Synthesis and Characterization of Iron (III)-Coordinated Amine

Oct 27, 2010 - ... shell: Equilibrium isotherm and kinetic studies. Equbal Ahmad Khan , Shahjahan , Tabrez Alam Khan. Journal of Molecular Liquids 201...
0 downloads 0 Views 2MB Size
Download PDF
12254

Ind. Eng. Chem. Res. 2010, 49, 12254–12262

Synthesis and Characterization of Iron(III)-Coordinated Amine-Modified Poly(glycidylmethacrylate)-Grafted Densified Cellulose and Its Applicability in Defluoridation from Industry Effluents Thayyath S. Anirudhan* and Padmajan S. Suchithra Department of Chemistry, UniVersity of Kerala, KariaVattom, TriVandrum 695 581, India

In this study, the development and characterization of a novel adsorbent, iron(III)-coordinated amine-modified poly(glycidylmethacrylate)-grafted densified cellulose (AM-Fe-PGDC), for the removal and recovery of fluoride ions from aqueous solutions is reported. The adsorbent was characterized by FTIR, X-ray diffraction, SEM, TG/DTG, surface area analyzer, and potentiometric titration. The effect of process parameters such as agitation time, concentration, pH, ionic strength, adsorbent dose, and temperature on the extent of fluoride adsorption was investigated. The adsorbent exhibits very high adsorption potential (>99.9%) at an optimum equilibrium pH 7.0. The nonlinear form of a pseudo second-order kinetic model and Langmuir isotherm model adequately described the experimental kinetic and equilibrium data. The thermodynamic parameters showed that the adsorption of fluoride onto AM-Fe-PGDC was feasible, spontaneous, and exothermic. Adsorption experiments were also conducted using a commercial anion exchanger, Duolite-A7, for comparison. Utility of the adsorbent was tested with a simulated industry wastewater sample. Adsorbed fluoride ions were desorbed effectively by 0.1 M HCl. Introduction As a beneficiary element in the human body for dental and bone health, fluoride in low concentrations is an essential nutrient in the human diet. Yet in high concentrations, it may lead to many health issues, such as neurological damage, fluorosis, osteoporosis, arthritis, cancer, brain damage, alzheimer syndrome, and thyroid disorder.1 Fluoride is the important chemical that causes large-scale health problems through drinking water exposure. While the current USEPA drinking water maximum fluoride contaminant level is 4.0 mg/L, the World Health Organization has set the guidance value as 1.5 mg/L.2 The presence of fluoride in drinking water above the permissible level and associated increased incidence of fluorosis among the people have been reported from all over the world including China, India, Pakistan, and Thailand.3 Recently, defluoridation from groundwater and wastewater has been paid more attention in some literature by different technologies like adsorption, precipitation, ion exchange, electrodialysis, reverse osmosis, and nanofiltration.4 Among these methods, adsorption is the most convenient, relatively simple, effective, economical, and appropriate for drinking water treatment, especially for small communities. Several conventional and nonconventional adsorbents like activated alumina, Fe/Al mixed hydroxides, ionexchange resin, limestone, polymer/alumina composites, solid industrial wastes like red mud, fly ash, and spent catalysts were studied for their fluoride adsorption capacities.5–9Graft copolymers based on natural polymer cellulose are finding increased use as adsorbents for the removal of metal and nonmetal ions and other toxic materials from water systems. Graft copolymers of cellulosic material have such advantages as chemical resistance, radiation stability, and low cost of production over conventional ion exchange. Lei et al.10 proved that the derivatives of custom assembled matrix such as TiO2 densified cellulose beads can be used to function as an anion exchanger by possessing a significant mechanical strength, stable hydro* To whom correspondence should be addressed. Tel.: +91 471 2418782. E-mail: [email protected].

dynamics, and comparable protein break through capacities. Because of the influence of the TiO2 (densifier), there is an increase in some of the properties of the adsorbent such as density, water content, volume shrinkage percentage, porosity, pore volume, pore radius, specific surface area, and mechanical strength.11 Embedding TiO2 on cellulose would affect the crystalline structure, making it more suitable for activation reaction.12 Polymers with ligands capable of coordinating with metals ions also have the potential to clean up wastewater and the recovery of metals. Munoz et al. succeeded in the preparation of iron loaded sponge for the adsorption of arsenic(V).13 The use of modified materials such as La(III)-loaded chelating resins and Na- and Al(III)-loaded Indion FR10 ion exchange resins have been reported to remove fluoride ions from wastewater.14,15 Moreover, in the natural environment, fluoride is strongly adsorbed onto surfaces of many aluminum and iron-containing minerals. Hence, iron-based adsorbents such as goethite, hydrous ferric oxide, magnetite, and granular ferric hydroxide have been developed and tested by some researchers in recent years to remove fluoride.4 In this work, we have prepared a novel adsorbent iron(III)-coordinated amine-modified poly(glycidylmethacrylate)-grafted TiO2 densified cellulose by means of simple graft copolymerization reaction followed by treatment with dimethylamine and iron(III) loading to utilize it for the effective removal of fluoride ions from aqueous solutions. Experimental Section Materials. All chemicals were analytical grade, and all stock solutions were prepared with distilled water. Fluoride stock solution was prepared using sodium fluoride obtained from E. Merck, India Ltd. Chlorobenzene, ferric chloride, sodium hydroxide, sodium chloride, sodium sulfate, sodium nitrate, and sodium carbonate were obtained from E. Merck, India Ltd. Reagent grade cellulose, carbon disulfide (CS2), methanol, benzene, glycidylmethacrylate (GMA), N,N′-methylenebisacrylamide (MBA), benzoyl peroxide (Bz2O2), polyvinyl alcohol (PVA), cyclohexane, alizarin complexone, sodium acetate,

10.1021/ie100809f  2010 American Chemical Society Published on Web 10/27/2010

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010 Scheme 1. Preparatory Route of AM-Fe-PGDC

lanthanum nitrate, butanol, acetone, acetic acid, and an anion exchanger Duolite-A7 (as a reference material) were purchased from Aldrich and were used as received. Preparation of Adsorbent. The cellulose (Cell) was first converted into cellulose xanthate. For that, 20 g of NaOH and 10 mL of CS2 were allowed to react with 40 g of cellulose. The product was then added to a mixture of 6.0% NaOH and 3.0 g of rutile (TiO2). This mixture was dispersed in 100 mL of chlorobenzene containing 100 mL of pump oil for 2 h at 90 °C. The resulting particles were washed successively using benzene and methanol. The decomposition of cellulose xanthate was completed by stirring in a solution of 50 mL of acetic acid and 100 mL of ethanol. After being washed with distilled water, the product densified cellulose (DC) was dried at 60 °C for 6 h. In an earlier work concerning the densification of cellulose using TiO2, researchers reported that the electrostatic interactions play an important role in immobilizing TiO2 into the cellulose matrix.12 Because the TiO2 particles are embedded in the internal matrix of cellulose, hydroxyl methyl groups of the cellulose skeleton retained for the interaction of GMA/MBA. Poly(glycidylmethacrylate)-grafted DC was prepared according to the procedure reported in our earlier publication.16 The preparation procedure of the adsorbent is presented in Scheme 1. To a suspension of DC (10 g in 100 mL of water) were added 9.6 mL of GMA and 0.8 g of MBA, and the reaction mixture was thermostatted at 30 °C in a thermostatted water bath after vigorous stirring. After 30 min, 0.1 g of Bz2O2 initiator was added followed by a mixture of isopropyl alcohol and cyclohexane in the ratio 1:12 (v/v). About 75 mL of 1% PVA was added, and the mixture was stirred at 80 °C for 3 h. The poly(GMA) grafted DC (PGDC) was separated from the reaction mixture and washed with methanol and dried in air. The percentage grafting (Pg) and grafting efficiency (%GE) were calculated by the following relationships given below: Pg ) weight of graft copolymer - weight of polymer backbone × 100 weight of polymer backbone

(1) %GE ) weight of graft copolymer - weight of polymer backbone × 100 weight of monomer charged

(2) Amino groups were introduced into the epoxy ring of the product by reaction with 150 mL of 40.0% dimethylamine solution at 70-80 °C for 4 h. The amine-modified PGDC (AMPGDC) was washed with distilled water and dried. Optimum reaction conditions for maximum loading of iron(III) on AMPGDC were obtained by batch adsorption isotherm experiments.

12255

0.1 g of AM-PGDC was placed in a 100 mL conical flask with glass stopper having 50 mL of iron(III) solution of different concentrations (25-400 mg/L), and the mixture was shaken in a shaker at 200 rpm. After 4 h of equilibrium, the suspension was sampled and centrifuged, and the supernatant was analyzed for iron(III) using a atomic absorption spectrophotometer (AAS). The effect of pH on iron(III) loading from iron(III) solutions was also studied by varying pH values (1.0-3.0). The uptakes of iron(III) were calculated from the decrease of iron(III) concentrations relative to those of initial concentration. The prepared iron(III) coordinated amine-modified PGDC (AM-FePGDC) was filtered, washed with distilled water, air-dried, and sieved into particle size with the average diameter 0.096 mm represented as (80-230) mesh, using standard sieves and kept in a desiccator. Adsorption Experiments. A stock solution of fluoride ions containing 1000 mg/L was prepared by dissolving an appropriate amount of NaF. Working standard solutions are prepared from stock solution by diluting it with deionized water. Batch experiments were conducted to study the extent of adsorption of fluoride on AM-Fe-PGDC under varying conditions. In general, 0.1 g of the adsorbent was combined with 50 mL of fluoride solution in 100 mL conical flasks. Initial pH of the suspension was adjusted using 0.1 M HCl and 0.1 M NaOH. The mixture was shaken in a water bath shaker at 200 rpm for 4 h and then centrifuged at 4000 rpm for 15 min. The supernatant solutions were analyzed for equilibrium fluoride concentration using spectrophotometric method-based colored complex with alizarin complexanone reagent in acetate buffer medium. The adsorbance of the colored complex was measured at 620 mm against a reagent blank.17 The amount of adsorbed fluoride was determined from the deference between fluoride concentrations before and after the adsorption procedure. Adsorption of fluoride on AM-Fe-PGADC was studied over the pH range 2.0-9.0 using 0.1 g adsorbent in 50 mL of 5 and 10 mg/L fluoride solutions at 30 °C. Batch experiments were conducted to determine the effect of adsorbent dose (0.05-0.5 mg) on fluoride removal by AM-Fe-PGDC under similar conditions. Kinetic study was carried out by using four different concentrations of fluoride solutions (10, 20, 30, and 40 mg/L). For the adsorption isotherm experiments, the initial solution pH was 7.0, while the initial F- concentration in the solution varied between 10 and 300 mg/L. To keep constant equilibrium condition, the pH of the fluoride solution is maintained at pH 7.0 for the entire contact time using known concentrations of NaOH and HCl solutions. The effect of ionic strength on the adsorption of F- was studied with different ionic strengths ranging from 0.001 to 0.1 M NaCl. To justify the validity of the Am-Fe-PGDC as an anion exchanger for fluoride adsorption, its adsorption capacity is compared to a commercial chloride form anion exchanger Duolite-A7. Also, the efficiency of AmFe-PGDC in the removal of fluoride was studied using industrial wastewater by varying the dose of adsorbent (0.05-0.35 g/L). Recovery of adsorbate and regeneration of adsorbent are the critical cost controlling process in wastewater treatment. The adsorbent loaded with fluoride was placed in the desorption medium (0.1 M NaOH, 0.1 M Na2CO3, 0.1 M NaNO3, 0.1 M NaCl, 0.1 M Na2SO4, and 0.1 M HCl solutions) and stirred at 200 rpm for 2 h at room temperature. The suspension was centrifuged, and the concentration of fluoride in the supernatant was measured. The amount of fluoride desorbed was calculated from the amount of fluoride adsorbed on the adsorbent and the final fluoride concentration in the desorption medium.

12256

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010

Table 1. Effect of Particle Size of DC on Grafting particle size (mm)

Pg (%)

GE (%)

0.595 0.250 0.125 0.105 0.096

10.75 22.54 34.30 42.17 48.20

9.10 19.04 28.95 35.60 40.73

Analysis of the Data. Each data point was taken as the average of three measurements with standard deviation of 3.0%. When the relative error exceeded 3.0%, the data were discarded, and a new experiment was conducted until the relative error falls within an acceptable range. The values of the kinetic and isotherm parameters were determined by a nonlinear regression analysis using ORIGIN program (version 7.5). Instrumentation. The FTIR spectra of the adsorbents were recorded on a Schimadzu FTIR spectrophotometer using KBr pellets. X-ray diffraction (XRD) patterns of the adsorbents were carried out on Siemens D 5005 X-ray unit using Ni-filtered Cu K∞ radiation. Scanning electrone micrographs (SEM) were obtained using a Polaron SC 7620 scanning electron microscope operated at 12 kV. A Metler Toledo thermal analyzer was used to study the thermal stability of the adsorbents. Density of the adsorbents was calculated by using nitrobenzene as displacing liquid using specific gravity bottle. The point of zero charge (pHpzc) of the adsorbent was determined by the potentiometric titration method.18 The amount of amino group and anion exchange capacity were determined using standard methods.19 A Systronic microprocessor pH meter (model µ-362) was used for pH measurements. A temperature-controlled water bath flask shaker (Lab line, India) was used for shaking all the solutions. The concentration of fluoride ions in aqueous solutions was determined using a UV-visible spectrophotometer (Jasco model V-530). A GBC Avanta (A 5450) AAS was used to determine iron(III) ions in aqueous solutions. Results and Discussion Effect of Particle Size of DC on Grafting. Because the particle size influences the grafting, the particle size is one of the important factors influencing the grafting of DC with GMA/ MBA. The grafting percentage and efficiency as a function of particle size of DC were examined over a particle size range of 0.096-0.595 mm. Table 1 shows that the particle size had a strong influence on GMA/MBA grafting onto DC. The grafting efficiency increases overall with an expected increase in surface area and an increase in the hydroxyl methyl group in the cellulose skeleton, which gets oxidized and becomes more susceptible to form radicals for GMA interaction. Iron Loading Efficiency. To optimize the pH for maximum iron loading in AM-PGDC, adsorption experiments were carried out by varying the solution pH over the range 1.5-3.5 with an initial iron(III) concentration of 10 mg/L. The results show that the quantity of iron(III) loaded AM-PGDC increases gradually between pH 1.0 and 3.0. Maximum iron(III) loading (>99.0%) was allowed at an initial solution pH 3.0 and decreased thereafter. This may due to the formation of stable iron(III) hydrolysis complexes such as Fe(OH)3 and Fe(OH)4- above pH 3.0, which deactivate the iron adsorption sites of the adsorbent. Figure 1 shows the adsorption of iron(III) ions onto AMPGDC of different particle size. As shown in the figure, smaller particle size provides greater adsorption of iron(III) ions onto AM-PGDC. This may be due to the fact that smaller size results in greater pressure drop, shortened mass transfer zone, and hence provides quicker rate of adsorption. As shown in Figure 1, in

Figure 1. Adsorption of iron(III) onto AM-PGDC of different particle size.

the low concentration range, the amount of iron(III) adsorbed on AM-PGDC sharply increased with an increase in equilibrium concentration, giving an indication of the high affinity of the binding sites for iron(III) ions. At high concentrations, the increase in quantities adsorbed is gradual as a result of an almost complete utilization of the active sites. The maximum loading efficiency of iron(III) onto AM-PGDC was found to be 50.81 mg/g. It can be assumed that the iron cations coordinated the -CH(OH)-CH2NH(CH3)2 group in the interstices of the AMPGDC within the micropores (Scheme 1). Adsorbent Characterization. SEM is becoming a more popular tool for research, because it offers a versatile approach to observe the surface morphology of adsorbents. When employing the SEM, the proper preparation of the sample for the observation is needed; nonconductive specimens tend to charge when scanned by the electron beam, and especially in secondary electron imaging mode, this causes scanning faults and other image artifacts. They are therefore usually coated with an ultrathin coating of electrically conducting material, commonly gold, deposited on the sample either by low vacuum sputter coating or by high vacuum evaporation. Coating prevents the accumulation of electric charge on the specimen during electron irradiation. In this work, the samples were placed in a chamber with applied vacuum and were sputtered with a thin layer of gold/palladium (Au/Pd) using the following conditions: voltage, 500 V; current, 10 mA; pressure, 200 mT; and time, 2 min. As shown in Figure 2, the surface morphologies of cell, AM-PGDC, and AM-Fe-PGDC are different from each other. It is observed that the plain cellulose surface is distinct flake, rough, and uneven due to intermolecular forces associated within the structure because of the strong intramolecular hydrogen bonds. For AM-PGDC, poly (GMA) grafting gives a smooth surface with visible micropores that can be attributed to the better adhesion of poly (GMA) to the surface due to its hydrophobic nature. The formation of micro pores, which may result from cross-linking caused by the opening of the epoxide ring of GMA, causes an increase in surface area and provides proof of grafting. The AM-Fe-PGDC surface exhibited a distinct fluffy appearance due to the incorporation of Fe(III) ions. The observed structural changes will positively affect the adsorption capacity of AM- Fe-PGDC. The XRD patterns of cell, DC, AM-PGDC, and AM-FePGDC are shown in Figure 3. In the XRD pattern of cell, the diffraction maxima at 2θ ) 22.4° with a d-value of 3.96 Å, which is assigned to the 002 crystalline plane. There are three more low intensity peaks in the XRD pattern of cell at 2θ positions 14.7°, 16.6°, and 33.9° with d-spacing of 6.02, 5.33, and 2.64 Å, respectively, which indicates the partial crystalline nature of cellulose, like all natural polymers. That the strong diffraction peaks at 2θ ) 27.3°, 36.1°, 39.2°, 41.2°, 44.0°, 54.4°, and 56.7° in the XRD pattern of DC can be indexed to the (110),

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010

12257

Figure 4. FTIR spectra of cell, DC, AM-PGDC, and AM-Fe-PGDC.

Figure 2. SEM images of cell, AM-PGDC, and AM-Fe-PGDC.

Figure 3. XRD spectra of cell, DC, AM-PGDC, and AM-Fe-PGDC.

(101), (200), (111), (210), (211), and (220) crystal planes of rutile TiO2 (PDF card 75-1755, JCPDS) indicates the presence of TiO2 as denser material for modification.20 The high intensity diffraction peaks at 30.0°, 36.0°, 39.1°, 41.2°, 44.0°, 54.3°, 56.5°, 62.7°, 64.0°, 68.9°, and 69.1° with d-spacing of 2.97, 2.49, 2.30, 2.19, 2.05, 1.68, 1.62, 1.48, 1.45, 1.36, and 1.35 Å, respectively, for AM-PGDC indicate that crystallinity increases after modification. This increase in crystallinity with graft

copolymerization and subsequent amination might be due to incorporation of bulkier group within the polymeric network, which in turn increases inter- and extra-molecular hydrogen bonding. Most of the diffraction peaks typical of TiO2 are visible in the XRD profile of AM-PGDC, thus confirming the stability of the adsorbent upon graft copolymerization followed by functionalization. The new peaks centered at 2θ ) 26.1° and 33.3° in the spectrum of AM-Fe-PGDC indicate the presence of iron. The FTIR spectra of cell, DC, AM-PGDC, and AM-Fe-PGDC are shown in Figure 4. As shown in the figure, all the samples share some common peaks of which the most predominant are 3424, 2926, 1461, and 891 cm-1 due to the hydrogen-bonded O-H stretching, C-H stretching, C-H bending, and the presence of glucosidic ring of cellulose, respectively. The peak appearing at 2846 cm-1 in the spectrum of DC, AM-PGDC, and AM-Fe-PGDC is attributed to the presence of Ti-O linkage. Moreover, the peaks at 721 and 632 cm-1 can be attributed to symmetric O-Ti-O stretching.21 The broad peak at 3424 cm-1, in the spectrum of AM-PGDC and AM-Fe-PGDC, may also be due to the overlapping of different vibrations of N-H and C-H groups. The peaks located at 1375, 1261, and 1155 cm-1 present in the spectrum of AM-PGDC and AM-Fe-PGDC are assigned to the C-H symmetric deformation of -CH2 group and the symmetric C-O-C stretching vibrations, respectively. The C-O stretching vibration of -C-OH group in cellulose at 1059 cm-1 shifts to 1021 cm-1 in AM-PGDC. Moreover, the peaks at 2770, 1630, 1460, and 650 cm-1 in the spectrum of AM-PGDC and AM-Fe-PGDC are due to NR3+ stretching, NR3+ asymmetric bending, the C-N stretching of -CH2N+HR2 group, and the methylene dioxy-C-O stretching, respectively. These data clearly indicate the presence of grafting. Also, the C-O-C stretching is observed as an overtone at 593 cm-1. Some overtones of β bonds in heterogeneous regions of cellulose are also observed in 400-500 cm-1 range. In AMFe-PGDC, a weak shoulder at around 1020 cm-1 is attributed to the stretching vibration of the Fe-N bond. TG/DTG curves for cell, DC, AM-PGDC, and AM-Fe-PGDC are shown in Figure 5. The TG curve of cell is characterized by two different temperature zones: (1) 240-330 °C and (2)

12258

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010

Figure 6. Effect of adsorbent dose for the adsorption of fluoride ions onto cell, AM-PGDC, and AM-Fe-PGDC.

Figure 5. TG/DTG curves of cell, DC, AM-PGDC, and AM-Fe-PGDC.

330-500 °C. At the initial stage of decomposition (T1 ) 290 °C), almost 61.1% is lost due to release of water followed by the breaking of glucosidic bond and carbon-carbon bond into a series of hydrocarbons, alcohols, aldehydes, acids, and others. At the second stage of decomposition (T2 ) 430 °C), about 94.8% of the initial dry weight is lost due to the decomposition of molecular compounds formed during the initial stage into large amounts of volatiles and solid char. The TG and DTG curves of DC show the decomposition and weight loss at two different stages. The first stage comes between 280 and 380 °C and the second between 460 and 520 °C. The weight loss at the first stage (T1 ) 332 °C) is about 42.0% due to splitting of cellulose structure and in the second stage decomposition (T2 ) 467 °C) of about 49.5% weight was lost leaving behind TiO2 residue and char. For AM-PGDC, the first stage of decomposition is in the range 280-420 °C and the second is between 480 and 640 °C. The weight loss at the first stage (T1 ) 337 °C) is about 43.8% due to splitting of cellulose structure and main chain scission, and in the second stage decomposition (T2 ) 533 °C) of about 55.3% weight was lost due to the pyrolytic depolymerization process. For AM-Fe-PGDC, the first stage of decomposition is in the range 280-360 °C and the second is between 360 and 580 °C. The weight loss at the first stage (T1 ) 320 °C) is about 38.2%, which may be due the pyrolytic decomposition of GMA polymer that was grafted on cellulose, and in the second stage of decomposition (T2 ) 460 °C) about

86.2% weight was lost. The thermal stability of AM-Fe-PGDC is greater when it is compared to cell, DC, and AM-PGDC. The values of pHzpc for cell, DC, AM-PGDC, and AM-FePGDC were found to be 3.0, 4.9, 6.2, and 7.8, respectively. The increase in pHzpc after modification indicates that the surface becomes more positive, which favors the adsorption of negatively charged ions on the adsorbent surface through electrostatic interaction. The values of BET surface area for cell, DC, AMPGDC, and AM-Fe-PGDC are 21.7, 14.6, 29.3, and 33.9 m2/g, respectively. The amounts of amino group and anion exchange capacity for AM-Fe-PGDC were found to be 1.98 and 1.69 mequiv/g, respectively. Effect of Adsorbent Dose. The effect of variation in adsorbent dose on the removal of fluoride from 50 mL aqueous solutions by cell, AM-PGDC, and AM-Fe-PGDC is shown in Figure 6. From the figure, it can be seen as the adsorbent concentration increases, the amount adsorbed increases for cell and AM-FePGDC, whereas AM-PGDC showed very little adsorption ( 1). The calculated RL values between 0.001 and 0.075 in the concentration range 10-300 mg/L and at different temperature conditions from 20 to 50 °C (data not given) indicate favorable adsorption of fluoride ions onto AMFe-PGDC. It can also be assumed that all the adsorption sites have equal affinities for molecules of the adsorbate and that the presence of adsorbed molecules at one site will not affect the adsorption of molecules at adjacent sites. The apparent equilibrium constant (Ko) value, which indicates the ability of adsorbent toward the adsorbate species, was calculated from the product of the Langmuir constants Qo and b,28 which increases with decrease in temperature. This suggests an increase in affinity of fluoride toward AM-Fe-PGDC surface at low temperatures. The performance of many adsorbent materials for the removal of fluoride from the aqueous solutions was reported in the literature. The maximum adsorption capacity Qo values of the adsorption of fluoride onto activated alumina,5 La-incorporated chitosan beads,29 iron(III)-tin(IV) mixed oxide,30 KMnO4modified activated carbon,31 hydrous ferric oxide,32 and crystalline Fe/Al oxides33 were reported to be 2.41, 4.72, 10.51, 15.90, 16.51, and 17.72 mg/g, respectively. Although a direct comparison is difficult due to the different physicochemical environments, the adsorption capacity of AM-Fe-PGDC of the present study (19.39 mg/g) is higher than the adsorbents previously reported for the uptake of fluoride from its aqueous solutions. Adsorption Thermodynamics. The exothermic nature of the adsorption of fluoride onto AM-Fe-PGDC can be explained on the basis of thermodynamic parameters such as change in Gibbs free energy [∆Go], enthalpy [∆Ho], and entropy [∆So], which are defined as the following equations for adsorptive reactions: ∆G0 ) -RT ln b ln b )

∆H0 ∆S0 R RT

(11) (12)

where R is the gas constant, and T is the temperature on the absolute scale. From the slope and intercept of the linear plot of ln b versus 1/T (figure not shown), the values of ∆Go, ∆Ho, and ∆So were calculated. The ∆Go values were found to be negative, indicating that the adsorption process is spontaneous in nature, and they varied from -30.1 to -32.9 kJ/mol in the temperature range 20-50 °C. The negative value of ∆H0 (-2.373 kJ/mol) indicates that the fluoride adsorption process is exothermic in nature. In an exothermic process, the total energy absorbed in bond breaking is less than the total energy released in bond making between fluoride and AM-Fe-PGDC, resulting in the release of extra energy in the form of heat. The positive value of ∆S° (94.680 J/K/mol) reflects the affinity of the AM-Fe-PGDC for the fluoride ions and confirms the increased randomness at the solid/solution interface during the adsorption of fluoride ions onto AM-Fe-PGDC surface. The results also showed that ∆Ho < T∆So at all temperatures. This indicates that the adsorption process is dominated by entropic rather than enthalpic changes. Desorption Studies. Desorption is an important process in adsorption studies. Desorption was done with 0.1 M NaOH,

0.1 M Na2CO3, 0.1 M NaNO3, 0.1 M NaCl, 0.1 M Na2SO4, and 0.1 M HCl solutions, which showed desorption efficiency of 73.8%, 48.1%, 27.8%, 53.1%, 83.5%, and 95.9%, respectively. The best results were observed in the case of 0.1 M HCl. The high desorption on acidic medium indeed reveals that the adsorption of fluoride ions onto AM-Fe-PGDC is purely electrostatic in nature. This means that HCl solution breaks down the interaction forces between fluoride ions and binding sites onto the surface of the AM-Fe-PGDC. Comparison with Commercial Adsorbent Duolite-A7. The validity of AM-Fe-PGDC as an anion exchanger for fluoride adsorption was justified by comparing its adsorption capacity with Duolite-A7, a commercial chloride form anion exchanger. It has an amine functionality and an anion exchange capacity of 13.90 mequiv/g. The isotherm data obtained with DuoliteA7 with different concentrations of fluoride ranging from 10.0 to 300.0 mg/L at 30 °C are shown in Figure 11. The calculated values of Langmuir parameters Qo and b were found to be 17.92 mg/g and 0.094 L/mg, respectively, which were comparable to those obtained from the AM-Fe-PGDC (Qo ) 19.39 mg/g and b ) 0.105 L/mg). The adsorption capacity of AM-Fe-PGDC is found to be fairly good. The economic viability of the adsorption process for the removal of fluoride ions from water and wastewater depends on the cost effectiveness as well as the availability of adsorbents. Duolite-A7, used in the present study for comparison, is available at a rate of about U.S. $31,000 tonnes-1. The precursor used in the present study cellulose was obtained at rate of about U.S. $4197 tonnes-1. After considering the expense for chemicals, electrical energy, and man power, the final developed adsorbent, AM-Fe-PGDC, would cost approximately U.S. $27,000 tonnes-1. Comparative cost and adsorption capacity of AM-Fe-PGDC with Duolite-A7 suggest that AM-Fe-PGDC can be used as an efficient adsorbent for the treatment of fluoridebased wastewaters. Test with Simulated Industry Wastewater. The adsorbent AM-Fe-PGDC was treated with simulated wastewater containing fluoride ions. The simulated fluoride industry wastewater sample34 contained, apart from fluoride (4.0 mg/L), other ions such as Mg2+ (12.5 mg/L), Ca2+ (75.0 mg/L), Na+ (105.0 mg/ L), K+ (10.0 mg/L), SO42- (90.0 mg/L), HCO3- (275.0 mg/L), Cl- (130.0 mg/L), and NO3- (25.0 mg/L). The experimental results for the influence of adsorbent dose on fluoride ions

Figure 11. Comparison of adsorption isotherms of the commercial adsorbent, Duolite, and AM-Fe-PGDC.

12262

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010

removal from wastewater show that for quantitative removal (>99.9%) of fluoride in 50 mL of simulated wastewater containing 4.0 mg/L of fluoride and several other ions, an adsorbent dosage of 4.0 g/L is sufficient, which is in accordance with the results obtained for batch adsorption studies. Conclusions In this study, a new adsorbent iron(III)-coordinated aminofunctionalized densified cellulose (AM-Fe-PGDC) was synthesized, and the adsorption performance of fluoride ions by AMFe-PGDC was investigated. The effects of initial fluoride concentration, agitation time, pH, adsorbent dose, ionic strength, and temperature on the removal of fluoride were studied. The maximum fluoride removal was observed at the initial optimum pH 7.0. The adsorption data were found to follow pseudo second-order kinetics. Langmuir, Freundlich, and RedlichPeterson equations were applied to the isotherm data, and the Langmuir model was found to be in better correlation with the experimental data. The maximum adsorption capacity for fluoride was found to be 19.39 mg/g. The computation of the parameters ∆H0, ∆S0, and ∆G0 indicated that the adsorption of fluoride with AM-Fe-PGDC was thermodynamically favorable and exothermic. Regeneration of the spent adsorbent was easily performed with 0.1 M HCl. The adsorption capacity of AMFe-PGDC for fluoride was found to be comparable to that of the commercial anion exchanger Duolite-A7. The adsorbent AM-Fe-PGDC was tested successfully with simulated wastewater containing fluoride ions. Acknowledgment We are grateful to the Professor and Head, Department of Chemistry, University of Kerala, Trivandrum, for providing laboratory facilities for this work. P.S.S. is thankful to CSIR, Government of India, New Delhi, India, for the award of a Senior Research Fellowship. Literature Cited (1) Mandal, S.; Mayadevi, S. Adsorption of Fluoride Ions by Zn-Al Layered Double Hydroxides. Appl. Clay Sci. 2008, 40, 54–62. (2) Tang, Y.; Guan, X.; Su, T.; Gao, N.; Wang, J. Fluoride Adsorption onto Activated Alumina: Modeling the Effects of pH and Some Competing Ions. Colloids Surf., A 2009, 337, 33–38. (3) Reardon, E. I.; Wang, Y. A Limestone Reactor for Fluoride Removal from Wastewaters. EnViron. Sci. Technol. 2000, 34, 3247–3253. (4) Tang, Y.; Guan, X.; Wang, J.; Gao, N.; McPhail, M. R.; Chusuei, C. C. Fluoride Adsorption onto Granular Ferric Hydroxide: Effects of Ionic Strength, pH, Surface Loading, and Major Co-Existing Anions. J. Hazard. Mater. 2009, 171, 774–779. (5) Ghorai, S.; Pant, K. K. Equilibrium, Kinetics and Breakthrough Studies for Adsorption of Fluoride on Activated Alumina. Sep. Purif. Technol. 2005, 42, 265–271. (6) Onyango, M. S.; Kojima, Y.; Aoyi, O.; Bernardo, E. C.; Matsuda, H. Adsorption Equilibrium Modeling and Solution Chemistry Dependence of Fluoride Removal from Water by Trivalent-Cation Exchanged Zeolite F-9. J. Colloid Interface Sci. 2004, 279, 341–350. (7) Chaturvedi, A. K.; Yadava, K. P.; Pathak, K. C.; Singh, V. N. Defluoridation of Water by Adsorption on Fly Ash. Water, Air, Soil Pollut. 1990, 49, 41–69. (8) Lai, Y. D.; Liu, J. C. Fluoride Removal from Water with Spent Catalyst. Sep. Sci. Technol. 1996, 31, 2791–2803. (9) Cengeloglu, Y.; Kir, E.; Ersoz, M. Removal of Fluoride from Aqueous Solution by Using Red Mud. Sep. Purif. Technol. 2002, 28, 81– 86. (10) Lei, Y. L.; Lin, D. Q.; Yao, S. J.; Zhu, Z. Q. Preparation of an Anion Exchanger Based on TiO2-Densified Cellulose Beads for Expanded Bed Adsorption. React. Funct. Polym. 2005, 62, 169–177.

(11) Miao, Z. J.; Lin, D. Q.; Yao, S. J. Preparation and Characterization of Cellulose-Stainless Steel Powder Composite Particles Customized for Expanded Bed Application. Ind. Eng. Chem. Res. 2005, 44, 8218–8222. (12) Lei, Y. L.; Lui, D. Q.; Yao, S. J.; Zhu, Z. Q. Preparation of an Anion Exchanger Based on TiO2-Densified Cellulose Beads for Expanded Bed Adsorption. React. Funct. Polym. 2005, 62, 169–197. (13) Munoz, J. A.; Gonzalo, A.; Valeinte, M. Kinetic and Dynamic Aspects of Arsenic Adsorption by Fe(III)-Loaded Sponge. J. Solution Chem. 2008, 37, 553–565. (14) Fang, L.; Ghimire, K. N.; Kuriyama, M.; Inoue, K.; Makino, K. Removal of Fluoride Using Some Lanthanum(III)-Loaded Adsorbents with Different Functional Groups and Polymer Matrices. J. Chem. Technol. Biotechnol. 2003, 78, 1038–1047. (15) Viswanathan, N.; Meenakshi, S. Role of Metal Ion Incorporation in Ion Exchange Resin on the Selectivity of Fluoride. J. Hazard. Mater. 2009, 162, 920–930. (16) Anirudhan, T. S.; Jalajamany, S.; Suchithra, P. S. Improved Performance of a Cellulose-Based Anion Exchanger with Tertiary Amine Functionality for the Adsorption of Chromium(VI)from Aqueous Solutions. Colloids Surf., A 2009, 335, 107–113. (17) Standard Methods for the Examination of Water and Wastewater, 20th ed.; APHA, AWWA, WEF: Washington, DC, 1998. (18) Schwarz, J. A.; Driscol, C. T.; Bhenet, A. K. The Zero Point of Charge of Silica-Alumina Oxide Suspensions. J. Colloid Interface Sci. 1984, 97, 55–61. (19) Zhu, X.; Alexandratos, S. D. Polystyrene-Supported Amines: Affinity for Mercury(II) as a Function of the Pendant Groups and the Hg(II) Counter Ion. Ind. Eng. Chem. Res. 2005, 44, 8605–8610. (20) Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. Anatase TiO2 Nanoparticles on Rutile TiO2 Nanorods: A Heterogeneous Nanostructure via Layer-by-Layer Assembly. Langmuir 2007, 23, 10916–10919. (21) Hadi, N. Modification of Titanium Surface Species of Titania by Attachment of Silica Nanoparticles. Mater. Sci. Eng. 2006, B 133, 49–54. (22) Tang, Y.; Guan, X.; Su, T.; Gao, N.; Wang, J. Fluoride adsorption onto activated alumina: Modeling the effects of pH and some competing ions. Colloids Surf., A 2009, 337, 33–38. (23) Ho, Y. S.; McKay, G. Pseudo-Second-Order Model for Sorption Process. Process Biochem. 1999, 34, 451–465. (24) Giles, C. H.; McEwan, T. H.; Nakhwa, S. N., III. A System of Classification of Solution Adsorption Isotherm. J. Chem. Soc. 1960, 4, 3973– 3993. (25) Langmuir, I. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. J. Am. Chem. Soc. 1918, 40, 1361–1403. (26) Freundlich, H. Uber die adsorption in losungen. J. Phys. Chem. 1906, 57, 387–470. (27) Redlich, O.; Peterson, D. L. A Useful Adsorption Isotherm. J. Phys. Chem. 1959, 63, 1024–1032. (28) Anirudhan, T. S.; Suchithra, P. S.; Rijith, S. Amine-Modified Polyacrylamide-Bentonite Composite for the Adsorption of Humic Acid in Aqueous Solutions. Colloids Surf., A 2008, 326, 147–156. (29) Thakre, D.; Jagtap, S.; Bansiwal, A.; Labhsetwar, N.; Rayalu, S. Synthesis of La-Incorporated Chitosan Beads for Fluoride Removal from Water. J. Fluorine Chem. 2010, 131, 373–377. (30) Biswas, K.; Gupta, K.; Ghosh, U. C. Adsorption of Fluoride by Hydrous Iron(III) Tin(IV) Bimetal Mixed Oxide from the Aqueous Solutions. Chem. Eng. J. 2009, 149, 196–206. (31) Daifullah, A. A. M.; Yakout, S. M.; Elreefy, S. A. Adsorption of Fluoride in Aqueous Solutions using KMnO4-Modified Activated Carbon Derived from Steam Pyrolysis of Rice Straw. J. Hazard. Mater. 2007, 147, 633–643. (32) Biswas, K.; Saha, S. K.; Ghosh, U. C. Adsorption of Fluoride from Aqueous Solution by a Synthetic Iron(III)- Aluminium(III) Mixed Oxide. Ind. Eng. Chem. Res. 2007, 46, 5346–5356. (33) Dey, S.; Goswami, S.; Ghosh, U. C. Hydrous Ferric Oxide (HFO)-A Scavenger for Fluoride from Contaminated Water. Water, Air, Soil Pollut. 2004, 158, 311–323. (34) Rao, S. M.; Mamatha, A. Water Quality in Sustainable Water Management. Curr. Sci. 2004, 87, 942–947.

ReceiVed for reView April 3, 2010 ReVised manuscript receiVed September 30, 2010 Accepted October 11, 2010 IE100809F