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−.
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).
MATERIALS AND METHODS
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.
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
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.
Effect of different anions
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.
Effect of different cations
Aniline removal in groundwater
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.
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.