Fluoride Removal from Aqueous Solutions By Adsorption with Coal Ash

Dynamic adsorption studies were performed to study the suitability of co al ash to remediate f luoride-contaminated water. A series of experiments were conducted with aqueous solutions containing 2. 5, 5, 10, 50 and100 mgF - / ℓ at 29 o C in a column packed with 500g of coal ash. The flow rate through the bed was 2 m ℓ /hr. The effects of contact time, initial adsorbate concent ration and sorbent particle size range on fluoride removal ef ficiency were studied. Better removal efficiencies were obtained with lower particle size ranges. The F - concentration in the effluent gradually decreased reaching 0 mg/ ℓ after 96-168 hours depending on the initial adsorbate concentration. The kinetic results showed that defluorination follows first order kinetics. An empirical formula was proposed to describe the relationship between the fluoride removal efficiency and the dependent operating variables with 95% confidence level.


Introduction
Fluoride is well recognized as an element of public health concern.Fluoride is present universally in almost every water (higher concentrations are found in ground water), earth crust, many minerals, rocks etc.It is also present in most of everyday needs, viz.toothpastes, drugs, cosmetics, chewing gums, mouthwashes, and so on.Though a small amount of it is beneficial for human health for preventing dental carries, it is very harmful when present in excess of 1 mg/ ℓ.World Health Organization (WHO) recommended maximum guideline limit of 1.5 mg/ℓ.It has been observed that with the intake of more than 6 mg/d of fluoride results in multidimensional health manifestations, the most common are skeletal abnormalities and dental fluorosis.Long term ingestion of excessive fluoride has a chronic effect on kidney.Various processes tried so far for the removal of excess fluoride from water are adsorption, ion exchange, precipitation, and membrane process.However, most of these methods have high operational and maintenance cost, low fluoride removal capacities, lack of selectivity for fluoride, undesirable effects on water quality, generation of large volumes of sludge and complicated procedures involved in the treatment.Adsorption techniques have been found to be useful means for the defluoridation of water.Researches were carried on different adsorbents, viz.activated carbon, processed bone char powder, activated alumina, magnesia, activated bauxite, fly ash, granular calcite, alum, lime, etc.The results indicated that fluoride adsorption rate and adsorption capacity of MOCA are far superior to that of activated alumina (AA), which was used as the base material for MOCA preparation.The maximum fluoride uptake capacity for MOCA and AA was found to be 2.85 and 1.08mgg _1 , respectively.The kinetic results showed that the fluoride sorption to MOCA followed pseudo-second-order kinetics with a PDF created with pdfFactory Pro trial version www.pdffactory.com,1997).In view of the advantages of fly ash as a low-cost sorbent, it seemed worth-while to study the feasibility of using coal ash in removing fluoride ion from aqueous solutions instead of costly adsorbents such as activated carbon, activated alumina, activated bauxite, etc.

Experimental Details 2.1 Materials
All the reagents used for the present study were of AR grade (BDH, Mumbai) and distilled water was used throughout.The stock solution of 100 mg/ℓ fluoride was prepared by dissolving 221 mg anhydrous NaF in 1 liter of distilled water.Thus 1 mℓ of this solution has 0.1 mg of fluoride.Dried coal ash was treated with 12M HCl at 80 °C for 4 hours, then treated with 5M NaOH to neutralize excess acid.The mixture was then filtered, washed and dried at 110 °C.The dried mass was grinded to two different particle size range (1-100µm) and (125-230µm).The results of its chemical analysis are as follows: SiO 2 24.5%, Al 2 O 3 12.1%, Fe 2 O 3 1.1%, CaO 36.3%,MgO 12.5%, C 13.5% (class C lignite and sub-bituminous coal (>10% CaO)).Slurry, prepared from 10g of the coal ash and 100 mℓ of water, had a pH of 12.0.

Experimental Set-up & Measurments
Dynamic adsorption experiments were conducted at room temperature (29 ± 0.5°C) in a column (6cm diameter and 40cm height) packed with 500g of coal ash.Synthetic fluoride solution was drawn into the column at a rate of 2 mℓ/hr from a separating funnel fixed at a height of 0.5m.Effluents were collected at regular intervals of time and analysed, using Where C o is the initial fluoride concentration in the aqueous solution and C is the residual fluoride concentration in the effluent treated wastewater.7.Each experiment with a solution of a given concentration was run in duplicate with fresh coal ash.

Kinetic Analysis
Theoretically, adsorption kinetics of fluoride onto solid particles is controlled by different mechanisms.These mechanisms involve diffusion or transport of fluoride from bulk solution to exterior surface of the adsorbent particle, adsorption of fluoride ions onto particle surfaces, movement of solute within the pores of particles, attachment of solute at sites on the interior surface of the adsorbent particles via sorption, complexation or intraparticle precipitation phenomena.The rate of sorption onto a solid surface depends upon a number of parameters such as structural properties of the sorbent, initial concentrations of the solute, and the interaction between the solute and the active sites of the sorbent (Oliveira et al., 2005).Therefore, kinetic data analysis for fluoride sorption onto coal ash was studied with the following models; Where, C(t) is the concentration of residual fluoride up to time t; k is the defluoridation rate constant; and n is a constant characterizing the order character of the process.On solving equation ( 2

Results and Discussion
Knowledge of the optimal conditions would herald a better design and modeling processes.Thus, the effect of some major parameters like contact time, particle size of adsorbent and concentration of fluoride ions of the uptake on adsorbent materials was investigated.The effect of contact time on the removal efficiency was studied by varying it from 24 to 168 hours.Complete retention of the fluoride by coal ash occurs after 96 hours for the lower F -concentrations (2.5 mg/ℓ) and after 144,168 hours for the higher F - concentrations (50 and 100 mg/ℓ) respectively.
The explanation for the high sorption capacity of coal ash with respect to the fluoride ion seems quite straightforward if one considers its high CaO content (36.2%) on the one hand, and a slow flow rate of the solution through the column (48 mℓ/24hr) on the other.The hydration of the oxide is responsible for the high pH of the water slurry of coal ash (pH exceeding 12).The resulting calcium hydroxide reacts with the fluoride to afford a sparingly soluble calcium fluoride.Strong sorptive capacity of residual carbon is also likely to contribute to this process.
The residual carbon samples have microporous surface area of 100 -200 m 2 g -1 carbonaceous material which is much larger than that of bulk coal ash (1to 6 m 2 g -1 ).Moreover large pores of the carbon are capable of accommodating oxygen molecules with the diameter of 300 pm, and can also provide good access to much smaller fluoride ion with the diameter of 172 pm.Inspection of the results presented in figure (3) show that the sorbent particle size range influenced the reduction of fluoride.The removal efficiency measured with 5 mg/L initial fluoride concentration after 24 hours contact time decreases with increasing sorbent particle size (best removals at small particle size ranges).In experiments performed under gravitational flow operation increases in sorbent particle size ranges allow larger flows, but at the same time it significantly decreases the effectiveness of the sorbent bed for the removal of fluoride.Figures (4 and 5) show a plot for ln(C/C 0 ) vs. time for initial fluoride concentrations of 5 and 100 mg/ℓ respectively.The linear behavior, confirm that the deflorination of aqueous solution by adsorption on coal ash follows a first order reaction kinetics.These findings are in agreement with the results obtained by Murugan and Subramanian (2006).A relationship for the estimation of the rate constant as a function of the initial fluoride concentration was proposed as;

Conclusions
Based on these studies, it is concluded that coal ash can be fruitfully utilized for the removal of fluoride from its aqueous solutions.A double mechanism is likely to operate in the retention of fluoride by coal ash: chemical binding by calcium hydroxide and physical sorption by residual carbon particles.The percentage of fluoride removal was found to be a function of contact time and initial solute concentration.The removal increased with time and with the increase of initial solute concentration.The removal kinetic follows a first order reaction rate.An equation for the estimation of the value of reaction rate constant as a function of the initial fluoride concentration was proposed.The developed empirical formula, for the removal efficiency represented the present experimental data with 95% confidence level with a correlation coefficient of 0.9698.

References
Bhargava and Killedar (2008) conducted batch adsorption studies to determine the effects of some parameters such as initial solute concentration and adsorbent dose on fluoride adsorption by fish bone charcoal.They found that the fluoride removal at any given solute concentration and adsorbent dose increased with time.The equilibrium fluoride concentration was found to be a function of the adsorbent dose and the initial fluoride concentration.Fly ash has been successfully used by several investigators for the removal of metal ions (Panday et al., 1985 ) and radionuclides (Aptak et al., 1996) ( from aqueous solutions, for treatment of wastewaters to remove organic compounds [Banerjee et al.and together with hydrated lime-for SO 2 removal from flue gases (Fernandez et al. ln(C/C o ) vs. t yields the value of the kinetic rate constant.

Figure
fluoride for different contact times.It was found that the removal of fluoride ions increases with the increase in contact time.The changes in the extent of removal might be due to the fact that initially all sorbent sites were vacant.Later, the fluoride uptake by PDF created with pdfFactory Pro trial version www.pdffactory.comshows that, the percentage removal of fluoride ion decreases with increasing the concentration of the initial fluoride ion concentration.
& b are constants, k is the rate constant (h -1 ) and C o is the initial fluoride concentration in mg/ℓ.An empirical formula for the defluorination efficiency, using coal ash, in terms of fluoride initial concentration and sorbent particle size was developed as follows: is the overall removal efficiency, a, b and c are constants; C 0 is the fluoride initial concentration and dp is the mean diameter of sorbent particles.STATISTICA software 9.0 is applied to evaluate the empirical constants of equations (4 & 5).An implicit nonlinear numerical method (Quasi-Newton) is applied as regression method.Initial values and step sizes are automatically allocated by the software.In this regression, second order (partial) derivatives of the loss PDF created with pdfFactory Pro trial version www.pdffactory.com is 3D surface plot for the effect of fluoride initial concentration and sorbent particle diameter on the overall removal efficiency.The equation representing the system is given as

FigureFigure ( 8 )
Figure (1) The change of fluoride removal efficiency with time for different fluoride initial concentration