Manganese oxide incorporated ferric oxide nanocomposites (MIFN): A novel adsorbent for effective removal of Cr(VI) from contaminated water

Journal of Water Process Engineering 7 (2015) 176–186

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Manganese oxide incorporated ferric oxide nanocomposites (MIFN): A novel adsorbent for effective removal of Cr(VI) from contaminated water Abir Ghosh a , Madhubonti Pal a , Krishna Biswas b , Uday Chand Ghosh a , Biswaranjan Manna a,∗ a b

Department of Chemistry, Presidency University, 86/1College Street, Kolkata 700073, India Maharaja Manindra Chandra College, 20, Ramkanto Bose Street, Kolkata 700003, India

a r t i c l e

i n f o

Article history: Received 21 March 2015 Received in revised form 9 June 2015 Accepted 15 June 2015 Keywords: Nanocomposites Chromium(VI) Adsorption Kinetics Isotherms Thermodynamics

a b s t r a c t Manganese oxide incorporated ferric oxide nanocomposite (MIFN), a novel adsorbent has been synthesized, characterized and explored for the removal of Cr(VI) from contaminated water. Surface elemental composition of semi-crystalline adsorbent was analyzed by SEM-EDX spectroscopy and was found to contain Fe (42.5%), Mn (8.1%) and O (49.4%), confirming Fe/Mn mole ratio of 5.0. BET surface area, particle diameter, pore size and total pore volume of the adsorbent were estimated as 186.28 m2 g−1 , 5–7 nm, 81.2 A0 and 0.31 cm3 g−1 respectively. The adsorption of Cr(VI) was highly pH sensitive, and maximum adsorption was achieved within pH 2.0–3.0. Adsorption kinetics were described well by pseudo 2nd order model and were followed by both film diffusion as well as intra-particle pore diffusion mechanism. Langmuir isotherm was recognized to be the best fit model and maximum monolayer adsorption capacity was estimated as 47.84 mg g−1 for Cr(VI) at pH 3.0 at 303 K. Thermodynamic analysis revealed that the adsorption was highly favorable, spontaneous and endothermic in nature. Adsorption of Cr(VI) was strongly inhibited by phosphate and sulphate, whereas fluoride, carbonate, bicarbonate and silicate have no significant interference. Adsorption efficiency of the spent adsorbent (MIFN) could be rejuvenated around 78–80% by 1.0 M NaOH and subsequently be reused. The present study revealed that MIFN could be an efficient adsorbent for scavenging Cr(VI) from contaminated water because of its high adsorption capacity and reusability. © 2015 Published by Elsevier Ltd.

1. Introduction Heavy metals, released in the environment through anthropogenic sources have been increasing constantly and their persistency as well as non-biodegradability pose a serious threat because of their carcinogenic and mutagenic effects to the living kingdom. Among the heavy metals, Cr(VI) has widely been used in electro plating industry for surface coating to enhance hardness and corrosion resistance properties of alloys [1]. It has also been extensively used as paints, pigments (chrome yellow, chrome red, chrome green) and wood preservatives. The tanning industries have been recognized as large contributor of Cr pollution to water sources where chrome alum and Cr(III) sulphate have

∗ Corresponding author at: District Institute of Education & Training, West Bengal, India. Fax: +91 3214256325. E-mail address: [email protected] (B. Manna). http://dx.doi.org/10.1016/j.jwpe.2015.06.008 2214-7144/© 2015 Published by Elsevier Ltd.

largely been used in tanning of leather by cross linking collagen fiver [2,3]. In India, tanning industries are responsible for releasing about 2000–32 000 tons of elemental Cr into the environment annually, imparting total Cr concentration up to 5000 mg L−1 [4,5], even though the recommended permissible limit in the released effluents is only 2 mg L−1 [4]. Cr(VI) oxide also find its application in manufacturing of magnetic tape and high performance audio tape. Additional input of Cr into the environment also derived from phosphate fertilizer (100 mg kg−1 ). Other sources of the Cr emission include oil and coal combustion, stainless steel welding, steel production, cement plants and cooling towers in which Cr(VI) is used as a rest inhibitor for their submerged moving parts [6]. Cr(VI), the predominant species above pH 2.5 [7] is found to be the more toxic and more mobile than Cr(III) and is considered to be extremely hazardous due to its carcinogenic and mutagenic effects, more particularly for human health [8,9]. The United State Environmental Protection Agency (USEPA) has identified Cr(VI) as one of the 17 chemicals posing the greatest threat to humans [10]. Con-

A. Ghosh et al. / Journal of Water Process Engineering 7 (2015) 176–186

sequently, the recommended maximum permissible limit of Cr(VI) in portable water is set to 0.05 mg L−1 (WHO). Thus, the attenuation of Cr(VI) level from aqueous streams or waste effluents is an important issue prior to discharge into the environment. Numerous technologies have been adopted so far concerning removal of Cr(VI) from superficial waters including chemical precipitation [11], ion-exchange [12], membrane separation [13,14], ultra filtration [15], reverse osmosis [16] and electrochemical coagulation [17]. The chemical precipitation has traditionally been used but, the major disadvantages associated with this process is the production of large volume of sludge imparting disposal problems. Ion-exchange is considered to be a better alternative, but it is not economically appealing because of its high operational costs. Other available treatment technologies, such as ultrafiltration, membrane separation and reverse osmosis require high capital investment and running cost [18,19]. Apart from those, surface adsorption technology has evolved as promising method and may be effective as well as economically viable depending on adsorption capacity, availability and reusability of the exhausted adsorbent. Various adsorbents had been studied for removal of Cr(VI) including bio-sorbents and natural as well as synthetic inorganic materials. The most studied adsorbent was found to be the activated carbon derived from various sources [20,21]. The inorganic materials used for Cr(VI) adsorption were activated red mud [22], akagonite [23], composite alginate–goethite beads [24], titanium oxide [25], stannic oxide [26], ferric oxide [27], Maghemite [28], zirconium oxide [29], nanostructured cerium oxide [30] and nanocomposites of various metal oxides [31–33] and reduced-grapheme oxide based supermagnetic nanocomposite [34]. Adsorption technology might be effective for removal of Cr(VI) when low cost adsorbents with high adsorptive potential are being used in combination with appropriate regeneration and recycling rendering the process economically viable and minimizing the sludge disposal problems. The primary objective of the present work is to synthesize a new material, which could be capable for scavenging Cr(VI) from contaminated water in a more efficient manner, compare to the reported works. In the present study, manganese oxide incorporated ferric oxide nanocomposites (MIFN) has been synthesized and characterized by various spectroscopic tools. Its efficiency in removing Cr(VI) from contaminated water has been explored by various reaction kinetics and equilibrium modeling. The effect of interfering ions which are commonly found in surface water and waste water has also been carried out. Finally, the technological and economical viability of the process has been assessed by considering its adsorption capacity, regeneration efficiency and reusability. 2. Materials and methods 2.1. Preparation of Cr(VI) and 1,5-diphenyl carbazide solution 2.83 g of potassium dichromate (K2 Cr2 O7 ) (AR grade, BDH, England) was dissolved in double distilled water and diluted to 1000 ml. The concentration of this stock Cr(VI) solution was 1000 mg L−1 and was used for the preparation of desired concentrations by appropriate dilution before the experiment. 25% 1, 5-diphenyl carbazide solution was prepared by dissolving 0.25 g 1, 5-diphenyl carbazide (AR grade, BDH, England) in 100 ml 1:1 acetone–water solution and was kept in refrigerator in absence of light prior to use for Cr(VI) analysis in samples. 2.2. Synthesis of MIFN The manganese oxide incorporated ferric oxide nanocomposites were synthesized by modified in situ chemical precipitation method [35]. Aqueous solution of 0.25 M MnCl2 and 0.25 M FeCl3

177

were mixed together in a reaction vessel in a controlled manner at the flow rate of 1:5 (v/v) followed by addition of 0.01 M NaOCl drop wise for oxidization of Mn2+ → Mn+3 with subsequent addition of 0.1 M NaOH to adjust pH at 6.0. The brown precipitate so formed was aged with mother liquid for 48 h and the solid mass was separated by filtration followed by washing with distilled water until free from chloride. It was then dried at 363 K for 12 h in an air oven and was sieved the agglomerates within the size of 140–290 ␮m. The method used for the preparation of iron-manganese mixed oxide nano materials was different from the previously reported methods using citrate gel [36], ultrasonication [37] and oxidation of Fe(II) by KMnO4 [38]. The strategy of the present method was in situ generation of Mn3+ /Mn4+ through oxidation of Mn2+ by NaOCl and simultaneous precipitation of the oxidized species with Fe(III) to form mixed hydroxide gel. This type of controlled oxidation–precipitation method from the mixed constituents might have the better possibility of incorporation for manganese ions into the crystals of iron oxide. 2.3. Analytical instruments UV–vis spectrophotometer (Hitachi, Model U-3210) was used for colorimetric analysis of Cr(VI) and ELICO-made pH meter (Model LI-127) was used for analyzing pH. X-ray diffraction (XRD) pattern of synthetic MIFN was taken using a Phillips X-ray diffractometer and Fourier transform infrared (FTIR) spectra were recorded using a Jasco 680 plus (Japan) spectrophotometer with a resolution of 2 cm−1 taking thin film of samples made by mixing with pure KBr in a ratio 3:100. Atomic force microscopic (AFM) images were recorded by a commercial Nanoscope III (Digital Instruments, Santa Barbara, CA) using optical beam deflection to monitor the displacement of a microfabricated silicon cantilever having a spring constant of 80 Nm−1 to visualize the topography of the oxide surface, particle distribution and surface roughness. Transmission electron micrograph (TEM) images were recorded by HR-TEMJEOLJEM-2100 electron microscope operated at 200 kV to estimate size of the particles and also to visualize the nature of agglomeration of synthetic oxides. The sample for the TEM image was dispersed in isopropanol by sonication, which was dropped to cast onto 200 meshes copper grids coated with a holey carbon film. Scanning electron microscopic (SEM) images with EDX (Tescan Vega, U.K.; model LSU+) was recorded for the sample spraying over the carbon tape to detect the surface morphology. Specific surface area (BET) was obtained from the nitrogen adsorption/desorption isotherm at 77 K using a fully automated Autosorb-1C gas adsorption system. The pH for zero-point charge (pHzpc) of MIFN was analyzed by pH metric titration [39]. 2.4. Experimental procedure 2.4.1. Estimation of Cr(VI) Cr(VI) solution was estimated spectrophotometrically using 0.25% 1,5-diphenyl carbazide solution in 50% acetone [40]. A definite volume of Cr(VI) solution was taken into a 25 ml volumetric flask followed by successive addition of 2.5 ml 2N sulphuric acid and 2 ml 0.25% 1,5-diphenyl carbazide solution and then the volume was made up to 25 ml with distilled water, allowed to stand for 15 min. Absorbance of the colored purple solution was measured spectrophotometrically at 540 nm with respect to the reagent blank having detection limit of 0.1–0.5 mg L−1 . 2.4.2. Batch sorption experiments 50 ml Cr(VI) solution of desired level of initial concentration (C0 mg L−1 ) at pH 3 was mixed with 0.05 g MIFN and agitated in a thermostat shaker with a shaking speed of 200 rpm at temperature of 288 K, 303 K and 318 K separately. After agitation for 3 h (equili-

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bration time), the reaction mixture was filtered through 0.45 ␮m membrane filter and filtrates were analyzed for the residual Cr(VI) concentration (Ce, mg L−1 ). The amount of Cr(VI) adsorbed (qe, mg g−1 ) at equilibrium was calculated thereon using the following equation (Eq. (1)): qe =

(Co − Ce ) V m

(1)

where C0 and Ce have their usual meanings as mentioned earlier, m is the mass of the adsorbent (g) and V is the volume (L) of the test solutions. Influence of pH on adsorption of Cr(VI) by MIFN, was conducted using two different concentrations of Cr(VI) (5 mg L−1 and 10 mg L−1 ) at initial pH (pHi ) within the range of 2.0–9.0 separately and optimum adsorption pH was estimated thereon. With a view to ascertain the reaction kinetics and the mechanism involved for adsorption of Cr(VI) on MIFN, the kinetic study has been carried out at pH 3.0 by varying initial concentrations ranging from 5 to 20 mg L−1 at 288 K, 303 K and 318 K. Isotherm modeling has also been performed by taking a wide range of Cr(VI) concentrations from 5 to 150 mg L−1 at a temperature of 288, 303 and 318 K separately at optimum pH maintaining experimental conditions as stated earlier and adsorption capacities (qe ) (mg g−1 ) were calculated using the mass balance Eq. (1) given above. For the effect of interfering ions, 50 ml Cr(VI) solution (C0 = 40 mg L−1 ) were agitated at 303 K with 0.05 g MIFN incorporating 1 mM foreign ions (fluoride, sulphate, silicate, bicarbonate, phosphate and carbonate) at optimum pH. 2.4.3. Desorption of Cr (VI) by NaOH solution With a view to estimate the economical viability of this adsorption technology, the regeneration efficiency of the exhausted adsorbent has been performed with the variation of NaOH concentrations from 0.2 to 1.0 M. Here, 0.5 g MIFN with adsorbed Cr(VI) of 47.9 mg g−1 was treated by 50 ml desired concentration of NaOH for 2 h at 303 K. The released Cr(VI) in leachate was estimated and after washing, the rejuvenated MIFN was being applied further for adsorption–desorption cycle as earlier. 3. Results and discussion 3.1. Characterization of MIFN The SEM–EDX analysis (Fig. 1) revealed that the elemental composition at the surface of MIFN were of Fe (42.5%), Mn (8.1%) and O (49.4%) indicating approximate Fe/Mn molar ratio of 5:1. This composition suggested that the co-precipitation of Fe(III) and oxidized Mn hydroxides from their respective chloride solutions occurred at the same rate due the almost identical ionic products of Fe(III) and Mn(III) hydroxides. The powder XRD patterns of synthetic MIFN nanocomposites and physical mixture of Fe2 O3 and MnO2 in Fe: Mn mole ratio of 5:1 were represented in (Fig. 2). The peaks at 28.210 , 31.290 , 33.20 , 35.720 , 40.90 , 43.380 , 53.900 , 57.450 , and 63.070 were obtained for physical mixture of Fe–Mn mixed oxides dried at 90 ◦ C (Fig. 2b) corresponding to the crystalline Fe2 O3 phase (33.20 , 35.720 , 40.90 , 43.380 , 53.900 , 63.070 ) and MnO2 phase (28.210 , 31.290 , 57.450 ) [35]. The XDR analysis of synthetic MIFN (Fig. 2a) revealed that the broad peak at 33.20 and moderate peaks at 40.90 , 43.380 , 53.900 , 63.070 can be assigned for Fe2 O3 phase whereas at 290 , 32.450 , 37.90 , 44.520 and 58.290 for Mn2 O3 phase derived from controlled oxidation of Mn2+ to Mn3+ by NaOCl. No distinct peaks were found at 36.10 and 60.240 indicating the absence of Mn3 O4 phase in the synthetic MIFN. This findings revealed that the synthetic MIFN was mainly consist of Fe2 O3 and Mn2 O3 phase, both of which were exist predominantly in +III oxidation states and agreed well with other researchers [35]. Moreover, lack of dis-

tinct peaks with sharp intensity indicates that MIFN is not well crystalline due to hydrous nature of the metal oxides composite. Additionally, the extent of crystallization depends on the Fe/Mn ratio and decreases in presence of one another during their in situ generation reflecting strong interaction between Fe and Mn oxides. Thus, the synthetic MIFN samples were semi-crystalline with single phase compound having formula Mnx Fe2 − x O3 ·yH2 O where x = 0.18 and was remained hydrated at 363 K [35,41]. However, the extent of crystallization was enhanced by incineration of MIFN at higher temperature (Fig. 2c) with the subsequent release of water of hydration leading to the condensation between hydrous metal oxides (2M − OH → M − O − M − + H2 O↑). The particle size was calculated from the XRD data using Scherrer’s Eq. (2): D=

(0.9) ˇ Cos 

(2)

where D is the particle diameter,  is the wavelength of X-ray used; ˇ and  are the peak width at half intensity in radian and half of the Bragg’s diffraction angle, respectively, and average dimensions of the particles were estimated <8.0 nm reflecting presence of nanoparticles in MIFN composites. The TEM analysis (Fig. 3) also revealed that the resultant MIFN composites were comprised of nanoparticles with average dimensions of 5–7 nm, which were aggregated together. The FTIR spectra of synthesized MIFN and pure Fe(III) oxide (Fig. 4) showed that the peaks at 3393 and 1416 cm−1 were associated with the stretching and bending vibration modes of O H bonds of lattice water molecules respectively. The peaks appeared at 995, 1090 and 1127 cm−1 were assigned corresponding to the bending vibration of hydroxyl groups (Fe–OH) and are responsible for the formation of inner sphere surface complexes. Moreover, peak obtained at 476 cm−1 was mainly due to the formation of Fe O Mn linkage, supporting incorporation of Mn in the crystal lattice of Fe(III) oxide [35]. Atomic Force Microscopic images demonstrated clearly that the surface topography of MIFN (Fig. 5) was rough and highly porous. The BET surface area, pore size and total pore volume of the adsorbent has also been measured and were found as 186.28 m2 g−1 , 81.2 A0 and 0.31 cm3 g−1 , respectively (Fig. 6). The zero point surface charge (pHzpc ) was estimated to be 7.2 which was found to be higher than the hydrous iron (III) oxide (6.2–6.7) and manganese substituted oxy hydroxide (6.7) [42]. The value of pHzpc indicated that the surface of MIFN should be positive at pHi < 7.2 and negative at pHi > 7.2. Thus, it is evident from the characteristic studies (Table 1) that the synthetic MIFN is poorly crystalline nanostructured material with high surface area and porosity that could be an efficient adsorbent for Cr(VI) from contaminated aqueous stream under acidic pH. 3.2. Adsorption of Cr (VI) by MIFN 3.2.1. Effect of initial pH The effect of pH on adsorption of Cr(VI) by MIFN have been investigated in a broad range of pH from 2.0 to 9.0 and the results were represented in Fig. 7. The adsorption was found to be highly pH dependent and maximum adsorption occurred within the pH of 2.0–3.0 and thereafter decreased sharply with the increase of pH. The removal of Cr(VI) decreased steadily from 99 to 60% with the increase of pH from 3.0 to 5.0 and thereafter reduced slowly to 25% with the decrease of pH at 9.0. The pH dependence of Cr(VI) adsorption onto MIFN can be understood by considering zero point surface charge (pHzpc ) of the adsorbent which was estimated as 7.2 ± 0.2 and prevalent species of Cr(VI) ions in the respective pH. The charge developed at the oxy or hydroxyl groups of MIFN surface may be either positive or negative for pH below or above pHzpc as a result of association or dissociation of protons. The speciation

A. Ghosh et al. / Journal of Water Process Engineering 7 (2015) 176–186

179

Fig. 1. SEM image of MIFN with EDX spectra.

of protonation leading to enhance adsorption of anionic Cr(VI) species (e.g., HCrO4 − , Cr2 O7 2− ) through electrostatic attraction, while above pHzpc the negative charge gradually developed at the solid surface with the increase of pH through deprotonation, allowing anionic Cr(VI) species to encounter columbic repulsion and subsequently the adsorption of Cr(VI) reduced drastically. Moreover, at higher pH, the high abundance of hydroxyl ions and its low negative charge in comparison to CrO4 2− ion would preferentially hindered the adsorption of Cr(VI). Thus, physicochemical adsorption through electrostatic attraction was the predominant mechanism of Cr(VI) removal from aqueous solution and maximum adsorption was achieved within pH 2.0–3.0. The result was found in close agreement with other researchers reported for various adsorbents such as rice bran [43], hydrous TiO2 [44], treated sawdust [45], activated carbon [46], synthetic magnetite [47] and fertilizer industry waste materials [48]. Fig. 2. XRD analysis of (a) synthesized MIFN dried at 363 K, (b) mixture of Fe2 O3 and MnO2 (5:1 w/w), (c) synthesised MIFN dried at 573 K.

of Cr(VI) in an inert environment without redox component indicates that Cr(VI) exist as HCrO4 − and Cr2 O7 2− in the ratio of 4:1 within the pH range 2–5.0, whereas it exists entirely as CrO4 2− within the pH of 7.5–14. All the three species co-exist in various proportions in the pH range 5–7.5, [29]. Thus, Cr(VI) has the most negative charge per species at pH values above 7.5 due to the presence of CrO4 2− only. With the lowering of pH below pHzpc , the solid surface become more and more positively charged as a result

3.2.2. Adsorption kinetics The kinetics of Cr(VI) adsorption over MIFN have been demonstrated with the variation of adsorbate concentrations from 5–20 mg L−1 at a temperature of 288, 308 and 318 K separately at pH 3.0. With a view to ascertain the adsorption kinetics with nonlinear least square fit method, the pseudo-first order (Eq. (3)) and the pseudo-second order (Eq. (4)) [49,50] model equations tested were as follows: qt = qe [1 − e−k1 t ]

Fig. 3. Transmission electron microscopic (TEM) image of MIFN composites.

(3)

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A. Ghosh et al. / Journal of Water Process Engineering 7 (2015) 176–186

10

-1

qe(mgg )

8

6

4

2

Fig. 4. FTIR spectra of (a) MIFN and (b) hydrated iron(III) oxide sample.

2

3

4

5

pH i

6

7

8

9

10

Fig. 7. Effect of pH on adsorption of Cr(VI) onto MIFN at 303K (䊏 10 mg L−1 , 䊉 5 mg L−1 ).

Table 1 Some physic-chemical characteristic of synthesized MIFN.

Fig. 5. AFM images of MIFN composites of different planes.

qt =

t × k2 × q2e 1 + (t × k2 × qe )

Chemical composition (%)

42.5% Fe, 8.1% Mn and 49.4% O

Probable formula Nature Particle diameter calculated from XRD peak pattern data Particle diameter obtained from the TEM image analysis BET surface area (m2 g−1 ) Pore size (nm) Pore volume (cm3 g−1 ) pHZPC

Mnx Fe2 − x O3 ·yH2 O Semi-crystalline 6–7 nm 5–8 nm 186.28 8.12 0.31 7.2 ± 0.2

(4)

where k1 (min−1 ), k2 (g mg−1 min−1 ), qt and qe represent the pseudo-first order, the pseudo-second order rate constants, amount of Cr(VI) adsorbed (mg g−1 ) at any time t (min) and at equilibrium, respectively. The experimental adsorption capacity (qt ) were found closer to the modeled qt of the pseudo-2nd order kinetics compared to the pseudo-1st order kinetics at studied concentrations and temperatures, supporting that the adsorption kinetics were fitted well by pseudo-2nd order model. The result showed (Fig. 8) very rapid uptake of Cr(VI) and more than 98% was adsorbed within 1 h followed by significantly slow adsorption. Equilibrium time was achieved within 3 h and was independent

of the initial concentration of Cr(VI). The initial fast adsorption was due to the rapid diffusion of solute through the boundary surface of liquid–solid interface as a consequence of electrostatic interaction, while the slow adsorption was presumably due to the enhanced columbic repulsion exerted between the adsorbed species and the adsorbate remained in solution and failed to cross over the diffusion barrier at solid–liquid boundary layer. The parameters estimated (Table 2) for these adsorption kinetic models based on regression coefficient (r2 ) at 303 K were found as 0.94, 0.93, 0.92 for pseudo-first order reaction and 0.98, 0.98, 0.99 for pseudo-second order reaction for 5, 10 and 20 mg L−1 Cr(VI),

Fig. 6. (A) The BET surface area and (B) Pore size distribution of MIFN.

A. Ghosh et al. / Journal of Water Process Engineering 7 (2015) 176–186 pseudo Secound order pseudo First order

5.5

Concentration 5mgL

181

-1

5.0 4.5 5.0

4.0

4.5

q e (mgg -1)

q e (mgg -1)

5.5

3.5

4.0 3.5 3.0

3.0

2.5 2.0

2.5

1.5 0

20

40

60

80

100

120

140

160

180

200

Time(min)

2.0 20

40

60

80

100

120

140

160

180

200

Time(min)

pseudo Secound order pseudo First order

11

Conc. 10 mgL -1

10

8

11 10

q e (mgg -1)

q e (mgg -1)

9

7 6

9 8 7 6 5 4

5

3 0

20

40

60

80

100

120

140

160

180

200

Time(min)

4 0

20

40

60

80

100

120

140

160

180

200

Time(min)

pseudo Secound order pseudo First order

Concentration 20 mgL-1

20 18

20 18

14 q e (mgg -1)

q e (mgg -1)

16

12 10

16 14 12 10 8 6

8

0

20

40

60

80

100

120

140

160

180

200

Time(min)

6 0

20

40

60

80

100

120

140

160

180

200

Time(min) Temparature

288K

303K

318K

Fig. 8. Non-linear pseudo 2nd order kinetic modeling on adsorption of Cr(VI) onto MIFN at different temperatures and concentrations.

respectively, suggesting that pseudo-second order was the best fit kinetic model. Moreover, rate of adsorption was obtained as a function of C0 and about six times higher value of k2 was achieved with the increase of C0 from 5 to 20 mg L−1 indicating rapid removal of Cr(VI) from its lower concentration. This is due to the greater

degree of freedom for distribution of solute over the surface of MIFN and is supposed to the involvement of chemical activation during adsorption process. Moreover, adsorption of Cr(VI) increases with temperature indicating that the adsorption is endothermic in nature.

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Table 2 Kinetic parameters on adsorption of Cr(VI) over MIFN with different initial concentrations and temperatures. 5.0 mg L−1

10.0 mg L−1

20.0 mg L−1

Kinetic model

Parameters

288 K

303 K

318 K

303 K

318 K

288 K

303 K

318 K

Pseudofirstorder

K1 (min−1 ) qe (mg g−1 ) r2 2

0.08 4.86 0.94 0.08

0.105 5 0.94 0.02

0.106 5.41 0.95 0.05

0.084 9.6 0.93 0.37

0.083 10.5 0.93 0.16

0.11 11.9 0.94 0.21

0.07 17.65 0.92 1.6

0.06 18.48 0.92 0.78

0.07 19.99 0.91 1.72

Pseudosecondorder

k2 (g mg−1 min−1 ) qe (mg g−1 ) r2 2

0.028 5.33 0.97 0.03

0.029 5.427 0.98 0.01

0.031 5.378 0.95 0.03

0.012 10.51 0.98 0.05

0.011 10.76 0.98 0.15

0.017 10.71 0.94 0.22

0.005 19.62 0.98 0.25

0.004 20.63 0.99 0.2

0.006 19.9 0.98 0.41

288 K

All values are average of three replicates.

Table 3 Kinetic parameters for adsorption mechanism of Cr (VI) by MNIFO at different concentrations and temperatures. Initial conc. of Cr(VI)

Diffusion coefficient (cm2 s−1 )

Temperature (K)

C0 (mg/L)

Film diffusion (10−8 )

Pore diffusion (10−11 )

10

288 303 318

4.65 4.94 5.44

5.7 4.8 4.29

20

288 303 318

3.07 5.34 7.08

9.02 8.43 7.91

All values are average of three replicates.

indicating that the adsorption of Cr(VI) onto MIFN was involved by both external film and internal pore diffusion mechanism. With a view to estimate the diffusion mechanism quantitatively, the film diffusion coefficient (DF ) and pore diffusion coefficient (DP ) were evaluated by Eqs. (6) and (7), respectively [51],

20 18 16



qt(mgg-1)

 



ln (1 − F) = ln 6/␲2 − DF /ro2 ␲2 t

14



12

F = 6/ro −Dp t/␲

10

6 4 2 0 2

4

t

6

8

1/2

1/2

(min

10

12

14

)

Fig. 9. Intraparticle diffusion mechanism on adsorption of Cr(VI) by MIFN at 303 K at different concentrations (mg L−1 ) (䊏 5.0, 䊉 10.0,  20.0).

Adsorption kinetics have great significance to evaluate the underlying mechanism and as a consequence the adsorption kinetic data were analyzed by intra-particle diffusion mechanism as stated by Weber and Morris, qt = kd × t 1/2 + I

(6) (7)

where F is the fraction of Cr(VI) adsorbed at time t (qt ) to the amount adsorbed at equilibrium (qe ) i.e., qt /qe and ro is the radius of MIFN particles by considering spherical in shape (1.07 × 10−3 cm). The film diffusion coefficient, DF (cm2 s−1 ) was estimated from slop of the linear plot of ln (1 − F) against t while pore diffusion coefficient, DP (cm2 s−1 ) was obtained from slop of the linear plot of F against t1/2 at temperatures of 288, 303 and 318 K separately (Table 3). The values of DF were obtained within the range of 10−6 –10−8 cm2 s−1 while, 10−11 –10−13 cm2 s−1 were achieved for DP in the studied concentrations and temperatures suggesting that the adsorption of Cr(VI) over MIFN was governed by both film and pore diffusion mechanism [52].

8

0

1/2

(5)

where kd is the intra-particle diffusion rate constant (mg g−1 min−1/2 ) and I is a constant related to the boundary layer thickness (mg g−1 ). The model indicates that if the plot of qt against t1/2 gives a straight line passing through the origin then the adsorption process will be controlled solely by intra-particle diffusion mechanism, while both film diffusion and intra-particle pore diffusion mechanism would favor for multi-linear plots. In our study, two distinct linear plots were obtained (Fig. 9)

3.2.3. Equilibrium modeling As the adsorption isotherms relate the amount of adsorbate attached over the surface of an adsorbent with equilibrium concentration at a definite temperature, the equilibrium data were analyzed by Langmuir and Freundlich isotherm models to understand the mechanism. The Langmuir isotherm equation (Eq. (8)) was developed based on assumption that the adsorption sites are homogeneous and adsorption takes place with monolayer surface coverage with adsorbate, while the Freundlich isotherm equation (Eq. (9)) was developed based on multilayer surface coverage with adsorbate and heterogeneity of adsorption sites as represented below: qe =

qm × KL × Ce (1 + KL × Ce )

(8)

1

qe = KF × Cen

(9)

A. Ghosh et al. / Journal of Water Process Engineering 7 (2015) 176–186 Table 4 Langmuir & Freundlich Isotherm parameters of Cr(VI) adsorption on MIFN at different temperatures at pH 3.0. Isotherm

Parameters

Temperatures 288 K

303 K

318 K

Langmuir

−1

qm (mg g ) KL (L mg−1 ) r2 2

36.8 0.59 0.99 1.12

47.84 0.6 0.97 8.3

52.3 0.21 0.98 12.1

Freundlich

KF [mg g−1 (L mg)1/n ] n r2 2

24.5 5.9 0.84 3.12

26.3 4.59 0.96 12.7

27.1 3.74 0.95 14.4

All values are average of three replicates.

Langmuir Modeling Freundlich Modeling

50

q e(mgg-1)

40

30

20

10

0 0

20

40

60

80

100

-1

C e (mgL ) Temperature

:

288K

is irreversible and (iii) >1.0, the reaction is unfavorable [69]. Inserting the values of KL and C0 in the above relation, the values of RL were found within the range of 0.0–1.0, suggesting highly favorable adsorption of Cr(VI) onto MIFN under experimental conditions. 3.2.4. Thermodynamic parameters Thermodynamic parameters including standard free energy change (G0 ), standard enthalpy change (H0 ) and standard entropy change (S0 ) associated with the adsorption of Cr(VI) were estimated to ascertain the efficiency of adsorption reaction. The change in standard free energy of adsorption were calculated assuming activity coefficient as unity for low solute concentration (Henry’s law) by the following equation: G0 = − 2.303RT log K c

where qm and KL are constants related to the monolayer adsorption capacity (mg g−1 ) and the energy of adsorption (L mg−1 ), respectively. The KF is the Freundlich adsorption capacity and 1/n is an arbitrary constant related to the adsorption intensity. The other terms such as Ce and qe have their usual significances. The data shown in Fig. 10 were analyzed by non-linear fit method with the Langmuir (Eq. (8)) and the Freundlich (Eq. (9)) models. The estimated parameters related to the above Eqs. (8) and (9) are given in Table 4 including regression coefficient (r2 ) and statistical error chi-square (␹2 ). It was evident that the equilibrium data were fitted well by the Langmuir model (Eq. (8)) than the Freundlich model (Eq. (9)). Moreover, the values of qm (36.80–52.3 mg g−1 ) obtained from isotherm model were almost identical with the experimental values suggesting that the adsorption isotherm followed the Langmuir model well. It also suggests that the sorption sites of MIFN surface are homogeneous containing fixed number of equivalent binding sites which are equally accessible by Cr(VI) ions. Moreover, the qm has increased with the increase in temperature, probably through the enhanced collision at elevated temperature resulting higher adsorption i.e., endothermic in nature. The estimated qm for MIFN was found to be higher than other nanomaterials reported in literature (Table 5). The present study indicates that MIFN could be one of the efficient materials for scavenging Cr(VI) from the contaminated water. The separation factor, RL (a dimensionless parameter) expressed as RL = 1/(1 + KL C0 ) where KL and C0 possess usual significances can be used to predict whether the adsorption is favorable or not. When the value of RL is equal to (i) 0–1.0, the reaction is favorable, (ii) 0.0, the reaction

60

183

303K

318K

Fig. 10. Isotherm models on adsorption of Cr(VI) by MIFN at different temperatures.

(10)

where Kc is the equilibrium adsorption constant, T is the absolute temperature in K and R is the real gas constant (8.314 J mol−1 K−1 ). The equilibrium adsorption constant Kc is almost identical with the Langmuir’s equilibrium constant KL when expressed in L mol−1 and may be expressed as qe /ce and known as adsorption affinity. Under identical experimental conditions, adsorption affinity may also be expressed as distribution coefficient Kd, which is defined as the ratio of the quantity of the Cr(VI) adsorbed per gram of MIFN to the amount of the Cr(VI) remained in solution at equilibrium and was related to H0 (kJ mol−1 ) and S0 (kJ mol−1 K−1 ) as a function of T (K) as represented below (Eq. (11)). log

qe S 0 = − Ce 2.303R



H 0 2.303R



1 T

(11)

Assuming S0 and H0 to be constant within the range of temperature studied, the values may be obtained from the intercept and slope of the linear plot of log (qe /ce ) versus 1/T. G0 (kJ mol−1 ), the change of free energy, at studied temperatures may be calculated using the following relation (Eq. (12)). 0

0

G = H − TS

0

(12) (r2

= 0.98) was A good linearity with high regression coefficient found (Figure not shown) and analyzed thermodynamic parameters were represented in Table 6. The negative values of G0 at different temperatures indicated the spontaneous nature of Cr(VI) adsorption over MIFN and the increase in magnitude of G0 (−1.26 to −4.13 KJ mol−1 ) values with the increase of temperature (288–318 K) revealed that the adsorption process is more favorable at higher temperature. Almost identical range of G0 values for adsorption of Cr(VI) on different adsorbents were reported by other researchers. The values of G0 for binding of Cr(VI) were reported as −0.44 KJ mol−1 at 303 K with bentonite [70], −3.72 to −4.81 KJ mol−1 with Al2 O3 [71,72], −6.26 to −8.96 KJ mol−1 at 293–323 K with Al–Mg mixed metal oxides [73,74] even though much higher magnitude of G0 (−40 KJ mol−1 ) was observed for modified clay materials [56]. Higher the negative values of G0 (−30 to −40 KJ mol−1 ), higher would be the possibility of binding Cr(VI) with the active sites of adsorbents through hydrogen bonding and subsequently, chemisorption would be favored to some extent [75]. In contrast, if the negative values of G0 are found to be lower than that of −18 KJ mol−1 , then the adsorption of Cr(VI) occurs predominantly through physisorption [75]. Moreover, the feasibility of ion-exchange reaction mechanism for adsorption of Cr(VI) over modified adsorbents can not be ruled out at the higher negative values of G0 . In the present study, the magnitudes of G0 were obtained in the range of −1.26 to −4.13 KJ mol−1 suggesting that physisorption would occur for Cr(VI) onto MIFN. The positive value of H0 confirms endothermic nature of adsorption, which agrees with the results obtained in adsorption kinetics and isotherms. Low value of H0 (4.63 KJ mol−1 ) further indi-

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cates that the binding of Cr(VI) onto MIFN mainly occurs through physisorption, as chemisorption is favoraed only at higher H0 (>40 KJ mol−1 ). Physisorption of Cr(VI) by zirconium oxide [29] and by FeMnO4 [62] were also reported by other researchers because of lower H0 values so obtained in their studies. An increase in randomness at the adsorbent/liquid interface during Cr(VI) adsorption is also confirmed by the positive S0 . Low value of S0 (13.56 J mol−1 K−1 ) also indicates that ion-exchange mechanism is not operating in the present study, because if it would happen then the anions released by Cr(VI) ions at the MIFN surface would definitely increase the entropy of the system and subsequently higher S0 would be expected. The activation energy has also been computed from logarithmic form of Arrhenius equation (Eq. (13)) by plotting ln k against 1/T and from the slope of the linear plot (r2 = 0.97), the activation energy has been estimated as 2.5945 KJ mol−1 . The activation energy for physical adsorption is usually less than 4.2 KJ mol−1 [76] and hence physical adsorption is involved in the present case. ln k = ln A +

 −E  a

Fig. 11. Effect of interfering ions on the adsorption of Cr (VI) on the MNFO surface.

(13)

RT

bon coated with quarternized poly(4-vinylpyridine) on the Cr(VI) adsorption. In contrast, Pakade et al. [78] reported that sulphate interference more strongly than phosphate on the Cr(VI) adsorption over ion imprinted polymer.

3.3. Effect of interfering ions The effect of interfering ions those are commonly found in surface water including fluoride, carbonate, bicarbonate, sulphate, silicate and phosphate has been investigated on adsorption of Cr(VI) over MIFN and results are shown in Fig. 11. It was found that phosphate and sulphate strongly interfere on adsorption of Cr(VI) whereas other ions have no significant interference. The decrease in Cr(VI) adsorption with the increase of sulphate and phosphate concentrations was mainly due to the higher affinity of these ions to the specific binding sites of adsorbent because of their similarity in structure with CrO4 −2 ion. The stronger interference of phosphate than sulphate is probably due to the higher negative charge density on former that facilitates adsorption at the positive charged sites of the adsorbent. Fang et al. [77] also found that phosphate competes more strongly than sulphate does for binding sites of activated car-

3.4. Regeneration Regeneration is the process for restoring the adsorptive properties of a spent adsorbent. Since optimum Cr(VI) adsorption was achieved within the pH range of 2–3, it might be expected that the adsorbed Cr(VI) ions could be desorbed from exhausted MIFN surface by enhancing pH of the desorbing medium. In this regard, desorption of Cr(VI) was carried out from spent MIFN [47.9 mg of Cr(VI) g−1 MIFN] by varying NaOH concentration. It has been found that the desorption efficiency increases from 40% to 78% with the increase of NaOH concentrations from 0.2 M to 1.0 M, which could be attributed to the enhanced abstraction of OH− ions that would facilitate higher release of Cr(VI) ions into the solution owing to the

Table 5 Efficiencies of some iron and manganese based nanosorbents used for Cr(VI) removal. Adsorbents

pH

Adsorption capacity (mg g−1 )

Reference

Maghemite Maghemite Magnetite Diamond supported magnetite Montmorillonite-supported magnetite Feroxyhyte-coated maghemite Mesoporous iron–iron oxide nanocomposites Surface modified nanozeolite A Magnetic Fe3O4/carbon nanotube Mixed magnetite and maghemaite MnFe2 O4 nanoparticles Fe/Mn mixed metal oxide nanocomposites Polypyrrole/Fe3 O4 magnetic nanocomposite Polypyrrole–polyaniline nanofibers Polypyrrole/Fe3 O4 nanocomposite Exfoliated polypyrrole-organically modified montmorillonite clay nanocomposite Sulfuric acid-modified avocado seed Metalic iron nanoparticles supported on polyaniline nanofibers Manganese oxide incorporated ferric oxide nanocomposite

4 2.5 4 2–2.5 2–2.6 2.5 6 3 2 2 3 2 2 2 2 2 2 2 3

1.62 19.2 1.21 11.4 15.3 25.8 34.1 14.16 47.98 2.4 3.87 26.3 169.5 (25 ◦ C) 227 230.17 119.3 333.33 434.78 47.8

[28] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [32] [63] [64] [65] [66] [67] [68] Present Work

Table 6 Thermodynamic parameters on adsorption of Cr(VI) by MIFN. Initial Cr(VI) concentration (C0 , mg L−1 )

H0 (kJ mol−1 )

S0 (J mol−1 K−1 )

40

0.463

13.56

G0 (kJ mol−1 ) at various Temperature (K) 288 −1.263

303 −1.286

318 −4.126

A. Ghosh et al. / Journal of Water Process Engineering 7 (2015) 176–186

formation of soluble sodium chromate, even though around 22% of the adsorbed Cr(VI) species are hardly to desorb under experimental conditions (1.0 M NaOH) and retain probably through the formation of strong chemical bond via chelation over interior surfaces of adsorbent. The process of regeneration and reuse was tested up to 5 cycles with a view to estimate the adsorption efficiency of rejuvenated adsorbent in each cycle. It has found that the adsorption efficiency was reduced from 78 to 50% from first to fifth cycle, indicating 4–5% loss of active adsorption sites on MIFN in successive cycle that can not be regained even by 1.0 M NaOH. Thus, MIFN retains its Cr(VI) removal efficiency up to 50% even after 5 consecutive adsorption–desorption cycle, making the process economically viable in attenuation of Cr(VI) from aqueous stream. 4. Conclusion Synthetic semi-crystalline and porous manganese incorporated ferric oxide nanocomposite (MIFN) is an efficient adsorbent for Cr(VI) removal from aqueous phase in acidic medium (pH 3.0). The Langmuir isotherm has been recognized as well fit model indicating homogeneous adsorption sites. The Langmuir adsorption capacity for Cr(VI) has been achieved as 47.6 mg g−1 for MIFN and that has increased with increasing temperature suggesting endothermic nature of adsorption. Both film diffusion and particle diffusion mechanisms operated simultaneously in this spontaneous adsorption process. The low cost of the adsorbent and its regeneration and reuse in multiple cycles make the process economically viable. Thus, MIFN could be an effective material for attenuation of Cr(VI), particularly from contaminated ground as well as waste water containing low level of coexisting phosphate and sulphate ions. Acknowledgement Authors are grateful to the Department of Science and Technology (New Delhi) for financial support of a project to work, and also to the Head, Department of Chemistry, Presidency University, Kolkata, India for laboratory facilities. References [1] J. Edwards, Coating and Surface Treatment Systems for Metals, Finishing Publication limited, ASM International, 1997, pp. 66–71. [2] K.J. Sreeram, T. Ramasami, Sustaining tanning process through conservation, recovery and better utilization of chromium, Resour. Conserv. Recycl. 38 (2003) 185–212. [3] E.M. Brown, A conformational study of collagen as effected by tanning procedures, J. Am. Leather Chem. Assoc. 92 (1997) 225–233. [4] P. Chandra, S. Sinha, U.N. Rai, Biomediation of Cr from water and soil by vascular aquatic plants In phytoremediation of soil and water contaminants, ACS symposium series 664, American Chemical Society, Washington, DC, 1997, pp. 274–282. [5] E.J. Rita, V. Ravisankar, Bioremidiation of chromium contamination – a review, Int. J. Res. Earth Environ. Sci. 1 (2014) 20–26. [6] N. Ballav, A. Maity, S.B. Mishra, High efficient removal of Cr(VI) using glycin doped polypyrole adsorbent from aqueous solution, Chem. Eng. J. 198 (2012) 536–546. [7] T.S. Anirudhan, S. Jalajamony, P.S. Suchithra, Improved performance of a cellulose-based anion exchanger with tertiary amine functionality for the adsorption of chromium(VI) from aqueous solutions, Colloids Surf. A: Physicochem. Eng. Aspects 335 (2009) 107–113. [8] D.A. Eastmond, J.T. MacGreor, R.S. Slasinski, Trivalent chromium: assessing the genotoxic risk of an essential trace element and widely used human and animal nutritional supplement, Crit. Rev. Toxicol. 38 (2008) 173–190. [9] D. Mohan, C.U. Pittman, Activated carbons and low cost adsorbents for remediation of tri and hexavalent chromium from water, J. Hazard. Mater. B137 (2006) 762–811. [10] USEPA, IRIS, Toxicological Review of Hexavalent Chromium, U.S. Environmental Protection Agency, Washington, DC, 2010, EPA/635/R-10/004A. [11] P. Cheryl, M.S. Booker, Reflections on hexavalent chromium: health hazards of an industrial heavy weigh, Environ. Health Perspect. 108 (2009) 402–407.

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