Insights into the crucial role of pH to achieve the dual goals of efficient fluoride removal and resource recovery from the photovoltaic manufacturing wastewater.

Spurred by the crises of energy shortage and climate change, new industries related to renewable energy have witnessed prosperity in recent years [1], [2]. Photovoltaic (PV) manufacturing, as an important sector of optoelectronics, is making enormous strides. The rapid development of the PV industry, however, results in the generation of complex and high-risk wastewater [3]. Prominently, the involvement of fluoride-based acid in the etching process of solar cell manufacturing gives rise to acidic wastewater containing high levels of fluoride, which can cause serious toxicity to natural ecosystems. For example, Chen et al. [4] reported that fluoride exposure induces oxidative stress in the fills of zebrafish. Wu et al. [5] indicated that fluoride toxicity can be broadcasted from polluted water sources to animals through food chains. For human beings, exposure to high levels of fluoride may lead to dental and skeletal fluorosis [6], [7]. On the other hand, fluorine plays a significant role in the manufacturing of semiconductors and optical equipment, as well as in the preparation of pharmaceuticals, pesticides, and insecticides [8]. These make fluorine an important resource deserving recovery. Therefore, it is urgent to develop technologies to achieve the dual goals of fluorine removal and recycling.

Currently, various technologies have been developed for fluoride removal, including chemical precipitation, adsorption, electrodialysis, ion exchange, nanofiltration, and reverse osmosis [9], [10], [11], [12], [13], [14]. In practice, chemical precipitation by adding calcium salt, such as CaCl₂ and Ca(OH)₂, to form calcium fluoride (CaF₂) is considered the most widely used method for fluoride-containing wastewater treatment, considering the merits of simplicity, low cost, and potential of resource recovery as fluorite. For example, Ho et al. [15] used concrete fines as precipitant to remove the fluoride from the synthetic wastewater and achieved a removal efficiency of > 90 %. Zeng et al. [16] also reported 96 % fluoride removal efficiency in a fluidized bed using the chemical precipitation method and realized recovery of high-purity fluorite.

To achieve the multiple goals of high fluoride removal, resource recovery and low cost, etc., the reaction conditions of chemical precipitation should be effectively controlled [17]. It has been reported that multitudinous conditions, such as initial fluoride concentration, Ca dosage, pH, and reactor configuration, should be considered [18], [19]. Among them, pH condition was a significant factor due to its direct linkage to precipitation reaction [20]. However, the information about the pH effect and its optimal levels for fluoride removal efficiency and precipitate recovery varies a lot. For example, You et al. [21] reported that the increased pH of wastewater from 3.03 to 8.09 enhanced fluoride removal performance with calcium sulfate dihydrate as the precipitant. Han et al. [22] reported that the preferred pH range for higher fluoride removal efficiency was 9.0 to 11.1. Yin et al. [23] reported a different pH value of 8.14 for CaF₂ recovery using response surface methodology. What factors determine the optimal pH for fluoride removal and the composition of the precipitate? On the other hand, it should be pointed out that most current studies focus on investigating the effect of pH on fluoride removal performance empirically through experimentation to obtain the scattered optimal pH, while rare information is available from the view of theoretical analysis to obtain a general result. With the guidance of theoretical analysis, it is believed that the range of optimal research can be narrowed to realize improved experimental efficiency and accuracy.

Therefore, the objective of this study is to investigate the effect of pH on fluoride removal from both theoretical and experimental views. The characteristics of precipitates and the recovery potential were also systematically analyzed. In detail, the optimal pH range was clarified theoretically by demonstrating the mathematical relationship among pH, reaction rate, and fluoride residual in the reaction system. Both continuous and batch tests were conducted to verify the theoretical results to achieve high fluoride removal efficiency with low precipitant invalidity by controlling pH. The properties of precipitates mediated by pH were also characterized to evaluate the recycling potential. This study is expected to deepen the insights into the crucial role of pH, and offer systematical optimization guidance for achieving the dual goals of efficient fluoride removal and resource recovery from the fluoride-containing manufacturing wastewater.

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