Fluoride Adsorption by Pumice from Aqueous Solutions

Drinking water provides many vital elements for the human body, but the presence of some dissolved elements more than permissible concentration can endanger human health. Among the dissolved elements in drinking water, fluoride is noticeable, because both the very low or very high concentrations have adverse health impacts such as dental caries. Therefore, fluoride concentration should be kept in acceptable levels. In this study Pumice was used for fluoride removal. It was found that Fluoride sorption kinetic was fitted by pseudo-second-order model. The maximum sorption capacity of Pumice was found to be 13.51 mg/g at laboratory temperature (24oC). Maximum sorption study occurred at pH= 3. Results of Isotherm showed the fluoride sorption has been well fitted with Freundlich isotherm model. This study has demonstrated that Pumice can be used as effective adsorbents for fluoride removal from aqueous solutions. The adsorbent prepared in this study was cheap and efficient in removal of fluoride than other adsorbents.


Introduction
Water always contains some dissolved material, suspended solids, and dissolved gasses.The presence of some of the dissolved materials are necessary for health, but, presence of them more than the permissible concentrations may lead to health problems.The concern about chemical ingredients in drinking water is different from that of microbial agents, because unlike chemical agents they need more time to show adverse impacts 1 .Similar to some natural elements, fluoride can enter the body by inhalation and ingestion of food and water and influence our health.Nearly 80% of fluoride entered into the body is excreted mainly via urine.

Experimental
All chemicals used in this study were purchased from Merck Co. Pumice was obtained from Azerbaijan, Iran.Fluoride stock solution was prepared by dissolving NaF in distilled water.Standards and fluoride spiked samples at required concentrations were prepared by appropriate dilution of the stock solution with deionized water.The concentration of fluoride in the solution was determined by UV-Visible spectrophotometer Lambda 25 (PerkinElmer).Fluoride analysis was carried out by following SPADNS method.A known volume of sample was taken in a 25 mL volumetric flask and addition of 5mL of SPADNS reagent to it.The volume was made up to 25 mL using distilled water and the absorbance was measured at 570 nm 29 .

Characterization of the Sorbent
The surface and textural morphology of Pumice was determined by scanning electron micrographs (SEM) using XL30 (Philips), Field emission LaBb scanning electron microscope.The physicochemical characteristics are determined by measuring point of zero charge (pH PZC ), by x-ray diffraction (XRD) using a Bruker AXS D8 Advance x-ray diffractometer and by XRF (X-ray fluorescence) analysis.

Preparation of Pumice
Pumice used in the study was obtained from Azerbaijan.To enhance the porosity of Pumice and also to remove its impurities, Pumice was pretreated in acid solution (HCl) at pH= 1, for 24 h at room temperature, being rinsed with double DW several times until effluent turbidity reached to lower than 0.1 NTU and then dried at 103 • for 36 h.Finally, the raw sample was grounded by a primary crusher (hammer type), and then sieved to particle size fraction of 30 mesh screen.

Fluoride Sorption Studies
Batch experiment was carried out to measure the adsorption characteristics of fluoride by the Pumice.The Pumice (0.8 g) was added to 150 mL of synthetic fluoride solutions of varying concentration (2-10 mg L -1 ).After equilibrium, samples were filtered and the filtrate was then analyzed for residual fluoride concentration.Amount of sobbed fluoride (q e (mg/g)) can be computed by following formula: where, C 0 and C e are initial and final concentrations of fluoride in solution in mg/L, V is the volume of solution (L) and M is the mass of sorbent (g).The effect of pH was investigated by performing the adsorption experiments at various pH in the range of 3-11 adjusted by addition of diluted HCl or NaOH solution.After adjusting pH at 3, affect of contact time (1-30 min) in concentrations of 2, 5, and 7.5 mg/L with 2 g/L of sorbent dosage was examined.The effects of competing anions (chloride, nitrate, sulfate, bicarbonate) on fluoride adsorption were investigated by performing fluoride sorption under a fixed fluoride concentration (2 mg/L) and initial competing anion concentrations of 0.1 M with sorbent dosage of 2 g/L.

Effect of Solution pH
pH of solution is an important factor controlling the surface charge of the adsorbent and the degree of ionization of the materials in the solution.To determine the optimum pH for the maximum removal of fluoride, the equilibrium sorption of fluoride (with initial fluoride concentration of 2 mg/L) was investigated over a pH range of 3-11.Other maintained conditions were: 20 min contact time, temperature of 24•C and adsorbent concentration of 2 g/L.It can be seen in Figure 1 as pH decreases the fluoride removal efficiency decreases.The fluoride sorption also increased due to the fact that for pH < pH pzc .Maximum fluoride adsorption was observed at a pH=3.The effect of pH on fluoride adsorption may be due to chemical and electrostatic interaction between the oxide surface and fluoride ion and also the availability of active sites on the oxide surfaces.Electrostatic interaction between Pumice and fluoride molecules in this pH range makes the surface of Pumice to become positively charged and therefore attracts the negative fluoride anions.The decrease in removal efficiency at pH greater than 3 can be attributed to the competition for the active sites by OH -ions and the electrostatic repulsion of anionic fluoride by the negatively charged Pumice surface.Similar observations of formation are presented elsewhere 14,[30][31] .

Pseudo-First Order Kinetic
Pseudo-first order kinetic is described by the following equation 32 :  , where q e and q t are the adsorption capacity of the adsorbate (mg/g) at equilibrium and at time t (min); k 1 is the pseudo-first-order rate constant.
the Eq. ( 2) at the boundary of q t =0 at t=0 and q t =q t at t=t, can converted Eq. ( 2) to the linear form as the following equation:

Pseudo-Second Order Kinetic
Pseudo-second order kinetic is expressed as the following equation: where k 2 is the pseudo-second-order rate constant (g/mg min).Integration of the Eq. ( 4) at the boundary of q t =0 at t=0 and q t =q t at t=t, can converted Eq. ( 4) to the linear form as follows: . 1 1 These equations have been used widely for the adsorption of an adsorbate from an aqueous solution.The best fit model was considered based on the regression coefficient (R 2 ) Percentage removal pH and the experimental q e .The rate of adsorption of fluoride was studied at different time intervals of 1-30 min.In kinetic studies, 250 mL of fluoride solution (5 and 7.5 mgL −1 ) with initial pH solution of 4.1 was agitated with Pumice (0.3 g/150cc).After a fixed time interval, the adsorbent was separated and the filtrate was analyzed to determine the equilibrium concentration of fluoride.Experiments were repeated for different periods until reaching the adsorption equilibrium.Kinetics of fluoride removal is shown in Figure 2. Parameters of kinetic equations are shown in Table 1.As shown in Table 1 the kinetic of fluoride removal by use of Pumice follows pseudo second order equation (R 2 >0.99).
. Comparison of pseudo-first-order and pseudo-second-order models parameters, and calculated q e (cal) and experimental q e (ac) values for different initial fluoride concentrations.
C 0 , mg/L Pseudo-first-order model Pseudo-second-order model q e (ac) q e (cal)

Competing Anions Studies
The adverse effect of various anions like Cl − , HCO 3 -, SO4 2 − , and PO 4 3-have been tested on fluoride adsorption rate under a fluoride concentration of 2 mgL −1 , and initial competing anion concentrations of 0.1 M with sorbent dosage of 2 g/L.As shown in Figure 3 the removal efficiency was decreased in the presence of anions.The anions reduced the fluoride removal in the order of PO 4 3->HCO 3 > SO 4 2− >Cl − .This can be explained by considering the stability of different anions associated with adsorbent 33 .This, indicates that Cl − was formed Time in minutes t/qt Time in minutes in outer-sphere surface complexes, while SO 4 2− was formed both in outer and inner-sphere surface complexes 34 .In contrast inner-spherically sorbing anion PO 4 3-can significantly be interfered by the sorption of fluoride at elevated concentrations where the sorption competition can be occurred for the limited amount of sorption sites on Pumice 35 .The decrease in fluoride adsorption rate in presence of carbonate was presumably due to the increase in pH of the solution 14 .

Effect of Contact Time and Initial Nitrate Concentration
Figure 4 shows the fluoride adsorption kinetic data, plotted as adsorption density (mg F/g pumice) as a function of time, for three different initial fluoride concentrations (2, 5, and 7.5 mg/L) with an initial pH=4.It was noticed that fluoride removal increased with time and then gradually decreased with lapse of time until saturation.The fluoride adsorption was initially speeded up to 20 min, and then it became slow.Therefore, the optimum contact time for adsorption of the fluoride was considered to be 20 min.Fluoride adsorption by Pumice increased when the initial fluoride concentration increased from 2, to 5 and 7.5 mg/L (Figure 4).This can be related to the increase in the driving force of the concentration gradient, as an increase in the initial fluoride concentration 35 .

Characterization of Pumice
A physicochemical characteristic of the Pumice sample by XRF (X-ray fluorescence) is shown in Table 2.The SiO 2 contents of Pumice are 74%.It appears that there may be a correlation between silica content and internal porosity in pumices.The detailed physicochemical characteristics of Pumice fractions are presented elsewhere 28,36 .The SEM images of Pumice are shown in Figure 5.As shown in Figure 5 the naked Pumice particles had a mean diameter of 126 nm.As seen from Figure 5 the sorbent does not possess any well defined porous structure (only few pores on the surface).EDX analyses were performed to determine the elemental constituents of Pumice (Figure 6).It shows that the presence of Si in large content appears in the spectrum other than the principal metal oxides elements.The weight percent of Si, Al, Fe and K were found to be 71.84,15.43, 7.12 and 5.61, respectively.
The X-ray diffraction pattern of Pumice is shown in Figure 7.The x-ray spectrum of Pumice shows more amorphous nature, which allows a better accessibility to fluoride and thus a better activity.
Adsorption isotherms are equations for description of equilibrium phase between sorbent and an aqueous solution.Freundlich models are empirical models for rough sorbents which are applicable for systems with a single dissolved material.Longmuir isotherm can be used for physical sorption in a single layer 37 .In the present study experimental data of adsorption equilibrium was described by use of Longmuir and Freundlich isotherms and could be shown as following: Equation of Longmuir isotherm can be expressed as: where q e is the amount of sorbed fraction in mass unit of sorbent (mg/gr), C e is equilibrium sorbate concentration after adsorption (mg/L), q m is adsorption capacity and b is the Langmuir constant related to energy of sorption 35 and can be commuted by plotting C e /q e vs. C e .The influence of adsorption isotherm shape has been discussed to examine whether adsorption is favorable in terms of R L , a dimensionless constant referred to as separation factor or equilibrium parameter.R L is defined by the following relationship: C 0 is the initial concentration.From the above factor, type of sorption can be indicated.The R L value indicates the shape of isotherm.R L values between 0 and 1 indicate favorable adsorption, while R L >1, R L = 1, and R L = 0 indicate unfavorable, linear, and irreversible adsorption isotherms, respectively 38 .Equation of Freundlich isotherm can be expressed as: ( 9) The equation is conveniently used in the linear form by taking the logarithm of both sides as: (  10  )   e F e c n k q log 1 log log   C e (mg/g) is equilibrium concentration; q e (mg/g) is capacity of sorption in moment of equilibrium and K f and n are Freundlich's constants 39 .K f represents amount of sorption tendency to the surface of sorbent.The higher values of K f represent the more tendency of sorption by sorbent 37 .The n value in the range of 2-10 indicates a favorable adsorption process 32 .Isothermal studies of fluoride removal are shown in Figure 8.
Table 3 presents the parameters of the Langmuir and Freudlich isotherms.It shows that the isotherm data has been fitted to the Freudlich model, which provides the best results for these sorts of curves based on the correlation coefficients (>0.995).The value of 1/n (0.34) lying between 0.1 and 1.0 and that of n (2.95) lying in the range 1-10 both confirmed the favorable conditions of sorption 40 .Also it is shown in Table 3 that the magnitude of the Langmuir constant 'b' has small values (0.07 L mg −1 ), which indicates a low heat of adsorption 29 .The values of R L calculated according to Eq. ( 7) to be 0.65.These R L values lying between 0 and 1 indicated favorable conditions for sorption.The values of q m obtained from Langmuir model is 13.51 mg/g.Satisfactory fitting of the Freundlich model to the adsorption isotherm suggests that the adsorption of fluoride involves the multilayer coverage on the surface of the Pumice and assumes an infinite supply of untouched adsorbent sites and tends to represent heterogeneous materials better than other models 41 .Table 3. Langmuir and Freundlich constants for the adsorption of fluoride on Pumice.

Figure 1 .
Figure 1.Effect of pH of solution on F removal.Conditions: adsorbent dose 2 g/L, initial fluoride concentration 2 mg/L, temperature 24•C and contact time 20 min.

Table 2 .
Percentages of the main constituents of Pumice found by XRF.
3 K 2 O Na 2 O Fe 2 O 3 CaO MgO Others Total