Degradation of conazole fungicides in water by electrochemical oxidation

Chemosphere 93 (2013) 2774–2781

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

Degradation of conazole fungicides in water by electrochemical oxidation J. Urzúa, C. González-Vargas, F. Sepúlveda, M.S. Ureta-Zañartu, R. Salazar ⇑ Departamento de Ciencias del Ambiente, Facultad de Química y Biología, Universidad de Santiago de Chile, USACH, Casilla 40, Correo 33, Santiago, Chile

h i g h l i g h t s  A complete mineralization of three conazole fungicides by electro oxidation is obtained.  The mineralization occurs at pH 7.  Degradation rate of fungicides depends of applied current density.  A pathway of mineralization of conazole fungicides is proposed.

a r t i c l e

i n f o

Article history: Received 22 April 2013 Received in revised form 4 September 2013 Accepted 6 September 2013 Available online 17 October 2013 Keywords: Anodic oxidation Conazole fungicides Boron-doped diamond electrode Hydroxyl radical Mineralization

a b s t r a c t The electrochemical oxidation (EO) treatment in water of three conazole fungicides, myclobutanil, triadimefon and propiconazole, has been carried out at constant current using a BDD/SS system. First, solutions of each fungicide were electrolyzed to assess the effect of the experimental parameters such as current, pH and fungicide concentration on the decay of each compound and total organic carbon abatement. Then a careful analysis of the degradation by-products was made by high performance liquid chromatography, ion chromatography and gas chromatography coupled with mass spectrometry in order to provide a detailed discussion on the original reaction pathways. Thus, during the degradation of conazole fungicides by the electrochemical oxidation process, aromatic intermediates, aliphatic carboxylic acids and Cl were detected prior to their complete mineralization to CO2 while NO 3 anions remained in the treated solution. This is an essential preliminary step towards the applicability of the EO processes for the treatment of wastewater containing conazole fungicides. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Conazoles are a diverse group of commercial fungicides commonly used in clinical and agricultural applications. They are applied as fungicides in fruit, vegetable, and cereal crop protection programs, and also in lawn care and wood preservation (Gupta et al., 1994). Medically, conazoles are widely used to treat local and systemic fungal and yeast infections (Georgopapadakou and Walsh, 1996). Their fungicidal activity consists in the inhibition of ergosterol synthesis, primarily by inhibiting the activity of lanosterol 14a-demethylase (Turner and Rodriguez, 1996). In Chile, conazole fungicides like myclobutanil and triadimefon are used to protect Vitis species, including the wine grape, from oidium, a disease caused by the fungus Uncinula necator, which leads to crop loss and poor wine quality (Calonnec et al., 2004).

⇑ Corresponding author. Tel.: +56 2 27181134. E-mail address: [email protected] (R. Salazar). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.09.035

Although all conazoles have anti-antifungal activity, some members of this family of chemicals have potential adverse health effects in humans and animals (Chen et al., 2008; Kjærstad et al., 2010), and they also induce liver or thyroid tumors and/or affect the reproductive system in experimental animals (Rockett et al., 2006; Hester and Nesnow, 2008). Triadimefon and propiconazole have been shown to induce hepatoadenomas in the liver of mice receiving the chemical in their feed (Ross et al., 2009), and triadimefon has been reported to induce chromosomal aberrations and micronuclei in a study on rat bone marrow (Iyer et al., 2010). In all these works, the authors found that conazoles have endocrine disruptive effects which occur at concentrations >30 lM (>10 mg L1). This concentration is indicated as the lowest concentration at which cytotoxicity was detected. For these reasons, there is great interest in the use of diverse methods with enough ability to mineralize conazole fungicides in aqueous media to avoid their dangerous accumulation in the aquatic environment.

J. Urzúa et al. / Chemosphere 93 (2013) 2774–2781

No studies about the degradation, elimination or mineralization of conazole fungicides have been reported in the literature. Recently, in our group, the degradation of triadimefon has been explored by electro-Fenton (EF) and photoelectro-Fenton processes (PEF), suggesting a pathway for the complete mineralization of this fungicide to CO2 (Salazar and Ureta-Zañartu, 2012). Understanding the mechanism of the oxidation of different organic pollutants like conazole fungicides is of great importance to develop better degradation processes for pollutants and their by-products. Chemically, conazole fungicides contain a triazole ring in their structure, a halogenated aromatic ring, and an alkyl chain or aliphatic heterorings. The oxidation of these compounds may produce aromatic by-products more dangerous than the initial fungicide, so new effective oxidation methods that can eliminate all organic pollutants present in water are required. A potential way to achieve better decontamination efficiency of wastewater with organic pollutants is to couple both traditional and electrochemical methods, in so-called electrochemical advanced oxidation processes (EAOPs) (Comninellis and Chen, 2010). The main electrochemical procedures used for the remediation of wastewater containing organic pollutants are electrocoagulation (EC), electrochemical oxidation (EO), EF, and photoassisted systems like PEF (Martínez-Huitle and Brillas, 2009). EO is actually the most widely used methodology, because it allows the treatment of highly toxic waste (Martínez-Huitle and Ferro, 2006). It is also called ‘‘electrochemical combustion’’ method because organic pollutants are completely mineralized, i.e., oxidized to CO2, water and inorganic salts, by direct reaction with physisorbed hydroxyl radicals (OH), electrogenerated from water discharge at the anode (M) (Sirés et al., 2006; Scialdone, 2009):

M þ H2 O ! Mð OHÞ þ Hþ þ e

ð1Þ

This radical is the second strongest oxidant known after fluorine, with a high standard potential (E0 = 2.80 V vs. SHE) that ensures its fast reaction with most organics, producing dehydrogenated or hydroxylated derivatives up to conversion into CO2. Both the electrochemical generation and chemical reactivity of heterogeneous M(OH) are dependent on the nature of the electrode material (Panizza and Cerisola, 2003, 2004). Thus, a non-active anode is required for the oxygen evolution reaction, because this kind of electrode interacts very weakly with OH, allowing the direct reaction of organics with M(OH) to give fully oxidized reaction products such as CO2 according to the reaction

aMð OHÞ þ R þ xe ! aM þ mCO2 þ nH2 O þ xHþ

ð2Þ

where R is an organic compound with m carbon atoms and without any heteroatom, which needs (2 m + n) oxygen atoms to be totally mineralized to CO2. The use of a boron doped diamond (BDD) thin film in EO provides total mineralization with high current efficiency for different organics in real wastewater (Kraft, 2007). The BDD anode is the best non-active electrode which shows that behavior (Panizza and Cerisola, 2005; Sirés et al., 2008; Guinea et al., 2009) so it has been proposed as the preferable anode for treating organics by EO. The aim of this work is to determine the optimal experimental conditions for EO at constant current density using a boron doped diamond/stainless steel system (BDD/SS) for the treatment of water containing concentrate solutions of three different conazole fungicides, myclobutanil, triadimefon and propiconazole. Moreover, the analytical determination of these compounds and their by-products or intermediates formed during the abatement process were identified and quantified.

2775

2. Experimental 2.1. Chemicals Myclobutanil ((RS)-2-(4-chlorophenyl)-2-(1H-1,2,4-triazol-1yl-methyl)hexanenitrile, C15H17ClN4, 99.4%) propiconazole ((2RS,4RS;2RS,4SR)-1-[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-ylmethyl]-1H-1,2,4-triazole, C15H17Cl2N3O2, 98.4%) and triadimefon ((RS)-1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4triazol-1-yl)butan-2-one, C14H16ClN3O2, 99.7%) were supplied by Sigma–Aldrich (Fluka, Pestanal) and used as received. Chemical structure and some characteristics of the fungicides studied are shown in Table 1. Fumaric, maleic, oxalic and oxamic acids were analytical grade from Sigma–Aldrich. Solutions of anhydrous sodium sulfate (analytical grade from Merck) prepared with distilled water previously deionized and with its pH adjusted with 1.0 M of analytical grade sulfuric acid or sodium hydroxide (both from Merck) were used as supporting electrolyte. Organic solvents and other chemicals used in the analysis procedure were either HPLC or analytical grade from Merck or Sigma–Aldrich. 2.2. Electrochemical system A volume of 0.1 L of fungicide solution was introduced into a single compartment electrolytic cell with constant stirring at 35 °C. A BDD thin film (on Si) electrode from Adamant Technologies was used as anode and an AISI 304 stainless steel (SS) plate as cathode, both of 5 cm2 geometric area, with 1.0 cm of interelectrode gap. Freshly prepared solutions of 100 mg L1 of each fungicide in 0.1 M Na2SO4 at pH 3.0, 7.0 and 10.0 were treated by EO. The effect of the applied current density (j) was tested at 15, 30, 50 and 80 mA cm2, resulting in average cell voltages of 7.8 (±0.3), 12.1 (±0.4), 13.0 (±0.3) and 15.3 (±0.5) V, respectively, for the fungicide solutions. The influence of pesticide concentration was examined for 50–100 mg L1 of myclobutanil, propiconazole and triadimefon, which correspond to the maximum solubility of this group of pesticide. All these solutions were prepared in 0.1 M Na2SO4 to provide enough ionic strength. Previously to the electrolysis, cyclic voltammetry experiments were performed to confirm that conazole fungicides studied in this work, did not suffer a direct reduction on SS cathode. On the other hand, conazole solutions were electrolyzed in a two-compartment cell containing the same electrodes: BDD/SS, showing that in the cathodic compartment no significant variations in the concentration of fungicides occurred during the electrolysis. Both voltammetric and galvanostatic experiments confirmed that the reactions on SS cathode did not change the concentration of the fungicides. 2.3. Apparatus and analytical procedures Constant current electrolyses were performed with an EHQ Power model PS3010 power supply, which also displayed the cell voltage. The pH was measured with an EXTECH model 321990 pH-meter. During the electrolysis, samples were always withdrawn at regular time intervals from the solution kept in the cell and then refrigerated until performing the analytical procedures. The degree of mineralization was monitored from total organic carbon (TOC) determined with a Vario TOC Select (Elementar) analyzer. Reproducible TOC values with an accuracy of ±1% were always found injecting 500 lL aliquots in the analyzer. From these data, the mineralization current efficiency (MCE) (Salazar et al., 2011) for each treated solution was then calculated from the following equation:

2776

MCEð%Þ ¼

J. Urzúa et al. / Chemosphere 93 (2013) 2774–2781



 nFV s ðDTOCÞexp  100 4:32  107 mIt



ð3Þ

where n is the number of electrons consumed in the mineralization of the respective fungicide, F is the Faraday constant (96 487 C mol1), Vs is the solution’s volume (L), D(TOC)exp is the experimental TOC decay (mg L1) evaluated as the difference between the initial value and that analyzed at time t, 4.32  107 is a conversion factor (3600 s h1  12 000 C mol1), I is the applied current (A) and t is the electrolysis time (h), and m corresponds to the number of carbon atoms present in the fungicide molecules, which are 15, 15 and 14 for myclobutanil, propiconazole and triadimefon, respectively. The n-values were taken according the mineralization reactions as 96 for myclobutanil (reaction 4), 86 for propiconazole (reaction 5) and 82 for triadimefon (reaction 6) considering the complete mineralization to CO2, Cl and NO 3 ion from the following reactions:

C15 H17 ClN4 þ 42H2 O ! 15CO2 þ 101Hþ þ 4NO3 þ Cl





þ 96e

ð4Þ 

C15 H17 Cl2 N3 O2 þ 37H2 O ! 15CO2 þ 91Hþ þ 3NO3 þ 2Cl 

þ 86e

ð5Þ

C14 H16 ClN3 O2 þ 35H2 O ! 14CO2 þ 86Hþ þ 3NO3 þ Cl 

þ 82e

ð6Þ

The decay of each fungicide with electrolysis time was followed by reversed-phase HPLC using a Hitachi LaChrome Elite chromatograph fitted with a Kromasil C-18 (4.6 mm (i.d)  250 mm) chromatographic column at room temperature, and coupled with a Hitachi L-2455 Elite LaChrome photodiode array detector set at k = 220 nm for myclobutanil and propiconazole and k = 277 nm for triadimefon. The analyses were carried out isocratically using 80:20 (v/v) methanol/water as mobile phase, at a flow rate of 0.5 mL min1. The aliphatic carboxylic acids obtained as intermediary products were identified and quantified by ion-exclusion chromatography using the same equipment mentioned before, but fitted with a Bio-Rad Aminex HPX 87H, 30 cm  7.8 mm (i.d.), column at 35 °C, at k = 210 nm. The mobile phase was 4 mM H2SO4 at 0.6 mL min1. The corresponding calibration curves were constructed using samples of pure acid. Absorption peaks with retention times (tR) of 8.5 min for myclobutanil, 8.9 min for propiconazole, 8.7 min for triadimefon, and 7.1 min for oxalic, 8.8 min for maleic, 10.1 min for oxamic,15.0 min for fumaric and 15.7 min for acetic acids were obtained in the corresponding chromatograms. Released inorganic ions were quantified by ionic chromatography by injecting 50 lL aliquots in a Waters 600

Table 1 Chemical structure and characteristics of the degraded conazole fungicides. Chemical structure

Commercial name

Chemical name

M (g mol1)

Myclobutanil

(RS)-2-(4-chlorophenyl)-2-(1H-1,2,4-triazol-1-ylmethyl)hexanenitrile

288.78

Propiconazole

(2RS,4RS;2RS,4SR)-1-[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-ylmethyl]-1H-1,2,4triazole

342.22

Triadimefon

(RS)-1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4-triazol-1-yl)butan-2-one

393.75

2777

J. Urzúa et al. / Chemosphere 93 (2013) 2774–2781

Another important factor to be considered in EO is the applied current density which regulates the generation rate of physisorbed hydroxyl radicals needed to oxidize the organics. As can be seen in Fig. 1a, the TOC of 100 mg L1 myclobutanil, corresponding to 62 mg L1 of TOC at pH 7.0 is poorly destroyed at 15 mA cm2 after 240 min, due to the low rate of BDD(OH) generation. The oxidation process is faster at 30 mA cm2, with a TOC reduction of 85% after 120 min of electrolysis (q = 3.0 A h L1), although almost total mineralization is achieved at 50 and 80 mA cm2 after higher q consumptions of 5 and 8 A h L1, respectively, after the same time. At current densities of 50 and 80 mA cm2 no difference in TOC decay was observed, but they caused a progressive consumption of electric charge, decreasing the MCE values from 50% applying 50 mA cm2 to 20% using 80 mA cm2, close to MCE values obtained applying 15 and 30 mA cm2 as shown in the inset of Fig. 1a. It is known that a series of parasite reactions occur in Na2SO4 media, increasing the electric charge consumption, q, with loss in MCE at higher current densities. In other words, the occurrence of non organic-oxidizing reactions involving hydroxyl radicals yields a relatively lower amount of organic oxidation events. The oxidation of BDD(OH) to O2 (reaction 7), the generation of other weaker oxidants such as ozone (reaction 8), peroxodisulfate ion (reaction 9) and H2O2 (reaction 10) and the destruction of OH with H2O2 in reaction 11 (Brillas et al., 2007; Sirés et al., 2007; Bezerra et al., 2012) are among the most common reactions that can be found. However, the MCE decrease is caused principally by oxygen evolution reaction at higher current densities.

MCE %

-1

TOC / mg L

30

40

20 10

30 0 0

20

50

100

150

200

250

40

50

Time / min

10 0 1.2

100

-1

The effect of the main experimental variables on the performance of the EO treatment of three conazole fungicides was first assessed by studying the mineralization trends to establish the best operational conditions. The initial pH was the first variable considered because it can exert a significant influence on the generation of hydroxyl radicals. Thus, solutions with 100 mg L1 of each fungicide were electrolyzed at initial pH 3.0, 7.0 and 10.0, with a current density of 50 mA cm2. No significant differences were found in the different studied pH values (data not shown). For 100 mg L1 of propiconazole fungicide, corresponding to 51.5 mg L1 of initial TOC, a continuous and quick TOC abatement was obtained for the fungicide at the three pH values (3.0, 7.0 and 10), reaching 90% mineralization after 120 min of electrolysis (q = 5 A h L1), evidencing that the pH does not play any role in the process. These results indicate that propiconazole and its organic by-products are destroyed by anodic oxidation at a similar rate at the three pH values using a BDD electrode. This behavior can be explained by the generation of a similar concentration of  OH by reaction (1) even at pH P 10 as reported by Martinez-Huitle et al. (2012). The same as with propiconazole, an overall mineralization >95% of TOC decay was achieved for myclobutanil and triadimefon under the same experimental conditions (pH 3.0, 7.0 and 10.0) after 3 h of electrolysis, confirming that the degradation of these conazoles and their by-products is practically pH-independent. Changes in the pH of solution at the end of electrolysis were to 3.36, 7.47 and 9.71 for initial pH 3.0, 7.0 and 10.0, respectively. This result is an important finding in view of the future scaling-up of this technology, because self-adjustment of pH simplifies the treatment and makes it more cost-affordable. Consequently, the following experiments were performed at pH 7.

40

50

[myclobutanil] / mg L

3.1. Effect of the experimental parameters on the mineralization of conazole fungicide solutions

50

a

60

3. Results and discussion

b

Ln ([C] 0 /[C] T)

chromatograph equipped with a 600 controller pump and a Waters  431 conductivity detector. The NO 3 and Cl concentrations in the electrolyzed solutions were determined using an IC-Pak AHR, 75 mm  4.6 mm (i.d.), anion column at 35 °C and a mobile phase of borate/gluconate at 1.0 mL min1. The possible formation of NH4+ was determined with an IC-Pak C M/D, 150 mm  3.9 mm (i.d.), cation column at 25 °C and 0.1 mM EDTA/3.0 mM HNO3 at 1.0 mL min1 as mobile phase. Aromatic intermediates were detected by GC–MS. Solutions under different experimental conditions were electrolyzed at short and long electrolysis times. They were then mixed until obtain 500 mL which were extracting three times with 30 mL of CH2Cl2 for each extraction with the purpose to identify as many reaction intermediates as possible. The collected organic solution (90 mL) was dried with anhydrous Na2SO4, filtered and completely evaporated in a rotary evaporator to obtain a pale yellow solid for the further identification. A gas chromatograph/mass selective detector (5890/5972) combination (Hewlett-Packard, Palo Alto, CA, USA) and a Hewlett-Packard 7673 autosampler were used for the analyses. The m/z range monitored was 45–400 with a scan rate of 1 scan/s; the normal electron energy was set at 70 eV. A Hewlett-Packard Ultra-1 column, 25 cm with 0.2 mm i.d. film thickness (Little Falls, Wilmington, Delaware, USA), was used. The temperature ramp was: 36 °C for 1 min, 5 °C min1 up to 300 °C and hold time 10 min. The temperature of the inlet, source and transfer line was 250, 230 and 280 °C, respectively. The use of the library of the equipment was used to compare spectra.

80

60

1.0 0.8 0.6 0.4 0.2 0.0 0

40

10

20

30

Time / min 20

0 0

50

100

150

200

250

Time / min Fig. 1. Effect of applied current density on (a) TOC abatement and (b) concentration decay for the EO of 0.1 L of 100 mg L1 myclobutanil solutions in 0.1 M Na2SO4 at 35 °C and pH 7 using the system BDD/SS. Applied current density: (h) 15 mA cm2, (j) 30 mA cm2, (d) 50 mA cm2, (N) 80 mA cm2. Inset panels (a) change in the mineralization current efficiency calculated from Eq. (3) vs. time. (b) Kinetic analysis assuming a pseudo first-order reaction. Data are the average of three independent experiments.

2778

J. Urzúa et al. / Chemosphere 93 (2013) 2774–2781

2BDDð OHÞ ! 2BDD þ O2 þ 2Hþ þ 2e

ð7Þ

a

60 þ

3H2 O ! O3 þ 6H þ 6e

18 15

MCE / %

ð8Þ 50 

þ 2H þ 2e

ð9Þ TOC / mg L

12 9 6 3

30

0 0

50

100

20

150

200

250

Time / min

10 0 1.0

b

100

80 -1

In the same way, Fig. 1b shows the decay of myclobutanil determined by HPLC experiments, for the same EO experiment described above, applying different electrolysis currents. A quicker removal for high applied j can be observed, with total disappearance of the fungicide in ca. 80 min using current densities of 50 and 80 mA cm2. At the same electrolysis time (80 min), 20 and 40 mg L1 of myclobutanil still remain applying 15 and 30 mA cm2, respectively. Finally, almost a complete decay of fungicide is obtained after 150–180 min. The inset of Fig. 2b and Table 2 show the linear correlations obtained in the corresponding kinetic analysis assuming a pseudo first-order reaction for myclobutanil when the current range is 15–50 mA cm2. At 80 mA cm2 the linearity is lost (square of regression coefficient (R2) = 0.971), mainly by oxygen evolution reaction and a lesser extent by competition with reactions (7)–(11). Thus, these data show a mild increase in the rate constants from 15 mA cm2 to 50 mA cm2 as can be seen in Table 2. This behavior suggests that in each EO experiment a constant amount of hydroxyl radicals is generated using a given applied current, reacting with myclobutanil until its complete disappearance. The same behavior was observed for the degradation of solutions containing equal amounts of propiconazole and triadimefon (100 mg L1) at pH 7.0 under similar experimental conditions. These results are summarized in Table 2. All these findings show that a current density of 50 mA cm2 is preferable for the EO treatment of conazole fungicide solutions, since it already allows reaching almost complete mineralization with high efficiency and reasonable energy consumption. As can be seen in Fig. 2a, TOC abatement of 100 mg L1 of propiconazole and triadimefon solutions (corresponding to 51 and 57 mg L1 of TOC, respectively) at pH 7.0 applying 50 mA cm2 reached almost a total mineralization after 90 min of electrolysis (q = 3.75 A h L1 for both fungicides), with a MCE close to 20% during the first hour (inset in Fig. 2a). A complete decay of the initial fungicide concentration was observed by HPLC experiments (Fig. 2b). The fungicide decay reaches 95% in 100 min, in the same way as TOC decay under similar experimental conditions. The inset of Fig. 2b shows the linear correlations obtained assuming a pseudo first order reaction for propiconazole and triadimefon with similar apparent rate constant values of 3.2  104 s1 (R2 = 0.998) and 2.9  104 s1 (R2 = 0.996), respectively, in both cases applying a current density of 50 mA cm2. Only small differences between TOC decay and fungicide decay were found, probably related to the different molecular structure of fungicides. Moreover, the fungicide is eliminated before total organic carbon, showing the formation of organic by-products from each compound, which are also rapidly destroyed by hydroxyl radicals, reaching a total decay of organic compounds to achieve complete mineralization, i.e., complete transformation into CO2 and H2O. In this work, the main by-products produced during EO of the conazole fungicides studied correspond to carboxylic acids, aromatic compounds, and inorganic ions. On the other hand, Fig. 2c shows the progressive TOC removal of myclobutanil from solutions at pH 7.0 electrolyzed at 50 mA cm2 with increasing fungicide content. For example, at q = 5 A h L1 (120 min) TOC is reduced by 86%, 85%, 78% and 75% at 25, 50, 75 and 100 mg L1, respectively. This trend indicates a decrease in

[Fungicide] / mg L

ð11Þ

60

0.8 0.6 0.4 0.2 0.0 0

40

10

20

30

Time / min

40

50

20

0

c

60

50 40

50 -1

H2 O2 þ  OH ! HO2 þ H2 O

ð10Þ

TOC / mg L

2H2 O ! H2 O2 þ 2Hþ þ 2e

40

Ln ([C]0/[C] T)

þ

MCE / %

!

S2 O2 8

-1

2HSO4

40

30 20 10

30 0

20

0

50

100

150

200

250

Time / min 10 0 0

50

100

150

200

250

Time / min Fig. 2. (a) TOC abatement and (b) concentration decay for the EO of 0.1 L of 100 mg L1 of propiconazole (d) and triadimefon (s) solutions. (c) TOC abatement of different initial myclobutanil concentration solutions. Experimental conditions: 0.1 M Na2SO4 at 35 °C and pH 7 applying 50 mA cm2 using the system BDD/SS. Inset panels (a) variation of mineralization current efficiency calculated from Eq. (3) vs. time. (b) Kinetic analysis assuming a pseudo first-order reaction. (c) Variation of mineralization current efficiency calculated from Eq. (3) vs. time. Initial myclobutanil content in (c): (s) 25 mg L1, (d) 50 mg L1, (h) 75 mg L1 and (j) 100 mg L1. Data are the average of three independent experiments.

the destruction of organic matter by a similar amount of generated hydroxyl radicals. However, the amount of TOC removed increases gradually, causing a concomitant rise in MCE (see inset in Fig. 2c). For the most concentrated solution the efficiency reaches 45% in the first 10 min of electrolysis and it is almost totally mineralized after 3 h. A similar behavior was verified for propiconazole and triadimefon solutions degraded under similar experimental conditions, evidencing that an increase of initial fungicide concentration is associated with an increase in the rate reaction, as expected for a first order reaction (Panizza and Cerisola, 2009). On the other hand,

J. Urzúa et al. / Chemosphere 93 (2013) 2774–2781 Table 2 Apparent rate constant considering a pseudo first order kinetic analysis, obtained for each fungicide separately. Data are the average of three independent experiments. Fungicide

japp (mA cm2)

kapp  104 (s1)

R2

Myclobutanil

15 30 50 80

1.82 2.77 3.64 3.70

0.991 0.993 0.995 0.970

Propiconazole

15 30 50 80

2.23 2.61 3.25 3.20

0.998 0.996 0.998 0.813

Triadimefon

15 30 50 80

1.54 2.11 2.88 2.23

0.995 0.993 0.996 0.913

Table 3 Aromatic compounds detected by GC–MS during the EO degradation of 0.1 L of 100 mg L1 of each conazole fungicide in 0.1 M Na2SO4 at pH 7.0, applying 50 mA cm2 at 25 °C using a BDD/SS system. Compound

Name

Molecular formula

m/za

1 2 3 4

Myclobutanil Propiconazole Triadimefon 4-chlorophenol

See Table 1 See Table 1 See Table 1 OH

288 341 293 128

5

Hydroquinone

Cl OH

109

6

2,4-dichlorophenol

OH OH

163

Cl

7

3,4-dihydroxy-chlorobenzene

Cl Cl

143

OH OH a

Negative ion with z = 1.

at higher organic compound concentrations, the oxidation rate of the pollutants with hydroxyl radicals increases, increasing the ratio between the fungicide and non-oxidizing parasite reaction rates like those of reactions (7)–(11). 3.2. Generated by- products detected during EO of conazole fungicides Intermediates and/or reaction products formed during the electrolysis of each fungicide applying 50 mA cm2 at pH 7.0 and

2779

35 °C, using the BDD/SS system, were followed by chromatographic techniques in order to identify and quantify some their main oxidation products. Thus, samples obtained at different electrolysis times were collected until accumulate a volume of 500 mL, then extracted with CH2Cl2 and finally concentrated by evaporation of the solvent. A yellow solid resulting was injected in a GC–MS apparatus. Aromatic intermediates detected during the electrolysis of each fungicide are described below and shown in Table 3. For myclobutanil solutions, MS spectra displayed the corresponding peaks associated with myclobutanil (1) and its most abundant fragments (m/z = 288, 179 and 152) at tR = 19.6 min, together with two additional peaks associated with the presence of 4-chlorophenol (4) (m/z = 128) at tR 14.7 min and for p-hydroquinone (5) (m/z = 109) at tR 13.7 min, which correspond to the main aromatic intermediates found for electrolyzed solutions of myclobutanil. On the other hand, 4-chlorophenol and p-hydroquinone, were confirmed and quantified by HPLC experiments using a pure standard and comparing their retention time and UV–Vis spectra, measured on the photodiode array detector. The 4-chlorophenol concentration reaches around 1.1 mg L1 at 30 min of electrolysis while hydroquinone is accumulated close to 0.8 mg L1 at same electrolysis time. After 3 h of electrolysis both, 4-chlorophenol and p-hydroquinone were not detected. With respect to propiconazole degradation, a similar MS spectrum was obtained, displaying the corresponding peaks associated with propiconazole (2) and its most abundant fragments (m/z = 341, 259 and 173) at tR = 20.6 min, together with four additional peaks associated with the main aromatic intermediates formed during electrolysis: 4-chlorophenol (m/z = 128) at tR = 14.7 min, 2,4-dichlorophenol (6) (m/z = 163) at tR = 13.4 min, 3,4-dihydroxy-chlorobenzene (7) (m/z = 143) at tR = 11.3 min, and hydroquinone (m/z = 109) at tR = 13.7 min. Finally, the MS spectra of triadimefon solutions displayed the corresponding peaks associated with triadimefon (3) and it most abundant fragments (m/ z = 293 and 196,) at tR = 14.4 min. Also, two additional peaks associated with the presence of 4-chlorophenol (m/z = 128) at tR = 14.7 min and for hydroquinone (m/z = 109) at tR = 13.7 min, were observed. The presence of all these aromatic intermediates explains the fact that the TOC abatement is slower than the decrease in concentration of each fungicide observed during the electrolysis. It is known that during the degradation of aromatic compounds occur the formation of short-linear carboxylic acids and then they are transformed into oxalic acid as ultimate product previous the complete mineralization to CO2 (Dirany et al., 2012). Thus, the carboxylic acids formed during the oxidation of the fungicides were determined by ion-exclusion chromatography. To identify each carboxylic acid seen in the chromatogram, samples of the most probable pure carboxylic acid and their mixtures were used as standards under the same experimental conditions. The evolution of carboxylic acid concentration during the electrolysis is shown in Fig. 3a–c. Oxalic, maleic, oxamic, fumaric and acetic acids were found in the propiconazole and triadimefon oxidation. For myclobutanil, acid acetic was not detected. Probably, fumaric and maleic acids come from the oxidation of the aryl moiety of the aromatics, and/or they could be formed by oxidation of chlorophenols and quinones, both detected by GC–MS. The oxidation of these acids may generate acetic acid, while oxamic acid may be formed when a part of the triazol ring is opened by the attack of OH. At the end of the electrolysis, oxalic and oxamic acids are the last carboxylic acids that are directly transformed into CO2 as expected (GarciaSegura and Brillas, 2011). As can be seen in Fig. 3a–c, a large accumulation of oxalic acid occurred up to 6.0–8.0 mg L1 at 50 min of the EO of the conazoles, which is rapidly removed by BDD(OH) radicals, disappearing in 150–180 min. A simple mass balance, for example at 180 min, reveals that the total contribution of the above acids to the TOC of electrolyzed myclobutanil, propiconazole and triadimefon solutions is only 2.1, 2.3 and 2.6 mg L1,

2780

J. Urzúa et al. / Chemosphere 93 (2013) 2774–2781

8

a

a

2.0

-1

1.0

[NO3 ] / mg L

1.5

-

6

4

2

0.5 0 0.0

b

5

1.8

4

1.5 -1

3

[Cl ] / mg L

2

-

[Carboxylic Acids] / mg L

-1

6

1

b

1.2 0.9 0.6

0 0.3 6

c

0.0

5 0

30

60

90

120

150

180

Time / min

4

 1 Fig. 4. Evolution of (a) NO of: 3 and (b) Cl detected during the EO of 100 mg L myclobutanil (j), propiconazole (d)and triadimefon (N) solutions. Experimental conditions: 0.1 M Na2SO4 at 35 °C and pH 7 applying 50 mA cm2.

3 2 1 0 0

30

60

90

120

150

180

Time / min Fig. 3. Evolution of (d) oxalic, (j) oxamic, (N) fumaric, (.) maleic and () acetic acids detected during the EO of 100 mg L1 of: (a) myclobutanil, (b) propiconazole and (c) triadimefon solutions under the same conditions of Fig. 1.

respectively, values lower than 4–5 mg L1 TOC determined for them (see Figs. 1 and 2 at 180 min). This means that a small proportion of other undetected hardly oxidizable by-products are also formed. The determination of other elements such as nitrogen and chlorine requires the presence of inorganic cations or anions. It is very  common to detect nitrogen compounds as NHþ 4 and NO3 ions in the mineralization of N-derivatives (Martínez-Huitle and Brillas, 2009; Alves et al., 2012). The presence of these ions and Cl was followed by ionic chromatography. The results indicate the presþ  ence of NO 3 and Cl , but NH4 was not detected, probably because the pH of the solution is slightly basic. Fig. 4a shows an accumula1 tion of NO at the end of the elec3 in the range of 1.5–2.0 mg L trolysis for the three studied pesticides. The concentration of NO 3 ions detected at the end of the electrolysis correspond to 10.3% of initial N for myclobutanil, 13.1% for propiconazole and 12.0% for triadimefon. Since the corresponding remaining oxamic

acid only contains 0.8–1.2% of the initial N (see Fig. 3), it can be inferred that a large fraction of the initial N of the fungicide molecules (ca. 3%) is loss as volatile N-species, mainly as N2 and/or a NOx. Also, Cl ion accumulates during the first 2 h of electrolysis (Fig. 4b) result of dechlorination of fungicide molecules during the mineralization process. Maximum Cl concentration of 0.15, 0.17 and 0.11 mg L1 for myclobutanil, propiconazole and triadimefon, respectively, were detected. After 2 h of electrolysis, a gradual disappearance of this ion was observed. The results showed that Cl ions are practically eliminated at the end of the experiments. This indicates that chloroaromatic rings in the fungicides are destroyed during the first 2 h of EO and their chlorine atoms are released in the form of Cl. However, this ion is unstable under these conditions since it is slowly oxidized on BDD producing Cl2 (Brillas et al., 2004). On the basis of the findings discussed above, the total mineralization of conazole fungicide studied in this work is by EO due to attack of OH could be involving three routes (i) the formation of aromatic intermediates such as 4-chlorophenol and hydroquinone (determined for the three fungicides studied) and the formation of 2,4-dichlorophenol and 3,4-dihydroxychlorobenzene in the case of  the electrooxidation of propiconazole, plus NO 3 and Cl ions. Aromatic compounds react with OH to produce a mixture of carbox ylic acids together with NO ions. (ii) A second route 3 and Cl involves the generation of acetic acid during the attack of OH onto alkyl group present in propiconazole and triadimefon. In the case of myclobutanil, acetic acid was not found. And (iii) a part of

J. Urzúa et al. / Chemosphere 93 (2013) 2774–2781

triazole ring gives oxamic acid and a large portion is loss as volatile N-species (N2 and/or a NOx). The carboxylic acids continue reacting until they turn into oxalic acid, and subsequently into CO2 plus H2O. The initial conazole fungicide molecule, either myclobutanil, propiconazole or triadimefon, becoming completely mineralized according to Eqs. (4)–(6). 4. Conclusions The results obtained in this work show that the EO process allowed an almost total mineralization with 94–96% TOC removal of conazole fungicides such as myclobutanil, propiconazole and triadimefon from solutions close to saturation at pH 7.0. The three compounds react at similar rates under similar experimental conditions. In order to achieve a good relation between the actual current consumed in the oxidation process and the ratio used in parallel reactions that reduce the yield of the process, the optimal current density was determined. The EO mechanism of the three conazole can be described by pseudofirst-order reactions whose rate constants are proportional to the current in the range of 15–50 mA cm2, in agreement with an increase of the adsorbed OH at the anode. At 80 mA cm2 a decrease of the rate constant was seen, which is attributed to parasite reactions. The degradation process of the conazole fungicides involves the formation of aromatic intermediates followed by the formation of aliphatic carboxylic acids, until they are completely mineralized to CO2. Initial Cl is present as chloride ions (Cl) while a small proportion of N is detected as nitrate (NO 3 ). Most of the initial N passes to the atmosphere as volatile compounds, which were not determined. In summary, these results allow the application of EO using a BDD/SS system for the degradation of this kind of persistent pollutants. Now it will be necessary to determine the optimum conditions not at the basic laboratory level, but on a larger scale. Acknowledgment The financial support of DICYT-USACH and FONDECYT under Project 11090275 is gratefully acknowledged. References Alves, S.A., Ferreira, T.C.R., Sabatini, N.S., Trientini, A.C.A., Migliorini, F.L., Baldan, M.R., Ferreira, N.G., Lanza, M.R.V., 2012. A comparative study of the electrochemical oxidation of the herbicide tebuthiuron using boron-doped diamond electrodes. Chemosphere 88, 155–160. Bezerra, J.H., Soares, M.M., Suely, N., Ribeiro da Silva, D., Martínez-Huitle, C.A., 2012. Application of electrochemical oxidation as alternative treatment of produced water generated by Brazilian petrochemical industry. Fuel Process. Technol. 96, 80–87. Brillas, E., Boye, B., Sirés, I., Garrido, J.A., Rodríguez, R.M., Arias, C., Cabot, P.-L., Comninellis, C., 2004. Electrochemical destruction of chlorophenoxy herbicides by anodic oxidation and electro-Fenton using a boron-doped diamond electrode. Electrochim. Acta 49, 4487–4496. Brillas, E., Baños, M.A., Skoumal, M., Cabot, P.L., Garrido, J.A., Rodríguez, R.M., 2007. Degradation of the herbicide 2,4-DP by anodic oxidation, electro-Fenton and photoelectro-Fenton using platinum and boron-doped diamond anodes. Chemosphere 68, 199–209. Calonnec, A., Cartolaro, P., Poupot, C., Dubourdieu, D., Darriet, P., 2004. Effects of uncinula necator on the yield and quality of grapes (vitis vinifera) and wine. Plant Pathol. 53, 434–445.

2781

Chen, P.-J., Moore, T., Nesnow, S., 2008. Cytotoxic effects of propiconazole and its metabolites in mouse and human hepatoma cells and primary mouse hepatocytes. Toxicol. in Vitro 22, 1476–1483. Comninellis, C., Chen, G., 2010. Electrochemistry for the Environment, first ed. Springer, New York. Dirany, A., Sirés, I., Oturan, N., Özcan, A., Oturan, M.A., 2012. Electrochemical treatment of the antibiotic sulfachloropyridazine: kinetics, reaction pathways, and toxicity evolution. Environ. Sci. Technol. 46, 4074–4082. Garcia-Segura, S., Brillas, E., 2011. Mineralization of the recalcitrant oxalic and oxamic acids by electrochemical advanced oxidation processes using a borondoped diamond anode. Water Res. 45, 2975–2984. Georgopapadakou, N.H., Walsh, T.J., 1996. Antifungal, agents: chemotherapeutic targets and immunologic strategies. Antimicrob. Agents Chemother. 40, 279– 291. Guinea, E., Centellas, F., Brillas, E., Cañizares, P., Sáez, C., Rodrigo, M.A., 2009. Electrocatalytic properties of diamond in the oxidation of a persistant pollutant. Appl. Catal. B: Environ. 89, 645–650. Gupta, A.K., Sauder, D.N., Shear, N.H., 1994. Antifungal agents: an overview. Part II. J. Am. Acad. Dermatol. 30, 911–933. Hester, S.D., Nesnow, S., 2008. Transcriptional responses in thyroid tissues from rats treated with a tumorigenic and a non-tumorigenic triazole conazole fungicide. Appl. Pharmacol. 227, 357–369. Iyer, V.V., Ovacik, M.A., Androulakis, I.P., Roth, C.M., Ierapetritou, M.G., 2010. Transcriptional and metabolic flux profiling of triadimefon effects on cultured hepatocytes. Toxicol. Appl. Pharmacol. 248, 165–177. Kjærstad, M.B., Taxvig, C., Nellemann, C., Vinggaard, A.M., Andersen, H.R., 2010. Endocrine disrupting effects in vitro of conazole antifungals used as pesticides and pharmaceuticals. Reprod. Toxicol. 30, 573–582. Kraft, A., 2007. Doped diamond: a compact review on a new, versatile electrode material. Int. J. Electrochem. Sci. 2, 355–385. Martínez-Huitle, C.A., Brillas, E., 2009. Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review. Appl. Catal. B: Environ. 87, 105–145. Martínez-Huitle, C.A., Ferro, S., 2006. Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes. Chem. Soc. Rev. 35, 1324–1340. Martinez-Huitle, C.A., Vieira dos Santos, E., Medeiros de Araujo, D., Panizza, M., 2012. Applicability of diamond electrode/anode to the electrochemical treatment of a real textile effluent. J. Electroanal. Chem. 674, 103–107. Panizza, M., Cerisola, G., 2003. Influence of anode material on the electrochemical oxidation of 2-naphthol. Part 1. Cyclic voltammetry and potential step experiments. Electrochim. Acta 48, 3491–3497. Panizza, M., Cerisola, G., 2004. Influence of anode material on the electrochemical oxidation of 2-naphthol. Part 2. Bulk electrolysis experiments. Electrochim. Acta 49, 3221–3226. Panizza, M., Cerisola, G., 2005. Application of diamond electrodes to electrochemical processes. Electrochim. Acta 51, 191–199. Panizza, M., Cerisola, G., 2009. Direct and mediated anodic oxidation of organic pollutants. Chem. Rev. 109, 6541–6569. Rockett, J.C., Narotsky, M.G., Thompson, K.E., Thillainadarajah, I., Blystone, C.R., Goetz, A.K., Ren, H., Best, D.S., Murrell, R.N., Nichols, H.P., Schmid, J.E., Wolf, D.C., Dix, D.J., 2006. Effect of conazole fungicides on reproductive development in the female rat. Reprod. Toxicol. 22, 647–658. Ross, J.A., Moore, T., Leavitt, S.A., 2009. In vivo mutagenicity of conazole fungicides correlates with tumorigenicity. Mutagenesis 24, 149–152. Salazar, R., Ureta-Zañartu, M.S., 2012. Mineralization of triadimefon fungicide in water by electro-Fenton and photo electro-Fenton. Water Air Soil Pollut., 4199– 4207. Salazar, R., Garcia-Segura, S., Ureta-Zañartu, M.S., Brillas, E., 2011. Degradation of disperse azo dyes from waters by solar photoelectro-Fenton. Electrochim. Acta 56, 6371–6379. Scialdone, O., 2009. Electrochemical oxidation of organic pollutants in water at metal oxide electrodes: a simple theoretical model including direct and indirect oxidation processes at the anodic surface. Electrochim. Acta 54, 6140–6147. Sirés, I., Cabot, P.L., Centellas, F., Garrido, J.A., Rodríguez, R.M., Arias, C., Brillas, E., 2006. Electrochemical degradation of clofibric acid in water by anodic oxidation. Comparative study with platinum and boron-doped diamond electrodes. Electrochim. Acta 52, 75–85. Sirés, I., Garrido, J.A., Rodríguez, R.M., Brillas, E., Oturan, N., Oturan, M.A., 2007. Catalytic behavior of the Fe3+/Fe2+ system in the electro-Fenton degradation of the antimicrobial chlorophene. Appl. Catal. B: Environ. 72, 382–394. Sirés, I., Brillas, E., Cerisola, G., Panizza, M., 2008. Comparative depollution of mecoprop aqueous solutions by electrochemical incineration using BDD and PbO2 as high oxidation power anodes. J. Electroanal. Chem. 613, 151–159. Turner, W.W., Rodriguez, M.J., 1996. Recent advances in the medicinal chemistry of antifungal agents. Curr. Pharmaceut. Des. 2, 209–224.