A facile synthesis of metal ion-imprinted graphene oxide/alginate hybrid biopolymeric beads for enhanced fluoride sorption

Abstract

The enhanced surface properties of graphene oxide (GO) illustrate its vital role in environmental remediation. However, it is difficult to use it in column studies as it causes a pressure drop during field applications. To overwhelm its drawbacks and to improve its adsorption feature, GO was synthesized in a usable bead form by interspersing GO within an alginate (Alg) polymeric matrix and cross-linked with La(III) ions (GOAlgLa). The synthesized GOAlgLa composite beads not only display good field applications but also demonstrate an extremely enhanced defluoridation capacity (DC) compared with GO and calcium cross-linked alginate beads (AlgCa). The DC of GOAlgLa, AlgCa composite beads and GO was found to be 6617, 618 and 2438 mg F⁻ kg⁻¹, respectively. To determine the effect of different influencing parameters, such as pH, contact time, competitor co-anions, temperature and initial fluoride concentration, studies were conducted in batch mode. The sorbents were analyzed using various characterization techniques such as FTIR, TEM, SEM, EDAX, and Raman analysis. The characteristics of the sorption process were investigated using Freundlich, Langmuir, and D–R isotherms. The value of the thermodynamic parameters indicates that the fluoride sorption onto GOAlgLa composite beads was endothermic and spontaneous in nature. GOAlgLa composite beads also reveal a good regenerability over repeated adsorption/desorption processes. The applicability of hybrid beads to field water sample indicate its adaptable nature at field conditions.

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1. Introduction

Removal of toxic ions from water has attracted considerable concern in worldwide, due to their adverse effects on human beings and environment. Fluoride causes adverse effects to human health when its concentration exceeds >1.5 mg L⁻¹ in drinking water. The excess fluoride levels in drinking water leads to different forms of fluorosis.¹ One of the foremost sources of fluoride ingestion by a human being is drinking water. The major sources of fluoride contaminants are fluoride bearing rocks/minerals and various industrial activities. Minerals such as fluorite, cryolite, ralstonite, and mica that contains fluoride prevalent components of sediments.² In developing countries, ground water is used for drinking, cooking, and domestic purposes; however, unfortunately, the fluoride concentrations are more than 1.5 mg L⁻¹ in several places in India and other foreign countries. Hence, it is necessary to provide water with safe fluoride levels to the common people to prevent them from devouring water contaminated with an excess amount of fluoride.

Numerous fluoride remediation technologies, including precipitation, ion-exchange, electrocoagulation, electrodialysis, nanofiltration, reverse osmosis, and adsorption,^3–9 have been developed. Among them, adsorption is believed to be a more simple, efficient, and universal option than the other reported techniques. The recycling of the adsorbent and the low generation of residues in this technique are some of the advantages over the other fluoride removal methods.^10–12 Researchers all over the world have paid plentiful efforts to developing a novel and efficient adsorbent for the removal of fluoride from aqueous solutions.

Graphene oxide has received considerable attention in recent years and shows a promising adsorption performance towards toxic ions owing its huge surface area, copious surface functional groups (hydroxyl and carboxyl groups), incredible dispersive properties in water, and easy modification.^13–19 Although GO possesses good adsorption efficiency for the removal of toxic ions, the application of GO in column studies is restricted due to its powdered nature, brittleness, and poor mechanical strength. To overcome such an industrial blockage, polymer assisted GO composite beads were prepared. The synthesized polymeric composite beads have unique properties such as hardness, inflexibility, and mold shrinkage, which help in the development of modern technology.^20–22

Recently, researchers have paid more attention to the biosorption process for environmental remediation because of its eco-friendly, non-toxic, biofunctional, biocompatible, and biodegradable properties.^23–25 Alginate is a biopolymer and it is a collection of two different monomers viz. (1 → 4) β-D-mannuronate and (1 → 4) α-L-guluronate and it is obtained from brown seaweeds. Alginate assisted composite adsorbents were extensively studied for toxic ion removal due its non-toxic, biodegradable and biocompatible nature.^26–30 It possesses very good affinity towards different metal ions to form ionotropic alginate metal complex beads. However, divalent cation cross-linked alginate composites beads are unstable in an acidic medium because the divalent metal ions can be easily replaced by H⁺ ions.³¹ To improve the adsorption capacity, reusability, and solidity, high valence metal ions (trivalent and tetravalent) are utilized as cross-linkers to develop the hybrid beads. Recently, the development of new adsorbents with selectivity and high performance towards fluoride was accomplished by the incorporation of high valence metal ions into the adsorbent materials.^32–40 Hence, we synthesized La(III) ion cross-linked alginate/graphene oxide composite beads for the sorption of fluoride.

The present investigation is aimed at the synthesis of economical, efficient, and eco-friendly composite beads by interspersing GO in Alg polymeric matrix and cross-linked with La(III) ion. The developed GOAlgLa composite beads were effectively utilized for fluoride sorption. To determine the effect of different influencing parameters such as competitor co-anions, pH, contact time, initial fluoride concentration, and temperature, batch mode was carried out. A comparative study was conducted for GO, AlgCa, and GOAlgLa composite beads to estimate the defluoridation capacity. The experimental results were fitted with various isotherm and thermodynamic parameters. The effective utilization of GOAlgLa composite beads in the field water sample was done at a nearby fluoride rife area in Dindigul district of Tamilnadu.

2. Experimental section

2.1. Materials

Sodium alginate was purchased from Himedia (India). Graphite powder was purchased from Central Drug House (India). Conc. H₂SO₄, NaNO₃, KMnO₄, H₂O₂, CaCl₂·2H₂O, LaCl₃·7H₂O and all other chemicals were purchased from Merck (India) and used as such without further purification. A fluoride solution containing 1000 mg L⁻¹ was prepared by dissolving 2.21 g of NaF (AR grade) in 1000 mL of double distilled water. The working solution of 20 mg L⁻¹ for the batch fluoride sorption process was then prepared by appropriate dilution of the stock solution.

2.2. Synthesis of GO

Graphene oxide was synthesized from natural graphite powder by a modified Hummers method.⁴¹ Briefly, 24 mL of conc. H₂SO₄ was added to 1 g of graphite powder. Then, 0.5 g of NaNO₃ was added into the reaction mixture, which was then cooled to 0 °C. 1.5 g of KMnO₄ was slowly added to the reaction mixture, and then the temperature was maintained at 20 °C. The content was heated to 35 °C and maintained for 45 min. Then, 46 mL distilled water was added to the reaction mixture and the temperature of the content was maintained at 98 °C for 15 min. The reaction was quenched by the addition of 144 mL of distilled water and 1 mL of a 30% H₂O₂ solution. The final product was cooled to room temperature and washed with a 0.1 M HCl solution several times and distilled water to remove metal ions and then dried.

2.3. Synthesis of AlgCa and GOAlgLa composite beads

About 0.5 g of GO was homogeneously dispersed in 100 mL of distilled water. To avoid agglomeration of GO in water during dispersed, the process was carried out by sonication and followed by mechanical stirring for about 3 h. To the dispersion solution, 2 g of sodium alginate was added and stirred vigorously for 3 h. This homogeneous solution was used to prepare the composite beads by dropping into 0.2 mol L⁻¹ of a LaCl₃·7H₂O aqueous solution to obtain GOAlgLa composite beads. AlgCa composite beads were synthesized by adding 2 g of sodium alginate to 100 mL of distilled water under vigorous stirring for 3 h. Then, the homogeneous alginate solution was dropped into 0.2 mol L⁻¹ of a CaCl₂·2H₂O solution to obtain AlgCa composite beads. Then, the formed AlgCa and GOAlgLa composite beads were kept in the mother solution for about 24 h to allow the cross-linking reaction to take place. The prepared AlgCa and GOAlgLa composite beads were separated from the solution, washed with distilled water and dried in a hot-air oven at 80 °C for 10 h. Finally, the dried AlgCa and GOAlgLa composite beads were used for defluoridation studies.

2.4. Adsorption experiments

Fluoride adsorption experiments were carried out in a batch mode in duplicate. About 0.1 g of GOAlgLa composite beads were added to an iodine flask containing 50 mL of a fluoride solution with an initial concentration of 20 mg L⁻¹ at 303 K. The contents were shaken in a thermostat shaker with a constant speed of 200 rpm. To evaluate the effect of contact time, the contents of the iodine flask were taken out at 10 min intervals from 10–100 min and then the solution was filtered and the final fluoride concentration was measured. The impact of the solution pH on the fluoride adsorption process was carried out by adjusting the solution pH to 3, 5, 7, 9, and 11 with a 0.1 M HCl/NaOH solution. The initial fluoride concentrations varied from 18, 20, 22, and 24 mg L⁻¹ at three various temperatures viz. 303, 313 and 323 K. The amount of fluoride removed by GOAlgLa composite beads was taken as the difference between the initial and equilibrium concentrations of fluoride in the solution, and the defluoridation capacity (mg F⁻ kg⁻¹) of GOAlgLa composite beads was obtained by the following equation.
where C_i is the initial fluoride concentration (mg L⁻¹), C_e is the equilibrium fluoride concentration (mg L⁻¹), V is the volume of the solution (L), and m is the mass of the adsorbent (g).

2.5. Analytical methods

The fluoride concentration present in the solution was measured using a Thermo Orion Benchtop multiparameter kit (Model: VERSA STAR92) using a fluoride ion selective electrode having a relative accuracy of ±1 significant digit, detection limit of 0.02 mg L⁻¹ and reproducibility of ±2%. The pH measurements were done with the same instrument (Model: VERSA STAR92) using a pH electrode (Orion 8157BNUMD). All other water quality parameters were investigated using standard methods.⁴²

2.6. Instrumentation studies

The FTIR spectra of the sorbents were obtained using a JASCO-460 plus spectrometer operated at a 1 cm⁻¹ resolution in the range of 400–4000 cm⁻¹ using KBr pellets. TEM images were recorded using a TEM CM 200 (Philips) model. The surface morphology of the hybrid beads were imagined by a Vega3 Tescan model scanning electron microscope (SEM). The change in the surface morphology of the fresh and fluoride sorbed composite beads was obtained by the SEM analysis. The energy dispersive X-ray analyzer (EDAX) of the composite beads was determined using a Bruker Nano GMBH model. The pH at the zero point of charge (pH_zpc) of the composite beads was measured using the pH drift method.⁴³

2.7. Statistical tools

The computations of the obtained experimental results were carried out using the Microcal Origin (Version 8.0) software. The best model and goodness of the fit were determined using the regression correlation coefficient (r), standard deviation (sd), and chi-square analysis (χ²).

3. Results and discussion

3.1. Effect of contact time on the DCs of the sorbents

The sorption performance of GO, AlgCa composite beads and GOAlgLa composite beads for fluoride removal in relation to contact time was carried out by varying the contact time from 10 to 100 min for a 50 mL of 20 mg L⁻¹ fluoride solution with a sorbent dosage of 0.1 g at a neutral pH.

The results are shown in Fig. 1. It was found that the equilibrium time for the sorption of fluoride onto AlgCa and GOAlgLa composite beads was 50 min, whereas the equilibrium time was 60 min for GO. The maximum DCs obtained at the equilibrium time for AlgCa, GO and GOAlgLa composite beads was found to be 618, 2438 and 6617 mg F⁻ kg⁻¹, respectively. Among the sorbents, GO and GOAlgLa composite beads possess higher DCs than AlgCa composite beads. Therefore, further studies were limited to GO and GOAlgLa composite beads.

Fig. 1 Effect of contact time on the DCs of GO and GOAlgLa composite beads.

3.2. Impact of solution pH on the fluoride sorption

The solution pH is one of the major factors that alter the net charge of the adsorbent, influencing the adsorption properties. Fig. 2 shows the impact of solution pH on fluoride removal by GO and GOAlgLa composite beads at various initial pH values between 3 and 11.

Fig. 2 Effect of solution pH on the DCs of GO and GOAlgLa composite beads.

The maximum DCs of GO and GOAlgLa composite beads was found to be 3085 and 7254 mg F⁻ kg⁻¹, respectively, at pH 3, after which the fluoride sorption efficiency of the sorbents decreased with increasing pH. In general, the adsorbent surface is negatively charged at pH > pH_zpc, and positively charged at pH < pH_zpc. The pH_zpc value of GOAlgLa composite beads was found to be 3.83 (inserted in Fig. 2), below which the surface of the sorbent possess a positive charge, favoring fluoride sorption. When the pH increases, the surface of the sorbent becomes less positively charged and the interaction between the fluoride and sorbent becomes less, changing to a repulsive force at pH > pH_zpc, resulting in a significant decrease in fluoride sorption. The change in the electrostatic force between the fluoride and the sorbent explains the impact of solution pH on fluoride removal.

3.3. Competitive sorption of fluoride on the sorbents

The co-existing competitive anions such as chloride, nitrate, sulphate, and bicarbonate ions that are usually present in drinking water might interfere with the fluoride adsorption process. Therefore, to analyze the effect of these anions on the DCs of GO and GOAlgLa composite beads, studies were carried out by taking 0.1 g of the sorbents, which were added to a 20 mg L⁻¹ fluoride solution along with 200 mg L⁻¹ of the competitive anions, and the results are presented in Fig. 3. The results reveal that the co-exiting anions, such as chloride, nitrate, and sulphate ions, slightly interfere with GO and GOAlgLa composite beads during fluoride removal, whereas bicarbonate shows a momentous effect on the DCs of GO and GOAlgLa hybrid beads.

Fig. 3 Effect of competitive anions on the DCs of GO and GOAlgLa composite beads.

The decline in the DCs of GO and GOAlgLa composite beads in the presence of HCO₃⁻ ions could be a competition between the HCO₃⁻ and fluoride ion for the active sites on the adsorbent surface, which is decided by the concentration, charge and size of the anions. In addition to this, HCO₃⁻ ions will increase the solution pH, which diminishes the active sites for fluoride sorption. Among the sorbents, GOAlgLa composite beads possess an enhanced DC compared with GO. Hence, the isotherm and field studies were restricted to GOAlgLa composite beads.

3.4. Characterization of GOAlgLa composite beads

FTIR spectra of GO, GOAlgLa composite beads, and fluoride sorbed GOAlgLa composite beads are shown in Fig. 4a–c, respectively. The FTIR spectrum of GO displays a peak at 1723 cm⁻¹, which is attributed to the C=O stretching of the –COOH group, and the bands present at 1410 and 1625 cm⁻¹ are due to the stretching vibration of C–O–H of phenolic groups and the (C=C) of aromatic rings, respectively.⁴⁴

Fig. 4 FTIR spectra of (a) GO, (b) GOAlgLa composite beads and (c) fluoride sorbed GOAlgLa composite beads.

In GOAlgLa composite beads, bands appeared at 1036, 1424, 1622, 2926, and 3425 cm⁻¹ are accredited to the C–O–C stretching, –CH₂ bending, –COO– asymmetric stretching, –CH₂ stretching, and –OH stretching vibrations, respectively.²⁰ The decrease in the intensity of the band at 3409 cm⁻¹ in the fluoride sorbed GOAlgLa composite beads is due to the exchangeable hydroxyl ions present in the composite beads being replaced by fluoride ions from the aqueous solution.

Raman spectroscopy is a widely used and definitive technique in the examination of the structural and electronic properties of GO. Fig. 5a and b displays the Raman spectra of GO and GOAlgLa composite beads, and the spectra possess two major peaks, namely, the D and G bands. The strong bands at ∼1330 and 1603 cm⁻¹ correspond to D and G bands, respectively. The D band is the edge-induced disordered state, related to the presence of sp³ defects, and the G band is associated with the in-plane vibration of the sp² (C=C) aromatic carbon structure.⁴⁵ Both the FTIR and Raman analysis results show that GO is highly dispersed in the alginate polymeric matrix.

Fig. 5 Raman spectra of (a) GO and (b) GOAlgLa composite beads.

The surface morphology of GO, full image of a GOAlgLa composite bead, close view of GOAlgLa composite bead and fluoride sorbed GOAlgLa composite beads were studied by SEM and the results are presented in Fig. 6a–d. The micrograph images of GO show a loosening of GO nano-sheets and their porous structure due to the opening of the planner carbon network. In GOAlgLa composite beads, alginate possesses a wide surface and acted as a bed/support for GO. The SEM image of GOAlgLa composite beads indicates that distinct porous structures are present in the composite beads and are expected to have more adsorption sites for the removal of fluoride. After fluoride sorption, the porous structure and grooves decreased in GOAlgLa composite beads, indicating fluoride sorption.

Fig. 6 SEM images of (a) GO and (b) overall shape of GOAlgLa composite bead. Close-view SEM images of (c) GOAlgLa composite beads and (d) fluoride sorbed GOAlgLa composite beads.

The fluoride sorption on GOAlgLa composite beads was also confirmed by EDAX analysis. GO contains C and H peaks (cf. Fig. 7a), GOAlgLa composite beads possesses C, H, and La peaks (cf. Fig. 7b) and the fluoride sorbed GOAlgLa composite beads possesses C, H, La, and F peaks, which confirms the adsorption of fluoride onto GOAlgLa composite beads (cf. Fig. 7c). The mapping image results are also given to confirm the fluoride sorption onto GOAlgLa composite beads. The corresponding colors represent the particular elements present in GOAlgLa composite beads. After fluoride sorption, the presence of the fluoride color with the other elements indicates that the fluoride sorption occurred on GOAlgLa composite beads.

Fig. 7 EDAX spectra of (a) GO, (b) GOAlgLa composite beads and (c) fluoride sorbed GOAlgLa composite beads.

The surface morphology of GOAlgLa composite beads was also analyzed by TEM analysis and the results are illustrated in Fig. 8. It indicates that the composite beads exhibited transparent and slightly aggregated morphologies with crinkles. In addition, GO was unidirectionally dispersed in the alginate polymeric matrix.

Fig. 8 TEM images of GOAlgLa composite beads.

3.5. Sorption isotherms

Sorption isotherm models were used to describe the solute–sorbent interaction and optimizing conditions for maximum sorption. To estimate the fluoride sorption efficiency of GOAlgLa composite beads, three significant isotherms viz. Freundlich,⁴⁶ Langmuir,⁴⁷ and Dubinin–Radushkevich (D–R)⁴⁸ were used. The linear plot of log q_e vs. log C_e signifies the applicability of Freundlich isotherm. The obtained 1/n, n and k_F values are presented in Table 1. The n values lie between 1 and 10 and 1/n values lies between 0 and 1, corresponding to favorable conditions for fluoride sorption. A linear plot of a Langmuir isotherm is acquired for GOAlgLa composite beads when C_e/q_e is plotted against C_e, which gives b and Q^o values from the intercept and slope, respectively. The calculated values of Q^o and b are listed in Table 1. The R_L values lie in the range between 0 and 1 and indicates a favorable sorption. The linear plot of ln q_e vs. ε² indicates the applicability of D–R isotherm.

Table 1 Isotherm parameters of GOAlgLa composite beads

Isotherms	Parameters	Temperature
		303 K	313 K	323 K
Freundlich	1/n	0.328 ± 0.020	0.379 ± 0.019	0.437 ± 0.008
	n	3.072 ± 0.005	3.416 ± 0.006	3.498 ± 0.005
	kF (mg g^−1) (L mg^−1)^1/n	4.897 ± 0.003	5.049 ± 0.010	5.169 ± 0.012
	r	0.991 ± 0.006	0.984 ± 0.015	0.987 ± 0.010
	sd	0.065 ± 0.007	0.071 ± 0.009	0.079 ± 0.004
	χ^2	1.6 × 10^−3 ± 0.008	2.0 × 10^−3 ± 0.011	2.1 × 10^−3 ± 0.007
Langmuir	Q^o (mg g^−1)	6.701 ± 0.002	6.869 ± 0.008	7.005 ± 0.005
	b (L g^−1)	1.864 ± 0.005	1.919 ± 0.008	2.175 ± 0.003
	RL	0.167 ± 0.003	0.176 ± 0.001	0.183 ± 0.008
	r	0.998 ± 0.001	0.997 ± 0.002	0.999 ± 0.001
	sd	0.009 ± 0.006	0.017 ± 0.009	0.011 ± 0.014
	χ^2	1.1 × 10^−3 ± 0.002	1.7 × 10^−4 ± 0.006	2.58 × 10^−4 ± 0.005
Dubinin–Radushkevich (D–R)	Xm (mg g^−1)	5.649 ± 0.022	5.764 ± 0.015	6.194 ± 0.017
	E (kJ mol^−1)	8.698 ± 0.031	9.064 ± 0.120	9.341 ± 0.045
	r	0.994 ± 0.005	0.990 ± 0.002	0.993 ± 0.007
	sd	0.038 ± 0.004	0.029 ± 0.009	0.047 ± 0.006
	χ^2	1.4 × 10^−3 ± 0.013	1.8 × 10^−3 ± 0.007	1.9 × 10^−3 ± 0.014

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The values of K_DR, X_m, and E are shown in Table 1. The E value ranging from 1.0 to 8.0 kJ mol⁻¹ indicates physical sorption, whereas E value ranging from 9.0 to 16.0 kJ mol⁻¹ indicates chemical sorption. The obtained E values are 8.698, 9.064 and 9.341 kJ mol⁻¹ for 303, 313 and 323 K, respectively, which indicate that the defluoridation mechanism of GOAlgLa composite beads is purely chemical in nature. The higher r values and lower sd values designate a suitable isotherm.

The best isotherm fit was identified using low chi-square (χ²) values. The calculated chi-square values are presented in Table 1. The best isotherm fit model follows the order: Langmuir > D–R > Freundlich for fluoride sorption onto GOAlgLa composite beads.

3.6. Thermodynamic studies

The thermodynamic parameters, namely, the standard free energy change (ΔG°), standard entropy change (ΔS°) and standard enthalpy (ΔH°), are commonly used to estimate the nature and feasibility of adsorption and are calculated by Khan and Singh method.⁴⁹ The obtained results are shown in Table 2. The ΔG° values are negative and increases with increasing adsorption temperature, which signifies the feasibility and spontaneity of fluoride sorption on GOAlgLa composite beads. The positive values of ΔH° indicate the endothermic nature of fluoride sorption. The positive value of ΔS° reveals the increase in the number of species and randomness at the solid–liquid interface.

Table 2 Thermodynamic parameters of GOAlgLa composite beads

Thermodynamic parameters	Temperature
ΔG° (kJ mol^−1)	303 K	−4.28
	313 K	−3.84
	323 K	−3.16
ΔH° (kJ mol^−1)		16.67
ΔS° (J K^−1 mol^−1)		57.00

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3.7. Mechanism of fluoride uptake on GOAlgLa composite beads

The graphical representation of fluoride removal by GOAlgLa hybrid beads is shown in Fig. 9. It shows that the fluoride removal by GOAlgLa composite beads is governed by both ion-exchange and electrostatic adsorption, and complexation mechanism. The surface of GO is positively charged with hydroxide as the negative counter anions. Therefore, the more electronegative fluoride ion easily replaces the hydroxyl anion by an ion-exchange mechanism. Second, the positively charged La³⁺ ions present in GOAlgLa composite beads attract the negatively charged fluoride ions by means of electrostatic adsorption and complexation mechanism.

Fig. 9 Feasible fluoride removal mechanism of GOAlgLa composite beads.

3.8. Desorption and reusability studies of GOAlgLa composite beads

In practical applications, the regeneration and recycling of the adsorbent are indispensible. After fluoride sorption studies, GOAlgLa composite beads were separated by simple filtration, washed with double distilled water, dried and reused for further adsorption–desorption cycles. During adsorption studies, 0.1 g of GOAlgLa composite beads were added to 50 mL of fluoride solution with an initial concentration of 20 mg L⁻¹ for 50 min at a neutral pH. After that, the fluoride sorbed GOAlgLa composite beads (13.30 mg L⁻¹) were separated by simple filtration, washed with double distilled water and left to dry. The dried GOAlgLa composite beads were then added to dilute NaOH solution (50 mL) with various concentrations ranging from 0.02 to 0.1 M. Desorption of fluoride from GOAlgLa composite beads was improved from 0.02 to 0.1 M. A maximum desorption efficiency of 92.6% (12.32 mg L⁻¹) of the composite beads was achieved with 0.1 M NaOH solution as the regenerant. From Fig. 10, it can be clearly observed that the fluoride sorption efficiency of GOAlgLa composite beads was decreased and was found to be 92.6% (12.32 mg L⁻¹), 84.4% (11.23 mg L⁻¹), 70.8% (9.42 mg L⁻¹), and 52.1% (6.93 mg L⁻¹) in fresh to four cycles, respectively. In each cycle, the fluoride removal efficiency was decreased gradually and after four cycles, the fluoride removal reached a minimum. It suggests that GOAlgLa composite beads could be regenerated and effectively used for upto four cycles.

Fig. 10 Reusability studies of GOAlgLa composite beads.

3.9. Removal of fluoride from field water sample

The application of GOAlgLa composite beads at field conditions was tested by collecting a bore-well water sample from a nearby fluoride widespread area. About 50 mL of fluoride contaminated water sample was shaked with 0.1 g of GOAlgLa composite beads for 60 min at room temperature and the results are shown in Table 3. The observed concentration of fluoride in the field water sample was 3.28 mg L⁻¹, which is higher than the value recommended by WHO i.e., >1.5 mg L⁻¹. After the treatment, fluoride concentration was greatly reduced to below the tolerance limit, showing the efficiency of GOAlgLa composite beads (cf. Table 3). The results also reveal the selectivity of the GOAlgLa composite beads towards fluoride, as the field water sample also contains a high concentration of other anions other than fluoride. It is interesting to note that the level of other water quality parameters has also been reduced to some extent. Hence, the GOAlgLa composite beads can be efficiently employed for defluoridation of water.

Table 3 Field trial results of GOAlgLa composite beads

Water quality parameters	Before treatment	After treatment
F^− (mg L^−1)	3.28	0.72
pH	7.68	7.75
Cl^− (mg L^−1)	617	486
Total hardness (mg L^−1)	376	351
Total dissolved solids (mg L^−1)	709	658

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3.10. Performance comparison of GOAlgLa composite beads with other adsorbents

A comparative review was carried out on GOAlgLa composite beads with other previous reported adsorbents. Table 4 shows that the fluoride adsorption capacity of GOAlgLa composite beads have appreciable adsorption capacities with the reported adsorbents.

Table 4 Fluoride adsorption capacity of different adsorbent materials

S. No.	Name of the adsorbent	Adsorption capacity (mg g^−1)	Reference
1	GOAlgLa composite beads	6.62	Present study
2	Bauxite clay	5.16	50
3	Zn/Al and Mg/Al intercalated sodium alginate	5.10	51
4	Al–Zr in cellulose matrix	5.76	52
5	Corn stover biochar (CSBC)	6.42	53
6	Magnetic corn stover biochar (MCSBC)	3.61	53
7	La(III) loaded hematite	0.36	54
8	La loaded zeolite	0.36	54
9	Fe–Al-impregnated granular ceramic	3.56	55
10	Calcium chloride modified natural zeolite	1.77	56
11	Magnesia-loaded fly ash cenospheres	6.00	57
12	La(III) incorporated carboxylated chitosan beads	4.71	58
13	Waste carbon slurry	4.31	59
14	La(III) cross-linked nano-hydroxyapatite alginate composite beads	3.72	21
15	Fe3O4 coated nano-hydroxyapatite gelatin composite	5.01	10
16	Chitosan supported lanthanum/zirconium mixed oxide composite	4.97	60
17	Activated carbon (Acacia farnesiana)	2.62	61
18	Siderite (modified)	5.46	62
19	Stilbite zeolite modified with Fe(III)	2.31	63
20	Cellulose@HAP nanocomposites	2.76	64
21	Hydrous bismuth oxides	1.93	65
22	HFO doped alginate beads	8.90	66
23	Zirconium–iron oxide	9.80	67
24	Zirconium phosphate	4.27	68
25	Aluminum (hydr)oxide coated pumice	7.87	69
26	Alginate impregnated alumina	17.00	34
27	KMnO4 modified carbon	15.90	70
28	Granular ceramic	12.12	71
29	Chemical treated laterite	37.90	72
30	Magnetic chitosan	22.49	73

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4. Conclusions

The hybrid biopolymeric composite beads were synthesized by incorporating GO into Alg polymeric matrix and followed by cross-linking with La(III) metal ions. The developed GOAlgLa composite beads showed a potential efficiency for the removal of fluoride from an aqueous solution. The DC of GOAlgLa composite beads decreased drastically when the solution pH was increased from 3 to 11. The presence of competitive anions such as chloride, nitrate, and sulphate ions do not considerably decrease the DC of GOAlgLa composite beads. However, bicarbonate ions retard the DC of GOAlgLa composite beads. The sorption isotherm process was better fit with the Langmuir isotherm than D–R and Freundlich models. GOAlgLa composite beads possess a higher DC than GO because they remove fluoride by ion-exchange, electrostatic adsorption and complexation. The thermodynamic parameter values indicate a spontaneous and endothermic nature for fluoride removal. The regeneration and reusability studies indicate that the performance of GOAlgLa composite beads for continuous applications. The results of the field application of GOAlgLa composite beads illustrates that the biopolymeric composite beads can be efficiently utilized for fluoride sorption and will make a platform to develop the defluoridation technology.

Acknowledgements

The authors are thankful to the Department of Science and Technology-Science and Engineering Research Board (No. SR/FT/CS-43/2011 dt. 24-05-2012), New Delhi, India for the provision of financial support to carry out this research study. The first author (K. Pandi) would like to thank the Council of Scientific and Industrial Research (CSIR), New Delhi, India for awarding the Senior Research Fellowship.

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