Degradation of Clopyralid by the Fenton Reaction - Industrial

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Degradation of Clopyralid by the Fenton Reaction Katharina Westphal, Ramona Saliger, Dennis Jag̈ er, Linda Teevs, and Ulf Prüße* Johann Heinrich von Thünen-Insitute, Institute of Agricultural Technology, Bundesallee 50, 38116 Braunschweig, Germany ABSTRACT: The degradation of clopyralid with the homogeneous and heterogeneous Fenton process was carried out under mild reaction conditions in aqueous solution. A complete degradation of clopyralid was obtained with the homogeneous Fenton reaction. Therefore, the influence of several parameters such as the initial concentrations of the catalyst, clopyralid, and H2O2 as well as the initial pH value on the homogeneous reaction was investigated. Several reaction intermediates and end products could be identified and allowed one to propose a degradation pathway of clopyralid. The heterogeneous degradation of clopyralid could only be achieved in the presence of HCl. Under these conditions, iron ions were detected in the reaction suspension, which indicates that the reaction is not heterogeneously but rather homogeneously catalyzed.

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INTRODUCTION The contamination of water with various types of pollutants is still a matter of great concern. Apart from inorganic contaminants like nitrates, cyanides, or heavy metals, the environment is strongly affected by chlorinated organic compounds such as chlorinated hydrocarbons and pesticides. These compounds are not readily degradable and show a high environmental persistence.1,2 Furthermore, they are spread globally and often have direct harmful effects on living organisms.3 Clopyralid (3,6-dichloropyridine-2-carboxylic acid) is a chlorinated pesticide from the chemical class of pyridine compounds. It is a growth regulator herbicide and has been used for the control of annual and perennial broadleaf weeds in certain crops and turf.4 Clopyralid is persistent with a half-life of up to 14 months5 and has a high water solubility of 1000 mg L−1.6 Moreover, it is very mobile in soils and has the potential to contaminate groundwater.5 Furthermore, clopyralid was found in surface drinking water of the northern Great Plains.7 Advanced oxidation processes (AOPs) show high potential for the removal of organic contaminants in waters and wastewaters. They generate highly reactive and nonselective hydroxyl radicals and operate at ambient reaction conditions (room temperature, atmospheric pressure, etc.).8 The Fenton reaction is the most commonly used AOP for the treatment of organic contaminants. In the classical homogeneous Fenton system, the generation of hydroxyl radicals is achieved by a mixture of H2O2 and soluble iron(II) salt according to the Haber−Weiss mechanism (eq 1).9−14 Fe 2 + + H 2O2 → Fe3 + + OH• + OH−

The advantages of the homogeneous Fenton reaction are due to the simple handling of this process and the good performance under mild conditions. The main drawbacks include a narrow pH range and the problem of catalyst separation. Such disadvantages could be avoided with heterogeneous catalysts. Typically, iron oxides like goethite, hematite, and magnetite as well as iron or iron oxides supported on silica, alumina, or other support materials were used for the heterogeneous Fenton reaction.17−22 In comparison to the homogeneous process, the degradation of organic compounds with heterogeneous catalysts is slower, and often iron leaching is detected.18,20,23 Therefore, it is still under discussion whether a real heterogeneous reaction occurs or whether the dissolved ferrous ions generate the hydroxyl radicals. The degradation of clopyralid received considerable attention in very recent years. A couple of studies described clopyralid degradation by electrochemical reduction,24,25 by photocatalysis,4,26,27 by the electro-Fenton reaction,28 by electronbeam treatment,29 by UV/H2O2 or O3 oxidation30 and by catalytic hydrodechlorination.31 Surprisingly, so far no study has been carried out on the degradation and mineralization of clopyralid by using the classical Fenton reaction. Hence, in the present paper, the degradation of clopyralid with the classical homogeneous Fenton process as well as with its heterogeneous counterpart was investigated. The influence of several parameters such as the initial concentrations of the catalyst, clopyralid, and H2O2 and the initial pH value on the homogeneous Fenton reaction was studied to deduce the general dependencies. Another aim of this work was to identify the reaction intermediates and end products of the degradation of clopyralid by the reaction. Finally, a degradation pathway in aqueous solution was proposed.

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The hydroxyl radicals react with organic contaminants by hydrogen abstraction, addition to C−C double bonds or to a π system, and redox reaction to generate organic radicals. These could be mineralized to CO2, H2O, and inorganic ions,9,15 although the degradation usually stops at short-chain carboxylic acids such as acetic, malonic, maleic, and oxalic acid.16 Because of the numerous attacks and reaction possibilities of radical reactions, it is difficult to predict a degradation mechanism. Therefore, most research work only investigated the degradation of various organic contaminants. © XXXX American Chemical Society

Special Issue: Recent Advances in Nanotechnology-based Water Purification Methods Received: October 16, 2012 Revised: January 23, 2013 Accepted: January 24, 2013

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dx.doi.org/10.1021/ie302769p | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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EXPERIMENTAL SECTION Preparation of the Heterogeneous Catalyst. The modified iron oxide catalyst was prepared by the precipitation method, as described in ref 17. FeSO4·7H2O (5 g) was dissolved under stirring in 50 mL of H2O. A 3 mol L−1 aqueous NaOH solution was added slowly to the solution to adjust the pH to 9. This solution was then agitated for 40 min, and the obtained precipitate was filtered, washed with H2O, and airdried overnight. Calcination was carried out in air for 2 h at 600 °C. Degradation Procedure. All experiments were performed at room temperature and atmospheric pressure in a batch glass reactor equipped with a magnetic stirrer. Clopyralid was dissolved by agitation (500 rpm) in 100 mL of deionized H2O. For the homogeneous Fenton reaction, a catalytic quantity of FeSO4·7H2O was added to the clopyralid solution. Likewise, iron oxide was used to carry out the reaction heterogeneously. When necessary, the pH was adjusted with 2 mol L−1 HCl or NaOH. The reaction was started by adding H2O2 (30 wt % solution) to the reaction mixture. At given time intervals during the reaction, 1 mL samplings were taken and mixed immediately with 0.25 mL of ethanol, which was used as a radical scavenger. In the case of the heterogeneous reaction, the samples additionally were filtered through 0.45 μm membrane filters to separate catalyst particles from the solution. Analytical Methods. High-performance liquid chromatography (HPLC) analysis was performed using different chromatographic columns (reversed-phase and ion-exchange columns) on a Shimadzu system equipped with a SPD-10AV UV−vis detector and an RI-Detector 8110 (Bischoff) refractive index detector. Chloride ions were determined by ion chromatography on a Metrohm 690 system with a Metrohm Metrosep anion dual 2 column. Ammonium ion measurements were carried out by a Lange LCK 304 ammonium−nitrogen test. Nitrate and nitrite ions were determined by HPLC using a ODS-Hypersil 120A (5 μm, 125 × 4.6 mm) column with a mobile phase (flow rate 2.1 mL min−1) of methanol and water (1:19), octylamine, and a pH of 6.5 adjusted with H3PO4 (85%). Optical emission spectroscopy by inductively coupled plasma (ICP-OES; iCAP 6000 Series ICP spectrometer, Thermo Scientific) was used for detection of dissolved iron in solution.

ferrous ions or H2O2. Degradation of clopyralid was only observed if both ferrous ions and H2O2 were present. Afterward, the influence of the catalyst concentration on the homogeneous degradation of clopyralid was investigated. The initial catalyst concentration was varied in the range of 28−190 mg L−1 (0.1−0.68 mmol L−1). It can be seen from Figure 1 that

Figure 1. Effect of the catalyst concentration on the degradation kinetics of clopyralid by a homogeneous Fenton process. Reaction conditions: 0.13 mmol L−1 clopyralid, 28, 95, and 190 mg L−1 (0.1, 0.34, and 0.68 mmol L−1) FeSO4·7H2O, 69.4 μL (6.8 mmol L−1) of H2O2, and pH 3.

a fast and complete degradation of clopyralid could be achieved with the homogeneous iron catalyst. Under these reaction conditions, the scavenger effect of the catalyst is not significant. The degradation rate was found to increase with increasing catalyst concentration. This meets the expectation that a higher catalyst dosage causes a faster catalytic degradation. The effect of the initial clopyralid concentration was examined between 0.13 and 1.30 mmol L−1 and is shown in Figure 2. Complete clopyralid conversion is reached after 90 min with an initial concentration of 0.13 mmol L−1 and in 330 min with an initial concentration of 0.52 mmol L−1. At an initial concentration of 1.30 mmol L−1, no complete conversion could be obtained during the reaction time.

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RESULTS AND DISCUSSION Degradation of Clopyralid by Homogeneous Fenton Reaction. Typically, the Fenton reaction is carried out at ambient temperature and atmospheric pressure. It is influenced significantly by the pH of the reaction medium and by the concentration of iron ions and H2O2. The best results can usually be obtained at a pH between 3 and 4. At higher pH, the catalyst tends to precipitate. At lower pH, on the one hand, the generation of hydroxyl radicals is slower and, on the other hand, H+ ions act as radical scavengers. Usually, the concentration of iron ions ranges from 0.1 to 15 mmol L−1 and the mass ratio of catalyst to H2O2 ranges from 1:5 to 1:10. Higher concentrations of catalyst and H2O2 cause a higher reaction rate. However, an excess of catalyst and H2O2 should be avoided because they act as radical scavengers.9,12 First of all, blank experiments were carried out that revealed that no significant clopyralid degradation could be observed within 24 h at pH 3 in clopyralid solutions containing solely clopyralid as well as in clopyralid solutions containing either

Figure 2. Effect of the clopyralid concentration on the degradation kinetics by a homogeneous Fenton process. Reaction conditions: 0.13, 0.52, and 1.3 mmol L−1 clopyralid, 28 mg L−1 (0.1 mmol L−1) FeSO4·7H2O, 69.4 μL (6.8 mmol L−1) of H2O2, and no pH adjustment. B



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min). According to these results, there is no significant dependence of the degradation of clopyralid on the H2O2 concentration. Therefore, the smallest amount of H2O2 (69.4 μL, 6.8 mmol L−1) is sufficient for the degradation of clopyralid. Further kinetic analysis indicated that clopyralid degradation by the homogeneous Fenton reaction can be described by a pseudo-first-order reaction model. Reaction rate constants of the smallest applied catalyst amount, i.e., 28 mg L−1, were calculated from the slope of ln(C/C0) versus time plots and were found to lie between 0.003 and 0.067 min−1, depending on the reaction conditions, i.e., the initial clopyralid concentration, dosed H2O2 amount, and pH value (Table 1).

The influence of the initial pH on the degradation of clopyralid was studied in the range of 1.6−6.2 (Figure 3). As

Table 1. Pseudo-First-Order Rate Constant for Clopyralid Degradation by the Homogeneous Fenton Reaction with 28 mg L−1 (0.1 mmol L−1) FeSO4·7H2O Depending on the Reaction Conditions Figure 3. Effect of the initial pH on the kinetics of clopyralid degradation by a homogeneous Fenton process. Reaction conditions: 0.52 mmol L−1 clopyralid, 28 mg L−1 (0.1 mmol L−1) FeSO4·7H2O, 69.4 μL (6.68 mmol L−1) of H2O2, pH 1.6, pH 3.1 (no pH adjustment), and pH 4.5 and 6.2.

mentioned above, the Fenton reaction is largely influenced by the pH of the reaction solution. The best results are usually obtained at a pH value of 3. This was confirmed by the present experiments. The fastest degradation of clopyralid was achieved at pH 3.1. At pH 4.5 and 6.2, a slower decomposition was observed, probably because of catalyst precipitation. Hence, the concentration of ferrous ion in solution decreased, which caused a diminished generation of hydroxyl radicals. At pH 1.6, an even slower degradation was observed. Furthermore, compared to the other pH values, there is no complete decomposition of clopyralid after 330 min. This is due to the fact that at low pH the generation of hydroxyl radicals is slower and H+ ions act as radical scavengers. Finally, the effect of the initial amount of H2O2 was analyzed between 69.4 and 278 μL (6.8−27.2 mmol L−1). For all three applied amounts, a similar behavior was found (Figure 4). The curves are almost identical, and complete degradation of clopyralid could be reached after the same amount of time (300

clopyralid (mmol L−1)

H2O2 (μL) [mmol L−1]

pH

k (min−1)

0.13 0.52 1.3 0.52 0.52 0.52 0.52 0.52

69.4 [6.8] 69.4 [6.8] 69.4 [6.8] 139 [13.6] 278 [27.2] 69.4 [6.8] 69.4 [6.8] 69.4 [6.8]

2.9 3.1 3.4 3.1 3.1 1.6 4.5 6.2

0.067 0.015 0.003 0.018 0.024 0.003 0.009 0.009

For example, at pH 3.1, degradation of 0.52 mmol L−1 clopyralid (equal to 100 mg L−1) with 28 mg L−1 FeSO4·7H2O (equal to 0.1 mmol L−1 Fe2+) and 69.4 μL of H2O2 (equal to 6.8 mmol L−1) proceeds with a rate constant of 0.015 min−1. Analysis of the Intermediates and Products. A general clopyralid degradation pathway is difficult to propose because of numerous attacks and reaction possibilities for the hydroxyl radicals. They mainly react by abstraction of hydrogen atoms from C−H, N−H, or O−H bonds and by addition to unsaturated bonds or aromatic rings.9,15 On this account, most research investigated only the degradation of organic molecules. However, in three of the studies on clopyralid degradation with AOPs, intermediates and reaction products have been identified. Sojic et al.4 reported a tentative photocatalytical degradation pathway of clopyralid. They could identify several pyridine-containing intermediates such as 3,6-dichloropyridin2-ol, 3,6-dichlorohydroxypyridine-2-carboxylic acid, and 3,3′,6,6′-tetrachloro-2,4′-bipyridine-2′-carboxylic acid. These intermediates undergo ring opening and release of chloride simultaneously and were finally degraded to CO2, H2O, HCl, and NH4+. Ö zcan et al.28 investigated the degradation pathway of clopyralid with the electro-Fenton process at 35 °C and pH 3. They reported about three dominant intermediates in the early stages of the reaction that they were not able to specify. Further on, they could identify a couple of short-chain carboxylic acids formed as intermediates, of which maleic, oxalic, and oxamic were produced in the highest concentrations. All short-chain carboxylic acids seem to be completely mineralized at higher reaction times. The clopyralid-bound chlorine was completely converted to chloride ions and almost 99% of the clopyralidbound nitrogen was converted to ammonium and traces of nitrate.

Figure 4. Effect of the H2O2 concentration on the kinetics of clopyralid degradation by a homogeneous Fenton process. Reaction conditions: 0.52 mmol L−1 clopyralid, 28 mg L−1 (0.1 mmol L−1) FeSO4·7H2O, 69.4, 139, and 278 μL (6.8, 13.6, and 27.2 mmol L−1) of H2O2, and no pH adjustment. C



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The results reported by Xu et al.29 during electron-beam treatment of clopyralid solutions are similar to those of Ö zcan et al.28 However, in contrast to the electro-Fenton reaction, with electron-beam treatment, complete degradation of shortchain carboxylic acids is difficult. Especially, oxalic acid seems to be recalcitrant under the applied conditions. Further on, as only small amounts of ammonium and nitrate were detected, the formation of nitrogen-containing organic compounds seems probable although this was not specified by the authors. In order to identify reaction intermediates and end products of the clopyralid degradation by the Fenton reaction, HPLC analysis was carried out in the present study. Only some of the generated compounds could be detected. Some of those, which could be analyzed, appear as reaction intermediates and others as stable end products. As can be seen from Figure 5, glycolic,

into inorganic anions, which remain in solution. As mineralization products of clopyralid degradation, the formation of NO2−, NO3−, and/or NH4+ as well as Cl− can be expected, as was already described for clopyralid degradation with other AOPs.4,28,29 Table 2 shows the results of inorganic Table 2. Inorganic Ion Concentration Analysis after Completion of the Clopyralid Degradation Reactiona inorganic ion −

NO3 NO2− NH4+ Cl−

detected ion concentration (mmol L−1) 0.00 0.00 0.03 1.04

a Reaction conditions: 0.52 mmol L−1 clopyralid, 28 mg L−1 (0.1 mmol L−1) FeSO4·7H2O, 69.4 μL (6.8 mmol L−1) of H2O2, and no pH adjustment.

ion analysis, which was carried out after the degradation reaction. Relating to the nitrogen in clopyralid, nitrate and nitrite could not be detected in the reaction solution. Only small amounts of ammonium (0.03 mmol L−1) were obtained, which correspond to about 6% of the initial nitrogen. This leads to the conclusion that the initial nitrogen in the clopyralid structure is converted mainly to oxamic acid without being oxidized further to inorganic ions. The obtained amounts of ammonium and oxamic acid correspond to 67% of the initial nitrogen in the clopyralid structure. The missing 33% of nitrogen suggests that at least one more nitrogen-containing compound is formed. The most probable fate of the chlorine atoms of the clopyralid structure is their conversion to chloride ions, which are released to the reaction solution. In agreement with that, the final concentration of chloride amounts to 1.04 mmol L−1. This equals 100% of the initial organic chlorine in the clopyralid structure. Degradation Pathway. According to the obtained results, a tentative degradation pathway by the homogeneous Fenton reaction in aqueous solution is proposed in Scheme 1. Presumably, the degradation starts with hydroxylation of the aromatic ring and the simultaneous release of chloride, as was also reported in refs 4, 28, and 29. The following ring opening will generate aliphatic carboxylic acids such as tartronic and succinic acid as well as short-chain carboxylic acids like glycolic, formic, acetic, oxalic, and oxamic acid. The aliphatic carboxylic acids will be further oxidized to short-chain carboxylic acids. Oxalic and oxamic acid form stable complexes with Fe3+ ions and, therefore, these acids are not further degraded under the present reaction conditions. Glycolic, acetic, and formic acid are finally converted to CO2 and H2O. Only small amounts of ammonium were detected. The initial nitrogen in the clopyralid structure is mainly converted to oxamic acid without being oxidized further to inorganic ions. However, only 67% of the initial nitrogen could be found (6% NH4+ and 61% oxamic acid), suggesting that there has to be at least one more nitrogen-containing compound, which could not be detected with the analytical methods used. Degradation of Clopyralid by a Heterogeneous Fenton Reaction. Lee et al.17 reported that the modified iron oxide calcined at 600 °C is a promising catalyst for heterogeneous Fenton applications. The degradation of phenol with this catalyst showed effectiveness similar to that of ferrous ions at pH 3. Moreover, no iron ions (ferrous and ferric) could be detected in the reaction solution. Therefore, this catalyst was

Figure 5. Time course of carboxylic acid concentration during the homogeneous Fenton process. Reaction conditions: 0.52 mmol L−1 clopyralid, 28 mg L−1 (0.1 mmol L−1) FeSO4·7H2O, 69.4 μL (6.8 mmol L−1) of H2O2, and no pH adjustment.

formic, tatronic, and acetic acid were obtained as reaction intermediates. The formation of these intermediates starts rapidly, and they have disappeared completely after 5.5 and 22 h, respectively, for the tatronic acid. Oxamic, oxalic, and succinic acid were obtained as end products after 22 h of reaction time. Succinic acid is formed in small quantities, while the reached amounts of oxamic and oxalic acid were much higher. Compared to the other identified acids, the formation of oxalic and oxamic acid is much slower. Both acids accumulate for 5.5 h and attain a steady concentration of 0.33 mmol L−1 (oxamic acid) and 0.23 mmol L−1 (oxalic acid). The obtained amount of oxamic acid in the present study comprises 61% of the initial nitrogen of the clopyralid structure, i.e., that most of the clopyralid nitrogen is converted to oxamic acid. According to the literature, oxalic and oxamic acid are not readily oxidized by the Fenton process. This is attributed to the fact that both form stable complexes with iron ions.16,32,33 Compared to the electro-Fenton study of Ö zcan et al.,28 two major differences in the degradation pathway of clopyralid by the two Fenton variants can be identified. On the one hand, maleic acid, which is the major intermediate under electroFenton conditions, was not produced in significant amounts in the present study. On the other hand, oxalic and oxamic acid, which are recalcitrant in the classical Fenton reaction studied here, are further degraded under electro-Fenton conditions. A complete mineralization reaction by hydroxyl radicals generally leads to CO2 and H2O. If existent, heteroatoms turn D



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measurements of the iron ion concentration by ICP-OES were carried out. In the reaction solution without the addition of HCl, no dissolved iron ions could be detected in the reaction suspension (detection limit: 0.0018 mmol L−1), while with the addition of HCl, 0.03 mmol L−1 of iron ions was detected. This confirms the suspicion that the catalyzing capacity stems from the dissolved iron ions and that thus the degradation of clopyralid is caused by a homogeneous reaction.

Scheme 1. Degradation Pathway by the Homogeneous Fenton Reaction in Aqueous Solution: (1) Clopyralid, (2) Dechlorinated Intermediate, (3) Tatronic Acid, (4) Succinic Acid, (5) Oxamic Acid, (6) Oxalic Acid, (7) Glycolic Acid, (8) Acetic Acid, and (9) Formic Acid

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CONCLUSIONS The homogeneous degradation of clopyralid by the Fenton reaction has been carried out successfully in aqueous solution. The influence of the initial concentration of the catalyst, clopyralid, and H2O2 as well as the initial pH value on the reaction was investigated. The degradation of clopyralid leads to the formation of several intermediates such as glycolic, formic, tatronic, and acetic acid. These acids are further degraded to CO2 and H2O. Oxalic and oxamic acid are able to form stable complexes with Fe3+ ions and could be obtained as end products after 22 h of reaction time. Moreover, the inorganic ammonium and chloride ions were identified as mineralization products. A heterogeneous degradation of clopyralid with the Fenton reaction could only be achieved by the addition of HCl to the reaction solution. This has the effect that iron ions are released from the heterogeneous catalyst with the result that the degradation of clopyralid is caused by a homogeneous reaction and not a heterogeneous reaction.

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AUTHOR INFORMATION

Corresponding Author

chosen for the heterogeneous degradation of clopyralid. As reported in refs 23 and 34, the addition of acids could lead to dissolution of iron ions from the heterogeneous catalyst. Hence, the heterogeneous reaction was carried out with and without the addition of HCl. Figure 6 shows the results of the heterogeneous degradation reaction of clopyralid. Complete degradation was achieved with the addition of HCl in 48 h, while without HCl, no decomposition of clopyralid could be obtained. This may be due to the fact that acids provoke dissolution of iron ions from the catalyst. The degradation is then caused by a homogeneous reaction and not by a heterogeneous reaction. To support this thesis,

*Tel.: +49 (0) 531 596 4270. Fax: +49 (0) 531 596 4199. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results received funding from the European Community’s Seventh Framework Programme [FP7/2007-2013] under Grant CP-FP 226524. For chloride ion analysis, we thank the Institute of Environmental and Sustainable Chemistry (Institute of Technology, University of Braunschweig, Braunschweig, Germany).

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Figure 6. Effect of the HCl addition on the clopyralid degradation by a heterogeneous Fenton process. Reaction conditions: 0.13 mmol L−1 clopyralid, 400 mg L−1 modified iron oxide, 69.4 μL (6.8 mmol L−1) of H2O2, pH 3.0 (with HCl), and pH 3.4 (without HCl). E



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Degradation of Clopyralid by the Fenton Reaction - Industrial

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