Advanced fluidized bed-ultrafiltration-reverse osmosis process for near-complete fluorine recovery from photovoltaic fluoride-laden wastewater: system parameters and mechanisms.

Graphical abstract

Introduction

F is one of the most important trace elements in the natural environment, predominantly occurring in the Earth’s crust in the form of fluoride minerals (Gao et al., 2021; Han et al., 2021). The environmental release of fluorine is chiefly propelled by industrial activities, notably in sectors such as aluminum smelting, glass manufacturing, fertilizer production, and semiconductor processing (Gao et al., 2024; Wu et al., 2024; Zhang et al., 2024; Zhao et al., 2017; Zhou et al., 2024). In recent years, the rapid expansion of the renewable energy sector, especially the photovoltaic and battery industries, has significantly increased the demand for fluoride-based chemicals (Vivar et al., 2024). The use of fluoride-based acids in etching procedures for manufacturing has generated substantial volumes of highly concentrated fluoride-laden acidic wastewater, raising environmental concerns (Won et al., 2012). Fluoride contamination in aquatic ecosystems can induce toxicity in plants, which, through bioaccumulation, can propagate across trophic levels and affect broader ecological systems (Yadav et al., 2019). On the other hand, fluorine ranks among the world’s most valuable mineral resources, due to its substantial industrial and economic significance (Dolbier, 2005; Fujiwara and O’Hagan, 2014; Smart, 2001). For example, fluorite serves as an indispensable intermediate in the fluorochemical industry, functioning as an essential precursor for synthesizing refrigerants, polytetrafluoroethylene plastics, and hydrofluoric acid (Du Boisson, 1994). High-purity fluorite is also an essential component in the production of advanced optical lenses, microscopy instruments, and gemstones (De Barra and Hill, 2000). Therefore, developing efficient water treatment technologies for fluorine resource recovery promises profound benefits for both environmental preservation and industrial utilization.

Compared to adsorption (Zhang et al., 2021), electrodialysis (Clímaco Patrocínio et al., 2019), ion exchange (Pillai et al., 2020), NF, and RO (Brião et al., 2019) technologies, chemical precipitation (Huang et al., 2017) is more notable for its ability to recover fluoride through precipitate calcination. In practical applications, precipitation is primarily achieved through a fluidized bed, which involves the addition of calcium salts, stirring of the mixed solution, and discharge of crystals. Zeng et al. (Zeng et al., 2019) demonstrated that utilizing a fluidized bed reactor with silica sand as a seed crystal serves as an efficient and economical approach. However, attaining low-cost and high-efficiency fluoride recovery through precipitation requires meticulous control over reaction parameters. This encompasses optimizing the calcium-to-fluoride ion concentration ratio, adjusting the pH, and maintaining ideal mixing conditions within the reactor. For instance, Yu et al. (Yu et al., 2024) investigated the role of pH in fluoride precipitation efficacy, while Yin et al. (Yin et al., 2016) used response surface methodology to demonstrate that multiple practical parameters impact fluoride precipitation efficiency. Reducing the fluoride ion concentration from hundreds of ppm to tens of ppm via fluidized bed precipitation is straightforward; however, achieving further fluoride recovery remains challenging.

Membrane filtration has evolved into a well-established technology for engineering applications (Liang et al., 2024; Wang et al., 2024; Xie et al., 2024). Although previous studies have examined the fluoride removal efficiency of discrete units such as precipitation, UF, and RO (Jia et al., 2015; Liu et al., 2022; Wang et al., 2020), they have failed to address the integration of these units into a cohesive and continuous process. The absence of sequential connections and recirculation control among unit effluents impedes a thorough assessment of the system’s long-term operational stability and efficiency. Furthermore, many studies rely on simulated fluoride-laden wastewater (Djouadi Belkada et al., 2018; Palahouane et al., 2023; Qiu et al., 2022). While these studies simplified experiments, they failed to capture the complexity of actual industrial effluents. Actual fluoride-laden wastewater often contains a multitude of ionic species and organic contaminants, which significantly affect both fluoride removal efficiency and the crystallization dynamics within treatment systems. Moreover, existing process optimization strategies are predominantly static (Clímaco Patrocínio et al., 2019; Yu et al., 2024), tailored to fixed operational conditions, and lack a theoretical framework for dynamic parameter adjustment. These studies also highlight the existing disparity between practical engineering applications and scientific research in the field of fluoride recovery by precipitation.

Therefore, this study proposes a novel integrated fluoride recovery technology, termed the FBUR process. Within the FBUR process, the fluidized bed system treats actual high-concentration fluoride wastewater, while membrane units concentrate low-concentration fluoride effluent. The concentrated brine from the membrane units is recirculated into the fluidized bed, reinitiating the crystallization and thereby enhancing the overall fluoride removal efficiency. In this way, fluoride resources were nearly completely recovered, and the entire system was able to achieve zero liquid discharge. The experimental results confirmed the feasibility of simultaneous water reuse and fluorite recovery from photovoltaic wastewater. This study emphasizes the critical importance of determining the optimal concentration levels of influents in sequential units. Furthermore, given that the fluidized bed effluent contains elevated levels of Ca²⁺, there is a potential risk of scaling in the downstream membrane system. The fouling mechanisms and performance characteristics of UF and RO membranes within the FBUR process were comprehensively characterized and systematically analyzed. Life cycle assessment (LCA) results indicate that the FBUR process has a highly positive environmental impact, while maintaining relatively low energy consumption and operating costs. This study offers a practical and efficient solution for concurrently recovering water and valuable minerals from industrial wastewater, thereby advancing resource recovery technologies and promoting sustainable wastewater management.

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Declaration of competing interest