Zeta Potential

The variation in fluoride uptake was in the order of surface charge on the silver nanoparticle-coated biomaterial scaffold (Ag Nps/Al-chitosan-alginate) (Kumar et al. 2016). The higher positive surface charge (c.a. 18.6 mV) at pH 4 coincided with the maximum percentage removal. Also at pH 5 the same surface charge (c.a.

18.6 mV) coincided with the maximum percentage removal, but the fluoride uptake declined. Further increase in pH led to a decline in surface charge, as well as a decline in uptake capacity. Similarly, adsorption capacity with the calcined product of Mg/Fe/La hydrotalcite declined with increase in pH (Wu et al. 2017). The zeta potential also declined with increase in pH. This was due to the electrostatic nature and competition from hydroxyl ions.

Effect of Coexisting Ions

Chloride, sulfate, nitrate, phosphate, hydrogen phosphate, carbonate, bicarbonate (Velazquez-Jimenez et al. 2014; Kumar et al. 2016; Mohan et al. 2012; Kundu et al. 2017; Ma et al. 2017; Liu, Cui, et al. 2016; Chen, Shu, et al. 2017; Wang, Yu, et al. 2017; Wu et al. 2016; Jin et al. 2016; Dhillon et al. 2015; Zhang, Li, et al. 2012; Tang and Zhang 2016), and arsenic (Jing et al. 2012) reduced the uptake of fluoride. Multivalent ions have more effect in the reduction of fluoride adsorption capacity as compared to monovalent anions (Zhang, Li, et al. 2012; Kundu et al. 2017; Wang, Yu, et al. 2017; Karmakar et al. 2016; Chen, Zhang, Li, et al. 2016; Jin et al. 2016). The larger effect of multivalent anions is attributed to the charge density or charge-to-ionic-radii (Liu, Cui, et al. 2016; Tang and Zhang 2016).

The effect of sulfate on the reduction of fluoride’s adsorption on akaganeite was more pronounced as compared to chloride and nitrate. The more pronounced effect of sulfate is attributed to the inner sphere complex formation by sulfate ions in comparison to outer sphere complex formation by chloride and nitrate (Kuang et al. 2017). In some cases, sulfate in addition to monovalent anions has also an insignificant effect on fluoride removal (Prabhu and Meenakshi 2014).

The effect of anions on fluoride adsorption capacity is also investigated on the basis of ionic radii in addition to charge-to-ionic radius ratio (Chen, Zhang, He, et al. 2016). The ionic radius of bicarbonate (1.56 A) was closer to the ionic radius of fluoride (1.33 A), as compared to that of sulfate (2.30 A). Hence, bicarbonate easily fits into the mineral arrangement of the adsorbent, i.e. apatite, which leads to a significant decline in adsorption of fluoride as compared to sulfate. Similarly, the chloride and nitrate have higher ionic radius (1.8 A) and lesser effect on the adsorption of fluoride.

The effect of bicarbonate on fluoride removal capacity is attributed to the increase in pH of the solution, which leads to the increased competition between hydroxide and fluoride ions (Prabhu and Meenakshi 2014). The adsorption capacity of fluoride with groundwater was less as compared to a synthetic solution (Jing et al. 2012). This is attributed to the fact that groundwater sample contained sulfate and carbonate, and competition from the anions led to decrease in adsorption capacity. Sulfate and carbonate reduced fluoride adsorption by competing with fluoride ions for active sites.

Effect of Surface Modification

The modification of the adsorbent led to increase in fluoride adsorption (Prabhu and Meenakshi 2014; Kuang et al. 2017; Chen et al. 2012). The modification of activated carbon with Zr(IV) led to reduction of adsorption capacity. However, the modification of activated carbon with Zr(IV) and oxalic acid led to increase in adsorption capacity (Velazquez-Jimenez et al. 2014). Zr(IV) alone on the surface of activated carbon acts as a Lewis acid center, and oxalic acid’s addition led to the conversion of active sites to the basic character. This enhanced the positive charge on Zr(IV) and enhanced the adsorption of fluoride. The unpaired electrons and it bond system of oxalate were the major reasons for conversion of active sites to basic sites. X-ray photoelectron (XPS) spectra of Zr(IV)-modified activated carbon and Zr(IV) and oxalate-modified activated carbon after fluoride adsorption showed an XPS peak at

182.5 and 183.3 eV, which suggests the presence of additional positive surface charge surrounding the Zr atoms.

The doping of titanium dioxide with iron enhanced the formation of hydroxyl groups, as suggested by XPS analysis (Chen et al. 2012). The adsorption of fluoride led to decrease in Ols intensity (530eV for O2- and 531.5 eV for OH) in XPS spectra. This suggested the exchange of OH during fluoride adsorption.

The doping positively affects only up to an optimum ratio, in terms of the adsorption capacity. The adsorption capacity increased with sulfate doping of hydroxyapatite till an S/P atomic ratio of 1:2. On further increase of the S/P atomic ratio (5:6 and 4:3), a new peak in the X-ray diffraction (XRD) spectra emerges, representing the distortion of the crystal structure, which leads to the decline in adsorption capacity.

The addition of cationic surfactants also enhanced the removal of fluoride (Prabhu and Meenakshi 2014). Cationic surfactants, i.e. hexadecylpyridinium chloride, dodecyltrimethyl ammonium bromide, and cetyltrimethyl ammonium bromide, enhanced the hydrophilic character of the hydroxyapatite (adsorbent) and increased the attraction of fluoride ions. The adsorption of hexadecylpyridinium chloride-modified hydroxyapatite was lower than that of dodecyltri methyl ammonium bromide- and cetyltrimethyl ammonium bromide-modified hydroxyapatite. This is attributed to the restriction of fluoride ions by electron-rich pyridinium groups to tertiary cationic groups.

Effect of Material

Adsorption varied with the synthesis route for preparation of the adsorbent (Zhang, Li, et al. 2012), the ratio of the precursors (Zhou et al. 2011), and terminative pH for precipitation of the precursor for preparation of the adsorbent (Chen et al. 2012). The calcined product of Li-Al layered double hydroxide prepared by the co-precipitation method showed more adsorption capacity than the material prepared by homogenous precipitation (Zhang, Li, et al. 2012). The fluoride ion adsorbs into the interlayer space. The calcination of the adsorbent prepared by homogenous precipitation led to incomplete decomposition of the interlayer, evidenced by Fourier-transform infrared spectroscopy (FT-IR). Hence, there is less adsorption of fluoride by the calcined product of the sample prepared by homogenous precipitation.

The terminative pH during precipitation of the precursors for the adsorbent material led to a change in adsorption capacity (Chen et al. 2012). The optimum terminative pH during synthesis of iron-doped titanium oxide for adsorption of fluoride was 5. In addition, the adsorbent prepared with terminative pH in an acidic pH environment depicted greater adsorption capacity than the adsorbents prepared with terminative pH in an alkaline environment. The XRD suggested the presence of crystalline FeOOH at a terminative pH of 7-9; however, at a terminative pH of 3-6, the crystalline phase of the FeOOH was not depicted.

The higher adsorption capacity of biochar as compared to activated carbon for fluoride is attributed to the opening of pores during the water contact in the adsorption process (Mohan et al. 2012). On contact with water, the swelling phenomenon led to widening of pores. This cannot be determined by the Brunauer-Emmett-Teller (BET) surface area analysis of the dry sample. This can be evidenced by the fact that more water is imbibed by the chars as compared to that measured by BET analysis.

Effect of Temperature

The increase in temperature led to increase (Wang, Yu, et al. 2017; Ma et al. 2017; Zhou et al. 2011; Prabhu and Meenakshi 2014) and decrease (Chen et al. 2012; Chen, Shu, et al. 2017; karmakar et al. 2016) in adsorption capacity. The increase in adsorption capacity of fluoride with mesoporous alumina is attributed to the increase in kinetic energy (Kundu et al. 2017). The XPS analysis for fluoride adsorption on Mg-Al-Zr triple metal composite suggests increase in fluoride adsorbed amount with temperature (Wang, Yu, et al. 2017).


The mechanism of adsorption was examined using different techniques. XRD of the polypyrrole/hydrous tin oxide after fluoride adsorption did not show any change

(Parashar et al. 2016). This suggests that fluoride adsorption is a physical phenomenon involving electrostatic interaction and ion exchange. The Arrhenius activation energy (20.05 kJ/mol) also suggested it to be a physisorption process. XRD of the material after fluoride adsorption on the silver nanoparticle-coated biomaterial scaffold (Ag Nps/Al-chitosan-alginate) explained the formation of a ralstonite-like compound with irreversible adsorption (Kumar et al. 2016).

In addition to XRD, FT-IR is also used to estimate the mechanism of adsorption (Velazquez-Jimenez et al. 2014; Chen, Zhang, He, et al. 2016; Wu et al. 2016). The FT-IR analysis of fluoride adsorption on Zr(IV) and oxalic acid-modified activated carbon suggests the interaction of zirconium ions with carboxylic groups (Velazquez-Jimenez et al. 2014). The sample after adsorption of fluoride with Zr(IV) and oxalate-modified activated carbon led to an FT-IR peak with sharper form at 3400cm-1 and had lower intensity than that of the adsorbent (Velazquez-Jimenez et al. 2014). This depicts involvement of the hydroxyl group in adsorption process. Vibrational changes also appeared in the range of 400-500cm_|. This is attributed to the fluoride complex with Zr-O groups. Fluoride adsorption occurs with the displacement of the hydroxide group from the Zr-oxalate complex. The chemical rearrangement of the COOH group leads to the formation of zirconium oxyfluoride. The XPS analysis showed that adsorption happened via ion exchange of hydroxide ions.

The XPS analysis for fluoride removal with zirconium phosphate suggested a stronger interaction between them, i.e. inner sphere complex formation (Zhang, Li, et al. 2017). The FIs of pure NaF in XPS spectra showed a peak at 684.9 eV as compared to 685.7eV in the case of F loaded in ZrP. This suggests the stronger adsorption efficiency between the sample and fluoride ions. In addition, the primitive zirconium phosphate contains two satellite peaks at 183.3eV (Zr 3d5/2) and 185.6eV (Zr 3d,/2), corresponding to charge transfer from the valence band of the ligand atom to the 4f orbital of the Zr atom. These peaks disappeared, and the appearance of new peaks at -184.4 and ~186.8eV corresponded to a newly formed complex. The large energy band shift of 1.1 eV suggests the formation of an inner sphere complex and formation of a new Zr-F complex.

The significant adsorption over the isoelectric point suggests the pH-independent mechanism (Kuang et al. 2017). The adsorption of fluoride with akaganeite-anchored graphene oxide declined with increase in chloride desorption. This suggests that ion exchange can be responsible for adsorption of fluoride on akaganeite-anchored graphene oxide. The goethite-anchored graphene oxide, however, showed similar ion exchange behavior with acetate ions rather than with chloride ions.

The ion exchange behavior in the case of fluoride adsorption with cetyltrimethyl ammonium bromide-modified hydroxyapatite is estimated on the basis of energy dispersive X-ray (EDX) analysis. The EDX analysis of the adsorbent after surface modification of hydroxyapatite with cetyltrimethyl ammonium bromide showed that there were no bromide peaks, which suggested the adsorption of fluoride with ion exchange, i.e. fluoride replacement with bromide (Prabhu and Meenakshi 2014).

The ion exchange behavior is also estimated by FT-IR analysis in the case of fluoride adsorption with hydroxyl aluminum oxalate (Wu et al. 2016). The FT-IR spectra depicted the interchange of fluoride with hydroxyl and oxalate groups (Wu et al. 2016). The FT-IR spectra after adsorption of fluoride showed disappearance of peaks at 3675 and 1715cm-1. Along with this, there was emergence of a peak at 600cm-1 attributed to Al-F stretching vibration. This suggests ion exchange with hydroxide and oxalate.

The mechanism of adsorption of fluoride onto the Li-Al layered double hydroxide was based on the memory effect (Zhang, Li, et al. 2012). The interlayer anion carbonate decomposed in Li-Al layered double hydroxide synthesized by the co-precipitation method. This is evidenced by the disappearance of FT-IR peaks at 1385 cm-1. However, the interlayer anions in the Li-Al layered double hydroxide prepared by the precipitation method did not disappear completely. Two small peaks were present at 1450-1350 cm-1.

The adsorption of fluoride with cerium-immobilized chitosan is governed by different mechanisms. The maximum adsorption capacity for fluoride with cerium-immobilized chitosan was at pH 3 (Zhu et al. 2017). At pH lower than 3, formation of the HF occurred, which led to the reduction of adsorption of fluoride ions. pH lower than 5.3 led to protonation of hydroxyl and amino groups. Alkaline pH led to decrease in adsorption. The fluoride Is peak in the XPS spectrum for cerium-immobilized chitosan after fluoride adsorption was higher (685.2 eV) than for NaF (684.5 eV). In addition, the increase in the Ce3+ state and decrease in the Ce4+ state explained the complexation of fluoride with cerium. The adsorption mechanism was electrostatic attraction, ion exchange with nitrate ions, and complexation with cerium ions. At pH higher than 5.3, only ion exchange and complexation played a major role in the removal of fluoride. qc (adsorption capacity at equilibrium) at pH =7 was more than half of qe at pH =5.3; hence, the major role was attributed to ion exchange rather than complexation.


The desorption of fluoride was achieved with sodium hydroxide (Chen et al. 2012; Chen, Shu, et al. 2017; Zhu et al. 2017; Dhillon et al. 2015; Zhang and Huang 2019), sodium carbonate (Wu et al. 2017), and sodium aluminate (Wang, Yu, et al. 2017). The high percentage of desorption suggests the physical nature of adsorption e.g. ion exchange (Chen, Shu, et al. 2017; Parashar et al. 2016). Adsorption capacity declined after adsorption in some cases (Wang, Yu, et al. 2017; Dhillon et al. 2015). The nondesorbed amount of the adsorbate is attributed to the chemically bound adsorbate species. Adsorption capacity increases with increase in sodium hydroxide strength in the case of regeneration of aluminum fumarate metal organic framework (karmakar et al. 2016).

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