Abstract

Polypyrrole (PPy)-based adsorbents have successfully been prepared via oxidative polymerization in aqueous media as a new adsorbent for the removal of arsenic ions in a batch equilibrium system. The prepared adsorbent was characterized by the Brunauer–Emmet–Teller (BET) surface analyzer, field emission scanning electron microscopy (FESEM), and attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR). The BET surface area and average particle size of the prepared PPy powder was 10.27 m2/g and ∼180–295 nm, respectively. Different adsorption parameters, such as adsorbent dosage, contact time, pH of the initial solution, temperature, initial ions and co-ions concentrations were investigated. The results showed that PPy powder acted as an effective sorbent for the removal of arsenic ions at the optimum conditions of pH 6.5 and a contact time of 6 h. The experimental data of PPy isotherms for arsenic ions followed the Freundlich isotherm model and kinetics data were well fitted to the pseudo-first-order model. Thermodynamically, the adsorption process was endothermic and spontaneous in nature. The FTIR and FESEM-EDX results also confirmed the presence of arsenic in adsorbents after adsorption. The presence of amine groups in PPy is believed to play the key role of adsorption of arsenic ions.

INTRODUCTION

The pollution of aqueous environments due to the contamination of various heavy metal ions is of great concern from a health point of view (Mohan & Pittman 2007; Zhang et al. 2016). Some heavy metal ions possess a long-term consequence on human health by disrupting the natural food chain via bioaccumulation (Peralta-Videa et al. 2009). Among the heavy metals, arsenic (As) has led to a massive epidemic of skin cancer, lung cancer, and even bladder cancer (Sazakli et al. 2015; Mahmud et al. 2016). The focal problem lies in the fact that even a lower concentration (<50 ppb) of arsenic chronic exposure may cause arsenic poisoning (Mohan & Pittman 2007; Bhowmick et al. 2014). Thus, various environmental protection agencies and the World Health Organization (WHO) set the lowering permissible limits of arsenic to below 10 ppb in drinking water (WHO 1998; US EPA 2001). Therefore, it is urgently required to find a simple, fast, and effective method to remove arsenic completely from drinking water. Currently, adsorption techniques are considered as the most suitable processes for arsenic removal due to their simplicity, stability, and cost-effectiveness (Mondal et al. 2013; Mahmud et al. 2016). In addition, some adsorption processes are sludge free where the loaded adsorbents can be regenerated for repeated use. From this perspective, polypyrrole (PPy)-based adsorbents provide a greater opportunity to adsorb various heavy metal ions from aqueous solutions owing to their ease of synthesis, operation, environmental stability, and regeneration (Deng & Bai 2004; Abdi et al. 2009). PPy has exhibited good prospects in adsorption application for its nitrogen atoms in the polymer chains (Abdi et al. 2009).

Some efforts have been made to adsorb heavy metal ions from aqueous solution by PPy-based composites or nanocomposites prepared by polymerization of pyrrole in the presence of different dopants and/or surfactants, biomaterials, nanomaterials, or nanofibers (magnetic nano Fe3O4, polyalanine, graphene, reduced graphene oxide, hierarchical porous nanomaterials, etc.) (Chandra & Kim 2011; Yao et al. 2011; Bhaumik et al. 2012; Karthik et al. 2015; Setshedi et al. 2015), but PPy has not been reported for the removal of arsenic ions from aqueous solution yet.

Thus, the present effort reports on the adsorption of arsenic ions from aqueous solution by PPy powder, a novel polymer-based adsorbent, synthesized from using pyrrole monomer and ferric chloride hexahydrate oxidant in aqueous solution at room temperature. The preparation technique of PPy plays a vital role towards higher adsorption efficiency (AE) of arsenic ions.

The effects of different parameters, such as adsorbent dosage, contact time, initial pH, temperature, and concentration of arsenic ions have been investigated as a function of operating conditions of adsorption. Sorption isotherms, kinetics, and thermodynamics on the adsorption of arsenic ions have been studied in detail. Finally, a plausible mechanism adsorption of arsenic ions by the prepared PPy-based adsorbents has been proposed.

MATERIALS AND METHODS

Materials

High purity chemicals were used in all reagents and stock solution preparation. The arsenic ions are As(V), purchased from Merck Millipore Arsenic ICP standard (product code: 170303). Freshly distilled pyrrole (Sigma-Aldrich) was used as the monomer and FeCl3.6H2O (Sigma-Aldrich) was used as an oxidant. HCl and NaOH (Merck) were used to control the pH of the working solution.

Synthesis of PPy

PPy-based adsorbent was prepared by chemical oxidative polymerization of pyrrole in aqueous solution using FeCl3.6H2O as an oxidant with a monomer to oxidant mole ratio of 1:1. The polymerization was carried out for 3 h at ambient temperature. The solution was filtered and washed several times with deionized water to remove excess reactants before drying at 65 °C for 24 h to collect the black PPy powder.

Characterization

The Brunauer–Emmet–Teller (BET) surface area analysis of the PPy adsorbent was done by using the surface analyzer (Sorptomatic Thermo Finnigan 1990, USA) via N2 adsorption at 77.40 K. The surface morphology was observed via field emission scanning electron microscopy (FESEM, Hitachi Brand, Model: SU 8220). The presence of metal ions was observed by FESEM equipped with EDX and the functional groups of PPy were observed via attenuated total reflectance–Fourier transform infrared spectrophotometer (ATR-FTIR, Perkin-Elmer, FTIR Spectrum 400). The wavelength was recorded in the region of 500 to 4,000 cm−1 at room temperature.

Batch adsorption experiments

The batch adsorption was carried out for the known concentration of arsenic ions with the prepared PPy-based adsorbents at various dosages. The adsorbent dosage used was from 0.05 g to 0.35 g for 50 mL of known concentration of arsenic ions. The optimum time and the adsorbent dosage were determined at room temperature with controlled pH. The initial and the final concentrations of As ions in aqueous solution were determined by inductively couple plasma-mass spectroscopy (ICP MS). The percentage removal of As ions was calculated according to the following equation:  
formula
(1)
where (mg/L) and Cf (mg/L) are the concentrations of metal ions at the initial and final times, respectively. The amount of arsenic ions adsorbed per unit mass of adsorbent at equilibrium, (mg/g), was calculated by the following equation:  
formula
(2)
where V is the volume of the solution in L, and m is the mass of adsorbent in g.

The effect of contact time on the adsorption of metal ions was carried out in steps of predetermined interval times (30 min) keeping other parameters constant. The optimum pH for adsorption of arsenic ions by PPy was determined experimentally in a wide pH range of 2.5–10.5 using 50 mL solutions of 1,000 ppb metal ions concentration. The effect of temperature on the adsorption of arsenic ions was studied using different initial concentrations of arsenic ions (1,000–10,000 ppb) at different temperatures (20, 40, and 60 °C). The pH, adsorption dosage, and the equilibrium time were kept constant. The effect of different ions like zinc and cadmium on arsenic ion adsorption by PPy was also studied at different concentrations of zinc and cadmium with arsenic in the solution at a pH of 6.5.

RESULTS AND DISCUSSION

PPy synthesis

The ratio of monomer (pyrrole) to oxidant (FeCl3.6H2O) plays a key role for synthesis of PPy as an effective adsorbent. Table 1 shows that the yield of PPy increased with an increase in the amount of oxidant at the same preparation conditions, but the AE of arsenic ions was found to be highly dependent on the specific ratios of monomer to oxidant.

Table 1

Monomer to oxidant ratio versus AE

Monomer: OxidantYield of PPy (g)Physical appearance before dryingAE of As (%)
1:0.25 0.1247 Very thin layer, glazy 85.4 
1:0.50 0.2986 Irregular thin leaves like, glazy, shiny 89.7 
1:1 0.6549 Irregular flakes, glazy, shiny, fine 99.8 
1:2 1.2172 Thick, muddy like, not shiny 88.3 
1:3 1.8831 Very thick compact layer 78.9 
Monomer: OxidantYield of PPy (g)Physical appearance before dryingAE of As (%)
1:0.25 0.1247 Very thin layer, glazy 85.4 
1:0.50 0.2986 Irregular thin leaves like, glazy, shiny 89.7 
1:1 0.6549 Irregular flakes, glazy, shiny, fine 99.8 
1:2 1.2172 Thick, muddy like, not shiny 88.3 
1:3 1.8831 Very thick compact layer 78.9 

Polymerization with a 1:1 ratio produces an irregular flaky, shiny and stable powder and gives the maximum AE (99.8%) for the adsorption of arsenic ions. On the other hand, polymerization with higher oxidant ratios (1:3 or 1:2) produces a very hard and compact polymer with lower AE for arsenic ions. Again, polymerization with lower oxidant ratio (1:0.5 or 1:0.25) produces a very thin and low amount. Among the different ratios of monomer to oxidant for the preparation of PPy adsorbents, the ratio of 1:1 has been chosen for the rest of the experiments as this ratio showed the maximum adsorption of arsenic ions under certain conditions.

Surface characterization

The surface area of the adsorbent is an important physical factor for the adsorption process of different materials. Table 2 shows the structural characteristics of the prepared PPy adsorbent. It was observed that the BET surface area, pore volume, and pore size of the prepared fine PPy powder were 10.27 m2/g, 0.0217 cm3/g, and 84.47 Å, respectively.

Table 2

BET analysis of the prepared PPy fine powder

BET surface area (m2/g) 10.27 
Pore volume (cm3/g) 0.0217 
Pore size (Å) 84.47 
Particle size (nm) ∼180–295 
BET surface area (m2/g) 10.27 
Pore volume (cm3/g) 0.0217 
Pore size (Å) 84.47 
Particle size (nm) ∼180–295 

The average particle size was measured at ∼180–295 nm by SEM micrograph (Figure 1). The cauliflower-like porous morphology of the prepared PPy fine powder was observed by FE-SEM (Figure 2(a)) which indicates the typical PPy formation. The morphology looks a little different after adsorption of arsenic ions by PPy (Figure 2(b)). However, EDX pattern exhibited strong evidence for adsorption of arsenic ions on the surface of PPy fine powder (Figure 2(c)).

Figure 1

Particle size measurement by FE-SEM micrograph.

Figure 1

Particle size measurement by FE-SEM micrograph.

Figure 2

FE-SEM micrograph of PPy (a) before adsorption, (b) after adsorption of arsenic ions, and (c) presence of As in EDX elements' mapping.

Figure 2

FE-SEM micrograph of PPy (a) before adsorption, (b) after adsorption of arsenic ions, and (c) presence of As in EDX elements' mapping.

Figure 3 shows the FTIR spectra of PPy (Figure 3(a)) before adsorption and (Figure 3(b)) after adsorption of arsenic ions. The characteristic peaks of pyrrole at 1,546 cm−1 (stretching mode of the -C-C- in the pyrrole ring) shifted to 1,528 cm−1 after arsenic ion adsorption. The bands at 1,041 cm−1 (-N-H in-plane bending mode) and 1,308 cm−1 (-C-N in-plane deformation) were also found to have shifted to 1,015 cm−1 and 1,288 cm−1, respectively, after adsorption of arsenic ions. The shift of the band positions of the -C-N- stretching mode of pyrrole and the -C-N in-plane deformation of pyrrole suggest the interaction of arsenic ion with nitrogen of the pyrrole ring. Again, the peaks observed at 902 cm−1 and 785 cm−1 (-C-H- out-plane deformation) shifted to 846 cm−1 and 762 cm−1, respectively, after arsenic adsorption. All these shifts clearly show the interaction of arsenic ions by the prepared PPy adsorbent through the probable coordination of arsenic ions to nitrogen in amine functional groups of PPy polymer. The possible complex metal ion adsorption by PPy is shown in Figure 4.

Figure 3

ATR-FTIR Spectra of PPy (a) before adsorption and (b) after adsorption of arsenic ions.

Figure 3

ATR-FTIR Spectra of PPy (a) before adsorption and (b) after adsorption of arsenic ions.

Figure 4

A schematic plausible mechanism of adsorption of arsenic ions by PPy fine powder.

Figure 4

A schematic plausible mechanism of adsorption of arsenic ions by PPy fine powder.

Effect of adsorbent dosage

Figure 5(a) shows the effect of adsorbent dosage (0.05–0.35 g) on the removal of arsenic ions. It was observed that the maximum AE (99.8%) was achieved using 0.2 g of PPy adsorbent. The removal efficiency of arsenic ions was considerably increased from 58.3% to 99.8% with the increase in adsorbent dosage from 0.05 g to 0.2 g. The removal efficiency remained unchanged when the dosage was increased beyond 0.2 g. The increase of the removal efficiency with the increase in adsorbent dosage is likely attributed to the availability of more active sites for the adsorption. However, a further increase in PPy dosage did not improve the removal efficiency, which may be due to the adsorption of almost all arsenic ions to the adsorbent and attaining equilibrium with the solution. Several other researchers have shown a very similar pattern of the effects of adsorbent dosage on different heavy metal ions (Katsoyiannis & Zouboulis 2002; Li et al. 2012). Thus, the optimum PPy dose of 0.2 g was chosen for the rest of the study on removal of arsenic ions.

Figure 5

Batch AE of arsenic ions: (a) effect of adsorbent dosage, (b) effect of pH, (c) effect on contact time, and (d) initial ions concentration.

Figure 5

Batch AE of arsenic ions: (a) effect of adsorbent dosage, (b) effect of pH, (c) effect on contact time, and (d) initial ions concentration.

Effect of initial pH of the solution

The initial pH of the solution is a key factor for the removal of different heavy metal ions from aqueous solution due to the impact of pH on the surface charge of the adsorbent and the degree of ionization and specificity of the adsorbate (Abdel-Halim & Al-Deyab 2011). In this study, a broad range of pH (2.5–10.5) was applied to determine the optimum pH for adsorption of arsenic ions while the literature suggests the optimum pH range of 5–7 by several polymeric and non-polymeric adsorbents (Chutia et al. 2009; Abdel-Halim & Al-Deyab 2011; Li et al. 2012). In this study, the optimum pH was found to be 6.5 (Figure 5(b)).

Effect of contact time

Figure 5(c) shows the effect of contact time and indicates that the maximum efficiency was found at a contact time of 6 h. It was observed that the removal efficiency was rapid within the first hour and then gradually increased over time up to 6 h, beyond which the AE was steady and reached equilibrium.

Effect of initial ion concentration and temperature

The effect of the initial concentration of arsenic ions on adsorption is shown in Figure 5(d). The adsorption efficiencies were high (>99%) at a lower initial arsenic ion concentration (1,000 ppb) in all three temperature zones. However, with the increase in the initial arsenic ion concentration from 1,000 ppb to 10,000 ppb, the efficiencies of arsenic ion adsorption were found to decrease in all temperature zones. This is probably due to the higher number of arsenic ions per unit mass of adsorbent. A similar temperature effect was observed for chromium ions uptake by PPy/magnetic nanocomposites (Bhaumik et al. 2011).

Effect of co-ions

Various metallic co-ions affect the adsorption behavior by their presence as competitive ions. The effect of co-ions, namely, zinc and cadmium, on the AE of arsenic ions indicated that these competitive ions had little affect on the adsorption of arsenic ions under the present optimum conditions (Table 3).

Table 3

Effects of various co-ions on the AE of arsenic ions

Concentration of As ions (ppb)Concentration of co-ions (ppb)
Removal of heavy metal ions (%)
ZnCdTotal co-ionsAsZnCd
1,000 – – 99.8 – – 
1,000 1,000 – 1,000 93.0 23.8 – 
1,000 – 1,000 1,000 92.1 – 24.9 
1,000 1,000 1,000 2,000 91.3 19.3 21.7 
Concentration of As ions (ppb)Concentration of co-ions (ppb)
Removal of heavy metal ions (%)
ZnCdTotal co-ionsAsZnCd
1,000 – – 99.8 – – 
1,000 1,000 – 1,000 93.0 23.8 – 
1,000 – 1,000 1,000 92.1 – 24.9 
1,000 1,000 1,000 2,000 91.3 19.3 21.7 

For the tri-metal solution with an equal concentration of As, Zn, and Cd ions, the corresponding AE was 91.3%, 19.3%, and 21.7%, respectively. This indicated that the PPy adsorbent displayed preferential adsorption of As over Cd and Zn ions. The different adsorption behaviors may be due to the nature of metal ions at certain pH and the interaction between the adsorbent and the adsorbate.

Adsorption isotherms

The results of adsorption data were used in two common models (Langmuir and Freundlich) for isotherm analysis by arsenic ion adsorption. The Langmuir model supposes that uptake of arsenic ions occurs on a homogeneous surface by monolayer adsorption. The equation of the Langmuir isotherm model is as follows:  
formula
(3)
where (mg/g) is the maximum adsorption capacity corresponding to complete monolayer coverage, (L/g) is a constant related to adsorption capacity and energy of adsorption. A straight line was obtained when was plotted against (Figure 6(a)).
Figure 6

Langmuir (a) and Freundlich (b) isotherms obtained for sorption of arsenic ions.

Figure 6

Langmuir (a) and Freundlich (b) isotherms obtained for sorption of arsenic ions.

For the linearized Freundlich adsorption isotherm based on adsorption on a heterogeneous surface, the expression is given as:  
formula
(4)
where (L/g) and n are the Freundlich constants, indicating the adsorption capacity and the adsorption intensity, respectively. A straight line was obtained when was plotted against and, n and were calculated from the slopes and intercepts (Figure 6(b)).

The results obtained from both the isotherm models for arsenic ions by PPy adsorbents have shown that the Freundlich isotherm model correlated better than the Langmuir isotherm model. Moreover, adsorption capacity for the adsorption of arsenic ions by the PPy adsorbent increased with an increase in temperature, which predicts that the adsorption process is endothermic (Li et al. 2016).

Adsorption kinetics

Kinetic studies of arsenic ions adsorption were thoroughly performed at 20 °C and a pH of 6.5 to observe the rate and mechanism of adsorption of arsenic ions using pseudo-first-order and pseudo-second-order kinetics. The maximum adsorption capacity (mg/g), the Lagergren rate constant of adsorption (min−1), and (mg/g), the amount of adsorption at time t (min), were calculated for arsenic ions from the pseudo-first-order rate model of Equation (5):  
formula
(5)
The values of and were determined from the intercepts and the slope of the plot of log () versus t (Figure 7(a)).
Figure 7

(a) Pseudo-first-order and (b) pseudo-second-order kinetic model for the removal of arsenic ions.

Figure 7

(a) Pseudo-first-order and (b) pseudo-second-order kinetic model for the removal of arsenic ions.

The pseudo-second-order rate model is expressed as follows:  
formula
(6)
where is the pseudo-second-order rate constant of adsorption (g/mg min), and were determined from the intercepts and the slope of the plot of versus t, as shown in Figure 7(b).

In the pseudo-first-order kinetic model, the correlation coefficients of are high and the calculated significantly resembles the experimental value, indicating that the pseudo-first-order model fits well to this adsorption process compared to the pseudo-second-order kinetic model at lower arsenic concentration. Similar findings on the kinetic study for the adsorption of As(III) from aqueous solution onto iron oxide impregnated activated alumina have been reported (Kuriakose et al. 2004).

Adsorption thermodynamics

The thermodynamic parameters ΔGo, ΔHo, and ΔSo for the adsorption of arsenic ions in aqueous solution onto PPy adsorbents were evaluated in this study at different temperatures (293, 313, and 333 K) by using the following equations:  
formula
(7)
 
formula
(8)
where ΔGo is Gibb's free energy, ΔSo is entropy, ΔHo is enthalpy, and R is the universal gas constant (8.314 J mol/K). The plot of lnkd against 1/T was made to measure the dependence of adsorption process on temperature by evaluating the thermodynamic parameters that are summarized at various temperatures in Table 4.
Table 4

Thermodynamic parameters of the removal of arsenic ions

T (K)Thermodynamic parameters
ΔG0 (kJ/mol)ΔH0 (kJ/mol)ΔS0 (J/K mol)
293 −17.996 12.376 103.7 
313 −20.069 
333 −22.142 
T (K)Thermodynamic parameters
ΔG0 (kJ/mol)ΔH0 (kJ/mol)ΔS0 (J/K mol)
293 −17.996 12.376 103.7 
313 −20.069 
333 −22.142 

The negative (ΔGo) value suggests that the adsorption process is a spontaneous process and thermodynamically favorable under the experimental conditions. The decrease of (ΔGo) with increase in temperature indicates more efficient adsorption at higher temperature. The enthalpy change (ΔHo) was found to be positive, indicating an endothermic nature for the adsorption. In the case of entropy, a positive entropy change (ΔSo) was found, suggesting an increase in the number of species and hence randomness in the interface sorption process. Similar results were reported in the literature for thermodynamic behaviors of arsenic ions by different polymeric adsorbent (Urbano et al. 2012).

The performance of the prepared PPy adsorbents to adsorption of arsenic ions were compared to the performance of other adsorbents, as shown in Table 5.

Table 5

Comparison of the performance of various adsorbents for arsenic ions adsorption

Types of adsorbentsCapacity
References
mg/g%
Iron oxide coated sand 0.136 – Thirunavukkarasu et al. (2005)  
Activated carbon – 15.8 Eisazadeh (2008)  
Conducting PPy – 7.4 Eisazadeh (2008)  
Composite of PPy with bentonite – 91.2 Eisazadeh (2008)  
Activated alumina 0.18 – Sing & Pant (2004)  
Activated red mud 0.94 – Altundogan et al. (2002)  
PPy/sawdust – 74.5 Ansari et al. (2008)  
Polyaniline/sawdust – 82.6 Ansari et al. (2008)  
Prepared PPy 1.91 99.8 Present study 
Types of adsorbentsCapacity
References
mg/g%
Iron oxide coated sand 0.136 – Thirunavukkarasu et al. (2005)  
Activated carbon – 15.8 Eisazadeh (2008)  
Conducting PPy – 7.4 Eisazadeh (2008)  
Composite of PPy with bentonite – 91.2 Eisazadeh (2008)  
Activated alumina 0.18 – Sing & Pant (2004)  
Activated red mud 0.94 – Altundogan et al. (2002)  
PPy/sawdust – 74.5 Ansari et al. (2008)  
Polyaniline/sawdust – 82.6 Ansari et al. (2008)  
Prepared PPy 1.91 99.8 Present study 

CONCLUSIONS

PPy-based adsorbents were synthesized via chemical oxidation of pyrrole which has exhibited almost complete adsorption of arsenic ions from aqueous solution. The adsorption performance of PPy is highly dependent on the preparation conditions of the mole ratio of monomer to oxidant. The batch adsorption conditions play a major role for adsorption of arsenic ions as they were highly pH and dose dependent. The presence of nitrogen atoms in PPy is mainly responsible for adsorption of arsenic ions as is evidenced from the study of FTIR and EDX. The thermodynamic constants, ΔGo, ΔHo, and ΔSo of the adsorption process showed that adsorption properties of arsenic ions were endothermic and spontaneous in nature. PPy fine powder appears to be a highly efficient adsorbent for the adsorption of arsenic ions from aqueous solution and offers the promise to be used commercially in the near future due to its high AE, low cost, and ease of preparation.

ACKNOWLEDGEMENTS

The authors would like to acknowledge a University of Malaya Research Grant (RP014A-15SUS) and University of Malaya PPP Grant (PG 109-2014A) for the financial support.

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