Parameters controlling the advanced oxidation degradation kinetics of nitroglycerin and pentaerythritol tetranitrate

2.1. Materials

Liquid NG, crystalline PETN, FeSO₄·7H₂O, and hydrogen peroxide (30% w/w) were of analytical grade. All solvents, including acetonitrile (ACN), ethanol, methanol, and hexane (Merck KGaA, Germany) were at HPLC grade. De-ionized water was used to prepare the aqueous solutions. All chemicals were used as received without any further purification.

2.2. Instruments

All photochemical reactions were performed in a home-built photo-reactor system, which was built following the procedure described elsewhere [15]. The concentrations of NG (liquid) and PETN were determined by using Agilent HPLC HP 1100 (Waldbronn, Germany) equipped with a diode array detector (DAD). The conditions of the HPLC measurements are listed in Table 1. An OAKTON pH meter, series 510 (Cole-Parmer Instrument Co., USA) was used to adjust the pH of the prepared solutions.

2.3. Methodology

In the oxidation of organic compounds (RH) by OH radical generated from Fenton’s reagent, the degradation kinetic study followed the pseudo-first-order model with an assumption that the concentration of hydroxyl radical was much higher than that of RH [17]. The equation is given by

where C_{RH (}t₎ and C_{RH (0)} are the concentrations of RH at any time t and at initial t=0 of the reactions, respectively, and k′_RH is the pseudo-first-order reaction rate constant. This model has also been applied in previous works for determination of k′_RH [11], [12], [13], [15], [16], [18].

3.1 Kinetics of NG and PETN degradation in various advanced oxidation systems

Figures 1 and 2 present the respective efficiencies of the degradation of NG and PETN in various oxidation systems (ENs/UV, ENs/H₂O₂; ENs/UV-H₂O₂, ENs/Fenton, and ENs/UV-Fenton). Clearly, the combination of the oxidative agents (i.e. H₂O₂ or Fenton) with UV radiation increased the efficiency of NG degradation (Figure 1). Therefore, the conversion effectiveness of NG in different systems followed the sequence (at 40 min) given by NG/UV (~6%)<NG/H₂O₂ (~20%)<NG/UV-H₂O₂ (~30%)<NG/Fenton (~60%)<NG/UV-Fenton (~100%). A similar trend for PETN degradation processes was also observed: PETN/UV (15%)<PETN/H₂O₂ (38.5%)<PETN/UV-H₂O₂ (50.3%)<PETN/Fenton (72.8%) <PETN/UV-Fenton (100%).

Figure 1:

Efficiency of NG removal vs. time in different advanced oxidation systems.

[NG]_o=0.228 mm; [Fe²⁺]₀=0.4 mm; [H₂O₂]_o=10 mm; pH=3; λ=245 nm; I_UV=1100 LUX, T=298 K.

Figure 2:

Efficiency of PETN removal vs. time in different advanced oxidation systems.

[PETN]_o=0.056 mm; [Fe²⁺]_o=0.4 mm; [H₂O₂]_o=10 mm; pH=3; λ=245 nm; I_UV=1100 LUX, T=298 K.

In the case of UV alone, the light radiation interacted directly with EN molecules to dissociate into species thanks to the photon energy. However, the yield of the reaction was very low [19] due to the low quantum yield of the direct photodecomposition of ENs. In the case of oxidative reagent H₂O₂, the conversion efficiency was higher, because it was a strong oxidative agent. Its oxidation potential (E_ox=1.78 V) was only lower than those of ozone (E_ox=2.07 V) and ˙OH (E_ox=2.80 V) [20]. However, EN-saturated compounds contained a nitrogen atom at the highest oxidation state (+5), so their ability to react with H₂O₂ proved to be limited. The combination of UV and H₂O₂ generated hydroxyl radical ˙OH due to the photolysis reaction of H₂O₂ [21], consequently increasing the effectiveness of the degradation reaction, as given by

In the presence of ferrous ions, hydrogen peroxide molecules were decomposed rapidly as a result of a catalytic activity [17], [22], as presented below.

Under UV exposure, there exist three simultaneous reactions: direct photolysis of ENs by UV radiation, direct oxidation of ENs by H₂O₂, and EN decomposition reaction by ˙OH radical generated from the photocatalytic activity.

or

It should be noted that [Fe³⁺(OH)⁻]²⁺ absorbs photons significantly stronger than H₂O₂ at the wavelength of 254 nm (molecular absorption coefficient of [Fe³⁺(OH)⁻]²⁺ is in the range of 1500–3500 (l·mol⁻¹·cm⁻¹) and that of H₂O₂ is 18.6 (l·mol⁻¹·cm⁻¹) [21]. Therefore, UV-Fenton is the most effective system for destroying ENs among the investigated AOPs.

The kinetics of NG and PETN decomposition in various advanced oxidation systems (ENs/AOPs) can be described by using the pseudo-first-order law, which is a general model for the oxidation of organic compounds, including some highly explosive compounds by ˙OH radical [12], [16], [17], [23]. The ˙OH radical is a strong and non-selective oxidative species that can react with many organic substances (the rate constant of 10⁸−10¹¹m⁻¹s⁻¹) [24]. The oxidation process of ENs by ˙OH radical also includes three main stages as follows:

where AOPs means advanced oxidation processes that can create hydroxyl radicals (e.g. UV-H₂O₂, Fenton, and UV-Fenton); ICi are the intermediates generated from the reaction of EN with ˙OH radical; and CO₂, H₂O, NO₃⁻ are the final products of the EN mineralization process. Stage (II) determines the rate of EN degradation. The reaction rate constants can be determined using Equation (1).

Figures 3 and 4 present the plots of –ln(C/C_o) against time (t) of the decomposition of NG and PETN. The pseudo-first-order reaction rate constant k′ was determined. The results are listed in Table 2.

Figure 3:

The plot of −ln(C/C_o) vs. time in different systems.

NG/UV-H₂O₂; NG/Fenton; NG/UV-Fenton; [NG]_o=0.228 mm; [Fe²⁺]_o=0.4 mm; [H₂O₂]_o=10 mm; pH=3; λ=245 nm; I_UV=1100 LUX, T=298 K.

Figure 4:

The −ln(C/C_o) vs. time in different systems.

PETN/UV-H₂O₂; PETN/Fenton; PETN/UV-Fenton; [PETN]_o=0.056 mm; [Fe²⁺]_o=0.4 mm, [H₂O₂]_o=10 mm; pH=3; λ=245 nm; I_UV=1100 LUX, T=298 K.

Many parameters affect the efficiency and kinetics of the advanced oxidation degradation of ENs, such as concentrations, pH, and temperature. Understanding how they influence the decomposition of EN explosives enables us to choose favorable conditions for the treatment of ENs.

3.2. Influence of H₂O₂ concentration

The influence of hydrogen peroxide concentration on the rate of NG degradation reaction is shown in Figure 5. Pseudo-first-order rate constants of NG and PETN decomposition were determined and collated in Table 2.

Figure 5:

Influence of initial H₂O₂ concentration on NG decomposition by different AOPs.

[NG]_o=0.228 mm; [Fe²⁺]_o=0.4 mm; pH=3; λ=245 nm; I_UV=1100 LUX, T=298 K.

We noticed that when H₂O₂ was added into the NG solutions (systems NG/H₂O₂ and NG/UV-H₂O₂), the rate of NG degradation slightly increased with the increase of H₂O₂ concentration (Figure 5 and Table 2). However, in Fenton and photo-Fenton systems, the reaction rate first increased with the addition of H₂O₂, and then reduced when the hydrogen peroxide concentration was 160 mm. A similar effect of hydrogen peroxide on the rate of PETN destruction was observed (Figure 5 and Table 2).

Higher initial hydrogen peroxide concentration enhanced reactions (2) and (3), after which it increased the amount of hydroxyl radical and increased the speed of reaction. However, when the concentration of hydrogen peroxide was in excess, it turned into a scavenger that consumed ˙OH radicals, resulting in the decrease of the ENs degradation rate [25] given by

Based on obtained results, H₂O₂ concentration of 10 mm was selected for further studies.

3.3. Influence of pH

The effect of pH on the oxidation of organic compounds by radiation (UV) or hydrogen peroxide is often insignificant. Hence, we considered the degradation efficiency varying with the change of pH from 2 to 5 in the EN/Fenton and EN/UV-Fenton systems (Figure 6).

Figure 6:

Effects of pH on the degradation of (A) NG and (B) PETN.

[NG]_o=0.228 mm; [PETN]_o=0.056 mm; [Fe²⁺]_o=0.4 mm; [H₂O₂]=10 mm; λ=245 nm; I_UV=1100 LUX, T=298 K.

The reaction rates gained the maximum value at pH 3 for decomposing both NG and PETN. The explanation can be given as follows: ferrous ion in Fenton (or UV-Fenton) served as a catalyst for the decomposition of H₂O₂ to generate hydroxyl radical, as shown in reactions (3) and (4). Due to the hydrolysis of ferrous ions, the oxidation resulting from the use of Fenton systems was more favorable in an acidic medium. However, pH<2.5 favors the formation of [Fe(H₂O)]²⁺ complexes [22], [25], [26], resulting in the decrease of free ferrous ions taking part in (3), which in turn, can reduce the production of hydroxyl radicals. Moreover, at low pH, the radical ˙OH can be easily scavenged by hydrogen peroxide (6), reducing the concentration of this active species that is mainly in charge of the oxidation reaction. The optimal range of pH for most Fenton reactions is around the value of 3. At higher pH, ferrous ion is not very stable, and can be easily oxidized to form ferric ion and then ferrous hydroxyl complexes [25] due to hydrolysis. These complexes cannot react with H₂O₂ to recycle initial ferrous ions, resulting in the decrease of ˙OH formation, which is then followed by the reduction of oxidation rate. The higher the pH level, the easier the occurrence of the hydrolysis process. At pH 5, most ferric ions are either hydroxyl complexes or even precipitates to form Fe(OH)₃, so the reaction rate sharply decreases. For studying other effects, experiments were carried out at pH 3.

3.4. Influence of temperature

Figure 7 presents the influence of temperature on EN decomposition by the Fenton and UV-Fenton systems. As can be seen, the increasing temperature in the range of 293–313 K had a promotional effect on all investigated advanced oxidation systems. The linear plots of ln(k) against 1/T indicated that the processes followed an Arrhenius-type behavior in the considered temperature range.

Figure 7:

Effects of temperature on the degradation of (A) NG and (B) PETN using Fenton and UV-Fenton.

[NG]_o=0.228 mm; [PETN]_o=0.056 mm [Fe²⁺]=0.4 mm; [H₂O₂]=10 mm; pH=3; λ=245 nm; I_UV=1100 LUX, T=298 K.

The activation energies and energetic parameters were calculated from Figure 7 and Eyring equation. The results are shown in Table 3. Obviously, among these advanced oxidation processes, UV-Fenton is the most effective system for the treatment of wastewater polluted by NG and other EN compounds, such as PETN [27].