Effects of operator parameters, anions and cations on the degradation of AY99 in an aqueous solution using Fenton’s reagent. Optimization and kinetics study

Abstract

In the present work, the decolourization of azo dye Acid Yellow 99 (AY99) in an aqueous solution by Fenton oxidation process was investigated. The results indicated that the initial pH, the nature of the acid to adjust pH, the temperature and the initial concentrations of the Fenton’s reagent FeSO₄ of H₂O₂ and of the AY99 played an important role on the dye decolourization. It was found that under optimum conditions, which have been determined to be pH = 3, [H₂O₂]₀ = 1.8 mmol/L, [Fe²⁺]₀ = 0.09 mmol/L, [AY]₀ = 0.06 mmol/L and T = 30°C, efficiency of degradation obtained after 40 min of reaction was about 95%. The decolourization of AY99 by the Fenton process followed the second-order reaction kinetics. The empirical kinetic equation for AY99 decolourization under the conditions of 0.03 to 0.15 mmol/L of Fe²⁺, 0.9 to 1.8 mmol/L of H₂O₂, 0.03 to 0.15 mmol/L of Acid Yellow and pH = 3 was found to follow the relation:

K = 6.39 × 10⁷ [AY] ^−2.8[H₂O₂]^2.29[FeSO₄]^0.837 exp (−7021.3/T).

The decolourization efficiency Ef (in percent) was 88.36%, 87.93%, 85.98% and 39.61% in the presence of H₂SO₄, HNO₃, HCl and H₃PO₄ acids, respectively. We can deduce that H₂SO₄, HNO₃ and HCl acids lead almost to the same efficiency (Ef), but in the presence of H₃PO₄, it is too low:

Ef (H₂SO₄) ~ Ef (HNO₃) > Ef (HCl) > > Ef (H₃PO₄).

Effects of additives such as inorganic salts (sodium chloride, calcium chloride, sodium sulphate and calcium sulphate) and cations (sodium sulphate, calcium sulphate, copper sulphate, manganese sulphate and magnesium sulphate) on the efficiency (Ef) and on the rate constant (k₂) of AY99 degradation were also studied under optimum conditions.

Ef (Na₂SO₄) > Ef (CaSO₄) > Ef (NaCl) > Ef (CaCl₂)

k₂ (CaCl₂) < k₂ (NaCl) < k₂ (CaSO₄) < k₂ (Na₂SO₄)

Ef (ZnSO₄) > Ef (MgSO₄) > Ef (Na₂SO₄) > Ef (CaSO₄) > Ef (CuSO₄) > Ef (MnSO₄)

k₂ (Na₂SO₄) < k₂ (MnSO₄) < k₂ (CuSO₄) < k₂ (CaSO₄) < k₂ (ZnSO₄) < k₂ (MgSO₄).

The presence of anions and cations in the aqueous solution decreases the decolourization efficiency and the rate constant of the degradation of AY99. This inhibition may be due to a complexation and a radical scavenging.

Background

Azo dyes are synthetic organic compounds widely used in textile dyeing, paper printing and other industrial processes such as the manufacture of pharmaceutical drugs, toys and foods.

Yearly, 800,000 tonne of dyes are produced in the world, and about 50% of them are azo dyes[1]. Coloured wastewaters from azo dye production processes and utilisation industries pose a major threat to the surrounding ecosystems due to the documented health hazards caused by toxicity and a potentially carcinogenic nature of such organic pollutants[2].

The traditional treatment techniques applied in textile wastewaters, such as coagulation/flocculation, membrane separation (ultra-filtration and reverse osmosis) or elimination by activated carbon adsorption[3–8], only do a phase transfer of the pollutant, and biological treatment is not a complete solution to the problem due to biological resistance[9, 10]. Recent progress in the removal of dyes has led to the development of advanced oxidation processes (AOPs). These processes have attracted wide interests in wastewater treatment since 1990. In principle, AOPs such as photocatalytic degradation[11–15], ozonation[16, 17], H₂O₂ photolysis (UV/H₂O₂)[17–21], Fenton’s reaction (H₂O₂/Fe²⁺)[22, 23] and photo-Fenton (H₂O₂/Fe²⁺/UV)[24–26] are based on the generation of the hydroxyl radical ^·OH in water. ^·OH is a highly reactive and non-selective oxidant which can initiate oxidative degradation reactions of refractory synthetic and natural organic compounds and is capable of mineralizing them ultimately to CO₂ and H₂O owing to their high oxidation potential (+2.80 eV versus NHE) in an aqueous solution[27, 28]. In recent years, different combinations of these methods were used to obtain the complete mineralization of pollutants[29–33].

The Fenton process is a homogeneous catalytic AOP using a mixture of hydrogen peroxide and ferrous ions[34]. Compared with other AOPs, Fenton’s reagent is relatively inexpensive, and the process is easy to operate. The mechanism that describes a Fenton’s process mainly includes the following reactions[35]:

$${\text{Fe}}^{2+}+H_2{O}_2\to {\text{Fe}}^{3+}+{\text{HO}}^{·}+{\text{OH}}^{-}{k}_1=76M^{-1}{s}^{-1}$$

(1)

$${\text{Fe}}^{3+}+H_2{O}_2\to {\text{Fe}}^{2+}+{\text{HOO}}^{·}+H^{+}{k}_2=0.01-0.02M^{-1}{s}^{-1}$$

(2)

$${\text{Fe}}^{3+}+{\text{HOO}}^{·}\to {\text{Fe}}^{2+}+H^{+}+{\text{O}}_2{k}_2=1.2\times {10}^6{M}^{-1}{s}^{-1}$$

(3)

This paper reports the findings of the decolourization of Acid Yellow 99 using Fenton’s reagent. Effects of the pH, the nature of the acid to adjust pH and the temperature were examined. The influence of the concentration of H₂O₂, Fe²⁺ and the dye were also explored. The relation between the reaction rate coefficient and the initial concentration of Acid Yellow 99, ferrous sulphate concentrations, hydrogen peroxide concentrations and operating temperature will subsequently be elucidated based on the obtained experimental data.

The influences of salts (such as sodium chloride, calcium chloride, sodium sulphate and calcium sulphate) on the degradation of Acid Yellow 99 (AY99) were examined. The effects of cations from salts (sodium sulphate, calcium sulphate, copper sulphate, manganese sulphate, zinc sulphate and magnesium sulphate) were also studied.

Methods

Materials

Acid Yellow 99 was purchased from Aldrich (Sigma-Aldrich Corporation, St. Louis, MO, USA) and was used as received. The molecular structure of Acid Yellow (C.I. 62055) is shown in Figure1. Hydrogen peroxide (30% w/w) was obtained from Merck (Merck & Co., Inc., White house Station, NJ, USA). Ferrous sulphate heptahydrate (≥99%) was supplied by Sigma-Aldrich. H₂SO₄, HCl, H₃PO₄ and HNO₃ were all obtained from Shanghai Chemical Reagents Co. (Shanghai, China). Sodium chloride, calcium chloride, sodium sulphate and calcium sulphate were supplied by Acros Organics (Geel, Belgium). Copper sulphate, manganese sulphate and magnesium sulphate were procured from Sigma-Aldrich.

Figure 1

Acid Yellow 99 chemical structure.

Procedure

The aqueous solutions of AY99 were prepared by dissolving the required amount in distilled water. The pH of the solution was adjusted by adding sulphuric acid (H₂SO₄) and measured by a pH meter (Eutech Instruments Pte. Ltd., Rajah Crescent, Singapore).

The experimental device consisted of a perfectly agitated reactor (reactor batch with a capacity of 1,000 mL) in which a volume (500 mL) of solution was studied. The temperature was controlled by a thermostatic bath (TECTRON BIO, SELECTA, Barcelona, Spain), and the agitation was carried out using a mechanical stirrer (Janke & Kunkel RW 20, Janke & Kunkel GmbH & Co. KG IKA-Labortechnik, Staufen, Germany). The dye decolourization was made by a Fenton’s reagent, which was composed of a mixture of FeSO₄ and H₂O₂. The necessary quantities of Fe²⁺ and H₂O₂ were added in the dye solution simultaneously. The concentration of the dye at different reaction times was determined by measuring the absorption intensity at${\lambda }_{max}=448nm$ using a UV-visible spectrophotometer (BOECOS24).

The efficiency of AY99 decolourization was calculated by using Equation 4:

$$\text{Degradation\hspace{0.17em}efficiency}=\frac{C_0-C_t}{C_0}\times 100\left(\%\right)$$

(4)

where C₀ is the initial concentration of AY99 and C _t is the concentration of AY99 at time t.

Results and discussion

Effect of operational parameters

Effect of initial pH

The pH of the wastewater controls the production rate of hydroxyl radical and the nature of iron species in the solution. Hence, pH is an important parameter for Fenton processes. In order to find the optimal pH at which the reaction mixture for the decolourization of AY99 can give the best efficiency, a series of experiments at different pH were conducted. The pH values used were 2, 2.5, 3, 3.5 and 4, the initial concentration of the dye was 0.06 mmol/L, the temperature was maintained at 30°C and the pH of the solutions was adjusted by concentrated (98%) sulphuric acid.

The effect of the initial pH on the treatment of AY99 is shown in Figure2. It was observed that the decolourization of AY99 was significantly influenced by the solution’s pH value, and the highest decolourization efficiency (88.36%) was achieved at pH = 3.0. These observations are in good agreement with published results[36–38].

Figure 2

Effect of pH on the decolourization of AY99 by Fenton oxidation process. Conditions: initial dye concentration, 0.06 mmol/L; Fe²⁺ concentration, 0.09 mmol/L; H₂O₂ concentration, 0.9 mmol/L; temperature, 30°C.

When pH is below 3, HO^· could be consumed by the scavenging effects of H⁺, which will limit the decolourization rate (Equation 5)[34]. According to Equations 1 and 2, the reaction could be slowed down because hydrogen peroxide can probably be stable by acquiring a proton to form an oxonium ion as H₃O₂⁺ in Equation 6.

$${\text{HO}}^{·}+H^{+}+e^{-}\to H_{2}O$$

(5)

$$H_2{O}_2+H^{+}\to H_3{O_2}^{+}$$

(6)

The oxonium ion makes hydrogen peroxide electrophilic to enhance its stability and presumably to reduce substantially the reactivity with ferrous ion[34]. For pH above 3, the generation of HO^· decreases due to the formation of iron ions, which tend to precipitate in the form Fe(OH)₃, as shown by reaction 7, and that is favoured at higher pH[35].

$${\text{Fe}}^{3+}+{3H}_{2}O\to \text{Fe}{\left(\text{OH}\right)}_3+{3H}^{+}$$

(7)

Following this precipitation, regeneration of Fe²⁺ from Fe³⁺ stops; then, the production of HO^· radicals decreases. However, the initial pH value should be between 2.5 and 3 to generate the maximum of radicals to oxidise organic matter.

Effect of temperature

The effect of temperature on the decolourization of AY99 was studied at 30°C, 40°C, 50°C, 60°C and 70°C with other test conditions at [AY]₀ = 0.06 mmol/L, [H₂O₂₀ = 0.9 mmol/L, [Fe²⁺₀ = 0.09 mmol/L and pH = 3.0. The results are shown in Figure3. It can be seen that the temperature exerts a strong effect on the decolourization efficiency of AY99, and the decolourization was accelerated by a rise in temperature. The decolourization efficiency increases from 45.00% to 89.88% as a consequence of increasing the temperature from 30°C to 70°C within 6 min. This is because higher temperature increased the reaction rate between hydrogen peroxide and any form of ferrous/ferric ion, thus increasing the rate of generation of oxidising species such as HO^· radical or high-valence iron species[36].

Figure 3

Effect of temperature on the decolourization of AY99 by Fenton oxidation process. Conditions: initial dye concentration, 0.06 mmol/L; Fe²⁺ concentration, 0.09 mmol/L; H₂O₂ concentration, 0.9 mmol/L; pH 3; temperature, 30°C.

Effect of FeSO ₄

To elucidate the role of Fe²⁺ ion on the degradation of AY99, a series of experiments was performed by varying the concentration of Fe²⁺ ions from 0.03 to 0.15 mmol/L for an adjusted H₂O₂ concentration of 0.9 mmol/L at pH = 3. The effect of Fe²⁺ on the decolourization of AY99 by Fenton is show in Figure4.

Figure 4

Effect of FeSO ₄ on the decolourization of AY99 by Fenton oxidation process. Conditions: initial dye concentration, 0.06 mmol/L; H₂O₂ concentration, 0.9 mmol/L; pH 3; temperature, 30°C.

It was observed that the decolourization efficiency increases from 25.74% to 69.51% as a consequence of increasing Fe²⁺ dosage from 0.03 to 0.15 mmol/L within 10 min. This is due to Fe²⁺ playing the role of a catalyst and initiating the decomposition of H₂O₂ to generate a very reactive HO^· in the process. Therefore, additional HO^· radicals were produced along with the increase of Fe²⁺ concentration. The maximum degradation efficiency (about 88.36%) was achieved at 0.09 mmol/L of Fe²⁺ within 70 min. The influence of higher Fe²⁺ concentration upon the degradation behaviour of AY99 may be explained by the redox reactions since HO^· radicals may be scavenged by the reaction with hydrogen peroxide or with another Fe²⁺ molecule, as shown below in Equation 8[35].

$${\text{Fe}}^{2+}+{\text{HO}}^{·}\to {\text{Fe}}^{3+}+{\text{HO}}^{-}$$

(82)

Effect of H ₂ O ₂ concentration

Hydrogen peroxide plays the role of an oxidising agent in the Fenton process. The effect of H₂O₂ dosage on the degradation of AY99 by Fenton process was investigated by varying initial concentration of H₂O₂ from 0.9 to 2.1 mmol/L, the results are shown in Figure5. It was observed that the decolourization efficiency increases from 85% to 95% as a consequence of increasing H₂O₂ dosage from 0.9 to 1.8 mmol/L within 40 min. This is due to the oxidation power of the Fenton process, which was improved by increasing the amount of HO^· radicals in the solution obtained from the decomposition of increasing hydrogen peroxide. The maximum decolourization efficiency of about 96% was achieved with 1.5 mol/L of H₂O₂ within 70 min.

Figure 5

Effect of H ₂ O ₂ concentration on the decolourization of AY99 by Fenton oxidation process. Conditions: initial dye concentration, 0.06 mM; Fe²⁺ concentration, 0.09 mmol/L; pH 3; temperature, 30°C.

Beyond critical concentrations, the degradation rate of AY99 decreased with the increasing H₂O₂ concentration due to the hydroxyl radical scavenging effect of H₂O₂[37] and the recombination of hydroxyl radicals according to Equations 9 and 10.

$$H_2{O}_2+{\text{HO}}^{·}\to {{\text{HO}}_2}^{·}+H_{2}O$$

(9)

$${\text{HO}}^{·}+{\text{HO}}^{·}\to H_2{O}_2$$

(10)

And the incremental generation of HOO^· also consumed HO^·.

$${{\text{HO}}_2}^{·}+{\text{HO}}^{·}\to H_{2}O+O_2$$

(11)

Effect of the Acid Yellow 99 concentration

The pollutant concentration is one of the important parameters in Fenton processes. Therefore, the effect of dye concentration on the degradation efficiency was investigated at different concentrations of AY99 and presented in Figure6. It was observed that the increase of dye concentration from 0.03 to 0.15 mmol/L decreases the decolourization from 97.5% to 67.26% within 25 min. This phenomenon can be explained by the fact that an increase in the initial concentration leads to an increase of the number of dye molecules and not the concentration of HO^· radicals, which is why the decolourization efficiency decreases. These results are in agreement with those found in the literature[36, 38].

Figure 6

Effect of AY99 concentration on the decolourization of AY99 by Fenton oxidation process. Conditions: H₂O₂ concentration, 1.8 mmol/L; Fe²⁺ concentration, 0.09 mmol/L; pH 3; temperature, 30°C.

Effect of the iron ion type

The nature of the catalyst ensures the smooth running of catalytic cycle. It is responsible for the production of HO^· radicals whose metal ions are the main actors. The AY99 decolourization has been studied in the presence of three different catalysts: FeSO₄, FeCl₃ and Fe(NO₃)₃. These experiments were carried out to compare the performance of Fe²⁺/H₂O₂ and Fe³⁺/H₂O₂ catalytic systems. The results presented in Figure7 show that the decolourization efficiency of Acid Yellow 99 at 30°C reached 98.5% within 120 min when the Fe³⁺ system was used and 95% within 40 min for the Fe²⁺ system. It was noted that these two systems were effective, but their kinetic speeds were different. The decolourization of Acid Yellow 99 by the system Fe²⁺/H₂O₂ was faster than that obtained by the system Fe³⁺/H₂O₂.

Figure 7

Kinetics of Acid Yellow 99 decolourization in the presence of different catalysts. C₀ = 0.06 mmol/L, [FeSO₄] = 0.09 mmol/L, [H₂O₂] = 1.8 mmol/L, T = 30°C, pH = 3.

The degradation of the dye was caused by hydroxyl radicals in the case of Fe²⁺/H₂O₂ (Fenton) and hydroperoxyl radicals in the case of the system Fe³⁺/H₂O₂. It is known that the radicals have a hydroperoxyl oxidising lower than that of hydroxyl radicals.

$${\text{ROO}}^{·}+H_2\text{O}\to \text{ROH}+{\text{HOO}}^{·}$$

(12)

$${\text{HOO}}^{·}+H_2{O}_2\to {\text{HO}}^{·}+H_{2}O+O_2$$

(13)

Several studies have shown that the oxidation of organic compounds is faster with the system Fe²⁺/H₂O₂ (Fenton) than with the system Fe³⁺/H₂O₂ (Fenton-like), and this concords with our results. This is due to the immediate formation of hydroxyl radicals in the case of Fe²⁺/H₂O₂ system. The mechanism of the oxidation of organic compounds[39] is shown in Figure8.

Figure 8

Mechanism of oxidation of organic compounds with Fe ³⁺ and H ₂ O ₂ .

Effect of anions

The inorganic ions inhibit Fenton and photo-Fenton oxidations of organic compounds; their reaction on iron makes complexes of ferrous and ferric ions less reactive for the Fenton reaction, and they can decrease the overall efficiency. In the presence of anions such as SO₄²⁻, NO₃⁻, Cl⁻ and PO₄³⁻, the rate of the reaction of H₂O₂ with ferrous ion is different. To evaluate the influence of these ions, the pH of an AY99 aqueous solution by different acids (H₂SO₄, HNO₃, HCl and H₃PO₄) was adjusted in similar operating conditions to those established previously. After 50 min of contact time, the decolourization efficiency was 88.36%, 87.93%, 85.98% and 39.61% in the presence of H₂SO₄, HNO₃, HCl and H₃PO₄ acids, respectively. 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 a complexation and a radical scavenging as shown in Equations 14 and 15[39–41]. Phosphate ions create competition between hydroxyl radicals and organics, leading to the inhibition of oxidation.

$${\text{HO}}^{·}+H_2{\text{PO}}_4^{-}\to H_2{\text{PO}4}^{·}+{\text{OH}}^{-}$$

(14)

$${\text{HO}}^{·}+{\text{PO}}_4^{3-}\to {\text{HO}}^{-}+{{\text{PO}}_4}^{2-}$$

(15)

According to complexation, effect of phosphate ions may undergo a complex reaction with ferrous and ferric ions, which impedes the reaction, causing hydroxyl radicals to be produced. The complexation reactions are shown by Equations 16 and 17.

$${\text{Fe}}^{2+}+H_2{\text{PO}}_4^{-}\to {\text{FeH}}_2{{\text{PO}}_4}^{+}$$

(16)

$${\text{Fe}}^{3+}+H_2{{\text{PO}}_4}^{-}\to {\text{FeH}}_2{{\text{PO}}_4}^{2+}$$

(17)

Kinetics study

Kinetics order

The degradation of dye by Fenton’s process was a complicated process due to the involvement of many side reactions and intermediates. The process could not be easily depicted by simple reaction kinetics. Several investigators have found that the Fenton reaction for dye degradation follows a pseudo-first-order kinetics[10]. Malik and Saha[36] investigated the oxidation of direct dyes via the Fenton reaction and found that the entire degradation reaction could be divided into a two-stage reaction. In the first stage, referred to as Fe²⁺/H₂O₂ stage, it was considered that the dyes decomposed quickly. The second stage of reaction was referred to as the Fe³⁺/H₂O₂ stage. The two stages have a pseudo-first-order kinetics.

Chan and Chu[42] have studied the degradation of atrazine by Fenton reagent. They showed that the reaction of degradation occurs in two stages and that these steps have a pseudo-second-order kinetics. Sheng-Pen Sun et al.[43] found that the degradation of the azo dye (orange G) by Fenton oxidation process in aqueous solutions occurs in one step and that the reaction kinetics follows a pseudo-second order.

Subsequently, the reaction order for the degradation of AY99 by Fenton reagent will be determined, and kinetics model will consider that it involves a single step. The reaction kinetics of a Fenton reaction on the degradation of dye can be described as:

$$-\frac{\mathit{\text{dC}}}{\mathit{\text{dt}}}=k{C}^m$$

(18)

where, C is AY99 concentration, m is the order of the reaction, t is the time and k is the reaction rate constant.

For a first-order reaction, the above equation after integration becomes:

$$\text{ln}\left(C_t\right)=\text{ln}\left(C_0\right)-k_{1}t$$

(19)

In which, C₀ is the initial AY99 concentration and C _t is the concentration at reaction time t. For a second-order reaction, the integrated equation becomes:

$$\frac{1}{C_t}=\frac{1}{C_O}+k_{2}t$$

(20)

The first- and second-order kinetics rate constants for the degradation of AY99 at different reaction conditions were obtained, and the results were shown in Table1. It was observed that the correlation coefficients obtained for the two models are different. The second-order kinetics shows higher correlation (R² > 0.85) than the first-order kinetics (R² > 0.7). It can be concluded that the degradation of AY99 by Fenton oxidation fits the second-order reaction kinetic of the type:

$$-\frac{dC}{dt}=k_2{C}^2$$

(21)

Table 1 First- and second-order kinetics rate constants for the decolourization of AY99 at different reaction conditions

To verify the feasibility of the proposed model, results derived from the model and those from raw data were compared and confirmed.

Rate equation for the degradation of AY99 using the homogeneous Fenton’s reagent

According to Equation 18, when plotting the function 1/C versus t, it is possible to determine the rate constant (k) (second order). Since Fenton’s reaction remains dependent on the initial concentrations of H₂O₂, Fe²⁺ and C₀, then these dependencies can be included in the expression for the rate constant (k₂) according to Equation 22:

$$k_2=A{{\left[\text{AY}\right]}_0}^n{{\left[H_2{\text{O}}_2\right]}_0}^m{{\left[{\text{FeSO}}_4\right]}_0}^p\text{exp}\left(-{\mathrm{E}}_{\mathrm{a}}/\text{RT}\right)$$

(22)

where A is the frequency factor, E_a is the apparent activation energy, T is the absolute temperature (in kelvin) and the constant R is the universal constant of ideal gazes (8.314 J/mol/K). [AY]₀, [H₂O₂]₀ and [FeSO₄]₀ are the three initial concentrations in millimole per liter and n, m and p are the orders of concentration dependence. These results show that the reaction rate is more dependent on the concentration of hydrogen peroxide than on the concentration of Fe²⁺. It is interesting to note that the order of concentration dependence on [H₂O₂]₀ is significantly larger than that on [FeSO₄]₀ for AY99 and that the frequency factor A depends on the pH of the aqueous medium in the Fenton process.

The Arrhenius expression showing the relationship between the reaction temperature and the specific reaction rate constant k is expressed as the following:

$$k_2=A\text{exp}\left(-\frac{E_a}{RT}\right)$$

(23)

where A is the pre-exponential (or frequency) factor, E _a is the activation energy (in Joule per mole), R is the ideal gas constant (8.314 J/mol/K) and T is the absolute temperature (in kelvin). Experimental data are plotted according to ln(k₂) versus 1/T. Good linear relationships exist between the plots of ln(k₂) and 1/T because the regression coefficient was higher than 0.99, as shown in Equation 24.

$$\text{ln}\left(k_2\right)=-7021.3\times (1/T)+23.687R^2=0.9954$$

(24)

Based on the slope (−E_a/R) and intercepts (ln A), E_a and A in Arrhenius form (Equation 23) were determined, i.e.,E_a = 58.375 kJ/mol and A = 1.937 × 1,010/min. Finally, the expression for the calculation of the constant of speed (k₂) for this process is:

$$k_2=6.39\times {10}^7{\left[{\text{AY}}_0\right]}^{-2.8}{\left[H_2{\text{O}}_2\right]}^{2.29}{\left[{\text{FeSO}}_4\right]}^{0.837}\text{exp}\left(-7021.3/T\right)$$

(25)

This equation is valid for the concentration range from 0.03 to 0.15 mmol/L for Fe²⁺, from 0.9 to 1.8 mmol/L for H₂O₂ and from 0.03 to 0.15 mmol/L for AY99. The resulting parity plot is shown in Figure9, giving a good agreement between the data computed from both equations.

Figure 9

Plot of k ₂ obtained from Equation 24 and predicted from Equation 21.

Effects of additives

Effects of inorganic anions

In the present study and under the optimum conditions (initial pH = 3, [H₂O₂]₀ = 1.8 mmol/L, [Fe²⁺]₀ = 0.09 mmol/L, concentration of dye [AY]₀ = 0.06 mmol/L and temperature = 303 K), the degradation of AY99 was investigated in the presence of certain inorganic anions over the range 0 to 5 mmol/L, which were added to the dye solution. Among different salts, sodium chloride, calcium chloride, sodium sulphate and calcium sulphate were chosen for this purpose. It was observed that the increase in sodium chloride and calcium chloride concentrations from 0 to 5 mmol/L decreases the decolourization efficiency from 88.36% to 52.18% and 40.91% within 20 min, respectively. At 40 min, the decolourization efficiency decreases about 33% for sodium chloride and about 45% for calcium chloride. This inhibition may be due to complexation and radical scavenging. As shown in Figure10, chloride ions create competition between hydroxyl radicals and AY99 dye, leading to the inhibition of degradation. Table2 shows the degradation rate constant of AY99 in the absence and in the presence of sodium chloride and calcium chloride. It can be seen that the rate constants decrease about 80% for increasing sodium chloride dosage from 0 to 5 mmol/L and about 90% for calcium chloride. This phenomenon can be explained by:

• a) The ferric reacts with chlorine radical Cl^·, which reacts rapidly with Fe(II) K = 5.9 × 10⁹/mol/s) to produce Cl⁻ and ferrous ions (Equation 26) [44–46].

  $${\text{Cl}}^{·}+{\text{Fe}}^{2+}\to {\text{Cl}}^{-}+{\text{Fe}}^{3+}$$

  (26)

Figure 10

Mechanism of oxidation of organic compounds with Fenton process in the presence of Cl ⁻ .

Table 2 Rate constants for the decolourization of AY99

The reaction rate constant (in per mole per second) of ferrous ions reacting with hydrogen peroxide to produce hydroxyl radicals is 53/mol/s, and the reaction rate constant of ferric ions reacting with hydrogen peroxide to produce ions is 0.02/mol/s.

• b) A decrease in the rate of generation of hydroxyl radicals because of the formation of chloro-Fe(III) complexes decreases the formation of perxo complexes [Fe^(III)H₂O₂]²⁺ and consequently decreases the generation rates of Fe(II) (Figure 11).

Figure 11

Decolourization efficiency of AY99 by Fenton oxidation process in the presence of different inorganic anions. Conditions: H₂O₂ concentration, 1.8 mmol/L; Fe²⁺ concentration, 0.09 mmol/L; pH 3; temperature, 30°C.

It should be noted that the formation of the dichloride anion radicals (Cl₂^·−) will not affect the oxidation rate of Fe(II) because this radical is a strong oxidant (E⁰ = 2.09 V), which reacts rapidly with Fe(II) species, and the formation of HO^· by the reaction of H₂O₂ with Fe(II) is the limiting step in the overall reaction rate of Fe(II) oxidation[44–46]. If we assume that FeCl⁺ can also be oxidised by H₂O₂ (Figure10), the rate constants for the reaction of H₂O₂ with FeCl⁺ and Fe²⁺ will be identical (55/mol/s) because the addition of chloride (and consequently the complexation of Fe(II) by Cl⁻) has no effect on the overall oxidation rate of Fe(II). The overall rate constant equations for AY99 degradation by Fenton process in the presence of sodium chloride and calcium chloride are:

$$k_2=6.39\times {10}^7{\left[{\text{AY}}_0\right]}^{-2.8}{\left[H_2{\text{O}}_2\right]}^{2.29}{\left[{\text{FeSO}}_4\right]}^{0.837}\text{exp}\left(-7021.3/\text{T}\right){\left[\text{NaCl}\right]}^{-0.7756}$$

(27)

$$k_2=6.39\times {10}^7{\left[{\text{AY}}_0\right]}^{-2.8}{\left[H_2{\text{O}}_2\right]}^{2.29}{\left[{\text{FeSO}}_4\right]}^{0.837}\text{exp}\left(-7021.3/\text{T}\right)-{\left[{\text{CaCl}}_2\right]}^{-0.889}$$

(28)

The effect of different sodium sulphate and calcium sulphate concentrations ranging from 0 to 5 mmol/L on the decolourization of 0.06 mmol/L AY99 solution is shown in Table2. Increasing of sodium sulphate and calcium sulphate concentrations will result in the decrease of the constant rate. This enhancement can be explained by the formation of SO₄⁻Fe(III) complex (Equations 29 and 30).

$${\text{Fe}}^{3+}+{{\text{SO}}_4}^{2-}\leftrightarrow {{\text{FeSO}}_4}^{+}$$

(29)

$${\text{Fe}}^{3+}+{{2\text{SO}}_4}^{2-}\leftrightarrow \text{Fe}{{\left({\text{SO}}_4\right)}_2}^{-}$$

(30)

Sulphate ions can also complex ferrous and ferric ions, which are responsible for the production of HO^· radicals and the regeneration of Fe²⁺ ions. As compared to the sulphate ion, the inhibiting effect of the chloride ion for the formation of the peroxocomplexes is less important (Figure11). The increase in sodium sulphate and calcium sulphate concentrations from 0 to 5 mmol/L decreases the decolourization efficiency from 88.36% to 81.86% and 79.48% within 20 min, respectively. The effect of various inorganic anions (concentration, 5 mmol/L) on the degradation of AY99 is in the following decreasing order:

the efficiency of degradation: Ef (Na₂SO₄) > Ef (CaSO₄) > Ef (NaCl) > Ef (CaCl₂)

the rate constant of degradation: k₂ (CaCl₂) < k₂ (NaCl) < k₂ (CaSO₄) < k₂ (Na₂SO₄)

Effect of cations

In order to compare the performance of different cations, additional experiments were performed with different sulphates. The cations of concern were Na⁺, Ca²⁺, Cu²⁺, Mn²⁺, Zn²⁺ and Mg²⁺ while maintaining the other operational conditions constant (hydrogen peroxide and AY99 concentrations were kept constant at 1.8 mmol/L and 0.06 mmol/L, respectively, initial pH = 3, temperature = 303 K and concentration of Fe²⁺ = 0.09 mmol/L).

Effect of copper sulphate

The influence of copper sulphate on the degradation of AY99 was studied for concentration in the range 0 to 8 mmol/L. The results are shown in Table2. It can be seen that the copper sulphate exerts a strong effect on the rate constant degradation of AY99.

The decolourization efficiency decreases from 88.36% to 60.63% as a consequence of increasing the copper sulphate concentration from 0 to 5 mmol/L within 20 min and from 95.08% to 80.34% within 40 min. The results showed that the difference in overall efficiency within 300 min for the different concentrations were negligible (<5%).

The rate constant in the absence of copper sulphate is three times larger than in the presence of 5 mmol/L copper sulphate and four times larger than in the presence of 8 mmol/L copper sulphate. The reaction mechanism and the catalytic cycle become complicated in presence of copper sulphate. This could be explained as Cu²⁺ ions reacting with H₂O₂ to produce less oxidative radicals such as H₂O^· in reactions 31 and 32[47, 48]:

$${\text{Cu}}^{2+}+H_2{\text{O}}_2\to {\text{Cu}}^{+}+H_2{\text{O}}^{·}+H^{+}$$

(31)

$${\text{Cu}}^{+}+H_2{\text{O}}_2\to {\text{Cu}}^{2+}+{\text{HO}}^{·}+{\text{HO}}^{-}$$

(32)

The overall rate constant equation for AY99 degradation by Fenton process in the presence of copper sulphate is:

$$k_2=6.39\times {10}^7{\left[{\text{AY}}_0\right]}^{-2.8}{\left[H_2{\text{O}}_2\right]}^{2.29}{\left[{\text{FeSO}}_4\right]}^{0.837}\text{exp}\left(-7021.3/\text{T}\right)-0.8485\left[{\text{CuSO}}_4\right]$$

(33)

Effect of manganese sulphate

The degradation of AY99 by Fenton process was investigated in the presence of different concentrations of manganese sulphate ranging from 0 to 8 mmol/L. The obtained results are presented in Table2. It was observed that the degradation efficiency and rate constant decreased with the increase in manganese sulphate concentration. The decolourization efficiency decreases from 88.36% to 49.36% as a consequence of increasing the manganese sulphate from 0 to 5 mmol/L within 20 min and from 95.08% to 83.59% within 140 min. It might to be that Mn²⁺ either reacts with H₂O₂ to decrease the amount of H₂O₂ available for HO^· radicals production or that it quenches the HO^· radicals directly. The overall rate constant equation for AY99 degradation by Fenton process in the presence of manganese sulphate is:

$$k_2=6.39\times {10}^7{\left[{\text{AY}}_0\right]}^{-2.8}{\left[H_2{\text{O}}_2\right]}^{2.29}{\left[{\text{FeSO}}_4\right]}^{0.837}\text{exp}\left(-7021.3/\text{T}\right){\left[{\text{MnSO}}_4\right]}^{-1.05}$$

(34)

Effect of zinc sulphate and magnesium sulphate

In order to investigate the effect induced from the addition of zinc sulphate and magnesium sulphate, the degradation of AY99 in the presence of 0 to 8 mmol/L zinc sulphate and 0 to 20 mmol/L magnesium sulphate was performed. It was observed that the increase of zinc sulphate and magnesium sulphate concentrations decreases the decolourization efficiency from 88.36% to 80.74% and 78.61%, respectively, within 20 min. After 80 min, for the different concentrations, the difference in overall efficiency was negligible.

Effect of sodium sulphate and calcium sulphate

As mentioned above, the increase in sodium sulphate and calcium sulphate concentrations from 0 to 5 mmol/L decreases the decolourization efficiency from 88.36% to 81.86% and 79.48% within 20 min, respectively. The effect of all these cations (concentration, 5 mmol/L) on the efficiency of the degradation of AY99 is listed in the following decreasing order:

Ef (ZnSO₄) > Ef (MgSO₄) > Ef (Na₂SO₄) > Ef (CaSO₄) > Ef (CuSO₄) > Ef (MnSO₄).

k₂ (Na₂SO₄) < k₂ (MnSO₄) < k₂ (CuSO₄) < k₂ (CaSO₄) < k₂ (ZnSO₄) < k₂ (MgSO₄).

Conclusions

The present study has investigated the degradation of Acid Yellow 99 by Fenton oxidation process. The test results showed that the solution’s pH value, the acid’s nature to adjust the pH, the initial H₂O₂ concentration, the initial Fe²⁺ concentration, the initial AY99 concentration and the temperature were the main factors that have strong influence on the degradation efficiency of Acid Yellow 99. The optimal values of the operating parameters during the oxidation of the Acid Yellow dye by Fenton process were found to be pH = 3, [H₂O₂]₀ = 1.8 mmol/L, [Fe²⁺]₀ = 0.09 mmol/L, [AY]₀ = 0.06 mmol/L and T = 30°C. The Fenton process for AY99 was found to follow pseudo-second-order kinetics, and the reaction rate constant was dominated by initial [H₂O₂]₀, [Fe²⁺]₀ and [AY]₀ concentrations. A kinetic model was proposed in the range of the operating conditions studied. The presence of cations and some inorganic anions in an aqueous solution decreases the decolourization efficiency and the rate constant of the degradation of AY99. This inhibition may be due to a complexation and a radical scavenging. In real wastewater, the phenomena will be more complicated; many anions and cations may be present simultaneously, and different interactions may occur.

Competing’ interest

The authors declare that they have no competing interests.

Authors’ information

Prof. Fadhel Ismail, Department of Process Engineering, University of Annaba, Annaba, Algeria Email: ismail.fadhel@univ-annaba.org.