Arising awareness of health hazards due to long-term exposure of fluoride has led researchers to seek for more innovative strategies to eliminate excess fluoride in drinking water. Fluoride-bearing chemicals in both natural and anthropogenic sources contaminate drinking water, which mainly cause for human fluoride ingestion. Hence, developing sustainable approaches toward alleviation is essential. Among many emerging techniques of defluoridation, nanotechnological approaches stand out owing to its high efficiency, and hence, as in many areas, nanotechnology for excess fluoride removal in water is gaining ground compared to other conventional adsorbents and process. The present review focuses on some of the advanced and recent nanoadsorbents including their strengths and shortcomings (e.g., CNT, LDH, graphene-based nanomaterials, and magnetic nanomaterials) and other processes involving nanotechnology while discussing basic aspects of hydrochemistry of fluoride and geological conditions leading for water fluoride contamination. Considering all the findings in survey, it is evident that developing more sustainable techniques is essential rather than conducting batch-type experiments solely.
Being an integral need of human survival, the quality and safety of water, particularly drinking water, is thoroughly concerned. The chemical composition of water is one of the basic factors monitored to make sure the access to safe drinking water of any community. Chemical composition depends on both anthropogenic sources and geogenic sources. While there are several chemical factors governing the quality of water, the concern over maintaining optimum fluoride concentration is gaining its ground in today’s world. Both developed countries and nondeveloped countries are now facing the repercussion of excess fluoride in drinking water. In developed countries, fluoride is deliberately added into drinking water systems to prevent dental carries and dental fluorosis; an issue which has been debatable for a long time since some authors have reported an opposite trend [
For many years, various types of methods have been studied for excess fluoride removal in water. A lot of research has been conducted on adsorbents while few studies have concentrated on filtration techniques as reverse osmosis and nanofiltration and electrochemical methods as capacitive deionization. The most traditional approach was the elimination through precipitation. However, all these methods have their own disadvantages and advantages when using in real-world applications.
Precipitation involves using aluminum sulphate and lime or calcium and phosphate compounds to precipitate out dissolved fluoride in water. Although it is widely used in community wise, the usage of high aluminum content is detrimental to human health. In addition, the efficiency of this removal method is not that high (around 70%) [
The other most diversified method can be given as adsorption in which various kinds of innovative adsorbents have evolved until today. Enumeration can be given as oxide materials, carbonaceous materials, ion exchange resins, biopolymer-based materials, soils, clays, and other low-cost materials. Many researchers have reported on different strategical endeavors on excess fluoride removal in water. Adsorption can be defined as the accumulation of a substance or material at an interface between the solid surface and the bathing solution [
Porosity, surface area, and the nature of the surface are the governing factors of adsorption. Proper physical and chemical modifications enhance the adsorption capacity of many adsorbents. Nanotechnology plays its role in this regard, and immense research work has been reported on utilizing nanotechnology in water remediation including removal of excess fluoride. Thus, this paper discusses various types of novel nanotechnological endeavors and their basic physical, chemical parameters, and the degree of possibility to use those in real-world applications.
However, most of these adsorbents have been assessed in batch-type experiments, and only few have been assessed on continues type experiments. The batch experiments reveal the design parameters as adsorption rate constant, equilibrium constants, mass transfer coefficient, etc. To evaluate the industrial applicability of the adsorbents, carrying out continuous type methods as fixed-bed adsorption process is essential [
Fluorine, having nine protons in its nucleus, is the most electronegative and reactive element in the periodic table. Thus, this pale yellow-green, irritant, and odorous gas does not exist in its natural state but instead forms ionic states or compounds with other chemicals in minerals as fluorspar, fluorapatite, and cryolite. When focusing on the abundance on earth’s surface, fluorine is ranked 13th [
While Sivasankar et al. [
Majority of research done on excess fluoride contamination in groundwater has suggested the corresponding factors as presence of fluorine-bearing minerals and rocks, circulation of water around rocks, withering of fluorine-rich rock fragments, long-time exposure, good depth, and other hydrological conditions as paleo groundwater circulation by fluoride-rich water near fault zones of granite terrain and upward flow of groundwaters revealed by isotope studies [
Fluoride is released from various types of human-involved processes. Usage of fertilizers as NPK, potash, and superphosphate releases considerable amounts of fluoride into soil and water. Waste management in this industry and others including aluminum, copper, and nickel production also release fluoride into the environment. Controlled addition of fluoride to dental products, food products, and pharmaceutical products also contributes to accumulation of fluoride in the environment. Other than these sources, fluoride accumulation in the environment occurs due to various industries that utilize fluorine-containing compounds. Hydrogen fluoride is used in manufacturing of aluminum and chlorofluorocarbons. Other fields where fluorides are involved are electronic industries, cleaning glass, tanning leathers, brick and tile work, petroleum industry, and uranium isotopes separating. For drinking water fluoridation, sodium fluoride is widely used. Other than that, it is used as an insecticide. Sodium hexafluorosilicate is also used in fluoridation of drinking water supplies. Calcium fluoride, on the other hand, is used in glass, ceramics, and fluorescent lamp industry, and sulfurehexafluoride is used as a gaseous dielectric medium in the electrical industry.
Fluoride can be a double-edged sword owing to both its beneficial and harmful impacts on human body. Fluoride is thought to reduce tooth decay although there are several controversial facts regarding the relationship between the level of fluoride and tooth decay. Research regarding the impact of fluoride on tooth decay suggests three ways of how fluoride mitigates the process of tooth decay. Development of the chemical structure of the enamel at the development stage, making the enamel more resistant to acid attacks, and enhancement of remineralization with an improved quality of crystals are the three proposed ways [
The WHO recommended value for fluoride content in drinking water is 1.5 mg/L while 1 mg/L is beneficial for human for preventing dental caries [
Being situated in the tropical band, Sri Lanka is inherited mainly with three climatic zones comprised of a dry zone with an annual rainfall of less than 1500 per annum, a wet zone where it has been reported as more than 2500 per annum, and an intermediate zone. The tropical climate of the country facilitates leaching out of fluoride from fluorine bearing rocks due to intense withering. Although the types of rocks and minerals are nearly same in both dry and wet zones, notably different climatic conditions and hydrological conditions have resulted a higher percentage of fluoride reported in the dry zone [
Among many methods documented in the literature, this paper targets for various kinds of possible nanotechnological approaches in removing excess fluoride in groundwater. Nanotechnology is a technology involving the manipulation of matter having at least one dimension in size range of 1 nm–100 nm. Due to their size, they are gifted with unusual chemical, physical, thermal, and electrical properties when compared to macro and microcounterparts. Principles such as high aspect ratio and quantum confinement are the underlying reasons for these discrepancies [
Layered double hydroxides which are also known as hydrotalcite-like compounds comprise of two-dimensional brucite-like positively charged layers. Excess positive charge occurring on each layer is attributed to cation replacement and oxidation in octahedra. Between these kinds of two layers, exchangeable anions are located to compensate the effect of excessive positive charge on each layer and this arrangement gives its layered type nature. Depending upon the nature of cations and the ratio between divalent and trivalent cations, a variety of LDHs are created [
In the study of Elhalil et al. [
Magnetic nanoparticles, used for versatile functions in many industries, possess variety of unique properties. Electrons in orbitals of atoms possess unique magnetic moments, and it gives rise to different magnetic properties of different type of materials. While paramagnetic state exhibits a temporary magnetism in presence of an external magnetic field (a small net crystal magnetization), ferromagnetic and antiferromagnetic states exhibit magnetism owing to individual magnetic moments without an external magnetic field below Curie temperature (for antiferromagnetic, net magnetic moment is zero). Ferrimagnets also possess magnetization [
Riahi et al. [
An efficient and novel adsorbent (superparamagnetic Fe3O4@Polyprrrole) in an electromagnetic coupling system has been evaluated for defluoridation in Weng et al.’s study [
Carbon nanotubes (CNTs), an allotrope of carbon, possess unusual characteristics which are valuable in nanotechnology. Further, owing to their capability of being functionalized by other chemicals and one-dimensional periodic structure, properties of CNT can easily be manipulated. Carbon nanotubes are best described as structures having cylindrical graphene structures capped with fullerinoid endcaps [
An adsorbent made by single-walled carbon nanotubes (SWCNTs) for excess fluoride removal was reported in a recent study [
Another study [
When an external pressure is applied to counteract the osmotic pressure through a semipermeable membrane, it allows impurities (certain solutes) to retain (concentrate) and water (permeate) to pass through; this phenomenon is called reverse osmosis (molecular separation from aqueous medium). In nanofiltration (NF), the membrane possesses nanometer-sized pores (1–10 nm) which are larger than pores in membranes used for reverse osmosis (RO). It allows divalent ions to remain and monovalent ions to pass through. Ability of processing large volumes and producing high output are some of the benefits of using nanofiltration. Ayoob et al. [
Graphene, a single layer of hexagonally arranged carbon allotrope (SP2), is now being widely used to construct sensors, transistors, and catalysts and deployed in environment pollution management [
Summary of fluoride removal capacities by different adsorbents.
No | Adsorbant | Special remarks | AC (mg/g) | Studies conducted | Ref |
---|---|---|---|---|---|
1 | MgO-MgFe2O3 binary oxide anchored on to GO (graphene oxide) | Adsorption mechanism: |
34 at pH 6.0 (303.15 K) | Pseudo-second-order kinetic equation |
[ |
|
|||||
2 | rGO/ZrO2 (continuous fixed bed column study in upward flow mode) | Hydrothermal route |
45.7 at pH 7 | Break through curve: Yoon–Nelson model (continuous bed type) | [ |
|
|||||
3 | 3D yttrium based-graphene oxide-sodium alginate hydrogel | Macro structure adsorbent which involves graphene oxide. A sol-gel process was used |
288.96 at pH 4.0 | Langmuir isotherm model |
[ |
|
|||||
4 | Hydrous iron (iii)-aluminum (iii) mixed oxide-graphene oxide composite (HIAGO) | Synthesized by a chemical precipitation method |
27.8 at pH 5 (318 K) | Best matched with Langmuir isotherm model and the pseudo-second-order kinetics endothermic reaction | [ |
|
|||||
5 | Hierarchical AlOOH@ reduced graphene oxide hybrid | Supercritical carbon dioxide assisted synthesis (eco-friendly) |
118.7 at pH 6.5 (high AC at lower F− concentration) | Best matched with Langmuir isotherm model and the pseudo-second-order kinetics | [ |
|
|||||
6 | Magnetic iron-aluminum oxide/graphene oxide nanoparticles | One step method of co -precipitation method |
64.72 at pH 6.5 | Best matched with Langmuir isotherm model and the pseudo-second-order kinetics. Saturation magnetization: |
[ |
|
|||||
7 | Al2O3-Fe3O4-expanded graphite nano-sandwich structure | Coprecipitate method resulting 60–80 nm Fe3O4 cubes and 20–50 nm Al2O3 spread on graphite surface |
2.19 at pH 2–10 | Best matched with Langmuir isotherm model and the pseudo-second-order kinetics. Saturation magnetization: |
[ |
|
|||||
8 | Hydrous CeO2-Fe3O4 decorated polyaniline fibers nanocomposite | A coprecipitation deposition on pre-synthesized polyaniline fibers over a large pH range (3–10) the removal rate was nearly constant giving the maximum as 94% at pH 3 | 93.46–117.64 over a pH range 3–10 | Best matched with Langmuir isotherm model and the pseudo-second-order kinetics. Exothermic reaction. Spontaneous reaction | [ |
|
|||||
9 | Activated carbon@SnO2 (biosorption process) | Activated carbon was synthesized via pyrolysis of sawdust and activation with phosphoric acid. A mixture of activated C and stannous chloride was sonicated followed by NH3 addition to obtain a gel like product. After washing the obtained product was calcined at 673 K for 2 hCrystal structure; rutile tetragonal tin oxide SnO2 nanoparticles embedded on amorphous carbon matrix (12 nm) |
4.60 at pH 6.5 | Best matched with Langmuir isotherm model and the pseudo-first-order kinetics. Endothermic and spontaneous reaction. Chemisorption may happen. Greater regeneration capability (removal % remained around 90% after 3 consecutive cycles) |
[ |
|
|||||
10 | Magnesium oxide entrapped polypyrrole hybrid nanocomposite | MgO nanoparticles were synthesized with constant mechanical stirring (1000 rpm) using MgCl2 and NH3. Polymer nanocomposite was synthesized by chemical oxidative polymerization using FeCl3 as an oxidant. | 4.32 | Best matched with Langmuir isotherm model fluoride removal is endothermic and spontaneous nature. |
[ |
|
|||||
11 | Mesoporous |
Extraction of Al from kaolinite using sulfuric acid instead of hydrochloric acid. Synthesis of mesoporous γAlOOH was performed in a hydrothermal synthesis using hexamethylenetetramine as the hydrolyzing agent |
Not given | BET surface area could be controlled from 6.3 to 192.5 m2·g−1 when varying reaction time and temperature. | [ |
|
|||||
12 | Chitosan-Fe-Al-Mn metal oxyhydroxides composite | Hydrous mixed-metal oxyhydroxides, loaded chitosan composite was made using laterite clay and waste from steel (low-cost materials) industry via coprecipitation method |
40.0 ± 0.5 at pH 6.7 | Best matched with Freundlich isotherm model (multilayer adsorption on heterogenous surface sites) and the pseudo-second-order kinetics with rapid adsorption (chemisorption is possible) |
[ |
|
|||||
13 | Highly efficient nano- adsorbent, Al(III)-Fe(III)-La(III) trimetallic oxide | Health risk of using Al solely has been eliminated by introducing Fe3+ in this study (although Fe3+ is widely used for fluoride adsorption some studies suggest low AC as 16.5 mg/g). La3+ is an excellent fluoride adsorbent. The presence of Fe3+ makes the separation easier via an external magnet |
Not given | Maximum fluoride could be removed up to 99.8% at pH 7.0, contact time: 60 min, adsorbent dose: 0.3 g/100 ml | [ |
|
|||||
14 | Hybrid Al2O3/Bio-TiO2 nanocomposite impregnated |
Titanium dioxide was synthesized using a bacteria |
1.9 | Both batch type and dip mode adsorption studies were carried out and, in both studies, adsorbent capacity decreased when the adsorbate concentration is increased. |
[ |
|
|||||
15 | Coconut-shell derived carbon/carbon nanotube composite | CNT were coated on coconut-shell charcoal using plasma enhanced chemical vapor deposition |
0.36 at pH 2 | Best matched with Langmuir isotherm model and the pseudo-second-order kinetics fluoride removal is endothermic and spontaneous nature. A field study was carried out to nalagonda water sample and obtained satisfactory results (initial adsorbent concentration is 10 g L−1 and equilibrium contact time was 3.5 h) | [ |
|
|||||
16 | Iron-oxide nanoparticles by green Synthesis. Method using |
Green synthesis (a solution of iron nitrate III was mixed with heated leaf extract under stirring and immediate color change from green to black indicates the nanoparticle formation) |
1.40 at pH 7 (higher than BGAC) | Study compared the removal percentage with granulated charcoal of bone (BGAC) |
[ |
|
|||||
17 | NaP : HAp nanocomposite | Hydroxyapatite was synthesized followed by composite making with NaP Zeolite |
66.66 mg/g | Analysis of several factors were carried out using the four factors box—Behnken design with three levels (optimum pH value was found to be 7 at 60 min with 5 mg/L of fluoride concentration and at 55°C) |
[ |
|
|||||
18 | Millisphere nanocomposite of La-Doped Li-Al layered double hydroxides supported by polymeric anion exchanger (LaLiAl-LDH@201) | Drawbacks of LDHs such as pressure drop and blockage were overcome by impregnating the LDHs into millimetric polystyrene anion exchanger |
75.7 | The adsorption kinetics could be fitted with both pseudo-first-order and pseudo-second-order models with approximately identical coefficients of determination. |
[ |
|
|||||
19 | Tetraethylenep-entamine functionalized nanomagnetic composite | nFe3O4@TEPA was produced using a polyol-media one-pot solvothermal method. Adsorption capacity was independent from solution pH (2–11). The effect of HF at pH below 5 (pHpzc) has not affected the AC. Avg. diameter: 20 nm. Higher saturation magnetization compared to other nanomagnetic polymers. |
163.9 | Best matched with Langmuir isotherm model. Endothermic and spontaneous saturation magnetization: 48.2 emu/g. Better reusability (composite could be used for at least 10 cycles with a loss of less than 2.8% upon recovery on average) | [ |
|
|||||
20 | Membrane capacitive deionization with nanoporous and microporous |
Nanoporous-activated C was synthesized from |
2.7554 at pH 7.2 |
Best matched with Langmuir isotherm model and the pseudo-first-order kinetics | [ |
|
|||||
21 | Electrically enhanced adsorption and green regeneration 1 for fluoride |
Blending of electrosorption and adsorbent adsorption |
115.2 at +1.2 V | Best matched with Langmuir isotherm model | [ |
|
|||||
22 | Millimeter-sized Mg-Al-LDH nanoflake impregnated magnetic alginate beads | Biobased sorbent |
32.4 at pH 5 | Best matched with Freundlich isotherm model and the pseudo-second-order kinetics with rapid adsorption | [ |
|
|||||
23 | Metal organic framework (Uio-66) | ZrCl4 [Zr6O4(OH)4(BDC)6]21 and terephthalic acid were dissolved in DMF solution and heated in an autoclave to synthesis Uio-66 |
31.09 pH range 6–9 | Best matched with Langmuir isotherm model and the pseudo-second-order kinetics. | [ |
Polymer grants intrigued qualities for nanocomposites and membranes. The greater flexibility, mechanical strength, selective transfer of chemicals, excellent film forming ability, high perm selectivity, and low cost are some of the properties that are beneficial in water purification applications [
The stem of a weed variety,
Various types of recent and novel fluoride removal techniques involving nanotechnology are summarized in this review. Further, contamination means have also been briefly elaborated. Focus has been given to novel adsorbents such as LDHs, magnetic nanoparticles, CNT, and other membrane techniques among many other possible techniques. However, given experimental results are valid under specific parameters including pH, other ions present, initial fluoride concentration, temperature, etc. Therefore, these methods should be tested in real-world application even though only few studies have succeeded so far. Other than that cost factor, simplicity and eco-friendliness of these nanomaterials should be evaluated in this regard.
The authors declare that there are no conflicts of interest regarding the publication of this paper.