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Folic Acid-Polyaniline Hybrid Hydrogel for Adsorption/Reduction of Chromium (VI) and Selective Adsorption of Anionic Dye from Water Sujoy Das, Priyadarshi Chakraborty, Radhakanta Ghosh, Susmita Paul, Sanjoy Mondal, Aditi Panja, and Arun K. Nandi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02342 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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ACS Sustainable Chemistry & Engineering

Folic Acid-Polyaniline Hybrid Hydrogel for Adsorption/Reduction of Chromium (VI) and Selective Adsorption of Anionic Dye from Water Sujoy Das, Priyadarshi Chakraborty, Radhakanta Ghosh, Susmita Paul, Sanjoy Mondal, Aditi Panja, and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India *For Correspondence: A. 913324734971

K.

Nandi,

email:

[email protected],

Telephone

No.

Abstract A porous 3D folic acid(F)-polyaniline(PANI) hybrid hydrogel (F-PANI), produced by in situ polymerization of aniline, exhibit highest compressive stress(15.1 kPa), 3D hierarchical network morphology with BET surface area 236 m2/g. Here PANI is present in emeraldine salt(ES) state which facilitates excellent adsorption of anionic pollutants. It exhibits an extremely high adsorption capacity for Cr(VI) and during adsorption Cr(VI) is reduced to Cr(III).The electrical impedance spectra of the Cr(VI) adsorbed xerogel, support the conversion of PANI chains from ES to pernigraniline base(PB) making the xerogel more resistive. It also selectively adsorbs anionic dyes, the adsorption capacity increases with decrease of pH. Both the adsorption data are found to be well explained through pseudosecond-order kinetic model and they obey Langmuir adsorption isotherm. F-PANI2 showed high adsorption capacities selectively towards anionic pollutants e.g. Cr(VI), eosine yellow, rose bengal, methyl orange and low adsorption capacities for Hg(II), Pb(II), rhodamineB, bismark brownY methylene blue and neutral red. The removal of Cr(VI) and anionic dyes are very much effective at neutral and acidic pH. After dye/Cr(VI) adsorption the Nyquist plot indicate significant decrease in the capacitance of xerogels. Cyclic experiments show that, FPANI xerogels can be effectively reused to remove Cr(VI) from different contaminated water. 1

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KEYWORDS: F-PANI xerogel, Cr(VI), anionic dye, impedance spectra. INTRODUCTION In this century, worldwide water pollution is a serious challenge to human civilization. The necessity of wastewater treatment has been increasing over the past decades, especially in developing countries, where many manufacturing industries are located.1 Because of the increasing concern on global environmental pollution problems, more and more researchers have dedicated their efforts to find suitable approaches to obtain good quality drinking water free from heavy metal ions, organic dye, and pathogenic bacteria, etc.2-10 Chromium ions and organic dyes are omnipresent in industrial processes, such as, electroplating, textile production, leather tanning, metal finishing, dyeing and so forth.11,12 Consequently, huge amount of aqueous chromium (VI) and organic dye wastes are discharged, posing profound threat to human health and environment.13-15 Cr(VI) is carcinogenic and is also harmful to lungs, liver and kidney.16 Eosin Yellow (EY) is a halogen containing red crystalline anionic dye and direct contact of this dye with the eye can cause seriously damage the cornea by destroying retinal ganglion cell.17 So, it is very important to prepare cost-effective environment-friendly materials to efficiently remove the chromium ions and organic dyes from waste water.4, 18-20 Soft materials, particularly gels, have received great attention in this direction because of their interesting material properties.2,21,22 Recently, soft gels have shown excellent competence for purifying polluted water by adsorbing heavy metal ions and organic dyes from polluted water without disintegrating.23-28 Folic acid (vitamin B9, F) is an important biomolecule containing carbonyl, carboxyl, primary amine, secondary amine and imine functional groups. Thus, F-based gels can effectively extract the heavy metal ions and dyes from the aqueous solution.29,30 Polyaniline (PANI)-based hydrogels are interesting materials 2


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because of their environmental friendliness, low price, good processability, and water insolubility.31 They serve as promising materials for supercapacitor and optoelectronic applications.32-34 In a previous work, we displayed that the folic acid-PANI xerogels have energy storage properties, and in situ grown silver nanoparticles elegantly improve the energy storage, and photoresponse capability of the gels.34 Also, they have incredible potential for purifying polluted water, because of the presence of PANI having amine, imine and emeraldine salt (ES) state which can serve as the chelating and adsorbing sites.35,36 In literature there are some reports of Cr(VI) removal from waste-water. Recently, Sarkar et al. prepared a redox-responsive copper(I) metallogel, which can effectively remove Cr(VI) from aqueous solution.37 Yu et al used chitosan/reduced graphene oxide/ montmorillonite composite hydrogels for effective Cr(VI) sorption.18 Samani et al removed chromium from aqueous solution using PANI-poly(ethylene glycol) composite.38 Wang, et al. fabricated polyaniline / α-zirconium phosphate composite to remove organic dyes from large volume of aqueous solutions.35 Reddy et al used a transparent hydrogel from an auxin–amino acid conjugate super hydrogelator for removal of entrapped dye.27 Okesola et al have used a small molecular pH-tolerant hydrogel for pH-dependent selective adsorption of dyes from aqueous solution.24 The recent reports therefore urge to develop an efficient gel based adsorbent being able to adsorb Cr(VI) and can also remove specific dyes from aqueous solution. Thus, it would be very attractive to fabricate a F-PANI-based hydrogel exhibiting both Cr(VI) and selective removal of dyes from wastewater. Herein, a novel 3D porous F-PANI hydrogel, in which F acts as a cross-linker to PANI chains is prepared by in-situ polymerization of aniline (Scheme S1). F-PANI hydrogels exhibit high specific surface area with 3D interconnected pores and PANI present here are in doped emeraldine salt (ES) state (hence, positively charged). It is found that the F-PANI xerogel can 3



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absorb selectively Cr(VI) and anionic dyes (eosine yellow, rose bengal, methyl orange) selectively from water while showing very little adsorption capacities towards cationic and neutral pollutants e.g. (Hg(II), Pb(II) rhodamine B, bismark brown Y methylene blue and neutral red) because of the electrostatic attraction between dichromate anions or anionic dyes with positively charged PANI. The pseudo-second order kinetics and Langmuir isotherm models are applied to understand adsorption mechanism of Cr(VI) and anionic dyes. EXPERIMENTAL SECTION Materials: Folic Acid (F) (SRL, Mumbai, India) was used as-received. Aniline monomer, sodium bicarbonate (NaHCO3), potassium dichromate (K2Cr2O7), mercury(II) chloride HgCl2, lead nitrate (Pb(NO3)2) and ammonium persulphate (APS) were purchased from Merck Chemicals, Mumbai and all the dyes purchased from Aldrich Chemical. Co., USA, were used as-received. Aniline monomer was distilled under reduced pressure prior to use. Preparation of F-PANI gels: F-PANI gels were synthesized using our previously reported method.34 F (20 mg) and NaHCO3 (7.6 mg) were dissolved in 1.5 mL and 1.0 mL water by mild heating to make two different F solutions. To these solutions, 1.5 and 2.0 mL of stock aniline solutions (0.8 mL of aniline (A) in 20 mL of 0.4 N H2SO4) were added; respectively to make a total volume of the mixture 3mL. Folic acid-aniline (FA) assembly (deep yellow) was found to be produced immediately in each case. Required quantity of APS was added to the FA assembly and was kept at 5o C for twenty four hours to complete the polymerization of aniline monomer. During polymerization PANI was produced and they turned into green colored, self-sustaining gels (F-PANI1 and F-PANI2 produced from 1.5 and 2 mL aniline, respectively). To remove excess ions and oligomeric impurities from these gels, the gels were dipped into distilled water for 7 days with alternate change of water after every 24 hr. Then, the gels were frozen in liquid nitrogen followed by drying under vacuum in a Freeze drier 4


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(Eyela, FDU-1200) to produce the F-PANI xerogels. The produced xerogels are termed as FPANI1 (WPANI= 0.72) and F-PANI2 (WPANI= 0.79), respectively where the weight fractions (WPANI) are measured from gravimetric technique during its preparation. Characterization: The morphology of the all xerogels was investigated by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). SEM and TEM images of the gels were observed through a FESEM instrument (JEOL, JSM 6700F) and TEM instrument (JEOL, model 2010EX). The specific surface area of xerogel was calculated from the N2 adsorption and desorption isotherms measured using Autosorb 1C instrument (Quantachrome, USA) at 77 K. UV-vis spectra of the hydrogels were recorded with a UV-Vis spectrophotometer (Hewlett-Packard, model 8453). Wide-angle X-ray scattering (WAXS) studies of all xerogels were performed with a Bruker AXS diffractomer (model D8 Advance) using a Lynx Eye detector. Xerogels were placed over glass slides and were scanned in the range of 2θ = 10-40° at a scan rate of 0.4 s/step with a step width of 0.02°. The mechanical properties of hydrogels were tested using Universal Testing Machine (Zwick Roell, Z005), fitted with a 10 N load cell in compression mode where the hydrogel samples (column shaped with a diameter of 15 mm and height of 15 mm) were placed between the self-levelling plates. Impedance spectra (IS) of the F-PANI2 xerogels, after Cr(VI) adsorption/reduction and dye adsorption were measured using a Solarton SI 1260 impedance analyzer (Solarton, London, U.K.) at 25 0C over the frequency range from 10 Hz to 1 MHz with ac perturbation of 10 mV at 0 V dc level. In the Nyquist plot, the imaginary part of impedance (-ZIm) was plotted on the +Y axis, and the real part of impedance (ZRe) was plotted on the X axis, representing a typical Cole-Cole plot in the complex impedance plane. The water uptake of the dried F-PANI2 xerogel was made immersing a certain weight of xerogel in Mili-Q water 5



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for 12 h at 25 oC After decanting the water carefully the swelled xerogel is weighed to get the water uptake and it was repeated for four times to get an average value. Cr(VI), Hg(II) and Pb(II) adsorption experiment The batch adsorption experiments of Cr(VI), Hg(II) and Pb(II) were performed in a 50 mL beaker taking 20 mg F-PANI xerogels into 20 mL of Cr(VI), Hg(II) and Pb(II) solutions in concentration 100 mg L-1 at pH 7 and the adsorption experiments were performed at 25 °C for 12 h. The amount adsorbed was measured by UV-Vis spectroscopy. We have used the Lambert Beer’s law to calculate the concentration of the adsorbate in the supernatant solution from their absorbance values at different times (Figure. S1). In another set, adsorption behavior for oxyanion Cr(VI) was determined using batch adsorption experiments where various concentrations of potassium dichromate (K2Cr2O7) solution were prepared. 20 mg of the F-PANI xerogels were added into 20 mL of Cr(VI) solution of desired concentration (10200 mg L-1) at different pH (2, 3, 5, 7 and 9) and the adsorption was performed at 25 °C for 4 h. After adsorption, the mixture was filtered and the filtrate was analyzed by UV-Vis spectrophotometer (Hewlett-Packard, model 8453) to determine the concentration of Cr(VI). The adsorption capacities were then calculated from equation 1 and 2:

in which qe is the equilibrium adsorption capacity (mg/g), Co and Ce are the initial and equilibrium concentration of the adsorbate in the solution phase (mg/L), respectively, V is the solution volume (L), and m is the adsorbent mass (g). We carried out the experiments both in pure water, and at controlled pH values adjusting pH with HCl and NaOH (both 0.1 M) 6


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solutions. The effect of pH on the Cr(VI) removal was performed with a range of pH 2-9. For adsorption kinetics studies, the initial test Cr(VI) solution at pH 2 was made at various time intervals. The xerogels after absorption were separated by centrifugation. For desorption experiments, it was transferred to 50 mL of 0.1 M HNO3 and then 0.2 M NaOH solution was added and was stirred for 60 min.16 The xerogels were then separated by centrifugation and was repeatedly washed with distilled water for recycling study. For this purpose the adsorbent’s efficiency for Cr(VI) uptake was repeated for five times in a similar fashion stated above to get an idea about its reusability. Dye adsorption For dye adsorption experiments, positively charged Rhodamine B (RB1, C20H6Br4Na2O5, MW 479, λmax : 543 nm); Bismark Brown Y (BY, C18H18N8·2HCl, MW 419, λmax: 468 nm), Methylene Blue (MB, C16H18ClN3S, MW 320, λmax: 663 nm); negatively charged Eosin Yellow (EY, C20H6Br4Na2O5, MW 692, λmax : 535 nm), Rose Bengal (RB2, C20H2Cl4I4Na2O5, MW 974, λmax : 546 nm), Methyl Orange (MO, C14H14N3NaO3S, MW 327, λmax: 465 nm) and neutral Neutral Red (NR, C15H17ClN4, MW 289, λmax : 530 nm) dyes were chosen. 20 mg of the F-PANI2 xerogel was added into 20 mL of dye solutions in desired concentration (100-230 mg L-1) at pH 7.0. The xerogels were allowed to adsorb dyes at 25°C until reaching equilibrium (30 mins). The equilibrium adsorption capacities were determined by equations (1) and (2). Selective adsorption of the F-PANI2 xerogel was investigated in mixtures of different organic dyes RB1, BY, MB, EY, RB2 and MO, using a batch adsorption experiment. Kinetic experiments were carried out with an initial dye concentration of 200 mg L-1 for all anionic dyes at 25°C to determine the minimum time required for adsorption to reach steady state. The concentrations of dyes after adsorption were measured at regular time intervals from 2 to 30 mins. For desorption studies, the dye adsorbed xerogel was separated 7



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by centrifugation, and it was then added into 50 mL of ethanol, followed by stirring for 60 min. The concentration of supernatant dye solution was measured by UV-Vis spectroscopy. Then, the adsorbent was reused for another adsorption-desorption cycle. To test the reusability of the gel, the adsorption-desorption cycle was repeated for five times following the same procedure discussed above. RESULT AND DISCUSSION The FESEM image (Figure 1a,b) of F-PANI1 and F-PANI2 exhibits hierarchical porous structures consisting of coral-like dendritic nanofibers. TEM images (Figure 1c,d) also reveal porous fibrillar network structure of the xerogels. Here folic acid acts as gelator, dopant as well as a supramolecular cross-linker of PANI chains; to produce 3D porous PANI hydrogel. Here increasing folic acid content causes the morphology more smooth and fibrous as evident from FESEM / TEM micrographs. The porosities of F-PANI1 and F-PANI2 xerogels were measured by the nitrogen adsorption-desorption measurements at 77 K, which shows type-IV adsorption isotherms with the presence of micro and meso-pores as evident from pore size distribution curve (Figure S2a,b). The hysteresis in Figure S2a indicates presence of interconnected and ink-bottle type pores in the xerogel. The xerogels also contain some macro pores due to the removal of entrapped solvents within the gel fibres.39 The BET surface area of F-PANI1 and F-PANI2 xerogels were measured to be 213 and 236 m2g-1, respectively. The pore volumes measured from BJH calculations are found to be 0.20 and 0.22 cm3 g-1 for F-PANI1 and F-PANI2, respectively; also, the pore diameter distribution curves indicate the pore diameters lie in the range of 1-15 nm (Figure S2b) indicating the formation of meso pores.

8


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(a)

200 nm

(b)

200 nm

(d)

(c)

Figure 1. FESEM images of (a) F-PANI1, (b) F-PANI2 gels and TEM images of (c) FPANI1, (d) F-PANI2 gels.

The high surface area and 3D interconnected fibers of the xerogels are capable of pollutant remover from waste water. The FTIR spectra of the xerogels and pure F are presented in Figure S3a. The sharp vibrational peaks in the region 3000-3600 cm-1for -OH and -NH2 groups of pure F powder combine to form a broad band at 3422 cm-1 in the xerogels, signifying supramolecular interaction between F with PANI. The characteristic stretching bands at 1569 cm−1 for γC=N for quinonoid rings, 1498 cm−1 for γC=C for benzonoid rings, 1295 cm−1 for γC-N the secondary aromatic amine, and 825 cm−1 for γC-H aromatic out of plane deformation vibration in all xerogel corresponds to the formation of PANI.40 The WAXS spectra (Figure S3b) of all the xerogels illustrate that two broad peaks at 2θ ~20 and ~25.2o, ascribing to the periodicity in the directions parallel and perpendicular to the PANI chains, respectively. In the xerogel the diffraction peaks of F-powder are found to be absent, 9



however, the peaks of pure PANI are retained with increasing broadness. This may be attributed to the π-π stacking between F and PANI. The formation of PANI is also confirmed from XPS study, the strong XPS signals at 286, 399 and 552 eV are assigned to C(1s), N(1s) and O(1s) respectively (Figure 2a). The N1s core-level spectrum (Figure 2b) of the xerogel are curve-fitted into three peak components with quinonoid imine (=N-) at 396.5 eV, benzonoid iimine (-NH-) at 398.7 eV and positively charged nitrogens (-N+-) at 400.4 and 401.9 eV,38,41 respectively, indicating that PANI is produced in doped emeraldine salt (ES) state.

F-PANI2

(a)

F-PANI2

(b)

C 1s

Intensity (a.u.)

N 1s

200

400

N 1s

-NH-

O 1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600

.+

-N=NH-

396

Binding energy (eV)

398

400

402

404

Binding energy (eV)

Figure 2. (a) XPS spectra of F-PANI2xerogel. (b)XPS core level spectra of the N 1s regions. Furthermore, the UV-vis spectra of both gels (Figure S3c) exhibit three absorbance peaks at 362 nm (π-π* transition), 428 nm (polaron band -π* band transition) and ~810 nm (π bandpolaron band transition) indicating the PANI produced is in the emeraldine salt state in the hydrogels. The high mechanical strength of the F-PANI gels was estimated by compressive stress-strain experiment and it shows linear elastic deformations during small compressive strain followed by inelastic hardening and densification (Figure S3d).34 Both the compressive stress and strain increase with increasing the PANI content and the compressive stress of FPANI1 and F-PANI2 hydrogels are 10.9 and 15.1 kPa, respectively. This is due to the increased amount of PANI chains causes increased coiling absorbing the higher amount of stress. So, robust F-PANI(ES) hydrogels are synthesised exhibiting high specific surface area 10


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and 3D interconnected pores capable to remove anionic pollutants eg. Cr(VI) and anionic dyes from wastewater (Scheme 1). As F-PANI2 xerogel have higher surface area (236 m2g-1) and more PANI than F-PANI-1 xerogel we have chosen F-PANI2 xerogel for the sorption experiments. The water uptake properties (swelling behaviour) of F-PANI2 xerogel were examined and the swelling ratio is 3.1±0.1. Considering the present experiment of using 20 mg F-PANI2 xerogel in 20 mL Cr(VI) solution only 0.06 gm water is adsorbed retaining 19.94 gm of water. So it is negligible amount of water loss for any practical purpose. Cr(VI) removal

F-PANI2 xerogel demonstrates the removal of oxyanionic pollutants [Cr2O72−, (148 mg/g)], however, the cations like Hg2+, Pb2+ are not at all removed ( only 6.3 mg/g for Hg2+ and 5.6 mg/g for Pb2+) (Figure 3a). This higher removal of Cr(VI) oxyanion may be explained using the electrostatic interaction between oppositely charged adsorbent (+ve) and adsorbate (-ve), however, it is not possible for the cations. As shown in Figure 3b, the F-PANI2 xerogel removes greater amount of Cr(VI) (148 mg/g) than that of F-PANI1 (97 mg/g) and pure PANI (68 mg/g). In pure PANI, which is in its doped state absorbs some Cr(VI) due to its interaction with radical cation and imine groups of PANI, but it lacks any porosity so surface required for adsorption of Cr(VI) is much lesser compared to F-PANI gels. But in F-PANI gels both amine and imine groups of F can additionally adsorb Cr(VI) together with the advent of porosity of the network structure of the xerogel. Cr(VI) removal efficiency of FPANI2 is greater than that of F-PANI1.

11



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Scheme 1 Separation mechanism of anionic pollutant from mixture.

The possible reason is high content of doped ES radical cations state of PANI, implying it can capture more oxyanion dichromate anion by electrostatic attraction. Also the surface area of F-PANI2 xerogel is higher than that of F-PANI1 giving more surface for adsorption. FPANI2 xerogel removes Cr(VI) for a wide pH range; lower pH facilitates the Cr(VI) removal to a higher extent with a higher rate (Figure 4a). For example, the F-PANI2 xerogel (1g /L) completely removes Cr(VI) (168.3 mg/L) from solutions at pH 2.0, but both the rate and extent of removal have gradually decreased with increasing pH of the medium. The bar diagram (Figure 4b) shows the gradual decrease of Cr(VI) removal efficiency with increasing pH of the medium. Figure 4b shows that, the F-PANI2 xerogels can adsorb Cr(VI) not only from acidic medium but also from neutral and basic medium. In basic media, Cr(VI) adsorbing capacity is found to be lowest, whereas in acidic media, it’s much better. Thus, Cr(VI) can be purified from the solution with a wide range of pH from 2.0 to 9.0.

12


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150

(a)

150

(b)

2-

Cr2O7 2+

Hg 2+ Pb blank

100

qt(mg/g)

100

qt(mg/g)

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50

50 Pure-PANI F-PANI1 F-PANI2

0

0 0

50

100

150

200

250

0

50

t(min)

100

150

200

250

t(min)

Figure 3. (a) Variation of adsorption capacity of heavy metal ions (Cr2O7-2, Hg2+,Pb2+) at pH 7 and 25 oC. (b) Adsorption capacity of Cr(VI) on pure PANI, F-PANI1 and FPANI2 xerogels at pH 7 and 25 oC.

The high adsorbance capacity of the xerogel is associated with the high BET surface area and the functional groups present on the surface of F-PANI2 xerogel. Folic acid has carbonyl, carboxyl acid, primary amine, and secondary amine groups, also PANI has large number of imine group and doping ES radical cations in its chains. At low pH the primary amines, secondary amine and imine groups are protonated to produce positively charged cations (for example –NH3+,–NH2+–, =NH+– groups) and electrostatic attraction occurs with Cr2O72− resulting an increase of removal efficiency. At basic pH all primary amine, secondary amine, and imines are not protonated, resulting in the lower adsorption through interaction with amino and imino groups of F-PANI2 xerogel.42 Moreover, F-PANI2 xerogel exhibits excellent Cr(VI) removal from the acidic, and neutral solution, illustrating that the F-PANI2 xerogel might have a wide practical application for the removal of Cr(VI) contamination from waste water.

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180

(a)

(b)

150 120

qt(mg/g)

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90 pH2 pH3 pH5 solution pH pH9

60 30 0 0

50

100

150

200

250

pH 2

pH 3

pH 5

pH 7

pH 9

t(min) Figure 4. (a) Cr (VI) removal by F-PANI2 xerogels (adsorbent dosage = 3.5 mg, V = 20 mL, T = 25°C, different pH. (b) Maximum Cr(VI) adsorption by F-PANI2 at different pH.

Adsorption kinetics

The adsorption kinetics of Cr(VI) onto F-PANI2 xerogel were conducted with the initial concentration of 20 mg/L K2Cr2O7, at pH 2, 5, 7 and 9. Primarily pseudo-first-order and pseudo-second order kinetic models are used to understand the adsorption behavior as shown in equation (3) and (4), respectively.43,44

Where qt is the adsorption capacity at time t (mg g-1) and qe is the adsorption capacity at equilibrium (saturation level), respectively. k1 (min−1) and k2 (g mg−1 min−1) are the pseudofirst-order rate constant and the pseudo-second order rate constant, respectively. Figure S4a,b illustrate the plots of log(qe-qt) vs t for pseudo first order and plots of t/qt vs t for pseudo second order for the absorption data at different pH. It is evident from the plots that the data are better fitted in Figure S4b than Figure S4a as evident from correlation coefficients (R2) values (Table-1). From the intercept and slope of the respective plots k2 and k1 are determined 14


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and they are summarized in Table-1. The pseudo second-order kinetic model is more suitable than that of pseudo first-order kinetics. Moreover, the qe value for pseudo-second-order data are well matching with experimental data (Figure 4b). It is evident from the Table-1 that qe values are much higher at acidic medium for both pseudo first and second order models, indicating the adsorption of Cr(VI) is facilitated by electrostatic interaction

Table 1. Kinetic Parameters for the Adsorption of Cr(VI) on F-PANI2 xerogel at different pH values

Adsorbate

Pseudo-first-order model

Pseudo-second-order model

F-PANI2

k1 (min-1)

qe (mg g-1)

R2

k2(g mg−1 min−1)

qe (mg g-1)

R2

pH 2

9.02×10-3

134.6±1.18

0.97656

1.02×10-3

171.5±1.9

0.99768

pH 5

9.68×10-3

95.9±1.28

0.94968

0.64 ×10-3

162.3±2.3

0.99446

pH 7

9.53×10-3

84.2±1.25

0.96142

0.60 ×10-3

152.6±2.7

0.99275

pH 9

8.23×10-3

45.1±1.17

0.97432

1.3×10-3

83.3±1.8

0.99592

. Adsorption Isotherm Study

To get deeper insight into the degree of the removal of Cr(VI), two well-known (Langmuir and Freundlich) adsorption isotherm models are used: these two models are sequentially expressed as eqs 5 and 6:45,46

15



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Page 16 of 38

Where Ce is the equilibrium concentration of the adsorbate (mg/L), qe is the equilibrium adsorption capacity of the pollutant adsorbed onto the F-PANI2 (mg/g), qm (mg/g) denoting maximum adsorption capacity, b (L/mg) is a constant; Kf and n are empirical constants of the Freundlich model. The Langmuir and Freundlich isotherms are displayed in Figures S4c, and S4d for the adsorption study of Cr(VI) at pH2. The associated equilibrium parameters are determined from the consequent linear fitting of both the plots and the results are presented in Table-2.

Table 2. Langmuir and Freundlich Isotherm Fitting Parameters for the Adsorption of Cr(VI) onto F-PANI2 xerogel

Langmuir isotherm model adsorbent

R2

F-PANI2

0.99513

qm (mg g-1) 171.2±3.2

Freundlich isotherm model b (L mg-1) 0.21

R2

Kf(mg g-1)

n

0.95695

68.2±6.4

4.8

The higher correlation coefficient (R2=0.99513) of the Langmuir model indicates that the adsorption data of Cr(VI) onto F-PANI2 xerogel fit better with the Langmuir isotherm model, manifesting that the adsorption process is mainly a monolayer adsorption. The low R2 value in Freundlich plot suggests that Cr(VI) adsorption onto F-PANI2 xerogel does not follow the Freundlich isotherm model. From Langmuir isotherm model the obtained qm value is 171.2 mg/g, the amount of Cr(VI) removal by F-PANI2 xerogels is greater than those of the recently reported adsorbents.(Table-3). The high adsorption capacity of F-PANI2 xerogel compared to PANI based materials is attributed to the high BET surface area and the electrostatic attraction between the adsorbent- adsorbate systems.

16


Page 17 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Comparison of Adsorption Capacities of F-PANI2 with Reported Values on Various Adsorbents for Cr(VI)

Adsorbent

qm (mg/g)

reference

Polyaniline doped with sulphuric acid

95

47

Polyaniline/Poly Ethylene Glycol

109

16

62.9

48

157

49

58

50

171.2

this work

Halloysite@Polyaniline Hybrid Nanotubes Polyaniline coated protonic titanate nanobelt (PANI/H-TNBs) Polyaniline-(Electrospun Polystyrene) Fibers F-PANI2 Cr(VI) removal mechanism

Cr(VI) adsorption on F-PANI2 xerogel exhibits an initial fast adsorption followed by a slower removal rate that gradually reached levelling indicating attainment of equilibrium. In acidic or neutral pH, the Cr(VI) ions start to diffuse into the porous F-PANI2 xerogels due to electrostatic attraction between it’s oxyanion and ES radical cations of F-PANI xerogel, when the reduction Cr(VI) to Cr(III) takes place. Simultaneously, the PANI (ES) state becomes oxidized to PANI pernigraniline base (PB) state.48 But, in basic solution all imine/amine groups are deprotonated and dedoped, so electrostatic attraction mechanism does not follow. Moreover, in alkali medium Cr(VI) is present as CrO42−,that has lower redox potential and may be reduced to Cr(III) easily.48 Here, folic acid does not leach out of the gels and remains present within the fibres. Therefore it reacts with PANI (PB) to produce back the PANI (ES), establishing equilibrium between the ES and PB form of PANI and it shifts towards ES at acidic medium and towards PB at basic medium. The XPS spectra of F-PANI2 xerogel after Cr2O72- adsorption/reduction are presented in Figure S5a and showing two energy bands at about 577 and 586 eV, which belongs to the binding energies of Cr 2p3/2 and Cr 2p1/2.The Cr 2p3/2 spectrum can be decovoluted into two peaks at about 577.35 and 578.7, which are 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

referred to Cr(III) and Cr(VI), respectively. Similarly the Cr 2p1/2 spectrum can be deconvoluted into 586.3 and 587.4 eVs, which are referred to Cr(III) and Cr(VI), respectively.51 From XPS spectrum, it indicates that the maximum amount (>60 at%) of Cr(VI) is reduced to Cr(III) after adsorption. The percentage of Cr(III) and Cr(VI) is 63.4 and 36.5 at %, respectively, after adsorption. The existence of both Cr(VI) and Cr(III) are attributed to the adsorption and reduction of Cr(VI) by PANI (ES) of F-PANI2 xerogel. After reduction of Cr(VI) by F-PANI (ES) state, it’s transformed into the neutral F-PANI (PB) state. This is evident from FTIR spectra of chromium adsorbed F-PANI xerogel (Figure S5b) where the quinonoid vibration peak of PANI (ES) chain in F-PANI at 1566 cm-1 has shifted to 1581 cm-1 showing a hump at 1635 cm-1 which is a characteristic vibration peak of PANI (PB).52 The shifting of the quinonoid adsorption peak to higher energy and the presence of PB characteristic peak as hump suggests the presence of an equilibrium mixture of PANI (PB) and PANI (ES) in Cr(VI) adsorbed xerogel. Hence the adsorption of Cr(VI) occurs mainly through electrostatic interaction of protonated species of F and PANI while at basic medium it is mainly Cr(III) that is adsorbed though the interaction mainly with the imine groups. It is really interesting that though the nature of both adsorbate and adsorbent changes during adsorption, still it obeys the Langmuir chemisorption model, indicating the adsorption process is similar type for both the Cr(VI) and Cr(III) ions. Reusability test of F-PANI2 xerogel with Cr(VI) solution at pH2 is presented in Figure S5c and it shows good results up to five cycles. Even after five cycles, the Cr(VI) removal efficiency of the F-PANI2 xerogel remains ~73% of the initial adsorption. In 2011, Bayramoglu and Arica synthesized poly(4vinyl pyridine-co-hydroxyethyl methacrylate) particles showing Cr(VI) good removal efficiency and good reusability (adsorption-desorption) upto 6 cycles.53 Wen et al. prepared PANI/H-TNBs showing moderate reusability (70% after 10 cycles).49 So, our synthesized FPANI2 xerogel shows good reusability for the removal of Cr(VI). 18


Page 18 of 38

Page 19 of 38

Impedance spectra

The electrical resistivity of PANI chains is related with their oxidation states. Here PANI is present in doped ES state and the dc-conductivity of F-PANI2 xerogel is 0.04 S/cm. Doped emeraldine salt (ES) state of PANI has low resistance value, whereas neutral pernigraniline base (PB) state of PANI has high resistance value. Adsorption of Cr(VI) by the F-PANI2 xerogel is supposed to change its electrical properties. To investigate this change, we have made impedance study of the F-PANI xerogel before and after the Cr(VI) removal experiment. The Nyquist plots of F-PANI2 and Cr(VI) adsorbed F-PANI2 [Cr(VI)@FPANI2] xerogel are presented in Figure. (5a and 5b). Both the samples exhibit semicircle indicating the existence of both resistive (R) and capacitive (C) features in the xerogel materials connected in a parallel mode. An equivalent circuit (inset, Figure 5b) is used to simulate the observed response. The equivalent circuit consists of a charge transfer resistance (Rct) and constant phase element (CPE) whose values are presented in Table-4. From the CPE values, the capacitance (C) values are calculated using the equation.

60

F-PANI2

(a)

4

6.0x10

45

Cr(VI)@F-PANI2

(b)

4

4.0x10

-ZIm(Ω )

-Zlm(Ω )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

4

2.0x10

15

0.0

0 0

30

60

90

0.0

120

ZRe(Ω )

4

3.0x10

4

6.0x10

4

9.0x10

5

1.2x10

ZRe(Ω)

Figure 5. Nyquist plots of before adsorption (a) F-PANI2 xerogel, and (b) after Cr(VI) adsorption at pH2 Cr(VI)@F-PANI2 at 25 oC. (Inset : Equivalent circuit for fitting impedance plot). 19



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Page 20 of 38

Table 4. Comparison of impedance parameters of F-PANI2 and Cr(VI)@F-PANI2

Samples

Rct (Ω)

CPE

Capacitance(F)

n

F-PANI2

135.4

33.81×10-6

12.5×10-6

0.844 6

Cr(VI)@F-PANI2

128.4×103

7.198×10-9

4.1×10-9

0.939 7

Where Q is the pre-factor of the CPE, and n is its exponent where CPE is related through the equation CPE-1 = Q.(iω)n; i being an imaginary factor, ω is the frequency. It is quite obvious that Cr(VI) adsorption by the xerogel (Cr(VI)@F-PANI2), drastically increases the Rct and decreases the CPE value. This is due to the high standard electrode potential value of Cr(VI) (1.37 V), which oxidizes low resistance PANI(ES) state to high resistance PANI(EB) state and itself reduces to Cr(III) concurrently. The high decrease of capacitance (3 orders) after adsorption is interesting and would be discussed later. Normal water and Industrial waste water: Cr(VI) removal

Further for the practical application, we have examined the removal efficiency of oxyanion Cr(VI) from ground water, pond water (Institute), river water (The Ganges, Kolkata) and industrial waste water (Tannery, Bantala, Kolkata) of the synthesized F-PANI2 xerogel. Here, we prepared the solutions by dissolving the Cr(VI) (1.5 mg K2Cr2O7) in 10 mL ground water, pond water, river water and industrial wastewater, separately and have studied the adsorption using UV-vis spectroscopy. F-PANI2 xerogel (1g/1L) removes the Cr(VI) >85% from all the water after four hour of adsorption. Table-5 shows that the as-synthesized adsorbent is able to remove 99% Cr(VI) from the Milli-Q water, while there is a slight decrease ∼95%,∼91%, ∼87% for river, pond and industrial waste water, respectively. No definite reason for the slight decrease in removal efficiency in waste waters are known and it may be due to the presence of different other ionic and impurity ingredients being get adsorbed in the F-PANI xerogel capturing the pores. The results suggest that the F-PANI2 20


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xerogel have exceptional potential as a suitable material for the purification Cr(VI) from contaminated water. Table 5. Cr(VI) removal efficiency from different solution using F-PANI2 xerogel

Solution

Maximum capacity (mgg-1)

Cr(VI) removal %

Milli-Q-water

148

98.6

Ground water

142.8

95.2

River water

142.1

94.7

Pond water

136.3

90.8

Industrial (tannery) water

130.2

86.8

Adsorption of hydrophilic dyes

F-PANI2 xerogel possess a three-dimensional porous network structure, which may make them suitable candidates for adsorption of dyes from waste water. To study the organic dye adsorption ability of cationic, anionic and neutral dyes are chosen and selective adsorption of any one type of dye from the mixture of two categories of dyes has also been investigated. We have made experiments with seven dyes: positively charged RB1, BY, MB; negatively charged EY, RB2, MO and neutral NR dyes (Figure S6). At first, the experiments are carried out on aqueous solution of all dye at pH 7. The preliminary results reveal that only anionic dyes are adsorbed by the F-PANI2 xerogel, whereas cationic and neutral dyes do not get adsorbed (Figure 6a). After addition of xerogel for 30 min, the coloured solutions of anionic dyes (EY, RB2 and MO) become colourless, while the colour of cationic dyes (RB1, BY and MB), and neutral dye (NR) solutions remains almost unchanged.

21


(a)

EY RB2 MO MB RB1 BY NR blank

210

qt(mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Maximum dye adsorption (mg/g)


140

70

0 0

10

20

30

40

t(min)

250

Page 22 of 38

pH 3 pH 7 pH 9

(b)

200 150 100 50 0

EY

RB2

MO

Figure 6. (a) Different dye removal by F-PANI2 xerogels at pH 7 and 25 oC. (b) Maximum dye adsorption of EY, RB2 and MO by F-PANI2 at different pH. The concentrated anionic dye solutions are mixed with xerogels, and after absorption for 30 min the aqueous supernatant was monitored by UV-vis spectroscopy, to determine the maximum dye adsorption value. We performed this experiment at pH 3, pH 7, and pH 9 at 25 °C, for a contact time of 30 min. At pH 3 the maximum possible dye adsorption values of the F-PANI2 xerogel for EY, RB2 and MO dyes are as high as 239, 206 and 173 mg/g (Figure 6b), whereas at pH 7 the maximum dye adsorption values are 210, 187 and 148 mg/g. At pH 9, the adsorption capacities for dye decline very abruptly (Figure 6b). At pH 3, the electrostatic attraction between the positively charged surface of F-PANI and anionic dye molecules is very high, leading to a maximum increase of dye removal by F-PANI2 xerogel. At pH 7 PANI in F-PANI xerogel still remains at ES state due to doping with folic acid causing an appreciable adsorption of the anionic dye, however, at pH 9 PANI remains completely in EB form causing a negligible adsorption of the anionic dye. Adsorption kinetics

The adsorption kinetics is monitored using the UV-Vis spectra of EY dye at various time intervals at pH 3 (Figure 7a). Here also two typical kinetic models, pseudo-first-order (Figure 22


Page 23 of 38

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S7) and pseudo-second-order (Figure 7b) are used to fit the experimental adsorption data. Furthermore, the UV-Vis spectra of RB2 and MO are monitored at various time intervals (Figure S8a, S9a). The corresponding pseudo-first-order and pseudo-second-order kinetic plots are shown in Figure (S8b, S8c) and Figure (S9b, S9c). The corresponding kinetic parameters of EY, RB2 and MO dyes are summarized in Table-6. The correlation coefficient (R2) of pseudo-second-order model (R2> 0.995) is more suitable to explain the adsorption kinetics of EY, RB2 and MO dyes onto F-PANI2 xerogel.

Table 6. Kinetics Parameters for the Adsorption of EY, RB2 and MO onto F-PANI2 xerogel

Pseudo-first-order model

Pseudo-second-order model

adsorbate

k1 (min-1)

qe (mg g-1)

R2

k2(g mg−1 min−1)

qe (mg g-1)

R2

EY

0.00324

275.4±5.3

0.97299

0.00385

243.3±2.1

0.99626

RB2

0.00328

264.4±3.4

0.94591

0.00165

217.5±1.9

0.99654

MO

0.00263

199.5±3.3

0.94784

0.00141

184.3±2.7

0.99848

Adsorption Isotherm

Here, the adsorption data of EY dye at pH3 are fitted in both Langmuir and Freundlich isotherms and the plots are shown in Figure 7c and 7d, respectively. The corresponding parameters qm, b, Kf and n are determined from these two isotherms, and the results are displayed in Table-7. The R2 value of the Langmuir model (0.995) is much better than that of the Freundlich model, suggesting that the adsorption of dyes onto F-PANI2 xerogel follows the Langmuir model. The adsorption capacity calculated from the Langmuir isotherm is 247.5 mg g−1, which is close to the experimental result (239 mg g−1). F-PANI2 xerogel shows faster and greater adsorption capacity of EY, which is higher than that of the adsorbents or 23



hydrogels reported in previous reports.54,55 Ansari et al. have synthesized polypyrrole (PPy) and polyaniline (PANI) on the surface of wood sawdust (SD) which shows adsorption of EY dye having qm = 5.57 and 5.9 mg g-1, respectively.54 In our previous work we observed that folic acid-chitosen gel exhibit the maximum adsorption of 25 mg g-1,29 and very recently Cheng et al used imidazolium-based surfactant gel and observed selective removal of EY dye with an efficiency in mM level.55 Hence, the high adsorption capacity of F-PANI2 xerogel is attributed to high porosity and strong electrostatic interaction. After adsorption, the SEM image (Figure S10) of F-PANI2 is observed, and the image clearly reveals that the EY dye is adsorbed on the surface of the gel fibres.

1.5

(a)

t = 0 min t = 1 min t = 5 min t = 10 min t = 15 min t = 20 min t = 25 min t = 30 min

Before After

1.0

0.15

(b)

2

EY : R = 0.99626

0.10

t/qt

Absorbance (a.u.)

2.0

0.05

0.5 0.00

0.0 400

450

500

550

0

600

5

Wavelength (nm) 0.8

(c)

10

15

20

25

30

t(min)

Experimental data Langmuir plot

2.4

(d)

Experimental data Freundlich plot

0.6 2.2

log qe

Ce/qe (g/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

0.4

0.2 2.0 0.0 0

50

100

150

200

0.0

Ce (mg/L)

0.5

1.0

1.5

2.0

2.5

log Ce

Figure 7. (a) UV-vis absorbance spectra with corresponding photo image (Inset, before and after removal) of EY dye in F-PANI2 xerogel. (b) pseudo second order kinetic plots of EY dye. (c,d) Langmuir and Freundlich isotherms for EY on F-PANI2 xerogel at pH3 and 25 oC. 24


Page 25 of 38

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Table 7. Langmuir and Freundlich Isotherm Fitting Parameters for the Adsorption of EY onto F-PANI2 xerogel

Langmuir isotherm model

Freundlich isotherm model

adsorbent

R2

qm (mg g-1)

b (L mg-1)

R2

Kf(mg g-1)

n

F-PANI2

0.9948

247.5±2.5

0.113

0.93228

81.2±7.2

4.31

The desorption of EY dye adsorbed in F-PANI2 xerogel is conducted in ethanol using UV-vis spectroscopy. The absorbance intensity of the EY dye in the supernatant increases with time (Figure 8a) and it is also evident visually. The reusability of adsorbent is an important factor for practical applications. So, the regeneration of the F-PANI2 xerogel and recycling tests are made to understand its reusability. Figure 8b shows that the removal efficiency of EY dye remains at 83% of its initial after five cycles, indicating the F-PANI2 has a very good recyclability for dye adsorption. In 2012 Wang et al. synthesized polyaniline/α-zirconium phosphate displaying anionic dye removal and the recycle study indicated that MO has removal efficiency 80% after 5 cycles of reuse.35 Recently, Arica et al. have prepared magnetic silica particles grafted with poly(glycidyl methacrylate) for removal of organic pollutant and exhibiting good reusability (89% retention after 8 cycles).56 So, the synthesized F-PANI2 xerogel has compareable reusability with other systems. Thus it shows great potential for anionic organic pollutant adsorption and is easily recyclable and reusable.

25



(a)

1.5

t = 1 min t = 5 min t = 10 min t = 20 min

100

Removal efficiency (%)

2.0

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

desorption

1.0

0.5

450

500

550

600

Wavelength (nm)

Page 26 of 38

(b)

80 60 40 20 0

1

2

3

4

5

Cycles

Figure 8. (a) Time dependent UV-Vis spectra of supernatant during desorption of dye from F-PANI2 in MeOH at 25 oC. (b) Reusability test of F-PANI2 xerogels for dye removal up to five cycles at 25 oC. Here, we also made impedance spectral (IS) studies before and after dye adsorption, to inspect the change in electrical properties. The Nyquist plots of F-PANI2 and DYE@FPANI2 are shown in Figure 9 and the corresponding equivalent circuit (inset Figure 9) is presented here. It is evident from the values presented in Table-8, that the Rct value increases and CPE value decreases after dye adsorption. This may be due to the non-conducting dye adsorbed on the surface of the F-PANI2 fibers giving rise to a thin non-conducting layer, causing increase of the resistance values and also attributing to the decrease of capacitance value for each dye. From a comparison of Table-4, and Table-8, it is clear that the resistance increases abruptly in the Cr(VI) adsorbed system compared to dye adsorbed system, and the reason of former is transformation of PANI(ES) to PANI(EB), but in later case no such decrease of Rct value is observed, probably that the PANI retains its ES form. The capacitance is the charge storing ability of a system and the same order decrease of capacitance in both the systems may be due to the blocking of pores by the adsorbates decreasing the effective surface area to same extent required for storing charges at an unit applied potential. 26


Page 27 of 38

400 F-PANI2 EY@F-PANI2 RB2@F-PANI2 MO@F-PANI2

300

-Zlm(Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

100

0 0

100

200

300

400

500

ZRe(Ω ) Figure 9. Nyquist plots of before dye adsorb F-PANI2 xerogel, and after dye adsorb tion at pH3 EY@F-PANI2, RB2@F-PANI2 and MO@F-PANI2 at 25 oC. (Inset : Equivalent circuit for fitting impedance plot).

Table 8. Comparison of impedance parameters of F-PANI2 and after dye adsorption

Samples

Rct (Ω)

CPE

Capacitance (F)

n

F-PANI2

135.4

33.81×10-6

12.5×10-6

0.844 6

EY@F-PANI2

491.9

2.793×10-9

9.08×10-10

0.9231

RB2@F-PANI2

470.9

26.76×10-9

11.02×10-9

0.933 7

MO@ F-PANI2

476.5

19.42×10-9

10.3×10-9

0.9491

Selective dye adsorption:

The selective dye adsorption of the xerogel was further examined by adsorption in the aqueous solutions containing two dyes with one positively charged, and other with negatively charged. In Figure 10a the selectivity of adsorption is shown for MO and MB, and the time dependent adsorption spectra indicate that MO is completely removed after 30 min of 27



adsorption. Figure S11a and Figure S11b show similar selective removal of EY from (EY + MB) mixture, and that of RB2 from (RB2 and MB) mixture. Also, we have studied dye adsorption of the aqueous solutions containing two dyes with one anionic charged (MO) and another neutral (NR) (Figure10b). F-PANI2 xerogel can selectively remove MO from (MO + NR) mixture. This result illustrates that the F-PANI2 xerogel can quickly and selectively remove anionic dyes from mixture of dyes or from wastewater.

t = 0 min t = 10 min t = 30 min

(a)

0.8

Absorbance (a.u.)

0.6

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

0.4

0.2

(b)

t = 0 min t = 10 min t = 30 min

0.6

0.4

0.2

0.0 400

500

600

700

0.0

800

400

Wavelength (nm)

500

600

700

Wavelength (nm)

Figure 10. (a) Selective removal of MO from MO + MB dye mixture solution in presence of F-PANI2 xerogel at pH7 (Inset : corresponding photo image before and after dye removal) (b) Selective removal of MO from MO + NR dye mixture solution in presence of F-PANI2 xerogel at pH7and 25oC. (Inset : corresponding photo image before and after dye removal). Conclusion

F-PANI2 xerogel has porous 3D hierarchically fibrillar network morphology showing a BET surface area of 236 m2/g. Uv-Vis and XPS spectra reveal that PANI produced in the hydrogels is in the ES state. The robustness of the F-PANI gels is evident from compressive stress-strain experiment. Because of the existence of amine and imine groups in the F-PANI gel fibers, the xerogel shows excellent adsorption towards Cr(VI) and anionic dyes (EY, RB2 and MO) at acidic medium. During the adsorption of Cr(VI) it is reduced to Cr(III) and PANI (ES) is transformed into PANI(PB) which in-turn transforms into PANI(ES) due to 28


Page 29 of 38

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presence of folic acid. Thus an equilibrium of PANI (ES) and PANI (PB) is established where a mixture of Cr(VI) and Cr(III) coexist as evident from XPS results. The adsorption kinetics of Cr(VI) follow the pseudo-second-order model, and adsorption isotherms on FPANI2 xerogel can be well-fitted by the Langmuir adsorption isotherm. The maximum adsorption capacity (qm) for Cr(VI) is 168.3 mg/g, which is much higher than that of other small molecular hydrogel-based adsorbents. The total adsorption capacity of F-PANI2 xerogel is 16.9 % (w/ w) for chromium ion of which 63.5% is Cr(III) and 36.5 % is Cr (VI). The results of Cr(VI) removal from different water samples suggest that the F-PANI2 xerogel have exceptional potential as a suitable material for the purification Cr(VI) from contaminated water.The change of both adsorbate and adsorbent nature during adsorption, still obeying the Langmuir adsorption isotherm, is really interesting. Furthermore, the selective anionic toxic dyes removal of the xerogel indicates its potential applications for adsorption, separation, and purification purposes. The adsorption isotherms of dye adsorption process are well-fitted with the Langmuir and pseudo-second-order models. Also, we have used impedance spectroscopy to investigate the differences observed when Cr(VI) and different dyes are adsorbed into the F-PANI2 xerogel. Cr(VI) adsorption causes oxidation of F-PANI2 xerogel, hence resistivity increases significantly. After Cr(VI)/dye adsorption the Nyquist plot also indicate significant decrease in the capacitance of xerogels due to blocking of pores decreasing the surface area for adsorption. Thus hydrogel prepared by F and PANI are promising material for both Cr(VI) and anionic dye removal applications for waste water purification. Acknowledgement:

We gratefully acknowledge CSIR grant No. 02(0241)/15/EMR-II for financial support. S.D R.G. S.M. and A.P. acknowledge CSIR and DST (Inspire), New Delhi for the fellowship. 29



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Supporting Information

Schematic presentation of F-PANI xerogel, BET surface area and pore size distribution of the F-PANI2 xerogel, FTIR, UV-vis spectra and WAXS data of pure PANI, F-PANI1 and FPANI2 gels; compressive stress/strain of F-PANI1 and F-PANI2, XPS and FTIR spectra of F-PANI2 xerogel after adsorption of Cr(VI), reusability test of F-PANI2 xerogels for Cr(VI) removal up to three cycles, chemical structures and abbreviation of different dyes, pseudofrist-order kinetic plots of EY dye, SEM image of the F-PANI2 xerogel after adsorption of EY, UV-vis absorbance spectra of RB2 and MO dye in F-PANI2 and pseudo frist and second order kinetic plots of RB2 and MO dye on F-PANI2 xerogel. UV-Vis spectra of (a) EY-MB and (b) RB2-MB during separation experiment. This information is available free of charge via the internet at http://pubs.acs.org.

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TOC Graphic

Folic acid-polyaniline(F-PANI) hydrogels selectively adsorb Cr(VI) and anionic dyes at lower pH; when Cr(VI) oxidizes PANI(ES) to PANI(PB) increasing electrical resistance.

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Reduction of

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