Introduction

The treatment of textile waste waters has always been a serious problem for these industries. These components sometimes are not biodegradable and are toxic enough for aquatic ecosystems [1]. Some classical methods such as adsorption on activated carbon, ozonation, reverse osmosis, ion exchange on synthetic adsorbent resins, flocculation, and decantation are available for removing dye from water. However, these methods have high operating cost [2] or are inefficient due to complex aromatic structure. Other alternatives to degrade recalcitrant organic pollutants are now being studied including advanced oxidation (AOPs) whose characteristic is to generate hydroxyl radicals (·OH), which are powerful oxidants [3].

These radicals are generated by Fenton’s reagent (Fe2+/H2O2), which has been the subject of many studies in the degradation of organic matter [4]. However, the disadvantage of the method lies in Fenton rejection of a significant amount of sludge Fe2+ and Fe3+ [5], which requires separation or removal, thus increasing the operational cost. To solve this problem, many studies have focused on the use of a modified Fenton process (Fenton-Like) to treat waste waters. A method which relies on the use iron incorporated in the recyclable compounds [6]. In this work, a new homogeneous Fenton-like system based on hydrogen peroxide and a recyclable Dawson-type heteropolyanion (Fig. 1) [7].

Fig. 1
figure 1

Polyhedral representation of the Dawson structure of polyanion

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(HFe2.5P2W18O62 23.H2O/H2O2) system was used for decolorization of Acid Orange 7 (AO7) (also known as Orange II).

Acid Orange 7 is one of the dyes that are produced in large amounts in the world. It is commonly used in pharmaceutical, food, and cosmetics industries. Acid Orange 7 is an azoique dye used in dyeing of silk and wool. For these qualities, Acid Orange 7 is much used in the textile industry. The molecular structure of azo-dye Orange II is shown in Fig. 2. Acid Orange 7 is not amenable to conventional biological treatment [8].

Fig. 2
figure 2

Chemical structure of Acid Orange 7 (AO7)

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The aim of the study is to examine the influence of different parameters such as the solution pH, the catalyst mass, the concentration of H2O2, and the initial dye concentration on the degradation of Acid Orange 7 by H2O2 using HFe2.5P2W18O6223.H2O as a catalyst. The mechanism of the reaction and catalytic stability were studied.

Experimental section

Synthesis of catalyst HFe2.5P2W18O62 23.H2O [9]

HFe2.5P2W18O62 23.H2O

5 g (1.088 mol) of H6P2W18O62 was dissolved in 20 mL of water at room temperature and 0.767 g of solid FeCl2,6H2O (3.26 mol) was then added. The mixture was then stirred for 10 min. Yellow powder was obtained after five days by slow evaporation.

H6P2W18O62 was prepared, starting from the substance mother (K6P2W18 O62, nH2O)6-, by extraction with ether in hydrochloric acid medium according to the methods described in the literature [10].

Behavior of AO7 oxidation using HFe2.5P2W18O62 23.H2O/H2O2 system

Reagents

The 4-(2-hydroxy-1-naphthylazo) benzene sulfonic acid sodium salt, commonly named orange II or Acid Orange 7 (AO7) (90%) was obtained from Aldrich (H2O2 35%, W/W) was purchased from Aldrich Chemical Company. All other reagents (NaOH or H2SO4) that used in this study were analytical grade.

Procedures and analysis

The experiments were performed in a reactor batch of 500 mL capacity. Various solutions of AO7 were prepared at different concentrations. They were then homogenized by stirring them until the dye is completely dissoluted of the dye. pH was adjusted using 0.1 N H2SO4 or NaOH aqueous solutions. In all experiments 100 mL of dyes (30 mg/L) solution containing the appropriate quantity of catalyst and H2O2 was magnetically stirred at room temperature. The AO7 decreasing concentration of dye was monitored by a 6705 UV visible spectrophotometer JENWAY. The Wave length corresponding to the maximum absorbance is given below: λ max = 486 nm [11].

The resolution of the wave length and bandwidth was 1 and 0.5 nm. The cell used during the experiments was made of 1 cm thick quartz.

Effects of operational parameters on AO7 oxidation

The oxidation of AO7 by H2O2 using (HFe2.5P2W18O61 23H2O) as a catalyst has been studied according to the following factors: initial pH of the solution, H2O2 concentration, mass of the catalyst (HFe2.5P2W18O61 23H2O), and dye concentration.

The oxidation efficiency (discoloration) was determined as it is shown below [12]:

$${\text{DE}} = \left[\left( {C_{i} - C_{f} } \right)/C_{i}\right] \times100$$
(1)

DE: discoloration efficiency; C i : Initial dye concentration; C f : final dye concentration.

Effect of solution pH

The pH value of dye solution has significant influence on the catalytic system efficiency because: (1) it can affect the catalyst stability and (2) it influences the catalytic reaction which controls the production rate of hydroxyl radicals [13]. To find the optimum pH for the decolorization of AO7, a series of experiments at initial pH value in the range 3–8 was conducted.

Figure 3 illustrates the effect of pH on the discoloration efficiency of AO7 in water. It was found that the discoloration efficiency of AO7 is strongly pH dependent. The optimal pH is about 4 giving discoloration efficiency equal to 94.25%. This result can be explained by the stability of the catalyst at this pH. Also, H2O2 molecules are unstable in alkaline solution and therefore, the degradation of dye decreases in alkaline solution [14].

Fig. 3
figure 3

Effect of solution pH on AO7 oxidation ([AO7]0 = 30 mg/L, [H2O2]0 = 0.11 mM, m (HFe2.5P2W18O62 23.H2O) = 0.01 g, T = 25 °C)

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The optimal value is chosen pH 4.

Effect of the nature of the acid used to adjust the pH

The oxidation of the dye by the Fenton-like process can be influenced by the presence of various ions such as SO4 2−, NO3−, Cl−, and PO4 3− [15]. To evaluate the influence of these anions, we adjusted the pH of an aqueous solution of AO7 by different acids H2SO4, HNO3, HCl, and H3PO4 in similar operating conditions to those previously established.

Figure 4 shows the effect of these acid ions (chloride, sulfate, nitrate, and phosphate) on the dye oxidation. It appears that the presence of phosphate ions inhibits the oxidation of the dye using HFe2.5P2W18O62 23.H2O/H2O2 system.

Fig. 4
figure 4

Effect of the nature of the acid used to adjust the pH (pH 4, [AO7]0 = 30 mg/L, [H2O2]0 = 0.11 mM, m (HFe2.5P2W18O62 23.H2O) = 0.01 g, T = 25 °C)

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Sulfate and nitrate ions have virtually no effect on the discoloration of AO7. However, depending on the nature of the acids, the discoloration efficiency is about 94.25, 94.35, 96.09, and 03.06% in the presence of H2SO4, HNO3, HCl, and H3PO4 acids, respectively. These results agree with those found at the degradation of other organic pollutants [16]. We can deduce that H2SO4, HNO3, and HCl acids lead almost to the same efficiency, but in the presence of H3PO4 it is too low. The inhibitory effect of phosphate ions may be due to the trapping of ·OH radicals.

Effect of catalyst mass

The effect of the catalyst mass was investigated, keeping operational parameters identical to those of the above-mentioned experiment. The following catalyst quantities have been used: 0.005; 0.008; 0.01; 0.02 g. The results are illustrated in Fig. 5.

Fig. 5
figure 5

Effect of catalyst mass on AO7 oxidation (pH 4, [AO7]0 = 30 mg/L, [H2O2]0 = 0.11 mM, T = 25 °C)

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These results show that the discoloration efficiency of AO7 oxidation increases when increasing the catalytic mass up to 0.01 g where the decolorization efficiency is optimal. Beyond this value, there is a decrease of the discoloration efficiency. The excess of the catalyst does not appear to ply a positive role in the AO7 oxidation using HFe2.5P2W18O62 23.H2O/H2O2 system. This result is in good agreement with literature [12, 13, 17]: the decrease of the discoloration efficiency of AO7 when increasing the mass of the catalyst can be explained by the presence of side reaction consuming the radicals hydroxyls. The mass of catalyst that gives the best result is 0.01 g.

Effect of H2O2 concentration

The effect of H2O2 concentration on AO7 decolorization was studied in the range 0.04–0.44 mmol. The results are illustrated in Fig. 6.

Fig. 6
figure 6

Effect of initial H2O2 concentration on AO7 oxidation (pH 4, [AO7]0 = 30 mg/L, m (HFe2.5P2W18O62 23.H2O) = 0.01 g, T = 25 °C)

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The results indicate that the degradation of AO7 was increased by increasing the concentration of H2O2 up to a value of concentration of H2O2 equal to 2 mM.

Decolorization of certain dyes, mainly azo, with activated hydrogen peroxide in homogeneous systems, using different soluble catalysts (heteropolyanions) was already studied [18].

The activation of hydrogen peroxide by homogeneous catalysts was attributed to the formation of highly active hydroxyl radicals [19]. High concentrated H2O2 solution undergoes self quenching of OH radicals, with formation of hydroperoxyl radicals HO2. Although HO2 is an effective oxidant itself, its oxidation potential is much lower than that of ·OH radicals [20].

$${\text{H}}_{2} {\text{O}}_{2} + {}^{ \cdot }{\text{OH}} \to {\text{H}}_{2} {\text{O}} + {\text{HO}}_{2}^{ \cdot } ,$$
(2)
$${\text{HO}}_{2}^{ \cdot } + {}^{ \cdot }{\text{OH}} \to {\text{H}}_{2} {\text{O}} + {\text{O}}_{ 2} ,$$
(3)
$${\text{OH}}^{ \cdot } +^{.} {\text{OH}} \to {\text{H}}_{2} {\text{O}}_{2} .$$
(4)

It can be postulated that H2O2 should be added at an optimum concentration to achieve the best degradation. Hence, 0.2 mM/L of H2O2 appears as an optimal.

Radical scavenging experiments and possible reaction mechanism

In general, ·OH radicals are known as a key active species in the catalytic oxidation process. ·OH radicals are powerful oxidants for many organic molecules.

Ethanol, methanol, and 2-propanol are known as OH scavengers [21–24]. These reagents were introduced into the reaction system to capture ·OH in the oxidation of AO7 process. The results are illustrated in Fig. 7.

Fig. 7
figure 7

Effect of addition of OH scavenger reagents on AO7 oxidation (pH 4, [AO7]0 = 30 mg/L, m (HFe2.5P2W18O62 23.H2O) = 0.01 g, T = 25 °C)

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These results show that the addition of ·OH scavenger reagents causes a sharp decrease in the degradation of AO7, and the discoloration efficiency of AO7 from 100% to 1.6, 2.08, and 2.53% in the presence of methanol, ethanol, and 2-propanol, respectively. According to the above discussion, it is furthermore confirmed that the degradation of AO7 molecules is dependent on the availability of ·OH radical species. ·OH radicals make a major contribution to degradation of AO7. OH scavenger reagents added consume ·OH radical species and less will be available for the degradation of the dye molecules, hence the lower discoloration efficiency.

Also, it was shown [25] that the action of H2O2 on a complex containing Fe3+ resulted in the reduction of Fe3+ to Fe2+ with apparition of HO·2.

The action of H2O2 on the complex of Fe2+ leads to the generation of hydroxyl radicals OH·. These hydroxyl radicals cause the degradation of the dye.

In agreement with the results found and the mechanism proposed below, we can propose the following mechanism:

$${\text{P}}_{2} {\text{W}}_{18} {\text{Fe}}^{3 + } + {\text{ H}}_{2} {\text{O}}_{2} \to {\text{P}}_{2} {\text{W}}_{18} {\text{Fe}}^{2 + } + {\text{ HO}}^{.}_{2} + {\text{ H}}^{ + } ,$$
(5)
$${\text{P}}_{2} {\text{W}}_{18} {\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{O}}_{2} \to {\text{P}}_{2} {\text{W}}_{18} {\text{Fe}}^{3 + } + \, 2^{.} {\text{OH}} .$$
(6)

The degradation of AO7 by H2O2 catalyzed with HFe2.5P2W18O62 23.H2O heteropolyanion can be well supported by the change in the UV–Vis absorption spectrum over the course of degradation (Fig. 8). The AO7 spectrum was characterized by the chromophore that contained azo linkage [26] in the visible range at 484 and 430 nm corresponding to the hydrazone and azo form. In the ultraviolet region, the absorbance peaks at 230 and 310 nm are due to the benzene and naphthalene rings, respectively. These four characteristic bands were markedly weakened during the degradation reaction, tending to disappear completely after 60 min, without the appearance of new absorption bands in the visible or ultraviolet regions due to destruction of the chromophoric and auxochromic structures by the homogeneous Fenton-like reaction.

Fig. 8
figure 8

UV-Vis spectra of AO7 water solutions during the treatment process with Fe(III)P2W18/H2O2 system (pH 4, [H2O2]0 = 0.2 mM, m (HFe2.5P2W18O62 23.H2O) = 0.01 g, [AO7] = 30 mg/L, T = 25 °C)

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Effect of the AO7 concentration

It is important from an application point of view to study the dependence of removal efficiency on the initial concentration of the colorant (AO7).

To determine the effect of AO7 concentration on discoloration efficiency, a series of experiments was realized between 5 and 50 mg/L. The variation of the discoloration efficiency of the AO7 dye with H2O2 and HFe2.5P2W18O61 23H2O catalyst as a function of initial dye concentration is represented in Fig. 9. The results show that the discoloration efficiency increases with increasing of dye concentration up to a value equal to 30 mg/L, but above it the discoloration efficiency decreases.

Fig. 9
figure 9

Effect of initial dye concentration on AO7 oxidation. (pH 4, [H2O2]0 = 0.2 mM, m (HFe2.5P2W18O62 23.H2O) = 0.01 g, T = 25 °C)

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This result can be explained as follows:

At low concentration of dye, the discoloration efficiency is lower (ED 39% for the concentration of AO7 5 mg/L). These results concur with those found in literature [27–29]. This phenomenon can be explained by the effect of H2O2 concentration is in excess compared to latter and can cause the scavenging effect (the reaction of H2O2 and OH· in aqueous solution).

  • At higher concentration of dye, the discoloration efficiency decreases (ED 48% for the concentration of AO7 50 mg/L). This phenomenon can be explained by the fact that increasing the initial concentration of dye leads to an increase in the number of molecules of (AO7), while the number of the radical’s hydroxyls remains constant (H2O2 concentration and catalyst kept constant), thereby causing a decrease in the discoloration efficiency [19, 27].

Stability and recycling of the catalyst

After reaction, we have tried to recover the catalyst P2W18Fe3+. The method used is its precipitation, as potassium salt, by adding a mass of KCl in the solution after complete discoloration in a basic medium. The resulting precipitate was characterized by infrared spectroscopy. The spectra are illustrated in Fig. 10.

Fig. 10
figure 10

IR spectra of HFe2.5P2W18O62 23.H2O and recovered HFe2.5P2W18O62 23.H2O

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The spectrum of the recovered heteropolyanion is identical to that of the initially added heteropolyanion. We can therefore conclude that the catalyst remains stable and robust after the reaction.

Figure 11 shows the catalytic performance of recovered catalyst after four cycles through the oxidation of AO7 dye, by hydrogen peroxide.

Fig. 11
figure 11

Catalytic performance of recovered catalyst after four cycles through the oxidation of AO7 dye by hydrogen peroxide. (pH 4, [H2O2]0 = 0.2 mM, m (HFe2.5P2W18O62 23.H2O) = 0.01 g, T = 25 °C, [AO7] = 30 mg/L)

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After second cycle, the catalyst did not exhibit significant loss of catalytic activity and the discoloration efficiency was 100. AO7 degradation decreases only from 100 to 95% in three cycles. In four cycles, AO7 degradation decreases from 100 to 89%.

This result is in good agreement with the literature [28]. The loss of activity can be explained by to poisoning of the active catalytic sites due to adsorbed organic species. However, this could be avoided by placing the catalyst in an oven at 180 °C, to enhance the organic solvent.

Conclusion

Fe(III)-phosphotunstate/H2O2 system is a new homogeneous Fenton-like system that is capable of oxidizing organic dye compounds. The experimental results show that the initial pH, the initial concentration of H2O2, Fe(III)P2W18, AO7 concentration, had a great influence on the degradation of AO7 dye. It is approximately 100% dye which has been eliminated the Fe(III)P2W18/H2O2 on the following operating conditions: pH 4, catalyst mass 0.01 g, [H2O2]0 = 2 m M. The molar ratio H2O2/AO7 = 0.02. The possibility of recovery of the catalyst after reaction is verified by adding a mass of KCl in the solution after complete discoloration. The stability of reused catalyst is confirmed after second cycle.

This system constitutes simple and effective method compared to those previously reported for the oxidation of Acid Orange 7 [29, 30].