CHAPTER 2 REVIEW OF LITERATURE

Transcription

1 17 CHAPTER 2 REVIEW OF LITERATURE 2.1 GENERAL Fluoride ion in water exhibits unique properties, as its concentration in optimum dose in drinking water is advantageous to health and excess concentration beyond the prescribed limits affects the health (Vekata Mohan et al 1995). High fluoride concentration in the ground water and surface water in many parts of the world is a problem, to be paid more attention. High fluoride in drinking water was reported from different geographical regions. The removal of the excess fluoride from waters and wastewaters is important in terms of protection of public health and environment. Defluoridation is the only practicable way to overcome the problem of excessive fluoride in drinking water. Several adsorbents were tried for the removal of fluoride from water and wastewater and are categorized in the following section. 2.2 DEFLUORIDATION OF WATER USING NATURALLY AVAILABLE WASTE MATERIAL Corn Cobs The adsorption of fluoride on corn cobs powder was investigated by Parmar et al (2006). Powdered corn cobs did not show remarkable adsorption but aluminum treated corn cobs had good adsorption capacity. The parameters studied include the contact time, concentration, temperature and ph. Near

2 18 neutral ph was identified as the optimum condition of the medium, and 90 to 120 minutes was the best contact time for maximum fluoride adsorption. The adsorption process was found to follow Freundlich isotherm. The study showed that Ca-treated corn cobs powder was found to be more effective as an adsorbent compared to Al-treated corn cobs powder. The rate of uptake of fluoride was very high with both the materials. The fluoride containing corn cobs powder can be dumped as a solid waste material in pits. The raw materials employed for the preparation of the substrate were cheap and easily available Tamarind Seed Murugan and Subramanian (2006) investigated tamarind seed, a household waste from the kitchen, for the sorptive removal of fluoride from synthetic aqueous solution as well as from field water supplies. Batch sorptive defluoridation was conducted under variable experimental conditions such as ph, agitation time, initial fluoride concentration, particle size and sorbent dose. Maximum defluoridation was achieved at ph 7.0. Defluoridation capacity decreased with increase in temperature and particle size and the adsorption followed first order kinetics and Langmuir adsorption isotherm. Desorption was carried out with 0.1N HCl and it was found to be 90 %. The surface and sorption characteristics were analyzed using FTIR and SEM techniques. For domestic and industrial applications, defluoridation with 100 % achievement and subsequent regeneration of adsorbent was performed with a household water filter and fixed bed column respectively. 2.3 CARBON BASED ADSORBENTS FOR FLUORIDE REMOVAL Commercial Activated Carbon Batch adsorption studies were undertaken to assess the suitability of commercially available activated charcoal (Tembhurkar and Dongre 2006) to

3 19 remediate fluoride- contaminated water. The effects of some of the major parameters of adsorption, viz. ph, and dose of adsorbent, rate of stirring, contact time and initial adsorbate concentration on fluoride removal efficiency were studied and optimized. Based on these studies, it was concluded that activated charcoal could be fruitfully utilized for the removal of fluoride. The uptake of fluoride ions was possible between ph of 2.0 and 8.0, however, ph of 2 gave maximum fluoride removal since neutralization of OH - ions by large number of H + ions takes place at less ph values. The percentage of fluoride removal was found to be a function of adsorbent dose and contact time at a given initial solute concentration. The removal increased with time and adsorbent dose. The study on defluoridation using activated carbon revealed that the equilibrium data fitted better to Langmuir isotherm than Frendulich isotherm. Emmanuel et al (2008) prepared commercial activated carbon (CAC) and indigenously prepared activated carbons (IPACs) from Pithacelobium dulce, Ipomoea batatas and Peltophorum ferrugineum for the removal of fluoride. The effects of various experimental parameters like ph, dose of the adsorbent, adsorbate concentration and contact time have been investigated using a batch adsorption technique. The results of the experiments have shown that the percentage of fluoride removal has increased with the increase in contact time and dose of adsorbent. On the contrary the percentage of removal has decreased with the increase in initial concentration of the standard fluoride solution. The adsorption process was found to be of first order with the intra particle diffusion as one of the rate determining steps. Among the adsorbents considered PLDC (pithacelobium dulce carbon) possessed the highest or the maximum adsorption capacity. The adsorption capacity and efficacy in the removal of fluoride were far greater than CAC.

4 Alum-impregnated Carbons Most of the carbon from different carbonaceous sources showed fluoride removal capacity after alum impregnation. High fluoride removal capacities of various types of activated carbon have been reported (Venkataraman 1960). Activated carbon prepared from cotton waste, coffee waste, coconut waste, paddy husk, corn cob, neem leaf etc. was tried for defluoridation. All these materials proved to be of academic interest only (Bulusu et al 1979). The activated carbon prepared by carbonization of coconut shell (CSC) in the presence of sulphuric acid was used for the removal of fluoride by Arulanantham et al (1992). The developed carbon after impregnation with Al (III) ions when used in wet condition showed a capacity for fluoride removal three times higher than the same material used after drying. The studies clearly showed that the coconut shell after alum impregnation was effective for the removal of fluoride from dilute aqueous solutions. The optimum ph for fluoride removal was wide for CSC as against for alumina. The regeneration of CSC with aluminum sulphate solution was found to be simple and problems associated with alumina were not encountered. The CSC could be prepared from waste coconut shell material with good bulk density comparable to that of coal based commercial activated carbons. In addition this carbon had good attritional characteristics Graphite Batch adsorption system using various grades of graphite as adsorbents was investigated (Karthikeyan and Elango 2008) to remove fluoride ions from aqueous solutions. The system variables studied include initial concentration of the sorbate, agitation time, adsorbent dose, ph, co-ions and temperature. Lower range of ph and high temperature ranges were found

5 21 as the optimum conditions for maximum fluoride adsorption by the adsorbents. The result gained from this study was well described by the Langmuir and Freundlich isotherms. On the basis of the kinetic and XRD studies a mechanism in which surface adsorption as well as intra-particle diffusion as rate limiting step has been proposed for the physisorption of fluoride ions onto graphite Waste Carbon Slurry Waste carbon slurry obtained from fuel oil based generators of fertilizer industry was investigated for adsorption of fluoride by Gupta et al (2007). They investigated the effect of contact time, ph, temperature and adsorbent dose on the extent of adsorption by carbon slurry. The contact time and ph for maximum fluoride uptake were found to be 1 h and 7.58 respectively. The experimental isotherm data were found to follow Langmuir equation more closely. From kinetic analysis, the adsorption was found to follow second-order mechanism. The performance of adsorbent was excellent as it removed fluoride even at low concentration, i.e. < 2 mg L -1. The practical utility of the column was tested by drawing breakthrough curves for fluoride adsorption from wastewater using carbon slurry, which showed an implemental breakthrough capacity of mg L Carbon Nanotubes Exploring the application of carbon nanotubes to adsorbing fluoride, a team led by Li et al (2003a) prepared aligned carbon nanotubes (ACNT), by the decomposition of xylene, catalysed by ferrocene. The authors found this material to adsorb 4.5 mg g -1 fluoride from 15 mg L -1 fluoride at ph 7. The adsorption isotherms generated under identical conditions for activated carbon, -Al 2 O 3, a typical soil and carbon nanotubes showed that the order of adsorption was: carbon nanotubes > soil > -Al 2 O 3 > activated carbon.

6 Carbonaceous Materials From Pyrolysis of Sewage Sludge Mendoza et al (2011) investigated the removal of fluoride ions by a carbonaceous material obtained from pyrolysis of sewage sludge. Effect of contact time, ph, and fluoride concentration were evaluated. Equilibrium was reached after 18 h of contact time and the maximum removal obtained at ph 7.06 ± 0.08, which corresponds to the zero charge point of the adsorbent. The maximum removal of 82.2 ± 0.5% of fluoride was found with 0.4 g L -1 and with 20 g L -1.The kinetic data of the process could be fitted to the pseudo second order and the intraparticle mass transfer diffusion models. The study on defluoridation using carbonaceous material revealed that the equilibrium data fitted to both Langmuir and Frendulich isotherms. 2.4 ION EXCHANGE RESINS AS ADSORBENTS FOR REMOVAL OF FLUORIDE Fluoride ion can be removed from water supplied with a strongly basic anion-exchange resin containing quarternary ammonium fuctional groups. The removal takes place according to the following reaction: Matrix N R 3 Cl - + F - Matrix N R 3 F - + Cl - (2.1) The fluoride ions replace the chloride ions of the resin. This process continues until all the sites on the resin are occupied. The resin is then back washed with water that is supersaturated with dissolved sodium chloride salt. New chloride ions then replace the fluoride ions leading to recharge of the resin and starting the process again. The driving force for the replacement of chloride ions from the resin is the stronger electronegativity of the fluoride ions.

7 23 Sundaram and Meenakshi (2009) developed organic-inorganic hybrid type ion exchangers for fluoride removal. Synthesized hybrids were characterized using FTIR studies. Batch adsorption studies were performed as a function of contact time, ph and influence of other interfering anions. All the ion exchangers possessed appreciable defluoridation capacity. The values of defluoridation capacity of polyacrylamide Al (III) phosphate (Al-Ex), polyacrylamide Ce(IV) phosphate (Ce-Ex), polyacrylamide Zr (IV) phosphate(zr-ex) were found to be 2144, 2290 and 2166 mg F - kg -1, respectively. Ce-Ex had slightly higher defluoridation capacity than Al-Ex and Zr-Ex. The defluoridation capacity of sorbents was significantly influenced by the ph of the medium and was altered in the presence of bicarbonate ions. The adsorption followed Freundlich isotherm. The nature of adsorption of all the exchangers studied was spontaneous and endothermic. The hybrid type sorbents had more scope for field applicability because of their usable form. The defluoridation capacity of a chelating resin, namely Indion FR 10 (IND), and Ceralite IRA 400 (CER), an anion-exchange resin, were compared under various equilibrating conditions for the identification of selective sorbent (Meenakshi and Viswanathan 2007). The results showed that chelating resin is more selective than an anion-exchange resin for fluoride removal. The surface morphology of resins before and after fluoride sorption was observed using scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) was used for the determination of functional groups responsible for fluoride sorption. The sorption process was found to be controlled by pseudo-second-order. The results of analysis of both the resins clearly established that the sorption process was spontaneous and exothermic and the equilibrium data agreed well with both Freundlich and Langmuir isotherms. The pseudo-second-order kinetic reaction model was found to be the best correlation of the data for fluoride removal on IND. It was found

8 24 to be an effective sorbent for fluoride removal than CER resin because of its specificity towards fluoride. It could be concluded that though fluoride is an anion it cannot be effectively exchanged using anion-exchange resins, as its concentration is lower than the other anions present in water. Hence, as an alternative, cation exchange / chelating type resins may be employed for selective sorption of fluoride, which removes fluoride by an adsorption / precipitation mechanism. 2.5 MINERAL BASED ADSORBENTS FOR FLUORIDE REMOVAL Low Cost Natural Minerals Fan et al (2003) investigated the adsorption kinetics and adsorption capacity of low cost materials at a low initial fluoride concentration. The experiments were carried out at a neutral ph, and radioisotope 18 F rather than 19 F was used since 18 F can be rapidly measured by measuring the radioactivity with a resolution of 1 X mg or 0.01 µci. The tested materials are hydroxyapatite, fluorospar, calcite, quartz and quartz activated by ferric ions. Their adsorption capacities followed the order: Hydroxyapatite > Fluorospar > Quartz activated using ferric ions > Calcite > Quartz The uptake of fluoride on hydroxyapatite was an ion-exchange process and followed the pseudo-first-and second-order equations. Calcite has been seen as a good adsorbent in fluoride removal and has been patented. However, the data suggested that its adsorption capacity was only better than quartz. Various low-cost materials like kaolinite, bentonite, charfines, lignite and nirmali seeds were investigated (Srimurali et al 1998) to assess

9 25 their capacity for removal of fluorides from water by batch adsorption studies. Studies were also conducted to determine optimum operating system parameters; such as contact time, ph, dose and size of the adsorbent. From the experimental investigation, the order of removal of fluoride from the test solution using low-cost materials was found to be bentonite > charfines > Kaolinite > lignite > nirmali seeds. At optimum system conditions, charfines and bentonite exhibited highest removal capacity (around 40%). The study indicated that removal of fluoride from water depended on the contact time, ph, and dose of the adsorbent. Removal of fluoride increased with time and approached more or less constant values. It decreased with the increasing ph of the test solution. Removal of fluoride increased with the decreasing size of the sorbent and with increasing sorbent dose. The study also revealed that chemical pre-treatment of the sorbents (charfines) was not effective in enhancing the fluoride removal capacity Synthetic Hydroxyapatite Sundaram et al (2008) have reported the advantages of nanohydroxyapatite (n-hap), a cost effective sorbent for fluoride removal, n-hap possessed a maximum deflouridation capacity (DC) of 1845 mg F - kg -1 which was comparable with that of activated alumina, a defluoridation agent commonly used in the indigenous defluoridation technology. A new mechanism of fluoride removal by n-hap was proposed in which it was established that this material removes fluoride by both ion-exchange and adsorption process. There was no significant influence of other co-anions like chloride, nitrate and sulphate on the DC of n-hap except bi-carbonate ions. The adsorption pattern followed both Langmuir and Freundlich isotherms, but better fitted to Langmuir isotherm model. The rate of reaction followed pseudo-second-order kinetics.

10 Granular Red Mud Red mud is a very fine material (particle size of which is generally below 75µ) and high specific surface area (10 m 2 gm -1 ) which is produced during the Bayer process for alumina production (Hind et al 1999). It is the insoluble product of bauxite digestion with sodium hydroxide at elevated temperature and pressure. It is mainly composed of iron oxides and has a variety of elements and mineralogical phases. The removal of fluoride from aqueous solution using the original and activated red mud forms has been studied by many researchers (Lopez et al 1998). The fluoride adsorption capacity of activated form has been found to be same as original form. The adsorption was highly dependent on ph. The possibility of removal of fluoride ion by using red mud was explained on the basis of the chemical nature and specific interaction with metal oxide surfaces (Yunus et al 2002). The removal of fluoride from water using granular red mud (GRM) was investigated by Tor et al (2009) using batch and column adsorption techniques. The main conclusions were the high capability of removing fluoride at low concentrations, satisfactory adsorption capacity in batch and column adsorption and fine reversibility to be regenerated rapidly for four cycles indicated that GRM could be used in fluoride adsorption. Batch experiments indicated that the time to attain equilibrium was 6 h and adsorption followed the pseudo-second-order kinetic model. Maximum removal of fluoride was achieved at a ph of 4.7. The adsorption of fluoride by GRM in batch systems could be described by the Freundlich isotherm, and the adsorption capacity was mg g -1. Higher fluoride sorption capacity was obtained using column experiments than using batch experiments.

11 Activated Titanium Rich Bauxite The potential of thermally activated titanium rich bauxite (TRB) for adsorptive removal of excess fluoride from drinking water was examined by Das et al (2005). Adsorption with respect to variation of ph, adsorbent dose, initial fluoride concentration, presence of interfering ions and heat treatment were investigated by batch equilibrium experiments. The optimum temperature of thermal activation for maximum adsorption capacity was found in between 300 and 450º C. Although uptake of fluoride was dependent on contact time, adsorbent dose, ph, and concentration of adsorbate the optimum ph for maximum uptake was found in between 5.5 and 6.5. The maximum adsorption capacity was found to be 3.8 mg g -1 at ph 6 with adsorbent dose and initial fluoride concentration of 1 g L -1 and 10 mg L -1 respectively. Adsorption of fluoride was fairly rapid in first min and thereafter increased slowly to reach the equilibrium in about h. The adsorption followed first order kinetics and the data fitted reasonably well to Langmuir and Freundlich isotherm models. The adsorption of fluoride was not greatly affected by the presence of common interfering ions indicating selectively of the material towards fluoride adsorption. Competitive effect of interfering ions could be minimized by proper selection of operating ph and fluoride could be removed to the desired level (<1.5 mg L -1 ) from contaminated water using appropriate dose of activated bauxite Plaster of Paris Batch sorption system using plaster of Paris as an adsorbent was investigated by Gopal and Elango (2007) to remove fluoride ions from aqueous solutions. Initial concentration of the sorbate, agitation time, adsorbent dose, ph, co-ions and temperature were discussed. Wide range of ph and low temperature ranges were found as the optimum conditions for maximum fluoride adsorption by the adsorbent. The results gained from this

12 28 study were extremely well described by the theoretical Freundlich and Langmuir isotherm. The higher enthalpy change for the adsorption process and XRD studies indicated that adsorption occurred through chemisorptions Clay Based Adsorbents Fluoride removal using China clay was studied by researchers (Chaturvedi et al 1988). Low fluoride concentration, high temperature and acidic ph were factors favouring the adsorption of fluoride. It was concluded that the alumina constituent of the China clay was responsible for fluoride adsorption. Ramdeni et al (2010) developed two types of natural clays for removal of fluoride in Sahara region. Water supply for people in the Sahara region was mainly assured by poor quality ground water which had excessive minerals, hardness and high concentration of fluoride. This lead to many teeth and bone diseases such as fluorosis. To eliminate the excess fluoride from the El Oued Souf City water supply located in the South East of the Algerian Sahara by retention process onto montmorillonite clay using potentiometric method two types of natural clays were tested. The first one contained a higher percentage of calcium (AC) and the second one without calcium (ANC). These adsorbents were activated chemically and thermally with temperatures ranging between 200 and 500ºC. Montmorillonite without calcium ANC-Na + was more efficient for deflouridation of Saharan water than montmorillonite with higher percentage of calcium ANC-Na +. They concluded that ANC-Na + could be a promising alternative sorbent for defluoridation of water Lateritic Ores Nickel laterites and chromite mine overburden usually contain high content of iron and small amounts of alumina, chromium, cobalt, nickel, manganese. Due to their high iron content in the form of goethite, some

13 29 studies have been reported for fluoride adsorption (Sarkar et al 2006). Recently Sujana et al (2009b) have compared the fluoride uptake capacity of various goethite containing geo materials of India. Effect of various experimental parameters such as time, temperature, ph, adsorbent and adsorbate concentration has been reported, the kinetic, isothermic and thermodynamic parameters were evaluated. Groundwater samples were also tested for fluoride removal Magnesium Oxide The application of Magnesium oxide for defluoridation is not a new one (Venkateswarlu and Rao 1953, 1954). The mechanism of removal of fluoride ions from water by magnesium oxide is as follows: Addition of magnesium oxide to fluoride-bearing water results in the hydration of magnesium oxide to magnesium hydroxide. MgO + H 2 O Mg (OH) 2 (2.2) The magnesium hydroxide formed in the above reaction combines with fluoride ions to form practically insoluble magnesium fluoride as: 2NaF + Mg (OH) 2 MgF 2 + 2NaOH (2.3) Precipitation of fluoride ions as insoluble magnesium fluoride lowers the fluoride ion concentration in water. It has been found that for a given mass of magnesium oxide, the amount of fluoride retained increases with concentration of fluoride ions in the spiked water samples. Further, at a given solution concentration, the amount of fluoride retained by magnesium oxide decreases in conjunction with calcium oxide (lime) Lanthanum Oxide Rare earth mineral based adsorbent viz. lanthanum oxide was investigated by Rao and Karthikeyan (2011) for defluoridation. Results of batch experiments indicated about 90 % removal in 30 min from a 4 mg L -1

14 30 synthetic fluoride solution. The effects of various parameters such as contact time, ph, initial concentration and sorbent dose on sorbent efficiency were investigated. Isothermal equilibrium sorption data suggested that sorption reaction followed the BET isotherm model involving multilayer sorption. Sorption capacity ranges from 0.5 to 2.5 mg g -1 depending upon the initial concentration and higher sorption capacities were accomplished at low ph values. Adsorbent showed negligible desorption of fluoride in distilled water. Alum was most effective regenerant than HCl and NaOH. Hence, 1 % alum solution was used for regeneration. Results of cyclic regeneration with alum indicated that the sorbent could be regenerated for ten cycles without significant loss of sorption capacity. Up flow column studies demonstrated the engineering application of lanthanum oxide for removal of fluoride in continuous flow column studies Hematite Modified with Aluminium Hydroxide Rios and Hernandez (2012) investigated the modification effects of hematite with aluminium hydroxide on the removal of fluoride ion from water using batch experiments. The authors concluded that the modified hematite is an efficient adsorbent, compared to unmodified hematite, for the removal of fluoride ion. The optimum ph range for maximum adsorption was between 2.34 and The maximum adsorption capacity was mg g -1. The kinetic sorption processes followed Elovich model and the isotherm conformed to Freundlich and Langmuir models. 2.6 REMOVAL OF EXCESS FLUORIDE FROM WATER USING ALUMINIUM COMPOUND BASED ADSORBENTS Waste Residue from Alum Manufacturing Process The ability of waste residue, generated from alum manufacturing process, to remove fluoride ion from water has been investigated (Nigusse et al 2007). The alum sludge a waste material from alum manufacture,

15 31 containing different metal oxides with a heterogeneous surface, has shown a superior adsorption capability for fluoride ion. Series of batch adsorption experiments were carried out to assess parameters that influence the adsorption process. The factors investigated include the effect of contact time, adsorbent dose, thermal pretreatment of the adsorbent, neutralization of the adsorbent, initial fluoride concentration, ph of the solution and effect of co-existing anions. The results revealed that low cost, locally available industrial waste material generated from aluminium sulphate manufacturing process was promising material to remove excess fluoride from water. Adsorption of fluoride was fairly rapid in first 5 min and thereafter increased slowly to reach the equilibrium in about 1h. Based on the results obtained it could be concluded that about 85% of fluoride was removed within the first 5 min at an optimum adsorbent dose of 16 g L -1 in ph range of less than 8 for initial fluoride concentration of 10 mg L -1. The adsorption followed second order kinetics. The adsorbent's fluoride removal efficiency was affected significantly with carbonate ion concentrations and little or no effect by other anions such as phosphates, chlorides, sulphates and nitrates Amorphous Fe / Al Mixed Hydroxides Sujana et al (2009) have reported that the effectiveness of amorphous iron and aluminum mixed hydroxides in removing fluoride from aqueous solutions. A series of mixed Fe / Al samples were prepared at room temperature by co-precipitating Fe and Al mixed salt solutions at ph 7.5. The compositions (Fe : Al molar ratio) of the oxides were varied as 1:0, 3:1, 2:1, 1:1 and 0:1. Batch adsorption studies for fluoride removal on these materials showed that the adsorption capacities of the materials were highly influenced by solution ph, temperature and initial fluoride concentration. The rate of adsorption was fast and equilibrium was attained within 2 h. The adsorption followed first-order kinetics. The sample with molar ratio 1 has shown maximum adsorption capacity of 91.7 mg g -1. The optimum ph range

16 32 for fluoride adsorption was found to be for the samples 1:0, 3:1 and 2:1, whereas it was in the range of for 1:1 and 0:1. The equilibrium data fitted to both Langmuir and Freundlich isotherm models and showed high adsorption capacities. 2.7 BONE CHARCOAL AS ADSORBENT FOR FLUORIDE REMOVAL The uptake nature of fluoride on bone surfaces is an early method of investigation of defluoridation of water supplies in 1937 itself (Smith et al 1937). The carbonate radical of the apatite comprising bone Ca (PO 4 ) 6.CaCO 3 was exchanged by fluoride ion, form insoluble fluorapatite on the bone surface (Benefield et al 1982). Ca (PO 4 ) 6.CaCO 3 + 2F - Ca (PO 4 ) 6.CaF + CO 3 2- (2.4) This process of treatment was further improved by carbonizing bone at temperature greater than 1100ºC, commonly known as bone char. Bone char possesses better ion exchange property for fluoride ion as compared to unprocessed bone. The column filled with bone char was regenerated by washing it with alkali solution. Batch adsorption studies were conducted to determine the effects of parameters such as initial solute concentration and adsorbent dose on fluoride adsorption by fish bone charcoal (Bhargava et al 2008). The fluoride removal was dependent on both the dose of adsorbent and the contact time, at any given initial solute concentration. For any dose of the adsorbent, the fluoride removal increased with increasing contact times. The rate of increase of fluoride removal was more significant up to 240 min contact time, while the rate significantly decreased beyond 240 min till an equilibrium condition was attained in

17 33 about 540 min. A model was developed to predict the equilibrium fluoride concentration for any given initial fluoride concentration and the adsorbent dose. Bone charcoal obtained from thermal activation of bones of goat was investigated by Bandyopadhyay et al (2009) for effective remediation of fluoride contaminated water. The crushed dried bones were thermally activated and sieved to separate the material into discrete size ranges (2.36 mm to 1.18 mm) used as adsorbent for the purpose. Bone char had good adsorption capacity for fluoride and as such this adsorbent showed excellent removal of fluoride from water. The adsorption of fluoride on the surface of the adsorbent was found to depend on ph of the solution as well as the concentration of the adsorbate. The adsorption of fluoride at neutral ph was found to be higher than those at acidic or alkaline range. It was found that with the increase in initial fluoride concentration up to certain limit, the percentage removal of fluoride increased with the adsorbent dose of 50 g L -1, but the percentage removal of fluoride decreased at same adsorbent dose, when initial fluoride concentration exceeded beyond the limit. Experimental equilibrium adsorption data obtained through batch study fitted reasonably well to both Langmuir and Freundlich isotherm models. 2.8 VARIOUS FORMS OF ALUMINA FOR FLUORIDE REMOVAL Gamma Alumina Aluminium compounds are known to have high potential for removal of fluoride from water. Gamma alumina, a purest form of alumina was investigated by Rao and Karthikeyan (2008) to assess its sorptive removal capacity of fluoride from water employing a synthetic fluoride solution of 4 mg L -1. The adsorbent exhibited rapid and high uptake of fluoride under experimental conditions. The effects of various parameters like

18 34 contact time, ph, initial fluoride concentration and sorbent dose were investigated. The fluoride uptake by gamma alumina initially increased with increase in ph from 3.0 to 4.0 and the removal increased from 84 % to 98 % and remained fairly constant up to ph 7.0. The results indicated the usefulness of the sorbent over a wide range of ph that was normally encountered in real field conditions. The presence of anions, in general, had a negative influence on sorptive uptake of fluoride by gamma alumina. The order of effect of anions on uptake of fluoride by gamma alumina in general was carbonates > bicarbonates > sulphates > chlorides. Results of cyclic regeneration with alum indicated the potential usefulness of the sorbent for nearly 10 cycles without significant loss of sorption capacity Mesoporous Alumina Lee et al (2010) prepared two different kinds of Mesoporous Alumina (MA) samples using aluminium tri-sec-butoxide in the presence of either cetyltrimethyl ammonium bromide (MA-1) or stearic acid (MA-2) as structure directing agent, and tested for adsorptive removal of fluoride in water. Both materials contained a worm-hole like mesopore structure, but exhibited different textural properties. The effectiveness of these materials for removal of fluoride ions in aqueous solution was evaluated in batch adsorption experiments by measuring adsorption capacities and kinetic parameters. Measured equilibrium adsorption data were fitted to the Langmuir model, and kinetics data were fitted to a pseudo-second-order. Mesoporous alumina demonstrated superior adsorption performances to gamma alumina in both sorption capacity and initial sorption rates Activated Alumina Activated alumina is regarded as excellent material for fluoride removal. However, ph and alkalinity were the factors which affect

19 35 the sorption capacity. The exhausted material could be regenerated by various treatments. The effect of other ions present in drinking water, like chlorides, sulphates and carbonates, over the defluoridation efficiency of activated alumina was minimum,even though the presence of bicarbonate ions showed considerable influence in the process of defluoridation. It is a granular, highly porous material consisting essentially of aluminum trihydrate. It is widely used as a commercial desiccant and in many gas drying processes. The crystal structure of alumina contains cation lattice discontinuities giving rise to localized areas of positive charge. This makes alumina attract various anionic species. Alumina has a high preference for fluoride compared to other anionic species, and hence is an attractive adsorbent. It also does not shrink, swell, soften nor disintegrate when immersed in water. The activated alumina was proposed for the first time for defluoridation of water for domestic use in the 1930s. Since then, the use of activated alumina has become a popular defluoridation method. The maximum adsorption capacity of activated alumina for fluoride was found to be 3.6 mg F g -1 of alumina (Shrivastava and Vani 2009) Alum-Impregnated Activated Alumina The alum-impregnated activated alumina (AIAA) for removal of fluoride from water through adsorption has been investigated by Tripathy et al (2006). All the experiments were carried out by batch mode. The effect of various parameters viz. contact time, ph effect (ph 2-8), adsorbent dose ( g L -1 ), and initial fluoride concentration (1-35 mg L -1 ) has been investigated to determine the adsorption capacity of AIAA. The removal of fluoride increased with increase in ph up to 6.5 then decreased with the increasing ph. The optimum ph was found to be 6.5, which is suitable for the potable purpose. Kinetic study showed that removal of fluoride was found to be very rapid during the initial period, i.e. most of the fluoride was

20 36 removed during min and reached a maximum of 92 % at 3 h. Alum impregnated activated alumina could remove fluoride effectively (up to 0.2 mg L -1 ) from water containing 20 mg L -1 fluoride. 2.9 TRI-CALCIUM PHOSPHATE AS ADSORBENT FOR FLUORIDE REMOVAL He et al (1995) compared Hydroxyapatite (HAP) and Tricalcium phosphate (TCP) for fluoride removal. Hydroxyapatite is a natural material that is strong in fixing fluoride. An X-ray diffraction analysis proved that the densitometric tracings of tricalcium phosphates (TCP) are similar to those of HAP. The study examined and compared the efficiency of defluoridation from drinking water by TCP under various ph level, temperature and contact time conditions. The defluoridation mechanism has been discussed in detail. The results showed that there was a close negative correlation between the defluoridation efficiency of TCP and the ph levels of raw water, positive correlation between defluoridation efficiency and both the temperature and the contact time, suggested that the defluoridation mechanism of TCP could be a complex chemical reaction. Static defluoridation of high fluoride (10-12 ppm) water by sixteen different combinations of tricalcium phosphate (TCP), bone char (BC), hydroxyapatite (HAP), and related substances has been investigated by He and Cao (1996). The defluoridation efficiency of relatively insoluble calcium phosphates was, TCP (87.0 %) > HAP (68.0 %) > BC (66.4 %), when they were used singly in a batch procedure for 24 h under routine conditions. When optimal amounts of free phosphate were added together with BC or HAP to high-fluoride water, the removal of fluoride reached 95 %. The bone char (BC)-mono calcium phosphate (MCP) system seemed to be the best combination for removal of fluoride from drinking water. With 300 mg BC plus 23 mg MCP per 100 ml of water, fluoride was reduced over

21 37 24 h from 10.4 mg L -1 to 0.6 mg L -1 by co- precipitation in the ph range Addition of calcium did not improve the defluoridation efficiency of calcium phosphate. Observations of the gel structure of tri-calcium phosphate during the process of manufacture suggested the possibility of its use as an adsorbent for fluorides (Adler et al 1938). Test towers have been used to prove the effectiveness of granular tri-calcium phosphate for the removal of fluorides from natural fluoride bearing water as well as from synthetic waters. The effect of particle size on capacity as well as the effect of fluoride concentration, ph and total hardness-fluoride ratio was recorded. Capacity tests showed tri-calcium phosphate of -20 to +40 mesh sizes to have approximately twice the efficiency of activated alumina reported by Fink and Lindsay and investigated by Swope and Hess (1937) CHEMICALLY MODIFIED ION EXCHANGER RESINS FOR FLUORIDE REMOVAL Indion FR 10 resin had sulphonic acid functional group (H + form) possessed appreciable defluoridation capacity and its capacity has been enhanced by chemical modification into Na + and Al 3+ forms by loading respective metal ions in H + forms of resin (Viswanathan and Meenakshi, 2009). The defluoridation capacity of Na + and Al 3+ forms were found to be 445 and 478 mg F - kg -1, respectively, whereas the defluoridation capacity of H + form was 265 mg F - kg -1 at 10 mg L -1 initial fluoride concentration. The defluoridation capacity of these sorbents was found to be independent of ph of the medium and unaltered in the presence of co-anions present in the medium. Aluminium (III) form had higher defluoridation capacity among the sorbents studied. The mechanism of fluoride removal by the sorbents is mainly controlled by chemisorptions. The sorption process followed Freundlich, Langmuir and Redlich-Peterson isotherms. The values of

22 38 thermodynamic parameters indicated that the fluoride removal was spontaneous and endothermic in nature. The rate of reaction of all the forms was controlled by pseudo-second-order and particle diffusion kinetic models. Field trial results indicated that these sorbents can be effectively used to remove the fluoride from water. The best eluent for the registration of the sorbents was identified as 0.1 M HCl. Indion FR 10 is a commercially available ion exchange resin with sulphonic acid functionality named as H + form had appreciable defluoridation capacity (DC). It has been chemically modified to La 3+, Fe 3+, Ce 3+, and Zr 4+ forms by incorporating respective metal ions into the resin in order to know their fluoride selectivity by measuring the DC of the respective resin ( Viswanathan and Meenakshi, 2008). The maximum DC of these chemically modified ion exchange resins suggested their higher selectivity towards fluoride than H + form which had the DC of only 275 mg F - kg -1 at 11 mg L -1 initial fluoride concentration. It was concluded that all modified resins possessed higher DC which in turn indicated their affinity to fluoride than the original resin. The DC of these sorbents was not influenced by ph of the medium and was slightly influenced in the presence of co-anions except bicarbonate. The sorption process followed Langmuir isotherm. The kinetics of the modified resins followed pseudo-second-order. Solangi et al (2010) described a convenient method for the modification of Amberlite XAD-4 resin by introducing thio urea (ATU) binding sites onto the aromatic rings. The modified (ATU) resin has been employed for the quantitative sorption of fluoride ions in batch as well as column experiments. The parameters (i.e. ph, contact time, etc.) were optimized and desorption of fluoride ions was fulfilled by using 0.01M HCl solution. It has been noticed that the modified resin had high efficiency for the removal of fluoride from water at a wide range of ph mainly at ph 7. The

23 39 resin can be regenerated several times with 0.01M HCl and may be used as an ion exchange material in filters for the removal of fluoride from drinking water. The study was extended to evaluate the efficacy of the resin towards the real samples of drinking water from the Thar Desert of Pakistan with high fluoride content. Amberlite XAD-4 has been modified by introducing amino group onto the aromatic ring for its application in fluoride remediation (Solangi et al 2009). The characteristics of the modified resin were studied by FTIR and elemental analysis techniques. It has been observed that the modified resin was efficient for the removal of fluoride ion from aqueous solution at various phs particularly at 9 ph. It has also been found that the resin was effective even in the presence of other anions such as Br -, NO - 2, NO - 3, HCO - 2-3, SO 4 ions. It was found to be a suitable adsorbent and could be applied for the removal of fluoride ion from the drinking water of Thar Desert AIM AND SCOPE OF THE WORK Activated alumina is regarded as an excellent and widely used adsorbent for defluoridation of drinking water in municipal supplies due to its surface area, adsorption effect, highly porous structure and high degree of surface reactivity. The presence of common anions presents in drinking water, like chlorides, sulphates, and carbonates except bicarbonate have little effect on the defluoridation efficiency of activated alumina. The maximum adsorption capacity of activated alumina for fluoride was found to be 3.6 mg F g -1 of alumina (Shrivastava and Vani 2009). Tri-calcium phosphate is another adsorbent that has gel structure and better characteristics than that of activated alumina. Irrespective of the ph of the influent water the ph of the treated water will move towards the neutral value. In addition tri-calcium phosphate is found to have

24 40 approximately twice the efficiency of activated alumina for fluoride. But the adsorbent is sparingly used in water treatment for fluoride removal from drinking water. Both the adsorbents show increased adsorption capacity in the powder form as the surface area and consequently more number of active sites are exposed. However, the adsorbents in the powdered form are not suitable for continuous column operations as they produce pressure drop. It was decided to agglomerate the above powdered adsorbents into granular form so that the adsorbents could be profitably used in column operations without loss in their capacity for fluoride removal. A suitable method could be agglomeration using of a cheap, readily available and non-toxic polymer. Polyvinyl acetate could be a better choice as it is soluble in simple organic solvents but insoluble in aqueous medium. Apart from the preparation of granular adsorbents from powdered activated alumina and tri-calcium phosphate by agglomeration it was also decided to prepare a superior adsorbent for fluoride removal. The better choice would be to chemically modify readily available granular ion exchange polymer resin so that its physical characteristics could be retained, simultaneously improving its capacity, ph tolerance and selectivity towards fluoride in the presence of common anions. Objectives: 1. There are no reports available with respect to agglomeration of adsorbents using polyvinyl acetate. 2. The present study is aimed at preparing agglomerated granular activated alumina and tri-calcium phosphate and evaluating the same in fluoride removal from aqueous solution.

25 41 3. Batch adsorption studies are proposed to optimize equilibration time, ph conditions and adsorbents dose for maximum removal of fluoride. Adsorption isotherm studies are used to characterize the equilibrium between the amount of adsorbate that accumulated on the adsorbent and the concentration of the adsorbate and to calculate adsorption capacity of the adsorbents. 4. The kinetic models namely pseudo-first order and pseudosecond order equations will be used to test the experimental data to examine the adsorption kinetics. 5. Column studies will be carried out in teflon column of 70 cm length and 1.5 cm dia. Optimum flow rate, bed height for maximum adsorption will be found out. Effect of common anions on the adsorption capacity of the adsorbents will be studied. 6. Regeneration studies are aimed to be done to find suitable regenerant and evaluate the concentration of the regenerant used for regeneration and number of cycles of operation the adsorbents could be subjected to. 7. SEM images of the adsorbent will be recorded to examine the morphology before and after adsorption. FT-IR spectroscopy analysis will be done on plain and chemically modified resin to analyse the functional groups that were originally present in the plain resin and the groups that have been replaced or newly introduced after chemical modification. 8. The performance of the three adsorbents will be compared to find out best among the three for applications in defluoridation techniques.