Fenton Oxidation Kinetics of Azo Dye Acid Light Yellow 2G Wastewater by Online Spectrophotometry

The online spectrophotometric technique was adopted to monitor the degradation of simulated Acid Light Yellow 2G (ALY 2G) solution with the Fenton oxidation process, and the kinetic process was also discussed. The effects of the initial concentration of H2O2 and Fe2SO4, pH value, and initial dye concentration on the degradation process were studied. The results showed that the ALY 2G can be degraded by Fenton oxidation, and the colour removal rate of Acid Light Yellow 2G was 94.66% after 300 s when the concentration of simulated wastewater was 20 mg/L, the dosage of Fe2+ was 0.1 mmol/L, the dosage of H2O2 was 0.6 mmol/L, and the pH was 3. The degradation process was divided into two stages: the first stage, the degradation rate is very fast; in the second stage, with the extension of reaction time, the increase of decolourization rate decreases. The first stage of the reaction accords with the first-order kinetics, and the reaction rate constant Kap is 0.04824 s -1. The intrinsic reaction rate constant of ALY 2G and hydroxyl in aqueous solution in the Fenton oxidation method is 0.55 × 109 M-1s-1.


INTRODUCTION
Textile and dyeing industries are one of the most important chemical industries. However, many printing and dyeing plants produce large amounts of high chroma wastewater during the production process (Xu et al. 2015b). Many aromatic agents, metals and chlorides contained in wastewater are toxic to aquatic organisms, human beings and even affect biosphere (Laszlo & Erzsebet 2008). Every year, 12% of synthetic dyes are run off during the production process, resulting in dye-containing wastewater with high chroma and chemical oxygen demand (COD), low biochemical oxygen demand, oxidation resistance and difficult biodegradation . Therefore, the most critical problem in the dyeing industry is how to treat visible pollutants contained in dye wastewater (Lee et al. 2006) to meet the industry emission standards.
Advanced oxidation processes (AOPs) have great potential for degrading organic pollutants in industrial wastewater. This oxidation mechanism produces strong oxidants, such as hydroxyl radicals (Cheng et al. 2016), which have high activity and are non-selective for decomposing organic pollutants into CO 2 , H 2 O, and inorganic salts in the water environment (Inmaculada et al. 2015). Fenton oxidation (Fe 2+ /H 2 O 2 /H + ) has received the intensive attention in wastewater treatment due to its superior degradation efficiency, rapid reaction speed and moderate investment (Azizi et al. 2015). Under weak acidic conditions, Fe 2+ is oxidized by H 2 O 2 to form Fe 3+ , hydroxyl (·OH) and OH − (Xu et al. 2015a), which produces highly reactive ·OH to destroy the molecular structure of organic dyes, thus achieves the decolourization effect of dye wastewater. The spectrophotometer can record the mass concentration change of the dye (Gao et al. 2019a, Gao et al. 2019b, Sibel et al. 2012, Xu et al. 2018) and monitor the instantaneous state of the dye decolourization during the Fenton oxidation process. Therefore, the experimental results are real-time and reliable with a very minor error.
In the present study, azo dye Acid Light Yellow 2G was selected as the target pollutant. We studied the effect of initial Fe 2+ , initial H 2 O 2 concentration, initial pH value of the solution and different dye concentrations on the degradation of ALY 2G by the Fenton method. The decolourization kinetics performance of Fenton oxidation was studied based on the experimental data. The kinetic model of azo dye degradation with Fenton's reagent was established. In this study, the online spectrophotometric system was used to monitor the degradation of Acid Light Yellow 2G. The kinetic analysis 418 Aifang Gao et al. Vol. 20, No. 1, 2021 • Nature Environment and Pollution Technology result was expected to provide basic experimental data for a deeper understanding of the Fenton oxidation process of wastewater containing ALY 2G dye.

Chemical Reagents
The structure of Acid Light Yellow 2G is shown in Fig. 1. ALY 2G was purchased from Shijiazhuang Dyestuffs Company (China) and the ALY 2G solution was prepared by dissolving a requisite quantity of dye in ultrapure water. Ferrous sulphate (FeSO 4 ·7H 2 O) and hydrogen peroxide (H 2 O 2 ) were purchased from Tianjin Damao Chemical Reagent Company, and sulfuric acid (H 2 SO 4 ) from Modern Chemical Reagent Company. They were of reagent analytical grade.

Apparatus Set-up
The online spectrophotometric system is shown in Fig. 2. Reaction section (degradation device) includes a digital magnetic stirrer apparatus (Shanghai Instrument Company, China), and a 500 mL beaker. Optical measuring part contains UV-Vis spectrometer (UNICO 2802, Shanghai, China), cycle peristaltic pump and cuvette (1 mL). The recording unit is a computer with the monitoring frequency of 12 min -1 during the oxidation process.

Experimental Procedure
Fenton oxidation process was performed in a 500 mL vessel. With the role of a peristaltic pump, the simulated dye wastewater was pumped into the cuvette of UV-Vis spectrophotometer. Absorbance at maximal absorption peak of dye was obtained by the spectrophotometer. The effects of FeSO 4 dosage, H 2 O 2 dosage, initial pH, and initial dye concentration on the degradation of Acid Light Yellow 2G were studied by single-factor experiments.

Feasibility Analysis of Online Spectrophotometric Technique
Online spectrophotometry method was applied to analyze the decolourization of ALY 2G dye in the Fenton process. The UV-Vis spectra of ALY 2G, H 2 SO 4 , Fe 2+ , and Fe 3+ are presented in Fig. 3. Azo dye ALY 2G has a maximum adsorption peak of 402 nm, which does not vary with the addition of H 2 SO 4 , Fe 2+ and Fe 3+ . Therefore, during the experiment, online spectrophotometry can be used to monitor ALY 2G wavelength at 402 nm. The standard equations and standard curves for dye concentration (C) and absorbance (A) are given in Fig. 4. The relationship of the absorbance (A) at 402 nm against concentration (C) of ALY 2G is A = 0.0309C + 0.0015 (R 2 = 0.9998).
Chemical Reagent Company. They were of reagent analytical grade.

et-up
ectrophotometric system is shown in Fig. 2. Reaction section (degradation device) ital magnetic stirrer apparatus (Shanghai Instrument Company, China), and a 500 mL l measuring part contains UV-Vis spectrometer (UNICO 2802, Shanghai, China), ic pump and cuvette (1 mL). The recording unit is a computer with the monitoring 2 min -1 during the oxidation process.   at maximal absorption peak of dye was obtained by the spectrophotometer. The e dosage, H 2 O 2 dosage, initial pH, and initial dye concentration on the degradation Yellow 2G were studied by single-factor experiments.

Feasibility Analysis of Online Spectrophotometric Technique
Online spectrophotometry method was applied to analyze the decolourization of AL Fenton process. The UV-Vis spectra of ALY 2G, H 2 SO 4 , Fe 2+ , and Fe 3+ are presente dye ALY 2G has a maximum adsorption peak of 402 nm, which does not vary with H 2 SO 4 , Fe 2+ and Fe 3+ . Therefore, during the experiment, online spectrophotometry monitor ALY 2G wavelength at 402 nm. The standard equations and standard concentration (C) and absorbance (A) are given in Fig. 4. The relationship of the ab 402 nm against concentration (C) of ALY 2G is A = 0.0309C+0.0015 (R 2 = 0.9998) Fig. 3: Comparison of dye UV-Vis spectra between dye and dye (+H 2 SO 4 +Fe 2+ +Fe 3+ ).

Single Factor Experiment
The effects of initial FeSO 4 dosage, initial H 2 O 2 dosage, pH value, and initial dye concentration on chroma removal have been discussed. When the reaction time reaches 300 s, the decolourization rate of the dye was calculated. The colour removal rate (R) is defined as given in Eqs. 1. C 0 and C represent the initial and the instant concentrations of the dye with reaction time, respectively.

Single Factor Experiment
The effects of initial FeSO 4 dosage, initial H 2 O 2 dosage, pH value, and initial dye concentration on chroma removal have been discussed. When the reaction time reaches 300 s, the decolourization rate of the dye was calculated. The colour removal rate (R) is defined as given in Eqs. 1. C 0 and C represent the initial and the instant concentrations of the dye with reaction time, respectively.  . This process results in the production of the hydroperoxyl radical (·OOH as a scavenger of hydroxyl radical) and then decreases the colour removal rate of dye (Sehested et al. 2003. In brief, we choose 0.6 mmol/L as an optimum H 2 O 2 concentration of the decolourization of ALY 2G in the Fenton oxidation process. The effect of solution pH: The pH of the solution plays an important role in dye degradation for the Fenton process. The influence of pH value on the decomposition of ALY 2G is illustrated in Fig. 7. The change trends of decolourization rate with various pH value are consistent. The decolourization rate increases from 57.12% to 94.66% as the pH value increases from 1.5 to 3. However, when the pH value further increases to 4, the decolourization rate decreases to be 89.39%. At the lower pH value(<3) the ·OH is consumed by the excessive hydrogen ion (·OH + H + + e -→ H 2 O), and on process. This reason is that because the excess ferrous ion competes with the dye the hydroxyl radical ·OH (Fe 2+ + ·OH → Fe 3+ + OH -) (Xu et al. 2015b). Therefore, propriate amount of Fe 2+ can improve the degradation effect of the ALY 2G dye. We he initial Fe 2+ concentration of 0.1 mmol/L as an optimum dosage for efficient n to ALY 2G aqueous solutions.   thus the decolourization rate is relatively small. When the solution (pH > 3), with the formation of the iron hydroxide complex, the hydrogen peroxide is decomposed and the ferrous ion catalyst is invalidated, which ultimately leads to a reduction in the oxidation ability of Fenton (Gao et al. 2014). Therefore, the pH value of 3 is considered to be the optimum value for the decolourization of azo dye ALY 2G in Fenton oxidation.
The effect of initial ALY 2G concentration: Fig. 8 shows the trend of the colour removal rate with various dye concentrations (conditions: [Fe 2+ ] = 0.1 mmol/L, [H 2 O 2 ] = 0.6 mmol/L, and pH = 3). Although the initial concentrations of the dye ALY 2G were different (from 10 to 40 mg/L), the chroma removal rate of dye can all reach more than 90% and the difference of all removal rates is very small after 300s. Moreover, it can be seen that the reaction rate gradually decreases as increasing dye concentration between 50s and 150s. The reason for this is that as the initial concentration of the Acid Light Yellow 2G dye solution increases, the number of dye molecules in the solution increases, whereas the amount of ·OH in the solution does not increase, which leads to a decrease in the reaction rate.

Reaction Kinetic Fitting Analysis
Kinetic process analysis is helpful to understand the Fenton oxidation process. Acid Light Yellow 2G was decolourized successfully in the experimental process, and the first stage of decolourization was analysed by first-order kinetics (Gao et al. 2019b). The first-order kinetics calculation formula (Eqs. 4-5) is as follow:

Reaction Kinetic Fitting Analysis
Kinetic process analysis is helpful to understand the 2G was decolourized successfully in the experimenta was analysed by first-order kinetics (Gao et al. 2019 (Eqs. 4-5) is as follow:

Reaction Kinetic Fitting Analysis
Kinetic process analysis is helpful to understand the 2G was decolourized successfully in the experimenta was analysed by first-order kinetics (Gao et al. 2019b (Eqs. 4-5) is as follow: Where, t is the reaction time; C is the instant d concentration.
Figs. 9-12 show the changes of ln (C 0 /C t ) with t different H 2 O 2 concentrations, different pH, and di ... (5) Where, t is the reaction time; C is the instant dye concentration; C 0 represents the initial dye concentration.
Figs. 9-12 show the changes of ln (C 0 /C t ) with time under different initial Fe 2+ concentrations, different H 2 O 2 concentrations, different pH, and different dye concentrations, respectively. The linear fitting results of the relationship curve between ln (C 0 /C t ) and time (t) are given in Table 1. The kinetic parameters are also listed in Table 1. The values of the kinetic parameters (the correlation coefficients R 2 ) are all above 0.94. It can be seen that the oxidation herefore, the pH value of 3 is considered to be the optimum value for the of azo dye ALY 2G in Fenton oxidation.  ugh the initial concentrations of the dye ALY 2G were different (from 10 to 40 mg/L), moval rate of dye can all reach more than 90% and the difference of all removal rates l after 300s. Moreover, it can be seen that the reaction rate gradually decreases as e concentration between 50s and 150s. The reason for this is that as the initial of the Acid Light Yellow 2G dye solution increases, the number of dye molecules in ncreases, whereas the amount of ·OH in the solution does not increase, which leads to the reaction rate. linear fitting results of the relationship curve between ln (C 0 /C t ) and time (t) are given The kinetic parameters are also listed in Table 1. The values of the kinetic parameters (the coefficients R 2 ) are all above 0.94. It can be seen that the oxidation process of Acid L 2G by Fenton method accords with first-order kinetics.  linear fitting results of the relationship curve between ln (C 0 /C t ) and time (t) are given The kinetic parameters are also listed in Table 1. The values of the kinetic parameters (the coefficients R 2 ) are all above 0.94. It can be seen that the oxidation process of Acid L 2G by Fenton method accords with first-order kinetics.   According to Eqs. (13) and (15) According to Eqs. (13) and (15) According to Eqs. (13) and (15) Kušić et al. 2006, Sibel et al. 2012, Sehested et al. 2003 The experimental results are shown in Fig. 13 The experimental results are shown in Fig. 13 The experimental results are shown in Fig. 13. [H 2 O 2 ]/K ap has ... (19) By fitting the first-order kinetics to the oxidation reaction process in the last stage, the results show that the correlation coefficients are all above 0.95. It can be seen that the fast stage meets the first-order kinetics, so that: The experimental results are shown in Fig. 13 The experimental results are shown in Fig. 13     process of Acid Light Yellow 2G by Fenton method accords with first-order kinetics.
First-order kinetics fitting is performed for the fast reaction stage. The fast stage of the Fenton oxidation process abides by first-order kinetics. Under the optimal reaction condition in Fenton oxidation, the correlation coefficient is 0.98 and the reaction rate K ap is 0.04824 s -1 .

Kinetics Study
The Fenton oxidation method uses a catalyst Fe 2+ and H 2 O 2 to undergo a redox reaction to form a highly active ·OH which can decompose the molecular structure of organic dyes. The reaction mechanism can be expressed by (Eqs. 6-14) (D represents dye molecules) (Gao et al. 2014, Gao et al. 2019b, Kušić et al. 2006, Sibel et al. 2012, Sehested et al. 2003

Kinetics Study
The Fenton oxidation method uses a catalyst Fe 2+ and H 2 O 2 to undergo a redox reaction to form a highly active ·OH which can decompose the molecular structure of organic dyes. The reaction mechanism can be expressed by (Eqs. 6-14) (D represents dye molecules) (Gao et al. 2014, Gao et al. 2019b, Kušić et al. 2006, Sibel et al. 2012, Sehested et al. 2003

Kinetics Study
The Fenton oxidation method uses a catalyst Fe 2+ and H 2 O 2 to undergo a redox reaction to form a highly active ·OH which can decompose the molecular structure of organic dyes. The reaction mechanism can be expressed by (Eqs. 6-14) (D represents dye molecules) (Gao et al. 2014, Gao et al. 2019b, Kušić et al. 2006, Sibel et al. 2012, Sehested et al. 2003

Kinetics Study
The Fenton oxidation method uses a catalyst Fe 2+ and H 2 O 2 to undergo a redox reaction to form a highly active ·OH which can decompose the molecular structure of organic dyes. The reaction mechanism can be expressed by (Eqs. 6-14) (D represents dye molecules) (Gao et al. 2014, Gao et al. 2019b, Kušić et al. 2006, Sibel et al. 2012, Sehested et al. 2003

Kinetics Study
The Fenton oxidation method uses a catalyst Fe 2+ and H 2 O 2 to undergo a redox reaction to form a highly active ·OH which can decompose the molecular structure of organic dyes. The reaction mechanism can be expressed by (Eqs. 6-14) (D represents dye molecules) (Gao et al. 2014, Gao et al. 2019b, Kušić et al. 2006, Sibel et al. 2012, Sehested et al. 2003

Kinetics Study
The Fenton oxidation method uses a catalyst Fe 2+ and H 2 O 2 to undergo a redox reaction to form a highly active ·OH which can decompose the molecular structure of organic dyes. The reaction mechanism can be expressed by (Eqs. 6-14) (D represents dye molecules) (Gao et al. 2014, Gao et al. 2019b, Kušić et al. 2006, Sibel et al. 2012, Sehested et al. 2003

Kinetics Study
The Fenton oxidation method uses a catalyst Fe 2+ and H 2 O 2 to undergo a redox reaction to form a highly active ·OH which can decompose the molecular structure of organic dyes. The reaction mechanism can be expressed by (Eqs. 6-14) (D represents dye molecules) (Gao et al. 2014, Gao et al. 2019b, Kušić et al. 2006, Sibel et al. 2012, Sehested et al. 2003  = 0.97133). According to intercept B, the intrinsic reaction rate constant of the dye concentration and ·OH in the aqueous solution is obtained (k 2 = 0.55 × 10 9 M -1 S -1 ).

CONCLUSION
In this study, the degradation of the azo dye Acid Light Yellow 2G by Fenton method and its influence factors (initial dye concentration, initial solution Fe 2+ concentration, initial H 2 O 2 concentration, and initial pH value) were studied. The following conclusions can be made. 1. The online spectrophotometric method was used to monitor the absorbance of dye Acid Light Yellow 2G with Fenton oxidation. This technique is accurate, feasible and fast. The Fenton oxidation process can be divided into two stages: a rapid degradation stage (t < 30s) and a slow degradation phase (t >30s).
2. The best experimental conditions for degradation of dyes are that FeSO 4 is 0.1 mmol/L, H 2 O 2 is 0.6 mmol/L, initial pH is 3, and when the dye concentration is 20 mg/L, the colour removal rate is 94.66%. Fenton oxidation process conforms to first-order reaction kinetics in the first stage. According to the formula K ap = ln (C 0 /C t ), the first-order rate constant is the linear fitting slope. The reaction rate constant K ap is 0.04824 s -1 under the best experimental conditions.
3. During the Fenton oxidation process, the intrinsic reaction rate constant of the ALY 2G dye and ·OH in aqueous solution was (k 2 = 0.55 × 10 9 M -1 S -1 ).