Removal of dinitrotoluene from petrochemical wastewater by Fenton oxidation, kinetics and the optimum experiment conditions
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The performance of Fenton oxidation processes with simultaneous application of two oxidants including (H2O2) and sodium persulfate (Na2S2O8), in the presence of iron (Fe2+) as a catalyst to oxide dinitrotoluene (DNT) was investigated. For optimization of processes, the effect of operational parameters such as solution pH, oxidant dosages, reaction time and initial DNT concentrations on their performance was evaluated. All experiments were conducted at room temperature (25 ± 2 °C). For all studied processes, the efficiency of pollutant removal was enhanced by increasing the oxidant dosages, while it witnessed a reduction with increasing the initial DNT concentration. The results showed that Fe2+/H2O2/Na2S2O8 system was more efficient in alkaline conditions and the oxidizing of DNT followed pseudo-first-order kinetics. At optimum conditions, the mineralization degree was higher than 86%. Moreover, high potential of DNT removal in a petrochemical effluent sample was observed. Fenton process in the presence of simultaneous application of Na2S2O8 and H2O2 can be used as an effective and promising method for petrochemical wastewaters treatment, due to high efficiency in contaminants removal and also high potential of mineralization.
KeywordsDinitrotoluene Fenton oxidation Sodium persulfate H2O2 Petrochemical wastewater
Recently, by an enhancement in legislation roles against water resources contamination, lots of studies have performed on improvement in water remediation approaches and development of cost-effective techniques for removal of organic pollutants from wastewater. DNT is an explosive nitroaromatic that can exist in the environment as six isomers containing approximately 80% of 2,4-DNT and 20% of 2,6-DNT and the other four forms including 2,3-DNT, 2,5-DNT, 3,4-DNT, and 3,5-DNT that make up only 5 percent of the technical grade .
DNT is widely used in munitions, polyurethane foams and as an intermediate in dyes, plastics, herbicides, and airbag of automobiles [2, 3]. It is obtained from nitration of toluene (or nitrotoluenes) and commonly found in surrounded environmental of petrochemical plants . Bradley et al.  showed that the indigenous microorganisms collected from munitions-contaminated soil can transform 2,4- and 2,6-DNT to amino-nitrointermediates within 70 days. Due to the moderate solubility and lower volatility of DNT, it may uptake and accumulate by plants but may lead to transportation of DNT to the surface and ground waters until degradation by light, oxygen, or biota . DNT is considered as a toxic chemical agent for many organisms, and the chronic exposures may have damaging effects on human health including blood, nervous system, liver, and kidney  DNT was introduced as a priority pollutant by the EPA. It was listed on the EPA’s Drinking Water Contaminant Candidate List 2 (probable human carcinogen) for possible regulation under the Safe Drinking Water Act . Therefore, it is indicated that the DNT releases to the water are considered as the main source of human exposure and an important environmental issue that must be resolved.
Fenton oxidation, as an Advanced Oxidation Process (AOP), was used successfully in treatment of different artificial wastewaters with various contaminants, like phenols [8, 9, 10, 11, 12, 13, 14, 15], nitrophenols [16, 17], chlorophenols [18, 19, 20], nitrotoluene [21, 22], and cresols . Fenton’s oxidation with ferrous iron (Fe2+) can generate strong oxidant hydroxyl-1 that led to degradation of recalcitrant organics at acidic conditions . It can promote the wastewater treatment efficiency by reducing the discharge of pollutants into the natural waters, minimizing the effluent toxicity and consequently, increasing the capability of post-biological treatments .
The most principal treatment techniques to DNT removal are included adsorption, advanced oxidation processes (AOPs), chemical reduction, and bioremediation . The important benefits of AOPs are its non-selectivity property and quick kinetic reactions . Fenton’s reagents (H2O2, Fe2+) have been known as the most applicable type of AOPs. Mohanty and Wei reported  the high degradation of 2,4-DNT, using Fenton’s reagent, by an optimum molar ratio of H2O2:Fe2+:2,4-DNT; 520:2.5:1. Li et al. applied UV/Fenton’s reagent to remove nitroaromatic substances, which found a direct relationship between the reaction rate and the number and position of the nitrogroups on the aromatic ring. Ho  investigated the photo-oxidation of 2,4-DNT in the presence of H2O2 and presented the reaction pathway which consists of dinitrobenzaldehyde and dinitrobenzene, corresponding to that reported by Larson et al. . Chen et al.  proposed that oxidative degradation of DNT isomers leads to o-, m-, p-mononitrotoluene (MNT), and 1,3-dinitrobenzene, respectively.
In this study, Fe2+/H2O2/Na2S2O8 process was applied for the removal of DNT and chemical oxygen demand (COD) from synthetic solution and real petrochemical wastewater sample. Petrochemical industries have a significant amount COD. Therefore, the experiments were carried out to find the optimal input variables. In order to achieve this goal, specific objectives were defined. Test was performed in a batch system to investigate the influence of experimental factors including pH, oxidant dosages, reaction time, and initial concentration of DNT and to find the optimum condition in simultaneous presence of H2O2 and Na2S2O8 in the presence of Fe2+ on removal of DNT from synthetic solution and original petrochemical wastewater sample. Also, the kinetic model of the DNT oxidation rate was determined.
2 Materials and methods
DNT (97%, Merck) was used as a pollutant. H2O2 (50%, Merck) and Na2S2O8 (99%, Merck) were applied as oxidants. FeSO4.7H2O (98%, Merck) was used as a catalyst. NaOH (98%, Merck) and HCl (98%, Merck) were applied to adjust solution pH. K2Cr2O7 (99%, Merck), (NH4)2 Fe (SO4)2·6H2O known as Mohr’s salt (99%, Merck) and C12H8N2.H2O (99%, Merck) were used for measuring COD.
Research was carried out on a different conditions such as varied DNT concentrations (40, 50, 100, 120 and 160 mg/l), solution pH (2, 3, 6, 7, 9 and 11), reaction times (20, 25, 30 and 60 min), oxidant dosages (0.1, 0.2, 0.3, 0.5 and 0.6 g), and fixed dosage of ferrous iron (0.3 g). All chemicals were in analytical grade, and aqueous solutions were prepared with analytical grade Milli-Q water (Millipore). A 100 mg/l DNT solution was prepared in distilled water to investigate the potential of used oxidants in DNT removal and find the optimum treatment conditions. An original sample of petrochemical wastewater was collected to examine the process efficiency of DNT removal. To assess the effects of the matrix, the blank samples were used. To identify the characteristics of original wastewater samples, several parameters were measured. Also, the GC/MS analysis was performed to measure DNT and its isomers concentrations in original samples (PerkinElmer, Norwalk, 7800, USA). The optimum flow rate of the carrier gas (He) was 25 ml/min, and the injection volume was 0.3 μl with thermal planning system: temperature = 80 °C, time = 6 S, speed = 30 °C/min 30).
3.1 Choice of optimal pH conditions
3.2 Influence of initial DNT concentration
3.3 Influence of Na2S2O8 dose on DNT oxidation
3.4 Influence of H2O2 dose on DNT oxidation
3.5 Influence of reaction time on DNT oxidation
Optimal conditions on DNT oxidation/removal from aqueous solution
Reaction time (min)
Initial concentration of DNT(mg/l)
Removal efficiency of DNT (%)
3.6 Fenton oxidation kinetics
The characteristics of Fenton oxidation kinetics in optimum conditions
reaction rate constant
Correlation coefficient (R2)
Y = 0.0084 X + 0.268
8.4 × 10−3
3.7 Treatment of original petrochemical wastewater samples
The characteristics of petrochemical wastewater
150 ± 20
3285 ± 40
1465.2 ± 15
1800 ± 10
603.1 ± 6
According to the results, the sample contains 1800 and 3285 mg/l DNT and COD, respectively. Also, the GC–MS analysis was performed to measure DNT and its isomers concentrations in original samples. Based on the results, the presence of some DNT isomers such as 2,4-DNT, 2,6-DNT, and MNT was confirmed. The results showed that removal efficiency of DNT and COD mineralization was 66.5 and 55.4%, respectively.
Results of study revealed that the obtained best removal efficiencies (Table 1) were in the order of alkaline and acidic and neutral conditions. Complete removal of 100 mg/l DNT and 89.1% mineralization of COD were obtained in alkaline conditions after 25 min reaction time.
pH is one of the most important factors that affected the rate of chemical reactions. It influences the oxidation of chemicals directly or indirectly and different methods of treatment process.
High removal efficiency in alkaline conditions may due to the simultaneous presence of oxidizing agents including sulfate and hydroxyl radicals [35, 36]. Zhao et al.  studied the characteristics of activated sulfate ions in removing PAHs from soil. The results showed that the pH values above 10 (especially 12) were effective in removing PAHs from soil . Also, Asgari et al.  studied the effect of pH on removal of pentachlorophenol from synthetic sewage by the microwave waves and sulfate ions. The results indicated that a higher eliminating efficiency of the contaminant was observed in alkaline pH . Also, Furman et al.  showed the similar results in the study of anisole, nitrobenzene, and hexachloroethane oxidizing by sulfate ions. Lin et al. , studying the oxidation of phenol with UV-activated persulfates, found that the process is more effective in alkaline conditions. These findings were concurred with other studies [35, 36, 39, 41, 42].
In acidic conditions, hydroxyl radicals are dominant, and very small amounts of sulfate radicals can be present. The main role of organic matter oxidation is hydroxyl radicals, which in comparison with sulfate radicals has lower oxidation power. Therefore, the process efficiency in acidic conditions is less than alkaline conditions. Moreover, in neutral conditions, the production rate of both radicals is very low which leads to decreasing mineralization. Furthermore, COD is consistently lower than the removal efficiency of DNT, indicating that together with mineralization to CO2, some intermediates were also generated. These findings were concurred with the study of Ai et al. .
Results of study showed that by increasing Na2S2O8 dosages, the decomposition rate of DNT and the mineralization of the organic matter will increase. It may be related to higher content of oxidizing sulfate radicals, which resulted in higher pollutant decomposition. Other studies have reported similar results in using Fenton’s advanced oxidation process to decompose different types of organic materials [35, 36, 44].
Also, the results of study showed that increasing the amount of H2O2 was effective, up to a certain value and at higher values no significant changes were observed in the process efficiency. The produced radicals in high concentrations of H2O2 can be used in H2O2 scavenging and hybrid open (combined) mechanisms. Subsequently, only a small amount of these chemically active components is reacted with the pollutant to be decomposed that was concurred with findings of other results [14, 45]. Other studies found that H2O2 would be effective up to a certain amount and in higher levels is either ineffective or has negative effects, due to the recombination of hydroxyl radicals [14, 46, 47]. Wu et al.  showed that increase in the H2O2 up to a critical concentration led to enhance the removal efficiency of humic substances. Murray and Parsons  studied the removal of natural organic matter from drinking water by the Fenton and Photo-Fenton process and showed that at the constant concentration of ferrous iron (Fe2+), increasing of hydrogen peroxide concentration, led to higher removal efficiency, but decreased dramatically after critical concentration.
The reaction time has a significant impact on the AOPs process. It is observed that high reaction time led to increase in the efficiency of DNT removal by the system. More reaction time may allow to make more reactions between pollutant and the oxidizing agent and increase the final efficiency. In this study, the high and rapid decomposition rate was observed due to the presence of both sulfate oxidation and hydroxyl radicals in the solution, both of which reacted simultaneously with organic matter and decomposed them. Sun et al.  showed that more oxidizing of dimethyl phthalate by Ag+/Na2S2O8/UV process can be achieved at higher reaction time. The same results obtained by studies including Asgari et al. , on the removal of pentachlorophenol using a combination of microwave/Na2S2O8, and Sun et al. to remove azo dye Acid Orange 7 by the interaction of heat, UV, and anions with common oxidants: persulfate, peroxymonosulfate, and hydrogen peroxide .
Based on the GC analysis, the presence of some DNT isomers such as 2,4-DNT, 2,6-DNT, and MNT was confirmed. He et al.  investigated the performance and kinetic parameters of Fenton oxidation of 2,4- and 2,6-DNT (DNT) in water–acetone mixtures and explosive contaminated soil washing-out solutions at a laboratory scale. Results demonstrated the following reaction pathway for 2,4-DNT primary degradation: 2,4-DNT → 2,4-dinitrobenzaldehyde → 2,4-dinitrobenzoic Acid → 1,3-dinitrobenzene → 3-nitrophenol and a series of radical propagation and termination reactions. Results of kinetics model reactions were concurred by Celin  revealed the high ability of first-order kinetic model for describing DNT Fenton’s degradation.
Fenton oxidation was a feasible method that was used to degrade DNT in aqueous solution. In the present research, removal of DNT and COD from a petrochemical wastewater by Na2S2O8/H2O2/Fe+ 2 could reach 100 and 89.1%, respectively, under the following optimal condition: the H2O2 and Na2S2O8 oxidant dosages of 0.2 and 0.5 g, respectively, the pH = 11.0 and the reaction time of 25 min. The oxidizing of DNT followed pseudo-first-order kinetics. The removal efficiency increased with H2O2 dosage but became reduced when H2O2 was in excess of 0.2 g. Moreover, higher concentration of Na2S2O8 up to critical concentration of 0.5 g led to higher degradation, since higher amounts reduced the removal efficiency.
The results showed that removal efficiency of DNT and COD mineralization was 66.5 and 55.4%, respectively. However, it could be suggested that these processes have a good potential for polluted or industrial wastewaters.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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