This paper presents a comparative review of arsenite (As(III)), arsenate (As(V)), and fluoride (F) for a better understanding of the conditions and factors during their adsorption with focus on (i) the isotherm adsorption models, (ii) effects of pH, (iii) effects of ionic strength, and (iv) effects of coexisting substances such as anions, cations, and natural organics matter. It provides an in-depth analysis of various methods of arsenite (As(III)), arsenate (As(V)), and fluoride (F-) removal by adsorption and the anions’ characteristics during the adsorption process. The surface area of the adsorbents does not contribute to the adsorption capacity of these anions but rather a combination of other physical and chemical properties. The adsorption capacity for the anions depends on the combination of all the factors: pH, ionic strength, coexisting substances, pore volume and particles size, surface modification, pretreatment of the adsorbents, and so forth. Extreme higher adsorption capacity can be obtained by the modification of the adsorbents. In general, pH has a greater influence on adsorption capacity at large, since it affects the ionic strength, coexisting anions such as bicarbonate, sulfate, and silica, the surface charges of the adsorbents, and the ionic species which can be present in the solution.

1. Introduction

Hazardous anions are another group of pollutants in drinking water in addition to metal ions and organics which are known to be toxic and carcinogenic. The presence of these anions in ground and surface waters has resulted in severe contamination and has caused adverse health effects. Among the toxic anions, arsenite (As(III)), arsenate (As(V)), and fluoride (F-) have shown concern in the treatment of wastewaters and drinking water. Adsorption process among several treatment technologies is applied for removal of fluoride (F-), arsenite (As(III)), and arsenate (As(V)) ions due to the availability of local material and being easy to operate and maintain and in general it is a low-cost technology and is suitable for use in developing countries. The adsorption process is influenced by several environmental conditions, such as pH, salt effect, or ionic strength, the presence of other anions, cations, and organic matter, and adsorbate physical and chemical properties which have been reported in previous studies [15]. However, no previous study was done to compare the two anions based on the influence factors such pH, salt effect, or ionic strength and the presence of other anions and their relationship on the adsorption isotherm. This review aims to present an evaluation of these parameters and their effects on arsenite (As(III)), arsenate (As(V)), and fluoride (F-) by adsorption process on different adsorbents. It also seeks to find the relationship between the anions and their influencing factors. Therefore, this review focuses on the isothermal adsorption models, the effects of the pH and ionic strength, and the impact of coexisting substances including anions and cations to show the characteristics of fluoride (F-), arsenite (As(III)), and arsenate (As(V)) anionic adsorption processes.

1.1. Arsenic

Arsenic is discharged into the environment by natural activities and anthropogenic activities [6, 7]. Arsenic occurs in both organic and inorganic forms. Arsenate (As(V)) and arsenite (As(III)) are the inorganic forms of arsenic which are considered to be more toxic and are more prevalent in water. The existence of arsenic species depends on the pH solution and redox conditions [8]. The ingestion of inorganic arsenic can result in both cancerous and noncancerous health effects. An arsenic concentration less than 0.05 mg/L in chronic exposure has been linked to skin diseases, neurological and cardiovascular system disorder, and skin, kidney, and lung cancer [6, 9, 10]. Arsenic affects physiological activities, such as the activities of essential cations, enzymes, and transcriptional events in cells [6, 9]. Elevated concentrations of arsenic in drinking water have also been reported to cause an increase in abortions and stillbirth [11].

The methods of arsenic removal from water include oxidation and filtration, adsorption, biological oxidation, coprecipitation, and membrane technologies [12]. Numerous arsenic treatment technologies require pH adjustment and are useful in removing arsenic in the pentavalent state. Thus oxidation is included as a pretreatment to convert As(III) to As(V) [13]. However, the process of coagulation and flocculation produces sludge, which makes it inefficient for As(III) removal [13]. Adsorption process with a wide range of adsorbents has been proven to be effective for the adsorption of arsenate (As(V)) and arsenite (As(III)) ions from water and has been reported by various researchers [1416].

1.2. Fluoride (F)

The contamination of fluoride in groundwater has been known worldwide to cause severe human health problems [17, 18]. Fluorides exist with iron, aluminum, and beryllium as fluoride ion (F-) in natural waters [19]. It is caused by the discharge of mineral sediments and industrial (production of phosphate fertilizers (3.8% fluorine), bricks, tiles, and ceramics) effluent which contains fluoride in receiving water bodies [19]. The inorganic fluorine compounds are used in industry for aluminum production and as a flux in the steel and glass fiber, fluorosilicic acid, sodium hexafluorosilicate, and sodium fluoride are used in public water fluoridation treatment [18]. Fluoride is also essential in human health as the component for normal mineralization of bones and formation of dental enamel [20].

Fluoride is beneficial in drinking water at levels up to 0.7 mg/L but is harmful above 1.5 mg/L, according to the World Health Organization limit [21]. The excessive intake of fluorides may result in dental and skeletal fluorosis [20]. Fluorosis is prevalent in more than 20 developed and developing nations [20, 21]. Fluorides can also cause arthritis, infertility osteoporosis, brain damage cancer, Alzheimer syndrome, thyroid disorder, and brittle bones [17, 21]. High fluoride concentrations are known to interfere with carbohydrates, lipids, proteins, vitamins and mineral metabolism, and gastrointestinal irritation when by initially, acting on the intestinal mucosa, and then form hydrofluoric acid in the stomach at a later stage [17, 20]. Fluoride can also cause kidney disease in both animals and humans and interfere with the functions of the brain and pineal gland [17].

Due to the toxic effects of fluoride in human health, various methods have been developed for the removal of excess fluoride ions from drinking water. Currently, the common methods include adsorption into activated alumina (AA), bone char, and clay [22], precipitation with lime, dolomite, and aluminum sulfate, Nalgonda technique [23], ion exchange and reverse osmosis, electrodialysis, and nanofiltration [22]. The coagulation, adsorption, and ion-exchange techniques are widely opted as defluoridation techniques practiced in fluoride endemic areas [23]. The Nalgonda technique and adsorption by bone char have been used in many developing countries like Tanzania and India [23]. However, the disadvantages of this technique were reported by some researchers, for example, higher residual aluminum concentration (2–7 mg/L) in treated water than WHO standard of 0.2 mg/L [17]. The membrane processes provide good-quality water but have a higher cost of operation [23].

2. Adsorption Method for Arsenic and Fluoride Removal

Adsorption methods have been proven to remove fluoride and arsenic up to 90% from water [16, 22]. However, in adsorption process, the constituents of adsorbents are at large accountable for the elimination of pollutants [24, 25]. Many adsorbents are used in adsorption process, but the most common ones in the treatment of both water and wastewater are activated carbon, activated alumina, ion-exchange resins, metal oxides, hydroxides, carbonates, and clays [16, 22]. The adsorption process is described through equilibrium isotherm which occurs when an adsorbate comes in contact with the adsorbent for a period of time. Thus, the concentration of adsorbate in the solution is balanced with the interface concentration [25]. Equilibrium isotherm is essential for the effective design of sorption systems [26].

The adsorption capacity of the various adsorbents is influenced by pH, the existence of other adsorbing anions, ionic strength, temperature, properties of the adsorbents, initial concentration of the adsorbates, and so forth [16, 17]. Experimental results showed that the pH solution is the main controlling parameter in the adsorption processes [27]. pH affects adsorption by affecting the surface charge on the adsorbent [4, 24, 28, 29]. However, the evaluation of pH effects is governed by the specific interactions between the ions and the adsorption sites [30]. The adsorption of anions decreases by increasing pH due to the higher concentration of competitive anions, such as OH and increases due to protonated surfaces. In aqueous solution, the adsorption process is primarily governed by the zero point charge (ZPC) of an adsorbent [31, 32]. At zero point charge, the pHzpc is the characteristic that determines the pH at which the adsorbent surface has net electrical neutrality. It is also where the acidic or basic functional groups no longer contribute to the pH of a solution [33]. At pH values above the ZPC, the surface has a net negative or anionic charge, and the surface would take part in cation attraction, as well as cation exchange reactions. The surface has a net positive charge at pH values below the ZPC. Therefore, it will attract anions USEPA [34]. Metal adsorption’s dependence on pH is related to the nature and ionic state of the functional group existing in the adsorbent and the metal chemistry in the solution [3537].

Many studies have been conducted on fluoride removal by using adsorption [22] and arsenic [16]. Moreover, pH has been reported to be a key factor affecting fluoride adsorption at the water adsorbent interface [38]. It has been reported that the removal of fluorine ions occurs between pH of 2.0 and 8.0 [27, 39, 40]. Also, the adsorption of arsenic species is highly dependent on pH due to its ability to exist as As(III) or As(IV) at different pH values. As(III) is highly removed at higher pH (basic) value, while As(IV) is removed in acidic pH and rapidly decreases in basic medium [4143].

The adsorption capacity of the adsorbents is also influenced by the physical and chemical characteristics of the adsorbent [16]. The surface area and total pore volume determine the adsorption capacity based on the results of the kinetics and equilibrium on adsorption experiments [4447]. The control of chemical and physical oxygen functional groups considerably affects their performance on the adsorption process [47]. However, the use of Extended X-ray Absorption Fine Structure (EXAFS), Fourier transform infrared spectroscopy (FTIR), and X-ray absorption near edge structure (XANES) has been employed for the removal of fluoride [48] and arsenic [49] to further study the adsorption mechanism and involvement of functional groups in different adsorbents. These have revealed the involvement of functional groups like hydroxyl and carbonates in the surface, which participate in the adsorption.

Different materials exist in water and wastewater, which have different absorption properties and may compete with each other for the limited amount of sites adsorption, and therefore the adsorption in the specific material is reduced [4, 50, 51]. These materials include anions and cations and organic matters [5155]. Anions such as sulfate, nitrate, carbonate, chloride, bicarbonate, and phosphate influence adsorption by adjustment of the electrostatic charge at the solid surface because of the same negative ions [11, 5661]. The effects of anions on fluoride removal were reported by [40, 6266]. Both direct and indirect effects of these anions are influenced by pH, anions’ concentrations, and intrinsic binding affinities [42, 59]. One of the common interfering anions is phosphates. Phosphates exist in four different forms depending on the pH value, phosphate ion (), dihydrogen phosphate ion (H2PO4−), and hydrogen phosphate ion () in dilute aqueous solution and in aqueous phosphoric acid (H3PO4(aq)) [60]. ions are more predominant in strongly basic conditions, ions in weakly basic conditions, H2PO4− ions in weak acidic condition, and aqueous H3PO4 [60]. In general, , , and Cl interfering ions have shown an insignificant effect on adsorption of these anions compared with and , which have a great impact in all anions adsorption [16, 67].

The effects of cations like Al3+, Fe3+, Mg2+, Ca2+, and Mn2+ in adsorption and their interference with the adsorption capacity of these anions have been reported by [56, 62, 6871]. The presence of cations like Ca2+ and Mg2+ increases the fluoride with increasing Ca2+ and Mg2+ concentration [72]. Ionic strength has been reported to increase with an increase in adsorption for arsenate [67] and it has no significant effect on the adsorption of fluoride [73].

Natural organic matter in water may delay sorption equilibrium and decrease the extent of arsenite and arsenate adsorption [16]. Natural organic matter contains a mixture of weak organic acids and organic compounds which do not have a clear chemical structure [50]. Fulvic acids (FA) and humic acids (HA) are hydrophobic in nature, and they represent almost 60% of the dissolved organic in aquatic systems [56]. Humic substances interfere with anionic adsorption through stable metal complex formations [23, 54, 56]. Due to the importance of these parameters in adsorption process, we evaluated the adsorptions isotherms, coexisting substances, the effects of pH, and ionic strength and their effects on arsenite (As(III)), arsenate (As(V)), and Fluoride (F-) removal on different adsorbents and aimed to find the relationship between them. The insight of anion adsorption in water basically confined to the adsorption of a single ion and on the specific adsorbent. Therefore this review summarizes the common characteristics of anions adsorption and explains the roles for the relevant forces and the adsorption control conditions and features of the adsorbents suitable for the arsenite (As(III)), arsenate (As(V)), and fluorine (F-) adsorption in water.

3. Adsorption Isotherm Studies on As(III) and As(V)

Freundlich isotherm, Langmuir isotherm, and BET isotherm are examples of the most frequently used isotherm models in adsorption process [25, 26, 37, 52]. In this review, only Freundlich and Langmuir were taken into consideration due to their wide applicability to gain an insight into the degree of the favorable adsorption [74].

Adsorption technology can reduce arsenic concentrations to less than 10 mg/L [75], and the common sorbents for arsenic removal are commercial and synthetic activated carbons, agricultural products and by-products, and industrial waste soils and constituents, for example, clay minerals, manganese dioxide, zeolite, activated alumina, ferrihydrite/iron and hydroxide/iron oxides, hydroxides, hydrotalcites, phosphates, and metal-based methods, for example, zerovalent iron [13, 16]. Iron-based sorbents (IBS) have been reported as promising adsorbents for arsenic removal, since they have higher affinity for arsenic under neutral conditions [76]. These include activated ferric oxide or ferric hydroxide and iron-coated sand [13]. The removal of arsenic by using iron-based adsorbents occurs through adsorption to a surface hydroxyl group, coprecipitation, and ion exchange [77]. However, with the iron oxides adsorbents, the removal of As(V) at neutral pH occurs through ligand exchange by the formation of monodentate complexes at low surface coverage, while at high surface coverage, As(V) species bind to the oxides through the formation of bidentate complexes while occupying two adsorption sites at the same time [78]. Few activated carbons are selective for the adsorption of As(III) and As(V) in concentrations <0.5 mg/L from water [51].

As(V) can be easily removed by a wide range of adsorbents compared to arsenic (III) (Table 1) and because arsenic (III) is easily adsorbed to arsenic (V) and is oxidized before or during the adsorption process. As(III) oxidation occurs naturally or by using iron, hydrogen peroxide, chlorine, hypochlorite, and manganese, as shown in a study conducted by Giles et al. [79]. Also, another reason is the fact that adsorption of arsenic significantly depends on arsenic speciation, which also depends on pH and potential. Modified adsorbents have been shown to have a higher capacity for arsenic adsorption (Figure 1) compared to unmodified adsorbents. The modified adsorbents include, but are not limited to, acid modified carbon black, modified red mud, chitosan-coated biosorbent, bimetal oxide magnetic nanomaterials (MnFe2O4 and CoFe2O4), polymeric Al/Fe modified montmorillonite, manganese oxide-coated alumina and synthetic siderite. During arsenic adsorption, surface area is insignificant (Figure 2) and does not contribute to the adsorption capacity of the adsorbents [30, 76, 80]. The equilibrium adsorption capacity depends on the concentration of ionic species in solution and the sorbent properties [30]. The choice of the adsorbents to agree with Langmuir or Freundlich is still unclear. However, a study by [81] suggested that if As(V) followed Freundlich isotherm on arsenic adsorption onto Iron-Zirconium Binary Oxide-Coated Sand (IZBOCS), it showed that the adsorption of As(V) followed multilayer and heterogeneous adsorption process. Langmuir model was the best fit for As(III) and As(V) adsorption on iron–aluminum hydroxide coated onto macroporous and, according to the author, the adsorptions occur through chemisorption.

Studies by using X-ray photoelectron spectroscopy (XPS) have revealed that As(III) was oxidized and absorbed in the form of As(V) on the surface of cupric oxide nanoparticles (CuO) [101]. FTIR spectroscopy has also been used to elucidate the arsenic adsorption mechanism on iron mineral oxide which involves ligand exchange reactions where the anions displace OH and H2O from the surface [102]. Fourier transform infrared (FTIR) spectra demonstrated that Ca–OH functional group was involved in As(V) removal of bone char and coprecipitation and ion exchange was involved before and after As(V) adsorption [103]. Another study suggested that the adsorption of As(III) occurs through electrostatic attraction on bismuth-impregnated biochar [104]. Similar results were obtained by [105] on arsenic adsorption on feldspars, but the electrostatic attraction was between the luminol function groups and predominant form of arsenate in low pH. Another EXAFS analysis concluded that the adsorption of As(III) and As(V) on iron–aluminium hydroxide coated onto macroporous was involved through formation of inner-sphere complexes to the iron hydroxide. However, for As(III) adsorption, the bidentate complex was formed, which corresponded with the pseudo-second-order kinetic model [106].

The adsorption of As(V) anions on activated carbon was associated with the concentration of hard acid functional groups and not the carbon surface area [30]. Similar results were reported by [80] on arsenic adsorption on hydrous ferric oxide. Figure 2 shows that adsorption capacity of various adsorbents does not depend on the surface area for arsenic removal. However, it has been reported that changes in the type of arsenic complex binding with the surface area depend on pH and the number of available sites for adsorption for specific adsorbents [80]. In arsenic ions adsorption, pore diffusion is important, since the particle size, solubility, pH, and metal ions concentration affect adsorption capacity and the adsorption capacity increases with a decrease in particle size [76, 107].

Hassan et al. [108] investigated arsenic removal on potassium hydroxide activated carbon-based apricot stone, calcium alginate beads, and calcium alginate/activated carbon composite beads. The adsorbents materials have surface areas of 1621 m2/g, 32.9 m2/g, and 733.6 m2/g and the adsorption capacity at 20°C was 26.3 mg/g, 39.4 mg/g, and 54.8 mg/g, respectively. The authors concluded that the As(V) adsorption depends largely on pH, surface chemistry of solid adsorbent, and textural properties of the adsorbate besides its surface area. Another study by using magnetic graphene oxide as an adsorbent for arsenic removal showed that initial concentration of arsenic and the adsorbent dosage were the major factors affecting the adsorption capacity [109].

A maximum adsorption capacity of 3.1 mg/g was reported for activated carbon prepared from oat hulls which has a specific area of 522 m2/g at initial arsenic concentrations of 25 to 200 μg/L [110]. Similarly, a low adsorption capacity for adsorbents with a high surface area was observed by Roy et al. [111] on thioglycolated sugarcane carbon (TSCC) as an adsorbent for arsenic removal. TSCC has a surface area of 5690 m2/g. The maximum reported As(III) and As(V) removal by TSCC was 85.01 and 83.82 μg/g, respectively, with an initial arsenic concentration of 1,500 μg/L [111].

Low surface areas with high adsorption capacity have also been observed in numerous research works. For example, the removal of As(V) and As(III) from water by a Zr(IV)-loaded orange waste gel had an initial concentration of 20 mg/L and surface area of 7.25 m2/g. The maximum adsorption capacity of the Zr(IV)-loaded sol-gel was 88 mg/g and 130 mg/g for As(V) and As(III), respectively [112]. Similar results were reported by [83] on chitosan-coated biosorbent and by [14] on zirconium polyacrylamide hybrid material (ZrPACM-43). A high adsorption capacity has also been reported for an adsorbent with a high surface area. For example, Liu et al. [113] investigated arsenic adsorption on Fe3O4-loaded activated carbon prepared from waste biomass with surface area of 349 m2/g; a very high adsorption capacity of 204.2 mg/g was observed with arsenic initial concentration of 40 mg/L [113]. With the reported observation, selection of adsorbent for arsenic adsorption should be based not only on surface area of the adsorbent but also on the combination of chemical properties of the adsorbent and adsorbate.

4. Effects of pH on Arsenic Removal

Removal of arsenic from water depends on pH as the adsorption capacity changes with changes in pH. The reduced trivalent form of arsenic (arsenite (As(III))) primarily exists in natural waters and anaerobic environment, while arsenate (As(V)), an oxidized pentavalent form, is found in aerobic environment conditions such as surface waters [79]. Arsenate species exist as , , and and arsenate species exist as As(OH)3, As, and As depending on the pH solution [114]. Chemical speciation affects arsenic removal in aqueous solution (USEPA) [115]. As(V) exists as a charged species in water across a wide range of pH while As(III) exists as a charged species across a much narrower range of pH [16, 42, 87, 89, 116, 117], and it has been reported that many adsorbents prefer to adsorb charged species [16]. Arsenite (As(III)) is uncharged (i.e., H3AsO3) at natural pH levels (6–9) (Figure 3); thus it it difficult to remove compared to charged arsenate (As(V)) (i.e., or ) (USEPA) [115]. As(III) is oxidized to As(V) by an oxidizing agent such as chlorine or permanganate in most arsenic treatment processes [79, 89]. Adsorbents like iron-based and iron minerals are characterized by their point of zero charges at a certain pH, where the mineral surface charge is equal to zero [42, 50, 84]. However, in other adsorbents, the strong adsorption of arsenic at pH > pHpzc indicates that the adsorption process is influenced by surface complexation and not electrostatic interaction [86].

Arsenate and arsenite adsorption onto activated alumina was governed by both the surface charge of activated alumina and the form of arsenic species in the water. Since alumina is sensitive to pH, adjustment of pH is done prior to treatments. Therefore, the adsorption of arsenite (As(III)) is much less than that of arsenate (As(V)) for activated alumina in most pH conditions, because arsenate is present in the negatively ionic state and arsenite is in nonionic state. For example, Singh and Pant [82] studied the removal of As(III) by activated alumina (AA), and the result showed that 94.4% maximum adsorption was reached in the pH range of 6.0–8.0 and it decreased at higher pH values [82]. Similar results were observed by [80, 87].

A study by Mamindy-Pajany et al. [118] on adsorption of As(V) on commercial hematite and goethite as a function of pH (2 to 12) and ionic strength showed that As(V) adsorption was higher at low pH value and insignificant at higher pH values. Similar results were reported by [119] on As(V) adsorption on goethite from aqueous solution and [120] on As(III) and As(V) adsorption on hematite, magnetite, and goethite. However, hematite was reported to be the appropriate adsorbent for As(V) removal in natural medium because it can exist at a wide pH range [99]. Mamindy-Pajany et al. [121] observed a similar trend on As(V) adsorption by hematite but, for As(III), the adsorption was highly dependent on initial pH, and a decrease in adsorption was observed at the pH value of 3.

The adsorption of As(III) and As(V) has also been reported to be affected by pH higher than 10. For example, a study of As(III) and As(V) ions to the Fe3O4 nanomaterial [70, 122] observed similar trend on an iron oxide-coated cement (IOCC) adsorption capacity of As(III) removal and [123] on activated carbon. Also, a high adsorption percentage was observed at higher pH for As(V) adsorption over activated charcoal and bone char adsorbents. Adsorption of As(V) has been reported to be maximum at low pH values. Ansari and Sadegh [123] reported a maximum adsorption of As(V) at pH of 3 and decreased with the decrease of pH. That is, adsorption by activated carbon is thought to be carried out through anion exchange process and physicochemical adsorption due to the highly porous structure of activated carbon. The surface of the activated carbon has a positive charge under acidic conditions, where these positive charges are balanced with their associated anions. Therefore, these anions can be exchanged with the other anionic species present in the solution. Arsenic (III) adsorption on synthetic siderite has also been related to electrostatic attraction as well as physicochemical reactions [124]. In general, As(III) is adsorbed at neutral pH (Figure 3) and As(V) at a wide range of pH (2–10) depending on the adsorbent material used. A similar trend on arsenic adsorption by various adsorbents was reported by [50, 57, 60, 103].

Another different result on effects of pH on As(V) has been reported by [83] on removal of arsenic (III) and arsenic (V) from aqueous medium using chitosan-coated biosorbent (CCB), whereby a decrease in pH increases As(V) adsorption and the mechanisms involved in As(V) removal were ionic attraction, nodule formation, and absorption.

5. Effects of Coexisting Anion, Cation, and Organic Matter on Arsenic

5.1. Effects of Coexisting Anions

Coexisting anions such as Cl, , , , and ions, in water sources, might compete with arsenic for active adsorption sites and significantly reduce the arsenic removal performance by the adsorbents. These coexisting anions have been reported to form inner-sphere complexes with arsenic and reduce the interaction between the two anions. The effects tend to be significant with a change in pH and with the increase of the anions concentration [3, 78].

5.1.1. Sulfate

Arsenic removal using KMnO4–Fe(II) showed that, in the presence of 50–100 mg/l of sulfate, adsorption of arsenic was reduced by 6.5–36.0% over pH 6–9. The reasons were the competition for surface sites, the weak affinity of arsenic to the adsorbent, and the mechanisms for the adsorption of arsenic and sulfate being not the same. Guan et al. [56] studied the adsorption of arsenic into hydrous ferric oxide, and the results showed a decreasing trend on adsorption of As(III) after the pH was less than 4 [58]. A similar trend was observed by [60] on As(V) removal by iron oxide and aluminum oxide and by [92] on arsenic removal by goethite and jarosite in acidic conditions. However, Maiti et al. [50] showed that sulfate ion did not have any effect on As(V) and As(III) ions adsorption due to higher adsorption affinity of treated laterite surface on As(V). A similar result was observed by [42, 125].

5.1.2. Phosphate

The effect of phosphate is different for both As(III) and As(V). The decrease in adsorption capacity of the adsorbents is more for As(III) at lower concentration and As(V) at higher concentration. However, the influence of phosphate is also affected by pH. Arsenic removal using KMnO4–Fe(II) showed that arsenic removal was reduced by 29.8%, 34.2%, and 47.3% at pH of 4, 5, and 6, respectively, in the presence of phosphate. The reasons were the competition of arsenate and phosphate for binding sites and inhibition of forming Fe precipitates and reduction of surface sites [56]. And, by increasing phosphate concentration to 2.5 mg/L, the arsenic removal was reduced by 59.6% [56]. As(III) and As(V) removal from water by copper oxide-incorporated mesoporous alumina showed that the presence of phosphate significantly reduces the As(V) adsorption due to the competition between phosphate and As(V) [45, 91]. Jeong et al. [60] reported that the increase of phosphate concentration decreases As(V) adsorption capacity on the adsorption of As(V) by Al2O3 compared to Fe2O3. A similar observation was reported by [89] on aluminum-loaded Shirasu-zeolite as an adsorbent. Arsenic adsorption was reduced by more than 20% when phosphate concentration increased by more than 0.2 mM on copper (II) oxide nanoparticles as adsorbents [57]. Maiti et al. [50] reported that arsenic was reduced to 72.9% removal when phosphate concentration was increased to 5 mg/L on synthetic siderite as an adsorbent for As(V) removal. However, at low concentration of 0.2 mg/L phosphate on Fe(VI)/Al(III) chloride salts, arsenic removal was reduced to 73% [61].

5.1.3. Silicate

In natural water, silicate originates from the weathering of minerals and is always dissolving and precipitating at the earth’s surface. Silicate is usually oxyanions in natural water, with their concentration between 0.45 and 14 mg Si/L [56, 60]. A 50% arsenite removal reduction was observed on Fe(VI), Fe(VI)/Fe(III), and Fe(VI)/Al(III) salt as adsorbent for arsenite removal in presence of 10 mg Si/L [61], whereas with 10 mg Si/L silicate concentration showed no effect with KMnO4–Fe(II) as adsorbent at pH 5, but adsorption was observed to be decreased by 42% at pH 9 [56]. Therefore effects of silicate on arsenic removal are more obvious with an increase in pH and can be associated with the weak silicic acid species distribution with pH [56]. Similar results were reported by [60] on adsorption of As(V) on Fe2O3 and Al2O3. Silicate in water has been reported to have no significant effects on adsorption of As(V) by cupric oxide nanoparticles but only slightly inhibited adsorption of As(III) [101]. 1 mg Si/L silicate concentration was reported to increase Arsenic adsorption due to electrostatic effects of Al2O3 compared to Fe2O3 adsorbent [60].

5.1.4. Bicarbonate and Carbonate

Dissolved carbonate exists as (bicarbonate) and carbonate () in groundwater and is likely to interfere with arsenic adsorption. Bicarbonate ions are known to form inner-sphere monodentate complexes with surface functional groups of Fe and Al hydroxides. With the existence of 400 mg/L of carbonate concentration in solutions, the percentage of As(III) adsorption decreases to 77%, as it was observed on treated laterite as adsorbents. This was due to the competition between and for positively charged adsorption sites [50]. According to Zhang et al. [42], the addition of decreased arsenic adsorption moderately on arsenite and arsenate adsorption by MnFe2O4 and CoFe2O4. The decrease of adsorption was due to the basic condition when Na2CO3 was added and formation of arsenic–carbonate complexes As(CO3)2−, As(CO3)(OH)2−, and AsCO3+ in the presence of high concentration of [42].

5.1.5. Chloride and Nitrate

Jeong et al. [60] showed that chloride and nitrate anions do not have visible effects on the adsorption of As(V) on Fe2O3 and Al2O3 [60]. This was due to the fact that complexes of chloride and nitrate with Fe2O3 and Al2O3 are much weaker than those between arsenate and Al2O3 or Fe2O3. Similar result was obtained by [61], where nitrate ions were increased from 1 to 10 mgL−1 concentrations on the arsenite removal by Fe(VI), Fe(VI)/Fe(III), and Fe(VI)/Al(III) salts, due to lack of competition between and the adsorbent for adsorption sites [61].

5.2. Effect of Cations

Ca2+ was reported to significantly enhance the adsorption capacity of modified red mud at pH 7.3 [69]. Increasing Ca2+ concentration from 0 mg/L to 40 mg/L resulted in an increase of 1 mg/g of arsenate adsorbed. Ca2+ was claimed to bond the modified red mud particle with arsenate and form a metal–arsenate complex or metal–H2O–arsenate complex [69]. Effects of Mg2+, Ca2+, and Fe2+ on arsenate As(V) and arsenite AS(III) were studied on nano zerovalent iron on activated carbon, and it was reported that the cations considerably increase the adsorption of arsenate As(V) by increasing the pH [3]. The existence of metal cations in the solution shifted the surface of the adsorbent to more positively charged nature, which on the other hand allowed the adsorbent to show higher affinity for arsenate anions [3]. Meanwhile, Fe2+ suppressed arsenite AS(III) adsorption by 4.1% at pH 3.5 and by 22% at pH 6.5 within the presence of Fe2+ [3]. The deprotonation of H3AsO3 at pH < 9 controlled the arsenite adsorption, whereas surface charge was not important. Fe2+ ions form complexes with arsenite in aqueous solution, and, as a result, the degree of deprotonation/dissociation is repressed, and the adsorption was reduced [3]. As(III) removal was increased with an increase of Mg2+, Ca2+, and Fe2+ concentration on iron oxide-coated cement (IOCC): the enhancement of As(III) adsorption onto the adsorbent surface was attributed to the formation of a surface species, whereby Mg2+, Ca2+, and Fe2+ ions cause the negative charges to be weak and act as a link between the adsorbent surface and the As(III) ions, as it was reported in the study conducted by Kundu and Gupta [70].

5.3. Effect of Organic Matter

Natural organic matter (NOM), which is a carbon source, comprises the combination of acidic organic molecules, which does not originate from a diversity of natural sources including sediments, water, and soil [126]. NOM concentration in natural waters ranges between 1 and 50 mg/l and may compete with arsenic for sorption sites [127]. The theory was proven by Guan et al. [56] on As(III) removal by KMnO4–Fe(II), whereby, due to the presence of the NOM, the adsorption of other solute decreased by competing for adsorption sites; hence the adsorption sites were reduced [56]. However, it was reported that the NOM adsorption for magnetite nanoparticles is through the interaction of organic functional groups OH and COOH and ligand exchange of surface hydroxyl groups, electrostatic attraction, and hydrogen bonding [127]. NOM has been found to assist in the As(III) oxidation to As (V) under alkaline conditions in the absence of both O2 and light [128]. It was observed that 8 mg/L of NOM reduces the adsorption of arsenic, whereas the adsorption increases with increasing pH in the pH range of 4.0–9.4 [128]. Another study on the adsorption of As(V) on alumina in the presence of fulvic acid (FA) showed that, at a pH between 3 and 7.5, FA reduces As(V) adsorption due to attraction forces and negatively charged surfaces were formed because of the deprotonation of functional groups at pH above pKa. Arsenic removal in the KMnO4–Fe(II) process was reported not to be affected at pH 4, but at pH 5 in 1 mg/L or 4 mg/L humic acid (HA), the adsorption dropped by 9% [56]. Arsenic removal was reported to be decreased by 24.8% at pH of 6 and by 58.4% at pH 7 in the presence of 1 mg/L HA [56].

5.4. Effects of Ionic Strength on Arsenic

The ionic strength has the ability to affect the binding of the adsorbed species [1, 54] and therefore compete for adsorption sites. By determining the effects of ionic strength, the inner-sphere and outer-sphere ion-surface complexes can be distinguished [129]. Outer-sphere complexes are predictable to be more vulnerable to ionic strength variations than inner-sphere complexes because the background electrolyte ions are positioned in the same plane for outer-sphere complexes [1]; therefore a decrease in the adsorption is observed when conversely the electrolyte concentration is increased due to competition for adsorption sites [130]. Ions that form inner-sphere complexes are straight synchronized to surface groups and may not compete or compete at lower percentage with electrolyte ions [130]. Thus, the adsorption is less affected by changing the ionic. In many cases of inner-sphere complex formation, the adsorption increases with increasing electrolyte concentration [130]. This effect is usually attributed to changes in the electric potential in the interface, whereby the electrostatic repulsion between the charged surface and the anion is decreased, and the adsorption is favored [130].

The effect of ionic strength on the As(V) sorption was studied at 0.02 and 0.15 mol dm−3 NaCl solutions goethite and jarosite and the result shows that As(V) sorption is independent of ionic strength [92]. The researchers suggested that As(V) adsorption on goethite could proceed via the formation of inner-sphere surface. References [92, 131] studied the effects of ionic strength in magnetite nanoparticles on arsenate and arsenite adsorption, and it was reported that the adsorption decreased by 4% by increasing the ionic strength from 0.01 to 0.1. Guo et al. [99] investigated the effect of background electrolyte (NaCl) concentration on synthetic siderite with 0.001–0.1 mol/L NaCl as background electrolyte. The result showed that As(V) removal was not affected by (NaCl). The increase of ionic strength was reported to increase As(V) adsorption onto TiO2 under alkaline conditions (pH 7.0–11.0). However, under acidic conditions, the adsorption of As(V) onto TiO2 decreased with increasing ionic strength in NaCl electrolyte [126]. Therefore ionic strength is also affected by pH of the solution.

6. Fluoride (F) Adsorption Isotherm and Adsorption Capacity

The adsorption behavior of fluoride by various adsorbents varies depending on the bonding between fluoride species and active sites on the surface of the adsorbent [132]. From the various adsorbents compared (Table 2), the Langmuir and Freundlich equations were the best fit for many adsorbents. The highest adsorption capacity for fluoride removal (400 mg/g) was observed on hydroxyl aluminum oxalate with a surface area of 68.34 m2/g [133] and (600 mg/g) for aluminum fumarate with a surface area of 1156 m2/g [134]. There is no general conclusion on how the type of adsorbent fits the chosen isotherm, since the extent of adsorption depends on many factors including the nature of the adsorbate and adsorbent, surface area, activation of the adsorbent, and experimental conditions. The isotherm may differ on the same fluoride concentration according to the type of the adsorbents. For example, [135] reported that Freundlich fitted the sol-gel-derived activated alumina modified with calcium oxide and Langmuir sol-gel-derived activated alumina modified with magnesium oxide.

One of the most widely used adsorbents on fluoride removal is activated alumina and it has widely been applied for fluoride adsorption [22]. However, the adsorption capacity of fluoride on activated alumina depends on pH at large and not on the surface areas of the adsorbent. This can be proven in Figure 4, whereby different adsorbents capacities were compared with their surface area. However, modified activated alumina has shown a considerate higher adsorption capacity compared to unmodified alumina even at high fluoride concentration. For example, [135] reported the removal of fluoride by activated alumina up to 96.23 mg/g for water with a fluoride concentration of 432 mg/L in a sol-gel-derived activated alumina modified with calcium oxide. The reason for the higher adsorption capacity was the alkalinity of the adsorbent and the technique used for modification of the activated alumina. Another study by using sol-gel alumina adsorbents was done on modified immobilized activated alumina (MIAA), and adsorption capacity of 0.76 mg/g was reported compared to 0.47 mg/g for activated charcoal on fluoride concentration of 12 mg/L [136]. Another comparison was made between alkoxide origin alumina and activated alumina on fluoride removal, and higher fluoride uptake was seen with alkoxide alumina because of Fe2O3 and SiO2, activated carbon pores, and the increased electropositivity of the material [4]. The effectiveness of modified material was also reported by [137] on magnetite modified with aluminum and lanthanum ions for the adsorption of fluoride. The adsorption of fluoride was reported to increase by 60% and 66% for the aluminum and lanthanum modified materials, respectively [137].

Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy are used to clarify the mechanism of fluoride adsorption [133]. Most of the FTIR studies on fluoride removal by different adsorbents have shown the involvement and the participation of the surface sites and potential functional groups in the adsorption, especially the hydroxyl group [138, 139]. It was observed that fluoride interacts with the OH and NH groups on surface aluminum-impregnated coconut fiber surface [140]. However, the adsorption of fluoride on Fe-impregnated chitosan was reported to occur due to ion exchange between fluoride and chloride [141]. Other studies using FTIR have revealed the formation of inner-spherically bonded complexes on γ−Fe2O3 nanoparticles with fluoride [142]. By using FTIR and XPS analyses, it was concluded that hydroxyl and the surface carbonates participate in the coexchange with fluoride ions on hierarchical magnesium oxide microspheres [143] and on magnesium oxide nanoplates adsorbents [144]. Another study revealed that the coexistence of C–O and C=O functional groups on lanthanum-loaded magnetic cationic hydrogel composite contributed to the fluoride adsorption by using FTIR [145].

Both Langmuir and Freundlich isotherm seem to fit well for adsorptions of these anions. However, the choice of the adsorbents to agree with Langmuir or Freundlich is still unclear. Higher adsorption capacity has been observed for fluoride ions compared to arsenic ions. Both modified adsorbents have been shown to have higher adsorption capacity compared to unmodified adsorbents, and, up to date, iron-based adsorbents (IBS) are still suitable material for arsenic adsorption and aluminum for fluoride.

This is the first report to show the relationship between the adsorption capacity and surface area of the adsorbents for arsenic and fluoride removal. It can be concluded that high surface area of the adsorbents does not often cause higher adsorption capacity on these anions’ removal but rather the chemical properties of the adsorbents. Modified adsorbents have been proven to be effective for the anions removal because modifications of adsorbents alter the physical and chemical properties of the adsorbents. For example, Gong et al. [132] studied different types of alumina and concluded that the acidity and basicity properties of the alumina significantly affect the fluoride adsorption and not the surface area.

Chen et al. [172] reported that adsorption of fluoride on PPy/TiO2 was not affected by surface area. PPy/TiO2 has a surface area of 95.71 m2/g, and its maximum adsorption capacity was 33.178 mg/g at 25°C with an initial fluoride concentration of 11.678 mg/L. The author concluded that physical adsorption was not the primary adsorption mechanism for the adsorption of fluoride on PPy/TiO2. Another study on adsorption of fluoride by a synthetic iron(III)–aluminum(III) mixed oxide with a specific surface area of 195.6 m2/g reached a maximum fluoride removal of 17.73 mg/g [173]. Zhang et al. [170] investigate the fluoride adsorption on micron-sized magnetic Fe3O4@Fe-Ti composite adsorbent (MMA) with a specific surface area of 99.2 m2/g and pore size of 0.38 cm3/g. The model followed Langmuir in isotherm study, and a high maximum adsorption value of 41.8 mg/g was observed [170]. Adsorption of fluoride on aluminum-impregnated coconut fiber ash (AICFA) was investigated by Mondal et al. [140]. AICFA has a surface area of 26.3 m2/g and 3.192 mg/g adsorption capacity. Another study reported 40.3 mg/g fluoride removal from drinking water by adsorption onto alum-impregnated activated alumina with a surface area of 176 m2/g [154]. Low adsorption capacity for adsorbents with the higher surface area was also reported. For example, a study on fluoride adsorption into nanoparticle goethite anchored onto graphene oxide (FeOOH + Ac/GO) with 202.60 m2/g reached a maximum adsorption capacity of 17.65 mg/g. Meanwhile, a rice spike like akaganeite anchored onto graphene oxide (FeOOH/GO) which has 255.24 m2/g reached 19.82 mg/g maximum adsorption capacity [174]. Another study reported a higher adsorption capacity of 93.84 mg/g when the fluoride concentration was 200 mg/L on Al(OH)3 nanoparticles modified hydroxyapatite (Al-HAP) nanowires with a surface area of 104.05 m2/g on fluoride removal [175].

7. Effects of pH on Fluoride Removal

The extent of fluoride adsorption is influenced by pH at large [4, 29]. Since the protonated surface is accountable for anions adsorption, the maximum fluoride adsorption for many adsorbents occurs at acidic pH [32, 63] and decreases at higher pH values. For adsorbents like activated alumina, fluoride adsorption is controlled by pH at point zero charges (pHzpc) [32, 73]. At a certain selected pH, the adsorption showed an increasing trend due to the positively charged alumina complexes AlF2+ and on fluoride removal by acidic alumina [32]. The fluoride adsorption decreased after pHzpc because the concentration of protonated surface sites decreases with increasing pH [4, 73], which causes strong competition of hydroxide ions [57]. A similar observation was reported by [66] on defluoridation of drinking water using chitosan-based mesoporous alumina. Nie et al. [146] reported maximum fluoride removal of 75% at pH of 5 and the adsorption decreased further as the pH increases on Al (III) modified calcium hydroxyapatite.

The adsorption of fluoride decreases below or above a certain pH value, as it has been reported by various researchers. For example, Li et al. [31] studied the effect of pH on amorphous alumina supported on carbon nanotubes. The result showed that the maximum fluoride adsorption occurred at pH 5.0–9.0 but decreased at pH less than 3.0 and more than 11.0. However, the distribution of F and HF on the surface was reported to be the reason for a decrease of fluoride at pH less than 5 on fluoride removal by aluminum-impregnated coconut fiber [140]. Similar results were reported by Tang et al. [63] on the adsorption of fluoride by granular ferric hydroxide, where at pH below 3, the HF were predominant.

Fluoride adsorption capacity was steady within a pH range of 2–11 on porous MgO nanoplates; however, it decreases abruptly at higher pH value above 12 [31, 144]. Tor et al. [148] also reported similar results in the removal of fluoride from water by using granular red mud (GRM). However, other studies reported maximum fluoride removal at neutral pH [29]. Huang et al. [165] observed that the maximum adsorption capacity by using protonated cross-linked chitosan particles was at pH 7. Moreover, a decrease below pH 7 and above was also observed on iron oxyhydroxide nanoparticles [176]. In acidic conditions, the decrease in adsorption capacity was caused by weak hydrofluoric acid. A similar observation was made by [177] on magnesium substituted hydroxyapatite absorbent at a pH below 3 by using different adsorbents as it was reported by [151, 154, 159, 161].

pH is the main parameter that affects the adsorption of these anions for both arsenic and fluoride. However, the adsorption of fluoride into various adsorbents has been reported to vary significantly at both high and low pH values due to the pH at point zero charges (pHzpc) which controls the adsorptions of adsorbents like activated alumina in fluoride adsorption and iron-based adsorbents for arsenic adsorption. The adsorption capacity of arsenic has been changed with changes in pH due to the chemical speciation of arsenic. Many adsorbents tend to remove charged species, which is As(V), which can exist at a wide range of pH compared to As(III).

8. Effects of Anion, Cation, and Organic Matter on Fluoride Removal

8.1. Effects of Anions

Drinking water contaminated with fluoride always exists with other coions like phosphate, bicarbonate, chloride, carbonate, sulfate, and nitrate [40, 62, 63, 65, 66]. In fluoride adsorption, it is expected that the presence of anions in solution would enhance coulombic repulsion forces between the anions and fluoride or would compete with fluoride for the active adsorption sites and therefore fluoride adsorption is reduced or increased [66, 135, 136, 178, 179]. Eskandarpour et al. [180] reported that, in fluoride adsorption, chloride and nitrate ions form outer-sphere complexes and sulfate and phosphate ions form inner-sphere complexes with binding surfaces. However, sulfate ions partially form outer-sphere complexes or inner-sphere complexes [180]. The adsorption of fluoride will increase due to an increase in ionic strength of the solution and weakening of lateral repulsion between adsorbed fluoride ions. For example, Chen et al. [40] investigated the presence of 20–200 mg/L salt solutions of chloride, nitrate, sulfate, carbonate, and phosphate in fluoride adsorption. Fluoride removal slightly increased in the presence of chloride and nitrate ions. The authors concluded that it was due to an increase in the ionic strength of the solution or weakening of lateral repulsion between adsorbed fluoride ions. Fluoride sorption was slightly decreased by sulfate ion due to the high coulombic repulsive forces, which reduce the probability of fluoride interactions with the active sites [40, 179].

Carbonate and phosphate ions showed a most significant effect on fluoride sorption due to the competition for the same active sites with fluoride and the high affinity and capacity for carbonate and phosphate ions [40]. The divalent nature of sulfate ion in solution may influence strong coulombic repulsive forces which lead to lessening fluoride interaction with the active sites. Similar results were reported by [38, 178, 179]. The effects of these anions on fluoride adsorption have also been reported to be different depending on experiment conditions which include pH, the anions concentrations, and the characteristics of the adsorbent. Recently, Zhang et al. [141] reported that the increasing concentrations of sulfate and carbonate to 200 mg/L cause a decrease in the fluoride adsorption by 43.24% and 18.77%, respectively, while 100 mg Cl/L increases the fluoride adsorption capacity of 0.58 mg/g by 2.53 mg/g with 5 mg of HCO3. For the former, the decrease was because of the electrostatic interactions of the anions and the adsorbent, and for the latter, the reaction of bicarbonate with ammonium acetate leads to the decrease of the ammonium acetate concentration.

Other researchers have reported a decrease depending on pH. At pH between 4.4 and 12, 500 mg/L of carbonate and bicarbonate ions concentration was reported to decrease the fluoride adsorption on acidic alumina [32]. Fluoride adsorption was reduced significantly as bicarbonate was acting as a pH buffering agent, and its existence in solution increases and buffered the pH; therefore, the adsorption of fluoride was decresed [4, 155, 166] Kamble et al. [4] reported that the addition of , , and ions increases pH of fluoride solution in adsorption of fluoride on alumina of alkoxide nature. Similar results were observed by [29, 147, 165, 181]. Adsorption of fluoride is more affected by the presence of phosphate (Figure 5) compared to other coexisting anions.

Changing of pH to alkaline condition (11.5) was reported to decrease the adsorption capacity of fluoride by more than 25% in the presence of carbonate [134]. The decrease in fluoride adsorption observed by the presence of , , and was due to the competition for active sorption sites or due to the change in pH or combination of them [4, 29, 147, 160, 161, 163167, 181], where the decrease of fluoride was related to anions competitions or effects of pH; however, in the same study, it was reported that sulfate did not have significant effect on fluoride removal [157]. A similar study was reported on MgO nanoplates [144].

The coexisting anions silicate and phosphate have been reported to have a negligible effect on the fluoride removal on a micron-sized magnetic Fe3O4@Fe-Ti composite adsorbent compared to 160 mg/L and 20 mg/L which decreased the adsorption efficiency [170]. Effects of phosphate on fluoride removal were also reported by [146]. The tendency of the anions to form inner-sphere complexes was reported by [28, 29, 32, 63, 170, 174, 175, 177, 178, 180183]. Chloride ions formed outer-sphere surface complexes and had a minor effect on fluoride adsorption; thus they are less absorbed on the absorbent surface [65]. Anion affinity for a surface site is associated with the physicochemical characteristics of the adsorption mechanism [40, 147, 179]. Another study demonstrated effects of coexisting ions on the removal of fluoride due to changing pH after salt addition and not the competitive adsorption of coexisting ions [184].

8.2. Effects of Cations

Only a few studies to date have reported the effects of cations on fluoride removal. At higher cations concentrations, effects become significant. Contradictory results have been presented for cations removal. Some researchers reported increased fluoride adsorption, while others reported a decrease in fluoride adsorption. An increase in adsorption capacity was observed with a higher concentration of magnesium and calcium due to an increase in surface positive charges and attraction of negatively charged ions onto various metal oxide surfaces on fluoride removal by nano magnesia (NM), as shown in the study conducted by Maliyekkal et al. [62]. Similar effects were observed by [157] at 400 mg/L of Ca2+ by using hydrated cement and on alumina cement granules [23]. Also, the formation of insoluble CaF2 and MgF2 was the reason for the increase of adsorption by granular ceramic at 200 mg/L and Ca2+ and Mg2+ concentration [40]; however, Mn2+ and Fe3+ at 200 mg/L were reported to decrease fluoride removal capacity up to 10% on nano magnesia [62]. Babaeivelni and Khodadoust [68] reported that the effects of calcium and magnesium ions on fluoride removal onto crystalline titanium dioxide were insignificant.

8.3. Effects of Organic Matter on Fluoride Adsorption

Natural organic matter (NOM) molecules consist of combinations of functional groups, including ester, phenolic, amino, nitroso, carboxylic, sulfhydryl, quinone, and hydroxyl, and most of them are negatively charged at neutral pH [23]. Thus NOM can compete with fluoride due to the principal anionic character together with high reactivity on both metals and surfaces [23]. The presence of NOM influences the fluoride adsorption either by decreasing the adsorption or increasing it. Only a few studies have presented the influence of NOM on fluoride adsorption, and the results presented vary at large.

The presence of humic acid was reported to cause a 50% reduction in fluoride removal efficiency by manganese oxide-coated alumina as an adsorbent; this was due to the blocking of active MnO2 sites by the larger humic acid molecule [151]. According to Ayoob and Gupta [23], the adsorption by using alumina cement granules was reduced with increasing NOM concentration because the adsorption capacity was dependent on the availability of the organic molecules and the inner surface of the adsorbent; thus the NOM can access mesopores of the adsorbent and the small molecules can access micropores. However, a study on carbon derived from Sargassum sp. by lanthanum indicated that there were insignificant effects on the adsorption of fluoride because the humic acid can be adsorbed on the surface of the activated carbon [166]. Samarghandi et al. [185] reported that, with 20 mg/L of NOM, fluoride adsorption by activated alumina was enhanced at a pH value of 5.5 to 6.

Adsorption of fluoride is more affected by the presence of phosphate compared to other coexisting anions. However, for arsenic adsorption, the influence of phosphate and other existing anions is affected by pH which controls the effects of arsenic to be adsorbed by the adsorbents if they are present in the water. Ionic strength can also interfere with the adsorption of these anions in the presence of other existing anions, and physicochemical characteristics of the adsorption mechanism affect the anion affinity for a surface site. Cations like Mg2+, Ca2+, and Fe2+ have been reported to increase the adsorption of the fluoride and arsenic by using different adsorbents. The presence of NOM influences the fluoride adsorption either by decreasing the adsorption or increasing it. However, pH also affects arsenic adsorption if NOM are also present by decreasing or increasing the extent of arsenic adsorption into the adsorbents.

8.4. Effects of Ionic Strength Fluoride

So far, very few studies have been done on the effects of ionic strength on fluoride removal. However, the importance of ionic strength lies in the fact that it can be used to distinguish whether inner-sphere or outer-sphere surface complexes are formed during the adsorption. It has been related to pH of the solution and characteristics of the adsorbent. Ionic strength has been reported to have insignificant effects on the fluoride removal by micron-sized magnetic Fe3O4@Fe-Ti composite and was confirmed to form an inner-sphere complex on the surface of the adsorbent [170]. However, the fluoride adsorption was inhibited by the increase of ionic strength, and therefore the adsorption process occurred through the outer-sphere complex adsorption mechanism on fluoride removal by carbon derived from Sargassum sp. by lanthanum [166]. Similar results were reported by [73] on fluoride adsorption onto granular ferric hydroxide and [73] on the adsorption of fluoride on activated alumina.

In general, presence of ionic strength can affect the adsorption of both arsenic and fluoride through the formation of inner-sphere or outer-sphere surface complexes. When the adsorption occurs through the inner sphere, there is less competition for adsorption sites; therefore, increasing the salt concentrations increases the adsorption. However, the ionic strength is also affected by pH of the solution when present with arsenic and fluoride anion in water.

9. Conclusion

Adsorption of both arsenic and fluoride is largely affected by environmental factors such as pH solution, ionic strength, and coexisting substances such as anions, cations, and organic matter. The adsorption capacity of the adsorbents depends not only on the surface area, pore volume, and particles size but also on a combination of all factors, surface chemistry, and pore structure. Specific area of the adsorbents does not contribute to the adsorption capacity on the removal of these anions from water. Therefore selection of the adsorbents for these anions removal should be based on a combination of all factors for the adsorbents and adsorbate. However, surface modification of the adsorbents increases the adsorption capacity of the adsorbents for removal of these anions due to the presence of more protonated surface sites which favor the removal of these anions. In general, pH has a greater influence on adsorption capacity at large, since it affects the ionic strength and the presence of coexisting anions such as bicarbonate, sulfate, and silica and it affects the surface charges of the adsorbents and the ionic species which can be present in the solution. Modified aluminum adsorbents have shown higher adsorption capacity for fluoride and modified iron oxide and aluminum minerals for arsenic. The adsorption of fluoride and arsenic is inhibited by the presence of phosphate followed by sulfate and silicate. Fourier transform infrared (FTIR) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy have revealed new functional groups on adsorbents surface, which participate in the adsorption of arsenic and fluoride.


This research did not receive any specific grant from funding agencies in the public or commercial or nonprofit organizations.

Conflicts of Interest

The authors declare that they have no conflicts of interest.