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Stannic chloride impregnated chitosan for defluoridation of water
Accepted Manuscript Title: Stannic chloride impregnated chitosan for defluoridation of water Authors: Shashikant Kahu, Anita Shekhawat, D. Saravanan, Ravin Jugade PII: DOI: Reference:
S0141-8130(16)32603-4 http://dx.doi.org/doi:10.1016/j.ijbiomac.2017.02.101 BIOMAC 7162
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
24-11-2016 28-1-2017 27-2-2017
Please cite this article as: Shashikant Kahu, Anita Shekhawat, D.Saravanan, Ravin Jugade, Stannic chloride impregnated chitosan for defluoridation of water, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2017.02.101 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Stannic chloride impregnated chitosan for defluoridation of water
Shashikant Kahua, Anita Shekhawata, D. Saravananb, Ravin Jugadea*
a
Department of Chemistry, R.T.M. Nagpur University, Nagpur-440033, India
b
Department of Chemistry, National College, Tiruchirappalli-620001, India.
*Corresponding author. Tel: +91 9420254377. E-mail address: [email protected]
Abstract Chitosan, a potent amino polysaccharide, has been impregnated with Sn(IV) chloride for effective adsorption of fluoride from water. The Sn(IV) chloride impregnated chitosan was synthesized using microwave assisted technique. The material was thoroughly characterized using FTIR, SEM, EDX and XRD. The increase in surface area and pore volume has been revealed from BET studies. Enhanced thermal stability of this material was ascertained by TGADTA studies. This Sn(IV) chloride impregnated chitosan(Sn-Ch) has been exploited for its defluoridation property. Various parameters like pH, amount of adsorbent, adsorption time etc have been optimized to achieve maximum defluoridation efficiency. Under optimum conditions, Sn-Ch was found to have adsorption capacity of 17. 63 mg/g. The equilibrium studies showed that the data fits well with Freundlich isotherm model. Thermodynamics and kinetics parameters have been evaluated. The material has been applied for the defluoridation of real water sample. It was found to be recyclable material and can be regenerated and reused multiple times adding a greener dimension.
Keywords: stannic chloride impregnated chitosan;adsorption; fluoride. 1.
Introduction
Fluorine is beneficial for bone strength. But high levels of fluoride exposure may adversely affect neurological disorders, allergic manifestations and gastrointestinal problems [1]. Consumption of fluoride beyond recommended level for a long period of time causes teeth and skeletal fluorosis. Fluoride induced nephrotoxicity has been observed in patients with renal insufficiency . Fluoride is endemic in most of the Asian countries[2]. Fluoride concentration is increasing day by day due to mineralization of rocks, various anthropogenic activities and rapid industrialization. Three basic defluoridation processes, namely adsorption[3], precipitation[4] and membrane separation[5] are primarily used in water treatment. Among them, precipitation process involves the high probability of contamination of drinking water due to unwanted chemicals. Membrane process involves constant maintenance and heavy prices. Hence, adsorption, being cost effective and ecofriendly, is the ultimate choice for defluoridation .Various adsorbents like activated alumina [6], chemically activated carbon[7], bone char[8], synthetic resins[9], kaolinite[10], and carbon nanotubes[11] have been used for fluoride adsorption. Nalgonda technique[12] which involves use of lime and alum has been in trend. However, this method involves fundamental problem of leaching of aluminium at high pH . Materials containing rare earth elements have been reported to have strong affinity towards fluoride [13]. This also involves higher costs of the materials. For synthesis of such materials, biopolymers like cellulose, chitin and chitosan have been proved to be the best matrices[14-16]. Chitosan has proved versatile for environmental, medicinal and industrial applications[17-19]. Chitosan has amino and hydroxyl groups which can be used for making its derivatives by crosslinking and impregnation method. These modifications increase the stability and properties of the adsorbent to adsorb the toxic elements from the water.Recently, use of multivalent metal ions like aluminium [20], magnesium [21]and zirconium[22]has been reported for enhancing the fluoride adsorption capacity. Metal ions impregnation on chitosan leads to selective adsorption of fluoride. Recently, assimilation of tin on chitin was reported by us for the fluoride removal[23]. In present work, we report impregnation of chitosan with stannic chloride for defluoridation application. The applicability of the adsorbent checked with water samples collected from Chandrapur district of Maharashtra, India, which has been identified as high fluoride prone area by Central Ground Water Board, India.
2.
Materials and Methods
2.1
Materials
Analytical grade reagents were used for the fluoride adsorption studies. Millipore water (Elix 3 Millipore unit) was used in preparation of the stock solution of fluoride. A 1000 mg /Lstock solution of fluoride was prepared using sodium fluoride (Merck, India) and stored in a polypropylene bottle. The working solutions of 5, 10 and 15 mg/L fluoride were prepared by appropriate dilutions in polypropylene volumetric flasks. Chitosan with 85% of degree of deacetylation and average molecular weight 120 kDa was supplied by Uniloid Bio-Chemicals India Limited, Hyderabad. All the reagents were of analytical grade and used without further purification. 2.2
Preparation of tin(IV) impregnated chitosan (Sn-Ch)
5 g chitosan was taken in two-naked round bottom flask. To it 50, mL ethanol was added and stirred to form slurry. 2 mL anhydrous Sn (IV) chloride was diluted to 25 mL with ethanol and added dropwise to the chitosan slurry with the help of dropping funnel. The mixture was stirred for 1 hour at room temperature and subjected to microwave irradiation for 2 min with an intermittent time interval of 30s. The microwave irradiation was carried out in domestic microwave oven (LG, India model MS2049) at 2450 MHz. The hydrochloric acid generated in reaction was neutralized with the help of 5% ammonia-ethanol (v/v) mixture. The residue was filtered and washed with double distilled water till negative test of chloride. The Sn-Ch adsorbent was dried in oven at 60 0C and sieved through 500 µ mesh. Fig.1 shows preparative schematic diagram of Sn-Ch. Fig. 1 Preparation of Sn-Ch. 2.3
Batch adsorption experiments
For batch absorption studies, 5-70 mg/L fluoride solutions were equilibrated with 100 mg Sn-Ch in stoppered polypropylene conical flasks at pH 6.0. Each system was stirred for 45 min. The amount of fluoride adsorbed (mg /g) on Sn-Ch at equilibrium (qe) can be given by-
qe
C0 Ce V W
Also the percent removal capacity was calculated as-
(1)
% Re moval
C0 Ce 100 C0
(2)
where C0 and Ce refer to the initial and equilibrium liquid phase concentrations in mg /L of fluoride, V is the volume of fluoridesolution in liter and W is the weight of Sn-Ch adsorbent in gram. All the batch adsorption experiments were performed with three replicates to obtain reliable results. 2.4
Physicochemical characterization of Sn-Ch adsorbent
Structural details of Sn-Ch could be explained on the basis of FT-IR spectra recorded using Bruker Alpha spectrometer in the wavelength range 500-4000 cm1. The XRD spectra were recorded by X-ray diffractometer system Righaku-Miniflex 300. Surface morphology of adsorbent was studied using Scanning Electron Microscope (SEM) model TESCAN VEGA 3 SBH. Energy dispersive X-ray (EDX) analysis was performed for elemental composition using X- ray analyzer Oxford INCA Energy 250 EDS System during SEM studies. Thermal analysis of Sn-Ch was carried out using Hitachi TG-DTA 7200 analyzer.The Brunauer–Emmet–Teller (BET) surface area estimation was carried out by nitrogen adsorption–desorption method on single point surface area analyzer model Smart Sorb 92/93. 2.5
Analysis
The concentration of fluoride was measured using ORION pH/ISE meter 710equipped with a calibratedfluoride ion electrode model ORION 9609 BNWP. The reproducibility of instrument was checked by using TISAB II buffer. Calibration of fluoride ion selective electrode was performed by using 1, 5 and 10 mg/L standard solutions of fluoride which gave an optimum slope between 90 and 100%. 3.
Results and discussion
3.1
Characterization of Sn-Ch
The FT-IR spectrum of chitosan (Fig. 2a) showed characteristic broad peak corresponding to O-1
-1
H and N-H stretching vibrations in the region 3300 cm and 3500 cm respectively. The N-H -1
bending vibration was observed around 1545 cm . The C-H and the C-O stretching bands were -1
-1
obtained around 2890 cm and 1017 cm respectively. In Sn-Ch, the peak at 817 cm−1 is assigned to Sn-O vibration and small peaks between 1600 cm-1 to 1900 cm-1 are attributed to SnOH vibrational modes (Fig. 2b)[24].
Fig. 2 FT-IR spectrum of (a) chitosan and (b) Sn-Ch. In XRD pattern, characteristic diffraction peaks of chitosan were observed at 2 = 19.940 and 21.990 (Fig. 3a) matching with the reported values. In Sn-Ch (Fig. 3b), the diffraction peaks at 2θ = 26.5° and 51.5° of Sn confirms impregnation of tin on chitosan [25]. After adsorption of fluoride, (Fig. 3c), the decrease in intensity of peaks at 2θ = 26.5° and 51.8° indicate deterioration of crystalinity due to interaction between Sn(IV) and fluoride ion. Fig. 3.Powder XRD pattern of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-.
SEM micrographs of chitosan, Sn-Ch and Sn-Ch with adsorbed fluoride were recorded to understand subsequent change in surface morphology (Fig. 4). The chitosan seems to have a smooth surface morphology. In case of Sn-Ch, it shows porous and rough morphology due to the impregnation of Sn(IV). After adsorption of fluoride, the surface of Sn-Ch became less porous. Fig. 4 SEM images of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-. The EDX spectrum of chitosan shows the peaks for carbon, oxygen and nitrogen (Fig. 5a). After impregnation with Sn(IV), a peak of Sn appeared in the range 3 to 4 eV which confirms the successful loading of Sn onto the chitosan matrix (Fig. 5b). The adsorption of fluoride was ascertained from the EDX spectrum which shows the presence of fluorine along with the other major peaks (Fig. 5c). Fig. 5 EDX spectra of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-. The TGA curve of chitosan showed two-step degradation (Fig. 6a). Initial degradation observed between 60 to 100 0C with weight loss of 5% which corresponds to dehydration. The second degradation occurred in the range of 250–350 0C and could be attributed to the degradation of the polysaccharide structure of the molecule, including the dehydration of polysaccharide rings and the decomposition of the acetylated and deacetylated units of chitosan[26]. At the end of 900 0C, chitosan looses 71% of its weight. In TGA curve of Sn-Ch weight loss has been observed at slightly lower temperature with same decomposition pattern as that of chitosan. In case of Sn-Ch,
the percent weight loss has been reduced substantially from 71% to 55%. This is mainly due to remnants of tin oxide in Sn-Ch at the end of 900 0C. In the DTA curve of chitosan, there is a strong exothermic peak at 288.66 0C which is due to the thermal decomposition of chitosan. This exothermic peak gets shifted to lower temperature in Sn-Ch. Apart from this,there are two endothermic peaks at 120 0C and 230 0C (Fig. 6b) which may be attributed to the degradation behaviour of Sn-Ch and formation of tin oxide.
Fig.6 (a)TGA curves and (b) DTA curves of chitosan and Sn-Ch.
BET surface area and pore volume of chitosan and Sn-Ch were measured by nitrogen adsorptiondesorption method. From Table 1, it is clear that the surface area and pore volume of Sn-Chare lower than that of chitosan. The decrease in surface area and pore volume of Sn-Chis mainly due to metal ion incorporation and crosslinking leading to decrease in pore volume which reduces the diffusion of N2 gas. Table 1 3.2
Batch adsorption experiments
50 mL of fluoride solutions with varying concentrations from 5 to 100 mg/L at pH 6.0 were equilibrated with 200 mg of Sn-Ch in stoppered conical flasks. The adsorption systems were stirred using magnetic stirrer at 298 K for 30 min. All the adsorption experiments were performed with three replicates. 3.3
Effect of contact time
Contact time plays important role in adsorption as it influences the adsorption efficiency. In the present study, defluoridation by Sn-Ch was studied at pH 6.0 by varying the contact time in the range 5-75 min with initial fluoride concentration of 10 mg/L equilibrating with 200 mg Sn-Ch at 298 K. The rate of adsorption of fluoride is higher in the beginning and reaches equilibrium in 30 min (Fig.7a).This is because of availability of more adsorption sites on Sn-Ch adsorbent in the beginning which are consequently occupied by fluoride ions. 3.4
Amount of adsorbent
The effect of Sn-Ch dosage on adsorption efficiency of fluoride was studied by varying the amount of adsorbent from 50 mg to 300 mg with initial fluoride concentration of 10 mg/L for contact time of 30 min at pH 6.0. From Fig.7b, it can be seen that upto 200 mg of Sn-Ch dose,
the uptake of fluoride ions reached its maxima. After 200 mg dose of Sn-Ch, there is no observable rise in % removal of fluoride. Hence, 200 mg of Sn-Ch dose per 50 mL of the solution was fixed for all the batch adsorption experiments. 3.5
Initial fluoride concentration
Adsorption experiments were performed at different initial fluoride concentrations ranging from 5 to 50 mg/L with fixed contact time (30 min), pH (6.0) and Sn-Ch dosage (200 mg). Up to 10 mg/L concentration of fluoride, the % removal was more than 90 and decreased rapidly thereafter due to sorbent saturation (Fig.7c). Thus 10 mg/L fluoride solution was used for further adsorption studies. 3.6 Role of pH The effect of pH onto Sn-Ch adsorbent for the defluoridation has been studied by varying pH from 2.0 to 8.0 with initial fluoride concentration of 10 mg/L equilibrating with 200 mg Sn-Ch at 298 K. At lower pH, fluoride ions exist in the form of hydrofluoric acid. Hence availability of free fluoride ions is less resulting into decrease in the defluoridation efficiency. Being smaller in size, fluoride ion leads to higher degree of solvation [27] which also affects the defluoridation process. It was found that adsorption of fluoride by Sn-Ch was maximum at pH 6.0 (Fig.7d). Fig. 7 Effect of (a) Contact time (adsorbent dose = 200mg, pH = 6.0, [F-]= 10 mg/L) (b) Adsorbent dose, (contact time = 30 min, pH = 6.0, [F-]= 10 mg/L) (c) Initial concentration (contact time = 30 min, adsorbent dose = 200 mg, pH 6.0) and (d) pH on adsorption efficiency (contact time = 30 min, adsorbent dose = 200 mg, [F-]= 10 mg/L) 3.7
Adsorption isotherms
The adsorption isotherm studies were carried out using 50 mL of varying concentrations of fluoride from 5 to 100 mg/L at pH 6, adsorbent dose of 200 mg at 298 K. Various parameters required in the isotherm models were obtained. The Langmuir adsorption isotherm model was studied to find out maximum adsorption capacity of adsorbent[28]. The plot of Ce/qe against Ce gives maximum adsorption capacity (q0) and also adsorption energy constant b (Fig. 8a). The maximum adsorption capacity of 17.63 mg/g was obtained which proves effective interaction between Sn-Ch adsorbent and fluoride ions. The value of RL was found to be 0.178 (Table 2) which is indicative of favorable adsorption.
Freundlich adsorption isotherm [29] parameters were obtained from logarithmic plot of qe against Ce. The values of constant kF and n have been depicted in Table 2. From the values of correlation coefficient and the plot of experimental qe along with qe values of both the isotherms against Ce (Fig. 8c) implies that the Freundlich isotherm model has good agreement with experimental qe with value for correlation coefficient close to 1. Thus, Freundlich isotherm is the best fit model for adsorption of fluoride by Sn-Ch. Fig. 8 Adsorption isotherms (contact time = 30 min, adsorbent dose = 200 mg, pH = 6.0) (a) Langmuir adsorption isotherm, (b) Freundlich adsorption isotherm and (c) Plot of qe versus Ce. 3.8
Kinetics of adsorption
Kinetics of adsorption of F- on to Sn-Ch was studied to understand the time dependency of adsorption process. Pseudo-first-order and pseudo-second-order kinetic models were studied to understand adsorption kinetics. The studies were carried out using 50 mL 10 mg/L fluoride solution at pH 6.0 at 298 K. It was equilibrated with 200 mg of Sn-Ch adsorbent for different time intervals of 5 to 75 min. The plot of log (qe - qt) against t gives pseudo-first-order rate constant (Fig.9a) while the plot of t/qt against t (Fig. 9b) gives pseudo-second-order rate constant. From Table 3 it is clear that the value of correlation coefficient is close to unity for pseudo-second-order model and hence a best fit model to describe the adsorption of fluoride by Sn-Ch. Weber–Morris model [30] was studied to understand intraparticle diffusion is the rate determining step or not. If the plot of qt verses t1/2 (Fig. 9c) passes through origin and islinear, then intraparticle diffusion is the only rate-limiting step. The slope gives the intraparticle rate constant kint(Table 3.) and non-zero intercept showed that diffusion is not the only rate-limiting step for adsorption of fluoride by Sn-Ch. Fig. 9 Kinetic studies (adsorbent dose = 200 mg, pH = 6.0, [F-]= 10 mg/L) (a) Pseudo-first-order kinetics, (b) Pseudo-second-order kinetics and (c) Intraparticle diffusion.
Table 3 3.9
Thermodynamics of adsorption
Effect of temperature on adsorption of fluoride by Sn-Ch was studied to obtain relevant thermodynamic parameters at 303 K, 313 K, 323 K and 333 K. The values of ∆H and ∆S (Table 4) were obtained from slope and intercept of the plot of ln K against 1/T (Fig. 10) respectively. The negative free energy change indicates the spontaneous nature, negative enthalpy change
indicates the exothermic nature of adsorption process while negative entropy change indicates the decrease in randomness of fluoride as it passes from solution to adsorbed state. Fig. 10 van't Hoff plot. Table 4
3.10
Column Studies
Column adsorption studies were performed in order to prove the applicability of Sn-Ch adsorbent for defluoridation of larger sample volumes. For this, a polypropylene column (30 cm length, 1 cm inner diameter) was packed with 1 g of Sn-Ch. 15 mg/L fluoride solution maintained at pH 6.0 was passed through the column at a flow rate of 5 mL/min. The eluent was collected after every 10 min interval and it was analyzed for fluoride content. The column eluent was also tested for possible leaching of tin but the results were negative. However, it was found that the eluent contain significant amount concentration of chloride confirming the ion exchange phenomenon between fluoride in the solution and chloride of the Sn-Ch. The results obtained from Fig.11 have been depicted in Table 5. Fig. 11 Breakthrough Curve. Table 5
3.11
Effect of co-anions
The real waste water contains variety of anions which compete for available active sites on adsorbent. The effect of these diverse anions on defluoridation capacity of Sn-Ch was studied with 10 mg/L fluoride solution and 100 mg/L co-anions such as SO42-, Cl-, NO3-, CO32-. From Fig 12, it is clear that chloride ions have higher interference in adsorption of fluoride by Sn-Ch adsorbent. However, these observations were obtained at ten times higher concentration of Cl- as compared to F-. Fig. 12 Effect of co-anions. 3.12
Regeneration and reusability of adsorbent
Various reagents such as sodium chloride, sodium nitrate, sodium sulphate and sodiumcarbonate were examined for desorption studies of Sn-Ch loaded with fluoride. The best results were obtained with 5% (v/v) sodium chloride solution (Fig. 13a). The chloride ions in NaCl causes ion exchange at this high concentration with the adsorbed fluoride ions of Sn-Ch leading to
desorption of fluoride ions. The regenerated Sn-Ch was tested for ten adsorption-desorption cycles and it was observed (Fig. 13b) that there is decrease in adsorption efficiency as compared to original Sn-Ch. Upto 5 cycles, the adsorption efficiency was found to be more than 90% which decreased subsequently after it. Fig. 13 (a) Effect of various reagents on desorption of F- loaded Sn-Ch and (b) Efficiency of regenerated Sn-Ch in succeeding adsorption cycle. 3.13
Mechanism of adsorption
Tin (IV) chloride binds amino groups of chitosan at room temperature. Exchange of chloride ions with fluoride ions (Fig.14) resulting into an increase in adsorption capacity of Sn-Ch. The ion exchange phenomenon was confirmed by performing test for chloride ions in the supernatant obtained after fluoride adsorption on Sn-Ch and the eluate in column studies. The presence of chloride ions on Sn-Ch was also detected in EDX spectra of adsorbent. The EDX spectrum clearly shows reduction in chloride peak with the introduction of fluoride peak after equilibration with fluoride.
Fig. 14 Adsorption mechanism of fluoride by Sn-Ch. 3.14 Field Study In order to confirm the applicability of the Sn-Ch for removal of fluoride, it was tested with ground water sample collected from Chandrapur district which is fluoride affected area in Central India. About 200 mg sorbent was added to 50 ml of fluoride containing water sample collected from Chandrapur region and kept for stirring at room temperature for 30 minutes. The results obtained for single adsorption are depicted in Table 6 demonstrating the ability of this material towards defluoridation. Table 6 3.15 Comparison with other related adsorbents The adsorption capacity is the most important parameter for describing an adsorbent and it distinguishes the adsorbent with the other adsorbents. A comparison of Sn-Ch with other adsorbents is mentioned in Table 7. Table 7 4.
Conclusion
Chitosan has been impregnated with tin(IV) chloride to give an excellent fluoride adsorbent SnCh. The polymeric chains found to get crosslinked by covalent bonding with Sn(IV) with the loss of HCl molecule. The resultant material has a number of chloride ions that can exchange with fluoride ions of the eluent. The Sn-Ch adsorbent shows high fluoride adsorption capacity of 17.63 mg/g. The adsorption process follows Freundlich adsorption isotherm. The kineticsstudies indicate that the experimental data fits well in pseudo-second order kinetics. Column studies clearly indicated that the material can be used to treat larger sample volumes. Applicability towards real sample, recycling and reusability are the most encouraging properties of Sn-Ch adsorbent.
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Figure captions Fig. 1 Preparation of Sn-Ch.
Fig. 2 FT-IR spectrum of (a) chitosan and (b) Sn-Ch.
Fig. 3.Powder XRD pattern of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-.
Fig. 4 SEM images of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-.
Fig. 5 EDX spectra of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-.
Fig.6 (a)TGA curves and (b) DTA curves of chitosan and Sn-Ch.
Fig. 7 Effect of (a) Contact time (adsorbent dose = 200mg, pH = 6.0, [F-]= 10 mg/L) (b) Adsorbent dose, (contact time = 30 min, pH = 6.0, [F-]= 10 mg/L) (c) Initial concentration (contact time = 30 min, adsorbent dose = 200 mg, pH 6.0) and (d) pH on adsorption efficiency (contact time = 30 min, adsorbent dose = 200 mg, [F-]= 10 mg/L)
Fig. 8 Adsorption isotherms (contact time = 30 min, adsorbent dose = 200 mg, pH = 6.0) (a) Langmuir adsorption isotherm, (b) Freundlich adsorption isotherm and (c) Plot of qe versus Ce.
Fig. 9 Kinetic studies (adsorbent dose = 200mg, pH = 6.0, [F-]= 10 mg/L) (a) Pseudo-first-order kinetics, (b) Pseudo-second-order kinetics and (c) Intraparticle diffusion.
Fig. 10 van't Hoff plot.
Fig. 11 Breakthrough Curve.
Fig. 12 Effect of co-anions.
Fig. 13 (a) Effect of various reagents on desorption of F- loaded Sn-Ch and (b) Efficiency of regenerated Sn-Ch in succeeding adsorption cycle.
Fig. 14 Adsorption mechanism of fluoride by Sn-Ch.
Table 1 Surface parameters Material
Surface Area
Pore Volume
(m2/g)
(cm3/g)
Chitosan
3.47
6.0 × 10-3
Sn-Ch
3.02
5.2 × 10-3
Table 2 Adsorption isotherm parameters Langmuir isotherm q0
b
(mg/g)
(L/mg)
17.63
0.112
Freundlich isotherm RL
r2
kF
n
r2
1.99
0.990
(mg/g/L ) 0.178
0.834
2.47
Table 3 Kinetic parameters Pseudo-first-order
Pseudo-second-order kinetics
Intraparticle diffusion
kinetics k1
r2
(1/min) 0.101
k2
r2
0.287
r2
(mg/g/min1/2)
(g/mg/min) 0.755
kint
0.999
0.072
0.673
eratureTemp
Table 4 Thermodynamic parameters Temperature
ΔG
ΔH
(kJ/mol) (kJ/mol)
303K
-7.52
313K
-4.13
323K
-2.70
333K
-1.62
- 65.83
ΔS (kJ/mol/K)
- 0.194
Table5 Column parameters Parameter
Result
Inlet F- concentration
15 mg/L
Breakthrough volume
1200 mL
Exhaustion volume
1800 mL
Breakthrough Capacity
18 mg/g
Exhaustion Capacity
27 mg/g
Degree of column utilization
66.67 %
Table 6 Field trial results of the Sn-Ch Parameter
Before Treatment
After Treatment
F- (mg/L)
2.73
0.06
SO42- (mg/L)
142.36
125.24
Cl-(mg/L)
242.28
197.43
Total hardness(mg/L)
369.87
223.16
Total dissolved solids(mg/L)
584.52
462.33
Table 7Comparison of Sn-Ch with reported adsorbents
Sorbents
Defluoridation
References
capacity (mg/g) Chitosan Al- impregnated chitosan biopolymer Lanthanum incorporated chitosan beads Zr (IV) entrapped chitosan Fe (III) carboxylated chitosan beads Sn-Ch
1.39
[31]
1.73
[32]
4.70
[33]
13.70
[34]
15.38
[35]
17.63
Present study
S0141-8130(16)32603-4 http://dx.doi.org/doi:10.1016/j.ijbiomac.2017.02.101 BIOMAC 7162
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
24-11-2016 28-1-2017 27-2-2017
Please cite this article as: Shashikant Kahu, Anita Shekhawat, D.Saravanan, Ravin Jugade, Stannic chloride impregnated chitosan for defluoridation of water, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2017.02.101 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Stannic chloride impregnated chitosan for defluoridation of water
Shashikant Kahua, Anita Shekhawata, D. Saravananb, Ravin Jugadea*
a
Department of Chemistry, R.T.M. Nagpur University, Nagpur-440033, India
b
Department of Chemistry, National College, Tiruchirappalli-620001, India.
*Corresponding author. Tel: +91 9420254377. E-mail address: [email protected]
Abstract Chitosan, a potent amino polysaccharide, has been impregnated with Sn(IV) chloride for effective adsorption of fluoride from water. The Sn(IV) chloride impregnated chitosan was synthesized using microwave assisted technique. The material was thoroughly characterized using FTIR, SEM, EDX and XRD. The increase in surface area and pore volume has been revealed from BET studies. Enhanced thermal stability of this material was ascertained by TGADTA studies. This Sn(IV) chloride impregnated chitosan(Sn-Ch) has been exploited for its defluoridation property. Various parameters like pH, amount of adsorbent, adsorption time etc have been optimized to achieve maximum defluoridation efficiency. Under optimum conditions, Sn-Ch was found to have adsorption capacity of 17. 63 mg/g. The equilibrium studies showed that the data fits well with Freundlich isotherm model. Thermodynamics and kinetics parameters have been evaluated. The material has been applied for the defluoridation of real water sample. It was found to be recyclable material and can be regenerated and reused multiple times adding a greener dimension.
Keywords: stannic chloride impregnated chitosan;adsorption; fluoride. 1.
Introduction
Fluorine is beneficial for bone strength. But high levels of fluoride exposure may adversely affect neurological disorders, allergic manifestations and gastrointestinal problems [1]. Consumption of fluoride beyond recommended level for a long period of time causes teeth and skeletal fluorosis. Fluoride induced nephrotoxicity has been observed in patients with renal insufficiency . Fluoride is endemic in most of the Asian countries[2]. Fluoride concentration is increasing day by day due to mineralization of rocks, various anthropogenic activities and rapid industrialization. Three basic defluoridation processes, namely adsorption[3], precipitation[4] and membrane separation[5] are primarily used in water treatment. Among them, precipitation process involves the high probability of contamination of drinking water due to unwanted chemicals. Membrane process involves constant maintenance and heavy prices. Hence, adsorption, being cost effective and ecofriendly, is the ultimate choice for defluoridation .Various adsorbents like activated alumina [6], chemically activated carbon[7], bone char[8], synthetic resins[9], kaolinite[10], and carbon nanotubes[11] have been used for fluoride adsorption. Nalgonda technique[12] which involves use of lime and alum has been in trend. However, this method involves fundamental problem of leaching of aluminium at high pH . Materials containing rare earth elements have been reported to have strong affinity towards fluoride [13]. This also involves higher costs of the materials. For synthesis of such materials, biopolymers like cellulose, chitin and chitosan have been proved to be the best matrices[14-16]. Chitosan has proved versatile for environmental, medicinal and industrial applications[17-19]. Chitosan has amino and hydroxyl groups which can be used for making its derivatives by crosslinking and impregnation method. These modifications increase the stability and properties of the adsorbent to adsorb the toxic elements from the water.Recently, use of multivalent metal ions like aluminium [20], magnesium [21]and zirconium[22]has been reported for enhancing the fluoride adsorption capacity. Metal ions impregnation on chitosan leads to selective adsorption of fluoride. Recently, assimilation of tin on chitin was reported by us for the fluoride removal[23]. In present work, we report impregnation of chitosan with stannic chloride for defluoridation application. The applicability of the adsorbent checked with water samples collected from Chandrapur district of Maharashtra, India, which has been identified as high fluoride prone area by Central Ground Water Board, India.
2.
Materials and Methods
2.1
Materials
Analytical grade reagents were used for the fluoride adsorption studies. Millipore water (Elix 3 Millipore unit) was used in preparation of the stock solution of fluoride. A 1000 mg /Lstock solution of fluoride was prepared using sodium fluoride (Merck, India) and stored in a polypropylene bottle. The working solutions of 5, 10 and 15 mg/L fluoride were prepared by appropriate dilutions in polypropylene volumetric flasks. Chitosan with 85% of degree of deacetylation and average molecular weight 120 kDa was supplied by Uniloid Bio-Chemicals India Limited, Hyderabad. All the reagents were of analytical grade and used without further purification. 2.2
Preparation of tin(IV) impregnated chitosan (Sn-Ch)
5 g chitosan was taken in two-naked round bottom flask. To it 50, mL ethanol was added and stirred to form slurry. 2 mL anhydrous Sn (IV) chloride was diluted to 25 mL with ethanol and added dropwise to the chitosan slurry with the help of dropping funnel. The mixture was stirred for 1 hour at room temperature and subjected to microwave irradiation for 2 min with an intermittent time interval of 30s. The microwave irradiation was carried out in domestic microwave oven (LG, India model MS2049) at 2450 MHz. The hydrochloric acid generated in reaction was neutralized with the help of 5% ammonia-ethanol (v/v) mixture. The residue was filtered and washed with double distilled water till negative test of chloride. The Sn-Ch adsorbent was dried in oven at 60 0C and sieved through 500 µ mesh. Fig.1 shows preparative schematic diagram of Sn-Ch. Fig. 1 Preparation of Sn-Ch. 2.3
Batch adsorption experiments
For batch absorption studies, 5-70 mg/L fluoride solutions were equilibrated with 100 mg Sn-Ch in stoppered polypropylene conical flasks at pH 6.0. Each system was stirred for 45 min. The amount of fluoride adsorbed (mg /g) on Sn-Ch at equilibrium (qe) can be given by-
qe
C0 Ce V W
Also the percent removal capacity was calculated as-
(1)
% Re moval
C0 Ce 100 C0
(2)
where C0 and Ce refer to the initial and equilibrium liquid phase concentrations in mg /L of fluoride, V is the volume of fluoridesolution in liter and W is the weight of Sn-Ch adsorbent in gram. All the batch adsorption experiments were performed with three replicates to obtain reliable results. 2.4
Physicochemical characterization of Sn-Ch adsorbent
Structural details of Sn-Ch could be explained on the basis of FT-IR spectra recorded using Bruker Alpha spectrometer in the wavelength range 500-4000 cm1. The XRD spectra were recorded by X-ray diffractometer system Righaku-Miniflex 300. Surface morphology of adsorbent was studied using Scanning Electron Microscope (SEM) model TESCAN VEGA 3 SBH. Energy dispersive X-ray (EDX) analysis was performed for elemental composition using X- ray analyzer Oxford INCA Energy 250 EDS System during SEM studies. Thermal analysis of Sn-Ch was carried out using Hitachi TG-DTA 7200 analyzer.The Brunauer–Emmet–Teller (BET) surface area estimation was carried out by nitrogen adsorption–desorption method on single point surface area analyzer model Smart Sorb 92/93. 2.5
Analysis
The concentration of fluoride was measured using ORION pH/ISE meter 710equipped with a calibratedfluoride ion electrode model ORION 9609 BNWP. The reproducibility of instrument was checked by using TISAB II buffer. Calibration of fluoride ion selective electrode was performed by using 1, 5 and 10 mg/L standard solutions of fluoride which gave an optimum slope between 90 and 100%. 3.
Results and discussion
3.1
Characterization of Sn-Ch
The FT-IR spectrum of chitosan (Fig. 2a) showed characteristic broad peak corresponding to O-1
-1
H and N-H stretching vibrations in the region 3300 cm and 3500 cm respectively. The N-H -1
bending vibration was observed around 1545 cm . The C-H and the C-O stretching bands were -1
-1
obtained around 2890 cm and 1017 cm respectively. In Sn-Ch, the peak at 817 cm−1 is assigned to Sn-O vibration and small peaks between 1600 cm-1 to 1900 cm-1 are attributed to SnOH vibrational modes (Fig. 2b)[24].
Fig. 2 FT-IR spectrum of (a) chitosan and (b) Sn-Ch. In XRD pattern, characteristic diffraction peaks of chitosan were observed at 2 = 19.940 and 21.990 (Fig. 3a) matching with the reported values. In Sn-Ch (Fig. 3b), the diffraction peaks at 2θ = 26.5° and 51.5° of Sn confirms impregnation of tin on chitosan [25]. After adsorption of fluoride, (Fig. 3c), the decrease in intensity of peaks at 2θ = 26.5° and 51.8° indicate deterioration of crystalinity due to interaction between Sn(IV) and fluoride ion. Fig. 3.Powder XRD pattern of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-.
SEM micrographs of chitosan, Sn-Ch and Sn-Ch with adsorbed fluoride were recorded to understand subsequent change in surface morphology (Fig. 4). The chitosan seems to have a smooth surface morphology. In case of Sn-Ch, it shows porous and rough morphology due to the impregnation of Sn(IV). After adsorption of fluoride, the surface of Sn-Ch became less porous. Fig. 4 SEM images of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-. The EDX spectrum of chitosan shows the peaks for carbon, oxygen and nitrogen (Fig. 5a). After impregnation with Sn(IV), a peak of Sn appeared in the range 3 to 4 eV which confirms the successful loading of Sn onto the chitosan matrix (Fig. 5b). The adsorption of fluoride was ascertained from the EDX spectrum which shows the presence of fluorine along with the other major peaks (Fig. 5c). Fig. 5 EDX spectra of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-. The TGA curve of chitosan showed two-step degradation (Fig. 6a). Initial degradation observed between 60 to 100 0C with weight loss of 5% which corresponds to dehydration. The second degradation occurred in the range of 250–350 0C and could be attributed to the degradation of the polysaccharide structure of the molecule, including the dehydration of polysaccharide rings and the decomposition of the acetylated and deacetylated units of chitosan[26]. At the end of 900 0C, chitosan looses 71% of its weight. In TGA curve of Sn-Ch weight loss has been observed at slightly lower temperature with same decomposition pattern as that of chitosan. In case of Sn-Ch,
the percent weight loss has been reduced substantially from 71% to 55%. This is mainly due to remnants of tin oxide in Sn-Ch at the end of 900 0C. In the DTA curve of chitosan, there is a strong exothermic peak at 288.66 0C which is due to the thermal decomposition of chitosan. This exothermic peak gets shifted to lower temperature in Sn-Ch. Apart from this,there are two endothermic peaks at 120 0C and 230 0C (Fig. 6b) which may be attributed to the degradation behaviour of Sn-Ch and formation of tin oxide.
Fig.6 (a)TGA curves and (b) DTA curves of chitosan and Sn-Ch.
BET surface area and pore volume of chitosan and Sn-Ch were measured by nitrogen adsorptiondesorption method. From Table 1, it is clear that the surface area and pore volume of Sn-Chare lower than that of chitosan. The decrease in surface area and pore volume of Sn-Chis mainly due to metal ion incorporation and crosslinking leading to decrease in pore volume which reduces the diffusion of N2 gas. Table 1 3.2
Batch adsorption experiments
50 mL of fluoride solutions with varying concentrations from 5 to 100 mg/L at pH 6.0 were equilibrated with 200 mg of Sn-Ch in stoppered conical flasks. The adsorption systems were stirred using magnetic stirrer at 298 K for 30 min. All the adsorption experiments were performed with three replicates. 3.3
Effect of contact time
Contact time plays important role in adsorption as it influences the adsorption efficiency. In the present study, defluoridation by Sn-Ch was studied at pH 6.0 by varying the contact time in the range 5-75 min with initial fluoride concentration of 10 mg/L equilibrating with 200 mg Sn-Ch at 298 K. The rate of adsorption of fluoride is higher in the beginning and reaches equilibrium in 30 min (Fig.7a).This is because of availability of more adsorption sites on Sn-Ch adsorbent in the beginning which are consequently occupied by fluoride ions. 3.4
Amount of adsorbent
The effect of Sn-Ch dosage on adsorption efficiency of fluoride was studied by varying the amount of adsorbent from 50 mg to 300 mg with initial fluoride concentration of 10 mg/L for contact time of 30 min at pH 6.0. From Fig.7b, it can be seen that upto 200 mg of Sn-Ch dose,
the uptake of fluoride ions reached its maxima. After 200 mg dose of Sn-Ch, there is no observable rise in % removal of fluoride. Hence, 200 mg of Sn-Ch dose per 50 mL of the solution was fixed for all the batch adsorption experiments. 3.5
Initial fluoride concentration
Adsorption experiments were performed at different initial fluoride concentrations ranging from 5 to 50 mg/L with fixed contact time (30 min), pH (6.0) and Sn-Ch dosage (200 mg). Up to 10 mg/L concentration of fluoride, the % removal was more than 90 and decreased rapidly thereafter due to sorbent saturation (Fig.7c). Thus 10 mg/L fluoride solution was used for further adsorption studies. 3.6 Role of pH The effect of pH onto Sn-Ch adsorbent for the defluoridation has been studied by varying pH from 2.0 to 8.0 with initial fluoride concentration of 10 mg/L equilibrating with 200 mg Sn-Ch at 298 K. At lower pH, fluoride ions exist in the form of hydrofluoric acid. Hence availability of free fluoride ions is less resulting into decrease in the defluoridation efficiency. Being smaller in size, fluoride ion leads to higher degree of solvation [27] which also affects the defluoridation process. It was found that adsorption of fluoride by Sn-Ch was maximum at pH 6.0 (Fig.7d). Fig. 7 Effect of (a) Contact time (adsorbent dose = 200mg, pH = 6.0, [F-]= 10 mg/L) (b) Adsorbent dose, (contact time = 30 min, pH = 6.0, [F-]= 10 mg/L) (c) Initial concentration (contact time = 30 min, adsorbent dose = 200 mg, pH 6.0) and (d) pH on adsorption efficiency (contact time = 30 min, adsorbent dose = 200 mg, [F-]= 10 mg/L) 3.7
Adsorption isotherms
The adsorption isotherm studies were carried out using 50 mL of varying concentrations of fluoride from 5 to 100 mg/L at pH 6, adsorbent dose of 200 mg at 298 K. Various parameters required in the isotherm models were obtained. The Langmuir adsorption isotherm model was studied to find out maximum adsorption capacity of adsorbent[28]. The plot of Ce/qe against Ce gives maximum adsorption capacity (q0) and also adsorption energy constant b (Fig. 8a). The maximum adsorption capacity of 17.63 mg/g was obtained which proves effective interaction between Sn-Ch adsorbent and fluoride ions. The value of RL was found to be 0.178 (Table 2) which is indicative of favorable adsorption.
Freundlich adsorption isotherm [29] parameters were obtained from logarithmic plot of qe against Ce. The values of constant kF and n have been depicted in Table 2. From the values of correlation coefficient and the plot of experimental qe along with qe values of both the isotherms against Ce (Fig. 8c) implies that the Freundlich isotherm model has good agreement with experimental qe with value for correlation coefficient close to 1. Thus, Freundlich isotherm is the best fit model for adsorption of fluoride by Sn-Ch. Fig. 8 Adsorption isotherms (contact time = 30 min, adsorbent dose = 200 mg, pH = 6.0) (a) Langmuir adsorption isotherm, (b) Freundlich adsorption isotherm and (c) Plot of qe versus Ce. 3.8
Kinetics of adsorption
Kinetics of adsorption of F- on to Sn-Ch was studied to understand the time dependency of adsorption process. Pseudo-first-order and pseudo-second-order kinetic models were studied to understand adsorption kinetics. The studies were carried out using 50 mL 10 mg/L fluoride solution at pH 6.0 at 298 K. It was equilibrated with 200 mg of Sn-Ch adsorbent for different time intervals of 5 to 75 min. The plot of log (qe - qt) against t gives pseudo-first-order rate constant (Fig.9a) while the plot of t/qt against t (Fig. 9b) gives pseudo-second-order rate constant. From Table 3 it is clear that the value of correlation coefficient is close to unity for pseudo-second-order model and hence a best fit model to describe the adsorption of fluoride by Sn-Ch. Weber–Morris model [30] was studied to understand intraparticle diffusion is the rate determining step or not. If the plot of qt verses t1/2 (Fig. 9c) passes through origin and islinear, then intraparticle diffusion is the only rate-limiting step. The slope gives the intraparticle rate constant kint(Table 3.) and non-zero intercept showed that diffusion is not the only rate-limiting step for adsorption of fluoride by Sn-Ch. Fig. 9 Kinetic studies (adsorbent dose = 200 mg, pH = 6.0, [F-]= 10 mg/L) (a) Pseudo-first-order kinetics, (b) Pseudo-second-order kinetics and (c) Intraparticle diffusion.
Table 3 3.9
Thermodynamics of adsorption
Effect of temperature on adsorption of fluoride by Sn-Ch was studied to obtain relevant thermodynamic parameters at 303 K, 313 K, 323 K and 333 K. The values of ∆H and ∆S (Table 4) were obtained from slope and intercept of the plot of ln K against 1/T (Fig. 10) respectively. The negative free energy change indicates the spontaneous nature, negative enthalpy change
indicates the exothermic nature of adsorption process while negative entropy change indicates the decrease in randomness of fluoride as it passes from solution to adsorbed state. Fig. 10 van't Hoff plot. Table 4
3.10
Column Studies
Column adsorption studies were performed in order to prove the applicability of Sn-Ch adsorbent for defluoridation of larger sample volumes. For this, a polypropylene column (30 cm length, 1 cm inner diameter) was packed with 1 g of Sn-Ch. 15 mg/L fluoride solution maintained at pH 6.0 was passed through the column at a flow rate of 5 mL/min. The eluent was collected after every 10 min interval and it was analyzed for fluoride content. The column eluent was also tested for possible leaching of tin but the results were negative. However, it was found that the eluent contain significant amount concentration of chloride confirming the ion exchange phenomenon between fluoride in the solution and chloride of the Sn-Ch. The results obtained from Fig.11 have been depicted in Table 5. Fig. 11 Breakthrough Curve. Table 5
3.11
Effect of co-anions
The real waste water contains variety of anions which compete for available active sites on adsorbent. The effect of these diverse anions on defluoridation capacity of Sn-Ch was studied with 10 mg/L fluoride solution and 100 mg/L co-anions such as SO42-, Cl-, NO3-, CO32-. From Fig 12, it is clear that chloride ions have higher interference in adsorption of fluoride by Sn-Ch adsorbent. However, these observations were obtained at ten times higher concentration of Cl- as compared to F-. Fig. 12 Effect of co-anions. 3.12
Regeneration and reusability of adsorbent
Various reagents such as sodium chloride, sodium nitrate, sodium sulphate and sodiumcarbonate were examined for desorption studies of Sn-Ch loaded with fluoride. The best results were obtained with 5% (v/v) sodium chloride solution (Fig. 13a). The chloride ions in NaCl causes ion exchange at this high concentration with the adsorbed fluoride ions of Sn-Ch leading to
desorption of fluoride ions. The regenerated Sn-Ch was tested for ten adsorption-desorption cycles and it was observed (Fig. 13b) that there is decrease in adsorption efficiency as compared to original Sn-Ch. Upto 5 cycles, the adsorption efficiency was found to be more than 90% which decreased subsequently after it. Fig. 13 (a) Effect of various reagents on desorption of F- loaded Sn-Ch and (b) Efficiency of regenerated Sn-Ch in succeeding adsorption cycle. 3.13
Mechanism of adsorption
Tin (IV) chloride binds amino groups of chitosan at room temperature. Exchange of chloride ions with fluoride ions (Fig.14) resulting into an increase in adsorption capacity of Sn-Ch. The ion exchange phenomenon was confirmed by performing test for chloride ions in the supernatant obtained after fluoride adsorption on Sn-Ch and the eluate in column studies. The presence of chloride ions on Sn-Ch was also detected in EDX spectra of adsorbent. The EDX spectrum clearly shows reduction in chloride peak with the introduction of fluoride peak after equilibration with fluoride.
Fig. 14 Adsorption mechanism of fluoride by Sn-Ch. 3.14 Field Study In order to confirm the applicability of the Sn-Ch for removal of fluoride, it was tested with ground water sample collected from Chandrapur district which is fluoride affected area in Central India. About 200 mg sorbent was added to 50 ml of fluoride containing water sample collected from Chandrapur region and kept for stirring at room temperature for 30 minutes. The results obtained for single adsorption are depicted in Table 6 demonstrating the ability of this material towards defluoridation. Table 6 3.15 Comparison with other related adsorbents The adsorption capacity is the most important parameter for describing an adsorbent and it distinguishes the adsorbent with the other adsorbents. A comparison of Sn-Ch with other adsorbents is mentioned in Table 7. Table 7 4.
Conclusion
Chitosan has been impregnated with tin(IV) chloride to give an excellent fluoride adsorbent SnCh. The polymeric chains found to get crosslinked by covalent bonding with Sn(IV) with the loss of HCl molecule. The resultant material has a number of chloride ions that can exchange with fluoride ions of the eluent. The Sn-Ch adsorbent shows high fluoride adsorption capacity of 17.63 mg/g. The adsorption process follows Freundlich adsorption isotherm. The kineticsstudies indicate that the experimental data fits well in pseudo-second order kinetics. Column studies clearly indicated that the material can be used to treat larger sample volumes. Applicability towards real sample, recycling and reusability are the most encouraging properties of Sn-Ch adsorbent.
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Figure captions Fig. 1 Preparation of Sn-Ch.
Fig. 2 FT-IR spectrum of (a) chitosan and (b) Sn-Ch.
Fig. 3.Powder XRD pattern of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-.
Fig. 4 SEM images of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-.
Fig. 5 EDX spectra of (a) chitosan, (b) Sn-Ch and (c) Sn-Ch with adsorbed F-.
Fig.6 (a)TGA curves and (b) DTA curves of chitosan and Sn-Ch.
Fig. 7 Effect of (a) Contact time (adsorbent dose = 200mg, pH = 6.0, [F-]= 10 mg/L) (b) Adsorbent dose, (contact time = 30 min, pH = 6.0, [F-]= 10 mg/L) (c) Initial concentration (contact time = 30 min, adsorbent dose = 200 mg, pH 6.0) and (d) pH on adsorption efficiency (contact time = 30 min, adsorbent dose = 200 mg, [F-]= 10 mg/L)
Fig. 8 Adsorption isotherms (contact time = 30 min, adsorbent dose = 200 mg, pH = 6.0) (a) Langmuir adsorption isotherm, (b) Freundlich adsorption isotherm and (c) Plot of qe versus Ce.
Fig. 9 Kinetic studies (adsorbent dose = 200mg, pH = 6.0, [F-]= 10 mg/L) (a) Pseudo-first-order kinetics, (b) Pseudo-second-order kinetics and (c) Intraparticle diffusion.
Fig. 10 van't Hoff plot.
Fig. 11 Breakthrough Curve.
Fig. 12 Effect of co-anions.
Fig. 13 (a) Effect of various reagents on desorption of F- loaded Sn-Ch and (b) Efficiency of regenerated Sn-Ch in succeeding adsorption cycle.
Fig. 14 Adsorption mechanism of fluoride by Sn-Ch.
Table 1 Surface parameters Material
Surface Area
Pore Volume
(m2/g)
(cm3/g)
Chitosan
3.47
6.0 × 10-3
Sn-Ch
3.02
5.2 × 10-3
Table 2 Adsorption isotherm parameters Langmuir isotherm q0
b
(mg/g)
(L/mg)
17.63
0.112
Freundlich isotherm RL
r2
kF
n
r2
1.99
0.990
(mg/g/L ) 0.178
0.834
2.47
Table 3 Kinetic parameters Pseudo-first-order
Pseudo-second-order kinetics
Intraparticle diffusion
kinetics k1
r2
(1/min) 0.101
k2
r2
0.287
r2
(mg/g/min1/2)
(g/mg/min) 0.755
kint
0.999
0.072
0.673
eratureTemp
Table 4 Thermodynamic parameters Temperature
ΔG
ΔH
(kJ/mol) (kJ/mol)
303K
-7.52
313K
-4.13
323K
-2.70
333K
-1.62
- 65.83
ΔS (kJ/mol/K)
- 0.194
Table5 Column parameters Parameter
Result
Inlet F- concentration
15 mg/L
Breakthrough volume
1200 mL
Exhaustion volume
1800 mL
Breakthrough Capacity
18 mg/g
Exhaustion Capacity
27 mg/g
Degree of column utilization
66.67 %
Table 6 Field trial results of the Sn-Ch Parameter
Before Treatment
After Treatment
F- (mg/L)
2.73
0.06
SO42- (mg/L)
142.36
125.24
Cl-(mg/L)
242.28
197.43
Total hardness(mg/L)
369.87
223.16
Total dissolved solids(mg/L)
584.52
462.33
Table 7Comparison of Sn-Ch with reported adsorbents
Sorbents
Defluoridation
References
capacity (mg/g) Chitosan Al- impregnated chitosan biopolymer Lanthanum incorporated chitosan beads Zr (IV) entrapped chitosan Fe (III) carboxylated chitosan beads Sn-Ch
1.39
[31]
1.73
[32]
4.70
[33]
13.70
[34]
15.38
[35]
17.63
Present study