m-Phenylenediamine-modified polypyrrole as an efficient adsorbent for removal of highly toxic hexavalent chromium in water

Accepted Manuscript Title: m-Phenylenediamine-modified polypyrrole as an efficient adsorbent for removal of highly toxic hexavalent chromium in water Authors: Nazia H. Kera, Madhumita Bhaumik, Kriveshini Pillay, Suprakas Sinha Ray, Arjun Maity PII: DOI: Reference:

S2352-4928(18)30013-8 https://doi.org/10.1016/j.mtcomm.2018.02.033 MTCOMM 309

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

8-1-2018 23-2-2018 25-2-2018

Please cite this article as: Nazia H.Kera, Madhumita Bhaumik, Kriveshini Pillay, Suprakas Sinha Ray, Arjun Maity, m-Phenylenediamine-modified polypyrrole as an efficient adsorbent for removal of highly toxic hexavalent chromium in water, Materials Today Communications https://doi.org/10.1016/j.mtcomm.2018.02.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

m-Phenylenediamine-modified polypyrrole as an efficient adsorbent for removal of highly toxic hexavalent chromium in water Nazia H Keraa,b, Madhumita Bhaumikb, Kriveshini Pillayb, Suprakas Sinha Raya,b*, Arjun Maitya,b* a

DST–CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research,

Pretoria, South Africa Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South

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b

Africa

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*Corresponding authors.

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E-mail addresses: [email protected] (A. Maity); [email protected]; [email protected] (S.S.Ray)

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Graphical abstract

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Highlights

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Novel PPy–mPD adsorbent was synthesised by in situ oxidative polymerisation

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Modification of PPy with mPD reduced the particle size and increased specific surface area

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Langmuir maximum adsorption of PPy–mPD for Cr(VI) was 526 mg/g at 25 °C at initial pH =2 Some of the Cr(VI) adsorbed by PPy–mPD was reduced to Cr(III)

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Selected co-existing ions in solution did not significantly affect the removal of Cr(VI)

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ABSTRACT: The aim of this work was to develop m-phenylenediamine (mPD)-modified polypyrrole (PPy–

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mPD) as an efficient adsorbent for the removal of highly toxic hexavalent chromium [Cr(VI)] from industrial wastewater. PPy–mPD was synthesised by in situ oxidative polymerisation and characterised using various

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techniques. The effects of pH, adsorbent dose, initial concentration, temperature, and co-existing ions on

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Cr(VI) removal by PPy–mPD were investigated in batch studies. The results showed that PPy–mPD had a high specific surface area of 183 m2/g, and a maximum Cr(VI) adsorption capacity of 526 mg/g at 25 °C and initial pH 2. XPS and IC–ICP–MS studies confirmed that some of the adsorbed Cr(VI) was reduced to Cr(III)

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by PPy–mPD. Cr(VI) removal by PPy–mPD followed pseudo-second-order kinetics, and the removal was highly selective for Cr(VI) in the presence of co-existing ions in solution. Desorption studies showed that Cr(VI) removal by PPy–mPD was decreased approximately 30% after third treatment cycle. In summary, PPy–mPD was shown to have potential as a novel adsorbent for the highly selective adsorption for Cr(VI)

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with high capacity although structural integrity of the polymer matrix would be needed if enhanced desorption characteristic is required for specific applications.

KEYWORDS: modified polypyrrole; hexavalent chromium adsorption; wastewater treatment

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1. Introduction Conducting polymers have received widespread interest for a variety of applications due to their high environmental stability and unique chemical, electrical, mechanical, electronic, optical, and magnetic

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properties [1–3]. Some of their potential applications include sensors, corrosion resistance, catalysis, energy storage, electrochemistry, electrochromic displays, electronic devices, and electromagnetic shielding [1–3].

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Recently, conducting polymers including polypyrrole (PPy), polyaniline (PANi), and polythiophene have

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been studied as adsorbents for removing heavy metals, organic compounds, dyes, and other toxic pollutants

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from contaminated water [4,5].

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Cr is a common heavy metal pollutant in wastewater, originating from industrial activities such as leather tanning, electroplating, wood treatment, steel and alloy manufacturing, and mining [6–8]. Two forms of Cr,

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hexavalent Cr [Cr(VI)] and trivalent Cr [Cr(III)], are stable in the environment [6–8]. Cr(VI) has a high solubility and mobility in the environment, while Cr(III) is relatively immobile as most Cr(III) compounds have low

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aqueous solubilities [6–8]. The toxicity of Cr(VI) is significantly higher. In trace amounts, Cr(III) is necessary

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for humans and other organisms [6–8], while exposure to Cr(VI) in water can damage the skin, liver, kidney, and gastrointestinal and immune systems [6–8]. The toxicity of Cr(VI) has led to the establishment of regulations for controlling the amounts of Cr allowed in various types of waters. For example, the USEPA

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set the maximum level of total Cr in drinking water to be 0.1 mg/L [6]. It is necessary to treat wastewater prior to its discharge into the environment, in order to conform to the stringent water quality standards regarding Cr(VI). As adsorption is a simple technology, adsorbents have been examined in numerous studies for removing Cr(VI) from water [6,9,10]. PPy has been identified as a potential adsorbent for this application because of its anion exchange capacity, owing to the high mobility of small dopant ions present in its structure [4,5,11–

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14]. Due to the spontaneous transfer of electrons from PPy to Cr(VI), PPy can also reduce Cr(VI) to much less toxic Cr(III) [4,5,11–14]. However, PPy particles tend to aggregate and do not disperse well in water; hence, the Cr(VI) adsorption capacities have been poor [15]. Previous studies have shown that modifying PPy with molecules containing desirable functional groups (through doping or copolymerisation) could increase the Cr(VI) adsorption capacity. For example, Ballav et al. synthesised glycine-doped PPy, which had

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a Cr(VI) adsorption capacity of 217.39 mg/g at 25 °C [16]. In a similar approach, Amalraj et al. prepared threonine-doped PPy and aspartic acid-doped PPy, with the respective adsorption capacities of 185.5 and 176.7 mg/g [17,18]. The enhanced Cr(VI) adsorption by these amino acid-doped PPy composites was

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attributed to interactions between anionic Cr species and the protonated amino moieties (NH3+) on the dopant molecules [16–18]. Bhaumik et al. synthesised PPy–PANi nanofibers with a Cr(VI) adsorption

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capacity of 227 mg/g [19]. The Cr(VI) removal mechanism was suggested to involve ion exchange between

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dopant Cl– ions in the conducting polymer chains and HCrO4− anions in the solution [19]. Karthik et al.

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synthesised PPy/chitosan that had an adsorption capacity of 78.61 mg/g for Cr(VI), in order to improve the

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Cr(VI) adsorption capacity of PPy and to overcome the limitations of using chitosan for water treatment applications [20]. Zhang et al. found that natural corncob-core sponge modified with PPy had a high

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mechanical stability and could be used for three adsorption–desorption cycles with no significant loss in Cr(VI) adsorption capacity [21]. Hu et al. prepared hollow PPy/polystyrene microspheres to hinder the

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agglomeration of particles [22]. Sall et al. prepared 4-amino-3-hydroxynaphthalene-1-sulfonic acid-doped

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PPy films by electrosynthesis, with a maximum adsorption capacity of 224 mg/g for Cr(VI) [23]. In this study, PPy was modified with m-phenylenediamine (mPD). mPD has two amino groups per molecule which we hypothesised significantly improve the adsorption capacity of mPD modified PPy for Cr(VI)

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adsorption. PPy–mPD was synthesised by in situ oxidative polymerisation, and its physicochemical properties were studied using different characterisation methods. Batch equilibrium and kinetic studies were carried out to evaluate the performance of PPy–mPD for removing Cr(VI) from water.

2. Methods and materials 2.1. Materials

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Pyrrole (Py) was purchased from Sigma-Aldrich in South Africa and distilled before usage. Fe(III) chloride (FeCl3), m-phenylenediamine (C6H8N2), and 1,5-diphenylcarbazide (C13H14N4O) were purchased from SigmaAldrich in South Africa and used as obtained. All reagent grade chemicals used in this study, including hydrochloric acid (HCl), sodium hydroxide (NaOH), and potassium dichromate (K2Cr2O7) were obtained from Minema Chemicals, South Africa. A stock solution (1000 ppm) of potassium dichromate (K 2Cr2O7) was

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diluted to prepare the Cr(VI) solutions for batch adsorption studies.

2.2. Synthesis of PPy–mPD

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The synthesis of PPy–mPD was carried out by in situ oxidative polymerisation of Py and mPD at room temperature (approximately 27 C) with FeCl3 as an oxidant. In a typical synthesis, mPD (0.8 g) was dissolved

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in deionised water (100 mL) in a conical flask. Py (0.8 mL) was injected into the conical flask and dispersed

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by mechanical stirring. FeCl3 (6 g) was dissolved in 100 mL of water and then added all at once to the conical

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flask containing the mixture of Py and mPD monomers. The contents of the conical flask were stirred for 5

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min, and then left standing for 24 h for the monomers to polymerise. The black product was recovered by vacuum filtration and washed firstly with deionised water and then with acetone until it became colourless.

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The product (~ 1.2 g) was dried for 24 h under vacuum at 60 °C. For comparison purposes, PPy homopolymer

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was also synthesised using a similar method but without using the mPD.

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2.3. Characterisation of PPy–mPD

The attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectra for PPy and PPy–mPD polymers were obtained in the 4000–400 cm-1 range by using a Spectrum 100 spectrometer (PerkinElmer,

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USA). The spectral resolution used was 4 cm–1 and the number of scans per spectrum was 16. The structure, surface morphology, and elemental composition of PPy–mPD were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX). For TEM analysis, the samples were dispersed in ethanol and dropped onto carbon-coated copper grids. Then, images were obtained from a JEOL–JEM 2100 microscope at 200 keV. SEM images were obtained by using an Auriga Cobra focused-ion beam (FIB) SEM device, and the samples were carbon-coated before analysis.

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X-ray diffraction (XRD) studies for PPy and PPy–mPD polymers were carried out by using an X’Pert PRO diffractometer (PANalytical, The Netherlands), which generates Cu Kα X-rays with wavelength of 1.5444 Å. The generator voltage and tube current were 45 kV and 40 mA, respectively, and the X-ray patterns were obtained for 2θ = 5–90°. XPS analysis was used to study the elemental composition of PPy–mPD before and after Cr(VI) removal from water. The spectra were obtained from a Thermo ESCALAB 250Xi X-ray

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photoelectron spectrometer which uses monochromatic Al Kα X-ray (1486.7 eV). Thermogravimetric analyses (TGA) of PPy and PPy–mPD were carried out by using a TGA Q500 system (TA Instruments, USA). The samples were heated in air (50 mL/min) from room temperature to 900 °C at a scan rate of 10 °C/min.

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An ASAP 2020 (Micromeritics, USA) instrument was used to analyse the surface area and porosity of PPy and PPy–mPD. Ion chromatography coupled to inductively coupled plasma mass spectroscopy (IC–ICP–MS,

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Thermo Fisher Scientific, USA) was used to determine the Cr speciation in aqueous solutions after Cr(VI)

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removal by PPy–mPD. To examine the effects of co-existing ions, the concentrations of Cu2+, Zn2+, Ni2+, and

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Co2+ in solution before and after Cr(VI) removal were obtained by ICP–MS (Thermo Fisher Scientific, USA)

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analysis. UV–Visible spectrophotometric analysis was carried out on a Lambda 750S spectrometer (Perkin

2.4. Batch adsorption studies

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2.4.1. Equilibrium studies

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Elmer, USA).

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Adsorption equilibrium studies were conducted to investigate the effect of adsorption parameters (such as initial solution pH, adsorbent dose, initial Cr(VI) concentration, temperature, and co-existing ions) on Cr(VI) removal by PPy–mPD. In a typical experiment, a given amount of PPy–mPD and Cr(VI) solution (50 mL) were

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placed into a plastic bottle and agitated for 24 h in a thermostatic shaker at 200 rpm. The solutions were then filtered with syringe membrane filters (0.45 μm) to remove the PPy–mPD and the Cr(VI) concentration of the filtrate obtained from UV–Visible spectrophotometric analysis by using the 1,5-diphenylcarbazide analysis method for Cr(VI). The percentage of Cr(VI) removal (i.e., adsorption) efficiency was calculated from equation (1):

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C0 -Ce ) ×100 C0

% removal = (

(1) where C0 (mg/L)

is the initial Cr(VI) concentration and Ce (mg/L) is the equilibrium concentration. To investigate the effect of pH on Cr(VI) removal, Cr(VI) solutions (100 mg/L) were adjusted to pH values from 2 to 12 by using NaOH and HCl solutions. The effect of PPy–mPD dose on Cr(VI) removal was studied by increasing the mass of PPy–mPD from 0.005 to 0.05 g, with the Cr(VI) concentration and pH fixed at 100

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mg/L and 2, respectively. For studying the effect of initial concentration, Cr(VI) solutions ranging from 100 to 600 mg/L at pH = 2 were

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prepared. Adsorption isotherm data for Cr(VI) removal by PPy–mPD polymer were obtained at 15, 25, 35, and 45 °C. The Cr(VI) adsorption capacity at equilibrium, qe, was calculated using equation (2): C0 -Ce )V m

qe = (

(2)

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where m is the mass of PPy–mPD in g and V is the volume in L. The thermodynamic parameters for the

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adsorption were obtained from the isotherm data.

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Selected ions (Cu2+, Zn2+, Ni2+, Co2+, Cl–, NO3–, PO43–, and SO42–, 20–100 mg/L) were used to examine the

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effect of co-existing ions on Cr(VI) removal by PPy–mPD adsorbent. After adjusting the Cr(VI) (100 mg/L)

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solution with the selected ions to pH = 2, the adsorbent and solution were shaken together for 24 h at 25

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°C.

2.4.2. Kinetic studies

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Four initial Cr(VI) concentrations (25, 50, 75, and 100 mg/L) were used to study the effect of contact time on the removal of Cr(VI) by PPy–mPD. In a typical experiment, PPy–mPD (0.5 g) was transferred into Cr(VI) solution (1 L, pH 2). An overhead stirrer was used to continuously stir the solution at 200 rpm, and the

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temperature was maintained at 25 °C. Samples (6 mL) were drawn at time zero and then at suitable time intervals after the addition of the PPy–mPD. After filtering the samples by using syringe membrane filters (0.45 μm), the filtrates were analysed to determine the Cr(VI) concentration. The adsorption capacity of the PPy–mPD polymer at time t, qt, was obtained using equation (3):

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C0 - Ct )V m

qt = (

(3) where Ct (mg/L) is the

concentration of Cr(VI) at time t. In addition, kinetics experiments were carried out at 15, 25, 35, and 45 °C with C0 = 100 mg/L to investigate the temperature dependence of the Cr(VI) removal, which was used to calculate the activation energy for Cr(VI) adsorption by PPy–mPD.

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2.4.3. Desorption studies After adsorption, Cr(VI) was desorbed from PPy–mPD by using either 0.1 M NaOH or 0.1 M NH4OH, to

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determine the potential of reusing this polymer. In a typical experiment, PPy–mPD (0.025 g) was shaken together with Cr(VI) solution (50 mL, 100 mg/L, pH 2) for 24 h at 25 °C. Cr(VI) was then desorbed by shaking the loaded PPy–mPD with one of the two desorbing solutions for 24 h at 25 °C. After desorption, the PPy–

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mPD was regenerated by shaking with 2 M HCl for 4 h. Four consecutive treatment cycles were conducted

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for each desorbing solution. PPy–mPD was recovered from solution after Cr(VI) adsorption and desorption

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and regeneration treatments by vacuum filtration.

3. Results and discussion

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3.1. Characterisation of PPy–mPD

The ATR-FTIR spectra for PPy and PPy–mPD are presented in Fig. 1A. Fig. 1A(a) shows bands characteristic

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of PPy at 1554, 1455, 1300, 1200, 1034, 897, and 795 cm–1; corresponding to the C=C Py ring stretching, C–

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N Py ring stretching, C–N stretching, C–H in-plane deformation, N–H in-plane deformation, C–H out-of-plane deformation, and C–H out-of-plane ring deformation, respectively [19,24,25]. Fig. 1A(b) shows that, after modification with mPD, the bands characteristic of PPy C=C and C–N ring stretching motions shifted to

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higher wavenumbers of 1588 and 1488 cm–1, respectively. The spectrum for PPy–mPD polymer has two broad bands at 3200 and 3340 cm–1 (Fig. 1A inset) corresponding to N–H stretch vibrations of –NH2 groups [26–28]. The PPy–mPD polymer also showed bands at 1625, 1530, and 1284 cm–1 that are assigned to the stretching of quinoid imines, stretching of benzenoid amines, and C–N stretching of mPD, respectively [Fig. 1A(b)] [26–28]. These results confirmed that PPy was successfully modified with mPD.

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100.5

(a) PPy (b) PPy/mPD

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Fig. 1. (A) ATR-FTIR spectra of (a) PPy and (b) PPy–mPD polymers. (B) XPS survey spectra of PPy–mPD before

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and after Cr(VI) adsorption. Cr 2p core level XPS spectra for PPy–mPD after removal of Cr(VI) at (C) pH 2 and

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(D) pH 7.

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The XPS survey spectra for PPy and PPy–mPD before and after Cr(VI) removal, presented in Fig. 1B, have peaks attributed to C1s, N1s, O1s, and Cl2p at 284, 400, 532, and 199 eV, respectively [29]. Additional peaks

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due to Cr at binding energies of 576 and 586 eV in Fig. 1B(b) and Fig. 1B(c) confirm adsorption of Cr(VI) by

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PPy–mPD polymer at initial solution pH of 2 and 7, respectively [29].

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The TEM and SEM images are compared in Fig. 2. Both PPy and PPy–mPD are composed of agglomerations of roughly spherical particles. However, the PPy–mPD particles are much smaller in size. This is similar to

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findings in previous studies where amino acids were used to modify PPy [16–18].

Fig. 2. TEM images of (A) PPy homopolymer and (B) PPy–mPD. SEM images of (C) PPy homopolymer and (D)

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PPy–mPD.

The BET specific surface area, average pore diameter, and average pore volume were 5 m 2/g, 49.45 nm, 0.01 cm3/g for PPy; and 183 m2/g, 11.67 nm, 0.46 cm3/g for PPy–mPD, respectively. The incorporation of mPD into PPy homopolymer therefore resulted in a significant increase in the BET surface area and average pore volume, plus a corresponding decrease in the average pore diameter. The increase in BET surface area

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correlates well to the decrease in particle size observed from the TEM and SEM images. The BET surface area of PPy–mPD is significantly higher than those of other PPy composites investigated for removing Cr(VI) from water, as shown in Table 1 [15,17,18,20,30–33].

Table 1. Comparison of the BET specific surface areas of PPy and PPy–mPD with other PPy-based adsorbents

BET specific surface area (m2/g) 5.30 183.2 3.06

PPy/2,5-diaminobenzene sulfonic acid PPy–PANI PPy/threonine PPy/aspartic acid PPy nanoclusters PPy/ titanium(IV)phosphate Capsular PPy hollow nanofibers

21.84 59.7 41.8 38.7 104 59.4 47.9

Reference This study This study [20] [30] [15] [17] [18] [31] [32] [33]

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PPy based adsorbent for Cr(VI) PPy PPy–mPD PPy/chitosan

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studied for Cr(VI) removal

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The EDX spectra for PPy–mPD before and after Cr(VI) removal, shown in Fig. 3A, contain bands due to C, N, O, and Cl at 0.27, 0.39, 0.52, and 2.61 keV, respectively. During oxidative polymerisation of Py, dopant Cl–

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anions from solution are incorporated to maintain electroneutrality by balancing the positive charge

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associated with nitrogen atoms [13,14,16]. The spectrum in Fig. 3A(b) has two additional peaks attributed to Cr at binding energies of 5.4 and 5.9 keV, confirming that Cr(VI) was adsorbed by PPy–mPD.

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The XRD patterns for PPy and PPy–mPD in Fig. 3B both show broad diffraction bands that are characteristic of amorphous structure [16,26,34]. After modification of PPy with mPD, the centre of the characteristic PPy broad band shifted from 2θ = 25° in Fig. 3B(a) to a lower angle of 23° [Fig. 3B(b)] [16,26,34]. Previous studies

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have suggested that the shifting of peaks to lower 2θ values indicates an increased spacing and amorphousness of the macromolecular chains in the polymer, both of which are conducive to the adsorption of ions [34]. The modification of PPy with mPD therefore increased the amorphousness, which may enhance the adsorption of Cr(VI) from water [34]. Fig. 3B(c) shows that Cr(VI) adsorption did not affect the structure of PPy–mPD polymer significantly.

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C (a) (a) PPymPD polymer (b) PPymPD polymer after Cr(VI) removal (pH 2)

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Counts

(a) PPy (b) PPymPD polymer (c) PPymPD polymer after Cr(VI) removal (pH 2)

(c)

Intensity / a.u.

90000

50000

(b) 40000 30000

(b)

20000

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(a) PPy (b) PPymPD polymer (c) PPymPD polymer after Cr(VI) removal (pH 2)

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Fig. 3. (A) EDX spectra of (a) PPy–mPD and (b) PPy–mPD after Cr(VI) adsorption. (B) XRD patterns and (C)

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TGA plots of (a) PPy, (b) as-prepared PPy–mPD, and (c) PPy–mPD polymer after Cr(VI) adsorption.

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TGA analysis was used to compare the thermal stability of the polymers before and after the adsorption.

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The TGA plots in Fig. 3C show a gradual loss of mass up to 200 °C. The decrease in mass at 100 °C may be due to water loss from the polymers, while that between 100 °C and 200 °C may be associated with the loss of oligomers with low molecular weights [15,33,35]. The decomposition of PPy and PPy–mPD occurred

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between 200 °C to 600 °C and was almost complete. The TGA plots indicate that the thermal stability of PPy was not adversely affected upon incorporating mPD. Fig. 3C(c) shows that the decomposition of PPy–mPD after Cr(VI) removal occurred at lower temperatures (between 200 °C and 420 °C), therefore the Cr(VI) adsorption decreased the thermal stability of PPy–mPD. This may be due to oxidation of the PPy–mPD by Cr(VI), or the adsorbed Cr ions undermining the structural integrity of the polymer [33]. However, these

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changes at higher temperatures do not directly affect the performance of PPy–mPD for water treatment at ambient temperatures.

3.2. Batch adsorption studies 3.2.1. Effect of pH

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The effect of pH on the removal of Cr(VI) by PPy–mPD is shown in Fig. 4A. The highest Cr(VI) removal was achieved at solution pH 2 (99%), gradually decreasing to 0% at pH 12. This change may be attributed to the pH-dependent Cr(VI) species present in solution and the surface charge of the PPy–mPD. The forms of Cr(VI)

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present in aqueous solution depends on the pH and the Cr(VI) concentration [6,8]. The dichromate ion, Cr2O72–, typically dominates in solutions at low pH values and relatively high Cr(VI) concentrations [6,8].

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Bichromate (HCrO4–) anions are the dominant form of Cr(VI) in solution at pH 2 with relatively low Cr(VI)

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concentration [6,8]. Meanwhile, the plot for zeta potential vs pH in Fig. 4B shows that the point of zero

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charge for PPy–mPD is at pH 7.4. Therefore, the polymer surface has a positive charge below pH 7.4 (possibly

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because of the protonation of the amino groups) and negative charge above it. At pH 2, the positively charged PPy–mPD surface attracts and adsorbs the Cr(VI) anions in solution. As the pH increases, there is

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less protonated amino group, while the additional OH– anions compete with Cr(VI) anions for adsorption

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sites. Therefore, the Cr(VI) removal by PPy–mPD decreases as pH increases.

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30 20

Zeta potential / mV

% Cr(VI) removal

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10 0 -10 -20 -30

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Initial solution (pH 2) pH 2 pH 3 pH 4 pH 5 pH 6 pH 7 pH 8 pH 9 pH 10 Cr(III) pH 11 pH 12

Cr(VI)

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Intensity / cps

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pH

pH

120

130

140

Time / s

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Fig. 4. (A) Effect of pH on Cr(VI) adsorption by PPy–mPD polymer, (B) plot to determine the pHPZC for PPy–

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mPD, and (C) IC-ICP-MS chromatograms showing chromium speciation in solutions at different pH values.

The Cr 2p core level XPS spectra for PPy–mPD after Cr(VI) adsorption at pH 2 and 7 are shown in Fig. 1C and

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Fig. 1D, respectively. The spectra showed two asymmetric bands arising from the Cr 2p1/2 and Cr 2p3/2 orbitals that were each deconvoluted into two symmetric bands. The bands at binding energies of 576.7

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and 586.5 eV for solution pH 2 and those at 576.5 and 586.0 eV for solution pH 7 were assigned to Cr(III), as in Cr2O3 [29,36,37]. The bands at binding energies of 577.9 and 587.5 eV for pH 2 and at 577.8 and 587.3 eV for pH 7 were assigned to Cr(VI), as in CrO3 [29,36,37]. The XPS studies therefore confirmed that Cr(VI)

A

and Cr(III) were both present on the surface of PPy–mPD after Cr(VI) removal at these two pH values. IC– ICP–MS studies were used to measure the Cr(VI) and Cr(III) concentrations of the solutions, and the chromatograms are presented in Fig. 4C. Cr(III) was found in solutions with pH = 2 and 3 at 10 and 3 mg/L concentrations after application of PPy–mPD, respectively, but not when pH = 4–12. These observations correlate well with the varying oxidising power of the Cr(VI) ions at different pH values. At pH 2, Cr(VI) exists

14

mainly as HCrO4– ions which have a high redox potential. These ions are reduced by PPy–mPD to Cr(III), as shown by equation (4) [8]: HCrO4– + 7H+ + 3e– ⇌ Cr3+ +4H2O

(4)

At pH > 6, Cr(VI) is mainly in the form of CrO42– anions, which have a redox potential lower than for the HCrO4– ions and therefore less easily reduced to Cr(III) according to equation (5) [8]: CrO42– + 4H2O + 3e– ⇌ Cr(OH)3 + 5OH–

IP T

(5)

The proposed mechanism for the removal of Cr(VI) by PPy–mPD at pH 2 and 7 is presented in Scheme 1. At pH 2, the HCrO4– anions in solution are adsorbed by the protonated amino groups on the surface of PPy–

SC R

mPD due to electrostatic interactions. Some of the adsorbed HCrO4– anions are then reduced by the polymer to Cr(III), as shown by equation (4) [8]. At pH 7, there are less adsorption sites (i.e., protonated amino

U

groups) on the surface of PPy–mPD, and OH- ions in the solution compete with Cr(VI) ions for these sites.

N

The result is a lowered capacity for Cr(VI) removal. In this case, PPy–mPD reduces also some of the adsorbed

A

CrO42– ions to Cr(III) according to equation (5) [8]. At initial solution pH 2, Cr(III) is present both in the solution

M

and on the PPy–mPD as Cr oxide, which has a low but not negligible solubility at low pH. For the initial solution pH 7, all the Cr(III) exists on the PPy–mPD as Cr oxide and not in the solution at all, as Cr oxide is

A

CC E

PT

ED

hardly soluble at this pH [8].

15

IP T SC R U N A

ED

3.2.2. Effect of adsorbent dose

M

Scheme 1. Proposed mechanism for Cr(VI) removal by PPy–mPD

According to Fig. S1 (supporting Information), the removal efficiency for Cr(VI) increased as the PPy–mPD

PT

dose was increased, due to the availability of more adsorption sites for Cr(VI). The minimum dose required

CC E

for removing 99% of Cr(VI) in solution (50 mL, 100 mg/L) was 25 mg.

3.2.3. Adsorption isotherm studies

A

The adsorption isotherms for Cr(VI) removal by PPy–mPD polymer are shown in Fig. 5A. The increase in Cr(VI) removal with temperature suggested that this adsorption process was endothermic. The isotherm data were fitted to both the Langmuir and Freundlich models. The Langmuir model and its linear form are shown in equations (6) and (7), respectively:

16

qe qm

bC

= 1+bCe

(6)

e

(7)

Ce 1 C = + e qe qm b 𝑞𝑚

where qm is the maximum

adsorption capacity in mg/g, and b is the Langmuir constant associated to the enthalpy of adsorption in L/mg. The dimensionless separation factor RL indicates whether the adsorption process is favourable. RL can be calculated from equation (8), which is derived from the Langmuir model. 1 1+ bC0

(8)

IP T

RL =

The Freundlich model and its linear form can be expressed as equations (9) and (10), respectively: qe = KF Ce 1/n

SC R

(9)

1

ln KF + n ln Ce

ln qe =

(10)

U

where the Freundlich constants, KF (mg/g) and n, are associated with the adsorption capacity and intensity,

N

respectively.

A

Fig. 5A shows the fitting results of the adsorption data to the non-linear forms of Langmuir and Freundlich

M

models, while those fitted to the linear forms are shown in Fig. 5B and S2 (Supporting Information), respectively. Table 2, which shows the isotherm parameters obtained from these figures, indicates that the

ED

data fits better to the Langmuir model than the Freundlich model, as the R2 values are higher and the Sy.x (the standard error of estimate) values are lower for the former model in both the linear and non-linear

PT

fittings. The RL values fall between 0 and 1, suggesting that Cr(VI) adsorption by PPy–mPD is favourable for all studied temperatures. The maximum adsorption capacity of PPy–mPD for Cr(VI) removal from water is

CC E

relatively high, when compared to PPy composites reported in literature, as presented in Table 3 [16– 21,23,30–32,38,39]. The high maximum adsorption capacity may be associated with the large BET surface

A

area and small particle size after incorporating mPD into the PPy.

17

800 1.2 700 1.0

Ce/qe / g/L

600

qe / mg/g

500 400 300 200

15 °C 25 °C Langmuir model fit

100

15 °C 25 °C 35 °C 45 °C

0.8 0.6 0.4 0.2

35 °C 45 °C Freundlich model fit

0.0 0

50

100

150

200

250

300

350

400

0

450

50

100

150

200

250

Ce / mg/L

Ce / mg/L 2.0

350

400

SC R

1.6

300

IP T

0

ln Kc

1.2

0.8

0.0032

0.0033

0.0034

0.0035

M

A

1/T / 1/K

N

0.0031

U

0.4

Fig. 5. (A) Adsorption equilibrium isotherms for Cr(VI) removal by PPy–mPD polymer, and fits to non-linear

ED

Langmuir and Freundlich isotherm models. (B) Fit of data to the linear form of the Langmuir isotherm model.

A

CC E

PT

(C) Plot to obtain thermodynamic parameters for Cr(VI) adsorption by PPy–mPD.

Table 2. Langmuir and Freundlich isotherm parameters for Cr(VI) adsorption by PPy–mPD

18

PT

CC E

A

35 °C

45 °C

526.3 0.2135 0.01748 0.9991

588.2 0.2464 0.01519 0.9988

666.7 0.375 0.01005 0.9993

491.7 0.7078

555.7 0.8628

645.5 0.1239

10.21 0.1155

13.62 0.15

13.39 0.1239

469.0–514.5 0.4504–0.9652

525.3–586.0 0.5284–1.197

615.6–675.3 0.7483–1.301

10 0.9649 7816 27.96

10 0.9554 13519 36.77

10 0.974 11590 34.04

12

12

196.1 5.32 0.8555

220.3 5.11 0.8150

257.4 4.94 0.7577

235.7 6.891

267.4 6.727

315.8 6.725

21.29 0.8848

26.37 0.9493

33.66 1.078

188.3–283.1 4.919–8.862

208.7–326.2 4.611–8.842

240.8–390.8 4.323–9.127

10 0.9077 19588 44.26

10 0.8881 33904 58.23

10 0.8528 65611 81

12

12

12

SC R

N A 19

IP T

25 °C

12

ED

Langmuir Linear qm 384.6 b 0.3171 RL 0.01186 R2 0.9997 Non-linear Best-fit values qm 373.3 b 0.8319 Std. Error qm 5.179 b 0.1034 95 % Confidence qm 361.8–384.8 Intervals b 0.6016–1.062 Goodness of Fit Degrees of Freedom 10 R² 0.9749 Absolute Sum of 2304 Sy.x 15.18 Squares Number of points Analysed 12 Freundlich Linear KF 164.4 n 6.26 2 R 0.8372 Non-linear Best-fit values KF 194.5 n 8.089 Std. Error KF 18.44 n 1.225 95 % Confidence KF 153.4–235.6 Intervals n 5.359–10.82 Goodness of Fit Degrees of Freedom 10 R² 0.8744 Absolute Sum of 11538 Sy.x 33.97 Squares Number of points Analysed 12 Units: qm: mg/g, b: L/mg, KF: mg/g

U

Temperature 15 °C

M

Isotherm model

Table 3. Comparison of the Cr(VI) adsorption capacity of PPy–mPD with other PPy composites Initial pH 4.2 2

Temperature (°C) 30 25

Reference [20] [30]

217.39 224

2 2

25 -

[16] [23]

176.7 227 185.5 180.44 31.64 302 61.80

2 2 2 5 2 2 2

30 25 30 30 25 25

[18] [19] [17] [31] [32] [39] [38]

84.7 526.3

3.5 2

25 25

SC R

IP T

Qm (mg/g) 78.61 303

[21] This study

U

Adsorbent Chitosan/PPy PPy/ 2,5-diaminobenzene sulfonic acid PPy/glycine PPy/4-amino-3hydroxynaphthalene-1-sulfonic acid PPy/aspartic acid PPy/polyaniline PPy/threonine PPy nanoclusters PPy/ titanium(IV)phosphate PPy/sepiolite nanofibers PPy/polyacrylonitrile core/shell nanofiber mat Corncob-core micro-sheets/PPy PPy–mPD

N

3.2.4. Thermodynamic parameters

A

The Gibbs free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) in Table 4 can be used

M

to characterise the thermodynamics of Cr(VI) adsorption by PPy–mPD. Their values were determined from the isotherm data by using equations (11) and (12) below:

e

∆S° R

-

∆H° RT

(12)

PT

lnKc =

(11)

ED

q

∆G° = – RTlnKc = − RTln(m Ce )

where Kc (L/mol), T (K), m (g/L), and R (J/mol/K) are the thermodynamic equilibrium constant, absolute

CC E

temperature, the adsorbent dose, and the ideal gas constant, respectively. ΔH° and ΔS° were calculated from the slope and intercept in the plot of ln Kc against 1/T in Fig. 5C, respectively. The negative ΔG° value for all temperatures suggested that Cr(VI) adsorption by PPy–mPD was spontaneous [40]. ΔG° also became

A

increasingly negative at higher temperatures, indicating that the adsorption Cr(VI) by PPy–mPD was more favourable at increased temperatures [40]. The positive ΔH° value confirmed that the adsorption process was endothermic [40]. The positive ΔS° value means that PPy–mPD has a high affinity for Cr(VI) and an increased randomness at the Cr(VI)–PPy–mPD interface. The latter may be due to the removal of solvent (water) molecules surrounding the hydrated Cr(VI) anions in the solution during adsorption [40].

20

Table 4. Thermodynamic parameters for Cr(VI) adsorption by PPy–mPD.

∆G° (kJ/mol) -0.530 -1.868 -3.207 -4.545

Temperature (°C) 15 25 35 45

∆H° (kJ/mol) 38.03

∆S° (kJ/mol/K) 0.1338

IP T

3.2.5. Adsorption kinetics

The effect of contact time on the adsorption of Cr(VI) by PPy–mPD was studied for four initial Cr(VI)

SC R

concentrations (25, 50, 75, and 100 mg/L), and the results are shown in Fig. 6A. Equilibrium was reached within 60, 150, 240, and 300 min, respectively. The kinetic data were fitted to the pseudo-first-order model

U

and pseudo-second-order model, shown in equations (13) and (14), respectively:

(13)

qt =

k2 q2e t 1+ k2 qe t

N

qt = qe ( 1- exp-k1t )

A

(14)

M

where qe (mg/g) is the equilibrium adsorption capacity, qt (mg/g) is the adsorption capacity at time t (min), k1 (1/min) is the pseudo-first-order rate constant, and k2 (g/mg/min) is the pseudo-second-order rate

and (16), respectively:

(15) (16)

CC E

t 1 1 = +q t qt k2 q2e e

k1 t 2.303

PT

log(qe - qt )= log qe -

ED

constant. Equations (13) and (14) can be further expressed in linear forms as presented by equations (15)

Fig. 6A presents the fit of the kinetic data to the nonlinear form of both models, while Fig. S3 and Fig. 6B show the fit to the linear forms, respectively. The kinetic parameters obtained from the fitting are given in

A

Table 5. The R2 values for the pseudo-second-order model were higher than those for the pseudo-first-order model. Moreover, the qe values for the pseudo-second-order fitting were similar to the experimental maximum adsorption capacity. These observations suggested that Cr(VI) removal by PPy–mPD followed pseudo-second-order kinetics, which is similar to previous studies of Cr(VI) adsorption by PPy-based adsorbents [16–20,23,30,32,38,39]. Table 5 also showed that the k2 values decreased as the initial Cr(VI)

21

initial concentration was increased. This suggests that the adsorption process is faster at lower initial concentrations. At low concentrations there is a higher degree of freedom for the distribution of the adsorbate ions onto the surface of the adsorbent. Liquid/solid adsorption typically involves three processes: film diffusion, intraparticle diffusion, and mass action [41]. Since mass action is usually a rapid process, the rate-limiting step is controlled by film diffusion

IP T

and/or intraparticle diffusion, which could be described by adsorption diffusion models [41]. The Weber– Morris adsorption diffusion model, presented in equation (17), was used to study the different steps in Cr(VI) adsorption by PPy–mPD. qt = ki t0.5 + C

SC R

(17)

where ki (mg/g/min0.5) is the intraparticle diffusion rate constant and C (mg/g) is the intercept that is related

U

to the thickness of the film layer [41,42]. Fitting of the kinetic data at different initial Cr(VI) concentrations

N

to the Weber–Morris model is presented in Fig. 6C. The sole rate limiting step is intraparticle diffusion when

A

the Weber–Morris plot is a straight line passing through the origin [41]. However, the plots in Fig. 6C do not

M

pass through the origin and can be separated into three straight line segments with varying slopes. Intraparticle diffusion is therefore not the rate-limiting step of Cr(VI) adsorption by PPy–mPD. Instead, the

ED

three linear sections in Fig. 6C represent the different adsorption processes which occur at different rates. The first section is due to film diffusion, which occurs faster than intraparticle diffusion (the second section

PT

of the plots) [42,43]. The third section represents the equilibrium stage of adsorption [43]. Table 6 presents

CC E

the ki and C values obtained for the three linear sections of each plot.

Table 5. Kinetic parameters for Cr(VI) adsorption by PPy–mPD .

A

Kinetic model

Pseudo-first-order Linear qe k1 R2 Non-linear Best-fit values qe k1

Initial Cr(VI) concentration 25 mg/L 50 mg/L

75 mg/L

100 mg/L

5.94 0.0152 0.9389

23.93 0.0170 0.9596

42.68 0.0143 0.9428

76.97 0.0138 0.8733

48.32 0.4565

95.09 0.2651

138.5 0.2098

178.9 0.1801

22

2.046 0.04268

2.77 0.02859

3.738 0.02439

45.86–50.79 0.2255–0.6875

90.88–99.29 0.1774–0.3529

132.8–144.2 0.1510–0.2686

171.2–186.5 0.1299–0.2302

26 -0.0402 882.1 5.825 28

26 0.622 2389 9.585 28

26 0.7649 4251 12.79 28

26 0.7819 7589 17.08 28

50.25 0.012572 31.74603 1.0000

100 0.003195 31.94888 1.0000

147.06 0.001531 33.11258 0.9999

IP T

49.55 0.01806

98.11 0.004959

143.7 0.002502

186.6 0.001561

1.228 0.006405

1.873 0.001018

2.325 0.0003879

3.117 0.0002329

47.03–52.08 0.004895–0.03123

94.26–102.0 0.002867–0.007051

139.0–148.5 0.001705–0.003300

180.2 -193.0 0.001082–0.002040

26 0.8741 2276 9.357 28

26 0.8878 3905 12.26 28

U

N

26 0.7525 1564 7.756 28

196.08 0.000754 28.98551 0.9996

CC E

PT

ED

M

Goodness of Fit Degrees of Freedom 26 R² 0.136 Absolute Sum of Squares 732.7 Sy.x 5.309 Number of points analysed 28 Units: qe: mg/g, k1: 1/min, k2: g/mg·min, h0: mg/g/min

SC R

1.2 0.1124

A

Std. Error qe k1 95% Confidence Intervals qe k1 Goodness of Fit Degrees of Freedom R² Absolute Sum of Squares Sy.x Number of points analysed Pseudo-second-order Linear qe k2 h0 R2 Non-linear Best-fit values qe k2 Std. Error qe k2 95 % Confidence Intervals qe k2

Table 6. Intraparticle diffusion model parameters for Cr(VI) adsorption by PPy–mPD

A

Cr(VI) initial concentration (mg/L) 25 50 75 100

ki (mg/g/min0.5) First Second

Third

C (mg/g) First

Second

Third

4.9622 9.552 15.093 23.435

0.0697 0.1292 0.512 0.8316

27.448 43.935 53.694 56.316

43.371 85.671 114.66 142.61

48.62 96.797 136.26 176.23

0.6492 1.0113 2.514 3.2684

23

220

10

200 180

8

160 -1

t/qt / min.g.mg

120 100 80 60 40 20

25 ppm 50 ppm Pseudo-1st-order model

0

75 ppm 100 ppm Pseudo-2nd-order model

25 ppm 50 ppm 75 ppm 100 ppm

6

4

2

0

-20 0

50

100

150

200

250

300

350

400

0

450

100

200

t / min

t / min 200 180

140

qt / mg/g

400

SC R

160

300

IP T

qt / mg/g

140

120 100 80

U

60 40

25 ppm

50 ppm

0 0

5

75 ppm

N

20

10 0.5

15

20

0.5

A

t / min

100 ppm

M

Fig. 6. (A) Effect of contact time on Cr(VI) removal by PPy–mPD for different initial concentrations, and fits to pseudo-first-order and pseudo-second-order kinetic models. (B) Fit of data to the linear form of the

ED

pseudo-second-order kinetic model. (C) Intra-particle diffusion model for adsorption of Cr(VI) by the PPy–

CC E

PT

mPD.

3.2.6. Activation energy of adsorption

A

The activation energy (Ea) for Cr(VI) adsorption by PPy–mPD was obtained from the temperature-dependent kinetic data by using the Arrhenius equation: ln k2 = ln A-

Ea RT

(18)

where k2 (g/mg/min) is the rate constant for the pseudo-second-order kinetic model, T (K) is the temperature, A is the frequency factor (g/mg/min), and R is the ideal gas constant (kJ/mol/K). The data in Fig. 7A were fit to the linear form of the pseudo-second-order kinetic model to obtain the k2 values at each

24

temperature, as shown in Table S1 (Supporting Information), showing that the rate of Cr(VI) removal by PPy–mPD increased with temperature. The corresponding Ea value was calculated to be 21 kJ/mol from the slope of the Arrhenius plot, as shown in Fig. 7B. This value suggests that Cr(VI) was adsorbed by PPy–mPD through chemisorption [40]. 220 -6.25

200 180

120 100 80

15 °C 25 °C 35 °C 45 °C

60 40 20 50

100

150

200

250

300

350

400

-7.00 -7.25 -7.50

0 0

-6.75

-7.75 0.0031

450

0.0032

SC R

lnk2 / mg/L/min

qt / mg/g

140

IP T

-6.50

160

0.0032

0.0033

0.0033

0.0034

0.0034

0.0035

0.0035

1/T / 1/K

U

t / min

N

Fig. 7. (A) Effect of temperature on the rate of Cr(VI) removal by PPy–mPD polymer for Cr(VI) initial

3.2.7. Effect of co-existing ions in solution

M

A

concentration of 100 mg/L. (B) Plot to determine the activation energy.

ED

In addition to high concentrations of Cr(VI), industrial wastewater from mining, chrome plating, tanning, etc. also contains many other ions. The effect of selected ions in solution on the removal of Cr(VI) by PPy–

PT

mPD was therefore studied. Fig. 8A shows the effect of Cu2+, Zn2+, Ni2+, and Co2+ cations and Cl–, NO3–, PO43–

CC E

, and SO42– anions. The four cations did not significantly affect Cr(VI) removal. The surface of PPy–mPD is positively charged at pH 2 due to protonation of amino groups and therefore does not attract and adsorb cations, as confirmed by ICP–MS analysis. Fig. 8A shows that Cl–, NO3– and PO43– anions also had a negligible

A

effect on the removal of Cr(VI). The removal efficiency was slightly affected by SO42– ions, decreasing to 93% when the SO42– concentration was 100 mg/L. Cl– and NO3– are low-affinity ligands that are adsorbed through the formation of weak outer sphere complexes, and they did not interfere with Cr(VI) adsorption by PPy– mPD [44]. In contrast, SO42– forms both inner sphere and outer sphere complexes. Therefore, it could occupy some of the adsorption sites and slightly reduce the Cr(VI) removal efficiency of PPy–mPD [45]. PO43– ions may not be adsorbed at the same sites as Cr(VI), and therefore did not reduce the removal Cr(VI) by PPy–

25

mPD [45]. Also, PPy-based adsorbents have much lower adsorption capacities for PO43– ions than other polymer based adsorbents such as polyacrylonitrile (PAN) nanofibers with immobilized La(OH) 3 nanorods [46–48]. The high selectivity of PPy–mPD for Cr(VI) can be attributed to the high positive redox potential of HCrO4– anions, the main form of Cr(VI) in solutions at low pH [8]. HCrO4– anions are easily reduced by PPy– mPD to Cr(III). Cl–, NO3–, SO42–, and PO43– anions have much weaker oxidising ability than HCrO4– anions, and

IP T

therefore, they do not lower the Cr(VI) removal efficiency.

3.2.8. Desorption studies

SC R

Desorption studies were used to assess the potential of recycling and reusing of PPy–mPD for Cr(VI) removal from water. Cr(VI) desorption from loaded PPy–mPD (0.025 g) was carried out with 0.1 M NaOH or 0.1 M

U

NH4OH solutions. After desorption, 2 M HCl was used to regenerate the PPy–mPD and desorb Cr(III). Four

N

treatment cycles were carried out. As shown in Fig. 8B, the removal efficiency of PPy–mPD decreased after

A

each cycle for both NaOH and NH4OH solutions. This may be due to over-oxidation of the PPy–mPD polymer

A

CC E

PT

ED

M

by Cr(VI) or degradation of the polymer chains as a result of the desorption and regeneration treatments.

26

(A)

80

60

40

20

0

None Co2+

2+

2+

Cu

Zn

Ni

2+

SO4

2-

Cl

-

PO3

4-

NO3

-

Multi

20 mg/L

40 mg/L

60 mg/L

80 mg/L

100

SC R

Ions coexisting with Cr(VI) in solution

IP T

% Cr(VI) removal

100

100 mg/L

(B)

U

0.1 M NaOH 0.1 M NH4OH

N A

60

40

M

% Cr(VI) removal

80

0 1

ED

20

2

3

4

CC E

adsorption cycles

PT

Cycle Fig. 8. (A) Effect of selected co-existing ions on the removal of Cr(VI) by PPy–mPD polymer and (B)

4. Conclusions

A

In this work we used a simple and effective oxidative polymerisation method for the synthesis of a novel PPy-mPD adsorbent. The incorporation of mPD into PPy backbone resulted in extremely high and selective adsorption capacity for Cr(VI) of 526 mg/g at 25 °C and at pH 2, a value that exceeds those of many reported PPy-based adsorbents. The observed adsorption efficiency was attributed to the presence of amino groups and a larger surface area of PPy-mPD. The thermodynamic parameters obtained from the isotherm data

27

confirmed that Cr(VI) removal by PPy–mPD was spontaneous and endothermic. Cr(VI) removal by PPy–mPD at pH 2 was proposed to involve adsorption of Cr(VI) anions by protonated amino groups on the PPy–mPD surface and reduction of Cr(VI) to Cr(III) by the adsorbent. The kinetic data for Cr(VI) removal by PPy–mPD fit the pseudo-second-order model well. The scale of the activation energy (21 kJ/mol) suggested chemisorption of Cr(VI) by PPy–mPD. Selected cations (Cu2+, Zn2+, Ni2+ and Co2+) and anions (Cl–, NO3–, PO43–

IP T

, and SO42–) did not significantly affect Cr(VI) removal by PPy–mPD. However, the removal efficiency of PPy– mPD for Cr(VI) decreased after each treatment cycle. Therefore, we can conclude that the modification of PPy with mPD resulted in a novel adsorbent having a high selectivity and adsorption capacity for Cr(VI).

SC R

However, further studies on recyclability are necessary to assess the potential application of this material

U

for treating Cr(VI)-containing wastewater.

N

Acknowledgements

A

The authors would like to acknowledge the National Research Foundation (NRF), Department of Science

M

and Technology (DST), and the Council for Scientific and Industrial Research (CSIR) of South Africa for financial support. The DST-CSIR National Centre for Nanostructured Materials characterisation facility is

PT

Supplementary data

ED

acknowledged for support and assistance with materials characterisation and analyses.

CC E

Effect of adsorbent dose on Cr(VI) adsorption by PPy–mPD, fit of data to the linear form of the Freundlich isotherm and the pseudo-first-order kinetic models, and kinetic parameters for Cr(VI) adsorption by PPy–

A

mPD.

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M. Ates, T. Karazehir, A. Sezai Sarac, Conducting polymers and their applications, Curr. Phys. Chem. 2 (2012) 224–240.

[2]

T.K. Das, S. Prusty, Review on conducting polymers and their applications, Polym. Plast. Technol. Eng. 51 (2012) 1487–1500.

28

[3]

G. Inzelt, M. Pineri, J.W. Schultze, M.A. Vorotyntsev, Electron and proton conducting polymers: recent developments and prospects, Electrochim. Acta. 45 (2000) 2403–2421.

[4]

Y. Huang, J. Li, X. Chen, X. Wang, Applications of conjugated polymer based composites in wastewater purification, RSC Adv. 4 (2014) 62160–62178.

[5]

H.N. Muhammad Ekramul Mahmud, A.K. Obidul Huq, R. B. Yahya, The removal of heavy metal ions

IP T

from wastewater/aqueous solution using polypyrrole-based adsorbents: A review, RSC Adv. 6 (2016) 14778–14791. [6]

Independent Environmental Technical Evaluation Group (IETEG), Chromium (VI) Handbook, CRC

[7]

SC R

Press, 2004.

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