FLUORIDE REMOVAL FROM AQUEOUS SOLUTION BY ALMOND SHELL ACTIVATED CARBON

Abstract

Fluoride (F-), the ion of the element fluorine, is one of the mineral elements present in nature that enters drinking water through underground aquifers and when present in high concentration it can lead to risks such as skeletal, dental, and nonskeletal fluorosis with morbidity such as decreased IQ and negative effects on brain development, tooth discolouration, and bone pain. The aim of the study, was to study the ability of activated carbon developed from almond shell (ACAS), to remove fluoride from an aqueous solution. We examined the effects on fluoride removal efficiency of contact time, initial F- concentration, pH, and ACAS mass. The equilibrium data were also examined with models including the Langmuir, Freundlich, and Temkin isotherms. The removal characteristics were validated through the use of different kinetic models for the estimation of the solute interaction and the nature of the biosorption. A contact time of 60 min, an adsorbent dosage of 0.6 g/L, and a pH of 3 were considered to be the optimal operational conditions. The biosorption of F- onto ACAS and the equilibrium data follow from the Langmuir adsorption isotherm with a maximum adsorption of 84.3 mg/g and a regression coefficient of R2=0.999. The kinetic studies showed that the system fitted the pseudo-second order kinetic model. Thus, this work gives new insights on the interaction of ACAS with F- in a reconstituted aqueous solution.

Headnote

ABSTRACT:

Fluoride (F-), the ion of the element fluorine, is one of the mineral elements present in nature that enters drinking water through underground aquifers and when present in high concentration it can lead to risks such as skeletal, dental, and nonskeletal fluorosis with morbidity such as decreased IQ and negative effects on brain development, tooth discolouration, and bone pain. The aim of the study, was to study the ability of activated carbon developed from almond shell (ACAS), to remove fluoride from an aqueous solution. We examined the effects on fluoride removal efficiency of contact time, initial F- concentration, pH, and ACAS mass. The equilibrium data were also examined with models including the Langmuir, Freundlich, and Temkin isotherms. The removal characteristics were validated through the use of different kinetic models for the estimation of the solute interaction and the nature of the biosorption. A contact time of 60 min, an adsorbent dosage of 0.6 g/L, and a pH of 3 were considered to be the optimal operational conditions. The biosorption of F- onto ACAS and the equilibrium data follow from the Langmuir adsorption isotherm with a maximum adsorption of 84.3 mg/g and a regression coefficient of R2=0.999. The kinetic studies showed that the system fitted the pseudo-second order kinetic model. Thus, this work gives new insights on the interaction of ACAS with F- in a reconstituted aqueous solution.

Keywords: Activated carbon; Almond shell; Fluoride; Kinetics; Isotherms.

(ProQuest: ... denotes formulae omitted.)

INTRODUCTION:

The sources of fluorine in water and soil are mostly geogenic and include several rock forming minerals.1-4 The load of the fluoride ion (F-) in water is also increased by various industries, such as those producing pesticides, ceramics, refrigerants, and aerosol propellants, Teflon cookware, and glassware.5,6 Fluoride is considered to be able to prevent dental caries by decreasing the rate of demineralization of dental enamel and reversing the progression of existing decay by promoting the rate of remineralization.7-9

Various values have been given for the permissible limit for the fluoride concentration in potable water with the World Health Organization (WHO) recommending a desirable upper limit of fluoride in drinking water of 1.5 mg/L.10-12 However, countries can set their own country standards and lower standards have been set of 0.6 mg/L in Senegal, West Africa, and 1 mg/L in India with a rider to the Indian limit of "the lesser the fluoride the better, as fluoride is injurious to health." In 2015, in the United States of America, a recommendation was made for community water systems that practice fluoridation of 0.7 mg/L, a reduction from the previous recommendation of 0.7-1.2 mg/L.13

A number of drinking water defluoridation techniques have been developed.14 Various traditional removal methods, such as precipitation, ion exchange, reverse osmosis, and oxidation, have been applied for the elimination of F- from waste water but they have limitations such as being highly expensive and lacking the capacity to remove F- at lower concentrations.15-17 Also, secondary pollution may occur with the generation of toxic sludge and complicated procedures may be involved in the treatment.18,19

Biosorption has been suggested as a prospective technique for the reduction of toxic metal ions from contaminated effluent streams. This method has also been considered to be an efficient and cost-effective approach as it utilizes extensively accessible biomass from nature.20 Among the numerous sorbent materials used for pollutant reduction, activated carbon, obtained from agricultural waste, has been identified as having excellent sorbent characteristics.2 Activated carbons can be used in a broad and growing range of environmental, health, safety, and industrial applications as they have extremely high surface areas, varied porous structures, large adsorption capacities, and fast adsorption kinetics.21 Many studies reveal that marine algae are one of the most important biosorbents with a high recovery for various pollutants, which results from the existence of distinct elemental groups in their cell membrane, like the hydroxyl, carboxyl, amino, and phosphate groups that play an essential role in the elimination of toxic contaminants from different industrial pollutants.22

In the present study, almond shells from almond waste were used as a Fremoval biosorbent due to the benefits of high efficiency, easy handling, costeffectiveness, and their ready availability. The aim of the study was to examine the potential of ACAS in the removal of fluoride from aqueous solutions including assessing the effect on the fluoride removal efficiency of the parameters of contact time, initial F- concentration, pH, and adsorbent dose.

MATERIAL AND METHODS

Adsorbent preparation: Almond shell was collected and washed several times with distilled water to remove adhered impurities from its surface. The biomass was dried at 105°C for 2 hr. The dried biomass was milled and sieved to 10-100 m particle size. The dried mass was subjected to a carbonization process at 200°C, then powdered well, and finally activated at a temperature of 500°C for a period of 1 hr.

Adsorbent characteristics: The specific surface area of ASAC was determined by the BET-N2 method based on nitrogen adsorption-desorption isotherms at 77K. Scanning electron microscopy (SEM) of the ASAC was carried out using a scanning electron microscope (EM3900M-KYKY). The FTIR spectra (Spectrum Two, Perkin Elmer) were recorded in the range of 400-4,000 cm-1 to find out the information regarding the bending vibrations and the stretching of the functional groups which are responsible for the adsorption process.

Sodium fluoride (NaF) was obtained from Merck. To prepare the primary solution of F (500 ppm), 0.1105 g NaF was dissolved into 500 mL distilled water and the next solutions are obtained by dilution.

Adsorption experiments: The examination of the removal characteristics of F" was conducted in a controlled reaction mixture volume of 200 mL by using ACAS as a removal agent and varying the biosorption factors of contact time, adsorbent dose, pH, initial F" strength, and biosorbent quantity. To prepare the stock solution of F" (500 mg/L), 0.1105 g NaF was dissolved into 500 mL distilled water and the studied concentrations of the F" solution were provided by diluting the stock solution. The pH of the solutions was adjusted to the desired value with 0.1M HCl and NaOH solutions. The adsorption experiments were carried out by the batch technique. For this, 100 mL of fluoride solution, in the concentration range of 2-20 mg/L, were transferred into an Erlenmeyer flask. Then, a certain amount of adsorbent (0.6 g/L) was added to the solution, and the mixture was agitated on a mechanical shaker at 150 rpm for 90 min. After reaching the equilibrium, the adsorbent was removed by vacuum filtration through 0.45 qm nitrocellulose membrane. The experiments were conducted with different process variables to estimate the sorption capability of ACAS, as a biosorbent, for F". The adsorbed quantity and the % removal of F" through ACAS were determined by using Equations 1 and 223

The Langmuir model is described using the following equation (Equation 3).

...

Where:

Ce = the equilibrium concentration of F- (mg/E

qe = the amount of F- adsorbed per unit weight of ACAS at equilibrium

B = the Langmuir constant representing the affinity of binding sites

qmax = the amount of F- adsorbed on ACAS at maximum adsorption capacity (mg/g)

The Freundlich model, which is based on the surface heterogeneity of the adsorbent, is described using the following equation (Equation 4)25"27

...

Where:

Kf = the Freundlich binding constant

Ce = the final concentration of fluoride ion

qe = the adsorption capacity for adsorbents (mg/L)

1/n = the surface heterogeneity constant

The Temkin model is described using the following equation (Equation 5).28

...

Where:

Kt = the Temkin adsorption potential (1/g)

Ce = the final concentration of fluoride ion

qe = the adsorption capacity for adsorbents (mg/L)

b = the Temkin constant

RESULTS

The SEM investigation was executed for the discovery of the surface characteristics and porosity of ACAS. The results obtained are shown in Figures 1A and 1B.

The SEM micrograph exposed that the ACAS is extremely heterogeneous in nature and irregular along with a porous and rough surface morphology.

The total pore value, the average diameter, and the specific surface of ACAS were calculated and determined to be 0.241 cm3/g, 1.73 nm, and 594.5 m2/g, using BET theory, respectively.

To understand the interaction between the functional groups on the surface of ACAS and F- were examined using FT-IR spectroscopy (Figure 2).

The intense absorption peaks at around 3429 cm-1 correspond to the O-H stretching. The peaks at 2927 cm-1 are attributed to the symmetric and asymmetric C-H stretching vibration of aliphatic acids. The peak observed at 1617 cm-1 is the stretching vibration of the bond due to non-ionic carboxyl groups (-COOH, - COOCH3). The peaks at 1100 and 1400 cm-1 are due to asymmetric and symmetric stretching vibrations of C=O in ionic carboxylic groups (-COO-).

The effect of pH on fluoride removal is shown in Figure 3. With increasing pH, the F removal decreased.

The effect of contact time on the removal of F- is shown in Figure 4. 81% Fremoval takes place in 60 min. The equilibrium was reached after 120 min.

Figure 5 shows the F- removal efficiency of ACAS at various adsorbent doses. It is evident from Figure 5 that the F- removal increased sharply with an increase in the dose adsorbent from 0.1 g to 1.0 g/L.

The effect of F- concentration on the adsorption of F- on to ACAS was investigated in the concentration range of 2-20 mg/L and is shown in Figure 6. The equilibrium adsorption capacity increased with an increase in the F- concentration.

Desorption isotherms: The adsorption of F- on ACAS was studied using three isotherm models, the Langmuir, Freundlich, and Temkin models. The adsorption isotherm models help in describing the adsorption mechanism between the adsorbate and the adsorbent. The Langmuir adsorption isotherm assumes that the adsorption takes place at specific homogeneous sites within the adsorbent and has found a successful application in many sorption processes of monolayer adsorption. In the aqueous phase processes, where the system does not fit as well, the three models of Langmuir, Freundlich, and Temkin were used to find a suitable mode for describing the process. The equilibrium data were was also evaluated for the isotherm models Table129-31

Kinetic studies: The pseudo-first order adsorption kinetics for the adsorption of the F- on ACAS was studied using the following equation, which assumes that one adsorbate molecule is attached to one binding site of the adsorbent (Equation 6).29

Where:

qe (mg/g) = the amount of adsorbed adsorbate at equilibrium

qt (mg/g) = the amount of adsorbed adsorbate at time t

ki (min-1) = the rate constant of pseudo-first order adsorption

A plot of Ln (qe-qt) versus t (the figure not shown) was used to determine the slope and intercept values. The adsorption kinetics can also be described using a pseudosecond order model, which assumes that one adsorbate molecule is adsorbed on to two active sites. The linear form of the pseudo-second order equation is given below (Equation 7).30

...

The intra-particle diffusion kinetics, as depicted by Weber and Morris, can be described by the following equation for the assessment of mass transfer and the ratedetermining steps (Equation 8).31-32

...

When the correlation coefficients were examined, the interaction between the surface of the ACAS and F- seemed to fit the pseudo-second order kinetic model (Table 3). This was decided based on the R2 and qe values; for greater R2 values (0.999) the qe values (76.35 mg/g) are closer to those of the experimental results (qexp) (71.42 mg/g). Also, the adsorption capacity values obtained from the pseudosecond order kinetic model are in good agreement with the experimental results.

DISCUSSION

According to Figure 3, with increasing pH, the F- removal decreased. This was probably due to an inappropriate surface charge and to competition for the adsorption sites because of the presence of excess anion in the alkaline conditions. However, there were no significant differences in F- removal. This finding is in agreement with the literature.26,27 About 81% of F- removal takes place in 60 min. The equilibrium was reached after 120 min (Figure 4). The fluoride removal process was incremental from 60 minutes to 120 minutes. Due to the small difference in the removal rate from 60 to 120 minutes, the contact time of 90 minutes was selected for the tests.

As shown in Figure 5, the F- removal increased sharply with an increase in the dose of adsorbent from 0.1 g/L to 1 g/L. The change in the rate of adsorption might be due to fact that initially all the adsorbent sites are vacant and the solute concentration gradient is very high.33,34 Later, the lower adsorption rate is due to a decrease in number of vacant sites on the adsorbent and a decrease in the F- concentrations.26,27 The decreased adsorption rate, particularly, toward the end of the experiments, indicates a possible monolayer formation of F-on the adsorbent surface. This may be attributed to a lack of the available active sites required for further uptake after attaining the equilibrium.35 This may be due to the availability of more ACAS sites as well as a greater availability of specific surfaces on the adsorbents.36 However, no significant changes in removal efficiency were observed beyond the ACAS dose of 0.6 g/L. Due to the conglomeration of ACAS particles, there is no increase in the effective surface area of ACAS with higher doses of ACAS. So, 0.6 g/L was considered to be the optimal dose for ACAS loading.

The results of the study showed that the equilibrium adsorption capacity increased with an increase in the F- concentration. Further increases in the F- concentration did not result in any significant changes in the removal efficiency (Figure 6). This is due to the fact that with increased F- concentrations the driving force for transfer also increases.37 At low concentrations there will be unoccupied active sites on the adsorbent surface.38

The Freundlich model is used to explain physical adsorptions taking place on the surface of heterogeneous adsorbents. It is assumed that each functional region on the surface of the adsorbent has a different adsorption potential. According to our findings, the regions where the adsorption events occur on the surface of the ACAS have the same potential making the system fit better with the Langmuir model. Since it fits the Langmuir model, the equilibrium was proven to be reached, at a constant temperature, between the concentration of F- in the environment and the concentration of the F- that was adsorbed as a single layer on the ACAS which was assumed to be homogeneous. Due to the better suitability of the Langmuir model, the saturation of the adsorbent surface was also proven. Moreover, with the finding that the material had a homogenous structure, the confirmation of the characterization was also established.30-32

The rate control mechanism depends on 3 possible steps during the adsorption process. The first one is the mass transfer to the outer surface at the early stages of the adsorption or film diffusion. This is followed by the second step of the reaction or constant rate step. The third and last step, involves the diffusion towards the inner parts of the pores where the adsorption amount is significantly decreased. These rate control mechanisms are explained with the pseudo-first order and the pseudo-second order kinetic models.39

If the adsorption rate determining step is based on the diffusion of the F- to the surface of the adsorbent, the system fits the pseudo-first order model. If the adsorption rate determining step is based on the interaction between the F- and the adsorbent, the system fits the pseudo-second order model. The first case is called a diffusion-controlled process and the second one is referred to as chemisorption. The treatment and exposure times are important to understand the steps affecting adsorption rates.38,39 The mechanisms controlling the adsorption process are mass transfer and chemical reactions. In the determination of these mechanisms, the pseudo-first and second order models were applied to the experimental data. When the kinetic data were analyzed it was found that the adsorption of F- on to ACAS was chemisorption without any diffusion restriction.40

CONCLUSIONS

We report here an effective strategy for the removal of F- using ACAS complexes. The ACAS was found to exhibit a F- removal efficiency of 99.2%. The adsorption of F- on ACAS was found to follow the Langmuir adsorption isotherm model and the pseudo-second order kinetics. The production of ACAS, within the scope of this study, is safe and cost-efficient, which makes this green adsorbent a good candidate for the removal of F- from water resources. This study represents the first F-adsorption study based on AC obtained from almond shell.

ACKNOWLEDGEMENT

The authors are grateful to the Zahedan University of Medical Sciences, Zahedan, Iran for the financial support to conduct this work with code 7842.

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