The Kinetic Model for Decolourization of Commercial Direct Blue 2 Azo Dye Aqueous Solution by the Fenton Process and the Effect of Inorganic Salts

The study of Fenton’s oxidation and degradation of Direct Blue 2 (DB2) as the commercial azo dye in synthetic aqueous solution has been accomplished. The optimum oxidative degradation reaction conditions were achieved as follows: pH = 3.50, [H2O2] = 1.1×10 -3 M, [Fe2+] = 1.0×10-4 M for [DB 2] = 1.0×10-4 M. Under optimal conditions, 80% of decolouration efficiency was carried out within 15 min of reaction. An engagement between the kinetics of the colour removal rates (ln k2) versus Lazo bond was carried out at the different pH levels. The colour removal rate was increased with decreasing of Lazo bond, in the order of pH: 3.5 > 5.0 > 2.5. The second-order kinetic model provided the best correlation of the data. Effects of various inorganic anions (such as Cl–, SO4 2-, CO3 2-, etc.) was studied to enhance the oxidation efficiency of Fenton reaction. Advanced oxidation technologies were developed in this study especially with dealing with contaminated textile wastewater over the use of chemical treatment.


INTRODUCTION
The textile industry is a major source of outflowing industrial wastewater due to more exhaustion of water during process operations. This industrial wastewater contains chemicals such as alkalis, acids, dyes, surfactants and matter high in biochemical oxygen demand (Razzak & Hossain 2016). As the textile industry uses more water than any other industry globally, virtually all wastewater discharged is highly polluted. Water consumption of an average-sized textile mill about 50 gals per kg of fabric manufactured daily (Luo et al. 2016). The most plentiful of these compounds are azo dyes, which exemplify 70% of the world dye product. Large volumes of industrial wastewater with high scales of azo dyes (about 250 mg.L -1 ) are every day vacuous by many industries around the world in the surface water. The stability and complexity of the dye structure make it more difficult to degrade when it is present in the textile wastewater (Garcia-Segura & Brillas 2016). Therefore, the mineralization of dyes generated by the textile industry is the main challenge and environmental concern (Holkar et al. 2016). There are several methods currently used to remove wastewater contamination in the fabric, but they are not universally applicable and are not cost-effective for all dyes (Nidheesh et al. 2013). In the last years, the problem of a high toxic level of wastewater has been tried by Advanced Oxidation Processes (AOPs) (Sharma et al. 2018). AOPs are based on the in-situ generation of hydroxyl radical (HO • , E° (HO • /H 2 O) = 2.80V) (Dewil et al. 2017). The Fenton system is one of the most used techniques to degrade different organic pollutants such as azo-dyes by hydroxyl free radical generated from the hydrogen peroxide molecules reduction with Fe 2+ ions at acidic pH  In Fenton oxidation process, hydroxyl free radical prefer to attack the azo bond (-N=N-) of the dye molecule by cleaving it to produce aromatic amines and inorganic ions such as NH 4 + (Trovó et al. 2016). For the treatment of industrial textile wastewater containing dyes; the AOPs are effective techniques for degradation of aromatic compounds consequent to the electrophilic aromatic exchange of HO • which then leads to open the aromatic ring (Mousset et al. 2014). The goal of the other treatment is reducing the chemical oxygen demand of the industrial textile wastewater. Typically, these two targets require various chemical reagents like H 2 O 2 and Fe 2+ coincide to either azo bond or chemical oxygen demand loadings (Dehghani et al. 2016). This manuscript reports the colour removal or COD removal kinetics of the DB 2 which contains diazo bond, by Fenton oxidation process. The goals of this study were: (1) to determine the best molar ratio of H 2 O 2 /Fe 2+ through Fenton oxidation process of DB 2 at optimum pH according to the colour removal kinetics with constant Fe 2+ and variable H 2 O 2 ; (2) at the optimum conditions, estimation of the effects of either azo bond loading factor (L azobond ) or COD loading factor (L COD ) at different pH values in relation to the colour removal kinetic classify and COD removal of DB2 by Fenton oxidation process; (3) at optimum conditions on degradation of DB2, study the effect of the inorganic anions such as sulphate, carbonate chloride and bicarbonate

Experimental Procedure
The colour removal of the azo dye DB 2 solutions was followed quantitatively by measuring the decrease in absorbance at max = 570 nm using (UV/VIS, Model SP-3000 OPTIMA) spectrophotometer. The chemical oxygen demand (COD) was determined by the method described in EPA method 410.4 (Luo et al. 2016, Razzak & Hossain 2016. H 2 O 2 was quantified spectrophotometrically as described by Nogueira (Nogueira et al. 2005). The degradation of DB 2 was carried out by the Fenton process using a batch reactor (total volume of 1 L) under constant agitation with a magnetic stirrer and room temperature ranged from 35±2°C. The experiments were conducted as the following:     5. The COD removal of DB 2, was studied at different L COD (1.0, 0.75, 0.5, and 0.25) at H 2 O 2 /Fe 2+ molar ratio equal to 11. Different H 2 O 2 concentrations: 1.4 × 10 -2 , 1.9 × 10 -2 , 2.8 × 10 -2 and 5.6 × 10 -2 M equivalent to L COD (1.0, 0.75, 0.5, and 0.25) were used for the COD removal, for the reason that the empirical COD concentration gained at 1.0 × 10 -4 M DB 2 solution was 224 mg O 2 L -1 (COD = 7.0 × 10 -3 M).
6. The effect of 1.0 % of inorganic salts on decolourization of DB 2 at (1.0 × 10 -4 M) was investigated. 10 g of inorganic salt was added to 1 L batch reactor for each experiment.
7. To ensure the removal of residual hydrogen peroxide H 2 O 2 , 100 μL of 1.0 M sodium sulphate Na 2 SO 3 solution, was added to all samples before the analysis by UV-Vis. Thus, the residual of H 2 O 2 was destroyed and Fenton reactions were stopped (Holkar et al. 2016). While, to measure the COD concentration in the treated water, the interference from residual H 2 O 2 was removed by addition of Na 2 CO 3 (20 g/L) and placed in a water bath at 90°C for 60 min (Wu andEnglehardt 2012, Nidheesh et al. 2013).

RESULTS AND DISCUSSION
Results presented here are based on the batch system of degradation of DB 2 by Fenton oxidation. The parameters for colour removal (decolourization) efficiencies such as loading azo bond factor (L azo bond ) or COD loading factor (L COD ) were studied; which are defined by Eqs. 4 and 5, respectively (Trovó et al. 2016, Sharma et al. 2018. In oxidation processes using the Fenton's reagent, the amount of oxygen O 2 available in H 2 O 2 must be measured to produce free hydroxyl radicals HO • responsible for the breakdown of the azo bond and the intermediate organic compounds (Sharma et al. 2018). Therefore, the dosage of H 2 O 2 required should be based on the initial L azo bond , L COD of DB 2, and O 2 supplied by H 2 O 2 , respectively.
Where, DB2 initial and COD initial are the initial concentration and the chemical oxygen demand of DB 2 dye, respectively. Dye decolourization efficiency was calculated as follows: (%) Dye colour removal efficiency = (1-C t / C 0 ) × 100 … (6) Where, C t and C 0 (mol.L -1 ) are the concentrations of DB 2 dye at reaction time t and 0, respectively. The chemical oxygen demand removal percentage was calculated as follows: Where, COD t and COD 0 are the chemical oxygen demand of DB 2 dye at reaction time t and 0, respectively.

Effect of the H 2 O 2 Dose on the Removal of DB 2
In the Fenton process, hydrogen peroxide plays an essential role in contaminant removal efficiency. Therefore, it was necessary to find the optimum hydrogen peroxide concentrations. The decolourization of 1.0 × 10 -4 M DB 2 was evaluated in the range of (0.22 × 10 -4 and 4.5 × 10 -3 M) H 2 O 2 and constant amount of ferrous iron (1 × 10 -4 M or 5.6 mg/L) to find the optimal oxidant dosage. The effect of hydrogen peroxide Where, COD t and COD0 are the chemical oxygen demand of DB 2 dye at reaction time t and 0, respectively.

Effect of the H2O2 dose on the removal of DB 2
In the Fenton process, hydrogen peroxide plays an essential role in contaminant removal efficiency. Therefore, it was necessary to find the optimum hydrogen peroxide concentrations.    Fig. 2. At the concentration of 0.22 × 10 -4 M, the colour removal of DB 2 was 24.0 % after 10 min of the reaction. However, the increment of peroxide dosage till 1.1 × 10 -3 M, a colour removal was reached at a higher level of 74.0%. The concentration of 1.1 × 10 -3 M H 2 O 2 was selected as the best concentration and used in all experiments to estimate the effects of Fe 2+ concentration on DB 2. At H 2 O 2 concentration of higher than 1.1 × 10 -3 M, the decolourization efficiency of dye solution showed little considerable efficiency, which may be due to the reaction of hydroxyl radicals with H 2 O 2, and scavenging of HO • radicals takes place as shown in Eq. 8 (Liu et al. 2017).

The kinetics of influence of H 2 O 2 concentration on DB 2 decolourization
Because of different side reactions occurring at the same time, the kinetic study of Fenton oxidation is highly complicated. Two models of kinetic studies were experimented to achieve the kinetics parameters. The first and second-order reaction, have been tested to fit the experimental data obtained from the colour removal experiments. The correlation coefficient (R 2 ) was used in the comparison of the two models. The data of Table 2 illustrate that the first-order model was not useful enough for proper parameter selection may due to the low correlation parameters, while the second-order reaction was much better. The results illustrated that the colour removal kinetics of DB 2 followed the second-order model very well. Fig. 3 shows that the DB 2 colour removal kinetic rates at constant iron ions of 1.0 × 10 -4 M increase with the H 2 O 2 concentration in two steps. The first step was at the small amount of H 2 O 2 ranged 0.22 × 10 -4 to 1.1 × 10 -3 M, the DB 2 colour removal kinetics increased slowly with a rate constant, k = 2 × 10 6 [H 2 O 2 ] + 416.97 and with higher correlation coefficient value of R 2 = 0.9715. The second step was at the concentration of H 2 O 2 was increased from 1.1 × 10 -3 to 4.5 × 10 -3 M, DB 2 colour removal kinetics was increased, and a rate constant K = 372437 [H 2 O 2 ] + 2014.7 and correlation coefficient R 2 was decreased to a value of 0.8561 (Fig. 3). The positive effect of DB 2 colour removal kinetics values was observed with a high concentration of H 2 O 2 may be due to the high production of hydroxyl free radical. When H 2 O 2 concentration was larger than 1.1 × 10 -3 M (Fig. 3), the DB 2 colour removal kinetics was linearly increased, the

Effect of Fe 2+ Doses on the Removal of DB 2
The influence of Fe 2+ stimulation on the removal of DB 2 was examined using the different Fe 2+ concentrations. The [Fe 2+ ] were in range of 1.0 × 10 -5 to 2.5 × 10 -4 M with constant H 2 O 2 concentration of 1.1 × 10 -3 M. Increase of the concentration of [Fe 2+ ] from 1.0 × 10 -5 to 1.0 × 10 -4 M had the positive effect on the removal rate for DB 2 (Fig. 4). The colour removal was increased from 22.0 % to74.0 % at 10 min of the reaction, that effect may be due to the activity of Fe 2+ in initiating the degradation of H 2 O 2 to generate hydroxyl free radicals as a part of Fenton process. These radicals can reacted with DB 2 instantly, that lead to DB 2 degradation (Lucas & Peres 2006). When the concentration of Fe 2+ was increased to higher than 1.0×10 -4 M, a slight increase in the decomposition rate may have occurred and that improved the function of Fe 2+ as a scavenger of HO * (Eq. 9). Hence, the optimum Fe 2+ concentration of the removal of DB 2 was selected as 1.0 × 10 -4 M.

The Kinetics of Influence of Fe 2+ Concentration on DB 2 Colour Removal
Two kinetic models were studied to estimate the effect of Fe 2+ concentrations on the decomposition kinetics of DB 2. Table  3 shows the kinetic parameters of the study. The regression coefficient (R 2 ) values of the second-order reaction were higher than the first-order and we concluded that the colour removal kinetics of DB 2 obeys to the second-order kinetics model. The correlation between the second-order kinetics of the DB 2 colour removal and different Fe 2+ concentrations (from 1.5 × 10 -5 to 2.5 × 10 -4 M) are presented in Fig. 5. Fig.  5 elucidates that the DB 2 colour removal kinetic average increase with the increase of Fe 2+ in two varied steps as well: (1) At low concentration of Fe 2+ ranged from 1.5 × 10 -5 to 1.0 × 10 -4 M, the DB 2 colour removal kinetics was increased clearly with a slope of 3.0 × 10 7 .
(2) At high concentration of Fe 2+ (from 1.5 × 10 -4 to 2.5 × 10 -4 M), DB 2 colour removal kinetics increased also but the slope (9.0 × 10 6 ) was decreased. At low colour removal kinetics in the second step in comparison with the first step, the proposition may lead to that Fe 2+ was higher than the need for consumption amount of HO • (Eq. 10). Thus, the amount of hydroxyl free radicals available to oxidize DB 2 dye became very low. The positive effect of Fe 2+ on the DB 2 colour removal kinetics assured that Fe 2+ stimulate by fast dissociation of H 2 O 2 into HO • radicals;   Dewil et al. 2017). These variations likely attributed to different proposed oxidation mechanisms during the Fenton oxidation process of different azo dyes. Table 2 and Table 3 show the results obtained in this study, the optimum concentrations of H 2 O 2 and Fe 2+ were 1.1×10 -3 M and 1.0×10 -4 M, respectively. Consequently, the experimental optimum H 2 O 2 /Fe 2+ molar ratio of 11 was selected for the next experiments.

Effect of pH and L azo bond on DB 2 Color Removal Kinetics
The pH effect on the Decolourization of DB 2 was achieved by a series of experiments conducted at three initial pH values 2.5, 3.5 and 5 (Fig. 6)   100% of Decolourization efficiency was achieved (Fig. 6).
The main reason is that more Fe(OH) + is formed, which has much higher activity compared to Fe 2+ in the Fenton process (Lopez-Alvarez et al. 2012, Trovó et al. 2016. Besides, at higher pH (pH 5), the precipitation of ferric hydroxide happen, causing the reduction in the dissolving Fe 3+ ions. Aside from, in such circumstances, H 2 O 2 is less stable, resulting in less HO • radicals formed, decreasing the removal efficiency of Fenton oxidation (Mousset et al. 2014. Therefore, the pH 3.5 was chosen the optimum pH of Fenton oxidation process. At molar ratio of H 2 O 2 /Fe 2+ equal to 11, the effect of four L azo bond operator values (0.25, 0.5, 0.75, and 1.0) was evaluated. Table 4 shows four levels of L azo bond and symbolized well by the second-order kinetic model. The results elucidate the significant difference in DB 2 colour removal rates at every pH and L azo bond . The results in Table 4 approved the selection of pH 3 in Decolourization rates of DB 2 dye. The data were in harmony with previous literatures for assessment of the colour removal of Amido black 10B and Terasil Red R (Dehghani et al. 2016). The best DB 2 colour removal rates were found at the smallest L azo bond factor of 0.25 and higher of H 2 O 2 with Fe 2+ Eq. (1), This may be attributed to HO • scavenging by H + ions; which e of the colour removal efficiency at pH 2.5 (Esteves et al. 2016. On the o the Decolourization efficiency of DB 2 rapidly increased with the increase in pH, at pH Decolourization efficiency was achieved (Fig. 6). The main reason is that more Fe(OH) has much higher activity compared to Fe 2+ in the Fenton process (Lopez-Alvarez et a 2016). Besides, at higher pH (pH 5), the precipitation of ferric hydroxide happen, causing dissolving Fe 3+ ions. Aside from, in such circumstances, H 2 O 2 is less stable, resulting formed, decreasing the removal efficiency of Fenton oxidation (Mousset et al. 2014 Table 4 shows four levels of L azo bond and symbolized well by the second-order kinetic elucidate the significant difference in DB 2 colour removal rates at every pH and L azo bond . 4 approved the selection of pH 3 in Decolourization rates of DB 2 dye. The data were in ha literatures for assessment of the colour removal of Amido black 10B and Terasil Red R (D The best DB 2 colour removal rates were found at the smallest L azo bond factor of 0.25 and to the smaller L azo bond factor represented a greater amount of H 2 O 2 concentration per coincides with the larger O2 concentration able to be used for oxidizing the DB 2 dye. Fig. 7 presents a linear relation between the second-order kinetic ln k2 value and the Lazo bond at each pH value. k 2 values, due to the smaller L azo bond factor represented a greater amount of H 2 O 2 concentration per mole of DB 2 and coincides with the larger O 2 concentration able to be used for oxidizing the DB 2 dye. Fig. 7 presents a linear relation between the second-order kinetic ln k 2 value and the L azo bond at each pH value.

COD Removal of RR120 at Different L COD Factors
Although homogeneous catalytic process decolorized the DB 2, the DB 2 azo dye was not completely mineralized. For this reason it is necessary to consider complementary information about the degradation of the organic pollutants (O'Dell 1939, Holkar et al. 2016. Chemical oxygen demand (COD) gives an average measure of the oxidation state of the organic by-products generated during the degradation of DB 2 (Orhon & Çokgör 1997) . Under the optimal conditions of pH 3.5 and H 2 O 2 /Fe 2+ ratio of 11, experiments using four varied L COD values (0.25, 0.5, 0.75, 1.0) were performed to examine the efficiency of Fenton reagent on COD removal kinetics of DB 2 (Fig. 8). The COD removal increases with decreasing L COD because the hypothetical amount of the concentration of H 2 O 2 and Fe 2+ was increased. At the

COD Removal of RR120 at Different L COD Factors
Although homogeneous catalytic process decolorized the DB 2, the DB 2 azo dye mineralized. For this reason it is necessary to consider complementary information abou the organic pollutants (O'Dell 1939, Holkar et al. 2016. Chemical oxygen demand (CO measure of the oxidation state of the organic by-products generated during the degradatio Çokgör 1997) . Under the optimal conditions of pH 3.5 and H 2 O 2 /Fe 2+ ratio of 11, exp varied L COD values (0.25, 0.5, 0.75, 1.0) were performed to examine the efficiency of Fen removal kinetics of DB 2 (Fig. 8). The COD removal increases with decreasing L COD bec amount of the concentration of H 2 O 2 and Fe 2+ was increased. At the maximum (L COD = 1. removed at 15 min while at the minimum (L COD = 0.25) 65% of COD was removed at t results approved that there was a residual amount of H 2 O 2 in solution after 15 min o reaction (Fig. 9). Although, all the Fe 2+ was transformed to Fe 3+ , and that may decrease maximum (L COD = 1.0) 41% of COD was removed at 15 min while at the minimum (L COD = 0.25) 65% of COD was removed at the same time. These results approved that there was a residual amount of H 2 O 2 in solution after 15 min of Fenton's oxidation reaction (Fig. 9). Although, all the Fe 2+ was transformed to Fe 3+ , and that may decrease the reaction rate of Fe 2+ with H 2 O 2 , low amount of H 2 O 2 was consumed (Fig. 9). The results of consuming the H 2 O 2 showed that the 82 % H 2 O 2 was consumed after 15 min at L COD = 1.0 while only about 43% H 2 O 2 was consumed at L COD = 0.25.

Effect of DB 2 Concentration on Colour Removal Kinetics
The colour removal efficiency at different concentrations of DB 2 was studied. The result was observed that the decolourization of dye increases with the decrease of primary DB 2 concentration (Fig. 10). As the concentration of dye decreased from 1.5×10 -4 M to 5.0×10 -5 M, the decolourization efficiency of dye increased from 43% to 93 % within the first 10 min of reaction. A decrease in the concentration of DB 2 dye reveals that lesser dye molecules will be available to scavenging by HO • radicals which lead to an increase in the colour removal efficiency of DB 2 (Javaid & Qazi 2019) . Table 5 represents the second-order kinetic rates of colour removal of DB 2 at various DB 2 concentrations. Also, Table   5 shows the effect of different [H 2 O 2 ]/[DB 2] ratios with an increase of colour removal kinetics combined with fact that the colour removal kinetics inversely proportional with DB 2 concentration in two varied steps.
A tenuous increase in colour removal kinetic rate (from 407 to 2604 M -1 min -1 ) occurred when the ratio H 2 O 2 /dye increases from 7.3 to 11. However, increasing the H 2 O 2 /dye ratio from 11 to 22, there was an acute increase in colour removal kinetic rate (from 2604 to 23415 M -1 min -1 ). Furthermore, the efficiency of DB2 colour removal increases with decreasing H 2 O 2 /DB 2 molar ratio, which points out that a higher concentration of DB 2 was removed by using a smaller dose of H 2 O 2 (Fig. 10).

The Influence of Inorganic Ions on DB 2 Colour Removal by Fenton Oxidation
The influence of some inorganic anions on the colour removal of DB 2 was tested at the optimum conditions. The experiments were designed to decompose (1.0 × 10 -4 M) DB 2 in the presence of 1.0 % of inorganic salt used in this study.
Due to the expectation of the existence of a large number of anions in industrial textile wastewater, consequently, we intend to assess the decolourization of DB 2 in high doses of selected inorganic salts. The existence of inorganic anions A tenuous increase in colour removal kinetic rate (from 407 to 2604 M -1 min -1 ) occurred when the ratio H2O2/dye increases from 7.3 to 11. However, increasing the H2O2/dye ratio from 11 to 22, there was an acute increase in colour removal kinetic rate (from 2604 to 23415 M -1 min -1 ).
Furthermore, the efficiency of DB2 colour removal increases with decreasing H2O2/DB 2 molar ratio, which points out that a higher concentration of DB 2 was removed by using a smaller dose of H2O2 (Fig. 10).    (Oliveira et al. 2015). In this research, the influence of carbonate, bicarbonate, sulphate and chloride on the Fenton process was evaluated. Fig. 11 shows the effect of the studied anions on the DB 2 degradation by Fenton oxidation process. Anions inhibit the degradation of DB 2 in the following order: HCO 3 -> CO 3 2-> Cl -> SO 4

2-
The addition of inorganic salts displayed various suppressed behaviours in Fenton process treatment. The influence of the addition of HCO 3 and CO 3 2may be attributed to a decrease in the average of production of HO • because of the formation of CO 3 •as shown in Eqs. (10) and (11). The radical CO 3 •is less reactive than HO • radicals. In the case of Cl -, it also has a great effect on the decomposition of DB 2 because it reacts with Fe 2+ forming complex and free radical less effective than the radical of hydroxyl as shown in Eqs. (12) and (13).

CONCLUSIONS
The parameters for loading azo bond factor (L azo bond ) or COD loading factor (L COD ) were highly effective parameters of colour removal (decolourization) efficiencies using the batch system by Fenton oxidation. The results obeyed that the overdosed H 2 O 2 was scavenging hydroxyl free radicals. The influence of the addition of HCO 3 and CO 3 2may be attributed to a decrease in the average of production of HO • because of the formation of CO 3 •-. The radical CO 3 •is less reactive than HO • radicals and the salts of the sulphate ion appear to have less effect on the Fenton process.

CONCLUSIONS
The parameters for loading azo bond factor (L azo bond ) or COD loading factor (L COD ) were highly effective parameters of colour removal (decolourization) efficiencies using the batch system by Fenton oxidation. The results obeyed that the overdosed H 2 O 2 was scavenging hydroxyl free radicals. The influence of the addition of HCO 3 and CO 3 2may be attributed to a decrease in the average of production of HO • because of the formation of CO 3 •-. The radical CO 3 •is less reactive than HO • radicals and the salts of the sulphate ion appear to have less effect on the Fenton process.