Fluoridated Water, Effects and Green Removal Techniques

Abstract

Fluoride is a naturally occurring mineral released by rocks into water, soil and air. It elicits dual effects to organisms. Its beneficial effects are effected through water fluoridation to adjust fluoride content in drinking water to acceptable levels that are deemed to prevent tooth decay. Moreover, fluoride itself may be dangerous at high levels. Excessive fluoride causes dental fluorosis and skeletal fluorosis or even severe form of fluorosis known as “crippling skeletal fluorosis,” characterized by muscle wasting, immobility and neurological problems. Studies on fluoride levels are important so as to protect organisms from the adverse effect of high fluoride exposure. Various conventional and sensor-based technologies have been applied, leading to the detection of fluoride in water systems across the globe, with some regions reporting levels above the World Health Organization (WHO) limits. This necessitates interventions to reduce the levels of fluoride in drinking water. Green technologies are emerging as viable options for fluoride remediation since they are associated with minimal environmental contamination. Knowledge on fluoride in the environment is a key and therefore, this chapter provides an overview of fluorides, their monitoring in the environment, benefits as well as health effects and removal technologies that range from conventional to green technologies.

1. Introduction

The topic on water security and its impact on our daily lives is of utmost concern because many countries are already experiencing water scarcity due to a myriad of reasons. Ironically, water covers about 71% of the earth’s surface. Worth noting is that of the world’s total surface area, only 1% is covered with fresh water supply present in various natural habitats, such as glaciers, lakes, rivers, groundwater, among others. Of this, only 3% exists as fresh water; 2.5% is unavailable and only 0.5% of the fresh water is available for use by living organisms [1]. The problem of limited supply of fresh water is further exacerbated by the high demand for water due to population growth/explosion, economic growth and changing use patterns, climate change and pollution. The latter can be attributed to primary and/or secondary sources of pollution, which can be from natural and/or manmade sources. These water pollutants include heavy metals, anions, organic matter, persistent organic pollutants (POPs), pathogens and other emerging contaminants such as personal care products (PCPs). According to the World Health Organization (WHO), about 3.4 million people die annually due to water-borne diseases, with reports indicating that over 65 million of the total population in India have been or are affected by fluoridated water [2]. Whereas, the 2018 edition of the United Nations World Water Development Report (UN WWDR) indicated that by 2050, nearly 6 billion people will suffer from clean water scarcity [3].

Of the various pollutants, fluoride is particularly of great concern since it’s a naturally available, necessary element for human life but excess intake can lead to adverse health effects [4]. Water fluoridation is a common but debatable practice globally due to the dual nature of fluoride having both positive and negative effects. Undoubtedly, fluoride is an essential trace element that has been used from time immemorial in toothpastes and water as a way to strengthen tooth enamel, however, too much of it can cause harm to the living system. In 2018, 73.0% of the US population on community water systems (207,426,535 people) had access to fluoridated water [5]. Fluoridation involves the addition of fluoride to a calibration amount of 1 mg/L. Some of the salts used in fluoridation include fluorosilicic acid (FSA), silicofluorides (SiFs) and sodium fluorosilicate (NaFSA). Fluoridation of water using sodium fluorosilicic acid and sodium fluorosilicate is reported to accelerate leaching of lead pipes, which increases chances of lead accumulation in children [6].

Studies in several parts of the sub-Saharan Africa (South Africa, Uganda, Kenya, Ethiopia, Sudan, Ghana, Niger, Malawi, Tanzania, Benin and Nigeria, among others) have recorded high fluoride levels in their groundwater [7]. This has been attributed to the geogenic rock structures that are mainly composed of fluorite or fluorspar (CaF₂), topaz (Al₂SiO₄(F,OH)₂), apatite (Ca₅(PO₄)₃(F,OH,Cl)), biotite (K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂), muscovite (KAl₂(AlSi₃O₁₀)(F,OH)₂) and cryolite (Na₃AlF₆). Fluorite is mainly mined in China, Western Europe and Mexico giving 4 million tons of the annual world production of the fluorite mineral. Very high fluoride levels have particularly been reported in East Africa regions due to the volcanic activities. Several regions in Kenya, specifically the Rift Valley and Central Kenya, have recorded the highest levels of naturally occurring fluoride in the world; up to 23.5 mg/L, which is way above 1.5 mg/L the World Health Organization (WHO) limit [8]. This has been attributed to the leaching of fluoride containing rocks. Sadly, in many residential homes in Kenya, County water supply is not adequate and/or is not available, hence it is supplemented by borehole water which in most cases is contaminated with fluoride ions. This has in turn led to exposure of millions of Kenyans living in these regions to the toxic effects of high fluoride ingestion through the use of untreated groundwater. Fluoride (F⁻) is anionic form of fluorine, which is a pale yellow-green univalent poisonous gas, being the 13th most abundant element, constituting about 0.06–0.09% of the earth crust with varied distribution within the environment. Fluorine is a highly reactive element of the halogen family, thus not found in free elemental state but forms organic and inorganic compounds called fluorides. Fluorine is generated through the electrolysis of hydrogen fluoride (HF), finding applications in nuclear power generation by use of uranium hexafluoride (UF6), dielectrics in electric power systems by use of sulfur hexafluoride (SF6), manufacture of fluorinated agents such as chlorine trifluoride (ClF₃) used as an oxidizer, iodine pentafluoride (IF₅) used as a solvent and fluoride salts, such as sodium fluoride, fluorosilicic acid and sodium fluorosilicate used in toothpaste and water fluoridation as well as polytetrafluoroethylene (PTFE) used in nonstick cooking utensils, among many other uses. Table 1 presents a summary of the chemical properties of fluorine [9].

Atomic number	9
Atomic mass	18.9984 g/mol
Electronegativity	4
Density	1.8*10^−3/cm^−3 at 20°C
Melting point	−219.6°C
Boiling point	−188°C
Electronic shell	[He]2s^22p^5
van der Waals radius	0.135 nm
Ionic radius	0.136 nm (−); 0.007 (+7)
Isotopes	2
Energy of 1st ionization	1680.6 KJ/mol
Standard potential	−2.87 V

Table 1.

Selected chemical properties of fluorine.

Fluoride distribution in water, air, soils, rocks and plants is 1.0–38.5 mg/L, 0.1–0.6 μg/L, 150–400 mg/kg, 100–2000 mg/kg and 0.01–42 mg/kg, respectively [10]. Fluoride contamination of the water bodies can occur through both natural and anthropogenic processes. Figure 1 indicates the distribution of fluoridated water in various regions in the world. The map shows that Africa has a considerable amount of fluoride contamination compared to the rest of the continents in the world. This is followed by Australia and South America (8%), Asia and North America (2%) and Europe (1%). Further reports indicate that 15% of Africa has groundwater, with fluoride concentration exceeding the recommended World Health Organization (WHO) limit of 1.5 mg/L [12].

Figure 1.

A probability Map of naturally occurring fluoride in groundwater exceeding the WHO guideline of 1.5 mg/L [11].

Groundwater is a major source of water in most developing countries, with over 400 million Africans depending on it for various functions (domestic/industrial). Several water pollution and remediation studies have mainly focused on heavy metals and/or volatile organic pollutants with little emphasis on toxic anions. The study on groundwater fluoridation is of utmost importance because of the dual nature of fluoride. For example, 0.5–1.0 mg/L of fluoride is beneficial for healthy development of bones and teeth, however, a concentration greater than 1.5 mg/L has been linked with several fluorosis and nonfluorosis diseases. A number of side effects dependent on the level of fluoride intoxication have been documented in the literature, broadly divided into osseous tissue and soft tissue effects. These include tooth discoloration, tooth decay, skeletal weakness, neurological problems, liver, kidney and thyroid damage, high blood pressure, acne (fluoroderma), gene and nervous system destruction and seizures. Whereas, liver and kidney are the target organs affected by chronic exposure to fluoride, the anion does not cause seizers but aggravates the effects putting seizure patients at a higher health risk [13]. In a study by Manji et al. (1986), 102 children born and reared in an area of rural Kenya with 2 mg/L fluoride in the drinking water indicated 100% dental fluorosis, 92% of all teeth exhibited a Thylstrup-Fejerskov index (TFI) score of ≥4 and 50% of the children had severe enamel damage [14].

This study aims to highlight water fluoridation and distribution, a recent research on the possible negative effects of excess fluoride intake on diverse species, different techniques for the defluoridation of water, such as electro-coagulation, adsorption and membrane processes. Focus will be made on the recent advances of green-based technologies in the treatment of water containing fluorides.

2. A case study of fluoride distribution in south eastern region, Kenya

The south eastern region consisting of Machakos, Makueni and Kitui counties is considered to be arid and semiarid lands (ASALs) and due to the limited amount of rainfall, the region relies on groundwater for irrigation as well as drinking. The region is also reported to contain metamorphic and volcanic rocks that contribute to the groundwater fluoride levels [15]. Drinking water is considered safe for human consumption if the levels of chemical components and physical parameters are within the set standards by the Kenya Bureau of Standards (KEBS) and globally by the WHO. Levels of F⁻ ranging from 0.6 to 7.17 mg/L have been reported in Makueni County [16] and 4.2 mg/L in Makindu district [17]. Irrigation of crops using this water is a potential pathway for exposure to fluoride and studies have shown that kale and maize pose a health risk to fluoride-related diseases. Concentrations as high as 700 mg/kg in kale and 3.47 mg/kg in soils were reported. The high levels of fluoride in soil and crops were attributed to the presence of apatite, muscovite and biotite, which are fluoride-rich minerals [18].

The earliest reports on fluoride levels in groundwater in Machakos County showed that water in Machakos district was highly contaminated with fluoride with levels up to 16.2 mg L⁻¹ [19]. Levels as high as 9.36 mg/L, exceeding the WHO limit of 1.5 mg/L, were reported in borehole water from Mlolongo area [20]. Another recent study in Mumbuni reported fluoride levels at concentrations of 0.252 mg/L and 0.214 for wells and borehole, respectively [21]. From these studies, it is evident that populations in south eastern region are at risk of developing fluoride-related diseases, which is corroborated by a study done in Athi River subcounty, Machakos, where the prevalence of dental fluorosis in children of 12–15 years was reported to be at 93.4% [22]. A recent study by our research group on the quality of borehole, wells and tap water in Machakos County showed that the residents of the area were ingesting fluoride polluted water. Twenty-nine (29) samples were analyzed for their physicochemical properties including fluorides and results showed that all the selected borehole water sources were fluoride-polluted with concentrations ranging from 2 to 7.5 mg/L. These values are very high compared to those of well water and tap water as they also exceeded the acceptable limits for fluoride in drinking water. The high fluoride levels can be explained by the fact that Machakos and its environs have been reported to contain volcanic rocks which could be containing fluoride ions. These fluoride ions get into the waters from weathering of the rocks and through leaching. Only two tap water samples exceeded the acceptable fluoride limits, which could be because the other tap water samples were treated. The fluoride levels of samples collected from selected boreholes, wells and taps in Machakos County are as shown in Table 2.

Water samples
		Borehole water samples							Well water samples					Tap water samples
Parameters	Units	BS3	BS4	BS6	BS7	BS8	BS9	BS10	WS1	WS2	WS3	WS4	WS5	TS1	TS2	TS3	TS4	TS5
pH	pH scale	8.05	7.28	7.16	7.41	7.86	8.92		6.26	7.24	7.05	6.84	7.32	6.85	7.38		7.9	8.24
Conductivity	μS/cm	819	638	3626	1572	4919	1036	982	403	3318	928	2387	1799	3294	3294	202	199.9	205
Total hardness	mg/L CaCO3	206	160	880	380	510	ND	ND	66	124	252	260	312	86	86	32	60	40
Chloride	mg/L Cl^−	27.99	33.99	439.8	225.9	644.8	71.48	66.98	21.99	294.9	99.97	279.9	204.9	299.9	299.9	13.99	13.99	13.99
Fluoride	mg/L F^−	3	2.5	3.5	3	6.5	7.5	6	2.75	3.5	2	2.5	2	5.75	5.75	0.4	0.55	0.75
Sulfate	mg/L SO4^2−	0.96	0.082	1520	48.84	90.57	63.417	46.56	0.237	1252.85	70.57	5.94	46.57	1172.84	1172.84	10.49	5.56	9.48
Nitrates	mg/L NO3^−	_	25.52	26.4	30.8	44	41.8	9.68	33.44	36.96	88	66	48.4	34.98	34.98	1.892	3.52	2.2

Table 2.

Physicochemical parameters of water samples collected from different sources in Machakos County in Kenya.

BS = borehole water sample WS = well water sample TS = tap water sample

There are few documented studies done in Kitui County to investigate fluoride levels in different water sources, thus calling for further studies to be conducted in the County to conclusively determine the water quality status of the different water sources therein. A physicochemical study done by Nzeve and Matata, 2021 on water samples collected from Kalundu stream and the dam in Kitui County reported fluoride levels between 0.47 mg/L and 1.40 mg/L, which were below the permissible limits set by the WHO [23]. Another water quality assessment undertaken by Wambua et al., 2022 to determine the levels of selected bacteriological and physicochemical parameters in three potable water sources, namely Kiembeni borehole, Mwitasyano River and Kalundu dam, indicated acceptable levels of fluorides ranging from 0.94 to 1.40 mg/L [24]. The lowest concentrations were recorded in the water samples collected from the dams, attributed to the fact that the dam is not static but moves downstream to feed Kalundu River [23]. On the other hand, the highest F⁻ levels were recorded in the borehole water samples which could have arisen as a result of the leaching of salts from weathering rocks. While the values show that the water is fluoridated, the levels are within acceptable limits and are an indication that the people who consume water from these sources are ingesting fluorides in low concentrations that is beneficial for the prevention of dental caries [25].

3. Determination of fluoride in the environment

Monitoring of pollutants in the environment is important to mitigate the harmful effects of the pollutants to living organisms. Several approaches and techniques exist for the detection and determination of fluorides in water. These include: fluoride ion meter or ion-selective electrodes (ISEs) and ion chromatography (IC) [18, 26], inductively coupled plasma optical emission spectrometry/mass spectrometry (ICP-OES/MS) [27], Fluorine-19 nuclear magnetic resonance spectroscopy (F-NMR) [28], molecular absorption spectrometry (MAS) [29] and gas chromatography-mass spectrometry (GC-MS) [30]. Most of these conventional techniques are not easy to use, are bulky, expensive, require long analysis time and may not be very reliable. The use of chemical sensors, especially functionalized metal organic frameworks (MOFs), has found significant application in the detection of fluorides. This is due to the porous nature and tunability of MOFs with well-positioned ligands that complex with various toxic pollutants. For example, Alhaddad and El-Sheikh, 2021 reported on the use of a salen-cobalt metal organic framework (Co(II)-MOF) to detect fluoride in real water samples [31]. In their study, the photoluminescence spectrum of Co(II)-MOF posted a red shift upon interacting with fluoride ions, even in the presence of other interferents posting limit of detection (LOD) and limit of quantification (LOQ) of 0.24 μg/L and 0.72 μg/L, respectively. Other MOFs-related studies are summarized in Table 3.

MOFs	LOD	Response range	Selectivity	References
Urea-functionalized MOFs	—	—	H2AsO4^− and F^−	[32]
NH2-MIL-53(Al)	0.31 μmol/L	0.5–100 μmol/L	F^−	[33]
NH2-MIL-101(Al)	0.05 μmol/L	0.05–8.0 μmol/L	F^−	[34]
NH2-(UiO-66)	0.229 mg/L	0–50 mg/L	F^−	[35]
Cu(II)-MOF	1.203 ppb	—	F^−	[36]
Y(III)-MOF nanoplates	8.5 ppb	0.05–8.0 mg/L	F^− and pH	[37]

Table 3.

Comparison of various MOFs-based chemosensors for the determination of fluoride ions.

Chemosensors also suffer certain drawbacks such as the use of supplementary imaging equipment and/or toxic organic solvents. On the other hand, biosensors are more eco-friendly, for example, the use of cell-free biosensor templates has been reported with very low LOD [38]. Toward on-site monitoring of fluoride ions using portable sensors, recently a study by Mukherjee et al., 2020 reported a portable mobile device based on CeO₂@ZrO₂ core-shell nanoparticles’ colorimetric detection of F⁻ in water. The sensor was very sensitive to fluoride in the range of 0.1–5 ppm [39].

4. Benefits and health effects of fluoride

4.1 Benefits of fluoride

Water fluoridation is the adjustment of water fluoride levels to 0.7 mg/L and 1.0 mg/L, with the aim of reducing severity and prevalence of dental caries. Since water intake depends on climatic conditions, as per the WHO guidelines and recommendations in the areas with a warm climate, the optimal fluoride concentration in drinking water should remain below 1 mg/L, while in cooler climates it could go up to 1.2 mg/L [40]. Naturally fluoridated water is beneficial to the human body since fluoride ion is considered an essential element in the development of strong bones, healthy teeth, protection from caries and prevention of osteoporosis [41]. Optimally fluoridated water is deemed practical, economical, effective and safe to prevent dental caries. Fluoride ions present in saliva impede enzymes that breed acid-producing bacteria, hence the tooth enamel is not eroded away [13].

Fluoride ions can also replace the hydroxyl group in hydroxyapatite to form fluoroapatite strengthening the tooth enamel and inhibit the formation of plaque [42]. Children born in areas with fluoridated water derive dual benefits from the water since their salivary glands are exposed to fluoride before tooth eruption hence acting as fluoride reservoirs by continually producing the fluoride and after tooth eruption, fluoridated water ingested is the main source of fluoride reducing tooth decay and promoting strong teeth formation [43]. The use of sodium fluoride (NaF) and sodium monofluorophosphate (Na₂FPO₃) in high doses of 20–30 mg/day to increase trabecular bone density has been used successfully to treat osteoporosis, which is age related. Studies done to determine the prevalence of bone fractures in Chinese populations residing in rural communities of various fluoride concentrations in drinking water reported a decrease in bone fracture which could be associated to an increase in bone mass [44].

4.2 Fluoride toxicity

Climatic conditions, duration of exposure and concentration of fluoride ion determine fluoride toxicity [45]. Studies postulate fluoride concentration of <0.5 mg/L to cause dental caries. Elevated fluoride levels of ≥4 mg/L cause fluorosis. Pearly white flecks on enamel of the teeth surface due to ingestion of fluoridated water are called dental fluorosis. Elevated fluoride levels of 4 mg/L cause dental fluorosis, a defect characterized by lack of mineralization, increased teeth porosity and gap formation. Mild fluorosis is exhibited by white spots on teeth surface, while severe fluorosis is indicated by yellow, brown or black spots [46]. A population estimate shows that 1.7 million people in China and 1 million people in India suffer from dental fluorosis, with recent research suggesting an effect on 70 million people around the globe [41]. Chronic health impact of fluoridated water is indicated by skeletal fluorosis, a condition characterized by increased bone mass and bone density due to overexposure to fluoride concentrations of 5–10 mg/L. Stiffness of bones, joint pains, muscle weakness, periodic pain and chronic fatigue are mild symptoms of the initial stage of skeletal fluorosis. Hardening and stiffening of joints or development of Poker back are clear indications of the intermediate stage, while concentrations >10 mg/L lead to difficulty in walking due to stiff joints and bending bones [8, 47].

Daily consumption of fluoridated groundwater whose fluoride concentration is 3 mg/L leads to the occurrence of skeletal fluorosis in children and adults [47]. In all continents, at least 25 countries have been reported to have endemic fluorosis [48]. Ingestion of fluoride doses over a short period of time leading to poisoning, nausea, abdominal pain, bloody vomiting and diarrhea are effects on the stomach [42]. Collapse with paleness, weakness, shallow breathing, weak heart sounds, wet, cold skin, cyanosis, dilated pupils, hypocalcemia and hyperkalemia are then manifested and in 2 to 4 h death may occur [49]. Other possible effects include muscle paralysis, carpopedal spasms and extremity spasms [13]. Wild herbivores, domestic animal and birds exposed to highly fluoridated water and fluoride in air have also been reported to suffer from dental, nonskeletal and skeletal fluorosis [50]. Plants have the ability to absorb high amounts of fluoride ions from the soil, air, water and store them in the shoot and leaves and have been reported to inhibit photosynthesis and other processes [51]. Consumption of these plants poses a human health risk [50].

4.3 Documented health effects of fluoride in Kenya

In Africa, dental fluorosis has been reported in many countries including South Africa, Tanzania, Uganda, Ethiopia, Kenya, Sudan, Niger, Nigeria, Benin, Ghana and Malawi. On the other hand, few countries, such as Kenya, Senegal, Tanzania and Ethiopia, have reported the prevalence of skeletal fluorosis [7]. Lake Baringo, which is one of the fresh water lakes in Kenya, was found to contain 55 mg/L of fluorides in a research that was conducted in 2021 by the Kenya Marine and Fisheries Research Institute (KEMFRI). This is about 35 times the permissible limit set by WHO. Majority of the residents around the lake who use the water for drinking were reported to complain about back pains, brittle and brown teeth and others showed bowed feet and paralysis. All these are classic symptoms of both dental and skeletal fluorosis, which are primary effects of prolonged ingestion of fluoridated water [52]. A number of studies that have been done on fluorosis in Kenya have shown that 80% of severe forms of fluorosis have been observed in people living along the Kenyan Rift Valley [53]. A study conducted in two health care facilities in Nakuru, a town in the Kenyan Rift Valley, reported the prevalence of dental fluorosis in 86% of the sample population, where 54% showed mild to moderate dental fluorosis and 32% had severe dental fluorosis [25]. This could be inferred to the fact that the area lies along the Rift Valley system, which has been known for volcanic rocks bearing fluoride ions [53].

In a survey conducted by Demarchi et al., 2022 in Nairobi suburbs in Kenya, 80% of the sample population suffered from fluorosis [54]. This could be explained by the fact that the majority of urban population in Kenya are not able to access treated drinking water. As an alternative, most of them depend on groundwater which could be polluted with fluorides. While the belt of the Great Eastern Africa Rift Valley is known for fluoride pollution, other areas outside this region have also been found to be fluoridated. One such area is the Bondo-Rarieda Area in the Kenyan part of the Lake Victoria Basin (LVB). Wambu et al., 2014 found that that 36% of children living in this area, who consume water from ground sources, such as shallow wells, water dams and boreholes from the area, could be at the risk of dental fluorosis [55]. Most of the studies conducted indicated that children are at a higher risk of suffering from fluorosis. In addition, the socioeconomic status is also a factor since it is a determinant of whether or not one is able to access treated drinking water [53].

5. Conventional fluoride removal/remediation techniques

Because of the adverse effects of fluorides, various technologies have been applied to reduce the levels in water. The main methods of defluoridation are adsorption, ion exchange, precipitation, coagulation, membrane processes, distillation and electrolysis. The choice of the method depends on conditions like area, concentration and availability of resources. Among all these techniques, adsorption methods have more advantages because of their greater accessibility, economical use, ease of operation and effectiveness in removing fluoride from water to the maximum extent [56].

5.1 Adsorption-based techniques

Gupta and Ali, 2013 defined adsorption as a stage-by-stage exchange process applied in the removal of substances from fluid phases like gases and liquids [57]. The adsorption process can be operated in physical, chemical and biological systems. Proponents of this technique are of the opinion that in addition to being efficient and economical, it also produces high-quality water [58]. Fluoride removal by adsorption normally follows a three-step process, viz. external mass transfer where the fluoride ions migrate from the bulk solution to the adsorbent surface, adsorption of the fluoride ions onto the adsorbent and finally, the interchanging of fluoride ions that have been adsorbed exchange with the structural elements inside adsorbent particles or their transfer into the inner pores of the adsorbent (intraparticle diffusion) [59], as represented in Figure 2.

Figure 2.

Flowchart summarizing the adsorption process.

The most commonly used adsorbents are activated alumina and activated carbon. Adsorption techniques are more applicable for fluoride removal in rural areas because various materials which are good adsorbents can be easily found at low cost. Studies are still underway to design adsorbents from materials that are available in most rural areas, pH independent and less affected by the presence of other ions in water [60, 61]. Discussed are different categories of adsorbent materials.

5.1.1 Carbon-based adsorbents

Carbon-based adsorbents can be used as natural or modified activated carbon containing a complex pore structure that helps increase the surface area available for the adsorption process [62]. Carbon-based materials can be used to prepare activated carbon via chemical or physical activation. Activated carbon can be synthesized from agricultural waste, such as rubber wood, sawdust, coconut shell, sugar beet bagasse, rice straw, bamboo, rattan sawdust and molasses, among others [63]. The synthesis of activated carbon follows several steps including; dehydration and carbonization by heating the carbon material in the absence of oxygen and finally activation by the addition of chemicals [64]. Modified activated carbon has a higher adsorption capacity as compared to unmodified activated carbon adsorbents. Modification of activated carbon is done on the surface of the adsorbents using chemicals by loading different metal hydroxides on activated carbon or treatment with acid to improve its sorption capacity [65]. Activated carbon from plant materials, such as Khat (Catha edulis) and Vitex negundo plant, has achieved adsorption efficiency of 73 and 89.2% for the removal of fluoride [64, 66]. Also in this category are magnetic carbon-based adsorbents that utilize a magnetic source supported on the surface of carbonaceous materials. These are majorly used for the purification of industrial waste water. These adsorbents have an edge over the other adsorbents, in that they enable the use of alternative and intensified equipment such as the magnetically stabilized fluidized beds, thus, offering new alternatives for water treatment [67].

5.1.2 Calcium-based adsorbents

Calcium-based adsorbents are highly stable and have a high affinity for fluoride ions since calcium binds to fluoride to form new compounds, thus enhancing fluoride removal. Examples of calcium-based adsorbents include the dicalcium phosphate (DCP), which can be prepared by the hydrothermal method and sodium calcium borate glass derived hydroxyapatite (G-HAP) prepared by the immersion of sodium calcium borate glass in 0.1 M dipotassium hydrogen phosphate (K₂HPO₄) solution in the ratio of 50 g/L for 7 days approximately [68]. In a study done by Chen et al. (2022), calcined eggshells were modified by aging treatment making CaO the main active component. The modified calcined eggshells showed great improvement for fluoride removal by about 29.2% [69]. Modification of biochar from daily manure with calcium enhanced the adsorption efficiency eight times more compared to neat biochar [70]. Bone char is another calcium-based adsorbent that has been widely studied, as it possesses hydroxyapatite (HAp) (Ca₁₀(PO₄)₆(OH₂)) with a capacity to remove fluoride through ion exchange as shown in Eq. 1 [71]. Its use though is limited because water treated with animal bones is thought not to be esthetically acceptable [72].

$$C{a}_{10}{\left(P{O}_4\right)}_6{\left(OH\right)}_2+{\text{2F}}^{-}\to C{a}_{10}{\left(P{O}_4\right)}_6{\text{F}}_2+{\text{2OH}}^{-}$$E1

5.1.3 Aluminum-based adsorbents

The most commonly used sorbent is the activated alumina, which is highly porous aluminum oxide with a large surface area. Several factors, such as the characteristics of the feed water, affect the efficiency and effectiveness of active alumina in the removal of fluorides. Active alumina exhibits an increased charge at pH below 6 and is very effective in the removal of fluoride ions. However, its defluoridation effectiveness is diminished at neutral pH [12]. Dehydration of aluminum hydroxide (Al(OH)₃) at a temperature range of 300–600°C leads to the formation of a highly porous aluminum oxide (Al₂0₃) with a large surface area. The alumina is greatly preferred for defluoridation due to its discontinuous cationic lattice. Defluoridation by activated alumina is strongly pH dependent, which is a demerit because at pH > 7, silicate and hydroxide become great competitors of fluoride ions and at pH less than 5, activated alumina loses its adsorptivity for it gets dissolved in an acidic environment. Regeneration is required after every 4–5 months and efficiency in fluoride removal decreases after regeneration [73, 74]. Table 4 shows selected studies that have used alumina in defluoridation.

Type of alumina	Contact time (min)	pH	Efficiency (%)	References
Manganese dioxide coated	180	5.5	98	[75]
Mesoporous aluminum oxide loaded on calcium oxide	15	3–11.5	90	[76]
Alum and lime	240	5.5–7.5	18–33	[13]
Magnesia/alumina	140	6.3–7.3	85	[77]
Magnesia amended activated alumina	180		95	[78]
Lanthanum oxide impregnated granular activated alumina	30–480	3.9–9.6	70.5–77.2	[79]
Alumina modified expanded graphite	120	3–7	94.4	[80]
Red mud	20	5–7	70–80	[81]
Lanthanum impregnated bauxite	20–250	5–7	99	[82]
Bauxite	15–120	5–6	94	[82]
Pyrophyllite	20	2.8–4.9	85	[83]

Table 4.

Use of alumina in defluoridation and percentage efficiency.

According to Zhao et al. there are several ways of enhancing the removal of fluorides by an alumina-based adsorbent. The use of sulfuric acid for the activation of alumina-based adsorbents can increase the surface area of alumina and as a result improve their adsorptive characteristics. The use of alumina-based adsorbents has a few disadvantages which include their decreasing efficiency in defluoridation, which is affected by water hardness and their solubility in high fluoride concentrations due to the formation of monomeric aluminum fluoride and aluminum hydroxyl fluoride complexes [59].

5.2 Coagulation-precipitation (Nalgonda technique)

Nalgonda technique involves the addition of aluminum salt, lime and bleaching powder in sequence followed by rapid mixing, then coagulation, sedimentation, filtration and disinfection. Nalgonda technique utilizes alum as a coagulant with lime and bleaching powder as disinfectants, hence it is simple and economical for fluoride removal from drinking water. The optimum pH range for maximum removal of fluoride is reported to be between 5.5 and 7.5. Nalgonda technique was used extensively in Nalgonda village in India under the Rajiv Gandhi National Drinking Water Mission (RGNDWM), though it has limitations. This method removes approximately 18–33% of fluoride through floccule formation while about 67–82% remains as the soluble fluoroaluminate (AlFX) complex ion which results in aluminum toxicity in drinking water due to a rise in the permissible Al limit of 0.2 mg Al/L [13]. Sludge disposal is another environmental health problem associated with Nalgonda technique of defluoridation [84].

5.3 Ion exchange

Ion exchange involves the removal of fluoride in water through replacement of ions that are held loosely by ion exchange resins with fluoride ion. There are two types of ion exchange resins according to the functional group that is attached to the polymer matrix. Anion exchange resins exchange negatively charged ions (like fluoride), whereas cation exchange resins exchange positively charged ions from the solution [85]. These resins have small porous beads and are insoluble in most organic solvents and water, making them suitable for fluoride removal. The ion exchange process of fluoride removal involves passing water through the bed of ion exchange resins that are usually polymeric. Polystyrene anion exchange resin and basic quaternary ammonium type are used to remove fluoride according to the reaction 2:

$$Res-N{R}^{+3}C{l}^{-}+{\text{F}}^{-}\to Res-N{R}^{+3}{\text{F}}^{-}+C{l}^{-}$$E2

The fluoride ions substitute the chloride ions of the resin. When all the sites of the resin are fully replaced by fluoride ions, it is regenerated by passing brine solution through the resin. Chloride ions then substitute the fluoride ions recharging the resin, hence it can be reused. Selective defluoridation with an efficiency of 90–95% has been reported using an ion exchange cyclic process. Though ion exchange techniques are excellent in defluoridation, the resin and its maintenance are costly, whereas the treated water has a high chloride concentration with low pH [45]. Modifications of the polymeric resin to increase fluoride removal efficiency include impregnation with various metals, viz. zirconium, titanium and iron, among others. Anion exchange resin impregnated with zirconium reported increased fluoride removal at 60% efficiency [72].

5.4 Membrane technologies

Commonly used membrane-based technologies that include ultrafiltration (UF), reverse osmosis (RO) and nanofiltration (NF) are among the best available defluoridation technologies [85]. Fluoride removal efficiency using reverse osmosis is more than 99%. The major setback in using reverse osmosis is that the technique is uneconomical for average income family and rural populations because it requires special equipment, specialized training for operators and electricity for its operation. Reverse osmosis is therefore normally used in rural sectors of the developing countries where energy and competent human resource are often unavailable [45]. However, membrane technologies can run on renewable energy sources like photovoltaic or wind turbines, which are helpful in reducing waste generation and minimizing carbon emissions to the atmosphere [86]. The other disadvantage of RO membranes is that they can discard a considerable amount of the feed water as a reject stream [87] since they reject ions based on their size and electrical charge [88].

6. Emerging technologies and green defluoridation techniques

6.1 Emerging technologies

There are quite a number of new emerging technologies reported for defluoridation that involve distillation, precipitation and a combination of principles. The Crystalactor® is one of the emerging technologies developed by a Dutch company that consists of a pellet reactor filled with suitable seed material to produce pellets of the target material through crystallization. Defluoridation is achieved by the formation of reusable, extremely low water content and highly purified calcium fluoride pellets. The Crystalactor® is a compact and low-cost technology compared to conventional precipitation techniques. The technology is recommended for the treatment of high fluoride waters (>10 mg/L) and to attain concentrations below 1 mg/L, a second treatment is often required [49]. The Memstill® technology is another technology that is a membrane-based distillation idea developed by the Netherlands Organization for Applied Scientific Research (TNO). This technology improves ecology and economy of the existing desalination technologies for brackish and sea water. The technology successfully removes other anions such as fluoride and arsenic. Memstill® technology combines multistage flush and multi-effect distillation modes using one membrane. In this technology, cold feed water takes up heat in the condenser channel through condensation of water vapor, then a small amount of (waste) heat is added, and flows countercurrently back via the membrane channel. The added heat evaporates water through the membrane which is discharged as cold condensate. The cooled brine is extra concentrated in a next module or disposed. The Memstill® technology is cheaper compared to reverse osmosis and distillation. Memstill® technology for small-scale applications using solar heat is yet to be achieved [89].

The water pyramid is employed in tropical rural areas. It uses solar energy to produce clean drinking water from saline, brackish or polluted water [13, 89]. The technology also removes fluoride by use of a water pyramid with a total area of 600 m² placed under favorable tropical conditions, to produce about 1250 liters of fresh water per day. The rate of production depends on climate, temperature, cloud cover and wind activity. Solar energy drives the desalination, while the energy required for pressuring the Water Pyramid® is obtained using solar cells combined with a battery backup system. Intermittent peak demands in electricity are accomplished using a small generator. The use of a porous membrane leads to purification of water using solar energy similar to the water pyramid. In this technique, humidity in the evaporation chamber is increased by water which sweats through the membrane and evaporates on the membrane surface. Temperature difference leads to condensation of pure water on the cooler surface of the system [49]. Intensity of solar radiation determines the quantity of water. Brine has to be drained periodically to avoid crystallization. Seawater and water contaminated with heavy metals, oil residue, boron and fluoride can be purified if the pH range is 5–11.

Boiling with brushite and calcite is another technology where fluoridated water is boiled with brushite (CaHPO₄.2H₂O) and calcite (CaCO₃) to form fluoroapatite, which results in defluoridation [49]. In situ treatment methods for fluoride removal rely on the dilution of fluoride concentration of groundwater. They include check dams, percolation tanks and recharge pots. Groundwater from wells closer to the check dams, percolation tanks and recharge pots have their water fluoride levels reduced through recharge, hence their quality improves compared to water in wells faraway [68]. These methods mainly rely on recharge using rainwater, hence may be limiting in arid and semiarid areas which have little to no rainfall.

6.2 Green technologies for defluoridation

Whereas, remediation technologies have continued to evolve, green remediation strategies based on the 12 principles of green chemistry have taken a center stage since they offer significant potential for increasing the net benefit of the detoxification process by cutting costs, yielding greater efficiencies at a reduced and/or no negative environmental impact. Technologies based on biological remediation capability of plants and microorganisms meet this need and play a vital role in remediation of pollutants. Biological methods, although require more time, are far cheaper than the physical- and chemical-based strategies [90]. Thus, efficient, applicable, low-cost and sustainable water treatment technologies are required to keep up with the surging demands of clean water supplies.

Green remediation techniques have become more significant in the recent past due to less environmental impact, low cost and higher social appeal as compared to other chemical-based conventional techniques. Moreover, the integration of several green remediation techniques may have a higher synergistic effect on remediation efficiency. Figure 3 indicates the growing number of publications in the application of eco-friendly remediation processes of pollutants from water, and defluoridation methodologies as reported in the Chemical Abstracts Service (CAS) SciFinder search done on May 08, 2023 for the last 22 years. A search done under the keywords, “green and sustainable chemistr” and “green remediation of fluorides in water” and fluoridation studies, indicated 38,284, 341,241 and 8783 related publications, respectively. From these statistics, it is clear that the number of fluoride-related studies and green remediation studies has increased tremendously over the years.

Figure 3.

Yearly statistics on studies related to green chemistry and fluoridation.

Depending on the type of adsorbent used, adsorption would qualify as a green remediation technique. Adsorption is a viable remediation technique with a myriad of advantages, viz. cost, effectiveness, higher accessibility, simplicity of design, large number of adsorbents, local availability and cheapness. However, it poses various disadvantages such as pH adjustment and decreased adsorption capacity with repeated use of the regenerated sorbent. Adsorption techniques are by far the most commonly used in the developing world due to their simplicity in application and low cost. Adsorbents can be derived from various naturally occurring materials including indigenous minerals and plants, however, they can also be synthetically derived. Effects of major adsorption parameters, viz. pH, dose of adsorbent, rate of stirring, contact time and initial concentration of adsorbate on pollutant removal, must be studied to determine the optimum sorption conditions. As indicated earlier, adsorbents can be natural-based (e.g., red mud, chitosan, bauxite, soil, leaves and bark of trees), carbon-based (e.g., activated carbon), iron-based (e.g., iron oxide-hydroxide nanoparticles, calcium-based (e.g., crushed limestone) and/or alumina/aluminum-based (e.g., gibbsite containing materials). Table 5 presents a summary of selected naturally available adsorbents and their respective optimum sorbent doses for removal of fluoride ions in water. The majority of these studies were reported to follow first-order rate mechanism and Freundlich or Langmuir isotherms.

Adsorbent	Optimum sorbent dose	Optimum pH range	Removal efficiency (%)	References
Brick powder	0.2–2 g/100 mL	6.0–8.0	54.4	[91]
	0.2–2 g/100 mL	8.0	56.8	[92]
Activated bagasse	4 g/L	6.0	56.4	[61]
Sawdust raw			49.8
Wheat straw raw			40.2
Hydrated cement	1 g	8.2	91	[93]
	10 g/L	3–10	92.37	[94]
Mechanochemically activated kaolinites	2.5 g	3	90	[95]
Tamarind seeds	2 g/L	7.0	100	[96]
Blue-green algae, Phormidium sp.	4.5 g	3.5–4.5	60.0	[97]
Banana peel	2 g	5	86.5	[98]
Tea leaves loaded with Al/Fe oxides	2 g/L	4.0–8.0	85	[99]
Neem leaves powder	5.0 g/L	5.0–7.0	80	[60]
Moringa oleifera seed’s powder	2.5 g/L	6–7	92.3	[100]
Maerua subcordata root powder	200 mg/L	—	66.2	[101]

Table 5.

List of some natural adsorbents and optimum conditions for the removal of fluoride ions.

Obijole et al. studied aluminosilicate activated clay hydrothermally treated for fluoride and pathogen removal from water at pH 5.8. The results indicated a maximum adsorption capacity of 1.75 mg/g with a 53% fluoride removal at 25 °C [102]. The study by Cherukumilli et al. investigated the use of minimally processed (dried/milled) bauxite ore as an adsorbent for remediating fluoride-contaminated groundwater with doses of ∼10–23 g/L effectively remediating 10 mg of fluoride ions per liter of water [103]. Ayoob et al. reported on carbonized form of the biomass of water hyacinth (Eichhornia crassipes), after thermal activation at 600 °C, and observed a removal capacity of 4.4 mg/g [57]. Recently, Alhendal et al. reported a hybrid filtration cell (HFC), which utilizes limestone and activated carbons, for fluoride removal from water, with results demonstrating that fluoride could be completely removed from artificial water when the HFC is run at pH of 5.0, initial fluoride concentration of 30 mg/L and adsorbent dosage of 30 mg/L [104].

Phytoremediation is the direct application of green plants and their associated microorganisms to stabilize or absorb contaminants. A number of phytoremediation studies for removal of fluoride ions from water have posted impressive results with very high removal efficiencies. Aquatic plants, viz. Pistia stratiotes, Eichhornia crassipes, and Spirodela polyrhiza that grow in natural water bodies, have also been shown to remove fluoride ions from water with removal efficiencies of 19.87, 12.71 and 19.23%, respectively [105]. Fluoride-resistant bacteria play a major role in bioremediation and biotransformation of fluoride ions to convert them as less available and less toxic form and effectively reduce the F⁻ by binding them with ionophores [106]. Bioremediation potential by Providencia vermicola (KX926492) bacteria at 82% at pH 7, 37°C has been reported [107]. Elsewhere, encapsulated active growing blue-green algae, Phormidium sp., was used with a 60% removal efficiency from 3.0 mg/L initial fluoride concentration [98].

In a recent study by Maghanga et al. results reported indicated that Maerua subcordata root powder (MSRP) is a viable plant in defluoridation with ≈68% fluoride ion removal efficiency [101]. Additional related studies have reported the use of Moringa oleifera seeds which can be used for treatment of water containing fluorides as well as removal of turbidity through biocoagulation [100]. Other biocoagulants include neem leaves, tealeaves, tamarind seeds, banana peels, among many others [60, 96, 98, 99]. Suneetha et al. indicated effective adsorption of fluorides using active carbon derived from Vitex negundo plant [64]. Interesting studies on tamarind leaves and fruit pulp have shown its ability to facilitate the detoxification of fluoride from the body, in addition to the removal of fluoride ions from water (https://www.biologicalmedicineinstitute.com/post/tamarind-fluoride).

pH is a crucial component in adsorption studies, as high adsorption efficiencies are obtained at specific pH values, as shown in Table 5. Specifically, most adsorbents have reported a maximum fluoride removal at pH of 3–8. For example, whereas fluoride removal by tamarind is favored at neutral pH [96], at higher pH, hydrated cement defluoridation capacity remarkably reduces due to competition between F- and OH- [94]. Most of the adsorption remediation studies adopted Batch flow experiments to test the viability for real field water samples with optimization of other parameters such as initial concentration of fluoride, water temperature, contact time, rate of stirring and adsorbent dosage to reach the highest removal of fluorides.

7. Conclusion

Clean water and sanitation is one of the sustainable development goals (SDGs) that the world aspires to achieve by 2030. The challenge to this goal is the continued pollution of the environment by both natural and anthropogenic sources, thus limiting the available clean water. The technological advancement for addressing this issue has not been felt in all regions of the world, with the developing nations suffering the most. Pollution of water sources with fluoride is a major threat to the human well-being because it has been demonstrated that high fluoride levels result in the most severe forms of skeletal fluorosis that manifest in the form of disability. As a way of eliminating these pollutants, various technologies have been proposed, with green technologies being the most attractive because of the minimized negative impact to the environment. Such technologies, especially those that utilize naturally available materials, are attractive and can be easily adopted in a local setup for water purification toward addressing any form of fluorosis. As such, this calls for enhanced investment in the form of research to provide solutions to rural populations in developing nations where clean water supply from the local governments may be a challenge.

Acknowledgments

Veronica Okello acknowledges the Organization for Women in Science for the Developing World (OWSD) for their support through the Early Career Fellowship Programme grant (Award Agreement Number 4500429516). Elizabeth Ndunda is grateful to the Royal Society for their support through the FLAIR fellowship programme grant (Grant Number FLR\R1\192054).

Conflict of interest

The authors declare no conflict of interest.