MAA)

Research Article pubs.acs.org/journal/ascecg

High-Performance Hg(II) Removal Using Thiol-Functionalized Polypyrrole (PPy/MAA) Composite and Effective Catalytic Activity of Hg(II)-Adsorbed Waste Material Raghunath Das,† Somnath Giri,† Anthony M. Muliwa,‡ and Arjun Maity*,†,§ †

Department of Civil and Chemical Engineering, University of South Africa (UNISA), Florida Campus, Cnr. Christiaan De Wet and Pioneer Ave., Florida Park, South Africa ‡ Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Private Bag X680, Pretoria, South Africa § DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, 1-Meiring Naude Road, Pretoria 0001, South Africa S Supporting Information *

ABSTRACT: Problems related with highly toxic mercury emissions from industrial effluents are one of the great concerns in the world of environmental science and technology. The primary aim of this present work is the remediation of hazardous pollutant Hg (II) from aqueous medium via a thiol-functionalized (mercaptoacetic acid) conducting polypyrrole (PPy/MAA) composite. The synthesized composite exhibited high Hg(II) adsorption capacity as the incorporated mercapto functionality plays a vital role for the strong binding affinity toward Hg(II) ions. To understand the adsorption properties of the developed polymer composite, a series of batch adsorption experiments were performed by altering the adsorption parameters. A maximum adsorption capacity (qmax) of 1736.8 mg/g at 25 °C was obtained using the Langmuir isotherm. The adsorption data showed better fitting to the pseudo-second-order rate equation for Hg(II) adsorption. A plausible adsorption mechanism is suggested on the basis of XPS results of major elements present in the adsorbent. To apply to catalytic reactions, the use of waste-derived mercury-adsorbed material (termed as PPy-MAA/Hg (II)) is also addressed here for further application of organic transformation. Here, we described the catalytic reaction with phenylacetylene in the presence of 5 mol % PPy-MAA/Hg (II) catalyst at 90 °C for 6 h. This method provides a straightforward formation of acetophenone in 55% yield. Thus, PPy/MAA not only effectively eliminates toxic Hg(II) ions from water but also is successfully applied for the catalytic organic transformation after adsorption. KEYWORDS: Thiol-functionalization, PPy/MAA composite, Adsorption, Mercury, Catalytic activity

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biological systems then other chemical forms.7 Trace amounts of Hg(II) ions can affect the central nervous system and cause serious damage to kidney and lung tissues.8 The United States Environmental Protection Agency (EPA) has suggested 0.002 mg/L (2 ppb) to be the maximum allowable limit for Hg(II) in drinking water.9 Therefore, it is imperative to eliminate even trace amounts of serious pollutants such as mercury from wastewater before discharging effluents into the environment in the interest of human safety. Several techniques have been developed to reduce the levels of mercury in contaminated industrial effluents, and these include chemical precipitation,10 ion exchange,11 solvent extraction,12 membrane filtration,13 coagulation,14 adsorption,15−17 and chemical reduction.18,19 Adsorption is regarded as a superior

INTRODUCTION Water pollution with highly toxic heavy metals such as Hg(II), Cr(VI), As(III), and Pb(II) has been a major environmental issue for several decades.1,2 Mercury is recognized as a one of the most hazardous metal ions relative to other toxic metals because of its poisonous and harmful effects on human health.3,4 Burning of petrochemical oil and coal, discharge of waste effluents from mining plants, printing industries, cement processings and manufacture of batteries are the main sources of mercury pollution in waterways. Mercury exists in several physical and chemical forms: elemental mercury (Hg0), divalent mercury (Hg2+), and monomethyl mercury cation (CH3Hg+).5 Divalent inorganic mercury (Hg2+) has been found to be a serious contaminant due to its high solubility and stability. It is reported that divalent mercury (Hg2+) may be easily transferred into higher toxic organometallic forms via biological methylation.6 Alkylmercury (CH3Hg+) has received considerable attention because of its low biodegradability and greater toxicity to © 2017 American Chemical Society

Received: February 14, 2017 Revised: June 21, 2017 Published: July 17, 2017 7524

DOI: 10.1021/acssuschemeng.7b00477 ACS Sustainable Chem. Eng. 2017, 5, 7524−7536

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ACS Sustainable Chemistry & Engineering

Py monomer at room temperature.34 In a typical reaction procedure, 0.8 mL (0.0114 mol) of Py monomer was injected into 60 mL of aqueous reaction mixture containing 1.6 mL (0.023 mol) of MAA in a 250 mL conical flask at 25 °C. The reaction mixture was stirred for 30 min to ensure complete dissolution, followed by the dropwise addition of 20 mL of 6.8 g (0.03 mol) of APS (oxidant) solution to initiate the polymerization reaction. During polymerization, the solution changed from colorless to dark gray indicating the formation of the PPy adduct. The reaction mixture was left for 6 h at ambient temperature to complete polymerization, after which the precipitate obtained was filtered and washed with ultrapure water followed by acetone several times (10 times). The resulting PPy/MAA composite was subsequently dried at 60 °C under vacuum. Characterization Techniques. The physicochemical properties of synthesized adsorbent before and after adsorption were determined by several characterization techniques. The surface morphology and elemental microanalysis (EDS) were analyzed using field emission scanning electron microscopy (Auriga FESEM, Carl Zeiss, Germany). A JEOL JEM-2100 was used for high resolution transmission and scanning transmission electron microscopy (HR-TEM; STEM). X-ray diffraction (XRD) spectra were recorded using a PANalytical X’Pert PRO-diffractometer using Cu Kα radiation (wavelength, λ = 1.5406 Å) at 40 kV and 40 mA. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra were obtained using a Spectrum 100 spectrometer (PerkinElmer, USA). Raman spectra of the composite (PPy/MAA) and Hg (II)-adsorbed composite material were acquired using Raman spectroscopy, PerkinElmer, USA. The binding energy of the elements (1s C, 1s O, 2p S, 4f Hg) was recorded by X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra device (Kratos, UK) using monochromatic Al Kα radiation. The Hg(II) concentration remaining in the medium after adsorption was determined using inductively coupled plasma-optical emission spectroscopy (ICPE-9000, Shimadzu, Japan). An Agilent VNMRS Premium Shield 600 MHz spectrometer was used to record NMR spectra of the pure compound (acetophenone), and chemical shifts (δ) were reported in parts per million (ppm) using TMS as the reference signal. Aluminum plates coated with Kieselgel 60 F-254 silica gel were used for thin layer chromatography (TLC), and visualization was done under UV light. The pure product was isolated through column chromatography, performed over 100−200 mesh silica gel using ethyl acetate/hexane solvent mixtures as the eluent. The data of NMR splitting patterns were presented as singlet (s) and multiplet (m). Adsorption Studies. A batch adsorption technique was applied to investigate the adsorption behaviors of Hg(II) from an aqueous solution using a PPy/MAA composite within the range from pH 2 to 8 at 25 °C. Mercury nitrate was used to prepare an aqueous stock solution of 1000 ppm of Hg(II) in the presence of 0.2 (M) HNO3. The desired Hg(II) concentration for the adsorption studies was obtained by diluting the stock solution using water. To explore the adsorption behavior, 20 mg of as-prepared PPy/MAA composite was loaded into 100 mL capped glass bottles with 50 mL of the target concentration of metal solutions started from 100 to 800 mg/L. The mixture in the glass bottles was agitated on a thermostatic incubator shaker (Separation Scientific, South Africa) at 200 rpm for 24 h. The adsorption temperature was maintained constant at 25 ± 0.2 °C, and the pH of the adsorption medium was adjusted and maintained at the target levels during the course of the experiment using 0.1 (M) HNO3 or NaOH. After 24 h, aliquots were withdrawn from the adsorption medium and filtered immediately through 0.45 μm membrane filters, and the remaining concentration of metal ions in the adsorption medium was determined using ICP-OES. The amount of adsorbed mercury on the polymer composite was determined by subtracting the residual Hg(II) concentration in the medium from its initial concentration. The adsorption capacity (qe) was calculated with the eq 1.

technique for wastewater purification because it is simple to setup, a wide range of adsorbents are available, and it is cost effective. Recently research efforts have been directed toward the development of new classes of chemically functionalized, low-cost adsorbents.20,21 Surface modification of adsorbents with sulfur, nitrogen, and oxygen functional groups have shown high binding affinity toward mercury ions. Current research has focused on development of adsorbents with sulfur-based ligands,22−26 such as thiol, thiourea, and thioether groups. These groups are known to enhance the removal efficiency of mercury ions from contaminated wastewater through soft Lewis acid-based interaction.27 Conducting polymer-based composite materials are emerging as promising adsorbents to purify the contaminated wastewater, and salient among these is polypyrrole (PPy). PPy is a wellknown conducting polymer that offers advantages including low cost, ease of polymerization, high electrical conductivity, environmental stability, and ease of functionalization.28,29 A survey of studies on the removal of Hg (II) shows the efficacy of PPy as an effective material for uptake of toxic mercury from water. Polymer-supported adsorbents including PPy matrixes are used as effective materials for separation of toxic mercury from a water medium. In one study, Chandra et al.9 obtained an Hg(II) adsorption capacity of 980 mg/g using a PPy-reduced graphene oxide composite. Sun et al.30 and Niu et al.31 obtained a mercury adsorption capacity of 1.43 mmol./g (286.8 mg/g) and 1.68 mmol./g (337 mg/g) using a silica gel-supported polymer composite. Moreover, Cai et al.32 and Kadirvelu et al.33 found an adsorption capacity of 41 and 55.6 mg/g using activated carbon for toxic Hg(II) ions removal, respectively. However, the superior performance of PPy over the other adsorbents comes with some limitations, in that this polymer has lower chelating functionality and lower surface area and pore volume. Notwithstanding, PPy is still a promising adsorbent where shortcomings can be overcome through optimal chemical modification with desirable functionality, resulting in many-fold improvement in adsorption capacity over the parent material. A further challenge, not limited to PPy, is the safe disposal of the adsorbed Hg (II) after it has been successfully recovered from contaminated water in order to avoid creating a secondary environmental pollution. Therefore, there is a need to increase not only the efficiency of the adsorbent composites, but also the capability to safely dispose of the pollutants recovered. Against this background, the present study was undertaken to develop PPy-based adsorbent-bearing thiol functional groups for separation of mercury ions from water resources. The study was initiated by the innovative idea to incorporate the mercaptan/ thiol group into the adsorbent for selective binding of mercury. PPy was modified using mercaptoacetic acid (MAA) based on greater kinetics and static adsorption capacity. One of the major concerns to us is the reusability of the mercury-adsorbed waste material. The recovery of the mercury adsorbed from contaminated waste was investigated following the organic catalytic transformation of phenylacetylene into acetophenone.

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EXPERIMENTAL SECTION

Materials. Pyrrole (Py) monomer and thioglycolic acid (mercaptoacetic acid; MAA), mercury(II) nitrate (Hg(NO3)2), ammonium persulfate (APS)m and phenylacetylene were all purchased from Sigma-Aldrich (U.S.A). Only analytical grade solvents (acetone, ethyl acetate, and hexane) and ultrapure water were used for this study. Synthesis of Mercaptoacetic Acid-Containing Polypyrrole (PPy/MAA) Composite. The thiol-functionalized polypyrrole composite (PPy/MAA) was prepared via chemical oxidative polymerization of the

qe (mg/g) =

(C0 − Ce)V W

(1)

where qe is adsorption capacity of PPy/MAA composite (in mg/g); C0 and Ce define the initial and equilibrium Hg(II) concentrations (mg/L) in the adsorption medium, respectively. The symbols V and W describe 7525


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ACS Sustainable Chemistry & Engineering the volume of tested metal ion solution (L) and weight of PPy/MAA (g), respectively. Four different initial metal ions concentrations (25, 50, 100, and 200 mg/L) have been selected for a kinetics study of Hg(II) adsorption. An amount of 0.4 g of adsorbent was placed into 1 L of experimental Hg(II) solution at optimize adsorption conditions (200 rpm, 25 °C, and pH 5.5). At a certain time intervals, 10 mL aliquots were sampled from the adsorption medium and filtered through 0.45 μm membrane filters. The quantity of Hg(II) present in the PPy/MAA composite per unit mass at time t (qt) was analyzed using eq 2. qt (mg/g) =

(C0 − C t)V W

Hg(II) adsorption (Figure 2f). The distribution of mercury in the adsorbed material is clearly shown in Figure 2g and jointly with sulfur in Figure 2h. FT-IR Spectral Analysis. To investigate functional groups present in the PPy/MAA composite and the change in surface functionality of the PPy/MAA composite after Hg(II) adsorption, FT-IR spectra in between 4000 and 600 cm−1 wavenumbers were recorded. Figure 3 shows the vibrational peaks at 3220, 2913, and 1696 cm−1 of pure PPy/MAA characteristic of O−H stretching, C−H stretching, and vibration of the carbonyl group of mercaptoacetic acid, respectively. Infrared spectra described the symmetrical and asymmetrical PPy ringstretching modes at 1544 and 1480 cm−1, respectively,35,36 as shown in Figure 3. The stretching band at 2636 cm−1 is identified as a S−H vibration which provides strong evidence supporting the presence of mercaptan groups in the polymer composite.37,38 In addition, the peak at 1255 cm−1 was assigned to the C−O stretching frequency of MAA and peaks at 1370 and 930 cm−1 to C−N stretching and C−H deformation vibrations, respectively. These features indicated that MAA had been successfully incorporated into the PPy polymer matrix. The IR spectrum (Figure 3) of post-adsorbed material (PPy-MAA/Hg(II)) was also studied to explain the adsorption mechanisms with the help of functionality change after Hg(II) adsorption. The PPy-MAA/Hg(II) material showed two major characteristic changes in the FT-IR spectrum. It was found that the O−H stretching region almost disappeared, and a broad, less intense peak was observed in the infrared region at 3274−3115 cm−1. Similarly, the strong, sharp peak of S−H vibrations at 2636 cm−1 disappeared, and a new absorption band was observed in the 2636−2456 cm−1 range. The IR peak shifts to lower regions, and the significant decrease in the intensity of the peaks is indicative of the strong binding of Hg(II) with thiol and acid functionality of the PPy/MAA composite. It is proposed that the changes observed in FT-IR spectra arise from the fact that mercury is a soft metal, with a strong tendency to bind the soft bases like thiol. Raman Study. Presence of thiol functional groups in the PPy/MAA composite was further investigated by Raman spectroscopy. The Raman spectra within 200−3000 cm−1 for the PPy/MAA composite and Hg-adsorbed PPy/MAA are shown in Figure 4. The Raman spectra in Figure 4 show the Raman band at 1376 cm−1 for the C−N stretching vibration of the polypyrrole ring.39 The peaks at 1591 cm−1 of PPy/MAA and 1596 cm−1 of (PPy-MAA/Hg(II)) are assigned to the CC stretching vibration.39 Before adsorption, PPy/MAA exhibits a Raman peak at 2532 cm−1 due to −S−H stretching.40 However, after Hg adsorption, the composite exhibits a band in the 2520−2665 cm−1 region for the Hg-adsorbed thiol group (−SH stretching frequency). The −SH peak broadening indicates the interaction between the thiol group and adsorbed Hg(II) ions. Powder XRD Pattern. Figure 5 depicts XRD spectra from the PPy/MAA composite before and after (PPy-MAA/Hg(II)) adsorption. The lattice pattern of the PPy/MAA composite exhibited a broad diffraction peak at 2θ = 20.5° (Figure 5a) due to the scattering from the polypyrrole chain at the interplanar spacing. The corresponding XRD pattern of pure PPy/MAA (Figure 5a) illustrates the amorphous state of the synthesized composite material. In Figure 5b, the Hg(II)-adsorbed PPy/MAA composite displays a broad peak within 2θ = 31−35°. The XRD peak observed at 2θ = 32.4 matches closely the (011) plane of HgO.41 It is possible that at the time of Hg(II) adsorption the

(2)

where Ct expresses Hg(II) concentration (mg/L) in the adsorption medium at any time (t); other symbols have the same meaning as above. Catalytic Performances of Waste-Derived PPy-MAA/Hg(II) Material as Catalyst. To investigate the catalytic activity, the PPy/MAA composite was treated with a 200 mg/L aqueous solution of the Hg(II) ion at pH 5.5 for 24 h at 25 °C, and the mercury-adsorbed material was washed thoroughly with ultrapure water and dried overnight at room temperature under vacuum. The weight percentage of Hg(II) loaded into the composite material (PPy/MAA) was calculated as detailed above. The adsorbed mercury material was subsequently used as a catalyst for the conversion of phenylacetylene to acetophenone. The following were loaded into a 25 mL round-bottomed flask: phenylacetylene (1 mmol, 0.1021 g), PPy-MAA/Hg(II) catalyst (5 mol %, 0.0333 g), and water (5 mL). The flask was connected to a condenser and maintained at 90 °C for 6 h. After that, ethyl acetate (3 mL × 20 mL) was used to extract the aqueous reaction mixture. The combined organic extract was washed with 20 mL saturated solution of sodium chloride. The organic layer was treated with anhydrous MgSO4 and concentrated under vacuum using a rotary evaporator. Column chromatography was used to purify the crude residue using 100−200 mesh silica gel assisted by 5% ethyl acetate in hexane solvent mixtures as an eluent to furnish the pure product acetophenone (55% yield, 0.061 g) as a colorless liquid. Column chromatography was used to purify the resulting acetophenone from the crude reaction mixtures, using TLC plates exposed to UV light.

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RESULTS AND DISCUSSION Characterization. FE-SEM and EDX. Figure 1a and b depict the FE-SEM images of the surface morphology of the PPy/MAA composite and after Hg(II) adsorption. As shown in Figure 1a, the surface morphology of the MAA-modified PPy composite was relatively smooth with a homogeneous granular size of approximately 200 nm diameter. In contrast, the surface morphology of the used adsorbent appeared rougher, possibly revealing the accumulation of mercury on its surface (Figure 1b). EDX confirmed the presence of carbon, nitrogen, oxygen, and sulfur in PPy/MAA as expected (Figure S1a, Supporting Information) and an additional mercury peak in the adsorbed material (Figure S1b, Supporting Information). Table S1 (Supporting Information) describes the surface elemental compositions (wt %) of the adsorbent (PPy/MAA) before and after Hg(II) adsorption. HR-TEM and STEM. The nanostructure of the pure adsorbent and Hg(II)-adsorbed composite was further examined using HR-TEM. As shown in Figure 2a, the pure adsorbent had a regular, spherical shape with a diameter of 150−200 nm. However, the surface of the Hg(II)-adsorbed material appeared to be large agglomerates structures (Figure 2b) with higher diameters (250−400 nm). Aggregation of water-solvated Hg(II) ions around the adsorbent surface creates size differences of the post-adsorbed material due to formation of bigger clumps at the time of the adsorption process. Elemental mapping in STEM (Figure 2c and e) revealed the homogeneous distribution of sulfur on the PPy/MAA composite before (Figure 2d) and after 7526


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Figure 1. FE-SEM image of PPy/MAA (a) before and (b) after adsorption of Hg(II).

formation of a surface Hg−O complex with −COOH groups present in the composite material are responsible for the broadening of the peak observed at 2θ = 32.4°. BET Surface Area. Nitrogen adsorption experiments were performed to determine the surface areas (m2/g), pore diameter (nm), and pore volume (cm3/g) of the neat PPy and its composite (PPy/MAA) (Figure S2, Supporting Information). BET results showed that the surface area of the as-prepared PPy/MAA composite calculated from the N2 adsorption isotherm was 12.86 m2/g, higher than pure PPy (7.91 m2/g).34 The pore diameter was calculated with the help of the BJH method, and the cumulative pore dimeter of PPy/MAA (40.83 nm) was almost twice that of pure PPy (20.75 nm). The pore volume of the PPy/MAA composite also increased from 0.0187 to 0.0849 cm3/g relative to PPy.

Equilibrium Adsorption Studies of Mercury Ions. Influence of pH. The pH of the adsorption medium plays an important role during metal ion removal. It not only affects the dominant form of the metal ion in the adsorption medium but also changes the surface functional properties of the adsorbents. Figure S3 (Supporting Information) summarizes the adsorption behavior of the PPy/MAA composite at different pH values. The results reveal that adsorption efficiency rises from 96.5% to 99.4% with increasing pH from 2.0 to 6.0. Mercury ions exist in different Hg aqueous species42,43 such as Hg2+, HgOH+, and Hg(OH)2 depending upon the pH values of the medium. At pH < 4, mercury ions exist predominantly as Hg2+ in solution, whereas HgOH+ and Hg(OH)2 dominate at pH 4−6 and Hg(OH)2 at pH > 6. Zeta potential values at various pH gave us a clear idea about the surface charge of the adsorbent. As shown in 7527


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Figure 2. HR-TEM image of PPy/MAA (a) before and (b) after adsorption. STEM image of PPy/MAA (c) before and (e) after adsorption of Hg(II). Homogeneous distribution of S atom in PPy/MAA (d) before and (f) after adsorption. (g) Distribution of adsorbed Hg(II). (h) Combined distribution of S and Hg(II) in the adsorbent.

Figure 3. FTIR spectra of PPy/MAA and PPy-MAA/Hg(II).

Figure S4 (Supporting Information), increasing pH from 2.0 to 10.0 significantly decreases the zeta potential of PPy/MAA from −6.5 to −22.9 mV. From the results (Figure S3, Supporting

Information), the observed low removal efficiency of PPy/MAA at very low and high pH is perhaps due to (i) competition in the exchange of Hg2+, H+, or H3O+ ions from the solution to the 7528


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maximum removal of Hg(II) from solution, the amount of composite used was varied from 5 to 25 mg in 50 mL of adsorption medium with C0 = 200 mg/L at pH 5.5. The percentage of Hg(II) adsorbed increased proportionally with adsorbent mass, and these results are shown in Figure S3b (Supporting Information). The surface area and active adsorption site, the two main factors on which the percentage adsorption of Hg(II) depends, significantly increase by the addition of more adsorbent onto the adsorption medium. However, after reaching equilibrium, the percentage adsorption of Hg(II) is not sufficiently affected by increasing the sorbent dosage over 20 mg. Therefore, this dosage value (0.02 g for 50 mL metal solution) was employed for further adsorption experiments. Influence of Initial Hg(II) Concentration and Static Adsorption Capacity. Initial concentration of Hg(II) in the adsorption medium also affects the removal percentage. The effect of mercury concentration on the adsorption study was investigated with a wide range of Hg(II) solutions from 100−800 mg/L under the optimized conditions of temperature (25 °C) and pH (5.5). From Figure S3c (Supporting Information), it is obvious that the amount of Hg(II) adsorption increases with Hg(II) concentration reaching equilibrium at higher Hg(II) concentrations. Initial Hg(II) concentration has a key role for enhancing the movement of metal ions from solution to the solid adsorbent surface. As a result, with increasing the initial concentration of metal ions, the interaction between adsorbate and adsorbent are also enhanced. Every adsorbent has a characteristic capacity to adsorb a fixed amount (maximum adsorption capacity) of particular ions at equilibrium.44−46 Langmuir47 and Freundlich,48 the two most popular adsorption models, were tested in an isotherm study to estimate the binding affinity of thiol-functionalized PPy/MAA to Hg(II) at concentrations ranging from 100 to 800 mg/L. The above two isotherm equations are expressed as follows:

Figure 4. Raman spectra of PPy/MAA and PPy-MAA/Hg(II).

Ce C 1 = + e qe bqm qm ln qe = ln kF +

1 ln Ce n

(3)

(4)

where qe (mg/g) represents the equilibrium quantity of Hg(II) adsorbed by PPy/MAA; Ce is equilibrium adsorbate concentration (mg/L) in solution; qm is the maximum binding capacity (mg/g); b is the Langmuir constant for monolayer adsorption (L/mg); kF is the Freundlich coefficient characteristic reflecting the adsorption capacity (mg/g); n signifies linearity index related to adsorption intensity. The adsorption data obtained from batch experimental results were correlated with a a linearized form of Langmuir and Freundlich isotherm equations, and the curves are shown in the inset of Figure 6. As shown in Table 1, the correlation coefficient (R2) of the Langmuir model (0.9988) was higher and thus better suited than that of the Freundlich isotherm model (0.9046). On the basis of the Langmuir model, the maximum Hg(II) adsorption capacity was 1736.8 mg/g at 25 °C. An excess number of available −SH chelating groups are responsible for the accumulation of large amounts of Hg(II) on the PPy/MAA surface. On the basis of the results (Table 1), it is obvious that the Langmuir isotherm appeared to be closer to the experimentally observed values than the Freundlich model. A comparison between synthesized PPy/MAA with that of other recently reported adsorbents reveals that the our composite has significantly higher adsorption capacity toward Hg(II)

Figure 5. XRD patterns of PPy/MAA (a) before and (b) after adsorption of Hg(II).

adsorbent surface and (ii) electrostatic repulsion between positively charged Hg2+ ions in solution with protonated surface functional groups in the adsorbent. Moreover, it was found that at moderately acidic pH (4.0−6.0) not only deprotonated surface functional groups of adsorbent would be free for binding the metal ions but also the PPy/MAA composite carried a maximum negative zeta potential value (Figure S4, Supporting Information). Therefore, these two combined factors make the adsorbent PPy/MAA able to achieve a maximum percentage (99.4%) of Hg(II) removal at pH 5.5. However, at higher initial pH (>6), the quantity of Hg(II) adsorbed declines due of formation of insoluble Hg(OH)2 as the dominant species in solution. It was expected that the binding strength would be reduced due to electrostatic interaction between the active groups in the adsorbent and neutral mercury species. However, neither precipitation of the metal hydroxide nor protonation of the functional group is expected at pH 5.5 and is therefore considered as an optimum pH for Hg(II) removal. Effect of Adsorbent Dosage for Hg(II) Removal. To determine the optimum quantity of PPy/MAA material for 7529


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where qe (mg/g) and qt (mg/g) are quantity of adsorbate at equilibrium and time t (min), respectively; k1 (1/min) is the rate constant. The kinetics data were fitted to eq 5, and calculated results are depicted in Figure S5 (Supporting Information) from the plot of log(qe − qt) versus t for Hg(II) ions. Table 3 summarizes the kinetic constant and correlation coefficient (R2) values. The calculated adsorption parameters not well matched with the experimental values. The correlation coefficient (R2) values (0.9075−0.9888) also showed lack of linearity. It is clear from Table 3 that the calculated adsorption data were not suitable to define the kinetics profile of Hg(II) adsorption with linear fitting of the pseudo-first-order model. Pseudo-Second-Order Kinetics. The pseudo-second-order kinetic model of Ho and McKay57 has been applied to investigate the adsorption kinetics t 1 t = + 2 qt q k 2qe e

where k2 (g/mg min) is the pseudo-second-order rate constant. Kinetics parameters of this model were calculated from the linear plot of t/qt vs t (Figure 7b). The correlation coefficients (R2) shown in Table 3 indicate a strong relationship between metal ion uptake with contact time for the removal of Hg(II) ions by PPy/MAA. In addition, the theoretically calculated qe values were similar to the experimentally determined qe(exp.) values. Therefore, comparing the two kinetic equations above reveals that the pseudo-second-order model is the most appropriate model for the above adsorption system. Intraparticle Diffusion. The Weber and Morris58 model for intraparticle diffusion was applied to determine the influence of diffusion in the adsorption process and can be expressed mathematically using eq 7

Figure 6. Adsorption isotherm of Hg(II) on PPy/MAA with different metal ion concentrations. (Inset) Best fitting of Langmuir and Freundlich isotherm models.

Table 1. Langmuir and Freundlich Isotherm Constants for Adsorption of Hg(II) Langmuir model qm (mg/g) b (L/mg) R2

Freundlich model kF (mg/g) n R2

1736.8 0.118 0.9988

310.52 2.54 0.9046

qt = k idt 1/2 + C

k1 t 2.303

(7) 1/2

where, kid (mg/g min ) is a measure of diffusion constant; C is directly proportional to boundary layer thickness. A plot of the quantity of adsorbate (qt) against the square root of the time (t1/2) gives the rate constant (slope of the plot) and boundary layer thickness (intercept of linear plot). Figure 7c displays the qt vs t1/2 plot for various initial concentrations of the Hg(II) solution, and it shows a multistep adsorption process. The initial sharp linear section implies an external diffusion, i.e., mass transfer from solution to solute phase. The second linear section represents the intraparticle diffusion of the adsorbate molecule within the micropores of the adsorbent networks. In the third phase, diffusion remains almost constant due to the exhaustion of pore volume in the adsorbent.38,59 Intraparticle diffusion parameters (kid and C) at different steps (first phase and second phase) are presented in Table 3. The rate parameters (kid) shifted to higher values with increasing initial concentration of Hg(II) ions. This may be attributed to high metal ions concentration; the ionic mobility of adsorbate is faster from the solution to solute phase. From the results (Table 3), it is obvious that adsorption data fit well the Weber−Morris equation with higher correlation coefficient values (R2 > 0.96). Therefore, the observed results indicate that not only external mass transfer but also intraparticle diffusion plays as a vital role for Hg(II) adsorption from aqueous media. Effect of Ionic Strength on Hg(II) Adsorption. The presence of dissolved salts has an effect on the Hg(II) adsorption capacity. Therefore, it is important to determine the performance of PPy/MAA as a function of adsorbent in two different conditions

(Table 2)49−54 making it an ideal candidate to remove Hg(II) from aqueous media. Adsorption Kinetics for Mercury Ions. Batch adsorption kinetics were studied to determine the optimal contact time for Hg(II) removal. The rate of Hg(II) adsorption was determined by employing the following experimental conditions: 0.4 g of adsorbent in 1 L of solution containing Hg(II) concentration ranging between 25 and 200 mg/L at pH 5.5 and 25 °C. Figure 7a shows that the rate of Hg(II) adsorption was rapid in the first 20 min of the adsorption study and then slowed with time until equilibrium was reached. Such rapid initial adsorption may have been due to the availability of active sites for metal binding on the adsorbent. As the number of vacant active adsorption sites decreases over time, so does the rate of adsorption. Although active sites are still available at equilibrium, further adsorption is prevented by repulsion forces between occupied sites on the adsorbent and incoming metal ions in the medium. To find out the mechanism of adsorption kinetics, two well-known rate equations, pseudo-first-order55 and pseudo-second-order,56 were applied. Pseudo-First-Order Kinetics. The linearized from of the pseudo-first-order equation proposed by Lagergren55 is mathematically described as follows: log(qe − qt) = log qe −

(6)

(5) 7530


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ACS Sustainable Chemistry & Engineering Table 2. Comparison of Maximum Adsorption Capacity against Other Adsorbents for Removal of Hg(II) ion Adsorbent materials

Adsorption Time

pH

Adsorption capacity (mg g−1)

Ref

Polypyrrole-reduced graphene oxide Chitosan-g-polyacrylamide Poly(2-aminothiazole) (3-mercaptopropyl) trimethoxysilane Mercapto-grafted rice straw (RS-GM) Polypyrrole (PPy) multilayer-laminated cellulose graphene oxide-L-cystine Graphene oxide-iron magnetic nanoparticles Thiol modified Fe3O4@SiO2 Fe3O4 nanoparticles capped with 3,4-dihydroxyphenethylcarbamodithioate Sulfur-functionalized silica microspheres Polypyrrole/mercaptoacetic acid (PPy/MAA)

3h 48 h 40 h 5 days 2 days 60 min − 180 min 10 min − 2 days 24 h

3.0 4.0 6.5 8.0 6.5 6.0 5.5−7.0 5.0 4.0 7.0 7.5 5.5

980 322.6 325.7 416 161.3 31.68 79.36 16.6 74 52.1 62.3 1736.8

9 15 25 37 38 49 50 51 52 52 54 present study

Figure 7. (a) Adsorption kinetics of Hg(II) at 25 °C with metal ion concentrations of 25, 50, 100, and 200 mg/L. (b) Pseudo-second-order and (c) intraparticle diffusion models of Hg(II) ions adsorption onto PPy/MAA composite.

Table 3. Kinetics Parameters for Adsorption of Hg(II) from Aqueous Medium Pseudo-first-order kinetic model Initial metal concentration (C0) (mg/L)

k1 (1/min) −2

8.5 × 10 3.1 × 10−2 1.4 × 10−2 1.1 × 10−2

25 50 100 200

qe (mg/g)

Pseudo-second-order kinetics model R

2

k2 (mg/g min) −4

51.06 0.9761 32.8 × 10 71.25 0.9888 12.1 × 10−4 165.69 0.9852 2.4 × 10−4 331.89 0.9075 0.9 × 10−4 Weber−Morris model (Intraparticle diffusion model)

Initial phase

qe (mg/g)

R2

59.2 121.9 256.4 500

0.9982 0.9983 0.9956 0.9935

Secondary phase

C0(mg/L)

Kip1

C1

R12

25 50 100 200

12.93 17.01 22.38 35.38

5.17 37.92 73.35 109.53

0.9736 0.9858 0.9798 0.9459

Kip2

C2

R2 2

0.156 0.25 10.28 15.73

55.23 74.3 96.38 196.86

0.9143 0.9254 0.9842 0.9865

Mechanism of Hg(II) Removal. There are several plausible mechanisms for metal ions adsorption using a functionalized adsorbent including electrostatic interaction, surface precipitation, and ion exchange processes.60,61 Physico-chemical characterization techniques like FE-SEM, EDX, FTIR, STEM, and XPS were used to explore the mechanisms involved in the removal of Hg(II) by PPy/MAA. It can be assumed that the mercapto-functionalized PPy adsorbent, containing electron-rich sulfur and oxygen atoms in −SH and −COOH groups, has a strong ability to bind Hg(II). On the basis of the Pearson’s concept, Hg(II) forms a compatible metal chelate complex

from the normal adsorption system to electrolytes present in the adsorption medium. To evaluate the ionic strength effect on Hg(II) adsorption, different KNO3 solutions with concentrations ranging from 0.01 to 1.0 M were introduced into the adsorption medium. There was no visible effect of salt concentration on the adsorption of Hg(II) (Figure S6, Supporting Information). Therefore, it can be concluded that the attraction between the sulfur-containing adsorbent (PPy/MAA) and Hg(II) ions is very effective as well as specific and unaffected by other significant effects of salt screening or attraction in the adsorption medium. 7531


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determine the characteristic changes that happened to the postadsorbed adsorbent. The different structural aspects between pure adsorbent and post-adsorbed materials help to determine the Hg(II) adsorption phenomenon. To provide evidence for the adsorption mechanism, binding energies of surface compositions of PPy/MAA and PPy-MAA/ Hg(II) were measured using XPS over the range of 0−1350 eV (Figure 8a). Spectra reveal the presence of Hg 4f signals after adsorption, which provides further evidence of the adsorption of Hg(II) onto PPy/MAA. Deconvolutions of the Hg 4f signal (Figure 8b) show two peaks, with binding energies at 100.05 and 104.04 eV, corresponding to Hg 4f7/2 and 4f5/2 signals, respectively.49,50,62 The binding energy of Hg 4f7/2 at 100.05 eV indicates an absence of Hg(0) (binding energy of Hg(0) 4f7/2 between 99.2 to 99.8 eV).59 The adsorbed Hg exists on the surface of the adsorbent as either Hg(I) or Hg(II) species. Figure 8c exhibits the deconvolution of C 1s spectra of the adsorbent (PPy/MAA) and post-adsorbed material (PPy-MAA/ Hg(II)). High-resolution spectra clearly indicate that C 1s could be divided into four major peaks due to four different bonding (C−C, C−N, C−S, and C−O) environments. Figure 8c indicates no major change in intensity, as well as binding energies of the C 1s peaks for PPy/MAA and PPy-MAA/Hg(II). In contrast, high-resolution S 2p spectra of PPy/MAA display remarkable changes during Hg(II) removal (Figure 8d). Deconvolution of S 2p shows two peaks at 162.45 eV for S 2p3/2 and at 163.4 eV for S 2p1/2. For post-adsorbed material,

through the coordination with sulfur and oxygen atoms. Therefore, electron donor functionality of mercaptoacetic acids, like thiol (−SH), carboxylic acid (−COOH) groups, contribute their lone pair of electrons to Hg(II) for strong soft acid−base interactions shown in Scheme 1. A comparison of Scheme 1. Schematic Representation of Possible Adsorption Mechanism of Hg(II) Ions onto PPy/MAA Composite

FTIR spectra of adsorbents with different functionalities (Figure 3) revealed peak shifts after adsorption. One noteworthy change involved the peak corresponding to the stretching vibration band of the S−H group of the adsorbent, which shifted from 2636 cm−1 to a broad infrared region of 2456−2636 cm−1. The peak broadening of the S−H stretching frequency suggests that the sulfur atom in the thiol functional group was involved in the Hg(II) adsorption process. Another one major change was observed in the −OH stretching frequency of the −COOH group of the adsorbent. In pure adsorbent (Figure 3), the strong stretching vibration band of O−H groups at approximately 3220 cm−1 almost disappeared after adsorption of Hg(II) to exhibit a broad and weak band in the infrared region of 3115−3274 cm−1. This characteristic spectral phenomenon further indicates that electrostatic interaction was involved between the positive metal ions (Hg(II)) and acid groups (−COOH) of the adsorbent. HR-TEM and FE-SEM studies

Scheme 2. Catalytic Transformation of Phenylacetylene to Acetophenone

Figure 8. (a) Full survey XPS spectra. (b) High resolution Hg 4f, (c) C 1s, (d) S 2p, and (e) O 1s spectra of PPy/MAA (i) before and (ii) after Hg(II) adsorption. 7532


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Figure 9. (a) 1H and (b) 13C NMR spectra of acetophenone.

the peak intensity of S atoms diminishes significantly, shifting toward higher binding energies (Figure 8d). This phenomenon indicates the development of strong interactions between sulfur

and mercury. Deconvolution of O 1s shows a peak at 531.89 eV attributed to the −COOH moiety of mercaptoacetic acid (Figure 8e). Peak intensity decreases markedly after adsorption, and the binding 7533


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under these reaction conditions. Modest deactivation of the catalyst was observed over catalytic cycles due to the leaching of mercury ions into the reaction medium.

energy value (530.91 eV) also shifts toward lower energy indicating the interaction between the oxygen atom of the acid group and mercury ions. On the basis of the FTIR and XPS results, it is clear that Hg(II) adsorption likely takes place by the strong interaction with sulfur and oxygen atoms of PPy/MAA. Catalytic Application of Hg(II)-Adsorbed Material. This aspect of the study explored the potential of using an Hg(II)-adsorbed composite (PPy-MAA/Hg(II)) in organic synthesis as a catalytic alternative, specifically for the transformation of phenyl acetylene to acetophenone (Scheme 2). Reusing PPy-MAA/Hg(II) would thus reduce the amount of waste generated from the remediation process.63,64 Treatment of 1 (Scheme 2) in the presence of 5 mol % PPy-MAA/Hg(II)-5 yielded 55% acetophenone (1a) in H2O as solvent at 90 °C for 6 h (Table S2, entry 1, Supporting Information). Furthermore, we have checked the catalytic activity of various Hg-loaded materials (PPy-MAA/Hg(II)-1, -2, -3, -4, and -5 with 10.87, 19.71, 33.2, 42.34, and 49.21 wt % of Hg, respectively) under similar conditions, affording the desired product in 44%, 48%, 51%, 54%, and 55% yields, respectively (Table S2, entries 1−5). Screening of the catalysts indicated that the PPy-MAA/Hg(II)-5 catalyst was proved to be the best choice among others to give the desired product in a 55% yield. The reaction mixture was charged with 1 mL of 6 (N) HCl to improve the conversion of the desired product under identical reaction conditions with a variety of catalysts (PPy-MAA/Hg(II)-1, -2, -3, -4, and -5). Satisfyingly, the PPy-MAA/Hg(II)-5 catalyst was found to be efficient to furnish product 1a in a 65% yield (Table S2, entry 6). However, the leaching test of the liquid phase was done after the reaction using ICP-OES, revealing that 0.02, 0.03, 1.58, 5.16, and 5.19 ppm of mercury had drifted to the solution from PPy-MAA/Hg(II)-1, -2, -3, -4, and -5 catalysts, respectively (Table S3, entries 1−5, Supporting Information). These results confirmed that leaching of mercury in water media enhances its catalytic activity (Caution! The aqueous part of the reaction was not discarded without f urther processing). However, no desired product was obtained in the absence of the PPy-MAA/Hg(II) catalyst (Table S2, entry 7). In addition, the reaction was performed with the PPy/MAA (50 mg) composite, and no trace of product was found (Table S2, entry 8). These experiments confirmed a definite role of Hg (II) for this transformation. However, optimization, substrates scopes, and general applicability of this method need to be explored further. Product 1a was confirmed by 1H and 13C NMR spectroscopy as shown in Figure 9. NMR data of acetophenone: 1H NMR (600 MHz, CDCl3) (Figure 9a): δ = 7.96−7.97 (m, 2Har), 7.5−7.58 (m, 1Har), 7.45−7.48 (m, 2Har), 2.61 (s, −CH3) ppm. 13C NMR (150 MHz, CDCl3) (Figure 9b): δ = 198.32, 137.43, 133.30, 128.79, 128.53, 26.81 ppm. Reusability of Catalyst. To check the reusability of the catalyst, the catalyst was subjected to five consecutive runs under the similar conditions previously described (Table S4, Supporting Information). The catalyst (PPy-MAA/Hg(II)-3) was separated from the reaction mixture through a simple filtration technique using 0.45 μm cellulose filter paper. After washing sequentially with deionized water (20 mL) and acetone (25 mL), the residue was dried under vacuum for 30 min at RT. The recovered material thus obtained was employed for the next catalytic experiments, affording the desired product in 41−57% yields in five consecutive runs, as summarized in Table S4. Additionally, a leaching test of the liquid phase of the reaction mixture was performed by ICP-OES analysis as given in Table S4. These results indicated that PPy-MAA/Hg(II)-3 was relatively unstable

■

CONCLUSION A PPy composite containing a thiol-functionalized chelating group was developed and tested for the remediation of toxic metal ion (Hg(II)) at a concentration range from 100 to 800 ppm. Results revealed excellent removal efficiency of Hg(II) by the synthesized adsorbent. The mercaptoacetic acid-containing thiol functionality played an important role in the removal of mercury due to the strong binding ability between soft acid mercury with soft base thiol. Adsorption data obeyed a pseudo-second-order rate equation, and a maximum adsorption capacity of PPy/MAA was calculated at 1736.8 mg/g at 25 °C using the Langmuir adsorption isotherm. In addition, post-adsorbed waste material containing a considerable amount of mercury played a vital catalytic role for the organic transformation reaction. The catalytic performance of the waste-derived mercury material for the transformation of phenylacetylene to acetophenone as an alternative route was encouraging, although further studies are required.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00477. EDX spectra of PPy/MAA and PPy-MAA/Hg(II), table of elemental composition, BET surface area measurement, effect of adsorption parameters, zeta potential, pseudofirst-order adsorption model, result of the effect of ionic strength in adsorption study, table of optimization of catalytic reaction, table of leaching study, table of reusability of the catalyst. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +27128412658. Fax: +27128413553. E-mails: maityarjun@ gmail.com and [email protected] (A. Maity). ORCID

Arjun Maity: 0000-0002-8057-5749 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS R.D., S.G., and A.M. are grateful to the University of South Africa (UNISA), National Research Foundation (NRF), Water Research Commission (WRC), Department of Science and Technology (DST), and Council for Scientific and Industrial Research (CSIR) of South Africa for financial support. The DST/CSIR National Centre for Nanostructured Materials characterization unit is also acknowledged for assisting with materials characterization.

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