Removal of Noble Metal Ions (Ag+) by Mercapto Group-Containing

Research Article pubs.acs.org/journal/ascecg

Removal of Noble Metal Ions (Ag+) by Mercapto Group-Containing Polypyrrole Matrix and Reusability of Its Waste Material in Environmental Applications Raghunath Das,† Somnath Giri,† Akebe Luther King Abia,‡ Baban Dhonge,§ and Arjun Maity*,†,§

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†

Department of Civil and Chemical Engineering, University of South Africa (UNISA), Florida Campus, Cnr. Christiaan De Wet and Pioneer Ave., Florida Park, South Africa ‡ Natural Resources and Environment, Council for Scientific and Industrial Research, 1-Meiring Naude Road, Pretoria 0001, 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: A novel approach was introduced to remove metal (Ag+) ions from aqueous solution and subsequently restate the metal-loaded materials for a number of environmentally friendly applications. A versatile adsorbent, polypyrrole with mercapto-functionalized chelating groups (PPy/ MAA), successfully adsorbed Ag+ ions through subsequent reduction to silver nanoparticles (Ag0 NPs) into the composite matrix. The as-prepared composite (PPy/MAA) and Agadsorbed PPy/MAA (PPy/MAA/Ag0) were fully characterized by FE-SEM, EDS, HR-(S)TEM, XRD, FTIR, BET, XPS, and zeta potential measurements. Batch adsorption results showed that the adsorption process can be explained well by a pseudosecond-order model. The maximum adsorption capacity calculated using a Langmuir isotherm model was 714.28 mg/ g at 25 °C. XRD, XPS, and HR-TEM analyses confirmed the presence of metallic silver nanoparticles on the surface of the composite matrix after the in situ reduction of Ag+ to Ag0. Among the applications tested, the metal-loaded waste (PPy/MAA/Ag0) was found to have antimicrobial activity, as it inhibited the growth of Escherichia coli, while pure adsorbent without silver showed no killing effect toward E. coli. PPy/MAA/Ag0 also played an important role in the catalytic reduction of 4-nitrophenol and also exhibited good sensitivity to NO2 in gas-sensing applications. Therefore, the developed PPy/MAA composite achieved 2-fold environmental benefits, not only remediating Ag+ from polluted waterways but also opening a new window for subsequently acting as an agent for antibacterial ability, catalytic activity, and gas-sensing efficiencies. KEYWORDS: Mercaptoacetic acid, Adsorption, Catalytic reduction, Antimicrobial activity, Gas sensing

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concentration of silver ions (Ag+) in drinking water can reach toxic levels in the human body. The concentration of silver in drinking water beyond the permissible limit (0.1 mg/L) causes serious diseases (skin, liver, and kidneys) and disorders (blood) in the human body.11 However, according to a survey, Ag concentration in sewage sludge ranged from 2 to 195 mg kg−1, which is quit harmful toward the environment.12 Silver concentration in sewage sludge also depends upon the sources and differs in various parts of the world. It is of serious concern that large volumes of industrial effluents containing silver ions

INTRODUCTION With the rapid expansion of industries and technology, environmental pollution has become a serious worldwide problem because of the negative impact on human health and other life systems.1,2 The release of industrial waste effluents into water bodies, especially heavy metal ions and organic dyes, has drawn serious concern due to the toxic effects on aquatic life and human beings. Noble heavy metals are widely used in innovative research areas such as catalysis, electronic devices, energy sources, and the chemical industry. Silver is one of the most important and precious noble metals because of its versatile properties in various applications such as electronics,3,4 laminations,5 catalysts,6,7 antimicrobial activity,8,9 and sensing materials.10 In spite of its beneficial use in daily life, a high © 2017 American Chemical Society

Received: December 16, 2016 Revised: January 25, 2017 Published: January 31, 2017 2711

DOI: 10.1021/acssuschemeng.6b03008 ACS Sustainable Chem. Eng. 2017, 5, 2711−2724

ACS Sustainable Chemistry & Engineering

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have been discarded into rivers, lakes, and ponds without suitable treatment. Therefore, the effective removal and recycling of silver ions from wastewater has received significant attention for health, environmental, and resource reasons.13,14 In recent years, research has focused on determining the presence of even trace amounts of toxic heavy metal ions in contaminated waters and to mitigate this problem by applying appropriate techniques to effluents in order to remove or minimize quantities within permissible limits before being discharged into water bodies. Among other methods, adsorption has been widely used to uptake the silver ions from aqueous media because of the high efficiency, low cost, simple operation, and easy design of the adsorbent. Polypyrrole (PPy) is considered one of the most effective adsorbent materials due to its outstanding properties such as its high conductivity, high stability, easy preparation, low cost, and biocompatibility.15,16 However, while polypyrrole has already been applied on a large scale to the adsorption of metal ions and dyes from aqueous solutions, the benefits of introducing suitable chelating agents into the polymeric chain has recently drawn great attention.17 The adsorption capacity for specific metal ions increases many fold, with selective accumulation of the complexing agent onto the surface of the adsorbent. Chelating agents with electronegative donor atoms like sulfur, nitrogen, and oxygen are most effective for the selective adsorption of heavy metal ions through their strong metalbinding capacities. Chelating agents with thiouera, 13,18 mercapto,19,20 and amine21 groups have extensively been used for the removal of heavy metals. The main goal of this study is to synthesize a novel mercapto-functionalized adsorbent, polypyrrole/mercaptoacetic acid (PPy/MAA) composite, via a simple chemical oxidative polymerization of pyrrole (Py) and mercaptoacetic acid (MAA) at room temperature. The authors have focused on the uptake of Ag+ ions from aqueous media, leading to the in situ reduction of Ag+ to Ag0 nanoparticles on the adsorbent surface. Adsorbent-containing mercapto groups play vital roles for the adsorption and subsequent reduction as well as stabilization of the newly developed nanoparticles through chelation. An important aspect of this study is to explore the novel use of waste-recovered silver-adsorbed materials in three different fields. First, as silver nanoparticles (Ag NPs) reportedly yield high catalytic activity,6,22,23 the silver-adsorbed PPy/MAA material in this study was tested for the reduction of 4-nitro phenol to 4-amino phenol in the presence of NaBH4, and the full conversion of product was obtained within 70 min of the reaction time. Second, silver is known to have antimicrobial effects upon fungi, bacteria, and viruses,24,25 and therefore, silver-containing materials were also used to control Escherichia coli (E. coli) and Staphylococcus epidermidis (S. epidermidis). In this present work, silver-adsorbed waste materials (PPy/MAA/ Ag) showed the killing effect on E. coli bacteria using 107 cfu within 30 min. Lastly, metal nanoparticles have also been incorporated into gas sensors to increase the sensitivity, selectivity, and response time to the experimental gases tested,26,27 so the usefulness of PPy/MAA/Ag as a gas sensor to NO2 gas was also tested. Therefore, this is the first report on the development of a mercapto-functionalized adsorbent for removal of silver ions (Ag+) followed by in situ reduction of Ag+ ions to Ag0 NPs and reuse of silver-adsorbed waste materials in different environmental (catalytic, antimicrobial, and gas sensing) application fields.

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EXPERIMENTAL SECTION

Materials. Pyrrole monomer (Py, 99%, Sigma-Aldrich, U.S.A.) was purified by vacuum distillation and stored at 4 °C prior to use. Ammonium persulfate (APS), mercaptoacetic acid (thioglycolic acid) were purchased from Sigma-Aldrich (U.S.A.). Silver nitrate and sodium borohydride were obtained from Sigma-Aldrich (U.S.A.). pNitrophenol, sodium bicarbonate, and dichloromethane were supplied by Sigma-Aldrich (U.S.A.). All other chemicals used in this study were of analytical grade and purchased from Sigma-Aldrich (U.S.A.). Synthesis of Mercaptoacetic Acid-Containing Polypyrrole (PPy/MAA) Composite. The PPy/MAA composite was synthesized via chemical oxidative polymerization at room temperature.28 The detailed synthetic procedure is given in the Supporting Information. A plausible explanation for polymerization of the Py monomer is based on the well-established oxidative mechanism indicating that ammonium persulfate (APS) generates free radical sites on the pyrrole backbone.28 These free radical sites react with another monomer to generate polymer (polypyrrole) as shown in Scheme S1. During polymerization of the Py monomer, PPy carries a positive charge on the hetero atoms at the time of the propagation stage in the polymerization reaction medium.28,29 Due to charge neutrality of N atoms, negatively charged mercaptoacetic acid dopants are introduced into polypyrrole networks through electrostatic interaction. Characterization. The synthesized materials were characterized using FE-SEM, HR-TEM, FTIR, XRD, FTIR, FE-SEM, HR-TE, UV− vis, NMR, BET surface area, and zeta potential analyzer. The instrumental details and techniques are provided in the Supporting Information. Adsorption Studies. The adsorption of silver ions (Ag+) from aqueous solution was conducted in batch experiments using the PPy/ MAA composite as an adsorbent at room temperature (25 °C). The detailed adsorption procedure is explained in the Supporting Information. Catalytic Activity Test of Silver-Adsorbed Waste Material (PPy/MAA/Ag). To investigate the catalytic reactivity of this silveradsorbed material for further application in organic synthesis, the reduction of 4-nitrophenol with sodium borohydride has been chosen as a model reaction. The detailed reaction procedure is described in the Supporting Information. Test for Antimicrobial Activity of PPy/MAA/Ag against Cultures of Escherichia coli. To test for the antimicrobial potential of Ag-adsorbed PPy/MAA waste material (PPy/MAA/Ag), a set of four samples was tested against E. coli. The samples consisted of the neat material (PPy/MAA) and silver-adsorbed PPy/MAA with different percentages of silver loading (11%, 32%, and 42%). The samples were labeled S0, S1, S2, and S3 containing 0%, 11%, 32%, and 42% silver loading, respectively. Each material (50 mg) was weighed into a 50 mL centrifuge tube containing 20 mL of sterile distilled water. Tubes were prepared in triplicate for each test material. Each tube was then inoculated with an overnight culture of E. coli (American Type Culture Collection; ATCC 25922) to obtain a final concentration of ∼107 colony forming units (CFU)/mL. A fifth set of three tubes containing distilled water and bacteria but no antimicrobial material was used as a control. Following inoculation, samples were immediately extracted (time zero) and plated on nutrient agar using the drop plate technique. Each tube was then sampled every 10 min for 60 min. Plates were incubated at 37 ± 0.2 °C for 24 h, after which they were examined for growth, and E. coli counts were recorded as CFU/mL. Gas Sensing Experiment. The sensing responses of neat adsorbent (PPy/MAA) as well as silver-adsorbed PPy/MAA (PPy/ MAA/Ag) devices were measured using a gas sensing station KSGAS6S (KENOSISTEC, Italy) at 25 °C. The sensing device was prepared by the drop casting method. The materials were ultrasonically dispersed in ethanol for 5 min, drop cast on the interdigitate electrode side of an alumina sensing strip, and dried at 60 °C for 1 h. The resistive response of the device to the various concentrations of NO2 gas was measured using a Keithley 6487 pico-ammeter/voltage source meter. The various concentration of NO2 gas in the sensing chamber was achieved by controlling the flow rate of synthetic air and 2712


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Figure 1. Morphology and elemental composition of PPy/MAA: FE-SEM image of PPy/MAA (a) before and (b) after adsorption of silver. Elemental mapping image of PPy/MAA (c) before and (d) after adsorption. EDS element composition analysis of PPy/MAA (e) before and (f) after silver adsorption. NO2 gas. The synthetic air was used to dilute the NO2 gas. The total flow rate of 500 sccm was maintained during the measurements. The response and recovery of the sensors were measured for 5 and 10 min for each concentration, respectively.

and elemental mapping image (Figure 1c,d) of C, N, O, S, and Ag for PPy/MAA and silver-adsorbed PPy/MAA. The existence of prominent silver peak in the EDS spectra (Figure 1f) and well distribution of silver onto the composite surface (Figure 1d) confirmed the successive adsorption of silver particles into the adsorbent surface. HR-TEM analysis (Figure 2a) showing a PPy/MAA composite revealed spherical particles with an average diameter of ∼200 nm. Figure 2b showed silver nanoparticles (Ag NPs) randomly dispersed on the surface of the PPy/MAA matrix. The inset of Figure 2b shows that the size distribution of Ag NPs on the composite matrix is broad, ranging from 10 to 50 nm. The interplanar spacing (d-spacing) of Ag crystallites is clearly observed in the well-defined lattice fringes of the Ag NPs in Figure 2c. The d-spacing of the Ag crystallites was measured as 0.230 nm (inset, Figure 2c), which is consistent with the reported value of the (111) crystallographic plane of cubic Ag.30,31 This result indicates that silver ions in the aqueous solution are adsorbed on the adsorbent surface and subsequently reduced to silver nanoparticles through electron

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RESULTS AND DISCUSSION Characterization. The morphologies of the as-prepared PPy/MAA composite before and after adsorption of silver ions were investigated by FE-SEM and HR-TEM analysis. As shown in Figure 1a, the typical FE-SEM image shows that the PPy/ MAA composite possesses a uniform distribution of granular/ spherical structures with relatively uniform diameters of around 150−200 nm. Figure 1b shows the FE-SEM images of the PPy/ MAA composite after adsorption of silver ions. The results appear markedly different from neat the PPy/MAA composite (Figure 1a). The presence of metallic crystal-like structures on the surface of the PPy/MAA composite suggests the accumulation of metal ions (silver) on the adsorbent surface. In addition, an even distribution of Ag adsorbed onto the composite surface is further confirmed with EDS (Figure 1e,f) 2713


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Figure 2. HR-TEM image of PPy/MAA (a) before and (b) after adsorption. (c) Lattice fringes of the adsorbed Ag at different magnification. STEM images of PPy/MAA (d) before and (f) after adsorption of silver. Homogeneous distribution of S atoms in PPy/MAA (e) before and (g) after adsorption. (h) Distribution of adsorbed Ag atom (i) composite map showing S and Ag on the adsorbent.

reach groups of the polymer-based composite. To better understand the distribution of silver nanoparticles on the adsorbent surface, STEM analysis was executed. Figure 2e and g displays the distribution of sulfur on the adsorbent before and after adsorption. Similarly, Figure 2h presents the dispersion of Ag throughout the adsorbent surface after adsorption. Figure 2i represents the STEM image where red spots indicate silver distribution and yellow spots indicate sulfur. Therefore, from HRTEM as well as STEM analyses, it is evident that the silver particles are randomly distributed throughout the composite matrix after adsorption. The structural properties of the functional groups of PPy/ MAA and PPy/MAA/Ag composites were further explored using FT-IR spectrometry, and results are shown in Figure S1(a) and (b) of the Supporting Information. Additional structural information on pure and silver ions trapped PPy/MAA surface was obtained by wide angle powder X-ray diffraction (XRD) analyses. Figure 3a presents the XRD pattern of the neat PPy/MAA composite before Ag + adsorption. The spectrum (Figure 3a) only exhibits a broad band at around 20.5°, which can be assigned to the amorphous phase of the polymer matrix. In Figure 3b, the PPy/MAA composite after silver ion sorption (PPy/MAA/Ag) shows five distinguishable sharp diffraction peaks at Bragg angles of 38.12°, 44.30°, 64.44°, 77.40°, and 81.7°, which exactly accord with the (111), (200), (220), (311), and (222) crystal planes of silver, respectively.32−34 From the XRD spectra, it is apparent that adsorbed Ag+ ions were reduced to metallic silver in the PPy/MAA composite. The above results provide direct evidence for the generation of metallic silver nanoparticles (Ag+ ions to Ag0) through reduction by the electron-rich

Figure 3. XRD patterns of PPy/MAA (a) before and (b) after adsorption of silver.

surface functionality of conducting polypyrrole networks-based composites. To analyze the surface area, pore volume, and pore diameter, the N2 adsorption−desorption measurements were carried out for PPy and PPy/MAA composites and are shown in Figure 4. On the basis of the BJH theory, the BET specific surface area, average pore diameter, and pore volume of the corresponding materials are summarized in Table S1 of the Supporting 2714


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did not yield any appreciable rise in adsorption efficiency due to the equilibrium of the adsorbent sites. Effect of Initial Silver Ion Concentration. The adsorption capacity of the PPy/MAA composite to different initial concentrations of silver solution is shown in Figure S2(c) of the Supporting Information. From the experimental results, it is evident that concurrent with an increase in the initial concentration of silver ions from 50 to 400 mg/L, the adsorption capacity of the PPy/MAA composite increases from 125 to 706 mg/g, respectively. This trend may be explained by the fact that a higher initial silver concentration provides a correspondingly higher driving force attracting metal ions from the solution to the surface of the adsorbent.37 On the other hand, the adsorption capacity of silver solutions between 400 and 600 mg/L remained almost constant at approximately 720 mg/g. This may be because the system reached equilibrium with the available adsorption sites saturated with fixed Ag+. Adsorption Kinetics. The rate of metal ion adsorption depends on the contact time between solid and liquid and on the diffusion processes. The effect of the contact time on the adsorption of silver ions by PPy/MAA, and the metal ion concentration-dependent kinetics, are evident in Figure 5, whereas Figure 5b illustrates temperature-dependent kinetics. Overall, both kinetic studies show that the silver ion adsorption efficiency was more rapid at an initial stage and also it increased rapidly with increasing metal ion concentration and higher temperatures before reaching equilibrium. In the case of 25 mg/L of silver solution, adsorption reached saturation after 15 min, whereas the 100 mg/L solution required 240 min before coming to an equilibrium point. Figure 5b shows that equilibrium is reached faster at higher temperatures of the adsorption medium. To get a better understanding of the adsorption rate and rate-controlling step of Ag+ adsorption onto the PPy/MAA composite, two well-known adsorption models, pseudo-firstorder38 (eq 1) and pseudo-second-order39 (eq 2) are used to fit experimental data.

Figure 4. Nitrogen adsorption and desorption isotherms of PPy and PPy/MAA composite with the corresponding pore-size distributions (inset).

Information. Following polymerization with MAA, the surface area of the PPy/MAA composite increased from 7.9 to 12.8 m2/g. The corresponding pore size distributions calculated from the desorption branch of nitrogen isotherms by the BJH method are presented in the inset of Figure 4. The cumulative pore volume of PPy is increased from 0.0187 to 0.0849 cm3/g, and the average pore diameter doubled from 20.75 to 40.83 nm after polymerization with MAA. Silver Ion Adsorption Property of the PPy/MAA Composite. Effect of the Solution pH on Silver Uptake. The pH of a solution plays an important role in metal ion adsorption because it affects not only the protonation of the functional groups of adsorbent but also the adsorptivity of the metal ions in solution. The percentage of adsorption of silver ions was investigated at a pH range between 1 and 6 but not above as silver ions precipitate above this value. As shown in Figure S2(a) of the Supporting Information, the adsorption capacity increased steadily above pH 1 reaching a maximum33,35,36 value at pH 5, decreasing at pH 6. It is likely that at higher pH, the deprotonation of mercaptoacetic acid takes place, and hence, the adsorption capacity of Ag+ ions was enhanced by the strong ionic interaction between the adsorbent and Ag+. The pH of the silver nitrate solution (200 mg/L) solution was 5.4, and this value was chosen as the working pH value for remainder of the study. Effect of Adsorbent Dosage. The effect of adsorbent dosage on the uptake of silver ions from the aqueous solution is shown in Figure S2(b) of the Supporting Information. It is clear that the adsorption efficiency increases with an increase in adsorbent (PPy/MAA) from 5 to 40 mg in 50 mL of 100 mg/L AgNO3 solution. At very low (5, 10 mg) adsorbent dosage, the percentage adsorption of Ag+ is very low. This is due to the lack of availability of the active sites on the adsorbent surface for silver adsorption. At 20 mg of adsorbent dosage, the efficiency of silver adoption was ∼98.8%, which is considered to be an equilibrium point. Further increase in adsorbent dosages

log(qe − qt) = log qe − t 1 t = + 2 qt qe k 2qe

k1 t 2.303

(1)

(2)

where qt and qe (mg/g) are the adsorption capacity at time t and equilibrium time, respectively, and k1 (min−1) and k2 (g mg−1/min) are the pseudo-first-order and the pseudo-secondorder model rate constants, respectively. The linear-fitting result of pseudo-second-order kinetic plot t/qt versus t is shown in Figure 5c, and the pseudo-first-order kinetic plot is presented in Figure S3 of the Supporting Information. The correlation coefficient (R2) value for the pseudo-first-order model was 0.8697−0.9554 at different concentrations of silver solution (from 25 to 100 mg/L), and the calculated qe value was significantly lower than the experimental value. Such a low correlation coefficient and qe value suggested the poor agreement of the pseudo-first-order model with the experimental data. The experimental data fitted well into the pseudosecond-order equation with correlation coefficients (R2) from 0.9995 to 1 at different silver solutions (from 25 to 100 mg/L), and the calculated value of qe was very close to the experimental result. The results show that the adsorption rates of Ag+ onto PPy/MAA can be considered as a pseudo-second-order adsorption process. The corresponding kinetic adsorption 2715


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Figure 5. Effect of contact time at (a) different initial concentration, (b) various temperature and kinetic curves of (c) pseudo-second-order, and (d) intraparticle diffusion model of silver ion adsorption onto PPy/MAA composite.

Table 1. Concentration-Dependent Kinetics Parameters for Adsorption of Silver Ions from Aqueous Media pseudo-first-order kinetic model initial metal concentration (C0) (mg/L)

k1 (min−1)

25 50 75 100

11.8 × 10−2 3.5 × 10−2 2.4 × 10−2 1.5 × 10−2

qe (mg/g) 22.66 62.20 88.41 130.49 Weber−Morris model

pseudo-second-order kinetics model k2 (mg g−1/min)

R2

0.8697 211.5 × 10−4 0.9554 21.5 × 10−4 0.9380 10.48 × 10−4 0.9731 4.12 × 10−4 (intraparticle diffusion model)

initial phase

qe (mg/g)

R2

62.5 126.5 188.6 256.4

1 0.9999 0.9999 0.9995

secondary phase

C0 (mg/L)

Kip1

C1

R21

Kip2

C2

R22

25 50 75 100

8.94 18.96 32.64 32.86

30.86 26.59 11.07 22.11

0.9764 0.9687 0.9702 0.9716

0.185 4.63 6.58 9.6

59.79 81.09 119.96 132.52

0.9754 0.9808 0.9854 0.9943

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Figure 6. (a) Adsorption isotherm of Ag+ ions onto the PPy/MAA composite. (b) Langmuir adsorption isotherm plots.

Table 2. Langmuir and Freundlich Constants for Adsorption of Silver Ions Langmuir model

Freundlich model 2

temp. (K)

qm (mg/g)

b (L/mg)

RL

R

kF (mg/g)

n

R2

298 308 318

714.28 769.23 909.09

0.311 0.345 0.579

0.06 0.05 0.03

0.9993 0.9998 0.9988

311.99 397.30 517.13

6.05 7.60 9.11

0.9073 0.9724 0.9788

Ce C 1 = + e qe bqm qm

parameters calculated from the above two kinetic models are listed in Table 1. In order to investigate the mechanism for Ag+ adsorption onto the PPy/MAA composite Weber−Morris intraparticle diffusion model40 (eq 3) can be introduced. qt = k idt

1/2

+C

ln qe = ln kF +

1 ln Ce n

(4)

(5)

where qe (mg/g) is the equilibrium adsorption capacity of the Ag+ adsorbed onto the PPy/MAA, Ce is the concentration of adsorbate at equilibrium (mg/L), and qm and b are Langmuir constants related to maximum adsorption capacity and binding energy of adsorption, respectively. Here, kF (mg/g) and 1/n are the characteristic Freundlich constants, which signify the adsorption capacity and adsorption intensity, respectively. The isotherms of the above two models are displayed in Figure 6b and Figure S4 of the Supporting Information. The related equilibrium parameters were determined from the corresponding linear fitting, and the results are listed in Table 2. It is evident that the adsorption isotherms become higher in consecutive order of 298, 308, and 318 K. From the experimental results, it can be seen that the higher temperature favors the adsorption process, and the process is endothermic in nature. The Langmuir isotherm model was established on the basis of the assumptions that the adsorption occurs onto the structurally homogeneous surface of the adsorbent with identical binding sites by monolayer adsorption without interaction between adsorbed molecules.45,46 As shown in Figure 6b, the experimental data is fitted well with the Langmuir isotherm model, and the value of qm changes from 714.28 to 909.09 (mg/g) with an increase in the temperature from 298 to 318 K. The correlation coefficient values (R2 = 0.999) show good fitting of the Langmuir isotherm to the experimental data at three different temperatures. Furthermore, the Langmuir constant, b, is used to calculate the feasibility of the adsorption process by mean of the dimensionless constant, called separation factor (RL). The mathematical expression of RL is repressed by the following eq (eq 6).47

(3)

where kid (mg g−1/min1/2) is an intraparticle diffusion rate constant and C is the thickness of boundary layer. The plot of qt versus t1/2 is presented in Figure 5d. It is evident that the experimental data are multilinear, including at least two linear segments. The first sharp phase represents the diffusion of adsorbate due to mass transfer from a metal solution to the interface space of the adsorbent and likely due to external diffusion.41,42 The second portion indicates a gradual adsorption step, corresponding with the diffusion of adsorbate molecules inside the pore of the adsorbent. This second step is also known as the intraparticle diffusion stage. The constants of the intraparticle model are listed in Table 1. From results (Table 1), it was observed that both phases fit well to the Weber−Morris model (R2 > 0.96). Therefore, intraparticle diffusion was the dominant rate-limiting process in silver ion adsorption on the PPy/MAA composite. Adsorption Isotherm. The adsorption equilibrium isotherm represents the mathematical relation of the amount of adsorbed target per gram of adsorbent to the equilibrium solution concentration at fixed temperature. Figure 6a shows the equilibrium isotherms for the adsorption of silver ions. The equilibrium adsorption data have been fitted into two wellknown adsorption isotherm equations, Langmuir and Freundlich, to discuss the equilibrium characteristics of the adsorption process. The linear forms of the Langmuir43 and Freundlich44 isotherms are depicted by eq 4 and eq 5, respectively. 2717


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Scheme 1. Schematic Representation of Possible Adsorption Mechanism of Silver Ions onto PPy/MAA Composite

RL =

1 1 + bC0

silver ion adsorption onto the PPy/MAA composite. The theoretical saturated capacity (qm) obtained from the Langmuir isotherm model (714.28 mg/g at 25 °C) was very much closer with the experimental saturated capacity. Adsorption Thermodynamics. In order to investigate the spontaneity of the adsorption of silver ions and the effect of temperature, adsorption experiments were performed at 298, 308, and 318 K using 200 mg/L silver ions solution. As evident from the above isotherm results (Figure 6a and Table 2), the adsorption process is endothermic since the increase in temperature leads to an increase in the adsorption capacities. Thermodynamic parameters can be evaluated using some wellknown equations described in the Supporting Information. The thermodynamic parameters for Ag+ adsorption onto the PPy/MAA composite are listed in Table S2, including the changes in Gibbs free energy (ΔG, kJ/mol), enthalpy (ΔH, kJ/ mol), and entropy (ΔS, kJ mol−1/K). All the ΔG values in Table S2 were negative, which indicates that the adsorption process is thermodynamically feasible and spontaneous. Moreover, the ΔG values decreased with an increase in temperature, indicating an increased trend in the degree of spontaneity and feasibility of adsorption of Ag+ onto PPy/ MMA composite. The positive value of ΔH (Table S2) confirmed that the adsorption process is endothermic in nature. Furthermore, the positive value of ΔS reveals the increase in disorderness and randomness at the solid solution interface during the fixation of silver ion on adsorbent surface. Adsorption Mechanism. According to evidence from XRD, HR-TEM, FE-SEM, EDS, and FTIR, the strong adsorption of Ag+ by the PPy/MAA composite can be explained by two different kinds of adsorption mechanisms: physical surface

(6)

where C0 (mg/L) is the initial concentration of Ag+ ions. Depending upon the value of separation factor (RL), there are four possibilities of the adsorption process. The values of separation factor (RL) within the range 0 < RL < 1 indicate a favorable process, for an unfavorable process, RL > 1, for a linear process, RL = 1, and for an irreversible adsorption process, RL = 0. As shown in Table 2, the RL values were from 0.06 to 0.03 for three different temperatures, which indicate that the silver adsorption on the PPy/MAA composite was a favorable process. The Freundlich model is based on the assumption of adsorption at multilayers and on energetically heterogeneous surfaces.44 Here, kF (mg/g) and n are characteristic Freundlich constants, which signify the adsorption efficiency and favorability of the adsorbent and adsorbate system. The values of kF and n obtained from the slope and intercept of the linear plots of the experimental data ln qe versus ln Ce at different temperatures (Figure S4, Supporting Information) are also listed in Table 2. The values of n were in the range from 1 to 10 (6.05 to 9.11), which provided evidence of a favorable adsorption process. According to the obtained results, the adsorption data of the metal ion on the PPy/MAA composite were fitted well with the Langmuir model compared to the Freundlich model, as designated by the very high values of the correlation coefficient (R2). In addition, the adsorption capacity of the Langmuir model (qm = 714.28 mg/g at 25 °C) was higher than that of the result obtained from the Freundlich model (kF = 311.9 mg/g). So, the Langmuir model is more appropriate and a good fit for 2718


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Figure 7. (a) Full scans of XPS spectra of PPy/MAA before and after adsorption of silver ions. High resolution (b) Ag 3d, (c) C 1s, (d) S 2p, and (e) O 1s spectra of PPy/MAA before and after Ag adsorption.

adsorption and redox adsorption.48 In physical adsorption, a porous adsorbent surface traditionally adsorbed the heavy metal ion through weak van der Waals forces between adsorbents and adsorbates molecules.49 On the other hand, the redox adsorption mainly occurs when adsorbents contain functional groups such as −NH2, −SH, −SO3H, −OH, and −COOH. Heavy metal ions (Ag+) with high standard reduction potential have a strong chelating ability with the above functional groups, which can be responsible for the reduction of initial adsorbed silver ions to the metallic silver nanoparticles during the sorption process50,51 (Scheme 1). The redox adsorption mechanism and the form of adsorbed silver on the PPy/ MAA surface could be verified by XRD analysis (Figure 3). The XRD spectrum (Figure 3b) exhibited five additional stronger peaks compared to the pure PPy/MAA composite for metallic Ag0 crystals, supporting the likely reduction of adsorbed Ag+ in the adsorbent surface by the redox adsorption mechanism. The HR-TEM image (Figure 2b and c) revealed the existence of a large amount of Ag nanoparticles on the surface of the adsorbent. The presence of silver particles on the adsorbent surface were also confirmed by the FE-SEM (Figure 1b) image and EDS (Figure 1f). The occurrence of redox sorption can be further confirmed by the FTIR spectra of PPy/MAA before and after adsorption of silver ions (Ag+) (Figure S1, Supporting Information). It is shown in Figure S1 of the Supporting Information that the intensity of the vibration band of the S−H group (2636 cm−1) became weak and broad after adsorption of silver ions, which suggests the chelating interaction between the −SH group and adsorbate molecules.

In order to further explore the adsorption mechanism and investigate the interaction between the adsorbate and adsorbent molecule, XPS analysis of the PPy/MAA composite was carried out before and after adsorption of silver. The complete XPS survey scans over the range of 0−1350 eV of the PPy/MAA composite before and after adsorption of silver ions are shown in Figure 7a. In addition, the characteristic peaks of C 1s (Figure 7c), S 2p (Figure 7d), and O 1s (Figure 7e) are detected at 284, 163, and 531 eV, respectively in the PPy/MAA composite before and after adsorption of silver ions. The presence of photoemission signals of Ag 3d3/2 and Ag 3d5/2 in the full scans survey indicate that a significant amount of Ag has been adsorbed into the PPy/MAA composite surface. The spectrum of Ag 3d (Figure 7b) displays that the two photoemission bands are located at 373.37 and 367.38 eV for Ag 3d3/2 and Ag 3d5/2, respectively. The XPS spectrum of C 1s of PPy/MAA before adsorption of Ag+ ions was deconvoluted into different signals and is shown in Figure 7c(i). The peak at 284.12 eV was attributed to C−C bond, and the peak at 285.72 eV corresponded to the C−N bond. The two photoemission peaks at 287.02 and 288.12 eV correspond to the C−S and C− O bonds, respectively. Figure 7c(ii) clearly shows that there is no major change in intensity or binding energies of the C 1s peaks after adsorption of Ag+ ions. The characteristic photoemission spectrum of S 2p shows a spin−orbital doublet (S 2p1/2 and S 2p3/2).52,53 The peak deconvolution of S 2p in Figure 7d(i) implies the two S chemical states: the first one at 162.45 eV for S 2p3/2 and the second one producing for S 2p1/2 at 163.4. It can be observed that the binding energy of the two 2719


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with time as shown in Figure 8. In contrast, the peak absorption intensity did not change markedly in the absence of PPy/ MAA/Ag, even after 24 h. This suggests that the PPy/MAA/Ag catalyst displayed a definite catalytic role for the reduction of 4NP under reaction conditions. Furthermore, the full conversion of the product was obtained within 70 min, as confirmed by the UV−visible spectrum (Figure 8). The color of the reaction mixture transformed from bright yellow to a completely faded color after 70 min, and a slow increase in the peak of 4-AP at 300 nm was observed in the UV−vis spectrum. In addition, aliquots of the reaction mixture were monitored by gas chromatography (GC), and the full conversion of the product was obtained after 70 min (Figure S7, Supporting Information). The product was further confirmed by 1H NMR spectroscopy as shown in Figure S8 of the Supporting Information; 1H NMR spectrum of 4-AP: (DMSO-d6, 400 MHz, ppm): δ = 8.32 (bs, 1H, −OH, i.e., for H1), 6.43−6.50 (m, 4H, Ar−H, i.e., for H2), 4.43 (bs, 2H, −NH2, i.e., for H3) ppm). The mechanism for the catalytic reduction of 4-NP is predicted to follow according to the reported literatures,56,57 as shown in Scheme 2. Initially, the hydrogen (H2) gas is liberated upon addition of NaBH4 into the aqueous suspension of 4-NP in the presence of Ag NPs. Then, adsorption of hydrogen on the surface of the silver nanoparticles generates the silver hydride complex. Due to the presence of an electron-rich conductive PPy-MAA matrix, electron-deficient 4-nitrophenolate ions attach to the catalyst surface of AgNPs. Thus, the intermediates are in close proximity to the hydride species, which reduces the nitro group of the 4-nitrophenolate ion to nitroso species, followed by further stepwise reduction to hydroxylamine intermediates and last 4-amino derivative. Finally, the catalyst surface is regenerated through desorption of 4-AP, and it can be reused for the next catalytic cycle. Microbial Activity of Silver-Adsorbed Waste Material. The antimicrobial activity of various silver-adsorbed PPy/MAA composites was investigated using E. coli (Gram-negative) bacteria following 60 min incubation time. The killing kinetics of assays was determined by the amount of viable cells remaining after treatment with the PPy/MAA/Ag composite. Antimicrobial assay results of each of the four samples (neat PPy/MAA and 11%, 32%, and 42% of Ag loaded in PPy/MAA) are shown in Figure 9. The bacterial growth curve (Figure 9) suggests that the bacterial growth was delayed with increasing the concentration of silver in the PPy/MAA composite. The growth of E. coli was completely inhibited by the PPy/MAA/Ag composite after 30 min exposure time (Figure S9, Supporting Information) at a Ag loading of 42%. Although the sample containing 32% Ag showed some effect, this was very minimal as up to 104 CFU/mL could still be counted on the agar plates after 60 min exposure. Neat PPy/MAA composite and 11% loading did not show any effect on E. coli bacteria. These antimicrobial assay results confirmed that the higher concentrations tested for the Ag-loaded PPy/MAA composite possessed enhanced antimicrobial activity compared pure PPy/MAA. Gas-Sensing Behavior of PPy/MAA/Ag. In order to investigate the effect of the Ag-adsorbed PPy/MAA composite upon NO2 gas detection, a systematic gas sensing experiment was carried out with different concentrations (20−100 ppm) of NO2 gas at room temperature. Figure 10 shows that the incorporation of Ag in the PPy/MAA composite enhances the response of the device. An increase in the current upon exposure to oxidizing gas reveals a typical p-type semi-

chemical states of S 2p increased from 162.45 to 162.65 and 163.40 to 163.76 eV (Figure 7d(ii)) after adsorption of silver. This indicates a strong interaction between the mercapto group (−SH) of the composite and uptake of silver ions. Thus, the XPS analysis reveals that the adsorption mechanism (Scheme 1) of silver ions onto the PPy/MAA composite surface is mainly through chelation of the mercapto group (−SH) with Ag+. Deconvolution of O 1s shows (Figure 7e(i)) a peak at 531.89 eV attributed to the −COOH moiety of mercaptoacetic acid. The peak intensity decreases significantly after adsorption, and binding energy values also shift toward lower energy (Figure 7e(ii)), indicating the interaction between the oxygen atom of the acid group and silver ions. From the zeta potential analysis (Figure S6, Supporting Information), it can be seen that the PPy/MAA composite carries a negative charge at a wide range of pH since the composite containing an acid group with increasing pH deprotonation takes place, which results in increasingly negative zeta values. Negative zeta values also suggest a strong ability of the adsorbent material for chelation with positive metal ions. Catalytic Reduction of 4-Nitrophenol by Ag-Adsorbed Waste Material. 4-Aminophenol (4-AP) has shown potential application in pharmaceuticals, whereas 4-nitrophenol (4-NP) is one of the most common and toxic pollutant dyes found in industrial wastewater. In past years, the catalytic reduction of 4NP has been extensively employed under metal nanoparticles with an excess amount of NaBH4 in aqueous reaction conditions.6,54,55 Herein, we demonstrated the reduction of 4NP to evaluate the catalytic efficiency of PPy/MAA/Ag waste material. Initially, the reaction was performed by reducing 4-NP with a 10-fold excess of NaBH4 in the presence of PPy/MAA/ Ag. The reaction was monitored by UV−visible spectrophotometry after certain time intervals, as depicted in Figure 8. After addition of NaBH4, the color of the 4-NP solution immediately changed from light yellow to bright yellow, and the λmax showed an absorbance peak at 400 nm due to the formation of the 4-nitrophenolate ion. However, after addition of PPy/MAA/Ag to the stirred reaction mixture, the intensity of the corresponding peak (λmax = 400 nm) slowly decreased

Figure 8. Typical time-resolved UV−vis spectra of the catalytic reduction of 4-nitrophenol (4-NP) in the presence of Ag-loaded adsorbent in aqueous medium. 2720


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ACS Sustainable Chemistry & Engineering Scheme 2. Plausible Mechanism for Reduction of 4-NP Using Waste-Based Ag NPs

and hence increases the hole concentration in the conduction band. However, NO2 has no or negligible affinity for ionized oxygen, so it interacts directly with the polymer. The physisorbed NO2 takes the electron from the valence band or shares it with ionized oxygen as follows:58 NO2 (gas) + e− ⇌ NO−2 (surf. adsorbed)

(7)

The chemisorbed NO2− releases the NO in the atmosphere by leaving behind an adsorbed O− on the surface, hence increasing the hole concentration for conduction to occur. Accordingly, the presence of Ag increases the surface area available for the physisorption of NO2, particularly in the presence of preadsorbed oxygen ions, facilitating the reduction of NO2 to NO gas at the interface between the Ag and polymer and leading to the high response observed.

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Figure 9. Killing kinetics of PPy/MAA and Ag-loaded PPy/MAA against Escherichia coli bacteria.

CONCLUSION In this article, we have demonstrated a novel approach to develop a mercapto group-containing polypyrrole derivative for remediation of environmentally toxic heavy metal ions (Ag+). In addition to the important adsorption of metal ions from polluted waterways, this work shows the potential to reuse the silver-adsorbed waste material for catalytic, gas sensing, and antimicrobial applications, not only in an environmentally friendly manner but also in an economically viable manner. Various characterization techniques (FESEM, EDS, HRTEM, STEM, XRD, FTIR, and XPS) confirmed the formation of a superior PPy/MAA adsorbent, with a strong affinity toward silver ions. The equilibrium adsorption capacity of silver ions on PPy/MAA reached up to 714.28 mg/g at 25 °C in the solution at 5.4 pH. The experimental adsorption results correlated well with the pseudo-second-order kinetics model, and the adsorption equilibrium followed the Langmuir isotherm model when compared to the Freundlich isotherm model. Negative ΔG and positive ΔH values for adsorption thermodynamic analysis revealed that the adsorption of Ag+ on

conducting behavior of the materials. The baseline current of the polymer was found to decrease after absorption of Ag due to the recombination of holes, the majority charge carrier from the PPy/MAA composite, and the free electron from the Ag particles. The response (Rres) was calculated as Rres = Ig/Ia, where Ig and Ia are current in presence and absence of NO2, respectively. Figure 10b shows the responses obtained with and without Ag incorporated in the PPy/MAA composite. The increase in Ag from 0% to 11% and 32% in the composite (PPy/MAA) enhances the sensor response from 1.4 to 4.2 and 7.2, respectively. Hence, the Ag particles enhance the activity of PPy/MAA by increasing the number of sites available for the NO2 adsorption and subsequent dissociation. In general, the surface defective site adsorbs the oxygen from the atmosphere. The physisorbed oxygen is further ionized to O2− at room temperature by obtaining an electron from the valence band 2721


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Figure 10. (a) Current variation, (b) dynamic sensing performance of pure adsorbent (PPy/MAA), and different percentage of silver-loaded adsorbent toward NO2 gas at concentrations ranging from 20 to 100 ppm under the operation temperature of 25 °C.

Notes

the PPy/MAA composite is a spontaneous and endothermic process. Furthermore, the PPy/MAA composite not only effectively adsorbed Ag+ from aqueous solutions but also converted the adsorbed Ag+ to metallic silver nanoparticles (Ag0) by chemical reduction. The newly formed nanoparticles might be stabilized by the chelation with electron-reach functionality (−SH, −COO−) of the composite matrix. The post-adsorption material containing a large amount of silver inhibited Escherichia coli (E. coli) bacterial growth, and thus, the waste silver-adsorbed material could be used as an antimicrobial agent in various applications. In addition, Ag-containing waste material played a vital catalytic role for the reduction of 4nitrophenol under standard reaction condition, and full conversion of the product was obtained within 70 min of the reaction. Moreover, the Ag-adsorbed waste material exhibited good sensitivity and response toward NO2 gas detection. Thus, the empirical results in this present work suggest that the PPy/ MAA composite can be used as a promising adsorbent for removal of toxic metal ions from contaminated water and should be further explored for their particular environmental applications.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors earnestly acknowledge the financial support from the University of South Africa (UNISA), the National Research Foundation, the Department of Science and Technology, Water Research Commission, and the Council for Scientific and Industrial Research (CSIR), South Africa, to carry out the reported investigation. The authors also gratefully acknowledge the DST/CSIR National Centre for Nanostructured Materials characterization unit for assisting with materials characterization.

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(1) Huang, M. R.; Li, S.; Li, X. G. Longan Shell as Novel Biomacromolecular Sorbent for Highly Selective Removal of Lead and Mercury Ions. J. Phys. Chem. B 2010, 114, 3534−3542. (2) Bailey, S. E.; Olin, T. J.; Bricka, R. M.; Adrian, D. D. A Review of Potentially Low-cost Sorbents for Heavy Metals. Water Res. 1999, 33, 2469−2479. (3) Ferry, D. K. Nanowires in Nanoelectronics. Science 2008, 319, 579−580. (4) Alshehri, A. H.; Jakubowska, M.; Młożniak, A.; Horaczek, M.; Rudka, D.; Free, C.; Carey, J. D. Enhanced Electrical Conductivity of Silver Nanoparticles for High Frequency Electronic Applications. ACS Appl. Mater. Interfaces 2012, 4 (12), 7007−7010. (5) Lasanta, T.; Olmos, M. E.; Laguna, A.; López-de-Luzuriaga, J. M.; Naumov, P. Making the Golden Connection: Reversible Mechanochemical and Vapochemical Switching of Luminescence from Bimetallic Gold-Silver Clusters Associated through Aurophilic Interactions. J. Am. Chem. Soc. 2011, 133, 16358−16361. (6) Baruah, B.; Gabriel, G. J.; Akbashev, M. J.; Booher, M. E. Facile Synthesis of Silver Nanoparticles Stabilized by Cationic Polynorbornenes and Their Catalytic Activity in 4-Nitrophenol Reduction. Langmuir 2013, 29, 4225−4234. (7) Hu, H.; Xin, J. H.; Hu, H. PAM/graphene/Ag Ternary Hydrogel: Synthesis, Characterization and Catalytic Application. J. Mater. Chem. A 2014, 2, 11319−11333. (8) Jain, J.; Arora, S.; Rajwade, J. M.; Omray, P.; Khandelwal, S.; Paknikar, K. M. Silver Nanoparticles in Therapeutics: Development of an Antimicrobial Gel Formulation for Topical Use. Mol. Pharmaceutics 2009, 6 (5), 1388−1401.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03008. Results of BET analyses (table), schematic representation for the synthesis of PPy/MAA, FTIR spectra of PPy/MAA before and after adsorption of silver ions, effect of adsorption parameters, pseudo-first-order model, Freundlich adsorption isotherm model, Van’t Hoff plot, zeta potential, and proton NMR spectrum. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +27128412658. Fax: +27128412229. E-mails: [email protected], [email protected]. ORCID

Arjun Maity: 0000-0002-8057-5749 2722


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Removal of Noble Metal Ions (Ag+) by Mercapto Group-Containing

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