Groundwater Defluoridation with Composite Oxyhydroxide Mineral Ores—The Case with Bauxite, a Systematic Review

Abstract

Fluoride contamination of groundwater is a significant concern because of its detrimental impact on human health. Adsorption using composite oxyhydroxide mineral ores such as bauxite has demonstrated feasibility as an environmental remediation technique for rural and disadvantaged communities due to its cost-effective, environmentally friendly, generally acceptable, and adaptive characteristics. The effects of the composition, pretreatment methods, and influencing factors or parameters on the adsorptive defluoridation of groundwater by bauxite, as well as the adsorption mechanisms and the environmental advantages of using composite mineral ore in the remediation of fluoride from groundwater, are highlighted. Generally, the results indicated that some raw mineral assemblage adsorbents and their pretreated versions are better than or practically similar to current commercial fluoride filters.

1. Introduction

Fluoride is necessary for the formation, proper functioning, and health of ossified and dental bodily structures [1,2,3], especially in children [4]. However, this substance’s elevated water levels (>1.5 mg/L) are detrimental to human health [5,6]. Health problems associated with high fluoride levels in drinking water include skeletal and dental fluorosis [7,8,9] and other diverse diseases and conditions [10,11,12,13,14,15,16,17]. Accordingly, many techniques have been introduced to remove fluoride from water. Each process is formulated to work in a particular set of socio-economic and environmental conditions. Critical factors include cost, contamination level, material obtainability, absorbent acceptability, and water quality. Hence, a delicate balance between economics and efficiency when applied to the actual field water samples is crucial [9].

Table 1. Comparison of standard defluoridation methods.

Methods	Main Properties	Merits	Drawbacks
Ion exchange	Non-destructive process using adsorbents	Highly rapid and efficient; easily used with other techniques; technologically tested and straightforward procedures.	May be expensive; replacement of media after multiple regenerations; matrix degrades with time and presents toxic solid waste; large volume.
Coagulation/precipitation	Uptake of the pollutants and separation of the products formed	It is highly efficient, economical, and technologically simple, adapted to high pollutant loads, and tested in actual conditions.	Expensive and affected by factors such as temperature, pH, presence of other ions, high residual aluminum, toxic sludge formation, and severe automation difficulty
Electro- coagulation	Use of electric current	Adaptation to different pollutant loads and different flow rates; pH control is not needed; generation of coagulants in situ; no secondary pollutants.	Release of aluminum; high cost of installation and maintenance; requires the addition of chemicals; anode passivation and sludge deposition of the electrodes; formation of sludge.
Membrane filtration	Non-destructive separation with semi-permeable barrier	Small space requirement; simple, rapid, and efficient; color and taste of treated water are unaffected; limited maintenance; no chemicals required; pH-independent.	High investment, maintenance, and operational costs; rapid membrane clogging; low throughput; removal of essential ions; requires pH correction of treated water.
Adsorption	Non-destructive process using an absorbent	Technologically simple operation; excellent treated effluent quality; environmentally friendly; cost-effective process; greater accessibility.	Rapid saturation and clogging of the reactors (regeneration costly); affected by interfering ions; without high adsorption capacity, pretreatment is required.

demonstrates some merits and drawbacks of the main defluoridation processes [9,18,19,20]. As seen in the table, all these processes can keep fluoride concentrations well within the allowable levels in potable water. However, many systems aimed at lowering high fluoride concentrations apply the adsorption technique because of its relatively low cost, simplicity of design, and ease of operation [21,22,23,24,25].

The composite mineral ores are cheaper adsorbents than the conventional activated alumina (AA) [26,27,28] and activated carbon (AC) [29,30,31,32,33] used for water remediation. The ore materials are more acceptable than other fluoride adsorbents, such as bone black (a form of AC) in some regions due to socio-religious reasons [26]. They are relatively abundant and ubiquitous, especially in parts of Africa, Asia, and other underdeveloped areas of the world [34], where high fluoride levels have been reported in groundwater. The primary minerals such as gibbsite in most composite ores are known adsorbents for numerous cations and anions [35].

Few reviews have included the use of minerals and their composite ores in their discussion of removing fluoride from water [9,20,21,36,37,38,39]. Although all these earlier works extensively reported the pretreatment methods; removal efficiencies and capacities; and other adsorption functionalities of fluoride removal adsorbents, little or no attempts were made to explain the impacts of the pretreatment techniques and the specific compositions of the minerals on the defluoridation process. The previous reviews did not elaborate on fluoride adsorption mechanisms of the mineral composites under different conditions. Although fluoride adsorption by oxyhydroxide ores is achieved primarily by oxides of Al and Fe [26], the minor oxides can also contribute immensely by modifying the fluoride adsorption behavior and defining the quality and safety of finished water products [26,40,41,42]. Moreover, the pretreatment processes can play a crucial role in determining the sorption performance of ore materials [26,43,44,45,46]. The removal efficiency, which indicates the maximum percentage of fluoride removal at optimal conditions [47], along with the other parameters, is critical in designing an adsorption reactor. Therefore, understanding the factors that generate optimum adsorbate removal and elucidating the sorption behavior and performance will engender schemes and designs for cost-effective fluoride removal from groundwater. Robust characterization of diversely sourced composite ore adsorbents will also aid in this regard. Consequently, these adsorption aspects and the technical barriers to the usage of bauxite and their solutions are at the core of the current work and are extensively explained.

Bauxite is one of the most known composite oxyhydroxide mineral ores in aqueous remediation (both as fluoride adsorbent [26,48,49,50] and raw material for producing other adsorbents including AA [51,52] and red mud [53], which are known fluoride adsorbents [51,52,54]). Hence, this paper aims to present a contemporary examination of composite oxyhydroxide mineral ore usage in fluoride adsorption from groundwater, focusing on bauxite. The impacts of composition, modification procedures, and the affecting variables or parameters on the adsorptive defluoridation of groundwater by bauxite, coupled with the adsorption mechanism and environmental benefits of employing composite mineral ore in fluoride removal from groundwater, are examined in this work. The findings in this review suggest that even raw or mildly processed bauxite can be a cost-competitive and acceptable substitute for conventional adsorbents. Therefore, oxyhydroxide ores promise to enhance the sustainable provision of potable water in rural communities where fluoride-contaminated groundwater may be the only water source.

2. The Composition and Pretreatment of Bauxite Ore and the Impact They Have on the Defluoridation of Groundwater

Many studies have established a robust framework for composite mineral ore-based fluoride adsorbents. Soil and ore materials and their derivatives have been extensively studied and employed as fluoride adsorbents [55]. Bauxite, as one of those materials, is used as an adsorbent in its raw or modified variants [48]. Understanding bauxite samples’ chemical and mineralogical compositions, their impact on fluoride adsorption, and the pretreatment techniques applied in bauxite modification are essential.

2.1. Characterization of Bauxite and the Effects of Minor Minerals on the Defluoridation Characteristics

An oxyhydroxide mineral contains hydrous oxide or hydroxide with the oxyhydroxyl (-OOH) radical and primarily comprises the cation Al or Fe but sometimes Mn, Zr, Ti, Ce, and La. For example, bauxite consists mainly of the aluminum minerals gibbsite (Al₂O₃·3H₂O), boehmite (γ-AlO(OH)), or diaspore (α-AlO(OH)), which consist of 30–60% of the ore [56]. Silica, goethite (α-FeO(OH)), hematite, anatase, and several minor minerals are also found in the ore.

Results from bauxite characterization in multiple studies with X-ray fluorescence (XRF) or X-ray diffraction (XRD) and other analytic tools are given in

Table 2. Elemental and mineralogical compositions of some bauxite samples used as fluoride adsorbents.

Chemical Composition (%)					Mineralogical Composition	References
Al2O3	Fe2O3	SiO2	TiO2	CaO
74.78	18.64	1.55	–	–	–	[43]
41.56–54.78	15.93	1.07–19.2	<3.34	<2.52	Gibbsite, hematite, and anatase	[26]
70.90	23.60	2.20	2.60	-	Gibbsite, hematite, anatase, and kaolinite	[58]
34.38	19.93	17.27	4.74	6.62	Diaspore, hematite, dolomite, quartz, and anatase	[57]
45.77	10.20	24.77	5.87	0.52	Diaspore, hematite, chlorite, and anatase	[57]
52.50	22.50	2.40	1.40	–	Gibbsite, hematite, goethite, anatase, and quartz	[59]
53.64	19.51	12.43	2.48	0.29	Boehmite, hematie, and kaolinite	[57]
59.90	1.89	9.31	11.60	0.45	Diaspore and anatase	[57]
43.30	14.20	2.20	1.80	–	Gibbsite, hematite, and kaolinite	[40]
30.33	14.30	15.00	1.57	0.76	–	[41]

. The principal oxides are those of Al, Fe, Si, and Ti. The mineralogical composition generally includes gibbsite, hematite, and anatase. The silica component is usually found in quartz and clay minerals such as kaolinite. However, in some bauxite ores, the primary aluminum oxide mineral is boehmite or diaspore, whereas goethite may be the leading iron oxide mineral. Cherukumilli et al. [26] noticed that the difference in aluminum, iron, and silicon contents of different bauxite samples did not affect the fluoride adsorption capacity. Analysis of results from another study [57] showed that the difference in fluoride adsorption bears little relation to the quantities of Al₂O₃ and Fe₂O₃. The results indicate that the minor minerals also significantly impact bauxite sorption behavior and largely contribute to the different sorption performances exhibited by composite mineral assemblage with similar major mineral compositions but dissimilar minor mineral proportions.

Substances such as CaO, MgO, or SO₃ found in the minerals, including calcite, dolomite, or gypsum, respectively, may occur in small but critical amounts in some bauxite samples. The presence of calcium carbonate adversely influences the fluoride removal rates [26]. Fluoride removal efficiencies for a synthetic solution and actual groundwater for bauxite from the Shahbalaghy mines exceeded the values for the sample from the Shomal-eYazd mines by more than 10% is referenced [57]. The composition of CaO from dolomite in the samples from Shomal-e-Yazd was around 23 times more than the one in Shahbalaghy’s specimen, indicating that the lower adsorption efficiency of the former could be partly due to the more significant amount of CaO in it. One other study [42] showed that gypsum (0.85 mgF/g) and magnesite 0.71 mgF/g), which contain a high level of CaO and MgO, respectively, exhibited lower fluoride adsorption capacities compared to bauxite (1.05 mgF/g) under similar experimental conditions. Again, these observations suggest that while the adsorption with composite ore is due to the major minerals, the minor minerals can ultimately modify the sorption characteristics.

The lower fluoride removal with bauxite samples containing a considerable amount of calcareous minerals may be ascribed to the dissolution of CaO in an aqueous medium according to the equation:

$$CaO+H_{2}O \to C{a}^{++}+2O{H}^{-}$$

(1)

It is well known that oxides of Group II metals generally react with water increasing the pH. The rise in pH is significant and leads to a higher final pH than the optimum. Cherukumilli et al. [26] noted an increase in the experimental and calculated final pH caused by the quantity of calcium oxide in the bauxite sample. A rise in pH above the optimum value hinders bauxite’s fluoride recovery, resulting in low adsorption efficiency. As the reader shall later see, this is due to adherence to Le Chatelier’s principle and competition between OH⁻ and F⁻ ions.

However, a pH lowering was observed in some cases of bauxite defluoridation of actual groundwater with alkaline pH [40,41,42,43]. Equations (2) and (3) show a synergistic effect between the covalent and aluminum oxides in bauxite:

$$A{l}_2{O}_3+2O{H}^{-}+3H_{2}O \to 2Al{\left(OH\right)}_4^{-}$$

(2)

$$S{O}_3+2H_{2}O \to O_{3}S{\left(OH\right)}^{-}+H_3{O}^{+}$$

(3)

In Equation (2), the reaction lowers the pH by consuming hydroxide ions. This reaction is possible due to the amphoteric nature of aluminum oxide. On the other hand, pH reduction is accomplished by the second reaction by introducing more hydronium ions into the solution. It is well known that most nonmetallic oxides are acidic in aqueous solutions. A similar pH lowering was observed with fluoride removal by gypsum, containing more sulfur oxide than calcium oxide [42]. Hence, it can be assumed that the pH lowering was due to the dominant effect of Equation (3) over Equation (1). It is essential to note that magnesite samples with a minimal quantity of sulfur and aluminum oxides exhibited a pH rise [42]. This pH increase can be attributed to the MgO in magnesite reacting similarly to CaO in Equation (1). However, it should be recalled that the sorption mechanisms of composite minerals assemblages are complicated. The adsorption mechanism of fluoride on bauxite will be elaborated on later in Section 3.

The preceding discussion further corroborates the statement that the specific composition of the ore influences its overall fluoride sorption behavior and defluoridation performances. Cherukumilli and associates [26] indicated that when surface capacities and affinities are comparable, fluoride removal is primarily influenced by the presence of minor minerals such as CaCO₃, which alter the equilibrium solution and influence sorption performance. However, the difference in degrees of pretreatment, solution matrices, and adsorption parameters can also affect specific adsorption characteristics. For instance, particle size may affect the adsorption capacity of adsorbents because large particle size fractions lead to a lower active sorption surface area.

2.2. Types and Influence of Composite Mineral Ore Pretreatment for Adsorption of Fluoride from Groundwater

It has been proven that samples of raw bauxite of diverse origins are capable of defluoridation of drinking water [26] (Table 3). However, many natural composite mineral ore samples require pretreatment or surface modification to improve their adsorption characteristics. The transformation involves incorporating metallic ions onto the surface of the adsorbent material, pretreating the adsorbent with heat, or both. Atasoy et al. [48] modified bauxite separately using sodium, magnesium, and heat; Malakootian et al. [57] and Ghosh and Mishra [44] pretreated bauxite with heat. Aluminum and calcination were used to modify bauxite in one process [60], while Vardhan and Srimurali [61] incorporated lanthanum onto the bauxite surface in another. These modification processes either activate or increase the original adsorbate adsorption sites.

The primary effect of incorporating metallic ions onto the surface of an adsorbent is the generation of more active contaminant sorption sites. Soft bases such as chalcogenides have high affinities for soft acids (metals) [62]. Similarly, the adsorbent modification with these metal ions (La³⁺, Al³⁺, Fe³⁺, or Zr⁴⁺) classified as hard acids promotes the metal–adsorbate interaction with the hard base fluoride ion [60] per the hard–soft acid–base (HSAB) theory. The use of divalent metals such as Ca²⁺, Mg²⁺, or Zn²⁺ is also reported in the literature. Generally, modification with metal ions aid in increasing the adsorbent adsorption capacity.

Table 3. Sources and pretreatment techniques of some bauxite samples applied in water defluoridation.

Source of Bauxite Sample (Short Name as Used in the Text)	Modification Method	References
Seydisehir Aluminum Process Plant, Eskisehir, Turkey (B-Mg-500) *	Thermal activation and metal (Mg) incorporation	[48]
Tailings, Ghana Bauxite Mining Company Limited, Awaso, Ghana (BXT) *	Thermal activation	[43]
High Aluminium Bauxite Ore, Ghana (AOCB) *	Metal (Al) incorporation	[63]
Mines in Visakhapatnam, Andhra Pradesh, India (JTTB) **	Thermal activation	[49]
Rawmin Mining and Industries Pvt. Ltd. Kolhapur, Maharashtra, India (KRB) **	None	[64]
Guinea (GRB) **, Ghana, USA, and India	None	[26]
Mine in Visakhapatnam, Andhra Pradesh, India (CTTB) **	Thermal activation	[50]
An aluminum mining company in the Western Region, Ghana (Gh-B) **	None	[58]
Jobhipat and Narma bauxite mines of Jharkhand, India (TRB) *	Thermal activation	[45]
Panchpatmali of Koraput district in Odisha, India (TTB) *	Thermal activation	[44]
Lichenya Plateau, Mulanje Mountain, Malawi (LRB) **	None	[65]
Texas, USA (URB) **	None	[66]
Sadr Abad, Iran (SATB) **	Thermal activation	[67]
Shomal-e-Yazd mines, Iran (SYTB) **	Thermal activation	[67]
Not given (RGB) **	Thermal activation	[68]
Mulanje Mountain in Mulanje district, Malawi (SMTTB) **	Thermal activation	[40,46]
Awaso Bauxite Mines, Ghana (GACB) *	Thermal activation and metal (Al) incorporation	[60]
Not given (SRB) **	None	[59]
Kwemashai, Usambara Mountains, Lushoto District, Tanzania (B-200) **	Thermal activation	[41,42,69,70]
Mahboobabad, India (LIB) *	Thermal activation and metal (La) incorporation	[61]
Mahboobabad, India (MTTB) **	Thermal activation	[61]

* Short names are from the source documents. ** Short names are given by the authors of the current work where the source articles provided none. Note that some samples are from the same locality but have been given different short names to simplify the discussion.

Table 3. Sources and pretreatment techniques of some bauxite samples applied in water defluoridation.

Source of Bauxite Sample (Short Name as Used in the Text)	Modification Method	References
Seydisehir Aluminum Process Plant, Eskisehir, Turkey (B-Mg-500) *	Thermal activation and metal (Mg) incorporation	[48]
Tailings, Ghana Bauxite Mining Company Limited, Awaso, Ghana (BXT) *	Thermal activation	[43]
High Aluminium Bauxite Ore, Ghana (AOCB) *	Metal (Al) incorporation	[63]
Mines in Visakhapatnam, Andhra Pradesh, India (JTTB) **	Thermal activation	[49]
Rawmin Mining and Industries Pvt. Ltd. Kolhapur, Maharashtra, India (KRB) **	None	[64]
Guinea (GRB) **, Ghana, USA, and India	None	[26]
Mine in Visakhapatnam, Andhra Pradesh, India (CTTB) **	Thermal activation	[50]
An aluminum mining company in the Western Region, Ghana (Gh-B) **	None	[58]
Jobhipat and Narma bauxite mines of Jharkhand, India (TRB) *	Thermal activation	[45]
Panchpatmali of Koraput district in Odisha, India (TTB) *	Thermal activation	[44]
Lichenya Plateau, Mulanje Mountain, Malawi (LRB) **	None	[65]
Texas, USA (URB) **	None	[66]
Sadr Abad, Iran (SATB) **	Thermal activation	[67]
Shomal-e-Yazd mines, Iran (SYTB) **	Thermal activation	[67]
Not given (RGB) **	Thermal activation	[68]
Mulanje Mountain in Mulanje district, Malawi (SMTTB) **	Thermal activation	[40,46]
Awaso Bauxite Mines, Ghana (GACB) *	Thermal activation and metal (Al) incorporation	[60]
Not given (SRB) **	None	[59]
Kwemashai, Usambara Mountains, Lushoto District, Tanzania (B-200) **	Thermal activation	[41,42,69,70]
Mahboobabad, India (LIB) *	Thermal activation and metal (La) incorporation	[61]
Mahboobabad, India (MTTB) **	Thermal activation	[61]

* Short names are from the source documents. ** Short names are given by the authors of the current work where the source articles provided none. Note that some samples are from the same locality but have been given different short names to simplify the discussion.

However, thermal heating (with or without the incorporation of metallic ions) is the dominant mode of modifying or pretreating bauxite (Table 3). Calcination serves several purposes: activation of original sorption sites, creation of new sorption sites, and facilitation of solid binding of incorporated metal ions to the surface of the adsorbent. There is a progressive change in the adsorbent’s physical and chemical characteristics with preheating temperatures, providing a theoretical basis for its activation. The decomposition of old minerals and the appearance of new ones are observed when composite mineral ores are heated.

As an illustration, one study [50] analyzed bauxite samples after heating them to 400 °C. Using TGA-MS and XRD, these researchers found a reduction in mass due to water loss between 250 °C and 300 °C. Between 300 °C and 400 °C, the crystalline gibbsite disappears, but the goethite, hematite, and anatase remain intact. Salifu et al. [60] reported a similar conversion of gibbsite to boehmite due to water loss. Boehmite was observed to have a greater surface area than gibbsite [45], indicating that dehydroxylation may cause an increase in the pores and fractures. This sequence of events explains a 15-fold increase in the surface area of bauxite on heating just above 300 °C [50]. However, some mass loss could be attributed to the evaporation of the physisorbed water on the material [46]. The changes in bauxite and the improvements in its sorption efficiency provided by thermal heating have been reported for other composite oxyhydroxide adsorbents with heat and other processes [71,72].

At intermediate heating temperatures (200–600 °C, depending on the specific sample), bauxite generally functions well as an adsorbent for fluoride, as seen by the inverted V-shaped curves in Figure 1. Defluoridation efficiencies of pristine and calcined (200 °C) bauxite samples showed a slight difference [46], attributed to the loss of water from the calcined bauxite, which meant an abundance in the active sites on gibbsite and kaolinite. This positive impact on bauxite fluoride sorption characteristics by moderate heating is evidenced via observations reported by Das et al. [45]. While investigating fluoride adsorption from drinking water by thermally activated titanium-rich bauxite, these investigators found mild activation temperatures (300–450 °C) were optimum. This temperature range exactly fits the interval for gibbsite conversion to boehmite by water loss. Another example is the increased sorption capability exhibited by bauxite modified with magnesium and heated at 500 °C (B–Mg–500) [48]. The B–Mg-500 acquires an enhanced fluoride sorption capacity due to the increased vacant and positively charged sites generated by water loss and firmer retention of magnesium ions on heating the modified bauxite adsorbent. A similar rise in the quantity of adsorbed fluoride with an initial increase in calcination temperature before leveling off or declining as the temperature rises further was observed with other natural minerals [73,74].

However, there is a drop in the sorption capacity for fluoride as the preheating temperature increases, probably caused by the dehydroxylation of some species that were structurally intact at lower temperatures. The decrease in fluoride sorption capacity at higher heating temperatures could be due to the structural deformation of species such as kaolinite [46] and hydrated iron (III) oxides [75]. These minerals feature prominently in the adsorption process at lower calcination temperatures. These changes may explain the 87% defluoridation removal at 500 °C compared to 95.3% at 200 °C [46]. The former temperature corresponds to the temperature at which kaolinite converts to metakaolinite [76].

Consequently, a diminishing adsorption benefit is accrued at a continuous increase in preheating temperature, probably due to the initiation of chemical reactions that yield substances that may hinder adsorption at very high modification temperatures. For example, anatase and gibbsite react to form tialite (Al₂TiO₅) while glass forms in bauxite at high preheating temperatures [77]. Some of these new substances could reduce the fluoride sorption capacity of composite mineral ores. Hence, while calcination leads to the formation of helpful adsorbent species at lower temperatures, it may hamper the sorption efficiency of mixed mineral ores at higher preheating temperatures.

Although high pretreatment temperatures may not be optimal for the best fluoride sorption efficiencies, some bauxite samples have shown excellent defluoridation characteristics after being calcined at high temperatures. For example, refractory-grade bauxite (RGB) has demonstrated promising defluoridation properties with favorable fluoride removal efficiency at pH 5.5 [68]. This encouraging performance can be explained by the exceptionally high alumina content of refractory grade bauxite, similar to AA. At higher temperatures (1300 °C–1650 °C), bauxite predominantly consists of corundum and mullite [78]. Furthermore, the surface areas of alumina generally increase as it transforms from one form to another through heating. For example, the γ-alumina (γ-Al₂O₃, the phase at 500 °C) can have a surface area of 400 m²/g [79], far greater than the surface area of gibbsite at <300 °C. A high surface area may provide a more significant number of active adsorption sites per unit area. Although a high surface area would be expected to give a higher adsorption efficiency, the process depends on the morphology of the adsorbent particles. For instance, the active adsorption sites of most porous solids primarily reside in the pore spaces [80], while coarse-grained materials tend to have significant intragranular porosity and active sites [81], resulting in an increased dependence on diffusive transport and slow exchange times. This observation indicates that active sites in micropores or narrow-neck pores may not be a consideration in determining the adsorption capacity of a sorbent, as this can hinder the solute transport processes [58]. Hence, the adsorption capacities and efficiencies are mainly due to the sorbents’ pore size, shape, and mineralogy. This observation explains why robust calcination may not engender high removal efficiency even though it may increase surface area.

The observed complexity of the impact of thermal pretreatment of composite mineral ores such as bauxite may also be explained by considering the varying transformation and coexistence of phases dictated by the composition of the composite ore material and the modification temperature. For instance, on increasing preheating temperature, the series of transformations: Al(OH)₃ → γ-AlOOH → γ-Al₂O₃ → δ-Al₂O₃ → θ-Al₂O₃ → α-Al₂O₃ may be obtained [82]. However, the transformation sequence may be affected by such factors as particle size, heating rate, impurities, and atmosphere because of the impact on the mechanics of the transformation [83,84,85]. Figure 2 illustrates different phases of aluminum oxides/hydroxides at different temperatures. Bauxite can either consist of gibbsite, boehmite, or diaspore or contain one of the other two minerals and diaspore [86]. In other words, the precursor aluminum oxides/hydroxides play a defining role in determining the composition of the alumina phase mixture at a given preheating temperature. For example, a bauxite sample composed of gibbsite and diaspore and preheated at 500 °C most likely would contain χ-Al₂O₃ and α-Al₂O₃, while a sample with boehmite and diaspore modified at the same temperature would generate a mixture of γ-Al₂O₃ and α-Al₂O₃. In general, the mechanism for the phase transformation is repeated dihydroxylation [87]. Adding the phase changes of other minerals such as kaolinite creates a complex web of phase coexistences which ultimately occasions the sophistication. This observation is so because all the individual phases exhibit different adsorption characteristics, including surface area and porosity at different temperatures. Hence, different bauxite samples have different adsorption capacities and removal efficiencies, even at the same preheating temperatures. These samples exhibit their highest fluoride removal efficiencies at different modification temperatures.

The significance of heat pretreatment in composite minerals ore modification needs not to be overemphasized as it has also been demonstrated in several studies, including those on RM (produced from bauxite), with identical improvements in adsorption characteristics [88,89]. Thus, it can be concluded that the principal role of the pretreatment/modification methods is to enhance adsorbent sorption characteristics. These improvements may include increased adsorption capacity, better regeneration property, preferential selectivity, or mitigating metals leaching from the original adsorbent material.

3. Mechanism of Fluoride Sorption on a Composite Metallic Oxyhydroxide Mineral Ore

The adsorption of fluoride on adsorbent particles can be summarized in three steps: (1) external mass transfer or diffusion of fluoride ions from the bulk heterogeneous solution across the boundary layers of adsorbent particles; (2) adsorption of fluoride ions onto the particle surfaces; and (3) intra-particle diffusion where adsorbed fluoride ions are conveyed to the internal surfaces of the porous adsorbent materials [18,21,47]. However, the effectiveness of the process is contingent on the nature of the adsorbent, among other factors.

A composite oxyhydroxide ore’s nature (composition) renders its adsorption characteristics suitable for adsorbate removal. Theoretically, substances with minerals of oxyhydroxide composition should have high contaminant sorption properties in aqueous solutions because of the difference in the number of coordinating metal ions for surface oxygens, enabling the removal of cations and anions by adsorption on the surface of the oxyhydroxide materials. Bauxite is primarily composed of oxides and hydroxides of Al and Fe; oxyhydroxides that possess excellent adsorption capacity for fluoride, as seen in several recent investigations [38,49,90,91].

The chemical characteristics of the composite oxyhydroxide ore and specific reactions at the metal oxide and hydroxide surface may be responsible for its pollutant removal from water. It has been reported that fluoride forms several complexes with medium/high oxidation state metals such as titanium, iron [92], and aluminum [93] in aqueous solutions. Hence, fluoride sorption on composite mineral assemblages is assumed to be due to the oxides of medium/high oxidation state metals, having high affinities for the fluoride ion. One study [94] reported that all metals involved in removing fluoride from an aqueous solution were trivalent. This high affinity of polyvalent hard metals for fluoride was also demonstrated by the XRD spectrum of another adsorbent of composite mineral assemblage derived from bauxite [95]. This affinity results in the replacement of OH⁻ groups in hydrated metal oxides by F⁻.

However, the fluoride ion cannot replace the hydroxide ion directly. The energy required for OH⁻ removal from the surface of the adsorbent is greater than the reactive potential of fluoride. Moreover, the concentration of fluoride may be low to cause direct replacement. The concentration dependence of fluoride sorption on the surface of aluminum oxide’s active sites has been investigated [96]. The findings showed that a higher fluoride concentration enhances its reactive potential, leading to the immediate replacement of hydroxide ions. However, this phenomenon depends on the specific adsorbent being used. For instance, the decomposition energy of OH⁻ in kaolinite is higher than in montmorillonite. While fluoride will react directly with active sites on the former at concentrations of >5 mg/L, it will not react with the latter even at levels up to 50 mg/L [96]. At higher concentrations, fluoride may not only replace OH⁻ but also precipitate metal ions in the interlayer spaces. This action may help explain the inherent difference in fluoride sorption efficiencies exhibited by different bauxite samples and other adsorbents of the same class.

Nevertheless, the fluoride sorption mechanism of composite oxyhydroxide mineral ores in aqueous media at relatively low fluoride concentrations can be explained using ligand exchange interactions. A parallel ligand exchange model was used to explain the specific adsorption of fluoride by goethite [97,98,99]. These reactions are demonstrated in Figure 3. The surfaces of the metal oxides become hydroxylated and develop charges under humid conditions. The specific adsorption of fluoride on metal oxides can be modeled by assuming ligand exchange reactions between the hydroxylated metals (Fe, Al, Mn, Ti, Zr, and La (including Ca, Mg, and Zn)) and the fluoride ions, culminating in the adsorption of the fluoride ion on the surface of the metal oxides. Metal oxides on the adsorbent’s surface are hydrolyzed, producing metal hydroxides and are protonated to –OH₂. The –OH₂− exclusively partakes in the ligand exchange at low fluoride levels, while the metal hydroxide provides an exchange site at high F− concentration [100].

Laboratory evidence suggests that surface complexation and penetrative diffusion may be responsible for fluoride adsorption on bauxite [40,60]. Bauxite samples’ initial rapid fluoride uptake may be attributed to external surface adsorption. The slower phase is due to intra-particle diffusion. Both outer and inner space complexations may be involved in fluoride adsorption. While the former interaction is mediated through physisorption and may not be precise in describing fluoride adsorption, the former embodies chemisorption definite to fluoride [100]. Experimental results suggested weak, outer-sphere electrostatic interactions minimally contribute to fluoride adsorption on bauxite in the pH range of 2–11 [26]. This finding agrees with the principal contribution of inner-sphere complexation for pure gibbsite [35]. Craig et al. [58] found no notable difference in fluoride adsorption capacity with a change in ionic strength for any aluminum-containing sorbents (including bauxite), which suggests that ionic strength does not influence the fluoride adsorption capacity. This behavior indicates a robust chemical interaction rather than a weaker electrostatic attraction [101]. Additionally, the Coulombic effect is contingent on surface charge [102]; hence, the fact that fluoride adsorption is indifferent to varying ionic strengths implies that electrostatics exhibit minimal impact compared to the intrinsic chemical energy on fluoride adsorption [101,102].

Fluoride adsorption on composite oxyhydroxide mineral ores may be specific or nonspecific. The chemisorption and physisorption of fluoride have been reported with another oxyhydroxide adsorbent [94]. The surfaces of metal oxyhydroxides in composite mineral ores exhibit positively charged sites at low pH. Hence, the negative fluoride ion may be adsorbed onto the metal oxides’ surfaces due to the coulombic attraction. An analogous involvement of an electrostatic attraction was reported for fluoride sorption by an adsorbent containing iron-aluminum binary oxyhydroxides [103]. While the dominant adsorption mechanism between the fluoride ions and the metal oxyhydroxide adsorbent surface at low pH may be coulombic, the indirect substitution of hydroxide groups with the fluoride ions on the adsorbent surface dominates at higher pH.

This fluoride sorption dependency on pH was reported with another adsorbent [104]. Hence, solution pH can be considered the principal factor controlling the adsorption at the water–adsorbent interface [97]. One experimental evidence is the reported >50% fall in fluoride recovery with a unit rise in pH more than the optimum pH [26]. However, the actual pH dependence is contingent on the adsorbent’s and adsorbate’s nature and the parameters and conditions under which adsorption is performed. Understanding the pH dependency and effects of other parameters on composite mineral ore adsorption of fluoride from groundwater is crucial for designing adsorption reactors.

4. Effect of Adsorption Parameters on Fluoride Removal from Water by a Composite Mineral Assemblage

Adsorption characteristics depend on physical parameters such as adsorption capacity, selectivity for the adsorbate ions, regenerability, compatibility, particle and pore size, initial adsorbate concentration, adsorbent dosage, pH of the solution, pH_ZPC of adsorbent, temperature, and contact time [38]. Hence, it is essential to understand the contribution of these factors/parameters for a complete comprehension of the contaminant adsorption on an adsorbent.

4.1. Effect of pH

Concerning pH, the several observations reported during the use of bauxite and bauxite-based adsorbents for fluoride removal from water can be summarized with the aid of the following equations:

$$M{\left(OH\right)}_x+Al{F}_n^{3-n} \to M{O}_{x}Al{F}_n^{3-n-x}+x{H}^{+}$$

(4)

$$M{\left(OH\right)}_x+H^{+} \to M{O}_x{H}_{x+1}^{+}$$

(5)

$$M{O}_x{H}_{x+1}^{+}+x{F}^{-}+\left(x-1\right)H^{+} \to M{F}_x+x{H}_{2}O$$

(6)

$$M{\left(OH\right)}_x+x{F}^{-} \to M{F}_x+xO{H}^{-}$$

(7)

$$M{O}_x{H}_{x+1}^{+}+2O{H}^{-} \to M{\left(OH\right)}_{x+1}^{-}+H_{2}O$$

(8)

The M represents a surface metal ion such as silicon, titanium, aluminum, and iron; x equals the charge on M; and ${AlF}_n^{3-n}$ denotes a negative aluminum–fluoride complex with n predominantly ranging from 4–6 [93]. These equations represent cation–anion-complex–adsorbent interactions, protonation, and deprotonation as ligand exchange mechanisms ensuing at the active sites on the surface of metal oxides in composite oxyhydroxide mineral ores.

Figure 4 shows the effect of initial pH on fluoride adsorption on bauxite, as reported by some studies. The highest fluoride recoveries for all bauxite samples are between pH 4.0–8.0. Furthermore, there are decreases in adsorption at lower and higher pH values. The primary minerals in composite oxyhydroxide mineral ores comprise the very reactive hydroxide ions in their crystal lattice. These ions play a significant role in determining the total charge appearing on the surfaces of the adsorbents. The zero point of charge (pH_ZPC) of some bauxite samples used to gather the data in Figure 3 is around or >pH 6 [58,59,61]. The knowledge of pH_ZPC (that is, the corresponding pH at which the surface charge of the mineral is zero) is of great significance for understanding adsorption on a solid surface. This pH is specific to the cations to which the surface oxygen atoms are bound in aqueous media [105]. Any pH less than this value should produce a net positive charge on the surface of the metal oxides in bauxite, whereas the opposite is true at higher pH. However, the charges are generally more pronounced further away from the pH_ZPC.

The formation of cation-fluoride complexes can explain the decline in fluoride removal at extremely acidic or very low pH. The cation–fluoride complexes (mainly aluminum fluoride complexes) are adsorbed onto the metal oxide surface, giving the surface a net negative charge (Equation (4)) at a very low pH, reducing any further fluoride adsorption. Aqueous cation–anion complexes influence the adsorption behavior of the anion, and their adsorption is pH-dependent, similar to those of the anion [106,107]. Their formation is probably due to reactions between gibbsite (or other aluminum oxyhydroxides) and the HF (having a pK_a of around 3.2), the latter of which forms due to bonding between the fluoride ion and the hydrogen ion that abounds under highly acidic conditions. Hence, there is a net reduction of free fluoride ions in the solution available for adsorption. Nevertheless, the moderately acidic or alkaline pH hinders their formation, stability, or affinity for the adsorbent surface, rendering these complexes to have no significant impact on the fluoride sorption under these higher pH conditions.

Some neutral sites on the bauxite surface are protonated at the medium pH range (4.0–8.0) (Equation (5)). That is, the hydroxide groups of hydrous metal oxides under these conditions follow the proteolytic reaction, and the surface is protonated. The protonation starts with binding the metal oxide/hydroxide particle with water because of the need to fulfill the oxygen coordination on the surface of the metal atoms [108,109,110,111]. The attached water molecules contain protons evenly distributed over the oxygen atoms on the particle’s surface. An equilibrium is established between the protons and the pH of the aqueous solution. At any pH below this pH_ZPC, there is an excess of hydrogen ions adsorbed by the metal oxide surface, charging it positively [105]. As reported by researchers, this results in the rapid adsorption of fluoride by the resultant charged sites and other neutral sites (Equations (6) and (7)), accompanied by the generation of excess hydroxide ions, increasing the pH. This increase in final pH has been experimentally confirmed [46,112]. The participation of neutral and protonated sites on the metal oxides, including gibbsite and hematite, during the adsorption process was also observed [113].

In contrast, as the pH becomes sufficiently higher than the pH_ZPC of bauxite, the metal surface acquires a negative charge, whereas the previously adsorbed hydrogen ions are converted to water molecules (Equation (8)). Many investigators reported a fall in fluoride removal efficiency by bauxite at high basic pH. As stated before, this reduction in recovery is a consequence of Le Chatelier’s principle, which simplifies as “An equilibrium solution tends to remove any added quantity of species already at equilibrium”. At high pH, hydroxide ions are abundant in the medium. Increasing the pH would add more OH⁻, rendering it difficult for ion exchange between fluoride and the hydroxide ions to occur, as the excess hydroxide ions adsorb on the bauxite surface, leaving the solution. Additionally, both anions exhibit similar charge and size and would compete, leading to repulsion between the negatively charged adsorbent surface and the fluoride ions, manifesting in the limited fluoride adsorption observed at high alkaline pH. The drop in fluoride removal efficiency for bauxite at high alkaline pH is well established in the literature (Figure 4).

4.2. Effect of Contact Time

After the adsorbent comes into contact with the adsorbate, an equilibrium is not immediately established. The time required for arriving at equilibrium (equilibrium contact time) depends on the adsorbent’s nature and the adsorbate’s concentration. With raw bauxite, Atasoy et al. [48] reported maximum fluoride adsorption in 3 h, reaching a constant value for the rest of the 24 h. This sudden change in removal efficiency can be attributed to vacant adsorbent sites and high solute concentration gradient before equilibrium and reduced sites after equilibrium. Other investigators gave a similar reason for the change in fluoride removal efficiency after rapid adsorption during the first few minutes or hours [50,64] (Figure 5).

The influence of the contact time on fluoride removal efficiency in increasing fluoride levels was investigated. One work [64] reported a fluoride removal fall of 93.8–86.6%, with a change in fluoride concentrations from 3 to 10 mg/L. Similar findings were reported in another study [114] when investigating the effect of contact time on montmorillonite abatement of fluoride from water. This occurrence will be further discussed in Section 4.4.

4.3. Effect of Adsorbent Dose

Several investigations have been conducted to understand the impact of adsorbent dose on the bauxite defluoridation of water. Atasoy et al. [48] observed a fall in the fluoride removal capacity from 0.30 mg/g to 0.05 mg/g with a rise in adsorbent dose from 2.5 g/L to 30g/L using raw bauxite at constant pH and initial fluoride concentration. Other studies have indicated similar observations of the reduced fluoride uptake of bauxite with increasing adsorbent dose [65,70] (Figure 6). This reduced loading capacity with increased bauxite quantity could be due to a higher adsorbent mass ratio to fluoride ions [70]. Investigating chemically activated bentonite fluoride removal from water, Kamble et al. [115] reported similar results. A high adsorbent dose results in lower equilibrium fluoride concentration and an excess of required active sites for the limited fluoride quantity, leading to a reduction in the sorption driving force and inefficient utilization of active sites, respectively. This “solid effect” at a fixed volume of solution engenders adsorption capacity drops with increased adsorbent dose [116].

Most studies on bauxite defluoridation of water have reported increased fluoride removal percentage with a rise in adsorbent dose. For instance, by keeping all other parameters constant and varying the adsorbent dose of thermally treated bauxite (TTB) from 5 to 30 g/L [44], the authors observed increased fluoride adsorption from 59.15 to 97.70%. Chaudhari and Sasane [64] reported a rise in fluoride adsorption from 70% to 90% at adsorbent doses of 4 g/L to 36 g/L, after which there was no significant change in fluoride adsorption with rising bauxite dose. Similar fluoride adsorption increases up to a certain level, and later leveling with increasing bauxite dose was observed by other researchers [42,45,48,65]. The initial fluoride adsorption increase is possibly due to the rise in the surface area caused by the augmented bauxite concentration, leading to an enhancement in the number of available active fluoride sorption sites.

The decline in marginal removal efficiency at an adsorbent dose higher than the optimum can be attributed to the lack of availability of enough fluoride ions for adsorption [45,65]. In other words, the drop in sorption efficiency after an initial elevation may be due to the possible decrease in the surface area resulting from clustering the adsorbent after the optimum adsorbent dose. A similar rise and subsequent leveling in adsorption efficiency were reported by Goswami and Purkait [117] while investigating pyrophyllite.

4.4. Effect of Initial Fluoride Concentration

There are great experimental supports for enhanced adsorption capacity with increased initial fluoride concentration. For example, Ghosh and Mishra [44] reported a fluoride uptake capacity increase from 0.74 to 3.11mg/g at an initial fluoride concentration of 5 to 25 mg/L, respectively. Similar findings that elevating the adsorbate concentration impacted the fluoride uptake capacity of bauxite positively were observed in other works [41,68]. However, all these authors pointed out that this behavior is due to the excess adsorbate efficient utilization of the available adsorption sites. This observation may be attributed to the proportional rise in uptake capacity with fluoride concentrations because of the increment in the concentration gradient’s driving force leading to the adsorbent’s enhanced fluoride loading capacity. In other words, the lower surface area to fluoride ions ratio at large initial adsorbate concentrations pushes equilibrium towards the solid phase. It can be assumed that the abundant adsorbate particles generate a more significant concentration gradient across the solid–solution interface causing a significant increase in the quantity of fluoride adsorbed per unit mass of the adsorbent [115]. This behavior has been exhibited by other adsorbents [115,118,119]. However, the occurrence may also be due to surface precipitation and adsorption at high sorbate concentrations [101,120,121].

It is difficult to correlate the relationship between the adsorbate and the adsorbent from the results of different studies because the experimental parameters and conditions used to derive the results are essentially dissimilar. However, the collision theory provides the theoretical basis for observing a rise in adsorption capacity with an increase in initial fluoride concentration. According to the theory, more collisions lead to more successful reactions, and a more significant number of particles will effectuate more effective collisions, as there is a higher probability of the adsorbate ions adsorbing on the adsorbent surface under the prevailing conditions. Thus, increasing the initial fluoride concentration raises the adsorption capacity of the adsorbent by ensuring a more significant number of successful interactions between the adsorbate molecules and the adsorbent functional groups until the active adsorbent sites are saturated.

Contrastingly, adsorption removal efficiency falls with increasing initial fluoride concentration when using bauxite adsorbent (Figure 7). For instance, Chaudhari and Sasane [64] indicated a fluoride removal percentage drop from 96.2 to 78.22% as the initial fluoride level rose from 3 to 15 mg/L. This occurrence is effectuated by the limited adsorbent sites at constant bauxite concentration for many fluoride ions at increased adsorbate concentration. This behavior indicates that large sorption capacities at high fluoride levels are not necessarily evidence of enhanced performance. In order words, because water defluoridation is a challenging and many-sided process, high adsorption capacity may not correspond to optimum fluoride removal efficiency [21]. The phenomenon of lowered removal efficiency with augmented initial fluoride concentrations has been reported in other works investigating bauxite defluoridation of water [41,44,48,68]. Other authors have reported similar observations for different adsorbents [117,122]. However, the phenomenon can be explained as the diminished sorption capacity of the adsorbent or the intense competition at the adsorbent surface among the excess adsorbate ions under the prevailing conditions.

Section 4.2 discussed the impact of contact time on removal efficiency at varying initial fluoride concentrations. It was noted that increasing fluoride concentrations show an inverse relationship with fluoride adsorption. Nevertheless, changing the initial fluoride concentration seems to have no significant influence on the equilibrium contact time. For instance, the fluoride adsorption efficiencies for initial fluoride concentrations of 10 and 15 mg/L were maximum at the same equilibrium contact time of approximately one hour [45]. Analogous occurrences were observed in other works [61,64]. Although this behavior is unexplained in the literature, it could be related to the intrinsic nature of the specific adsorbent.

4.5. Effect of Coexisting Ions

There are several critical process parameters; however, adsorbent selectivity is the most crucial under actual conditions at a water-treatment plant because of the presence of co-ions in the water, limiting adsorption capacity significantly on the active sites of adsorbents [123]. The adverse effect is caused by the competition between the fluoride and other ions in the water for the active adsorption sites [124]. Interestingly, Al₂O₃ can efficiently adsorb cations and anions, as noted earlier [35]. The impact on fluoride adsorption due to co-ions was investigated [44]. The interferences offered to fluoride adsorption by three cations: Al³⁺, Mg²⁺, and Na⁺ vary directly with the size of the cations and inversely to the charge on them. That is, aluminum ions delivered the most interference to fluoride removal. Besides the competition between ions for adsorption sites, the intense reduction in fluoride removal caused by Al³⁺ can be attributed to the higher affinity of trivalent ions for fluoride. As stated in the text, this metal ion forms complexes with fluoride, removing the free anion from the solution, resulting in less available fluoride ions for adsorption.

On the other hand, anions provide more restriction to fluoride adsorption than the cations due to the more intense competition for active adsorption sites because of their similar negative charge to fluoride. Among the three anions: Cl⁻, SO₄^2−, and PO₄³⁻, the phosphate ion is the most detrimental to fluoride adsorption, with less than 10% fluoride removal at a low concentration of 0.01 M PO₄³⁻ [44]. Chloride appears almost indifferent to fluoride removal, even at relatively high concentrations. These findings agree with results from another work, which indicated that this could reflect the relative affinities of these ions for the bauxite surface [60].

The impact of these anions can be explained by assuming that trivalent, PO₄³⁻, ions adsorb on bauxite principally as inner-sphere complexes; HCO₃⁻ and SO₄²⁻, as both inner-sphere and outer-sphere complexes; and monovalent, Cl⁻ and NO₃⁻, ions forming only outer-sphere complexes with lesser impact [125,126,127,128]. The high impact of HCO₃⁻ and SO₄²⁻ on fluoride adsorption efficiency was also reported in other works [59,61]. Therefore, the bauxite’s useful lifetime is expected to be reduced in groundwater with a high concentration of these ions [60].

Despite changes in the overall efficiency of bauxite, the results gathered from many research works relative to co-occurring ions should be favorable at higher pH ranges practical to potable groundwater standards. The concentrations of phosphate used in these works were far higher than those found naturally in groundwater. A phosphate level of 0.01 mM (typical phosphate concentration in groundwater) shows a negligible effect on the fluoride removal efficiency of GACB [60]. The conversion of monoanion to dianion in H₃PO₄ causes no significant impact, even though the second pKa is 7.21, and a change in sulfate protonation does not occur [129].

5. Regenerability and Environmental Concerns

Based on economic and environmental considerations, regeneration is a critical stage in the adsorption cycle as adsorbent disposal poses severe challenges to pollutant remediation using adsorption processes. Several regeneration techniques have been employed, but chemical treatments are common for natural materials and mineral ores [45,61,122]. These techniques are influenced by the solution pH, solubility of adsorbates, and charge of adsorbents.

The process of regenerating exhausted potential adsorbents is still unclear, and there are insufficient data on composite ore material regeneration. The regeneration capabilities of a few bauxite samples have been investigated [45,61,63], with mixed results. In one study [63], the capacity to regenerate aluminum oxide-coated bauxite (AOCB) by recoating with the oxide after fluoride adsorption yielded inconsequential results after the first regeneration. Other bauxite samples exhibited successful regeneration with chemical treatments, with one sample achieving 98 % desorption of fluoride-laden TRB with 0.015 M NaOH (pH = 11.1) [45]. Furthermore, the fluoride is reversibly adsorbed on the surface of TRB, making the material easier to recycle in the future. Another study [61] found that 4% NaOH regenerated LIB had a 95% desorption rate and that three regeneration cycles could still remove the fluoride to the allowed levels. Hence, it is concluded that chemical treatment with NaOH is suitable for regenerating bauxite adsorbents. However, the decreased effectiveness of each consecutive regenerated bauxite sample is of more significant consideration. This decreased efficiency may be due to the NaOH’s chemical alteration and damage on the adsorbent surface. It might also be due to adsorption, which blocks pores and restricts passage between the external solution and the internal adsorbent region, reducing the regeneration efficacy [130].

The Toxicity Characteristic Leaching Procedure (TCLP) is a chemical analytical method for identifying hazardous waste components. It simulates landfill leaching, generating a rating that may determine the appropriate waste management approach to be applied in a particular situation. A fluoride leach level of >150 mg/L is dangerous, but using TCLP protocol in their investigation, Buamah et al. [63] measured only 22 mg/L of leached fluoride from saturated AOCB, indicating that the fluoride-loaded adsorbent could be safely disposed of at a landfill site. Moreover, it was indicated that the adsorption of fluoride onto gibbsite and thermally activated bauxite is not thermodynamically reversible, indicating the various interactions of the fluoride-laden bauxite adsorbent would not significantly impact water safety [49].

A primary technical barrier to employing bauxite samples to remove fluoride from drinking water is the pH dependence of the adsorbents, with optimum removal efficiency at around pH 6. Hence, the pH-dependent challenge could be solved by acidification to pH 6 using carbon dioxide or hydrochloric acid, which is financially feasible [49,50]. However, in their study, Cherukumilli et al. [50] observed Al concentrations of about 250–620 ppb (>200 ppb, the US EPA acceptable level) in a particular bauxite sample using acidification and 100 °C preheating. However, it fell considerably below the EPA guidelines at preheating temperature 300 °C and acidification. Mn approached or exceeded the EPA limit in the product water at a preheating temperature of 300 °C and bulk acidification [50]. Because boehmite is generally less soluble than gibbsite [131], it is assumed that the lower Al leaching dissolution in 300 °C pretreated bauxite is due to the conversion of gibbsite to boehmite [50]. Hence, the best scenario would be heating bauxite slightly above 300 °C and continuous acidification. Moreover, As is found at levels above the WHO-MC using EPA leaching protocol; however, given the extreme nature of the protocol, As is likely not to generate colossal concern with water treatment using bauxite [49]. Further, Thole et al. [69] investigated the effect of particle size on loading capacity and water quality. They observed that while the loading capacity increases with a decrease in particle size, the parameters, such as color, alkalinity, hardness, and residual sulfate content, seemed to rise with particle size decline. It was earlier stated that SO₄²⁻ and PO₄³⁻ negatively impact the adsorption capacity of fluoride on bauxite.

Considering the regenerative potential and negligible fluoride-leaching levels of some bauxite samples, they could be promising adsorbents that are safe for disposal as stable solid wastes after being used. However, the possibility of phosphate and sulfate occurring in groundwater requires attention in adsorption reactor design. The levels of Al and As, as well as the degree of color, pH, alkalinity, hardness, and residual sulfate content during particle size reduction, must all be monitored for their optimization when treated with bauxite.

6. Comparison between Bauxite Adsorbents and AA

Raw or mildly treated bauxite provides distinct benefits over conventional adsorbents for removing F^—. Adsorption from this category of adsorbents is a classic example of Pearson’s hard–soft acid–base theory because hard Lewis bases such as F^— and anions have better interactions with hard Lewis acids such as Al³⁺ and Fe³⁺, which prefer them to soft bases. Table 4 summarizes the adsorption characteristics of several bauxite samples towards F^— removal from water. The most commonly used commercial adsorbents are AA and AC [13]. Other commercial adsorbents include TiO₂, ArsenXP^np-A33E (A33E), granular activated carbon (GAC), and granular ferric hydroxide (GFH).

In comparison, some composite mineral adsorbents and their modified versions are usually better than or comparable with current commercial fluoride removal filters, adsorbents, and conventional techniques. Although some bauxite samples combine various excellent adsorption functionalities for actual groundwater defluoridation, such as high removal efficiencies and rapid removal rates, their adsorption capacities are relatively low and restricted to a narrow operational pH range. Nevertheless, the adsorption functionalities of most bauxite samples are suited to reduce fluoride concentration below the permissible levels of 1.5 mg/L, and the pH range of maximum fluoride adsorption is similar to that of drinking water. Moreover, producing AA filter media (USD 1800/tonne) costs around 60 times more than raw bauxite ore (USD 30/tonne) [26,27,28], and the composite ore deposit abundance in fluoride-contaminated regions, bauxite may prove to be a cost-effective adsorbent for fluoride sequestration from groundwater. The price of AC was estimated between USD 1440–5000/tonne, depending on the method and material used [29,30,31,32,33], and can rise to USD 20,000/tonne for acid-activated metal ion sequestering carbon [29]. Additionally, AC generally has a relatively small adsorption capacity for fluoride [37].

Further, the fluoride removal rates and adsorption capacities with TiO₂, A33E, GAC, and GFH in actual groundwater are low [132]. As a result, substituting minimally processed bauxite ore for commercial adsorbents might result in a fluoride remediation method that is (a) more effective in terms of USD/volume of water treated, (b) affordable for low-income earners, and (c) more widely available in afflicted regions [26]. To summarize, bauxite samples have enhanced adsorption properties and appear to be promising low-cost environmental candidates for extracting fluoride ions from groundwater in rural and disadvantaged communities.

Table 4. Comparative evaluation of selected bauxite adsorbents and AA used for F⁻ removal from water.

Material	[F−]0 (mg/L)	pH	Contact Time (h)	Ads Dose (g/L)	Max Rem. (%)	Max Ads. Cap(mg/g)	Kinetic/Isothermal Models	Ref
AA	4GW	7.8	24	6.67	>99	1.08	Langmuir	[58]
ACG	2–10	3–11.5	0–140	0–12.5	99	3.91–0.806	Langmuir	[133]
AC	30	2	1	15	73	33.3	Freundlich	[134]
GAC	2.5	~7	7	5	35.20	0.18	PSO/	[132]
TiO2	2.5 (2.4GW)	~7(8GW)	7	5	98.10 (49.6GW)	4.96 (~0.18GW)	PSO/BET	[132]
A33E	2.5	~7	7	5	50.40	–	PSO/Freundlich	[132]
GFH	2.5	~7	7	5	14.60	–	–	[132]
BXT	10.81 GW	8.1	24	20	70.4	–	–	[43]
B-Mg-500	3	7.6	3	5	55	~0.18	Freundlich	[48]
AOCB	40–60SGW	7	24	12	~55.13	1.1	Langmuir & Freundlich	[63]
KRB	3–15	6	1.5	32	96.2	–	Langmuir	[64]
GRB	10SGW	6	3	~10	>85	1.7 *	Freundlich	[26]
Gh-B	10.07	5.3	24	6.67	~45	0.83	Langmuir	[58]
TRB	10	5.5–6.5	1.5	4	90	3.6	PFO/Langmuir & Freundlich	[45]
TTB	5–25	2–8	3	5–30	97.7	3.01	Langmuir	[44]
LRB	6.17 GW	8.12	1.5	8	96.1	0.27	PFO/Freundlich	[65]
URB	10	7	3	10	-	3.15	Freundlich	[66]
SATB	6	7	3	25	75.31	0.63	Freundlich	[67]
SYTB	2.74GW	6.9–7.7	3	30	59.9	–	Freundlich	[67]
SATB	6	7	3	25	51.21	0.44	Freundlich	[67]
SYTB	2.74GW	6.9–7.7	3	30	36.68	–	Freundlich	[67]
SMTTB	8	4	24	12.5	93.8	–	PFO/Langmuir	[46]
SMTTB	7.5 max (GW)	9.5max	24	12.5	–	–	PFO/Langmuir	[46]
GACB	5SGW	6–7	176	10	>90	12.29 *	PSO/Freundlich	[60]
SRB	4.85 GW	5–7	2	6	62	5.16 *	PFO/Langmuir	[59]
LIB	20	6.5–8.5	2	2	99	18.18 *	PSO/Langmuir	[61]

Max = maximum; ads = adsorption; rem = removal; GW = groundwater; SGW = simulated groundwater; * = isothermal capacity; PFO = pseudo-first-order; PSO = pseudo-second-order; Ref = reference; ACG = alumina cement granules; A33E = ArsenXP^np-A33E; GAC = granular activated carbon; GFH = granular ferric hydroxide.

Table 4. Comparative evaluation of selected bauxite adsorbents and AA used for F⁻ removal from water.

Material	[F−]0 (mg/L)	pH	Contact Time (h)	Ads Dose (g/L)	Max Rem. (%)	Max Ads. Cap(mg/g)	Kinetic/Isothermal Models	Ref
AA	4GW	7.8	24	6.67	>99	1.08	Langmuir	[58]
ACG	2–10	3–11.5	0–140	0–12.5	99	3.91–0.806	Langmuir	[133]
AC	30	2	1	15	73	33.3	Freundlich	[134]
GAC	2.5	~7	7	5	35.20	0.18	PSO/	[132]
TiO2	2.5 (2.4GW)	~7(8GW)	7	5	98.10 (49.6GW)	4.96 (~0.18GW)	PSO/BET	[132]
A33E	2.5	~7	7	5	50.40	–	PSO/Freundlich	[132]
GFH	2.5	~7	7	5	14.60	–	–	[132]
BXT	10.81 GW	8.1	24	20	70.4	–	–	[43]
B-Mg-500	3	7.6	3	5	55	~0.18	Freundlich	[48]
AOCB	40–60SGW	7	24	12	~55.13	1.1	Langmuir & Freundlich	[63]
KRB	3–15	6	1.5	32	96.2	–	Langmuir	[64]
GRB	10SGW	6	3	~10	>85	1.7 *	Freundlich	[26]
Gh-B	10.07	5.3	24	6.67	~45	0.83	Langmuir	[58]
TRB	10	5.5–6.5	1.5	4	90	3.6	PFO/Langmuir & Freundlich	[45]
TTB	5–25	2–8	3	5–30	97.7	3.01	Langmuir	[44]
LRB	6.17 GW	8.12	1.5	8	96.1	0.27	PFO/Freundlich	[65]
URB	10	7	3	10	-	3.15	Freundlich	[66]
SATB	6	7	3	25	75.31	0.63	Freundlich	[67]
SYTB	2.74GW	6.9–7.7	3	30	59.9	–	Freundlich	[67]
SATB	6	7	3	25	51.21	0.44	Freundlich	[67]
SYTB	2.74GW	6.9–7.7	3	30	36.68	–	Freundlich	[67]
SMTTB	8	4	24	12.5	93.8	–	PFO/Langmuir	[46]
SMTTB	7.5 max (GW)	9.5max	24	12.5	–	–	PFO/Langmuir	[46]
GACB	5SGW	6–7	176	10	>90	12.29 *	PSO/Freundlich	[60]
SRB	4.85 GW	5–7	2	6	62	5.16 *	PFO/Langmuir	[59]
LIB	20	6.5–8.5	2	2	99	18.18 *	PSO/Langmuir	[61]

Max = maximum; ads = adsorption; rem = removal; GW = groundwater; SGW = simulated groundwater; * = isothermal capacity; PFO = pseudo-first-order; PSO = pseudo-second-order; Ref = reference; ACG = alumina cement granules; A33E = ArsenXP^np-A33E; GAC = granular activated carbon; GFH = granular ferric hydroxide.

7. Conclusions and Prospect

Recent developments in using composite mineral assemblage to recover fluoride from polluted groundwater via adsorption are discussed in this work. Considerable energies have been expended for composite ore materials research in environmental remediation, especially the defluoridation of groundwater. Composite oxyhydroxide mineral ores are one of the most promising natural materials for reducing environmental degradation, notably elemental sequestration from aqueous media. The characterization, pretreatment methods, adsorption mechanisms, influencing factors, regenerability, and the environmental repercussions of using composite oxyhydroxide mineral ores in the defluoridation of groundwater were emphasized.

Regarding fluoride adsorption, the primary minerals in bauxite are oxides/hydroxides of Al, Fe, and Si; however, minor minerals such as dolomite, calcite, gypsum, and magnesite play a prominent role in modifying the overall adsorption functionalities of the ore. The primary pretreatment technique for bauxite is calcination which can be done alone or in combination with metal incorporation. The principal role of the modification processes is to enhance adsorbent sorption characteristics via activating original sorption sites, creating new sorption sites, and facilitating a solid binding of incorporated metal ions to the surface of the adsorbent. The primary fluoride adsorption mechanism of bauxite is ligand exchange between F⁻ and OH⁻, with the –OH₂− and metal hydroxide taking part in the process at low and high fluoride levels, respectively. The dominant adsorption mechanism is a strong chemical interaction (especially inner-sphere complexation). Outer-sphere complexation and intraparticle diffusion may also occur. At low pH, the fluoride adsorption mechanism may include electrostatic attraction due to the positive charge of the adsorbent surface; at higher pH, the indirect substitution of hydroxide groups with the fluoride ions on the adsorbent surface dominates. Chemical treatment with NaOH appears appropriate for the regeneration of bauxite samples. Studies on the effect of different parameters and conditions indicate that bauxite is cost-effective, safe, and environmentally friendly for groundwater treatment with composite ore. However, because of the relatively high effect of SO₄²⁻ and PO₄³⁻ on the adsorption capacity, the levels of leached Al and As, and the impact of particles size on loading capacity and water quality, robust monitoring of the water quality should be carried out at all stages of the defluoridation process.

Nevertheless, a host of challenges remained unsolved. Addressing these problems will require thoroughly investigating the significant gaps and proffering effective strategies to remedy them. One primary problem faced by bauxite and other composite mineral assemblages is their relatively low adsorption capacities. A principal solution is to pretreat the material with relatively expensive heating procedures. As a result, there is a pressing need for study into the scalability of composite mineral ore adsorbent manufacturing and industrial use at a cheap cost and with ease of operation while preserving and improving active sites.

Furthermore, most laboratory works assessing the efficacy of composite oxyhydroxide mineral ores in removing fluoride have used environmentally relevant fluoride concentrations; nevertheless, it is not unusual for water to include several contaminants. Hence, it is proper to explore the efficacy of composite minerals ore with simultaneous multiple-pollutant-contaminated aqueous media. This review revealed that the regeneration of bauxite adsorbent is not fully understood, and the small amount of research conducted on the subject has shown mixed results. Although chemical treatment with NaOH seems suitable for the regeneration of bauxite samples, the absence of a proper understanding of bauxite adsorbent regeneration and reusability necessitates detailed research. To fully appreciate the adsorption mechanism of bauxite and other composite mineral ores, removal of fluoride from aqueous media, mechanistic modeling, and continuous flow research should also be explored.

This research aimed to show that composite oxyhydroxide mineral ores may be used as low-cost alternatives to their more costly conventional equivalents for defluoridation groundwater, particularly in rural areas. With the information provided herein, this research should serve as a comprehensive review of studies on the use of bauxite in fluoride removal, as well as a compelling incentive to continue developing composite mineral ore materials for future and sustainable environmental remediation research, especially in the field of aqueous pollution mitigation.