Spectroscopic signature of branched polyaniline nanotubules decorated with nanospheres as an adsorbent for chromium

Accepted Manuscript Title: Spectroscopic signature of Branched Polyaniline nanotubules decorated with nanospheres as an adsorbent for chromium Authors: Ronak Bhatt, P. Padmaja PII: DOI: Reference:

S2213-3437(18)30666-3 https://doi.org/10.1016/j.jece.2018.10.057 JECE 2743

To appear in: Received date: Revised date: Accepted date:

29-7-2018 29-9-2018 26-10-2018

Please cite this article as: Bhatt R, Padmaja P, Spectroscopic signature of Branched Polyaniline nanotubules decorated with nanospheres as an adsorbent for chromium, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.10.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Spectroscopic signature of Branched Polyaniline nanotubules decorated with nanospheres as an adsorbent for chromium Ronak Bhatt† and P. Padmaja*† †Department

of Chemistry, Faculty of Science, The M. S. University of Baroda, Vadodara.

*Corresponding author. Tel. +91-265-2795552; Fax +91-265-2795552;

GRAPHICAL ABSTRACT

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Highlights

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E-mail address: [email protected] (Padmaja P. Sudhakar).

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   

Synthesis of Branched Polyaniline tubules decorated with nano spheres(PANI). The nano-architectures formed by self assembly. Chromium could be effectively adsorbed by PANI in a wide pH range of 2-6. PANI selectively adsorbed Chromium from chrome plating effluent.

Abstract Branched Polyaniline tubules decorated with nano polyaniline structures (PANI) were obtained by self-assembly during slow polymerization in strongly acidic conditions at 5°C. The 1

surface area of PANI was found to be 37.6716 m²/g with a pore size of 149.6430 Å corresponding to a maximum pore volume of 0.08255 cm³/g. Thermogravimetric analysis indicated a three step weight loss attributed to loss of water, chloride and degradation of PANI. The potential of PANI in the acid doped form was investigated for Cr6+ ion removal. The effects of operating parameters including pH, contact time and Cr6+ concentration were examined by batch adsorption experiments. Optimum adsorption occurred at pH 5 with Langmuir monolayer adsorption capacity Qmax of 182mg/g. The adsorption process was diffusion controlled and was

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governed by pseudo second-order kinetics. The adsorption data were well fitted with Freundlich, Halsey and Elovich isotherm models indicating interactive and multilayer nature of the

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adsorption process. The applicability of PANI for removal of chromium from plating effluent was demonstrated. Key words

Polyaniline tubules; Polyaniline spheres; self assembly; hexavalent Chromium; Chrome plating

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effluent

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1. Introduction

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Chromium enters the different water supplies through industrial discharges, hazardous waste site leachates and from the erosion of natural deposits. Two distinct forms of chromium

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occur in these water sources; the non-toxic trivalent chromium (Cr3+) an essential trace nutrient, and hexavalent chromium (Cr6+), a toxic species that can cause great harm to biota and the

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surrounding environment. The toxicity of Cr6+ serves as the basis of setting the (total) chromium standard for drinking water at 0.05 mg L-1 [1]. Simultaneous adsorption and reduction of toxic

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Cr6+ oxyanion is considered as the most convenient and effective method for removal of chromium. The use of conductive polymers for the removal of Cr6+ has gained interest due to their reversibility and high efficiency. Polyaniline (PANI) is one of the most extensively

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investigated conducting polymer because of its good stability, environment friendliness and can be synthesized in large scale at low cost [2–4]. The Cr6+ can be effectively reduced to Cr3+ with the electron transfer from the leucoemeraldine (LB) or emeraldine base (EB) states to

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permigraaniline (PB) state of PANI. PANI is typically produced by oxidative polymerization of aniline using a strong oxidizing agent such as ammonium persulfate (APS) in a strongly acidic solution such as 1 M HCl [5,6]. PANI, synthesized by this technique, possessed a granular morphology, characterized by irregular agglomerates and high conductivity. However, under certain conditions, PANI can form supramolecular ordered nanostructures (nanotubes, nanorods, nanofibers, and nanospheres), the formation and structural characteristics of which have been extensively studied and reported [24-26]. Wan and co-workers have fabricated cylindrical micro2

or nanotubes of PANI through the polymerization oxidation of aniline in the presence of capping agents such as carboxylic acids [7], C60(OSO3H6) [8], naphthalene sulphonic acid [9] camphor sulfonic acid [9] and salicylic acid [10]. Very recently, cylindrical nanotubes of PANI have also been produced in the presence of mineral acids [11–13], amino acids [14], manganese oxide [15] as well as in aqueous solution [16,17]. One dimensional (1D) PANI sub-microtubes with rectangular cross sections were prepared by oxidative polymerization of aniline in dilute SDS solution at room temperature (25°C) [18].

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Guo et al. used one-dimensional polyaniline (1D PANI) nanowire/tubes with rough surface prepared by simple chemical oxidation for rapid removal of Cr6+ and its reduction to

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Cr3+ [19]. PANI tubules decorated with nano PANI structures have not been reported hitherto. The objective of the present investigation was to fabricate self assembled polyanilne structures comprising of tubules with nanospheres that would provide an amenable platform for adsorption

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and reduction of hexavalent chromium.

Scheme 1 Different form of PANI

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2. Materials and Methods 2.1 Materials

All the reagents used were of Analytical Grade (AR) grade. Aniline [Renkem, India],

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Ammonium persulphate and Hydorchloric acid (Lab India, INDIA) were used as such without further purification. 2.2 Synthesis of PANI tubules decorated with nano PANI Polyaniline was prepared by adopting the method of Stejskal and Gilbert [20] with slight modifications. Briefly, 100mL of 0.25M aqueous solution of ammonium persulfate (NH4)2S2O8 and 0.20M solution of aniline were added to 100mL of 1M HCl at 5°C. Polymerization was allowed to proceed overnight in a refrigerator. The green precipitate that was formed was 3

separated by filtration, washed with 1M HCl followed by acetone until clear filtrate was obtained. The precipitate was further dried in a vacuum oven at room temperature (30°C) for 6 hours. The resulting green colored PANI was investigated for its adsorption potential towards Cr6+. 2.3 Characterization The synthesized PANI was characterized by various techniques. The point of zero charge (pzc) and the zeta potential of PANI at different pH was measured using ZetaPlus

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90Plus/BI-MAS Brook Haven model Zeta sizer. Thermo Finnigan EA 1112 Series Flash Elemental Analyzer with TCD detector was used for CHNS elemental analysis, where in the

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furnace was set to a temperature of ~900°C. Scanning electron microscope (SEM) (model JEOL JSM-5610LV) and Transmission Electron Microscope (TEM) - Tecnai 20 from Philips, Holland were used to observe the surface morphology of PANI and Cr6+ loaded PANI. Jasco V630 model UV spectrophotometer was used for measuring UV spectra of polyaniline. Photoluminescence

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spectra was taken using Jasco FP-6300 Spectrofluorimeter. The FTIR spectra were collected

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using Perkin Elmer RX1 model in the wave number range of 400-4000 cm-1. Raman analysis

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was done in the 4000-150 cm-1 spectral range using a MiniRam Portable Raman Spectrometer from B&WTek. Thermogravimetric analysis (TGA) was done using EXSTAR6000 TG/DTA

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6300 model instrument with 7.5mg of PANI in the temperature range 30 to 900°C at a heating rate of 10°C per minute. Differential Scanning Calorimetry (DSC) thermograms were recorded

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in a nitrogen stream of 50mL/min-1 with a ramp of 10°C min-1 from 30 to 500°C using 822EN Mettler Toledo Model. XRD analysis of the powder samples at 2 theta 0°-90° were performed

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with a PANalytical Diffractometer (Bruker) using Cu-Kα radiation (λ= 0.15418 nm) at a voltage of 45 kV. The oxidation state of chromium in polyaniline after adsorption was investigated by recording an EPR spectrum on a JEO, Japan JES-FA200 ESR Spectrometer with X and Q band.

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Bonding energies of C, O and N on the surface of PANI and after PANI-Cr was determined by Kratos AXIS Ultra HSA X-ray photoelectron spectrometer. All XPS spectra were presented charge balanced and energy referenced to C1s at 284.6eV. Chemical states of O, N and C species

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were determined from the charge corrected hi-resolution scans. The teledyne Leemn lab pro DG high dispersion ICP having Solid-state detector technology was used for estimation of total chromium content. 2.4 Preparation of metal solution Stock solution of 1000 ppm Cr6+ was prepared by dissolving 2.8317 g of potassium dichromate (99.92% assay) in 1L of double distilled water. Working standards were prepared by diluting different volumes of the stock solution to obtain the desired concentration. 4

2.5 Batch sorption Experiments Batch adsorption experiments were conducted at 30°C by agitating 0.05g PANI with 25mL of Cr6+ solution of desired concentration at optimum pH in 100mL stoppered conical flask using a thermostated rotary mechanical shaker at 200 rpm for 4 h at 30°C except for pH optimization experiments. The pH range at which the maximum chromium uptake occurred was determined by varying the initial pH of the solution in the range 1 to 11 using NaOH or H2SO4. The optimum equilibrium time was determined by varying the contact time in the range of 60 to

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240 min. In each experiment, after agitation the contents of the flasks were filtered and remaining concentration of chromium was determined by Inductive Couple Plasma Instrument.

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The Isotherm and kinetics models investigated are described in Table S1 and Table S2. 2.6 Desorption

For desorption studies, 0.05g of PANI preloaded with known amount of Cr6+ was equilibrated with 25mL of 0.1 M NaOH, H2SO4, HCl and HNO3 solutions for 120 min. The

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contents of the flasks were filtered and the chromium content in the filtrate was determined by

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ICP. Three consecutive adsorption-desorption cycles were performed to test the reusability of

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PANI.

2.7 Potential of PANI in treating chrome plating effluent of PANI for removal of chromium from a Chrome plating industry

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The potential

effluent, was investigated. The effluent was used directly after filtration. The effluent

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composition before and after equilibration with 0.05 g PANI was determined by ICP-OES. The data from this experiment was used to calculate the distribution coefficient (Kd) and selectivity

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coefficient (β).

The selectivity coefficient (β)was calculated using Eq.(1)

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𝛽 = 𝐾𝑑1 /𝐾𝑑2

(1)

Where Kd1and Kd2 are the distribution coefficients of metal ion under study and the interfering metal ion respectively. Kd was calculated using Eq. (2) 𝐾𝑑 = 𝑄𝑒 /𝐶𝑒

(2)

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Where Qe represents the amount of Cr6+ adsorbed per gram of adsorbent calculated using Eq. (3) 𝑄𝑒 = (𝐶𝑖 − 𝐶𝑒 )/𝑚

(3)

Where m represents the mass of adsorbent in g/L. Ci and Ce (ppm) are the initial and equilibrium (after adsorption) concentrations of Cr6+ in solution, respectively 3. Results and discussion 3.1 Zeta Potential 5

The values of zeta potential for PANI were positive at all pH conditions in the range 1 to 10 (Figure S1), with a pzc value of 10.3 indicating that PANI backbone acquired a positive charge and was a polycation. The fact that the positive charges were not neutralised even at pH 10 indicates a highly protonated PANI [21]. 3.2 Elemental and Surface area Analysis Elemental analyses of PANI was performed by CHN microanalysis and SEM EDX techniques. The comparable percentages of nitrogen in both EDX and CHN analyses (Table 1

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and Figure S2) in PANI indicated the presence of the nitrogen backbone in the polymer chain [22]. The presence of chromium manifested with decrease in nitrogen and carbon percentage in

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PANI-Cr. However, there was a significant increase in oxygen content due to the adsorption of HCrO4-. In general, the C/N ratio is ~6 for aniline units in PANI. Surface area, pore volume, and pore diameter of the composite biosorbent were determined with a BET instrument by means of adsorption of ultra-pure nitrogen at -196°C (Figure 1). The surface area of PANI was found to

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be 37.6716 m²/g. A pore size of 149.6430 Å corresponding to a maximum pore volume of

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0.08255 cm³/g was obtained from the Barrett-Joyner-Halenda (BJH) pore size distribution curve. PANI- Adsorption PANI- Desorption

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80

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60 50 40

30

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Quantity Adsorbed (cm 3/g STP)

70

20

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10 0

0.0

0.1

0.2

0.3

0.4 0.5 0.6 0.7 Relative Pressure (P/Po)

0.8

0.9

1.0

Figure 1 Isotherm Linear Plot (BET)

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Table 1 EDX and CHN microanalyses PANI

PANI-Cr6+

Element

Weight% Atomic% CHN

Weight% Atomic% CHN

Carbon

70.59

73.68

52.1734 43.93

55.84

39.4373

Nitrogen

9.42

16.53

9.337

1.99

6.5525

Hydrogen 6

4.5927

1.83

4.1437

0.2856

Sulfur Oxygen

4.63

3.92

Chloride

15.36

5.87

39.73

37.91

14.51

4.26

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Chromium

0

3.3 TEM Analysis

The PANI TEM image (Figure 2) showed interconnected tubules decorated with PANI spheres of size 25-45 nm. The Debye-Scherrer rings indicate that majority of the tubules and

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spheres have self assembled in an ordered pattern.

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Figure 2 TEM images of (a) Nano PANI (b) Tubules decorated with spheres and (c) SAED spectrum of PANI

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3.4 UV–Vis analysis of PANI and PANICr6+

Figure 3 UV-Vis spectra of (a) PANI and (b) PANICr6+

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The absorption peak observed at 330 nm in PANI (Figure 3) was probably due to the π– π* transition in the benzenoid ring [23], while the absorption peak at 399 nm can be assigned to aniline oligomeric intermediates containing phenazine-like structures [24–26] viz. the π- π* transition of the benzenoid rings and polaron bands in emeraldine salt state [27]. The broad shoulder at around 471 nm was due to polaron- π* transition [28–31]. The pi-bipolaron transition of emeraldine salt was associated with absorption at 770 nm. The plateau at about 800–1000 nm also indicated emeraldine salt state of PANI [27]. The broadening extended towards the near

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infra-red region indicated an increased concentration of polarons leading to the easier delocalization of π-electrons that was characteristic of highly conductive polymers [32].

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3.5 Photoluminescence analysis

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Figure 4 Photoluminescence spectra of PANI

The emission peak at 348 nm (Figure 4) can be attributed to ᴨ→ᴨ* transition of

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benzenoid unit while the peak at 401 nm could be attributed to fully reduced form of benzenoid species in leucoemeraldine form of PANI [33,34]. The emission at 471 nm could be attributed to S0→S1 transition while a shoulder at 500 nm was indicative of quinoid unit in PANI .The vibronic peaks that appeared at lower energy (∼500-600 nm)

confirmed the presence of

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benzenoid and quinoid units in PANI specifically arranged in a proper order amenable for the formation of excitons[35]. 3.6 IR Spectra analysis

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525

%Transmission

668

PANICr6+

4000

3600

3200

2800

2400

2000

1800 1600 Cm-1

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1457

1113 1048

1241

1650

PANI

1400

1200

1000

800

600

400

Figure 5 FTIR Spectra of PANI and PANICr6+

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Figure 5 shows the FTIR spectra of PANI and PANICr6+. The characteristic peaks

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observed in the FT-IR spectra and their assignments are given in Table S3. The peak located at 1048 cm-1 suggested the presence of sulfonate groups attached to the aromatic rings corroborated

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by elemental analysis [36,37]. The band observed at ∼1457 cm-1 supported the presence of

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ortho-linked aniline constitutional units in the oligomers [36]. The band at ∼1650 cm-1 could be attributed to C=C and C=N stretching vibrations in a phenazine-like segment [38]. The

1415 1464 1592

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900

Intensity (a.u)

1163 1293

180

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Intensity (a.u)

1000

650

409

600

833

800 700

(b)

200

160

140

1562

(a)

1100

1399

1200

305 348

3.7 Raman Analysis

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characteristic Cr-O stretching was observed in PANI-Cr at 668 and 525 cm-1.

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500

100

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400

0

500

1000

1500

2000

2500

Raman Shift (cm-1)

3000

3500

0

500

1000

1500

2000

2500

3000

3500

Raman Shift (cm-1)

Figure 6 Raman Spectra of (a) PANI and (b) PANICr6+ Figure 6 shows the Raman spectra of PANI and PANI- Cr. The characteristic bands of emeraldine salt were observed in the Raman spectra of PANI samples. In pristine PANI sample, the Raman peaks appeared at ∼1163 cm-1, ∼1293 cm-1, ∼1464 cm-1 and 1592 cm-1, 9

corresponding to the vibrations of C-H benzenoid or quinoid stretching, C-N benzenoid stretching, C=N and C=C quinoid stretching, respectively [39,40]. The intensity of the bands was reduced in Cr-PANI. The band at 1592 cm-1 and 1562 cm-1 PANI-Cr can be attributed to the C=C deformation of semiquinoid rings and C=C quinoid stretching; the band at 1415 cm-1 can be assigned to the C=N stretching of semiquinoid rings. The peak at 1399 cm-1 in PANI-Cr6+ was characteristic of the N-H bending deformation of protonated amine; the band at 833 cm-1 was attributed to the PANI ring deformation in Q-ring; the peak at 650 cm-1 was associated with

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amine deformation in B while the band at 409 cm-1 could be assigned to the out-of-plane C-H wagging in the bipolaronic and CNC torsion of polaronic structures in Q-ring [41–45].

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Characteristic peaks of Cr were observed at 305.53, 348.38 and 548 cm–1 attributed to two Eg vibrations and A1g symmetry respectively[21,46–48]. 3.8 TGA and DSC 250.0 100.0 20.00

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200.0 0.0 10.00

150.0

-100.0

-200.0

-300.0

50.0

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-20.00

TG ug

-10.00 DTA uV

DTG ug/min

100.0

N

0.00

-400.0

0.0 -30.00

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-500.0

-40.00

-100.0

-50.00

-150.0

-60.00

-600.0

-700.0

-800.0

100.0

200.0

300.0

400.0 500.0 Temp Cel

600.0

700.0

800.0

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-50.0

900.0

Figure 7 TGA and DSC analysis of PANI

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The first mass loss between 25°C to 100°C was attributed to the loss of water contained in the polymer. The weight loss at 100-200°C could be attributed to further loss of moisture and

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the elimination of chloride ion. This was followed by continuous weight loss due to decomposition of PANI. DSC analysis of PANI was characterized through a broad endothermic transition, starting at approximately 52°C and centered at approximately 92°C corresponding to its glass transition temperature (Figure 7) [49]. Exothermic peaks at 188°C and 337°C were due

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to oxidative degradation of PANI. 3.9 XRD analysis of PANI and PANI-Cr6+

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PANI

Pristine PANI exhibited four peaks

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Figure 8 XRD spectra of PANI and PANICr6+

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20.1

15.1

25.3

9.5

PANICr6+

at ~9.5°, ~15.1°, ~20.1° and ~25.3° [50]

corresponding to the diffractions of (001), (010), (100) and (110) crystallographic planes of

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PANI in its emeraldine salt form(Figure 8). The peak at a 2θ value of 15.1° was attributed to the

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periodicity parallel to the polymeric chain in emeraldine salt [51]. The peak centered at a 2θ value of 20.1° was due to emeraldine base. The reflections at 25.3° were the most intense which

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arose from the periodicity perpendicular to the polymer chain and from the crystallinity of

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polymer. [52] The d-spacing of 3.45 Å associated with the diffraction peak at 25.3° corresponded to the face-to-face inter chain stacking distance between phenyl rings. The diffraction peaks

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indicated that PANI polymer was partially crystalline but predominantly amorphous. The XRD pattern of PANI after adsorption of Cr6+ exhibited only one broad hump indicating that PANI had become completely amorphous[53].The adsorption of chromium would have affected the

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molecular arrangement and crystallinity of PANI resulting in a disordered structure.

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3.10 ESR spectra of PANI and PANICr6+

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Figure 9 ESR Spectra of PANI and PANICr6+

Pristine PANI showed an ESR signal with a g value of 2.00272 (Figure 9) indicating the

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presence of polaron. Poly [semi-quinone radical cations] or polarons are present as charge carriers[54]. This value is characteristic of emeraldine salt. However a slight shift from, ge =

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2.00273 suggested that the spins in PANI were delocalized over some repeating units[55].The g

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value of Cr was related to exchange interactions between the bound chromium species [56].

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3.11 XPS

The pristine PANI and PANI after equilibration with chromium were characterized by

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XPS (Figure 10) and the observed peaks with their assignments are tabulated in Table 2.

Figure 10 XPS spectra of PANI and PANI-Cr6+ 12

Table 2 XPS analysis of PANI and PANI-Cr6+ Sample name

284.604

2.229

27701.2 76.0

C 1s

286.283

2.241

8765.8

O 1s

532.776

2.853

19529.8 100.0

N 1s

399.253

2.622

5106.6

74.4

N 1s

401.147

3.141

1757.3

25.6

Cl 2p

197.329

2.391

1771.6

Cl 2p

200.216

2.371

1273.1

Cl 2p

199.156

2.317

1163.6

Cl 2p

201.862

1.94

C 1s

284.604

2.019

C 1s

285.785

2.378

O 1s

531.474

2.805

O 1s

533.13

2.732

N 1s

398.658

N 1s

399.677

N 1s

400.724

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24.0

36.5 26.2

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24

644.2

13.3

14035.9 71.3

4728.1

26.1

1.285

273.7

31.7

1.204

346.8

40.2

1.014

241.6

28

Cr 2p3/2 577.226

3.055

5487.1

44

Cr 2p1/2 586.469

2.993

2985

23.9

Cr 2p3/2 579.785

3.384

2483.5

19.9

Cr 2p1/2 589.04

2.948

1513.5

12.1

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28.7

N

5650

13392.5 73.9

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PANICr6+

Percentage

C 1s

M

PANI

Binding Energy (eV) FWHM eV Area

C 1s : The peak at binding energy 284.6 eV was attributed to C-C or C-H bonds at the

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backbone of the polymer. The 286.2 eV peak can be attributed to the carbon atoms bonded to polaronic and bipolaronic-type nitrogen atoms. The peak at 285.7 eV in PANI-Cr was attributed to C-N/ C=N bonds [57].

N 1s : In PANI the 399.2eV peak was assigned to benzenoid

amine while the 401.1eV peak was assigned to protonated nitrogen atoms of iminium ion [58].

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The N1s XPS data of PANI-Cr could be deconvoluted into 3 peaks located at 398.6eV, 399.7 eV and 400.7 eV. The first peak was assigned to quinoid imine, the second one to benzenoid amine and the third one to protonated nitrogen atoms of iminium ion [58]. The shift to higher binding energy as compared to PANI was indicative of interaction of polyaniline with chromium. The Cr6+ was reduced to Cr3+ by the oxidation of benzenoid amine to quinoid imine resulting in the transformation of PANI from EB to PB form. Cr 2p : The binding energy peaks at 577.2 and 13

579.7eV corresponded to Cr-OH/Cr-Cl of trivalent hydroxy species of chromium or chloride [59–61]. No Cr6+ was detected suggesting that all Cr6+ was converted to Cr3+. Cl 2p : The Cl 2p peaks were fitted with spin-orbit doublets (Cl 2p1/2 and Cl 2p3/2, with area ratio ∼1:2 and BE separation of ∼1.6 to 1.8 eV). The Cl2p3/2 component at ∼197.3 eV represented anionic Cl while the peak around 200.2 eV could be attributed to covalently bonded chlorine [62]. O1s : The presence of oxygen can be attributed to superficial oxidation or presence of water [63].

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4. Adsorption Studies 4.1 Effect of pH

The effect of pH on the adsorption process is presented in Figure 11 wherein the

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adsorption was determined over the pH range 1-11 using 0.05 g PANI and 100 ppm chromium solution. Chromium can exist in various ionic forms at different pH values. Hence, the initial solution pH strongly influenced its adsorption. It was observed that maximum adsorption of Cr6+ occurred at pH 2–6 where zeta potential was highly positive. Cr6+ predominantly exists in its

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anionic HCrO4- form in this pH range. The state of amine/ imine groups of PANI is also pH

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dependent and are protonated resulting in electrostatic interactions between the protonated

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amine/imine groups of PANI and negatively charged Cr6+. The nanospheres and the tubules provided abundant protonated sites for chromium adsorption.

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100 90 80

60 50 40

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% Uptake

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70

30 20

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10

0

0

2

4

6 pH

8

10

12

Figure 11 Effect of pH

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Volume = 25mL, Amount adsorbent = 0.05 g, (Cr6+)= 100 ppm: pH = 1 to 11, contact time =4 h, Temperature = 30°C.

4.2 Effect of Variation of Dose of PANI The effect of sorbent dose was also investigated (Figure S3). Different amounts (0.01–0.1 g) of PANI was suspended in 25mL Cr6+ solution (100 ppm) under optimized conditions. It was observed that the adsorption percentage of Cr6+ onto PANI increased with increasing adsorbent

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dose and reached saturation limit at 0.05 g. This could be due to greater availability of surface area with increased adsorbent dose. 4.3 Effect of Initial Cr6+ concentration 100 98

94

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% Upatke

96

92

88 40

60

80

100

120 140 160 Concetration (ppm)

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90

180

200

220

Figure 12 Effect of Concentration

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Volume = 25mL, Amount adsorbent = 0.05g, (Cr6+) = 60ppm to 2000ppm pH = 5, contact time =

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4 h, Temp. = 30°C

The effect of metal ion concentration on the uptake behavior of PANI was studied in the

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concentration range 60-2000 ppm of Cr6+ (Figure 12). The percent uptake was found to decrease

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with increase in concentration of Cr6+. However qe increased with increase in the initial concentration of Cr6+ solution. The data was fitted with several isotherm models (Supporting

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data - Table S1) and isotherm constants for the adsorption of Cr6+ by PANI are presented in Table 3. The adsorption data for PANI gave reasonably high correlation coefficient values for all the models studied. From the correlation coefficients (Table 3), the Freundlich model simulated

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the experimental data better than the Langmuir model suggesting that monolayer adsorption was not taking place on PANI. The Freundlich constant, n was more than unity implying that the

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adsorption intensity was favorable over the entire range of concentrations studied and also suggests an interactive nature for the Cr6+ adsorption process [64]. The maximum removal capacity of Cr6+ by PANI calculated from Langmuir isotherm model was 182 mg/g. From Figure

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S4 and S5, the fitting of Temkin model indicated favorable adsorption of Cr6+ onto adsorbent under study. The high regression values for Halsey model suggested multilayer adsorption. The low fitting of the Langmuir adsorption isotherm of Cr6+ on PANI could be due to the low saturation of the adsorbent surface of PANI. This has been reported to occur on the surface of non-porous or macroporous adsorbents. Upon completion of the monolayer adsorption process, more layers may begin to form with the increase in the relative pressure of the adsorbate (the increase in the concentration of Cr6+ relative to the saturation vapor pressure)[65]. The negative 15

values of nFH and low values of KFH (Table 3) of FH isotherm implied that the Flory-Huggins model cannot be used to describe the adsorption data. The high correlation coefficient of Elovich isotherm further suggested multilayer adsorption. Table 3 Isotherm constants for Cr6+ adsorption on PANI

F-H isotherm

Elovich

52.2516

5.7081

0.9937

0.0151

KL (L.mmol-1)

qm (mg.g-1)

∆G (kJ.mol-1)

r2

SD

0.0365

182.48

-8.6077

0.9943

0.2031

BT

-∆H (kJ.mol-1)

KT (L.mmol-1)

r2

SD

19.2166

0.0266

7.5046

0.9981

kH (L.g-1)

nH

r2

SD

52.2516

-5.7081

0.9937

0.0348

NFH

r2

SD

KFH

-0.6126

0.9923

qm (mg.g-1)

KE (L.mmol-1)

24.2131

8.4259

IP T

SD

SC R

Halsey

r2

U

Temkin

N

0.0293

N

Langmuir

Kf (L.g-1)

A

Freundlich

Constants

2.5367

0.0009

r2

SD

Lnqm

0.9917

0.2039

3.1869

M

Isotherms

4.4 Sorption Kinetics

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The time dependent Cr6+ adsorption on PANI was further investigated(Figure S6). The experimental results showed increase in Cr6+ removal with time. It was observed that the rate of

PT

uptake was very high initially, with around 91% of Cr6+ being removed in about 60 minutes and equilibrium was achieved in 240 minutes. The removal rate was high initially due to the presence

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of free binding sites on PANI which gradually got occupied with time and hence there was a decrease in rate of adsorption. The order of adsorption of chromium was investigated by fitting the kinetic data to different kinetic models (Supporting Data Table S2). The rate constants for different kinetic models are presented in Table 4. The correlation coefficients for pseudo second

A

order model were found to be high and the calculated qe values were closer to those obtained experimentally. This indicated that surface complexation may be the rate-limiting step which involves valence forces through sharing or exchanging of electron between adsorbent and adsorbate. Figure S7 and S8 show that the value of r2 was higher for pseudo first order kinetic model than pseudo second order model, but the calculated qe values were much lesser than those observed from experiment. The liquid film diffusion and Elovich equations were used to 16

determine the mass transport and chemical adsorption mechanism of Cr6+. The reasonably good correlation coefficients confirmed the existence and importance of diffusion-control in the transport/adsorption mechanism for Cr6+ adsorption by PANI. The plot of qt versus t1/2 did not pass through origin suggesting that intraparticle diffusion was not the only rate controlling step but some degree of the boundary layer diffusion also controlled the adsorption process and that the overall rate of the metal adsorption process appeared to be controlled by more than one-step. The reasonably good correlation coefficient for Bangham equation indicated that the diffusion of

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Cr6+ into the pores of the adsorbent under study also controlled the adsorption process. As the adsorption kinetics and isotherm followed less familiar relationships such as the Lagergren,

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Liquid film diffusion model (LFD) as well as Elovich kinetic models and Freundlich and Halsey isotherm models the mechanism of uptake of chromium seems to be complex and multilayered.

Isotherm

Constants

Experimental qt (mg/g)

49.505

ED

Lagergren

PT

Liquid film diffusion model

CC E

Intraparticle

Bangham

SD

0.9994

0.0484 SD

r2

qe (mg/g) K

r2

SD

0.0092

0.0193

0.9818

0.0967

Kfd

r2

SD

0.0036

0.9818

0.0420

B

Alpha

0.4721

6.3E+17 0.9786

M

Elovich

0.0023

r2

A

50.865

K

N

qt (mg/g)

Pseudo 2 order

U

Table 4 Kinetic constants for Cr6+ adsorption on PANI

0.2462

K

r2

SD

0.4172

0.9911

0.1222

Km

Alpha

r2

SD

-24.519

0.2529

0.9517

0.0195

4.5 Thermodynamic studies

A

The chromium uptake decreased with increase in temperature from 30°C to 70°C (Figure

S9) indicating that the metal uptake onto PANI was dependent on temperature and that the adsorption process was exothermic. This decrease in the uptake capacity with increase in temperature might be due to decrease in the thickness of the boundary layer with increased tendency of the metal ion to escape into the solution. The free energy change during the adsorption process was negative for the experimental range of temperatures (Figure 13 and Table 17

5) indicating the feasibility and spontaneity of the adsorption process [66]. The negative value of enthalpy change indicated that the adsorption process was exothermic [67]. The negative entropy change corresponded to decreasing system randomness at the solid-liquid interface during the adsorption process. 3.5

3.0

IP T

LnKd

2.5

2.0

1.0 -0.00330

-0.00325

-0.00320

-0.00315

1/T

SC R

1.5

-0.00310

Figure 13 Thermodynamics plot for adsorption of Cr6+ on PANI ΔH

(kJ/mole) (kJ/mole) -7.9351

313

-5.6516

323

-3.3682

333

-1.0847

343

1.1987

M

303

ΔS

N

ΔG

(J/moleK)

A

T (K)

U

Table 5 Thermodynamics constants

-143.293

ED

-77.124

PT

4.6 Application of PANI as adsorbent to chrome Plating Effluent The composition of the effluent was analyzed by ICP. The results of the potential of

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PANI as adsorbent for chrome plating effluent with percent extraction of major metal ions present in the effluent, their Kd values and selectivity coefficient of chromium over other metals are shown in Figure 14. The distribution coefficient was highest for chromium when PANI was used as adsorbent. PANI did not prove to be a good adsorbent for the removal of other metal ions

A

as they predominantly exist as cations. PANI showed highest selectivity towards chromium. However, further investigations are warranted in this direction.

18

Co =38.8, Ce=36.3 %=6.5, Kd=0.1, S=14.7

Mg

Co =1.1, Ce=1.0 %=9.1, Kd=0.1, S=10.4 Co =15.0, Ce=13.6 %=9.3, Kd=0.1, S=10.1

Hg Al

Actual Concentration (Co) Remaining concentration (Ce)

Co =10.1, Ce=9.0 %=10.9, Kd=0.1, S=8.7

Cu

Co =236.3, Ce=208.8 %=11.6, Kd=0.1, S=8.1

Zn Co =14.0, Ce=11.0 %=21.4, Kd=0.3, S=4.4

Ni

Co =4.4, Ce=2.1 %=52.3, Kd=0.7, S=1.8 Co =0.5, Ce=0.2 %=53.9, Kd=0.7, S=1.8

Pb Cd

Co =62.5, Ce=5.8 %=90.7, Kd=1.1

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Cr

IP T

Co =232.0, Ce=121.5 %=47.6, Kd=0.6, S=2.0

Fe

% = % Extraction, Kd=Distribution ratio Kd, S=Selectivity coefficient for Cr Figure 14 Behavior of PANI under study to chrome Plating Effluent

U

4.7 Mechanism

N

Literature studies reveal that the oligomeric micro/nanostructures were persumably formed during aniline oxidation by self-assembly of the phenazine like reaction intermediates

A

due to the interplay of π-π interactions, hydrogen bonding and hydrophobic interactions. These

M

would act as templates for the adsorption of N-phenylphenazines formed in solution during the induction period. The subsequent growth of PANI chains from stacked N-phenylphenazines

ED

could give rise to the walls of the nanotubes [17]. PANI being a polycation resulted in a positively charged membrane that was not permeable to the anilinium cation, due to the electrostatic repulsive interactions. The oxidative polymerization of aniline on the PANI

PT

membrane proceeded with the transfer of electrons from aniline molecules to the peroxydisulfate through the conducting PANI membrane[68].The electrostatic repulsion between protonated

CC E

polymer chains facilitated the ordered arrangement of nanostructures on the rectangular tubes [69]. The proposed mechanism for the uptake of Cr6+ by PANI is shown in Scheme 2. Firstly, the surface of the adsorbent gets protonated and forms positively charged amine functional groups at

A

lower pH. These protonated amine groups form surface complexes with the negatively charged Cr6+ (HCrO4-) through electrostatic attraction followed by reduction to Cr3+ . The reduction process was corroborated by the fact that the presence of Cr6+ on PANI was neither indicated by the XPS Cr2p spectra nor by diphenyl carbazide spectrophotometric method. The Cr6+ was reduced to Cr3+ by the oxidation of benzenoid amine to quinoid imine indicated by FTIR, XPS and Raman signals corresponding to quinoid amine [70]. The oxidation was evidenced by the presence of imine nitrogen in PANI-Cr as well as the reduction in amine nitrogen content in XPS 19

spectra. Ion-exchange with protonated imine nitrogens in emeraldine salt facilitated the reduced Cr6+(Cr3+) to bind onto PANI. The Cr3+ can also be further bound on the surface of PANI by chelation with imino groups [71,72]. An electron in sp3 orbit of nitrogen atom of partially or fully oxidized forms of polyaniline makes a co-ordinate bond with positively charged Cr3+. Comparing the observations of other researchers and our results, it was felt that when mesoporous or nano structured adsorbents are used, the internal porous structure sites play a role in the uptake of chromium, resulting in multilayered adsorption. The self assembled tubules and

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spheres could be providing active surface for adsorption and complete reduction of hexavalent chromium. It thus becomes important to study other multilayer models before coming to a

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conclusion. The PANI based adsorbents reported in the literature have been compared in Table S4. The removal of chromium could be performed at pH 5 while the optimum pH for removal of chromium by PANI based adsorbents reported in literature was 1 to 4.5 except for Polyaniline/poly(ethylene glycol) composite ( pH 5), Sulfuric acid doped Polyaniline (pH 6) and However, it is difficult to compare the adsorption efficiency of the

U

PANI-G10( pH 6.5).

A

CC E

PT

ED

M

A

adsorption capacity was calculated at different pH.

N

adsorbents as different kinetic and isotherm model fits were obtained by various researchers and

Scheme 2 Mechanism of chromium removal by PANI 5. Conclusion Nano sized PANI was used for effective removal of Cr6+ from water. The removal of Cr6+ was governed by electrostatic interactions followed by reduction which was corroborated by Raman, FT-IR, ESR and XPS techniques. The film, tubules and spheres could be providing 20

active surface for adsorption and complete reduction of hexavalent chromium. The adsorption of Cr6+ was completed in 240 min and was governed by pseudo second-order kinetics which involved multilayered adsorption process. The adsorption data of chromium on PANI were well fitted with Freundlich, Halsey and Elovich isotherm models. However it was not possible to regenerate PANI due to strong binding of Cr3+ with PANI. PANI showed highest selectivity towards chromium in chrome plating effluent . Acknowledgements

carry out this work. Ronak Bhatt is thankful to GNFC for giving support.

[1]

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The Isotherm and Kinetic Models used with their formulae are given in Tables S1 and S2 respectively. FTIR Frequencies with their assignments are given in Tables S3. The Comparison

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of adsorption capacity of PANI with literature reported adsorbents is in Table S4. Zeta potential, SEM/EDX, Effect of Dose, Linear fits and Back Calculation of Isotherm model, Effect of Time,

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Linear fit and Back Calculation of Kinetic models and Thermodynamics plots are mentioned as Fig. S1, Fig. S2, Fig. S3, Fig. S4, Fig.S5, Fig. S6, Fig. S7, Fig. S8 and Fig.S9 respectively. This

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Material is available free of charge via Journal Website.

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