Eggshell Powder as an Adsorbent for Removal of Fluoride from Aqueous Solution: Equilibrium, Kinetic and Thermodynamic Studies

A new medium, eggshell powder has been developed for fluoride removal from aqueous solution. Fluoride adsorption was studied in a batch system where adsorption was found to be pH dependent with maximum removal efficiency at 6.0. The experimental data was more satisfactorily fitted with Langmuir isotherm model. The kinetics and the factor controlling adsorption process fully accepted by pseudo-second-order model were also discussed. E a was found to be 45.98 kJmol-1 by using Arrhenius equation, indicating chemisorption nature of fluoride onto eggshell powder. Thermodynamic study showed spontaneous nature and feasibility of the adsorption process with negative enthalpy (∆H 0) value also supported the exothermic nature. Batch experiments were performed to study the applicability of the adsorbent by using fluoride contaminated water collected from affected areas. These results indicate that eggshell powder can be used as an effective, low-cost adsorbent to remove fluoride from aqueous solution as well as groundwater.


Introduction
Fluorine, the most electronegative element is distributed ubiquitously as fluorides in nature 1 .Water is the major medium of fluoride intake by humans 2 .So the problems associated with the excess fluoride in drinking water are highly endemic and widespread in country like India 3 .Many availablewater treatment technologies such as electro-dialysis 4 , Donnan dialysis 5 and reverse osmosis 6 which are usually too expensive have been reported by previous researchers.The excessive fluoride intake leads to the loss of calcium from the tooth matrix, aggravating cavity formation throughout life 7 .Severe chronic and cumulative over exposure can cause the incurable cripping of skeletal fluorosis 8 .After recommendations of WHO, a number of countries legislated to set 1.5 mg/L as the maximum contaminant level (MCL) for fluoride in drinking waters 2 .The traditional method of removing high

Introduction
High concentrations of fluoride occurring naturally in ground water have caused widespread fluorosis (both dental and skeletal) throughout many parts of the world.The excessive fluoride intake leads to the loss of calcium from the tooth matrix aggravating cavity formation throughout life (Sujana and Anand, 2010).Severe chronic and cumulative over expose can cause the incurable cripping of skeletal fluorosis (UNICEF 2004).The dental or skeletal fluorosis is irreversible and no treatment exists (Sujana and Anand 2010).The removal of fluoride from water is one of the most important issues due to the effect on human health and environment (Ma et al., 2007).The optimum fluoride level in drinking water for general good health set by WHO (1993) is considered to be less than 1.5 mg/L).But many epidemiological studies have shown possible adverse effects of the longterm ingestion of fluoride, even in the places where people are consuming water with fluoride content of 1.5 mg/L (Sujana and Anand, 2010).Membrane filtration (Ndiaye et al., 2005), precipitation (Parthasarathy et al., 1986), nanofiltration (Simons et al., 1993), ion-exchange (Ruixia et al., 2002), electrocoagualation flotation (Hu et al., 2005) and adsorption (Mohapatro et al., 2004) have been used for fluoride removal.Among these methods, adsorption techniques is the most effective and widely used methods, because its universal has a low maintenance cost, and is applicable for the removal of fluoride even at low concentration (Chen et al., 2010).In recent years, to find out cost effective alternative for removing high fluoride content from waters many geomaterials have been tested (Wang and Reardon 2001;Mohapatro et al., 2004;Hiemstra et al., 2000;and Sujana et al., 2009).In the present study we tried to explore the fluoride removal efficiency of activated rice husk ash (ARHA) and its potentiality compare study with activated silica gel (ASiG).

Introduction
Fluoride is one of the most potent groundwater pollutant 1 .Excess intake (>1.5 mgL -1 ) may cause fluorosis (dental, skeletal and non-skeletal) along with various neurological complication.Removal of fluoride from the ground water is challenging task among the scientists.Various technologies already developed to remove fluoride from drinking water by coagulation, membrane filtration, ion exchange, etc.But due to high cost for processing such technology is unfit for developing country.Therefore there is a great need for environmental friendly and low cost technology.One such low cost technology is adsorption and which is effective for removal of fluoride 1 .Many low cost adsorbents such as Tamarind seed 1 ; Tamarind gel; Duck weed spirodela polyrrhiza were used for removal of fluoride.

INTRODUCTION
The removal of fluoride ions from aqueous solutions is one of the most important environmental concerns due to their high

Introduction
Fluorine is one of the strong electronegative elements and its gaseous form is tremendous powerful oxidizing agent.It exists in underground water as fluoride ion (F − ).However, natural abundance of fluorine ranges from 0.06% to 0.09% by weight in the earth crust [1].
Fluoride is mainly toxic to the human body when it exceeds the threshold limit of 1.There is a gap in knowledge about the carbonized and chemically treated forms.But classical batch adsorption technique is unable to provide fine optimization.To overcome such a problem by taking the help of computerize optimization process called Response Surface Methodology (RSM), in this study lemon leaf was chosen for fluoride adsorption as dried powder (LLD-1), carbonized form (LLD-2) and chemically treated (LLD-3) together to establish new adsorbents for defluoridation.Due to carbonization high specific surface area occurred in the adsorbent and due to chemical treatment, more binding sites appear which are responsible for more fluoride adsorption than naturally occurring materials.
It is well known that consumption of lemon leaf is one of the most common fruit grown mainly in all tropical countries, including India.In fresh samples, high levels of calcium occur in the vacuoles and especially the inner tangenital walls of epidermal and sub-epidermal cells near the gap of the abscission zone.Calcium containing crystals (calcium oxalate) is also abundant in vacuoles of the cortex parenchyma and leaf blade sides [10].In 2004, Storey and Leig explained citrus leaves accumulate large amounts of calcium in palisade, spongy mesophyll and crystal containing idioblast cells.
toxicity and impact on human health.They can occur in the environment due to both natural as well as anthropogenic activities.Since fluoride is present in several naturally occurring minerals and geo-chemical deposits, it can be ultimately leached out by rainwater and spreads into the environment through soils and water streams and finally accumulates along the food chain which causes human health hazards.On the other hand, many industries such as semiconductor industry, fertilizer industry and processes like electrolysis of alumina also contribute to fluoride pollution.The major health hazards caused by fluoride are dental fluorosis, mottling of teeth, skeletal fluorosis and deformation of bones in children as well as in adults [1].Drinking water containing fluoride may be beneficial or detrimental depending upon its concentration and total amount consumed in the range of 1.5 to 6.5 mg/L against its critical limit of 1.5 mg/L in drinking water [4].Therefore, there is an urgent need to remove fluoride from drinking water.The common methods for removing fluoride ions from water and wastewater include chemical 5 mg/L [2].The excess intake of fluoride may cause fluorosis (dental and skeletal), neurological damage [3] decreasing growth and intelligence [4].There is a tremendous demand for for removal of fluoride from drinking water.In recent years, various plant materials like coconut shell [4], bone char, [5] tamarind seed, neem and kikar leaves [6], Barmuda grass [7] neem charcoal [8], Moringa oleifera seed [9] have also been used as adsorbents for defluoridation.

ABSTRACT
In this work, the feasibility of employing calcareous soil to remove fluoride ions from its aqueous solutions was investigated under batch mode.The influence of solution pH, sorbent dose, initial fluoride concentration, contact time, stirring rate and temperature on the removal process were investigated.The equilibrium adsorption data were analyzed using Langmuir, Freundlich and Temkin isotherm models.The kinetics of fluoride ion was discussed by pseudo-first-order, pseudo-second-order and intraparticle diffusion models.It was shown that the adsorption of fluoride ions could be described by the pseudosecond-order kinetic model.Thermodynamic parameters such as Gibbs free energy (G o ), enthalpy(H o) and the entropy change of sorption (S o ) have also been evaluated and it has been found that the adsorption process was spontaneous, feasible and exothermic in nature.A six-layered feed forward neural network with back propagation training algorithm was developed using thirty-one experimental data sets obtained from laboratory batch study.The ANN predicted results were compared with the experimental results of the laboratory test.Regeneration study indicates that about 81.4% fluorides can regenerate from the adsorbent.It was concluded that calcareous soil has potential for application as an effective adsorbent for removal of fluoride ions from aqueous solution.