Study of Fluoride Affinity by Zirconium ImpregnatedWalnut Shell Carbon in Aqueous Phase : Kinetic and Isotherm Evaluation

is paper examines the kinetics of �uoride removal from water by the adsorbent zirconium-impregnated walnut-shell carbon (�IWSC), exploring the mechanisms involved. e dependence of the adsorption of �uoride on the pH of the solution has been studied to achieve the optimumpHvalue and a better understanding of the adsorptionmechanism.epresence of bicarbonate ions in aqueous solution was found to affect the �uoride removal indicating that these anions compete with the sorption of �uoride on adsorbents. e kinetic pro�le has been modeled using pseudo-�rst-order model, pseudo-second-order model, and intraparticle diffusion model. e kinetic sorption pro�les offered excellent �t with pseudo-second-order model. Adsorption isotherms have been modeled by Langmuir, Freundlich, and Temkin equations, and their constants were determined. e equilibrium adsorption data were �tted reasonably well for Freundlich isotherm model. �RD and SEM patterns of the �IWSC were recorded to get better insight into the mechanism of adsorption process.


Introduction
Fluoride ion exists in natural waters and it is an essential micronutrient in human in preventing dental carries and in facilitating the mineralization of hard tissues.It takes at a recommended range of concentration (1.0-1.5 mg/L).Higher level of �uoride in ground water is a huge problem in all the countries including the USA, the Asian and the African countries [1].It has become a global issue.In India, although water resources are high, there is an acute scarcity of safe drinking water due to acceptable water quality.According to the World Health Organization (WHO), the maximum acceptable concentration of �uoride in drinking water lies below 1.5 mg/L [2].e incidence of high �uoride content in ground water (>1.5 mg/L) is common in both shallow and deeper water bearing [3,4].Fluoride contamination in ground water arises both from natural causes and anthropogenic activities.Fluoride is a persistent and nondegradable poison that accumulates in soil, plants, wild life, and humans.e presence of �uoride ion in portable water has adverse effect on human health.Fluoride is affected by positive charged calcium in teeth and bones due to its strong electronegativity, which results in dental, skeletal and nonskeletal form of �uorosis in children as well as adults [5,6].e dental and skeletal �uorosis is irreversible and no treatment exists.e only remedy is prevention by keeping �uoride intake within the safe limits.
e excess �uoride (>1.5 mg/L) in drinking water can be removed by various de�uoridation techniques.De�uoridation technologies can be divided into following important categories, namely, chemical precipitation [7,8], membrane processes [9], adsorption and ion exchange [10] and Nalgonda technique [11].e adsorption process is the most commonly used technique for the �uoride removal from drinking water.e adsorption process is a widely acceptable pollution removal technique because of its ease of operation and cost-effectiveness.Large numbers of adsorbents have been studied for the removal of �uoride from water based on alumina [12], other minerals and metal oxides [13][14][15], clays [15][16][17], carbons [18], zeolites [19], agricultural wastes [20,21], and so forth.ough the removal of �uoride using zirconium-impregnated cashew nut shell carbon [21], zirconium-impregnated ground nut shell carbon [22] has been reported earlier, the author felt that the use of ZIWSC could de�nitely give a new dimension in the �eld of de�uo-ridation� hence de�uoridation experiments were carried out using the ZIWSC.
In this study, waste walnut shell was utilized as the raw material for the production of activation carbon, which was impregnated with zirconium oxychloride by chemical methods, and its adsorption capacity for �uoride ions was evaluated.Johns et al. [23] utilized granular activated carbon (GACs) made from walnut hull to successfully remove higher levels of benzene, toluene, methanol, acetonotrile, acetone, and 1,4-dioxane from an aqueous mixture rather than commercial GACs.e effects of contact time, dosage, solution pH, �uoride concentration, and reaction temperature on �uoride were studied in details.

Preparation of Activated
Carbon.e walnut shell was taken from local natural resources.e materials were cleaned and dried to constant weight at room temperature and then activated as per the procedure [24].About 100 g of the crushed walnut shell was kept for 3 hours in a lowtemperature muffle furnace at 573-673 K. e carbonized material was taken out of the muffle furnace, cooled, powdered, and kept in a beaker and 200 mL of concentrated sulphuric acid was gradually added to it and the contents were stirred continuously to ensure thorough mixing.e activated carbon was then cooled and le overnight and washed free of acid and dried at 383 K for 2 hours, and then sieved using various mesh sizes.Subsequently the activated carbon was further immersed in 2 N NaOH solution and washed free of alkali, providing the desired adsorbent for impregnation.
Zirconium ion impregnation was carried out by adding 5% ZrOCl 2 solution with activated carbon (solution/solid ratio = 2 : 1) and the mixture was kept for three days at room temperature (298 K). e impregnated carbon was then �ltered, rinsed to con�rm the effluent free from zirconium, dried in an oven at 380 K and subsequently used for de�uoridation studies.e de�uoridation capacity of ZIWSC was investigated by pursuing the batch equilibrium and kinetic experiments.e reagents used in this present study are of analytical grade.A �uoride ion stock solution (100 mg/L) was prepared and other �uoride test solutions were prepared by subsequent dilution of the stock solution.All the experiments were carried out at room temperature.Fluoride ion concentration was measured with a speci�c ion selective electrode by use of total ionic strength adjustment buffer (TISAB) solution to maintain pH 5 to 5.5 and to eliminate the interference effect of complexing ions [25].Fluoride ion concentration and pH of the samples were measured by Orion ion selective electrode equipment.All the other water quality parameters were analysed by using standard methods [26].

Sorption
Experiments.e sorption isotherm and kinetics experiments were performed by batch adsorption experiments and were carried out by mixing 1.5 gm of sorbent with 100 mL of sodium �uoride containing 3 mg/L as initial �uoride concentration.e mixture was agitated in a thermostatic shaker at a speed of 200 rpm at room temperature.e de�uoridation studies were conducted for the optimization of various experimental conditions like contact time, pH, initial �uoride concentration, and in�uence of coions with �xed dosage.Kinetic studies of sorbent were carried out in a temperature-controlled mechanical shaker.e effect of different initial �uoride concentrations, namely, 2, 4, 6, 8 and 10 mg/L at four different temperatures, namely, 303, 313, 323 and 333 K on sorption rate were studied by keeping the mass of sorbent as 1.5 gm and volume of solution as 100 mL in neutral pH.e pH at zero point charge (pH zpc ) of sorbent was measured using the pH dri method [27].e pH of the solution was adjusted by using 0.01 mol/L sodium hydroxide or hydrochloric acid.Nitrogen was bubbled through the solution at 30 ∘ C to remove the dissolved carbon dioxide.50 mg of the adsorbent was added to 50 mL of the solution.Aer stabilization, the �nal pH was recorded.e graph of �nal pH versus initial pH was used to determine the zero point charge of the activated carbon.

Characterisation of Sorbents. e X-ray diffraction
(XRD) pattern of the impregnated carbon was obtained using a Bruker AXS D8 Advance, Inst ID: OCPL/ARD/26-002 Xray diffractometer.Examination of adsorption with Scanning Electron Microscope (SEM) with HITACHI-S-3400N model �tted with an energy dispersive X-ray analyzer (EDAX) allows a qualitative detection and localization of elements in the adsorption.e SEM enables a direct observation of the surface microstructures of the fresh and �uoride-adsorbed adsorbents.

Effect of Contact Time and Concentration.
In order to determine the equilibration time for maximum adsorption of �uoride and to know the kinetics of adsorption process, the adsorption of �uoride on ZIWSC was studied as a function of contact time.It was noticed that the �uoride removal increased with the time.e adsorbent exhibited an initial gradual uptake of �uoride followed by a slower removal rate that gradually reached an equilibrium condition.Nearly 50% removal of �uoride was achieved, �rst 60 min of contact time and gradual increasing of removal occurred in the following 180 min.erefore 180 minutes was �xed as minimum contact time for the maximum de�uoridation of the sorbent.e results compared with the other adsorbents to get good removal [21,22,28].

Effect of Adsorbent Dose on the Adsorption
Process.e effect of adsorbent dosage on the adsorption of �uoride for the initial concentration of 3.0 mg/L was studied.Adsorbent dosage varied from 0.5 gm to 2.50 gm.It is seen that the adsorption increased with the increase in the dose of adsorbent.e maximum removal percentage was exhibited at a dosage of 1.5 gm adsorbent per 100 mL.Further addition of the adsorbent did not show a considerable increase in de�uoridation, may be attributed to two reasons.e increase in sorbent dose at constant sorbent concentration and volume will lead to unsaturation of sorption sites through the sorption process [29,30], and secondly may be due to particulate interaction such as aggregation resulting from high-sorbent dose [31].

Effect of pH on the Adsorption of Fluoride
. e pH of the aqueous solution is an important variable, which controls the adsorption at the water-adsorbent interfaces.erefore, the adsorption of �uoride on the ZIWSC was examined at different pH values ranging from 3 to 12.It can be seen from the investigation that �uoride removal decreases with increasing pH, and the removal of �uoride is maximum 94% (ZIWSC) and 81% (WSC) at pH 3 and is minimum 22% (ZIWSC) and 18% (WSC) at pH 12. is can be explained by the change of surface charge of the adsorbent.It is known that in highly acidic medium, the surface of adsorbent is highly protonated while it is neutralized and tended to have negative charge in alkaline medium.erefore, high efficiency in acidic medium can be attributed to the gradual increase in attractive forces and low efficiency in alkaline medium can be explained by the repulsion between the negatively charged surface and �uoride.e pH zpc of WSC and ZIWSC are 5.2 and 4.6, respectively.However the percentage of �uoride removal by ZIWSC was higher than WSC in all the pH ranges studied.

Effect of Particle Size. e de�uoridation experiments
were conducted using WSC with �ve different particle sizes, namely <53, 53-106, 106-212, 212-300, and 300-426 microns.As the adsorption process is a surface phenomenon, the de�uoridation efficiency is increased with decreasing of surface of the sample due to larger surface area.erefore, with a given mass of walnut shell, smaller particle size would increase surface area availability hence the number of sites increased.Hence the material with particle size of <53 microns has been chosen for further experiments.All the forthcoming discussion is based on the experimental result using this sample.

3.�. Assessment of In�uence of Interferin� �o Ions
. �e�uoridation studies of ZIWSC were carried out in the presence of common ions like sulphate, chloride, bicarbonate, and nitrate, which are normally present in water, was experimentally veri�ed.e concentration of coexisting ions was varied from 50 mg/L to 500 mg/L with an initial �uoride concentration of 10 mg/L at neutral pH.It was inferred that there was no remarkable in�uence on the removal of �uoride in presence of Cl − , SO 4 2− , and nitrate.However, the presence of bicarbonate ion resulted in the decrease of percentage from 85.67% to 54.94%.is may be due to the competition of bicarbonate ions with �uoride for sorption sites.Similar trend was reported while studying montmorrillonite as a sorbent for �uoride removal [32].
e Langmuir isotherm model assumes that adsorption sites are energetically the same with monolayer formation and is generally expressed as A plot of   /  against   should yield a straight line with Q and b obtained from the intercept and the slope.e value Q is 3.53 mg/g and Langmuir constant  is 3.11 (L/mg).e results are summarized in Table 1.e essential characteristics of the Langmuir isotherms can be expressed in terms of dimensionless constant separation factor or equilibrium parameter, R, which is de�ned as [36] where  0 is the initial �uoride concentration and b is the Langmuir isotherm constant.e feasibility criteria of the process can be judged from R values as follows: Value Type   1 unfavorable, e R values in Table 2 are also indicating favorable sorption of �uoride on to the adsorbent.
e Freundlich adsorption isotherm equation can be represented as follows: A plot of ln   against ln   should yield a straight line with slope 1/n and intercept ln   .It is found that the related correlation coefficient  2 value for the Freundlich models is 0.982.Another equation used in the analysis of isotherm was the Temkin model is given as where  1 = /.Temkin sorption contains a factor that explicity takes into the account adsorption species and adsorbent interaction.e heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to adsorbate/adsorbate interaction [37].is isotherm assumes that (i) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent and adsorbate interactions, and that (ii) the adsorption is characterized by a uniform distribution of binding energy [38].A plot of   versus ln  enables the determination of the isotherm constants  1 and   from the slope and intercept, respectively.  is the equilibrium binding constant (L/mol) corresponding to the maximum binding energy, and the constant  1 is related to the heat of adsorption.A linear relationship between   and ln   indicates the applicability of this model to understand the adsorption mechanism.Temkin isotherm for the adsorption of �uoride and the corresponding constant are represented in Table 1.e higher  2 values Freundlich of over Langmuir and Temkin isotherm indicated the suitability of Freundlich isotherm than the Langmuir and Temkin isotherm.

ermodynamic
Investigations.e effect of temperature has a major in�uence in the sorption process and hence the sorption of ZICNSC was monitored at four different temperatures 303, 313, 323, and 333 K under the optimized condition and thermodynamic parameters, namely, standard free energy change (ΔG ∘ ), standard enthalpy change (ΔH ∘ ) and standard entropy change (ΔS ∘ ) were calculated [39,40] and presented in Table 3. e negative values of ΔG ∘ indicated the spontaneity of the sorption reaction.e positive values of ΔH ∘ indicated the endothermic nature of the sorption process.e positive value of ΔS ∘ showed the increasing randomness at the solid/liquid interface during sorption of �uoride.Its also indicates the increased disorder in the system with changes in the hydration of adsorbing �uoride ions [41].
3.8.Adsorption Kinetics.e adsorption kinetics was studied with initial �uoride concentration 2, 4, 6, 8, and 10 mg/L.e kinetics analysis of adsorption data is based on reaction kinetics of pseudo-�rst and pseudo-second-order mechanisms.euptake of �uoride on zirconium impregnated adsorbent occurred rapidly, and reached equilibrium within 180 min.e kinetics of adsorption was analyzed by using different rate equation.A pseudo-�rst-order rate expression or the Lagergren rate equation [42] is expressed as where   and   are the �uoride on adsorbent (mg g −1 ) at equilibrium and at time t, respectively, and  1 (min −1 ) is �rst-order rate constant.A plot of ln (  −   ) against time (t) should yield a straight line and the rate constant  1 is evaluated from the slope.e linear form of pseudo-second-order expression [43], is expressed as 2 can be determined by plotting   against  of (6) gives the straight line with higher correlation coefficient  2 values, which is higher than that observed with pseudo-�rst-order model indicating the applicability of the pseudosecond-order model and the values are shown in Table 4. From the model, the values of   increased with increase in initial concentration and it also increased with increasing in Temperature.e values of rate constant (k) have also increased with temperature indicating chemisorptions.
e adsorption process believed to follow a complex phenomenon is accompanied by both the surface and the pore diffusion, but to different extents.e extend of a particular diffusion to the total process may be estimated by a plot following the Weber and Morris equation [44] which is expressed as where   , the date of pore diffusion, is obtained from the slope of the linear portion of the   versus √ .e Weber and Morris plot reveals an initial curved portion (indicating of boundary layer effect, that is, surface adherence) followed by the linear portion yields (indicating of intraparticle or pore diffusion).e slope of the linear portion yields the   value, while intercept of the plot signi�es the extent of the boundary layer effect.e larger the intercept, the greater is the contribution of the surface adherence [45,46] in the rate limiting steps.It is found that   value increases with increased concentration and in lower temperature (Table 4).
3.9.Mechanism of Fluoride Sorption.e �uoride removal by ZIWSC was governed by adsorption mechanism.e surface acquired positive charge at lower pH values and hence the �uoride sorption at this pH level was mainly due to electrostatic attraction between the positive charged surface and negatively charged �uoride ions and chemisorption dominated.As the pH is increased slowly, the surface acquired negative charges, physisorption dominated and hence percentage removal of �uoride was decreased.e slight enhancement of �uoride removal by ZIWSC over WSC may be due to sorption by Zirconium, adsorption by physical forces and �uoride ion, a Lewis base, coordinates strongly with the Zirconium species adsorbed on WSC, which are Lewis acid sites: e chemisorption mechanism of �uoride uptake, involves an exchange of the chloride and the hydroxide of the ZrO(OH)Cl species adsorbed on WSC by �uorides, leading to the formation of ZrOF 2 .

Desorption and Reuse Potential.
Any adsorbent is economically viable if the adsorbent can be regenerated and reused in many cycles of operation.For checking the desorption capacity of the sorbent, the material was subjected to an adsorption at an initial �uoride concentration of 3 mg/L.e exhausted ZIWSC was regenerated using HCl and NaOH.NaOH is better regenerated than HCl.e concentrations were ranging from 0% to 10%.At 2.5% NaOH concentration, ZIWSC had desorbed almost 96.2% of �uoride.To test the adsorption potential of regenerated ZIWSC, two more cycles of adsorption-desorption studies were carried out by maintaining the initial conditions the same.In third cycle, the adsorbent capacity has shown 28.00%.However, in the fourth cycle, adsorption capacity was observed as 5%.More tests have to be conducted to determine the exact life cycle of the adsorbent.

Field Study.
In order to gain the practical utility of the studied carbonaceous adsorbents, batch studies were preformed to evaluate their viability for real �eld application.e samples from �uorosis affected areas of Tirunelveli district, India having higher concentrations of �uoride were collected and adsorption studies were performed (adsorbent: 1.5 g; temperature: 303 K; agitation: 200 rpm; pH: 7).e consolidated experimental results for all the water samples studied with the function of adsorbents are depicted in Table 5. e sorption data phenomena were found to be dependent on the concentration of �uoride in the water sample used.It has been observed that all the water quality parameters show marked improvement.
3.12.Instrumental Studies.For understanding the nature of �uoride sorption X-ray and FTIR studies were performed  using the raw and treated adsorbents.Powder X-ray diffraction was carried out on the raw and �uoride treated ZIWSC samples.e XRD patterns for treated adsorbents showed signi�cant changes.e XRD data of the treated ZIWSC provided evidence of slight modi�cation over the crystal cleavages.e intensity of the peak due to the hkl plane 010 of the monoclinic crystal system of ZIWSC disappeared aer the �uoride adsorption on its surface.is is possible due to the lattice dislocation in the crystal system.e X-ray diffraction patterns of raw and �uoride treated material are given in Figures 1 and 2. e surface condition and the existence of �uoride onto ZIWSC were con�rmed by the S�M with �D�X analysis.

Conclusions
e impregnated walnut shell acts as a reasonably good adsorbent for the removal of �uoride from aqueous solution.e optimum pH for removal was found to be 3.0, at which �uoride removal was 94%.e removal increased with the increase in the adsorbate concentration.An increasing equilibrium adsorption capacity with the rise in temperature indicated that the nature of adsorption process is endothermic, which is further supported by the thermodynamic parameters calculated from the Langmuir isotherm at various temperatures.e adsorption process was found to follow the Freundlich adsorption isotherm model and Pseudo-secondorder.Compared to the various other sorbents reported in the literature, the impregnated walnut shell in this study shows very good promise for practical applicability.However, more studies are needed to optimize the system from the regeneration point of view and to investigate the economic aspects.

F 3 :Figures 3
Figures 3(a) and 3(b) show the SEM images before and aer �uoride sorption with ZIWSC.e changes in the surface morphology of adsorption before and aer �uoride treatment indicate �uoride sorption on ZIWSC.e E�A� T 1: Linear isotherm parameters of �uoride adsorption on ZIWSC.

T 4 :
Kinetic parameters of values of ZIWSC.