Removal of Fluoride from Aqueous Solutions Using Chitosan Cryogels

In this study, crosslinked chitosan cryogels (QE) and chitosan’s cryogels modified with iron (QEFe) were synthesized. They were characterized by BET, PZC, FTIR, and XPS spectroscopies. Results show a specific surface area of 36.67 and 29.17mg and 7.0 and 6.1 of PZC for the cryogels QE and QEFe, respectively. FTIR results show the characteristic bands of amino and hydroxyl groups, while in the XPS analysis, interactions between iron and oxygen with fluorine were observed. The removal of fluoride at temperatures of 303, 313, and 323K in cryogels was tested. The Ho model is the best fit for the experimental data, suggesting that there is a chemisorption process involved in the removal of fluoride. The Langmuir-Freundlich model is the best to represent the behavior of the cryogels, and it is used to sorbents with heterogeneous surfaces. A maximum fluoride adsorption capacity of 280 and 295mg F/g for QE and QEFe, respectively, at 303 K was obtained, showing that the removal of fluoride is favored by the iron incorporated in the polymer matrix of the cryogels. The thermodynamic parameters were obtained for both cryogels, where the values of ΔH∘ and ΔG∘ indicate that both systems are endothermic and nonspontaneous.


Introduction
The most abundant halogen in the Earth's crust is fluorine, with a concentration close to 0.3 g/kg.Diverse minerals contain fluorine in the form of fluorides [1,2].Aqueous solubilization of molecules containing fluorine occurs through geochemical processes.The regional geomorphic composition determines the amount of fluorine that reaches the aquifers [1][2][3][4][5].The presence of fluorine affects the water quality for human consumption.Fluorine is highly reactive and therefore forms compounds with practically all the elements.Biologically active compounds are likely to incorporate fluorine when ingested in water for long periods.Epidemiological studies prove detrimental to the bone tissue (bones and teeth) and the spinal cord and brain.The degeneration of neurons and alteration of the metabolism of free radicals in the liver, kidney, and heart and lesions on the thyroid and the endocrine glands interfere with DNA synthesis, and metabolic disorders in soft tissues and some cancers are also associated with the exposure to fluorine [1][2][3][4][5][6][7][8][9][10][11][12][13][14].The World Health Organization (WHO) classifies fluorine as a public health risk and recommends a maximum concentration of 1.5 mg/L of fluorine in drinking water.Globally, inhabitants in some regions of China, India, Mexico, and the United States are affected by higher concentrations [1, 2, 6-8, 12, 13].
In an effort to remove fluorine in water, diverse techniques have been studied, with absorption being the more versatile due to its high efficiency, sludge minimization, and regeneration of the sorbent material [6,10,[15][16][17][18].In recent years, biopolymers such as chitosan have proved to be effective for fluorine removal.Chitosan is a nontoxic, biodegradable, and biocompatible material [7].Among the fluoride removal studies that have been used, sorbents from 2 Journal of Chemistry chitosan inferred that these biosorbents offer satisfactory results in terms of fluoride elimination.In addition, chitosan is obtained from natural sources that are an ecologically low cost material [19][20][21][22][23].The biopolymer structure consists of amino and hydroxyl groups which are mainly responsible for the adsorption of various ions [11,16,24,25].Also, limited swelling of chitosan in water causes low availability of its functional groups during sorption.Chemical and physical modifications improve the readiness of contact sites on chitosan [26,27].The cryogel synthesis route allows the structural transformations supplying adequate stability and strength to the material.Cryogels are porous materials that can provide a better contact surface between the functional groups of chitosan and different ions in aqueous solutions [28][29][30][31][32][33], so these materials are a suitable alternative for removal of fluoride ions in aqueous solutions.
The aim of this work was to synthesize chitosan's cryogels crosslinked with ethylene glycol diglycidyl ether (EGDE) and its modification with iron, in order to use them in the removal of fluoride from aqueous solutions prepared in the laboratory.A comparative assessment of the modified cryogels (QEFe) and unmodified cryogels (QE) was performed to determine the sorption capacity of each material and observe whether introducing Fe 3+ into the chitosan polymeric matrix had a significant impact on the adsorption phenomenon.

Synthesis of Crosslinked Chitosan Cryogels (QE).
Two solutions dissolving chitosan (Q) in 100 mL of acetic acid (3%) were prepared.Afterwards, the ethylene glycol diglycidyl ether as a cross-linking agent (E) was added at 1% relative to the chitosan weight under inert atmosphere and constant stirring during a 6-hour period.Each one of the solutions was dripped into liquid nitrogen and the obtained spheres were freeze-dried by using a lyophilizer CHRIST BETA 2-8 LD to 0018 mbar and 190 K for 17 hours.Finally, QE cryogels were washed with 1.5% acetic acid and then with deionized water until the rinse water's pH was equal to the pH of the deionized water.

Synthesis of Chitosan's Cryogels Modified with Iron (QEFe).
The total QE cryogels obtained by one of the solutions in the synthesis of crosslinked chitosan cryogels were treated with 300 mL of solution 0.1 M FeCl 3 during a 24-hour period, with constant stirring at 303 K. Subsequently, the modified beads were washed with ethanol for removing the unreacted FeCl 3 , followed by washing with deionized water until the rinse water pH was equal to the pH of the deionized water [26,34].1).Furthermore, the effect of different fluoride initial concentrations was studied using 2 mg/L to 1000 mg/L at 303 K, 313 K, and 323 K.The adsorbent mass (0.002 g) was kept constant as well as pH value with a study solution volume of 8 mL.Fluoride ions in the solutions were quantified by using a Cole-Parmer 27504-14 selective ion electrode and a Thermo Scientific Orion 4-Star potentiometer.All experiments were triplicated under identical conditions.Finally, experimental data was fitted to mathematical models with nonlinear regression, using the Origin 8.1 program.

Characterization of Cryogels.
The BET method was used with a Belsorp-max equipment to determine the specific surface area of cryogels.Before the analyses, the samples were subject to a degassing process in a Belprep II equipment, at a temperature of 373 K for 2 hours in order to remove impurities and water molecules contained in the material structure.Subsequently, each sample was weighed and placed in the Belsorp-max to carry out the determination of specific area through process of adsorption and desorption of N 2 gas at 77 K and 76.5 kPa.
The PZC was determined by means of contacting 0.5 g of the solid sample of cryogels (QE and QEFe) with 20 mL of NaCl solution 0.1 M at different pHs.The initial pH values of the solutions were adjusted using HCl or NaOH 0.1 M. The suspensions were then shaken for 24 hours (200 rpm and at 303 K), after which the pH value of the supernatant for each sample was measured.The difference between the final and initial pH (ΔpH = pH  −pH  ) was plotted versus pH  and the point of intersection of the resulting curve at which ΔpH = 0 was the pH PZC value.
A FTIR spectrometer Variant 600 with an ATR-coupled device was used in order to identify the presence of the functional groups in chitosan-modified molecule by crosslinking in the adsorbent materials before (QEFe and QE) and after (QEFe/F − and QE/F − ) sorption.Sixteen scans were performed in a range of 4000-500 cm −1 with a resolution of 4 cm −1 .An XPS spectrometer Thermo K-Alpha Scientific was also used to further verify the existence of study species in the materials, and the spectra were subjected to a mathematical deconvolution and an analysis of presented chemical interactions by using the Origin 8.1 program and the http://lasurface.comdatabase.

Characterization of Cryogels.
The specific surface area of cryogels was determined by the BET method.The specific surface areas for QE and QEFe cryogels were 36.67 m 2 g −1 and 29.17 m 2 g −1 , respectively.
In the infrared spectrum (Figure 2), vibrations of the different functional groups at the chitosan can be observed.In 3433 cm −1 band one is observed due to vibration of NH 2 and in 2922 cm −1 a band is observed due to vibration of OH; at 1633 cm −1 and 1420 cm −1 the vibration signals are present of NH 2 and OH, respectively.The spectra of the QE and QEFe cryogels have higher Q spectrum bands, virtually unchanged, showing that these functional groups are present in the cryogels.However, a decrease in signal intensity can be observed in the QEFe cryogel corresponding to the amino groups, which may indicate that the added iron is attached mainly by these functional groups.The cryogels spectra after fluoride sorption (QE/F − and QEFe/F − ) show a decrease in the intensity of signal corresponding to the amino and hydroxyl groups.The hydroxyl group was the most decreased.This suggests that these groups are related to the removal of fluoride ions [35,36].
The cryogels were analyzed by XPS before (QEFe and QE) and after (QEFe/F − and QE/F − ) the sorption of fluoride ions.C, O, N, Fe, and F were identified in both samples.Figures 3 and 4 show the XPS results before and after the sorption of each material, where the predominant peaks C1s and O1s and a lower intensity peak N1s can be observed.These can be attributed to the amino groups of chitosan.In the case of QEFe cryogel, the presence of Fe2p can also be observed.After the sorption, the peak of F1s appears to show an average binding energy of about 690 eV, meaning that fluoride was adsorbed on the surface of both cryogels [37,38].
Figures 5, 6, and 7 show the deconvolution of the O1s and Fe2p spectrums after the sorption with QE and QEFe cryogels.Here, it can be observed that fluoride ions are interacting with the atoms of iron and oxygen in the materials; that is, fluorine is showing bonds with the hydroxyl group of the cryogels, which can be hydrogen bonds.In the case of the QEFe cryogel, it is also forming links with the iron present in the modified cryogel.These interactions may be because the QEFe cryogel presents a higher sorption capacity than the cryogel QE [39,40].
Figure 8 shows the PZC results.When the pH of the solution is lower than the PZC, the total charge of the solid surface is positive, while if the pH of the solution is greater than the PZC, the surface is negatively charged [41,42].The cryogel QEFe showed a less slightly acidic PZC compared to the cryogel QE (6.1 and 7.0, resp.), which can indicate that, in the first one, the content of total acid groups is greater than the content of basic groups.
The pH of the washing solution of cryogels (QE and QEFe) is pH 5 and according to the PZC, both cryogels have a positive charge on their surface; because of this, the removal mechanism of fluorine in the QE cryogel is mainly a physical adsorption by the formation of hydrogen bonds between fluoride and the hydroxyl group and in the QEFe cryogel is a physical and chemical adsorption mainly due to interactions between iron and fluoride (Figure 9) [21,34,43,44].

Sorption Kinetics.
The quality of the sorbent material is evaluated according to the capacity to retain certain species on its surface.For the purpose, it is customary to determine the amount of species removed per unit of solid phase () and the following equation is used: where   and   are the initial and final concentrations (mgL −1 ), respectively,  is the total volume (L), and  is the amount of solid added (g).
In general, the removal of ions from aqueous solutions depends on the chemical mechanisms that involve the interaction of ions with the active groups present in the sorbent.Therefore, the kinetics show the steps through which the sorption takes place.In this paper, Lagergren, Elovich, and Ho models have been considered.
The mathematical expression for the kinetics of pseudofirst-order or Lagergren model is based on the assumption   that each ion is assigned a sorption site of the sorbent material and it is represented [12,[45][46][47]: where  1 is the sorption constant for pseudo-first-order model (min −1 ),   is the mg of F − adsorbed/g of sorbent anytime, and   is the amount adsorbed on the equilibrium (mgg −1 ).
The mathematical expression of Elovich model was developed for heterogeneous sorption processes of gas in solids.However, it has been successfully applied in processes of sorption for contaminants in solution, and the next equation describes this model [48][49][50][51][52][53]: where  is the initial sorption rate (mg g −1 h −1 ),  is the desorption constant (gmg −1 ),  is time (h), and  is the amount of solute sorbed (mgg −1 ).The Ho model was developed by Ho and McKay; it is assumed that the sorbate is adsorbed by two active sites in the sorbent.The next equation describes this model [45,46,52,53]: where  2 is the pseudo-second-order constant (gmg −1 min −1 ),   is the amount adsorbed anytime (or time, ) (mg g −1 ), and   is the amount adsorbed on the equilibrium (mg g −1 ).The fluoride sorption occurs in the first 5 minutes of the test for both cryogels (Figure 10) and then begins to stabilize until equilibrium is reached.It was determined that the equilibrium time was reached approximately in 480 minutes for both cryogels.After reaching equilibrium, there were no significant changes in the quantity of fluorine captured by the materials.Furthermore, the QEFe cryogel exhibited a greater sorption capacity at the three temperatures studied than the QE cryogel.
After the sorption kinetics, an adjustment of the experimental data for the Lagergren, Elovich, and Ho models by means of nonlinear regression using the Origin 8.1 program was performed.Table 1 provides a summary of the obtained parameters.
The QE cryogel shows a higher  2 coefficient in the Lagergren model at 303 K, whereas at 313 K and 323 K the coefficient is higher in the Elovich model.The QEFe cryogel shows a higher correlation coefficient for Ho model at 303 K and 313 K and for the Lagergren model at 323 K.According to Figure 11, the Ho model is what best fits the experimental data, suggesting that there is a chemisorption process involved in the removal of fluoride.As a result the model that best describes the behavior of the systems is the Ho model.
In other articles materials of chitosan have been used in order to remove fluoride and under similar conditions used in this study, such is the case of the chitosan beads modified by carboxylation followed by chelation with Fe 3+ (4.23 mg F − /g), the chitosan modified with a mixed of rare earths (3.72 mg F − /g), a bioadsorbent of titanium(IV) hydrate based on chitosan template (11.44 mg F − /g), and an Aldoping chitosan-Fe(III) hydrogel (3.9 mg F − /g).The sorption capacity in equilibrium time is similar to or lower than sorption capacity obtained in this work: 13.10 mg F − /g and 23.20 mg F − /g at 303 K for the QE and QEFe cryogels, respectively [13,34,54,55].

Sorption Isotherms.
Over time, several mathematical models have been developed for the treatment of experimental data using the models of Langmuir, Freundlich, and Langmuir-Freundlich.The Freundlich model is an empirical model that can be applied to a nonideal adsorption in heterogeneous surfaces, as well as multilayer adsorption.The Freundlich model is expressed by the following equation [12,13,[46][47][48]: where   is the adsorption capacity at equilibrium (mgg −1 ),   is the constant for Freundlich's isotherm (mgg −1 ),   is the ion concentration in the solution at equilibrium (mgL −1 ), 1/ is the quotient related to adsorption intensity, and  is the dimensionless exponent of the Freundlich isotherm.The Langmuir model is probably the best-known and the most widely applied adsorption isotherm.This isotherm assumes that a solid has a limited adsorption capacity; the molecules are adsorbed on well-defined and energetically equivalent sites and are far enough away from each other to not allow interaction between the adsorbed molecules in adjacent sites.Each site can retain only one molecule of adsorbate and the sorption leads to the deposition of a layer of solute molecules on the surface of sorbent.The model can be represented by the following equation [46,[48][49][50]56]: where   is the adsorption capacity at equilibrium (mgg −1 ),   is the monolayer adsorption capacity (mgg −1 ),  is the constant for Langmuir's isotherm (Lmg −1 ), and   is the ion concentration in the solution at equilibrium (mgL −1 ).Langmuir also considered the case of a molecule occupying two sites and it produces the Sips or Langmuir-Freundlich isotherm.The Langmuir-Freundlich name is derived from  the limiting behavior of the equation.At low sorbate concentrations it effectively reduces to a Freundlich isotherm.At high sorbate concentrations, it predicts the monolayer adsorption characteristic of the Langmuir isotherm [46,[57][58][59][60].This isotherm can be determined by the following equation: where   is the adsorption capacity at equilibrium (mgg −1 ),  LF is the constant of Langmuir-Freundlich's isotherm [(Lmg −1 )  LF ],   is the ion concentration in the solution at equilibrium (mgL −1 ),  LF is the constant of Langmuir-Freundlich's isotherm (mg 1− LF L 3 LF g −1 ), and  LF is the dimensionless exponent of Langmuir-Freundlich's isotherm.The isotherms do not show appreciable changes in adsorption capacity in a concentration of 500 mg/L of fluorine (Figures 12 and 13).In addition, it can be observed that the sorption capacities for both cryogels increase as the temperature increases; that is, the removal of fluoride ions was favored at higher temperatures, indicating an endothermic adsorption [15,61].
Table 2 shows a summary of the values obtained after the adjustment of the experimental data to the models of Freundlich, Langmuir, and Langmuir-Freundlich, obtained by nonlinear regression and using the program Origin 8.1.Figures 14 and 15 show the data adjustment of these three models at the temperature of 323 K.
The identification of an adequate isotherm model can be carried out by means of the analysis of chi-square ( 2 ), which uses the experimental data and the adjusted data by the isotherms models: where   and   are the adsorption capacities obtained by the isotherm models and the experimental adsorption capacity (mgg −1 ), respectively.When low values of  2 are observed, the experimental data has a good fit to the model used [15,17,34,40,44,50,53,62].
A maximum fluoride adsorption capacity of 280 mg F − /g for QE and 295 mg F − /g for QEFe at 303 K was obtained, showing that the removal of fluoride is favored by the iron incorporated in the polymer matrix of the cryogels.The coefficients of determination are very similar in the Langmuir and Langmuir-Freundlich models (Table 2).The Chi-square test shows that the Langmuir and Langmuir-Freundlich models can adequately represent the experimental data because the  [47,63]; this is consistent with the proposed removal mechanism in Section 3.1.

Thermodynamic Parameters.
The thermodynamic parameters are used to estimate the feasibility of the adsorption process as well as the effect of temperature.The change in the standard Gibbs free energy (Δ ∘ ) determines if a process is spontaneous or not.The Van't Hoff equation is used to obtain graphically the values Δ ∘ and Δ ∘ .This equation arises from the equation of Gibbs free energy as follows: A plotting of ln   in the -axis and  −1 on the -axis should be lineal and the intercept would be equal to Δ ∘ /, while the slope would be numerically equal to −Δ ∘ /, where  (8.314 Jmol −1 K −1 ) is the universal gas constant.Meanwhile   is determined as follows: where  Ae is equilibrium adsorbate concentration on the surface of the adsorbent and  Se is the equilibrium concentration in the solution.
Table 3 shows the results of the thermodynamic parameters.Δ ∘ suggests an increase in the disorder of the system and a more widespread disorder can be expected during sorption.The values of Δ ∘ indicate that these systems are endothermic, and because Δ ∘ is below 40 kJ/mol this suggests a coexistence of physical and chemical adsorption.Moreover, Δ ∘ suggest that the system is not spontaneous, so it is necessary to provide energy to the system [7,13,15,16,46,52,66,67].

Conclusions
The X-ray photoelectron spectroscopy shows that fluoride groups interact with oxygen groups in the QE cryogel.Therefore, the fluoride presents links with the hydroxyl groups of the cryogels, which can be hydrogen bonds.Meanwhile, the QEFe cryogel shows interactions between iron and oxygen in the material, so the fluoride ions also form links with the iron by means of hydrogen bonds.The QEFe cryogel shows a higher sorption capacity for fluorides than the QE cryogel.
The QE and QEFe cryogels have a similar adsorption capacity.However, the QEFe cryogel shows the highest adsorption capacity.The sorption kinetics show that the Ho model fits the data better than the Elovich and Lagergren models, which suggests that the removal mechanism also involves chemical adsorption.The sorption isotherms show that the Langmuir-Freundlich model best represents the experimental data and the behavior of cryogels.The point of zero charge shows that on the surface of the cryogels it has predominantly positive charges; because of this, the removal mechanism of fluorine in the QE cryogel is mainly the physical adsorption type and in the QEFe cryogel is a physical and chemical adsorption.Finally, the thermodynamic parameters show that the systems are endothermic and nonspontaneous and there is a coexistence of physical and chemical adsorption.