Introduction

The textile industry’s production of dyestuffs creates large volumes of waste water. The World Bank estimates that 17–20% of all industrial water pollution emanates from textile dyeing and treatment processes (Kant 2012). There are serious concerns about the discharge of dye-contaminated waste waters into the environment from both aesthetic and environmental perspectives; the color of the dye is often visible in the waste water, even at concentrations as low as 1 mg L−1 (Fernández et al. 2004).

Acid Yellow 99 (AY99) is an anionic dye used in the paint, printing, and textile industries. It is appreciated for its color fastness and weather stability, and is used to dye nylon, wool, and silk (Elwakeel et al. 2016). However, AY99 can cause hard skin and eye irritation (Elwakeel et al. 2016). In addition, the inhalation of Acid Yellow 99 may irritate the respiratory tract, and its ingestion causes irritation to the gastrointestinal tract (Elwakeel et al. 2016).

Various physical and chemical treatment processes have been used for dyes in waste water. Among the methods applied to AY99 is decolorization, such as adsorption with coir pith. It was found that 1 g of coir pith can adsorb 442.13 mg of AY99. This adsorption process is found to be pH dependent; the optimum pH value for the process is 2.0, and it follows the Langmuir–Freundlich dual isotherm model (Khan et al. 2011).

The adsorption of AY99 onto dodecylethyldimethylammonium (DEDMA)-sepiolite was investigated in aqueous solution (Ozcan and Ozcan 2008). The photocatalytic decolorization of Acid Yellow 99 solutions under the action of solar radiation using ZnO as a photocatalyst has also been studied (Pitchaimuthu et al. 2014). 98.0% decolorization of an AY99 dye solution was achieved within 180 min using this method. The extraction of AY99 by a liquid surfactant membrane was also studied (Bahloul et al. 2014). The membrane consisted of the surfactant SPAN 80 (sorbitan monooleate) and the extractant Aliquat 366. Cyclohexane was used as the thinner, and sulfuric acid as the internal phase. Extraction yields of 99.09–99.12% were obtained experimentally.

Recently, the development of inexpensive water remediation technologies has attracted considerable interest (Kant 2012). One such method may be the oxidation of organic textile dyes by hydrogen peroxide using polyoxometalates (POMs) as catalysts. H2O2 is an environmentally benign oxidant that is cheap, readily available, and produces a nontoxic substance—water—during oxidation (Qiu and Liu 2005). Polyoxometalates (POMs) are a diverse class of anionic metal oxide clusters that contain early transition metals in their highest oxidation states (Pope 1983). These compounds, which are minerals, are generally easy to synthesize from relatively environmentally friendly simple reagents. Many POMs are known to be effective catalysts of the oxidation of a wide range of organic compounds. They are also potentially applicable in the photodegradation, degradation, and mineralization of organic pollutants in waste water (Hu and Burns 2002; Troupis et al. 2009; Lee and Sedlak 2009; Bechiri et al. 2013). Dawson-type heteropolyanions (Fig. 1) are a family of interesting and selective recyclable POM catalysts (Gustavo et al. 2008).

Fig. 1
figure 1

Polyhedral representation of the Dawson structure of polyanion (Pope 1983; Carraro and Gross 2014)

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Given the discussion above, we decided to study the catalytic oxidation of Acid Yellow 99 in contaminated water by H2O2 using the recyclable Dawson-type iron-substituted heteropolyanion H1.5Fe1.5P2W12Mo6O61, 22H2O (P2W12Mo6 Fe3+) as catalyst. We examined how parameters such as the initial pH, the initial H2O2 concentration, the mass of the catalyst, and the initial dye concentration affect the oxidation of AY99, and the results are reported in the present paper.

Experimental section

Reagents

The catalyst HFe1.5P2W12Mo6O61, 22H2O was prepared starting from H6P2W12Mo6O62.24H2O according to the protocol described in Bechiri et al. (2014).

The heteropolyanion precursor H6P2W12Mo6O62 24H2O was synthesized according to the procedure outlined in Ciabrini et al. (1983). The purity of the synthesized heteropolyanions was confirmed by 31P NMR spectroscopy (Bechiri et al. 2014).

The anionic dye Acid Yellow 99 was supplied by Sigma–Aldrich (St. Louis, MO, USA). The chemical structure and other characteristics of this dye are shown in Table 1.

H2O2 35% (w/w) was obtained from Merck (Darmstadt, Germany).

Table 1 Physical and chemical properties of the dye AY99
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NaOH, H2SO4, and ammonium heptamolybdate (NH4)6Mo7·4H2O were procured from Sigma–Aldrich. Potassium iodide (KI) was supplied by Riedel-de Haën (Seelze, Germany).

Procedure and analysis

The experiments were performed in a 500-mL batch reactor. Various solutions of AY99 were prepared at different concentrations, which were then homogenized by stirring until the dye was completely dissolved. The pH was adjusted using 0.1 N H2SO4 or NaOH. In all of the experiments, 100 mL of a 30 mg L−1 dye solution containing the appropriate quantities of catalyst and H2O2 were magnetically stirred at room temperature. The decrease in the concentration of AY99 was monitored by a model 6705 Jenway UV visible spectrophotometer (Cole-Parmer, London, UK); the wavelength of maximum absorbance λ max was 450 nm (Pitchaimuthu et al. 2014).

Wavelength and bandwidth resolutions were 1 and 0.5 nm. The cell used during the experiments was made of 1-cm-thick quartz.

The concentration of H2O2 was determined spectrophotometrically using potassium iodide and ammonuim heptamolybdate salt as the catalyst (Kormann et al. 1988).

The oxidation of AY99 by H2O2 using H1.5Fe1.5P2W12Mo6O61, 22H2O as catalyst was studied by monitoring the effects of varying the initial pH of the solution, the H2O2 concentration, the mass of the catalyst, and the dye concentration on the oxidation reaction. The oxidation efficiency (i.e., the discolorization efficiency) was determined via the equation (Sasmaz et al. 2011)

$$ {\text{DE}} = \left[ {\left( {C_{\text{i}} - C_{\text{f}} } \right)/C_{\text{i}} } \right] \times 100, $$
(1)

where DE is the discolorization efficiency, C i is the initial dye concentration, and C f is the final dye concentration.

Results and discussion

Effect of the solution pH

The effect of pH on the discolorization efficiency was investigated by varying the pH from 3 to 8. At a strongly acidic pH (<3), there is a risk that the catalyst will dimerize, while the catalyst is likely to deteriorate when the pH is above 8 (Bechiri et al. 2012).

The results presented in Fig. 2 show that the optimum pH value for the oxidation of AY99 by H2O2 using the iron-substituted Wells–Dawson-type heteropolyanion HFe2.5P2W12Mo6O62.22H2O as catalyst is 3. This result agrees with those reported in Bechiri et al. (2014), in which the oxidation of an azo dye (methyl violet) by H2O2 in aqueous solution using a Dawson-type heteropolyanion (HFe2.5P2W12Mo6O62.22H2O) as catalyst was studied; in that work, the optimum pH was found to be about 3.

Fig. 2
figure 2

Effect of solution pH on AY99 oxidation (C 0 = 30 mg/L; [H2O2] = 0.06 mM; [H1.5Fe1.5P2W12Mo6O61, 22H2O] = 0.23 mM)

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This result can be explained by the enhanced stability of the catalyst at this pH (Bechiri et al. 2013). It has also been shown that the catalytic efficiency of the Fe3+/H2O2 system during the oxidation of organic dyes is highest at pH 3 (Strukul 1992). H2O2 molecules are unstable in alkaline solution, so dye degradation decreases in alkaline solution (Gould et al. 2005).

Possible reaction mechanism

Figure 3 shows the evolution of the H2O2 concentration during the oxidation catalyzed by (HFe2.5P2W12Mo6O62.22H2O)/H2O2. The H2O2 concentration decreases during the oxidation process, suggesting that HFe2.5P2W12Mo6O62.22H2O catalyzes the decomposition of H2O2. The action of H2O2 on the Fe2+ complex leads to the generation of hydroxyl radicals (OH·). These hydroxyl radicals degrade the dye (Bechiri et al. 2013).

Fig. 3
figure 3

Evolution of the H2O2 concentration during the oxidation of AY99 as catalyzed by (H1.5Fe1.5P2W12Mo6O61, 22H2O)/H2O2 (pH 3; catalyst mass = 0.05 g; [H2O2] = 0.06 mM)

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Therefore, we can propose the following mechanism:

$$ \left( {{\text{P}}_{2} {\text{W}}_{12} {\text{Mo}}_{6} } \right){\text{Fe}}^{3 + } {\text{ + H}}_{2} {\text{O}}_{2} \to \left( {{\text{P}}_{2} {\text{W}}_{12} {\text{Mo}}_{6} } \right){\text{Fe}}^{2 + } {\text{ + HO}}_{2}^{ \cdot } $$
(2)
$$ \left( {{\text{P}}_{2} {\text{W}}_{12} {\text{Mo}}_{6} } \right){\text{Fe}}^{2 + } {\text{ + H}}_{2} {\text{O}}_{2} \to \left( {{\text{P}}_{2} {\text{W}}_{12} {\text{Mo}}_{6} } \right){\text{Fe}}^{3 + } {\text{ + 2OH}}^{ \cdot } $$
(3)

Effect of the catalyst concentration

The mass of the catalyst is a critical parameter in catalytic oxidation processes (Bechiri et al. 2012). Thus, the effect of varying the catalyst mass on the AY99 decolorization efficiency was investigated in this work by varying the concentration of P2W12Mo6Fe3+ from 0 to 0.45 mM. The results are shown in Fig. 4. The concentration of hydrogen peroxide was fixed at 0.006 mM and the AY99 concentration was 30 mg/L.

Fig. 4
figure 4

Effect of the catalyst concentration on AY99 oxidation (pH 3; C 0 = 30 mg L−1; [H2O2] = 0.06 mM)

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It is clear from the results that the AY99 decolorization efficiency increases as the catalyst mass is increased up to a certain level. This is due to the fact that P2W12Mo6Fe3+ plays a very important role in the decomposition of H2O2 into hydroxyl radicals. The low degradation capacity at low P2W12Mo6 Fe3+ concentrations is probably due to the relative scarcity of OH· radicals. As the concentration of the catalyst increases, the number of radicals increases, increasing the decolorization efficiency; however, when the catalyst concentration exceeds a certain value, the decolorization efficiency begins to decrease. This decrease in AY99 decolorization efficiency at large catalyst concentrations can be explained by the occurrence of reaction (4) below, which becomes competitive with the AY99 oxidation reaction at high P2W12Mo6Fe3+ concentrations:

$$ {\text{P}}_{2} {\text{W}}_{12} {\text{Mo}}_{6} {\text{Fe}}^{2 + } {\text{ + OH}}^{ \cdot } \to {\text{P}}_{2} {\text{W}}_{12} {\text{Mo}}_{6} {\text{Fe}}^{3 + } {\text{ + OH}}^{ - } . $$
(4)

Consequently, a P2W12Mo6Fe3+ concentration of 0.23 mM was chosen for AY99 oxidation.

Effect of the initial H2O2 concentration

The initial H2O2 concentration is an important influence on the degradation of AY99 and thus the decolorization efficiency. The effect of this parameter was studied by varying the initial H2O2 concentration from 0.006 to 0.1 mM under the following optimal conditions: pH 3, [NBB] = 30 mg L−1, [catalyst] = 0.23 mM.

Based on the results shown in Fig. 5, the optimal initial H2O2 concentration for the degradation of 30 mg/L AY99 is about 0.03 mM.

Fig. 5
figure 5

Effect of the initial H2O2 concentration on AY99 oxidation ([H1.5Fe1.5P2W12Mo6O61, 22H2O] = 0.23 mM; pH 3; C 0 = 30 mg L−1)

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The activation of hydrogen peroxide by homogeneous catalysts has been attributed to the formation of highly active hydroxyl radicals (Anipsitakis and Dionysiou 2004). A highly concentrated H2O2 solution (0.03–0.1 mM) presents self-quenching of OH· radicals and the formation of hydroperoxyl radicals (HO2·). Although HO2· is an effective oxidant, it is a less effective oxidant than OH·, so the AY99 decolorization efficiency drops when the concentration of H2O2 exceeds a threshold value (Liu et al. 2005).

$$ {\text{H}}_{ 2} {\text{O}}_{ 2} + {\text{OH}}^{ \cdot } \to {\text{H}}_{ 2} {\text{O }} + {\text{HO}}_{2}^{ \cdot } \left( {k { = 2} . 7 \times 1 0^{ 7} {\text{mol}}^{ - 1} {\text{L s}}^{ - 1} } \right) $$
(5)
$$ {\text{HO}}_{2}^{ \cdot } + {\text{OH}}^{ \cdot } \to {\text{H}}_{ 2} {\text{O }} + {\text{O}}_{ 2} \left( {k { = 0} . 7 1 \times 1 0^{ 1 0} {\text{mol}}^{ - 1} {\text{L s}}^{ - 1} } \right) $$
(6)
$$ {\text{OH}}^{ \cdot } + {\text{OH}}^{ \cdot } \to {\text{H}}_{ 2} {\text{O}}_{ 2} \left( {k { = 5} . 2 \times 1 0^{ 9} {\text{mol}}^{ - 1} {\text{L s}}^{ - 1} } \right) . $$
(7)

Effect of the AY99 concentration

From Fig. 6, it is clear that the discolorization efficiency decreases with increasing initial concentration of the dye. This result is in good agreement with results reported in the literature, and can be explained by the fact that increasing the initial dye concentration increases the ratio of AY99 molecules to hydroxyl radicals (as the concentrations of H2O2 and the catalyst are fixed), thereby decreasing the discolorization efficiency (Ramirez et al. 2007; Li et al. 2006; Ji et al. 2011).

Fig. 6
figure 6

Effect of the initial dye concentration on AY99 oxidation (pH 3; [H1.5Fe1.5P2W12Mo6O61, 22H2O] = 0.23 mM; [H2O2] = 0.03 mM)

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Monitoring the catalyzed oxidative degradation of AY99 over time using visible spectroscopy

Under the optimal experimental conditions determined above (pH 3; [H2O2] = 0.03 mM; C 0 = 30 mg L−1; [H1.5Fe1.5P2W12Mo6O61, 22H2O] = 0.23 mM; T = 25 °C), we used visible spectroscopy to follow the catalyzed oxidative degradation of AY99 over time. The results are depicted in Fig. 7. The figure shows that the catalytic oxidation process is fast: the color of the dye disappears quickly. The azo dye absorbance peak at λ = 450 nm that was used to monitor the discolorization decreases in intensity over time and disappears completely 60 min after the start of the reaction.

Fig. 7
figure 7

Application of visible spectroscopy to monitor the catalyzed oxidative degradation of the dye AY99 (pH 3; [H1.5Fe1.5P2W12Mo6O61, 22H2O] = 0.23 mM; [H2O2] = 0.03 mM)

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The decrease in this peak is caused by the cleavage of –N=N– bonds due to attack by hydroxyl radicals (Pitchaimuthu et al. 2014).

Conclusion

Textile dyes are a large group of organic compounds that can have undesirable effects when released into the environment. In this work, the oxidation of the azo dye AY99 by H2O2 in aqueous solution and in the presence of the iron-substituted Dawson-type heteropolyanion catalyst HFe2.5P2W12Mo6O62.22H2O was studied. The optimal parameter values for this oxidation were found to be: initial pH 3; [H2O2]0 = 0.03 mM; [catalyst] = 0.23 mM. Note that it is easy to separate and recover (i.e., recycle) this catalyst so that it can be used again. To do this, the catalyst is precipitated as its potassium salt by adding KCl to the solution following complete discolorization. The catalyst can also be separated out by extraction using a liquid surfactant membrane (Bechiri et al. 2008).