Remediation Techniques Used for Removal of Fluoride from Groundwater: A Concise Review

Due to the increase in industrialization, climate change, and urbanization, the quality of drinking water is declining day by day. Fluoride levels in drinking water have seen a remarkable increase in several places across the world in the last decade. The accumulation of fluoride in an aqueous environment is caused by both natural as well as anthropogenic sources. The main concern regarding fluoride contamination is dental and bones related issues. According to WHO guidelines, the permissible limit of fluoride in drinking water is 1.5 mg/L. Fluoride contamination causes a detrimental effect on the health of the people. This article highlights the distribution of fluoride in India, as well as the many routes by which it enters the human body and probable fluoride metabolism inside the body. The numerous strategies used to remediate fluoride from water, like precipitation/coagulation, ion exchange, membrane separation, electro-separation, and adsorption are elaborated. Amongst these techniques, adsorption is one of the best-optimized techniques involving sustainable materials like biochars, which are cost-effective, highly efficient, and environmentally friendly adsorbents.


Introduction
Water is a precious natural source and forms the basis of the existence of life.Water possesses unique physiochemical properties, which attributes to its applications in various metabolic cycles in a living organism [1].Due to the excessive discharge of pollutants present on the surface of the water and their seepage into groundwater, water pollution is rapidly expanding in both developed and developing countries such as India [2].Due to the high concentration of fluoride in groundwater and surface water, contamination from fluoride poses a serious danger to the provision of clean drinking water [3].Due to their high electronegativity, they are often present as inorganic and organic fluorides in the environment.Depending upon the concentration of fluoride intake in the human body, it can be both beneficial and harmful.For example, a low concentration of fluoride (approximately 0.5 mg/L) is beneficial for the calcification of dental enamel, while concentrations above the permissible limit (> 1.5 mg/L) cause fluorosis, often referred to as dental and skeletal fluorosis [4].Water is the major and also the largest contributor to fluoride intake by humans [5].
Consistent consumption of fluoride-rich water causes several health concerns, including osteosclerosis, low haemoglobin levels, stomach pain, and muscle weakness (hardening and calcification of the bones).Additionally, it has been associated with decreased IQ, DNA structural abnormalities, neurotoxic effects, and the rare bone malignancy osteosarcoma [6].There are around twenty-five countries in the world where fluorosis is extensively distributed [7].According to the report of the United Nations International Children's Emergency Fund (UNICEF), it was found that fluorosis was common in seventeen states of India by the year 1999 [8].In India, presently several semi-arid areas of certain states that are found to be severely affected by high fluoride concentrations and are recognized as fluoride risk zones are Tamil Nadu, Assam, Chhattisgarh, Gujarat, Jharkhand, Karnataka, Punjab, Kerala, Maharashtra, Andhra Pradesh, Bihar, Haryana, Madhya Pradesh, Rajasthan, Uttar Pradesh, New Delhi and Telangana [9].
Fluoride is naturally present in groundwater due to fluoride-containing minerals present in the rocks, sediments, and soil.The leaching from fluoride-containing minerals such as sellaite, cryolite, fluorspar, fluorapatite, topaz, biotite, and tremolite is a significant contributor to fluoride contamination of groundwater.Anthropogenic sources which include discharge from glass-manufacturing industries, semiconductor industry, metallurgy, and fertilizer industry, underground storage tanks, runoff from agricultural fields, improper disposal of solid waste, etc. also add fluoride content to groundwater and soil as shown in figure 1 [10].

Figure 1. Sources of fluoride in an aqueous environment
Due to the alarming signs of fluoride accumulation in the human body, defluorination is essential to avert fluorosis and to make water safe for drinking purposes.There is focus on the fluoride metabolism inside the body, as well as the numerous strategies used to remediate fluoride from water, like precipitation/coagulation, ion exchange, membrane separation, electro-separation, and adsorption.The one of the most relevant methods for the fluoride removal is adsorption.The adsorption method has been broadly adopted as a cost-effective and efficient technique for the removal of fluoride due to easy availability of biomass for the synthesis of adsorbents, its easy handling and cost-effectiveness.

Fluoride Metabolism
Fluoride is primarily consumed through the oral route.80-90% of the fluoride that is consumed is absorbed through the alimentary tract.A portion of the fluoride consumed is absorbed in the abdomen, and the amount of absorption is influenced by the pH of the stomach.The stomach's pH affects the absorption of fluoride in the abdomen.Higher acidity (low pH) of stomach contents increases fluoride absorption [11].An increase in the concentration of acid in the stomach converts fluoride into hydrogen fluoride (HF), while fluoride that is not absorbed in the stomach is then absorbed in the small intestine, where it is not affected by pH [12,13].Numerous factors such as the presence of food inside the abdomen, the pH of the stomach, the chemical formula of consumed fluoride, calcium, aluminium, and magnesium compounds are responsible for the absorption of fluoride [14].The unabsorbed fluoride is defecated through excretion.Fluoride metabolism is depicted in figure 2.

Figure 2. Fluoride metabolism in the human body
Once absorbed, fluoride is circulated quickly throughout the body.Accumulation of fluoride in calcium-rich areas like bones and teeth is reversible and contains 99% of body fluoride.When the level of fluoride in plasma falls, it is released back slowly [15].Soft tissues contain approximately 1% of body fluoride.The amount of fluoride found in the bones also depends on the age and sex of an individual [16].The path through which fluoride is excreted from the body is chiefly via urine.Scientific studies have revealed that about 45% of the absorbed fluoride is excreted in the urine of children and 65% by adults [17].About 10% of the fluoride is excreted through faeces [18].

Defluorination methods
Reduced fluoride levels in drinking water are essential for reducing the negative effects of fluoride on human health.To keep fluoride levels in the human body at optimal levels, populations afflicted by fluoride poisoning must adopt alternative water sources and improve their nutritional standards.Surface water is greatly polluted with biological and chemical pollutants, their treatment and disinfection are essential before using it for drinking purposes.These cumbersome treatment processes associated with them lead to high operating costs and thus restrict their application in rural areas.Rainwater could prove to be a useful solution for providing a clean water supply for irrigation purposes at a low cost, however, rainfall is not uniformly distributed and requires high storage capacity in domestic households.The other substitute to combat fluorosis is to check and make necessary changes in the diet of the people.Taking an ample quantity of calcium reduces the chances of dental fluorosis [19].Besides, vitamin C also helps in reducing and preventing fluorosis to a certain level [20].
Conventional methods of fluoride remediation consume a lot of time, need a lot of effort, and are cost-intensive, due to which they are not suitable for scaling-up fluoride removal.Therefore, adopting an economical, environmentally friendly, and cost-efficient technology is necessary to reduce this environmental pollution.Defluorination is a practicable substitute and is usually the state-of-the-art method to reduce fluoride contamination in drinking water.It is "the downward adjustment of the level of fluoride in water to the optimum level".Various defluorination techniques implemented for fluoride remediation from groundwater include coagulation/precipitation, membrane separation (reverse osmosis, electro-dialysis, and nanofiltration), ion exchange, and adsorption [21][22][23][24][25][26][27][28][29][30][31][32][33][34].The practical applicability of these defluorination techniques relies on local needs, availability, fluoride concentration, type of source, and prevailing geographical and economic conditions.

Precipitation/coagulation
Fluoride can be easily neutralized with opposite charged particles.There are some known coagulating agents such as, alum, aluminium salts, lime, etc, that are often mixed in fluoride-contaminated water to precipitate out fluoride.
In this method, fluoride gets precipitated as insoluble fluorapatite by the addition of chemicals to raw water.Precipitation takes place by adding lime and coagulation takes place by adding alum [21].Adsorption of fluoride occurs on an aluminium precipitate surface.The other name of this process is the Nalgonda technique [29].Various coagulants can be used in place of alum, and aluminium salts, such as ferric hydroxide, and zirconium chloride [30].The primary advantage of this strategy is that it is very efficient and may be utilized in various practical ways.The chief drawback is that it is not a costeffective technique.

Ion-exchange
The ion exchange method makes use of a strongly basic anion exchange resin usually containing quaternary ammonium ion functionality with chloride ions.The fluoride ions present in the water get easily exchanged with chloride ions held on the resin owing to their strong electronegativity [3].The efficiency of this method can reach up to 95%.As understandable, the removal capacity decreases with an increase in pH, while the presence of bicarbonate ions presented the highest interference with fluoride ions.However, the main disadvantage of this technique is the high operational cost [23,31].

Membrane filtration
Membrane filtration is essentially a physical process where the contaminants are removed by applying pressure to overcome the osmotic pressure posed due to the semi-permeable membrane.There are processes like reverse osmosis, nanofiltration, and electrodialysis which are commonly used these days.
Reverse Osmosis (RO) as the name indicates works opposite of natural osmosis.It has become a viable option for supplying the public with clean drinking water [24].This technique removes fluoride more than 90% and provides steady water quality; however, it is expensive and even removes some important ions from our bodies.Over some time, the chances of developing microbial growth on the membrane are high, thus such processes require regular maintenance.Nanofiltration (NF) is another fluoride purification technique that decreases the hardness of water using high retention capacity membranes for charged particles like bivalent ions.The main difference between nanofiltration and reverse osmosis is regarding pore size.These nano filters consist of bigger pores due to which the solvent and solute can enter easily [25].As a result, less pressure is required to prevent the loss of some essential minerals.The technique is mainly used for drinking and wastewater treatment, but it is highly expensive [25].
Similarly, the electro-membrane process has been developed for the demineralization of water.Here, in place of pressure, an electric potential is applied to remove ions from water with the help of ionexchange membranes.These membranes permit the ions to pass through, but not water [26].Electrodialysis is a highly effective process but is unsuited for application in rural areas, and water with high salinity and total dissolved solids (TDS).

Electrocoagulation
Electrocoagulation (EC) removes the ions like fluoride from water using three basic processes, viz., electrochemistry, coagulation, and flotation [27].In this procedure, electrodes that have been submerged in water are subjected to direct current.The electro-dissolution of the sacrificial anode and the reduction of water takes place yielding coagulant species.These coagulant species then remove the contaminants by forming flocs which are eventually separated from the water.As a result, less sludge is produced because no coagulants or flocculants are used in the treatment process.Figure 3 illustrates the general mechanism which occurs during the EC process involving the production of coagulants, coagulation, flocculation, precipitation, and/or flotation [31].During an EC process, the electric current which flows through the electrodes immersed in the solution causes the dissolution of the anode to produce Al 3+ ions.At the same time, the cathode produces hydroxide (OH − ) ions and hydrogen (H2) gas from the water through the reduction reaction [31].The electrode reactions which occur during the process are shown in equation (5) (which occurs at the anode), equation ( 6) (occurring at the cathode), and equation ( 7) (in the bulk solution) [31].

Figure 3. Fluoride removal via electrocoagulation
The H2 bubbles float in the reactor.The Al 3+ ions further react with OH -ions as shown in Eq. ( 7) to form a solid Al(OH)3 precipitate (Figure .3).These precipitates combine with fluoride to form flocs.The Al(OH)3 floc is supposed to adsorb F − strongly as shown by equation ( 8): Finally, these particles are removed by sedimentation or flotation.It is a good technique for fluoride removal but is dependent on electricity.

Adsorption
Adsorption is a process in which atoms, molecules, or ions get attached to a surface [22,28].Adsorption occurs in pores and depends on various factors such as temperature, pressure, surface area, nature of adsorbate and adsorbent, particle size, contact time, etc.In this method, the polluted water is allowed to pass through a contact bed containing defluorination material that adsorbs fluoride either by ion exchange or chemical and physical forces [32,33].The mechanism involved in the process of adsorption consists of transportation, adsorption of fluoride ions, and intraparticle diffusion.Adsorption is a frequently used method for fluoride removal because it is cheap, easy to operate, environmentally friendly, and highly efficient but it is pH-dependent and produces a large amount of sludge [34].

Adsorbents
A good adsorbent should be inexpensive and easily regenerated.Its capacity to adsorb fluoride should be high.Various adsorbents which are reported from the scientific studies conducted by different researchers include rice husk, calcite, activated sawdust, activated alumina, activated carbon, activated coconut shell carbon and activated fly ash, groundnut shell, coffee husk, magnesia, serpentine, zeolites, bone charcoal, nanoparticles, etc. [35].Industrial products have also been tested for fluoride removal because these materials are cheap and are available in considerable quantities, but adsorbent regeneration is difficult.Though a variety of commercial adsorbents are available, their widespread use is often constrained due to inadequate accessibility and high cost.Therefore, researchers [36] have developed alternative low-cost yet efficient adsorbents by using agricultural and industrial wastes.
Biochar is a heterogeneous low-cost, environmentally friendly, and effective adsorbent that is obtained from the thermo-chemical decomposition of biomass.It is used for removing pollutants because of its high adsorption capacity, large specific surface area (SSA), pH ranging from neutral to alkaline, excellent ion-exchange capacity, micro-porosity, and high amount of fixed carbon [37].Due to industrialization, urbanization, an increase in population, and making eco-friendly surroundings, the utilization of biochar is becoming more significant these days.The characteristics and composition of biochar depend upon temperature and the type of biomass used as feedstock.Biochar can be prepared from different feedstocks such as crop residues, agricultural wastes, kitchen wastes, municipal solid wastes, industrial wastes, and sewage sludge.The product charcoal obtained can be used as a source of fuel.Biochar can be used to reduce environmental pollution.Feedstock and pyrolysis temperature mainly affect the presence of acidic as well as basic functional groups present on the surface of biochar.In general, the oxygen-containing functional groups are acidic like carboxyl (-COOH) while the groups containing lone pair of electrons are basic and do not adsorb fluoride [38].
In comparison to the various low-priced methods available for water treatment, biochar technology has numerous advantages that make it attractive and suitable for use in various parts of countries like India.It is an environmentally benign, cost-effective, and renewable adsorbent prepared from biomass, thus apt for low socioeconomic communities, removes all types of biological, chemical, and physical contaminants from water, helps to sequester carbon in the soil, and reduces greenhouse gas emissions.The preparation and applications of biochar are represented in figure 4.

Figure 4. Preparation and applications of biochar
It has been found that biochar has the potential to remove various pollutants such as heavy metals, fluoride, nitrate, and phosphate [39].Activation of biochar increases the porosity, adsorption capacity, and surface area of biochar [40].
On the whole, the process of adsorption is user-friendly, cheap, eco-friendly, does not require a high level of scientific and technical expertise, and removes approximately 90% of fluoride but is highly dependent on pH and requires regeneration after every 4-5 months.The advantages and limitations of various methods of fluoride removal are given in table 1.
• No chemicals are used.
• Water quality remains the same.
• Ions do not interfere.
• Essential ions are removed from the water.• Huge quantity of water is wasted as saline solution. [24] Nano-filtration • High productivity.
• Chemicals are not required.
• No interference of ions is observed.
• Susceptible to membrane degradation.
• Even the useful ions are removed from water. [25] Electrodialysis • Highly efficient.
• Low chemical requirement.
• More resistant to fouling.
• Requires high pre-treatment.
• Useful ions are not removed.
• Various pollutants can be removed.
• Sacrificial electrodes have to be replaced regularly.• A large amount of electrical power is required.• Aluminium can be released in water during the process. [27]
• Easy to be operated.
• Regeneration is possible.
• Most commonly employed method for fluoride removal.• Various adsorbents are easily available.
• A high amount of sludge is produced which is a big problem.• pH dependent and works best in the pH range of 5-6.

Challenges and future prospects
Water is a vital natural source for sustaining life.An increase in urbanization and population has led to the overexploitation of groundwater.High fluoride concentrations have been reported in various parts of India, which is a matter of serious concern.In India, especially the rural population which has low socioeconomic status is unaware of the adverse effects of fluoride on human health.Less coordination and association between the local population and groundwater management authorities, insufficient medical facilities in the fluoride-affected regions, high cost involved in the treatment of fluoride remedial techniques, shortage of low-cost and area-based fluoride remedial methods, variation in fluoride concentration due to climate change, and incomplete information regarding fluoride affected areas are some issues of concern.Therefore, there should be proper utilization of groundwater resources in the future.Government should develop certain policies and guidelines to label risk zones having fluoride-contaminated water.Strict penalties should be imposed on the people who disobey the rules regarding the contamination of groundwater resources.Various awareness programs should be organized to educate the rural population about the harmful effects of fluoride.
Fluorosis continues to be a common problem.High fluoride intake may result in dental, skeletal, and non-skeletal fluorosis both in humans and animals.Consequently, fluoride removal from drinking water is mandatory so that every individual can be provided with safe water for drinking and cooking purposes.
In this article, we have highlighted the distribution of fluoride in India, as well as the many routes by which it enters the human body.We have also enlightened the fluoride metabolism inside the body, as well as the numerous strategies used to remediate fluoride from water, like precipitation/coagulation, ion exchange, membrane separation, electro-separation, and adsorption.At last, we have discussed the most relevant method for the fluoride removal, viz.adsorption.The adsorption method is more suitable because it is cheap, efficient, easy to operate, convenient, environmentally benign, and can be easily implemented in rural areas.

Table 1 .
A comparison of different defluorination techniques