A STUDY ON DEFLUORIDATION CAPACITY OF BETEL NUT COIR CHARCOAL FROM AQUEOUS SOLUTIONS

Article-14 Poll Res. 30 (4) : 75-5 (2011) Copyright EM International A STUDY ON DEFLUORIDATION CAPACITY OF BETEL NUT COIR CHARCOAL FROM AQUEOUS SOLUTIONS SUTAPA CHAKRABARTY * AND H.P. SARMA 1 *Department of Environmental Science, Gauhati University, Guwahati 781014, India 1 Department of Environmental Science, Gauhati University, Guwahati 781014, India (Received 7 June, 2011; accepted 13 August, 2011) ABSTRACT This study evaluated the effectiveness of betel nut coir charcoal as a potential adsorbent for defluoridation of synthetic aqueous solution following batch method of operation. The efficiency of the adsorbent was measured at different temperature settings with respect to initial fluoride concentration, contact time, and ph of the solution. The process achieved equilibrium at 180 minutes with maximum defluoridation efficiency of 92%. The isotherm can be conformed to either Langmuir and Freundlich model at different temperature. The process follows pseudo 2 nd order rate with both boundary layer effect and pore diffusion mechanism. KEY WORDS : Betel nut coir charcoal; Defluoridation; Langmuir isotherm; Freundlich isotherm; adsorption kinetics; pseudo 2 nd order rate. INTRODUCTION Fluorine is the lightest member of the halogen group of elements and the most electronegative of all the elements. In most potable water sources (TDS <500 mg/l), the fluoride ion (F - ) comprises over 95% of the total fluoride present, and the magnesiumfluoride complex (MgF + ) is typically the next most prevalent form (Edmunds and Smedley 2005; Doull et al., 2006). Fluoride can also form strong complexes in combination with aluminum, boron, beryllium, ferric iron, silica, uranium, and vanadium, but either these constituents are not present or the conditions necessary for their stability are not usually reached in natural waters (Hem, 1985). Fluorite (CaF 2 ), fluorapatite (Ca 5 (PO 4 ) 3 F), micas and amphiboles (where F- substitutes for OH- within the mineral structures) cryolite (Na 3 AlF 6 ), villiaumite (NaF), topaz (Al 2 (SiO 4 )F 2 ), are the common fluoride bearing minerals found in nature; besides certain clays may also contain some fluoride. These minerals are found primarily in certain igneous and metamorphic rocks and, to a lesser extent, the sedimentary deposits derived from such rocks. However, sedimentary formations may also contain fluorine-enriched clays and fluorapatite that precipitated from phosphate-rich waters. Corresponding author : Email: sutapa.chk@gmail.com Fluorine may be released into the environment via industrial aerosols, which include emissions from brickkilns, aluminum smelters, iron and steel production, fossil fuel burning, ceramic industries and phosphate fertilizers plants. The high incidence of skeletal and dental fluorosis, caused by excessive content of fluoride in drinking water, is one of the main health problems in many developing countries. In developed countries the fluoride content in drinking water is controlled using advanced defluoridation technologies like reverse osmosis, electrodialysis, ion exchange, and activated alumina adsorption processes. However, most of these techniques are not available in developing countries because of their cost. Moreover, drinking water supplies are often absent in rural areas of these countries. Thus, the reduction of fluoride content in water still presents a big problem in the developing countries. Suitable techniques for the water treatment should be designed for small communities and based on low-cost processes. In the recent years considerable attention has been devoted to develop cost effective novel adsorbent with high adsorption capacities. Among these adsorbents, activated alumina is the most widely used because of its availability and

76 CHAKRABARTY AND SARMA inexpensiveness. However, it requires frequent regeneration because of its low adsorption capacity at neutral ph, which results in increased difficulty for operation and it also leads to increased dissolved aluminum in treated water. Anionic clay adsorbents have also attracted increasing attention due to its simple synthesis procedure and easy regenerability. Materials like coconut shell carbon (Arulanantham et al., 1992), activated carbon (Muthukumaran et al., 1995; Mariappan et al., 2002; Sivabalan et al., 2002), activated alumina (Kumar, 1995; Li et al., 2001), bone char (Killedar and Bhargava, 1993) and ion exchange resins (Shrivastava and Deshmukh, 1994) have been used as adsorbents. So far, we have not come across the literature on defluoridation of fluoride contaminated water sources with betel nut coir. In the present study, removal of fluoride by adsorption on to a new low cost material like thermally activated betel nut coir charcoal has been investigated. EXPERIMENTAL Reagents: Fluoride stock solutions for adorption study were prepared from AR grade sodium fluoride (Merck). Double distilled water was used for preparing the standard solutions. Fluoride analysis was carried out by following SPADNS method and the colour absorbance was measured at 570 nm on UV-VIS spectrophotometer (Shimadzu, model 1240). The desired ph of the solutions was maintained by adding HNO 3 and NaOH solutions. ph of the solution was measured by digital ph meter. Synthesis of Adsorbent The Betel nut coir was collected from local market. They were then washed with water to remove dust and other impurities and dried in the sun. They were then kept in a muffle furnace for about 2-3 h at 200-300 O C at which all the materials were completely carbonized. The carbonized materials were cooled and powered and kept in an air tight container. The charcoal powder were sieved and retain on a 400 mesh (APHA, 1998) (Jamode et al. 2004) (Kumar et al. 2008). Adsorption Experiments Batch adsorption experiments were performed for determination of equilibrium time and kinetics and identifications of isotherm. Fluoride stock solutions of 100 mg/l concentration was prepared by dissolving 0.221g of NaF in 1L of double distilled water at 25 0 C (room temperature) (APHA,1998). The experimental standard solutions were prepared by diluting the stock solutions appropriately. Adsorption studies were performed at different conditions viz., contact time (15-180min), ph (2-8), initial fluoride ion concentration (2-10 mg/l) and temperature (25 o C, 30 o C, 40 o C, 55 o C). The adsorbent dose was maintained at 0.5g/100mL std. solution. The mixture was agitated in a mechanical shaker (model) for about 1 hr and placed in an incubator for about 30 min until equilibrium was attained. The solutions were then filtered using Whatman 42 filter paper and the fluoride content was measured. Each experiments was conducted three times and average values are reported. Adsorption capacity of the betel nut coir charcoal was estimated using the formula Q e (mg/g) = (Co-Ce) *V / M where Q e is the amount of adsorbed fluoride onto per unit weight of the biomass in mg/g, V is the volume of solution treated in litre, C o is the initial concentration of fluoride ion in mg/l, C e is the residual fluoride ion concentration in mg/l and m is the mass of adsorbent in g/l (Mandal, et al, 2009). Adsorption isotherm was studied at four different temperatures (25 o C, 30 o C, 40 o C, 55 o C) as stated above with 10 mg/l fluoride ion concentration, adsorbent dosage of 0.5g/100 ml, ph=6 and contact time of 180 min. Adsorption kinetics was studied on 10 mg/l fluoride standard solution, with an adsorbent dose of 0.5g/100 ml solution, ph=6 and contact time of 15-180 min at an interval of every 15 minutes. RESULTS AND DISCUSSION The XRD pattern of the BNC before defluoridation and after defluoridation is given in Fig 1a and Fig1b. Fig. 1a shows two peaks at the 2q positions 23.0013 and 28.0141 with corresponding d-values 3.8667 and 3.18515 respectively while in case of Fig 1b peaks were observed at the 2q positions 11.9120, 26.2416, and 89.3854 with corresponding d- values 7.42966, 3.39613, 1.09526 respectively. Effect of contact time The effect of contact time on Fluoride adsorption on betel nut coir charcoal was studied and the results were shown in Fig. 2. From Fig. 2, it was found that the adsorption

A STUDY ON DEFLUORIDATION CAPACITY OF BETEL NUT COIR CHARCOAL 77 Fig 1a. XRD pattern of BNC before defluoridation 40 o C, and 80-68% at 55 o C, but at higher concentrations adsorption was achieved slowly with a success rate of 78-70% at 25 o C, 70-61% at 30 o C, 65-59% at 40 o C, and 65-60% at 55 o C. At lower concentrations, all fluoride ions present in the solution would interact with binding sites and then facilitated more than 60% adsorption. At higher concentration, more fluoride ions are left unabsorbed in the solution due to the saturation of binding sites. This appears due to the increase in the number of ions competing for available binding sites in the biomass. Effect of solution ph Fig 1b. XRD pattern of BNC after defluoridation quantity of Fluoride ion on BNC charcoal increases as the contact time increased. The adsorption of fluoride onto BNC was rapid for first 30 minutes (71% at 25 o C, 60% at 30 o C,53% at 40 o C, and 47% at 55 o C) and equilibrium was nearly reached after 180 minutes (94% at 25 o C, 86% at 30 o C, 80% at 40 o C, and 64% at 55 o C). Hence, in the present study, 180 minutes was chosen as the equilibrium time. Basically the removal rate of sorbate is rapid, but it gradually decreases with time until it reaches equilibrium. The rate of percent fluoride removal is higher in the beginning due to the larger surface area of the adsorbent being available for the adsorption of the fluoride ion. It is also relevant that, since active sorption sites in a system have a fixed number and each active sites can adsorb only one ion in a monolayer, the fluoride uptake by the sorbent surface will be rapid initially, slowing down as the competition for decreasing availability of active sites intensifies by the fluoride ions remaining in the solution. Effect of initial fluoride ion concentration Adsorption experiments with BNC charcoal were conducted for solutions containing 2mg/L to 10 mg/l fluoride ion. As seen in the Fig. 3, at lower concentrations of fluoride ion (2 mg/l 6mg/L), adsorption was successfully completed rapidly to about 92-81% at 25 o C, 88-75% at 30 o C, 81-67% at The ph of the solution is perhaps the most important parameter for adsorption. To understand the adsorption mechanism, the adsorption of fluoride as a function of ph was measured and the results are shown in Fig 4. Fluoride removal recorded its maximum values at ph 6.0. There was an increase in adsorption capacity of the biomass with increase in ph from 2.0 to 6.0 and showed a marginal decreasing trend beyond ph 6.0 upto ph 8.0. This is attributed to the fact that at lower ph the negative charge at the activated surface of the BNC charcoal is neutralized by the greater hydrogen ion concentration thus facilitating the diffusion of the negatively charged fluoride ions onto the increased active surface of the charcoal. Bhargava and Killedar, 1993, made similar observations while working with fishbone charcoal. Isotherm Study Langmuir Isotherm Defluoridation capacity of BNC was evaluated using the Langmuir and Freundlich adsorption isotherms. Langmuir equation which is valid for monolayer sorption onto a surface with a finite number of identical sites which are homogeneously distributed over the adsorbent surface is given by the equation: q e = q 1 + max bc bc where qe is the amount of fluoride adsorbed per unit weight of the sorbent (mg g 1 ), Ce is the equilibrium concentration of fluoride in solution (mg L 1 ), q max is the amount of adsorbate at complete monolayer coverage (mg g 1 ) and gives the maximum sorption capacity of sorbent and b (Lmg 1 ) is Langmuir isotherm constant that relates to the eq eq

78 CHAKRABARTY AND SARMA energy of adsorption. Langmuir isotherm constants, viz., l max and b were calculated from the slope and intercept of the linear plot of Ce/qe vs Ce (Fig. 5) (Mandal et al., 2009)( Ma et al., 2009). The essential characteristics of Langmuir can be explained in terms of dimensionless constant separation factor (R L ), defined by: 1 R L = 1 + bco Where b is the Langmuir constant and C o is the initial concentration of fluoride ion. The value of R L indicated the type of Langmuir isotherm to be irreversible (R L =0), favourable ( 0 < R L < 1 ), linear (R L =1) or unfavourable ( R L >1). The R L were found to be 0.205 to 0.049 for concentrations of 2 10 mg/ LF/(Table 1). Freundlich isotherm The Freundlich isotherm in its linear form is represented by: log qe = log K f + 1/n log Ce, where qe is the amount of fluoride adsorbed per unit weight of the sorbent (mg g -1 ), Ce is the equilibrium concentration of fluoride in solution (mg L -1 ), K f is a measure of adsorption capacity and 1/n is the adsorption intensity. Freundlich isotherm constants of the sorbent were calculated from the slope and intercept of the linear plot log qe vs log Ce as shown in Fig. 6 (Jamode et al., 2004) (Kumar et al., 2008). The higher values of R 2 (the regression coefficients) indicate the applicability of both Langmuir and Freundlich isotherm. (a) (c) (b) (d) Fig 2. (a, b, c, d.) Effect of contact time at 25,30,40, and 55 o C respectively. Table 1. Isotherm model and their parameters at different temperature Isotherm Model Constants values of isotherm constants at different temperature 25 C 30 C 40 C 55 C Langmuir b 1.93 1.86 0.91 1.82 qmax -0.256-0.207 0.94-0.19 R 2 0.994 0.981 0.992 0.994 Freundlich Kf -4.58-3.05-2.34-2.34 n 0.83 1.12 1.27 1.25 R 2 0.951 0.976 0.959 0.983

A STUDY ON DEFLUORIDATION CAPACITY OF BETEL NUT COIR CHARCOAL 79 Adsorption Kinetics of Fluoride To understand the kinetics of the process, the data was fitted to different rate equations. A general rate expression takes the form dq/dt= K n (q e -q t ) eq(a) with n being the rate of the process. Reaction-based models A pseudo-first-order rate expression, or the Lagergren rate equation, is expressed as ln (q e -q t ) = ln q e K 1 t where q e and q t are the amount of fluoride on BNC (mg g -1 ) at equilibrium and at time t, respectively, and K 1 (min -1 ) is the first-order rate constant (Sarkar et al., 2006). A plot of ln(q e -q t ) against time (t) should yield a straight line (Fig 7), and the rate constant K 1 is evaluated from the slope. In the present investigation K 1 values for different studied temperature condition have been shown in Table 2. A pseudo-second-order expression may be derived from eq (A) as t/q t = (1/K 2 )(1/q e2 )+ t/q e the boundary conditions of t = 0 to t = t and q t = 0 to q t = q t can be applied. A plot of t/q t against t should yield a straight line (Fig 8), with K 2 (g mg -1 min -1 ), the second-order rate constant, obtained from the intercept. The K 2 values have been shown in Table 2. Diffusion-based models In a solid liquid sorption process the transfer of Table 2. Kinetic parameters at different temperature Kinetic model Temperature Rate constant R2 pseudo 1st order 25 C K1= -46.03 0.961 30 C K1= -50.51 0.971 40 C K1= -40.13 0.817 55 C K1= -50.91 0.968 Pseudo 2 nd Order Temperature Rate constant R2 25 C K2=6.82 0.997 30 C K2= 10.26 0.994 40 C K2= 15.30 0.979 55 C K2= 11.27 0.997 Pore diffusion Temperature Rate constant R2 model 25 C Ki=16.73 0.979 30 C Ki= 14.91 0.995 40 C Ki=13.94 0.95 55 C Ki=21.96 0.989 solute is characterized by pore diffusion or particle diffusion control. The pore diffusion model used here was proposed by Weber and Morris. The linear form of the equation is represented by: qt = K i t 0.5, where K i is the intraparticle rate constant (mg g -1 min -0.5 ). The slope of the plot of qt against t 0.5 will give the value of intra particle rate constant as shown in Fig 9. The K i values are shown in Table 2. CONCLUSION The betel nut coir charcoal is found to be an efficient (a) (c) (b) (d) Fig. 3(a, b, c, d.) Effect of Initial fluoride ion concentration at 25,30,40, and 55 o C respectively.

80 CHAKRABARTY AND SARMA (a) (c) (b) (d) Fig 4(a, b, c, d). Effect of ph at 25, 30, 40, and 55 o C respectively. adsorbent for the defluoridation of contaminated drinking water sources. The adsorbent was successful in removal of fluoride ions from aqueous solution of 2-10 mg/l fluoride concentration with about 92% to 70% efficiency at 25 o C, showing a decreasing adsorption trend with the increase in initial fluoride concentration. Adsorption equilibrium was achieved within 180 minutes. It was observed that the adsorption was ph dependent with maximum adsorption achieved at ph 6.0. Both Langmuir and Freundlich isotherm models fits well to the adsorption mechanism. Although regression coefficient of both pseudo first order and pseudo second order plot indicates adherence of both the rate laws but higher regression value of second order plot than the pseudo first order reaction indicates that the adsorption follows the second order rate law. The Webber and Morris plot reveals an initial curved path indicating boundary layer effect followed by a linear path indicating pore diffusion. REFERENCES APHA, A.W.W.A.W.P.C.F., 1998. Standard Methods for the Examination of Water and Wastewater. 20th ed. American Public Health Association, Washington, DC. Doull J., Boekelheide K., Farishian B.G, Isaacson R.L., Klotz J.B., Kumar J.V. et al 2006. Fluoride in drinking water: a scientific review of EPA s standards, Committee on Fluoride in Drinking Water, Board on Environmental Studies and Toxicology, Division on Earth and Life Sciences, National Research Council of the National Academies. National Academies Press, Washington, DC, p 530. (http://www.nap.edu) Edmunds,W.M. and Smedley, P.L. 2005. Fluoride in natural waters. In: Selinus O (ed) Essentials of medical geology. Elsevier Academic Press, Burlington, MA, pp 301 329. Hem, J.D. 1985. The study and interpretation of the chemical characteristics of natural water, 3rd edn. U.S. Geological Survey Water-Supply, Paper 2254 Killedar, D.J. and Bhargava, D.S. 1993. Effects of stirring rate and temperature on fluoride removal by fishbone charcoal. Ind. J. Environ. Hlth. 35: 81-87. Kumar, S. 1995. Studies on desorption of fluoride from activated alumina. Ind. J. Environ. Protect.. 16: 50-53. Kumar, S., Gupta, A., Yadav, J.P. 2008. Removal of fluoride by thermally activated carbon prepared from neem (Azadirachta indica) and kikar (Acacia arabica) leaves. J Env Biol, 29(2) : 227-232. Li Y.H., Wang S., Cao A., Zhao D., Zhang X., Xu C., Luan Z., Ruan D., Liang J., Wu D. and Wie B. 2001. Adsorption of fluoride from water by amorphous alumina supported on carbon nanotubes. Chem. Phys. Lett. 322: 1-5. Ma, Y., Wang, S.G., Fan, M., Gong, W.X., Gao B. Y, 2009. Characteristics and defluoridation performance of granular activated carbons coated with manganese oxides J Haz. Mat. 168: 1140-1146. Mandal S., Mayadevi S. 2009. Defluoridation of water using assynthesized Zn/Al/Cl anionic clay adsorbent: Equilibrium and regeneration studies, J Haz Mat. 167: 873-878. Mariappan P., Yegnaraman V. and Vasudevan T. 2002. Defluoridation of water using low cost activated carbons. Ind. J. Environ. Protect. 22: 154-160. Sarkar M., Banerjee A., Pramanick P. P., Kinetics and Mechanism of Fluoride Removal Using Laterite Ind. Eng. Chem. Res., 2006, 45(17): 5920 5927. Shrivastava, P.K. and Deshmukh, A. 1994. Defluoridation of water with natural zeolite. J. IPHE. 4: 11-14. Sivabalan, R., Rengaraj, S., Arabindoo B. and Murugesan, V. 2002. Fluoride uptake characteristics of activated carbon from agriculture-waste. J. Sci. Indst. Res. 61: 1039-1045.