Environ Sci Pollut Res DOI 10.1007/s11356-014-2668-3
RESEARCH ARTICLE
Degradation of 1,4-dioxane in water with heat- and Fe2+-activated persulfate oxidation Long Zhao & Hong Hou & Ayuko Fujii & Masaaki Hosomi & Fasheng Li
Received: 12 August 2013 / Accepted: 19 February 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract This research investigated the 1,4-dioxane (1,4-D) degradation efficiency and rate during persulfate oxidation at different temperatures, with and without Fe2+ addition, also considering the effect of pH and persulfate concentration on the oxidation of 1,4-D. Degradation pathways for 1,4-D have also been proposed based on the decomposition intermediates and by-products. The results indicate that 1,4-D was completely degraded with heat-activated persulfate oxidation within 3–80 h. The kinetics of the 1,4-D degradation process fitted well to a pseudo-first-order reaction model. Temperature was identified as the most important factor influencing the 1,4-D degradation rate during the oxidation process. As the temperature increased from 40 to 60 °C, the degradation rate improved significantly. At 40 °C, the addition of Fe2+ also increased the 1,4-D degradation rate. Interestingly, at 50 and 60 °C, the 1,4-D degradation rate decreased slightly with the addition of Fe2+. This reduced degradation rate may be attributed to the rapid conversion of Fe2+ to Fe3+ and the production of an Fe(OH)3 precipitate which limited the ultimate oxidizing capability of persulfate with Fe2+ under higher temperatures. Higher persulfate concentrations led to higher 1,4-D
Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-2668-3) contains supplementary material, which is available to authorized users. L. Zhao : H. Hou (*) : F. Li State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Dayangfang 8, Beijing 100012, People’s Republic of China e-mail: [email protected] A. Fujii : M. Hosomi Department of Chemical Engineering, Faculty of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan
degradation rates, but pH adjustment had no significant effect on the 1,4-D degradation rate. The identification of intermediates and by-products in the aqueous and gas phases showed that acetaldehyde, acetic acid, glycolaldehyde, glycolic acid, carbon dioxide, and hydrogen ion were generated during the persulfate oxidation process. A carbon balance analysis showed that 96 and 93 % of the carbon from the 1,4-D degradation were recovered as by-products with and without Fe2+ addition, respectively. Overall, persulfate oxidation of 1,4-D is promising as an economical and highly efficient technology for treatment of 1,4-D-contaminated water. Keyword 1,4-Dioxane . Persulfate . Ferrous ion . Arrhenius equation . Degradation pathways
Introduction 1,4-Dioxane (1,4-diethylene dioxide, C4H8O2, referred to as 1,4-D) is an organic compound that is used as a solvent in a wide range of industrial organic products (e.g., paints, varnishes, inks, and dyes). It is also present as a by-product in many consumer products (e.g., cleaning products, cosmetics, shampoos, and laundry detergents) (Stickney et al. 2003). The improper disposal of industrial waste and accidental solvent spills have resulted in 1,4-D contamination of subsurface water. The United States Environmental Protection Agency (USEPA) and the International Agency for Research on Cancer (IARC) have classified 1,4-D as a Class 2B carcinogen (INCHEM website 1999; IARC 1999). The tolerable limit for 1,4-D in drinking water is currently 50 μg L−1 based on draft guidelines proposed by the World Health Organization (WHO) (World Health Organization 2005). Because of its effect on both the environment and human health, 1,4-D has been extensively investigated.
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Common remedial technologies such as adsorption and air stripping are not cost effective for removal from 1,4-D-contaminated water because 1,4-D has a high water solubility (4.31×105 mg L−1), low octanol-water partition coefficient (log Kow =−0.27), and low vapor pressure (37 mmHg at 25 °C) (Lesage et al. 1990; Zenker et al. 2003). Further, 1,4D is recalcitrant to microbial degradation because of its heterocyclic structure (Adams et al. 1994), so that conventional water treatment techniques are limited in their application for the treatment of 1,4-D-contaminated water. Advanced oxidation processes (AOPs) have aroused considerable interest as alternatives for treating 1,4-Dcontaminated water, with processes such as TiO2-based photocatalytic oxidation (Coleman et al. 2007), Fentonlike degradation (Son et al. 2009), oxidative reactions using sonication (Beckett and Hua 2003; Son et al. 2006), and the use of ozone or hydrogen peroxide (O3/H2O2) (Suh and Mohseni 2004). Although some AOP technologies have been highly effective in the removal of 1,4-D from water (Zenker et al. 2003), most AOPs are not cost effective because of equipment capacities, operating costs that involve the consumption of expensive chemicals such as O3 and H2O2, and expensive unit processes such as continuous UV irradiation (Stefan and Bolton 1998; Adams et al. 1994; Maurino et al. 1997). Sodium persulfate has drawn increasing attention as an alternative oxidant in the chemical oxidation of contaminants (Huang et al. 2002; Huang et al. 2005; Liang et al. 2003; Liang et al. 2007) as it has several potential advantages for water treatment. The persulfate anion (S2O82−) is a strong oxidizing agent with a redox potential of 2.01 V, which is comparable to ozone (E0 =2.07 V) or hydrogen peroxide (E0 =1.78 V) (Huang et al. 2002). Persulfate has a number of advantages over other oxidants, as it is a solid at room temperature, can be easily stored and transported, is stable, has high stability and high aqueous solubility, and is relatively low cost (Lau et al. 2007). Persulfate can also be reduced to sulfate anions as shown in Eq. (1). These features make persulfate a promising choice for environmental remediation applications. Persulfate oxidation at room temperature is not usually very effective, so persulfate is commonly used with UV light or under high temperatures to initiate/enhance its radical oxidation mechanisms. Sulfate radicals (SO4−▪), which have a higher redox potential of 2.6 V and can be formed through photolytic reactions or the heat decomposition of persulfate (Eq. (2)), are capable of decomposing most organic contaminants (Huang et al. 2005). The decomposition of S2O82− in the presence of a transition metal activator (e.g., Fe2+) also leads to the formation of SO4−▪. The overall reaction between S2O82− and Fe2+ is described in Eq. (3) (House 1962). Furthermore, hydroxyl radicals (OH▪) with a redox potential of
2.8 V can be also produced by the reaction between SO4−▪ and water, as shown in Eq. (4). − 2 S2 O2− 8 þ 2e →2SO4
ð1Þ
−▪ S2 O2− 8 þ heat→2SO4
ð2Þ
2− Fe2þ þ S2 O8 2− →Fe3þ þ SO−▪ 4 þ SO4
ð3Þ
▪ −▪ SO−▪ 4 þ H2 O→HO þ HSO4
ð4Þ
In recent years, the persulfate oxidation process has been applied to the degradation of various organic pollutants (Huang et al. 2002; Liang et al. 2004b, 2008; Xu and Li 2010; Rastogi, et al. 2009), showing that it can mineralize organic compounds and allow them to reach very low concentrations at low cost. Consequently, persulfate oxidation is expected to be a promising alternative to conventional 1,4-D degradation technologies. However, the degradation of 1,4-D using persulfate oxidation has not been investigated to date. Therefore, this study was conducted to examine (1) the rate and efficiency of 1,4-D degradation by heat- and Fe2+-activated persulfate oxidation under different experimental conditions, (2) the effect of pH and various persulfate concentrations on the oxidation of 1,4-D, and (3) the probable degradation pathways of 1,4-D by persulfate based on identified decomposition intermediates and by-products.
Materials and methods Materials A 1,000 mg L−1 1,4-D stock solution was prepared by dissolving reagent grade 1,4-D (99.5 %, Wako Pure Chemical Industries, Ltd., Osaka, Japan) in nanopure deionized water (R = 18 MΩ cm−1) from a Millipore Milli-Q system. A 100 mg L−1 solution was prepared by diluting the stock solution, and this was used in all experiments. Sodium persulfate (Na2S2O8, 98 %) and ferrous sulfate heptahydrate (FeSO4 ·7H2O, 99 %) were purchased from Junsei Chemical Industries, Ltd. (Tokyo, Japan) and Kanto Chemical Industries, Ltd. (Tokyo, Japan), respectively.
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Experimental procedures Batch experiments were performed in graduated, stoppered vials (75 mL). To evaluate the degradation of 1,4-D in the presence of sodium persulfate concentration, 48 mL of the reaction solution, prepared in pure water, was placed in the 75-mL test vial and held overnight in a constant temperature incubator at 40, 50, or 60 °C. Then, 1 mL of 1,4-D solution (100 mg L−1) and 1 mL of Na2S2O8 solution (mainly at 5,000 mg L−1 but some trials at 1,250 or 2,500 mg L−1) were added to make a total volume of 50 mL. For the experiments in the presence of Fe2+, 0.25 g of FeSO4 ·7H2O was also added. Then, all the vials were sealed with butyl alcohol and an aluminum cap, shaken for 5 min. Finally, the vials were returned to the incubator again at 40, 50, or 60 °C, which allowed the reaction to begin. A control test was also conducted at 40 °C without Na2S2O8 addition. At various time intervals, 1-mL samples were taken from the vials using a syringe and then filtered through a 0.45-μm membrane filter before analysis. After each experiment, the loss in volume of the reaction solution was checked, and the concentration of 1,4D during the oxidation process was not affected. The headspace/gas phase in the vials were also sampled via a syringe and analyzed. The overall experimental scheme is shown in Fig. S1. A range of experiments were conducted to investigate the effects of various parameters on the 1,4-D degradation efficiency and rate. The experimental conditions for Runs A to O are shown in Table 1. To determine the effect of temperature on the degradation efficiency and rate of 1,4-D, three different temperatures, 40, 50, and 60 °C controlled by an incubator were used in Runs A to C. Runs D to F examined the effects of Fe2+ at the same three temperatures on the degradation efficiency and rate of 1,4-D. Additional experiments were conducted with pH adjustment (Run G) and various persulfate concentrations (Runs H to O) to examine the influence of pH and persulfate concentration on the 1,4-D degradation efficiency and rate. Analysis of 1,4-dioxane and intermediates The concentration of 1,4-D and major intermediates in the reaction solution was determined by gas chromatography (GC) using an HP-1 capillary column (Agilent, 60-m length×0.32-mm ID×5-μm film) equipped with a flame ionization detector (FID) (GC-2014, SPL-2014, Shimadzu, Japan) and a mass selective detector (HP 5973). The GC was operated with the following conditions: 1-μL samples were injected with a split ratio of 10:1 and an inlet temperature of 250 °C; the flow rate was constant at 2.22 mL min−1 with He as the carrier gas; the oven temperature program started at 40 °C for 5 min, then ramped to 150 °C at 10 °C min−1, held at
150 °C for 5 min. The detector temperatures were maintained at 250 °C. A calibration curve was established using 1,4-D solutions ranging from 0.01 to 100 mg L−1, and this showed a high linear correlation coefficient (R2 =0.997), with limits of detection and quantification at 0.0036 and 0.01 mg L−1, respectively. The headspace gas sampled during the experiments was analyzed on a GC (GC-8A, Shimadzu, Japan) equipped with a thermal conductivity detector (TCD). A packed column (SUS, 2 m, OD 4 Φ, ID 3 Φ, active carbon, mesh 60/80) was used at a constant temperature of 110 °C with 600 μL of each sample injected. The injector and detector temperatures were both set at 100 °C. Argon was used as the carrier gas, and the injector pressure was 160 kPa. The inorganic carbon (IC) and non-purgeable organic carbon (NPOC) in the reaction solution were measured using an ultraviolet persulfate oxidation total organic carbon (TOC, Shimadzu, Japan) analyzer with an infrared detector (Phoenix 8000TM, TekMar Dohrmann, USA). A pH meter (Orion, Model 52A, USA) was used to determine pH. Selected experiments were conducted in duplicate or triplicate, and the data were averaged.
Results and discussion Heat-activated persulfate oxidation of 1,4-D under different temperatures Temperature has been shown to be the most important parameter influencing the degradation rate of organic pollutants by persulfate (Huang et al. 2002, 2005; Oh et al. 2009). In this study, 1,4-D oxidation by persulfate under different temperatures was evaluated, and the results are presented in Fig. 1a. At 40 °C, 1,4-D was completely oxidized by persulfate over 80 h. In contrast, the 1,4-D concentration measured during the control experiment (without persulfate) did not change relative to the initial concentration. As the temperature increased, the degradation rate of 1,4-D by persulfate increased significantly, with complete oxidation of 1,4D by persulfate observed in 8 and 3 h at 50 and 60 °C, respectively. These results indicate that, after heating, persulfate can be effectively activated to SO 4 − ▪ (Eq. (2)), leading to the increase in 1,4-D oxidation at elevated temperatures. These results are consistent with the persulfate oxidation of polyvinyl alcohol and aniline reported by Oh et al. (2009) and Xie et al. (2012), respectively. Statistically, the oxidation of 1,4-D fitted well to a pseudofirst-order kinetic reaction model, as shown in Fig. 1b and Eq. (5) with correlation coefficient R2 values of 0.98, 1.0, and 0.97 at 40, 50, and 60 °C, respectively. The oxidation of 1,4-D
Environ Sci Pollut Res Table 1 Experimental conditions and pseudo-first-order parameters in the heat- and Fe2+-activated persulfate oxidation of 1,4-dioxane (1,4-D) in different runs Run
Experimental conditions
pHinitial
1,4-D mg L−1
S2O82− mg L−1
Fe2+ mg L−1
S2O82−-/Fe2+ ratio
Temperature ºC
Reaction timea
A B C D E F
100
5,000
0 0 0 1,000 1,000 1,000
50/0b 50/0 50/0 5 5 5
40 50 60 40 50 60
80 8 3 51 9 4
2.81 2.86 2.89 2.30 2.31 2.31
G H I J K L M N O
100 100
5,000 1,250
0 0 1,000 0 1,000 0 1,000 0 1,000
50/0 12.5/0 1.25 12.5/0 1.25 25/0 2.5 25/0 2.5
60 50 50 60 60 50 50 60 60
3 74 –e 11 –e 28 –e 5 –e
2.30c NAd NA NA NA NA NA NA NA
a
2,500
pHfinal
Pseudo-first-order parameters Rate constant K h−1
Correlation coefficient R2
2.33 2.20 2.31 2.02 2.04 2.05
0.062 0.68 2.2 0.10 0.60 1.5
0.98 1.0 0.97 0.98 0.99 0.97
1.71 NA NA NA NA NA NA NA NA
1.9 0.073 – 0.47 – 0.19 – 1.0 –
0.97 1.0 – 0.96 – 0.99 – 0.99 –
Time in hours, from the beginning of 1,4-D oxidation to its complete degradation
b
Concentration ratios
c
Adjusted pH prior to oxidation
d
Not available
e
Not determined as 1,4-D could not be completely degraded
by heat-activated persulfate is shown as Runs A to C in Table 1. C ¼ C 0 expð−ktÞ
ð5Þ
where C is the concentration of 1,4-D at time [t], C0 is the initial concentration of 1,4-D, and k is the pseudo-first-order reaction rate constant, which is obtained from the slope of the line after plotting ln(C0 /C) against time (Xu and Li 2010). The reaction rate constants for 1,4-D increased with increased temperature, with values of 0.062, 0.68, and 2.2 at 40, 50, and 60 °C, respectively. It appears that the degradation rate of 1,4D by persulfate at 40 °C was approximately an order of magnitude lower than at 50 °C, while the degradation rate of 1,4-D at 60 °C was nearly three times higher than at 50 °C. Using the pseudo-first-order rate constants at different temperatures, the apparent activation energy for 1,4-D degradation by persulfate was calculated using the Arrhenius equation (Eq. (6)). k ¼ Aexpð−E=RT Þ
ð6Þ
where A is the pre-exponential (or frequency) factor, E is the apparent activation energy (J mol−1), R is the ideal gas constant (8.314 J mol−1 K−1), and T is the reaction temperature
(K). A good linear relationship was obtained from the Arrhenius plot of ln (k) versus 1,000/T (Fig. 2a). The apparent activation energy was calculated to be 155 kJ mol−1 (R2 = 0.94), which indicates that the 1,4-D degradation in aqueous persulfate solution requires only moderate activation energy, as the activation energy for the thermal reaction has a normal range of 60–250 kJ mol−1 (Mortimer 2008). Xu and Li (2010) determined that a relatively low activation energy of 92.2 kJ mol−1 was required to degrade the azo dye Orange G. Liang et al. (2004a, 2007) reported activation energies of around 108–130 kJ mol−1 for the degradation of trichloroethylene and 1,1,1-trichloroethane. The different activation energies may be related to the chemical structures of the different target compounds.
Iron(II)-activated persulfate oxidation of 1,4-D at different temperatures Iron(II) is one of the main species able to catalyze persulfate to produce SO4−▪, as described in Eq. (3), and its presence can significantly enhance the degradation efficiency of pollutants (Anipsitakis and Dionysiou 2004; Oh et al. 2009). The 1,4-D oxidation by Fe2+-activated persulfate under different temperatures is shown in Fig. 1c. As the experiments with Fe2+, the
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a
c
b
d
Fig. 1 Effect of temperature on 1,4-dioxane oxidation by heat-activated persulfate in the absence and presence of Fe2+
1,4-D degradation rate by Fe2+-activated persulfate oxidation increased with increasing temperature. At 40 °C, 1,4-D was completely oxidized by Fe2+-activated persulfate in 51 h, while the control test (Fe2+ without persulfate) did not change the 1,4-D concentration. As the temperature increased to 50 and 60 °C, complete oxidation of 1,4-D by Fe2+-activated persulfate was observed in 9 and 4 h, respectively (Runs D– F, Table 1). Interestingly, the addition of Fe2+ significantly decreased the degradation time for all the 1,4-D being removed at 40 °C, compared to the experiments without Fe2+ (Fig. 1a, c), but a slight increase in degradation time was observed at 50 and 60 °C. It appears that more SO4−▪ is generated by Fe2+-activated persulfate (via Eq. (3)) as reported by Kolthoff et al. (1951) and House (1962), and this increases the 1,4-D degradation rate at 40 °C. As the added concentrations of Fe 2+ change from 1,000 mg L −1 to 500 and 1,500 mg L−1, the degradation rate of 1,4-D from water was not significantly varied. It is indicated that the Fe2+ concentration seems not the key factor during this oxidation process of 1,4-D. The corresponding mechanism of this result needs to be further studied.
The effect of Fe2+-activated persulfate on the 1,4-D degradation rate was influenced by the higher temperatures at 50 and 60 °C. This might occur as the higher temperature can accelerate the reaction between Fe2+ and SO4−▪, forming Fe3+, and the Fe3+ then reacts with OH− to form Fe(OH)3 precipitate which might inhibit the oxidation of 1,4-D by SO4−▪. This possible reaction scheme is described in Eqs. (7) and (8), below. Besides ferric hydroxide, there were no other sulphate salts confirmed. 3þ Fe2þ þ SO−▪ þ SO2− 4 →Fe 4
ð7Þ
Fe3þ þ 3OH− →FeðOHÞ3
ð8Þ
This difference can be seen in the color of the 1,4-D during oxidation solutions under different experimental conditions. As shown in Fig. S2b, the color of 1,4-D oxidation solution at 60 °C was much darker than at 40 °C, indicating that more
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2
Variation of pH before and after reaction 2+
Fe addition 2+ No Fe addition
1
a 0 y=-18.7x+57.0
lnk
2
R =0.94
-1 y=-14.2x+43.1 2
R =0.94
-2 -3 2.95
3.00
3.05
3.10
3.15
3.20
3.25
-1
1000/T [K ] 1 No pH adjustment pH adjustment
0
b
ln (C/C0)
-1 -2
K=1.9 2 R =0.97
-3 K=2.2
-4
2
R =0.97
-5 0.0
0.5
1.0
1.5
2.0
2.5
Time [hours] Fig. 2 Arrhenius plot of 1,4-dioxane oxidation between 40 and 60 °C with and without Fe2+ and the effect of pH on the oxidation of 1,4dioxane by persulfate
The measured initial and final pH values for the oxidation of 1,4-D using heat- and Fe2+-activated persulfate (Runs A to F) are shown in Table 1. During the 1,4-D degradation process, the pH values in Runs A to F decreased by 0.3–0.7 pH units because of the production of organic acid or removal of OH▪ to form Fe(OH)3 during 1,4-D degradation. The initial and final pH values of Runs A, B, and C were slightly higher than for Runs D, E, and F, due to the Fe2+ addition in Runs D to F. This suggests that more sulfuric acid could be produced as described in Eq. (7) because of the Fe2+ in the reaction solution. There was no significant pH variation with the different temperatures (Table 1), as the initial pH values were almost the same for Runs A to C (2.81–2.89) and Runs D to F (2.30–2.31). Thus, the pH values were not temperature dependent in the persulfate oxidation of 1,4-D with heat and Fe2+ activation. To determine whether the 1,4-D degradation efficiency and rate were affected by changing the pH, a trial at 60 °C without Fe2+ addition was adjusted to a pH of 2.30 before oxidation (Run G), which is the same pH as in the Fe2+ addition experiments (Runs D to F). The 1,4-D degradation with pH adjustment also fits the pseudo-first-order reaction model as shown in Fig. 2b. There was no significant difference in the 1,4-D degradation rate with and without pH adjustment, as the 1,4-D was completely degraded within 3 h in both conditions, and the pseudo-first-order rate constants in Runs C and G were 2.2 and 1.9, respectively (Table 1). These results show that pH adjustment is not necessary for the heat-activated persulfate oxidation of 1,4-D under the experimental conditions investigated. Effect of persulfate concentration
Fe(OH)3 precipitate was present in the reaction solution. This Fe(OH)3 could affect the oxidation of 1,4-D by Fe2+-activated persulfate, and thus temperature also has an important role in 1,4-D degradation by Fe2+-activated persulfate. The oxidation of 1,4-D by Fe2+-activated persulfate also fitted well to a pseudo-first-order kinetic reaction model (Fig. 1d and Eq. (5)) as described above. The statistical correlations with the pseudo-first-order model gave correlation coefficient R2 values of 0.98, 0.99, and 0.97 at 40, 50, and 60 °C, respectively. As with the oxidation of 1,4-D without Fe2+ addition, the pseudo-first-order reaction rate constants (k) with Fe2+-activated persulfate conditions also increase with increasing temperature. Based on the pseudo-first-order rate constants, the activation energy for 1,4-D oxidation by Fe2+activated persulfate was calculated as 117 kJ mol−1 (R2 =0.94) (Fig. 2a). This activation energy, lower than that for 1,4-D oxidation without Fe2+ addition (155 kJ mol−1), implies that the reaction was more easily to occur.
Figure 3 shows the overall degradation rate for 1,4-D against time at 60 °C with different persulfate concentrations with and without Fe2+ addition. In the absence of Fe2+ (Fig. 3a), the 1,4D was completely degraded within a maximum of 11 h. The persulfate concentrations were positively correlated with the oxidation rate, and as the persulfate concentrations increased from 1,250 mg L−1 to 2,500 and 5,000 mg L−1, the reaction rates approximately doubled with each concentration increase. The experimental conditions and results are shown as Runs C, J, and N in Table 1. These results indicate that the concentration trend was consistent with the reaction temperature trend as increasing these different parameters both improved the degradation rate of 1,4-D during persulfate oxidation. In the presence of Fe2+ (Fig. 3b), 1,4-D was not completely oxidized when the persulfate concentration was 2,500 mg L−1 or lower, as shown in Runs F, K, and O in Table 1. As shown in Fig. 3b, the concentration of 1,4-D decreased to 50– 60 mg L−1 within 20 h and remained in that range thereafter.
Environ Sci Pollut Res 80
a
100
60 -1
80
2+
No Fe addition
60
2+
IC(Fe addition) 2+ NPOC(Fe addition) IC NPOC
70
-1
1250mg L -1 2500mg L -1 5000mg L
Concentration [mg L ]
-1
1,4-dioxane concentrations [mg L ]
120
40 20
50 40 30 20 10
0
0
2
4
6
8
10
12
0 0
Time [hours]
1
2
3
4
Time [hours]
b
-1
1,4-dioxane concentrations [mg L ]
100
Fig. 4 Effect of Fe2+ on the inorganic carbon (IC) and non-purgeable organic carbon (NPOC) concentrations during 1,4-dioxane oxidation
-1
2500 mg L -1 5000 mg L
80
Degradation pathways and by-products of 1,4-D
2+
Fe addition
60
40
20
0
0
20
40
60
80
100
120
140
160
Time [hours]
Fig. 3 Effect of persulfate concentration on 1,4-dioxane oxidation with and without Fe2+
This indicates that Fe2+ may act as a scavenger for the SO4−▪ in accordance with Eq. (7), limiting the oxidation of 1,4-D in the presence of excess Fe2+. The S2O82−/Fe2+ ratio was 5:1 when 5,000 mg L−1 persulfate was added (Table 1), but this ratio decreased to 2.5:1 and 1.25:1 as the persulfate concentrations decreased to 2,500 and 1,250 mg L−1, respectively. It is speculated that the incomplete degradation of 1,4-D may be due to the scavenging of SO4−▪ in the presence of excess of Fe2+. Similar results were reported by Liang et al. (2008) during the persulfate degradation of benzene, toluene, ethylbenzene, and xylene in the presence of Fe2+. Similar trends were observed for the degradation of 1,4-D under different persulfate concentrations with and without Fe2+ addition when the reaction temperature was 50 °C, with incomplete degradation of 1,4-D observed for persulfate concentrations decreasing from 5,000 mg L-1 to 2,500 and 1,250 mg L-1 in the presence of Fe2+. The experimental conditions are shown as Runs B, E, H, I, L, and M in Table 1. From these results, an optimum persulfate concentration of 5,000 mg L−1 was identified.
During the degradation of 1,4-D at 60 °C with and without Fe2+, the concentrations of inorganic carbon (IC) and nonpurgeable organic carbon (NPOC) in the reaction liquid were measured by a TOC analyzer. As shown in Fig. 4, there were no significant differences in the concentrations of IC and NPOC with and without Fe2+. The major intermediates were identified as glycolaldehyde, glycolic acid, oxalic acid, acetaldehyde, and acetic acid, while CO2 and H2 were identified in the headspace. Carbon balance studies showed that 30 and 16 % of the 1,4-D were mineralized to CO2 by persulfate in the presence and absence of Fe2+, respectively (Table 2). The IC and NPOC accounted for 7.3 and 59 %, respectively, of the carbon balance with Fe2+ addition and 6.6 and 70 % of the carbon balance without Fe 2+ addition, respectively.
Table 2 Carbon balance during 1,4-dioxane degradation process Phase
Liquid phase
Gaseous phase
By-products
Glycolaldehyde (NPOCa) Glycolic acid (NPOC) Oxalic acid (NPOC) Acetaldehyde (NPOC) Ethylic acid (NPOC) ICb CO2 H2
Total by-products a
Non-purgeable organic carbon
b
Inorganic carbon
Carbon balance [%] Fe2+ addition
No Fe2+ addition
59
70
7.3 30 – 96
6.6 16 – 93
Environ Sci Pollut Res Fig. 5 Proposed degradation pathways for the mineralization of 1,4-dioxane
O
SO4-
O
O
HO
O
1,4-dioxane
α–oxyl radical
a
1,4-dioxanyl radical Ethylene glycol
HO
OH
b H
OH
H+ Glycoaldehyde
HO
O
H+ H
O
OH
Glycolic acid
HO
O
Ethanol
Acetaldehyde
OH H
O
Acetic acid
OH
Glyoxylic acid
O
O
OH
OH
O
O
CO2
Oxalic acid
Statistically, the total carbon balances with and without Fe2+ addition were 96 and 93 %, respectively, which indicates that Fe2+ may have a role in accelerating the mineralization of 1,4-D during persulfate oxidation. From the analytical results, we propose two degradation pathways for 1,4-D by heat- and Fe2+-activated persulfate. In both pathways, the 1,4-D is first attacked by SO4−▪ and ▪ OH forming the 1,4-dioxanyl radical. This reacts with oxygen forming peroxyl radicals, which are transformed into α-oxyl radical as the precursor of the primary intermediates (Choi et al. 2010). Following two different routes (a and b), this α-oxyl radical is either turned into glycolaldehyde and glycolic acid, or acetaldehyde and acetic acid by an intramolecular reaction and several oxidation steps. Finally, intermediates such as oxalic acid and acetic acid are mineralized to CO2 via common cellular metabolic pathways. The proposed degradation pathways are shown in Fig. 5. One of the proposed degradation pathways (route b) was also reported by Mahendra et al. (2007) for the bioremediation and electrochemical AOP of 1,4-D. The other degradation pathway (route a) has not previously been reported in the literature. There may be some differences in the reaction mechanisms of 1,4-D between heat-/Fe2+-activated persulfate oxidation and other oxidation processes. Further study is needed to examine the degradation mechanisms in more depth. Nevertheless, heat- and Fe2+-activated persulfate oxidation of 1,4-D via these pathways is not expected to cause an accumulation of toxic compounds in the environment.
Conclusion In this study, the 1,4-D degradation efficiency and rate by heat- and Fe2+-activated persulfate oxidation were investigated under different experimental conditions. The effects of pH adjustment and different persulfate concentrations were also examined. The results showed that 1,4-D was completely degraded in an aqueous solution within 3–80 h under different temperatures. Increasing the temperature generally increased the degradation rate because of the increased production of free radicals. With the addition of Fe2+, a much faster 1,4-D degradation rate was observed at 40 °C compared to the equivalent trial without Fe2+. However, a slower 1,4-D degradation rate was observed when the reaction temperature increased to 50 and 60 °C, possibly due to the production of Fe(OH)3 precipitate which was enhanced at higher temperatures. There is no need to adjust the pH prior to persulfate oxidation, since no significant difference in 1,4-D degradation rate was observed with or without pH adjustment. The optimal persulfate concentration was determined to be 5,000 mg L−1. Decreasing the persulfate concentration reduced the degradation rate of 1,4-D in the absence of Fe2+ and resulted in incomplete oxidation of 1,4-D in the presence of Fe2+. Two degradation pathways were proposed for 1,4-D. Acetaldehyde, acetic acid, glycolaldehyde, and glycolic acid were confirmed as the major intermediates, and these were converted to carbon dioxide and hydrogen ion as final degradation products. Further studies, including scaling up of the system will help establish a novel, economical, and high
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oxidation efficiency AOP technology using persulfate for removal of 1,4-D from water. Acknowledgments This study was supported by Scientific Research Funding from the Ministry of the Environment, Japan, the National Natural Science Foundation of China (Grant No. 41301342), and the Special Environmental Protection Foundation for Public Welfare Project of China (201009032, 2007KYYW03).
References Adams CD, Scanlan PA, Secrlat ND (1994) Oxidation and biodegradability enhancement of 1,4-dioxane using hydrogen peroxide and ozone. Environ Sci Technol 28:1812–1818 Anipsitakis GP, Dionysiou DD (2004) Radical generation by the interaction of transition metals with common oxidants. Environ Sci Technol 38:3705–3712 Beckett MA, Hua I (2003) Enhanced sonochemical decomposition of 1, 4-dioxane by ferrous iron. Water Res 37:2372–2376 Choi JY, Lee YJ, Shin J et al (2010) Anodic oxidation of 1,4-dioxane on boron-doped diamond electrodes for wastewater treatment. J Hazard Mater 179:762–768 Coleman HM, Vimonses V, Leslie G et al (2007) Degradation of 1,4dioxane in water using TiO2 based photocatalytic and H2O2/UV processes. J Hazard Mater 146:496–501 House DA (1962) Kinetics and mechanism of oxidations by peroxydisulfate. Chem Rev 62:185–203 Huang KC, Couttenye RA, Hoag GE (2002) Kinetics of heat-assisted persulfate oxidation of methyl tert-butyl ether (MTBE). Chemosphere 49:413–420 Huang KC, Zhao Z, Hoag GE et al (2005) Degradation of volatile organic compounds with thermally activated persulfate oxidation. Chemosphere 61:551–560 IARC (1999) Monograph on 1,4-Dioxane. International Agency for Research on Cancer. Lyon, France INCHEM Website (1999) Summaries & Evaluations 1,4-dioxane (Group 2B). http://www.inchem.org/documents/iarc/vol71/019-dioxane. html Kolthoff IM, Medalia AI, Raaen HP (1951) The reaction between ferrous iron and peroxides. IV. Reaction with potassium persulfate. J Am Chem Soc 73:1733–1739 Lau TK, Chu W, Graham NJD (2007) The aqueous degradation of butylatedhydroxyanisole by UV/S2O8-2: study of reaction mechanisms via dimerization and mineralization. Environ Sci Technol 41: 613–619 Lesage S, Jackson RE, Priddle MW et al (1990) Occurrence and fate of organic solvent residues in anoxic groundwater at the Gloucester landfill, Canada. Environ Sci Technol 24:559–566 Liang C, Bruell CJ, Markey MC et al (2003) Thermally activated persulfate oxidation of trichloroethylene (TCE) and 1,1,1-trichloroethane
(TCA) in aqueous systems and soil slurries. Soil Sediment Contam 12:207–228 Liang C, Bruell CJ, Marley MC et al (2004a) Persulfate oxidation for in situ remediation of TCE. I. Activated by ferrous ion with and without a persulfate–thiosulfate redox couple. Chemosphere 55:1213–1223 Liang C, Bruell CJ, Marley MC et al (2004b) Persulfate oxidation for in situ remediation of TCE.II. Activated by chelated ferrous ion. Chemosphere 55:1225–1233 Liang C, Wang ZS, Bruell CJ (2007) Influence of pH on persulfate oxidation of TCE at ambient temperatures. Chemosphere 66:106– 113 Liang C, Huang CF, Chen YJ (2008) Potential for activated persulfate degradation of BTEX contamination. Water Res 42:4091–4100 Mahendra S, Petzold CJ, Baidoo EE et al (2007) Identification of the intermediates of in vivo oxidation of 1,4-dioxane by monooxygenase-containing bacteria. Environ Sci Technol 41: 7330–7336 Maurino V, Calza P, Minero C (1997) Light-assisted 1,4-dioxane degradation. Chemosphere 30:2675–2688 Mortimer RG (2008) Physical chemistry. Elsevier Academic, Burlington Oh SY, Kim HW, Park JM et al (2009) Oxidation of polyvinyl alcohol by persulfate activated with heat, Fe2+, and zero-valent iron. J Hazard Mater 168:346–351 Rastogi A, Al-Abed SR, Dionysiou DD (2009) Sulfate radical-based ferrous peroxymonosulfate oxidative system for PCBs degradation in aqueous and sediment systems. Appl Catal B Environ 85:171– 179 Son HS, Choi SB, Khan E et al (2006) Removal of 1,4-dioxane from water using sonication: effect of adding oxidants on the degradation kinetics. Water Res 40:692–698 Son HS, Im JK, Zoh KD (2009) A Fenton-like degradation mechanism for 1,4-dioxane using zero-valent iron (Fe0) and UV light. Water Res 43:1457–1463 Stefan MI, Bolton JR (1998) Mechanism of the degradation of 1,4dioxane in the dilute aqueous solution using the UV/hydrogen peroxide process. Environ Sci Technol 32:1588–1595 Stickney JA, Sager SL, Clarkson JR et al (2003) An updated evaluation of the carcinogenic potential of 1,4-dioxane. Regul Toxicol Pharmacol 38:183–195 Suh JH, Mohseni M (2004) A study on the relationship between biodegradability enhancement and oxidation of 1,4-dioxane using ozone and hydrogen peroxide. Water Res 38:2596–2604 World Health Organization (2005) 1,4-Dioxane in drinking-water. Background Document for Development of WHO Guidelines for Drinking-Water Quality. WHO/sde/wsh/05.08/120, Geneva. Xie XF, Zhang YQ, Huang WL et al (2012) Degradation kinetics and mechanism of aniline by heat-assisted persulfate oxidation. J Environ Sci 24(5):821–826 Xu XR, Li XZ (2010) Degradation of azo dye Orange G in aqueous solutions by persulfate with ferrous ion. Sep Purif Technol 72:105– 111 Zenker MJ, Borden RC, Barlaz MA (2003) Occurrence and treatment of 1,4-dioxane in aqueous environments. Environ Eng Sci 20:423–432
RESEARCH ARTICLE
Degradation of 1,4-dioxane in water with heat- and Fe2+-activated persulfate oxidation Long Zhao & Hong Hou & Ayuko Fujii & Masaaki Hosomi & Fasheng Li
Received: 12 August 2013 / Accepted: 19 February 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract This research investigated the 1,4-dioxane (1,4-D) degradation efficiency and rate during persulfate oxidation at different temperatures, with and without Fe2+ addition, also considering the effect of pH and persulfate concentration on the oxidation of 1,4-D. Degradation pathways for 1,4-D have also been proposed based on the decomposition intermediates and by-products. The results indicate that 1,4-D was completely degraded with heat-activated persulfate oxidation within 3–80 h. The kinetics of the 1,4-D degradation process fitted well to a pseudo-first-order reaction model. Temperature was identified as the most important factor influencing the 1,4-D degradation rate during the oxidation process. As the temperature increased from 40 to 60 °C, the degradation rate improved significantly. At 40 °C, the addition of Fe2+ also increased the 1,4-D degradation rate. Interestingly, at 50 and 60 °C, the 1,4-D degradation rate decreased slightly with the addition of Fe2+. This reduced degradation rate may be attributed to the rapid conversion of Fe2+ to Fe3+ and the production of an Fe(OH)3 precipitate which limited the ultimate oxidizing capability of persulfate with Fe2+ under higher temperatures. Higher persulfate concentrations led to higher 1,4-D
Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-2668-3) contains supplementary material, which is available to authorized users. L. Zhao : H. Hou (*) : F. Li State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Dayangfang 8, Beijing 100012, People’s Republic of China e-mail: [email protected] A. Fujii : M. Hosomi Department of Chemical Engineering, Faculty of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan
degradation rates, but pH adjustment had no significant effect on the 1,4-D degradation rate. The identification of intermediates and by-products in the aqueous and gas phases showed that acetaldehyde, acetic acid, glycolaldehyde, glycolic acid, carbon dioxide, and hydrogen ion were generated during the persulfate oxidation process. A carbon balance analysis showed that 96 and 93 % of the carbon from the 1,4-D degradation were recovered as by-products with and without Fe2+ addition, respectively. Overall, persulfate oxidation of 1,4-D is promising as an economical and highly efficient technology for treatment of 1,4-D-contaminated water. Keyword 1,4-Dioxane . Persulfate . Ferrous ion . Arrhenius equation . Degradation pathways
Introduction 1,4-Dioxane (1,4-diethylene dioxide, C4H8O2, referred to as 1,4-D) is an organic compound that is used as a solvent in a wide range of industrial organic products (e.g., paints, varnishes, inks, and dyes). It is also present as a by-product in many consumer products (e.g., cleaning products, cosmetics, shampoos, and laundry detergents) (Stickney et al. 2003). The improper disposal of industrial waste and accidental solvent spills have resulted in 1,4-D contamination of subsurface water. The United States Environmental Protection Agency (USEPA) and the International Agency for Research on Cancer (IARC) have classified 1,4-D as a Class 2B carcinogen (INCHEM website 1999; IARC 1999). The tolerable limit for 1,4-D in drinking water is currently 50 μg L−1 based on draft guidelines proposed by the World Health Organization (WHO) (World Health Organization 2005). Because of its effect on both the environment and human health, 1,4-D has been extensively investigated.
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Common remedial technologies such as adsorption and air stripping are not cost effective for removal from 1,4-D-contaminated water because 1,4-D has a high water solubility (4.31×105 mg L−1), low octanol-water partition coefficient (log Kow =−0.27), and low vapor pressure (37 mmHg at 25 °C) (Lesage et al. 1990; Zenker et al. 2003). Further, 1,4D is recalcitrant to microbial degradation because of its heterocyclic structure (Adams et al. 1994), so that conventional water treatment techniques are limited in their application for the treatment of 1,4-D-contaminated water. Advanced oxidation processes (AOPs) have aroused considerable interest as alternatives for treating 1,4-Dcontaminated water, with processes such as TiO2-based photocatalytic oxidation (Coleman et al. 2007), Fentonlike degradation (Son et al. 2009), oxidative reactions using sonication (Beckett and Hua 2003; Son et al. 2006), and the use of ozone or hydrogen peroxide (O3/H2O2) (Suh and Mohseni 2004). Although some AOP technologies have been highly effective in the removal of 1,4-D from water (Zenker et al. 2003), most AOPs are not cost effective because of equipment capacities, operating costs that involve the consumption of expensive chemicals such as O3 and H2O2, and expensive unit processes such as continuous UV irradiation (Stefan and Bolton 1998; Adams et al. 1994; Maurino et al. 1997). Sodium persulfate has drawn increasing attention as an alternative oxidant in the chemical oxidation of contaminants (Huang et al. 2002; Huang et al. 2005; Liang et al. 2003; Liang et al. 2007) as it has several potential advantages for water treatment. The persulfate anion (S2O82−) is a strong oxidizing agent with a redox potential of 2.01 V, which is comparable to ozone (E0 =2.07 V) or hydrogen peroxide (E0 =1.78 V) (Huang et al. 2002). Persulfate has a number of advantages over other oxidants, as it is a solid at room temperature, can be easily stored and transported, is stable, has high stability and high aqueous solubility, and is relatively low cost (Lau et al. 2007). Persulfate can also be reduced to sulfate anions as shown in Eq. (1). These features make persulfate a promising choice for environmental remediation applications. Persulfate oxidation at room temperature is not usually very effective, so persulfate is commonly used with UV light or under high temperatures to initiate/enhance its radical oxidation mechanisms. Sulfate radicals (SO4−▪), which have a higher redox potential of 2.6 V and can be formed through photolytic reactions or the heat decomposition of persulfate (Eq. (2)), are capable of decomposing most organic contaminants (Huang et al. 2005). The decomposition of S2O82− in the presence of a transition metal activator (e.g., Fe2+) also leads to the formation of SO4−▪. The overall reaction between S2O82− and Fe2+ is described in Eq. (3) (House 1962). Furthermore, hydroxyl radicals (OH▪) with a redox potential of
2.8 V can be also produced by the reaction between SO4−▪ and water, as shown in Eq. (4). − 2 S2 O2− 8 þ 2e →2SO4
ð1Þ
−▪ S2 O2− 8 þ heat→2SO4
ð2Þ
2− Fe2þ þ S2 O8 2− →Fe3þ þ SO−▪ 4 þ SO4
ð3Þ
▪ −▪ SO−▪ 4 þ H2 O→HO þ HSO4
ð4Þ
In recent years, the persulfate oxidation process has been applied to the degradation of various organic pollutants (Huang et al. 2002; Liang et al. 2004b, 2008; Xu and Li 2010; Rastogi, et al. 2009), showing that it can mineralize organic compounds and allow them to reach very low concentrations at low cost. Consequently, persulfate oxidation is expected to be a promising alternative to conventional 1,4-D degradation technologies. However, the degradation of 1,4-D using persulfate oxidation has not been investigated to date. Therefore, this study was conducted to examine (1) the rate and efficiency of 1,4-D degradation by heat- and Fe2+-activated persulfate oxidation under different experimental conditions, (2) the effect of pH and various persulfate concentrations on the oxidation of 1,4-D, and (3) the probable degradation pathways of 1,4-D by persulfate based on identified decomposition intermediates and by-products.
Materials and methods Materials A 1,000 mg L−1 1,4-D stock solution was prepared by dissolving reagent grade 1,4-D (99.5 %, Wako Pure Chemical Industries, Ltd., Osaka, Japan) in nanopure deionized water (R = 18 MΩ cm−1) from a Millipore Milli-Q system. A 100 mg L−1 solution was prepared by diluting the stock solution, and this was used in all experiments. Sodium persulfate (Na2S2O8, 98 %) and ferrous sulfate heptahydrate (FeSO4 ·7H2O, 99 %) were purchased from Junsei Chemical Industries, Ltd. (Tokyo, Japan) and Kanto Chemical Industries, Ltd. (Tokyo, Japan), respectively.
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Experimental procedures Batch experiments were performed in graduated, stoppered vials (75 mL). To evaluate the degradation of 1,4-D in the presence of sodium persulfate concentration, 48 mL of the reaction solution, prepared in pure water, was placed in the 75-mL test vial and held overnight in a constant temperature incubator at 40, 50, or 60 °C. Then, 1 mL of 1,4-D solution (100 mg L−1) and 1 mL of Na2S2O8 solution (mainly at 5,000 mg L−1 but some trials at 1,250 or 2,500 mg L−1) were added to make a total volume of 50 mL. For the experiments in the presence of Fe2+, 0.25 g of FeSO4 ·7H2O was also added. Then, all the vials were sealed with butyl alcohol and an aluminum cap, shaken for 5 min. Finally, the vials were returned to the incubator again at 40, 50, or 60 °C, which allowed the reaction to begin. A control test was also conducted at 40 °C without Na2S2O8 addition. At various time intervals, 1-mL samples were taken from the vials using a syringe and then filtered through a 0.45-μm membrane filter before analysis. After each experiment, the loss in volume of the reaction solution was checked, and the concentration of 1,4D during the oxidation process was not affected. The headspace/gas phase in the vials were also sampled via a syringe and analyzed. The overall experimental scheme is shown in Fig. S1. A range of experiments were conducted to investigate the effects of various parameters on the 1,4-D degradation efficiency and rate. The experimental conditions for Runs A to O are shown in Table 1. To determine the effect of temperature on the degradation efficiency and rate of 1,4-D, three different temperatures, 40, 50, and 60 °C controlled by an incubator were used in Runs A to C. Runs D to F examined the effects of Fe2+ at the same three temperatures on the degradation efficiency and rate of 1,4-D. Additional experiments were conducted with pH adjustment (Run G) and various persulfate concentrations (Runs H to O) to examine the influence of pH and persulfate concentration on the 1,4-D degradation efficiency and rate. Analysis of 1,4-dioxane and intermediates The concentration of 1,4-D and major intermediates in the reaction solution was determined by gas chromatography (GC) using an HP-1 capillary column (Agilent, 60-m length×0.32-mm ID×5-μm film) equipped with a flame ionization detector (FID) (GC-2014, SPL-2014, Shimadzu, Japan) and a mass selective detector (HP 5973). The GC was operated with the following conditions: 1-μL samples were injected with a split ratio of 10:1 and an inlet temperature of 250 °C; the flow rate was constant at 2.22 mL min−1 with He as the carrier gas; the oven temperature program started at 40 °C for 5 min, then ramped to 150 °C at 10 °C min−1, held at
150 °C for 5 min. The detector temperatures were maintained at 250 °C. A calibration curve was established using 1,4-D solutions ranging from 0.01 to 100 mg L−1, and this showed a high linear correlation coefficient (R2 =0.997), with limits of detection and quantification at 0.0036 and 0.01 mg L−1, respectively. The headspace gas sampled during the experiments was analyzed on a GC (GC-8A, Shimadzu, Japan) equipped with a thermal conductivity detector (TCD). A packed column (SUS, 2 m, OD 4 Φ, ID 3 Φ, active carbon, mesh 60/80) was used at a constant temperature of 110 °C with 600 μL of each sample injected. The injector and detector temperatures were both set at 100 °C. Argon was used as the carrier gas, and the injector pressure was 160 kPa. The inorganic carbon (IC) and non-purgeable organic carbon (NPOC) in the reaction solution were measured using an ultraviolet persulfate oxidation total organic carbon (TOC, Shimadzu, Japan) analyzer with an infrared detector (Phoenix 8000TM, TekMar Dohrmann, USA). A pH meter (Orion, Model 52A, USA) was used to determine pH. Selected experiments were conducted in duplicate or triplicate, and the data were averaged.
Results and discussion Heat-activated persulfate oxidation of 1,4-D under different temperatures Temperature has been shown to be the most important parameter influencing the degradation rate of organic pollutants by persulfate (Huang et al. 2002, 2005; Oh et al. 2009). In this study, 1,4-D oxidation by persulfate under different temperatures was evaluated, and the results are presented in Fig. 1a. At 40 °C, 1,4-D was completely oxidized by persulfate over 80 h. In contrast, the 1,4-D concentration measured during the control experiment (without persulfate) did not change relative to the initial concentration. As the temperature increased, the degradation rate of 1,4-D by persulfate increased significantly, with complete oxidation of 1,4D by persulfate observed in 8 and 3 h at 50 and 60 °C, respectively. These results indicate that, after heating, persulfate can be effectively activated to SO 4 − ▪ (Eq. (2)), leading to the increase in 1,4-D oxidation at elevated temperatures. These results are consistent with the persulfate oxidation of polyvinyl alcohol and aniline reported by Oh et al. (2009) and Xie et al. (2012), respectively. Statistically, the oxidation of 1,4-D fitted well to a pseudofirst-order kinetic reaction model, as shown in Fig. 1b and Eq. (5) with correlation coefficient R2 values of 0.98, 1.0, and 0.97 at 40, 50, and 60 °C, respectively. The oxidation of 1,4-D
Environ Sci Pollut Res Table 1 Experimental conditions and pseudo-first-order parameters in the heat- and Fe2+-activated persulfate oxidation of 1,4-dioxane (1,4-D) in different runs Run
Experimental conditions
pHinitial
1,4-D mg L−1
S2O82− mg L−1
Fe2+ mg L−1
S2O82−-/Fe2+ ratio
Temperature ºC
Reaction timea
A B C D E F
100
5,000
0 0 0 1,000 1,000 1,000
50/0b 50/0 50/0 5 5 5
40 50 60 40 50 60
80 8 3 51 9 4
2.81 2.86 2.89 2.30 2.31 2.31
G H I J K L M N O
100 100
5,000 1,250
0 0 1,000 0 1,000 0 1,000 0 1,000
50/0 12.5/0 1.25 12.5/0 1.25 25/0 2.5 25/0 2.5
60 50 50 60 60 50 50 60 60
3 74 –e 11 –e 28 –e 5 –e
2.30c NAd NA NA NA NA NA NA NA
a
2,500
pHfinal
Pseudo-first-order parameters Rate constant K h−1
Correlation coefficient R2
2.33 2.20 2.31 2.02 2.04 2.05
0.062 0.68 2.2 0.10 0.60 1.5
0.98 1.0 0.97 0.98 0.99 0.97
1.71 NA NA NA NA NA NA NA NA
1.9 0.073 – 0.47 – 0.19 – 1.0 –
0.97 1.0 – 0.96 – 0.99 – 0.99 –
Time in hours, from the beginning of 1,4-D oxidation to its complete degradation
b
Concentration ratios
c
Adjusted pH prior to oxidation
d
Not available
e
Not determined as 1,4-D could not be completely degraded
by heat-activated persulfate is shown as Runs A to C in Table 1. C ¼ C 0 expð−ktÞ
ð5Þ
where C is the concentration of 1,4-D at time [t], C0 is the initial concentration of 1,4-D, and k is the pseudo-first-order reaction rate constant, which is obtained from the slope of the line after plotting ln(C0 /C) against time (Xu and Li 2010). The reaction rate constants for 1,4-D increased with increased temperature, with values of 0.062, 0.68, and 2.2 at 40, 50, and 60 °C, respectively. It appears that the degradation rate of 1,4D by persulfate at 40 °C was approximately an order of magnitude lower than at 50 °C, while the degradation rate of 1,4-D at 60 °C was nearly three times higher than at 50 °C. Using the pseudo-first-order rate constants at different temperatures, the apparent activation energy for 1,4-D degradation by persulfate was calculated using the Arrhenius equation (Eq. (6)). k ¼ Aexpð−E=RT Þ
ð6Þ
where A is the pre-exponential (or frequency) factor, E is the apparent activation energy (J mol−1), R is the ideal gas constant (8.314 J mol−1 K−1), and T is the reaction temperature
(K). A good linear relationship was obtained from the Arrhenius plot of ln (k) versus 1,000/T (Fig. 2a). The apparent activation energy was calculated to be 155 kJ mol−1 (R2 = 0.94), which indicates that the 1,4-D degradation in aqueous persulfate solution requires only moderate activation energy, as the activation energy for the thermal reaction has a normal range of 60–250 kJ mol−1 (Mortimer 2008). Xu and Li (2010) determined that a relatively low activation energy of 92.2 kJ mol−1 was required to degrade the azo dye Orange G. Liang et al. (2004a, 2007) reported activation energies of around 108–130 kJ mol−1 for the degradation of trichloroethylene and 1,1,1-trichloroethane. The different activation energies may be related to the chemical structures of the different target compounds.
Iron(II)-activated persulfate oxidation of 1,4-D at different temperatures Iron(II) is one of the main species able to catalyze persulfate to produce SO4−▪, as described in Eq. (3), and its presence can significantly enhance the degradation efficiency of pollutants (Anipsitakis and Dionysiou 2004; Oh et al. 2009). The 1,4-D oxidation by Fe2+-activated persulfate under different temperatures is shown in Fig. 1c. As the experiments with Fe2+, the
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a
c
b
d
Fig. 1 Effect of temperature on 1,4-dioxane oxidation by heat-activated persulfate in the absence and presence of Fe2+
1,4-D degradation rate by Fe2+-activated persulfate oxidation increased with increasing temperature. At 40 °C, 1,4-D was completely oxidized by Fe2+-activated persulfate in 51 h, while the control test (Fe2+ without persulfate) did not change the 1,4-D concentration. As the temperature increased to 50 and 60 °C, complete oxidation of 1,4-D by Fe2+-activated persulfate was observed in 9 and 4 h, respectively (Runs D– F, Table 1). Interestingly, the addition of Fe2+ significantly decreased the degradation time for all the 1,4-D being removed at 40 °C, compared to the experiments without Fe2+ (Fig. 1a, c), but a slight increase in degradation time was observed at 50 and 60 °C. It appears that more SO4−▪ is generated by Fe2+-activated persulfate (via Eq. (3)) as reported by Kolthoff et al. (1951) and House (1962), and this increases the 1,4-D degradation rate at 40 °C. As the added concentrations of Fe 2+ change from 1,000 mg L −1 to 500 and 1,500 mg L−1, the degradation rate of 1,4-D from water was not significantly varied. It is indicated that the Fe2+ concentration seems not the key factor during this oxidation process of 1,4-D. The corresponding mechanism of this result needs to be further studied.
The effect of Fe2+-activated persulfate on the 1,4-D degradation rate was influenced by the higher temperatures at 50 and 60 °C. This might occur as the higher temperature can accelerate the reaction between Fe2+ and SO4−▪, forming Fe3+, and the Fe3+ then reacts with OH− to form Fe(OH)3 precipitate which might inhibit the oxidation of 1,4-D by SO4−▪. This possible reaction scheme is described in Eqs. (7) and (8), below. Besides ferric hydroxide, there were no other sulphate salts confirmed. 3þ Fe2þ þ SO−▪ þ SO2− 4 →Fe 4
ð7Þ
Fe3þ þ 3OH− →FeðOHÞ3
ð8Þ
This difference can be seen in the color of the 1,4-D during oxidation solutions under different experimental conditions. As shown in Fig. S2b, the color of 1,4-D oxidation solution at 60 °C was much darker than at 40 °C, indicating that more
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2
Variation of pH before and after reaction 2+
Fe addition 2+ No Fe addition
1
a 0 y=-18.7x+57.0
lnk
2
R =0.94
-1 y=-14.2x+43.1 2
R =0.94
-2 -3 2.95
3.00
3.05
3.10
3.15
3.20
3.25
-1
1000/T [K ] 1 No pH adjustment pH adjustment
0
b
ln (C/C0)
-1 -2
K=1.9 2 R =0.97
-3 K=2.2
-4
2
R =0.97
-5 0.0
0.5
1.0
1.5
2.0
2.5
Time [hours] Fig. 2 Arrhenius plot of 1,4-dioxane oxidation between 40 and 60 °C with and without Fe2+ and the effect of pH on the oxidation of 1,4dioxane by persulfate
The measured initial and final pH values for the oxidation of 1,4-D using heat- and Fe2+-activated persulfate (Runs A to F) are shown in Table 1. During the 1,4-D degradation process, the pH values in Runs A to F decreased by 0.3–0.7 pH units because of the production of organic acid or removal of OH▪ to form Fe(OH)3 during 1,4-D degradation. The initial and final pH values of Runs A, B, and C were slightly higher than for Runs D, E, and F, due to the Fe2+ addition in Runs D to F. This suggests that more sulfuric acid could be produced as described in Eq. (7) because of the Fe2+ in the reaction solution. There was no significant pH variation with the different temperatures (Table 1), as the initial pH values were almost the same for Runs A to C (2.81–2.89) and Runs D to F (2.30–2.31). Thus, the pH values were not temperature dependent in the persulfate oxidation of 1,4-D with heat and Fe2+ activation. To determine whether the 1,4-D degradation efficiency and rate were affected by changing the pH, a trial at 60 °C without Fe2+ addition was adjusted to a pH of 2.30 before oxidation (Run G), which is the same pH as in the Fe2+ addition experiments (Runs D to F). The 1,4-D degradation with pH adjustment also fits the pseudo-first-order reaction model as shown in Fig. 2b. There was no significant difference in the 1,4-D degradation rate with and without pH adjustment, as the 1,4-D was completely degraded within 3 h in both conditions, and the pseudo-first-order rate constants in Runs C and G were 2.2 and 1.9, respectively (Table 1). These results show that pH adjustment is not necessary for the heat-activated persulfate oxidation of 1,4-D under the experimental conditions investigated. Effect of persulfate concentration
Fe(OH)3 precipitate was present in the reaction solution. This Fe(OH)3 could affect the oxidation of 1,4-D by Fe2+-activated persulfate, and thus temperature also has an important role in 1,4-D degradation by Fe2+-activated persulfate. The oxidation of 1,4-D by Fe2+-activated persulfate also fitted well to a pseudo-first-order kinetic reaction model (Fig. 1d and Eq. (5)) as described above. The statistical correlations with the pseudo-first-order model gave correlation coefficient R2 values of 0.98, 0.99, and 0.97 at 40, 50, and 60 °C, respectively. As with the oxidation of 1,4-D without Fe2+ addition, the pseudo-first-order reaction rate constants (k) with Fe2+-activated persulfate conditions also increase with increasing temperature. Based on the pseudo-first-order rate constants, the activation energy for 1,4-D oxidation by Fe2+activated persulfate was calculated as 117 kJ mol−1 (R2 =0.94) (Fig. 2a). This activation energy, lower than that for 1,4-D oxidation without Fe2+ addition (155 kJ mol−1), implies that the reaction was more easily to occur.
Figure 3 shows the overall degradation rate for 1,4-D against time at 60 °C with different persulfate concentrations with and without Fe2+ addition. In the absence of Fe2+ (Fig. 3a), the 1,4D was completely degraded within a maximum of 11 h. The persulfate concentrations were positively correlated with the oxidation rate, and as the persulfate concentrations increased from 1,250 mg L−1 to 2,500 and 5,000 mg L−1, the reaction rates approximately doubled with each concentration increase. The experimental conditions and results are shown as Runs C, J, and N in Table 1. These results indicate that the concentration trend was consistent with the reaction temperature trend as increasing these different parameters both improved the degradation rate of 1,4-D during persulfate oxidation. In the presence of Fe2+ (Fig. 3b), 1,4-D was not completely oxidized when the persulfate concentration was 2,500 mg L−1 or lower, as shown in Runs F, K, and O in Table 1. As shown in Fig. 3b, the concentration of 1,4-D decreased to 50– 60 mg L−1 within 20 h and remained in that range thereafter.
Environ Sci Pollut Res 80
a
100
60 -1
80
2+
No Fe addition
60
2+
IC(Fe addition) 2+ NPOC(Fe addition) IC NPOC
70
-1
1250mg L -1 2500mg L -1 5000mg L
Concentration [mg L ]
-1
1,4-dioxane concentrations [mg L ]
120
40 20
50 40 30 20 10
0
0
2
4
6
8
10
12
0 0
Time [hours]
1
2
3
4
Time [hours]
b
-1
1,4-dioxane concentrations [mg L ]
100
Fig. 4 Effect of Fe2+ on the inorganic carbon (IC) and non-purgeable organic carbon (NPOC) concentrations during 1,4-dioxane oxidation
-1
2500 mg L -1 5000 mg L
80
Degradation pathways and by-products of 1,4-D
2+
Fe addition
60
40
20
0
0
20
40
60
80
100
120
140
160
Time [hours]
Fig. 3 Effect of persulfate concentration on 1,4-dioxane oxidation with and without Fe2+
This indicates that Fe2+ may act as a scavenger for the SO4−▪ in accordance with Eq. (7), limiting the oxidation of 1,4-D in the presence of excess Fe2+. The S2O82−/Fe2+ ratio was 5:1 when 5,000 mg L−1 persulfate was added (Table 1), but this ratio decreased to 2.5:1 and 1.25:1 as the persulfate concentrations decreased to 2,500 and 1,250 mg L−1, respectively. It is speculated that the incomplete degradation of 1,4-D may be due to the scavenging of SO4−▪ in the presence of excess of Fe2+. Similar results were reported by Liang et al. (2008) during the persulfate degradation of benzene, toluene, ethylbenzene, and xylene in the presence of Fe2+. Similar trends were observed for the degradation of 1,4-D under different persulfate concentrations with and without Fe2+ addition when the reaction temperature was 50 °C, with incomplete degradation of 1,4-D observed for persulfate concentrations decreasing from 5,000 mg L-1 to 2,500 and 1,250 mg L-1 in the presence of Fe2+. The experimental conditions are shown as Runs B, E, H, I, L, and M in Table 1. From these results, an optimum persulfate concentration of 5,000 mg L−1 was identified.
During the degradation of 1,4-D at 60 °C with and without Fe2+, the concentrations of inorganic carbon (IC) and nonpurgeable organic carbon (NPOC) in the reaction liquid were measured by a TOC analyzer. As shown in Fig. 4, there were no significant differences in the concentrations of IC and NPOC with and without Fe2+. The major intermediates were identified as glycolaldehyde, glycolic acid, oxalic acid, acetaldehyde, and acetic acid, while CO2 and H2 were identified in the headspace. Carbon balance studies showed that 30 and 16 % of the 1,4-D were mineralized to CO2 by persulfate in the presence and absence of Fe2+, respectively (Table 2). The IC and NPOC accounted for 7.3 and 59 %, respectively, of the carbon balance with Fe2+ addition and 6.6 and 70 % of the carbon balance without Fe 2+ addition, respectively.
Table 2 Carbon balance during 1,4-dioxane degradation process Phase
Liquid phase
Gaseous phase
By-products
Glycolaldehyde (NPOCa) Glycolic acid (NPOC) Oxalic acid (NPOC) Acetaldehyde (NPOC) Ethylic acid (NPOC) ICb CO2 H2
Total by-products a
Non-purgeable organic carbon
b
Inorganic carbon
Carbon balance [%] Fe2+ addition
No Fe2+ addition
59
70
7.3 30 – 96
6.6 16 – 93
Environ Sci Pollut Res Fig. 5 Proposed degradation pathways for the mineralization of 1,4-dioxane
O
SO4-
O
O
HO
O
1,4-dioxane
α–oxyl radical
a
1,4-dioxanyl radical Ethylene glycol
HO
OH
b H
OH
H+ Glycoaldehyde
HO
O
H+ H
O
OH
Glycolic acid
HO
O
Ethanol
Acetaldehyde
OH H
O
Acetic acid
OH
Glyoxylic acid
O
O
OH
OH
O
O
CO2
Oxalic acid
Statistically, the total carbon balances with and without Fe2+ addition were 96 and 93 %, respectively, which indicates that Fe2+ may have a role in accelerating the mineralization of 1,4-D during persulfate oxidation. From the analytical results, we propose two degradation pathways for 1,4-D by heat- and Fe2+-activated persulfate. In both pathways, the 1,4-D is first attacked by SO4−▪ and ▪ OH forming the 1,4-dioxanyl radical. This reacts with oxygen forming peroxyl radicals, which are transformed into α-oxyl radical as the precursor of the primary intermediates (Choi et al. 2010). Following two different routes (a and b), this α-oxyl radical is either turned into glycolaldehyde and glycolic acid, or acetaldehyde and acetic acid by an intramolecular reaction and several oxidation steps. Finally, intermediates such as oxalic acid and acetic acid are mineralized to CO2 via common cellular metabolic pathways. The proposed degradation pathways are shown in Fig. 5. One of the proposed degradation pathways (route b) was also reported by Mahendra et al. (2007) for the bioremediation and electrochemical AOP of 1,4-D. The other degradation pathway (route a) has not previously been reported in the literature. There may be some differences in the reaction mechanisms of 1,4-D between heat-/Fe2+-activated persulfate oxidation and other oxidation processes. Further study is needed to examine the degradation mechanisms in more depth. Nevertheless, heat- and Fe2+-activated persulfate oxidation of 1,4-D via these pathways is not expected to cause an accumulation of toxic compounds in the environment.
Conclusion In this study, the 1,4-D degradation efficiency and rate by heat- and Fe2+-activated persulfate oxidation were investigated under different experimental conditions. The effects of pH adjustment and different persulfate concentrations were also examined. The results showed that 1,4-D was completely degraded in an aqueous solution within 3–80 h under different temperatures. Increasing the temperature generally increased the degradation rate because of the increased production of free radicals. With the addition of Fe2+, a much faster 1,4-D degradation rate was observed at 40 °C compared to the equivalent trial without Fe2+. However, a slower 1,4-D degradation rate was observed when the reaction temperature increased to 50 and 60 °C, possibly due to the production of Fe(OH)3 precipitate which was enhanced at higher temperatures. There is no need to adjust the pH prior to persulfate oxidation, since no significant difference in 1,4-D degradation rate was observed with or without pH adjustment. The optimal persulfate concentration was determined to be 5,000 mg L−1. Decreasing the persulfate concentration reduced the degradation rate of 1,4-D in the absence of Fe2+ and resulted in incomplete oxidation of 1,4-D in the presence of Fe2+. Two degradation pathways were proposed for 1,4-D. Acetaldehyde, acetic acid, glycolaldehyde, and glycolic acid were confirmed as the major intermediates, and these were converted to carbon dioxide and hydrogen ion as final degradation products. Further studies, including scaling up of the system will help establish a novel, economical, and high
Environ Sci Pollut Res
oxidation efficiency AOP technology using persulfate for removal of 1,4-D from water. Acknowledgments This study was supported by Scientific Research Funding from the Ministry of the Environment, Japan, the National Natural Science Foundation of China (Grant No. 41301342), and the Special Environmental Protection Foundation for Public Welfare Project of China (201009032, 2007KYYW03).
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