Î²-Cyclodextrin

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Fabrication of La3+ Impregnated Chitosan/β-Cyclodextrin Biopolymeric Materials for Effective Utilization of Chromate and Fluoride Adsorption in Single Systems Jayaram Preethi and Sankaran Meenakshi* Department of Chemistry, The Gandhigram Rural Institute-Deemed University Gandhigram, Dindigul, Tamil Nadu 624 302, India S Supporting Information *

ABSTRACT: Lanthanum impregnated chitosan/β-cyclodextrin (CS− La−βCD) composite was prepared using a facile in situ fabrication method and successfully evaluated for the adsorption of CrO42− and F−. The pristine and treated CS−La−βCD was characterized using Fourier transform infrared, scanning electron microscopy, energy dispersive X-ray spectroscopy with mapping, X-ray diffraction, and thermal gravametric analytical techniques. The optimum conditions for the uptake of CrO42− and F− were investigated as a function of shaking time, dosage, initial ion concentration, pH, and predatory ions in batch experiments. The equilibrium data were fitted using Langmuir, Freundlich, and Dubinin−Radushkevich models for the defluoridation and sorption of chromium onto CS−La−βCD. The adsorption kinetics of CrO42− and F− followed the pseudo-second order model. The maximum Langmuir adsorption capacity was found to be 91.58 mg/g and 8.14 mg/g for CrO42− and F−, respectively at 303 K. Sequential adsorption and desorption studies were carried out up to five cycles using 0.1 M of NaOH as an eluent to check the reusability of the adsorbents.

1. INTRODUCTION Water is tainted with heterogeneous contaminations which include inorganic oxy anions, overwhelming heavy metals, fluoride, and organics such as pesticides, dyes, phenols, oil, and grease. Fluoride contamination is found to be prevalent throughout the world due to its geological abundance. Naturally, fluoride is exposed to the environment through fluorine-containing rocks such as shale, syenite, and granite.1 Moreover, the effluents discharged from various industrial processes such as electroplating, semiconductor processing, glass, and ceramic manufacturing units are the other anthropogenic sources of fluoride.2 It is an essential element for the hardening of teeth and bones in the trace level of 0.5− 1.5 mg/L, but when the intake of fluoride exceeds its limits, it causes dental and skeletal fluorosis. 3,4 The maximum permissible limit for fluoride recommended by the World Health Organization is 1.5 mg/L. Heavy metals are found to be most widespread pollutants which are released through various industrial actions. Among them chromium took precedence on the list of toxic pollutants by the United States Environmental Protection Agency (USEPA). Chromium is determined as a human carcinogen and also causes bronchitis, liver dysfunction, and ulcer.5 The maximum permissible limit for chromium and fluoride recommended by World Health Organization is 0.05 and 1.5 mg/L, respectively. The semiconductor industries and surface wafer etching units discharge the chromium and fluoride containing effluents due to the usage of chemicals such as hydrofluoric acid and chromic acid.3,6 Hence, it is necessary to treat the effluents before it is discarded into the © XXXX American Chemical Society

water streams. So far, various technologies such as adsorption, electrochemical precipitation, ion exchange, nano filtration, and reverse osmosis have been adopted for the detoxification of chromium and fluoride contamination.7,8 Adsorption is found to be a superior technique among the reviewed techniques since it is effective, requires the least economic consideration, and is easy to handle. Over the past few decades chitosan has played an indispensable role in the field of water treatment. Chitosan is obtained from the exoskeleton of the crustaceans shells through the alkaline deacetylation process. The polysaccharide chain in the chitosan is enriched with multiple functional groups, namely, hydroxyl and amine groups which make it valuable as a unique adsorbent.9 β-Cyclodextrin is a truncated cone-shaped oligosaccharide composed of seven membered α(1- 4)-linked D-glucopyranose units. βCD is featured with a inner hydrophobic cavity and the hydrophilic external phase. The hydroxyl groups present in both interior and exterior of the cavity tend to form complexes with metal ions.10 Owing to the unique properties, βCD is grafted or chemically modified with different molecules such as graphene oxide, silica, metal oxides, layered double hydroxides, and magnetic nanoparticles. Sikder et al. established the thermodynamic, kinetic, and isotherm parameters of the chitosan modified with carboxymethyl β-cyclodextrin entrapped with zerovalent iron nanoparticles on the uptake of Cu(II) and Received: October 11, 2017 Accepted: December 29, 2017

A

DOI: 10.1021/acs.jced.7b00889 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. X-ray diffraction patterns of (A) chitosan, (B) β-cyclodextrin, (C) lanthanum chloride, (D) CS−βCD, (E) CS−La−βCD, (F) Cr−CS− La−βCD, and (G) F−CS−La−βCD.

Cr(VI).11 Graphene oxide−α-cyclodextrin−polypyrrole nanocomposites12 and zerovalent iron impregnated chitosan− caboxymethyl−β-cyclodextrin composite beads13 were utilized for chromium and arsenic adsorption with the capacities of 606.06 mg/g, 18.51 mg/g (As(III)), and 13.51 mg/g (As(V)) respectively. Wilson et al.14 reported β-cyclodextrin−chitosan− gluteraldehyde terpolymers as a potential adsorbent for arsenate and p-nitrophenolate anions and analyzed each anion with Sips isotherm. In the present study, chitosan is grafted with β-cyclodextrin along with the incorporation of lanthanum using gluteraldehyde as a cross-linker and the prepared lanthanum incorporated chitosan−β-cyclodextrin (CS− La−βCD) composite was applied for the adsorption studies. The effects of various influencing parameters toward the adsorption of CrO42−/F− ions were optimized by batch method. The isotherm and kinetic parameters were evaluated for fluoride and chromate ion adsorption studies at different temperatures. The regeneration studies were demonstrated for the technology transfer in the field of water treatment.

mixed for 90 min to ensure the homogeneous dispersion. The mixture was added with 5% gluteraldehyde and stirred vigorously until the mixture turned into orange-red hydrogel. Then the hydrogel was kept to a refrigerator at 4 o C for 24 h to undergo a complete cross-linking reaction. Further the mixture was allowed to attain room temperature, and then washed thoroughly with distilled water to remove excess gluteraldehyde. The obtained product was dried at 60 °C for 9 h in a hot air oven. The dried material was finely ground with mortar and pestle and used for further studies. The same procedure was followed for the preparation of chitosan−β-cyclodextrin composite without the addition of lanthanum for comparison studies. 2.3. Characterization of Adsorbent. Physiochemical properties of CS−La−βCD before and after sorption were characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), and thermal gravimetric analysis (TGA). FTIR spectra were recorded for pristine and treated samples using the KBr pellet method in the range of 400 to 4000 cm−1 in JASCO-460 FT-IR spectrometer. The surface morphologies were examined using the scanning electron microscope (Vega3Tescan model) equipped with energy dispersive X-ray analyzer. XRD patterns were obtained at 25 °C on X’per PRO model-PANalytical using Cu Kα radiation. TGA was measured using a SEIKO model TG-DTA 6200 instrument at the heating rate of 10 °C/min under N2 atmosphere. Expandable ion analyzer EA 940 (Orion, USA) with ion selective fluoride electrode BN 9609 (Orion, USA) and pH electrode was used for the quantitative analysis of fluoride and pH measurements, respectively. The residual concentration of Cr(VI) was measured with UV−vis spectrophotometer (Pharo, Merck 300) using 1,5-diphenyl carbazide method. 2.4. Sorption Experiments. The sorption experiments were carried out by taking 50 mL of chromium(VI)/fluoride ion solution with 0.1 g of CS−La−βCD using a thermostated

2. MATERIALS AND METHODS 2.1. Materials. Chitosan (deacetylation: 85% molecular weight: 25 kDa) was purchased from Pelican Biotech and Chemicals Laboratories, Kerala, India. β-Cyclodextrin, lanthanum chloride heptahydrate, potassium dichromate, sodium fluoride, and gluteraldehyde were procured from Merck, Mumbai. Sodium hydroxide and hydrochloric acid were obtained from Central Drug House, New Delhi. All the chemicals were used without further purification. The working solutions were prepared from an appropriate dilution of 1000 mg/L stock solutions of dichromate and fluoride using double distilled water. 2.2. Synthesis of Lanthanum Incorporated Chitosan−β-Cyclodextrin Composite. Chitosan flakes (2 g) were dissolved in 2% (v/v) acetic acid of 200 mL and stirred for 6 h at room temperature. 1 g of β-cyclodextrin and 0.25 M lanthanum solution were added to the chitosan solution and B



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orbital shaker at 200 rpm. The effects of concentrations of each CrO42−/F− with the fixed dosage were optimized by varying initial concentration. The pH experiments were performed in the 2−11 range. The acid and base concentrations were adjusted with 0.1 M HCl/NaOH solution. Isotherm experiments were done at 30, 40, and 50 °C with 100 mg of CS− La−βCD for 30 min. In the kinetic studies, five series of experiments were carried out at various concentrations, such as, 100, 125, 150, 175, and 200 mg/L, and 12, 14, 16, 18, and 20 mg/L for CrO42− and F−, respectively. All the experiments were carried out in duplicate, and the average values were used for calculation. The adsorption densities of both CrO42− and F− were calculated using following equations. removal percentage =

Co − Ce × 100 Co

(1)

v w

(2)

sorption capacity qe = (Co − Ce) ×

Figure 2. FTIR spectra of (A) CS−βCD, (B) CS−La−βCD, (C) Cr− CS−La−βCD, and (D) F−CS−La−βCD.

where Co and Ce are the initial and final concentration of the CrO42−/F− (mg/L), qe is the equilibrium adsorption capacity (mg/g), V is the volume of solution (L), W is the weight of adsorbent (g).

functional groups and the amine groups on the surface.18 The band at 3431 cm−1 of CS−La−βCD was shifted to 3417 cm−1 and the widening of band at 3447 cm−1 due to the intermolecular hydrogen-bonding interactions of hydrochromate anion and fluoride with the adsorbent, respectively. In Figure 2D the metal−fluoride interactions were depicted in the range of 900−500 cm−1, and a weak band appearing at 767 cm−1 indicated the hydrogen-bonding interactions of fluoride with the CS−La−βCD.19 3.3. SEM with EDAX Analysis and SEM mapping. The SEM micrographs of (A) CS-β-CD (B) CS- La-βCD (C) CrCS−La−βCD (D) F-CS−La−βCD were represented in Figure 3. The grooved surface of CS-β-CD was modified like the scales of fish sprouting from the layered structure after the assimilation of lanthanum which provides high surface area. After the adsorption experiments the treated specimens exhibited the uniform and clear surface which is different from its pristine form. The modification of surface composition was determined using energy dispersive X-ray elemental analyses. In Figure 3F the occurrence of the lanthanum peak depicted the successful incorporation of lanthanum onto CS−βCD. The EDAX analyses were also recorded for the spent sorbents; the appearance of new Cr and F peaks along with C, N, O, and La demonstrated the adsorption onto CS− La−βCD and is shown in Figure 3G,H. The distribution of individual elements of CS−La−βCD was mapped using SEM mapping analysis and is shown in Figure 4. The emergence of yellow signals indicated the successful incorporation of lanthanum. 3.4. Thermal Analysis. The thermal characteristics of pristine and spent CS−La−βCD materials were assessed using thermogravimetric analysis (TGA). In Figure 5A. Four thermal degradation steps were observed. The first degradation step was due to the evaporation of water molecules bound onto the adsorbent. The second thermal event corresponds (190−420 °C) to the destruction of cross-linking among the chitosan, βcyclodextrin and lanthanum moieties with the major weight loss of 39.3%. The denaturization of skeletal structures of chitosan and β-cyclodextrin occurred at a high temperature of 420−550 °C. The last thermal event was because of the degradation of lanthanum metal of CS−La−βCD. The prominent weight loss

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Studies. The X-ray diffraction patterns of (A) chitosan, (B) β-cyclodextrin, (C) lanthanum chloride, (D) CS−βCD, (E) CS−La−βCD, (F) Cr−CS− La−βCD, and (G) F−CS−La−βCD were displayed in Figure 1. The peaks at 15.18(120) and 26.5(013) (JCPDS No. 391894) were the characteristic peaks of chitosan. The 2θ values at 12.48, 12.72, 15.44, 17.98, and 18.82 corresponded to the βcyclodextrin peaks (JCPDS No. 53-1903). In the XRD pattern of CS-β-CD the respective 2θ = 26.5 of chitosan was shifted to 22.43 and the 2θ = 12.48, 18.82 of β-cyclodextrin were shifted to 12.95 and 20.51 and the intensities of the peaks were lowered due the formation of the CS−βCD complex.15 The new peaks at 16.68 and 38.18 indicated the incorporation of lanthanum onto the CS−βCD matrix. In the Cr−CS−La−βCD the characteristic peaks in the range of 2θ = 10 to 35 were shifted and the intensities were diminished due to the adsorption of CrO42−. The new peak at 28.31(JCPDS No. 33-0704) was observed in the 100% intensity which may be due to the formation of La−F bonding16 (Figure 1G). 3.2. FTIR Analysis. The FTIR spectra of (A) CS-β-CD (B) CS−La−βCD (C) Cr-CS−La−βCD and (D) F-CS−La−βCD were shown in Figure 2. A broad peak at 3430 cm−1 is typically assigned to the O−H and − NH stretching vibrations of βcyclodextrin and chitosan (Figure 2A). The peaks at 2925 and 2921 cm−1 corresponded to CH2 asymmetric stretching vibrations. The C−O−C skeletal stretching vibration and C− OH stretching vibration were observed at 1154 and 1028 cm−1 for β-cyclodextrin.17 The bands at 1640 and 1030 cm−1 were due to C−O and C−N stretching vibrations of acetamide group of chitosan. The distinct peaks observed at 516 and 462 cm−1 were attributed to the La−O stretching vibration which exhibited that lanthanum was successfully incorporated into the CS-β-CD (Figure 2B). The peaks observed at 1641 cm−1 and 1078 cm−1 (C−O), was shifted to 1636 and 1063 cm−1, respectively, and the intensities of the peaks were diminished after the reaction with lanthanum in CS−La−βCD. In Figure 2C a new characteristic peak was observed at 2369 cm−1 indicating the interaction of CrO42− with oxygen-containing C



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Figure 3. SEM micrographs of (A) CS−βCD, (B) CS−La−βCD, (C) Cr−CS−La−βCD, (D) F−CS−La−βCD, and EDAX spectra of (E) CS−βCD, (F) CS−La−βCD, (G) Cr−CS−La−βCD, and (H) F−CS−La−βCD.

of 46.2% at 330 to 450 °C was depicted for the chromium treated CS−La−βCD in the third thermal event Figure 5B. 3.5. Influence of Acid−Base Stability and Presumed Removal Mechanism. The pH of aqueous media is an important parameter which influences the surface charge of the sorbent, speciation of the adsorbate molecules, and contributes an indispensable role in the removal mechanism. The percentage removal of CS−La−βCD for CrO42− was found to be decreased, while the pH from 2 to 11 was increased, and the results are shown in Figure 6. Speciation of chromate salt is highly governed by the solute ion pH; therefore. Cr2O72−, CrO42−, HCrO4−, were the existing forms of hexavalent chromium20 and at below pH 4 hydrogen tetraoxochromate anion (HCrO4−) is the prevalent ionic form. At low pH the ternary nitrogen of chitosan and hydroxyl groups of βcyclodextrin and chitosan were protonated and consequently followed by the electrostatic association with HCrO4− anions.

Through the electron-rich functional groups present on the surface of CS−La−βCD there is a possibility of a reduction and oxidation reaction taking place with the transfer of electrons at low pH. Hence it is stated that the CS−La−βCD preferably adsorbs HCrO4− rather than various ionic forms of hexavalent chromium. Fluoride adsorption is found to decrease with an increase in pH. At lower pH values of 2−4, where the fluoride removal was found to be more electrostatic adsorption, the percentage of removal was 98.4%, and thereafter the removal was gradually decreased to 45.6%. At higher pH the hydroxyl ion concentrations were increased which compete for the surface active sites, therefore the removal percentage was decreased for defluoridation21 and detoxification of chromium studies.22 On the other hand the heteroatoms present in the biopolymer matrix tend to form intramolecular hydrogen bonding with both HCrO4− and F− ions. The zero point charge of the D



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surface is negatively charged which leads to a decline in the adsorption capacity. According to the HSAB principle fluoride and lanthanum were categorized as hard base and acid correspondingly; they tend to form La−F bonding. The mechanistic pathway on the removal of CrO42−/F− was schematically represented in Scheme 1. The plausible Scheme 1. Mechanistic Pathway for CS−La−βCD toward the Removal of CrO42−/F−

Figure 4. Mapping images of CS−La−βCD.

mechanism for the removal of fluoride was a combination of hydrogen bonding and electrostatic force of attraction. In addition to that, a reduction reaction also takes place in the chromium removal mechanism. 3.6. Influence of Predator Ions. The complicated composition of groundwater and wastewater made the detoxification of CrO42−/F− a great task. The sorption capacity of CS−La−βCD onto the removal of CrO42−/F− was revealed in the presence of SO42−, HCO3−, NO3−, PO43− and Cl−. The experiments were carried out with the 200 mg/L of co-ions with the optimized dosage, initial concentration, and shaking time in the presence of CrO42−/F−. Figure 7 suggested that

Figure 5. TGA profiles of (A) CS−La−βCD, (B) Cr−CS−La−βCD, and (C) F−CS−La−βCD.

Figure 6. Effect of pH toward the removal of CrO42− and F− using CS−La−βCD [Co(CrO42−) = 100; Co(F−) = 10 mg/L; dosage, 0.1 g; time, 30 min; temp, 303 K]. Figure 7. Effect of predatory ions toward the removal of CrO42−/F− using CS−La−βCD. [Co (CrO42−) = 100; Co(F−) = 10 mg/L; dosage, 0.1 g; time, 30 min; temp, 303 K; co-anion concentration, 200 mg/L].

sorbent was determined using the pH drift method. CS−LaβCD showed a zero point charge of 4.56, pH < 4.56 provides a potential platform for effective sorption, above this value the E



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Figure 8. (A) Regeneration studies of CS−La−βCD on CrO42− removal [Co, 100 mg/L; dosage, 0.1 g; adsorption time, 30 min; eluent, 0.1 M NaOH; desorption time, 120 min; temp, 303 K]. (B) Regeneration studies of CS−La−βCD on F− removal [Co, 10 mg/L; dosage, 0.1 g; adsorption time, 30 min; eluent, 0.1 M NaOH; desorption time, 120 min; temp, 303 K].

chloride, phosphate, nitrate, and sulfate showed little impact on the detoxification of chromium. Meanwhile the defluoridation percentage was slightly altered with the interference of SO42−, NO 3− , PO 4 3−, and Cl − . Sulfate and bicarbonate ions significantly altered the defluoridation percentage which is due to their similar ionic radii of fluoride.23 The repulsive forces that persist between these ions lower the defluoridation capacity. In the case of chromium, bicarbonate ions released hydroxyl ions in water to compete for active sites. 3.7. Regeneration Studies. Recycle studies were performed to explore the sustainability, reusability, and costeffectiveness of the CS−La−βCD as shown in Figure 8. The 0.1 g of spent sorbent was treated with 0.1 M NaOH and was subjected to shaking for about 2 h, then it washed with doubly distilled water to remove the excess alkali. The desorbed sorbent was tested up to five consecutive cycles for the sorption of CrO42− and F−. The degradation of removal capacities of CS−La−βCD was observed in both cases after each successive cycle due to the loss of weight in the sorbent. In the first cycle adsorption of CrO42− was about 47.5 mg/g and after the fifth cycle it was about 35.2 mg/g. The same trend was followed in the fluoride cycle that the adsorption capacity was decreased from 4.5 mg/g to 2.8 mg/g after five successive cycles. 3.8. Thermodynamic Stability. The various thermodynamic parameters, namely, standard Gibbs free energy change ΔG°, standard entropy change ΔS°, and standard enthalpy change ΔH° were evaluated using the following relationship.24,25 ΔGo = −RT ln Ko

Table 1. Isotherm Models of CS−La−βCD on the removal of CrO42− and F− thermodynamic parameters −1

ΔG° (kJmol )

ΔH° (kJmol−1) ΔS° (kJ mol−1 K−1)

ΔS o ΔH o − R RT

F−

−8.13 −7.65 −6.86 27.42 0.06

−4.57 −4.55 −4.48 5.96 0.05

positive value of ΔH° 27.42 and 5.96 (kJ/mol) implied that the adsorption on CS−La−βCD was an endothermically driven process for Cr(VI) and fluoride, respectively. The positive values of entropy indicated the randomness in the sorbent solution interface. 3.9. Effect of Contact Time and Kinetic Modeling. To define the adsorption capacities and rate of the reaction of CS−βCD and CS−La−βCD on removal of CrO42−/F−, the batch experiments were carried out as a function of time until the state of equilibrium was reached, and the results were shown in Figure S1A and S1B. The effect of defluoridation capacity and detoxification of chromium were demonstrated using 10 mg/L and 100 mg/L of respective initial concentrations with the 0.1 g dosage of each adsorbent by varying the time between 5 and 60 min. It was found that the removal increased with increase in time. The maximum sorption capacity was found to be quick within 15 min, and then equilibrium was attained slowly after 30 min for all the experiments. This scenario was due to the availability of active sites that could easily encounter the highly dispersed adsorbate molecules. On the other hand, an increase in time leads to a slow down of the adsorption phenomenon because of saturation of sorbent active sites. The defluoridation capacities of CS−βCD and CS−La−βCD were found to be 2.8 and 4.9 mg/g, and the adsorption capacities on the removal of CrO42− were 17.1 and 48.4 mg/g. Because of the high adsorption capacity, the sorption experiments were limited to CS− La−βCD. The adsorption kinetics was studied using pseudo-first-order and pseudo-second-order models30,31 and their expressions are as follows:

(3)

Van’t Hoff isotherm:

ln Ko =

303 K 313 K 323 K

CrO42−

(4)

where R is universal gas constant (8.314 J/mol K−1), Ko is thermodynamic equilibrium constant, and T is experimental temperature (K). These temperature-dependent studies were carried out at 303, 313, and 323 K. The negative value of ΔG° presumed that the spontaneous and feasibility of the adsorption process for both CrO42− and F− and the values were represented in Table 1. The ΔG° values were decreased with increase in temperature and lies between in the range of 0−20 (kJ/mol) depicted the physical nature of the adsorption. The

log(qe − qt ) = log qe − kad F

t 2.303

(5)



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Table 2. Isotherm Parameters of CS−La−βCD on the Removal of CrO42−/F−

Parameters

303

1/n n KF(mg g−1)(L mg−1)1/n r sd χ2

0.3142 3.1824 26.1782 0.9787 0.0142 0.0489

Q0(mg g−1) B (L g−1) RL r sd χ2

91.5751 0.1224 0.4948 0.9346 0.0186 0.1991

kDR (mol2 J−2) Xm (mg g−1) E (kJ mol−1) r χ2

3.19 × 10−06 73.2838 0.0396 0.7064 0.6421

t 1 t = + qt h qe

CrO42−

F−

Temperature (K)

Temperature (K)

313

323

Freundlich Isotherm 0.2780 0.2078 3.5967 4.8132 31.5617 36.7333 0.9523 0.9715 0.0201 0.0193 0.1137 0.6781 Langmuir Isotherm 96.9932 97.9432 0.1697 0.5100 0.4924 0.4780 0.8881 0.8744 0.0203 0.0237 0.4298 1.1468 Dubinin−Radushkevich Isotherm 1.96 × 10−06 3.65 × 10−07 73.4760 73.9167 0.0608 0.1170 0.7082 0.8351 1.0409 0.9833

303

313

323

0.2987 3.3480 3.6498 0.9829 0.0080 0.0489

0.2960 3.3782 3.8506 0.9774 0.0095 0.1137

0.2819 35462 4.1307 0.9985 0.0193 0.0109

8.1446 0.5544 0.2894 0.9615 0.0186 0.0026

8.3236 0.6108 0.2938 0.9634 0.0203 0.0026

8.3354 0.7361 0.2942 0.9945 0.0237 0.0015

4.70 × 10−07 6.6265 1.0310 0.8890 0.0068

3.92 × 10−07 6.8378 1.1290 0.8780 0.0075

3.21 × 10−07 7.0529 1.2470 0.9061 0.0056

(a) Langmuir,26 (b) Freundlich,27 and (c) Dubinin−Radushkevich28 isotherms. The various parameters evaluated from these equations endowed important information on the adsorption mechanism, adsorption capacity, and surface chemistry. The linear form of the equation and their respective plots were represented in Table S1. The Langmuir parameters KL, (Langmuir constant), and Q0 are energy of adsorption and maximum adsorption capacity for the monolayer coverage. Weber and Chakkravorti derived the significant parameter RL which is dimensionless equilibrium parameter.29 From this value we can conceive that the adsorption nature is unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). The RL values between 0 and 1 indicates the favorable conditions for adsorption of CrO42−/F− onto CS−La−βCD. The Qo values for CrO42− and F− increasing with an increase in temperature demonstrated the adsorption reaction is an endothermically driven process. The calculated isotherm data for adsorption of CrO42− and F− onto CS−La−βCD are represented in Table 2. KF is a Freundlich constant, which relates that the bonding energy was increased with an increase in temperature, denotes the endothermic nature of adsorption. The value of n > 1 and 1/n < 1 indicated the favorable conditions for the adsorption. In the D-R isotherm, the values of EDR depict the physical nature of adsorption. The best fit of the isotherm data was assessed on the values of correlation coefficient (r) calculated from the nonlinear regression plots and the standard deviation values. On the basis of the agreement with the abovementioned criteria the best fit orders are Freundlich > Langmuir > D-R and Freundlich ∼ Langmuir > D-R for CrO42− and F− removal onto CS−La−βCD, respectively. The performance of CS−La−βCD was compared with the related sorbents which are reported previously which is represented in Table 3.The comparative assessment authenticated the efficacy of lanthanum incorporated chitosan/β-cyclodextrin composite toward the removal of CrO42−/F−.

(6)

The kinetic parameters derived from the plot of log (qe − qt) vs t and t/qt vs t. The pseudo-second-order model showed the better fitness of data due to the higher values of the correlation coefficient, and the lower standard deviation showed the linear fit of the data as compared to the pseudo-first-order model. The rate constants and other parameters for the kinetics of CrO42− and F− removal were tabulated in Tables S2 and S3. In addition to this the rate of adsorption rises due to the diffusion process which is a consequence of the reaction steps: (a) the sorbent molecules are transported from the bulk of the solution to the external surface; (b) film diffusion is followed by intraparticle diffusion; (c) particles move to the interior sites of the sorbent. The intraparticle diffusion rate constants ki and pore diffusion constants kp were calculated from the slope of their corresponding plots of qt vs t1/2, ln(1 − Ct/Ce) vs t, respectively. From the higher r2 values, particle and pore models depict that diffusion process would influence the rate of the reaction. 3.10. Effect of Initial Concentration and Isotherm Studies. The initial concentration is one of the indispensible parameters to characterize the adsorption reaction. The removal efficiencies were tested with different CrO42−concentrations (50−200 mg/L) and F− concentrations (10−20 mg/L) with 50 mL of adsorbate solution with 0.1 g of CS−La−βCD and optimized contact time of 30 min. Because of the dynamic collisions between the solid and liquid interface, the removal capacity was increased with the increase in initial concentration for both CrO42− and F− represented in Figure S2. The removal efficiencies were decreased after 100 mg/L and 10 mg/L of CrO42−/F−, respectively, due to the limited number of active sites of the adsorbent. To establish the interaction behavior of the adsorption reaction between per unit mass of the adsorbent and the equilibrium concentration of adsorbate molecules present in the bulk of the solution. The isotherm data were modeled using G


Journal of Chemical & Engineering Data Table 3. Comparative Assessment of Adsorption Capacity of CS−La−βCD with Already Reported Materials

adsorbent

1. 2.

lanthanum incorporated chitosan beads chitosan supported mixed metal oxide beads La(III) encapsulated silica gel/chitosan composite lanthanum impregnated silica gel Al−Zr impregnated cellulose beads Zr−Mn composite Alginate entrapped Fe(III) Zr(IV) binary mixed oxide Fe−Al-impregnated granular ceramic adsorbent CoFe2O4/ activated carbon composite carbon nanotube supported ceria nano particles magnetic cyclodextrin−chitosan/ graphene oxide (CCGO) Chitosan lanthanum loaded chitosan−βcyclodextrin composite

3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

CrO42−

F− 4.70 6.4

6.12

ref 32 33

35 19 36 37

3.56

38

83.33 26.8

39 40

67.66

41

91.58

8.14

present study

4. CONCLUSIONS CS−βCD was successfully incorporated with lanthanum and showed the enhanced detoxification of chromium and defluoridation studies. The maximum Langmuir adsorption capacities were 97.67 and 8.44 (mg/g) at 323 K for CrO42− and F −, respectively. Among the various isotherm models Freundlich and Langmuir−Freundlich were more appropriate with higher correlation coefficient values for CrO42−/F− removal expressed by the physical nature of adsorption. The rate of the reaction was followed by pseudo-second-order kinetics, and also adsorption dynamics indicated the endothermic spontaneous nature of adsorption. The higher adsorption capacity and the recycle and recovery studies revealed that CS−La−βCD is a promising sorbent for practical implication to meet the industrial needs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00889. Effect of contact time and initial concentration; linear forms of isotherm equation and their plots; kinetic models of CS−La−βCD toward CrO42−and F− removal (DOCX)

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REFERENCES

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34 3.80 5.76 3.05 0.98

ACKNOWLEDGMENTS

J.P. is grateful to the Department of Chemistry, The Gandhigram Rural Institute-DU, for providing facilities to carry out this work.

adsorption capacity (mg/g) S. No.

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Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-451-2452371. Fax: +91-451-2454466. E-mail: [email protected]. ORCID

Sankaran Meenakshi: 0000-0003-3350-2325 Notes

The authors declare no competing financial interest. H



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