Aniline is widespread in groundwater and of great toxicity. Advanced oxidation processes, such as the ferrous iron (Fe2+)-activated persulfate process, have been proven to be effective for organic pollutants. However, few studies have focused on the effects of coexisting ions on the degradation of aniline. In this study, the degradation efficiency of aniline and the effects of common inorganic ions (CO32−, PO43−, HCO3, SO42−, NO3, Na+, K+, Mg2+, and Ca2+) on aniline degradation were examined. Under the optimum operating conditions, 86.33% aniline degradation (C0 = 11 mmol/L) was observed within 60 min. The effects of cations on aniline degradation were negligible. Anions decreased the removal efficiency of aniline because of the radicals generated by the reaction between sulfate radical or hydroxyl radical and these anions. As the concentrations of PO43−, CO32−, SO42−, HCO3, and NO3 increased from 0 mmol/L to 5 mmol/L, the removal efficiency of aniline decreased to 19.72%, 24.56%, 66.76%, 68.76%, and 82.42%, respectively. The order of inhibitory effects was PO43− > CO32− > >SO42− > HCO3 > >NO3.

INTRODUCTION

Aniline widely exists in wastewater discharged from chemical industries, such as polymer, antioxidant, pesticide, and dye industries (Matsushita et al. 2005). It comprises a benzene ring to which one amino group is attached. This structure causes it to become a carcinogen, gastrointestinal toxicant, and kidney toxicant (Anotai et al. 2011). Although many regulations restrict its discharge, it is still detected in groundwater, especially after a sudden industrial accident (Zhu et al. 2007).

Traditional remediation methods for aniline-contaminated water, including physical (Tanhaei et al. 2014), chemical (Gomes et al. 2008), and biological (Jin et al. 2012) methods, have certain limitations because of the feature of groundwater. Advanced oxidation processes (AOPs) have been developed for the treatment of organic contaminants (Wang & Xu 2012). AOPs utilize active radicals, such as •OH and SO4• produced by several physical chemical reactions, to degrade pollutants (Liang & Su 2009; Zheng et al. 2014). Although AOPs cannot thoroughly mineralize contaminants (Antoniou et al. 2010) due to the relatively low radical yields and the complex water matrix, the AOPs may degrade most of the organic pollutants.(Huang & Huang 2009, Sun et al. 2015) and decrease the secondary pollution (Chu et al. 2015).

In AOPs, sodium persulfate (Na2S2O8) can be chemically (by transition metal ions, such as Fe2+, Ag+, and Co2+) or thermally activated, and produces SO4•, which is a stronger oxidant (Eθ = +2.6 v) than S2O82− (Eθ = +2.01 v) (Hussain et al. 2012). SO4• has a longer half-life than •OH (Liu et al. 2014), and could degrade organic pollutants over a wide pH range (Oh et al. 2009). The activated reactions are as follows: 
formula
1
 
formula
2
 
formula
3
Ferrous ion is one of the most common inorganic ions in groundwater. Fe2+-activated persulfate processing requires lower activation energy (62.16 kJ/mol) than thermal activation (140.7 kJ/mol) to produce SO4• (Zhang et al. 2014). Thus, Fe2+ is a promising activator agent (Wu et al. 2014). 
formula
4
Nevertheless, the existence of common inorganic ions in groundwater may influence the efficiency of AOPs. Previous studies have shown that major anions can affect the effectiveness of Fenton advanced oxidation treatment. Siedlecka et al. (2007) reported that the addition of chloride or phosphate ions decreased the efficiency of Fenton treatment of methyl t-butyl ether-contaminated water. Ratanatamskul et al. (2010) reported that inorganic ions, including chloride ions, dihydrogen phosphate ions, and nitrate ions, can decrease the removal of nitrobenzene by the fluidized-bed Fenton process. Wu et al. (2014) studied the system of persulfate activated by citric acid-chelated-ferrous ion, and observed that the trichloroethene (TCE) degradation rate decreased as the Cl and HCO3 anions increased. However, studies on the effects of coexisting ions on the remediation of aniline-contaminated groundwater by Fe2+-activated persulfate are rare. Thus, this study aimed to explore the optimum reaction concentrations of Fe2+ and persulfate for efficient aniline degradation, and observe the influence of common groundwater ions on aniline degradation.

MATERIALS AND METHODS

Chemicals

Sodium persulfate, ferrous nitrate, sodium carbonate, sodium bicarbonate, sodium sulfate, sodium phosphate, sodium nitrate, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, aniline and methanol were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). All solutions were prepared with ultrapure water purified by a Milli-Q system.

Batch oxidation experiments

The experiments were conducted in cylindrical glass bottles in a shaking water bath at 20 °C, and pH was controlled at 7.0 by H2SO4 (0.1 mol/L) and NaOH (0.1 mol/L). Appropriate volumes of stock aniline and ferrous ion solution were placed into the reactor, and then sodium persulfate solutions were added to initiate the reaction. The final solution volumes were set at 100 mL. Aliquots of 1,000 μL were collected at selected time intervals. Methyl alcohol (MA), a common scavenger (the reaction kinetic constant of MA and SO4• or •OH are 1.6–7.7 × 107s−1 and 1.2–2.8 × 109 s−1 (Liang et al. 2013)), were mixed with the samples to quench the reaction. The solutions were immediately passed through a 0.22 μm membrane filter (Oh et al. 2009; Liu et al. 2014) for aniline analysis.

Different species and dosages of inorganic salts were dissolved in ultrapure water and mixed with aniline solution to investigate the effect of coexisting ions. Groundwater was also applied in the oxidized system to demonstrate the influence of anions in groundwater. Experiments were conducted in duplicate or triplicate. The data points without error bars are the mean of duplicate experiments. The error bars in the bar chart represent ± one standard deviation from the mean of triplicate data.

Analytical methods

The concentration of residual aniline in the aqueous phase was measured using high performance liquid chromatography (HPLC) (Agilent 1200, USA) equipped with a C18 column (150 mmol/L × 4.6 mmol/L, 5 μm) and UV detector at λ = 262 nm. The mobile phase was composed of acetonitrile and buffer salts (65/35, v/v) with a flow rate of 1.0 mL/min. pH was measured by a YSI pH100 multi-parameter tester (OH, USA).

RESULTS AND DISCUSSION

Effect of initial concentration of Fe2+ and persulfate

To determine effect of the Fe2+ dose on aniline oxidation, experiments were carried out at a fixed sodium persulfate concentration (4.4 mmol/L) at pH 7.0 and 20 °C. The results are shown in Figure 1(a). Without Fe2+, the degradation efficiency was only 19.05% over a 60 min reaction. With the addition of Fe2+ (1.1–3.3 mmol/L), the removal efficiency of aniline increased to 62.56–86.33%. The addition of more Fe2+ generated more SO4 to oxidize more aniline. However, the amount of SO4 did not increase further when the initial Fe2+ concentration reached 4.4 mmol/L (corresponding to a degradation ratio of 73.38%). When Fe2+ was added in excess, excess SO4 species were yielded, which may be directly scavenged by the remaining Fe2+ rather than oxidize aniline (Liang et al. 2009). 
formula
5
Figure 1

Effect of Fe2+ and persulfate concentration on aniline degradation: (a) Fe2+, [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 0–4.4 mmol/L; (b) persulfate, [Aniline]0 = 11 mmol/L; [Persulfate] = 0–5.5 mmol/L; [Fe2+] = 3.3 mmol/L.

Figure 1

Effect of Fe2+ and persulfate concentration on aniline degradation: (a) Fe2+, [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 0–4.4 mmol/L; (b) persulfate, [Aniline]0 = 11 mmol/L; [Persulfate] = 0–5.5 mmol/L; [Fe2+] = 3.3 mmol/L.

In addition, aniline degradation mainly occurred in the first 2 min of reaction. These results demonstrated that the activation of S2O82− by Fe2+ was a fast process, i.e., SO4 radical was generated instantly.

The effect of persulfate concentration was analyzed at 11 mmol/L aniline and 3.3 mmol/L Fe2+ at pH 7.0. The results are shown in Figure 1(b). Persulfate at 0, 2.2, 3.3, 4.4, and 5.5 mmol/L resulted in degradation efficiencies of 15.72, 80.68, 85.64, 86.33, and 82.76% in 60 min, respectively. When the persulfate concentration increased from 0–4.4 mmol/L, the removal efficiencies of aniline were enhanced to more than 65%. The highest degradation efficiency was 86.33% at 4.4 mmol/L persulfate. However, the aniline removal efficiency decreased when a higher concentration of persulfate (5.5 mmol/L) was applied. With excessive persulfate, a large quantity of SO4 was produced within a short time and quenched by themselves (Xu & Li 2010). Excessive S2O82− also resulted in competition for SO4 with the target contaminant (Lau et al. 2007). 
formula
6
 
formula
7
Under the condition in this study, the optimum Fe2+ and PS concentrations were 3.3 and 4.4 mmol/L, respectively, and this condition was applied in subsequent experiments.

Effect of different anions

To determine the effect of anions on aniline degradation by Fe2+-activated persulfate, five types of anions (HCO3, NO3, SO42−, CO32−, and PO43−) at different concentrations were added into the system. The results are shown in Figure 2. Figures 2(a)2(d) indicate that HCO3, SO42−, CO32−, and PO43− had negative effects on the degradation of aniline. Figure 2(e) indicates that the influence of NO3 on the degradation of aniline was not significant. As the anion concentration increased to 10 mmol/L, the degradation efficiencies of aniline decreased to 69.15, 59.83, 19.19, 17.32, and 82.47% for HCO3, SO42−, PO43−, CO32−, and NO3, respectively. However, the removal efficiency of aniline was 86.33% without any added anions in the solution.
Figure 2

Effect of common anions on aniline degradation by Fe2+ activated persulfate: (a) CO32−; (b) PO43−; (c) HCO3; (d) SO42−; and (e) NO3. [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 3.3 mmol/L.

Figure 2

Effect of common anions on aniline degradation by Fe2+ activated persulfate: (a) CO32−; (b) PO43−; (c) HCO3; (d) SO42−; and (e) NO3. [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 3.3 mmol/L.

As shown in Figure 2(a), the degradation of aniline decreased with increasing CO32− concentration. The aniline removal efficiencies ranged from 66.36% to 17.32% at 1 to 10 mmol/L CO32−, respectively. Previous studies reported that SO4• can react with H2O and OH to generate •OH at different pH levels (Liang et al. 2007). SO4• starts to convert into •OH quickly at the solution pH > 8.5. As the solution pH > 10.7, •OH will become the main radical in the reaction system (Dogliotti & Hayon 1967). The initial pH value was 11.4 in the reaction systems when we added 2 mmol/L CO32−. Thus, •OH was the major radical for aniline degradation. Radicals may be scavenged by the anions in solutions, and the formation of the corresponding anion radicals has also been demonstrated (Roshani & Karpel vel Leitner 2011). These corresponding anion radicals can oxidize organic compounds at different rates (Neta et al. 1988). Under this condition, CO32− is the radical quencher of •OH (Buxton et al. 1988). 
formula
8
Figure 2(a) also shows the effect of different dosages of CO32− on aniline degradation. At low CO32− concentration, CO3 may compete with •OH, which could lead to the decrease in the degradation efficiency of aniline because CO3 is a poorer oxidant than SO4• (Wu et al. 2014). When the concentration increased to 10 mmol/L, the anion was possibly converted into an insoluble compound with Fe2+ in solution, which resulted in the reduction in Fe2+ concentration. Given that Fe2+ was applied to activate the persulfate to generate •OH or SO4•, lower utilization of Fe2+ could reduce the quantity of radicals and then decrease the removal efficiency.
In reaction systems with PO43− (pH 11.6) and HCO3 (pH 8.1), the degradation efficiencies of aniline decreased from 86.33% to 55.30% and 72.49%, respectively, at an anion concentration of 2 mmol/L. For solutions containing PO43− and HCO3, the major radicals decomposing aniline were •OH and SO4•, respectively. Quenching of •OH or SO4• occurred and generated free radicals with lower oxidizing ability than •OH or SO4• (Neta et al. 1988). 
formula
9
 
formula
10
As shown in Figure 2(b), when the concentration of PO43− was low, such as 1 mmol/L, the degradation percentage of aniline decreased from 86.33 to 64.50%. At high PO43− concentration of 5 mmol/L, the removal efficiency of aniline was 19.72%. Besides the scavenging of •OH and SO4, Fe2+ and Fe3+ may form an insoluble compound with PO43−, such as FePO4 and Fe3(PO4)2, with respective solubility products (Ksp) of 4 × 10−27 and 1 × 10−36. The precipitation of Fe2+ resulted in low utilization of Fe2+ to activate persulfate and decreased the removal of aniline.

As shown in Figure 2(d), the degradation efficiency of aniline decreased from 72.49 to 59.83% as the concentration of SO42− increased from 1 to 10 mmol/L. The inhibitory effect of SO42− on aniline removal was slightly larger than that of HCO3 and much smaller than those of CO32− and PO43−. According to chemical equilibrium theory, the accumulation of reaction products inhibited the forward reaction. Based on Equation (4), the addition of SO42− inhibited the forward reaction between S2O82− and Fe2+ and reduced the generation of SO4•. Thus, the additional SO42− inhibited aniline removal. SO42− could not react with SO4• or •OH nor precipitate with Fe2+. Therefore, the inhibitory effect of SO42− was smaller than that of CO32− and PO43−.

The effect of NO3 on aniline degradation was not significant (Figure 2(e)). The removal efficiency of aniline was between 86.33 and 82.47% when the nitrate concentrations varied from 0 to 10 mmol/L. NO3 did not form insoluble compounds with Fe2+ or Fe3+, and the reaction between Fe2+ and persulfate was not suppressed. In addition, NO3 did not react with SO4• or •OH, so the removal efficiency of aniline was not inhibited.

In the Fe2+-activated persulfate system, coexisting anions inhibited the removal of aniline. As shown in Figure 3, different anions affected aniline degradation at different degrees. The order of inhibitory effect was PO43− > CO32− > >SO42−> HCO3 > >NO3. The removal efficiencies of aniline were 68.76, 66.76, 19.72, 24.65, and 82.42% with HCO3, SO42−, PO43−, CO32−, and NO3 at 5 mmol/L, respectively. Quenching of SO4 or •OH by PO43−, CO32−, and HCO3 generated radicals corresponding to these anions. These radicals had lower oxidizing ability than SO4• or •OH, resulting in the decline of the aniline removal. Besides that, PO43− and CO32− could also form insoluble compounds with Fe2+ (such as FeCO3 and Fe3(PO4)2) and lower the utilization of Fe2+, which lower the generation of SO4•. Hence, CO32− and PO43− had larger inhibitory effects than SO42−, HCO3, and NO3. According to chemical equilibrium theory, addition of SO42− inhibited the generation of SO4• and then decreased the removal of aniline in the system. NO3 did not undergo the reactions mentioned above, so the inhibitory effect of NO3 on the aniline removal was the smallest of all anions.
Figure 3

Effect of different anions on degradation of aniline at [Anions] = 5 mmol/L. [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 3.3 mmol/L.

Figure 3

Effect of different anions on degradation of aniline at [Anions] = 5 mmol/L. [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 3.3 mmol/L.

Effect of different cations

Four different types of cations (Na+, K+, Mg2+, and Ca2+) at two concentrations (10 and 50 mmol/L) were added in the reaction system. Figure 4 shows that these cations had no significant effects on aniline degradation. Although the concentration of added cations increased to 10 and 50 mmol/L, the removal efficiency of aniline deceased only less than 5%. This finding indicated that all these cations could not activate persulfate to produce the SO4•, or inhibited the yields of SO4• and •OH. These results were consistent with the findings of previous studies, which demonstrated that persulfate can be activated only by transition metal ions (Men+) to form SO4• (Wu et al. 2014).
Figure 4

Effect of common cations on degradation of aniline. [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 3.3 mmol/L.

Figure 4

Effect of common cations on degradation of aniline. [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 3.3 mmol/L.

Aniline removal in groundwater

Experiments were conducted to study the degradation of aniline in real groundwater and ultrapure water. Properties of the groundwater sample are: pH: 7.24; total dissolved solids (TDS): 830 mg/L; CO32−: 0.2 mmol/L; HCO3: 1.6 mmol/L; SO42−: 1.74 mmol/L; Cl: 1.58 mmol/L; and NO3: 0.62 mmol/L. Figure 5 shows the comparison of Fe2+-activated persulfate oxidation of aniline in groundwater and ultrapure water at 20 °C. It can be seen that aniline removal efficiency in groundwater was 53.05% within 60 min which was much lower than that in ultrapure water (86.33%). The anions existed in groundwater, such as CO32−, HCO3 and SO42−, hindered the degradation of aniline in Fe2+-activated persulfate system. When the concentrations of CO32−, HCO3 and SO42− were 1 mmol/L, 2 mmol/L and 2 mmol/L in ultrapure water, the aniline removal efficiency reduced tp 19.97, 13.85 and 17.18% respectively. Comparatively, although the concentration of each anion in groundwater was lower than that in the ultrapure water, the total concentration was higher, and the inhibition influence of the anions on aniline removal efficiency was enhanced to 33.28%.
Figure 5

Comparison of degradation of aniline in groundwater and ultrapure water. [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 3.3 mmol/L.

Figure 5

Comparison of degradation of aniline in groundwater and ultrapure water. [Aniline]0 = 11 mmol/L; [Persulfate] = 4.4 mmol/L; [Fe2+] = 3.3 mmol/L.

CONCLUSIONS

Aniline degradation was investigated via Fe2+-activated persulfate under different ion conditions. The major conclusions included the following. (1) In the aqueous solution system, the highest removal efficiency of aniline (C0 = 11 mmol/L) reached 83.66% within 60 min. The degradation efficiency decreased by further addition of either persulfate or Fe2+. (2) An inhibitory effect was found with the addition of CO32−, PO43−, SO42−, HCO3, and NO3. The order of the inhibition was PO43− > CO32− > >SO42− > HCO3 > >NO3. The reasons included: (i) quenching of SO4• or •OH by CO32−, PO43−, SO42−, and HCO3 generated free radicals with lower oxidizing ability than SO4• and •OH, which decreased the removal efficiency of aniline; (ii) given that CO32− and PO43− could form insoluble compounds with Fe2+, which further decreased the aniline degradation efficiency; (iii) according to chemical equilibrium theory, addition of SO42− may retard the generation of SO4•, thereby reducing the removal of aniline; and (iv) the inhibition of NO3 on aniline removal was negligible. (3) Cations (Na+, K+, Mg2+, Ca2+) common in groundwater also had minimal and negligible effects on the degradation efficiency of aniline.

ACKNOWLEDGEMENTS

This study was funded by Specific Research of the Environmental Nonprofit Research, No. 2013A073 and the National Natural Science Foundation of China, No. 41302184, as well as interdisciplinary research project of Jilin University.

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