Removal of Fluoride from Groundwater by Carbonised Punica granatum Carbon ( “ CPGC ” ) Bio-Adsorbent

This study applies the development and application of low cost, Punica granatum bio-adsorbent for the removal of fluoride in groundwater. The batch adsorption study was carried out to analyze the defluoridation by contact time variation, adsorbent dose, adsorbate concentration, adsorbent particle size and presence of co-anions at neutral pH. The analysis of the isotherm equilibrium data using the Langmuir and Freundlich equations by linear methods showed that the data fitted better with Freundlich model (R2 > 0.980). Prepared adsorbent showed enhanced removal of fluoride concentration by 78.1% at equilibrium contact time of 75 minutes. Carbonised Punica granatum Carbon (CPGC) seeds showed a high affinity for fluoride ions compared with other conventional adsorbents. Therefore, it can be considered as a potentially “good”, low-cost bio-adsorbent for de-fluoridation of water compared to other bio-adsorbent.


Introduction
Water is one of the most important elements for all forms of life and is indispensable to the maintenance of life on earth.Safe drinking water is the primary need of every human being.Pure water is scarce and is not easily available at all.Water may be contaminated by natural sources or industrial effluents.One such contaminant is fluoride.bent, it is significant to choose the suitable matrix materials used for loading of rare metals and consequently affect the adsorption behaviour for fluoride anion directly.
The biomass produced in the agricultural industry is one of the most promising candidates for this application for its effectiveness, availability and abundance.In this aspect, orange juice residue has been used as the matrix material to load rare metal ions, showing quite excellent adsorption properties for anions including fluoride, arsenic and phosphate [36] [37].While for the wide application of this technique, more biomass should be sieved to develop highly cost-effective adsorbents in fluoride purification.
In this study, a bio-adsorbent CPGC was developed for fluoride removal with a kind of easily available biowaste, Pomegranate seeds, which can be an alternative to the costly de-fluoridation products such as ion exchange resins and electro dialysis.In India, 810 × 10 3 metric ton Pomegranate fruit is consumed every year, and lots of seed waste is disposed, causing a severe problem in the community.So, for the environmental interest, this bio-waste could be a low-cost adsorbent and remove fluoride from groundwater.The aim of the present study is to evaluate the feasibility of using pomegranate seed waste for fluoride removal from groundwater and the effect of common experimental parameters, such as the initial pH value of the solution, equilibrium concentration of fluoride, contact time, adsorbent dose and coexisting anions.

Adsorbent Preparation
The Punica granatum seed (powdered sample), common name, Pomegranate, was purchased from market.Then the material was dried at 378 -383 K for 24 hours.It was washed with doubly distilled water to remove the free acid and dried at the same temperature for 3 hours.Later, the dried adsorbent was carbonised thermally in Muffle furnace between 1073 K to 1084 K temperature ranges.The resulting product was cooled to room temperature and sieved to the desired particle sizes, namely, <55, 55 -106, 106 -150, 150 -225 and 225 -305 micron.Finally, the product was stored in vacuum desiccators until required for treatment.The physiochemical properties of the adsorbent are listed in Table 1.

Batch Adsorption Study
The batch adsorption de-fluoridation study was conducted for the optimization of various experimental conditions like contact time, initial fluoride concentration, adsorbent dose, particle size and influence of co-ions with fixed dosage.The mixture was agitated in a thermostatic shaker at a speed of 250 rpm at room temperature.The reagents used in this present study are of analytical grade.A fluoride ion stock solution (100 mg/l) was prepared and other fluoride test solutions were prepared by subsequent dilution of the stock solution.Analytical grade sodium chloride, sodium nitrate, sodium sulphate and sodium phosphate were used to prepare 1.0 mol/L stock solutions, and mixed at an arbitrary volume ratio to adjust the concentration of the coexisting anions in the solution.Analytical grade sodium citrate and sodium nitrate were employed to prepare the total ion strength adjusting buffer (TISAB) solution for the pre-treatment of all sampling solutions before testing by using ion selective electrode of fluoride.
All the experiments were carried out at room temperature.Fluoride ion concentration was measured with a specific ion selective electrode by use of total ionic strength adjustment buffer II (TISAB II) solution to maintain pH 5 -5.5 and to eliminate the interference effect of complexing ions [38].The pH of samples was measured by Orion ion selective equipment.All other water quality parameters were analyzed by using standard methods [39].Effect of different initial fluoride concentrations viz., 2, 4, 6, 8 and 10 mg/l were studied by keeping the mass of sorbent at 0.75 g and volume of solution at 100 ml in neutral pH.
The fluoride concentration retained in the adsorbent phase, q e (mg/g), is calculated by Equation ( 1).
( ) where q e is the amount of fluoride adsorbed (mg/g); V is the volume of solution; c o and c e are the initial and residual concentration at equilibrium (mg/l), respectively, of fluoride and W is the weight (g) of the adsorbent.

Effect of Contact Time and Initial Fluoride Concentration
Contact time plays a very important role in adsorption dynamics.The effect of contact time on adsorption of fluoride onto CPGC is shown in Figure 1.Batch adsorption studies using the concentrations 2, 3, 4, 6, 8 and 10 mg/l of fluoride solution with 0.75 g of the adsorbent were carried out at 298 K as a function of time to evaluate the de-fluoridation and adsorption rate constants.The adsorption of fluoride increases with time and gradually attains equilibrium after 75 minutes.From Figure 1, the time to reach equilibrium conditions appears to be independent of initial fluoride concentrations.Therefore, 75 minutes was fixed as minimum contact time for the maximum de-fluoridation of the sorbent.The adsorption of fluoride decreased from 88% to 47% by increasing fluoride concentration from 2.0 to 10.0 mg/l.Further, it was observed that the removal curves are smooth and continuous indicating the possibility of the formation of monolayer coverage of fluoride ion at the interface of adsorbent.

Effect of Particle Size
The de-fluoridation experiments were conducted using CPGC with five different particle sizes viz.55, 55 -106, 106 -150, 150 -225 and 225 -303 μm.As the adsorption process is a surface phenomenon, the de-fluoridation efficiency of the sample with 55 μm registered high de-fluoridation efficiency due to larger surface area (Figure 2).The variation in the percentages of fluoride removal by the sample with different particle sizes was studied.
Hence, the material with particle size of 55 μm has been chosen for further experiments.Higher percentage of adsorption by CPGC with smaller particle size is due to the availability of more specific surface area on the adsorbent surface.

Effect of Adsorbent Dose
One of the parameters that strongly affect the adsorption capacity is the concentration of the adsorbents.The effect of adsorbent dosage on the removal of fluoride from ground water was studied at neutral pH and fluoride concentration of 3 mg/l for 150 min.Figure 3 indicates that, increasing adsorbent dosage increases fluoride uptake, indicating that for a fluoride removal an optimum CPGC dosage of 0.75 g/L was required.The results further illustrates that the fluoride removal efficiency increases up to this optimum dosage beyond which fluoride uptake has no significant change with CPGC dosage.The increase in the adsorbent dosage provides greater surface area and hence increases the adsorption.However, by increasing the adsorbent dosage beyond the optimum dosage, there may be a slight improvement in fluoride removal efficiency but the adsorption capacity decreases.

Effect of Interfering Co-Ions
The effects of coexisting anions such as sulphate, nitrate, chloride and bicarbonate on fluoride adsorption by the CPGC adsorbent were examined and the results are given in Figure 4.
Chloride and nitrate did not perceptibly interfere with fluoride removal even at a concentration of 500 mg/l, while sulphate began to show some adverse effects when the 2 4 SO − concentration increases.However, bicar- bonate showed great competitive adsorption with fluoride.The fluoride adsorption amount decreased quickly from 78.1 to 52% with the increase of bicarbonate concentration 0 -500 mg/l.This may be attributed to the competition of bicarbonate ions with the fluoride ions at the active site, on the surface of the sorbents.The selective nature of the fluoride by the sorbent depends on size, charge, polarizability, electro negativity difference, etc.The order of interference for fluoride removal was in the following order,

Adsorption Isotherms
The sorption isotherms express the specific relation between the concentration of sorbate and its degree to  accumulation onto sorbent surface.The fluoride sorption capacity of CPGC at 298 K was evaluated using Langmuir and Freundlich isotherm models.
The experimental data was applied in accordance with the linearized form of the Langmuir isotherm model.It can be represented as Equation ( 2): where q m is q e for a complete monolayer (mg/g); K a is the adsorption equilibrium constant (cm 3 /mg).To evaluate the adsorption capacity for a particular range of adsorbate concentration, the aforementioned equation can be used in the following linear Equation (3): 1 1 The constants q m and K a in Equation ( 2) can be determined from the slope and intercept of the linear plot/graph of c e /q e versus c e in Figure 5. Results indicate that the adsorbent CPGC has a high affinity for fluoride adsorption under these conditions.The present data fit well in Langmuir isotherm model with R 2 > 0.970.The average monolayer adsorption capacity (q m ) obtained for CPGC is 1.68 mg/g.
Freundlich adsorption isotherm is based on adsorption on heterogeneous surface, and is the earliest known relationship describing the adsorption equilibrium and is given as Equation ( 4): where x is the amount of solute adsorbed (mg), m is the mass of adsorbent used (g), K f and 1/n are empirical constants, indicating the adsorption capacity and adsorption intensity, respectively.Above Equation ( 4) may be converted to a linear form (5) by taking logarithms: The values of K f and 1/n were obtained from the slope and intercept of the plot between log(q e ) and log(c e ).The Freundlich equation deals with physicochemical adsorption on heterogeneous surfaces.Linearized form of the Freundlich equation is demonstrated in Figure 6.Adsorption data fitted well in linearized form of model equation (R 2 > 0.980) which indicated the acceptability of the model.

Conclusion
Among various bio-waste materials, Punica granatum seeds were first time utilised in de-fluoridation of groundwater.In the present study, the fluoride is removed from aqueous solutions using Carbonised Punica granatum Carbon (CPGC) seeds as bio-adsorbent.It can be concluded that CPGC has good properties for the sorption of fluoride ions from aqueous solutions.The equilibrium time for removal of fluoride concentration is determined to be 75 min.The fluoride saturation capacity of CPGC is 1.68 mg F − /g at room temperature.The best fitting adsorption isotherm is Freundlich model (R 2 > 0.980), which indicates that fluoride bio-adsorption onto CPGC is characterized by chemisorptions on heterogeneous surfaces.The particle size is an important parameter that affects the sorption for fluoride on CPGC since the sorption of these ions increases as the particle size decreases.Finally, this low-cost material can be employed as an adsorbent for fluoride removal from groundwater, in particular in domestic systems where fluoride related problems exist.

Figure 1 .
Figure 1.Effect of contact time and fluoride concentration on fluoride removal.

Figure 2 .
Figure 2. Effect of particle size on fluoride removal.

Figure 4 .
Figure 4. Variation in adsorption capacity for different concentrations of co-ions.

Table 1 .
The physicochemical properties of CPGC adsorbent.