Synthesis, characterization and As(III) adsorption behavior of β-cyclodextrin modified hydrous ferric oxide

Journal of Industrial and Engineering Chemistry 20 (2014) 1741–1751

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Synthesis, characterization and As(III) adsorption behavior of b-cyclodextrin modified hydrous ferric oxide Indranil Saha a,d, Kaushik Gupta a, Sudipta Chakraborty b, Debashis Chatterjee c, Uday Chand Ghosh a,* a

Department of Chemistry and Biochemistry, Presidency University, 86/1 College Street, Kolkata, 700073, India Department of Chemistry, Kanchrapara College, N-24-Parganas, PIN-743145, India Department of Chemistry (Analytical), University of Kalyani, Nadia, PIN-741235, India d Department of Chemistry, Sripat Singh College, Jiaganj, Murshidabad, PIN-742123, India b c

A R T I C L E I N F O

Article history: Received 11 January 2013 Accepted 21 August 2013 Available online 1 September 2013 Keywords: Arsenic Adsorption b-cyclodextrin Hydrous ferric oxide Groundwater.

A B S T R A C T

This study investigates the adsorption of As(III) on b-cyclodextrin modified hydrous ferric oxide (HCC). This is characterized by XRD, FESEM, AFM, XPS, BET, surface site concentration and FTIR. The modification of hydrous ferric oxide (HFO) surface by b-cyclodextrin provides ample –OH groups which in turn increase As(III) adsorption on HCC compared to HFO. The adsorption remains almost constant in pH range 3–8 which decreases at higher pH (>8) and followed monolayer and pseudo first order kinetics. It is spontaneous at 303 K with increasing entropy and decreasing enthalpy. Thus HCC is found to be more efficient adsorbent than HFO. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction The metalloid arsenic (As) which has been inflicting havoc on mankind for the past few decades and has assumed global proportions transcending geographical boundaries and communities, has targeted water which is the support system of a civilization as the medium for its transportation into the environment and biological systems which explains the countless deaths it has left behind in its wake [1]. The cases of As poisoning through drinking water can be traced long back in the world history. In 1898, the first cases of skin cancer were observed among population consuming As contaminated water in Poland [2]. Since then it has been conclusively established that unabated ingestion of As via drinking water causes a variety of carcinogenic effects of skin, liver, kidney and other organs which ultimately leads to death [3,4]. All these incidents have led to World Health Organization (WHO) and other national agencies to fix the permissible limit to 10 mg L1 of As in drinking water [5]. The principle source of As contamination is groundwater and the majority of rural and semi urban population in vast parts of the globe depend on this form of water for their drinking water and

* Corresponding author. Tel.: +91 33 2241 3893; fax: +91 33 2241 3893. E-mail address: [email protected] (U.C. Ghosh).

other household needs. The problem associated with As contamination is severe in Bangladesh and major parts of West Bengal, India [6,7]. In West Bengal, more than 6 million people from 12 districts covering 111 blocks are worst affected and consuming drinking water with As concentration >50 mg L1 [8–10]. Arsenic occurs in groundwater mainly in trivalent and pentavalent oxidation states. As(III) is much more toxic soluble and mobile than As(V) [11,12]. Furthermore, inorganic As has been reported to be about 100 times more toxic than the corresponding organic form [13]. The chemical characteristics such as pH and redox potential govern the As speciation in groundwater. As(V) (as H2AsO4 and HAsO42) is the predominant form of As in oxic waters, while As(III) occurs predominantly as H3AsO30 and H2AsO3 in reduced environments [14]. This type of speciation naturally has a profound effect on its removal/remediation processes. For example, As(III) is a neutral species in the pH ranges of natural waters which accounts for the fact that it does not thermodynamically interact with other elements, making more difficult to remove from water using various treatment processes. Above pH 8.0, As(III) dissociates into negatively charged ionic forms, making this state more reactive to treatment processes. Among the various treatment processes/technologies developed and refined over the years for As remediation, adsorption has emerged as a particularly attractive candidate for As eradication chiefly due to its low cost and simplicity. Since As has a high

1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.08.026

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affinity toward Fe, the development of Fe based adsorbents has received considerable attentions in recent years to treat As contaminated drinking water [15–19]. Among these, Fe based inorganic hybrid adsorbents, especially binary metal oxides have shown enhanced efficacy in removing trace concentrations of As from contaminated groundwater [20–22]. The hydrous ferric oxide (HFO) is a promising adsorbent for both As(III) and As(V) [19]. Modification of HFO surface using cyclodextrin increases its adsorption capacity compared to that of pure HFO. Among three forms (a, b and g), b-cyclodextrin is a well-known toroidal oligosaccharide which is non-toxic, easily available, relatively inexpensive and widely used in separation studies and drug delivery [23,24]. It is composed of seven a-(1,4)-linked glycosyl units and contains several –OH groups which can provide several binding sites for metal oxides [23]. Cyclodextrin usually forms inclusion complex with different inorganic and organic moieties [25–29]. Cyclodextrin or cyclodextrin based derivative inclusion complex with different metal oxides is applied in adsorption technology for removal of organic and inorganic water pollutants. Due to the presence of large number of surface –OH groups they possess high adsorption capacity through either surface complexation or hydrogen bond formation [30– 34]. The aim of this contribution is to modify the surface of HFO by b-cyclodextrin and to study its adsorption capacity toward As(III) compared to pure HFO. We have synthesized this novel hybrid material by simple precipitation technique to apply in removal of As(III) from aqueous solution. The present investigations provide promising results demonstrating that HFO modified by b-cyclodextrin would be a better scavenger of As(III) from contaminated groundwater as against conventional pure HFO. 2. Materials and methods 2.1. Chemicals All reagents used were of analytical grade and used without further purification. Arsenic (III) oxide (As2O3), ferric chloride (FeCl3), sodium chloride (NaCl), silver diethyl dithiocarbamate (SDDC), potassium iodide, sodium borohydride and b-cyclodextrin were obtained from Aldrich (USA), Merck (India) and Loba Chemie (India). 2.2. Preparation of hydrous ferric oxide-cyclodextrin composite (HCC) A ferric chloride solution was prepared in 0.1 M HCl and mixed with an aqueous solution of b-cyclodextrin in a 1:10 weight ratio. The mixture was warmed at 90 8C accompanied by gradual addition of dilute (1:1) NH3 solution with constant stirring till the pH reached around 7. The dark brown precipitate formed was aged for 48 h, filtered, and washed repeatedly with deionized water. The filtered mass obtained was dried at 100 8C into an air oven. The fully dried mass was ground in a mortar and sieved to obtain a particle size between 60–100 mesh (250–150 microns) and subsequently used for adsorption studies. 2.3. Arsenic solutions A standard stock As(III) solution (1000 mg L1) was prepared by dissolving 0.1320 g of As2O3 in 10 mL of 4% (w/v) sodium hydroxide, acidified with 2.0 mL of concentrated hydrochloric acid, and diluted to 100 mL with deionized water. The working solutions of required As(III) concentrations were made by diluting the stock with 0.2% (v/v) hydrochloric acid. The stock solution was prepared freshly after every 3 days and frozen to prevent oxidation of As(III) to As(V).

3. Analytical methods 3.1. Arsenic analysis The concentration of As in solution was analyzed (in triplicates) by UV–VIS spectrophotometer (Hitachi model 3210) using the procedure described in Standard Methods for Examination of Water and Wastewater [35]. Here, the dissolved inorganic arsenic in samples was determined by adding hydrochloric acid (32%, v/v), potassium iodide (10%, w/v), and sodium borohydride (3%, w/v). The arsine (AsH3) gas generated was absorbed in silver diethyl dithiocarbamate (SDDC) solution in chloroform solvent, and the absorbance was measured at 520 nm against a blank and compared with the standard curve for the value. The detection limit of the used method is 10 mg L1 with precision of 5%. The mean values of these measurements are taken into account. 3.2. Characterization of HCC The X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), Xray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area method and Fourier transform infrared (FTIR) spectroscopy were used to characterize HFO-cyclodextrin composite (HCC). X-ray diffraction (XRD) patterns of the material was recorded using an X-ray powder diffractometer (Philips Analytical PW-1710) equipped with Cu-Ka radiation at a scanning speed 0.48 min1 between the angle 108 and 708 operating at voltage 40 kV and applied potential current 30 mA. Atomic force microscopic (AFM) images were taken by a commercial Nanoscope III (Digital Instruments, Santa Barbara, CA) using optical beam deflection to monitor the displacement of a micro-fabricated silicon cantilever having a spring constant of 80 Nm1 to visualize the topography of the composite surface. The composite before and after As(III) adsorption were analyzed by X-ray photoelectron spectroscope (XPS) using an angle resolved photoemission system (ARPES) and equipped with a twin anode non-monochromatic Xray source for performing XPS measurements providing Al Ka (1253.7 eV) radiations. All binding energies were referenced to the neutral C 1s peak at 284.7 eV to compensate for the surface charging effects. The surface area of the HCC and HFO were analyzed by Brunauer–Emmett–Teller (BET) method by N2 gas adsorption at 77 K using high-speed surface analyzer (model: ASAP 2000, Norcross, USA). The pHZPC (pH for zero surface charge) was determined by pH metric titration following the method described by Babic et al. [36]. Surface site concentrations of HFO and HCC were estimated following the fast titration method described by Mayant et al. [37]. 4. Batch experimental program 4.1. Batch As(III) adsorption experiment The adsorption experiments were carried out by a batch method. Appropriate stock solutions of As(III) were prepared by dissolving arsenic(III) oxide (As2O3) in deionized water from which the necessary As(III) solutions were prepared (concentrations: 5 and 10 mg L1) in 0.01 M NaCl. 0.01 M NaCl was used as background electrolyte during the course of the experiments. The adsorption experiments with varying pH range (3–11) and adsorption kinetics at pH 7.0 were conducted at 303 K in open atmosphere. The agitation speed used was 300  10 rpm. Detailed experimental conditions are listed in Table 1. In 200 ml polythene bottle, 25 mg of HCC were equilibrated with 50 ml 0.01 M NaCl as background electrolyte containing As(III) for 120 min. The pH adjustments have been made with 0.1 M HCl and/or NaOH. The pH

I. Saha et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 1741–1751 Table 1 Experimental conditions of adsorption kinetics, edge and isotherm experiments.

pH [HCC] (g L1) [As(III)]0 (mg L1) Time (min)

Adsorption kinetics

Adsorption edge

Adsorption isotherm

7.0 (0.1) 0.5 5, 10 0–120

3–11 1.0 10 120

7.0 (0.1) 0.5 10–100 120

[HCC] = concentration of HCC, [As(III)]0 = initial As(III) concentration.

(pHf) of the suspension was recorded by pH-meter (model LI-127, ELICO, India). Then the suspensions were filtered using 0.45 mm membrane filters, and the filtrates were analyzed for As by UV–VIS spectrophotometer (Hitachi model 3210) following Standard Methods for Examination of Water and Wastewater [35]. The As adsorption capacity was calculated using the relation [(Ci–Cf)V]/w, where Ci and Cf are the initial and the filtered solution arsenic concentrations (mg L1), V is the solution volume (L), and w is the mass (g) of the adsorbent added. 5. Result and discussion 5.1. Characterization of adsorbent (HCC) 5.1.1. SEM analysis Fig. 1 shows the FESEM image of HCC (Fig. 1A). The image indicates that the particles do not have any definite surface morphology; however the surface of the materials is quite smooth in nature. The EDAX spectrum (Fig. 1B) of the material shows the surface composition of the material. Three main elements O, Fe and C are present on the surface. The spectrum indicates the absence of impurities on the composite surface.

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5.1.2. AFM analysis of HCC Fig. 2 shows the AFM images (A–D) of pure HFO and HCC. Comparison of the 2D view of HFO and HCC shows that the surface of pure HFO contains high degree of roughness. After modifying the surface of HFO with b-cyclodextrin, the surface becomes partially smooth in nature which is in agreement with the FESEM image of the composite material (HCC). Particle size distribution curves of HFO and HCC obtained from AFM analysis are shown in Fig. 2E and F. The curves show that the average particle size of HFO is 150 nm and that of HCC is 200 nm. 5.1.3. FTIR analysis Fig. 3 shows the FTIR spectra of HFO, HCC and b-Cyclodextrin (b-CD). The peak patterns of HCC (spectrum C) are similar to that of pure HFO. However the HCC contains some additional peaks at 2926 cm1 and 1158 cm1. These peaks are common with those of b-CD, (spectrum B) which are attributed due to the C–H and C–O stretching mode of vibrations respectively [38–40]. It indicates that the surface of HCC is coated with b-CD. The peak at 690 cm1 in the FTIR spectrum of HCC is due to the presence of Fe–O bonds (comparable with the FTIR spectra of pure HFO in Fig. 3 and two peaks at 3400 cm1 and 1625 cm1are due to the –O–H stretching and bending mode of vibrations respectively which are attributed to the presence of surface hydroxyl groups and also adsorbed water molecules on the surface [41]. The FTIR analysis of HCC clearly indicates that HFO has been coated with b-CD. 5.1.4. XPS analysis Fig. 4 shows the XPS spectra of HCC. The peak at 711.9 eV (spectrum B) is due to the 2p electron of Fe and peak at 532.0 eV (spectrum C) value indicates the O 1s electron. The peak at 286.1 eV showed the presence of C–OH/C–O–C bonds on the HCC surface which clearly reveals that HFO was modified with bcyclodextrin [42]. 5.1.5. XRD analysis The XRD pattern of HCC sample presented in Fig. 5 indicates its poorly crystalline nature. Peaks at 82u values 26.5, 35.4, 39.1 indicate that the iron oxide present in the sample is of hematite variety. The peaks position agrees well with that reported by the Joint Committee on Powder Diffraction Standards (JCPDS) database (No. 3-863 and 21-1272).

Fig. 1. (A) Field emission scanning electron microscopy (FESEM) of HCC (B) EDAX spectra of HCC.

5.1.6. BET surface area analysis The Brunauer–Emmett–Teller (BET) method provides a convenient method to determine the specific surface area of a material. BET analysis was done for both HFO and HCC, shown in Fig. 6A and B respectively and it was found that while the BET surface area of HFO was 16.28 m2 g1 with pore volume 0.0206 cm3 g1, HCC presented us with a remarkably low value of 0.05 m2 g1. This should not be surprising as modification of HFO with bcyclodextrin drastically increases the number of surface –OH groups. As discussed earlier, this results in heightened adsorption capacity of the composite as evidenced by the considerable increase of the number of sorption sites per nm2 of HCC as compared to HFO. However this phenomenon appreciably diminishes the surface area as observed by the low BET surface area of HCC. This type of observation has already been corroborated by Wilson et al. [43]. Formation of such inclusion complexes also results in the blocking of pores as evidenced by the insignificantly low pore volume of HCC as compared to HFO (Plot not shown). Ponchel et al. [44] explain this trend by correlating this phenomenon with the covering of the sorption sites by organic moieties such as cyclodextrin immobilized on the inorganic surface, which deters the N2 molecules from accessing the binding sites.

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Fig. 2. (A) Atomic force microscopy (AFM) image of HFO, (B) AFM image of HCC; (C) size variation curve of HFO and (D) size variation curve of HCC. Particle size distribution curves are shown for (E) HFO and (F) HCC.

5.1.7. Estimation of zero point surface charge (pHZPC) The zero point charge or ZPC is the pH value of a solution in equilibrium with a particle whose net charge from all sources is zero. The ZPC of oxides serves as a convenient reference point in predicting how the surface charge and potential depend on the pH [45,46]. Zero point surface charge (pHZPC) of both HCC and HFO was estimated following the method described by Babic et al. [36]. When the solution pH remains below the pHZPC value the surface of

the material remains predominantly positively charged, it can be designated as S–OH2+ and above the pHZPC value the surface is predominantly negatively charged which can be designated as S– O. The pHZPCvalue of HFO was found at 7.1 and that of HCC at 7.9. The ZPC value of HFO was reported previously within the range 6.8–7.7 [21] depending on its nature. When HFO was grafted with b-cyclodextrin the ZPC value of the pure oxide (HFO) was shifted to slightly higher pH value as given in Fig. 6. This is due to the increase

I. Saha et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 1741–1751

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6

Intesity (cps)

4

2

0 10

20

30

40

Fig. 3. FTIR spectra of (A) HFO, (B) b-CD and (C) HCC.

50 o

of hydroxyl group on the sample surface. Increase of pHZPC value from the HFO to HCC indicates that pure HFO was modified by bcyclodextrin. 5.1.8. Estimation of surface site concentration Calculation of Gran’s function (direct fast acid base titration method) generated the surface site concentrations of HFO and HCC which were 1.65 and 600 adsorption sites nm2 respectively [37]. The values indicate the remarkable multiplicity of surface hydroxyl groups in HCC compared to HFO.

60

70

80

Fig. 5. X-Ray diffraction plot of HCC.

5.1.9. Effect of pH on As(III) adsorption Fig. 7 shows the percentage of As(III) adsorbed on both HCC and HFO as a function of final solution pH (pHf). It was found that As(III) adsorption slightly decreased up to about pHf 8.0 for both the adsorbents. Above this pHf value the adsorption decreased markedly up to pH 11.0. At pHf 7.0, 66.85% As(III) uptake was observed for pure HFO whereas for HCC 81.5% of the total As(III)

A

44000

2000

B

Intensity (a.u.)

1000

500

42000

41000

40000

39000

38000 0 0

200

400

600

800

700

Binding Energy (eV)

710

720

730

Binding Energy (eV)

C 50000

45000

Intensity (a.u.)

Intensity (a.u.)

43000

1500

40000

35000

30000

520

525

90

2

530

535

540

545

Binding Energy (eV) Fig. 4. Photoelectron spectroscopic (XPS) analysis of (A) HCC, (B) Fe 2p spectrum and (C) O 1s spectrum.

740

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11 10 9

pHf

8 7 6 5

: HC C : HFO

4 3 2 2

4

6

8

10

12

pHi Fig. 7. Plot of initial solution pH versus final solution pH after shaking with HFO and HCC for estimation of pHZPC of HFO and HCC (Adsorbent dose: 0.5 g L1, I = 0.01 M, T = 303 K).

: HCC : HFO

90

% of As(III) adsorbed

80

70

60

50

40

Fig. 6. Plot of BET Surface area for (A) HFO and (B) HCC. 2

was removed from solution. When the final solution pH was raised to 9.0 the As(III) adsorption dropped down to 55% for HCC. This trend is quite akin to the typical trend shown by As(III) adsorption of different adsorbents reported previously [22,47]. The pHZPC value of HCC lied within the range 7.6–7.9 indicating that below this pH range the surface charge of HCC was positive and above it surface becomes negative. At low pH the most dominant form of As(III) species present in the solution is As(OH)3 (pk1 = 9.2). Due to the presence of large number of surface –OH groups the neutral As(OH)3 species form strong H-bond with HCC surface at low pH region. At higher pH (pH > 9) values there must be some columbic repulsion between the solute and negatively charged adsorbent surface for which the adsorption percentage was decreased at high pH values [48]. 5.1.10. As(III) adsorption kinetics As(III) adsorption kinetics of both pure HFO and HCC were studied at T = 300 K and the data obtained are shown in Fig. 8. The result showed that maximum As (III) adsorption takes place within first 30 min of equilibration. After that the adsorption rate gradually decreases, which is due to the lack of availability of adsorption sites owing to the surface coverage by As(III) species. The model equations used for interpreting the kinetics data for arsenic adsorption were pseudo-first order [49], pseudo-second order [50] power function. The relevant equations [51] for the mechanism of As(III) adsorption on both HCC and HFO are given by Eqs. (1) and (2). Pseudo-first order : qt ¼ qe ½1  expðk1  tÞ

(1)

3

4

5

6

7

8

9

10

11

pHf Fig. 8. Plot of As(III) adsorption percentage as a function of final solution pHf(T = 303 K, [As(III)]0 = 10 mg L1, [HCC] = 1.0 g L1, agitation time = 120 min, agitation speed: 300  10 rpm, I = 0.01 M NaCl).

Pseudo  second order : qt ¼ t  k2  q2e =ð1 þ k2  qe  tÞ

(2)

where k1 (min1) and k2 (g mg1 min1) are the pseudo-first order and the pseudo-second order rate constants, respectively; qt and qe have their usual significance and described elsewhere. The kinetic parameters obtained by nonlinear analysis of data are summarized in Table 2. Comparison of nonlinear regression coefficient (x2) values indicates that the adsorption reaction on HCC followed the pseudo-first order rate equation but in case of HFO it followed the pseudo-second order rate equation. Thus, it can be conferred that the adsorption mechanisms are different for two different adsorbents. The values of pseudo-first order rate constants (k1) indicate that the rate of adsorption was higher at low solute concentration which is due to the fact that at low solute concentration number of available adsorption sites (HCC) per solute species is large. 5.1.11. As(III) adsorption isotherm Fig. 9 demonstrates the value of qe (mg g1) versus Ce (mg L1) at pHi = 7.0 and T = 303 K, which were analyzed separately using isotherm equations viz. the Langmuir (Eq. (3)) and the Freundlich (Eq. (4)) to predict the nature of reaction equilibrium. Langmuir isotherm : qe ¼ ðqm  K a  C e Þ=ð1 þ K a  C e Þ

(3)

I. Saha et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 1741–1751

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Table 2 Kinetic parameters estimated for the adsorption of As(III) on HCC and HFO at pH = 7.0 and T = 303 K. As(III) concentration (mg L1)

Adsorbent

5

HCC HFO HCC HFO

10

1

k1 (min

)

0.262  0.009 0.102  0.007 0.195  0.009 0.143  0.012

2

1

qe (mg g

)

4.418  0.022 3.973  0.072 12.290  0.093 12.124  0.219

R

x2

k2 (g mg1 min1)

qe (mg g1)

R2

x2

0.999 0.989 0.997 0.986

0.003 0.022 0.051 0.243

0.028  0.004 0.031  0.0032 0.125  0.016 0.017  0.001

13.07  0.236 4.45  0.083 4.62  0.053 13.24  0.156

0.990 0.993 0.995 0.997

0.168 0.013 0.010 0.059

related to the adsorption capacity (mg g1) and adsorption intensity, respectively. The isotherm parameters obtained after analyzing the isotherm data for both HFO and HCC are summarized in Table 4. Values of x2 (given in Table 2) indicate that for both the adsorbents Langmuir equation is the best fit model. Maximum monolayer adsorption capacity of HCC is twice (66.9 mg g1) than that of HFO (33.3 mg g1) [52]. This suggests that HCC can act as a novel material for scavenging As(III) from contaminated groundwater. A comparative study of As(III) adsorption by similar iron based adsorbents reported previously is included in Table 3.

A

14 12

-1

qe (mgg )

10 8 6 4 -1

: 10 m gL : 5 m g L -1

2

5.1.12. Energy of adsorption The equilibrium isotherm data shown as points in Fig. 10 have been analyzed by the Dubinin–Radushkevich (D–R) Equation (Eq. (5)) [56] to evaluate the adsorption energy.

0 0

20

40

60

80

10 0

120

t (m in .)

lnQ e ¼ lnQ m  K DR  e2

B

14

(5)

where Qe and Qmare the equilibrium and saturated adsorption capacities in (mol kg1), respectively, and KDR is a constant related to the free energy (mol2 kJ2) of adsorption. e, the Polanyi potential, is expressed by the Eq. (6) below.

12 10

-1

qe (mgg )

Pseudo-second order qt = t k2 qe2/(1 + k2 qe t)

Pseudo-first order qt = qe [1  exp(k1  t)]

8

e ¼ RT lnf1 þ ð1=C e Þg

6

where Ce has its usual meaning described elsewhere; R and T, respectively, are the molar gas constant in kJ mol1 K1 and absolute temperature in K. The Qmand KDR parameters, respectively, were evaluated from the intercept and slope of the plot of lnQe versus e2(Fig. 11), If the mean energy (E, kJ mol1) of adsorption, which can be calculated by computation of KDR (kJ mol1) in the following relation, ranges from 8.0 to 16.0, the adsorption should be of chemical nature but the value below this range also includes the physical nature of the adsorption process [57,58]. Equation (7) relates the mean free energy (E) of adsorption with KDR:

4

: 10 mgL -1 : 5 mgL

2

-1

0 0

20

40

60

80

100

12 0

t ( min.) Fig. 9. (A) Adsorption kinetics of As(III) on HCC at initial As(III) concentrations (T = 303 K, pHi = 7.0). (*), 5 mg L1; (&), 10 mg L1; –––– pseudo first order; - - - - pseudo second order. The symbols represent the experimental data, and the lines predicted the model fits. (B) Similar plot for HFO having similar conditions - - - - pseudo first order; –––– pseudo second order. (T = 303 K, pHi = 7.0  0.1, [HCC] = 0.5 g L1, agitation time = 120 min, agitation speed: 300  10 rpm, I = 0.01 M NaCl). ð1=nÞ

Freundlich isotherm : qe ¼ K F  Ce

(4)

where qm and Ka are the Langmuir constants related to monolayer adsorption capacity (mg g1) and adsorption equilibrium constant (L mg1), respectively. KF and n are the Freundlich constants

(6)

E ¼ ð2 K DR Þ0:5

(7)

The estimation of the KDR value for the present adsorption process showed that it is 7.93 kJ mol1. 5.1.13. Thermodynamic parameters Thermodynamic parameters for the present adsorption study were calculated using the standard relations available in the literature [59] and the data points shown in Fig. 12. The values of the standard enthalpy change (DH˚) and the standard entropy

Table 3 Comparison of Langmuir adsorption capacity of HCC for As(III) with literature values. Sorbent material

pH

Langmuir adsorption capacity (mg g1)

References

HCC Crystalline hydrous ferric oxide Crystalline hydrous titanium oxide Nanoscale zero valent iron Hydrous iron(III)-tin(IV) binary mixed oxide

7.0 (0.1) 7.0 7.0 7.0 7.0

66.9 33.3 31.7 2.4 43.8

Present work [20] [53] [54,55] [21]

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0.0

A

-0.5

25

-1

lnQe /mg g

-1

qe (mg g )

-1.0 20

15

: Experimental : Langmuir ------ : Freundlich

10

-1.5

-2.0

-2.5

5 0

5

10

15

20

25

C e (mg L

-1

30

35

40

-3.0

) 350

400

450

500

B

40

550

600

650

700

2

Fig. 11. D–R isotherm plot of As(III) adsorption on HCC. 35

7.0

25 20

6.5

15

: Experimental : Freundlich ------: Langmuir

10 5

lnCs/CL

-1

qe (mg g )

30

6.0

0 0

10

20

30

Ce(mg L

-1

5.5

40

)

Fig. 10. Equilibrium data for As(III) sorption by (A) HFO and (B) HCC. —, Freundlich; ––––, Langmuir. The symbols represent the experimental data, and the lines predicted the model fit. (T = 303 K, pHi = 7.0  0.1, [HCC] = 0.5 g L1, agitation time = 120 min, agitation speed: 300  10 rpm, I = 0.01 M NaCl).

5.0

0.00 30

0.003 1

0.003 2

0.003 3

0.0034

0.003 5

1/T Fig. 12. Thermodynamic parameters of HCC.

change (DS˚) were estimated from the plot (ln Cs/CL versus 1/T) shown in Fig. 12, which are given in Table 5. The slope of the line is positive which means that the As(III) adsorption reaction with HCC is exothermic in nature (i.e. DH˚ = ve). The value of standard entropy change for the present adsorption process is positive. This indicates that the randomness of the system increased during the adsorption reaction and this is quite common in case of general

adsorption processes. Standard Gibb’s free energy values were calculated to be negative for As(III) adsorption reaction on HCC at all the four temperatures. Thus, the adsorption reaction of As(III) species on HCC was spontaneous under the studied experimental conditions.

Table 4 Isotherm parameters evaluated for As(III) adsorption on HCC at pH=7.0 and T=303 K. Isotherm parameters Freundlich qe = KFCe(1/n)

Langmuir qe = qmKaCe/(1 + KaCe) 2

2

Adsorbent

R

x

HCC HFO

0.984 0.957

4.054 3.438

1

qm (mg g

)

66.96  9.16 33.55  2.573

Ka(L mg

1

)

0.035  0.01 0.132  0.03

R2

x2

KF (mg g1)(L mg1)1/n

n

0.958 0.902

10.392 7.867

3.818  1.277 7.176  1.533

1.567  0.239 2.574  0.465

Table 5 Thermodynamic parameters evaluated for As(III) adsorption on HCC at pH = 7.0. As(III) Concentration (mg L1)

DH˚ (kJ mol1)

DS˚ (kJ mol1 K1)

DG˚(kJ mol1) 288 K

5

4.2

0.007

6.504

303 K 6.624

318 K 6.774

333 K 6.864

I. Saha et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 1741–1751

80

1749

: KOH : NaOH

75 70 65 60

% of Desorption

55 50 45 40 35 30 25 20 15 Fig. 15. FTIR spectra of (A) pure HCC and (B) As(III) adsorbed HCC.

10 5 0 0.0

0.2

0.4

0.6

0.8

1.0

Base Concentration (M) Fig. 13. Desorption of arsenic from the As(III)-adsorbed HCC. (T = 303 K, pHi = 7.0  0.1, [HCC] = 0.5 g L1, agitation time = 120 min, agitation speed: 300  10 rpm, I = 0.01 M NaCl).

5 .0

A

5.1.15. Effect of some competing ions Effect of co-occurring ions (sulphate, phosphate) on As(III) adsorption was studied and is shown in Fig. 14. It was found that both sulphate and phosphate interfere with the As(III) adsorption on HCC markedly. Between the two ions, phosphate showed more marked interference than sulphate. In presence of 10 mg L1 phosphate the As(III) adsorption was nearly 0.4 mg g1.

4 .5 4 .0

-1

qe(mg.g )

3 .5 3 .0 2 .5 2 .0 1 .5 1 .0 0 .5 0

5

10

15

20

25

30

2-

[SO 4 ]/[A s(III)] 5

B

4

-1

qe(mg.g )

3

2

1

0

0.0

5.1.14. Desorption study Desorption of adsorbed As from any material is very much essential for regeneration and reuse purpose. For the present case of desorption, it was carried out using NaOH and KOH as desorbing agents. The result obtained, shown in the Fig. 13, indicates that 75% of the adsorbed As is desorbed by treating with 1 M NaOH and 30% with 1 M KOH. Thus, NaOH always showed greater desorption ability than KOH. Desorption study with highly concentrated base (>1 M) was not performed considering the possibility of dissolution of adsorbent (HFO based).

0.5

1.0

1 .5

2 .0

3-

[P O 4 ]/[A s(II I)] Fig. 14. Effect of some co-occurring ions (A) SO42, (B) PO43 on As(III) adsorption of HCC (T = 303 K, pHi = 7.0  0.1, [HCC] = 0.5 g L1, agitation time = 120 min, agitation speed: 300  10 rpm, I = 0.01 M NaCl).

5.1.16. Adsorption mechanism Considering the effect of changes of initial solution pH on the As(III) adsorption, energy of adsorption study, comparison of the FTIR spectra of pure and As(III) adsorbed HCC and also from the study of desorption a mechanism of As(III) adsorption on HCC under the present experimental condition were developed. The increased adsorption on HCC compared to HFO is a resulting effect of physical and chemical adsorption where b-cyclodextrin provides a large number of –OH sites for adsorption. The initial pH of the solutions after As(III) adsorption were shifted to a very small extent which also indicated the weak bond formation. Comparison of FTIR spectra of pure HCC (Fig. 3C) with As(III) adsorbed HCC (Fig. 15) gives a concrete idea about the adsorption mechanism. The intensity of –OH stretching mode of vibration is lowered to a large extent which clearly indicates that the –OH bond strength of HCC was decreased after As(III) adsorption. This is possible only when a weak hydrogen bond formation occurs between the surface –OH group of HCC and As(III) species. For HCC the total number of –OH groups present per unit area are much higher than HFO (as HCC provides more reactive –OH sites) which facilitates effortless binding with As(III) through H-bonding which is not the case with HFO. Intensity of Fe–O bond also decreased indicating the weakening of Fe–O bond. This observation indicated that the entrapped hydrated iron oxide particles within the cavities of b-CD also form weak H-bond with As(III) species entered into those cavities. On the basis of the above discussion a schematic diagram of As(III) adsorption by HCC has been shown in Fig. 16.

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I. Saha et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 1741–1751

Fig. 16. Schematic diagram of As(III) adsorption on HCC.

6. Conclusion Hydrous ferric oxide (HFO) has proven to be a potent adsorbent for dissolved As(III) in contaminated water. However, surface modification of HFO with b-cyclodextrin by a simple and economical route shows large enhancement of As(III) scavenging power. The systematic As(III) adsorption by this material has shown that the optimum pH and equilibrium contact time are 7.0  0.1 and 120 min, respectively. The pseudo first-order equation describes the kinetic data (pH, 7.0  0.1; temperature, 303  1.6 K) well. The equilibrium data (pH, 7.0  0.2; temperatures (1.6 K), 288, 303, 318 and 333) fit very well with the Langmuir isotherm model. The Langmuir monolayer adsorption capacity is 66.96  9.16 (mg of As per g. of HCC) at 303 K, and that increases with increasing temperature. The adsorption reaction is spontaneous (DG˚ = negative) and exothermic (DH˚ = negative), and it takes place with increasing entropy (DS˚ = 0.007). The energy (kJ mol1) of As(III) adsorption and the FTIR analysis have suggested that the As(III) adsorption on HCC is of both physisorption as well as chemisorption in nature. The As(III) adsorption by HCC is negatively influenced by phosphate and sulphate ions. The regeneration of As(III) adsorbed material is possible maximum up to 75% with 1 M NaOH solution. Acknowledgments The authors are grateful to the Head, Department of Chemistry and the Vice-Chancellor, Presidency University, Kolkata, India for providing laboratory facilities. Authors also acknowledge Dr. A. Nayak (Associate Professor), Department of Physics, Presidency University, Kolkata for XRD analysis, Prof. N.R. Roy and Mr. Harishankar Biswas, Surface Physics Division, Saha Institute of Nuclear Physics, Kolkata, India for AFM analysis; Mr. Swachchha Majumdar, Principal Scientist, Ceramic Membrane Division, Central Glass and Ceramic Research Institute, Kolkata, India for FESEM and EDAX analysis; Mr. Jayanta Das and his supervisor, Prof. Krishnakumar S R Menon, ARPES (Angle Resolved Photoemission Spectroscopy) laboratory, Surface Physics Division, Saha Institute of Nuclear Physics, Kolkata, India for XPS analysis of the samples. References [1] P.L. Smedley, D.G. Kinniburgh, Appl. Geochem. 17 (2002) 517. [2] J.A. Ritchie, N.Z.J. Sci. 4 (1961) 218. [3] L. Wang, A.S.C. Chen, N, Tong, C.T. Coonfare, EPA/600/R-07/017, U.S. EPA, Water Supply and Water Resources Division, National Risk Management Research Laboratory, Cincinnati, OH, 2007.

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