Efficient fluoride recovery from wastewater using mesoporous Ca-based nanomaterials derived from municipal solid waste incineration fly ash via fluidized bed crystallization.

In the semiconductor industry, the intensive use of ultrapure water in combination with hydrofluoric acid for wafer back grinding and etching generates fluoride-rich wastewater with concentrations exceeding 1000?mg/L [1], [2]. The discharge of such fluoride-rich wastewater into aquatic ecosystems poses significant environmental hazards, prompting increased research efforts to develop effective treatment and recovery solutions for these effluents [3], [4]. Although fluoride has beneficial effects on human health, such as promoting bone development, excessive exposure can lead to adverse health outcomes, including renal impairment, DNA disruption, skeletal fragility, and neurological complications [4], [5]. Recognizing the health risks associated with excessive fluoride exposure, global authorities, such as the World Health Organization (WHO), have set stringent limits for fluoride levels in drinking water, capping it at less than 1.5?mg/L [6]. Regarding environmental exposure, the standard limit for fluoride emissions in effluents is set at 15.0?mg/L [5], [7]. These regulations underscore the urgency for innovative technologies that can mitigate the impact of fluoride emissions on the environment while recovering fluoride for beneficial reuse, aligning with the principles of environmental and energy sustainability and promoting a circular economy.

In recent years, several techniques have been explored for the removal of fluoride from wastewater, including chemical precipitation [8], [9], adsorption [7], [10], electrocoagulation [11], homogeneous fluidized bed crystallization (FHBC) [12], and heterogeneous fluidized bed crystallization (FBC) [13]. Each method has its advantages and limitations. Chemical precipitation, especially with calcium, aluminum, magnesium, and zirconium-based reagents, is effective at removing fluoride at high concentrations by forming calcium fluoride (CaF₂) precipitates [9], [14], [15]. However, this approach generates large volumes of sludge, which poses significant disposal and handling challenges. In the FHBC process, nucleation and crystallization occur within the bulk solution without the use of a silica-based carrier, resulting in small and dispersed CaF₂ particles that are difficult to recover, and often produce excessive sludge. In contrast, FBC is a heterogeneous process that uses active silica-based carriers or granular calcite in a reaction column, which facilitates the crystallization of CaF₂ on the carrier surface [16]. However, traditional FBC processes still face issues such as the production of suspended solids (SS) and challenges with sludge separation.

Traditionally, silica sand has been used as a carrier in the FBC process to minimize SS and sludge production [17]. This method has proven effective in reducing sludge formation and in removing various anionic pollutants, such as boron [17], phosphorous [18], and fluoride [19]. Aldaco et al. [16] demonstrated that using silica sand and granular calcite as carriers in the FBC process achieved a crystallization efficiency of approximately 70.0?%. However, increasing the particle size of the carrier can reduce the process’s overall efficiency. Zeng et al. [19] studied fluoride removal and CaF₂ recovery in an FBC system using calcite and silica as carriers and found that using an optimal carrier size of 0.5 – 1.0?mm and 30?g of material resulted in a 93.79?% fluoride removal efficiency and an 89.45?% fluoride recovery ratio.

While these results are promising, the use of silica sand in FBC systems is limited by its low surface area and non-porous structure, which restricts the number of nucleation sites available for CaF₂ crystallization in the presence of calcium ions (Ca²⁺). Recent research has investigated the use of mesoporous materials with high surface areas and tunable pore structures as potential carriers for fluoride removal. Materials such as zirconium pyrophosphate [20], titania-alumina composites [21], goethite (?-FeOOH) [22], and gamma-Al₂O₃/gamma-Fe₂O₃ composites [23] have shown significant potential for enhancing fluoride adsorption. However, the application of these mesoporous materials as carriers in the FBC process for fluoride recovery has been underexplored. Unlike traditional silica sand or calcite carriers, mesoporous materials offer more nucleation sites, which can improve the efficiency of CaF₂ crystallization. With growing demand for sustainable and cost-effective wastewater treatment technologies, mesoporous Ca-based nanocomposites (MCN) derived from municipal solid waste incineration (MSWI) fly ash show significant promise. These materials can enhance fluoride recovery by facilitating rapid and efficient crystallization, particularly in high-fluoride wastewater from industrial wastewater.

Municipal solid waste incineration (MSWI) fly ash contains valuable constituents such as CaO, SiO₂, K₂O, Na₂O, Fe₂O₄, and Al₂O₃, which are crucial for producing cements [24] and geopolymers [25]. Despite its potential, MSWI fly ash contains high levels of chloride, which accelerate metal corrosion in concrete, posing challenges for cement-based applications. Traditional approaches to mitigate these issues include simple washing [26] and hydrothermal treatment [27] to facilitate dichlorination. To address this, our previous study introduced ultrasonic treatment and acid washing to develop mesoporous Ca-based composite, which can be employed to convert MSWI fly ash into mesoporous Ca-based nanomaterial (MCN) [28]. The MCN is eco-friendly, with a high surface area, porosity, and tunable surface roughness. Moreover, the MCN shows promise as a carrier in the FBC process for efficiently recovering fluoride ions as CaF₂ crystals from wastewater.

This research pioneers the use of MCN derived from MSWI fly ash as an innovative carrier in the FBC process, enhancing fluoride removal efficiency and crystallization while reducing SS in effluents. As illustrated in Fig. 1, the process begins with the leaching of MSWI fly ash using an ultrasonic-assisted leaching method with ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA–Na) for effective dichlorination and impurity removal. The leached MSWI fly ash undergoes several treatments to become MCN granules with enhanced porosity and improved surface area. Utilizing MCN as a carrier in the FBC process offers high affinity for CaF₂ crystallization, facilitating fluoride recovery from wastewater. The outcomes from this study validate the potential of MCN produced from MSWI fly ash to efficiently recover fluoride in the form of CaF₂ crystals from semiconductor wastewater, leading to a new discovery in developing sustainable technologies for environmental remediation. This development offers promising prospects for the simultaneous recovery of fluoride and other inorganic substances from industrial wastes, making a significant step forward in waste management and resource recovery strategies.