Application Progress of New Adsorption Materials for Removing Fluorine from Water

Abstract

A large amount of fluorine−containing wastewater was produced with the rapid development of the industry. Excessive fluoride content in water will not only endanger ecological security but also pose threat to human health. In this study, common new adsorbents for defluorination, such as metal−based adsorbents, natural adsorbents, and nanomaterial adsorbents were reviewed for its physicochemical properties and fluorine removal efficiency. The fluorine removal mechanism of different adsorbents was introduced in detail, and the future work of the removal of fluorine using novel adsorbents was proposed. This research also introduces the application of the coupling of the adsorption method with the technology of chemical precipitation, filtration, and super−magnetic separation to treat high concentration of fluoride wastewater. A good selection of process combinations according to different needs can achieve high−efficiency defluorination in water. Finally, some existing problems of practical operation of fluorine using removal materials in the environment are summarized, hoping to contribute to the future research of fluorine removal materials.

1. Introduction

Fluorine is the 13th element (625 mg kg⁻¹) in the Earth’s crust, and it is one of the essential trace elements for human. Fluorine is widely found in nature and is very important to human health [1]. Fluorine and fluoride are currently the most effective substances for preventing caries and mineralization [2,3,4,5], and moderate fluoride intake is beneficial to the calcification of tooth enamel and bone [6]. However, excessive intake of fluoride can cause harm to the human body, which can initially appear as dental fluorosis. If humans are exposed to very high doses of fluorine, it may cause skeletal fluorosis and affect the development of the nervous system [7,8,9,10]. In addition, there are many areas in China with high levels of fluoride in the environment, so the fluoride pollution of water became one of the major environmental problems that need to be solved urgently in China. Over 200 million people worldwide drink water with excessive fluoride concentrations, and the incidence of fluorosis in various countries and regions is also increasing year by year [11]. In China, the standard for drinking water quality (GB5749−2006) stipulates that the fluoride content in drinking water should not exceed 1.0 mg L⁻¹ [12].

Fluorine−containing chemical raw materials are widely used in electronics, photovoltaics, aluminum smelting, phosphate fertilizer, steel, organic fluorine advanced lubricating oil, oxygen difluoride, hydrazine fluoride, and the industrial production of other rocket propellants. With the vigorous development of the industry, high−concentration fluorine−containing wastewater is also produced in large quantities. For example, the mass concentration of fluoride ion (F⁻) in the wastewater of quartz products [13] can reach 50~200 mg L⁻¹, and the concentration of F⁻ in the flotation wastewater of aluminum electrolysis waste cathode [14] can reach 19,000 mg L⁻¹. The generated high−fluorine wastewater, waste gas, waste residue discharge, and the slow dissolution and release of fluorine−containing rocks cause serious fluorine pollution [15,16,17]. Fluorine pollution will not only pollute the soil and form a solid source of pollution, but also cause toxicity to plants, inhibit the metabolism of crops, and reduce yield. The intake of fluoride by humans is mainly through drinking, and the World Health Organization lists fluoride as one of the pollutants in human drinking water [18].

Currently, the methods widely used to remove fluorine in water mainly include the coagulation method [19,20], ion exchange method [21,22], electrodialysis method [23,24], membrane filtration method [25,26], and adsorption method [27,28]. The coagulation method is widely used, but the dosage of reagents requires precise control. After the treatment is completed, a large amount of hazardous waste will be generated, and chemical substances will remain in the water after treatment, which requires secondary treatment. The ion exchange method has the advantages of simple design and flexible operation, but it has low selectivity and requires frequent adsorbent regeneration or replacement. Electrodialysis and membrane filtration have better treatment results, need no chemical reagents, and no waste pollution is generated. However, they do not have ion selectivity, so the beneficial components to the human are also removed. The adsorption method has strong selectivity, high removal efficiency, and simple design and operation. The adsorption method is typically thought to offer the highest research value when considering economic and environmental concerns. The adsorption material with excellent adsorption performance is the critical factor for the fluorine removal efficiency, which always aroused the attention of many scholars. At present, the research content mainly focuses on the adsorption and removal of fluoride ions in water by materials with high adsorption capacity, renewable energy, and no secondary pollution. Natural minerals, nanomaterials, carbon−based material, layered double hydroxides, agricultural and forestry wastes, and other adsorbents are modified to have the advantages of surface physical adsorption and pore size diffusion, which is conducive to obtaining a larger specific surface area, pore size, and more active functional sites, thereby improving the adsorption efficiency. Because of the high concentration of F⁻ in industrial wastewater or other water in high fluoride areas, a single defluorination method often cannot meet the discharge conditions. As an essential part of advanced treatment, the adsorption method can form a combined process with chemical precipitation, coagulation precipitation, and other methods [29,30,31,32,33,34], which can further reduce the F⁻ concentration of the effluent from the previous treatment and improve the stability of the effluent F⁻ concentration.

Based on the existing research results in the field of fluoride removal, this paper summarizes the adsorption performance, fluorine removal mechanism, and modification methods of fluoride removal materials, such as metal−based adsorbents, natural adsorbents, nanomaterial adsorbents, carbon−based adsorbents, composite metallization, layered double hydroxide, and agricultural and forestry waste. The application and research of the adsorption method, chemical precipitation method, filtration, and ultramagnetic separation technology coupling treatment of high fluoride water were introduced. Finally, some existing problems of the practical operation of fluorine removal materials in the environment are summarized, and the future development of fluorine removal materials is prospected to provide reference for researchers in the field of fluorine removal materials.

2. New Defluorination Adsorbent and Its Mechanism

Adsorption is a commonly used water treatment method that mainly removes F⁻ from water through adsorption, ion exchange, or surface complexation [35]. It is one of the most effective and reliable methods for fluorine removal. The adsorption process of F− on solid particles usually comprises three basic steps [36]: (a) F⁻ diffuses or transports from solution through the boundary layer around the adsorbent particles to the outer surface of the adsorbent, known as molecular diffusion or membrane diffusion; (b) the adsorption of F⁻ on the particle surface; (c) the exchange of F⁻ on the surface with the substance in the adsorbent particle, or the transfer to the inner surface of the adsorbent particle, is called intra−particle diffusion. The commonly used adsorbents include aluminum−based adsorbents, calcium−based adsorbents, iron−based adsorbents, natural adsorbents, carbon−based adsorbents, composite metal compounds, layered double hydroxide adsorbents, agricultural and forestry wastes, etc. [37]. Given the adsorption mechanism of F⁻ on the adsorbent, the researchers modified the adsorbent as the above−mentioned materials, and the synthesized new fluorine adsorbent was used to remove fluorine in water to obtain a good removal effect of fluorine.

2.1. Metal−Based Adsorbents

2.1.1. Aluminum−Based Adsorbent

Activated alumina is widely used in groundwater treatment processes because of its high selectivity, high efficiency, and easy operation in removing fluoride. R. M. et al. [38] used a modified sol−gel method to prepare nano−porous activated alumina particles. Studies showed that the material has more than 90% fluorine removal efficiency and good regeneration ability. The activated alumina treated with alkali can still maintain a certain adsorption capacity after ten use cycles. Xu et al. [39] prepared modified activated alumina by impregnating alumina with ferric sulfate, found that its maximum adsorption capacity for F⁻ was 16.78 mg g⁻¹, and studied the adsorption mechanism.

The activation of sulfuric acid can significantly increase the surface area of alumina, and the alumina−based adsorbents modified by metal oxides have the cumulative adsorption characteristics of alumina and externally supported metal oxides. For this characteristic of alumina, Kumari et al. [40] prepared calcium−zirconium modified acid−activated alumina by a combined process of sulfuric acid activation and calcium−zirconium oxide loading, and the maximum adsorption capacity was 216 mg g⁻¹. Figure 1 summarizes the calcium and zirconium−modified acid−activated alumina (CAZ) for the adsorptive removal of fluoride in water. Figure 1a,b was the flow diagram of prepared adsorbent. The Al₂O₃ and CaO was added into acid solution ageing for 12 h. Then, the ZrO₂ particle was spiked into the acid solution and stirred for 24 h. Then the white powder of CAZ was obtained after dry. The results of SEM−EDS indicate the loading of the calcium and zirconium on acidically activated alumina (Figure 1c,d). The result of FTIR and XRD confirmed the successful adsorption of fluorine by CAZ (Figure 1e,f). The adsorption kinetics of fluorine by CAZ was pseudo second−order kinetics, and the adsorption isotherm was the Langmuir isotherm model compared with the Freundlich model (Figure 1g,h). The mechanism of electrostatic attraction and ion exchange was the main adsorption mechanism of fluorine by CAZ (Figure 1i). These results indicate that CAZ could be a promising adsorbent for the defluoridation in wastewater. In the future, increasing the specific surface area and anti−interference ability of materials will still be the research direction of aluminum−based adsorbents.

2.1.2. Iron−Based Adsorbent

Nano−zero valent iron has high reactivity to remove heavy metals, halogenated organic pollutants, and other inorganic pollutants, and it can also be used in defluorination in water. Li et al. [41] used calcined banana peel biochar and grape pomace to synthesize biochar loaded with nano−zero valent iron (BC−nZVI). In the fluorine removal process of BC−nZVI, the zero−valent iron will first react with oxygen in water to form hydrated iron oxide. The hydroxide ions on the hydrated iron oxide and F⁻ produce an ion exchange to form a precipitate to remove F⁻. The results show that the maximum adsorption capacity of the synthesized biochar material was 56.82 mg g⁻¹.

In recent years, sulfide nanoscale zero−valent iron (S−nZVI) became a promising defluorination agent. To investigate the effect of different sulfur reagents and additional procedures on the defluorination reactivity of S−nZVI, Cao et al. [42] synthesized S−nZVI through various sulfidation methods and conducted defluorination studies on florfenicol (FF) and found that FeO and sulfur contents in the co−sulfurization and post−sulfurization methods significantly affected the properties and reactivity. When the FeO content is similar, the defluorination rate of the sulfide particles after S−nZVI decreases slightly with the decrease in the sulfur content. The higher the [S/Fe] particle, the higher the defluorination rate. Wu et al. [43] prepared nanoscale Fe₃O₄ with the co−precipitation method. Under acidic conditions, due to the increase in specific surface area, the positive charge on the surface is further attracted to F⁻, and the removal efficiency of fluorine is the highest at pH 3, which is 80.03%, the adsorption capacity reached 2.1834 mg g⁻¹. To further explore the surface properties of different hematite and their adsorption properties for fluorine, Xiang et al. [44] prepared three kinds of hematite by hydrothermal synthesis, burning goethite and lepidocrocite. The research shows that the three’s surface fractal degree and surface−active hydroxyl density increase in turn. The former two are monolayer adsorption on a homogeneous surface, and the latter is multi−layer adsorption with the largest adsorption capacity, and the maximum adsorption capacity is 10.54 mg g⁻¹. At present, the side reaction of FeO and H₂O is still a major problem affecting the defluorination of iron−based adsorbents. In the future, we should focus on and strengthen the research on this aspect.

2.1.3. Calcium−Based Adsorbent

Calcium−based adsorbents have good biocompatibility and F⁻ affinity with the human body, and researchers conducted extensive research on various calcium−based fluoride scavengers. Yang et al. [45] synthesized dicalcium phosphate (DCP) by a hydrothermal method. DCP has strong stability and strong F⁻ affinity. In the range of pH 6~10, the effect of pH value and coexisting anions on the adsorption capacity is negligible, and the maximum adsorption capacity is 66.72 mg g⁻¹. It was found that F⁻ easily migrated from the solution to the surface of the DCP sample and generated new species by chemical reaction or the dissolution–precipitation mechanism. Li et al. [41] prepared mesoporous calcium oxide hollow spheres (MHS−CaO) with the wet impregnation method. When the pH = 3~11, the removal efficiency of MHS−CaO for fluorine can reach more than 80%, and it has good anti−interference ability. Cl⁻, CO₃²⁻, and SO₄²⁻ ions have little effect on the fluorine adsorption process. The reaction process of fluorine adsorption by MHS−CaO conformed to the pseudo−second−order kinetic equation, which belonged to chemical adsorption, and the maximum adsorption capacity was 181.96 mg g⁻¹.

2.2. Natural Adsorbent

2.2.1. Zeolite

Many natural adsorption materials were used to remove fluorine in water to seek a low−cost, simple, and environmentally friendly adsorption technology. The main component of zeolite is the positively charged aluminum colloid formed by the hydrolysis of alumina, which has a certain adsorption effect on the highly negative F⁻. Zeolite will also adsorb some Ca²⁺ and Mg²⁺ in the defluorination process and reduce the hardness of water simultaneously. However, there are impurities in natural zeolite, which block the pores and affect the adsorption, and the adsorption capacity is low. Therefore, scholars at home and abroad focus their research on zeolite modification. Javier A. et al. [46] combined two natural adsorbents, chitosan and zeolite, in different proportions to explore their fluoride removal effects. The adsorption results show a chemical interaction between the zeolite and the chitosan components, and the adsorption performance is better than that of the simple physical mixture of the precursor. The higher the content of zeolite in the composite material, the greater the dispersion of adsorption centers and the better the adsorption effect. Rahmani A. et al. [47] modified natural zeolites with Fe³⁺ and Al³⁺, respectively, and the results show that the zeolite modified by Al³⁺ had higher adsorption performance. The experiment also found that acidic conditions are more suitable for F⁻ removal. The presence of bicarbonate will increase the pH and reduce the adsorption site of fluorine, which is a disadvantage for the adsorption of fluorine. After the adsorption adsorbent is saturated, it can be regenerated using alum or aluminum sulfate solution.

2.2.2. Natural Minerals

At present, natural mineral defluorination materials mainly include alumina, bauxite, manganese ore, montmorillonite, clay, zeolite, and their modified products. Nabbou N. et al. [48] explored the fluorine removal mechanism of kaolinite and found that there is an electrostatic interaction between the positive charge on the surface of natural clay and the negatively charged F⁻, which makes the natural clay adsorb F⁻. Lu et al. [49] pointed out that the fluorine removal rate of kaolinite was higher at a low pH and high pH, and the adsorption capacity could reach 8.0244 mg g⁻¹ at pH = 1.5. In addition, when the simulated wastewater with a pH value of 13 was treated, the fluorine removal rate of the adsorbent could reach 82.44% when the dosage was 10 g L⁻¹, which was related to the change in the interlayer spacing of kaolinite under the action of strong alkali.

However, most natural materials are not very efficient in removing fluorine, and the ability to remove fluorine can be increased by modifying or compounding with various minerals. Wang et al. [50] used modified layered clay, layered carbonaceous minerals, amorphous silica, etc., as raw materials to prepare lightweight porous adsorbent materials to treat quartz−purified wastewater. The adsorbent is suitable for a slightly alkaline environment. When the dosage is 20 g L⁻¹, the mass concentration of F⁻ can be reduced from 3590.00 mg L⁻¹ to 118.47 mg L⁻¹ under suitable conditions for 90 min. The maximum adsorption capacity can reach 308.86 mg g⁻¹, the adsorption effect is good after regeneration by hydrochloric acid, and the fluorine removal rate remains high after five times of recycling.

2.2.3. Other Modified Natural Adsorbents

A preliminary study on the fluorine adsorption properties of pumice was conducted by Malakootian et al. [51]. The main components in pumice are SiO₂ and Al₂O₃ (mass fractions are 61.5% and 15.49%, respectively) and have a unique pore structure, so they have a certain adsorption performance for fluorine. Under the optimized reaction conditions, the adsorption efficiency can reach 85.75%. At present, there are few reports on the fluorine adsorption performance of pumice, but pumice has a wide range of sources and low prices, so it has certain development prospects. Sengupta et al. [52] prepared a new type of ferric hydroxide dispersed in a chitosan matrix (CHI−FCA) by dispersing the new ferric hydroxide particles and nano−calcium oxide in the chitosan matrix using the impregnation method. The characterization results show that the adsorption mechanism of CHI−FCA for fluorine may be an ion exchange mechanism combining physical adsorption and chemical adsorption. The F⁻ undergoes ion exchange with hydroxide ions in Fe(OH)₃ and reacts with calcium oxide nanoparticles to generate more stable CaF₂. Compared with literature reports on other related adsorbents, CHI−FCA has a synergistic improvement in F⁻ adsorption.

2.3. Nanomaterial Adsorbents

Nano−adsorbents have the advantages of small size, large specific surface area, short adsorption time, high adsorption capacity, and show excellent adsorption performance in the adsorption treatment of fluorine−containing wastewater [53]. At present, the nano−adsorption materials used to remove F⁻ from water mainly include metal−based nanomaterials, magnetic nanomaterials, carbon−based nanomaterials, and other nanomaterials. Fallahf et al. [54] prepared a TiO₂−modified β−cyclodextrin nanocomposite, and the functionalized TiO₂ nanoparticles can remove fluoride ions from water with a batch technique. Furthermore, such nanoparticles can be regenerated in NaOH solution and reused three times. Zaidi et al. [55] synthesized Ce−Al (1:1, 1:3, 1:6, and 1:9) bimetallic oxide nanoparticles using a simple co−precipitation method at room temperature for fluorine removal in aqueous solutions. The research results show that the cerium−aluminum bimetallic oxides with a molar concentration ratio of 1:6 show good fluorine removal ability. Fluoride ions were adsorbed on the surface of cerium−aluminum binary metal oxide nanoparticles within 1 h, and the maximum adsorption amount was at pH = 2.4. This nanomaterial exhibits extremely high adsorption capacity, and the adsorption capacity can reach 384.6 mg g⁻¹.

2.4. Carbon−Based Adsorbents

Activated carbon (AC) is a vital carbon material with an extremely high specific surface area and highly developed internal microporosity and functional groups, so it was used as an adsorbent for a long time to remove various pollutants in water. Bakhta et al. [56] prepared jujube residues into activated carbon (AC) and found that the priority chemical species for fluoride adsorption was in the order of Al > Co > Ca > Mg, and the dipping time of 2 h was the best value for the study of aluminum loading. At the same time, the influence of the impregnation operating parameters on the removal of fluorine was explored, and it was found that the removal rate could reach 80% when the impregnation time was 2 h, and the content of the loading aluminum was 5% (wt%). Thakur R. S. et al. [57] prepared modified pinecone−activated carbon as a fluoride removal agent, and the adsorption process depended on the chemical bond between the adsorbent and the surface of the material, and the maximum adsorption amount was 1.12 mg g⁻¹. In view of the shortcomings of traditional impregnation methods, such as low metal loading and large chemical consumption. Pang et al. [58] proposed a new method for preparing Zr−activated carbon fibers (Zr−ACF) by drop coating. Ion exchange and electrostatic attraction are the main adsorption mechanisms of Zr−ACF. The drop−coating method can load Zr onto the ACF more efficiently, and the fluorine adsorption capacity of the drop−coating method can reach 5.5 times that of the dipping method under the same conditions. The drop−coating method can improve the adsorption efficiency of the adsorbent and is a promising modification method for defluorination adsorbents. Currently, carbon−based adsorption materials are not put into commercial use on a large scale. In the future, we should focus on exploring methods to reduce the cost of their synthesis and modification.

2.5. Composite Metallization

Chaudhary et al. [59] explored the removal efficiency of fluorine using the chitosan Fe−Al−Mn metal oxide composite adsorbent. X−ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis showed that the exchange of fluorine with metal hydroxyl functional groups (Me−OH) might be the key factor for the adsorption of fluorine by the composite adsorbent material. It was also confirmed that the interaction between the polymer material and the inorganic composite material helps improve the composite material’s overall adsorption capacity. The maximum adsorption capacity for fluorine is 40 mg g⁻¹. Wang et al. [60] synthesized Fe−La composites that could simultaneously remove fluorine and phosphorus. The XRD and TEM mapping result indicated Fe−La composites were synthesized after the hydrothermal reaction (Figure 2a,b). The weakened band of M−OH, after the adsorption of fluorine shown in Figure 2c, indicated that the hydroxyl groups of metal oxides took part in the adsorption of fluoride. Figure 2d confirmed the adsorption of fluoride by Fe−La composites. The adsorption process of fluoride by Fe−La was a good correlation with pseudo−second−order kinetics and the adsorption isotherm (Figure 2e,f). The remove mechanism of fluorine in water using Fe−La composites was ion exchange (Figure 2g).

2.6. Layered Double Hydroxide

Layered double hydroxide (LDHs) can rebuild their layered structure after adsorbing various anions, have a certain potential for fluorine removal, and do not generate a large amount of sludge. These are considered to be the next generation of anion exchangers. Taking advantage of the memory effect of some LDHs, Liu et al. [61] used the calcined product of Mg/Al LDHs (CLDHMA) as a fluoride scavenger and studied the adsorption mechanism. The XRD spectrum indicated that the material recovered the layered structure of LDHs through structural reconstruction (Figure 3a), and the FT−IR results of prepared CLDHMA show competitive ion−fluoride ion entering the interlayer (Figure 3b). It is found that the adsorption process was followed by the pseudo−second−order kinetic model, which belongs to chemical adsorption, and the memory effect of the material is the main reason for fluorine removal (Figure 3c). The study of the adsorption isotherm of fluorine using CLDHMA was a good correlation with Langmuir models, indicating that the removal of fluorine using fluorine was a single−layer molecular adsorption (Figure 3d). Figure 3e shows that the adsorption mechanism of fluorine in water using CLDHMA was through a complete structural reconstruction of the material by utilizing the “memory effect” of calcined LDHs.

Li et al. [62] prepared Mg/Al−LDH composites with a core/shell/shell hierarchical structure. The mechanism of adsorption and removal of fluorine is that the anion exchange occurs between the interlayer nitrate and F⁻ in the solution, the metal hydroxyl group on the surface of the material is complexed with the surface of F⁻, and the fluoride is complexed and retained by the outer sphere. It was confirmed that the adsorption rate could be accelerated by introducing mesopores in LDHs. Therefore, modifying various structures of LDHs to improve their adsorption performance for the removal of fluorine in water will still be a future research direction.

2.7. Agricultural and Forestry Waste

Making agricultural and forestry wastes into adsorbents can create economic value and reduce waste disposal costs. Ibrahim et al. [63] modified citrus pomace, and the adsorption capacity was 12.6 mg g⁻¹ in the environment of pH = 4. With the increase in pH, the negative charge on the surface of the adsorbent also increases, which reduces the adsorption of F⁻ due to electrostatic repulsion, the enhancement of OH⁻ ions also increases the competition for adsorption sites and reduces the adsorption of F⁻. Choong et al. [21] used palm shell waste as raw material to make biochar modified with magnesium silicate and explored the fluorine removal performance of modified biochar magnesium silicate (MgSiO₃)−modified PSAC and MPSAC, and the fluorine adsorption capacities before and after modification were found to be 116 mg g⁻¹ and 150 mg g⁻¹, respectively. The characterization results show that the adsorption mechanism of F⁻ on MPSAC may be completed by the electrostatic attraction and ion exchange occurring on the adsorbent surface: F⁻ can be adsorbed on the surface of MPSAC through electrostatic adsorption or ion exchange reaction with available hydroxyl groups attached to Mg to generate Mg−F bonds. MPSAC has a strong regeneration performance, and the F⁻ removal efficiency of MPSAC is still higher than 60% after five cycles of NaOH treatment. Agricultural and forestry waste materials are easy to obtain and safe to use in the water fluoride removal process. More attention should be paid to exploring such bio−sorbents, and methods to improve the efficiency of fluoride removal and their large−scale application should be studied.

2.8. Comparison of Different Adsorbents

Metal−based adsorbents have the best treatment effect, and their composition and structure have certain controllability, which was always a hot topic in adsorbent research. However, the cost of metal−based adsorbents is relatively high at present, and coexisting anions easily interfere with fluorine removal. In addition to metal−based substances, other adsorbents can also improve the adsorption performance by loading metal ions. Natural mineral adsorbents are mostly made of natural materials and have advantages in terms of cost and safety, but their selective adsorption capacity for fluoride ions is low. The modification treatment helps to improve the adsorption efficiency of the adsorbent. Adsorbent materials based on agricultural and forestry wastes are generally modified. With the increasing environmental safety requirements, reusing agricultural and forestry wastes as fluoride removal agents is of great significance. Table 1 summarizes the adsorption capacity and adsorption conditions of various new and different adsorbents for defluorination.

3. Defluorination by Adsorption Method Coupled with Other Processes

3.1. Chemical Precipitation−Adsorption Method

The chemical precipitation method is suitable for treating high−concentration fluorine−containing wastewater and can be used as a primary treatment process. However, the treatment effect of the chemical precipitation method is unstable, and an adsorption device can be added as an advanced treatment section. Jiang et al. [80] took the feldspar beneficiation wastewater with a fluoride ion mass concentration of 1064 mg L⁻¹ as the treatment object, selected slaked lime industrial calcium chloride as the precipitant, and added seed fluorite to improve the removal effect. Magnesium–aluminum hydrotalcite was selected as the adsorbent, and the mass concentration of fluorine was reduced to 1.43 mg L⁻¹ after treatment, which met the emission standard. Adding the adsorption section can improve the stability of the effluent, and compared with the simple adsorption method, the process has a broader range of applications. The combined flow chart is shown in Figure 4, in which the formation of calcium fluoride has a great influence on the fluoride removal effect. During operation, it is necessary to pay attention to the effluent and sedimentation of the chemical precipitation section to avoid calcium fluoride adhering to the surface of the adsorbent and affecting the subsequent adsorption process.

3.2. Filtration−Adsorption Method

The filtration step is to clarify the water quality, and at the same time, it can also be pretreated for the subsequent adsorption work so that the adsorbent is in a more suitable adsorption condition. Xue et al. [81] aimed at the problem that the fluoride ion mass concentration (3~5 mg L⁻¹) in the external drainage of a coal chemical industry cycle in Henan exceeded emission standards and multi−media filtration, and activated alumina defluorination devices were added, the flow chart is shown in Figure 5. The multi−media filter uses anthracite quartz sand as the filter material to remove impurities and suspended solids in the water to improve the service life of activated alumina. After the activated alumina is saturated with adsorption, it is regenerated by soaking in an aluminum sulfate solution, and the mass concentration of fluoride ions in the effluent after treatment is ≤1 mg L⁻¹. The filtration–adsorption process is suitable for water quality and low fluoride ion concentration. In this process, attention should be paid to selecting filter material types and particle sizes to suit the adsorbent.

3.3. Super−Magnetic Separation Technology−Adsorption Method

The super−magnetic separation technology is to add magnetic seeds while adding a coagulant to the water. The non−magnetic pollutants in the water form magnetic flocs through the magnetic coagulation reaction, and the super−magnetic separator is used to strengthen the solid–liquid separation to remove fluorine. He et al. [82] used the super−magnetic separation–adsorption method to treat high−concentration fluorine−containing wastewater, and the fluoride ion mass concentration could be reduced from 816 mg L⁻¹ to below 1 mg L⁻¹. The super−magnetic separation process is fast and efficient, the subsequent sludge treatment is simplified, and the removal rate of fluoride ions can be significantly improved by adsorption. The precipitant in this process is CaCl₂, the coagulant is polyaluminum chloride (PAC), the coagulant is polyacrylamide (PAM), and the adsorbent is carbon−based apatite, and the fluoride removal rate can reach more than 95% under alkaline conditions, which saves the cost of coagulant. The flow chart is shown in Figure 6.

4. Conclusions

This paper summarizes the research on the use of the adsorption method to remove fluoride in water, and finds that when the natural adsorbent material is not modified, the adsorption effect is poor when treating fluorine wastewater due to low activity, and the amount of adsorption and fluoride removal can be effectively increased after modification, with less dosage and a high fluoride removal rate. The nano−adsorbent has strong activity and large adsorption capacity, but the disadvantage is that the solid–liquid separation is difficult because of its small particle size, which easily causes secondary pollution. The source of carbon−based materials is wide and easy to obtain, and the fluorine removal amount is greatly increased after modification and introduction of groups. Although the fluoride removal effect of industrial waste is not good, it can effectively increase its fluoride removal amount by compounding with other materials. Most of the research was aimed at improving the fluorine removal efficiency and adsorption capacity of the adsorbent, ignoring the research on the practical feasibility of the adsorbent to be used on a commercial scale, and the following points were put forward:

• (a). Recyclability. In the current research, there are only a few works of literature on the cyclic regeneration of adsorbents. More detailed regeneration studies should be conducted on adsorbents with lower regeneration efficiency to improve their fluorine absorption efficiency and regeneration efficiency while reducing the cost and waste generated by the adsorption process, thereby improving the economic viability of the process.

• (b). Suitability. A new type of adsorbent may show a high fluorine removal rate in laboratory studies, but it will be affected by many factors in practical application, which may significantly reduce its fluorine removal efficiency. Practical research on adsorbents, such as enhancing hydraulic conductivity, should be strengthened to enhance the practical value of these adsorbents on a commercial scale.

• (c). Environmental Safety. Research on the environmental safety of adsorbents should be strengthened because both of emerging nano−adsorbent materials and traditional F adsorbents may cause secondary pollution of environment.

Author Contributions

Conceptualization, M.−M.Z. and Q.W.; validation, L.S.N.K. and R.−N.Y.; formal analysis, R.−J.Z. and X.−Y.L.; investigation, R.−J.Z., X.−Y.L. and P.H.; re−sources, Q.W.; data curation, M.−M.Z. and Q.W.; writing—original draft preparation, M.−M.Z.; writing—review and editing, M.−M.Z., Q.W. and L.S.N.K.; visualization, M.−M.Z., Q.W. and L.S.N.K.; supervision, Q.W.; project administration, Q.W.; funding acquisition, Q.W. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

The authors would like to thank the anonymous reviewers for their valuable comments and suggestions on previous versions of this paper.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

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Figure 1. Adsorption of fluoride by CAZ. (a,b) Flow diagram of the preparation of CAZ; (c,d) the SEM−EDS result of CAZ after the adsorption of fluoride by CAZ; (e) FI−IR result of CAZ; (f) XRD result of CAZ; (g) adsorption isotherm of fluorine by CAZ; (h) adsorption kinetics of fluorine by CAZ; and (i) adsorption mechanism of fluoride by CAZ [40].

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Figure 2. Adsorption of fluoride by Fe−La composites. (a) XRD result of Fe−La composites; (b) TEM mapping result of Fe−La composites; (c) FI−IR result of Fe−La composites; (d) XPS result of Fe−La composites; (e) adsorption kinetics of fluorine by Fe−La composites; (f) adsorption isotherm of fluorine by Fe−La composites; and (g) adsorption mechanism of fluoride by Fe−La composites [60].

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Figure 3. Adsorption of fluoride by CLDHMA. (a) XRD result of CLDHMA; (b) FI−IR result of CLDHMA; (c) adsorption kinetics of fluorine by CLDHMA; (d) adsorption isotherm of fluorine by CLDHMA; and (e) adsorption mechanism of fluoride by CLDHMA [61].

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Figure 4. Flow chart of chemical precipitation–adsorption process.

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Figure 5. Flow chart of filtration−adsorption process.

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Figure 6. Flow chart of ultra−magnetic separation–adsorption process.

Table 1

Summary of adsorption capacity of different adsorbents for fluoride in water.

Adsorbent	Adsorption Capacity(mg g−1)	Concentration Range(mg L−1)	Adsorption Time(h)	pH	Adsorption Isotherms	Adsorption Kinetic	References
Ferric sulfate solution modified activated alumina	16.78	20	12	5 ± 0.2	Langmuir	pseudo−secondorder kinetics	[39]
Calcium zirconium modified acid activated alumina	216	30	3	6	Langmuir	pseudo−secondorder kinetics	[40]
Dicalcium phosphate	66.72	50	4	3~10	Langmuir	pseudo−secondorder kinetics	[45]
Nanoscale iron oxide	2.1834	5	2.5	3	Freundlich	pseudo−secondorder kinetics	[43]
Non−calcined synthetic hydroxyapatite	6.30	10	24	5	Langmuir	−	[46]
Kaolin	0.556	5	2	4~6	Freundlich	pseudosecond−order	[48]
Modified citrus biochar	12.6	5~30	3	4	Langmuir	pseudo−secondorder kinetics	[63]
Palm shell activated carbon powder	116	5~125	2	5	Langmuir	pseudo−secondorder kinetics	[21]
Magnesium silicate−modified palm shell−activated carbon powder	150	5~125	2	5	Freundlich	pseudosecond−order	[21]
Pine cone activated carbon	1.12	6	1	6.5~7.5	Langmuir	pseudo−secondorder kinetics	[57]
Zirconium modified activated carbon fiber	20	28.50	6	5	Langmuir	pseudo−secondorder kinetics	[58]
Mg Al−LDH composite material	28.51	10	24	3	Langmuir	−	[62]
Chitosan−Fe−Al−Mn composite adsorbent	40 ± 0.5	10	0.75	3.5~8.5	Freundlich	pseudosecond−order	[59]
Fe−La composites	27.42	10	−	4~6	Freundlich	pseudosecond−order	[60]
Ce−AlOOH	88	100	2	1~3	Freundlich	pseudosecond−order	[64]
Fe3O4 @ Ce(OH)3 nanomaterials	59.52	100	0.33	3.5	Langmuir	pseudo−secondorder kinetics	[65]
Modified activated magnesium oxide	88.50	5~10	5	4~9.5	Langmuir	pseudo−secondorder kinetics	[66]
Rice husk modified silica nanoparticles	28.50	10	1	8	Freundlich	pseudosecond−order	[67]
Citric acid modified activated carbon	1.65	5	4	2	D−R	pseudosecond−order	[68]
Camellia seed shell biochar	11.04	10	3	3~9	Langmuir	pseudo−secondorder kinetics	[69]
Modified crushed oyster shell biochar	10	10	−	4~10	Freundlich	−	[70]
Al(OH)3−hydroxyapatite nanosheets	194.2	200	2	3~9	Freundlich	pseudosecond−order	[71]
Ti(IV) modified plantain	17.2	2	1	2	Langmuir	pseudo−secondorder kinetics	[72]
Rice husk ash silica	25.641	−	1.5	5	Langmuir	pseudo−secondorder kinetics	[73]
Zr−modified grape pomace	7.54	19.91	1	3	Langmuir	−	[74]
Aluminum−modified jujube mud biochar	13.03	10	1	5~10	Langmuir	−	[56]
Modified FeCl3−lemon sheet−activated carbon	4.926~9.709	1000	4	6.6	Langmuir	pseudo−secondorder kinetics	[75]
Alumina powder	9.28	50~500	20	8. 1	Langmuir	pseudo−secondorder kinetics	[76]
Serpentine−clay composite particles	−	5~15	2	6.5~7.0	Langmuir	pseudo−secondorder kinetics	[77]
Iron−aluminum layered double hydroxide	87.09	−	2	3~10	Freundlich	pseudosecond−order	[78]
Cerium−iron bimetallic oxide adsorbent	60.97	10	0.67	2.9~10.1	Langmuir	pseudo−secondorder kinetics	[79]

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