2,5-diaminobenzene sulfonic acid composite

Accepted Manuscript Selective removal of Cr(VI) from aqueous solution by polypyrrole/2,5-diaminobenzene sulfonic acid composite Nazia H. Kera, Madhumita Bhaumik, Niladri Ballav, Kriveshini Pillay, Suprakas Sinha Ray, Arjun Maity PII: DOI: Reference:

S0021-9797(16)30296-X http://dx.doi.org/10.1016/j.jcis.2016.05.011 YJCIS 21255

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

16 February 2016 7 May 2016 10 May 2016

Please cite this article as: N.H. Kera, M. Bhaumik, N. Ballav, K. Pillay, S.S. Ray, A. Maity, Selective removal of Cr(VI) from aqueous solution by polypyrrole/2,5-diaminobenzene sulfonic acid composite, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.05.011

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Selective removal of Cr(VI) from aqueous solution by polypyrrole/2,5diaminobenzene sulfonic acid composite Nazia H Keraa,b, Madhumita Bhaumikb, Niladri Ballavb, Kriveshini Pillayb, Suprakas Sinha Raya,b*, Arjun Maitya,b* a

DST-CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria, 0001, South Africa b Department of Applied Chemistry, University of Johannesburg, Doornfontein, 2028, Johannesburg, South Africa

__________________________________________________________________________

___________________________________________________________________________ *Corresponding author: Tel.: +27 12 841 2658; fax: +27 12 841 3553 (A. Maity) and +27128412388; fax: +27 12 841 2135 (S. SinhaRay). E-mail addresses: [email protected], [email protected] (A. Maity) and [email protected] (S. SinhaRay). 1

Abstract A polypyrrole/2,5-diaminobenzenesulfonic acid (PPy/DABSA) composite, synthesised by the in situ oxidative polymerization of pyrrole in the presence of DABSA, was studied as an adsorbent for the removal of Cr(VI) from aqueous solution. The structure and morphology of the composite were investigated by ATR-FTIR, FE-SEM, EDX, TGA, XRD and XPS studies. The adsorption of Cr(VI) by PPy/DABSA composite was highly pH dependent and optimum removal was achieved at pH 2. Adsorption of Cr(VI) was confirmed by EDX and XPS studies. The isotherm data fitted the linear Langmuir model well, with a maximum adsorption capacity of 303 mg/g at 25 °C. Thermodynamic parameters (∆G°, ∆H° and ∆S°) were calculated using isotherm data and confirmed that the adsorption process was spontaneous and endothermic. Adsorption kinetics was best described by the pseudo-secondorder model. The activation energy of the adsorption process suggested that Cr(VI) was chemisorbed by PPy/DABSA composite. PPy/DABSA composite could be used for three consecutive adsorption-desorption cycles without loss of its original adsorption capacity. Highly selective removal of Cr(VI) was observed even when co-existing ions such as Cu2+, Zn2+, Ni2+, Cl-, SO42- and NO3- were present in the solution. In summary, the potential for remediating industrial wastewater contaminated Cr(VI) using PPy/DABSA composite has been demonstrated.

Keywords: Polypyrrole; 2,5-diaminobenzenesulfonic Hexavalent chromium; Equilibrium; Isotherms; Kinetics 2

acid;

Composite;

Adsorption;

1. Introduction

Extensive mining and industrial activities release heavy metal ions into the environment where they can bioaccumulate in organisms and biomagnify in the food chain. Chromium (Cr) is a major heavy metal pollutant that commonly occurs in wastewater in industries where it is used for a range of products and applications including leather tanning, chrome plating, wood preservation, manufacturing of alloys, dyes, pigments and paints [1,2]. In the aquatic environment, Cr exists mainly in the trivalent [Cr(III)] and hexavalent [Cr(VI)] forms [1,2]. Cr(III) is neither toxic nor carcinogenic and may be a trace element essential for humans and animals [1,2]. Cr(VI), on the other hand, is highly toxic and mobile in the environment [1,2]. Cr(VI) is a potent mutagen and potential carcinogen that causes a range of disorders in the body that affect the skin, lungs, liver, kidneys and stomach [1–3]. The World Health Organisation (WHO), therefore, recommends maximum allowable limits of 0.05 mg/L and 0.1 mg/L for Cr in potable and surface waters, respectively, for water to be safe for drinking and suitable for other uses [4,5]. Therefore, there is a need to develop technologies that effectively remove Cr(VI) from industrial wastewater to below the allowable levels. Conventional water treatment processes employed for Cr(VI) removal such as chemical precipitation, flotation, ion exchange, and electrochemical deposition often fail to reduce Cr(VI) concentrations to the low levels required by water quality standards and have a number of other limitations and disadvantages [6–8]. Among these processes, adsorption has been identified as an ideal technique for the removal of Cr(VI) from water because with the proper design and selection of adsorbents, the process can be made efficient, effective, economically viable, robust and environmentally friendly [7,9]. Activated carbon (AC), the most widely studied adsorbent for Cr(VI) removal from water, is expensive and lacks selectivity [9,10]. Low-cost adsorbents, obtained from natural sources or as waste from industrial and agricultural processes, have shown low adsorption capacities for Cr(VI) 3

removal from water and have other disadvantages such as being difficult to regenerate for reuse [6,9]. Since their discovery, conducting polymers have gained considerable attention for a range of applications because of their attractive electrical and electrochemical properties, good stability, non-toxicity, ease of production and relatively low cost [11,12]. Recently, interest in the use of conducting polymers as adsorbents for water treatment applications has also developed [13–16]. Polypyrrole (PPy), the most extensively studied conducting polymer, has been investigated for the removal of Cr(VI) from water because of its porous structure, tunable morphology, unique redox chemistry, insolubility in water, ion-exchange capacity and ability to effectively reduce Cr(VI) to Cr(III) [13,14,17]. The anion exchange capability of PPy is caused by dopants that are incorporated into the polymer chain to balance the positively charged nitrogen atoms that arise during the polymerization of Py [5]. However, PPy does not disperse well in water and tends to form irregular agglomerates during synthesis because of strong π-π* interactions between the chains [18,19]. To prevent agglomeration and increase surface area for Cr(VI) removal from water, researchers have synthesised PPy composites [14]. These composites have been synthesised in various ways including modification of PPy with functional groups or dopants, copolymerization with suitable compounds and incorporation of functional nanomaterials to form polymer composites [14]. Yao et al. [20] obtained a maximum capacity of 180.43 mg/g for Cr(VI) removal from water by PPy nanoclusters prepared using Fe3O4 as a template and oxidant. Various PPy-coated substrates, such as carbon black, sawdust, titanium, halloysite nanotubes and cellulose fibers, have been studied for the removal of Cr(VI) from aqueous solution [21–25]. Bhaumik et al. [5] synthesised magnetic PPy coated Fe3O4 nanoparticles with an adsorption capacity of 169.49 mg/g for Cr(VI), which could easily be separated from treated water using an external magnetic field. They later reported a maximum adsorption capacity of 227 mg/g for PPypolyaniline (PPy/PANI) nanofibers for Cr(VI) removal [10]. The incorporation of PANI

4

enhanced Cr(VI) adsorption by increasing the surface area and providing imine and amine groups that

can chelate

metal

ions

and

adsorb anionic

metal

species

[10].

Polyacrylonitrile/PPy (PAN/PPy) nanofibers, prepared via electrospinning followed by in situ polymerization of Py, showed an adsorption capacity of 61.80 mg/g at 25 °C for Cr(VI) removal from solution [26]. Karthik and Meenakshi [27] obtained a maximum adsorption capacity of 78.61 mg/g at 30 °C for the removal of Cr(VI) from water by PPy/chitosan composite. PPy/clay nanocomposites were also studied for Cr(VI) removal from aqueous solution, as reported by Setshedi et al. [28] and Yao et al. [29]. Maximum Cr(VI) adsorption capacities of 119.3 mg/g (at 25 °C) and 72.81 mg/g (at 30 °C) were obtained for PPyorganically modified montmorillonite nanocomposite and PPy/palygorskite composite respectively [28,29]. PPy/attapulgite core-shell nanocomposite with an adsorption capacity of 48.45 mg/g at 25 °C for Cr(VI) was reported by Chen et al [30]. Ballav et al. [15] obtained a maximum adsorption capacity for Cr(VI) of 217 mg/g at 25 °C by using glycine doped PPy. In this work, PPy was modified using 2,5-diaminobenzenesulfonic acid (DABSA) to improve its adsorption capacity for removal of Cr(VI) in aqueous solution. DABSA was incorporated into PPy homopolymer to introduce additional functional groups for Cr(VI) adsorption in the form of amino groups. The sulfonic groups in DABSA were also expected to contribute to the stability of the polymer particles formed [16]. The PPy/DABSA composite was easily synthesized by in situ chemical oxidative polymerization and characterised using various techniques including attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), field emission scanning electron microscopy (FE-SEM), high-resolution transmission

electron

spectroscopy

(HR-TEM),

X-ray

diffraction

(XRD),

X-ray

photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX) and Brunauer-Emmett-Teller (BET) surface area analysis. Batch experiments were conducted to investigate the effect of various parameters such as initial solution pH, adsorbent dose,

5

temperature, contact time and co-existing ions on the removal of Cr(VI) by the PPy/DABSA composite.

2. Experimental procedures 2.1. Materials

Pyrrole monomer (Py), obtained from Sigma-Aldrich, South Africa, was purified by vacuum distillation before use. Reagent grade anhydrous iron (III) chloride (FeCl3), 2,5diaminobenzenesulfonic acid (DABSA), hydrochloric acid, sodium hydroxide, potassium dichromate (K2Cr2O7) and 1,5-diphenylcarbazide were also obtained from Sigma-Aldrich, South Africa, and used as received. Cr(VI) stock solution (1000 mg/L) was prepared by dissolving an appropriate amount of potassium dichromate (K2Cr2O7) in deionised water.

2.2. Synthesis of PPy/DABSA adsorbent

PPy/DABSA composite was synthesized by the in situ chemical oxidative polymerization of Py and DABSA by using FeCl3 as an oxidant. In this method, 0.3 g of DABSA was dissolved in 120 mL of deionised water. Then 0.6 mL of Py was added using a syringe, and the mixture was stirred at room temperature. Thereafter, 6.6 g of FeCl3 was added and the mixture was stirred for 15 minutes and then left overnight without stirring for polymerization to occur. The mixture was subsequently filtered to obtain the polymer product which was then washed thoroughly with deionised water and acetone. The PPy/DABSA composite was dried at 60 °C until a constant mass of 1 g was obtained. PPy homopolymer was also synthesized for comparison using a similar method described in a previous study [18].

2.3. Characterization of PPy/DABSA composite

6

The ATR-FTIR spectra of PPy homopolymer and PPy/DABSA composite were obtained using a PerkinElmer Spectrum 100 spectrometer (PerkinElmer, USA) in the range of 650 – 4000 cm-1 with the spectral resolution and number of scans per spectrum set at 4 cm1

and 16 respectively. FE-SEM was used to investigate the surface morphology of

PPy/DABSA composite and PPy homopolymer. FE-SEM images and EDX spectra were obtained on a JEOL-JSM 7500F microscope. TGA experiments of PPy/DABSA composite and PPy homopolymer were carried out in air at a flow rate of 50 mL/min with a heating rate of 10 °C/min on a TGA Q500 (TA Instruments, USA). BET surface area measurements were obtained from a Micromeritics ASAP 2020 gas adsorption apparatus (Micromeritics, USA) by using the low-temperature N2 adsorption−desorption technique. XRD studies were carried out on a PANalytical X’Pert PRO-diffractometer (PANalytical, The Netherlands) using Cu Kα radiation with a wavelength of 1.5406 Å with variable slits at 45 kV/40 mA for 2θ values ranging from 5° to 90°. To determine the elemental composition of PPy/DABSA composite and the oxidation state of adsorbed Cr on the adsorbent, XPS studies were carried out on a Kratos Axis Ultra device with an Al monochromatic X-ray source (1486.6 eV). The point of zero charge (PZC) for PPy/DABSA composite was determined using a Zeta-Sizer, Malvern Ltd., UK.

2.4. Batch adsorption studies

2.4.1. Adsorption equilibrium and kinetic experiments

The Cr(VI) solutions used for adsorption studies were prepared by diluting the stock solution (1000 mg/L). Adsorption equilibrium studies were carried out in a thermostatic shaker set at 200 rpm for 24 h by contacting 0.05 g of PPy/DABSA composite with 50 mL of Cr(VI) solution in plastic bottles. The effect of pH on Cr(VI) adsorption was studied by varying the initial pH of the 100 mg/L Cr(VI) solution from 2 to 11. The pH of Cr(VI) 7

solution was adjusted by using either HCl or NaOH solutions of concentrations 1-0.1 M. The Cr(VI) concentration was measured using a UV–visible spectrophotometer (Perkin Elmer Lambda 35, USA) by applying the 1,5-diphenylcarbazide method [31,32]. The Cr(VI) percentage removal or adsorption efficiency was calculated using equation (1):

(1) where

and

are the initial and equilibrium pollutant concentrations in mg/L,

respectively. The effect of PPy/DABSA composite dose on Cr(VI) adsorption was studied by varying the mass of the adsorbent from 0.01 to 0.1 g. The procedure was similar to that used for pH effect experiments. The concentration of Cr(VI) solution used was 100 mg/L, the volume was 50 mL and the pH 2. Adsorption isotherm experiments were carried out at 15 °C, 25 °C, 35 °C and 45 °C to investigate the effect of temperature on Cr(VI) adsorption. The residual Cr(VI) concentration was varied from 100 to 600 mg/L for each experiment. The adsorption capacity of the adsorbent at equilibrium was calculated using equation (2): (2) where

is the equilibrium amount of pollutant adsorbed per unit mass of adsorbent (mg/g),

(g) is the mass of the adsorbent and

(L) is the sample volume. Thermodynamic

parameters associated with the adsorption process such as the standard Gibbs free energy change (∆G°), enthalpy change (∆H°) and entropy change (∆S°) were also obtained from isotherm data. Adsorption kinetics experiments were carried out to investigate the effect of contact time and the rate of Cr(VI) adsorption at three initial Cr(VI) concentrations of 50, 75 and 100 mg/L. In a typical kinetics experiment, 1 g of PPy/DABSA composite was added to 1 L Cr(VI) solution at pH 2 in a temperature-controlled water bath and the mixture was agitated at 200 rpm using an overhead stirrer. At time zero and then at pre-selected time 8

intervals, samples were collected, filtered using a membrane filter and analysed for Cr(VI) as previously described. The capacity of the adsorbent

at time

was obtained using

equation (3): (3) where qt (mg/g) is the amount of Cr(VI) adsorbed per unit mass of adsorbent at time t and Ct (mg/L) is the concentration of the pollutant at time . To determine the activation energy for the adsorption process, kinetic experiments were also performed at 15 °C, 25 °C, 35 °C and 45 °C with a fixed Cr(VI) concentration of 100 mg/L and an initial solution pH of 2.

2.4.2. Adsorption-desorption experiments

Adsorption-desorption experiments were conducted to study the regeneration and reusability of spent PPy/DABSA composite. In a typical experiment, 0.05 g of PPy/DABSA composite was loaded with Cr(VI) by contacting with 50 mL of 100 mg/L Cr(VI) solution at pH 2. The Cr(VI)-loaded adsorbent was then treated with 50 mL of NH4OH solutions of concentrations varying from 0.05 to 2.0 M by shaking for 24 h. The solutions were filtered and analysed for the residual Cr(VI) concentrations. Regeneration of spent PPy/DABSA composite was carried out by transferring the adsorbent to 50 mL of a 2M HCl solution and shaking for 5 h. Four consecutive adsorption-desorption cycles were carried out using the same adsorbent to study the reusability of PPy/DABSA composite for Cr(VI) removal from water.

2.4.3. Effect of co-existing ions The effect of co-existing cations, Cu2+, Zn2+ and Ni2+, and anions, Cl-, NO3 - and SO42, in solution on Cr(VI) adsorption by PPy/DABSA composite was also investigated. In a typical experiment, 50 mL of 100 mg/L Cr(VI) solutions containing the ion of interest was 9

contacted with 0.05 g of PPy/DABSA composite by shaking in a thermostatic shaker at 25 °C. The concentration of the co-existing ions in the Cr(VI) solution was varied from 20 to 100 mg/L. After equilibrium was reached the solutions were filtered and the Cr(VI) concentration of the solutions determined as previously described.

3. Results and discussion

3.1. Characterization of the PPy/DABSA composite before and after Cr(VI) adsorption

ATR-FTIR analyses were carried out to study the chemical composition of PPy and PPy/DABSA composite, before and after Cr(VI) removal from aqueous solution, as shown in Fig. 1A. The FTIR spectrum for PPy (Fig. 1A(a)) contains bands at 1540, 1453, 1093 and 964 cm-1 which can be assigned to the C=C, C-N and C-H stretching vibrations, and the C-H out-of-plane deformation, respectively [33,34]. The adsorption band at 1294 cm-1 can be assigned to the C-N in-plane deformation modes [34,35]. The FTIR spectrum for PPy/DABSA composite (Fig. 1A(b)) shows that the band positions caused by C=C and C-N stretching vibrations shifted to higher wavenumber values of 1571 and 1491 cm-1, respectively after incorporation of DABSA. Because dopants affect the skeletal vibrations of the PPy ring, this finding suggests that DABSA was incorporated as a dopant into the polymer [34]. The FTIR spectrum for PPy/DABSA composite shows additional bands at 1023 and 1050 cm-1 which may be assigned to asymmetric and symmetric S=O stretching vibrations, respectively, from the sulfonic acid group on the DABSA molecules [36]. The band at 701 cm-1 may be assigned to the stretching vibration of the C-S bond [36]. These additional bands indicate the presence of sulfonic acid groups (-SO3H) in the polymer and confirm the incorporation of DABSA into the PPy homopolymer. After Cr(VI) removal by PPy/DABSA composite (Fig. 1A(c)), the bands associated with the C=C and C-N stretching

10

vibrations at 1571 and 1491 cm-1 shifted to higher values of 1585 and 1498 cm-1, respectively which may have been caused by the interaction between PPy/DABSA and Cr(VI) anions [5]. XPS analysis was carried out to compare the elemental composition of the PPy/DABSA composite before and after Cr(VI) removal. The survey spectra obtained are shown in Fig. 1B. The spectrum obtained for the PPy/DABSA composite (Fig. 1B(a)) contained bands for C1s (284.31 eV), N1s (398.98 eV), O1s (531.68 eV), Cl2p (197.52) and S2p (167.96 eV). It has been reported that the XPS spectrum for PPy showed peaks due to Cls (285 eV), Nls (400 eV), Ols (530 eV) and Cl2p (199 eV) [37–39]. The presence of S2p band in the PPy/DABSA composite confirmed the incorporation of DABSA into the PPy matrix [38,40–42]. The survey spectrum for the PPy/DABSA composite after Cr(VI) removal (Fig. 1B(b)) contained additional peaks at 577.59 eV and 585.77 eV due to Cr2p, confirming Cr(VI) adsorption by the composite [10,15,40]. The surface morphologies of PPy and PPy/DABSA composite were observed using FE-SEM and are shown in Fig. 2. Parts A and B of Fig. 2 show that both PPy and PPy/DABSA composite samples were composed agglomerations of nearly spherical particles. However, agglomerations of both large and small particles were observed in the PPy/DABSA composite sample. EDX spectra were used to compare the elemental composition of PPy and PPy/DABSA composite (before and after Cr(VI) adsorption) as shown in Fig. 3A. The spectrum for PPy (Fig. 3A(a)) shows peaks for C (0.27 keV), N (0.39 keV) and Cl (2.61 keV), which was incorporated as a dopant from the polymerizing solution. A peak for O (0.52 keV) which may have been caused by the partial oxidation of the polymer was also observed [43]. The PPy/DABSA composite spectrum (Fig. 3A(b)) contains peaks associated with C (0.27 keV), N (0.39 keV), O (0.52 keV) and S (2.31 keV). This observation confirms the incorporation of DABSA into the PPy homopolymer. It can also be observed that a smaller peak associated with Cl at 2.61 keV in the PPy/DABSA composite than that observed for

11

PPy, which suggests that DABSA replaced Cl as a dopant in the polymer. After Cr(VI) removal, the spectrum for PPy/DABSA composite (Fig. 3A(c)) contained additional peaks at 5.4 keV and 5.9 keV caused by Cr confirming the adsorption of Cr(VI) by the composite. The XRD patterns obtained for PPy and PPy/DABSA composite are shown in Fig. 3B. A broad peak at approximately 2θ = 25° was observed for PPy, which is a characteristic peak of amorphous PPy [15,44]. For PPy/DABSA, the characteristic peak shifted to a lower 2θ value of 23° which may be due to the incorporation of DABSA into the PPy homopolymer backbone [44]. TGA plots obtained for PPy and PPy/DABSA composite are shown in Fig. 3C. The initial weight loss which occurred up to 100 °C corresponds to the loss of water from the material [24]. Both PPy and PPy/DABSA composite showed broad decomposition over the temperature range between 200 and 600 °C and the nature of TGA thermograms for PPy and PPy/DABSA composite suggest that the structures of the polymers are very similar. This shows that the incorporation of DABSA into the PPy homopolymer did not affect the structure of the polymer deleteriously and the composite can be used for water treatment applications carried out in an open air atmosphere at temperatures of up to 200 °C. The specific surface area, average pore diameter and average pore volume, obtained using the BET method from the N2 adsorption-desorption curves, were found to be 21.8361 m2/g, 40.8586 nm and 0.034864 cm3/g for PPy/DABSA and 21.15 m2/g, 20.038 nm and 0.0073 cm3/g, for PPy, respectively. PPy/DABSA is therefore mesoporous in nature since the diameter of the pores are in the range 10-50 nm [19]. The specific surface area obtained for PPy/DABSA is similar in value to that obtained for PPy. This suggests that the enhanced performance of PPy/DABSA for Cr(VI) removal from aqueous solution is not due to an increase in surface area and may be due to an increase in the pore volume and the presence of amino groups due to the incorporation of DABSA into the PPy homopolymer.

12

3.2. Effect of pH on the removal of Cr(VI) by PPy/DABSA composite

Solution pH affects both the surface charge of an adsorbent and the speciation of metal ions. The effect of initial solution pH on Cr(VI) adsorption by the PPy homopolymer and PPy/DABSA composite was therefore studied and is illustrated in Fig. 4A. Maximum Cr(VI) removal (~100%) was achieved at pH 2 by PPy/DABSA composite while the PPy homopolymer only removed 32.24% of the Cr(VI). The higher removal efficiency observed for PPy/DABSA may be due to the introduction of amino groups in the composite due to the incorporation of DABSA. The Cr(VI) removal efficiency decreased as the solution pH was increased from 2 to 11 for both the PPy and PPy/DABSA adsorbents. Cr(VI) speciation in solution is known to be highly pH dependent [1,2]. Bichromate (HCrO4-) ions are the main Cr(VI) species present in solutions with pH values of 2 to 6 whereas chromate (CrO42-) species dominate at pH values higher than 6 [1,2,15]. The higher Cr(VI) removal efficiencies at lower pH values may be attributed to the surface properties of the PPy/DABSA composite. The pH at the point of zero charge (pHPZC) for PPy/DABSA composite was found to be 3 from the plot of zeta potential (mV) against pH (Fig. 4C). The adsorption of anions occurs when solution pH < pHPZC while cations are adsorbed when solution pH > pHPZC [45]. At solution pH values < 3, the surface of the PPy/DABSA is positively charged (positive zeta potential at solution pH < 3), because of the protonation of the amino groups in the composite, and therefore adsorbs the oppositely charged HCrO4 - through electrostatic interactions. At solution pH values > 3, the surface of PPy/DABSA composite is negatively charged because of the deprotonation of amino groups in the composite. PPy/DABSA composite then has a lower affinity for HCrO4- and CrO42- ions resulting in a decrease in Cr(VI) removal. At higher pH values, increased competition between Cr(VI) species and OHanions for adsorption sites is also observed. The proposed mechanism for the removal of Cr(VI) from aqueous solution at pH 2 by PPy/DABSA composite is presented in Scheme 1. 13

Because Cr(VI) removal by PPy/DABSA composite was found to be optimal at pH 2, all further adsorption experiments were conducted at this pH. The results obtained by XPS analysis, which was carried out to determine the oxidation state of the Cr adsorbed on PPy/DABSA composite, are shown in Fig. 1C. The XPS spectrum contains two peaks at 576 eV and 586 eV which may be attributed to the binding energies of the Cr2p3/2 and Cr2p1/2 orbitals, respectively [40,46]. The position of these bands are similar to those observed for Cr(III) in Cr2O3 [40,46]. This finding shows that Cr(III) is present on the surface of PPy/DABSA composite after Cr(VI) adsorption, which suggests that adsorbed Cr(VI) was reduced to Cr(III) by the electron rich polymeric species in PPy/DABSA composite [10,15,27].

3.3. Effect of adsorbent dose

The effect of adsorbent dose was studied in order to obtain the adsorption efficiency of PPy/DABSA composite for a given initial concentration of Cr(VI) to determine the minimum mass required for complete Cr(VI) removal. The effect of PPy/DABSA absorbent dose on the removal of Cr(VI) is shown in Fig. 4B. The Cr(VI) removal efficiency increased from 58% to 100% as the PPy/DABSA dose was increased from 0.01 to 0.05 g because of the availability of more active sites for Cr(VI) adsorption. Thereafter, the removal efficiency remained constant with an increase in PPy/DABSA composite dose from 0.06 to 0.1 g because the amount of Cr(VI) was the limiting factor. The optimum adsorbent dose of 0.05 g of PPy/DABSA composite for the treatment of 50 mL of a 100 mg/L Cr(VI) solution (equivalent to 1 g/L) was therefore used for subsequent adsorption studies. A comparison of the adsorbent dose used for Cr(VI) removal from aqueous solution by PPy/DABSA to other PPy based composites reported in literature is presented in Table 1. It can be seen that

14

PPy/DABSA composite is more efficient for the removal of Cr(VI) from aqueous solution than the other PPy based composites.

3.4. Adsorption isotherm

Adsorption isotherms are essential for understanding the interactions between the adsorbent and adsorbate for the design and operation of an effective adsorption system. The effect of temperature on the adsorption of Cr(VI) by PPy/DABSA composite was therefore studied at 15 °C, 25 °C, 35 °C and 45 °C and the results are shown in Fig. 5A. The figure shows that Cr(VI) adsorption by PPy/DABSA composite increased as temperature increased, which suggests that Cr(VI) adsorption by PPy/DABSA composite is an endothermic process. The isotherm data obtained was further analysed by using two common isotherm models, viz. the Langmuir and Freundlich models (non-linear), which are presented by equations (4) and (5), respectively:

(4) (5) where qm (mg/g) is the Langmuir maximum adsorption capacity, b (L/mg) is the Langmuir constant related to the energy of adsorption, KF (mg/g) is the Freundlich constant related to the adsorption capacity and 1/n is the Freundlich constant related to the intensity of adsorption. The linearized forms of the Langmuir and Freundlich models are presented in equations (6) and (7), respectively: (6) (7) The favourability of an adsorption process can be described by the dimensionless separation factor (RL), which can be defined by equation (8): 15

(8) where C0 (mg/L) is the initial Cr(VI) concentration and b (L/mg) is the Langmuir constant related to the energy of adsorption. The plots obtained for the non-linear and linearized Langmuir and Freundlich models are, respectively, shown in Figs. 5A, 5B and S1 (supporting information) and the isotherm parameters calculated are summarized in Table 2. The correlation coefficient (R2) values obtained are higher for the linearized Langmuir model than for the linearized Freundlich model, suggesting that the Langmuir model describes the isotherm data better than the Freundlich model. However, the opposite is true in the case of the non-linear fitting of the data, in which R2 values obtained for the Freundlich model are higher than those obtained for the Langmuir model. The results obtained from the non-linear fitting suggest that the data fits the Freundlich model better than the Langmuir model. It should be noted that even though the linear forms of isotherm models are often used to analyse of adsorption data, the linearization of equations results in errors in estimating adsorption parameters and fit distortion [47,48]. Additionally, the Freundlich model describes adsorption on a heterogeneous surface with sites of varying affinity for an adsorbate and may be more suitable for describing the adsorption of Cr(VI) by the PPy/DABSA composite than the Langmuir model which describes monolayer adsorption at equivalent adsorption sites on the adsorbent surface [47]. Therefore, the Freundlich model provides a better description of the adsorption isotherm data than the Langmuir model because of its underlying assumptions and the inherent limitations of using linearized forms of equations for data analysis [47]. The RL values obtained from the linear Langmuir fitting of the isotherm data fall between 0 and 1, which suggests that the adsorption of Cr(VI) by PPy/DABSA composite is favourable over the range of temperatures studied. Table 3 compares the adsorption capacity of PPy/DABSA with other adsorbents reported in literature for Cr(VI) adsorption [5,10,15,20,22,24,26–30,49–53]. The high 16

adsorption capacity of PPy/DABSA composite for Cr(VI) can be attributed to the addition of amine functional groups because of the incorporation of DABSA into PPy homopolymer [10,15,54]. PPy/DABSA is therefore a promising adsorbent for the removal of Cr(VI) from industrial wastewater because of its high adsorption capacity and relatively easy and low-cost synthesis.

3.5. Thermodynamic study Thermodynamic parameters associated with the adsorption process, ∆G°, ∆H° and ∆S°, were calculated from adsorption isotherm data by using equations (9) and (10):

(9) (10) where R (J/mol/K) is the gas constant, m is the adsorbent dose (g/L), T is temperature (K), Kc is the equilibrium constant and the ratio mqe/Ce is the adsorption affinity. The values of ∆H° and ∆S° were calculated from the slope and intercept of the plot of ln(mqe/Ce) versus 1/T, as shown in Fig. 5C. ∆G° was calculated by using equation (9). The thermodynamic parameters obtained are shown in Table 4. The positive value of ∆H° confirms the endothermic nature of the adsorption process [55,56]. The positive value of ∆S° may be attributed to increased disorder at the solid-liquid interface during adsorption [55,56]. The decreasingly negative values of ∆G° as temperature increases suggests the spontaneity of the adsorption process [55,56].

3.6. Adsorption kinetics

An adsorbent must have both a high adsorption capacity and exhibit rapid adsorption kinetics to be deemed feasible for use in industry for wastewater treatment. The effect of 17

contact time on Cr(VI) adsorption by PPy/DABSA composite for three initial Cr(VI) concentrations: 50, 75 and 100 mg/L was therefore studied and is shown in Fig. 6A. The figure shows that Cr(VI) adsorption increased with an increase in contact time and with an increase in initial concentration. Equilibrium was reached within 90 minutes for the initial Cr(VI) concentration of 50 mg/L. The time required to reach equilibrium increased to 120 and 270 minutes when the concentration was increased to 75 and 100 mg/L, respectively. To investigate the kinetics of adsorption and understand the underlying mechanism, the pseudofirst-order and pseudo-second-order kinetic models, represented by equations (11) and (12), respectively, were employed to fit the kinetic data.

(11) (12) where qe (mg/g) and qt (mg/g) are the adsorption capacities at equilibrium and at time t (min) and k1 (1/min) and k2 (g/mg/min) are the pseudo-first-order and pseudo-second-order rate constants, respectively. The linearized forms of the pseudo-first-order and pseudo-second-order kinetic models are represented by equations (13) and (14) respectively: (13) (14) Based on equation (14), the initial sorption rate, h0, (mg/g/min) can be defined as presented in equation (15): (15) The fits of the kinetic data to the non-linear pseudo-first-order and pseudo-second-order kinetic models, linearized pseudo-second-order and pseudo-first-order models are shown in Figs. 6A, 6B and S2 (supporting information), respectively. The values of the rate constants 18

and other parameters calculated from the non-linear and linear regression of the plots are presented in Table 5. The value of the R2 indicates which kinetic model fits the data best. The R2 values obtained for the pseudo-second-order model were higher than those obtained for the pseudo-first-order model. The qe values obtained from the pseudo-second-order model were also in close agreement to those obtained experimentally. These results show that Cr(VI) adsorption by PPy/DABSA composite followed pseudo-second-order kinetics. Table 5 also shows that the values of the pseudo-second-order rate constant k2 decreased with an increase in the initial Cr(VI) concentration. This finding implies that the removal of Cr(VI) by PPy/DABSA was faster for the lower Cr(VI) initial concentration of 50 mg/L than for the higher concentrations of 75 and 100 mg/L. This observation was confirmed by the initial sorption rate, h0, which decreased from 21.0526 to 10.02 mg/g/min as the Cr(VI) concentration was increased from 50 to 100 mg/L. The rate-determining step of Cr(VI) adsorption using the PPy/DABSA composite was studied by the intra-particle diffusion model of Weber and Morris [10,57] expressed in equation (16): (16) where ki (mg/g/min0.5) is the intra-particle diffusion rate constant and C (mg/g) is the intercept related to the boundary layer thickness. Fig. 6C shows plots of the intra-particle diffusion model for the three Cr(VI) initial concentrations studied. The figure shows that the curve has three different linear regions. The first region, in which rapid Cr(VI) removal occurs, may be attributed to the boundary film diffusion process [10,28]. Cr(VI) removal in the second region of the plot occurs more gradually because of the intra-particle pore diffusion effect [10,28]. The final region of the curves corresponds to the equilibrium stage [10,28]. The values of the intra-particle diffusion rate constants, ki, and intercepts, C, were calculated for each region from the slopes and intercepts of the curve and are presented in Table 6.

3.7. Determination of the adsorption activation energy 19

The results obtained for the effect of temperature and time on the rate of Cr(VI) removal by PPy/DABSA composite are shown in Fig. 7A. Experiments were carried out at 15 °C, 25 °C, 35 °C and 45 °C with an initial Cr(VI) concentration of 100 mg/L and adsorbent dose of 1 g/L. Fig. 7A shows that the rate of Cr(VI) removal by PPy/DABSA composite increased as the temperature was increased. The activation energy (Ea) for the adsorption process was obtained by using the linearized Arrhenius equation (17):

(17) where k (g/mg/min) is the adsorption rate constant at temperature T(K), A is the frequency factor (g/mg/min), Ea (kJ/mol) is the activation energy of the adsorption process and R is the gas constant (0.008314 kJ/mol/K). The rate constants, k2, in Table S1 (supporting information) were obtained for each of the four temperatures by fitting the data in Fig. 7A to the linearized form of the pseudo-second-order kinetic model as shown in Fig. S3 (supporting information). The slope of the plot of lnk2 versus 1/T, as shown in Fig. 7B, was used to calculate the Ea of the adsorption process. A high Ea value of 48.59 kJ/mol was obtained which suggests that chemically controlled steps are involved in the adsorption of Cr(VI) by PPy/DABSA composite [55,56].

3.8. Effect of co-existing ions on the removal of Cr(VI) by PPy/DABSA composite

Cr(VI) containing industrial wastewater also has a large number of other ions that can compete with Cr(VI) for adsorption sites and lower its percentage removal. The effect of selected ions including Cu2+, Zn2+, Ni2+, Cl-, NO3- and SO42- on the removal of Cr(VI) by PPy/DABSA composite was therefore studied and the results are presented in Fig. 8A. The figure shows that both anions and cations in solution at varying concentrations do not significantly affect the removal of Cr(VI) by PPy/DABSA composite. These results can be explained by the surface properties of PPy/DABSA composite. At low pH, cations in solution 20

are repelled from the positively charged surface of PPy/DABSA composite, and therefore do not affect Cr(VI) removal as expected. Anions such as Cl-, NO3 - and SO42-, on the other hand, are expected to compete with Cr(VI) ions for positively charged adsorption sites on PPy/DABSA composite because they are negatively charged (HCrO4-). The fact that this is not observed as seen from Fig. 8A could be due to weak interactions occurring between adsorption sites on PPy/DABSA composite and Cl-, NO3- and SO42- anions in solution [10,28]. The high selectivity of PPy/DABSA composite for Cr(VI) has also been attributed by Setshedi et al. [28] to the reduction of Cr(VI) to Cr(III) by the electron-rich PPy polymer which, according to Le Chatelier’s principle, shifts the equilibrium toward the forward direction, thus promoting further adsorption and reduction of Cr(VI). Because Cl-, NO3 - and SO42- are weaker oxidising agents than HCrO4- (Cr(VI)) they are not reduced by PPy/DABSA composite and therefore do not affect Cr(VI) adsorption [28].

3.9. Desorption/regeneration studies

The regeneration and reusability of spent adsorbent are important factors affecting the economic feasibility of large scale adsorption processes such as the treatment of industrial wastewater. Fig. S4 (supplementary figure) shows the results obtained for the desorption of adsorbed Cr(VI) (100 mg/g PPy/DABSA composite) from 0.05 g PPy/DABSA composite carried out using NH4OH solutions of varying concentrations (0.05 – 2M). As the concentration of the NH4OH solutions was increased from 0.05 to 2 M, the desorption efficiency increased from 2.03 to 4.23%. The percentage of Cr(VI) desorbed appeared to be very low and was attributed to the reduction of adsorbed Cr(VI) to Cr(III) species by electron-rich polymeric species in PPy/DABSA composite [10,15,27]. This phenomenon has also been observed for other PPy-based adsorbents used for Cr(VI) removal from water reported in the literature [10,15,22,27]. To regenerate the adsorbent and desorb any Cr(III)

21

species, the PPy/DABSA composite was then treated with 50 mL of 2M HCl and was subjected to a number of adsorption-desorption cycles as shown in Fig. 8B. The figure shows that the removal efficiency remained at 99% for three cycles and then decreased to 92 % after the fourth cycle. The decrease in adsorption capacity have been caused by loss of the ability of PPy/DABSA composite to reduce Cr(VI) due to desorption and regeneration treatments. It was also observed that the % of Cr(VI) desorbed increased with the number of cycles, which suggests that the ability of PPy/DABSA composite to reduce Cr(VI) to Cr(III) was lowered after each cycle. In conclusion, PPy/DABSA composite can therefore be used for three cycles without significant loss of adsorption efficiency.

4. Conclusions

The modification of PPy with DABSA was accomplished by the in situ chemical oxidative polymerization of pyrrole and 2,5-diaminobenzenesulfonic acid. The incorporation of DABSA into the PPy homopolymer was confirmed by ATR-FTIR, XPS, EDX and XRD studies. The PPy/DABSA composite obtained exhibited a high adsorption capacity of 303 mg/g (25 °C) for Cr(VI) in aqueous solution at pH 2. Cr(VI) removal by PPy/DABSA occurred due to electrostatic interactions between Cr(VI) anions in the solution at pH 2 and the protonated amino groups present in the composite (due to the incorporation of DABSA) and the subsequent reduction of Cr(VI) to Cr(III) by the electron-rich PPy polymer. The PPy/DABSA composite also demonstrated a high selectivity for Cr(VI) in aqueous solution. Thermodynamic parameters, obtained using adsorption isotherm data, confirmed that the adsorption process was endothermic and spontaneous. Kinetic data revealed that Cr(VI) adsorption by PPy/DABSA composite followed pseudo-second-order kinetics. The activation energy for the adsorption process, obtained using kinetic data, suggested chemisorption of Cr(VI) onto PPy/DABSA composite. The PPy/DABSA composite could also be used for

22

three adsorption-desorption-regeneration cycles without significant loss of adsorption capacity. The modification of PPy with DABSA therefore enhanced the adsorption capacity of the conducting polymer and resulted in a highly selective adsorbent for Cr(VI). Column studies and cost studies are required to further assess its potential for use in the treatment of industrial wastewater containing Cr(VI). The application of conducting polymer based composites for water treatment is therefore promising and the modification of conducting polymers and their development will be studied further. Acknowledgements The authors would like to acknowledge the National Research Foundation, the Department of Science and Technology and the Council for Scientific and Industrial Research (CSIR), South Africa, for financial support. The characterization unit at the DST-CSIR National Centre for Nanostructured Materials is acknowledged for assisting with materials characterization.

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Figure captions Fig. 1. (A) ATR-FTIR spectra of (a) PPy, (b) PPy/DABSA composite and (c) PPy/DABSA composite after Cr(VI) adsorption, (B) XPS survey spectra of PPy/DABSA composite before and after Cr(VI) adsorption and (C) XPS spectrum of PPy/DABSA composite after Cr(VI) adsorption showing Cr2p region Fig. 2. FE-SEM images of (A) PPy homopolymer and (B) PPy/DABSA composite. Fig. 3. (A) EDX spectra of (a) PPy (b) PPy/DABSA composite and c) PPy/DABSA composite after Cr(VI) adsorption, (B) XRD patterns for a) PPy and b) PPy/DABSA composite and (C) TGA plots for (a) PPy and (b) PPy/DABSA composite. Fig. 4. (A) Effect of pH on Cr(VI) adsorption by (a) PPy homopolymer and (b) PPy/DABSA, (B) Effect of adsorbent dose on Cr(VI) adsorption by PPy/DABSA composite and (C) Plot to determine the pHPZC for PPy/DABSA composite. Fig. 5. (A) Adsorption equilibrium isotherms for Cr(VI) removal by the PPy/DABSA composite and fit of data to non-linear Langmuir and Freundlich isotherm models; (B) Fit of data to linearized Langmuir isotherm model and (C) Plot to obtain thermodynamic parameters for Cr(VI) adsorption by the PPy/DABSA composite. Fig. 6. (A) Effect of contact time on Cr(VI) removal by the PPy/DABSA composite for different initial concentrations and fit of data to pseudo-first-order and pseudo-second-order

26

kinetic models, (B) Fit of data to linearized pseudo-second-order kinetic model and (C) Intraparticle diffusion model for adsorption of Cr(VI) by the PPy/DABSA composite. Fig. 7. (A) Effect of temperature and time on Cr(VI) removal by the PPy/DABSA composite for Cr(VI) initial concentration of 100 mg/L and (B) Plot to determine the activation energy for Cr(VI) adsorption by the PPy/DABSA composite. Fig. 8. Effect of co-existing ions on the removal of Cr(VI) by PPy/DABSA composite; and (B) Adsorption-desorption cycles. Scheme 1. Possible mechanism for Cr(VI) removal by PPy/DABSA composite. Table captions Table 1 Comparison of the adsorbent dose used for Cr(VI) removal by PPy/DABSA composite to other PPy based adsorbents Table 2 Langmuir and Freundlich isotherm parameters for Cr(VI) adsorption by PPy/DABSA composite. Table 3 Comparison of adsorption capacity of PPy/DABSA composite with other adsorbents used for Cr(VI) removal. Table 4 Thermodynamic parameters for Cr(VI) adsorption by PPy/DABSA composite. Table 5 Kinetic parameters for Cr(VI) adsorption by PPy/DABSA composite. Table 6 Intra-particle diffusion model parameters for Cr(VI) adsorption by PPy/DABSA composite. Table S1 Kinetic parameters for Cr(VI) adsorption by PPy/DABSA composite at different temperatures.

27

12000

(A)

(B) 1050 1309

1498

11000

(c) 1023

1585

(b) 1491

1050

1300

1023

9000 Cr2p3/2 8000

701

Intensity / cps

Intensity / a.u

1571

1540 1453

Cr2p1/2

(b)

7000

C1s

6000 5000 4000

1294 1093

O1s

10000

N1s

(a)

3000

964

2000

(a) PPy (b) PPy/DABSA (c) PPy/DABSA after Cr(VI) adsorption

(a)

Cl2p S2p

(a) PPy/DABSA before Cr(VI) adsorption (b) PPy/DABSA after Cr(VI) adsorption

1000 0

1800

1600

1400

1200

-1

1000

800

1200

1000

800

Wavenumber / cm 1600

600

400

200

Binding energy / eV

(C) Cr2p3/2

1400

Cr2p1/2

Intensity / cps

2000

1200

1000

800

600

590

585

580

575

570

Binding energy / eV

Fig. 1. (A) ATR-FTIR spectra of (a) PPy, (b) PPy/DABSA composite and (c) PPy/DABSA composite after Cr(VI) adsorption, (B) XPS survey spectra of PPy/DABSA composite before and after Cr(VI) adsorption and (C) XPS spectrum of PPy/DABSA composite after Cr(VI) adsorption showing Cr2p region.

28

0

(A)

(B)

Fig. 2. FE-SEM images of (A) PPy homopolymer and (B) PPy/DABSA composite.

29

3250 3000

4000

(A)

C

(B)

(a) PPy (b) PPy/Dabsa (c) PPy/Dabsa after Cr(VI) adsorption

2750 2500

(a)

3500

3000

1750 1500 S

O

1250

(b) 2500

2000

1500 1000 750 500

1000

(b) Cr

N

Cl

Al

250

(c)

(a) PPy (b) PPy/DABSA

500 Cr

0 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0

6.5

10

Energy (keV)

20

30

40

50

60

70

80

90

 / degree

(C)

100 90 80

Weight retained / %

counts

(a)

Cl

2000

Intensity / a.u.

2250

70 60 50 40 30 20 (a) PPy (b) PPy/DABSA

10

(b) (a)

0 100

200

300

400

500

600

700

800

900

o

Temperature / C

Fig. 3. (A) EDX spectra of (a) PPy (b) PPy/DABSA composite and (c) PPy/DABSA composite after Cr(VI) adsorption, (B) XRD patterns for (a) PPy and (b) PPy/DABSA composite and (C) TGA plots for (a) PPy and (b) PPy/DABSA composite.

30

100

(A)

100

(B) 100

(a) PPy (b) PPy/Dabsa

% Cr(VI) removal

90

60

40

80

70

20

0 1

2

3

4

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6

7

8

9

10

11

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(a)

(b)

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13

0.00

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0.04

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0.08

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5 0

Zeta potential / mV

% Cr(VI) removal

80

-5 -10 -15 -20 -25 -30 2

3

4

5

6

7

8

9

10

11

pH

Fig. 4. (A) Effect of pH on Cr(VI) adsorption by (a) PPy homopolymer and (b) PPy/DABSA, (B) Effect of adsorbent dose on Cr(VI) adsorption by PPy/DABSA composite and (C) Plot to determine the pHPZC for PPy/DABSA composite.

31

0.10

1.8

(A)

400

(B)

1.6 350

1.4 300

Ce/qe / g/L

200

1.0 0.8

o

15 C o 25 C o 35 C o 45 C Freundlich model fit Langmuir model fit

150 100 50

0.6

0.2

0 0

50

100

150

200

250

300

350

o

15 C o 25 C o 35 C o 45 C

0.4

400

Ce / mg/L

0.0 0

50

100

150

200

250

300

350

Ce / mg/L

2.0

(C) 1.6

1.2

ln Kc

qe / mg/g

1.2 250

0.8

0.4

0.0

0.0031

0.0032

0.0033

0.0034

0.0035

1/T / 1/K

Fig. 5. (A) Adsorption equilibrium isotherms for Cr(VI) removal by the PPy/DABSA composite and fit of data to non-linear Langmuir and Freundlich isotherm models; (B) Fit of data to linearized Langmuir isotherm model and (C) Plot to obtain thermodynamic parameters for Cr(VI) adsorption by the PPy/DABSA composite.

32

400

100

8

(A)

(B) 7

80

6

t/qt / (min.g.mg )

90

-1

60 50 40 50 mg/L 75 mg/L 100 mg/L Pseudo-first-order model fit Pseudo-second-order model fit

30 20

5 4 3 2 50 mg/L 75 mg/L 100 mg/L

1

10 0

0 0

50

100

150

200

250

300

350

0

400

50

100

150

200

100

250

300

350

t / min

t / min

(C)

90 80 70

qt / (mg/g)

qt / (mg/g)

70

60 50 40 30 50 mg/L 75 mg/L 100 mg/L

20 10 0 0

2

4

6

8

10

t

1/2

/ min

12

14

16

18

20

1/2

Fig. 6. (A) Effect of contact time on Cr(VI) removal by the PPy/DABSA composite for different initial concentrations and fit of data to pseudo-first-order and pseudo-second-order kinetic models, (B) Fit of data to linearized pseudo-second-order kinetic model and (C) Intraparticle diffusion model for adsorption of Cr(VI) by the PPy/DABSA composite.

33

400

(A) 100 90 80

qt / (mg/g)

70 60 50 40 30 o

15 C o 25 C o 35 C o 45 C

20 10 0 0

50

100

150

200

250

300

350

400

t / min

(B) -5.5

lnk2 / (mg/L/min)

-6.0

-6.5

-7.0

-7.5

0.0031

0.0032

0.0033

0.0034

0.0035

1/T / (1/K)

Fig. 7. (A) Effect of temperature and time on Cr(VI) removal by the PPy/DABSA composite for Cr(VI) initial concentration of 100 mg/L and (B) Plot to determine the activation energy for Cr(VI) adsorption by the PPy/DABSA composite.

34

100

(A)

Cr(VI) uptake / (mg/g)

80 2+

Cu 2+ Ni 2+ Zn NO3

60

-

Cl 2SO4

40

Multi

20

0 0

20

40

60

80

100

Initial concentration of co-existing ions / (mg/L)

% Adsorption/desorption of Cr(VI)

100 (B)

% Adsorption % Desorption

80

60

40

20

0 1

2

3

Cycle

4

5

Fig. 8. Effect of co-existing ions on the removal of Cr(VI) by PPy/DABSA composite; and (B) Adsorption-desorption cycles.

35

Scheme 1. Possible mechanism for Cr(VI) removal by PPy/DABSA composite. 36

Table 1 Comparison of the adsorbent dose used for Cr(VI) removal by PPy/DABSA composite to other PPy based adsorbents Adsorbent

Adsorbent

Concentration of

Removal

Reference

dose (g/L)

Cr(VI) (mg/L)

efficiency (%)

PPy/wood sawdust

24

100

100

[22]

Polyacrylonitrile/PPy

6.67

200

84.5

[26]

10

100

100

[25]

PPy/palygorskite

0.6

10

100

[29]

PPy/attapulgite

3

100

100

[30]

PPy/DABSA composite

1

100

100

[Present work]

nanofibers PPy/cellulose fibre composite

37

Table 2 Langmuir and Freundlich isotherm parameters for Cr(VI) adsorption by PPy/DABSA composite. Isotherm model 15 °C Langmuir Linear qm 222.22 b 0.149 RL 0.025 R2 0.9987 Non-linear Best-fit values qm 202.1 b 5.148 Std. Error qm 6.896 b 2.208 95 % Confidence q 186.5 – 217.7 m Intervals b 0.1521 – 10.14 Goodness of Fit Degrees of Freedom 9 R² 0.7174 Absolute Sum of Squares 4088 Sy.x 21.31 Number of points Analyzed 11 Freundlich Linear KF 124.14 n 9.90 R2 0.9904 Non-linear Best-fit values 126.7 KF 10.36 n Std. Error KF 2.457 n 0.417 95 % Confidence K 121.1 – 132.2 F Intervals n 9.414 – 11.30 Goodness of Fit Degrees of Freedom 9 R² 0.9903 Absolute Sum of Squares 140.7 Sy.x 3.954 Number of points Analyzed 11 Units: qm: mg/g, b: L/mg, KF: mg/g

25 °C

Temperature 35 °C

45 °C

303.03 0.559 0.007 0.9997

333.33 0.556 0.007 0.9996

384.62 0.510 0.007 0.9985

280.6 18.54

300.3 82.52

349.7 75.95

15.30 11.94

17.99 59.58

13.14 17.03

246.0 – 315.2 -8.464 – 45.54

259.6 – 341.0 -52.26 – 217.3

320.0 – 379.5 37.43 – 114.5

9 0.6337 18111 44.86

9 0.6233 25480 53.21

9 0.8862 10963 34.9

11

11

11

173.26 9.38 0.7105

197.57 10.29 0.6561

239.32 10.83 0.7862

200.1 12.57

222.5 12.77

249.8 12.39

18.52 3.213

16.40 2.693

12.65 1.754

158.2 – 242.0 5.301 – 19.83

185.4 – 259.6 6.675 – 18.86

221.2 – 278.4 8.428 – 16.36

9 0.7911 10330 33.88

9 0.8486 10238 33.73

9 0.8881 10783 34.61

11

11

11

38

Table 3 Comparison of adsorption capacity of PPy/DABSA composite with other adsorbents used for Cr(VI) removal. Adsorbent

Qm (mg/g)

Optimum pH

Reference

PPy nanoclusters

180.43

5

[20]

PPy/wood sawdust

3.4

5

[22]

Bismuth hollow nanospheres

9.42

5

[49]

PPy/halloysite nanotube clay

149.25 – 227.27

2

[24]

PPy/Fe3O4

169.4 – 243.9

2

[5]

Bi2O3 nanotubes

79

PPy/polyaniline nanofibres

227 - 294

2

[10]

Polyacrylonitrile/PPy nanofibers

61.80 – 74.91

2

[26]

α-Fe2O3

29.52

5

[51]

PPy/chitosan

78.61

4.2

[27]

BiOBr nanostructures

63.5

PPy/organically modified

119.3 – 209.6

2

[28]

Ti-Fe kaolinite composite

23.47

3

[53]

PPy/palygorskite

65 - 86

5

[29]

PPy/attapulgite

48.45

3

[30]

PPy/glycine

217.39 – 232.55

2

[15]

PPy/DABSA composite

222.22 – 384.62

2

[Present work]

[50]

[52]

montmorillonite clay

39

Table 4 Thermodynamic parameters for Cr(VI) adsorption by PPy/DABSA composite. Temperature (°C)

∆G° (kJ/mol)

∆H° (kJ/mol)

∆S° (kJ/mol/K)

15

-0.294

41.50

0.1462

25

-2.648

35

-3.455

45

-4.854

40

Table 5 Kinetic parameters for Cr(VI) adsorption by PPy/DABSA composite. Kinetic model 50 mg/L Pseudo-first-order Linear qe 7.91 k1 0.0207 2 R 0.9016 Non-linear Best-fit values qe 48.67 k1 0.1886 Std. Error qe 0.8882 k1 0.02448 95 % Confidence q 46.82 – 50.52 e Intervals k1 0.1377 – 0.2395 Goodness of Fit Degrees of Freedom 21 R² 0.8440 Absolute Sum of Squares 293.80 Sy.x 3.740 Number of points Analyzed 23 Pseudo-second-order Linear qe 50.2513 k2 0.00834 h0 21.0526 R2 1 Non-linear Best-fit values qe 50.48 k2 0.006692 Std. Error qe 0.8119 k2 0.001091 95 % Confidence q 48.79 – 52.17 e Intervals k2 0.004422 – 0.008962 Goodness of Fit Degrees of Freedom 21 R² 0.9055 Absolute Sum of Squares 178.1 Sy.x 2.912 Number of points Analyzed 23 Units: qe: mg/g, k1: 1/min, k2: g/mg.min, h0: mg/g/min

Initial Concentration 75 mg/L Concentration

100 mg/L

19.95 0.0216 0.9563

53.95 0.0182 0.964

72.01 0.1716

91.11 0.07875

1.428 0.02340

2.670 0.01312

69.04 – 74.98 0.1230 – 0.2203

85.56 – 96.67 0.05146 – 0.01060

21 0.8312 750.90 5.980

21 0.7844 2262 10.38

23

23

75.7576 0.00363 20.8333 0.9999

101.01 0.00098 10.02 0.9993

75.05 0.003887

97.85 0.001191

1.242 0.0006159

2.428 0.0002119

72.47 – 77.64 0.002605 – 0.005168

92.80 – 102.9 0.0007503 – 0.001632

21 0.9095 402.7 4.379

21 0.8933 1120 7.302

23

23

41

Table 6 Intra-particle diffusion model parameters for Cr(VI) adsorption by PPy/DABSA composite.

Regions ki (mg/g/min0.5) C

50 mg/L First Second 7.8844 1.7099 13.373 35.267

Cr(VI) initial concentration 75 mg/L Third First Second Third 0.0484 11.304 2.9884 0.2458 49.122 19.273 47.711 70.651

Graphical Abstract

42

100 mg/L First Second 9.1942 5.1917 24.352 40.776

Third 0.5617 87.572

43