Magnetic nanomaterials as an effective absorbent material for removal of fluoride concentration in water: a review

CNTs

carbon nanotubes

MNPs

magnetic nanoparticles

NPs

nanoparticles

pH

potential hydrogen

RO

reverse osmosis

ROS

reactive oxygen species

Water is the most abundant and precious resource on Earth, but less than 1% of fresh water is available for humans and other lives (Grey et al. 2013). As per the World Health Organization (WHO), more than 760 million individuals were deprived of the supply of satisfactory drinking water in 2011 (Ali et al. 2023). Water prices are increasing wherever there is access to clean water as a result of rising energy costs, a growing population, and environmental issues (Levin et al. 2002; Panhwar et al. 2021). Furthermore, growing amounts of clean water resources are revealing evidence of pollution, particularly by emerging contaminations such as personal care products and pharmaceuticals (Houtman 2010).

Elemental fluorine can be found only in fluoride mineral complexes rather than in nature because it is the most electronegative and reactive element. Fluorides make up between 0.06 and 0.09% of the crust of the Earth. Fluoride existing intake water is either harmful or beneficial to humans but depends merely on the recommended concentration (Han et al. 2021). According to WHO guidelines for safe water, the concentration of fluoride should not exceed 1.5 mg/L. Therefore, it is quite an excess level to get rid of fluoride from drinking water (Craig et al. 2015).

The flame spray pyrolysis approach used aluminum acetylacetonate as a reactant, which is dissolved in methanol to form Al₂O₃ nanoparticles (NPs) for the purpose of eliminating fluoride from water by utilizing NPs. Fluoride removal methods have included micron-sized adsorbents made of aluminum. Metal oxide NPs, on the other hand, have been shown by a number of researchers to be more effective at removing fluoride from aqueous solutions in recent years (Craig et al. 2015). Reverse osmosis (RO), nanofiltration, electrodialysis, and Donnan dialysis are examples of conventional methods that are used to remove fluoride from water. These methods, on the other hand, have a number of major drawbacks, and the most significant of which are that they consume a lot of energy, are imperfect, and are less adaptable to an extensive range of fluoride amounts (El Batouti et al. 2021). To effectively remove fluoride from water, the study by Riahi et al. (2015) discusses the synthesis and characterization of grafting Fe₃O₄ NPs with zirconium (IV)-metalloporphyrin. The created NPs demonstrated strong fluoride selectivity and adsorption capability, making them an attractive choice for water treatment applications. In addition to being extremely adaptable for use in situ, nano-iron particles can offer cost-effective solutions to difficult environmental cleanup issues (Chen et al. 2003). In the study by Pan et al. (2013), to enhance the removal of fluoride from water, hydrous zirconium oxide NPs assisted by a polystyrene anion exchanger were created and described. The large capacity for adsorption and stability of the synthesized NPs point to their potential use in water treatment procedures (Panhwar et al. 2022). Iron oxide–hydroxide NPs were utilized to remove fluoride from water. The strong adsorption capability and effectiveness of the produced NPs in removing fluoride make them a good choice for application in water defluoridation (Raul et al. 2012).

In lower concentrations, fluoride was shown to have beneficial effects on bones and overall health, but at greater concentrations, fluoride caused fluorosis, which had an impact on bodily tissue cells. However, dental and skeletal fluorosis are the main health concerns linked to excessive consumption of fluoride concentration (Prabhu & Meenakshi 2014). Despite the fact that man-made sources of fluorine in the environment have sometimes been the source of human health disorders, the majority of fluoride-related health problems are caused by consuming fluoride from natural sources (Han et al. 2021). Researchers are exploring new techniques to eliminate excess fluoride from potable water as people become more aware of the risks of long-term fluoride exposure (Waugh et al. 2016). Drinking water is contaminated by fluoride-containing substances from both natural and man-made sources, and this is how most people consume fluoride (Aoun et al. 2018). The majority of fluoride in rain comes from marine aerosols, volcanic emissions, and anthropogenic inputs (Shaji et al. 2023). While various chemical elements influence water quality, maintaining an optimal fluoride concentration is becoming increasingly important in today's society (Bharti et al. 2017). Both industrialized and developing nations are currently feeling the impacts of too much fluoride in their water supply (Sellers 2004). Fluoride is purposefully added to drinking water sources in industrialized countries to lower fluorosis and dental cavities, although this technique has long been challenged since some researchers have found evidence of the opposite tendency (Zulfiqar et al. 2020). In addition, industrial waste phosphate compounds instead of pharmaceutical quality have been identified as the source of these artificial fluoridation supplies (Raul et al. 2012).

The high surface area, ease of separation from water, high pollutant removal efficiency, targeted removal of specific molecules, and the possibility of reuse are all advantages of using MNPs (Sharma et al. 2018). Surface modifications can be made using a variety of synthesis techniques to improve the effectiveness of MNPs in water decontamination (Ali et al. 2016). MNPs have the potential to be used in biomedical applications like medication delivery and magnetic resonance imaging as well as environmental services like the cleanup of heavy metals and organic contaminants (Wang et al. 2019).

Surface and groundwater resources are contaminated due to urbanization and industrialization. The entry of various species in water such as arsenic, fluorine, nitrite, nitrate, sulfate, iron, selenium, manganese, and heavy metals may cause health and environmental problems (Dixit et al. 2003). Fluorine is a component of many minerals and is found as fluorides throughout the crust of the Earth. Fluoride in both surface and groundwater is extremely important due to its direct or indirect impact on health (Hu et al. 2021). The Earth's crust contains the lightest halogen element, fluorine, which is also movable at high temperatures (Raja & Neelakantan 2021). The water used for food and beverages without treatment is responsible for fluorosis (Matloob 2011; Hu et al. 2021). Some of the geogenic fluorides such as fluorite, biotite, and phlogopite in groundwater are found in rocks and sediments. High levels of electrical conductivity (EC), bicarbonate, and alkalinity regulate how much fluoride dissolves in groundwater (Ali et al. 2018). Both beneficial and bad impacts on health can be attributed to fluoride in drinking water. Fluorosis outbreaks occur by consuming drinking water with high concentration of fluoride (Viswanathan et al. 2009). Those who consume water with high fluoride concentrations typically live in underdeveloped nations where access to potable water is not always assured (Otal et al. 2021). Optimum ingestion of fluoride can lower tooth cavities and encourage bone growth. Less or excess consumption may cause dental diseases (Qasemi et al. 2019; Toolabi et al. 2021). The high level of fluoride in groundwater is due to water–rock interactions and mineral weathering (Adimalla 2020). In the case of groundwater predominantly traveling via fissures in crystalline rocks, the residence time has an impact on dissolved fluoride levels in a number of field studies. In some cases, this effect has a direct relationship between the quantity of fluoride and the depth at which a sample of water was taken (Beg et al. 2023). Fluoride has a significant concentration in soils, which is hazardous to plants, animals, and microbes. At a range of 2500 m, at the predominant downwind of the industrial region, the amounts of total fluoride and water-soluble fluoride in the surface soils declined with the increasing distance (Wang et al. 2019). According to the findings of Wang et al. (2019), the contribution of air (5%) and fertilizers from the chemical industries (3%), as well as dust falls (69%), make up the majority of anthropogenic sources of soil fluorine. Phosphate rock, phosphogypsum, and surface soils with higher fluorine contents from the chemical industries were the major causes of the high fluorine concentration of dust falls (Wang et al. 2019).

The WHO established the fluoride in drinking water standard at 0.50–1.5 mg/L to improve sanitation facilities and lessen negative effects. The concentration of fluoride in drinking water should be between 0.5 and 1.5 mg/L; however, depending on temperature, it might be as high as 1 mg/L (WHO 2011). Based on daily water use (2 L)., the WHO recommends a fluoride content of 1.5 mg/L in drinking water as the upper allowable level. While establishing national limits for fluoride, it is crucial to take into account regional cuisines as well as meteorological variables that affect how much water is consumed. However, the water consumption rate is higher during summer and lower in winter (Kimambo et al. 2019). The greater concentration (>10 mg/L) is frequently linked to thyroid, hypertension, infertility, cancer, arthritis, and neurological problems. Fluoride in drinking water can have harmful consequences, including tooth loss (<0.50 mg/L), fluorosis (1.50–5 mg/L), and skeletal fluorosis (5–40 mg/L) (Ali et al. 2018; Kimambo et al. 2019; Toolabi et al. 2021). There is no evidence to support the claim that the 1984-set guideline value of 1.5 mg/L was incorrect. Dental fluorosis becomes more likely at 1.5 mg/L, while skeletal fluorosis develops at considerably higher amounts. The number is higher than the 0.5–1.0 mg/L range typically advised for artificial fluoridation of water systems. It is crucial to think about establishing a local guideline at a concentration lower than 1.5 mg/L if intakes are higher than 6 mg/day (WHO 2011). In the last couple of decades, much research has been conducted on fluoride, but it needs more study to provide safe water in developing countries.

In Pakistan, groundwater is the major natural resource used for household and agricultural purposes. Fluoride pollution in Pakistani groundwater is serious, with concentrations ranging from 0.05 to 49.3 mg/L as presented in Table 1. In some places, the concentration level is alarming and above the permissible limit. This is mainly due to the unsustainable consumption of groundwater for resident activities, which leads to the deterioration of freshwater quality. Expanding urban areas and extensive mining are also major environmental concerns in underdeveloped countries (Rashid et al. 2018). The literature already in existence demonstrated that skeletal and dental fluorosis was discovered to be documented in nations like China, India, and Pakistan (Aziz et al. 2021). It is also noted that fluoride contamination of groundwater and surface water is caused by both nature and human activity (Thivya et al. 2017). Moreover, sedimentary deposits, geothermal rocks, acidic igneous rocks, and volcanic rocks all contain fluoride. At high pH levels, the -OH anion combines with the fluoride-containing mineral in clay, and an exchange between the two occurs (Regenspurg et al. 2022), then fluoride is released when bicarbonate (HCO₃) and fluorite (CaF₂) combine. The lengthier the groundwater residence time, the more time is available for the reaction to take place. A high temperature encourages the weathering of rocks, which further raises the fluoride content of groundwater (Regenspurg et al. 2022). Other man-made causes of fluoride are agriculture runoffs, industrial sewage, mining operations, and municipal products, which lead to health issues in residents.

Owing to expensive treatment and lack of awareness, preventing skeletal fluorosis is a topic of serious concern. The emphasis of exploration and development must be focused on the advancement of inexpensive and easy treatment of skeletal fluorosis, and at the same time, water should be free from this element. Furthermore, Government/Non-Governmental Organizations can create public awareness and play an important role in controlling this major threat. Survey camps should be arranged to create public awareness about the effects of fluorosis disease and the significance of drinking clean and safe water. In rural and urban areas where water is contaminated with elements that lead to skeletal fluorosis and there is no other choice of freshwater, then medical health insurance should be given to the community to mitigate its impacts (Srivastava & Flora 2020).

The main mode for a person to inhale fluoride is through the consumption of water on a regular basis. According to the existing literature, 200 million individuals worldwide use water with excessive fluoride concentrations (Huang et al. 2017; Yadav et al. 2019). An extreme amount of fluoride consumption can be toxic. Acute toxicity of fluoride may lead to poisoning if one or more doses are taken in a short time. Herein, the high concentration implies a concentration beyond 1.5 mg/L. Nonetheless, consuming fluoride within the allowed range (i.e., 0.5 to 1.5 mg/L) may aid in promoting bone growth. Under acute toxic conditions, the stomach is affected first. Since the stomach's pH is acidic, the fluoride can react with hydrogen to form hydrogen fluoride that can impact mucous membranes present in the stomach (Yadav et al. 2019). Fluoride exposure in animals has shown symptoms based on the dosage. Chronic gastritis of the stomach has been shown to occur at a fluoride amount of 190 mg/L or greater (Huang et al. 2017). The initial symptoms of the acute toxic effect of fluoride start from abdominal pain, nausea, vomiting with blood, and diarrhea. This condition further leads to collapse with paleness, severe weakness, low breathing, slow heartbeat, feeling cold, dilated pupils, hypocalcemia, cyanosis, and hyperkalemia. This severe condition may lead to death within 2–4 h. Additional effects may include paralysis of muscles, extremity spasms, and carpopedal spasms, and probable toxic dose (PTD) is 5 mg of fluorine per kilogram of body mass. This PTD can be life threatening and requires instant treatment (Ullah et al. 2017).

Numerous processes contribute to the toxicity of fluoride; however, oxidative stress is an important process that may start the toxicity of fluoride. Moreover, it causes the emergence of reactive oxygen species (ROS), such as hydrogen peroxide, hydroxyl, and superoxide anion radicals (Regenspurg et al. 2022). The defense mechanism of the human body can neutralize ROS development. Fluoride can also produce peroxynitrite, and once superoxide ions and peroxynitrite rejoin, it takes binding affinity with the metalloproteins’ cofactors and thiol molecule, and the nitrosyl adducts must be made and accumulate in the endoplasm (Ullah et al. 2017). Fluoride's increased electronegativity increases its reactivity, and as a result, it forms strong bonds of hydrogen with the -OH and -NH moieties. Consequently, oxidation with a variety of biomolecules may induce chronic diseases such as cancer, fluorosis, cardiovascular disease, and muscular degeneration (Srivastava & Flora 2020).

Fluorite, topaz, cryolite, kaolinite, montmorillonite, fluorapatite, and vermiculite are the major fluoride-containing minerals found on the Earth's crust. These minerals are responsible for destroying the water quality in the context of fluoride via different geochemical processes (Rahman et al. 2020). It is important to note that the pH, hardness, and alkalinity of water impact how fluoride dissolves in water. Technically, the Na and Ca ions in the water have an effect on the thermodynamic concentration of fluoride in groundwater (Rahman et al. 2020).

Indeed, dental fluorosis is taken into account globally by fluoride intake (Choubisa 2018; Martinez-Mier 2018). During enamel forming, disturbance of enamel is most common which leads to dental fluorosis. The development of the enamel on the crowns of permanent teeth occurs throughout the first 6 years of life, which is crucial for preventing dental fluorosis. The enamel is filled with proteins and is porous, thick, and less transparent (DenBesten & Li 2011; Choubisa 2018). To reduce the effects of toothpaste containing fluoride, some guidelines are recommended as shown in Table 2. In this way, the probability of exposure to fluorosis and the development of caries is reduced significantly (Wong et al. 2011; Choubisa 2018).

Despite the wide availability of fluoride in our daily life, the causes of acute toxicity due to fluoride are very rare today compared to the first half of the 21st century. Fluoride was employed as a rat poison and pesticide in the first decade of the 21st century (Kanduti et al. 2016). The physical appearance of fluoride was frequently mixed up with wheat flour, powdered sugar, and other white powder food products that are used in the kitchen. In Oregon State Hospital, a fluoride poisoning event happened in 1942 (Augustsson & Berger 2014). Four hundred and twenty persons suffered severe poisoning when powdered milk was accidentally substituted for sodium fluoride during the preparation of scrambled eggs, and 47 people faced fatal poisoning. This incident was regarded as massive poisoning due to fluoride (Clements 2005). Nowadays, poisoning due to fluoride is mostly because of unsupervised consumption of products for dental care, oral hygiene, and the excessive fluoride levels in the water (Clements 2005).

Since toxins have permeated aquifers and been carried to sources of drinking water, contaminated underground water and surface water pose a serious threat to water resources. To address this problem, a new technique for alternative water treatment must be developed (Panhwar et al. 2021). Yet, there has been little research into the potential of synthetic nanomaterials to be efficient adsorbents for purifying water (Tofighy & Mohammadi 2011), filters (Brady-Estévez et al. 2008), disinfectants (Li et al. 2008; Dankovich & Gray 2011), and reactionary agents (Chen et al. 2003; Crane & Scott 2012), even though they exhibit extensive water treatment and environmental improvement (Qu et al. 2013). Hence, compared to nanotechnology uses in the fields of health and electronics, not much advancement has been made. According to numerous areas based on bench-scale, full-scale, and pilot investigations, the use of nanotechnology in drinking water treatment and contaminant remediation appears promising (Ren et al. 2013; Mellor et al. 2015).

Nowadays, several innovative uses for nanomaterials are employed to identify and remove fluoride. Most of the current applications correspond to the developmental progress of present innovations, e.g. decrease in the electrical gadget size as innovative portable instruments, etc. (Qu et al. 2013). However, technology and nanomaterials aid in water protection, food, and groundwater contamination. Mainly, nanotechnology approaches have been used for removing excess fluoride in groundwater (Guerra et al. 2018). Nanotechnology is involved in manipulating material having at least in size of one dimension ranging from 1 to 100 nm. Compared to micro-counter and macro components, NPs exhibit remarkable physical, chemical, electrical, and thermal capabilities. Mainly, high-level quantum restriction and aspect ratio have been underlined for these differences (Khan et al. 2019). Without exception, the reactivity was also increased (Liu et al. 2018). Compared to the macro-counterpart, NPs play a valuable role when moving on to the water treatment application (Indermitte et al. 2014). Researchers have developed more creative methods to eliminate excess fluoride in potable water as a result of growing health risks brought on by fluoride disorders that have been there for a while (Grandjean 2019).

As a result, fluoride-carrying compounds from both natural and man-made sources polluted drinking water, which led to fluoride consumption by people. Thus, it is crucial to create sustainable techniques for mitigation (Tolkou et al. 2021). As nanotechnological strategies emerge as one of the many prospective defluorination methods due to their high effectiveness, they are advancing more quickly than other established traditional adsorbents and procedures in many regions for the removal of excess fluoride from groundwater (Habuda-Stanić et al. 2014).

In the ion-exchange techniques, using the synthetic polymer, fluoride ions from water have been eliminated. Usually, those synthetic polymers are said to be cation and anion exchange resins. These resins are produced commercially by reacting base chemicals. In many circumstances, they are uneconomical because they have been used at a deficient concentration level. The major benefit of this technique over others is its simplicity, cheap price, and effectiveness. This is why this method is one of the most implemented over other methods (Chen et al. 2012).

Adsorption is a process of eliminating dissolved fluoride ions from water under active surfaces of an adsorbent. It is worth mentioning that the adsorption process is the most economical, easy operation, and readily accessible, but on the other hand, it shows a limited removal efficiency (Chai et al. 2013). Further, a specific condition is required for the removal of dissolved ions. Many adsorbents have already been tried for these uses; the three most common ones are bone char, activated alumina, and activated carbon (Kennedy & Arias-Paic 2020). The material used in the process of absorption can also be recycled and reused, making the most ideal method that can be used in underdeveloped rural areas. The added benefit of using this technology is that decentralized water supply systems can use it (Naimušin & Januševičius 2023).

The precipitation method relies on adding chemicals to cause precipitation, which then settles by gravity. In common practice, aluminum salt has been added to perform the precipitation. It is a very easy technique that separates several pollutants from water. Precipitation is a useful technique for the purification of water and wastewater. Fluoride may be precipitated out of drinking water at a low cost using calcium. The current methods of precipitation result in a significant volume of sludge with high water content (Sun et al. 2019).

In the membrane-based defluoridation, hydraulic pressure is inserted into the system by the high-pressure centrifugal pump. The pump that converts mechanical energy into pressure energy forces water to pass through a semi-permeable membrane that ultimately leaves salt on the other hand. In addition, diffusion is also taking place by a concentration gradient. The membrane-based separation of fluoride ions is considered an energy-intensive process (Muqeet et al. 2023).

Fluoride ions have been eliminated from water on a massive scale using coagulation (physicochemical process). It is a well-established technique taken as efficient and cost-effective. However, the ultimate sludge produced after the process completion is considered a challenge for the practitioners.

Other methods of fluoride elimination from water include electro dialysis and electrocoagulation (EC). Among all the methods described earlier, absorption is the most economical and effective method; however, recent literature has shown that several absorbents were only efficient in low concentrations (10–100 mg F⁻/L). It has been reported that the membrane filtration is only confined to curing initial fluoride concentration (IFC) at 1000 ppm, and electro-based treatments are limited to <1,000 ppm and consume large amounts of electricity, which in turn leads to higher operational costs. This discussion leads to the point that precipitation is the most economical and has high efficiency (>90%) of fluoride removal in water (Chakrabortty et al. 2013).

Carbon nanotubes (CNTs) have a unique property that is useful in nanotechnology. These materials have a lot of potential in the field of nanotechnology since environmental pollution is getting worse every year. CNTs may prove to be a valuable absorbent material that can replace activated carbon in many ways. They have needle-like shapes with concentric graphite carbon covered by fullerene. CNT shows a very high efficiency for Pb⁺² removal after oxidation with nitric acid. Compared to activated carbon and other fluoride absorbers, aligned nanotubes of carbon have a greater fluoride absorption capacity (Yang et al. 2018). Single-walled nanotubes of carbon are allotropes of carbon that consist of a diameter in the range of nanometers. Multiwalled CNTs have multiple single-wall CNTs that are joined together by van der Waal forces. The outstanding properties of CNTs include their high heat conductivity, high EC, and high tensile strength. These properties occur as a result of bonds between carbon atoms (Otal et al. 2021). It has been reported that CNTs have an outer diameter, inner diameter, length, and intertubular distance of about 2–20 nm, 1–3 nm, 1 μm, and 340 pm, respectively. The multiwalled CNTs available commercially are processed to form a carboxylic group. Functionalized multiwalled carbon nanotubes are sonicated with sodium dodecyl sulfate (SDS) so that the SDS gets completely absorbed into the CNT tubes (Khizar et al. 2022).

However, the application of magnetite Fe₃O₄ NPs and surface-functionalized composites for fluoride removal has received considerable attention in the last decades. Coprecipitation, hydrothermal, solvochemical, microemulsion, sonochemical, and other methods are used to produce magnetite NPs (Zhao et al. 2010). The coprecipitation approach is one of the most efficient methods for obtaining magnetite NPs. Fe²⁺ and Fe³⁺ stoichiometric ratios were employed as an iron source, producing Fe₃O₄ NPs in an alkaline media (Mohseni-Bandpi et al. 2015). The synthesized NPs’ size, shape, and composition were determined by the salt utilized, Fe²⁺ and Fe³⁺ stoichiometric ratio, temperature, the type of stabilizing agent used, and the pH values of the reaction media in this technique of synthesis (Panhwar et al. 2019; Ajinkya et al. 2020). The reaction temperature in this technique of synthesis is typically 20–90 °C (Mukherji et al. 2018). In an autoclave, the hydrothermal process is frequently used to create single mineral crystals under hot water under high pressure (Yang & Park 2019). The hydrothermal process differs from the coprecipitation method in that it uses a single ferrous precursor rather than a stoichiometric combination. The reaction temperature is typically 100–320 °C in this technique of synthesizing (Ali et al. 2016). Solvochemical synthesis is analogous to hydrothermal synthesis, but instead of water, an organic solvent is utilized as a medium of dispersion. The structure of the particles may be changed to a large level by the solvochemical synthesis. Hydrophobic particles can also be produced, in addition to morphological control (Sharma et al. 2019; Jaldurgam et al. 2021).

Minju et al. (2015) used a modified sol-gel technique to produce a magnesium oxide (MgO)-coated magnetite (Fe₃O₄) NP with magnetite properties for measuring the fluoride scavenging potential. It was reported that the adsorption capacity was 10.96 mg/g for initial fluoride concentrations up to 10 mg/L; the adsorbent demonstrated excellent fluoride scavenging capacity.

The magnetite nano-sized adsorbent utilizing hydrous aluminum oxide embedded with Fe₃O₄ NPs was created by Zhao et al. (2010) to investigate the elimination of fluoride from an aqueous solution. The highest reported adsorption capacity was 88.48 mg/g at room temperature and neutral pH. The adsorbents can easily be extracted from sample solutions using an external magnetite field, which is a particular advantage of this defluoridation technique. According to Zhao et al. (2010), the substance has three desirable features: quick defluoridation treatment, high adsorption capacity, and ease of manufacture, all of which can meet the demand for practical use to treat large volumes of high fluoride-contaminated water. Bhaumik et al. (2011) and Ijaz et al. (2022) prepared in situ polymerization to make a polypyrrole (PPy)/Fe₃O₄ magnetite nanocomposite adsorbent and reported that the nanocomposites had a fluoride adsorption capability of 17.6–22.3 mg/g.

In addition, one of the most fluoride ion-selective magnetite sorbents is Fe₃O₄ NPs rooted in a network of Zr (IV) complexed poly (acrylamide) (Su 2017). The adsorption effectiveness of the material is 124.5 mg/g. Figure 3 displays a graphic picture of the magnetite separation of NPs when fluoride adsorption is completed. Compared to traditional separation methods, magnetite separation has shown to be a promising approach for solid–liquid phase separation with several benefits such as high speed, accuracy, simplicity, high-level absorption capability, and effective solid–liquid separation (Thakur et al. 2014).

The activated alumina was mainly used as the adsorbent for fluoride removal (Goswami & Purkait 2012); it is widely accessible, is inexpensive, and has a high capacity for adsorption at low pH levels, which eventually encourages aluminum dissolution. As a result of that interaction, the metal oxides (Fe₂O₃ and TiO₂) were functionalized with the trivalent aluminum cation (Al³⁺) to create 1:1:1 trimetallic oxide for the photodegradation and defluoridation processes. The aluminum cation increases the efficiency of the ferric ion (Fe³⁺) and titanium cation (Ti⁴⁺) in bimetallic systems. Bhaumik et al. (2011) and Mukherjee et al. (2019) developed the magnetic adsorption process as a cost-effective water treatment process. Therefore, a novel nanocomposite was prepared and characterized using combined magnetic property and Fe₃O₄ NPs for fluoride removal in contaminated water. As a result, the synthesized nanocomposite material was very effective for defluoridation in water. The starting concentrations, temperature, pH, and adsorbent dosage all had an impact on how quickly the fluoride was absorbed. The adsorption procedure used an ion-exchange mechanism and was heat absorbing in nature.

In addition, other anions including nitrate, phosphate, and chloride have been used in studies to measure fluoride adsorption, and these ions have not interfered. However, 97% of adsorbed fluoride on the PPy/Fe₃O₄ magnetic nanocomposite was desorbed at the range of pH 12 (Bhaumik et al. 2011). To demonstrate the reusability of nanocomposite in the removal of fluoride, the adsorbent retained the initial quantity of adsorption following the completion of the adsorption–desorption cycle. The spontaneity and endothermic character of the fluoride adsorption were further supported by thermodynamic parameters. Simultaneously, an ion-exchange mechanism is used to advance fluoride adsorption. Moreover, it has reported a green and cost-effective magnetic composite using defatted jojoba meal. The developed approach has a higher water defluoridation capacity of 43.47 mg g1 from fluoride concentration (Sharma et al. 2018).

Magnetic γ − Fe₂O₃ NPs are synthesized by the coprecipitation method. These particles possess surprisingly high efficiency for the specific fluoride elimination from drinking water by applying batch absorption study. Around 3.65 mg/g of fluoride could be removed, and this capacity was highly dependent on the starting pH of the solution. The removal level is high as 95% occurred in acidic to neutral pH. However, it has been shown that fluoride adsorption was influenced by the pH of the aqueous phase, with absorption being larger at lower pH levels. The functional groups involved in the adsorption were found using Fourier transforms infrared spectroscopy, which also revealed the interactions between the targeted fluoride and γ-Fe₂O₃ particles (Zhao et al. 2010; Jayarathna et al. 2015).

Further, Fe₃O₄ NPs were prepared and embedded with hydrous aluminum oxide for the adsorbent process to eliminate excessive fluoride concentration from an aqueous solution. This adsorbent combines the benefits of MNPs and hydrous aluminum oxide floc with magnetic separability and strong affinity toward removing fluoride, providing typical merit with easy preparation, high adsorption capacity, and easy separation from the particular sample by the use of an external magnetic field. The adsorption capability has been analyzed by the Langmuir equation (88.48 mg/g at pH 6.5). However, the main factors influencing fluoride removal, such as IFC, solution pH, temperature, co-existing anions, and adsorption time, were studied. It has been observed that the treatment process avoided the slow adsorbent with the bed passing process and exited the excellent potential in treatment with a large volume sample of fluoride-contaminated water (Zhao et al. 2010).

MNPs have recently been used by researchers working in the fields of biosorption and adsorption to remove pollutants from aqueous solutions. By a coprecipitation method, chitosan was saturated with MNPs to create a hybrid adsorbent of (Fe₃O₄–chitosan). After characterizing the adsorbent's physical, chemical, and structural characteristics, its efficacy in removing fluoride from the water was assessed (Nethaji et al. 2013). However, the operational factors, such as adsorbent dosages, pH, fluoride concentration, contact time, and temperature, influencing the adsorption procedure were studied (Kakavandi et al. 2014). The Fe₃O₄–chitosan, however, was easily regenerated through acid treatment, according to the results. There is a possibility of removing fluoride from water using a magnetic composite made of Fe₃O₄–chitosan. For the purpose of swiftly and easily separating fluoride from water, Fe₃O₄–chitosan has been thought to be an effective adsorbent. Also, it is quite effective and creates no secondary pollutants (Mohseni-Bandpi et al. 2015).

This brief review highlights the occurrence of fluoride as a common contaminant in drinking water, contributing to various diseases such as dental fluorosis, skeletal fluorosis, muscular damage, fatigue, and joint-related problems. Herein, traditional water treatment techniques like coagulation, ion exchange, adsorption, and membrane separation have been employed to mitigate contamination, and the emergence of nanomaterials, particularly magnetic NPs, has opened up new possibilities. Among others, the adsorption of fluoride using magnetic NPs has been found to be a more promising method. The removal efficiency of 97% has been reported. The use of an external magnetic field makes it a simpler method to remove the adsorbents from sample solutions, which is a particular advantage of this defluoridation technique. Fe₃O₄–chitosan is also regarded as a suitable adsorbent for removing fluoride from drinking water since it can be quickly and easily separated. The treatment process avoided the slow adsorbent with the bed passing process and exploited the excellent potential of treatment with a large volume of fluoride-contaminated water. The review also revealed that coatings with certain green organic materials can be used for the adsorption of contamination from water samples.

We offer sincere thanks to the department of Civil Engineering National University of Science and Technology, Balochistan Campus, Quetta, Pakistan, for their kind support and encouragement.

All relevant data are included in the paper or its Supplementary Information.

The authors declare that they have no competing interests regarding the authorship, research, and publishing of this review paper.