3.1 Effect of experimental conditions on Metolcarb degradation
3.1.1 Initial concentration of pesticide
The evolution in function of time of a chemical reaction can be influenced by kinetic factors such as the concentration of a chemical product. So a kinetic factor allows to reach during time a determined state of progress of reaction, such as the case of electrolysis of pesticide by an advanced oxidation process.
The treatment of Metolcarb solutions of two different concentrations 10− 4 M and 3 × 10− 4 M by electro Fenton process is studied in acid medium, in presence of sodium sulphate (Na2SO4) like a carrier electrolyte, at current intensity of 500 mA, at pH = 3, for 90 min and at room temperature.
Figure 1 shows result curves, [Mcb] = f (t) during electrochemical treatment of insecticide Metolcarb. It is noted that by increasing the initial concentration of the pollutant to be treated, the rate of its degradation decreases under the experimental conditions previously described. In consequence, the concentration as a kinetic factor is modified the duration of evolution of the electrolysis reaction of Metolcarb by the electro Fenton process.
3.1.2 Intensity of applied current
The intensity of electric current has an effect in general on the concentration of an aqueous electrolytic solution and therefore on its electrolysis in function of time. In fact, intensity of applied current influences on the dissociation of the organic matter during its electrochemical treatment.
The degradation of Metolcarb by electro Fenton process and in acidic solution was investigated by HPLC. By chromatography, Metolcarb appeared at a retention time of 4.4 min. To examine the effect of current intensity on the rate of electrolytic decomposition of this pesticide, we are monitored its concentrations at the following applied current intensities, 100 mA, 500 mA and 800 mA and for 90 min. The experimental results are given in Fig. 2.
Concentration - time curves of Metolcarb pesticide present the changes of its concentrations at the three current intensities with its different degradation rates (Fig. 3).
By exploiting the obtained curves, we note that the augmentation of applied current intensity from 100 mA to 800 mA leads to faster degradation kinetics. These results cause almost total degradation of Metolcarb after electrolysis duration of 60 min at 100 mA, 25 min at 500 mA and 15 min at 800 mA. Thus, it proves an improvement in the efficiency of the electrochemical treatment. Graphical representations of Fig. 3 showed that the degradation of Metolcarb by electro Fenton process followed pseudo first order reaction kinetics.
3.2 Absolute rate constant of Metolcarb degradation by hydroxyl radicals
The hydroxyl radical •OH is a very powerful oxidant, it is the species having the highest oxidizing power after the fluorine. This hydroxyl radical has the possibility to attack organic and inorganic substrates. Their reaction with Metolcarb pesticide as an organic matter is an important step in the oxidative degradation. The rate constant of this reaction can be determined by the competitive kinetics method by competing the constant substrate to be determined with a reference compound which kinetic constant is known [26–28]. The following equation shows the rate expression of Metolcarb with hydroxyl radicals (Eq. (1)).
d[Metolcarb]/dt = kapp × [Metolcarb] = kabs × [HO•] × [Metolcarb] (1)
In Eq. (1), kapp is the apparent rate constant of Metolcarb degradation by hydroxyl radicals and kabs is the absolute rate constant. The absolute kinetic constant of the electrolysis reaction of Metolcarb (Mcb) is determined by the competitive kinetics with benzoic acid (BA), which its absolute rate constant with hydroxyl radicals is known, kabs = 4.3 × 109 M− 1 s− 1 . Figure 4 presents the graphical representation of the straight line Ln ([Mcb]0 / [Mcb]t) in function of Ln ([BA]0 / [BA]t).
According to these results, it is found that the value of the absolute kinetic constant kabs of the reaction of hydroxyl radicals with Metolcarb is equal to 3.59 × 109 M− 1 s− 1. This value indicates that the magnitude of the kinetic constants generally obtained for the reactions between the aromatic compounds and the hydroxyl radicals belongs to a high range and that the interactions of this pesticide reaction are rapid.
3.3 Mineralization of Metolcarb
Organic matter can be distinguished from mineral matter, so carbon plays a central role. For that, the chemical molecule can pass through decomposition steps under defined experimental conditions.
To characterize and follow the evolution of the mineralization of insecticide Metolcarb by electro Fenton process in aqueous and acidic medium, we are studied the progression of the total organic carbon (TOC) during its electrolysis reaction. Indeed, at different current intensities, we are measured the TOC values of the pesticide solutions and for 90 min. Figure 5 presents the evolution of TOC values in function of time for the electrolyzed pollutant.
These results are highlighted the efficiency of this electrochemical process to reach an advanced stage of Metolcarb mineralization. The obtained curves show progressive diminution in values of TOC versus time from 100 mA to 800 mA and for 90 min. Figure 5 proves that in the first 30 min, TOC removal is faster at 800 mA than at 500 mA and at 100 mA. The obtained TOC values during 3 hours of Metolcarb electrolysis are given in Fig. 6.
It can be seen from this figure that after 3 hours the rate of the mineralization of Metolcarb has been improved. Thus, at 100 mA the percentage of the decomposition of Metolcarb by electro Fenton is changed gradually from 26.39–36.21% and at 800 mA from 53.78–58.84%. Organic matter such as the case of Metolcarb pesticide is consisting of different pure bodies. Then, its chemical composition provides the proportions of its decomposition by measuring parameters such as TOC and COD.
3.4 By-products of Metolcarb degradation
3.4.1 Identification of aromatic intermediates
It is notable, in advance, to highlight and detect organic compounds such as carbon molecules as intermediates of reaction media. In general, electrolytic reactions are important for the degradation of pesticides. In experimental conditions that are described above, LC–MS analysis were used to determine the aromatic by-products of Metolcarb during its electrolysis by electro Fenton process. Indeed, hydroxyl radicals are reacted with insecticide carbamate evidently, and then various by-products are formed in solution. Table 1presents the obtained compounds and its m/z reports.
Table 1 Molecular compounds of intermediates formed during electrolysis of Metolcarb by electro Fenton process.
The fragmentation of organic compounds is observed when these molecules are subjected to internal and external energies. Peaks of the fragmentation make it therefore possible to identify the different molecular structures of the matter. Under our experimental conditions, benzoquinone was determined by HPLC analysis. The other cyclic and aromatic compounds are formed during the electrochemical treatment of Metolcarb (tR = 4.14 min) by the electro Fenton process and are identified by LC-MS. Other by products and components of the reaction medium of pesticide electrolysis could be characterized by another chromatographic method.
3.4.2 Determination of carboxylic acids
In general, carboxylic acids are abundantly found in nature under several forms. Acids that derive from chemical reactions such as reduction and oxidation of pesticides can be detected and can also express the degradation of the insecticide. Indeed, several studies were concerned the detection of different acids during the mineralization of pesticides in aqueous solutions [30–32]. In this study, we have tried to investigate the formation of carboxylic acids throughout the electrolysis of Metolcarb by electro Fenton process (Fig. 7).
At the experimental and interactional level, the electrolysis medium of Metolcarb by electro Fenton process being oxidizing, the functional groups alcohols are oxidized to aldehydes, themselves converted to carboxylic acids. In the case of this carbamate and according to results presented in Table 1, hydroxymethyl acetate was identified by LC / MS chromatography. We think that this carbonyl compound is converted to a carboxylic acid. As it is seen in Fig. 7, acetic acid is detected by Ion Chromatography (IC) during 90 min of the electrochemical treatment of Metolcarb. This acid was started to form at the beginning of the reaction. This curve shows that its evolution was fairly rapid and it was reached its maximum at 30 min.
3.4.3 Detection of nitrate ions
Compounds that are given by the decomposition of hydrocarbons carrying functional aldehyde, alcohol or carbamate groups can be ions such as nitrate ions . NO3− ions are powerful eutrophicants and they can be resulted in situations of interactions in aqueous media. In this part, we are noted the formation of nitrate ions during the electrolysis of Metolcarb by electro Fenton. These anions were detected by Ion chromatography (IC). The curve of Fig. 8 is attributed to the evolution of nitrate ions in this reaction versus time.
The detection of these ions experimentally as it is observed in Fig. 8 indicated that the nitrogen atom of the structure of Metolcarb can be converted to nitrate ions during the oxidation of this carbamate by electro Fenton. Thus, at the first 30 min the quantity of the detected ions is superior than at 60 min and at 75 min. The organic products, the carboxylic acid and the nitrate ions which are detected by different chromatographic methods make it possible to present the mechanism of mineralization of Metolcarb.
3.5 Metolcarb degradation mechanism
In order to develop the monitoring of the degradation and mineralization of Metolcarb as an organic compound, we are analyzed the interaction between the generated hydroxyl radicals and this pesticide during its electrolysis by the advanced oxidation process electro Fenton. The diverse transformations and reaction steps are highlighted and allow to characterize the oxidation of Metolcarb in acidic aqueous medium. Thus, these observations were revealed breaking and changing of chemical bonds as well as addition and formation of functional groups under the influence of experimental effects. In Fig. 9, we are proposed the mechanism of electrochemical degradation of Metolcarb.
The pathways of this mechanism are based on the determination of the above described aliphatic and aromatic by-products of Metolcarb. In pathway 1, the decomposition of Metolcarb by electrolysis gives the formation of 5-methylphenyl methylene carbamate by its deprotonation. This insecticide can be oxidized by the hydroxyl radicals to an unstable radical as noted in pathway 2, which is converted after other reactions to an intermediate and there to 5-methylphenol. Indeed, the addition of the hydroxyl groups to the benzene ring is seen in various processing steps of this pesticide. The mechanism continues from 5-methylbenzene-1,2,3,4-tetraol to lead successively to the formation of hydroquinone and then benzoquinone. On the other hand, the generation of carboxylic acids is the result of the oxidative opening of the aromatic ring. The electrolysis of an organic acid under the effect of the electric current allows it to be transformed into a carboxylate ion being unstable and it provides the carbon dioxide by its decarboxylation. Therefore, we note that Metolcarb can decompose electrolytically to various elements, while its biodegradation will be investigated.
3.6 Biodegradation of electrolyzed Metolcarb solution
The degree of decomposition of an organic substance and the time required for its decomposition and its sensitivity to the reaction medium can express its biodegradability.
BOD5 of a pesticide is used to determine the ability of this chemical product to react with the active clay used as an environmental medium of significant ecological character. However, the measurement of oxygen consumption is a suitable measure to assess its biodegradation.
Generally we follow the biodegradation of Metolcarb by testing the evolution of the solution during this study. clay is a locally abundant material, very diverse and treated before its utilization in this research part. Thus the solution of Metolcarb electrolyzed by the electro Fenton process is mixed with the reagents described in 2.5. This experimental technique allows to the measurements of the Biological Oxygen Demand (BOD5) of Metolcarb which having certain physico chemical properties while having others. The biodegradation of Metolcarb is investigated after its electrochemical treatment at current intensities 100 mA and 800 mA and during 3 hours. The results of the measurements in this paragraph correspond to the determination of the ratios of the Chemical oxygen demand and the Biological Oxygen Demand. Then, the found COD / BOD5 ratios indicate the biodegradability of Metolcarb using the same experimental method.These ratios are 1.4 and 0.2 at current intensities 100 mA and 800 mA respectively. It is then noted that the two ratios are less than 5. We deduce that the Metolcarb is biodegradable after its two electrolyses by electro Fenton process. Indeed, by increasing the electric current, the COD / BOD5 ratio decreases and Metolcarb is more biodegradable.