The treatment of aniline in aqueous solutions by gamma irradiation

2.1 Materials

The aniline, the hydrochloric acid naphthalene ethylenediamine (C₁₂H₁₄N₂·2HCl, molecular weight of 259.20 g/mol), and the sulfamic acid ammonium (H₆N₂O₃S,molecular weight of 114.12 g/mol) were purchased from Aladdin Industrial Inc.(China). The hydrogen peroxide, the sodium hydroxide, the ammonia, the carbon tetrachloride and the sulfuric acid were obtained from Nanjing Chemical Reagent Co. Ltd. All of the chemicals were of analytical grade or the higher purity available from the supplier. The water used in all of the experiments was ultrapure water.

2.2 Radiolysis experiments

Aniline solutions were irradiated in a 10 ml centrifuge tube with the designated initial concentration at room temperature. Each group of samples was irradiated for different time periods. The reference samples were non-irradiated samples. In addition, three groups of parallel samples were set up for guaranteeing the statistics of data. Some of the results are listed in Table 1.

Data of the parallel samples were used to calculate the standard deviation. Table 1 indicates that the standard deviation was small enough so that the measuring method and all of the instruments were thought to be available. In order to conduct the experiments efficiently, three groups of parallel samples were set up in the tests as are described in parts 3.2, 3.4 and 3.5 of this article.

All irradiation experiments were performed at a gamma irradiator which was located at the Center of Irradiation Research, Nanjing University of Aeronautics and Astronautics (NUAA). The experiments used a ⁶⁰Co-γ source with an activity level of about 1.48 × 10¹⁶ Bq for irradiation. A silver dichromate dosimeter was used for detecting absorbed doses with an absorbed dose rate of 0.78 kGy/h. The initial pH was adjusted by using 1 mol/L H₂SO₄ or NaOH solutions.

2.3 The analytical instruments and methods

3.1 The determination of the standard curve

To measure the aniline concentration by the UV-Vis spectrophotometer, the hydrochloric acid naphthalene ethylenediamine method [20] was referred, and the absorbances of samples with different known concentrations of aniline (5, 10, 20, 50, 100 and 200 mg/L) were measured by UV-vis spectrophotometer. The relation between the absorbance at 545 nm and the known concentration of aniline is shown in Figure 1.

Table 3:

The analyzed compounds identified by the GC/MS.

Symbol	RT (min)	Molecular Weight	Compounds	Structural formula
A	6.18	93	Aniline
B	13.95	94	Phenol
C	4.18	106	Ethylbenzene

Figure 1:

The relation between the absorbance and the concentration of aniline.

The absorbance was measured after diluting each sample 10 times. The result after the linear fitting is shown in eq. 2):

Where Y stands for the absorbance; X stands for the concentration of aniline; R² stands for linear correlation coefficient. Good linearity as shown in eq. 2 indicates that eq. 2 can be used to determine the aniline concentration for the following irradiated samples.

3.2 The degradation kinetics of aniline solution by gamma irradiation

By using eq. 2 the concentration of aniline in each irradiated sample was calculated. The relationships between the degradation efficiency of aniline and its corresponding absorbed dose are shown in Figure 2.

Figure 2:

The degradation efficiency along with the absorbed doses (Initial concentrations of aniline were 25, 50, 75, 100 and 200 mg/L).

The results indicate that the initial concentration of aniline and the absorbed dose influences its degradation efficiency. For the samples with the same initial concentration of aniline, the degradation efficiency of aniline increases rapidly with the increase of the absorbed dose. However, the degradation efficiency decreases with the increasing initial concentration of aniline, because the number of the intermediate products increases with the increase of the initial concentration of aniline, and the intermediate products also take part in the degradation reaction. Thus, the intermediate products act as competitors with aniline. The degradation efficiencies for initial aniline concentrations of 25, 50, 75, 100 and 200 mg/L are 100 %, 94 %, 85 %, 82 % and 70 %, respectively.

The reaction velocity is defined as the concentration change per unit dose in a selected dose interval. The radiation-induced degradation of many pollutants is known to observe pseudo first-order behavior [21–24]. The kinetics models were listed in the eq. 3 [24]:

where C₀ stands for the initial concentration of aniline (mg/L); C stands for the concentration of aniline at a dose of D (mg/L). D is the absorbed dose during the irradiation (kGy), and d stands for the pseudo first-order kinetic constant used to describe the degradation rate (kGy^–1).

C, C₀, and D are known to calculate the d (dose constant, kGy^–1). Table 2 lists the d value at different initial concentrations. The results show that the d value decreases with the increase of the initial concentration. This suggests that the aniline degradation rate is lower at a high initial concentration than that at a low initial concentration. In addition, the correlation coefficients suggest that the degradation of aniline follows pseudo first-order kinetics [24].

Some degradation rate constants of other degradation methods are shown in various professional journals. Jin Anotai et al [25]. indicated that when the experimental condition (0.01 M aniline, 1.07×10⁻³M Fe²⁺, 74 g SiO₂/L, current 4 A, pH 3.2) was set up, the reaction rate of aniline reached 0.053 M/min by the Electro-Fenton process and 0.029 M/min by the Fluidized-bed Fenton process. Ming-Chun Lu et al. [26] indicated the rate constant of aniline by photo-degradation at different initial concentrations. When the initial concentration of aniline ranged from 0.047 mM to 0.80 mM, the rate constant decreased from 0.1950 to 0.0017 (mM min)^–1.

3.3 The effect of the initial pH of the aniline solution

1 mol/L of sulfuric acid and sodium hydroxide solution were used for adjusting the pH of the aniline solution ranged from 2 to 11. The relationship between the removal efficiency of COD and the initial pH are shown in Figure 3.

Figure 3:

The removal efficiency of COD along with the change of pH. (a) The initial concentration of aniline was 25 mg/L. (b) The initial concentration of aniline was 50 mg/L. (c) The initial concentration of aniline was 100 mg/L.

The removal efficiency of COD was calculated following eq. (4):

Where COD₀ stands for the initial COD value of the aniline solution, and COD stands for the COD value after irradiation.

Figure 3 shows a similar trend which indicates that the removal efficiency of COD is higher in an alkaline environment than in an acidic environment at a low absorbed dose of 7.8 kGy, which was observed in the initial concentrations of 25, 50 and 100 mg/L. On the contrary, at a high absorbed-dose rate of 15.6 kGy, the removal efficiency of COD was lower in an alkaline environment than in an acidic environment. The main reason for this is that aniline tends to exist in a molecular form instead of an ion form in alkaline solution. This is helpful for the aniline-degradation process. Too, hydroxyl radical is easily produced in an alkaline environment. With the absorbed dose increases, organic carbon is converted into inorganic carbon which exists in the forms of HCO₃^-, and CO₃^2- in an alkaline solution. These react with hydroxyl radicals, resulting in a decrease in the removal efficiency of COD (eq. 5), eq. 6, 27]. In an acidic solution, however, inorganic carbon escapes in the form of CO₂ and has no influence on the concentration of hydroxyl radicals [28].

When researchers have treated aniline pollution through biological means, optical density (OD) is commonly used to represent the degradation rate of aniline. This procedure performs the same function as COD in the irradiation–degradation process. Junmin Li et al. [3] found that when the pH of aniline solution ranged from 3 to 13, the suitable pH for strain HSA6 was wide (from 5 to 11) and the optimal pH was 6 whose OD value reached 2.6 in 40 h.

3.4 The role of an hydroxyl radical

When ultrapure water is irradiated by gamma ray, many kinds of active substances generate in the solution (eq. 7, 29, 30].

The hydroxyl radical, hydrogen radical and hydrated electrons have an effect on the degradation of organic pollutants. In the experiments described here, sodium bicarbonate was chosen as the scavenger of the hydroxyl radical to allow for discussion on the influence of hydroxyl radicals on the degradation process. The influence of the sodium bicarbonate addition is shown in Figure 4.

Figure 4:

The influence of sodium bicarbonate addition.

One can see in Figure 4 that with the increase of the concentration of the sodium bicarbonate addition, the removal efficiency of COD decreased significantly. The reason for this is that when sodium bicarbonate is added to the solution, the HCO₃^- competes with the aniline to react to the hydroxyl radical (eq. 6, 27], leading to the decrease of COD removal efficiency. This indicates that hydroxyl radicals play a major role in the degradation of aniline.

3.5 The enhancement of aniline degradation by irradiation with the addition of H₂O₂

In the present study, the enhancement of the H₂O₂ addition was investigated by analyzing the change of the COD removal efficiency in the presence of H₂O₂, whose initial concentrations in the samples were 0, 100, 200, 500 and 1000 mg/L. The results are shown in Figure 5.

Figure 5:

The enhancement of the removal efficiency by using hydrogen peroxide. (a) The initial concentration of aniline was 100 mg/L. (b) The initial concentration of aniline was 200 mg/L.

Figure 5 shows that the removal efficiency of COD by gamma irradiation improved significantly in the presence of H₂O₂. This can be explained by the fact that the addition of H₂O₂ in the solution improves the yield of hydroxyl radicals by eq. 8 and eq. 9 [30–32]. The removal of COD is facilitated by the increase of hydroxyl radicals.

In comparison with other methods of treating aniline pollution, Jin Anotai et al [33]. reported on the effectiveness of using the ordinary Fenton route. He said that when the concentration of aniline was 0.01 M, the removal efficiency of COD was steady, around 14 %, with the addition of 0.01 M Fe²⁺ and 0.3 M H₂O₂. He also noted that under the same conditions, the removal efficiency of COD was steady, around 40 %, by using the electro-Fenton method.

3.6 The intermediate products formation

To confirm the species of organic intermediates forming in the radiolytic decomposition of aniline, the samples were analyzed by the GC/MS. Since the samples had no water requirements, carbon tetrachloride was used to extract the aniline and its intermediates from the aqueous solutions. The decomposition by products are identified and listed in Table 3.

3.7 The organic nitrogen analysis

In the present study, an ammonia nitrogen rapid tester and an ion chromatograph were used to analyze the concentration change of the ammonium ion and the nitrite and nitrate. The concentration of the aniline was 200 mg/L and the absorbed doses were 7.8, 15.6 and 23.4 kGy. The results are shown in Figure 6, Table 4 and Table 5.

Figure 6:

The relation between the concentration of ammonium and absorbed dose.

The concentration of the ammonium ion was measured by the ammonia nitrogen rapid tester. Figure 6 shows that with the increase of the absorbed dose, the concentration of the ammonium ion increased significantly. This indicates that with the formation of the hydroxyl radical, and aniline’s subsequent reaction to it, the hydroxyl radical replaces the amidogen and nitrogen appears as the ammonium ion in aqueous solutions [34].

The ion chromatograph was used to verify the existence of nitrate and nitrite. In the result of ion chromatograph analysis, peak area has a positive correlation with the concentration, thus, the peak area was used to represent the concentration of nitrate and nitrite. Table 4 shows the increase of the absorbed dose along with the concentration of nitrite, which at first increased and then slowly decreased. Table 5 illustrates how the nitrate peaked at 14 min. This discovery showed that with the increase of the absorbed dose, nitrate in aqueous solution manifested itself.

The results cited above indicate that when aniline reacts with radicals, nitrogen presents itself as the ammonium ion, and nitrate and nitrite appear and remain in aqueous solutions [35].

3.8 The decomposition pathways

The results analyzed by the GC/MS, ammonia-nitrogen rapid tester and the ion chromatograph have formed this project’s proposed pathways for the decomposition of aniline, as shown in Figure 7. In gamma irradiation, the hydroxyl radical generates in aqueous solution, and it reacts with the aniline, resulting in the formation of phenol (B, 36, 37]. The amidogen is substituted and oxidized, and then exists as an ammonium ion, nitrate and nitrite in aqueous solution [35]. In addition, the gamma ray attacks the bond between the amidogen and the aromatic ring, resulting in the formation of an aromatic ring with a single electron. Ethyl then reacts with the aromatic ring and ethylbenzene (C) forms. Finally, the aromatic ring mineralizes into CO₂, H₂O, etc.

Figure 7:

The proposed pathway for aniline decomposition by gamma irradiation in aqueous solution.

In a previous study, Knight J A [34] investigated the products of aniline by gamma radiolysis, and identified some complicated cross-linked products of aniline. He discovered that the reaction of the amino group was about 14 times greater than that of the phenyl ring. He did not, however, discuss the changes that can take place in organic nitrogen.