Nanocatalysis using metal nanoparticles constitutes one of the emerging technologies for destructive oxidation of organics such as dyes. This paper deals with the degradation of acid red-26 (AR-26), an azo dye by hexacyanoferrate (abbreviated as HCF) (III) using iridium nanoparticles. UV-vis spectroscopy has been employed to obtain the details of the oxidative degradation of the selected dye. The effect of various operational parameters such as HCF(III) concentration, pH, initial dye concentration, catalyst and temperature was investigated systematically at the λmax, 507 nm, of the reaction mixture. Degradation kinetics follows the first order kinetic model with respect to AR-26 and Ir nano concentrations, while with respect to the HCF(III) concentration reaction it follows first order kinetics at lower concentrations, tending towards zero order at higher concentrations. Thermodynamic parameters have been calculated by studying the reaction rate at four different temperatures. The UV-vis, high performance liquid chromatography (HPLC), liquid chromatography–mass spectrometry (LC-MS) analysis of degradation products showed the formation of carboxylic acid and substituted carboxylic acids as major degradation products, which are simple and less hazardous compounds. The big advantage of the present method is the recovery and reuse of iridium nanoparticles. Moreover, turnover frequencies for each catalytic cycle have been determined, indicating the long life span of Ir nanoparticles. Thus, the finding is a novel and highly economical alternative for environmental safety against pollution by dyes, and extendable for other contaminants as well.

INTRODUCTION

Metal nanoparticles possess unique catalytic behavior under mild conditions and are considered as catalysts of choice due to their fascinating properties, green nature and ability to catalyze selective reactions at low temperatures (Karlik & Biffis 2001; Stowell & Korgel 2005; Hassan et al. 2011). Partly owing to their increased surface-to-volume ratio and new electronic properties from quantum confinement, nanoparticles often have new or superior catalytic activity compared with their corresponding bulk materials. Despite their application for various tasks, nanoparticles possess tremendous potential as catalysts in various types of pollution control studies, especially aquatic pollution (Pradhan et al. 2001; Cortie & van der Lingen 2002; Zhang 2003; Poursaberi et al. 2012). The organic dyes found extensively in industrial waste water produced by the dyestuff manufacturing, dyeing, printing and textile industries, are a major source of toxic contaminants in the form of colored wastewater. This causes damage to both aquatic life and human beings due to its carcinogenic and mutagenic effects (Daneshwar et al. 2004, 2005; Qin et al. 2009). Azo dyes containing one or more azo (–N=N–) groups, which account for nearly 50% of all dyes produced, are commonly used because of their chemical stability and versatility (Bokare et al. 2008). Treatment of dye waste water has been achieved by many methods such as adsorption, chemical degradation, coagulation, filtration, and ozone treatment, and most recently, advanced oxidation processes, each having their characteristic advantages and disadvantages (Samarghandi et al. 2012).

Nanocatalysis using metal nanoparticles constitutes one of the emerging technologies for destructive oxidation of organics such as dyes. Among the different transition metals used as catalysts, the interest in iridium based nanocatalysts is increasing due to their high activity, stability and selectivity under different reaction conditions (Kundu & Liang 2011). The use of nano iridium has already shown tremendous potential for degradation of amino acids (Goel & Sharma 2012a; Goel & Sharma 2012b), methyl orange (Goel et al. 2012) and orange II (Goel et al. 2014). Extending our research to acid red-26 (AR-26), an azo dye extensively used in the dyeing and printing industries, the present work highlights the catalytic oxidative degradation of AR-26 by HCF(III) as an oxidizing agent in the presence of Ir nanoparticles. The main objective of this work is to test the catalytic activity of Ir nanoparticles for the degradation of AR-26, and to confirm the relevance of this process to treat azo dyes. Furthermore, the influence of some important reaction conditions such as pH, oxidant concentration, AR-26 concentration, Ir nano concentration and temperature on AR-26 degradation has been investigated. The recovery and reuse of iridium nanoparticles make the present method simple, green, reproducible and cost effective.

EXPERIMENTAL

Materials

Azo dye AR-26 (the chemical structure is shown in Figure 1) was purchased from Loba Chemie (Loba Chemie Pvt. Ltd, Mumbai, India). All the chemicals and reagents used were of analytical grade. Deionized water was used throughout this study. The measurements of the pH of the reaction mixture were carried out with a digital pH meter (Systronics μ pH system 361), which was adjusted by using solutions of KH2PO4 and NaOH. Ir-nano were synthesized according to the literature reported earlier (Goel & Rani 2012) by the wet reduction method using polyvinylpyrrolidone (PVP) and polyoxyethylene (23) lauryl ether (POLE) as protecting agents after the reduction of the precursor salt, IrCl3.3H2O, by methanol. Average particle size of PVP-stabilized Ir-nano and POLE-stabilized Ir-nano is 4 ± 0.5 nm and 21.21 ± 1.3 nm, respectively. Due to its smaller size, PVP-stabilized Ir-nano has been used as the catalyst in the kinetic study.
Figure 1

Chemical structure of AR-26.

Figure 1

Chemical structure of AR-26.

Experimental procedures

The kinetic experiments were conducted at optimum pH (9.0) and constant temperature (40 ± 0.1 °C). All experiments were carried out in 100 ml-stoppered iodine flasks, which were placed in a thermostated water-bath provided with agitation. Each experimental run was performed by adding the pre-determined amount of each reactant in the iodine flask. The reaction was initiated by injecting the solution of AR-26 into the aforementioned reaction mixture. Samples were withdrawn from the reaction mixture at preset time intervals using a pipette and were analyzed immediately. The progress of the reaction was measured spectrophotometrically (Systronics −117) with a spectrometric quartz cell (1 cm path length) at 507 nm, corresponding to the λmax of the reaction mixture. It was verified that there is negligible interference from other species present in the reaction mixture at this wavelength. The initial rate was calculated by the plot of absorbance vs time using the plane mirror method, which is one of the common methods used for the evaluation of initial rates in terms of (da/dt)i, where (da/dt)i denotes the initial change in the absorbance with time. The absorbance of the reaction mixture is directly proportional to the concentration of dye.

Analytical methods

The reaction mixture was kept at atmospheric conditions for 24 hrs and the products were extracted with ethyl acetate. The degradation of dye was assessed using UV-vis spectral analysis and identification of degradation products was carried out by high performance liquid chromatography (HPLC) (Shimadzu, LC-2010CHT) and liquid chromatography–mass spectrometry (LC-MS) analysis (Bruker, ELITE). The mobile phase consisted of acetonitrile: water at (60:40) at a flow rate of 300 μL/min.

RESULTS AND DISCUSSION

Kinetic study

In order to investigate the mechanism of degradation and potential rate controlling steps, the kinetic behavior of oxidative degradation has been studied at a constant pH and temperature at different concentrations of one reactant, keeping the concentration of the others constant.

Effect of pH

The pH of solutions is a key parameter in the present method as it controls the generation of hydroxyl ions. The effect of pH on the rate was investigated in the pH range 6.0 to 10.5. Figure 2 shows the maximum degradation rate at pH 9.0. In an alkaline medium, OH reacts with dye to extract protons and form anionic species, D2− in equilibrium (Goel et al. 2014). At high pH values, the rate of degradation may decline due to columbic repulsion between the anionic dye surface and the hydroxyl anion, hence they do not have the opportunity to react with the dye molecules (Davis & Huang 1989; Rupa et al. 2007). 
formula
1
Figure 2

Effect of pH on the degradation of AR-26 by HCF(III). Experimental conditions: AR-26 concentration = 3.0 × 10−5 mol dm−3; HCF concentration = 3.0 × 10−6 mol dm−3; Ir nano concentration = 1.004 × 10−7 mol dm−3; temperature = 40 ± 0.1 °C.

Figure 2

Effect of pH on the degradation of AR-26 by HCF(III). Experimental conditions: AR-26 concentration = 3.0 × 10−5 mol dm−3; HCF concentration = 3.0 × 10−6 mol dm−3; Ir nano concentration = 1.004 × 10−7 mol dm−3; temperature = 40 ± 0.1 °C.

Effect of HCF(III) concentration

The degradation of AR-26 follows first order kinetics at lower concentrations, tending towards zero order at higher concentrations with respect to HCF(III) concentration, showing the influence of HCF(III) concentration on the rate of degradation. The concentration of HCF(III) was varied from 1 × 10−6 to 9 × 10−6 mol dm−3 and the results are shown in Figure 3.
Figure 3

Effect of HCF(III) concentration on the degradation of AR-26 by HCF (III). Experimental conditions: AR-26 concentration = 3.0 × 10−5 mol dm−3; Ir nano concentration = 1.004 × 10−7 mol dm−3; pH= 9.0; temperature = 40 ± 0.1 °C.

Figure 3

Effect of HCF(III) concentration on the degradation of AR-26 by HCF (III). Experimental conditions: AR-26 concentration = 3.0 × 10−5 mol dm−3; Ir nano concentration = 1.004 × 10−7 mol dm−3; pH= 9.0; temperature = 40 ± 0.1 °C.

Effect of AR-26 concentration

It is important from an application point of view to study the dependence of the initial concentration of dye on the rate of degradation. The effect of the variation of dye concentration was examined by varying the concentration of AR-26 from 1 × 10−5 to 8 × 10−5 mol dm−3. Figure 4 shows the first order dependence of rate on the concentration of substrate. It can be observed that initially at a low substrate concentration, reaction rates are low as compared to a high concentration.
Figure 4

Effect of AR-26 concentration on the degradation of AR-14 by HCF(III). Experimental conditions: HCF(III) concentration = 3.0 × 10−6 mol dm−3; Ir nano concentration = 1.004 × 10−7 mol dm−3; pH = 9.0; temperature = 40 ± 0.1 °C.

Figure 4

Effect of AR-26 concentration on the degradation of AR-14 by HCF(III). Experimental conditions: HCF(III) concentration = 3.0 × 10−6 mol dm−3; Ir nano concentration = 1.004 × 10−7 mol dm−3; pH = 9.0; temperature = 40 ± 0.1 °C.

Effect of Ir nano concentration

The concentration of catalyst may also affect the rate of degradation of the dye and therefore, the concentration of Ir nanoparticles was varied many times, from 0.2 × 10−7 to 1.204 × 10−7 mol dm−3. A gradual increase in rate with Ir nano concentration reveals first order kinetics with respect to the Ir nano concentration, as shown in Figure 5. The increase in rate means that the rate of reaction may have been controlled by the mass of the catalyst.
Figure 5

Effect of Ir nano concentration on the degradation of AR-26 by HCF(III). Experimental conditions: AR-26 concentration = 3.0 × 10−5 mol dm−3; HCF (III) concentration = 3.0 × 10−6 mol dm−3; pH = 9.0; temperature = 40 ± 0.1 °C.

Figure 5

Effect of Ir nano concentration on the degradation of AR-26 by HCF(III). Experimental conditions: AR-26 concentration = 3.0 × 10−5 mol dm−3; HCF (III) concentration = 3.0 × 10−6 mol dm−3; pH = 9.0; temperature = 40 ± 0.1 °C.

Effect of the addition of electrolytes

Investigation of the interference of electrolytes on the degradation of AR-26 has been conducted with NaCl, KCl, and Na2SO4. The concentration of electrolytes was varied between 0 and 4 wt%, and all other experimental conditions were kept constant. The effect of the presence of different electrolyte dosages on the reaction rate is shown in Table 1. It was observed that the reaction rate increased with an increase in concentration of NaCl and KCl, which may be due to increase in ionic strength. On the contrary, the addition of Na2SO4 resulted in a decreased degradation rate, which may be attributed to the common ion effect of sulphate ions formed during the reaction.

Table 1

Effect of the addition of electrolyters

  (−da/dt)i × 103 (min−1)
 
% wt. of electrolyte KCl NaCl Na2SO4 
2.8 2.8 2.8 
3.0 3.0. 2.6 
3.2 3.2 2.4 
3.6 3.4 2.0 
4.0 3.8 1.6 
  (−da/dt)i × 103 (min−1)
 
% wt. of electrolyte KCl NaCl Na2SO4 
2.8 2.8 2.8 
3.0 3.0. 2.6 
3.2 3.2 2.4 
3.6 3.4 2.0 
4.0 3.8 1.6 

Effect of temperature and calculation of thermodynamic parameters

The effect of temperature on the degradation was studied at different temperatures, i.e., 40, 45, 50 and 55 °C. Increasing temperature had a positive effect on the degradation rate. This can be explained by the degradation being accelerated by raising the temperature, which improved the generation rate of OH ions and therefore enhanced the degradation of AR-26. The activation energy, Ea, was computed with the Arrhenius equation: 
formula
2
The Arrhenius plot of the degradation rate of AR-26 is shown in Figure 6. It can be seen that a good linear relationship exists in the plot. Values for the activation energy, Ea, and the pre-exponential factor (A) were obtained from the Arrhenius plot (Table 2). Thermodynamic calculations were also performed to investigate the impacts of temperature in detail. Thermodynamic parameters including enthalpy of activation (ΔH#), entropy of activation (ΔS#) and energy of formation (ΔF#) have been calculated in order to evaluate the degradation process. With this aim, the following equations were used: 
formula
3
 
formula
4
 
formula
5
Table 2

Thermodynamic parameters

Parameter Values 
Ea (J mol−14.14 × 104 
Δ H# (J mol−13.87 × 104 
Δ S# e.u. −26.47 
Δ F# (J mol−17.43 × 104 
A (l mol−1 s−12.98 × 107 
Parameter Values 
Ea (J mol−14.14 × 104 
Δ H# (J mol−13.87 × 104 
Δ S# e.u. −26.47 
Δ F# (J mol−17.43 × 104 
A (l mol−1 s−12.98 × 107 
Figure 6

Arrhenius plot for initial degradation rate of AR-26 by [HCF (III) ]. Experimental conditions: [AR-26] = 3.0 × 10−5 mol dm−3; [HCF(III)] = 3.0 × 10−5 mol dm−6; [Ir nano] = 1.004 × 10−7 mol dm−3; pH = 9.0.

Figure 6

Arrhenius plot for initial degradation rate of AR-26 by [HCF (III) ]. Experimental conditions: [AR-26] = 3.0 × 10−5 mol dm−3; [HCF(III)] = 3.0 × 10−5 mol dm−6; [Ir nano] = 1.004 × 10−7 mol dm−3; pH = 9.0.

Calculated values of ΔH#, ΔS#, and ΔF# are summarized in Table 2. The data in Table 2 reveal low energy of activation for the reaction, which implies good catalytic activity of Ir-nano. The negative entropy of activation shows the formation of polar species during the reaction.

Effect of iridium nanoparticle particle size

The effect of iridium nanoparticle size on the rate of degradation of AR-26 was also studied under similar experimental conditions. A graph plotted showing absorbance vs time for Ir(III), an Ir-nano with a particle size of 21.21 ± 1.3 nm (POLE-stabilized Ir-nano) and an Ir-nano with a particle size of 4 ± 0.5 nm (PVP-stabilized Ir-nano) in Figure 7 shows that the degradation rate is maximum for the reaction catalyzed by Ir-nano with a particle size of 4 ± 0.5 nm compared to other catalysts. This reveals that the catalytic activity of nanoparticles of a smaller size (4 ± 0.5 nm) is greater than that of particles of a larger size (21.21 ± 1.3 nm) and Ir(III). This may be due to the large surface area to volume ratio of these particles.
Figure 7

Comparative kinetics of AR-26 degradation using Ir nano with size 4 ± 0.5 nm, Ir nano with size 21.21 ± 1.3 nm and Ir(III) as catalyst.

Figure 7

Comparative kinetics of AR-26 degradation using Ir nano with size 4 ± 0.5 nm, Ir nano with size 21.21 ± 1.3 nm and Ir(III) as catalyst.

Efficiency of recycled catalyst

The economy of the present oxidative degradation process, which is the number of times a catalyst can be reused without sacrificing its efficiency, was investigated by determining the lifespan of Ir nanoparticles. These nanoparticles were recovered and reused for three consecutive cycles. After the first degradation cycle, the treated reaction mixture was centrifuged. The obtained nanoparticles were washed thoroughly with double distilled water five to six times and were further reused as a catalyst in the kinetic study. The same method was used for two successive cycles with fixed experimental conditions. The results are presented in Table 3. It was found that the rate of reaction decreased with each successive cycle. The stepwise decrease in the catalytic efficiency is, however, due to the continuous agglomeration of Ir nanoparticles after each cycle.

Table 3

Rate of degradation of reaction mixture for three consecutive cycles

No. of cycles (dn/dt) × 10−18 (molecules/s) Particle size by XRD (nm) TOF (s−1
Before recovery 64.68 4.5 6.38 
After I cycle 50.83 6.16 3.30 
After II cycle 39.26 28.14 0.56 
After III cycle 32.34 42.25 0.31 
No. of cycles (dn/dt) × 10−18 (molecules/s) Particle size by XRD (nm) TOF (s−1
Before recovery 64.68 4.5 6.38 
After I cycle 50.83 6.16 3.30 
After II cycle 39.26 28.14 0.56 
After III cycle 32.34 42.25 0.31 

Turnover frequency

Turnover frequency (TOF) is defined as the number of molecules ‘n’ reacted at each available catalytic site per unit time ‘t’ (Stowell & Korgel 2005). 
formula
6
where Nact is the number of active sites

Assuming all surface atoms to be active, then Nact=Ap/AUC n, where Ap is the surface area of the average particle size, AUC is the surface area of an Ir unit cell face, and n is the number of Ir atoms in a unit cell face. Only a small portion of surface Ir atoms can actually act as catalytically active sites, as many will be bonded to capping ligands and unavailable for catalysis. In practice, it is common to take the total number of surface atoms as the number of catalytic sites when the value is not known. The TOF is a specific reaction rate based on the number of active sites. It is the most fundamental definition of reaction rate, since it is the frequency at which molecules react on an active site. Turnover frequencies have units of molecules/site-second or s−1. Values of TOF for commercial catalysts are typically in the range of about 10−3 to 10 s−1, while TOF values for enzymes are in the range of 102 to 104 s−1 (Bartholomew & Farrauto 2006).

The values of TOF given in Table 3 clearly reveal that the catalytic activity of Ir nanoparticles decreased significantly with each successive cycle. Ir nanoparticles increase in size (because of Ostwald ripening) after each cycle, which was accounted for by TOF calculations in each cycle.

Identification of degradation products

Degradation of AR-26 was monitored by UV-vis spectroscopy (Figure 8). UV-vis spectral analysis of the reaction mixture showed an absorption maximum at 507 nm characteristic of the chromophore containing an azo linkage of the dye molecule in the solution. The disappearance of this band and the formation of new bands at 245 and 273 nm supports the complete degradation of dye by HCF(III).
Figure 8

UV-vis spectra of AR-26 and extracted product.

Figure 8

UV-vis spectra of AR-26 and extracted product.

The HPLC chromatogram for the pure dye sample and extracted product is given in Figure 9. Figure 9(b) shows one major peak with retention time at 3.432 min and two minor peaks having retention time at 6.703 and 10.886 min for extracted product, which are totally different from that of pure dye, which has only one peak at 2.005 min as shown in Figure 9(a). Carboxylic acids and substituted carboxylic acids were identified as the major degradation products from LC-MS (Figure 10).
Figure 9

(a) HPLC chromatogram of AR-26 (b) Degradation products of AR-26.

Figure 9

(a) HPLC chromatogram of AR-26 (b) Degradation products of AR-26.

Figure 10

LC-MS spectra of degraded product of AR-26.

Figure 10

LC-MS spectra of degraded product of AR-26.

Degradation pathway

It is assumed that in an alkaline medium, azo dye exists as an anion, which forms a loosely bonded complex with iridium nanoparticles. This complex slowly reacts with the HCF(III) ion, resulting in products along with Irn and Fe(CN)64− (Goel & Lasyal 2015). On the basis of the previously reported work and experimental data, a tentative degradation pathway has been suggested for AR-26 (Figure 11). Azo dye can be cleaved both symmetrically and asymmetrically, with an active site available for the excitation of the molecule, followed by cleavage processes, resulting in ring opening. The presence of the hydroxyl ion in the aqueous phase enhances the cleavage compared to the hydrogen ion. Regarding the cleavage mechanism, the initial oxidative activation of dyes results in the formation of the carbonium ion followed by the nucleophilic attack of water on this cationic species (Gnanamani et al. 2005). The hetero atoms (S and N) present in the dye are transformed into sulphate (SO42−) and nitrogen (N2), respectively. The azo bond the (C–N=N–bond) is the area of attack in the degradation processes. The formation of carboxylic acids corresponds to the opening of aromatic and naphthalene rings.
Figure 11

Tentative degradation pathway for AR-26. The products postulated are based on the analysis of LC-MS data only.

Figure 11

Tentative degradation pathway for AR-26. The products postulated are based on the analysis of LC-MS data only.

CONCLUSION

AR-26, an azo dye, can be easily degraded by HCF(III) in the presence of Ir nanoparticles as a catalyst. The reaction kinetics are sensitive to operational parameters, pH, oxidant concentration, AR-26 concentration and temperature and followed the first order kinetic model with respect to the concentration of HCF(III), AR-26 and Ir-nano. LC-MS of the extracted product revealed the cleavage of the azo linkage to produce carboxylic acids and substituted carboxylic acids as the major degradation products.

Hexacyanoferrate(III), a mild oxidizing agent with appreciable stability in the solution, has been considered as the oxidant in the degradation of AR-26. The present study confirms the ability of HCF(III) to degrade the textile dye AR-26 using Ir nanoparticles as a catalyst, thus suggesting its application in the degradation of dye-bearing wastewater. Ir nanoparticles once again proved to be an efficient catalyst for the degradation of azo dyes, as they demonstrated an enhanced degradation rate compared to the Ir precursor due to their large surface area to volume ratio. Moreover, these nanoparticles can be recovered and reused showing a significantly long life span with sustained reactivity, making them potential candidates for dye degradation technologies. The present method seems to be a better alternative for the removal of dyes from wastewaters, considering its sufficient efficiency and economical benefits. This study could be extended to catalytic oxidation of other hazardous materials such as phenols.

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