ABSTRACT

Kinetics of homogeneous degradation of Eosin Y (EY), also known as Acid red 87 (CI 45380), are studied, mostly using Fenton’s process, at 30˚C by monitoring its absorbance at 517 nm (λ_max of EY). This process is one of the advanced oxidation processes (AOPs). Mixture of H₂O₂ and Fe(II) ion in acetate buffer medium (pH 2.74 - 4.56) generates hydroxyl free radicals (^•OH) which attack the dye molecules, resulting in degradation of the dye molecules. Results show that the initial rate of EY degradation decreases with the increasing of solution pH because of removal of kinetically important Fe (iron) species through formation of ferric hydroxide. On the other hand, the rate increases with increasing the concentrations of H₂O₂, Fe(II) and EY at low solution pH. The initial rate increases with increasing of concentration of H₂O₂ and, subsequently remains unaffected with further increase of its concentration at a constant Fe(II) concentration because of the enhanced scavenging environment created by H₂O₂ at its higher concentration. The initial rate also increases with increasing of concentration of Fe(II) at a constant H₂O₂ concentration and remains unaffected with its further increase. EY concentration also enhances the initial rate at low pH. However, the initial rate is significantly enhanced by UV light. This is because of formation of additional hydroxyl radicals through excitation of the dye molecules by UV light. During the period of experiment, EY in aqueous solution alone hardly suffered any degradation. Degradation mechanism of EY by the Fenton and photo-Fenton’s processes is also discussed. Statistical analysis was used to validate the experimental results. Low values of the standard deviation for both the initial rate and % degradation indicated the consistency of the experimental data.

Keywords:

Eosin Y, Fenton’s Process, Pseudo First-Order, Initial Rate, Degradation

1. Introduction

Synthetic dyes are widely used in dyeing, painting, leather making, printing, paper making, cosmetics, photography, and coating [1] [2] . Wastewater especially from dyeing/textile industries contains about 15% of the total dye that is discharged into the nearby water bodies without adequate treatment resulting in water pollution [3] . Water pollution is common in developing countries. One example is Bangladesh. Dyes in wastewaters are non-biodegradable. Phase transfer of pollutants from aqueous system into sludge takes place when attempts are made to remove them by coagulation/flocculation, membrane separation (ultrafiltration, reverse osmosis) or adsorption on activated carbon [4] . To overcome such a problem, advanced oxidation processes (AOPs) were developed to mineralize the organic pollutants. The mineralized components are mainly CO₂, H₂O and inorganic ions and/or biodegradable compounds [5] . AOPs are considered as promising methods for the treatment of industrial wastewater since they are environmentally friendly.

Eosin Y (EY), a synthetic heterocyclic compound, is a red fluorescent dye in the form of triclinic crystals. All forms of Eosins are bromine derivatives of fluorescein. They are used in dyeing/textiles, ink manufacturing, in coloring cosmetics and gasoline and as a toner. EY is a stable dye and has been used as a catalyst for photocatalytic degradation of some aromatic compounds e.g., arenediazonium salts. The dye shows redox properties with redox potential of 1.1 V (vs. SCE) for the pair EY⁺/EY^* [6] . Wastewater containing EY causes serious environmental problems due to its stability and dark color [6] . Moreover, Muruganandham and Swaminathan [7] reported that the dye, EY, was potentially hazardous to human health.

Among the AOPs, the homogeneous Fenton’s reactions [8] , comprising reactions of ionic iron (Fe²⁺/Fe³⁺) and hydrogen peroxide (H₂O₂), appear to be a promising method to be applied to dye mineralization. This is because of its simple operation, fast initial rate of reaction, low toxicity and it is environment friendly. However, the formation of iron sludge and its effectiveness at acidic pH are likely to be the barrier of its wide applications. AOPs generate hydroxyl radical which is the most reactive oxidizing species in water treatment, with an oxidation potential between 2.8 V (pH = 0) and 1.95 V (pH = 14) vs. SCE (saturated calomel electrode) [9] . The hydroxyl radical is very non-selective in nature and rapidly reacts with numerous organic species with relatively higher rate constants (10⁸ - 10¹⁰ M⁻¹∙s⁻¹) resulting in formation of mineralized products like, CO₂, H₂O and inorganic ions and/or biodegradable compounds [10] [11] .

In the present study, Eosin Y, a dye widely used in many industries, has been taken as a model compound. The homogeneous Fenton’s process has been used to investigate the effects of some kinetic parameters, such as concentrations of H₂O₂, Fe(II), EY and solution pH on its initial rates of its degradation in aqueous system. The dye is degraded in the dark but the rate of degradation and extent of degradation are enhanced in the presence of UV light.

2. Experimental

2.1. Materials

Reagent grade (RG) Eosin Y (EY) was purchased from the local market and used without further purification. Analar grade (AR) Mohr’s salt, FeSO₄∙(NH₄)₂SO₄∙6H₂O, hydrogen peroxide (RG), sodium hydroxide (RG), sulfuric acid (RG), oxalic acid (AR), potassium permanganate (RG), potassium nitrate (RG), glacial acetic acid (RG), sodium acetate (RG), and 1,10-phenanthroline were purchased from BDH. Deionized water was used throughout the experiment.

2.2. Experimental Procedures

2.2.1. Preparation of Dye Solution

2.58 × 10⁻³ M stock solution of EY was made by dissolving 0.42 g dye in a 250 mL volumetric flask with deionized water. The flask was then wrapped with aluminum foil and stored in the dark. Concentrations of the samples of the EY solutions during experiments were determined by measuring the absorbance at 517 nm (λ_max of EY) using its molar absorption coefficient (ε_EY = 5.46 × 10⁴ L∙mol⁻¹∙cm⁻¹ at 30˚C). To calculate the absorption coefficient of EY at 517 nm, five different concentrations of EY were prepared in acetate buffer (pH 4.60) and their absorbance was monitored at 517 nm. The obtained absorbance was plotted against their respective concentrations that showed a straight line which passed through the origin (y = mx, m stands for slope). Beer’s law was obeyed over about 2 orders of magnitude of EY concentration. The absorption coefficient of EY was then calculated from the slope of the straight line by using A = εcl equation. (where ε stands for absorption coefficient, A for absorbance, c for concentration and l for cell path length that is 1). Concentration of the EY stock solution was 2.58 × 10⁻³ M.

2.2.2. Preparation of Fe(II), H₂O₂ and Acetate Buffer Solutions

A stock solution of Fe(II) was prepared from requisite amount of Mohr’s salt in a 250 mL volumetric flask by using deionized water containing sufficient amount of sulfuric acid. The permanganate solution was standardized by standard oxalic acid solution. A stock solution of H₂O₂ solution was prepared through dilution from its 28% solution in a 250 mL volumetric flask and then standardized by the standard permanganate solution. Acetate buffer solution was prepared through mixing of separately prepared acetic acid and sodium acetate solutions. Dilute sulfuric acid or sodium hydroxide solution was added to the buffer solution to adjust the solution pH. Spectrophotometric analysis of the Mohr’s salt stock solution, using 1,10-phenanthroline in acetate buffer (pH 4.60) at 510 nm (λ_max of Fe(II)-phenanthroline complex), verified the concentration of Fe(II) in the solution. Here absorption coefficient of the Fe(II)-phenanthroline complex was calculated from the slope of the straight line of absorbance vs. concentration of Fe(II)/1,10-phenanthroline plot by using A = εcl equation. The calculated absorption coefficient (ε) value was found to be 1.11 × 10⁴ L∙mol⁻¹∙cm⁻¹ at 30˚C which nicely satisfies the literature value, 1.10 × 10⁴ L∙mol⁻¹∙cm⁻¹ [12] .

2.2.3. Degradation of EY by Fenton’s Process

A typical experiment was carried out in the following way: 40 mL reaction mixture containing dye solution, acetate buffer solution of known pH and Fe(II) solution was taken in a flask and kept it in a water bath (30˚C). The pH of the mixture was measured at 30˚C by using a pH meter (Orion, Japan). The reaction was started by adding 10 mL H₂O₂ solution which was also kept at 30˚C. The well agitated reaction mixture was quickly taken into 1-cm quartz cell (Sigma Aldrich) which was kept in the thermostat compartment of the spectrophotometer (Shimadzu UV-160A, Japan). The absorbance of the solution was measured at 517 nm at different time intervals e.g., 1, 2, 3, 5 min.

The degradation of EY by photo-Fenton’s process was carried out in the presence of UV light in a specially designed light house.

2.3. Determination of Initial Rates

Pseudo first-order kinetics provides rather accurate rate data than generally realized i.e. second-order kinetics [13] [14] . Concentration of Eosin Y (EY) was measured at different time intervals e.g., 1, 2, 3, 5 min and the pseudo first-order rate constant (k¢, min⁻¹) was obtained from the slope of Ln A vs. time (min) plot. The initial rates were calculated from the observed pseudo first order rate constants, according to the following Equation (1) and Equation (2):

(1)

(2)

where, R_i = initial rate (mol∙L⁻¹∙min⁻¹),

A_o = absorbance at t = 0,

= molar absorption coefficient (L∙mol⁻¹∙cm⁻¹),

= cell path length (cm) = 1 cm.

2.4. Determination of % Degradation

For each set of experiment, the initial absorbance of desired EY concentration was measured at 517 nm, in the presence of all the reagents except H₂O₂ where aqueous solution of all the reagents except EY, was as reference. After that a new solution of the desired EY concentration was prepared with all the reagents and reaction started just after addition of H₂O₂. According to the present experimental conditions, the reaction time was 30 min. After 30 min, the absorbance was measured with the same aqueous reagents solution as a reference. In each experiment, the % degradation was calculated according to the following Equation (3):

(3)

Here, A_o = absorbance at time, t = 0 min.

A_t = absorbance at t = 30 min.

2.5. Statistical Analysis

To validate the experimental results, statistical analysis, such as standard deviation was calculated to quantify the amount of variation or dispersion of a set of data values. A low standard deviation value indicates that the data points tend to be close to the mean of the set, while a high value indicates that the data points are spread out over a wider range of values.

Standard deviation was calculated for each set of experiment through carrying out at least three times, and calculated according to the following Equation (4):

(4)

= standard deviation,

= each value of data-set,

= arithmetic mean of the data,

= total number of data points.

3. Results and Discussion

3.1. General

Prior to discuss the effects of some kinetic parameters on the initial rates of degradation of Eosin Y by Fenton’s process, it is desirable to focus the characteristics of the dye, EY, on aqueous solution. Eosin Y is an anionic azo dye. It is available as Na-salt (Figure 1). Its spectrum in aqueous solution shows four peaks at 517 nm, 342 nm, 301 nm and 254 nm with molar absorption coefficients 5.46 × 10⁴ L∙mol⁻¹∙cm⁻¹, 0.40 × 10⁴ L∙mol⁻¹∙cm⁻¹, 1.03 × 10⁴ L∙mol⁻¹∙cm⁻¹ and 2.20 × 10⁴ L∙mol⁻¹∙cm⁻¹ respectively (Figure 2). The peak at 517 nm is the most intense and this peak was taken as the λ_max, at which degradation (decrease in absorbance) of EY was studied.

The conjugation effect in EY, because of the polycyclic aromatic chromophore, produces the most intense peak at 517 nm. Its peak at 301 nm arises due to the presence of the carbonyl group (-C=O) in the aromatic ring. The relatively weak peak at 342 nm is ascribed to the presence of the carboxylic group in the aromatic rings. The peak at 254 nm arises is due to the presence of benzene ring in the structure of EY. As abovementioned, the presence of the polycyclic aromatic rings is responsible for the most intense peak of EY at 517 nm which is visible range. Therefore, disappearance of color of the EY indicates the degradation of the polycyclic aromatic rings resulting in formation of smaller components and ions, such as CO₂, Br⁻, Na⁺ etc.

In normal solution (pH ~ 6) EY remains mostly as B (EY²⁻), as the pH decreases the species C (HEY⁻) is formed and on the further decrease the species D (H₂EY) will be formed as shown in Equation (5) and Figure 3.

(5)

The situation is shown below:

In this study, EY has been used to represent any of the forms of EY (Figure 3) although the predominant species at the experimental pH (3.78) is form C. As shown in Figure 4, the absorbance of EY increases with increasing of its solution pH. This is because of the enhancing in conjugation of the aromatic rings of EY structure through formation of carboxylate as well as hydroxylate anions at higher pH that favors the excitation (Equation (5) and Figure 3). However, protonation of the EY²⁻ (D) at low pH (Figure 4) reduces the conjugation which needs higher energy for excitation.

A mixture of H₂O₂ and Fe(II) ions in the aqueous solution is known as Fenton’s reagent. It has been experimentally proven that Fenton’s reagent generates hydroxyl free radicals (^•OH) which attack organic molecules non-selectively resulting in mineralization of the parent molecules [15] . Reactivity of Fenton’s reagent mainly depends on the

rate of formation of reactive species, for example, ^•OH but irrespective of the EY species.

In our previous studies [16] , we observed no significant difference between % decolorization and % degradation of Ponceau S (PS) by using ZnO-mediated photodegradation through measuring disappearance of PS color and TOC (total organic carbon). Of course, decolorization of organic dyes is faster than their photodegradation. Obviously, the photodecolorization of organic dyes is related to their degradation. Accordingly, studies on photodecolorization gave a measure of the photodegradation of the dyes. Rate of degradation is only depends on the rate of formation of hydroxyl free radicals which depends on the kinetic parameters, for example, solution pH, concentrations of H₂O₂, Fe(II) and EY. Therefore, it has considered that the disappearance of color (decolorization) of EY solution is proportional to its degradation. According to the present experimental results, the effective solution pH for significant degradation of EY would be 2.74.

3.2. Factors Affecting the Initial Rates of Degradation of EY by Fenton’s Process

3.2.1. Effect of pH on Initial Rate of Degradation of EY

Hydrogen peroxide undergoes auto decomposition at low pH [15] . On the other hand, Fe(III) form hydroxide species through hydrolysis reaction from pH ~ 2.8 resulting in removing of kinetically important Fe species [17] . All these contribute to the lowering of initial rate with rise in pH of the solution (Figure 5). Thus the degradation of EY by Fe(II) + H₂O₂ could be studied only in a narrow range of hydrogen ion concentration. The decrease in initial rate with pH is typical for a Fenton’s process (Figure 5) [18] .

The minimum decrease in the rate is around pH 4, above which it increases with pH. This can be explained by considering the values of the slopes at pH 4 and 4.5 (Figure 5). The decrease in the initial rate has been attributed to the formation of colloidal ferric species that decompose H₂O₂ [19] [20] . On the other hand, if the pH is too low, ^•OH radicals are destroyed according to the Equation (6):

(6)

3.2.2. Effect of H₂O₂ Concentration on Initial Rate of Degradation of EY

H₂O₂ generates hydroxyl free radical (^•OH) in the presence of Fe(II) ion. Accordingly,

the concentration of hydroxyl radical (^•OH) is expected to be proportional to the concentration of hydrogen peroxide.

At a constant temperature and for a fixed amount of EY and Fe(II), and at a fixed pH, the initial rate of degradation of EY (Figure 6) increases with the increase in H₂O₂ concentration. However, above a certain concentration of H₂O₂, ~ 10.00 × 10⁻³ M, the initial rate remains almost unaffected with further increase in H₂O₂ concentration at pH 3.78, [EY]: 2.58 × 10⁻⁵ M and [Fe(II)]: 2.00 × 10⁻⁴ M. The degradation of EY also increases with the increase in H₂O₂ concentration, but virtually becomes independent of its concentration under these conditions (Table 1).

In Fenton’s process, hydrogen peroxide generates hydroxyl free radical (^•OH) through reaction with Fe(II), however, at high H₂O₂ concentration, hydroxyl radical efficiently reacts with H₂O₂ and produces hydrogen dioxide radical (HO₂^•), also ^•OH radicals generated at high concentration react with HO₂^• or dimerize to H₂O₂. It is obvious that HO₂^• radicals are less reactive than ^•OH radicals, therefore leading to negligible contribution in degradation of the dye molecules [18] [21] .

3.2.3. Effect of Fe(II) Ion Concentration on Initial Rate of Degradation of EY

The formation of hydroxyl radical (^•OH) is also directly proportional to the concentration of Fe(II) ion. Under constant temperature and for fixed concentrations of EY, H₂O₂ and at a constant solution pH, the initial rate of degradation of EY increases with the increase in concentration of Fe(II) (Figure 7).

The pattern of this increase appears to be unaffected by H₂O₂ concentration until the H₂O₂ concentration is more than ~16 × 10⁻³ M. Three folds increase in H₂O₂ concentration seems to initiate another reaction path in which the hydroxyl radical (^•OH) may react with H₂O₂ that can be shown by the Equation (7):

(7)

The generated ^•O₂H radicals react with EY, but at a slow rate. The ultimate degradation of EY does not change significantly even if Fe(II) concentration is raised to 4.00 × 10⁻⁴ M at pH 3.78 and at higher H₂O₂ concentration (Table 1).

3.2.4. Effect of EY Concentration on Initial Rate of Degradation of EY

The variation of EY concentration (Figure 8) was investigated using Fenton’s process at 7.90 × 10⁻³ M and 15.8 × 10⁻³ M H₂O₂ and photo-Fenton’s process at 7.90 × 10⁻³ M H₂O₂. The kinetic parameters, such as concentrations of EY, H₂O₂ and pH, for the

Fenton and photo-Fenton’s processes were the same only an ultraviolet light source (UV_A) was used in the photo-Fenton’s process.

As shown from Figure 8, H₂O₂ concentration has the major influence on the initial rate of degradation of EY at definite concentrations of Fe(II) and EY, and at a fixed solution pH. If the H₂O₂ concentration is below the optimum value, in the present case when [Fe(II)]: 2.00 × 10⁻⁴ M and [H₂O₂]: 7.90 × 10⁻³ M, lower EY concentration facilitates the following reaction as shown in Equation (8):

(8)

However when the dye concentration is >2.00 × 10⁻⁵ M, the expected reaction, i.e. the degradation of dye starts becoming favorable. The use of UV_A (350 - 400 nm light) is responsible to change the situation, that is, increases the initial rate with the increase of EY concentration. This is because the UV light facilitates, primarily, the excitation of some dye molecules resulting in formation of additional ^•OH radicals. Moreover some dye molecules, however insignificant, may be directly degraded through the generation of dye molecular radicals by the UV light. All these enhance the initial rate of UV-assisted degradation of EY.

It is to be noted that UV_A play a significant role on degradation of EY at [H₂O₂]: 7.90 × 10⁻³ M, [Fe(II)]: 2.00 × 10⁻⁴ M and at pH 3.78 (Table 2). The degradation was observed about 74% with 1.24 × 10⁻⁵ M EY concentration within 30 min. Further increase in EY concentration appears to decrease the degradation (Table 2). One of the important reasons for this is the increase of inner filter effect by the increased numbers of EY species in solution. This diminishes the UV_A photons that usually enhanced the concentration of ^•OH radicals through excitation of the EY species [22] .

3.2.5. Effect of H₂O₂ Concentration on Initial Rate of UV Light-Assisted Degradation of EY

As the UV light can excite dyes, an additional route to formation of (^•OH) hydroxyl radical is created [23] [24] .

In a previous study [25] , it was found that UV_B (λ < 350 nm) decolorized EY to some extent. In the present case, EY did not seem to undergo noticeable degradation with UV_A up to 30 minutes. However when H₂O₂ at a relatively high concentration was used, there was noticeable degradation of EY in the absence of Fe(II) at pH ~ 6. The variation of initial rate with H₂O₂ concentration is linear as shown in Figure 9. The intercept with the best fit plot (r² = 0.9980) (Figure 9) clearly indicates some degradation of EY by UV_A is occurred but not detectable within 30 minutes by the present experimental technique.

3.3. Mechanism of Degradation of EY

In the aqueous acidic environment, in the presence of an organic substance, here EY and Fe(II) species, H₂O₂ initiates a set of complex redox reactions [26] [27] [28] [29]

[30] . The overall reactions are shown by the following Equation (9) and Equation (10):

(9)

(10)

H₂O₂ reacts with Fe(II) to generate hydroxyl free radical (^•OH) and Fe(II) converted into Fe(III) as shown in Equation (9)). Then Fe(III) reacts with H₂O to form hydrogen dioxide radical (^•O₂H) (Equation (10)). The generated ^•OH radical reacts with the aqueous dye molecules (EY) to form their molecular radicals (^•EY) (Equation (11)) and converted into their cations through reaction with Fe(III) species (Equation (12)).

(11)

(12)

Numerous competing reactions, involving Fe(II), Fe(III), H₂O₂, ^•OH, ^•O₂H radicals derived from the substances, are possible. The following typical reactions may also take place:

(13)

(14)

Here the ^•OH radical further reacts with H₂O₂ to form ^•O₂H radical (Equation (13)) and the generated ^•O₂H reacts with Fe(III) resulting in diminishing of the reactive oxidizing species (Equation (14)).

In the presence of UV_A, the following important processes can be occurred:

(15)

(16)

Here H₂O₂ produces ^•OH radical under UV irradiation (Equation (15)) and the dye molecules converted into either their molecular radical (^•EY) and/or their cations (EY⁺) (Equation (16)).

Finally, degradation products (e.g., CO₂, ,) are formed through reactions between the ^•OH radical and/or other oxidizing species and EY/^•EY/EY⁺ as shown in Equation (17)).

(17)

EY stands for H₂EY, HEY⁻, EY²⁻. Fe(II) and Fe(III) are respective free ions and/or any other of these forms.

4. Conclusions

The present work reveals that the initial rate of degradation of Eosin Y (EY) dye depends on the concentrations of H₂O₂, Fe(II), EY and the solution pH. Degradation of EY is significantly regulated by its solution pH. Most of the EY species in solution remain as negatively charged species at solution pH ~ 6. The pH of EY solution was kept constant by sodium acetate buffer solution. The buffer did not appear to interfere with the normal rate of H₂O₂ or Fe(II).

Low solution pH (2.74) shows highest initial rate as well as degradation at [H₂O₂]: 2.37 × 10⁻⁴; [Fe(III)]: 2.00 × 10⁻⁴; [EY]: 2.58 × 10⁻⁵ M, however, both the initial rate and degradation decrease with increasing of solution pH. This is because of removing of kinetically important iron (Fe) species through formation of ferric hydroxide. H₂O₂ also increases both the initial rate and degradation up to a certain concentration, after that virtually no effect was observed. This is because of scavenging of the hydroxyl radicals by the H₂O₂ at its higher concentration. Both the initial rate and degradation were significantly enhanced by using of photo-Fenton’s process. Fenton’s process showed the initial rate: 7.35 × 10⁻⁷ mol∙L⁻¹∙min⁻¹ and degradation: 27.62% at [H₂O₂]: 7.90 × 10⁻³ M; [Fe(II)]: 2.00 × 10⁻⁴ M; [EY]: 2.58 × 10⁻⁵ M and pH: 3.78 within 30 min reaction time, while photo-Fenton’s process showed the initial rate: 11.51 × 10⁻⁷ mol∙L⁻¹∙min⁻¹ and degradation: 54.32% under the same experimental conditions.

The disappearance of color of EY solution and its mineralization to CO₂, Br^?, Na⁺ should take place at different rates [31] . High concentration of only H₂O₂ degrades EY solution significantly by UV_A. This point is the fact that degradation of dyes in the textile factory effluents can be achieved simply with the help of H₂O₂ and sunlight (containing UV_A) at relatively low pH e.g., 3.0.

Acknowledgements

The authors acknowledge the Ministry of Science and Technology of the People’s Republic of Bangladesh for financial support to carry out this work under the project “Photoelectrochemical splitting of water into hydrogen using solar light”.

Cite this paper

Hossain, A., Rayhan, A.B.M.S., Raihan, M.J., Nargis, A., Ismail, I.M.I., Habib, A. and Mahmood, A.J. (2016) Kinetics of Degradation of Eosin Y by One of the Advanced Oxidation Processes (AOPs)―Fenton’s Process. American Journal of Analytical Chemistry, 7, 863-879. http://dx.doi.org/10.4236/ajac.2016.712074

References