Fluorine is a chemical element with atomic number 9 and is a halogen element belonging to group 17 of the periodic table. Molecular fluorine (F₂: gaseous state) is an irritant and explosive toxic gas and is very unstable at room temperature, hence, is not found in nature [16]. Most of it exists as the highly electronegative fluorine ion (F⁻) or as a fluorine compound. Fluorine is a non-metallic element that is chemically very active and reacts with most substances to form compounds, thus playing an important role in industry, medicine, and the environment [17]. Representative examples of fluorine compounds include hydrogen fluoride (HF), sodium fluoride (NaF), and calcium fluoride (CaF₂). These fluorides are mainly used in toothpaste that protects teeth and bones, dental treatments, and as a catalyst for chemical reactions in the chemical industry [18]. In this way, fluorine compounds play an important role in various industrial processes.

From a general environmental perspective, fluorine is an element with a hazard index that ranks 13^th in the terrestrial region and 12^th in the biosphere, and is widely distributed, making it difficult to decompose naturally in the environment [19]. Most of the fluorine in the atmosphere mainly exists in the form of fluoride compounds, and the content is very low, at about 0.04–1.8 μg/L [20]. Additionally, fluorine in seawater varies depending on the geology of the surrounding area, marine biological activity, and other environmental factors, but is generally present at a concentration of about 1.0–1.3 mg/L [21]. However, groundwater passing through geological and rock layers containing fluorine compounds has a high fluoride ion content of 9–10 mg/L [21].

Depending on its concentration, fluoride can be either toxic or beneficial to humans. For instance, when consumed in an appropriate amount of 1 mg/L or less, fluoride has a beneficial effect on the human body by helping to strengthen teeth and form bones [22]. However, ingesting excessive amounts of fluoride over a long period of time or ingesting more than 4 mg/L of fluoride in a short period of time can cause white spots on teeth, damage to teeth and bones, and dental disorders [23]. Additionally, exposure to high concentrations of fluorine is known to increase the bond strength of bones, increasing the risk of fractures, and further damage the structure of bones and the nervous system, causing functional disorders (Table 1) [7, 8].

Fluoride-containing wastewater mainly originates from industrial activities, agricultural activities, and sometimes from the natural environment [13]. In particular, it is generated during manufacturing or chemical treatment processes in various industries. As industrial activities increase, the use of fluorine increases, resulting in the production of large amounts of fluorine-containing wastewater [3]. Representative industries include coal industry, aluminum smelting, printing and dyeing processes, electronics industry, semiconductors and displays, concrete parts manufacturing, steel industry, and LCD manufacturing industry [4, 24].

If we delve into the history of fluoride-containing wastewater, it traces back to the onset of industrial processes using fluorine alongside the progress in industrialization during the mid-19^th century [21]. During that earlier era, fluoride containing wastewater began to be generated when fluoride was used in processes such as metal smelting, glass manufacturing, and aluminum fluoride smelting. Subsequently, with the development of the chemical industry in the early 20^th century, fluoride compounds started being employed as solvents [25]. Calcium fluoride (CaF₂) found diverse applications, including its use in processes involving aluminum, uranium, and other fluoride compounds, leading to the substantial generation of fluorine wastewater [4]. Currently, the major industries that produce fluoride-containing wastewater are semiconductor manufacturing process, steel manufacturing process, and glass manufacturing process [26]. In the following section, these industries and processes involved in fluoride wastewater generation are discussed in detail.

Discharge of industrial wastewater containing high concentrations of fluoride can cause enormous problems for living organisms and the environment [39]. Fluoride is toxic and can have negative effects not only on water pollution but also on the ecosystem. In particular, it has a harmful effect on aquatic life, and invertebrates and small fish tend to react sensitively to fluoride concentrations [40]. Fluoride sequesters calcium ions in the blood of aquatic organisms, causing a significant decrease in calcium, which affects the formation and maintenance of bones and skeletons, resulting in damage to the skeletons of aquatic organisms and reducing growth and survival rates [41]. Moreover, as it has the potential to impact higher organisms within the food chain, creating concern about disruption of the ecosystem equilibrium.

Fluoride can accumulate continuously over a long period, causing serious pollution problems in the environment. In particular, groundwater and soil contamination caused by fluoride continues to concentrate due to continuously occurring emissions [42]. Increased fluoride levels in groundwater can have serious impacts on a community’s drinking water supply as well as health issues. In addition, the increase in fluoride concentration in the soil is directly absorbed by crops, causing food safety problems. Moreover, due to the uneven distribution of fluoride in soil depending on depth, it can affect physical properties, thereby reducing soil biodiversity [39].

Until recently, bulk sludge discharged after coagulation/sedimentation treatment, an existing fluoride treatment technology, was mainly recycled as cement raw material [43]. The utilization of fluoride sludge was limited owing to its high impurity and moisture levels, along with the inclusion of hazardous heavy metal constituents contributed by the coagulants or wastewater. However, due to the recent expansion of such industries, the amount of inorganic sludge generated has also been increasing, and due to the limited capacity of domestic cement plants, it is disposed of directly in landfills [44]. As this situation continues, secondary soil pollution is becoming increasingly evident, necessitating treatment methods with better fluoride recovery and recycling potential.

In addition, if we consider the overall worldwide growth of key industrial sectors, namely semiconductors, electrical/electronics, and steel, all are showing an upward trend [2]. Fluoride, essential for industrial cleaning and surface treatment, is in short supply domestically in many countries, thus mostly dependent on imports. Relying on imports makes fluoride more sensitive to changes in international market prices [45]. If prices rise, a burden will be placed on domestic production and use, with the risk of supply being interrupted due to changes in economic conditions or conflicts with exporting countries [46]. Furthermore, there is a potential weakening of domestic fluoride production technology, along with a decrease in investments towards hydrofluoric acid or fluoride production technology and research and development. This reduction may compromise the technological competitiveness of domestic industries. Considering this trend, problems in strengthening fluoride concentration regulations and securing fluorine resources are expected to arise in the future. Hence, there is a need for technological advancement for the efficient treatment of fluoride while concurrently extracting and utilizing it as a range of valuable resources.

The most commonly used technology for treating wastewater containing fluoride ions is the coagulation/sedimentation method, which involves adding a coagulant and a polymer coagulant aid to the wastewater to form fluoride compound floc that can be separated through sedimentation [13]. This process is attractive for fluoride removal due to its low energy requirement, comparatively simple design, and ease of operation (Fig. 1). In most cases, calcium ions are added to form CaF₂, which is then allowed to settle and removed. Calcium compounds are widely used for this purpose because they are economical and can effectively remove fluoride at high concentrations [47]. Due to this reason, this method was first applied in many developing and under-developed countries to provide economic means for supplying fluoride-free water.

The reaction to precipitate fluoride in the form of CaF₂ using different Ca-based reactants are shown in the following Eq. (3) to (5) [48].

The efficiency of coagulation/precipitation method is affected by the solubility of the fluoride compound formed during the process, which is also affected by temperature, pH, reactant concentration, ionic strength, and other factors like common ion effects [13]. In this method, theoretical effluent fluoride concentration based on the chemical reaction is often not achieved despite adding the required concentration of calcium ions. Hence, excess amount of calcium ion, greater than its stochiometric value, is regularly added to reach desired fluoride release concentration [49]. However, when an excessive amount of chemicals is added and if the amount of wastewater to be treated is large, the amount of waste generated is excessive, causing efficiency and secondary pollution problems.

One of the solutions that can reduce processing time and increase reaction efficiency without the need for excessive calcium compounds is the addition of PAC (poly aluminum chloride) and alum as flocculating aids. The following is the fluoride treatment mechanism by gradually introducing slaked lime and alum (Eq. 6–9) [50].

Step 1: Alkali neutralization

Step 2: Coagulation, Precipitation

Step 3: Acid neutralization (H₂SO₄)

This can also be achieved by another approach i.e., by slacked lime and high concentration alum which has a different reaction mechanism as shown below (Eq. 10–15):

Step 1: Alkali neutralization

Step 2: Coagulation, Precipitation

Step 3: Acid neutralization (HCl)

Hence, regulatory fluoride concentration can be achieved by using this method, but it may lead to an increase in the requirement for secondary coagulants, and corresponding facilities must be considered from an economic perspective [13, 47]. In addition, most of the inorganic sludge generated through this process is disposed of after treatment in landfills, which may cause secondary pollution presence of toxic components [51]. Another issue faced by this process in potential release of aluminium ions along with treated water, which can create health problems in human, including disease like Alzheimer’s over long-term exposure.

Electrocoagulation (EC) method is a method in which an electric field is applied to the electrolyte to generate metal ions, which coagulate impurities and bind the material to form a precipitate [52]. This method is a widely used wastewater treatment technology that integrates the advantages of coagulation and electrochemical methods, capable of removing not only fluoride ions but also various pollutants such as COD and heavy metals [53].

For fluoride removal process, an aluminium electrode plate is employed in the EC device to effectively eliminate fluorine ions. This choice of electrode material is primarily driven by the favorable chemical interaction between fluorine and aluminium, coupled with aluminium’s convenient solubility, non-toxic nature, and reliable performance [54]. The reaction mechanism for the fluorine ion removal process when using an aluminium electrode is given below (Eq. 16–19):

The reaction at the EC cathode produces hydroxide ions (Eq. 16), which can be used to control pH fluctuations due to cationic hydrolysis. Due to the application of an electric field, negatively charged fluorine ions move near the anode, resulting in a high concentration of fluoride at the anode [55]. Moreover, the anodic current initiates the dissolution of aluminium into aluminium ions (Eq. 17), facilitating the breakdown of the metal electrode. Simultaneously, as aluminium dissolves, it disrupts suspended particles at a suitable pH level, causing coagulation or precipitation, and the adsorption of contaminants. This process results in the formation of metal hydroxide, as depicted in Eq. 18–19. Through this mechanism by the formation of Al(OH)₃ during electrocoagulation process, fluorine ions are removed as elucidated in the reaction scheme [56]. EC removes pollutants using the principle of collecting and separating particles using electrical force, which is particularly useful for removing fine particles (approximately 96% or more), enabling efficient fluoride removal.

Electrocoagulation is an excellent method for removing a wide range of contaminants, including fluoride, over a broad range of concentrations (Fig. 1). It does not require additional chemicals for coagulation, unlike some conventional treatment methods, reducing operational costs and minimizing chemical handling risks. The formation of precipitates during EC process causes minimal sludge production, which reduces disposal costs and environmental impact of secondary pollution due to sludge disposal [57]. Above all, EC systems can be easily adjusted to accommodate changes in water quality and flow rates, making them suitable for both centralized and decentralized water treatment applications. However, the cost associated with energy (electricity) consumption and high cost of installation and maintenance is one of the major limitations of this technology [58]. Other problems associated with this process are related electrode fouling, which reduces electrochemical reaction performance, and the possibility of toxic metal ion leaching from the electrodes which could further pollute the effluent coming out of the EC system. Additional consideration needs to be given to the optimization of several important process parameters affecting the EC process prior to its installation as the environmental conditions can vary depending on source and characteristics of the targeted wastewater [55]. Another problem faced during practical application of EC technology is that it could became difficult to maintain optimal conditions due to poor control and maintenance over long period of operation, which could lead to drastic reduction in fluoride removal efficiency over time even though initial parameter optimization had given significantly better performance [59]. Due to such challenges, the industrial application of EC technology for fluoride removal from actual industrial wastewater has taken a hit, despite having many proven studies showing its superior performance in the literature. Future research objectives related to electrocoagulation include the optimization of electrode materials, operating conditions, and reactor configurations, which could help in improving process efficiency and consistency.

Adsorption-based pollutant removal is an essential step in the purification and remediation of environmental pollutions [60]. The adherence of molecules, atoms, or ions from a gas, liquid, or dissolved solid to a surface is known as adsorption. This same principle of adhering pollutants from liquid phase onto a solid surface, known as adsorbent, can be applied for treatment of contaminated water and wastewater [61, 62]. The adsorption process is highly effective, simple to apply and its versatility and nonspecific nature towards pollutants makes its application in various industrial and environmental processes suitable [63]. From the immense amount of literature found on this topic, it is clear that adsorption has been successfully used for the removal of both organic and inorganic pollutants. Some examples of organic pollutants removal using adsorption are volatile organic compounds (VOCs), pesticides, pharmaceuticals, and dyes, whereas inorganic contaminants such as heavy metals and metalloids are also removed effectively [11]. Activated carbon, silica gel, zeolites, and other kinds of resins are common adsorbents used in industry to remove pollutants from their effluent stream [64]. These materials are very effective in removing pollutants from gas or liquid streams because of their large surface areas and unique surface characteristics suited for adsorption [65].

The process of pollutant removal by adsorption typically involves the following steps. Firstly, contact between the pollutant and adsorbent. This can be achieved through various methods such as passing the fluid through a packed bed of adsorbent, mixing the adsorbent with the liquid phase, or exposing the gas phase (in case of gaseous pollutants) to a porous adsorbent medium [66]. Following contact, the pollutant molecules present in the fluid or gas adhere to the surface of the adsorbent through physical or chemical interactions. The nature of these interactions depends on factors such as the properties of the adsorbent and the chemical composition of the pollutants [67, 68]. As the adsorption process continues, a state of equilibrium is reached where the rate of adsorption equals the rate of desorption [69]. At this point, the concentration of pollutants in the fluid or gas phase remains constant, and the adsorbent becomes saturated with pollutants, necessitating regeneration or disposal of the adsorbent. Desorption involves removing the captured pollutants from the surface of the adsorbent, typically by heating, flushing with a desorbing agent (e.g., acid or alkaline solution, etc.), or subjecting the adsorbent to vacuum conditions [70]. In many cases, the adsorbent can be regenerated and reused multiple times, reducing operational costs and environmental impact. Regeneration methods vary depending on the adsorbent and the type of pollutants being removed.

With regards to fluoride removal, commonly used adsorbents reported in the literature include activated alumina, activated carbon, zeolites, and various modified materials like metal oxides and biochar. Different biosorbents such as plant materials, agriculture residue, algal biomass, and microorganisms have also been found to be effective in fluoride removal [11, 63, 71]. Adsorption offers high removal efficiency for fluoride ions from wastewater, often achieving levels below the permissible limits set by regulatory bodies such as the World Health Organization (WHO). The various factors that affect adsorption process are pH, temperature, pollutant concentration, adsorbent dosage, etc., which are optimized to obtain the best result [11]. Targeting certain pollutants, such as fluoride, using adsorption is possible without adversely affecting other water components. Because of its adaptability, treatment processes may be tailored to the specific characteristics of each wastewater sample. Moreover, unlike many other technologies, adsorption is effective even for low fluoride concentration, which makes this technology suited for such wastewater as well. Due to the inherent simplicity of the adsorption process compared to other water treatment technologies, adsorption-based systems are not difficult to operate or maintain even at a large-scale operation [46, 66]. The regeneration of the adsorbent bed following its saturation with fluoride is also a simple process involving the passing of a regenerating solution (acid or alkaline liquid) through it. Regeneration extends the lifespan of adsorbents and reduces operating costs by preventing its early replacement before complete use [46]. Although from the conventional knowledge, it is clear that adsorption is a low-cost technology in comparison to membrane process and ion exchange method, proper cost-benefit analysis needs to be undertaken before making any decision. The key elements affecting the cost of adsorption process, i.e., type of adsorbent used and the size/scale of the treatment system, can help to determine if this process is suited or not as an ideal treatment system for a particular situation [64, 65].

The most common challenge of the adsorption process is the requirement of additional resources (chemicals) for regenerating adsorbents, and the need for further treatment and disposal of the secondary waste stream generated subsequent to this regeneration [60]. Regeneration methods can also decrease the adsorbent’s efficiency over time, necessitating eventual replacement and separate disposal of the original adsorbent materials. Moreover, the co-pollutants present in wastewater along with fluoride may sometime hinders its adsorption, as those other ions may have a higher affinity towards the adsorbent. This can lead to reduced efficiency in the presence of such competing ions [72]. This selectivity issue may require pre-treatment steps or the use of hybrid adsorbents to enhance fluoride removal. Scaling up adsorption processes from laboratory or pilot-scale to full industrial applications is also a challenging task involving efficient reactor design, flow dynamics, and the availability of suitable adsorbent materials in large quantities at a cheap cost (Fig. 1). Due to this reason, despite having so many new adsorbents reported in the literature, only a few well-known materials are often used at an industrial scale. Additionally, the long-term stability and performance of adsorbents in real-world conditions need to be thoroughly evaluated to ensure consistent and reliable fluoride removal over an extended period.

Ion exchange refers to a chemical process that exchanges ions contained in solutions for other ions and is mainly performed selectively using ion exchange resins [73]. This is achieved through a reaction that adsorbs certain ions and releases other ions instead. Ion exchange resins are typically composed of organic polymer beads containing positively or negatively charged functional groups. This process is used in different industries for various purposes, including water purification, metal extraction, dehydration using anion exchange resins, production of chemical products, pharmaceutical manufacturing, and wastewater treatment [74, 75].

Generally, to remove fluorine in wastewater, anion exchange resin is used. It mainly uses water-soluble polystyrene-based anion exchange resin with –OH functional group and hydrogen resin (H) to remove the negatively charged ion, i.e., F⁻ in this case [76]. The chemical reaction when adsorbing fluorine ions is as follows (Eq. 20).

In this reaction, the anion exchange resin adsorbs fluorine ions (F⁻) and releases (or removes) OH⁻, another anion presents in the resin. This same process can also be used to adsorb and capture Cl⁻, NO³⁻, and CN⁻. During fluoride-containing water or wastewater treatment, water is generally passed through a resin bed, fluoride ions in the water phase are attracted to the resin and replaced with other anions (generally OH⁻), leading to the formation of fluoride-free water. This process of fluoride capture from water continues until an equilibrium is reached, where the resin is saturated with fluoride ions and can no longer effectively remove additional fluoride from the water [77]. Once the resin becomes saturated with fluoride ions, it needs to be regenerated to restore its ion exchange capacity. Regeneration usually involves passing a solution containing a high concentration of the exchangeable ions (such as chloride ions) through the resin bed, displacing the fluoride ions and restoring the resin’s capacity to remove fluoride from the water [76]. Ion exchange resin-based fluoride removal processes are highly effective (Fig. 1), and capable of reducing fluoride concentration in water to below regulatory limits (as low as 0.7–1.5 mg/L). However, their effectiveness is dependent on a multitude of factors including the resin type, fluoride concentration, water chemistry, presence of other ions, flow rate, contact time, and resin regeneration frequency. Cost wise, ion exchange resin-based systems are on the higher side due to their high installation and maintenance cost, particularly as they require periodic resin regeneration and replacement [78]. Another limitation of this technology is that the ion exchange process is highly sensitive to any fluctuation in pH and water flow rate, which could negatively affect the process efficiency. Hence, constant monitoring of these key process parameters along with proper process control is needed to achieve desired performance (Fig. 1). Various wastes are generated during the large-scale operation of this system, such as spent resin and regeneration chemicals, along with concentrated fluoride streams recovered during regeneration [78]. These compounds require careful consideration for their safe disposal to avoid any further environmental damage. Future research avenues include continuing modification of ion exchange resins and exploring new resin chemistries to enhance fluoride removal efficiency, regeneration capacity, and overall process economics.

Fluidized bed crystallization (FBC) is an advanced and efficient technology for fluoride removal and recovery from contaminated industrial wastewater [79]. In this method, fluoride ions are converted to their crystalline form by reacting with suitable chemicals (e.g., calcium) for their separation from aqueous solutions. In order to perform crystallization method, first a supersaturated solution of calcium and fluoride is prepared, which is then passed into a fluidized bed reactor containing seed crystals in the form a bed [4, 80]. These seed crystals aid in the crystallization process by acting as nucleation sites for calcium fluoride (CaF₂) crystal formation. The crystallization process consists of three steps: crystal nucleation, growth of the resulting crystals, and crystal aggregation. Nuclei are generated before crystal growth, and this process consists of two stages: primary nucleation and secondary nucleation [81]. Primary nucleation is a stage in which the solute molecules dissolved in the solution self-assemble to form nuclei. The primary nucleation stage involves homogeneous nucleation, in which crystals are created in the solution, and heterogeneous nucleation, in which nuclei are formed on the surface [82].

Secondary nucleation refers to the phenomenon in which crystal nuclei are created from already formed crystals [83]. Afterwards, the crystal grows around the nucleus, and at this time, a large amount of solute diffuses to the interface where the crystal can grow [84]. Following this, a surface aggregation process occurs where the crystal and solute combine. The surface integration process is largely divided into two stages. In the first step, the solute at the solid-liquid interface is adsorbed onto the crystal surface. The second step involves a process in which crystal growth spreads on the active growth surface of the crystal. After the crystal grows, Ostwald ripening occurs [83]. In the equilibrium state of dissolution and aggregation, the surface energy of the particles acts as a driving force, so that small particles have a fast dissolution rate, and large particles have a slow dissolution rate, causing the small particles to be absorbed by the large particles [85]. In other words, among the generated crystals, the particles that are smaller than those of the dispersed system become smaller, and in the crystals grown in this way, large crystals undergo agglomeration of crystals that become larger, and a crystal crushing process occurs in which small crystals become smaller and integrate into larger crystals [83].

The crystallization process involves the growth of calcium fluoride crystals on the surface of seed crystals within the metastable supersaturated zone, lying between the solubility and supersolubility curves of calcium fluoride [80, 86]. Remarkably, this mechanism enables calcium fluoride to precipitate as crystals without the spontaneous generation of microcrystals, distinguishing it from conventional coagulation/precipitation methods [3, 4]. Unlike coagulation/precipitation methods, the calcium fluoride crystallization method produces minimal inorganic sludge and yields high-purity CaF₂ crystals. These crystals, characterized by their relatively low moisture content and high purity compared to sludge produced by coagulation/flocculation, hold significant potential for various applications in various industries such as steel manufacturing [87].

In addition to high fluoride removal efficiency, other major advantage of this process is its high selectivity towards fluoride without significantly affecting other ions (or getting affected by them) in the wastewater, minimizing the production of undesired byproducts [4, 88]. Another benefit of this system is that it can be operated continuously, allowing for consistent and reliable fluoride removal over extended periods, making it highly applicable in industries generating a continuous stream of fluoride-containing wastewater. The primary challenge of fluidized bed crystallization process for fluoride removal is the influence of process parameters such as pH, temperature, reactants concentrations (relative ratios), and flow rate [3]. Efficient operation of FBC system requires careful control of these parameters to maintain optimal conditions for crystal growth. The choice of seed (size, type etc.) also affects the process efficiency, and the selection of suitable seed material is key for achieving desired outcomes [80]. Moreover, the cost of chemicals and seed materials should also be taken into consideration before selecting this process.

Recent research trends are focusing not only on fluoride removal but also on the regeneration and recycling of crystals. In particular, the recycling of fluoride-free crystals is an effective strategy for cost reduction and sustainable operations, and the development of recyclable crystals has the advantage of increasing the application of environmentally friendly technologies and improving resource efficiency (Fig. 1) [4]. However, research related to the purpose of recycling through fluoride crystallization is insufficient and shows the possibility of continued development.

Membrane technologies offer promising solutions for fluoride removal due to their efficiency, scalability, and ability to operate without the need for extensive chemical treatments [3]. Several membrane-based methods are employed for fluoride removal, including reverse osmosis (RO), nanofiltration (NF), electrodialysis, and pervaporation [89, 90]. These methods are briefly described in the following sections.

Reverse Osmosis (RO) is a widely utilized membrane technology for fluoride removal, including full-scale installation in important industries generating fluoride containing wastewater [91]. It operates by applying pressure to a solution to force water molecules through a semi-permeable membrane while leaving behind fluoride ions and other contaminants. RO membranes have fine pores that reject fluoride ions, resulting in purified water on one side and concentrated fluoride solution on the other [92]. In addition to fluoride, reverse osmosis systems can successfully remove a wide range of other water contaminants, including heavy metals, dissolved solids, bacteria, and viruses, providing comprehensive water purification [91]. This makes RO suited for the treatment of even ground water contaminated with fluoride. Moreover, due to its compact size, it takes up comparatively less space and is suited for both household and commercial applications. Although RO is effective, it requires high energy input, and fouling of the membrane surface by scaling and organic matter can reduce efficiency over time [93]. Another major drawback of the RO process is the generation of large quantities of wastewater, known as reject or brine water. As for fluoride-containing wastewater treatment, reject water generated from RO process will contain a concentrated amount of fluoride along with other pollutants, requiring their secondary treatment before disposal [94]. The initial installation cost of an RO system is also high; however, due to the low operating cost and long-term usability of the system, this one-time cost seems to be justified.

Nanofiltration (NF) membranes operate in a similar mechanism similar to RO but have larger pore sizes, allowing for the partial retention of multivalent ions like calcium and magnesium while rejecting monovalent ions like fluoride [3]. NF is particularly effective in treating water with lower fluoride concentrations and is less energy-intensive compared to RO [95]. Although NF membranes are susceptible to fouling and require regular maintenance, they are less prone to fouling compared to RO membranes, leading to reduced maintenance requirements and longer membrane lifespan. Another advantage over RO, nanofiltration generates lesser amount of reject water, that too typically less concentrated, making its disposal more manageable [96]. However, the pollutant removal efficiency of the nanofiltration process is lower than that of other conventional methods, which is one of the concerns for its commercial application. Furthermore, this process necessitates proper process control and through monitoring compared with other simpler methods to maintain effective treatment [97]. In this regard, suitable system design and operation are essential to achieve desired fluoride standards.

Electrodialysis is a membrane-based process involving transport of ions through semipermeable membranes using an applied electric field [98, 99]. This method is highly suited for treating high fluoride-containing wastewater, such as those generated by the semiconductor industry [100]. The relatively low membrane fouling in electrodialysis compared with other membrane-based processes provides additional advantage for continuous industrial wastewater treatment. The main challenge faced by this technology is the complex nature of this system and the need for maintaining proper control over process parameters [101]. Due to such critical issues, this method is not often the method of choice for a particular industry to treat fluoride-containing wastewater as other method such as reverse osmosis. Another issue with this method is the requirement for additional energy to create electrical field necessary for transmembrane transport of ions [102]. This increase in energy requirement raises the operating cost of electrodialysis process. Furthermore, the inherent complexity of its design and configuration involving multiple membrane stacks and electrodes makes its operation, monitoring, and maintenance difficult.

Pervaporation is another membrane separation process that can be used for fluoride removal from water and wastewater, is based on the principle of combining permeation and evaporation [103]. In this process, volatile compounds can be separated out of solutions through a selective membrane. The mechanism involves applying vacuum or introducing a flow of purge gas on one side of a dense membrane, through which the volatile compounds in a liquid flow diffuse [104]. This movement of volatile compounds causes separation of fluoride ions and other contaminants from the liquid stream. Among all the commonly used membrane separation techniques, this technology has not been widely applied so far [105]. However, this technology shows great promise for this application, particularly due to its simplicity and energy-efficient nature.

There are various deciding factors behind selecting a particular membrane technology for fluoride removal, some of which are fluoride concentration in water, water characteristics (co-pollutants), energy consideration including its availability, and cost-wise considerations (Fig. 1) [3]. Moreover, considering the possibility of membrane fouling, pretreatment of wastewater may be necessary in some cases in order to maintain proper membrane functioning [106]. The target for future research and development in this field includes efforts to improve the affordability, sustainability (e.g., reducing energy consumption), and efficiency (e.g., optimizing system design) of membrane-based processes in order to expand the use of these technologies in environmental remediation and water treatment projects.

Despite having many research and review papers on this topic, there are still numerous areas that remain either underexplored or entirely unaddressed in the context of fluoride removal. Investigating the potential utilization of biological mechanisms to remove fluoride from water/wastewater, such as bioaccumulation or microbial-mediated precipitation, represents one such unexplored area. Specific research under biological treatment may focus on exploring suitable microorganisms, optimizing environmental factors, and comprehending the microbial processes underlying fluoride sequestration. Another prospective area for future research involves the exploration of hybrid treatment systems by integrating two or more treatment methods (e.g., adsorption together with membrane filtration or ion exchange) to address particular water quality issues and improve fluoride removal efficiency in a synergistic way. Combining more than one method could potentially overcome many challenges associated with individual treatment methods and are also better equipped to provide effluent with low fluoride concentration to meet the regulatory standards. Additionally, investigating methods for valorizing high-fluoride-containing waste streams produced during the water treatment process represents another lesser explored research area. Presently, only fluidized bed crystallization provides a fluoride-rich product subsequent to wastewater treatment, which can be reutilized in other industrial processes. Conversely, the other treatment technologies discussed in this paper generate highly concentrated fluoride-containing effluents that need to be disposed of carefully. Instead of mere disposal, fluoride-containing waste steam should be explored for possible resource recovery strategy by integrating with suitable method. This approach could enable recovery of valuable materials (such metals and fluoride) from waste streams or the conversion of waste into usable products, thereby improving process sustainability and economics. Furthermore, a potential future research direction can be employing cutting-edge modeling and simulation methods to optimize process parameters, create effective treatment systems, and predict the performance of fluoride removal technologies under various operating conditions. It is also imperative to assess the scalability and validate the efficacy of fluoride removal systems through field research and pilot-scale demonstration programs to identify possible obstacles to full-scale implementation.