ADSORPTION OF FLUORIDE ON CHITOSAN IN AQUEOUS SOLUTIONS: DETERMINATION OF ADSORPTION KINETICS

Abstract

The fluoride ion (F) is widely used in different industries, including the semi-conductor, power plant, and glass product industries, and is released into the environment through effluents. Accordingly, the effectiveness of chitosan in the adsorption of F from water was evaluated in this study. At different concentrations of F, the significance of contact duration, pH, and adsorbent dose was examined in laboratory conditions. To understand the mode of adsorption and its kinetics, the Langmuir, Freundlich, and Temkin isotherms and the pseudo-first-order, the pseudosecond order, and the intraparticle diffusion model equations for the equilibrium adsorption data were analyzed. The results for the adsorption of F onto chitosan were best described by the Langmuir isotherm and the pseudo-second order models with the correlation values for both being 0.997. A contact time of 180 min, an adsorbent dosage of 5 g/L, and a pH of 7 were considered to be the optimal operational conditions and gave a fluoride removal efficiency of 87%. We conclude that, fluoride adsorption by chitosan can effectively remove fluoride from water.

Headnote

ABSTRACT: The fluoride ion (F) is widely used in different industries, including the semi-conductor, power plant, and glass product industries, and is released into the environment through effluents. Accordingly, the effectiveness of chitosan in the adsorption of F from water was evaluated in this study. At different concentrations of F, the significance of contact duration, pH, and adsorbent dose was examined in laboratory conditions. To understand the mode of adsorption and its kinetics, the Langmuir, Freundlich, and Temkin isotherms and the pseudo-first-order, the pseudosecond order, and the intraparticle diffusion model equations for the equilibrium adsorption data were analyzed. The results for the adsorption of F onto chitosan were best described by the Langmuir isotherm and the pseudo-second order models with the correlation values for both being 0.997. A contact time of 180 min, an adsorbent dosage of 5 g/L, and a pH of 7 were considered to be the optimal operational conditions and gave a fluoride removal efficiency of 87%. We conclude that, fluoride adsorption by chitosan can effectively remove fluoride from water.

Key words: Adsorption; Chitosan; Fluoride; Isotherms, Kinetics.

(ProQuest: ... denotes formulae omitted.)

INTRODUCTION

Fluorine, a natural element, can be found as the fluoride ion (F) in minerals, systems of natural water, and geological sediments and this element can enter the food chain through the consumption of water or vegetation.1,2 Fluoride is released into the environment via effluents and industries.3 The discharge of these effluents to surface waters can lead to the contamination of underground water. The concentration of fluoride ions in underground waters exceeds the acceptable limits in many areas around the world.4-6

There is a considerable literature on the biological effects of fluoride including on dental fluorosis, skeletal fluorosis, and non-fluorosis including the effects on IQ in children.7-10 The WHO recommends an upper limit of F in drinking water of 1.5 mg/ L.11

Among the different methods for removing F from water, adsorption is the one most commonly used. The selection criteria for an adsorbent include the adsorption capacity and the economic aspects.12 Studies on the defluoridation of water have considered various adsorbents including fluorapatite (FAP)/calcite,12 activated alumina,13 leaf powders,14 enriched titanium bockcite,15 ferric oxide,16 limestone powder,17 and Azolla filiculoides.18 Moreover, in 2011 Miretzky and Cirelli assessed fluoride removal from water using chitosan derivatives.19 They concluded that chloride, bicarbonate, carbonate, and sulfate ions have a major influence on fluoride uptake in drinking water and that these anions are therefore in competition with fluoride sorption.19 With this background in mind, the main purpose of the current study was to evaluate the efficiency of chitosan in fluoride adsorption.

MATERIALS AND METHODS

Substances: Sigma Co. supplied chitosan, while NaOH, H2SO4, and fluoride solution were provided by Merck Co. (Germany).

Experimental procedures: The effects of the operational parameters of pH (3-11), adsorbent dosage (1-5 g/L), F concentration (5-25 mg/L), and contact time (15-240 min) were studied consecutively, with an experimental design that examined one factor at a time. For the analysis, the samples were passed through a filter (Whatman, 0.45 pm), and the fluoride adsorption was determined using a spectrophotometer at 570 nm20 The adsorption capacity was determined using Equation 1.14

... Equation 1

Where:

q denotes fluoride ion (F) adsorption on the adsorbent (mg/g)

Co and Ce are the initial and final F concentrations (mg/L)

M refers to the adsorbent mass (g)

V shows the solution volume (L)

RESULTS

Effect of pH on F removal: Using an adsorbent dose of 2 g/L, an initial F concentration of 5 mg/L, and a contact time of 120 min, the maximum fluoride removal, an adsorption efficiency of 24% corresponding to an adsorption capacity of 0.25 mg/g, was achieved with a pH of 7 (Figure 1).

Effect of contact time on fluoride removal: Using a pH of 7, an adsorbent dose of 2 g/L, and an initial F concentration of 5 mg/L, the removal efficiency increased as the contact time increased with the highest removal efficiencies (59% and 60%) being observed at the contact times of 180 and 240 minutes (Figure 2). The adsorption capacity was 1.35 mg/g at a contact time of 180 minutes.

Effect of adsorbent dose on fluoride removal: F removal was enhanced as the chitosan dosage increased. Using a pH of 7, a contact time of 180 min, and an initial F concentration of 5 mg/L, the removal efficiency was 12.8% for an adsorbent concentration of 1 g/L and increased to 87% at an adsorbent dose of 5 g/L (Figure 3).

Effect offluoride concentration on fluoride removal: The F removal efficiencies for initial F concentrations of 5, 10, 15, 20, and 20 mg/L were 88%, 86.6%, 69.3%, 52.5%, and 50.4%, respectively. The adsorption capacities for these F concentrations were 0.88, 1.732, 2.08, 2.1, and 2.52 mg/g, respectively (Figure 4).

Adsorption models: For the analysis of the equilibrium data for F adsorption onto the chitosan, the Langmuir, Freundlich, and Temkin models were used. They are listed below:21-27

Langmuir model:

...

where:

q~ = the mass of fluoride per unit mass of sorbent (mglg)

q,~ = the monolayer sorption capacity

b = the Langmuir constant related to the free energy of sorption equilibrium concentration of fluoride in the solution (mgIL) after biosorption

ce= the equilibrium concentration of adsorbate (mgIL)

Freundlich model:

...

where:

q~ the amount of fluoride adsorbed per unit weight of the sorbent (mg/g)

KF = the Freundlich capacity factor and a measure of biosorption capacity

1/n = the equilibrium concentration of fluoride in the solution (mgIL) after biosorption

Ce = the equilibrium concentration of adsorbate (mgIL)

Temkin model: qe = ß ln a + ß ln Ce

where:

qe = the amount of fluoride adsorbed per unit weight of the sorbent at equilibrium (mg/

ß = constant related to heat of sorption (J/mol)

a = Temkin isotherm equilibrium binding constant (L/g)

Ce = the equilibrium concentration of adsorbate (mg/L)

The results of the isotherm parameters/constants are displayed in Table 1. Based on the correlation coefficient for all the isotherm models studied, the Langmuir model gave the highest R2 value, 0.997, showing that this model best described the adsorption of F onto chitosan.

The Langmuir plots for F adsorption onto chitosan are shown in Figure 5.

Adsorption kinetics: Adsorption kinetics experiments were conducted to study the impact of contact time and assess the dynamic characteristics. The ability of three kinetic equations to fit the experimental data was examined. These models, the pseudo-first-order kinetic equation, the pseudo-second-order kinetic equation, and the intraparticle diffusion model, are listed below 21-27

Pseudo-first order model: ...

where:

qe = the amount of fluoride ion adsorbed (mg/g) at equilibrium (min)

qt = the amount of fluoride ion adsorbed (mg/g) at time t (min)

ki = the rate constant of the pseudo-first-order kinetic model (min-1). The values of k1 can be calculated from the plots of ln (qe -qt) versus t for the equation.

Pseudo-second order model:

...

where:

qe = the amount of fluoride ion adsorbed (mg/g) at equilibrium (min)

qt = the amount of fluoride ion adsorbed (mg/g) at time t (min)

k2 = the rate constant (g mg-1 min-1) of the pseudo-second-order kinetic model for adsorption. The slope and intercept of the linear plots of t/qt against t yield the values of 1/qe and 1/ k2qe2 for the equation.

Intraparticle diffusion model:

...

where:

qt = the amount of fluoride ion adsorbed (mg/g) at time t (min)

k3 = the intra-particle diffusion rate constant (mg g-1 min-1), which can be calculated from the slope of the linear plots of qt versus t1/2.

C = the intercept (mg/g)

The F adsorption kinetic parameters onto chitosan are shown in Table 2.

Based on the findings, a greater consistency was found between the pseudo-secondorder model and the experimental findings with respect to the higher correlation coefficients (R2=0.997) (Figure 6). The maximum adsorption capacity of F calculated by pseudo-second order kinetic model was 1.25 mg/g.

DISCUSSION

It is apparent in Figure 2 that the percentage removal of F was increased by increasing the chitosan dose. This is consistent with the report by Zazouli et al. in 2009 that increasing the dose of the adsorbent sorghum from 2 to 14 g/L resulted in an increase in adsorption capacity from 4.2 to 9.5 mg/g and for which the reason given was an increase in the number of available adsorption sites.26

Contact time is also an important factor in the adsorption of contaminants. Figure 4 shows that the adsorption of F onto the chitosan reached to the equilibrium in 180 min. During the adsorption of F, the F molecules initially reach the boundary layer; they then have to diffuse into the adsorbent surface, and, finally, they have to diffuse into the porous structure of the chitosan.28 Therefore, the adsorption of F by chitosan takes a relatively long time.

With increasing F concentration, the removal percentage is decreased, but the absorption capacity is increased. This means that the adsorption is highly dependent on the initial concentration of F. At lower initial concentrations of F, the ratio of the initial number of F molecules to the available surface area is low and subsequently the fractional adsorption becomes independent of the initial concentration.29 However, at higher initial concentrations of F, the ratio of the initial number of F molecules to the available surface is greater with relatively fewer sites available of adsorption. Thus, the percentage removal of F is dependent upon the initial F concentration.28

With increasing pH, the F removal decreased, which it is probably be due to an inappropriate surface charge and to competition for adsorption sites because of excess anion at alkaline conditions. However, there were no significant differences in F removal. This finding is in agreement with the literature.21,26

The value of RL for the adsorption of F indicated that the adsorption behavior of chitosan was favorable for the fluoride ion (RL<1). According to a linear regression method, the Freundlich and Temkin isotherms showed a poor suitability for modelling the adsorption of chitosan in comparison to the Langmuir isotherm. The validity of the Langmuir model suggests that the F uptake was due to a monolayer coverage of solute particles onto the surface of the chitosan and that the adsorption of each molecule had an equal activation energy. This result similar to that of the studies by Craig30 and Kusrini.31

CONCLUSION

The application of inexpensive adsorbents or agricultural and industrial wastes can be a suitable strategy for removing pollutants. In the current study, F elimination by chitosan was examined. The results for the adsorption of F onto chitosan were best described by the Langmuir isotherm and the pseudo-second order models with the correlation values for both being 0.997. According to the results of the current study, the adsorption process with chitosan can be considered to be an effective, reliable, viable, and inexpensive method for removing F from water. This adsorbent can decrease the dose of F to less than 1.5 mg/L in optimal conditions, which is acceptable in most countries.

ACKNOWLEDGMENTS

The researchers express their gratitude to Zahedan Academic University of Medical Sciences for their grant and financial support of this research proposal with Code 8742. We are also grateful to the Ahvaz Jundishapur University of Medical Sciences for supporting the study.

References

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