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Kinetic and thermodynamic studies of fenton oxidative decolorization of methylene blue

The need for light intensity has made dye degradation very costly for industry. In this work, Fenton reagent was used for the efficient degradation of an aqueous solution of dye without the need for a light source. The influences of the pH of the media, the initial concentrations of Fe2+, H2O2, and methylene blue (MB) dye; in addition to temperature on the oxidation of MB dye were studied. The optimum amounts of the Fenton reagent were 4mM of Fe2+ and 70mM of H2O2 at 20 mg/L of dye. The optimum ratio of 0.05 of Fe2+/H2O2 was found to give the best result for the decolorization of dye. The Fenton process was effective at pH 3 with a maximum dye decolorization efficiency of 98.8% within 30 min of reaction, corresponding to a COD removal of 85%. The decolorization process was thermodynamically feasible, spontaneous, and endothermic. The activation energy (Ea) was 33.6 kJ/mol suggesting that the degradation reaction proceeded with a low energy barrier.


Introduction
The use of dye in industry is increasing and thus the amount of industrial wastewater that is produced from different processes including the paper and printing industries (Pang and Abdullah, 2013). Wastewater stemming from processes such as dyeing has continued to have an adverse impact (Saha et al., 2012). Therefore, their toxicity and recalcitrance to degradation pose a huge challenge to removal technologies. To combat this menace of the pollution problem, it seems more desirable to reduce dyes to a harmless form before discharging into the mainstream.
Different actions are widely utilized to treat the dye-containing wastewater, including physical, such as membrane treatment (Pang and Abdullah, 2013), adsorption (Bello et al., 2008;Saleh, 2020;Saleh and Ali, 2018), chemicals such as coagulation (Dalvand et al., 2011), and chemical oxidation (Turhan et al., 2012) or those enhanced with light (Liu et al., 2013), electricity (Bensalah et al., 2013), ultrasonic (Ghauch et al., 2016), biological methods (Mezohegyi et al.,2008), and the combination of several methods (Prato-Garcia et al., 2013). However, there are some disadvantages, such as the cost and time required, recycling, etc (Arulkumar et al., 2011). On the other hand, some processes like chemical methods could end up with negative side products adding more pollution to the environment due to the use of reagents (Pang and Abdullah, 2013). Also, although biological degradation is cheap, it is ineffective owing to toxicity and the recalcitrant nature of dyes to biospecies (Ji et al., 2011).
The scientific and research community have shown great interest in the cleaning of industrial effluents using methods called "Advanced Oxidation Processes (AOPs)", not only as a result of environmental concern tackled by legislative characteristics but also due to the thriving of advanced oxidation schemes to overcome pollutant 'phase transfer' issues. AOPs offer an extremely reactive oxidant, specifically hydroxyl radicals (HO⋅), which are capable of destroying organic pollutants. Among various AOPs, the use of the Fenton process is an effective method for organic pollutant oxidation. Fenton's reagent is considered as homogeneous catalytic oxidation method using hydrogen peroxide, as well as iron (II) ions in acidic media, to cause a complex redox reaction for producing HO⋅ with an oxidation potential of about 2.8 V (Metcalf and Eddy, 2003). These are strong oxidants compared to ozone 2.08 V and hydrogen peroxide 1.78 V.
The overall reaction is: The HO⋅ is capable of quickly degrading organic pollutants (RH) and causing decomposition of the molecules by H-abstraction, as well as addition into C─C unsaturated bonds In this study, we aimed at investigating the efficiency of degrading aqueous solutions of MB dye by a new Fenton process with the optimization of several experimental conditions. The results obtained prove that this reported procedure is cost effective and promising for the efficient treatment of industrial effluents.

Fenton experiments
The decolorization of MB dye was carried out in a batch operation. The dye solution was prepared in deionized water. The batch tests of the Fenton degradation process were performed in a 500 mL glass beaker at different experimental conditions including a Fenton dosage of 10-80 mM H 2 O 2 , 2-5mM FeSO 4 , (pH ¼ 2-7), and temperature of 298-318 K. The temperature was controlled through a thermostat. For each experiment, the reactor was loaded with 250 mL of MB aqueous solution, to which 5 mL of the ferrous catalyst was added, the pH value was adjusted with 0.1 M H 2 SO 4 or 0.1 M NaOH under stirring. The reactions were initiated by adding 5 mL of H 2 O 2 and the time was recorded. The reaction was halted by using 0.5 mL of 10 M NaOH (Xiang-Rong et al., 2004). The absorbance of the dye solution was taken and recorded before and after each oxidative degradation experiment. All of the results were estimated by obtaining the numerical mean of the results of triplicate experiments.
The UV-Vis spectrum of each MB solution was determined by a Cecil 7200 model spectrophotometer with a quartz cuvette 1 cm in length. The range from 200 to 800 nm was scanned and a calibration curve was created using the maximum absorbance wavelength of the dye solution. On the other hand, the MB degradation was computed by Eq. (3) below.
where A 0 and A t are the absorbance of MB after treatment time t in minutes, at corresponding t ax respectively. The expanse of the mineralization of the MB by the Fenton process was determined by chemical oxygen demand (COD) measurements. COD was determined by the closed reflux method as outlined in the standard method (APHA, 1998). The mineralization of MB was assessed for COD reduction of the treated solutions.

pH effect on fenton oxidation
Fenton oxidation processes are dependent on pH which plays a key role in the HO⋅ generation mechanism in the Fenton reactions as indicated in Eq. (1). The HO⋅ are efficiently formed in acidic media (Neamtu et al., 2003). The influence of pH on the decolorization of MB dye by the Fenton procedure is depicted in Figure 1a. Clearly, pH has a considerable effect on the dye degradation. At pH above 5, very weak color degradation of less than 10% was produced. At high pH, ferrous ions are not stable and form ferric ions with a tendency to generate a colloidal ferric hydroxo complex [Fe(OH) 3 ] (Ensign et al., 2003). The production of HO⋅ gets slower due to the development of this species which impeded further reaction of Fe 2þ and H 2 O 2 . At a pH of 3-5, the degradation percentage obtained was greater than 80%. Under these conditions, sufficient HO⋅ is formed and Fe 2þ is more soluble in aqueous media at a pH from 2-5 (Arslan-Alaton et al., 2008). In this work, higher degradation was observed at pH 3, where degradation was 93.9% at 60 min. The optimal pH of 3 obtained in this work was in close agreement with previous studies (Saeedah, 2013;Jafari et al., 2014). However, when the pH dropped to 2, a substantial decline in degradation efficiency was noticed compared to that of pH 3. At pH 2, the decolorization percentage was 56.6%. This occurrence could be explained by the high excess of hydrogen ions, behaving as an HO⋅ radical scavenger according to the reaction scheme (4) below.

Influence of H 2 O 2 concentrations
H 2 O 2 plays an important role as an oxidizing reagent in the presence of iron. To make the Fenton process compete with other processes, it is necessary that the applications depict a cost-effective operation that involves a better control of H 2 O 2 dosages. The aim of the test was to optimize the Fenton degradation process to obtain the highest performance under an optimum H 2 O 2 dosage. The effect of H 2 O 2 concentrations on the process was studied in H 2 O 2 concentrations of 10 mM and 80 mM while keeping the ferrous ion dose, pH of solution and temperature at 4 mM, 3 and 298 K respectively. There is a correlation between the degradation efficiency of the dye and H 2 O 2 concentrations in the Fenton process as shown in Figure 1b below. The results designate that the degradation efficiency appreciated from 74.3 to 96.3% with an increase in H 2 O 2 concentration from 10 mM to 30 mM after 60 min of the reaction. The intensification in the color degradation efficiency of the Fenton process was due to an increase in HO⋅ concentration by the addition of H 2 O 2 . The further enhancement from 30 mM to 40 mM initiated no noteworthy change in color degradation, as it only improved by 0.1%. Doubling the concentration of H 2 O 2 to 80 mM only improved the efficiency by 1.2%. The highest degradation efficiency of 97.6% was achieved with a 70 mM H 2 O 2 dose which was 0.1% higher than that obtained at an H 2 O 2 dose of 80mM. This small difference could be explained by the fact that at high H 2 O 2 concentrations, scavenging of HO⋅ will take place and the result is the formation of perhydroxyl radicals, HO 2 ⋅, which are less reactive than HO⋅ and thus affect the degradation of the dye (Bergendahl and Thies, 2004). An optimal H 2 O 2 dose of 50 mM for the removal of methyl violet was reported with a degradation efficiency of 95.5% (Saeedah, 2013) while the best color degradation efficiency of 99.0% with 44 mM H 2 O 2 for the removal of Basic Blue 3 was reported (Elham and Mina, 2014).

Effect of Fe 2þ concentration
The concentrations of ferrous ions affect the Fenton process. Generally, advanced oxidation of organic pollutants is more efficient when ferrous ions are at 2-5 mM concentrations. This range is sufficient for HO⋅ to be produced where Fe 2þ ions are still highly soluble at a pH of 2-5 (Arslan-Alaton et al., 2008;Askarniya et al., 2020). Iron solubility is one of the obvious aspects of Fenton oxidation since the rate of HO⋅ formation is proportional to Fe 2þ concentration. The effect of the addition of Fe 2þ was studied. Different doses of ferrous catalyst from 2 to 5 mM were tested as shown in Figure 2. No remarkable difference was observed in degradation with increasing ferrous concentrations. The addition of Fe 2þ from around 2 mM-4 mM raised color degradation from 97.0% to 97.6 %. However, at 4 mM-5 mM, color degradation remained constant with a further increase of Fe 2þ concentrations. The lower removal efficiency of Fe 2þ at low concentrations was due to the lower HO⋅ radical formation for oxidation. On the other hand, the excess of Fe 2þ ions scavenged the formed HO⋅ which can decrease the color degradation.
The results obtained were in line with the literature report (Tarr, 2003), in which an increase in iron (II) salt concentration increases the removal rate of the organic pollutants but only to the point where the subsequent addition of Fe becomes insignificant. The concentration of Fe 2þ 4 mM or 5 mM can be used as an optimum dosage within this treatment. The results were in line with the findings of (Agustina and Ang, 2012) in which 4mM was stated as the optimal ferrous dose in the degradation of Reactive Blue 4.

Fenton ratio
Both the iron (II) ion (Fe 2þ ) and H 2 O 2 do not react solely to form HO⋅ but also are scavengers of HO⋅. ] on the removal of MB dye, the fixed concentration of 4 mM of ferrous ion and 20 ppm of the dye was studied at different hydrogen peroxide concentrations to give molar ratios 1:2.5-1:20. In this work, it was observed that the best molar ratio is at the value 1:17.5, Figure 3. The results were in close agreement with what the literature reported on Fenton treatment regarding the degradation of azo dyes which identified that the effective condition was established at an iron-to -hydrogen peroxide ratio of 1:20 (Neamtu et al., 2003;Matyszczak et al., 2020).

Influence of MB concentrations
Dye concentrations are a very crucial specification in wastewater management. The influence of initial MB dye concentrations on its decolonization was investigated with in a range of 10-50 ppm of MB dye. The impact of the dye concentration is illustrated in Figure 4. It is evident that the percentage of color degradation drops with a rise in the dye concentrations. Increasing the initial concentration of MB from 10 to 50 ppm decreases the color degradation from 95.6% to 84.2% after 60 min of reaction. The degradation efficiencies were 95.6% (10 ppm), 92.8% (20 ppm), 90.9% (30 ppm), 89.1 % (40 ppm) and 84.2 % (50 ppm) after 1h reaction time (Wang et al., 2019;Lucas and Peres, 2006).

Influence of temperature and time
Most commercial applications of Fenton Reagent occur at temperatures between 293 and 313 K. A temperature range of 298-318K was investigated to obtain the effect of temperature on the Fenton treatment. There was a significant temperature effect on the removal of MB efficiency as presented in Figure 5. The removal efficacy of the MB dye was 85-90% after 5 min of treatment for all temperatures. The degradation efficiency of MB was enhanced from around 85 to about 98.8% by increasing temperature from 298 to 318K after 30 min of the oxidation process. This is because the high temperature could increase the reaction rate between H 2 O 2 and iron (II) ion, thus increasing the rate of HO⋅ formation. The results implied that increasing the temperature from 298K to 318K had a positive effect on MB dye degradation. At 318K, percentage of color degradation was 98.8% after 30 min. A further increase in temperature of 10K caused a slight decrease in color degradation from 99.8% (318K) to 95.0% (318K). In this study, the optimal temperature of 45 C (318K) was obtained in the degradation of MB dye as opposed to some literature reports (Guedes et al., 2003;Ramirez et al., 2005) in which around 30 C was reported as optimal temperature for Fenton degradation. Also, 50 C was stated on dyes decolorization by Fenton-like reaction (Medien and Khalil, 2010). However, results in this study were in agreement with the report by Dutta et al. (2001) which stated no remarkable change in the expanse of color removal between 40 C and 50 C. There are conflicting reports in the literature, including conflicting reports as regards optimal temperatures, what is common in that degradation efficiency depreciated above the optimal value since H 2 O 2 decomposed to O 2 and H 2 O was noticeable at a temperature >40 C.

Decolorization kinetics
The oxidation of organic pollutants by the HO⋅ radicals can be represented by several reaction mechanisms. General rate law of the key organic molecule is: The term Ox i represents oxidants other than HO⋅ such as (hydroperoxyl radical HOO⋅). In order to simplify; the HO⋅ can be regarded as the only active oxidant. The concentration of HO⋅ cannot be measured directly. It is considered to be constant under certain reaction conditions, leading to the pseudo-first order reaction where HO⋅ concentration is a part of the apparent rate constant k app (Saeedah, 2013).
The absorbance of the detected compound is proportional to its concentration according to the Lambert-Beer equation and as such the absorbance can be used in the rate expression instead of concentration.
The degradation kinetics of MB by the Fenton process was studied at different contact times varying from 5 to 30 min at different temperatures 298, 308,318 & 318 K. Assuming pseudo-first-order reaction kinetics for MB dye decolorization in water, the decolorization rate constant,k d , was computed from the slope of the logarithmic plot of color (Absorbance A) vs. treatment time t as presented in Eq. (8). A pseudo first order kinetic plot (Figure 6a) was utilized to define the degradation of the MB dye in this study as reported in previous work (Arslan et al., 2001). The pseudo second order model was also tested as proposed by Ho and McKay (1998) using the following equation. The plot is as shown in Figure 6b.   A e is the equilibrium absorbance of the dye. The pseudo second order rate constant was obtained from the plot of t=A t versust.The data obtained were presented in Table 1. In order to fit the best experimental data, the correlation coefficients of both models were compared. The rate constants obtained from kinetic plots of the pseudo first order model were higher than that of the pseudo second order model. Temperature has a significant impact on the pseudo first order rate constants. Increasing the temperature from 298 to 318K increased k 1 from 0.0302 to 0.0674 min À1 (the rate constant increased by a factor of 2). A further increase in temperature by 10K decreased k 1 from 0.0674 to 0.0420 min À1 . The pseudo second order rate values did not follow a specific trend with a high temperature from about 298.15 to 318.15 K. The retention of negative value for k 2 indicated that the experimental data could not be best described by the pseudo second order kinetic model. Given that the preexponential factor of the Arrhenius equation is always positive, the rate constant is never negative. The coefficients of correlation of the kinetic study of the pseudo-first order are very close to unity and are more than the pseudo second order values. Thus, the results of the experiments indicated that degradation of MB dye by Fenton process could be best described by the pseudo-first-order kinetic model. This was in close agreement with the studies conducted by previous researchers (Arslan and Balcioglu, 2001;Abou-Gamra, 2014). The reported method is comparable to the literature, Table 2.   Turhan et al., (2012) (1)Information about thermodynamic studies was not supplied.

Oxidation of dye mainly by TiO 2 -UV irradiation
Effects of experimental conditions on the photo catalytic process &kinetics of the degradation process were determined. Marziyeh et al., (2012) (1)The thermodynamics of the photo catalytic process was not studied.

>90
(2)Formation of by-products Activated Carbon Dye removal by adsorption Using: (a) Java Plum leaves Experimental conditions, isothermal and thermodynamic studies were used to determine the effectiveness of the adsorption process 93.45 Gnana et al., (2014) (1)Degradation kinetics was not studied.
(2)Regeneration difficulties (b) periwinkle shell Experimental conditions, kinetic and isothermal studies were used to verify the effectiveness of the adsorption process. To study the effect of reaction temperature on the MB removal, kinetic data were collected at four different temperatures (298, 308, and 318 K). The temperature necessity of the kinetic values of the Fenton oxidation was found by using the Arrhenius Eq. (10) below: From the plot of the natural logarithm of k against 1/T, with the straight line was obtained. From the slope of the line E a was deduced. Rate constants may also be related to energy changes by the general form of Erying-Polanyi equation (from transition state theory) somewhat resembles the Arrhenius equation as shown in Eq. (11).
where ΔG z is the free energy activation and ΔG z ¼ ΔH z À TΔS z The linear Eyring-Polanyi Equation is given below: The plot of Inðk 1 =TÞversus 1=Tgives a straight line showing slope À ΔH z =R from which activation enthalpy is calculated. The intercept Inðk B =hÞ þ ΔS z =R can be used to find activation entropy. The activation energy (E a ) of the Fenton process was computed from the slope of the Arrhenius plot in Figure 7a to be 31.5 kJ/mol. This low value suggested that the oxidative reaction proceeded with a low energy barrier. The other thermodynamic parameters like activation enthalpy (ΔH z ) and activation entropy (ΔS z ) were found from Erying-Polanyi plot in Figure 7b respectively. The enthalpy of activation (ΔH z ) was found to be 29.0 kJ/mol and the positive value of the enthalpy showed that the oxidative reaction was endothermic. The entropy of activation was 196J/mol and this positive value implied that transition state was highly disordered associated with the ground state. Translational and rotational, as well as vibrational degrees of freedom, can be liberated as they go from ground states to transition states. Therefore, the oxidative degradation reaction preceded rapidly and the reaction was favorable. The values of the Gibb's free energy of activation (ΔG z ), which is wellthought-out as a driving force of the Fenton reaction, were calculated from the enthalpy and the entropy of activation at different varying temperatures (El Haddad et al., 2013). The values were found to be -29.4, -31.3 and -33.3 kJ/mol at the temperatures 298.15, 308.15 and 318.15 K. The retention of negative numbers of the free energy of activation implied that the oxidative reaction using the Fenton reagent for the degradation of MB was spontaneous and the extent of the spontaneity of the reaction increased by increasing the temperature. This result is in line with the proposal that the reaction mechanisms were energetically stable with a rate of Fenton reaction enhanced by the increase in the temperature until the optimum temperature was reached.

Conclusion
In this work, colour removal was investigated with the influence of process parameters including the pH and temperature of the media and the concentrations of FeSO 4, H 2 O 2 and the dye. The process efficacy was studied and established based on the UV-Vis and COD measurements. The optimal conditions included: 70mM of H 2 O 2 , 4mM of FeSO 4 and a 318 K temperature, in addition to a pH of 3.0 and an initial MB concentration of around 20 mg/L after 30 min. The results indicated that the Fenton process was operative as a color degradation efficiency of 98.8% was attained within 30 min. This corresponds to a COD removal of 85%. Under these optimal conditions, the pseudo-first order removal rate constants are found from batch tests results. The thermodynamic parameters of the Fenton oxidation process were examined and the results showed that the Fenton process is highly spontaneous and endothermic.

Author contribution statement
Abdur-Rahim A. Giwa: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data.
Isah A. Bello: Performed the experiments; Analyzed and interpreted the data.
Abdullahi B. Olabintan, Olugbenga S. Bello: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.
Tawfik A. Saleh: Contributed reagents, materials, analysis tools or data.

Funding statement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.