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Removal of 1,4-dioxane from water using sonication: Effect of adding oxidants on the degradation kinetics
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40 (2006) 692– 698
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Removal of 1,4-dioxane from water using sonication: Effect of adding oxidants on the degradation kinetics Hyun-Seok Sona, Seok-Bong Choib, Eakalak Khanc, Kyung-Duk Zoha, a
Institute of Health and Environment, School of Public Health, Seoul National University, Seoul 110-799, Republic of Korea Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, ND 58105, USA c Department of Civil Engineering and Construction, North Dakota State University, Fargo, ND 58105, USA b
ar t ic l e i n f o
A B S T R A C T
Article history:
This research investigates the effect of adding oxidants such as Fe0, Fe2+ and S2O2 8 in the
Received 30 June 2005
sonication of 1,4-dioxane (1,4-D). The results indicate that the degradation pattern of 1,4-D
Received in revised form
kinetically could be divided into three steps (initiation, acceleration, and stabilization), with
24 October 2005
the first two steps predominating. The initiation step agreed with zero order rate model,
Accepted 18 November 2005
while the acceleration step was the pseudo-first order. In the presence of HCO 3 as a radical
Available online 19 January 2006
scavenger, the degradations of 1,4-D and TOC were suppressed, indicating that OH radical
Keywords:
is an important factor in the sonolysis, especially at the acceleration step. The overall
1, 4-dioxane
degradation efficiency of 79.0% in the sonolysis of 1,4-D was achieved within 200 min.
Sonication
were individually combined with sonication, the total While Fe0, Fe2+ and S2O2 8
HCO 3
degradation efficiency of 1,4-D increased 18.6%, 19.1% and 16.5% after 200 min, respectively.
Fe2+
The addition of oxidants not only increased the rate constant in the acceleration step, but
Fe0
also changed the kinetic model from zero to pseudo-first order at the initiation step. The addition of oxidants such as Fe2+, Fe0 and S2O2 8 in the sonication of 1,4-D also improved the
S2O2 8
mineralization of 1,4-D. However, the degradation efficiencies of 1,4-D and TOC were not statistically different (p ¼ 0.709, ANOVA) with different oxidants such as Fe2+, Fe0 and S2O2 8 . & 2005 Elsevier Ltd. All rights reserved.
1.
Introduction
1,4-dioxane (C4H8O2, 1,4-D) is currently used as a solvent in several industries such as manufacturing of plastics, pesticide and paints. Several studies reported that 1,4-D has been detected in surface and groundwater, and considered as an emerging water contaminant (Beckett and Hua, 2000). Some common consumer products such as detergents and shampoo also contain low levels of 1,4-D. Several studies reported that 1,4-D has been detected in surface and groundwater in Japan, Canada and the United States (Suzanne et al., 1990). The US EPA and International Agency for Research on Cancer (IARC) classified 1,4-D as a probable human carcinogen (INCHEM website, 1999). Corresponding author. Tel.: +82 2 740 8891; fax: +82 2 745 9104.
E-mail address: [email protected] (K.-D. Zoh). 0043-1354/$ - see front matter & 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.11.046
Since 1,4-D has a high water solubility of 4.31 105 mg/L, low Kow value of 100.27 and low vapor pressure (37 mmHg at 25 1C), adsorption and air stripping technology are not cost effective for removal from 1,4-D-contaminated water (Suzanne et al., 1990; Zenker et al., 2003). 1,4-D is also recalcitrant to microbial degradation due to its heterocyclic structure (Adams et al., 1994). Therefore, as alternatives, advance oxidation processes (AOPs) have been investigated as the alterative methods for treating 1,4-D (Stefan and Bolton, 1998). Among AOP processes, the sonolytic treatment of 1,4-D has been recently studied. Beckett and Hua (2000) studied the degradation of 1,4-D using ultrasound sonication, and reported increases in the biodegradability of 1,4-D after sonolysis. They found that 1,4-D degradation kinetics could
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be well-described with the pseudo-first order model at three frequencies: 205, 618 and 1017 kHz, tested. Organic pollutants in sonolytic treatment are degraded by two factors—pyrolysis and oxidation by OH radical. To increase the efficiency of sonolysis, first, improving the transmission of energy can be used by the control of frequency. A change on the frequency may amplify the cavity procedure produced by ultrasonic irradiation, which may result in increasing thermal degradation efficiency. However, such the increase of the physical parameters hinders the field application of the sonolysis process due to technical problems such as noise, energy efficiency and cost. Secondly, the addition of oxidants, such as Fe2+, may increase OH radical generation during sonolysis (Beckett and Hua, 2003). However, most studies on the sonolytic degradation of organic contaminants have relied on the use of high frequencies and power settings (Hiskia et al., 2001), making the reaction rates too fast for estimating the degradation mechanism accurately. In this study, the sonication of 1,4-D at a low frequency (20 kHz) was investigated in order to identify the roles of the OH radical and the degradation mechanism. The effect of OH radical in the sonication of 1,4-D was investigated by adding HCO 3 as a radical scavenger. The effect of adding oxidants on the degradation efficiency, such as Fe0, Fe2+ and S2O2 8 kinetics model and degradation mechanism of 1,4-D sonolysis were also examined.
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heptahydrate (Merck), hydrogen peroxide (Merck), sodium persulfate (J.T. Baker), iron powder (325 mesh, Alfa Aesar) and sodium carbonate (HACH) were used as received.
2.2.
Materials and methods
2.1.
Materials
A 1,4-D solution was prepared by diluting a 1000 mg/L stock solution. The stock solution was prepared by dissolving reagent grade 1,4-D (Aldrich) in Nanopure deionization water (R ¼ 18 MO cm1, Barnstead). The initial solutions of 1,4-D of 100 mg/L were used in all experiments. Ferrous sulfate
Experimental set-up
Fig. 1 shows schematic diagram of the sonolytic reactor. All experiments were batch type and were performed using a sonicator (Sonics & Materials, VC600-1, USA) with an ultrasonic transducer of horn type, which has an active acoustical vibration area of 19.6 cm2, fixed frequency of 20 kHz and maximum input power of 600 W. Air as a dissolved gas was provided in the solution in order to facilitate cavitation and to prevent uncontrolled incoming air into the reactor (Prasad et al., 1994; Wheat and Tumeo, 1997). The solution was sparged for 3 min using air at a flow rate of 5 mL/min (monitored by a rotameter), and then stopped for the next 3 min. This aeration scheme was repeated continuously throughout the entire run. Sparging study in the absence of sonication was performed using 100 mg/L solution of 1,4-D for 2 h. The temperature was controlled at 25 1C by circulating water around the reactor. During that time period, the maximum loss of 1,4-dioxane was found to be 0.67%, indicating that there is no volatilization loss. For all experiments, the reactor was irradiated at 20 kHz of frequency and 300 W of power.
2.3.
2.
693
Experimental procedure
In order to qualitatively estimate the contribution of OH radical in the sonolysis of 1,4-D, the experiment using 0.082 mM of HCO 3 , which is known as a radical scavenger (Beckett and Hua, 2003; Hiskia et al., 2001), was performed. The effect of adding oxidants in the sonolysis of 1,4-D was as the oxidants also investigated using Fe2+, Fe0 and S2O2 8 individually at 0.5 mg/L for both experiments with and without sonication. In case of experiment in the presence of
Fig. 1 – Schematic diagram of sonolytic reactor system: (a) sonicator system, (b) sectional view of reactor, and (c) outline of sonolytic reactor system.
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Fe2+ without sonication, additional experiments were performed with and without H2O2 (0.01 mM).
2.4.
Analysis
1,4-D was analyzed using a purge and trap sample concentrator (TekMar Dohrmann 3100, TekMar Dohrmann, USA) equipped with an autosampler (TekMar Dohrmann 2016) and gas chromatograph (Hewlett Packard 6890, USA) equipped with a Hewlett Packard-5MS 5% phenyl methyl siloxane capillary column (30 m 0.25 mm 0.25 mm) and a mass selective detector (Hewlett Packard 5973). The temperature condition of purge and trap was 150 1C for line, 30 1C for purge ready, 70 1C for purge and 280 1C for bake, respectively. Purge and trap was performed with helium as a carrier gas with 14 and 10 min for purge and bake time, respectively. The gas chromatograph-mass selective detector was operated under the following conditions—10:1 split ratio, the carrier gas at 1.2 mL/min, electron ionization at 71 eV and mass scan range of 35–550 m/z. The temperature was programmed with holding 40 1C for 5 min to 250 1C at 10 1C/min, followed by 3 min hold at 250 1C for the oven and 150 1C for the inlet. TOC was measured using an ultraviolet persulfate oxidation TOC analyzer with an infrared detector (Phoenix 8000TM, TekMar Dohrmann) and a pH meter (Orion, Model 52A, USA) was used to determine pH.
3.
Results and discussion
3.1.
Role of OH radical in the sonication of 1,4-D
First, in order to identify the contribution of OH radical in the sonolytic degradation of 1,4-D, the role of OH radical were examined in the absence and presence of HCO 3 . Fig. 2 presents the percent reduction of 1,4-D during these experiments. As shown in Fig. 2, the degradation curves can be mainly divided into three regions (initiation, acceleration and stabilization steps), but the most degradation of 1,4-D was observed in two regions (initiation and acceleration steps). The degradation curves of Fig. 2 can be estimated with the sigmoidal model based on the best fits by trial and error using Eq. (1): y ¼ y0 þ
a 1 þ ðx=x0 Þb
,
(1)
where y is the degradation percent of 1,4-D and y0 is the intercept, x0 is the time at the inflection point (the time dividing between the first two stages), and a and b are dependent variables. This sigmoidal model (Eq. (1)) was used to divide the curves in Fig. 2 mainly into two stages—the initiation and the acceleration step. By fitting the data into Eq. (1), R2Eq1 value (the conformity to the sigmoidal model) of each degradation curve in the runs without HCO 3 and with HCO3 at 0 and 100 min were obtained, and found to be 0.981, 0.969 and 0.965, respectively. The R2Eq1 values obtained from Eq. (1) also provided the estimation of reaction time in each step, and this estimated reaction time in each step is shown in Table 1.
40 (2006) 692– 698
As shown in Fig. 2, the total percent degradation values during the initiation steps were not significantly different (p ¼ 0.107, two-tailed t-test) between these two cases. However, significant differences (p ¼ 0.009, two-tailed t-test) were observed between the percent degradation values at the acceleration steps. Similar statistical results were obtained between the trials without HCO 3 and with HCO3 added at 100 min (p ¼ 0.525 and 0.007, two-tailed t-test) at the initiation steps. HCO 3 is well-known OH radical scavenger as shown in Eq. (2): HCO 3 þ OH ! HCO3 þ OH .
(2)
Since HCO 3 is an ion, it will scavenge free radicals predominantly in the bulk water phase. Water-soluble compounds that are less volatile will be signicantly inuenced by the bicarbonate concentration (Kiwi. et al., 2000; Liao et al., 2001). Since 1,4-D is non-volatile, the bicarbonate can compete with 1,4-D for available free radicals in the interfacial and bulk region and decrease its decomposition efficiency. Fig. 2 shows the contribution of HCO 3 in the role of OH radical scavenger during sonolysis. As shown in Fig. 2, the effect of radical scavenging by HCO 3 mainly occurred during the acceleration step, and suggesting most OH radical in the sonication is generated at the acceleration step. In this study, pH changes during the sonication reactions were also measured. It was found that the decrease of pH (5.9–4.8) was observed in the run without HCO 3 . This maybe due to the production of nitrous acid (HNO2) and nitric acid (HNO3) during the sonication of water. Adewuyi (2001) reported that the pH decreases during the sonolysis of water with air due to the production of HNO2 and HNO3 by the various reactions with N2 and OH radical. Next, no change of pH occurred at the initial step in the run with HCO 3 added at 0 min, and a slight increase of pH (6.0–6.8) was observed at the acceleration step (after 120 min) in the run with HCO 3 . It is also found that pH increased during the reaction with the addition of HCO 3 at 100 min from 6.0 to 8.0. These results can be summarized as follows—OH radicals are mainly produced at the acceleration step, which react with HCO 3 to produce OH ion, resulting in the increase in pH of the solution. However, at the initiation step, the production of OH radical is insufficient for the reaction with HCO 3 , as shown in Eq. (2). Instead, the presence of HCO 3 may act as a buffer in the solution, resulting in almost no change of pH. The results in Fig. 2 and the pH changes during the sonication can induce that the reaction between OH radical and HCO 3 mainly occurs at the acceleration step rather than at the initial step. The sonication of 1,4-D at the initiation step may proceed by a thermal reaction, while the degradation of 1,4-D is dominated by OH radical reaction at the acceleration step. As shown in Fig. 2, thermal degradation and chemical oxidation contributed approximately 35% and 44% to the degradation of 1,4-D according to the result of the sonication without HCO 3. Linear regression analysis was performed in order to determine the conformity to zero order or the pseudo-first order model in each step, based on Eq. (1). R20, and R21 stand for the linear regression coefficients for zero order and the
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pseudo-first order model in each step, respectively. Table 1 shows the results of rate constants, and R20 and R21 values fitted by zero and first order model estimation at the initiation and acceleration steps. Table 1 shows that the initiation step in case of both without HCO 3 and with HCO3 added at 0 min follows the zero order rate model, and both acceleration steps followed the pseudo-first order rate model, respectively. However, the data in the experiment with HCO 3 added at 100 min fitted better to the zero order rate model for both initiation (R20 ¼ 0.975) and acceleration (R20 ¼ 0.991) steps. Fig. 2 also shows that approximately 20 min time shortening in the beginning time for the acceleration step was observed in case with HCO 3 addition at 100 min comparing with the condition without HCO 3 . Since the initiation step in the sonication-only takes approximately 100 min, shortening the initiation step can minimize the total reaction time to
100
Without HCO3HCO3- addition at 0 min HCO3- addition at 100 min
(C0-Ct)/C0x100 (%,1,4-D)
80
60
40
20
0 0
50
100 Time (min)
150
200
Fig. 2 – Effect of HCO 3 on the sonolysis of 1,4-D.
695
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achieve maximum sonication efficiency. Different supplemental oxidants can be added to achieve this purpose.
3.2.
Effect of adding oxidants
As stated above, the contribution of OH radical is minimal than at the initiation step, and the kinetics followed zero order rate model regardless of adding HCO 3 . In the next were experiment, the oxidants such as Fe2+, Fe0 and S2O2 8 added in the sonication of 1,4-D in order to improve the degradation efficiency of 1,4-D by increasing OH radical, especially at the initiation step. First, Fig. 3 shows the sonolytic degradation effect of 1,4-D and TOC when Fe2+ was added as the oxidant. Compared with sonication-only condition, the total degradation efficiency of 1,4-D and TOC in case of sonication with Fe2+ condition was 98% and 95% during 200 min of reaction time, respectively. The result indicates that adding Fe2+ in the sonication enhanced the degradation efficiency of 1,4-D. In addition to Fig. 3, Table 2 shows the kinetic models, and the rate constants at the initiation and acceleration step. As shown in Table 2, while the kinetic model followed zero order at the initiation step, and pseudo-first order at the acceleration step, the kinetic models in both steps in case of sonication with Fe2+ were fitted better to the pseudo-first order rate model. This result indicates that, in the presence of Fe2+, the main factor for 1,4-D degradation at the initiation step as well as the acceleration step is OH radical, and the increase in the degradation rate was achieved by the increase of OH radical by adding Fe2+, especially at the initiation step. OH radical can be produced by the oxidation of Fe2+ and reduction of Fe3+ with sonication, as described in Eqs. (3) and (4) (Kavitha and Palanivelu, 2004; Liang et al., 2004): Fe2þ þ H2 O2 þ sonication ! Fe3þ þ OH þ OH ,
(3)
Fe3þ þ H2 O þ sonication ! Fe2þ þ Hþ þ OH .
(4)
As shown in Eq. (3), in case of sonication with Fe2+, H2O2 is needed in order to enhance the production of OH radical.
Table 1 – Zero (k0) and pseudo-first order (k1) rate constants, and coefficient of determination (R2) at different steps in the sonolysis of 1,4-D under different HCO 3 addition scenarios Condition
Stepa
Zero order
Pseudo-first order
k0 ( 101 M/min)
R20
k1 ( 103 min1)
R21
Without HCO 3
Overall Initiation (o100 min) Acceleration (o115 min)
6.74 2.22 26.04
0.8218 0.9697 0.7669
13.20 3.87 60.81
0.8070 0.8584 0.9853
HCO 3 added at 0 min
Overall Initiation (o120 min) Acceleration (o140 min)
6.10 2.17 28.43
0.9234 0.9632 0.8762
11.22 3.75 36.34
0.8397 0.9037 0.9998
HCO 3 added at 100 min
Overall Initiation (o80 min) Acceleration (o115 min)
5.12 2.22 13.11
0.9186 0.9746 0.9908
4.12 3.84 10.24
0.8877 0.9360 0.9335
a
Time for changing between the initiation step and acceleration step were calculated by Eq. (1).
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case of sonication with Fe2+, resulting in the inhibition of Fe(OH)3 sludge production. Next, Fig. 4 shows the effect of adding Fe0 in the sonication of 1,4-D. As shown in Fig. 4(a), compared with sonication without Fe0, the presence of Fe0 in the sonication process augmented total degradation of 1,4-D from 78% to 97% during 200 min of reaction time. In case of the Fe0-only condition, however, 1,4-D degradation was only 13% during the same period. TOC degradation during sonication of 1,4-D with and without Fe0 was 94% and 71%, respectively, and only 7% at the Fe0-only condition. Table 2 also shows the kinetics and rate constants in case of reaction involving Fe0. While the kinetics at the initiation steps in the sonication-only condition are fitted better to zero order, the data in sonication with Fe0 are fitted better to pseudo-first order at both initiation and acceleration steps. The presence of Fe0 in the sonication process may improve the production of the OH radical, as shown in Eqs. (7) and (8), resulting in the increase in the degradation efficiency of 1,4-D (Arnold and Roberts, 2000; Mantha et al., 2001):
100 Sonication Sonication + Fe2+ Fe2+ + H2O2 Fe2+
(C0-Ct)/C0x 100 (%, 1,4-D)
80
60
40
20
0 0
50
(a)
100 Time (min)
150
200
100 Sonication Sonication + Fe2+ Fe2+ + H2O2 Fe2+
(C0-Ct)/C0 x 100 (%, TOC)
80
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Fe0 þ 2H2 O þ sonication ! Fe2þ þ H2 þ 2OH ,
(7)
2Fe0 þ O2 þ 2H2 O þ sonication ! 2Fe2þ þ 2H2 þ 4OH .
(8)
Interestingly, the degradation of 1,4-D in case of sonication with Fe0 at the initiation step was slower than that in sonication without Fe0 until 100 min of reaction time, as shown in Fig. 4(a). This implicates that the presence of Fe0 inhibited the sonication of 1,4-D at the initiation step. Several researchers (Arnold and Roberts, 2000; Kiwi et al., 2000) reported that Fe0 could be oxidized to Fe2+ by OH radical as shown in Eq. (9):
60
40
20
Fe0 þ OH ! Fe2þ þ OH þ e
0 0 (b)
ðinitiation stepÞ:
(9)
0
50
100 Time (min)
150
200
Fig. 3 – Effect of adding Fe2+ on the (a) sonolytic degradation and (b) mineralization of 1,4-D.
However, H2O2 can be produced by OH radical in the sonication process, as shown in Eq. (5) (Adewuyi, 2001): OH þ OH þ sonication ! H2 O2 .
(5) 2+
Fig. 3(a) also shows that, while in case of Fe only condition, the degradation of 1,4-D and TOC was only 15% and 9% in 200 min of reaction time, in case of Fe2+ with H2O2 condition, the degradation of 1,4-D and TOC increased to 35% and 37%, respectively. The condition in the presence of Fe2+ and H2O2 without sonication is called Fenton reaction, as shown in Eq. (6): Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
(6)
By comparing four reactions in Fig. 3, the sonication with Fe2+ is a more effective process than Fenton or the sonicationonly process. Also, the precipitation of Fe(OH)3 sludge, which is the important drawback in the Fenton process, was not produced in the sonication with Fe2+. This result indicates that the chain reactions of Eqs. (3) and (4) may occur in the
Fe can hinder the degradation of 1,4-D in sonolytic reaction by competing with 1,4-D in the reaction with OH radical, especially at the initiation step. However, the degradation efficiency of 1,4-D at the acceleration step was higher in case of sonication with Fe0 than in case of sonication without Fe0. This result indicates that the main form of Fe at the acceleration step is Fe2+ by the oxidation of Fe0, and the increase in the degradation of 1,4-D at the acceleration step with Fe0 is due to the same reason of Fe2+ presence in the sonication, as shown in Eq. (3). Next, Fig. 5 shows the effect of adding S2O2 8 as an oxidant on the degradation of 1-4-D and TOC in the sonication of 1,4D. Compared with sonication-only condition, the degradation efficiency of 1,4-D and TOC in the sonication with S2O2 8 was enhanced by 16.5% and 17.9% after 200 min of reaction time, respectively. The kinetics at the initiation step in the sonication with S2O28 also changed to the pseudo-first order rate model, similar to the reactions of sonication with Fe2+ and Fe0. This indicates that OH radical is also produced by the reactions between sonication and S2O2 8 , as shown in Eqs. (10) and (11): S2 O2 8 þ sonication ! 2SO4 ,
(10)
2SO 4 þ H2 O þ sonication ! HSO4 þ OH ,
(11)
S2 O2 8 þ dioxane ! 2SO4 þ dioxane .
(12)
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Table 2 – Effect of adding oxidants for sonication Oxidants
Step (min)
Zero order
Pseudo-first order
k0 101 [M1min1]
R20
k1 [ 103 min1]
R21
Sonication only
Overall Initiation (o100 min) Acceleration (o115 min)
6.74 2.22 26.0
0.822 0.970 0.767
13.20 3.88 60.81
0.807 0.858 0.985
Sonication+Fe2+
Overall Initiation (o100 min) Acceleration (o120 min)
5.09 5.35 16.42
0.754 0.888 0.798
20.42 8.53 116.40
0.913 0.952 0.990
Sonication+Fe0
Overall Initiation (o 140 min) Acceleration (o160 min)
5.08 2.51 27.56
0.807 0.913 0.796
18.26 3.35 118.7
0.898 0.935 0.954
Sonication+S2O2 8
Overall Initiation (o 120 min) Acceleration (o 140 min)
4.91 3.22 21.63
0.920 0.869 0.800
17.04 4.64 116.8
0.908 0.952 0.985
k0 is zero order, k1 is pseudo-first order rate constant, and R2 is coefficient of determination.
100
100 Sonication Sonication + Fe0 Fe0
80 (C0-Ct)/C0 x 100 (%,1,4-D)
(C0-Ct)/C0 x 100(%,1,4-D)
80
60
40
20
0
50
(a)
100 Time (min)
150
40
20
0
200
50
(a)
100 Time (min)
150
200
150
200
100
100 Sonication Sonication + Fe0 Fe0
Sonication Sonication + S2O82S2O82-
80 (C0-Ct)/C0 x 100 (%, TOC)
80 (C0-Ct)/C0 x 100 (%,TOC)
60
0
0
60
40
20
60
40
20
0
0 0 (b)
Sonication Sonication + S2O82S2O82-
50
100 Time (min)
150
0
200
Fig. 4 – Effect of adding Fe0 on the (a) sonolytic degradation and (b) mineralization of 1,4-D.
(b)
50
100 Time (min)
Fig. 5 – Effect of adding S2O2 on the (a) sonolytic 8 degradation and (b) mineralization of 1,4-D.
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As shown in Fig. 5, 1,4-D and TOC degradation in the presence of S2O2 8 -only were 34% and 32% during 200 min of reaction time. This indicates that 1,4-D can be directly degraded without the help of sonication as by the oxidation of S2O2 8 shown in Eq. (12) (Liang et al., 2004; Price et al., 1996). Statistically, the total degradation efficiencies of 1,4-D and TOC were significantly different between the conditions of sonication with and without oxidants (p ¼ 0.001, ANOVA). However, the degradation efficiencies were not statistically different among the kinds of oxidants (p ¼ 0.709, MANOVA).
4.
Conclusions
In this study, the role of OH radicals in the sonolysis of 1,4-D was investigated using HCO 3 and several oxidants. The results are summarized as follows: 1. The sonolytic degradation of 1,4-D fitted the sigmoidal pattern, which could be divided primarily into initiation and acceleration steps. From the experiment using HCO 3, it was found that OH radical is a main factor in the sonication of 1,4-D especially at the acceleration step. 2. The degradation trend at the acceleration step was also not significantly different between two conditions without HCO 3 , and with HCO3 added at 0 min. However, the rapid decrease of 1,4-D degradation was observed at the acceleration step in case with HCO 3 at 100 min of reaction time. 3. The degradation of 1,4-D at the initiation step followed the zero order rate model, while the acceleration step fitted to the pseudo-first order. During 200 min of sonication-only, total 1,4-D degradation was 79%, out of which 44% was degraded at the acceleration step. in the 4. By adding oxidants such as Fe2+, Fe0 and S2O2 8 sonication of 1,4-D facilitated production of OH radical, reduced the reaction time of the initiation step and changed the kinetics at the initiation step from zero order to pseudofirst order. Adding oxidants not only made OH radical the main degradation factor at the initiation step, but also augmented the degradation efficiency of 1,4-D and TOC. 5. The addition of oxidants such as Fe2+, Fe0 and S2O2 8 in the sonication of 1,4-D also improved the mineralization of 1,4-D; however, the degradation efficiencies were not statistically different among the kinds of oxidants. This study found that OH radical is the important factor in the sonolytic degradation of 1,4-D, and the addition of oxidants can facilitate the production of OH radicals, reduce the time of the initiation step and change the rate from zero to pseudo-first order. Several researches suggested that the increase of physical factors such as frequency in the sonication process could reduce the reaction time at the initiation step (Hill et al., 1997; Maurino et al., 1997). However, as investigated in this study, adding oxidants in the sonication process increased the OH radical, strong oxidant as well as the important factor to operate AOP processes and achieved similar results with increase by the physical factors.
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R E F E R E N C E S
Adams, C.D., Scanlan, P.A., Secrlat, N.D., 1994. Oxidation and biodegradability enhancement of 1,4-Dioxane using hydrogen peroxide and ozone. Environ. Sci. Technol. 28, 1812–1818. Adewuyi, Y.G., 2001. Sonochemistry: environmental science and engineering applications. Ind. Eng. Chem. Res. 40, 4681–4715. Arnold, W.A., Roberts, A.L., 2000. Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles. Environ. Sci. Technol. 34, 1794–1805. Beckett, M.A., Hua, I., 2000. Elucidation of the 1,4-dioxane decomposition pathway at discrete ultrasonic frequencies. Environ. Sci. Technol. 34, 3944–3953. Beckett, M.A., Hua, I., 2003. Enhanced sonochemical decomposition of 1,4-dioxane by ferrous iron. Water Res. 37, 2372–2376. Hill, R.R., Jeffs, G.E., Roberts, D.R., 1997. Photocatalytic degradation of 1,4-dioxane in aqueous solution. J. Photochem. Photobiol. A: Chem. 108, 55–58. Hiskia, A., Ecke, M., Troupes, A., Kokopakis, A., Hennig, H., Papaconstantinou, E., 2001. Sonolytic, photolytic, and photocatalytic decomposition of atrazine in the presence of polyoxometalates. Environ. Sci. Technol. 35, 2358–2364. INCHEM website, 1999. Summaries & evaluations 1,4-dioxane (group 2B). http://www.inchem.org/documents/iarc/vol71/019dioxane.html. Kavitha, V., Palanivelu, K., 2004. The role of ferrous ion in Fenton and photo-Fenton process for the degradation of phenol. Chemosphere 55, 1235–1243. Kiwi, J., Lopez, A., Nadtochenko, V., 2000. Mechanism and kinetics of the OH-radical intervention during Fenton oxidation in the presence of a significant amount of radical scavenger (Cl). Environ. Sci. Technol. 34, 2162–2168. Liang, C., Bruell, C.J., Marley, M.C., Sperry, K.L., 2004. Persulfate oxidation for in situ remediation of TCE. II Activated by chelated ferrous ion. Chemosphere 55, 1225–1233. Liao, C.H., Kang, S.F., Wu, F.A., 2001. Hydroxyl radical scavenging role of chloride and bicarbonate ions in the H2O2/UV process. Chemosphere 44, 1193–1200. Mantha, R., Taylor, K.E., Biswas, N., Bewtra, J.K., 2001. A continuous system for Fe0 reduction of nitrobenzene in synthetic wastewater. Environ. Sci. Technol. 35, 3231–3236. Maurino, V., Calza, P., Minero, C., Pelizzetti, E., Vincernti, M., 1997. Light-assisted 1,4-dioxane degradation. Chemosphere 35, 2675–2688. Prasad, N.D.V., Rajan, R., Kumar, R., Gandhi, K.S., Arakeri, V.H., Chandrasekaran, S., 1994. Modeling of a batch sonochemical reactor. Chem. Eng. Sci. 49, 877–888. Price, G.J., Clifton, A.A., Keen, F., 1996. Ultrasonically enhanced persulfate oxidation of polyethylene surfaces. Polymer 37, 5825–5829. Stefan, M.I., Bolton, J.R., 1998. Mechanism of the degradation of 1,4-dioxane in dilute aqueous solution using the UV/hydrogen peroxide process. Environ. Sci. Technol. 32, 1588–1595. Suzanne, L., Richard, E.J., Mark, W.P., Peter, G.R., 1990. Occurrence and fate of organic solvent residues in anoxic groundwater at the Gloucester Landfill, Canada. Environ. Sci. Technol. 24, 559–566. Wheat, P.E., Tumeo, M.A., 1997. Ultrasound induced aqueous polycyclic aromatic hydrocarbon reactivity. Ultrason. Sonochem. 4, 55–59. Zenker, M.J., Borden, R.C., Barlaz, M.A., 2003. Occurrence and treatment of 1,4-dioxane in aqueous environments. Environ. Eng. Sci. 20, 423–432.
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Removal of 1,4-dioxane from water using sonication: Effect of adding oxidants on the degradation kinetics Hyun-Seok Sona, Seok-Bong Choib, Eakalak Khanc, Kyung-Duk Zoha, a
Institute of Health and Environment, School of Public Health, Seoul National University, Seoul 110-799, Republic of Korea Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, ND 58105, USA c Department of Civil Engineering and Construction, North Dakota State University, Fargo, ND 58105, USA b
ar t ic l e i n f o
A B S T R A C T
Article history:
This research investigates the effect of adding oxidants such as Fe0, Fe2+ and S2O2 8 in the
Received 30 June 2005
sonication of 1,4-dioxane (1,4-D). The results indicate that the degradation pattern of 1,4-D
Received in revised form
kinetically could be divided into three steps (initiation, acceleration, and stabilization), with
24 October 2005
the first two steps predominating. The initiation step agreed with zero order rate model,
Accepted 18 November 2005
while the acceleration step was the pseudo-first order. In the presence of HCO 3 as a radical
Available online 19 January 2006
scavenger, the degradations of 1,4-D and TOC were suppressed, indicating that OH radical
Keywords:
is an important factor in the sonolysis, especially at the acceleration step. The overall
1, 4-dioxane
degradation efficiency of 79.0% in the sonolysis of 1,4-D was achieved within 200 min.
Sonication
were individually combined with sonication, the total While Fe0, Fe2+ and S2O2 8
HCO 3
degradation efficiency of 1,4-D increased 18.6%, 19.1% and 16.5% after 200 min, respectively.
Fe2+
The addition of oxidants not only increased the rate constant in the acceleration step, but
Fe0
also changed the kinetic model from zero to pseudo-first order at the initiation step. The addition of oxidants such as Fe2+, Fe0 and S2O2 8 in the sonication of 1,4-D also improved the
S2O2 8
mineralization of 1,4-D. However, the degradation efficiencies of 1,4-D and TOC were not statistically different (p ¼ 0.709, ANOVA) with different oxidants such as Fe2+, Fe0 and S2O2 8 . & 2005 Elsevier Ltd. All rights reserved.
1.
Introduction
1,4-dioxane (C4H8O2, 1,4-D) is currently used as a solvent in several industries such as manufacturing of plastics, pesticide and paints. Several studies reported that 1,4-D has been detected in surface and groundwater, and considered as an emerging water contaminant (Beckett and Hua, 2000). Some common consumer products such as detergents and shampoo also contain low levels of 1,4-D. Several studies reported that 1,4-D has been detected in surface and groundwater in Japan, Canada and the United States (Suzanne et al., 1990). The US EPA and International Agency for Research on Cancer (IARC) classified 1,4-D as a probable human carcinogen (INCHEM website, 1999). Corresponding author. Tel.: +82 2 740 8891; fax: +82 2 745 9104.
E-mail address: [email protected] (K.-D. Zoh). 0043-1354/$ - see front matter & 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.11.046
Since 1,4-D has a high water solubility of 4.31 105 mg/L, low Kow value of 100.27 and low vapor pressure (37 mmHg at 25 1C), adsorption and air stripping technology are not cost effective for removal from 1,4-D-contaminated water (Suzanne et al., 1990; Zenker et al., 2003). 1,4-D is also recalcitrant to microbial degradation due to its heterocyclic structure (Adams et al., 1994). Therefore, as alternatives, advance oxidation processes (AOPs) have been investigated as the alterative methods for treating 1,4-D (Stefan and Bolton, 1998). Among AOP processes, the sonolytic treatment of 1,4-D has been recently studied. Beckett and Hua (2000) studied the degradation of 1,4-D using ultrasound sonication, and reported increases in the biodegradability of 1,4-D after sonolysis. They found that 1,4-D degradation kinetics could
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be well-described with the pseudo-first order model at three frequencies: 205, 618 and 1017 kHz, tested. Organic pollutants in sonolytic treatment are degraded by two factors—pyrolysis and oxidation by OH radical. To increase the efficiency of sonolysis, first, improving the transmission of energy can be used by the control of frequency. A change on the frequency may amplify the cavity procedure produced by ultrasonic irradiation, which may result in increasing thermal degradation efficiency. However, such the increase of the physical parameters hinders the field application of the sonolysis process due to technical problems such as noise, energy efficiency and cost. Secondly, the addition of oxidants, such as Fe2+, may increase OH radical generation during sonolysis (Beckett and Hua, 2003). However, most studies on the sonolytic degradation of organic contaminants have relied on the use of high frequencies and power settings (Hiskia et al., 2001), making the reaction rates too fast for estimating the degradation mechanism accurately. In this study, the sonication of 1,4-D at a low frequency (20 kHz) was investigated in order to identify the roles of the OH radical and the degradation mechanism. The effect of OH radical in the sonication of 1,4-D was investigated by adding HCO 3 as a radical scavenger. The effect of adding oxidants on the degradation efficiency, such as Fe0, Fe2+ and S2O2 8 kinetics model and degradation mechanism of 1,4-D sonolysis were also examined.
40 (20 06) 69 2 – 698
heptahydrate (Merck), hydrogen peroxide (Merck), sodium persulfate (J.T. Baker), iron powder (325 mesh, Alfa Aesar) and sodium carbonate (HACH) were used as received.
2.2.
Materials and methods
2.1.
Materials
A 1,4-D solution was prepared by diluting a 1000 mg/L stock solution. The stock solution was prepared by dissolving reagent grade 1,4-D (Aldrich) in Nanopure deionization water (R ¼ 18 MO cm1, Barnstead). The initial solutions of 1,4-D of 100 mg/L were used in all experiments. Ferrous sulfate
Experimental set-up
Fig. 1 shows schematic diagram of the sonolytic reactor. All experiments were batch type and were performed using a sonicator (Sonics & Materials, VC600-1, USA) with an ultrasonic transducer of horn type, which has an active acoustical vibration area of 19.6 cm2, fixed frequency of 20 kHz and maximum input power of 600 W. Air as a dissolved gas was provided in the solution in order to facilitate cavitation and to prevent uncontrolled incoming air into the reactor (Prasad et al., 1994; Wheat and Tumeo, 1997). The solution was sparged for 3 min using air at a flow rate of 5 mL/min (monitored by a rotameter), and then stopped for the next 3 min. This aeration scheme was repeated continuously throughout the entire run. Sparging study in the absence of sonication was performed using 100 mg/L solution of 1,4-D for 2 h. The temperature was controlled at 25 1C by circulating water around the reactor. During that time period, the maximum loss of 1,4-dioxane was found to be 0.67%, indicating that there is no volatilization loss. For all experiments, the reactor was irradiated at 20 kHz of frequency and 300 W of power.
2.3.
2.
693
Experimental procedure
In order to qualitatively estimate the contribution of OH radical in the sonolysis of 1,4-D, the experiment using 0.082 mM of HCO 3 , which is known as a radical scavenger (Beckett and Hua, 2003; Hiskia et al., 2001), was performed. The effect of adding oxidants in the sonolysis of 1,4-D was as the oxidants also investigated using Fe2+, Fe0 and S2O2 8 individually at 0.5 mg/L for both experiments with and without sonication. In case of experiment in the presence of
Fig. 1 – Schematic diagram of sonolytic reactor system: (a) sonicator system, (b) sectional view of reactor, and (c) outline of sonolytic reactor system.
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Fe2+ without sonication, additional experiments were performed with and without H2O2 (0.01 mM).
2.4.
Analysis
1,4-D was analyzed using a purge and trap sample concentrator (TekMar Dohrmann 3100, TekMar Dohrmann, USA) equipped with an autosampler (TekMar Dohrmann 2016) and gas chromatograph (Hewlett Packard 6890, USA) equipped with a Hewlett Packard-5MS 5% phenyl methyl siloxane capillary column (30 m 0.25 mm 0.25 mm) and a mass selective detector (Hewlett Packard 5973). The temperature condition of purge and trap was 150 1C for line, 30 1C for purge ready, 70 1C for purge and 280 1C for bake, respectively. Purge and trap was performed with helium as a carrier gas with 14 and 10 min for purge and bake time, respectively. The gas chromatograph-mass selective detector was operated under the following conditions—10:1 split ratio, the carrier gas at 1.2 mL/min, electron ionization at 71 eV and mass scan range of 35–550 m/z. The temperature was programmed with holding 40 1C for 5 min to 250 1C at 10 1C/min, followed by 3 min hold at 250 1C for the oven and 150 1C for the inlet. TOC was measured using an ultraviolet persulfate oxidation TOC analyzer with an infrared detector (Phoenix 8000TM, TekMar Dohrmann) and a pH meter (Orion, Model 52A, USA) was used to determine pH.
3.
Results and discussion
3.1.
Role of OH radical in the sonication of 1,4-D
First, in order to identify the contribution of OH radical in the sonolytic degradation of 1,4-D, the role of OH radical were examined in the absence and presence of HCO 3 . Fig. 2 presents the percent reduction of 1,4-D during these experiments. As shown in Fig. 2, the degradation curves can be mainly divided into three regions (initiation, acceleration and stabilization steps), but the most degradation of 1,4-D was observed in two regions (initiation and acceleration steps). The degradation curves of Fig. 2 can be estimated with the sigmoidal model based on the best fits by trial and error using Eq. (1): y ¼ y0 þ
a 1 þ ðx=x0 Þb
,
(1)
where y is the degradation percent of 1,4-D and y0 is the intercept, x0 is the time at the inflection point (the time dividing between the first two stages), and a and b are dependent variables. This sigmoidal model (Eq. (1)) was used to divide the curves in Fig. 2 mainly into two stages—the initiation and the acceleration step. By fitting the data into Eq. (1), R2Eq1 value (the conformity to the sigmoidal model) of each degradation curve in the runs without HCO 3 and with HCO3 at 0 and 100 min were obtained, and found to be 0.981, 0.969 and 0.965, respectively. The R2Eq1 values obtained from Eq. (1) also provided the estimation of reaction time in each step, and this estimated reaction time in each step is shown in Table 1.
40 (2006) 692– 698
As shown in Fig. 2, the total percent degradation values during the initiation steps were not significantly different (p ¼ 0.107, two-tailed t-test) between these two cases. However, significant differences (p ¼ 0.009, two-tailed t-test) were observed between the percent degradation values at the acceleration steps. Similar statistical results were obtained between the trials without HCO 3 and with HCO3 added at 100 min (p ¼ 0.525 and 0.007, two-tailed t-test) at the initiation steps. HCO 3 is well-known OH radical scavenger as shown in Eq. (2): HCO 3 þ OH ! HCO3 þ OH .
(2)
Since HCO 3 is an ion, it will scavenge free radicals predominantly in the bulk water phase. Water-soluble compounds that are less volatile will be signicantly inuenced by the bicarbonate concentration (Kiwi. et al., 2000; Liao et al., 2001). Since 1,4-D is non-volatile, the bicarbonate can compete with 1,4-D for available free radicals in the interfacial and bulk region and decrease its decomposition efficiency. Fig. 2 shows the contribution of HCO 3 in the role of OH radical scavenger during sonolysis. As shown in Fig. 2, the effect of radical scavenging by HCO 3 mainly occurred during the acceleration step, and suggesting most OH radical in the sonication is generated at the acceleration step. In this study, pH changes during the sonication reactions were also measured. It was found that the decrease of pH (5.9–4.8) was observed in the run without HCO 3 . This maybe due to the production of nitrous acid (HNO2) and nitric acid (HNO3) during the sonication of water. Adewuyi (2001) reported that the pH decreases during the sonolysis of water with air due to the production of HNO2 and HNO3 by the various reactions with N2 and OH radical. Next, no change of pH occurred at the initial step in the run with HCO 3 added at 0 min, and a slight increase of pH (6.0–6.8) was observed at the acceleration step (after 120 min) in the run with HCO 3 . It is also found that pH increased during the reaction with the addition of HCO 3 at 100 min from 6.0 to 8.0. These results can be summarized as follows—OH radicals are mainly produced at the acceleration step, which react with HCO 3 to produce OH ion, resulting in the increase in pH of the solution. However, at the initiation step, the production of OH radical is insufficient for the reaction with HCO 3 , as shown in Eq. (2). Instead, the presence of HCO 3 may act as a buffer in the solution, resulting in almost no change of pH. The results in Fig. 2 and the pH changes during the sonication can induce that the reaction between OH radical and HCO 3 mainly occurs at the acceleration step rather than at the initial step. The sonication of 1,4-D at the initiation step may proceed by a thermal reaction, while the degradation of 1,4-D is dominated by OH radical reaction at the acceleration step. As shown in Fig. 2, thermal degradation and chemical oxidation contributed approximately 35% and 44% to the degradation of 1,4-D according to the result of the sonication without HCO 3. Linear regression analysis was performed in order to determine the conformity to zero order or the pseudo-first order model in each step, based on Eq. (1). R20, and R21 stand for the linear regression coefficients for zero order and the
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pseudo-first order model in each step, respectively. Table 1 shows the results of rate constants, and R20 and R21 values fitted by zero and first order model estimation at the initiation and acceleration steps. Table 1 shows that the initiation step in case of both without HCO 3 and with HCO3 added at 0 min follows the zero order rate model, and both acceleration steps followed the pseudo-first order rate model, respectively. However, the data in the experiment with HCO 3 added at 100 min fitted better to the zero order rate model for both initiation (R20 ¼ 0.975) and acceleration (R20 ¼ 0.991) steps. Fig. 2 also shows that approximately 20 min time shortening in the beginning time for the acceleration step was observed in case with HCO 3 addition at 100 min comparing with the condition without HCO 3 . Since the initiation step in the sonication-only takes approximately 100 min, shortening the initiation step can minimize the total reaction time to
100
Without HCO3HCO3- addition at 0 min HCO3- addition at 100 min
(C0-Ct)/C0x100 (%,1,4-D)
80
60
40
20
0 0
50
100 Time (min)
150
200
Fig. 2 – Effect of HCO 3 on the sonolysis of 1,4-D.
695
40 (20 06) 69 2 – 698
achieve maximum sonication efficiency. Different supplemental oxidants can be added to achieve this purpose.
3.2.
Effect of adding oxidants
As stated above, the contribution of OH radical is minimal than at the initiation step, and the kinetics followed zero order rate model regardless of adding HCO 3 . In the next were experiment, the oxidants such as Fe2+, Fe0 and S2O2 8 added in the sonication of 1,4-D in order to improve the degradation efficiency of 1,4-D by increasing OH radical, especially at the initiation step. First, Fig. 3 shows the sonolytic degradation effect of 1,4-D and TOC when Fe2+ was added as the oxidant. Compared with sonication-only condition, the total degradation efficiency of 1,4-D and TOC in case of sonication with Fe2+ condition was 98% and 95% during 200 min of reaction time, respectively. The result indicates that adding Fe2+ in the sonication enhanced the degradation efficiency of 1,4-D. In addition to Fig. 3, Table 2 shows the kinetic models, and the rate constants at the initiation and acceleration step. As shown in Table 2, while the kinetic model followed zero order at the initiation step, and pseudo-first order at the acceleration step, the kinetic models in both steps in case of sonication with Fe2+ were fitted better to the pseudo-first order rate model. This result indicates that, in the presence of Fe2+, the main factor for 1,4-D degradation at the initiation step as well as the acceleration step is OH radical, and the increase in the degradation rate was achieved by the increase of OH radical by adding Fe2+, especially at the initiation step. OH radical can be produced by the oxidation of Fe2+ and reduction of Fe3+ with sonication, as described in Eqs. (3) and (4) (Kavitha and Palanivelu, 2004; Liang et al., 2004): Fe2þ þ H2 O2 þ sonication ! Fe3þ þ OH þ OH ,
(3)
Fe3þ þ H2 O þ sonication ! Fe2þ þ Hþ þ OH .
(4)
As shown in Eq. (3), in case of sonication with Fe2+, H2O2 is needed in order to enhance the production of OH radical.
Table 1 – Zero (k0) and pseudo-first order (k1) rate constants, and coefficient of determination (R2) at different steps in the sonolysis of 1,4-D under different HCO 3 addition scenarios Condition
Stepa
Zero order
Pseudo-first order
k0 ( 101 M/min)
R20
k1 ( 103 min1)
R21
Without HCO 3
Overall Initiation (o100 min) Acceleration (o115 min)
6.74 2.22 26.04
0.8218 0.9697 0.7669
13.20 3.87 60.81
0.8070 0.8584 0.9853
HCO 3 added at 0 min
Overall Initiation (o120 min) Acceleration (o140 min)
6.10 2.17 28.43
0.9234 0.9632 0.8762
11.22 3.75 36.34
0.8397 0.9037 0.9998
HCO 3 added at 100 min
Overall Initiation (o80 min) Acceleration (o115 min)
5.12 2.22 13.11
0.9186 0.9746 0.9908
4.12 3.84 10.24
0.8877 0.9360 0.9335
a
Time for changing between the initiation step and acceleration step were calculated by Eq. (1).
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case of sonication with Fe2+, resulting in the inhibition of Fe(OH)3 sludge production. Next, Fig. 4 shows the effect of adding Fe0 in the sonication of 1,4-D. As shown in Fig. 4(a), compared with sonication without Fe0, the presence of Fe0 in the sonication process augmented total degradation of 1,4-D from 78% to 97% during 200 min of reaction time. In case of the Fe0-only condition, however, 1,4-D degradation was only 13% during the same period. TOC degradation during sonication of 1,4-D with and without Fe0 was 94% and 71%, respectively, and only 7% at the Fe0-only condition. Table 2 also shows the kinetics and rate constants in case of reaction involving Fe0. While the kinetics at the initiation steps in the sonication-only condition are fitted better to zero order, the data in sonication with Fe0 are fitted better to pseudo-first order at both initiation and acceleration steps. The presence of Fe0 in the sonication process may improve the production of the OH radical, as shown in Eqs. (7) and (8), resulting in the increase in the degradation efficiency of 1,4-D (Arnold and Roberts, 2000; Mantha et al., 2001):
100 Sonication Sonication + Fe2+ Fe2+ + H2O2 Fe2+
(C0-Ct)/C0x 100 (%, 1,4-D)
80
60
40
20
0 0
50
(a)
100 Time (min)
150
200
100 Sonication Sonication + Fe2+ Fe2+ + H2O2 Fe2+
(C0-Ct)/C0 x 100 (%, TOC)
80
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Fe0 þ 2H2 O þ sonication ! Fe2þ þ H2 þ 2OH ,
(7)
2Fe0 þ O2 þ 2H2 O þ sonication ! 2Fe2þ þ 2H2 þ 4OH .
(8)
Interestingly, the degradation of 1,4-D in case of sonication with Fe0 at the initiation step was slower than that in sonication without Fe0 until 100 min of reaction time, as shown in Fig. 4(a). This implicates that the presence of Fe0 inhibited the sonication of 1,4-D at the initiation step. Several researchers (Arnold and Roberts, 2000; Kiwi et al., 2000) reported that Fe0 could be oxidized to Fe2+ by OH radical as shown in Eq. (9):
60
40
20
Fe0 þ OH ! Fe2þ þ OH þ e
0 0 (b)
ðinitiation stepÞ:
(9)
0
50
100 Time (min)
150
200
Fig. 3 – Effect of adding Fe2+ on the (a) sonolytic degradation and (b) mineralization of 1,4-D.
However, H2O2 can be produced by OH radical in the sonication process, as shown in Eq. (5) (Adewuyi, 2001): OH þ OH þ sonication ! H2 O2 .
(5) 2+
Fig. 3(a) also shows that, while in case of Fe only condition, the degradation of 1,4-D and TOC was only 15% and 9% in 200 min of reaction time, in case of Fe2+ with H2O2 condition, the degradation of 1,4-D and TOC increased to 35% and 37%, respectively. The condition in the presence of Fe2+ and H2O2 without sonication is called Fenton reaction, as shown in Eq. (6): Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
(6)
By comparing four reactions in Fig. 3, the sonication with Fe2+ is a more effective process than Fenton or the sonicationonly process. Also, the precipitation of Fe(OH)3 sludge, which is the important drawback in the Fenton process, was not produced in the sonication with Fe2+. This result indicates that the chain reactions of Eqs. (3) and (4) may occur in the
Fe can hinder the degradation of 1,4-D in sonolytic reaction by competing with 1,4-D in the reaction with OH radical, especially at the initiation step. However, the degradation efficiency of 1,4-D at the acceleration step was higher in case of sonication with Fe0 than in case of sonication without Fe0. This result indicates that the main form of Fe at the acceleration step is Fe2+ by the oxidation of Fe0, and the increase in the degradation of 1,4-D at the acceleration step with Fe0 is due to the same reason of Fe2+ presence in the sonication, as shown in Eq. (3). Next, Fig. 5 shows the effect of adding S2O2 8 as an oxidant on the degradation of 1-4-D and TOC in the sonication of 1,4D. Compared with sonication-only condition, the degradation efficiency of 1,4-D and TOC in the sonication with S2O2 8 was enhanced by 16.5% and 17.9% after 200 min of reaction time, respectively. The kinetics at the initiation step in the sonication with S2O28 also changed to the pseudo-first order rate model, similar to the reactions of sonication with Fe2+ and Fe0. This indicates that OH radical is also produced by the reactions between sonication and S2O2 8 , as shown in Eqs. (10) and (11): S2 O2 8 þ sonication ! 2SO4 ,
(10)
2SO 4 þ H2 O þ sonication ! HSO4 þ OH ,
(11)
S2 O2 8 þ dioxane ! 2SO4 þ dioxane .
(12)
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Table 2 – Effect of adding oxidants for sonication Oxidants
Step (min)
Zero order
Pseudo-first order
k0 101 [M1min1]
R20
k1 [ 103 min1]
R21
Sonication only
Overall Initiation (o100 min) Acceleration (o115 min)
6.74 2.22 26.0
0.822 0.970 0.767
13.20 3.88 60.81
0.807 0.858 0.985
Sonication+Fe2+
Overall Initiation (o100 min) Acceleration (o120 min)
5.09 5.35 16.42
0.754 0.888 0.798
20.42 8.53 116.40
0.913 0.952 0.990
Sonication+Fe0
Overall Initiation (o 140 min) Acceleration (o160 min)
5.08 2.51 27.56
0.807 0.913 0.796
18.26 3.35 118.7
0.898 0.935 0.954
Sonication+S2O2 8
Overall Initiation (o 120 min) Acceleration (o 140 min)
4.91 3.22 21.63
0.920 0.869 0.800
17.04 4.64 116.8
0.908 0.952 0.985
k0 is zero order, k1 is pseudo-first order rate constant, and R2 is coefficient of determination.
100
100 Sonication Sonication + Fe0 Fe0
80 (C0-Ct)/C0 x 100 (%,1,4-D)
(C0-Ct)/C0 x 100(%,1,4-D)
80
60
40
20
0
50
(a)
100 Time (min)
150
40
20
0
200
50
(a)
100 Time (min)
150
200
150
200
100
100 Sonication Sonication + Fe0 Fe0
Sonication Sonication + S2O82S2O82-
80 (C0-Ct)/C0 x 100 (%, TOC)
80 (C0-Ct)/C0 x 100 (%,TOC)
60
0
0
60
40
20
60
40
20
0
0 0 (b)
Sonication Sonication + S2O82S2O82-
50
100 Time (min)
150
0
200
Fig. 4 – Effect of adding Fe0 on the (a) sonolytic degradation and (b) mineralization of 1,4-D.
(b)
50
100 Time (min)
Fig. 5 – Effect of adding S2O2 on the (a) sonolytic 8 degradation and (b) mineralization of 1,4-D.
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WA T E R R E S E A R C H
As shown in Fig. 5, 1,4-D and TOC degradation in the presence of S2O2 8 -only were 34% and 32% during 200 min of reaction time. This indicates that 1,4-D can be directly degraded without the help of sonication as by the oxidation of S2O2 8 shown in Eq. (12) (Liang et al., 2004; Price et al., 1996). Statistically, the total degradation efficiencies of 1,4-D and TOC were significantly different between the conditions of sonication with and without oxidants (p ¼ 0.001, ANOVA). However, the degradation efficiencies were not statistically different among the kinds of oxidants (p ¼ 0.709, MANOVA).
4.
Conclusions
In this study, the role of OH radicals in the sonolysis of 1,4-D was investigated using HCO 3 and several oxidants. The results are summarized as follows: 1. The sonolytic degradation of 1,4-D fitted the sigmoidal pattern, which could be divided primarily into initiation and acceleration steps. From the experiment using HCO 3, it was found that OH radical is a main factor in the sonication of 1,4-D especially at the acceleration step. 2. The degradation trend at the acceleration step was also not significantly different between two conditions without HCO 3 , and with HCO3 added at 0 min. However, the rapid decrease of 1,4-D degradation was observed at the acceleration step in case with HCO 3 at 100 min of reaction time. 3. The degradation of 1,4-D at the initiation step followed the zero order rate model, while the acceleration step fitted to the pseudo-first order. During 200 min of sonication-only, total 1,4-D degradation was 79%, out of which 44% was degraded at the acceleration step. in the 4. By adding oxidants such as Fe2+, Fe0 and S2O2 8 sonication of 1,4-D facilitated production of OH radical, reduced the reaction time of the initiation step and changed the kinetics at the initiation step from zero order to pseudofirst order. Adding oxidants not only made OH radical the main degradation factor at the initiation step, but also augmented the degradation efficiency of 1,4-D and TOC. 5. The addition of oxidants such as Fe2+, Fe0 and S2O2 8 in the sonication of 1,4-D also improved the mineralization of 1,4-D; however, the degradation efficiencies were not statistically different among the kinds of oxidants. This study found that OH radical is the important factor in the sonolytic degradation of 1,4-D, and the addition of oxidants can facilitate the production of OH radicals, reduce the time of the initiation step and change the rate from zero to pseudo-first order. Several researches suggested that the increase of physical factors such as frequency in the sonication process could reduce the reaction time at the initiation step (Hill et al., 1997; Maurino et al., 1997). However, as investigated in this study, adding oxidants in the sonication process increased the OH radical, strong oxidant as well as the important factor to operate AOP processes and achieved similar results with increase by the physical factors.
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