ADSORPTIVE REMOVAL OF TRACE CONCENTRATION OF FLUORIDE FROM WATER USING CERIUM LOADED DRIED ORANGE JUICE RESIDUE

Two adsorbents for fluoride ion removal were developed from dried orange juice residue (DOJR) after loading Ce(III) and Ce(IV) in this study. The characterization of adsorbents was done by an energy dispersive X-ray spectroscopy, X-ray diffraction analysis, electron microscopy and chemical analysis. The experimental results indicated that the fluoride removal efficiency of both the adsorbents was influenced significantly by pH and the optimum operating pH was found to be 4. The equilibrium data were well fitted with Langmuir isotherm model and the maximum adsorption capacity of Ce(III)-DOJR and Ce(IV)-DOJR for fluoride were evaluated to be 0.67 and 1.22 mmol/g, respectively. Leakage of cerium from Ce(IV)-DOJR was negligible for trace concentration of fluoride but it was significant for high concentration of fluoride at pH below 3 resulting fluoride precipitation. Therefore, cerium-loaded DOJR investigated in this study can be considered as an efficient, environmentally benign and promising alternative for the treatment of fluoride contaminated with trace amount of fluoride ion.


INTRODUCTION
The contamination of surface and ground water occur due to natural as well as anthropogenic processes.Naturally, the pollution of water with fluoride occurs due to weathering of fluoride rich rocks and minerals whereas anthropogenic pollution takes place by the discharge of industrial waste water generated from semiconductor, plating and other high tech industries (Shen et al. 2003).Fluoride ion concentration below 1 mg/L is considered to be beneficial for our dental health.However, consumption of high concentration of fluoride for long period of time can cause mottling of teeth and lesions of endocrine glands, liver, thyroid and other organs in addition to dental and skeletal fluorosis.It is estimated that more than 70 million people are suffering from fluorosis globally due to consumption of fluoride contaminated water (Viswanathan & Meenakshi 2008, Paudyal et al. 2011).In order to minimize the risk, WHO has recommended the tolerance limit of 1.5 mg/L of fluoride in drinking water (WHO 1993).Therefore, it is necessary to lower the elevated amount of fluoride in water.
Nowadays, there has been a considerable increase in the prevalence of dental and skeletal fluorosis among population around the fluoride polluted area (Meenakshi & Viswanathan 2007).Moreover, intake of fluoride from sources other than drinking water such as foodstuffs, toothpaste, fluoridated milk, dietary supplements, fluoridated salt, mouth rises and brick tea are another cause of fluorosis in our body.The highly preferable alternative for the preventive measure is to find a supply of drinking water with safe fluoride levels and one of such option is defluoridation (Liao & Shi 2005).Most commonly used methods for defluoridation of water are chemical precipitation, membrane process, ion exchange and surface adsorption.The surface adsorption using chemically modified waste biomass material seems to be more promising because of its easy operation and affordable cost (Biswas et al. 2009, Zhao et al. 2008).
The orange juice residue obtained after juicing is one of the pectin rich biomass that can be easily converted into cation exchanger by basic hydrolysis reaction with lime water (Paudyal et al. 2013).In the present work the cattle food of orange juice residue abbreviated as dried orange juice, DOJR, was utilized for the preparation of adsorbents for fluoride by loading Ce(III) and Ce(IV).The fluoride removal potential of cerium-loaded DOJR was investigated batch wise.The effects of varying parameters such as pH and initial fluoride concentration for the removal of fluoride from water were investigated using both the adsorbents.

Chemicals and instrumentation
The stock solution of fluoride was prepared from NaF purchased from Wako Chemical Co. Ltd.Japan.The CeCl 3 .7H 2 O purchased from Wako Chemical and CeSO 4 .4H 2 O purchased from Sigma Aldrich were used to prepare Ce(III) and Ce(IV) solution, respectively, for loading reaction.The pH of the solution was measured using pH meter (TOA DKK, HM-30R pH meter).The surface structure of DOJR before and after Ce(III) loading was analysed by using electron microscope.An elemental analysis of DOJR before and after Ce(IV) loading were performed by using an Energy Dispersive X-ray spectrometer (Shimadzu model, EDX-800HS).The concentration of metal ion was measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Shimadzu model ICPS 8100) where as that of fluoride was analyzed by ion chromatography (Dionex model ICS 1500).

Preparation of cerium loaded DOJR
The chemical modification of DOJR was carried out according to the procedure described elsewhere (Paudyal et al. 2012).In the present study, three grams of DOJR was mixed with 500 mL of 0.1 M Ce(III) solution at pH around 3.4 and shacked for 24 h at 30 °C for the immobilization of Ce(III) ion instead of Zr(IV).Then it was washed with distilled water until neutral pH was achieved and was dried in convection oven at 70 °C for two days.The material prepared was termed as Ce(III) loaded DOJR (i.e., Ce(III)-DOJR) hereafter.Similarly, Ce(IV) sulphate solution was used in order to prepare the Ce(IV) loaded DOJR (Ce(IV)-DOJR).During the loading reaction, Ca 2+ ions present in the dried orange juice residue (DOJR) undergoes cation exchange reaction with the loaded Ce(III) or Ce(IV) ions and the coordinated hydroxyl group present in both types of cerium loaded DOJR were expected to be substituted with fluoride during adsorption also shown in Scheme 1.

Batch adsorption studies
The adsorption experiments for cerium loaded DOJR towards fluoride ion was carried out at varying pH (1.5 to 11.5), contact time and fluoride ions concentration using batch adsorption method.In a typical adsorption experiment, 10 mg adsorbent and 15 mL of fluoride solution were mixed in sealed glass bottle and shaken for 24 h at 30 °C.The solid-liquid mixture was separated by filtration and the filtrate was analyzed using ion chromatography.The adsorption percentage and capacity were evaluated by using following relationship Where, C i and C e are initial and equilibrium concentration (mmol/L) respectively.W is weight of the adsorbent (g) and V is volume of solution (L).

Elemental analysis of DOJR
For the qualitative determination of the presence of various elements in the tested sample of adsorbents, an energy dispersive X-ray spectroscopic (EDX) technique was used.The EDX spectra of DOJR and DOJR after Ce(IV) loading is presented in Figs 1(a) and 1(b).It was observed from the EDX spectra of DOJR that it contained the elemental peaks of C (0.28 keV), O (0.32 keV), Na (1.09 keV), Si (1.78), P (2.04 keV), S (2.32 keV), K (2.89 keV), Ca (3.68 and 4.12 keV) and Fe (6.41 keV) as shown in Fig. 1(a).After Ce(IV) loading, new peaks corresponding to cerium element were also observed at the energy value of 4.29, 4.81, 5.24, 5.62, 6.01 and 6.27 keV in addition to the peaks of DOJR as shown in Fig. 1(b) which strongly suggest the effective loading of Ce(IV) onto DOJR.
In addition to this, the intensity of elemental peaks of Ca at 3.68 keV and 4.12 keV were decreased sharply after Ce(IV) loading.The peaks corresponds to Fe at energy value around 6.41 keV had nearly disappeared in Ce(IV) loaded DOJR as shown in Fig. 1(b).These results strongly suggested that, the metal substitution reaction of Ca and Fe occurred with Ce during cerium loading reaction via cation exchange mechanism as demonstrated in Scheme 1.  Biswas et al. (2009) for the removal of fluoride ion, respectively, from Zr(IV) loaded SRP and Fe(III)-Sn(IV) bi-metallic oxide adsorbents.Because of high oxidation state of Ce(IV) followed by Ce(III), more active sites for fluoride ion were created in Ce(IV)-DOJR and hence adsorption capability of Ce(IV)-DOJR was found to be higher than Ce(III)-DOJR.

Fig. 4. Adsorption isotherm of cerium loaded DOJR for fluoride ion
The experimental data were modeled by using well known Freundlich and Langmuir isotherm model.The linear form of the Freundlich isotherm model is represented by the equation 3 as (Freundlich 1906).
log q e = log K F + ( 1/n ) log C e (3) Where, K F and n are Freundlich constant related to adsorption capacity and adsorption intensity of the adsorbent, respectively.The values of K F and 1/n were estimated from the intercept and slope, respectively, of the linear plot of experimental data of log Q e vs log C e as shown in Fig. 5(a).
Similarly, a linear form of the Langmuir isotherm model is given by the equation 4 as (Langmuir 1916).
Where, C e (mmol/L) and q e (mmol /g) are equilibrium concentration and amount of adsorption, respectively, while q max and b are maximum loading capacity and adsorption equilibrium constant.The values of q m and b were evaluated from the slope and intercept of straight line plot of C e /q e versus C e as shown in Fig. 5(b).The calculated values of Langmuir and Freundlich isotherm constants are listed in Table 1.Although total amount of cerium content in Ce(III)-DOJR (0.91 mmol/g) and Ce(IV)-DOJR (0.89 mmol/g) were nearly the same, adsorption potential of fluoride using Ce(IV)-DOJR was higher than Ce(III)-DOJR which was due to the development of more anion exchange site in Ce(IV)-DOJR owed to higher oxidation state of Ce(IV).Because of the higher value of correlation coefficient obtained for Langmuir isotherm model (R 2 > 0.99) than Freundlich isotherm model (R 2 not higher than 0.93), the Langmuir isotherm model is more suitable to describe adsorptions behavior of fluoride ion onto both the adsorbents tested.Therefore, Langmuir type monolayer formation of fluoride on the surface of both the adsorbents took place during adsorption process.
Fig. 1.An energy dispersive X-ray (EDX) spectra of (a) dried orange juice residue (DOJR) and (b) Ce(IV)-DOJR Surface analysis of DOJR and Ce(III)-DOJR The morphological microstructures and surface characteristics of DOJR and Ce(III)-DOJR were observed with electron microscope as shown in Figs 2(a) and 2(b), respectively.The surface morphology of DOJR had a smooth, golden yellow appearance with low microporosity as shown in Fig. 2(a), while on the surfaces of Ce(III)-DOJR some irregular cracks or deformation were

Fig. 5 .
Fig. 5. Modeling of isotherm data using (a) Langmuir and (b) Freundlich adsorption isotherms Effect of fluoride ion and pH for adsorbent stabilityThe stability of investigated adsorbent is very important for the application purpose.The XRD spectra of Ce(IV)-DOJR at two different condition, that is, first after treating low concentration of fluoride at optimal pH and second after treating high concentration of fluoride at lower pH were recorded to investigate the adsorbent stability and the results are shown in Figs 6(a) and 6(b), respectively.

Fig. 6 .
Fig. 6.XRD spectra of (a) Ce(IV)-DOJR before and after treating low concentration of fluoride (25 mg/L) at 4.05 pH and (b) Ce(IV)-DOJR before and after treating high concentration of fluoride (250 mg/L) at 2.03 pH