually, the soil remediation effect of zeolite-based composite materials is better than pure zeolite materials due to the synergy between the components. Targeting different types of pollutants in the soil requires the dedicated zeolite materials for the treatment, and the microscopic interaction mechanism between zeolite materials and pollutants are also various. For cationic contaminants, such as heavy metal ions, zeolite with low Si/Al ratio and large pore size are effective amendments, because the soil remediation effect directly correlated to the CEC in the used materials. As most of the zeolites have a net negative structural charge, which typically results in little or no affinity for anionic species. Therefore, pure zeolite materials are rarely used for anionic pollutants remediation. While combining zeolite with other materials, such as nZVI, both the cationic and anionic pollutants in the soil can be effectively removed. Moreover, organic pollutants can also be removed by zeolite as the organic pollutants can be physically adsorbed within the zeolite channels through Van der Waals forces or hydrogen bonding. The adsorption capacity of zeolite materials on the organic matter is heavily dependent on the zeolites' pore sizes and hydrophobic properties. Usually, zeolites with a high Si/Al ratio and large pore size can be used to immobilize organic pollutants more effectively. We concluded this review with some perspectives. We think that elucidating the remediation mechanism down to the atomic-scale and optimizing the green chemistry synthesis process for zeolites are two urgent approaches that need to be considered seriously soon.