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Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesb20
Degradation of alachlor using an enhanced sono-Fenton process with efficient Fenton's reagent dosages a
a
Chikang Wang & Zonghan Liu a
Department of Environmental Engineering and Health, Yuanpei University of Medical Technology, Hsinchu, Taiwan Published online: 21 May 2015.
Click for updates To cite this article: Chikang Wang & Zonghan Liu (2015) Degradation of alachlor using an enhanced sono-Fenton process with efficient Fenton's reagent dosages, Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes, 50:7, 504-513, DOI: 10.1080/03601234.2015.1018763 To link to this article: http://dx.doi.org/10.1080/03601234.2015.1018763
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Journal of Environmental Science and Health, Part B (2015) 50, 504–513 Copyright © Taylor & Francis Group, LLC ISSN: 0360-1234 (Print); 1532-4109 (Online) DOI: 10.1080/03601234.2015.1018763
Degradation of alachlor using an enhanced sono-Fenton process with efficient Fenton’s reagent dosages CHIKANG WANG and ZONGHAN LIU
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Department of Environmental Engineering and Health, Yuanpei University of Medical Technology, Hsinchu, Taiwan
In this study, an enhanced sono-Fenton process for the degradation of alachlor is presented. At high ultrasonic power, low pH, and in the presence of adequate Fenton’s reagent dosages, alachlor degradation can reach nearly 100%. The toxicity of treated alachlor wastewater, which was measured by changes in cell viability, slightly decreased after the Fenton or ultrasound/H2O2 process and significantly decreased after the enhanced sono-Fenton process. A satisfactory relationship was observed between the total organic carbon removal and cell viability increment, indicating that alachlor mineralization is a key step in reducing the toxicity of the solution. The formation of alachlor degradation byproducts was observed during the oxidation process, in which the first step was the substitution of a chloride by a hydroxyl group. In conclusion, the enhanced sono-Fenton process was effective in the degradation and detoxification of alachlor within a short reaction time. Thus, the treated wastewater can then be passed through a biological treatment unit for further treatment. Keywords: Alachlor, cell viability, degradation, enhanced sono-Fenton process, ultrasonic powers.
Introduction The use of pesticides to inhibit the growth of insects, fungi, and weeds was common in most countries for several years. However, pesticides can acidify soil and destroy organic substrates in the soil, which decreases agricultural productivity. In addition, residual pesticides in soil not only appear in vegetable and fruit products but can also be transferred to other areas through the underground water system. Alachlor (2-chloro-20 ,60 -diethyl-N-methoxymethylacetanilide, C14H20ClNO2), which has been commercially used since 1969, is a member of the chloroacetanilide family of herbicides used in the United States, Europe, Japan, and Taiwan to inhibit the appearance of herbaceous plants and weeds in cotton, brassicas, maize, rapeseed, peanut, radish, soy bean, and sugar cane fields. Alachlor is a known carcinogen according to the United States Environmental Protection Agency (USEPA) (Group B2) and is moderately toxic to aquatic organisms.[1] Furthermore, alachlor is a highly toxic endocrine-disrupting chemical with half-life of more than 70 and 30 days in soil and Address correspondence to Chikang Wang, Department of Environmental Engineering and Health, Yuanpei University of Medical Technology, Hsinchu 300, Taiwan; E-mail: ckwang@mail. ypu.edu.tw Received December 2, 2014. Color versions of one or more figures in this article can be found online at www.tandfonline.com/lesb.
water, respectively.[2] Since alachlor is toxic to many organisms, conventional biological remediation processes are not suitable for removing this herbicide from contaminated water. Therefore, alternative treatment methods are required. Among several available treatment methods, advanced oxidation processes (AOPs) are the most promising alternatives for treating herbicides because they involve the generation of hydroxyl radicals (OH), which are nonselective and highly reactive oxidants. The sonochemical process, a combination of ultrasound and chemicals or oxidation processes, is an AOP used to degrade refractory compounds.[3–6] The ultrasonic process possesses two degradation mechanisms for organic compounds: (1) indirect oxidation by OH in the aqueous phase, and (2) direct oxidation by thermal cleavage inside the ultrasonic cavitation bubbles.[6] Many ultrasonic methods, including the ultrasound/ H2O2 and sono-Fenton process, have been implemented to degrade alachlor.[6–8] These studies were conducted using a batch chemical dosing method, and most of the studies were performed in small reactors (0.1–0.3 L). Similar results from these studies showed that excess amounts of H2O2 and Fe2C were impracticable for increasing the degradation of pollutants. Hence, an enhanced sono-Fenton process, in which H2O2 and Fe2C were dosed into the reactor drop-by-drop (namely, a continuous dosing mode), was designed in this study to degrade alachlor. The objectives of this study were to investigate the effects of initial pH, Fenton’s reagent dose, temperature, ultrasonic power,
Degradation of alachlor using an enhanced sono-Fenton process and the presence of six anions on alachlor degradation. In addition, this study presents toxicity profiles measured by cell viability before and after treatment and proposes possible degradation intermediates.
Materials and methods
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Standards and reagents Analytical grade alachlor (99.8%) was purchased from Sigma-Aldrich. The other chemical reagents used in this study included H2SO4 (>97%), NaOH (>97%), FeSO4 7H2O (>99.5%), and an aqueous solution of the purest grade of commercially available H2O2 (30%, w/w in water). These chemicals were used without further purification. Potassium hydrogen phthalate (C8H5KO4) and nhexane (C6H14) were used as standard chemicals to determine the total organic carbon (TOC) concentration and isolate alachlor from aqueous solutions via a liquid–liquid extraction procedure.
Experimental apparatus and design Figure 1 shows the schematic diagram of the sono-Fenton reactor. A sonicator (Microson VCX 750, Newtown, CT, USA) was equipped with an S sealed converter (Model CV 33, 63.5 mm in diameter and 183 mm in length) and a titanium probe tip (Part. No. 630-0210, 25 mm in diameter and 122 mm in length). This sonicator was operated at 20 kHz in a cylindrical reactor (working volume of 1 L with a cooling jacket), and a circulating temperature controller was used to maintain the reaction temperature (15– 50 C). The reactor was equipped with pH and oxidation reduction potential (ORP) meters (Suntex PC-3200, Taiwan) to monitor the pH and ORP profiles. Other reaction parameters were designed as follows: pH 3–9, H2O2 dosing rates of 1–4 mg min¡1, Fe2C dosage rates of 5–30 mg L¡1,
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and ultrasonic powers of 0–100 Watts (W). The effects of six anions, SO42¡, NO3¡, CO32¡, CH3COO¡, SO32¡, and Cl¡, on the degradation of alachlor during the enhanced sono-Fenton process were also investigated. A concentration of 50 mg L¡1 was used for all anions. The degradation of 1,000 mL of aqueous alachlor (initial concentration of 50 mg L¡1 with a theoretical TOC concentration of 31.14 mg L¡1) was conducted in the reactor, and the reaction pH was pre-adjusted using 0.1-N H2SO4 and 0.1-N NaOH. The reaction parameters for the ultrasound/Fenton processes are shown in Table 1. In this study, H2O2 and Fe2C were pre-adjusted to the desired concentrations before adding them to the reactor dropwise using a micro-pump and a continuous process. The flow rate used for H2O2 and Fe2C was 0.5 mL min¡1; thus, the total added volume over 60 min was 60 mL. The reactor was aerated at 0.2 L min¡1 during the reaction to provide sufficient dissolved oxygen. Analytical methods Alachlor was detected using a gas chromatography/flame ionization detector (GC/FID-Varian GC 3400, Mulgarve, Victoria, Australia) equipped with a DB-1 column (30 m £ 0.53 mm inner diameter (i.d.), 1.50 mm). Before analysis, each collected alachlor sample (10 mL) was extracted by shaking at a mixing speed of 150 rpm for 30 min with 1 mL of n-hexane. After extraction, 1 mL of the upper solvent layer was injected into GC/FID in the splitless mode. The GC oven was programed as follows: 50 C (holding time 10 min) to 250 C (holding time 3 min) at a rate of 10 C min¡1. The injector and detector temperatures were 200 C and 290 C, respectively. Nitrogen gas was used as the carrier gas (15 mL min¡1), and hydrogen gas (33 mL min¡1) and air (400 mL min¡1) were used for GC/FID. The method detection limit of alachlor by GC/FID was 0.022 mg L¡1, and the recovery test result was 97.0 § 3.6%. The qualification of alachlor and alachlor
Fig. 1. Schematic diagram of the experimental apparatus designed for alachlor degradation.
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Table 1. Design of reaction parameters in the decomposition of alachlor by sono-Fenton processes. Item
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Sono-Fenton
Parameters
Range
Alachlor (mg/L) pH Fe2C addition (mg L¡1) H2O2 addition (mg min¡1) 2¡ Inorganic anions (SO4 、NO3¡、CO32¡、CH3COO¡、SO32¡、Cl¡) Temp. ( C) Ultrasonic energy (W)
50 3, 5, 7, 9 0–30 1, 2, 3, 4 50 mg L¡1 for each anion test 15, 20, 30, 40, 50 0, 25, 50, 75, 100
byproducts was performed using a gas chromatographer equipped with a mass spectrometer (GC-MS-QP2010, Shimadzu, Japan) with a DB-5 MS column (length: 30 m, thickness: 0.25 mm, and diameter: 0.25 mm) in GC oven. The extraction procedures were identical to those described above for GC/FID. The GC oven temperature was increased from 50 C (holding time 1 min) to 180 C at a rate of 30 C min¡1 and then to 280 C at a rate of 10 C min¡1 (holding time 10 min). The injector and detector temperatures were 250 C and 280 C, respectively. High purity helium (99.99%) was used as the carrier gas (1.5 mL min¡1), and the sample was analyzed in the splitless mode. Mass spectra were obtained by electron-impact (EI) at 70 eV using the full-scan mode. The alachlor spectra determined for GC/FID and GC/MS are shown in Fig. 2. The H2O2 concentration was measured using a titration method with KI. The maximum detected H2O2 concentration during 60 min of experimentation without addition of H2O2 (with ultrasound only) was 2.1 mg L¡1. Alachlor mineralization was investigated by determining TOC concentration using a TOC analyzer (TOC-500, Shimadzu, Japan). Each sample collected during the reaction was analyzed in triplicate to ensure that the TOC and alachlor concentration results were accurate. If the data for the three analyzed results were significantly different (more
than 5%), then one of the results was withdrawn and the sample was analyzed again to ensure that the results were accurate. The toxicity of alachlor samples (before and after treatments) was determined by assessing cell viability, and the viability was determined by cell counting, as described below. First, water samples were sterilized by filtration through a 0.25-mm Millipore membrane filter (Millipore, Bedford, MA, USA). The rat liver cell line Clone-9 was maintained in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% (v/v) fetal bovine serum, 100 U mL¡1 penicillin, and 100 mg mL¡1 streptomycin at 37 C in a humidified incubator under 5% CO2. Confluent cultures were passaged by trypsinization. The cells were washed twice with warm DMEM (without phenol red) and then treated in a serum-free medium. In all the experiments, the cells were treated with alachlor water samples for 24 h before and after the sono-Fenton-like treatment. Next, cell viability was determined using a blue formazan assay, in which colorless 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) was metabolized to a blue product by mitochondrial dehydrogenases. Absorbance was recorded at 540 nm using a SpectraMAX 340, and the data are expressed as the mean percentage of viable cells compared with the control.
Fig. 2. The GC/FID and GC/MS spectrum of alachlor (retention time: 19.367 min).
Degradation of alachlor using an enhanced sono-Fenton process
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Results and discussion
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pH effects Figure 3 shows the alachlor degradation and TOC removal results and the ratio of TOC removal/alachlor degradation following the enhanced sono-Fenton process at different initial pH levels. In these experiments, 2-mg H2O2 min¡1, 5-mg L¡1 Fe2C, 100 W, and 20 C were used (Fig. 3). More alachlor was degraded at pH 3 than at other pH levels. The percentage of alachlor degradation at pH values of 3, 5, 7, and 9 were 44.7%, 31.2%, 30.1%, and 20.3%, respectively. The removal of TOC at different pH levels was comparable with the alachlor degradation percentages, with TOC removal percentages of 14.9%, 9.5%, 8.1% and 6.2%, respectively. Based on this observation, the ratio of TOC removal/alachlor degradation was calculated (Fig. 3), which varied from 0.27 to 0.34. This ratio could be used to understand the formation of byproducts during the oxidation process, and the low ratio reveals that most of the degraded alachlor was still present in the aqueous phase and was oxidized to byproducts. The ratio of TOC removal/alachlor degradation at pH 7 was lower than at other pH conditions, which potentially explained the higher number of OH radical recombination events resulting in the formation of H2O2 at pH 5– 8. Therefore, the number of OH radicals available for the desired degradation reaction decreased.[6] The treatment efficiency of alachlor at pH 3 (Fig. 3) was less than 50%, which was better than that at other pH levels. The low treatment efficiency could be correlated with an inadequate addition of Fenton’s reagent. In addition, when the pH value is more than 4, the produced Fe3C from the Fenton reaction easily precipitates to decrease OH formation. Therefore, an initial pH of 3 was used in the following experiments, and the effects of providing Fe2C and H2O2 were investigated.
Fig. 3. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process with a H2O2 dosing rate of 2 mg min¡1 and 5 mg L¡1 Fe2C at 20 C and pH between 3 and 9.
Fig. 4. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process at pH 3, a H2O2 dosing rate of 2 mg min¡1, Fe2C dosing rates of 5 to 30 mg L¡1, and at 20 C.
Effects of Fenton’s reagent Figure 4 shows the result of alachlor degradation and mineralization and the ratio of TOC removal/alachlor degradation at different Fe2C concentrations. The alachlor degradation increased significantly when the Fe2C concentration increased from 5 mg L¡1 to 20 mg L¡1 (the percentage of alachlor degraded was 99.7%). Higher additions of Fe2C resulted in greater OH formation; therefore the degradation results in the presence of 20 mg L¡1 Fe2C (Fig. 4) were greater than in the presence of 5 or 10 mg L¡1 Fe2C. However, when 30 mg L¡1 of Fe2C was used, the degradation and mineralization of alachlor was slightly lower than that at 20 mg L¡1 Fe2C. This result could be explained by the observations made by Li and Song,[9] who observed that excess Fe2C reacts with OH to produce Fe3C and subsequently quenches the OH oxidation reaction. The ratios of TOC removal/alachlor degradation in Fig. 4 varied from 0.21 to 0.47, and the maximum ratio was observed at 20 mg L¡1 Fe2C. This result indicated that 47% of the removed alachlor was mineralized as CO2, and the other 53% remained in solution with different types of organics. The minimum removal occurred at 10 mg L¡1 Fe2C, which was slightly lower than that at 5 mg L¡1 Fe2C. At a concentration of 10 mg L¡1 Fe2C, more than 88% of alachlor was degraded, and only 18.3% of TOC was removed. At 5 mg L¡1 Fe2C, 44.7% and 14.9% of alachlor was degraded and mineralized, respectively. Therefore, addition of 10 mg L¡1 Fe2C resulted in the formation of more OH, which resulted in the degradation of most of the alachlor. However, this increase in OH did not fully destroy or mineralize alachlor to form CO2. Therefore, TOC removal could result from thermal cleavage inside cavitation bubbles.[4,10] € Li and Song[9] and Ozdemir et al.[11] proposed that increasing the H2O2 concentration in the sono-Fenton process increases the degradation efficiency of organic compounds, but excess H2O2 would react with OH and
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Fig. 5. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process at pH 3, an Fe2C dosing rate of 20 mg L¡1, H2O2 dosing rates of 1 to 4 mg min¡1, and at 20 C.
decrease the degradation efficiency. Based on the above description, the addition of adequate concentrations of H2O2 is a key parameter when conducting the ultrasound/ Fe2C/H2O2 process. In this study, the degradation of alachlor exceeded 82% for four different H2O2 dosing rates, and the alachlor degradation efficiency increased with increase in H2O2 dosing rate (Fig. 5). Maximum alachlor degradation was observed at 2- and 3-mg H2O2 min¡1. However, the TOC removal at 2 mg H2O2 min¡1 was much higher than at other H2O2 dosing rates. Decrease in mineralization at 3 mg H2O2 min¡1 could be explained by the reaction of OH radicals with H2O2 (Eq. (1)), which occurs faster than reactions between OH radicals and organic pollutants[12]: OH C H2 O2 ! H2 O C HO2
(1)
Therefore, 2-mg H2O2 min¡1 is generally considered adequate for increasing the formation of OH for alachlor oxidation and for further mineralizing alachlor to CO2. The operational cost is considered in Fig. 5, which indicates that the addition of even 1-mg H2O2 min¡1 was less expensive than the addition at 2 mg H2O2 min¡1. In addition, mineralization accounted for only half of the alachlor degradation for 2-mg H2O2 min¡1. As H2O2 increased to 3 or 4 mg min¡1, higher operational costs and lower TOC removals were observed. Therefore, 20 mg L¡1 Fe2C and 2 mg H2O2 min¡1 are considered optimal. Effects of temperature Grcic et al.[13] proposed that increasing the temperature could increase the production of cavitation bubbles in an ultrasonic system, thereby increasing the thermal cleavage of organic compounds. However, most ultrasonic systems are equipped with a temperature controller to maintain
Fig. 6. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process at pH 3, a H2O2 dosing rate of 2 mg min¡1, an Fe2C dosing rate of 20 mg L¡1, and at 15 to 50 C.
temperature at an appropriate level to avoid the generation of excess heat in solution.[14] High temperatures can lead to sedimentation of Fe2C and the self-decomposition of H2O2, reducing the reaction rate or degradation efficiencies of organic compounds.[15] Therefore, the reaction temperature is an important parameter in the sonochemical treatment of organic compounds. Figure 6 shows the results of alachlor degradation and mineralization by the enhanced sono-Fenton process. These results indicate that the degradation and mineralization efficiencies of alachlor at 15 C were lower than at other temperatures and that 20 C was the optimal temperature for this system. In addition, the degradation efficiencies of alachlor decreased from 99.7 to 92.4% when the reaction temperatures increased from 20 to 40 C. However, difference in efficiencies among these temperatures was insignificant. During the reactions, changes in pH recorded by pH meter were between 3.0 and 3.2 for five different temperatures. Therefore, the effects of temperature on pH were neglected. In addition, the ORP values (data not shown) obtained from the reactions indicated that the maximum ORP value at 20 C was 605 mV. This value was higher than the values at 15 C and 30–50 C (ORP values were between 506 and 567 mV). The initial ORP value for all runs was approximately 270 mV. This high ORP value indicated that setting the temperature at 20 C has a higher oxidation power to oxidize alachlor. Although the higher reaction temperatures were useful for forming more cavitation bubbles for degrading organic pollutants, increase in temperature increased the vapor pressure of water and volatile solutes inside the cavitation bubbles. Therefore, the collapse of cavity was more cushioned than at the lower bulk temperatures. Therefore, increase in temperature would decrease the degradation efficiencies of organic pollutants.[16,17] This effect potentially caused the lower sonochemical degradation rate observed at higher reaction temperatures. Figure 6 also shows the ratio of TOC removal/alachlor degradation by
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Degradation of alachlor using an enhanced sono-Fenton process an enhanced sono-Fenton process at different temperatures, which increased from 0.37 (15 C) to 0.47 (20 C) before gently decreasing to 0.30 (50 C).
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Effects of ultrasonic power Ultrasonic power, which affects the production of OH radicals from water or H2O2 isolation and the formation of cavitation bubbles, is a well-known and important operational parameter in sonochemical processes.[4,6,17] Frontistis and Mantzavinos[18] and Li et al.[19] used the sonoFenton process to treat wastewater containing organic compounds and observed that greater ultrasonic power in the sono-Fenton process effectively enhanced the degradation efficiencies of organic compounds. This increase in efficiency resulted from the increased production of cavitation bubbles (namely, a micro oxidation reactor for oxidizing organic compounds), which occurred when higher ultrasonic powers were introduced into the system. In addition, this study investigated the effects of ultrasonic power on the degradation and mineralization of alachlor, as shown in Table 2. Our earlier study[20] proposed that the degradation of alachlor by ultrasound (100 W) was only 6.5% effective with 3.6% TOC removal after 60 min, which indicated that using ultrasound alone was not effective for degrading alachlor. The ultrasonic power designed at 0 W in Table 2 should be defined as the Fenton reaction (only Fe2C and H2O2 without ultrasound). After 60 min of reaction, 70.6% and 10.5% of alachlor were degraded and mineralized, respectively. The ratio of TOC removal/alachlor degradation was 0.15, indicating that only 15% of the removed alachlor was mineralized as CO2. As the ultrasonic power increased from 25 to 100 W, the degradation efficiencies of alachlor increased from 73.0 to 99.7%. The mineralization of alachlor presented comparable results, in which the mineralization efficiency of alachlor increased with increase in ultrasonic power. The highest TOC removal was 46.7%, which occurred at an ultrasonic power of 100 W. This result can be explained by the increased production of OH radicals and cavitation bubbles at higher ultrasonic powers.[21]
However, the effect of ultrasonic power at low power inputs (25 W) was insignificant. Chen and Huang[22] demonstrated that the destruction rate of nitrotoluene increased proportionately with the increasing power intensity, which could be interpreted as a significant increase in the number of bubbles near the emitting surface due to the sharply increasing power intensity. Thus, an increase in ultrasonic intensity will lead to greater sonochemical effects in collapsing bubbles. Increasing the ultrasonic intensity has been shown to increase the degradation rate of organic compounds.[23,24] The degradation of alachlor and removal of TOC at 25 W after 60 min were 73.0% and 13.4%, respectively (Table 2). These values were slightly higher than those observed from Fenton’s reaction. When the ultrasonic power was greater than 50 W, the ultrasonic wave was sufficient for isolating Fe-OOH2C; therefore, the degradation efficiency of alachlor increased. Hence, an appropriate ultrasonic power, 100 W, is necessary for enhancing the sono-Fenton process. In this study, the operational costs of the Fenton and the enhanced sono-Fenton processes for removing 1-kg alachlor were calculated, which are shown in Table 2. In the lab-scale sono-Fenton system, the two main operational costs include the consumption of electricity for operating a sonicator and the cost of fundamental chemicals used (including Fe2C, H2O2, NaOH, and H2SO4). Therefore, when the Fenton’s reaction method was used, the sonicator did not operate or consume electricity. The only operational cost when using the Fenton method was the chemical usage. Removing 1 kg of alachlor by a Fenton reaction costs USD306.7. When ultrasound was performed, the electricity consumed by sonicator was determined (the electric bill was calculated based on the standard electric rates in Taiwan). The results shown in Table 2 indicate that the operational cost for removing 1 kg of alachlor increased from USD411.7 to 513.3 as the degradation efficiencies increased when ultrasound was combined with the Fenton process (ultrasonic power from 25 to 75 W, and the reaction time for calculating total costs was 60 min). When the ultrasonic power was 100 W, a synergetic effect occurred between the Fenton’s reagent and ultrasound, and the necessary treatment duration was
Table 2. Results of alachlor degradation, TOC removal, the TOC removal/alachlor degradation ratio, and the operational costs when using the continuous dosing mode ultrasound/Fe2C/H2O2 process with different ultrasonic powers. Ultrasonic power (W) 0 25 50 75 100 *
Alachlor degradation (%)
TOC removal (%)
TOC removal/ alachlor degradation
Operational cost (USD kg¡1)
70.6 73.0 80.6 89.8 99.7
10.5 13.4 14.4 21.2 46.9
0.15 0.18 0.18 0.24 0.47
306.7* 411.7* 475.2* 513.3* 273.3**
Reaction time for calculating the total cost was 60 min. Reaction time for calculating the total cost was 10 min.
**
510 reduced to less than 10 min. Therefore, the operational cost was reduced to USD273.3 (the reaction time for calculating the total cost was 10 min). This result is a good example of the benefits of the synergetic effect between the Fenton’s reagent and the ultrasound, which saves treatment time and operational costs and enhances the treatment efficiency for degrading refractory compounds.
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Effects of anion addition The optimal operational conditions for removing 50 mg L¡1 alachlor occurred at pH 3 and 20 C with 2-mg H2O2 min¡1, 20-mg L¡1 Fe2C, and 100 W. Under these conditions, 99.7% of the alachlor was degraded within 10 min of reaction and 46.7% of TOC was removed after 60 min of reaction. However, anions are present in real wastewaters; therefore, the effects of anions should be determined. Zhang et al.[25] proposed that the presence of CO32¡ decreased the decolorization rate of C.I. Acid Orange 7. When the CO32¡ concentration was 48.2 mg L¡1, the decolorization rate and the color removal were only 50% of the values observed in the experiment without any anions. Therefore, in this study, the same concentration (50 mg L¡1) was used for the six anions to investigate the effects of anion type. Li and Song[9] investigated the effects of PO43¡, Cl¡, and SO42¡ on the decolorization of AR97 dye wastewater using a combination of ultrasound and Fenton reagents, and found that anions would react with OH radicals. This reaction consumes OH radicals in the solution phase, which decreases the decolorization rate. In this study, six anions, SO32¡, CH3COO¡, Cl¡, CO32¡, SO42¡, and NO3¡, were individually added to the solution. As shown in Fig. 7, the presence of CO32¡ significantly decreased the degradation efficiency of alachlor from nearly 100% (blank test, without anions) to only 14.9%. This observation is comparable to that of Qiang et al.,[21] and can be explained by Eq. (2). In this case, CO32¡ rapidly reacts with OH and quenches the OH, which directly inhibits alachlor degradation. The
Fig. 7. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process at pH 3, a H2O2 dosing rate of 2 mg min¡1, an Fe2C dosing rate of 20 mg L¡1, and at 20 C with different anions.
C. Wang and Z. Liu effects of inorganic anions decreased as follows: CO32– (14.9%) > NO3¡ (47.1%) > CH3COO¡ (60.9%) > Cl¡ (70.7%) > SO32¡ (72.2%) > SO42¡ (77.2%): CO3 2 ¡ C OH ! CO3 ¡ C OH ¡ :
(2)
The results shown in this study are comparable with those of most studies.[26,27] Sun et al.[27] proposed the occurrence of reactions between anions and OH and found that the effects of various inorganic anions decreased in the following order: SO32¡ > CH3COO¡ > Cl¡ > CO32– > HCO3¡ > SO42¡ > NO3¡; SO32¡. These anions significantly inhibited the degradation of pollutants in the ultrasonic system, potentially because SO32¡ either reacted with H2O2 to decrease the amount of H2O2 available for OH production or quenched OH directly, as shown in Eqs. (3) and (4). In this study, H2O2 was dosed into the reactor drop-by-drop. Thus, the H2O2 easily reacted with Fe2C to produce OH, and the effects of Eq. (3) could be ignored. This study used FeSO4 to prepare Fe2C in the sono-Fenton system and indicated that the SO42¡ anion was always present in the water sample. The presence of SO42¡ under all conditions could explain why adding additional SO32¡ and SO42¡ resulted in lower inhibition of alachlor degradation than other anions. However, the effects of SO32¡ and SO42¡ are still important because approximately 25% of the alachlor degradation was inhibited, ���SO3 2 ¡ C H2 O2 ! SO4 2 ¡ C H2 O; ���
(3)
���SO3 2 ¡ C OH ! SO3 ¡ C OH ¡ :���
(4)
Possible formation of byproducts Bagal and Gogate[6] observed that 2 hydroxy-20 60 -diethylN-methyl acetanilide (m/z 221) is one major by-product of alachlor degradation from the substitution of a chloride by a hydroxyl group. Other compounds include 2-chloro20 -60 -diethyl N-methyl acetanilide (m/z 239), 2-acetyl-6ethyl-N-(methoxymethyl) acetanilide (m/z 249), and N(2,6-diethylphenyl) methyleneamine (m/z 161). In this study, the substitution of a chloride by a hydroxyl group was also observed as the first step in alachlor degradation; thus, 2 hydroxy-20 60 -diethyl-N-methyl acetanilide (compound I) was the first by-product that appeared during the first two minutes. Qiang et al.[21] proposed that 2-chloro20 ,60 -ethylacetanilide was the possible byproduct, which appeared during alachlor ozonation. This by-product was formed by the attack of OH on CH2OCH3 groups. In this study, two byproducts, 2-chloro-20 ,60 -ethyl-N-methyl acetanilide (compound IV) and 2-chloro-20 ,60 -
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Degradation of alachlor using an enhanced sono-Fenton process
Fig. 8. Possible byproducts formed during alachlor degradation by ¢OH.
Fig. 9. MS spectra of the degradation byproducts.
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ethylacetanilide (compound V), were observed after 5 min. These byproducts could be explained by the initial OH attack on the -OCH3 group and by further attack on the CH3 group. In addition, these compounds could form the second type of major byproducts. As proposed in the study conducted by Qiang et al.,[21] this study proposes that the following possible byproduct appears during alachlor degradation: 2-chloro-20 ,60 -acetyl-acetanilide. This byproduct was formed from the addition of an oxygen group, which resulted from OH attack. However, this mechanism of oxygen addition may be insufficient for reducing the toxicity of wastewater because the molecular structure of this byproduct was almost identical to that of the original alachlor. Compound III may correspond to 2chloro-20 ,60 -diethyl-N-methoxyethyl-benzenamine, which could form from the deoxygenation of an acetanilide group. However, this compound has not been previously
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reported. Based on the identified byproducts, a possible alachlor degradation pathway is proposed, and the MS spectra of degradation byproducts are shown in Figs. 8 and 9.
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Cell viability The above results show that wastewater containing alachlor can be effectively degraded using the enhanced sonoFenton process. However, half of the carbon content remains in the water after treatment. The remaining unmineralized carbon content may increase or decrease the toxicity. Therefore, changes in toxicity must be investigated in addition to degradation efficiencies. In this study, cell viability is used to evaluate changes in wastewater toxicity. Figure 10(a) shows the cell viability test results for the untreated alachlor wastewater (item A) and the treated wastewater (after 60 min by ultrasound (item B), Fenton process (item C), ultrasound/H2O2 (item D), sono-Fenton process (item E), and blank (RO water, cell viability was defined as 100%, item F)). Figure 10(a) shows that the ultrasound or Fenton process slightly enhanced the cell viability from 70% (untreated alachlor solution) to 71% and 73%, respectively. When the alachlor was treated using the enhanced sono-Fenton process, the cell viability increased to 90%, indicating that the oxidation process could significantly reduce the toxicity of alachlor. Figure 10(b) shows the relationship between TOC removal and the cell viability profiles. A relationship was noted between TOC removal and cell viability, in which an increase in cell viability resulted from removing TOC. Therefore, the mineralization of alachlor is an important reaction for reducing toxicity.
Fig. 11. Comparison of cell viability, alachlor degradation, and TOC removal by the enhanced sono-Fenton process at different Fe2C dosages.
Figure 11 shows the results of cell viability, alachlor degradation, and TOC removal by an enhanced sono-Fenton process at different Fe2C concentrations. After a 60min reaction, Fig. 11 shows that 16.2, 43.8, 88, 99.7, and 100% of the alachlor was degraded and 12.2, 14.9, 18.3, 46.8, and 42.8 of TOC was removed at Fe2C concentrations of 0, 5, 10, 20, and 30 mg L¡1, respectively. The cell viability profiles initially increased with Fe2C addition, with the cell viability reaching up to 90% at 20 mg L¡1 Fe2C, and slightly decreased to 88% at an Fe2C concentration of 30 mg L¡1. Based on the above observations, a suitable dosage of Fe2C can effectively enhance alachlor degradation and reduce the toxicity when using an enhanced sono-Fenton process. Since the toxicity of the alachlor wastewater was reduced during the enhanced sono-Fenton process, this wastewater can be further treated using biological methods. Therefore, this enhanced sono-Fenton process can be considered as a “Green Technology.”
Conclusions
Fig. 10. (a) Cell viability profiles of (A) untreated alachlor wastewater and treated wastewater after 60 min by (B) ultrasound, (C) Fenton process, (D) ultrasound/H2O2, (E) ultrasound/ Fe2C/H2O2 process, and (F) blank (RO water). (b) Relationship between the TOC removal and cell viability in the presence of alachlor.
Alachlor degradation was strongly affected by pH, H2O2, and Fe2C dosage, temperature, ultrasonic power, and the presence of anions. The experimental results indicated that the optimal Fe2C and H2O2 doses are 20 mg L¡1 and 2 mg min¡1, respectively, for an initial alachlor concentration of 50 mg L¡1 at pH 3 and 20 C with an ultrasonic power of 100 W. Complete degradation of 50 mg L¡1 of alachlor was achieved within 10 min of using the enhanced sonoFenton reaction under optimum conditions. Five possible byproducts were observed in this study, and the substitution of chloride by a hydroxyl group could be the first step in alachlor oxidation. The experimental findings of this study indicate that higher ultrasonic powers lower operational costs by enhancing alachlor degradation and
Degradation of alachlor using an enhanced sono-Fenton process reducing the toxicity of wastewater. The CO32¡ anion significantly repressed the degradation of alachlor, and the effects of the anions decreased in the following order: CO32– > NO3¡ > CH3COO¡ > Cl¡ > SO32¡ > SO42¡. The application of the enhanced sono-Fenton process for degrading alachlor-contaminated wastewater can provide information required for designing industrial scale sonoFenton processes and for treating wastewater contaminated with pesticides.
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Funding The financial support of the National Science Council, Republic of China (No. 101-2221-E-264-005) and the Ministry of Science and Technology, Republic of China (Grant 101-2221-E-264-005 and 103-2221-E-264-001-MY2) are gratefully appreciated.
References [1] Gernjak, W.; Fuerhacker, M.; Fern andez-Iba~ nez, P.; Blanco, J.; Malato, S. Solar photo-Fenton treatment-process parameters and process control. Appl. Catal. B. 2006, 64(1–2), 121–130. [2] Katsumata, H.; Kaneco, S.; Suzuki, T.; Ohta, K.; Yobiko, Y. Photo-Fenton degradation of alachlor in the presence of citrate solution. J. Photochem. Photobiol. A. 2006, 180(1–2), 38–45. [3] Casadonte, D.J. Jr.; Flores, M.; Petrier, C. The use of pulsed ultrasound technology to improve environmental remediation: a comparative study. Environ. Technol. 2005, 26(12), 1411–1418. [4] Joseph, C.G.; Puma, G.L.; Bono, A.; Krishnaiah, D. Sonophotocatalysis in advanced oxidation process: a short review. Ultrason. Sonochem. 2009, 16(5), 583–589. [5] Joseph, C.G.; Puma, G.L.; Bono, A.; Taufiq-Yap, Y.H.; Krishnaiah, D. Operating parameters and synergistic effects of combining ultrasound and ultraviolet irradiation in the degradation of 2,4,6-trichlorophenol. Desalination 2011, 276(1–3), 303–309. [6] Bagal, M.V.; Gogate, P.R. Sonochemical degradation of alachlor in the presence of process intensifying additives. Sep. Purif. Technol. 2012, 90, 92–100. [7] Wayment, D.G.; Casadonte, D.J.M. Jr. Frequency effect on the sonochemical remediation of alachlor. Ultrason. Sonochem. 2002, 9(5), 251–257. [8] Torres, R.A.; Mosteo, R.; Petrier, C.; Pulgarin, C. Experimental design approach to the optimization of ultrasonic degradation of alachlor and enhancement of treated water biodegradability. Ultrason. Sonochem. 2009, 16(3), 425–430. [9] Li, J.T.; Song, Y.L. Degradation of AR 97 aqueous solution by combination of ultrasound and Fenton reagent. Environ. Prog. Sustain. 2010, 29(1), 101–106. [10] Mason, T.J.; Lorimer, J.P.; Bates, D.M. Quantifying sonochemistry: casting some light on a “black art.” Ultrasonics 1992, 30(1), 40–42.
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€ € [11] Ozdemir, C.; Oden, M.K.; S¸ ahinkaya, S.; G€ uc¸ l€ u, D. The sonochemical decolorisation of textile azo dye CI reactive orange 127. Color. Technol. 2011, 127(4), 268–273. [12] Weng, C.H.; Lin, Y.T.; Chang, C.K.; Liu, N. Decolourization of direct blue 15 by Fenton/ultrasonic process using a zero-valent iron aggregate catalyst. Ultrason. Sonochem. 2013, 20(3), 970–977. [13] Grcic, I.; Obradovic, M.; Vujevic, D.; Koprivanac, N. Sono-Fenton oxidation of formic acid/formate ions in an aqueous solution: from an experimental design to the mechanistic modelling. Chem. Eng. J. 2010, 164(1), 196–207. [14] Torres, R.A.; Abdelmalek, F.; Combet, E.; Petrier, C.; Pulgarin, C. A comparative study of ultrasonic cavitation and Fenton’s reagent for bisphenol: a degradation in deionised and natural waters. J. Hazard. Mater. 2007, 146(3), 546–551. [15] Farias, J.; Albizzati, E.D.; Alfano, O.M. Kinetic study of the photo-Fenton degradation of formic acid: combined effects of temperature and iron concentration. Catal. Today 2009, 144(1–2), 117–123. [16] Meshram, S.P.; Tayade, D.T.; Ingle, P.D.; Jolhe, P.D.; Diwate, B. B.; Biswas, S.B. Ultrasonic cavitation induced degradation of Congo red in aqueous solutions. Chem. Eng. Res. Bull. 2010, 14(2), 119–123. [17] Ma, Y.S. Short Review: current trends and future challenges in the application of sono-Fenton oxidation for wastewater treatment. Sustain. Environ. Res. 2012, 22(5), 271–278. [18] Frontistis, Z.; Mantzavinos D. Sonodegradation of 17a-ethynylestradiol in environmentally relevant matrices: laboratory-scale kinetic studies. Ultrason. Sonochem. 2012, 19(1), 77–84. [19] Li, Y.; Hsieh, W.P.; Mahmudov, R.; Wei, X.; Huang, C.P. Combined ultrasound and Fenton (US-Fenton) process for the treatment of ammunition wastewater. J. Hazard. Mater. 2013, 244–245, 403–411. [20] Wang, C.K.; Liu, C.H. Decontamination of alachlor herbicide wastewater by a continuous dosing mode ultrasound/Fe2C/H2O2 process. J. Environ. Sci. 2014, 26(6), 1332–1339. [21] Qiang, Z.; Liu, C.; Dong, B.; Zhang, Y. Degradation mechanism of alachlor during direct ozonation and O3/H2O2 advanced oxidation process. Chemosphere 2010, 78(5), 517–526. [22] Chen, W.S.; Huang, G.C. Sonochemical decomposition of di-nitrotoluenes and tri-nitrotoluene in wastewater. J. Hazard. Mater. 2009, 169(1–3), 868–874. [23] Guo, Z.; Zheng, Z.; Zheng, S.; Hu, W.; Feng, R. Effect of various sono-oxidation parameters on the removal of aqueous 2,4-dinitrophenol. Ultrason. Sonochem. 2005, 12(6), 461–465. [24] Katsumata, H.; Kaneco, S.; Suzuki, T.; Ohta, K.; Yobiko, Y. Sonochemical degradation of 2,3,7,8-tetrachlorodibenzo-p-dioxins in aqueous solution with Fe(III)/UV system. Chemosphere 2007, 69(8), 1261–1266. [25] Zhang, H.; Fu, H.; Zhang, D. Degradation of C.I. acid orange 7 by ultrasound-enhanced heterogeneous Fenton-like process. J. Hazard. Mater. 2009, 172(2–3), 654–660. [26] Bhatkhande, D.S.; Pangarkar, V.G.; Beenackers, A.A.C.M. Photocatalytic degradation for environmental applications – a review. J. Chem. Technol. Biotechnol. 2002, 77(1), 102–116. [27] Sun, J.H.; Sun, S.P.; Sun, J.Y.; Sun, R.X.; Qiao, L.P.; Guo, H.Q.; Fan, M.H. Degradation of azo dye acid black 1 using low concentration iron of Fenton process facilitated by ultrasonic irradiation. Ultrason. Sonochem. 2007, 14(6), 761–766.
Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesb20
Degradation of alachlor using an enhanced sono-Fenton process with efficient Fenton's reagent dosages a
a
Chikang Wang & Zonghan Liu a
Department of Environmental Engineering and Health, Yuanpei University of Medical Technology, Hsinchu, Taiwan Published online: 21 May 2015.
Click for updates To cite this article: Chikang Wang & Zonghan Liu (2015) Degradation of alachlor using an enhanced sono-Fenton process with efficient Fenton's reagent dosages, Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes, 50:7, 504-513, DOI: 10.1080/03601234.2015.1018763 To link to this article: http://dx.doi.org/10.1080/03601234.2015.1018763
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Journal of Environmental Science and Health, Part B (2015) 50, 504–513 Copyright © Taylor & Francis Group, LLC ISSN: 0360-1234 (Print); 1532-4109 (Online) DOI: 10.1080/03601234.2015.1018763
Degradation of alachlor using an enhanced sono-Fenton process with efficient Fenton’s reagent dosages CHIKANG WANG and ZONGHAN LIU
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Department of Environmental Engineering and Health, Yuanpei University of Medical Technology, Hsinchu, Taiwan
In this study, an enhanced sono-Fenton process for the degradation of alachlor is presented. At high ultrasonic power, low pH, and in the presence of adequate Fenton’s reagent dosages, alachlor degradation can reach nearly 100%. The toxicity of treated alachlor wastewater, which was measured by changes in cell viability, slightly decreased after the Fenton or ultrasound/H2O2 process and significantly decreased after the enhanced sono-Fenton process. A satisfactory relationship was observed between the total organic carbon removal and cell viability increment, indicating that alachlor mineralization is a key step in reducing the toxicity of the solution. The formation of alachlor degradation byproducts was observed during the oxidation process, in which the first step was the substitution of a chloride by a hydroxyl group. In conclusion, the enhanced sono-Fenton process was effective in the degradation and detoxification of alachlor within a short reaction time. Thus, the treated wastewater can then be passed through a biological treatment unit for further treatment. Keywords: Alachlor, cell viability, degradation, enhanced sono-Fenton process, ultrasonic powers.
Introduction The use of pesticides to inhibit the growth of insects, fungi, and weeds was common in most countries for several years. However, pesticides can acidify soil and destroy organic substrates in the soil, which decreases agricultural productivity. In addition, residual pesticides in soil not only appear in vegetable and fruit products but can also be transferred to other areas through the underground water system. Alachlor (2-chloro-20 ,60 -diethyl-N-methoxymethylacetanilide, C14H20ClNO2), which has been commercially used since 1969, is a member of the chloroacetanilide family of herbicides used in the United States, Europe, Japan, and Taiwan to inhibit the appearance of herbaceous plants and weeds in cotton, brassicas, maize, rapeseed, peanut, radish, soy bean, and sugar cane fields. Alachlor is a known carcinogen according to the United States Environmental Protection Agency (USEPA) (Group B2) and is moderately toxic to aquatic organisms.[1] Furthermore, alachlor is a highly toxic endocrine-disrupting chemical with half-life of more than 70 and 30 days in soil and Address correspondence to Chikang Wang, Department of Environmental Engineering and Health, Yuanpei University of Medical Technology, Hsinchu 300, Taiwan; E-mail: ckwang@mail. ypu.edu.tw Received December 2, 2014. Color versions of one or more figures in this article can be found online at www.tandfonline.com/lesb.
water, respectively.[2] Since alachlor is toxic to many organisms, conventional biological remediation processes are not suitable for removing this herbicide from contaminated water. Therefore, alternative treatment methods are required. Among several available treatment methods, advanced oxidation processes (AOPs) are the most promising alternatives for treating herbicides because they involve the generation of hydroxyl radicals (OH), which are nonselective and highly reactive oxidants. The sonochemical process, a combination of ultrasound and chemicals or oxidation processes, is an AOP used to degrade refractory compounds.[3–6] The ultrasonic process possesses two degradation mechanisms for organic compounds: (1) indirect oxidation by OH in the aqueous phase, and (2) direct oxidation by thermal cleavage inside the ultrasonic cavitation bubbles.[6] Many ultrasonic methods, including the ultrasound/ H2O2 and sono-Fenton process, have been implemented to degrade alachlor.[6–8] These studies were conducted using a batch chemical dosing method, and most of the studies were performed in small reactors (0.1–0.3 L). Similar results from these studies showed that excess amounts of H2O2 and Fe2C were impracticable for increasing the degradation of pollutants. Hence, an enhanced sono-Fenton process, in which H2O2 and Fe2C were dosed into the reactor drop-by-drop (namely, a continuous dosing mode), was designed in this study to degrade alachlor. The objectives of this study were to investigate the effects of initial pH, Fenton’s reagent dose, temperature, ultrasonic power,
Degradation of alachlor using an enhanced sono-Fenton process and the presence of six anions on alachlor degradation. In addition, this study presents toxicity profiles measured by cell viability before and after treatment and proposes possible degradation intermediates.
Materials and methods
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Standards and reagents Analytical grade alachlor (99.8%) was purchased from Sigma-Aldrich. The other chemical reagents used in this study included H2SO4 (>97%), NaOH (>97%), FeSO4 7H2O (>99.5%), and an aqueous solution of the purest grade of commercially available H2O2 (30%, w/w in water). These chemicals were used without further purification. Potassium hydrogen phthalate (C8H5KO4) and nhexane (C6H14) were used as standard chemicals to determine the total organic carbon (TOC) concentration and isolate alachlor from aqueous solutions via a liquid–liquid extraction procedure.
Experimental apparatus and design Figure 1 shows the schematic diagram of the sono-Fenton reactor. A sonicator (Microson VCX 750, Newtown, CT, USA) was equipped with an S sealed converter (Model CV 33, 63.5 mm in diameter and 183 mm in length) and a titanium probe tip (Part. No. 630-0210, 25 mm in diameter and 122 mm in length). This sonicator was operated at 20 kHz in a cylindrical reactor (working volume of 1 L with a cooling jacket), and a circulating temperature controller was used to maintain the reaction temperature (15– 50 C). The reactor was equipped with pH and oxidation reduction potential (ORP) meters (Suntex PC-3200, Taiwan) to monitor the pH and ORP profiles. Other reaction parameters were designed as follows: pH 3–9, H2O2 dosing rates of 1–4 mg min¡1, Fe2C dosage rates of 5–30 mg L¡1,
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and ultrasonic powers of 0–100 Watts (W). The effects of six anions, SO42¡, NO3¡, CO32¡, CH3COO¡, SO32¡, and Cl¡, on the degradation of alachlor during the enhanced sono-Fenton process were also investigated. A concentration of 50 mg L¡1 was used for all anions. The degradation of 1,000 mL of aqueous alachlor (initial concentration of 50 mg L¡1 with a theoretical TOC concentration of 31.14 mg L¡1) was conducted in the reactor, and the reaction pH was pre-adjusted using 0.1-N H2SO4 and 0.1-N NaOH. The reaction parameters for the ultrasound/Fenton processes are shown in Table 1. In this study, H2O2 and Fe2C were pre-adjusted to the desired concentrations before adding them to the reactor dropwise using a micro-pump and a continuous process. The flow rate used for H2O2 and Fe2C was 0.5 mL min¡1; thus, the total added volume over 60 min was 60 mL. The reactor was aerated at 0.2 L min¡1 during the reaction to provide sufficient dissolved oxygen. Analytical methods Alachlor was detected using a gas chromatography/flame ionization detector (GC/FID-Varian GC 3400, Mulgarve, Victoria, Australia) equipped with a DB-1 column (30 m £ 0.53 mm inner diameter (i.d.), 1.50 mm). Before analysis, each collected alachlor sample (10 mL) was extracted by shaking at a mixing speed of 150 rpm for 30 min with 1 mL of n-hexane. After extraction, 1 mL of the upper solvent layer was injected into GC/FID in the splitless mode. The GC oven was programed as follows: 50 C (holding time 10 min) to 250 C (holding time 3 min) at a rate of 10 C min¡1. The injector and detector temperatures were 200 C and 290 C, respectively. Nitrogen gas was used as the carrier gas (15 mL min¡1), and hydrogen gas (33 mL min¡1) and air (400 mL min¡1) were used for GC/FID. The method detection limit of alachlor by GC/FID was 0.022 mg L¡1, and the recovery test result was 97.0 § 3.6%. The qualification of alachlor and alachlor
Fig. 1. Schematic diagram of the experimental apparatus designed for alachlor degradation.
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Table 1. Design of reaction parameters in the decomposition of alachlor by sono-Fenton processes. Item
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Sono-Fenton
Parameters
Range
Alachlor (mg/L) pH Fe2C addition (mg L¡1) H2O2 addition (mg min¡1) 2¡ Inorganic anions (SO4 、NO3¡、CO32¡、CH3COO¡、SO32¡、Cl¡) Temp. ( C) Ultrasonic energy (W)
50 3, 5, 7, 9 0–30 1, 2, 3, 4 50 mg L¡1 for each anion test 15, 20, 30, 40, 50 0, 25, 50, 75, 100
byproducts was performed using a gas chromatographer equipped with a mass spectrometer (GC-MS-QP2010, Shimadzu, Japan) with a DB-5 MS column (length: 30 m, thickness: 0.25 mm, and diameter: 0.25 mm) in GC oven. The extraction procedures were identical to those described above for GC/FID. The GC oven temperature was increased from 50 C (holding time 1 min) to 180 C at a rate of 30 C min¡1 and then to 280 C at a rate of 10 C min¡1 (holding time 10 min). The injector and detector temperatures were 250 C and 280 C, respectively. High purity helium (99.99%) was used as the carrier gas (1.5 mL min¡1), and the sample was analyzed in the splitless mode. Mass spectra were obtained by electron-impact (EI) at 70 eV using the full-scan mode. The alachlor spectra determined for GC/FID and GC/MS are shown in Fig. 2. The H2O2 concentration was measured using a titration method with KI. The maximum detected H2O2 concentration during 60 min of experimentation without addition of H2O2 (with ultrasound only) was 2.1 mg L¡1. Alachlor mineralization was investigated by determining TOC concentration using a TOC analyzer (TOC-500, Shimadzu, Japan). Each sample collected during the reaction was analyzed in triplicate to ensure that the TOC and alachlor concentration results were accurate. If the data for the three analyzed results were significantly different (more
than 5%), then one of the results was withdrawn and the sample was analyzed again to ensure that the results were accurate. The toxicity of alachlor samples (before and after treatments) was determined by assessing cell viability, and the viability was determined by cell counting, as described below. First, water samples were sterilized by filtration through a 0.25-mm Millipore membrane filter (Millipore, Bedford, MA, USA). The rat liver cell line Clone-9 was maintained in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% (v/v) fetal bovine serum, 100 U mL¡1 penicillin, and 100 mg mL¡1 streptomycin at 37 C in a humidified incubator under 5% CO2. Confluent cultures were passaged by trypsinization. The cells were washed twice with warm DMEM (without phenol red) and then treated in a serum-free medium. In all the experiments, the cells were treated with alachlor water samples for 24 h before and after the sono-Fenton-like treatment. Next, cell viability was determined using a blue formazan assay, in which colorless 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) was metabolized to a blue product by mitochondrial dehydrogenases. Absorbance was recorded at 540 nm using a SpectraMAX 340, and the data are expressed as the mean percentage of viable cells compared with the control.
Fig. 2. The GC/FID and GC/MS spectrum of alachlor (retention time: 19.367 min).
Degradation of alachlor using an enhanced sono-Fenton process
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Results and discussion
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pH effects Figure 3 shows the alachlor degradation and TOC removal results and the ratio of TOC removal/alachlor degradation following the enhanced sono-Fenton process at different initial pH levels. In these experiments, 2-mg H2O2 min¡1, 5-mg L¡1 Fe2C, 100 W, and 20 C were used (Fig. 3). More alachlor was degraded at pH 3 than at other pH levels. The percentage of alachlor degradation at pH values of 3, 5, 7, and 9 were 44.7%, 31.2%, 30.1%, and 20.3%, respectively. The removal of TOC at different pH levels was comparable with the alachlor degradation percentages, with TOC removal percentages of 14.9%, 9.5%, 8.1% and 6.2%, respectively. Based on this observation, the ratio of TOC removal/alachlor degradation was calculated (Fig. 3), which varied from 0.27 to 0.34. This ratio could be used to understand the formation of byproducts during the oxidation process, and the low ratio reveals that most of the degraded alachlor was still present in the aqueous phase and was oxidized to byproducts. The ratio of TOC removal/alachlor degradation at pH 7 was lower than at other pH conditions, which potentially explained the higher number of OH radical recombination events resulting in the formation of H2O2 at pH 5– 8. Therefore, the number of OH radicals available for the desired degradation reaction decreased.[6] The treatment efficiency of alachlor at pH 3 (Fig. 3) was less than 50%, which was better than that at other pH levels. The low treatment efficiency could be correlated with an inadequate addition of Fenton’s reagent. In addition, when the pH value is more than 4, the produced Fe3C from the Fenton reaction easily precipitates to decrease OH formation. Therefore, an initial pH of 3 was used in the following experiments, and the effects of providing Fe2C and H2O2 were investigated.
Fig. 3. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process with a H2O2 dosing rate of 2 mg min¡1 and 5 mg L¡1 Fe2C at 20 C and pH between 3 and 9.
Fig. 4. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process at pH 3, a H2O2 dosing rate of 2 mg min¡1, Fe2C dosing rates of 5 to 30 mg L¡1, and at 20 C.
Effects of Fenton’s reagent Figure 4 shows the result of alachlor degradation and mineralization and the ratio of TOC removal/alachlor degradation at different Fe2C concentrations. The alachlor degradation increased significantly when the Fe2C concentration increased from 5 mg L¡1 to 20 mg L¡1 (the percentage of alachlor degraded was 99.7%). Higher additions of Fe2C resulted in greater OH formation; therefore the degradation results in the presence of 20 mg L¡1 Fe2C (Fig. 4) were greater than in the presence of 5 or 10 mg L¡1 Fe2C. However, when 30 mg L¡1 of Fe2C was used, the degradation and mineralization of alachlor was slightly lower than that at 20 mg L¡1 Fe2C. This result could be explained by the observations made by Li and Song,[9] who observed that excess Fe2C reacts with OH to produce Fe3C and subsequently quenches the OH oxidation reaction. The ratios of TOC removal/alachlor degradation in Fig. 4 varied from 0.21 to 0.47, and the maximum ratio was observed at 20 mg L¡1 Fe2C. This result indicated that 47% of the removed alachlor was mineralized as CO2, and the other 53% remained in solution with different types of organics. The minimum removal occurred at 10 mg L¡1 Fe2C, which was slightly lower than that at 5 mg L¡1 Fe2C. At a concentration of 10 mg L¡1 Fe2C, more than 88% of alachlor was degraded, and only 18.3% of TOC was removed. At 5 mg L¡1 Fe2C, 44.7% and 14.9% of alachlor was degraded and mineralized, respectively. Therefore, addition of 10 mg L¡1 Fe2C resulted in the formation of more OH, which resulted in the degradation of most of the alachlor. However, this increase in OH did not fully destroy or mineralize alachlor to form CO2. Therefore, TOC removal could result from thermal cleavage inside cavitation bubbles.[4,10] € Li and Song[9] and Ozdemir et al.[11] proposed that increasing the H2O2 concentration in the sono-Fenton process increases the degradation efficiency of organic compounds, but excess H2O2 would react with OH and
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Fig. 5. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process at pH 3, an Fe2C dosing rate of 20 mg L¡1, H2O2 dosing rates of 1 to 4 mg min¡1, and at 20 C.
decrease the degradation efficiency. Based on the above description, the addition of adequate concentrations of H2O2 is a key parameter when conducting the ultrasound/ Fe2C/H2O2 process. In this study, the degradation of alachlor exceeded 82% for four different H2O2 dosing rates, and the alachlor degradation efficiency increased with increase in H2O2 dosing rate (Fig. 5). Maximum alachlor degradation was observed at 2- and 3-mg H2O2 min¡1. However, the TOC removal at 2 mg H2O2 min¡1 was much higher than at other H2O2 dosing rates. Decrease in mineralization at 3 mg H2O2 min¡1 could be explained by the reaction of OH radicals with H2O2 (Eq. (1)), which occurs faster than reactions between OH radicals and organic pollutants[12]: OH C H2 O2 ! H2 O C HO2
(1)
Therefore, 2-mg H2O2 min¡1 is generally considered adequate for increasing the formation of OH for alachlor oxidation and for further mineralizing alachlor to CO2. The operational cost is considered in Fig. 5, which indicates that the addition of even 1-mg H2O2 min¡1 was less expensive than the addition at 2 mg H2O2 min¡1. In addition, mineralization accounted for only half of the alachlor degradation for 2-mg H2O2 min¡1. As H2O2 increased to 3 or 4 mg min¡1, higher operational costs and lower TOC removals were observed. Therefore, 20 mg L¡1 Fe2C and 2 mg H2O2 min¡1 are considered optimal. Effects of temperature Grcic et al.[13] proposed that increasing the temperature could increase the production of cavitation bubbles in an ultrasonic system, thereby increasing the thermal cleavage of organic compounds. However, most ultrasonic systems are equipped with a temperature controller to maintain
Fig. 6. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process at pH 3, a H2O2 dosing rate of 2 mg min¡1, an Fe2C dosing rate of 20 mg L¡1, and at 15 to 50 C.
temperature at an appropriate level to avoid the generation of excess heat in solution.[14] High temperatures can lead to sedimentation of Fe2C and the self-decomposition of H2O2, reducing the reaction rate or degradation efficiencies of organic compounds.[15] Therefore, the reaction temperature is an important parameter in the sonochemical treatment of organic compounds. Figure 6 shows the results of alachlor degradation and mineralization by the enhanced sono-Fenton process. These results indicate that the degradation and mineralization efficiencies of alachlor at 15 C were lower than at other temperatures and that 20 C was the optimal temperature for this system. In addition, the degradation efficiencies of alachlor decreased from 99.7 to 92.4% when the reaction temperatures increased from 20 to 40 C. However, difference in efficiencies among these temperatures was insignificant. During the reactions, changes in pH recorded by pH meter were between 3.0 and 3.2 for five different temperatures. Therefore, the effects of temperature on pH were neglected. In addition, the ORP values (data not shown) obtained from the reactions indicated that the maximum ORP value at 20 C was 605 mV. This value was higher than the values at 15 C and 30–50 C (ORP values were between 506 and 567 mV). The initial ORP value for all runs was approximately 270 mV. This high ORP value indicated that setting the temperature at 20 C has a higher oxidation power to oxidize alachlor. Although the higher reaction temperatures were useful for forming more cavitation bubbles for degrading organic pollutants, increase in temperature increased the vapor pressure of water and volatile solutes inside the cavitation bubbles. Therefore, the collapse of cavity was more cushioned than at the lower bulk temperatures. Therefore, increase in temperature would decrease the degradation efficiencies of organic pollutants.[16,17] This effect potentially caused the lower sonochemical degradation rate observed at higher reaction temperatures. Figure 6 also shows the ratio of TOC removal/alachlor degradation by
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Degradation of alachlor using an enhanced sono-Fenton process an enhanced sono-Fenton process at different temperatures, which increased from 0.37 (15 C) to 0.47 (20 C) before gently decreasing to 0.30 (50 C).
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Effects of ultrasonic power Ultrasonic power, which affects the production of OH radicals from water or H2O2 isolation and the formation of cavitation bubbles, is a well-known and important operational parameter in sonochemical processes.[4,6,17] Frontistis and Mantzavinos[18] and Li et al.[19] used the sonoFenton process to treat wastewater containing organic compounds and observed that greater ultrasonic power in the sono-Fenton process effectively enhanced the degradation efficiencies of organic compounds. This increase in efficiency resulted from the increased production of cavitation bubbles (namely, a micro oxidation reactor for oxidizing organic compounds), which occurred when higher ultrasonic powers were introduced into the system. In addition, this study investigated the effects of ultrasonic power on the degradation and mineralization of alachlor, as shown in Table 2. Our earlier study[20] proposed that the degradation of alachlor by ultrasound (100 W) was only 6.5% effective with 3.6% TOC removal after 60 min, which indicated that using ultrasound alone was not effective for degrading alachlor. The ultrasonic power designed at 0 W in Table 2 should be defined as the Fenton reaction (only Fe2C and H2O2 without ultrasound). After 60 min of reaction, 70.6% and 10.5% of alachlor were degraded and mineralized, respectively. The ratio of TOC removal/alachlor degradation was 0.15, indicating that only 15% of the removed alachlor was mineralized as CO2. As the ultrasonic power increased from 25 to 100 W, the degradation efficiencies of alachlor increased from 73.0 to 99.7%. The mineralization of alachlor presented comparable results, in which the mineralization efficiency of alachlor increased with increase in ultrasonic power. The highest TOC removal was 46.7%, which occurred at an ultrasonic power of 100 W. This result can be explained by the increased production of OH radicals and cavitation bubbles at higher ultrasonic powers.[21]
However, the effect of ultrasonic power at low power inputs (25 W) was insignificant. Chen and Huang[22] demonstrated that the destruction rate of nitrotoluene increased proportionately with the increasing power intensity, which could be interpreted as a significant increase in the number of bubbles near the emitting surface due to the sharply increasing power intensity. Thus, an increase in ultrasonic intensity will lead to greater sonochemical effects in collapsing bubbles. Increasing the ultrasonic intensity has been shown to increase the degradation rate of organic compounds.[23,24] The degradation of alachlor and removal of TOC at 25 W after 60 min were 73.0% and 13.4%, respectively (Table 2). These values were slightly higher than those observed from Fenton’s reaction. When the ultrasonic power was greater than 50 W, the ultrasonic wave was sufficient for isolating Fe-OOH2C; therefore, the degradation efficiency of alachlor increased. Hence, an appropriate ultrasonic power, 100 W, is necessary for enhancing the sono-Fenton process. In this study, the operational costs of the Fenton and the enhanced sono-Fenton processes for removing 1-kg alachlor were calculated, which are shown in Table 2. In the lab-scale sono-Fenton system, the two main operational costs include the consumption of electricity for operating a sonicator and the cost of fundamental chemicals used (including Fe2C, H2O2, NaOH, and H2SO4). Therefore, when the Fenton’s reaction method was used, the sonicator did not operate or consume electricity. The only operational cost when using the Fenton method was the chemical usage. Removing 1 kg of alachlor by a Fenton reaction costs USD306.7. When ultrasound was performed, the electricity consumed by sonicator was determined (the electric bill was calculated based on the standard electric rates in Taiwan). The results shown in Table 2 indicate that the operational cost for removing 1 kg of alachlor increased from USD411.7 to 513.3 as the degradation efficiencies increased when ultrasound was combined with the Fenton process (ultrasonic power from 25 to 75 W, and the reaction time for calculating total costs was 60 min). When the ultrasonic power was 100 W, a synergetic effect occurred between the Fenton’s reagent and ultrasound, and the necessary treatment duration was
Table 2. Results of alachlor degradation, TOC removal, the TOC removal/alachlor degradation ratio, and the operational costs when using the continuous dosing mode ultrasound/Fe2C/H2O2 process with different ultrasonic powers. Ultrasonic power (W) 0 25 50 75 100 *
Alachlor degradation (%)
TOC removal (%)
TOC removal/ alachlor degradation
Operational cost (USD kg¡1)
70.6 73.0 80.6 89.8 99.7
10.5 13.4 14.4 21.2 46.9
0.15 0.18 0.18 0.24 0.47
306.7* 411.7* 475.2* 513.3* 273.3**
Reaction time for calculating the total cost was 60 min. Reaction time for calculating the total cost was 10 min.
**
510 reduced to less than 10 min. Therefore, the operational cost was reduced to USD273.3 (the reaction time for calculating the total cost was 10 min). This result is a good example of the benefits of the synergetic effect between the Fenton’s reagent and the ultrasound, which saves treatment time and operational costs and enhances the treatment efficiency for degrading refractory compounds.
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Effects of anion addition The optimal operational conditions for removing 50 mg L¡1 alachlor occurred at pH 3 and 20 C with 2-mg H2O2 min¡1, 20-mg L¡1 Fe2C, and 100 W. Under these conditions, 99.7% of the alachlor was degraded within 10 min of reaction and 46.7% of TOC was removed after 60 min of reaction. However, anions are present in real wastewaters; therefore, the effects of anions should be determined. Zhang et al.[25] proposed that the presence of CO32¡ decreased the decolorization rate of C.I. Acid Orange 7. When the CO32¡ concentration was 48.2 mg L¡1, the decolorization rate and the color removal were only 50% of the values observed in the experiment without any anions. Therefore, in this study, the same concentration (50 mg L¡1) was used for the six anions to investigate the effects of anion type. Li and Song[9] investigated the effects of PO43¡, Cl¡, and SO42¡ on the decolorization of AR97 dye wastewater using a combination of ultrasound and Fenton reagents, and found that anions would react with OH radicals. This reaction consumes OH radicals in the solution phase, which decreases the decolorization rate. In this study, six anions, SO32¡, CH3COO¡, Cl¡, CO32¡, SO42¡, and NO3¡, were individually added to the solution. As shown in Fig. 7, the presence of CO32¡ significantly decreased the degradation efficiency of alachlor from nearly 100% (blank test, without anions) to only 14.9%. This observation is comparable to that of Qiang et al.,[21] and can be explained by Eq. (2). In this case, CO32¡ rapidly reacts with OH and quenches the OH, which directly inhibits alachlor degradation. The
Fig. 7. Degradation and mineralization of alachlor and the ratio of TOC removal/alachlor removal by the enhanced sono-Fenton process at pH 3, a H2O2 dosing rate of 2 mg min¡1, an Fe2C dosing rate of 20 mg L¡1, and at 20 C with different anions.
C. Wang and Z. Liu effects of inorganic anions decreased as follows: CO32– (14.9%) > NO3¡ (47.1%) > CH3COO¡ (60.9%) > Cl¡ (70.7%) > SO32¡ (72.2%) > SO42¡ (77.2%): CO3 2 ¡ C OH ! CO3 ¡ C OH ¡ :
(2)
The results shown in this study are comparable with those of most studies.[26,27] Sun et al.[27] proposed the occurrence of reactions between anions and OH and found that the effects of various inorganic anions decreased in the following order: SO32¡ > CH3COO¡ > Cl¡ > CO32– > HCO3¡ > SO42¡ > NO3¡; SO32¡. These anions significantly inhibited the degradation of pollutants in the ultrasonic system, potentially because SO32¡ either reacted with H2O2 to decrease the amount of H2O2 available for OH production or quenched OH directly, as shown in Eqs. (3) and (4). In this study, H2O2 was dosed into the reactor drop-by-drop. Thus, the H2O2 easily reacted with Fe2C to produce OH, and the effects of Eq. (3) could be ignored. This study used FeSO4 to prepare Fe2C in the sono-Fenton system and indicated that the SO42¡ anion was always present in the water sample. The presence of SO42¡ under all conditions could explain why adding additional SO32¡ and SO42¡ resulted in lower inhibition of alachlor degradation than other anions. However, the effects of SO32¡ and SO42¡ are still important because approximately 25% of the alachlor degradation was inhibited, ���SO3 2 ¡ C H2 O2 ! SO4 2 ¡ C H2 O; ���
(3)
���SO3 2 ¡ C OH ! SO3 ¡ C OH ¡ :���
(4)
Possible formation of byproducts Bagal and Gogate[6] observed that 2 hydroxy-20 60 -diethylN-methyl acetanilide (m/z 221) is one major by-product of alachlor degradation from the substitution of a chloride by a hydroxyl group. Other compounds include 2-chloro20 -60 -diethyl N-methyl acetanilide (m/z 239), 2-acetyl-6ethyl-N-(methoxymethyl) acetanilide (m/z 249), and N(2,6-diethylphenyl) methyleneamine (m/z 161). In this study, the substitution of a chloride by a hydroxyl group was also observed as the first step in alachlor degradation; thus, 2 hydroxy-20 60 -diethyl-N-methyl acetanilide (compound I) was the first by-product that appeared during the first two minutes. Qiang et al.[21] proposed that 2-chloro20 ,60 -ethylacetanilide was the possible byproduct, which appeared during alachlor ozonation. This by-product was formed by the attack of OH on CH2OCH3 groups. In this study, two byproducts, 2-chloro-20 ,60 -ethyl-N-methyl acetanilide (compound IV) and 2-chloro-20 ,60 -
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Degradation of alachlor using an enhanced sono-Fenton process
Fig. 8. Possible byproducts formed during alachlor degradation by ¢OH.
Fig. 9. MS spectra of the degradation byproducts.
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ethylacetanilide (compound V), were observed after 5 min. These byproducts could be explained by the initial OH attack on the -OCH3 group and by further attack on the CH3 group. In addition, these compounds could form the second type of major byproducts. As proposed in the study conducted by Qiang et al.,[21] this study proposes that the following possible byproduct appears during alachlor degradation: 2-chloro-20 ,60 -acetyl-acetanilide. This byproduct was formed from the addition of an oxygen group, which resulted from OH attack. However, this mechanism of oxygen addition may be insufficient for reducing the toxicity of wastewater because the molecular structure of this byproduct was almost identical to that of the original alachlor. Compound III may correspond to 2chloro-20 ,60 -diethyl-N-methoxyethyl-benzenamine, which could form from the deoxygenation of an acetanilide group. However, this compound has not been previously
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C. Wang and Z. Liu
reported. Based on the identified byproducts, a possible alachlor degradation pathway is proposed, and the MS spectra of degradation byproducts are shown in Figs. 8 and 9.
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Cell viability The above results show that wastewater containing alachlor can be effectively degraded using the enhanced sonoFenton process. However, half of the carbon content remains in the water after treatment. The remaining unmineralized carbon content may increase or decrease the toxicity. Therefore, changes in toxicity must be investigated in addition to degradation efficiencies. In this study, cell viability is used to evaluate changes in wastewater toxicity. Figure 10(a) shows the cell viability test results for the untreated alachlor wastewater (item A) and the treated wastewater (after 60 min by ultrasound (item B), Fenton process (item C), ultrasound/H2O2 (item D), sono-Fenton process (item E), and blank (RO water, cell viability was defined as 100%, item F)). Figure 10(a) shows that the ultrasound or Fenton process slightly enhanced the cell viability from 70% (untreated alachlor solution) to 71% and 73%, respectively. When the alachlor was treated using the enhanced sono-Fenton process, the cell viability increased to 90%, indicating that the oxidation process could significantly reduce the toxicity of alachlor. Figure 10(b) shows the relationship between TOC removal and the cell viability profiles. A relationship was noted between TOC removal and cell viability, in which an increase in cell viability resulted from removing TOC. Therefore, the mineralization of alachlor is an important reaction for reducing toxicity.
Fig. 11. Comparison of cell viability, alachlor degradation, and TOC removal by the enhanced sono-Fenton process at different Fe2C dosages.
Figure 11 shows the results of cell viability, alachlor degradation, and TOC removal by an enhanced sono-Fenton process at different Fe2C concentrations. After a 60min reaction, Fig. 11 shows that 16.2, 43.8, 88, 99.7, and 100% of the alachlor was degraded and 12.2, 14.9, 18.3, 46.8, and 42.8 of TOC was removed at Fe2C concentrations of 0, 5, 10, 20, and 30 mg L¡1, respectively. The cell viability profiles initially increased with Fe2C addition, with the cell viability reaching up to 90% at 20 mg L¡1 Fe2C, and slightly decreased to 88% at an Fe2C concentration of 30 mg L¡1. Based on the above observations, a suitable dosage of Fe2C can effectively enhance alachlor degradation and reduce the toxicity when using an enhanced sono-Fenton process. Since the toxicity of the alachlor wastewater was reduced during the enhanced sono-Fenton process, this wastewater can be further treated using biological methods. Therefore, this enhanced sono-Fenton process can be considered as a “Green Technology.”
Conclusions
Fig. 10. (a) Cell viability profiles of (A) untreated alachlor wastewater and treated wastewater after 60 min by (B) ultrasound, (C) Fenton process, (D) ultrasound/H2O2, (E) ultrasound/ Fe2C/H2O2 process, and (F) blank (RO water). (b) Relationship between the TOC removal and cell viability in the presence of alachlor.
Alachlor degradation was strongly affected by pH, H2O2, and Fe2C dosage, temperature, ultrasonic power, and the presence of anions. The experimental results indicated that the optimal Fe2C and H2O2 doses are 20 mg L¡1 and 2 mg min¡1, respectively, for an initial alachlor concentration of 50 mg L¡1 at pH 3 and 20 C with an ultrasonic power of 100 W. Complete degradation of 50 mg L¡1 of alachlor was achieved within 10 min of using the enhanced sonoFenton reaction under optimum conditions. Five possible byproducts were observed in this study, and the substitution of chloride by a hydroxyl group could be the first step in alachlor oxidation. The experimental findings of this study indicate that higher ultrasonic powers lower operational costs by enhancing alachlor degradation and
Degradation of alachlor using an enhanced sono-Fenton process reducing the toxicity of wastewater. The CO32¡ anion significantly repressed the degradation of alachlor, and the effects of the anions decreased in the following order: CO32– > NO3¡ > CH3COO¡ > Cl¡ > SO32¡ > SO42¡. The application of the enhanced sono-Fenton process for degrading alachlor-contaminated wastewater can provide information required for designing industrial scale sonoFenton processes and for treating wastewater contaminated with pesticides.
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Funding The financial support of the National Science Council, Republic of China (No. 101-2221-E-264-005) and the Ministry of Science and Technology, Republic of China (Grant 101-2221-E-264-005 and 103-2221-E-264-001-MY2) are gratefully appreciated.
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