Fluoride is a detrimental element to humans and most animals, due to its extreme corrosivity, toxicity and carcinogenicity. This poses a significant threat to safety, especially when consuming fluoride-bearing water or being exposed to solid waste with fluoride over a long period of time.
Due to the increasingly strict carbon emissions requirements and the rapid growth of the new energy industry, the strategic importance significance of lithium resource has reached unprecedented heights level (Bailey, 1977), playing a crucial role in modern power batteries. Lepidolite is widely recognized as one of the primary lithium (Li) sources for Li extraction (Zhang et al., 2019). The process of Li extraction in the form of lithium carbonate from lepidolite mainly involves direct leaching in acidic or alkaline medium, or water leaching after roasting with salt assistance (Mulwanda et al., 2023; Mulwanda et al., 2021). Fluoride-bearing wastewater was inevitably produced from hydrometallurgical process of Li extraction, leading to the surrounding environmental risk. Therefore, it is essential to develop efficient methods for the treatment of the fluoride-bearing wastewater from lepidolite hydrometallurgical process.
Presently, the primary processes for the treatment of the fluoride-bearing wastewater treating wastewater include adsorption (Bhatnagar et al., 2011; Guo et al., 2023; He et al., 2020; Shao et al., 2021), precipitation (Lacson et al., 2021; Tsuchiya et al., 2020; Wang et al., 2021), ion exchange (Chen et al., 2023; Yu et al., 2021; Zhang et al., 2022), and electrodialysis (Annouar et al., 2004; Gmar et al., 2015; Keri et al., 2011). Among the frequently employed methods for eliminating fluoride, the adsorption approach has been extensively investigated and presents a highly promising method for removing fluoride from conventional domestic and industrial wastewater due to its simplicity and environmentally friendly natures. As reported, fluoride ions are eliminated through ion exchange or chemical reactions with the active groups or compound ions of the adsorbent during the adsorption (Gómez-Hortigüela et al., 2013). The dominating types of fluoride adsorbents are known as zeolite (Arcibar-Orozco et al., 2020; Velazquez-Peña et al., 2017), activated alumina (De Paula et al., 2023; Yu et al., 2023), hydroxyapatite (Huang et al., 2023; Jeyaseelan et al., 2023; Rathnayake et al., 2022), and activated carbon (Rashid and Bezbaruah, 2020).
Numerous modified materials have been explored and developed to remove fluoride from water bodies (Dar and Kurella, 2023). An innovative LMC was successfully developed by the modification of seaweed biomaterial using lanthanum ions. The resulted adsorbent demonstrated fluoride adsorption with a wider pH ranging and the concentration of fluoride from 20.0 mg/L to 2.0 mg/L (Yu et al., 2015). Artocarpus hirsutus sawdust impregnated with ferric hydroxide and activated alumina (SFAA) was found to be effective for arsenic and fluoride removal from groundwater. The experimental results showed the concentration of fluoride reduced to 5.2 mg/L (Dhanasekaran and Sahu, 2021). Also, the applicability of zirconium ion impregnated coconut fiber carbon (ZICFC) as an adsorbent for fluoride removal from water was investigated. The experimental results showed the concentration of fluoride reduced to 1.4 mg/L (Sathish et al., 2008). Among these adsorbents, zeolite, in particular, has been extensively studied as it is of wide availability, high capacity for fluoride removal, and long periodicity for adsorption (Janas et al., 2009). However, because the presence of impurities in natural zeolite gives rise to several challenges, including blockages, limited pore connectivity, and a negatively charged surface. These factors are all detrimental to the effective adsorption of anions (Lu et al., 2022). The modifications of zeolite are usually required in order to promote properties of zeolite in catalysis and adsorption (Sun et al., 2011). Therefore, the prevalent method for enhancing its adsorption performance is to modify natural zeolite by loading multivalent active metal ions (Gómez-Hortigüela et al., 2013). It has been discovered that the adsorption capacity of natural zeolite from a mining area in northern Ethiopia was 0.47 mg/g (Gómez-Hortigüela et al., 2013), but after modification with Mg2+, Al3+, and Ti4+, its fluoride removal effectiveness significantly increased (Ma et al., 2018). Many studies on the effectiveness of natural and modified zeolites for fluoride removal have been conducted in the past two decades. With many modified zeolites, fluoride ions can be effectively removed, achieving adsorption amounts ranging from 1.44 to 22.2 mg/g (Lu et al., 2022).
Even though several adsorbents performed well at fluoride removal, the research are required further to reduce fluoride levels from water within the permissible level of 1 mg/L in primarily polluted drinking water. Furthermore, it is rough for adsorbents in the removal of fluoride from wastewater in a short time. The treatment of the fluoride-bearing wastewater from lepidolite hydrometallurgical process is facing a greater challenge compared to traditional fluoride-bearing wastewater due to the presence of co-existed metal ions including Li+, Rb+ and Ca2+, as well as anions such as SiO32− and SO42−. Currently, there is a lack of research in this specific area. In this study, a novel Al@zeolite was synthesized and characterized, and then the adsorption of fluoride from the fluoride-bearing wastewater from lepidolite hydrometallurgical process by the synthesized Al@zeolite was investigated. The outcome of this research will provide valuable technical support for a safe treatment of fluoride-bearing wastewater from Li extraction process.