Fenton Oxidation of Methyl Violet in Aqueous Solution

In this study, oxidative discoloration of methyl violet (MV) dye in aqueous solution has been studied using Fenton (Fe/H2O2) process.eparameters such as concentration of Fe, H2O2,MV, temperature, andCl − and SO4 2− ions that affected of discoloration in Fenton process were investigated.e rate of degradation is dependent on initial concentration of Fe ion, initial concentration of H2O2, and pH of media. Discoloration of MV was increased by increasing the temperature of reaction. Optimized condition was determined and it was found that the obtained efficiency was about 95.5% aer 15 minutes of reaction at pH 3. TOC of dye sample, before and aer the oxidation process, was determined. TOC removal indicates partial and signi�cant mineralization of MV dye. e results of experiments showed that degradation of MV dye in Fenton oxidation can be described with a pseudo-irstorder kinetic model. e thermodynamic constants of the Fenton oxidation process were evaluated. e results implied that the oxidation process was feasible, spontaneous, and endothermic.


Introduction
Wastewaters from textile and dye industries are highly colored.ese wastewaters are a large problem for conventional treatment plants in the entire world.Direct discharge of textile industry wastewater into the receiving media causes serious environmental pollution by imparting intensive color and toxicity to the aquatic environment [1].Methyl violet is a triarylmethane dye, a mutagen and mitotic poison; therefore, concerns exist regarding the ecological impact of the release of methyl violet into the environment.Methyl violet has been used in vast quantities for textile and paper dyeing, and 15% of such dyes produced worldwide are released to the environment in wastewater.Numerous methods have been developed to treat methyl violet pollution.e traditional treatment techniques applied in textile wastewaters, include coagulation/�occulation [2], electrocoagulation [3], ozonation [4], oxidation [5], and adsorption [6][7][8][9][10].Adsorption only does a phase transfer of the pollutant.e biological treatment is not a complete solution to the problem due to the biological resistance of some dyes [11,12]; therefore, the resource to advanced oxidation processes (AOPs), like Fenton and photo-Fenton processes, could be a good option to treat and eliminate textile dyes.Homogenous Fenton reaction (Fe 2+ /H 2 O 2 ) is one of the most important processes to generate hydroxyl radicals OH • [13][14][15][16][17][18][19][20].In classic Fenton chemistry, the reaction between hydrogen peroxide and Fe 2+ in an acidic aqueous solution is generally recognized to produce hydroxyl radicals.e generally accepted free radical chain mechanism for the Fenton reaction is shown as below [21,22]: e main objective of this study is to analyze the feasibility of decolorization and mineralization of methyl violet dye by Fenton processes.e in�uences of different operational parameters (H 2 O 2 concentration, Fe 2+ , MV concentration, and temperature) which affect the efficiency of Fenton reaction have been investigated.e kinetics and thermodynamic parameters of the process also were determined.

Experimental
2.1.Material and Methods.All the reagents used in the experiments were in analytical grade (Merck) and were used without further puri�cation.All the experiments were conducted at room temperature.Methyl violet dye (Tris (4-(dimethylamino)phenyl) methylium chloride, MV, C.I. 42535, MW 393.95) was used as the contaminant.Figure 1 displays the molecular structure and UV-visible spectra of MV dye.3.0 × 10 −5 stock solution of MV was prepared, and working solutions were prepared by the dilution.e dye oxidation was achieved by Fenton's reagent which was composed of a mixture of FeSO 4 ⋅7H 2 O and H 2 O 2 30%.e necessary quantities of Fe 2+ and H 2 O 2 were added simultaneously in the dye solution.All experiments were conducted in a 500 mL thermostated batch glass reactor equipped with the magnetic stirrer.e kinetics of oxidation was followed by taking samples at regular time intervals.e residual concentration of the MV in the solution at different times of sampling was determined.e residual concentration of the dye was deducted from the calibration curves which were produced at wavelength corresponding to the maximum of absorbance (585 nm) on a UV-visible spectrophotometer apparatus (Shimadzu 160 A). e cells used were in quartz 1 cm thick.e discoloration efficiency of the dye (X) with respect to its initial concentration is calculated as where [MV] 0 and [MV] are the initial and appropriate concentration of dye at any reactions time t, respectively.

Results and Discussion
present study, the in�uence of different iron concentrations  [16].However, at high H 2 O 2 concentration, efficiency of dye removal showed no signi�cant efficiency, which is due to the recombination of hydroxyl radicals, and scavenging of OH radicals will occur, which can be expressed by ( 2) and (6).In Fenton process of MV, the decolorization efficiency is not signi�cantly different at the end.More than 70% of the oxidation of MV by Fenton reaction falls down at 15 min of reaction.

Effect of Ratio of [H 2 O 2 ]/[Fe 2+
].To observe the high uptake of dye by Fenton oxidation process, optimal initial [H 2 O 2 ]/[Fe 2+ ] ratio on the degradation of MV dye was investigated.e different ratios (1-400) of [H 2 O 2 ]/[Fe 2+ ] for optimum oxidation of dyes were reported from the literature [23].e results indicate that the amount of degradation of MV increases when the ratio of [H 2 O 2 ]/[Fe 2+ ] was 5.For bigger value of [H 2 O 2 ]/[Fe 2+ ], it is visible, that the MV degradation decreases.is is due to the fact that at higher H 2 O 2 concentration, scavenging of OH radicals will occur, decreasing the MV decolorization.

Effect of Dye Concentration
. e effect of initial concentration of MV dye was investigated, since pollutant concentration is an important parameter in wastewater treatment.e in�uence of dye concentration is shown in Figure 4.
From the �gure, it can be noted that when the initial dye concentration increases, the yield of decolorization decreases [24,25].is phenomenon can be explained by the fact that an increase in the initial concentration leads to increasing the number of dye molecules.e number of hydroxyl radicals remains the same.Concentrations of H 2 O 2 and Fe 2+ do not change, which causes a decrease in efficiency of discoloration.When the dye concentration is low, the concentration of H 2 O 2 is in excess compared to the latter and traps the OH • radicals.On the other hand, intermediate products increase.Figure 5 shows that below 15 min, performance is affected by relatively low temperature.Aer 15 min of reaction, the yield of discoloration is not greatly affected by the temperature in the interval studies.For real wastewater treatment, 35 ∘ C to 45 ∘ C can be considered as a good range of temperature giving an acceptable performance superior to 80%.Beyond this temperature, there is a slight reduction in yield.e phenomenon may be due to the decomposition of H 2 O 2 at relatively high temperatures (8).is is consistent with the results found in the literature [26,27] 3.6.Effect of pH on Decolorization.e aqueous pH has a major effect on the efficiency of Fenton's treatment.When dye is treated with Fenton's reagent, it may be that the reactant H 2 O 2 added might not be sufficiently utilized.is would lead to the residual of H 2 O 2 in treated dye waste.Hydrogen peroxide, being a mild oxidant, might affect the subsequent biological process.ereby residual H 2 O 2 was measured.e reaction was done for 60 min under controlled pH condition with constant dose of Fe 2+ (1.0 × 10 −4 M) and H 2 O 2 (5.0 × 10 −2 M).It is apparent that the extent of decolorization decreases with the increase in pH, and at pH 3.0 almost >95% color removal was observed (Figure 6).e main reason is that at a low pH more Fe(OH) + is formed, which has much  higher activity compared to Fe 2+ in Fenton's oxidation.Also, the generated OH • radicals may be scavenged by the excess H + ions [19].Also at the higher pH, H 2 O 2 loses its oxidizing potential.e formation of ferrous and ferric oxyhydroxides under pH values of more than 4.0 inhibits the reaction between Fe 2+ and H 2 O 2 .erefore, the low amount of OH • radical generation can be the reason.erefore, the pH 3.0 is the optimum pH for Fenton oxidation process [28][29][30][31].

Mineralization Study.
It is known that reaction intermediates can form during the oxidation of dyes and some of them could be long-lived and even more toxic than their parent compounds.erefore, it is necessary to understand the mineralization degree of the dye to evaluate the degradation level applied by Fenton process.Extent of mineralization of the dye by Fenton's process can be evaluated by measuring total organic carbon (TOC).To determine the change in the TOC of reaction medium, initial TOC (pure dye solution) and the TOC of a sample at different intervals during the reaction were measured.TOC reduction was determined as follows: where TOC  and TOC 0 (mg L −1 ) are values at time (t) and at time (0), respectively.58.5% TOC reduction is achieved for MV dye in 1 h (MV = 3.0 × 10 −5 , Fe 2+ = 1.0 × 10 −4 , H 2 O 2 = 5.0 × 10 −2 M, and pH = 3), which indicates the partial mineralization of dyes [32][33][34].Figure 7 shows the TOC removal of MV dye by Fenton oxidation process.e results of TOC removal clearly indicate that the reaction does not go to completion.In fact, aer 60 min of reaction, about 58.5% of the initial organic carbon had been transformed into CO 2 , which implied the existence of impurity and other organic compounds in the solution.is suggests the presence of residual organic products even aer 60 min of reaction, con�rming the noticeable degradation of the examined dye.

Conclusion
From the results of Fenton oxidation studies of MV, a model compound of textile wastewaters, the following conclusions can be drawn.
(1) e optimal parameters for Fenton process are (2) Fenton process only complete decolorize MV dye, but also partially mineralize the MV dye.
(3) e rate of Fenton oxidation of MV is �rst fast (15 min) and then is very slow.

3. 1 . 6 F 2 :
Effect of Ferrous Dosage.e concentration of Fe 2+ is one of the critical parameters in Fenton processes.In the Effect of concentration of Fe 2+ on the decolorization of MV by Fenton process ([MV]

3. 5 .
Effect of Temperature.Temperature affects the reaction between H 2 O 2 and Fe 2+ , and therefore, it should in�uence the dye degradation.Experiments were performed by varying the temperature from 20 ∘ C to 70 ∘ C. Figure5illustrates the effect of temperature on the reaction of MV discoloration according to time.It may be noted that the temperature has a great effect on the initial rate of MV discoloration.

9 F 6 :
Effect of pH on the decolorization of MV by Fenton process.

3. 8 .
Effect of Cl − and  4 2− on Fenton Effectiveness.Cl − and SO 4 2− are common coexisting anions with dyes in wastewater; therefore, the effect of Cl − and SO 4 2− ions on MV removal by Fenton process was investigated.It was found that the presence of Cl − at the concentration range of 0-0.02 mol L −1 did not have a signi�cant effect on removing MV. e effect of SO 4 2− on the removal of MV was signi�cant at the concentration range of 0-0.001 mol L −1 .e removal of MV decreased at a concentration of 0.01 mol L −1 of SO 4 2− .e removal of MV decreased to 52% for concentration of 0.01 mol L −1 of SO 4 2− .
Concentration. e initial concentration of H 2 O 2 plays an important role in the Fenton process.Oxidation of dyes by Fenton process is carried out by OH • radicals that are directly produced from the reaction between H 2 O 2 and Fe 2+ .To determine the concentration of H 2 O 2 giving the maximum MV discoloration efficiency, experiments were conducted, and results obtained are represented in Figure3.e discoloration efficiency according to the time for different concentrations of H 2 O 2 shows that the dye degradation yield increases with increasing concentration of H 2 O 2 .For the Fenton process, the addition of H 2 O 2 from 5.0 × 10 −3 -1.0 × 10 −1 M increases the decolorization from 75% to 90% at 15 min of contact time.e increase in the decolorization is due to the increase in hydroxyl radical concentration by the addition of H 2 O 2 [15]oncentration of hydrogen peroxide is �xed a 0.05 M, and dye concentration is 3.0 × 10 −5 M. It can be seen from results, MV degradation increased with increasing Fe 2+ concentrations.is is due to the fact that Fe 2+ plays a very important role in initiating the decompositions of H 2 O 2 to generate the OH • in the Fenton process.When the concentrations of Fe 2+ and OH • are high, Fe 2+ can react with the OH • according to(5).elowerdegradationcapacity of Fe 2+ at small concentration is probably due to the lowest OH • radicals production of variable for oxidation[15].3.2.Effect of H 2 O 2