1 Introduction

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 [1].

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 [4]. Bradley et al. [5] 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 [6]. 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 [7] 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 [7]. 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 [23]. Fenton’s oxidation with ferrous iron (Fe2+) can generate strong oxidant hydroxyl-1 that led to degradation of recalcitrant organics at acidic conditions [24]. 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 [25].

The most principal treatment techniques to DNT removal are included adsorption, advanced oxidation processes (AOPs), chemical reduction, and bioremediation [26]. The important benefits of AOPs are its non-selectivity property and quick kinetic reactions [27]. Fenton’s reagents (H2O2, Fe2+) have been known as the most applicable type of AOPs. Mohanty and Wei reported [22] 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 [28] 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. [29]. Chen et al. [30] proposed that oxidative degradation of DNT isomers leads to o-, m-, p-mononitrotoluene (MNT), and 1,3-dinitrobenzene, respectively.

AOPs were used for the removal of DNT from wastewater [16,17,18, 22, 31], but none of them have discussed Na2S2O8/H2O2/Fe+2 system.

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).

The kinetic constants of Fenton oxidation to remove DNT were investigated and determined. Pseudo-first-order kinetic model was made to determine the kinetic constants and represented by the following reaction [32]:

$$\frac{dC}{dt} = -\,KC^{ \propto }$$

where C was the DNT concentration, α was the order of the reaction, K was the reaction rate constant, and t was the time. For a first-order reaction, the equation changed to

$$\ln \left( {Ct|C0} \right) = -\,Kt$$

where C0 and Ct were the initial and time t DNT concentrations, respectively. According to the equation, a plot of ln(C/C0) against t will yield to a straight line with a slope of k.

Theoretical oxygen demand that is defined as a required dosage of oxygen for oxidation of compound to end products including CO2, NH3, H2PO4 −, SO42−, and H2O was obtained using standard method in the literature [33]. Removal efficiency of aromatic compounds was monitored by COD measurements. It was determined by closed reflux-titrimetric method according to the standard methods [34]. The COD removal efficiency of DNT compounds in wastewater (η) was obtained using the following equation:

$$\eta = \frac{{{\text{COD initial}} - {\text{COD final}} }}{\text{COD intial}} \times 100$$

The DNT removal efficiency (β) from wastewater and synthetic solutions was calculated using the following equation:

$$\beta = \frac{{{\text{DNT initial}} - {\text{DNT final}} }}{\text{DNT intial}} \times 100$$

3 Results

3.1 Choice of optimal pH conditions

Figure 2 shows the effect of pH variation (pH 2, 3,6,7, 9 and 11) in initial DNT concentration of 100 mg/L, 0.2 g H2O2 and 0.5 g Na2S2O, 0.3 g/Fe+ 2 over 60 min) on the removal efficiency of DNT by Fenton process of Na2S2O8/H2O2/Fe+ 2. Results showed the significant effect of pH on process efficiency. Maximum (100%) and minimum (63.7%) removal efficiencies of DNT were obtained at pH values of 11 and 3, respectively. In pH above 6, the removal efficiency increases significantly and complete elimination was obtained in pH 11. Therefore, this point was selected as an optimum pH value. A similar trend was also observed in COD removal efficiency. Based on the results presented in Fig. 1, it can be stated that the efficiency of COD removal in both acidic and alkaline conditions was higher than neutral conditions.

Fig. 1
figure 1

Influence of Na2S2O8/H2O2/Fe+2 on DNT oxidation/removal in different pHs (reaction time: 60 min, H2O2 = 0.2 g, Na2S2O8 = 0.5 g, Fe+ 2 = 0.3 g)

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3.2 Influence of initial DNT concentration

Figure 2 shows the results of experiments to assess the effect of the initial concentration on the removal of DNT. The DNT concentration varied between 40 and 160 mg/l. According to Fig. 2, increasing in the DNT initial concentration led to a reduction in their removal efficiency. At an initial concentration of 100 mg/l, there is a dramatical reduction in removal efficiency. So, it was selected as an optimal initial concentration of DNT. The same trend was observed for COD removal efficiency.

Fig. 2
figure 2

Influence of DNT initial concentrations (mg/l) on DNT oxidation/removal (reaction time: 60 min, Na2S2O8 = 0.5 g, H2O2 = 0.2 g, pH = 11, Fe+2 = 0.3 g)

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3.3 Influence of Na2S2O8 dose on DNT oxidation

The effect of different dosages of Na2S2O8 (0.1–0.6 g) was investigated on the removal efficiency of DNT and COD mineralization. The results are shown in Fig. 3. It is observed that by increasing the concentration of oxidant, the process efficiency increased significantly. Results indicated that with an increase in the amount of oxidant from 0.1 to 0.3 g, the DNT removal efficiency and COD mineralization increased to 66.8 and 57.2%, respectively. Findings also showed that under the experimental conditions, (initial concentration of 100 mg/L DNT and 0.5 g of Na2S2O8), complete DNT elimination was observed. So, the 0.5 g/l was selected as the optimal dosage of Na2S2O8. This amount of oxidant can reduce 86% of solution COD (Fig. 3).

Fig. 3
figure 3

Influence of Na2S2O8/H2O2/Fe+2 on DNT oxidation/removal in different Na2S2O8 dosage ((reaction time: 60 min, H2O2 = 0.2 g, pH = 11, Fe+2 = 0.3 g)

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In the Na2S2O8/H2O2/Fe+2 system, persulfate is a strong chemical oxidant (Eo = 2.01 V) which can be activated by transition metal ions such as Fe2+ under acidic pH to form a sulfate radical (SO −24 ), which in comparison with persulfate, is a stronger oxidizing agent (Eo = 2.60 V) [2, 22]. It is demonstrated that the persulfate ions have high reaction ability in 3 < pH > 10 [35]. Persulfate ions as an oxidant agent have an important role in the decomposition of organic compounds; (1) production of sulfate radicals that have high oxidation and reduction potential, (2) in alkaline conditions, sulfate radicals interacted with water molecules and hydroxyl ions (Eq. 1) and transformed into radical hydroxyl, which itself play a significant role in the decomposition of organic compounds.

$${\text{SO}}_{4}^{ - 0} + {\text{OH}}^{ - } \to {\text{OH}}^{0} + {\text{SO}}_{4}^{ - 2}$$
(1)

The production of free hydroxyl and sulfate radicals is presented in Eqs. 2–13.

$$^{ - 3} {\text{OS}}{-}{\text{O}}{-}{\text{O}}{-}{\text{SO}}^{3 - } + {\text{H}}_{2} {\text{O}} \to \left[ {^{ - 3} {\text{OS}}{-}{\text{O}}{-}{\text{O}}^{ - } } \right] + {\text{SO}}_{4}^{2 - } + 2{\text{H}}^{ + }$$
(2)
$$\left[ {^{ - 3} {\text{OS}}{-}{\text{O}}{-}{\text{O}}^{ - } } \right] + {\text{H}}_{2} {\text{O}} \to {\text{H}}{-}{\text{O}}{-}{\text{O}}^{ - } + {\text{SO}}_{4}^{2 - } + {\text{H}}^{ + }$$
(3)
$$^{ - 3} {\text{OS}}{-}{\text{O}}{-}{\text{O}}{-}{\text{SO}}^{3 - } + 2{\text{H}}_{2} {\text{O}} \to {\text{HO}}^{2 - } + 2{\text{SO}}_{4}^{2 - } + 3{\text{H}}^{ + }$$
(4)
$${\text{HO}}_{2 - } +^{ - 3} {\text{OS}}{-}{\text{O}}{-}{\text{O}}{-}{\text{SO}}^{3 - } \to {\text{SO}}_{4}^{ \cdot - } + {\text{SO}}_{4}^{2 - } + {\text{H}}^{ + } + {\text{O}}_{2}^{ \cdot - }$$
(5)
$$2{\text{S}}_{2} {\text{O}}_{8}^{2 - } + 2{\text{H}}_{2} {\text{O}} \to 3{\text{SO}}_{3}^{2 - } + {\text{SO}}_{4}^{ \cdot - } + {\text{O}}_{2}^{ \cdot - } + 4{\text{H}}^{ + }$$
(6)
$${\text{SO}}_{4}^{ \cdot - } + {\text{OH}}^{ - } \to {\text{SO}}_{4}^{2 - } + {\text{OH}}^{ \cdot }$$
(7)
$${\text{SO}}_{4}^{ \cdot - } + {\text{OH}}^{ - } \to {\text{SO}}_{4}^{2 - } + {\text{OH}}^{ \cdot }$$
(8)
$${\text{SO}}_{4}^{ \cdot - } + {\text{OH}}^{ - } \to {\text{OH}}^{ \cdot } + {\text{SO}}_{4}^{2 - }$$
(9)
$${\text{OH}}^{ \cdot } + {\text{OH}}^{ \cdot } \to {\text{H}}_{2} {\text{O}}_{2}$$
(10)
$${\text{S}}_{2} {\text{O}}_{8}^{2 - } + 2{\text{H}}_{2} {\text{O}} \to 2{\text{HSO}}_{4 - } + {\text{H}}_{2} {\text{O}}_{2}$$
(11)
$${\text{H}}_{2} {\text{O}}_{2} + {\text{OH}}^{ - } \leftrightarrow {\text{ HO}}_{2 - } + {\text{H}}_{2} {\text{O}}$$
(12)
$${\text{H}}_{2} {\text{O}}_{2} + {\text{HO}}_{2 - } \to {\text{O}}_{2}^{ \cdot - } + {\text{OH}}^{ \cdot } + {\text{H}}_{2} {\text{O}}$$
(13)

3.4 Influence of H2O2 dose on DNT oxidation

The effect of different amounts of H2O2 (0.1–0.6 g) on the efficiency of DNT removal at optimal conditions (pH = 11, 0.5 g Na2S2O8) is shown in Fig. 4. Results revealed that the removal efficiency of DNT was increased with increasing H2O2 dosage, but it was reduced when the oxidant dosage exceeded 0.2 g. Complete elimination of DNT and COD mineralization was obtained by application of 0.2 g H2O2. Therefore, the amount of 0.2 g of this oxidant was considered as an optimum amount for the subsequent experiments.

Fig. 4
figure 4

Influence of Na2S2O8/H2O2/Fe+2 on DNT oxidation/removal in different H2O2 dosage (reaction time: 60 min, Na2S2O8 = 0.5 g, pH = 11, Fe+ 2 = 0.3 g)

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3.5 Influence of reaction time on DNT oxidation

Figure 5 shows the effect of reaction time on the removal efficiency of DNT by Na2S2O8/H2O2/Fe+ 2 in optimum pH and oxidants dosage. Results showed that the process efficiency for removing pollutants was increased with increase in reaction time. So, by increasing the reaction time to 20 min, the removal rates of DNT and COD were increased to 65.4 and 49.8%, respectively. Findings also revealed that under 25 min TIME, degradation of DNT and COD can reach to 100 and 86.9%, respectively, representing complete elimination of DNT from solution. It is also clear that the removal efficiency was almost constant after 25 min and no significant changes were made. So, 25 min was chosen as an optimal time for process. At the end of experiments, the optimal conditions are summarized in Table 1.

Fig. 5
figure 5

Influence of Na2S2O8/H2O2/Fe+2 on DNT oxidation/removal in different reaction times (H2O2 = 0.2 g, Na2S2O8 = 0.5 g, pH = 11, Fe+2 = 0.3 g)

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Table 1 Optimal conditions on DNT oxidation/removal from aqueous solution
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3.6 Fenton oxidation kinetics

In this study, the reaction rate constants and the correlation coefficients indicated that the degradation of DNT followed first-order kinetics. The values of first-order kinetics parameters for the removal of DNT by processes Na2S2O8/H2O2/Fe+2 are presented in Table 2. The constant coefficients of the reaction rate for the process were 8.4 × 10−3. The correlation coefficient value of model was 0.974.

Table 2 The characteristics of Fenton oxidation kinetics in optimum conditions
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3.7 Treatment of original petrochemical wastewater samples

The performance of Na2S2O8/H2O2/Fe+ 2 was evaluated under optimal conditions to remove DNT from original petrochemical wastewater samples. The characteristics of sample wastewater are summarized in Table 3.

Table 3 The characteristics of petrochemical wastewater
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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.

4 Discussion

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. [37] 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 [37]. Also, Asgari et al. [38] 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 [38]. Also, Furman et al. [39] showed the similar results in the study of anisole, nitrobenzene, and hexachloroethane oxidizing by sulfate ions. Lin et al. [40], 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. [43].

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. [48] showed that increase in the H2O2 up to a critical concentration led to enhance the removal efficiency of humic substances. Murray and Parsons [49] 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. [50] 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. [38], 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 [51].

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. [2] 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 [52] revealed the high ability of first-order kinetic model for describing DNT Fenton’s degradation.

5 Conclusion

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.