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Degradation of alachlor with zero-valent iron activating persulfate oxidation
Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 379–385
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Degradation of alachlor with zero-valent iron activating persulfate oxidation Qiongfang Wang a, Yisheng Shao a,b,∗, Naiyun Gao a, Wenhai Chu a, Jing Deng a, Xiang Shen c, Xian Lu a, Yanping Zhu a, Xingya Wei a a b c
State Key Laboratory of Pollution Control Reuse, Tongji University, Mingjing Building, 1239 Siping Road, Shanghai 200092, China China Academy of Urban Planning & Design, No. 5 West Road of Che Gong Zhuang, Beijing 100037, China Department of Bridge Engineering, Tongji University, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 8 September 2015 Revised 24 February 2016 Accepted 17 March 2016 Available online 6 April 2016 Keywords: Zero-valent iron (ZVI) Oxidation Persulfate (PS) Alachlor Degradation
a b s t r a c t Alachlor, a commonly used herbicide in agriculture, has been selected as a target pollutant to evaluate the oxidative performance of zero-valent iron (ZVI) coupling with persulfate (PS) for the first time. Compared with Fe3+ and Fe2+ , ZVI achieved the best degradation effect. Bench-scale kinetics tests were conducted to demonstrate the impacts of several key factors controlling the treatment performance, including ZVI dosage, PS dosage, initial pH, temperature, nature organic matter (NOM), citrate and anions. The alachlor degradation followed a pseudo-first-order kinetics pattern and was effective in a broader pH range. The optimum ZVI to PS molar ratio was found to be 2:1. Heat could facilitate production of sulfate radicals and thus enhance the alachlor degradation. Sodium citrate as a chelating agent at an appropriate concentration could improve the alachlor decay in the Fe2+ +PS and ZVI+PS system. The removal was strongly − inhibited in system added NOM, anions of Cl− or HCO− 3 while it was not obvious impacted by adding NO3 or SO24− . Seven proposed degradation pathways were evaluated, using Liquid Chromatography–Mass Spectroscopy (LC–MS) analysis. In conclusion, ZVI+PS can be as a potential technology for purifying alachlorpolluted water. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Alachlor (2-chloro-N-2,6-diethylphenyl-N-(methoxymethyl) acetamide), a well-known pre- and post-emergence herbicide from the chloroacetanilide family, has been commonly used to control the annual grasses and many broad-leaf weeds in crops [1]. Alachlor has been classified as the carcinogen of B2 group by the Environmental Protection Agency (EPA) and has also been known as a highly toxic endocrine disrupting chemical where the permissible maximum concentration in drinking water is 20 μg/L [1,2]. As chlorine endocrine disruptor, its toxic and genotoxic effects may cause cancer and mutagenicity in laboratory animal and even contribute to infertility [3]. Alachlor has a half-life in soil of 7–38 days [4], and can leach beyond the root zone and migrate to natural waters under certain conditions [5,6]. Therefore, alachlor was frequently detected in the surface water and ground water [7]. Also, alachlor has a low ∗ Corresponding author at: State Key Laboratory of Pollution Control Reuse, Tongji University, Mingjing Building, 1239 Siping Road, Shanghai 20 0 092, China. Tel.: +86 2165982691. E-mail address: yishengshao20 0 [email protected] (Y. Shao).
molecular weight (269.77 Da), a high solubility (240 g/mL, at 25 °C). So the biological pre-treatment [8] and the traditional coagulation– sedimentation–filtration treatment processes in drinking water treatment plants, present the poor treatability for alachlor [9,10]. Therefore, there is urgent need to seek an efficient and economical technique to eliminate alachlor. Chemical oxidation processes like catalytic and non-catalytic wet air oxidation [11] and advanced oxidation processes (AOPs), including photochemical (O3 /UV and H2 O2 /UV) [9] and photocatalytical processes (TiO2 /UV) [12], Fenton, photo-Fenton [3], ozonation [13], ultrasonic oxidation [14] and PS [15] have shown successful results dealing with these recalcitrant compounds. AOPs have shown successful results dealing with the recalcitrant compounds, since it has many advantages such as relative high solubility and stability at room temperature, non-selectively widespread reactivity with environmental containments [16]. PS can be activated by heat [17], UV [18] and transition metal ions [19], generating a stronger and non-selective oxidant sulfate radical (SO·− 4 , E0 = 2.6 V). Owing to the advantages of cost effectiveness, high activity and the environmentally friendly nature, Fe2+ has been commonly selected as the activator of PS to generate SO·− 4 in practical
http://dx.doi.org/10.1016/j.jtice.2016.03.038 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Q. Wang et al. / Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 379–385 Table 1 The physicochemical properties of alachlor. Chemicals formula C14 H20 ClNO2
Chemical structure
Molecular weight (g/mol)
Water solubility (g/mL, 25 °C)
269.77
240
application [20]. However, a high concentration of Fe2+ is required in the iron activated PS system since Fe2+ is hard to be regenerated after conversion to Fe3+ [21]. The excessive Fe2+ can act as a scavenger of SO·− 4 and iron sludge generated at the end of the treatment is not reusable requiring additional treatment and disposal [22]. ZVI is non-toxic, cheap and easily obtained. It can not only be the alternative source of Fe2+ , but also recycle Fe3+ on its surface and reduce the precipitation of iron hydroxides during the reaction [23]. The use of ZVI overcomes the disadvantage of Fe2+ and has the advantage of avoiding the addition of other anions by ferrous salts [24]. ZVI can produce Fe2+ by the following reactions:
F e0 → F e2+ + 2e−1
(1)
F e0 + H2 O + 0.5O2 → F e2+ + 2OH −
(2)
F e0 + H2 O → F e2+ + 2OH − + H2
(3)
3+ S2 O28− + F e2+ → SO24− + SO·− 4 + Fe
(4)
2+ SO·− → SO24− + F e3+ 4 + Fe
(5)
2.2. Procedures All the tests were performed in 250 mL glass vessels with 200 mL simulated alachlor-contaminated water. When the PS and ZVI were added into the solution, the mechanical stirrer was switched on to maintain the solution well mixed. As the effect of temperature was considered, the vessels were installed in a water bath apparatus (SHZ-B, Shanghai Yuejin Medical Instruments Co., Ltd.), providing a desirable reaction temperature. At designated time intervals (0, 2, 4, 6, 10, 15, 30, 45, 60 min), 0.8 mL sample was taken out from each replicate vessel and put into 0.2 mL ethanol to quench the oxidation induced by any residual oxidant. After filtering, the filtrate was used for the detection of the residual. All the experiments were carried out in duplicate to ensure accurate data acquisition and interpretation. 2.3. Analytical methods The alachlor concentration was examined by using a high performance liquid chromatography (HPLC, Waters 2010, USA) equipped with a Symmetry C18 column by using a UV–vis detector (Waters 2489) at the absorption wavelength of 200 nm. The details of the alachlor analyses were summarized in the supplementary material (Text S2). 3. Results and discussion
F e0 + F e3+ → 2F e2+
(6)
F e0 + 2H + → F e2+ + H2
(7)
F e0 + S2 O28− → F e2+ + 2SO24−
(8)
To data, ZVI+PS system has been reported to remove some important organic pollutants such as p-nitorphenol [25], dyes (acid orange Ⅱ and methyl orange) [24,26] and bisphenol A [27]. However, to the best of our knowledge, very limited information on the ZVI+PS oxidation of herbicides in water is available. The object of this study was to investigate the performance of ZVI for PS activation. Then, several key influencing factors including initial ZVI dosage, PS dose, initial pH, citrate, NOM and common coexisting ions on the alachlor degradation were evaluated. Finally, the preliminary degradation mechanistic information was provided through the products identification. 2. Experimental 2.1. Materials The alachlor was supplied by Sigma-Aldrich (St. Louis, Missouri, USA). The physicochemical properties of alachlor were shown in Table 1. Sources of other chemicals and reagents used in the study were summarized in the supplementary material (Text S1).
3.1. Effect of initial ZVI dosage In this study, three different forms of iron (Fe3+ , Fe2+ and ZVI) were used to test the formation of SO·− 4 . The detailed discussion was shown in the supplementary material (Text S3 and Fig. S1). ZVI +PS system achieved the optimal performance in the degradation of alachlor compared with Fe3+ +PS and Fe2+ +PS system. Also, noted that ZVI alone did not have significant influence on the degradation of alachlor and less than 18% of alachlor was degraded under the PS alone (Fig. S2). Fig. 1a illustrated the alachlor removal under different ZVI to PS molar ratio. As observed, the alachlor removal was significantly influenced by the molar ratio of ZVI to PS and the best alachlor removal was obtained at the molar ratio of 2:1 for ZVI to PS. Of note, the alachlor degradation well followed a pseudo-first-order kinetics pattern (R2 > 0.95) with any particular ZVI dosage in 60 min (Fig. 1b). The overall rate law and the degradation efficiency for the alachlor degradation could be expressed as Eqs. S1 and S2. The pseudo-first-order reaction rate constant increased from 0.0711 to 0.1271 min−1 when the molar ratio of ZVI to PS increased from 0.5 to 2, but gradually decreased to 0.0671 min−1 as the molar ratio further increased to 4. The significant enhancement in degradation efficiency appeared to be attributed to production of more active radical species after introduction of the catalyst [19]. Excess ZVI dosage likely provided too much Fe2+ which could scavenge SO·− 4 produced in the ZVI +PS system (Eq. 5), leading to reduce the overall degradation efficiency. In general, the optimum
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Fig. 1. Effect of initial ZVI dosage on alachor degradation in the ZVI+PS system. Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 1.0 mmol/L; no pH adjustment; 30 °C.
Fig. 2. Effect of initial PS dosage on alachor degradation in the ZVI+PS system. Experimental conditions: [Alachlor] = 5 mg/L; [ZVI] = 2.0 mmol/L; no pH adjustment; 30 °C.
ZVI to PS molar ratio was 2:1 and the oxidation was complete almost in 60 min. 3.2. Effect of PS dosage Fig. 2 showed the alachlor degradation by ZVI activated PS at different PS dosages. Also, the results demonstrated that the oxidation process well followed a pseudo-first-order kinetics pattern (R2 > 0.95) with any particular PS dosage in 60 min. As shown in the insert graph of Fig. 2b, when the catalyst dosage was fixed at 2.0 mmol/L, the pseudo-first-order degradation rate constant (kobs ) increased from 8.02 × 102 to 8.49× 102 min−1 as PS increasing from 0.5 to 1.0 mmol/L, and then decreased to 4.61 × 102 min−1 when PS dosage further increased to 3.0 mmol/L. With the increase of PS dosage, large amounts of SO·− 4 were generated due to the high level of S2 O8 2− (Eq. (4)). Though some SO·− 4 were scavenged by Fe2+ (Eq. (5)), more generated Fe3+ could produce more Fe2+ (Eq. (6)). Then, lots of SO·− 4 were formed with the high concentration of S2 O8 2− and Fe2+ , leading an increasing removal efficiency of alachlor. However, high level of SO·− 4 could also be consumed by itself and S2 O8 2− (Eqs. (9) and (10)), causing the decreasing kobs with further increasing dosage of PS. ·− 2− 8 −1 −1 SO·− 4 + SO4 → S2 O8 k = 4 × 10 M s 2− ·− 5 −1 −1 S2 O28− + SO·− 4 → SO4 + S2 O8 k = 6.1 × 10 M s
(9) (10)
3.3. Effect of initial pH From Fig. 3, it could be seen that acidic circumstance favored the degradation of alachlor. The reason might be ascribed to the increase of the formation rate of Fe2+ (Eq. (7)), which could generate more SO·− 4 in the ZVI+PS system. The ZVI+PS system was effective for the alachlor degradation over a broad pH range of 1.5– 6.7, in which more than 99% of alachlor was decomposed within
Fig. 3. Effect of initial pH on alachlor degradation in the ZVI+PS system. The insert of the table was the initial and final pH value of each setting point. Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 0.5 mmol/L; [ZVI] = 1.0 mmol/L; 30 °C.
60 min. Although the alachlor removal efficiency under different pH (1.5–6.7) over 60 min were close, their degradation rates were significantly different. Especially, more than 89.39% alachlor was degraded in 4 min at pH of 1.5 and more than 97% alachlor was removed in 15 min at pH 3.0. So the efficient degradation of alachlor was obtained in 30 min at the range of 1.5–3.0. This finding was in agreement with a previous study that a low pH (pH 3.0) favored the degradation of 4-chlorophenol in the Fe/PS system [28]. When the reaction time were 4 and 6 min, the removal rates of alachlor were 94.23%, 94.29% at pH 1.5 and 95.99 %, 97.10% at pH 3.0. Although the increase of H+ in acidic condition might result in the rapid formation of Fe2+ by H+ (Eq. (7)), the acid-catalyzation (Eqs. (11) and (12)) could also accelerate the
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Fig. 4. Effect of temperature on degradation of alachlor. a.in only PS system; b in ZVI+PS system. The insert of the figure is Arrhenius plot – lnkobs vs. (1/T) x 103 .Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 0.5 mmol/L; [ZVI] = 1.0 mmol/L; no pH adjustment.
·− formation of SO·− in the ZVI+PS system might 4 . Excess SO4 2− promote the scavenging of SO·− 4 by itself (Eq. (9)) and S2 O8 (Eq. (10)), resulting in the reduction of the alachlor degradation.
S2 O28− + H + → H S2 O− 8
(11)
2− ·− + H S2 O− 8 → H + SO4 + SO4
(12)
Alkaline conditions remarkably inhibited the degradation, the removal rate decreased from 94.36% to 29.08% when the pH increased from 6.7 to 10. Because Fe2+ and Fe3+ were almost insoluble at such a high pH, the iron ions rapidly formed iron hydroxides and escaped from the reaction system. In addition, the presence of SO·− 4 could result in radical interconversion reactions and produce hydroxyl radicals (Eqs. (13) and (14)) [29]. The generated OH· could be readily scavenged by SO·− 4 that was abundant in the ZVI+PS system. 2− − · SO·− 4 + OH → SO4 + OH
(13)
SO·− 4
(14)
+ H2 O →
SO24−
+ OH · + H +
The pH of ultrapure water and original alachlor solution alone was 6.4 and 6.7, respectively. The final pH of each experiment setting pH were also detected and shown in the insert table of Fig. 3. All the solutions evolved into acidity after 60 min. Moreover, the eventual pH in the PS alone and ZVI alone system was 5.8 and 6.8, respectively. So it was easy to understand that the addition of PS greatly changed the reaction environment. Then some acids might be produced in the ZVI+PS system, leading a more acidic condition. 3.4. Effect of temperature The effect of temperature on the degradation was shown in Fig. 4. Both kinetics and thermodynamics parameters for the degradation processes were displayed in Table S1. Also, the degradation process followed the pseudo-first-order kinetics pattern. The degradation efficiency of alachlor with PS alone was 17.97%, 25.68%, 38.95% and 77.55% at different temperature 30, 40, 50 and 60 °C, respectively. Obviously, due to the increasing production of SO·− 4 in the system with temperature rising, the degradation rate of alachlor increased. As expected, the ZVI +PS system could achieve a higher alachlor degradation rate than the PS alone at any temperature. For example, the degradation rate could achieve 94.3% in ZVI+PS system at 60 °C within 15 min, which was about 3 times higher than that of PS alone system under th identical conditions. Also, the activation energy was calculated through Arrhenius equation in Text S5 and Table S1.
Fig. 5. Effect of NOM on the degradation of alachlor. Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 0.5 mmol/L; [ZVI] = 1.0 mmol/L; no pH adjustment; 30 °C.
3.5. Effects of citrate and NOM dosage Citrate as chelating agent had been used to enhance Fe2+ +PS oxidation of certain pollutant [30], because the chelating agent could increase Fe2+ solubility at neutral pH. Rastogi et al. [20] have also used the citrate acid (CA) as the chelating agents to stabilize iron ions in solution and accelerate the generation of reactive oxidants. Both the degradation in Fe2+ +PS and ZVI+PS systems were promoted by citrate and CA shown in Fig. S3 and discussed in Text S6. Different dosages of NOM (0–5 mg/L) and citrate dosage were added in the samples to elucidate their effects on the degradation of alachlor, respectively. As displayed in Fig. 5, the NOM greatly inhibited the degradation of alachlor in the ZVI+PS system, with the removal rate of 97.8%, 90.0%, 79.7% and 78.6% at the dosage of 0, 1, 3 and 5 mg/L, respectively. Because NOM as the organics could compete for the radicals participating in the main degradation reaction, leading to the alachlor degradation slow down. 3.6. Effects of anions The reaction activity of PS might be influenced by the background anions in natural water. In this study, the efficiency of degradation alachlor influenced by different anions, such as NO− 3, SO24− , Cl− and HCO− with different dosages were considered, the 3 results were shown in Fig. 6. As shown, the lower concentration of Cl− (1 and 10 mmol/L) posed an unapparent effect to the degradation of alachlor. However, the oxidation process was greatly suppressed with the increasing concentration of Cl− . This might be accounted for that Cl− react with SO·− to generate large 4 ·− amounts of free radicals containing chlorine, such as Cl· and Cl2 − (Eqs. (15) and (16)). The Cl could compete with the alachlor for
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− 2− Fig. 6. Effect of anions on the degradation of alachlor. a. Cl− ; b. HCO− 3 ; c. NO3 ; d. SO4 . Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 0.5 mmol/L; [ZVI] = 1.0 mmol/L; no pH adjustment; 30 °C.
− the consumption of SO·− 4 when at a high concentration of Cl .
SO·− 4
−
+ Cl →
SO24−
+ Cl
·
·−
C l − + C l · → Cl 2
(15)
(16)
The same result was also reported by Tan et al. [31], who employed PS to degrade the antipyrine. Previous research also demonstrated that the interference effect of Cl− had correlation with generation rate of SO·− 4 [32]. So when the concentration of Cl− was 100 mmol/L, SO·− was quenched to a great extent. 4 Compared with Cl− , the inhibition of the HCO− 3 contributed to the ZVI+PS system was more significant. Three reasons accounted for this: (1) when the HCO− 3 was added into the water, the solution turned acidic to alkaline environment, in which condition the alachlor degradation rate decreased discussed in the chapter 3.3. ·− (2) HCO− 3 played a role as the quencher of SO4 . (3) There was a dissociation equilibrium of HCO− in the water, in which HCO− 3 3 and 2− 2− 2 + CO3 coexisted, CO3 could combine Fe to form precipitation. So the removal rate was strongly inhibited in system added anion of − Cl- or HCO− 3 , while it was not obviously impacted by adding NO3 2− or SO4 discussed in Text S7. 3.7. Proposed degradation pathways The proposed degradation pathways were shown in Fig. 7. It had been observed that the degradation of alachlor followed seven main proposed pathways (from I to VII) during the oxidation process. In the degradation pathways of I, II and III, the attacked site of alachlor could occur on the ethyl, N-methoxymethyl, Nchloroacetyl groups or the benzene ring. The ethyl-side chain could be oxidized to an acetyl group and yield compound 1 or compound 4. Then, oxidation of the ethyl chain of compound 1 or compound 4 would yield compound 5. Besides the oxidation of the aryl–ethyl group, cleavage of the N-methoxymethyl group was a significant feature of environmental degradation of alachlor [33].
By analogy, compound 9 could also be produced during the oxidation of alachlor. Successively, both oxidation of the aryl-ethyl group of compound 9 and N-dealkylation of compound 6 would yield compound 7. Cyclization was an important pathway in photo degradation and photocatalytic degradation of alachlor [34]. In this study, cyclization was initiated by N-dealkylation of alachlor and compound 9 transformed into compound 10. Further oxidation of compound 10 or cyclization of compound 7 gave rise to compound 8. However, the cleavage of the N-chloroacetyl moiety tended to occur and thus compound 11 could be generated. Further cyclization of compound 11 produced compounds 12 and 13. Also, alachlor could lose CH3 OH and cyclization was occurred to form compound 14. Then, the dechlorination led the compound 14 to transform into compound 15 by losing CH3 OCl. The compound 15 could continue lose CH3 or C2 H4 to form compound 16 or compound 17. Also, the compound 14 could be dehydrated to form cyclization compound 18 which could generate compound 19 with further dechlorination. The benzene ring or the aryl-ethyl group might also be attacked during the oxidation, producing compounds 20, 21, 22 and 23, according the proposed pathways of VI and VII. Somich et al. [35] proposed that the benzene ring cleavage could occur during the degradation of alachlor by ozone, and thus formic, acetic, propionic and oxalic acids were generated. Qiang et al. [36] had undertaken some research about degradation mechanism of alachlor by ozonation (E0 = 2.01 V) and OH · (E0 = 2.8 V), proposing the degradation pathways like II, III, IV, VI and VII. The electrophilic attack of ozone on the benzene ring or the aryl–ethyl group would produce compounds III and IV which were also detected during photocatalytic oxidation of alachlor [34]. Also, Li et al. [37] had considered the decomposition of alachlor by ozonation, showing the degradation pathway I. Also, as shown · in the Eq. (13), SO·− 4 (E0 = 2.6 V) could transform into OH . While in the combined system, oxidation potential of two free radicals was close to each other. Thereby, it was easy to understand the proposed pathways occurred in this research considering the two reasons.
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Fig. 7. Proposed pathways for alachlor degradation.
4. Conclusion This study explored ZVI as an alternative iron source to activate PS and degrade alachlor, a representative herbicide in water. The principal oxidant responsible for the oxidation was sulfate radicals produced from persulfate activated by ZVI gradually releasing Fe2+ . Compared with Fe2+ , ZVI can minimize the SO·− 4 scavenging and thus improve the iron reuse efficiency by reduction of Fe3+ to Fe2+ . The alachlor was degraded by a synesthetic effect by combination of ZVI and PS in the oxidation process. The reaction rate was significantly influenced by factors such as ZVI dosage, PS dosage, initial pH, temperature, NOM, citrate and anions. The intermediate products were investigated, proposing seven degradation pathways. This study showed encouraging results, and demonstrated that the ZVI activated persulfate oxidation was a promising technology for control of the water pollution caused by alachlor. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no. 51178321), the National Major Project of Science & Technology Ministry of China (nos. 2012ZX07403-001, 2012ZX07403-0 02 and 20 08ZX07421–0 02), the Specialized Research Fund for the Doctoral Program of Higher
Education (no. 20120 072110 050), the research and development Project of Ministry of Housing and Urban-Rural Development (no. 2009-K7-4) and the Fundamental Research Funds for the Central Universities (no. 0400219279). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2016.03.038. References [1] National Primary Drinking Water Regulations. United States Environmental Protection Agency (EPA). Retrieved 4. September 2015. [2] Bagal MV, Gogate PR. Sonochemical degradation of alachlor in the presence of process intensifying additives. Sep Purif Technol 2012;90:92–100. [3] Ballesteros Martín MM, Sánchez Pérez JA, García Sánchez JL, Montes de Oca L, Casas López JL, Oller I, et al. Degradation of alachlor and pyrimethanil by combined photo-Fenton and biological oxidation. J Hazard Mater 2008;155:342–9. [4] Conway RW. Environmental risk analysis for chemicals. 1st ed. New York: Van Nostrand Reinhold; 1982. [5] US Environmental Protection Agency Alachlor. Rev Environ Contam Toxicol 1988;104:11. [6] Chesters G, Simsiman GV, Levy J, Alhajjar BJ, Fathulla RN, Harkin JM. Environmental fate of alachlor and metolachlor. Rev Environ Contam Toxocol 1989;110:1–74. [7] Ritter WF. Pesticide contamination of ground water in the United States-a review. J Environ Sci Health B 1990;25:1–29.
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Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Degradation of alachlor with zero-valent iron activating persulfate oxidation Qiongfang Wang a, Yisheng Shao a,b,∗, Naiyun Gao a, Wenhai Chu a, Jing Deng a, Xiang Shen c, Xian Lu a, Yanping Zhu a, Xingya Wei a a b c
State Key Laboratory of Pollution Control Reuse, Tongji University, Mingjing Building, 1239 Siping Road, Shanghai 200092, China China Academy of Urban Planning & Design, No. 5 West Road of Che Gong Zhuang, Beijing 100037, China Department of Bridge Engineering, Tongji University, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 8 September 2015 Revised 24 February 2016 Accepted 17 March 2016 Available online 6 April 2016 Keywords: Zero-valent iron (ZVI) Oxidation Persulfate (PS) Alachlor Degradation
a b s t r a c t Alachlor, a commonly used herbicide in agriculture, has been selected as a target pollutant to evaluate the oxidative performance of zero-valent iron (ZVI) coupling with persulfate (PS) for the first time. Compared with Fe3+ and Fe2+ , ZVI achieved the best degradation effect. Bench-scale kinetics tests were conducted to demonstrate the impacts of several key factors controlling the treatment performance, including ZVI dosage, PS dosage, initial pH, temperature, nature organic matter (NOM), citrate and anions. The alachlor degradation followed a pseudo-first-order kinetics pattern and was effective in a broader pH range. The optimum ZVI to PS molar ratio was found to be 2:1. Heat could facilitate production of sulfate radicals and thus enhance the alachlor degradation. Sodium citrate as a chelating agent at an appropriate concentration could improve the alachlor decay in the Fe2+ +PS and ZVI+PS system. The removal was strongly − inhibited in system added NOM, anions of Cl− or HCO− 3 while it was not obvious impacted by adding NO3 or SO24− . Seven proposed degradation pathways were evaluated, using Liquid Chromatography–Mass Spectroscopy (LC–MS) analysis. In conclusion, ZVI+PS can be as a potential technology for purifying alachlorpolluted water. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Alachlor (2-chloro-N-2,6-diethylphenyl-N-(methoxymethyl) acetamide), a well-known pre- and post-emergence herbicide from the chloroacetanilide family, has been commonly used to control the annual grasses and many broad-leaf weeds in crops [1]. Alachlor has been classified as the carcinogen of B2 group by the Environmental Protection Agency (EPA) and has also been known as a highly toxic endocrine disrupting chemical where the permissible maximum concentration in drinking water is 20 μg/L [1,2]. As chlorine endocrine disruptor, its toxic and genotoxic effects may cause cancer and mutagenicity in laboratory animal and even contribute to infertility [3]. Alachlor has a half-life in soil of 7–38 days [4], and can leach beyond the root zone and migrate to natural waters under certain conditions [5,6]. Therefore, alachlor was frequently detected in the surface water and ground water [7]. Also, alachlor has a low ∗ Corresponding author at: State Key Laboratory of Pollution Control Reuse, Tongji University, Mingjing Building, 1239 Siping Road, Shanghai 20 0 092, China. Tel.: +86 2165982691. E-mail address: yishengshao20 0 [email protected] (Y. Shao).
molecular weight (269.77 Da), a high solubility (240 g/mL, at 25 °C). So the biological pre-treatment [8] and the traditional coagulation– sedimentation–filtration treatment processes in drinking water treatment plants, present the poor treatability for alachlor [9,10]. Therefore, there is urgent need to seek an efficient and economical technique to eliminate alachlor. Chemical oxidation processes like catalytic and non-catalytic wet air oxidation [11] and advanced oxidation processes (AOPs), including photochemical (O3 /UV and H2 O2 /UV) [9] and photocatalytical processes (TiO2 /UV) [12], Fenton, photo-Fenton [3], ozonation [13], ultrasonic oxidation [14] and PS [15] have shown successful results dealing with these recalcitrant compounds. AOPs have shown successful results dealing with the recalcitrant compounds, since it has many advantages such as relative high solubility and stability at room temperature, non-selectively widespread reactivity with environmental containments [16]. PS can be activated by heat [17], UV [18] and transition metal ions [19], generating a stronger and non-selective oxidant sulfate radical (SO·− 4 , E0 = 2.6 V). Owing to the advantages of cost effectiveness, high activity and the environmentally friendly nature, Fe2+ has been commonly selected as the activator of PS to generate SO·− 4 in practical
http://dx.doi.org/10.1016/j.jtice.2016.03.038 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Q. Wang et al. / Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 379–385 Table 1 The physicochemical properties of alachlor. Chemicals formula C14 H20 ClNO2
Chemical structure
Molecular weight (g/mol)
Water solubility (g/mL, 25 °C)
269.77
240
application [20]. However, a high concentration of Fe2+ is required in the iron activated PS system since Fe2+ is hard to be regenerated after conversion to Fe3+ [21]. The excessive Fe2+ can act as a scavenger of SO·− 4 and iron sludge generated at the end of the treatment is not reusable requiring additional treatment and disposal [22]. ZVI is non-toxic, cheap and easily obtained. It can not only be the alternative source of Fe2+ , but also recycle Fe3+ on its surface and reduce the precipitation of iron hydroxides during the reaction [23]. The use of ZVI overcomes the disadvantage of Fe2+ and has the advantage of avoiding the addition of other anions by ferrous salts [24]. ZVI can produce Fe2+ by the following reactions:
F e0 → F e2+ + 2e−1
(1)
F e0 + H2 O + 0.5O2 → F e2+ + 2OH −
(2)
F e0 + H2 O → F e2+ + 2OH − + H2
(3)
3+ S2 O28− + F e2+ → SO24− + SO·− 4 + Fe
(4)
2+ SO·− → SO24− + F e3+ 4 + Fe
(5)
2.2. Procedures All the tests were performed in 250 mL glass vessels with 200 mL simulated alachlor-contaminated water. When the PS and ZVI were added into the solution, the mechanical stirrer was switched on to maintain the solution well mixed. As the effect of temperature was considered, the vessels were installed in a water bath apparatus (SHZ-B, Shanghai Yuejin Medical Instruments Co., Ltd.), providing a desirable reaction temperature. At designated time intervals (0, 2, 4, 6, 10, 15, 30, 45, 60 min), 0.8 mL sample was taken out from each replicate vessel and put into 0.2 mL ethanol to quench the oxidation induced by any residual oxidant. After filtering, the filtrate was used for the detection of the residual. All the experiments were carried out in duplicate to ensure accurate data acquisition and interpretation. 2.3. Analytical methods The alachlor concentration was examined by using a high performance liquid chromatography (HPLC, Waters 2010, USA) equipped with a Symmetry C18 column by using a UV–vis detector (Waters 2489) at the absorption wavelength of 200 nm. The details of the alachlor analyses were summarized in the supplementary material (Text S2). 3. Results and discussion
F e0 + F e3+ → 2F e2+
(6)
F e0 + 2H + → F e2+ + H2
(7)
F e0 + S2 O28− → F e2+ + 2SO24−
(8)
To data, ZVI+PS system has been reported to remove some important organic pollutants such as p-nitorphenol [25], dyes (acid orange Ⅱ and methyl orange) [24,26] and bisphenol A [27]. However, to the best of our knowledge, very limited information on the ZVI+PS oxidation of herbicides in water is available. The object of this study was to investigate the performance of ZVI for PS activation. Then, several key influencing factors including initial ZVI dosage, PS dose, initial pH, citrate, NOM and common coexisting ions on the alachlor degradation were evaluated. Finally, the preliminary degradation mechanistic information was provided through the products identification. 2. Experimental 2.1. Materials The alachlor was supplied by Sigma-Aldrich (St. Louis, Missouri, USA). The physicochemical properties of alachlor were shown in Table 1. Sources of other chemicals and reagents used in the study were summarized in the supplementary material (Text S1).
3.1. Effect of initial ZVI dosage In this study, three different forms of iron (Fe3+ , Fe2+ and ZVI) were used to test the formation of SO·− 4 . The detailed discussion was shown in the supplementary material (Text S3 and Fig. S1). ZVI +PS system achieved the optimal performance in the degradation of alachlor compared with Fe3+ +PS and Fe2+ +PS system. Also, noted that ZVI alone did not have significant influence on the degradation of alachlor and less than 18% of alachlor was degraded under the PS alone (Fig. S2). Fig. 1a illustrated the alachlor removal under different ZVI to PS molar ratio. As observed, the alachlor removal was significantly influenced by the molar ratio of ZVI to PS and the best alachlor removal was obtained at the molar ratio of 2:1 for ZVI to PS. Of note, the alachlor degradation well followed a pseudo-first-order kinetics pattern (R2 > 0.95) with any particular ZVI dosage in 60 min (Fig. 1b). The overall rate law and the degradation efficiency for the alachlor degradation could be expressed as Eqs. S1 and S2. The pseudo-first-order reaction rate constant increased from 0.0711 to 0.1271 min−1 when the molar ratio of ZVI to PS increased from 0.5 to 2, but gradually decreased to 0.0671 min−1 as the molar ratio further increased to 4. The significant enhancement in degradation efficiency appeared to be attributed to production of more active radical species after introduction of the catalyst [19]. Excess ZVI dosage likely provided too much Fe2+ which could scavenge SO·− 4 produced in the ZVI +PS system (Eq. 5), leading to reduce the overall degradation efficiency. In general, the optimum
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Fig. 1. Effect of initial ZVI dosage on alachor degradation in the ZVI+PS system. Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 1.0 mmol/L; no pH adjustment; 30 °C.
Fig. 2. Effect of initial PS dosage on alachor degradation in the ZVI+PS system. Experimental conditions: [Alachlor] = 5 mg/L; [ZVI] = 2.0 mmol/L; no pH adjustment; 30 °C.
ZVI to PS molar ratio was 2:1 and the oxidation was complete almost in 60 min. 3.2. Effect of PS dosage Fig. 2 showed the alachlor degradation by ZVI activated PS at different PS dosages. Also, the results demonstrated that the oxidation process well followed a pseudo-first-order kinetics pattern (R2 > 0.95) with any particular PS dosage in 60 min. As shown in the insert graph of Fig. 2b, when the catalyst dosage was fixed at 2.0 mmol/L, the pseudo-first-order degradation rate constant (kobs ) increased from 8.02 × 102 to 8.49× 102 min−1 as PS increasing from 0.5 to 1.0 mmol/L, and then decreased to 4.61 × 102 min−1 when PS dosage further increased to 3.0 mmol/L. With the increase of PS dosage, large amounts of SO·− 4 were generated due to the high level of S2 O8 2− (Eq. (4)). Though some SO·− 4 were scavenged by Fe2+ (Eq. (5)), more generated Fe3+ could produce more Fe2+ (Eq. (6)). Then, lots of SO·− 4 were formed with the high concentration of S2 O8 2− and Fe2+ , leading an increasing removal efficiency of alachlor. However, high level of SO·− 4 could also be consumed by itself and S2 O8 2− (Eqs. (9) and (10)), causing the decreasing kobs with further increasing dosage of PS. ·− 2− 8 −1 −1 SO·− 4 + SO4 → S2 O8 k = 4 × 10 M s 2− ·− 5 −1 −1 S2 O28− + SO·− 4 → SO4 + S2 O8 k = 6.1 × 10 M s
(9) (10)
3.3. Effect of initial pH From Fig. 3, it could be seen that acidic circumstance favored the degradation of alachlor. The reason might be ascribed to the increase of the formation rate of Fe2+ (Eq. (7)), which could generate more SO·− 4 in the ZVI+PS system. The ZVI+PS system was effective for the alachlor degradation over a broad pH range of 1.5– 6.7, in which more than 99% of alachlor was decomposed within
Fig. 3. Effect of initial pH on alachlor degradation in the ZVI+PS system. The insert of the table was the initial and final pH value of each setting point. Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 0.5 mmol/L; [ZVI] = 1.0 mmol/L; 30 °C.
60 min. Although the alachlor removal efficiency under different pH (1.5–6.7) over 60 min were close, their degradation rates were significantly different. Especially, more than 89.39% alachlor was degraded in 4 min at pH of 1.5 and more than 97% alachlor was removed in 15 min at pH 3.0. So the efficient degradation of alachlor was obtained in 30 min at the range of 1.5–3.0. This finding was in agreement with a previous study that a low pH (pH 3.0) favored the degradation of 4-chlorophenol in the Fe/PS system [28]. When the reaction time were 4 and 6 min, the removal rates of alachlor were 94.23%, 94.29% at pH 1.5 and 95.99 %, 97.10% at pH 3.0. Although the increase of H+ in acidic condition might result in the rapid formation of Fe2+ by H+ (Eq. (7)), the acid-catalyzation (Eqs. (11) and (12)) could also accelerate the
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Fig. 4. Effect of temperature on degradation of alachlor. a.in only PS system; b in ZVI+PS system. The insert of the figure is Arrhenius plot – lnkobs vs. (1/T) x 103 .Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 0.5 mmol/L; [ZVI] = 1.0 mmol/L; no pH adjustment.
·− formation of SO·− in the ZVI+PS system might 4 . Excess SO4 2− promote the scavenging of SO·− 4 by itself (Eq. (9)) and S2 O8 (Eq. (10)), resulting in the reduction of the alachlor degradation.
S2 O28− + H + → H S2 O− 8
(11)
2− ·− + H S2 O− 8 → H + SO4 + SO4
(12)
Alkaline conditions remarkably inhibited the degradation, the removal rate decreased from 94.36% to 29.08% when the pH increased from 6.7 to 10. Because Fe2+ and Fe3+ were almost insoluble at such a high pH, the iron ions rapidly formed iron hydroxides and escaped from the reaction system. In addition, the presence of SO·− 4 could result in radical interconversion reactions and produce hydroxyl radicals (Eqs. (13) and (14)) [29]. The generated OH· could be readily scavenged by SO·− 4 that was abundant in the ZVI+PS system. 2− − · SO·− 4 + OH → SO4 + OH
(13)
SO·− 4
(14)
+ H2 O →
SO24−
+ OH · + H +
The pH of ultrapure water and original alachlor solution alone was 6.4 and 6.7, respectively. The final pH of each experiment setting pH were also detected and shown in the insert table of Fig. 3. All the solutions evolved into acidity after 60 min. Moreover, the eventual pH in the PS alone and ZVI alone system was 5.8 and 6.8, respectively. So it was easy to understand that the addition of PS greatly changed the reaction environment. Then some acids might be produced in the ZVI+PS system, leading a more acidic condition. 3.4. Effect of temperature The effect of temperature on the degradation was shown in Fig. 4. Both kinetics and thermodynamics parameters for the degradation processes were displayed in Table S1. Also, the degradation process followed the pseudo-first-order kinetics pattern. The degradation efficiency of alachlor with PS alone was 17.97%, 25.68%, 38.95% and 77.55% at different temperature 30, 40, 50 and 60 °C, respectively. Obviously, due to the increasing production of SO·− 4 in the system with temperature rising, the degradation rate of alachlor increased. As expected, the ZVI +PS system could achieve a higher alachlor degradation rate than the PS alone at any temperature. For example, the degradation rate could achieve 94.3% in ZVI+PS system at 60 °C within 15 min, which was about 3 times higher than that of PS alone system under th identical conditions. Also, the activation energy was calculated through Arrhenius equation in Text S5 and Table S1.
Fig. 5. Effect of NOM on the degradation of alachlor. Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 0.5 mmol/L; [ZVI] = 1.0 mmol/L; no pH adjustment; 30 °C.
3.5. Effects of citrate and NOM dosage Citrate as chelating agent had been used to enhance Fe2+ +PS oxidation of certain pollutant [30], because the chelating agent could increase Fe2+ solubility at neutral pH. Rastogi et al. [20] have also used the citrate acid (CA) as the chelating agents to stabilize iron ions in solution and accelerate the generation of reactive oxidants. Both the degradation in Fe2+ +PS and ZVI+PS systems were promoted by citrate and CA shown in Fig. S3 and discussed in Text S6. Different dosages of NOM (0–5 mg/L) and citrate dosage were added in the samples to elucidate their effects on the degradation of alachlor, respectively. As displayed in Fig. 5, the NOM greatly inhibited the degradation of alachlor in the ZVI+PS system, with the removal rate of 97.8%, 90.0%, 79.7% and 78.6% at the dosage of 0, 1, 3 and 5 mg/L, respectively. Because NOM as the organics could compete for the radicals participating in the main degradation reaction, leading to the alachlor degradation slow down. 3.6. Effects of anions The reaction activity of PS might be influenced by the background anions in natural water. In this study, the efficiency of degradation alachlor influenced by different anions, such as NO− 3, SO24− , Cl− and HCO− with different dosages were considered, the 3 results were shown in Fig. 6. As shown, the lower concentration of Cl− (1 and 10 mmol/L) posed an unapparent effect to the degradation of alachlor. However, the oxidation process was greatly suppressed with the increasing concentration of Cl− . This might be accounted for that Cl− react with SO·− to generate large 4 ·− amounts of free radicals containing chlorine, such as Cl· and Cl2 − (Eqs. (15) and (16)). The Cl could compete with the alachlor for
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− 2− Fig. 6. Effect of anions on the degradation of alachlor. a. Cl− ; b. HCO− 3 ; c. NO3 ; d. SO4 . Experimental conditions: [Alachlor] = 5 mg/L; [PS] = 0.5 mmol/L; [ZVI] = 1.0 mmol/L; no pH adjustment; 30 °C.
− the consumption of SO·− 4 when at a high concentration of Cl .
SO·− 4
−
+ Cl →
SO24−
+ Cl
·
·−
C l − + C l · → Cl 2
(15)
(16)
The same result was also reported by Tan et al. [31], who employed PS to degrade the antipyrine. Previous research also demonstrated that the interference effect of Cl− had correlation with generation rate of SO·− 4 [32]. So when the concentration of Cl− was 100 mmol/L, SO·− was quenched to a great extent. 4 Compared with Cl− , the inhibition of the HCO− 3 contributed to the ZVI+PS system was more significant. Three reasons accounted for this: (1) when the HCO− 3 was added into the water, the solution turned acidic to alkaline environment, in which condition the alachlor degradation rate decreased discussed in the chapter 3.3. ·− (2) HCO− 3 played a role as the quencher of SO4 . (3) There was a dissociation equilibrium of HCO− in the water, in which HCO− 3 3 and 2− 2− 2 + CO3 coexisted, CO3 could combine Fe to form precipitation. So the removal rate was strongly inhibited in system added anion of − Cl- or HCO− 3 , while it was not obviously impacted by adding NO3 2− or SO4 discussed in Text S7. 3.7. Proposed degradation pathways The proposed degradation pathways were shown in Fig. 7. It had been observed that the degradation of alachlor followed seven main proposed pathways (from I to VII) during the oxidation process. In the degradation pathways of I, II and III, the attacked site of alachlor could occur on the ethyl, N-methoxymethyl, Nchloroacetyl groups or the benzene ring. The ethyl-side chain could be oxidized to an acetyl group and yield compound 1 or compound 4. Then, oxidation of the ethyl chain of compound 1 or compound 4 would yield compound 5. Besides the oxidation of the aryl–ethyl group, cleavage of the N-methoxymethyl group was a significant feature of environmental degradation of alachlor [33].
By analogy, compound 9 could also be produced during the oxidation of alachlor. Successively, both oxidation of the aryl-ethyl group of compound 9 and N-dealkylation of compound 6 would yield compound 7. Cyclization was an important pathway in photo degradation and photocatalytic degradation of alachlor [34]. In this study, cyclization was initiated by N-dealkylation of alachlor and compound 9 transformed into compound 10. Further oxidation of compound 10 or cyclization of compound 7 gave rise to compound 8. However, the cleavage of the N-chloroacetyl moiety tended to occur and thus compound 11 could be generated. Further cyclization of compound 11 produced compounds 12 and 13. Also, alachlor could lose CH3 OH and cyclization was occurred to form compound 14. Then, the dechlorination led the compound 14 to transform into compound 15 by losing CH3 OCl. The compound 15 could continue lose CH3 or C2 H4 to form compound 16 or compound 17. Also, the compound 14 could be dehydrated to form cyclization compound 18 which could generate compound 19 with further dechlorination. The benzene ring or the aryl-ethyl group might also be attacked during the oxidation, producing compounds 20, 21, 22 and 23, according the proposed pathways of VI and VII. Somich et al. [35] proposed that the benzene ring cleavage could occur during the degradation of alachlor by ozone, and thus formic, acetic, propionic and oxalic acids were generated. Qiang et al. [36] had undertaken some research about degradation mechanism of alachlor by ozonation (E0 = 2.01 V) and OH · (E0 = 2.8 V), proposing the degradation pathways like II, III, IV, VI and VII. The electrophilic attack of ozone on the benzene ring or the aryl–ethyl group would produce compounds III and IV which were also detected during photocatalytic oxidation of alachlor [34]. Also, Li et al. [37] had considered the decomposition of alachlor by ozonation, showing the degradation pathway I. Also, as shown · in the Eq. (13), SO·− 4 (E0 = 2.6 V) could transform into OH . While in the combined system, oxidation potential of two free radicals was close to each other. Thereby, it was easy to understand the proposed pathways occurred in this research considering the two reasons.
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Fig. 7. Proposed pathways for alachlor degradation.
4. Conclusion This study explored ZVI as an alternative iron source to activate PS and degrade alachlor, a representative herbicide in water. The principal oxidant responsible for the oxidation was sulfate radicals produced from persulfate activated by ZVI gradually releasing Fe2+ . Compared with Fe2+ , ZVI can minimize the SO·− 4 scavenging and thus improve the iron reuse efficiency by reduction of Fe3+ to Fe2+ . The alachlor was degraded by a synesthetic effect by combination of ZVI and PS in the oxidation process. The reaction rate was significantly influenced by factors such as ZVI dosage, PS dosage, initial pH, temperature, NOM, citrate and anions. The intermediate products were investigated, proposing seven degradation pathways. This study showed encouraging results, and demonstrated that the ZVI activated persulfate oxidation was a promising technology for control of the water pollution caused by alachlor. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no. 51178321), the National Major Project of Science & Technology Ministry of China (nos. 2012ZX07403-001, 2012ZX07403-0 02 and 20 08ZX07421–0 02), the Specialized Research Fund for the Doctoral Program of Higher
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