Efficient removal of toxic hexavalent chromium from aqueous solution using threonine doped polypyrrole nanocomposite

Journal of Water Process Engineering 13 (2016) 88–99

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Efficient removal of toxic hexavalent chromium from aqueous solution using threonine doped polypyrrole nanocomposite Augustine Amalraj, M. Kalai Selvi, A. Rajeswari, E. Jackcina Stobel Christy, Anitha Pius ∗ Department of Chemistry, The Gandhigram Rural Institute – Deemed University Gandhigram, Dindigul 624 302, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 18 May 2016 Received in revised form 4 August 2016 Accepted 19 August 2016 Keywords: Threonine Polypyrrole Hexavalent chromium Adsorption Isotherm

a b s t r a c t A new threonine doped polypyrrole (Thr-PPy) was prepared via in situ polymerization of pyrrole with threonine for the removal of Cr (VI) from aqueous solutions. The prepared Thr-PPy was characterized using FT-IR, SEM with EDS, XRD, XPS and BET methods. The adsorption experiments were carried out in batch mode to optimize various parameters like contact time, initial concentration, pH, adsorbent dose, coexisting ions and temperature that influence the adsorption rate. Langmuir, Freundlich, DubininRadushkevich and Temkin adsorption isotherm models were applied to describe isotherm constants. Equilibrium data obeyed well with the Langmuir isotherm model with maximum adsorption capacity of 185.5 mg/g. Thermodynamic studies revealed that the nature of adsorption is spontaneous and endothermic. The results of the kinetic experiments show that Cr (VI) adsorption on Thr-PPy follow pseudo-second-order and intra-particle diffusion models. The mechanism of Cr (VI) adsorption by ThrPPy was governed by the ionic interaction between NH3 + of Thr and HCrO4 − ions. Thr-PPy can be reused successfully for the removal of Cr (VI) four times without loss of its removal efficiency. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Water pollution by heavy metal ions is considered to be one of the major environmental issues due to the adverse effects of heavy metals on ecosystem and on human health. Chromium is one of the dangerous metal pollutants, generally introduced into water bodies from many industrial processes, such as leather tanning, electroplating, textiles and dyeing, ceramic manufacture, metal processing, wood preservation, chromium salt industry, water cooling, film and pigment manufacture, chemical industries, and mining operation. Chromium exists in two main stable oxidation states, namely Cr (III) and Cr (VI), in aqueous systems, in which, Cr (VI) is generally considered to pose great human health risk because it is more toxic, soluble and mobile than Cr (III). Chromium (VI) species are 100 times more toxic than Cr (III) species. Several Cr (VI) compounds act as carcinogens, mutagens and teratogens in biological systems [1–6]. Subsequently, WHO set the maximum permissible limits of Cr (VI) in potable water, inland surface water and industrial wastewater and they are 0.05, 0.1 and 0.25 mg/L, respectively [7]. A sequence of in vitro and in vivo studies have demonstrated that Cr (VI) brings an oxidative stress through better

∗ Corresponding author. E-mail address: [email protected] (A. Pius). http://dx.doi.org/10.1016/j.jwpe.2016.08.013 2214-7144/© 2016 Elsevier Ltd. All rights reserved.

production of reactive oxygen species (ROS) leading to genomic DNA damage and oxidative deterioration of lipids and proteins [4,8]. The purification methods have to avoid generation of secondary waste and to evolve materials that can be recycled and easily used on an industrial scale. Various treatment technologies such as ion exchange, reverse osmosis, electrolytic removal, liquid-liquid interaction, adsorption, chemical precipitation, membrane filtration and solvent extraction have been reported for the removal of Cr (VI) from water or waste water [1,9,10]. However, most of these technologies are associated with high operational and maintenance cost, incomplete metal removal, high energy requirement and generation of toxic residual metal sludge that pose a disposal problem. Comparatively, to handle a large volume of wastewater with low concentration of Cr (VI), adsorption is considered as the simplest and the most cost effective method [10–12]. Many kinds of adsorbents for wastewater treatment have been developed, such as activated carbon [13–15], activated alumina [16], coated silica gel [17], biosorbents [18–20], modified resins [21,22] and metal oxides [23,24]. Most of these adsorbents have very low chromium adsorption capacity and slow process dynamics. So there is a necessity to develop novel adsorbents with high capacity and fast kinetics for the removal of Cr (VI) from water. In recent years, conducting polymer nanocomposites have attracted special attention in the fields of nanoscience and nanotechnology due to their large surface area, low cost, high efficiency

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and regeneration ability. Preparation of conducting polymers is a single step reaction using very few low cost starting materials and definitely cost effectiveness is an advantage. Polypyrrole (PPy), a conducting polymer, is one of the most promising materials for the removal of toxic ions, especially Cr (VI) ions from water/waste water. To enhance maximum adsorption capacity, various functional groups, including amine, carboxylate, hydroxyl, sulfate, phosphate and amide groups have been used to modify the polymeric nanocomposites (NCs) [4,10,25–29]. Among these functional groups, amino- functionalized polymers would be expected to be efficient ones for removing heavy metals. Amino-functionalized polymeric nanocomposites are expected to be efficient ones rather than other functional groups such as carboxylate, hydroxyl, sulfate, phosphate etc., for removing Cr (VI). Threonine is one of the amino acids, easily doped with polypyrrole. There is no study on the removal of Cr (VI) using Thr-PPy. Under acidic conditions, amino groups are easier to be protonated and electrostatic attraction can happen between –NH3 + and anions as in Eq. (1) [4,30]. Here, –NH3 + and HCrO4 − are taken as representatives: –NH3 + + HCrO4 − → –NH3 + -------HCrO4 −

(1)

To the best of our knowledge, there is no study on the removal of Cr (VI) using Thr doped PPy. In the present study, Thr-PPy was prepared and characterized by using different analytical methods like FT-IR, SEM with EDS, XRD, XPS and BET. Thr-PPy was applied for the removal of Cr (VI) from an aqueous solution. The effects of solution pH, contact time, initial Cr (VI) concentration, competition from coexisting ions, desorption and reusability were tested in batch experiments. The adsorption isotherms and kinetics were also studied.

89

Scheme 1. Preparation of PPy and Thr-PPy.

2.4. Adsorption experiments Batch technique was selected to attain the equilibrium and kinetic data. About 0.1 g of adsorbent was added to 50 mL of solution containing 50 mg/L as initial chromium concentration. The mixture was shaken in a thermostated shaker at a speed of 200 rpm at room temperature. The solution was then filtered and the residual chromium concentration was measured using AAS. The adsorption capacity of the adsorbent was studied at different conditions like contact time, initial concentration, adsorbent dose, pH and the effect of other common ions present in the water. 3. Results and discussion 3.1. Structural characterization of Thr-PPy

2. Materials and methods 2.1. Chemicals Pyrrole (Py) monomer, Ammonium peroxydisulfate [APS] ((NH4 )2 S2 O8 ), threonine (Thr) and potassium dichromate (K2 Cr2 O7 ) were purchased from Sigma-Aldrich, India. All other chemicals used in this study were of analytical grade and were supplied by Sigma-Aldrich, India.

2.2. Preparation of the PPy and Thr-PPy

FT-IR spectra were used to characterize the chemical structure of PPy (Fig. 1 (a)), Thr-PPy before (Fig. 1 (b)) and after adsorption of Cr (VI) (Fig. 1 (c)). The peak at 1459 cm−1 is due to stretching mode of C N in the pyrrole ring [27,31]. The bands around 1180 and 1038 cm−1 are attributed for C H in plane vibration and C H in plane bending mode vibrations respectively [32,33]. The peaks at 3124, 1679 and 1050 cm−1 are C O symmetric, asymmetric stretching vibrations and vibration of C N in NH3 group of zwitter ionic threonine respectively [27,34]. The N H bending vibration peak at 1548 cm−1 was intensified and shifted to 1560 cm−1 after Cr (VI) adsorption on Thr-PPy [27]. The peaks at 790 and 927 cm−1

Polypyrrole (PPy) and threonine doped polypyrrole (Thr-PPy) were prepared by the chemical oxidative, in situ polymerization of pyrrole monomer and with threonine respectively in the presence of FeCl3 as an oxidant as described by Ballav et al. with suitable modifications (Scheme 1) [27].

2.3. Characterization and analysis Fourier transform infra-red (FT-IR) spectra were recorded before and after adsorption on the adsorbents by JASCO FT/IR460 plus instrument. The surface morphology of the adsorbent, before and after adsorption was determined using scanning electron microscope (SEM). Elemental analysis was carried out by using energy dispersive analyzer unit (EDS) attached with SEM (Vega3Tescan, Brucker). The crystalline natures of the adsorbents were determined using X-ray diffraction (XRD) (Xpert-Pro). Chromium concentrations in solutions were determined by using Atomic Absorption Spectrometer (AAS) (Perkin – Elmer A Analyst 100)

Fig. 1. FT-IR spectra of (a) PPy (b) Thr-PPy before and (c) Thr-PPy after adsorption with Cr (VI).

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Fig. 2. XRD pattern of (a) PPy (b) Thr-PPy before and (c) Thr-PPy after adsorption with Cr (VI).

are intensified after adsorption of Cr (VI), this is attributed to CrO and Cr O bonds of HCrO4 − which also suggest that Cr (VI) was adsorbed onto Thr-PPy [33,35]. Most of the peaks are intense and shifted to increasing wave number after Cr (VI) adsorption indicating a possible interaction between the Thr-PPy with the Cr (VI) ions. This may be due to ␲-electrons of the polymer skeleton [27]. XRD patterns of PPy and Thr-PPy, before and after adsorption of Cr (VI) ions were also recorded and are presented in Fig. 2. In XRD patterns broad peaks were found for both PPy and Thr-PPy nanocomposites before and after adsorption at 2␪ ≈ 22◦ for pyrrole which is the characteristic peak of amorphous PPy [36]. A new small peak appeared at 2␪ ≈ 41◦ in the case of Thr-PPy (Fig. 2 (b)) which is

attributed to Thr [37]. Fig. 2 (c) shows another new peak at 2␪ ≈ 45◦ which corresponds to the adsorbed chromium [38]. SEM micrographs and particle size histogram of adsorbents before and after adsorption of Cr (VI) are shown in Fig. 3. Fig. 3 (a) shows a mixture of spherical and rod like structures of PPy with random distribution and high aggregation with sizes ranging between 300 and 500 nm. SEM image of Thr-PPy (Fig. 3 (b)) shows agglomerated spherical particles in the range of 50–300 nm. After the addition of Thr, sizes of the nanocomposites were considerably decreased [27]. The surface changes in the SEM micrographs of the Thr-PPy before and after chromium treatment indicate structural changes in the adsorbents (Fig. 3 (b) and (c)). Particle size histograms are clear evidence that the size distribution varied from 50 nm to 300 nm with an average diameter of 192 ± 3 nm for ThrPPy before adsorption (Fig. 3 (d)) and in the case of Cr (VI) adsorbed Thr-PPy, the particle size increased and the distribution varied from 150 nm to 350 nm with an average diameter of 245 ± 2 nm (Fig. 3 (e)). This is further supported by EDS analysis (Fig. 4), which provides direct evidence for the adsorption of chromium onto Thr-PPy polymeric nanocomposite. The EDS spectra of chromium adsorbed Thr-PPy show the presence of chromium peaks and EDS mapping images are used to explain the absence, presence and distribution of chromium before and after treatment respectively (Fig. 4). The density of red spots for chromium as seen in the EDS images confirm that chromium adsorption has occurred onto the Thr-PPy nanocomposite. X-ray photo electron spectroscopy survey was conducted to explore the surface chemistry of Thr-PPy and Cr (VI) adsorbed Thr-PPy. The survey spectra was recorded and shown in Fig. 5 (a). The predominant elements in both spectra were observed as C1s, N1 s and O1 s at 285, 399 and 532 eV respectively. The two new peaks at 577.6 eV and 587.6 eV of Thr-PPy-Cr (VI) was assigned

Fig. 3. SEM micrograph of (a) PPy, (b) Thr-PPy before and (c) Thr-PPy after adsorption with Cr (VI), (d) particle size histogram of Thr-PPy before and (e) Thr-PPy after adsorption with Cr (VI).

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Fig. 4. EDS spectra of (a) Thr-PPy before and (b) Thr-PPy after adsorption with Cr (VI), EDS mapping images of (c) Thr-PPy before and (d) Thr-PPy after adsorption with Cr (VI).

Fig. 5. (a) XPS survey scan of Thr-PPy and Thr-PPy-Cr (VI); and (b) XPS of Cr 2p spectra.

Scheme 2. Possible adsorption mechanism of Cr (VI) by the Thr-PPy.

as the chromium peak, which was also high resolved in Fig. 5 (b) indicating the successful adsorption of chromium on Thr-PPy. The energy bands at 577.6 eV and 587.6 eV are corresponding to the binding energies of Cr (2p1/2 ) and Cr (2p3/2 ) orbitals respec-

tively indicated the presence of both Cr (III) and Cr (VI) species on the adsorbent surface. The presence of Cr (VI) on the ThrPPy surface due to the ionic interaction between Thr-PPy and Cr (VI) as presented in Scheme 2. The existence of Cr (III) on the

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Fig. 6. Effect of various parameters (a) pH, (b) contact time (c) adsorbent dose and (d) initial concentration of Cr (VI) on adsorption capacity of the Thr-PPy.

adsorbent surface suggested that some portion of adsorbed Cr (VI) reduced to Cr (III) by electron rich PPy of Thr-PPy [27,39]. The Brunauer–Emmett–Teller (BET) surfaces of the PPy and Thr-PPy, before and after adsorption of Cr (VI) were determined by performing N2 physical adsorption–desorption studies at 77.3 K and is found to be 12.5 m2 /g, 41.8 m2 /g and 22.79 m2 /g respectively. The enhancement of surface area of the Thr-PPy compared to the PPy would suggest improved performance in Cr (VI) removal. After Cr (VI) adsorption on the Thr-PPy, the surface area reduction was observed, which is due to the absorbed chromium [40]. 3.2. Effect of pH In the adsorption process, initial pH of the solution is one of the most important parameters for metal ion adsorption on the surface of an adsorbent. The effect of pH on adsorption of Cr (VI) by PPy, Thr-PPy was investigated to find out the optimum pH for maximum adsorption capacity. Hence adsorption of chromium ions onto the adsorbents was analyzed at different initial pH levels from 2 to 11, keeping constant all other parameters. The dependence of Cr (VI) adsorption on the pH can be explained from the perspective of surface chemistry in an aqueous phase. The surfaces of Thr-PPy are generally covered with amino groups that vary in forms at different pH levels. The common forms of hexavalent chromium are dichromate (Cr2 O7 2− ), chromate (CrO4 2− ) and hydrogen chromate (HCrO4 − ). These forms depend on pH of the solution and concentration. In the solution pH range of 1–6, chromium ions can exist in different forms such as HCrO4 − , Cr2 O7 2− , Cr3 O10 2− , Cr4 O13 2− [33].

Generally, below pH 6.0, HCrO4 − is the major chromium species and above pH 6.0 it is CrO4 2− . At low pH, the amine group of threonine is protonated, which increases the adsorption of negatively charged HCrO4 − ion through ionic interaction (Eq. (1)) as shown in Scheme 2. The effect of solution pH on Cr (VI) adsorption by PPy and Thr-PPy is shown in Fig. 6 (a). It is observed that Cr (VI) adsorption capacity decreased gradually with increase in pH from 2 to 11 for both the adsorbents. In particular, 99.3% adsorption capacity was achieved on Thr-PPy at pH 2 whereas for PPy it was only 83.1% at the same experimental conditions. The adsorption of Cr (VI) on the surface of Thr-PPy may be due to the electrostatic interaction between the positive electric charge of protonated amine group of threonine and the negative electric charge of HCrO4 − ions as shown in Scheme 2. In the case of PPy, the adsorption process occurred via the ion exchange property of PPy by replacing the doped Cl− ions with HCrO4 − ions [27,41]. It was also found from Fig. 6 (a) that at higher pH the adsorption capacity of Thr-PPy was lower than PPy, which may be due to excess concentration of OH− ions competing with HCrO4 − . So, protonation of –NH2 is weakened, resulting in the decline of Cr (VI) removal efficiency [4]. The maximum adsorption capacity of Cr (VI) onto Thr-PPy (99.3%) was achieved at pH 2. Based on the pH results, all further adsorption studies were carried out at pH 2 only. 3.3. Effect of contact time The adsorption capacity of adsorbent Thr-PPy was determined by varying the contact time between 20 and 100 min. About 0.1 g

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Table 1 Isotherms with linear forms and their plots. Isotherms Freundlich Langmuir-I

1/n

qe = kF Ce qe = (Q 0 bCe )/(1 + bCe )

Langmuir-II Langmuir-III Langmuir-IV Dubinin–Radushkevich Temkin

Qe = Xm exp(−kDR ε2 ) qe = (RT BT )ln(kT Ce )

of the adsorbent was mixed with 50 mL of 50 mg/L initial Cr (VI) solution. The contents were shaken thoroughly using a mechanical shaker at 200 rpm and the contents were filtered and analyzed for Cr (VI). As it is evident from Fig. 6 (b), the adsorption capacity of Thr-PPy reached saturation after 60 min. Hence, 60 min was fixed as the contact time for the adsorbent Thr-PPy for further studies.

Linear form

Plot

log qe = logkF + 1/n log Ce Ce /qe = (1/Q 0 b) + (Ce /Q0 ) 1 1 1 1 =[ 0 ] + 0 qe Q b Ce Q 1 qe qe = Q 0 - [ ] b Ce qe = bQ 0 -bqe Ce ln qe = lnXm − kDR ε2 qe = BT lnkT + BT lnCe

log qe vs log Ce Ce /qe vs Ce 1 1 vs qe Ce qe qe vs Ce qe vsqe Ce ln qe vsε2 qe vs ln Ce

Table 2 Isotherm parameters of Cr (VI) removal on Thr-PPy obtained at different temperatures. Isotherms

Parameters

303 K

313 K

323 K

Langmuir-I

Qo (mg/g) b (L/g) RL R SD ␹2

185.529 0.452 0.0086 0.999 0.0003 0.0088

204.918 0.493 0.0080 0.999 0.0003 0.0744

236.407 0.538 0.0074 0.999 0.0002 0.0134

Langmuir-II

Qo (mg/g) b (L/g) RL R SD ␹2

176.056 0.5022 0.0079 0.999 0.0003 0.0409

194.553 0.5337 0.0074 0.999 0.0002 0.1447

222.717 0.5862 0.0068 0.999 0.0003 0.0398

Langmuir-III

Qo (mg/g) b (L/g) RL R SD ␹2

182.175 0.477 0.0083 0.998 3.1519 0.0129

200.715 0.510 0.0078 0.997 3.2449 0.1024

232.717 0.553 0.0072 0.996 3.9307 0.0170

Langmuir-IV

Qo (mg/g) b (L/g) RL R SD ␹2

182.700 0.475 0.0084 0.998 1.4400 0.0120

200.466 0.510 0.0078 0.997 1.6826 0.1168

232.925 0.551 0.0072 0.996 2.2016 0.0160

Freundlich

1/n n kF(mg/g)(L/mg) 1/n R SD ␹2

0.630 1.586 54.068 0.993 0.0368 0.4140

0.670 1.492 62.816 0.996 0.0293 0.2477

0.719 1.390 78.793 0.997 0.0233 0.1542

Dubinin–Radushkevich

k DR(mol 2 /J X m(mg/g) E (KJ/mol) R SD ␹2

1.203E-07 102.681 2.039 0.961 0.2012 2.3028

9.631E-08 106.860 2.278 0.968 0.1829 1.9709

7.344E-08 113.142 2.609 0.974 0.1673 1.6011

Temkin

kT (L/g) BT(KJ/mol) R SD ␹2

43.285 38.689 0.993 5.2530 43.5499

45.158 43.148 0.995 4.6500 50.9741

92.584 44.002 0.987 7.2153 68.0127

3.4. Effect of Thr-PPy dose To examine the effect of Thr-PPy dosage on the adsorption of Cr (VI) ions, studies were conducted with varied Thr-PPy dosage ranging from 0.5 g/L to 3.0 g/L in 50 mg/L Cr (VI) solution while the solution pH was kept at 2 and contact time was fixed as 60 min. From Fig. 6 (c), it was observed that the adsorption capacity changed from 54% at a dose of 0.5 g/L to 99% at a dose of 2.0 g/L and remained unchanged thereafter. This performance is due to the fact that at increased adsorbent dosage, there are more available active adsorption sites for the Cr (VI) to adhere, hence a high percentage of adsorption capacity. From the investigation, 2.0 g/L of Thr-PPy was found to be the optimum dosage for the removal of 99% of Cr (VI) ions and hence in the following experiments the dosage of Thr-PPy was fixed to 2.0 g/L. 3.5. Effect of initial Cr (VI) ions concentration The effect of different initial concentration on the adsorption capacity of Cr (VI) ions was carried out using solutions containing 50–250 mg/L of Cr (VI) ions. All other parameters are kept at optimum conditions. Fig. 6 (d) shows the influence of initial concentration on the removal of Cr (VI) ions by Thr-PPy composite. When the initial Cr (VI) concentrations were increased from 50 to 250 mg/L, the removal efficiency slightly decreased (99.3–98.3%). At higher concentration, some Cr (VI) ions are left unabsorbed in the solution due to the saturation of binding sites. Thr-PPy can be used as an efficient adsorbent to remove Cr (VI) up to 100 mg/L without Cr (VI) residue (Fig. 6 (d)). Thus Thr-PPy composites can be utilized effectively for the removal of Cr (VI) from waste water. 3.6. Adsorption isotherms Adsorption isotherms play an important role in predicting the ability of an adsorbent to remove a pollutant. They express the specific relation between the concentration of adsorbate and its degree of accumulation onto the adsorbent surface at constant temperature. The Cr (VI) adsorption capacity of Thr-PPy had been evaluated using four different isotherms namely Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherms. The isotherms were performed at pH 2, and at temperatures of 303, 313 and 323 K. 3.6.1. Langmuir isotherm Langmuir isotherm [42] model has four types and are listed in Table 1 and the linear plots given in Fig. 7. This isotherm model

2

)

predicts that a monomolecular layer is formed when adsorption takes place without any interaction between the adsorbed ions. The adsorption capacity (Q◦) is the amount of adsorbate at complete monolayer coverage (mg/g), it gives the maximum adsorption capacity of adsorbent and b (L/mg) is the Langmuir isotherm constant that relates to the energy of adsorption. The respective values of Q◦ and b were determined and are presented in Table 2. From Table 2 it is clear that higher R values for the adsorption of Cr (VI) on Thr-PPy were shown by Type I and Type II Langmuir isotherms. To find out the probability of isotherm, the important characteristics of Langmuir isotherm can be expressed in terms of a

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Fig. 7. Isotherm plots of (a) Langmuir-I, (b) Langmuir-II, (c) Langmuir-III and (d) Langmuir-IV.

dimensionless constant separation factor or equilibrium parameter, RL [43] RL = 1/(1 + bC 0 )

(2)

Where b is the Langmuir isotherm constant and C0 is the initial concentration of Cr (VI) (mg/L). The RL values between 0 and 1 indicate favorable adsorption for all the temperatures studied. The values of RL are given in Table 2. 3.6.2. Freundlich isotherm Freundlich isotherm assumes that the up taken or the adsorbed ions occur on a heterogeneous surface by multilayer adsorption and that the amount of adsorbate increases infinitely with an increase in concentration. The linear form of Freundlich isotherm [44] is represented in Table 1. qe is the amount of Cr (VI) adsorbed per unit weight of the adsorbent at equilibrium (mg/g). Ce is the equilibrium concentration of Cr (VI) in solution (mg/L), kF is a measure of adsorption capacity and 1/n is the adsorption intensity. The Freundlich isotherm constants kF and n were calculated from the slope and intercept of the plot of log qe versus log Ce (Fig. 8 (a)) and are presented in Table 2. The values of 1/n lie between 0.1 and 1.0 and the n value are lying in the range of 1–10 confirm the favorable conditions for adsorption [45]. 3.6.3. Dubinin-Radushkevich (D-R) isotherm The Dubinin-Radushkevich (D-R) isotherm helps in understanding the type of adsorption from the data of Cr (VI) both in the adsorbent and in the solution at equilibrium. The linear plot of

ln qe vs ε2 indicates the applicability of D-R isotherm (Fig. 8 (b)). The values of KDR , Xm and E are shown in Table 2. The magnitude of mean free energy of adsorption E gives information about the type of adsorption mechanism. The E values are less than 8 kJ/mol which indicate that the Cr (VI) removal is governed by adsorption mechanism [46]. 3.6.4. Temkin isotherm Temkin isotherm model [47] is applicable to evaluate the adsorption potentials of the applied adsorbents for adsorbate ions. The linear form of the Temkin model is given Table 1 and shown in Fig. 8 (c). BT is the adsorption heat (KJ/mol) and kT is the equilibrium binding constant (L/g) corresponding to the maximum binding energy. The linear plot of qe vs ln Ce for the adsorption system indicated the applicability of Temkin isotherm. The values of BT and kT were determined, respectively, from the slope and intercept of the plot (Table 2). High values of kT and BT indicate fast adsorption and formation of strong bond between Cr (VI) and Thr-PPy [48]. 3.7. ␹2 –analysis To identify a suitable isotherm model for the adsorption of Cr (VI) on Thr-PPy composite, 2 –analysis has been carried out. The equivalent mathematical statement is

 2

 =

(qe − qe, m )2 qe, m

 (3)

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Fig. 8. Isotherm plots of (a) Freundlich, (b) Dubinin-Radushkevich (D-R) and (c) Temkin.

Where qe,m is equilibrium capacity obtained by calculating from the model (mg/g) and qe is experimental data of the equilibrium capacity (mg/g). If data from the model are similar to the experimental data, 2 will be a small number, while if they differ, 2 will be a bigger number. Therefore, it is also necessary to analyze the data set using the non-linear 2 test to confirm the best fit isotherm for the adsorption system [49]. The results of 2 analysis are presented in Table 2. From the 2 values the best fit for the adsorption of Cr (VI) on Thr-PPy is in the following order: Langmuir I > Langmuir III > Langmuir IV > Langmuir II > Freundlich > D-R isotherm > Temkin isotherm. The lower 2 values of Langmuir isotherm indicate its better applicability for the Cr (VI) adsorption on Thr-PPy.

Table 3 Thermodynamic parameters obtained at different temperatures for Cr (VI) adsorption on Thr-PPy. Temperature (K)

Go (kJ/mol)

Ho (kJ/mol)

So (kJ/mol/K)

303 313 323

−3.59 −3.26 −2.84

14.98

0.04

ferent temperatures and extrapolating to zero Ce according to the method suggested by Khan and Singh [50]. The adsorption distribution coefficient may be expressed in terms of H 0 and S 0 as a function of temperature:

3.8. Thermodynamic treatment of the adsorption process Thermodynamic parameters associated with the adsorption, viz., standard free energy change (G0 ), standard enthalpy change (H 0 ) and standard entropy change (S 0 ) were calculated as follows. The free energy of adsorption process, considering the adsorption equilibrium coefficient K0 , is given by the equation G0 = −RTlnK0

(4)

where G0 is the standard free energy of adsorption (kJ/mol), T is the temperature in Kelvin and R is the universal gas constant (8.314 J/mol/K). The adsorption distribution coefficient K0 was determined from the slope of the plot ln (qe /Ce ) against Ce at dif-

ln K0 = (S 0 /R)- (H 0 /RT )

(5)

where H 0 is the standard enthalpy change (kJ/mol) and S 0 is the standard entropy change (kJ/mol/K). The values of H 0 and S 0 can be obtained from the slope and intercept of a plot of ln K0 against 1/T. The calculated values of thermodynamic parameters are shown in Table 3. The negative values of G0 express the spontaneous nature of Cr (VI) adsorption onto Thr-PPy. The value of H 0 is positive indicating that the adsorption process is endothermic. The positive value of S 0 shows increased randomness at the solid/solution interfaces during Cr (VI) adsorption.

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Table 4 Kinetic parameters of various Cr (VI) concentrations onto Thr-PPy. Models

Parameters

Thr-PPy

Concentration (mg/L)

150

200

250

Pseudo first order model

k1 (1/min) R SD

4.189E-2 0.998 1.621E-2

4.422E-2 0.998 2.505E-2

4.548E-2 0.990 5.060E-2

Pseudo second order model

qe (mg/g) k2 (g/mg/min) h (mg/g/min) R SD

74.850 7.315E-3 40.984 0.999 3.980E-3

99.900 4.970E-3 49.603 0.999 3.460E-3

124.533 3.913E-3 60.680 0.999 3.310E-3

Particle diffusion

kp (1/min) R SD

4.188E-2 0.999 3.736E-2

4.420E-2 0.997 5.773E-2

4.546E-2 0.990 1.165E-1

Intra-particle diffusion

ki (mg/g/min0.5 ) R SD

1.949 0.996 2.909E-2

2.805 0.998 3.451E-2

3.467 0.999 1.981E-2

3.9. Adsorption kinetic models

order rate model with the correlation coefficients record higher than 0.999 in the present work.

Two main types of adsorption kinetic models, namely reaction – based and diffusion – based models were adopted to fit the experimental data [51]. The study of adsorption dynamic describes the solute uptake rate and evidently this rate controls the residence time of adsorbate uptake at the solid/solution interface. A relatively higher R value indicates that the model successfully describes the kinetics of adsorption of Cr (VI) on the adsorbent.

3.9.2. Diffusion-based models For a solid-liquid adsorption process, the solute transfer is usually characterized either by particle diffusion or by intra-particle diffusion control. An equation for the particle diffusion controlled adsorption process [55] is given below ln(1 − Ct /Ce ) = − kp t

3.9.1. Reaction based model The most commonly used pseudo-first-order and pseudo second-order model were employed to explain the solid/liquid adsorption. A simple pseudo first order kinetic model [52] is given as log(qe − qt ) = log qe − kl t/2.303

(6)

where qt is the amount of Cr (VI) on the surface of Thr-PPy at time t (mg/g) and k1 is the equilibrium rate constant of the pseudo firstorder adsorption (per min). The linear plots of log (qe −qt ) against t give straight lines indicating the applicability of pseudo-first-order model. The slope of the straight line plot of log (qe −qt ) against t with respect to adsorption gives the values of the pseudo-first-order rate constant (k1 ) and R which are listed in Table 4. In addition, the pseudo-second-order model is also widely used and the most popular linear form of pseudo second order model is [53] t /qt = 1/h + t/qe

(7)

where qt = qe 2 k2t/(1 + qe k2 t), the amount (mg/g) of Cr (VI) on the surface of Thr-PPy at any point of time t, k2 is the pseudo-secondorder rate constant (g/mg/min), qe is the amount of Cr (VI) ion adsorbed at equilibrium (mg/g) and the initial adsorption rate, h = 2 k2qe (mg/g/min). The values of qe (1/slope), k2 (slope2 /intercept) and h (1/intercept) of the pseudo-second-order equation can be found out experimentally by plotting t/qt against t. The values of qe , k2 , h and R of pseudo-second-order model were obtained from the plots of t/qt vs. t for Cr (VI) adsorption and are presented in Table 4. The higher R values obtained for pseudo-second-order model than pseudo-first-order model show better applicability of pseudo-second-order model. Cr (VI) adsorption on Thr-PPy is a chemisorption process, involving chemical bonding between active sites of adsorbent and adsorbate valance forces as suggested by an earlier study [54]. This indicates that the kinetic modeling of the Cr (VI) adsorption onto Thr-PPy well followed the pseudo-second-

(8)

where kp is the particle rate constant (per min) which is obtained from the slope of the plot between ln(1 − Ct /Ce ) and t. Weber et al. [56] proposed a theory which refers intra-particle diffusion model and its equation is qt = ki t1/2

(9)

where ki is the intra-particle rate constant (mg/g/min0.5 ). The slope of the plot of qt against t1/2 will give the value of intra-particle rate constant. The straight line plots of ln(1 − Ct /Ce ) vs. t and qt vs. t1/2 indicate the applicability of particle and intra-particle diffusion models respectively. The kp , ki and R values for both particle and intraparticle diffusion models are given in Table 4. The R values of both particle and intra particle diffusion models are almost comparable and suggest that the Cr (VI) distribution on Thr-PPy nanocomposites follow both the models. 3.10. Fitness of the kinetic models Assessment of the employed kinetic models for fitting the adsorption data was made with standard deviation (SD) and the model which possess lower values of SD show better fit to adsorption data. The SD values of Thr-PPy for all the kinetic models are summarized in Table 4. Smaller SD values were observed for the pseudo-second-order and intra-particle diffusion models which indicate that these two models are significant in the Cr (VI) adsorption process and suggest the adsorption of Cr (VI) ion onto the pores of Thr-PPy. 3.11. Effect of coexisting ions Generally, natural ground water contains several other ions viz., Cl− , NO3 − , HCO3 − ,SO4 2− , CO3 2− , PO4 3− , Ca2+ and Mg2+ ions and industrial waste water contains some other ions like Cu2+ , Zn2+ and Ni2+ , which may compete for the active sites in the adsorbent along

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97

Fig. 9. Effects of co-ions on the adsorption capacity of Thr-PPy.

Fig. 10. Regeneration of Thr-PPy for six cycles.

with chromium. Therefore, it is essential to investigate the competitive influence of coexisting ions. Fig. 9 shows the percentage of adsorption capacity of Thr-PPy composite in the presence of coexisting ions with a fixed initial concentration of 200 mg/L keeping all other parameters, such as contact time, pH and dosage as constant. It is observed that the presence of both cations and anions have no significant competitive influence on the adsorption capacity of Cr (VI) onto the Thr-PPy composite. The results suggest that Thr-PPy can effectively remove Cr (VI) in the presence of other common ions. So it is concluded that the removal of Cr (VI) by Thr-PPy is selective in nature.

removal efficiency of the Thr-PPy in each cycle of adsorption. It can be seen in Fig. 10 that the removal efficiency remains unchanged for the first four cycles and was above 97% but the removal efficiency was reduced to 69% and 40% in the following fifth and sixth cycle respectively. This result suggests that the Thr-PPy can be reused successfully for the removal of Cr (VI) for four times without loss of its removal efficiency.

3.12. Regeneration study Regeneration is one of the most important factors from environmental and economical perspective to suggest the applicability of adsorbents for Cr (VI) removal. Desorption efficiency of the Thr-PPy was evaluated by employing adsorption-desorption experiments. 14.5% of the adsorbed Cr (VI) only was extracted by using 1 M NaOH. The desorption efficiency of Cr (VI) seems to be very low and this is due to the reduction of adsorbed Cr (VI) to Cr (III) which could not be desorbed upon treatment with NaOH solution. But it was achieved by treating the adsorbent with 2 M HCl. The regeneration was achieved by initially alkali treatment (1 M NaOH) and followed by acid treatment with 2 M HCl. Fig. 10 clearly shows the

3.13. Comparison of maximum adsorption capacity of different adsorbents with Thr-PPy The maximum adsorption capacities of Thr-PPy with various polymer based adsorbents examined for the removal of Cr (VI) reported in the literature were summarized in Table 5. As shown in Table 5, Thr-PPy had a much higher maximum adsorption capacity compared to other adsorbents except few of adsorbents reported in the literature. It is very promising material for the removal of Cr (VI) in wastewater. 4. Conclusions Threonine doped polypyrrole was successfully prepared by in situ chemical polymerization technique and it was characterized using FT-IR, SEM with EDS and XRD studies. Thr-PPy is found to be an effective adsorbent for the removal of toxic hexavalent

Table 5 Comparison of adsorption capacity of the Thr-PPy with other reported adsorbents for Cr (VI) removal. Adsorbent

Initial concentration of Cr (VI) (mg/L)

Equilibrium time (min)

Qo (mg/g)

pH

References

Ethylenediamine-functionalized Fe3 O4 magnetic polymer Poly (N-methylaniline) Activated carbon coated with quaternized poly (4-vinylpyridine PANi/PEG composite Hierarchical porous PPy nanoclusters Aminated poly acrylonitrile fiber PPy/Fe3 O4 magnetic NCs PPy-OMMT NC PANi/Humic acid Polyacrylamide grafted sawdust Polymeric Fe/Zr pillered montmorillonite Polyethyleneglycomethacrylate −co–vinylimidamidazole microspheres PVA-PEI magnetic microspheres PGMA Fe 2-chloracetone cross linked poly (4-vinyl pyridines) Chitosan coated poly 3-methylthiophene Functionalized pyridine copolymer with amide groups Thr-PPy

50 100 18 50 3.4 50 200 200 15 100 50 80 50 30 22 200 13 50

60 40 1440 30 30 240 30 180 120 240 120 20 8 30 1440 200 120 60

61.4 125.0 53.7 109.9 180.4 20.7 169.4 119.3 150.0 45.0 22.4 108.7 88.4 149.9 142.9 127.6 82.5 185.5

2.5 2.0 2.3 5.0 5.0 2.4 2.0 2.0 5.0 3.0 3.0 2.0 2.0 2.0 3.0 2.0 3.0 2.0

[4] [6] [12] [29] [32] [35] [41] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] Present Study

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chromium from aqueous solution. The adsorption capacity of ThrPPy is highly pH dependent and the optimum adsorption occurred at pH 2. The adsorption pattern follows Langmuir isotherm. The positive H 0 value indicates the endothermic nature of the adsorption and negative G0 values indicate that the adsorption process of Cr (VI) is a spontaneous process, whereas the positive S 0 value suggests good affinity of Cr (VI) towards Thr-PPy. The kinetic process demonstrates that the adsorption process follows pseudosecond-order and intra-particle diffusion models. The adsorption process for the removal of Cr (VI) is governed by the ionic interaction between NH3 + of Thr and HCrO4 − ions. There is no influence of other coexisting ions like Cl− , NO3 − , HCO3 − , SO4 2− , CO3 2− , PO4 3− , Ca2+ , Mg2+ Cu2+ , Zn2+ and Ni2+ . Thr-PPy can be reused successfully for the removal of Cr (VI) up to four times without loss of its removal efficiency. Thr-PPy has certain advantages like easy preparation, low cost and high adsorption capacity, when compared to many other materials. Acknowledgements The authors thank the authorities of Gandhigram Rural Institute – Deemed University for the encouragement. References [1] D. Mohan, C.U. Pittman Jr., Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water, J. Hazard. Mater. B 137 (2006) 762–811. [2] V. Sarin, T.S. Singh, K.K. Pant, Thermodynamic and breakthrough column studies for the selective sorption of chromium from industrial effluent on activated eucalyptus bark, Bioresour. Technol. 97 (2006) 1986–1993. [3] Y.Q. Xing, X.M. Chen, D.H. Wang, Electrically regenerated ion exchange for removal and recovery of Cr (VI) from wastewater, Environ. Sci. Technol. 41 (2007) 1439–1443. [4] Z. Yong-Gang, S. Hao-Yu, P. Sheng-Dong, H. Mei-Qin, Synthesis, characterization and properties of ethylenediamine-functionalized Fe3 O4 magnetic polymers for removal of Cr (VI) in wastewater, J. Hazard. Mater. 182 (2010) 295–302. [5] N. Tewari, P. Vasudevan, B.K. Guha, Study on biosorption of Cr (VI) by Mucor hiemalis, Biochem. Eng. J. 23 (2005) 185–192. [6] H. Javadian, Adsorption performance of suitable nanostructured novel composite adsorbent of poly (N-methylaniline) for removal of heavy metal from aqueous solutions, J. Ind. Eng. Chem. 20 (2014) 4344–4352. [7] Guidelines for Drinking-Water Quality, 3rd ed., World Health Organization, Geneva, 2006. [8] R.P. Farrell, R.J. Judd, P.A. Lay, N.E. Dixon, R.S.U. Baker, A.M. Bonin, Chromium (V) induced cleavage of DNA: are chromium (V) complexes the active carcinogens in chromium (VI)-induced cancers? Chem. Res. Toxicol. 2 (1989) 227–229. [9] B. Preetha, T. Viruthagiri, Batch and continuous biosorption of chromium (VI) by Rhizopus arrhizus, Sep. Purif. Technol. 57 (2007) 126–133. [10] M. Bhaumik, K. Setshedi, A. Maity, M.S. Onyango, Chromium (VI) removal from water using fixed bed column of polypyrrole/Fe3 O4 nanocomposite, Sep. Purif. Technol. 110 (2013) 11–19. [11] E. Pehlivan, S. Cetin, Sorption of Cr (VI) ions on two Lewatit-anion exchange resins and their quantitative determination using UV–visible spectrophotometer, J. Hazard. Mater. 163 (2009) 448–453. [12] J. Fang, Z. Gu, D. Gang, C. Liu, E.S. Ilton, B. Deng, Cr (VI) removal from aqueous solution by activated carbon coated with quartinized poly(4-vinylpyridine), Environ. Sci. Technol. 41 (2007) 4748–4753. [13] Y. Sun, Q. Yue, Y. Mao, B. Gao, Y. Gao, L. Huang, Enhanced adsorption of chromium onto activated carbon by microwave-assisted H3 PO4 mixed with Fe/Al/Mn activation, J. Hazard. Mater. 265 (2014) 191–200. [14] H. Zhang, Y. Tang, D. Cai, X. Liu, X. Wang, Q. Huang, Z. Yu, Hexavalent chromium removal from aqueous solution by algal bloom residue derived activated carbon: equilibrium and kinetic studies, J. Hazard. Mater. 181 (2010) 801–808. [15] Z.A. AL-Othman, R. Ali, Mu. Naushad, Hexavalent chromium removal from aqueous medium by activated carbon prepared from peanut shell: adsorption kinetics, equilibrium and thermodynamic studies, Chem. Eng. J. 184 (2012) 238–247. [16] N. Sankararamakrishnan, M. Jaiswal, N. Verma, Composite nano floral clusters of Carbon Nano tubes and Activated alumina: an efficient sorbent for heavy metal removal, Chem. Eng. J. 235 (2014) 1–9. [17] J. Guo, Y. Li, R. Dai, Y. Lan, Rapid reduction of Cr (VI) coupling with efficient removal of total chromium in the coexistence of Zn (0) and silica gel, J. Hazard. Mater. 243 (2012) 265–271.

[18] N.A. Fathy, S.T. El-Wakeel, R.R. Abd El-Latif, Biosorption and desorption studies on chromium (VI) by novel biosorbents of raw rutin and rutin resin, J.Environ. Chem. Eng. 3 (2) (2015) 1137–1145. [19] R.K. Gautam, A. Mudhoo, G. Lofrano, M.C. Chattopadhyaya, Biomass-derived biosorbents for metal ions sequestration: adsorbent modification and activation methods and adsorbent regeneration, J. Environ. Chem. Eng. 2 (2014) 239–259. [20] Z.A. AL-Othman, Mu Naushad, R. Ali, Kinetic, equilibrium isotherm and thermodynamic studies of Cr(VI) adsorption onto low-cost adsorbent developed from peanut shell activated with phosphoric acid, Environ. Sci. Pollut. Res. 20 (2013) 3351–3365. [21] Mu. Naushad, Z.A. AL-Othman, G. Sharma, Inamuddin, Kinetics, isotherm and thermodynamic investigations for the adsorption of Co(II) ion onto crystal violet modified amberlite IR-120 resin, Ionics 21 (2015) 1453–1459. [22] Mu. Naushad, Z.A. ALOthman, Separation of toxic Pb2+ metal from aqueous solution using strongly acidic cation-exchange resin: analytical applications for the removal of metal ions from pharmaceutical formulation, Desalin. Water Treat. 53 (2015) 2158–2166. [23] Z. Zhang, M. Liao, H. Zeng, S. Xu, X. Liu, J. Du, P. Zhu, Q. Huang, Temperature effect on chromium (VI) removal by Mg/Al mixed metal oxides as adsorbents, Appl. Clay Sci. 102 (2014) 246–253. [24] A.A. Alqadami, Mu Naushad, M.A. Abdalla, T. Ahamad, Z.A. Alothman, S.M. Alshehri, Synthesis and characterization of Fe3 O4 @TSC nanocomposite: highly efficient removal of toxic metal ions from aqueous medium, RSC Adv. 6 (2016) 22679–22689. [25] M. Bhaumik, T.Y. Leswifi, A. Maity, V.V. Srinivasu, M.S. Onyango, Removal of fluoride from aqueous solution by ploypyrrole/Fe3 O4 magnetic nanocomposite, J. Hazard. Mater. 186 (2011) 150–159. [26] Y. Sag, Biosorption of heavy metals by fungal biomass and modeling of fungal biosorption: a review, Sep. Purif. Methods 30 (2001) 1–48. [27] N. Ballav, A. Maity, S.B. Mishra, High efficient removal of chromium (VI) using glycine doped polypyrrole adsorbent from aqueous solution, Chem. Eng. J. 198–199 (2012) 536–546. [28] M. Bhaumik, A. Maity, V.V. Srinivasu, M.S. Onyango, Removal of hexavalent chromium from aqueous solution using polypyrrole-polyaniline nanofibers, Chem. Eng. J. 181–182 (2012) 323–333. [29] M.R. Samani, S.M. Borghei, A. Olad, M.J. Chaichi, Removal of chromium from aqueous solution using polyaniline–Poly ethylene glycol composite, J. Hazard. Mater. 184 (2010) 248–254. [30] S.B. Deng, Y.P. Ting, Polyethylenimine-modified fungal biomass as a high-capacity biosorbent for Cr (VI) anions: sorption capacity and uptake mechanisms, Environ. Sci. Technol. 39 (2005) 8490–8496. [31] A. Maity, S. Shina Ray, Highly conductive core – shell nanocomposites of poly N-vinylcarbazole – polypyrrole with multiwallcarbon nanotubes, Macromol. Rapid Commun. 29 (2008) 1582–1587. [32] T. Yao, T. Cui, J. Wu, Q. Chen, S. Lu, K. Sun, Preparation of hierarchical porous polypyrrole nanoclusters and their application for removal of Cr (VI) ions in aqueous solution, Polym. Chem. 2 (2011) 2893–2899. [33] N. Ballav, H.J. Choi, S.B. Mishra, A. Maity, Synthesis, characterization of Fe3 O4 @glycine doped polypyrrole magnetic nanocomposites and their potential performance to remove toxic Cr (VI), J. Ind. Eng. Chem. 20 (2014) 4085–4093. [34] C. Wang, S. Liu, L. Liu, X. Bai, Synthesis of cobalt – aluminate spinels via glycine chelated precursor, Mater. Chem. Phys. 96 (2006) 361–370. [35] S. Deng, R. Bai, Removal of trivalent and hexavalent chromium with aminated polyacrylonitrile fibers: performance and mechanisms, Water Res. 38 (2004) 2424–2432. [36] R. Partch, S.G. Gangoli, E. Matijevic, W. Cai, S. Arajs, Surface induced polymerisation of pyrrole on iron (III) and cerium (IV) oxide particles, J.Colloid Interface Sci. 144 (1991) 27–35. [37] K. Pal, A.K. Banthia, D.K. Majumdar, Polyvinyl alcohol – glycine composite membrane: preparation, characterization, drug release and cytocompatibility studies, Biomed. Mater. 1 (2006) 49–55. [38] S. Kalidhasan, A.S. Krishna Kumar, V. Rajesh, N. Rajesh, Enhanced adsorption of hexavalent chromium arising out of an admirable interaction between a synthetic polymer and an ionic liquid, Chem. Eng. J. 222 (2013) 454–463. [39] H. Qi, S. Wang, H. Liu, Y. Gao, T. Wang, Y. Huang, Synthesis of an organic-inorganic polypyrrole/titanium(IV) biphosphate hybrid for Cr(VI) removal, J. Mol. Liq. 215 (2016) 402–409. [40] M. Bhaumik, S. Agarwal, V.K. Gupta, A. Maity, Enhanced removal of Cr(VI) from aqueous solutions using polypyrrole wrapped oxidized MWCNTs nanocomposites adsorbent, J.Colloid Interface Sci. 470 (2016) 257–267. [41] M. Bhaumik, A. Maity, V.V. Srinivasu, M.S. Onyango, Enhanced removal of Cr (VI) from aqueous solution using polypyrrole/Fe3 O4 magnetic nanocomposites, J. Hazard. Mater. 190 (2011) 381–390. [42] I. Langmuir, The constitution and fundamental properties of solids and liquids, J. Am. Chem. Soc. 38 (1916) 2221–2295. [43] T.W. Weber, R.K. Chakravorti, Pore and solid diffusion models for fixed bed adsorbers, J. Am. Inst. Chem. Eng. 20 (1974) 228–238. [44] H.M.F. Freundlich, Uber die adsorption in losungen, Z. Phys. Chem. 57A (1906) 385–470. [45] V. Sivasankar, S. Rajkumar, S. Murugesh, A. Darchen, Influence of shaking or stirring dynamic methods in the defluoridation behavior of activated tamarind fruit shell carbon, Chem. Eng. J. 197 (2012) 162–172. [46] B.P. Bering, M.M. Dubinin, V.V. Serpenski, On thermodynamics of adsorption on micropores, J. Colloid Interf. Sci. 38 (1972) 185–194.

A. Amalraj et al. / Journal of Water Process Engineering 13 (2016) 88–99 [47] M.I. Temkin, V. Pyzhev, Kinetic of ammonia synthesis on promoted iron catalyst, Acta physiochim. USSR. 12 (1940) 327–356. [48] M.N. Sepehr, A. Amrane, K.A. Karimaian, M. Zarrabi, H.R. Ghaffari, Potential of waste pumice and surface modified pumice for hexavalent chromium removal: characterization equilibrium, thermodynamic and kinetic study, J. Taiwan Inst. Chem. Eng. 45 (2014) 635–647. [49] Y.S. Ho, Selection of optimum sorption isotherm, Carbon 42 (2004) 2115–2116. [50] A.A. Khan, R.P. Singh, Adsorption thermodynamics of carbofuran on Sn (IV) arsenosilicate in H+ , Na+ and Ca2+ forms, Colloids Surf. 24 (1987) 33–42. [51] Y.S. Ho, J.C.Y. Ng, G. McKay, Kinetics of pollutant sorption by biosorbents: review, Sep. Purif. Methods 29 (2000) 189–232. [52] S. Lagergren, Zur theorie der sogenannten adsorption gelöster stoffe, K. Sven. Vetenskapsakad. Handl. 24 (1898) 1–39. [53] Y.S. Ho, Second order kinetic model for the sorption of cadmium onto tree fern: a comparison of linear and non linear methods, Water Res. 40 (2006) 119–125. [54] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem. 34 (1999) 451–465. [55] M. Chanda, K.F. O’Driscoll, G.L. Rempel, Sorption of phenolics onto cross-linked poly (4-vinyl pyridine), React. Polym. 1 (1983) 281–293. [56] W.J. Weber, J.C. Morris, Equilibria and capacities for adsorption on carbon, J. Sanitary Eng. Div. 90 (1964) 79–91. [57] K.Z. Setshedi, M. Bhaumik, S. Songwane, M.S. Onyango, A. Maity, Exfoliated polypyrrole-organically modified montmorillonite clay nanocomposite as a potential adsorbent for Cr(VI) removal, Chem. Eng. J. 222 (2013) 186–197.

99

[58] Q. Li, L. Sun, Y. Zhang, Y. Qian, J. Zhai, Characteristics of equilibrium, kinetics studies for adsorption of Hg (II) and Cr (VI) by polyaniline/humic acid composite, Desalination 266 (2011) 188–194. [59] C. Raji, T.S. Anirudhan, Batch Cr (VI) removal by polyacrylamide grafted sawdust: kinetics and thermodynamics, Water Res. 32 (1998) 3772–3780. [60] J. Zhou, P. Wu, Z. Dang, N. Zhu, P. Li, J. Wu, X. Wang, Polymeric Fe/Zr pillared montmorillonite for the removal of Cr (VI) from aqueous solutions, Chem. Eng. J. 162 (2010) 1035–1044. [61] E. Uquzdogan, E.B. Denkbas, O.S. Kabasakal, The use of polyethylene glycolmethacrylate-co-vinylimidazole (PEGMA-co-VI) microspheres for the removal of nickel (II) and chromium (VI) ions, J. Hazard. Mater. 177 (2010) 119–125. [62] X. Sun, L. Yang, Q. Li, Z. Liu, T. Dong, H. Liu, Polyethylenimine-functionalized poly(vinyl alcohol) magnetic microspheres as a novel adsorbent for rapid removal of Cr (VI) from aqueous solution, Chem. Eng. J. 262 (2015) 101–108. [63] D. Duranoglu, I.G.B. Kaya, U. Beker, B.F. Senkal, Synthesis and adsorption properties of polymeric and polymer-based hybrid adsorbent for hexavalent chromium removal, Chem. Eng. J. 181-182 (2012) 103–112. [64] V. Neagu, S. Mikhalovsky, Removal of hexavalent chromium by new quaternized cross linked poly(4-vinylpyridines), J. Hazard. Mater. 183 (2010) 533–540. [65] S. Hena, Removal of chromium hexavalent ion from aqueous solutions using biopolymer chitosan coated with poly 3-methyl thiophene polymer, J. Hazard. Mater. 181 (2010) 474–479. [66] V. Neagu, Removal of Cr (VI) onto functionalized pyridine copolymer with amide groups, J. Hazard. Mater. 171 (2009) 410–416.