Chemosphere 117 (2014) 596–603

Contents lists available at ScienceDirect

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

Electrochemical oxidation of imazapyr with BDD electrode in titanium substrate F.L. Souza a, T.Q. Teodoro a, V.M. Vasconcelos a, F.L. Migliorini b, P.C.F. Lima Gomes c, N.G. Ferreira b, M.R. Baldan b, R.L.A. Haiduke a, M.R.V. Lanza a,⇑ a b c

Instituto de Química de São Carlos, Universidade de São Paulo, P.O. Box 780, São Carlos, SP, Brazil Instituto Nacional de pesquisas Espaciais, 12201-970 São José dos Campos, SP, Brazil Escola de Engenharia de São Carlos, Universidade de São Paulo, P.O. Box 780, São Carlos, SP, Brazil

h i g h l i g h t s  The treatment with boron-doped diamond anodes is able to completely degrade imazapyr.  The wastewater toxicity can be reduced by electrochemical treatment.  The major degradation intermediates are detected and identified by LC–MS/MS analyses.  Formic, acetic and butyric acids are the main non aromatic by-products.

a r t i c l e

i n f o

Article history: Received 1 July 2014 Received in revised form 11 September 2014 Accepted 15 September 2014

Handling Editor: E. Brillas Keywords: Imazapyr Wastewater treatment Electrochemical oxidation Boron-doped diamond anodes Herbicide

a b s t r a c t In this work we have studied the treatment of imazapyr by electrochemical oxidation with boron-doped diamond anode. Electrochemical degradation experiments were performed in a one-compartment cell containing 0.45 L of commercial formulations of herbicide in the pH range 3.0–10.0 by applying a density current between 10 and 150 mA cm2 and in the temperature range 25–45 °C. The maximum current efficiencies were obtained at lower current densities since the electrochemical system is under mass transfer control. The mineralization rate increased in acid medium and at higher temperatures. The treatment was able to completely degrade imazapyr in the range 4.6–100.0 mg L1, although the current charge required rises along with the increasing initial concentration of the herbicide. Toxicity analysis with the bioluminescent bacterium Vibrio fischeri showed that at higher pollutant concentrations the toxicity was reduced after the electrochemical treatment. To clarify the reaction pathway for imazapyr mineralization by OH radicals, LC–MS/MS analyses we performed together with a theoretical study. Ions analysis showed the formation of high levels of ammonium in the cathode. The main final products of the electrochemical oxidation of imazapyr with diamond thin film electrodes are formic, acetic and butyric acids. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Regarding the concerns about the occurrence of pollutants in wastewater, surface and groundwater, pesticides are considered as one of the main environmental concerns by regulatory agencies (Cohen et al., 1986). These substances exhibit bio-recalcitrant properties, high toxicity, and persist in the environment. However, the rapid expansion of modern agricultural technologies in recent decades, in addition to the increased pest resistance and the lack of strict regulations, has resulted in a steady increase in the ⇑ Corresponding author. Tel.: +55 1633739968. E-mail address: [email protected] (M.R.V. Lanza). http://dx.doi.org/10.1016/j.chemosphere.2014.09.051 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

production of these compounds (Rahman, 2013). In this context, Brazil is one of the largest worldwide consumers of pesticides (Pedlowski et al., 2012). The unsafe use of these products has largely contributed to environmental contamination and its accidental or intentional ingestion has also become a major public health problem (Caldas, 2011). The present research focuses on one herbicide, in particular: an imidazolinone compound, imazapyr ((RS)-2-(4-methyl-5-oxo-4propan-2-yl-1H-imidazol-2-yl)pyridine-3-carboxylic acid). This is a low-molecular weight, water soluble and commercially available simple compound (Cong and Wu, 2007). Imazapyr is widely used as a non-selective wide-spectrum herbicide in non-crop situations, as well as in forestry, grasslands (Ramezani et al., 2010) and

F.L. Souza et al. / Chemosphere 117 (2014) 596–603

leguminous crops, such as rice (Kaloumenos et al., 2013), soybean (Alister and Kogan, 2005) and sunflower (Elezovic et al., 2012). This compound inhibits the plant enzyme acetohydroxyacid synthase, which interferes in DNA and protein synthesis inhibiting the cell growth (Harir et al., 2008). On the other hand, imazapyr is a persistent herbicide with high soil mobility and large water solubility. Therefore, it can penetrate deep into the groundwater causing potential negative environmental effects. Consequently, considerable efforts have been devoted to the development of efficient and cost effective treatment methods. Most of the studies developed are based on the photodegradation and heterogeneous photocatalytic oxidation process using catalysts such as TiO2 and UV light (Pizarro et al., 2005; Carrier et al., 2006; Quivet et al., 2006). However, the main limitation for the use of titania-based materials is the tough catalyst recovery when used in the slurry form. In addition, the development of a more reliable, low-cost photocatalyst and the modeling of the reactor should be further explored (Ahmed et al., 2011). The electrolytic technologies have proven to be a viable option for wastewater treatment compared to other advanced oxidation process. The robustness and efficiency of these technologies were mainly achieved through advances in electrodic materials and reactor designs for industrial applications (Marselli et al., 2003; Rodrigo et al., 2010). Among the anodic material available, the electrodes comprising boron-doped diamond film supported on titanium (BDD/Ti) have been extensively studied for the treatment of various refractory organic compounds, e.g., dyes (Migliorini et al., 2011; Sales Solano et al., 2013), pharmaceutical compounds (Vidales et al., 2012; Cavalcanti et al., 2013) and pesticides (Sirés et al., 2008; Alves et al., 2012). The results have shown high efficiency, attributed to the material characteristics such as the high overpotential for water electrolysis, excellent chemical and electrochemical stability, satisfactory conductivity and the ability to generate other oxidants depending on the wastewater composition and the operation conditions (Panizza and Cerisola, 2009). Hence, our paper aims to study the electrochemical oxidation of imazapyr with diamond electrodes with particular emphasis on the role of the operating conditions such as the applied current density, pH, temperature and initial pollutant concentration. Finally, the primary intermediate products formed during the degradation of imazapyr were analyzed and the reaction pathways in the electrolytic process were elucidated through theoretical calculations and comparisons to the literature reported. 2. Experimental 2.1. Preparation of BDD electrodes The boron doped diamond electrode was grown on Ti substrate (25 mm  25 mm  0.5 mm dimensions) by hot filament chemical vapor deposition (HFCVD) technique. The doping control was obtained from an additional H2 gas flux passing through a bubbler containing a solution of B2O3 dissolved in CH3OH with the B/C ratio of 5000 ppm. The films were deposited for 8 h. Top-view SEM images of BDD films were obtained from a JEOL JSM-5310 microscope system at 20 kV accelerating voltage. The diamond quality as well as its boron level was analyzed by Micro-Raman scattering spectroscopy (Renishaw microscope system 2000) in backscattering configuration using Ar incident laser at 514.5 nm. The spectra covered the range from 300 to 3500 cm1.

597

through liquid chromatography coupled to ultraviolet (LC–UV Shimadzu) using a C-18 column (5 lm, 250 mm  4.6 mm) maintained at 30 °C and flow rate of 1.0 ml min1. Acetonitrile and water (30:70) was used as mobile phase, the water pH was adjusted to 2.80 using H3PO4. The detection wavelength was 254 nm. The acid intermediates were analyzed at 190 nm. The ion exchange column used was the SUPELCOGELTM H Carbohydrate Columns (Sigma–Aldrich) whereas a diluted H3PO4 solution (0.022 M) was employed as the solvent. The TOC concentration was monitored using a Shimadzu – VCPN. Measurements of pH were carried out with a pHB 500 Ion pH-meter. Anion and cation concentrations were analyzed using a DIONEX ICS- 1000 ion chromatograph equipped with a DS 56 conductivity detector, using AS4A-SC column (4 mm  250 mm anion-exchange column for anion analysis) and a CSRS column (4 mm  250 mm cationexchange column for cation analysis). The mobile phase was 1.7 mM NaHCO3 and 1.8 mM K2CO3 for anion analyses and 9 mM H2SO4 for cation analyses. Degradation intermediates were analyzed using a LC coupled to a mass spectrometer using electrospray ionization source. A hybrid mass spectrometer was used, an ABSCIEX QTRAP 5500 (quadrupoleion-trap) in positive mode. The LC system consisted of an Agilent 1260, a C-18 Agilent Poroshell 120 (2.7 lm, 50 mm  3.0 mm) column was used to separate the analytes. A gradient separation was used starting at 95% of A (water with 0.1% of formic acid) and 5% B (acetonitrile with 0.1% of formic acid), increase to 65% B and 35% B after 6 min, remaining for 3 min. Thereafter, returned to initial condition in 11 min, chromatographic run total time. The injection volume and flow rate used were 10 lL and 0.6 mL min1, respectively. A selected reaction monitored (SRM) and independent data acquisition (IDA) criteria were used whereas monitored in the SRM mode using three transitions. The IDA criteria threshold chosen was 10,000 cps, reaching the threshold, an enhanced product-ion scans started using three different collision energies. An online solid phase extraction (SPE) was used as sample preparation technique to concentrate the intermediates and eliminate the interferences. Samples were only filtered in 0.22 lm nylon filter and direct injected in an Oasis HLB SPE column (25 lm, 20 mm  2.1 mm) using a Shimadzu 10A pump at flow rate of 0.6 mL min1. After injected the samples were loaded in the SPE column for 3 min using water as mobile phase. Thereafter, through a column switching six-port valve, the pre-concentrated analytes were eluted to the chromatographic in a backflush mode, following the LC–MS/MS analysis described above. 2.3. Electrochemical cell The commercial herbicide Gramizap (Citromax S/A, Brazil), containing 0.26% m/v of imazapyr was used in the degradation experiments. The galvanostatic electrolyses were carried out in a single-compartment electrochemical cell containing 450 cm3 of solution. Potassium sulphate 0.1 M was used as supporting electrolyte. A heat exchanger coupled with a controlled thermostatic bath (Ethik technology, Brazil) was used to maintain the temperature at the desired set point. More details about the electrochemical system and cell configuration are described (Alves et al., 2012). The Boron-doped diamond films (BDD/Ti) (4.15 cm2 of geometric area) were employed as anode and a platinum spiral as cathode. All electrochemical measurements were carried out using a potentiostat/ galvanostat AUTOLAB model PGSTAT 302 (Eco Chimie) coupled with a BRTS-10A current booster, controlled by GPES software.

2.2. Apparatus and analysis procedures

2.4. V. Fischeri assay

All samples were filtered with 0.45 lm nylon filters (Millipore) before analysis. The pesticide concentrations were quantified

Lyophilized V. fischeri (NRRL B-11177- BLX-104 Lyo 5) and the activation solutions were obtained from Biolux LYOÒ test kit. A

598

F.L. Souza et al. / Chemosphere 117 (2014) 596–603

solution of 2% NaCl in pH 7 was used as control solution. The samples after electrolysis were prepared at conductivity of 50 mS cm1 and pH 6–8.5, prior to the analysis. The luminescence was measured using a LUMIStherm automatic incubation block (LUMIStox 300 DR LANGE, GERMANY) according to technical requirements of DIN 38412 L34 and L341. Half-maximal effective concentration (EC50) values were calculated from the dose-response curve. Automatic color correction was also done when necessary. 3. Results and discussion 3.1. Morphologic and structural characterization of the BDD electrode Fig. 1 presents the Raman scattering spectra and the top-view SEM image of the diamond film deposited on the Ti substrate is represented in inset which corresponds to a general view of the electrode surface morphology. The image in Fig. 1 shows that the BDD/Ti film grew with a continuous and uniform surface morphology. The film is very adherent, without cracks or delamination. In general, the deposited BDD films consist of (1 1 1) and (1 0 0) faces. The BDD morphology observed is associated to microcystalline grans with dominant (1 1 1) faces which is characteristic of highly boron doped samples. The presence of a Raman peak at around 1332 cm1 was verified, which corresponds to the diamond firstorder phonon vibration. We also observed the emergent band at 1200 cm1, characteristic of the boron doped diamond films, which is attributed to the induced disorder in the diamond structure due to the boron incorporation (Ager, 1995; Zhang, 1996; May et al., 2008). The band at around 1580 cm1 can be attributed to band G (graphitic phases). The appearance of a band at around 500 cm1 is also observed and is attributed to the vibration of boron pairs in diamond lattice (May et al., 2008). 3.2. Effect of experimental parameters on imazapyr degradation Commercial herbicide Gramizap 0.26% (Citromax S/A, Brazil) was diluted (76% v/v) in order to obtain about 100 mg L1 of imazapyr. The solution was initially electrolyzed at 10, 50, 100 and 150 mA cm2, at 25 °C, in the presence of 0.1 M K2SO4 as supporting electrolyte and its natural pH (6.8). Fig. 2 shows the imazapyr concentration and TOC reduction as a function of time for different values of current densities. Fig. 2a indicates that an increase in j from 10 mA cm2 (Q = 0.36 kA h L1) to 100 mA cm2

Fig. 1. Scanning electron micrographs of diamond film grown on titanium substrate with 5000 ppm B/C and (b) Raman spectra of diamond film grown on titanium substrate.

Fig. 2. Normalized (a) imazapyr concentration and (b) TOC as a function of time for different values of current densities: (j) 10 mA cm2, (d) 50 mA cm-2, (N) 100 mA cm2 and (.) 150 mA cm2. Conditions: pH 6.8, T 25 °C, and 0.1 M K2SO4.

(Q = 3.6 kA h L1) causes an improvement in imazapyr degradation from 14% to 35%, while at 150 mA cm2 (Q = 5.46 kA h L1) only 13% was removed. As can be seen in Fig. 2b, a similar pattern was found for TOC mineralization. Moreover, TOC decays slower than the imazapyr concentration, attaining only 15% at 100 mA cm2. Such a lower mineralization in comparison to oxidation compounds can be attributed to the fact that the mineralization is the last step of the oxidation process in which there are hard oxidation compounds. Regarding reaction rates, the calculated kinetic constants value demonstrate a pseudo first-order reaction. Imazapyr concentration decreases when j increases from 10 to 100 mA cm2 (k = 8.9  104 min1, 2.2  103 min1 and 2.3 103 min1 for 10, 50, 100 mA cm2 respectively). This behavior could be explained by a concomitant OH production and that accelerates the oxidation rate of all organic compounds present in the medium. In addition to the hydroxyl radicals, many oxidants (peroxide hydrogen, ozone, peroxosulphate) are produced on the electrode surface and extend the oxidation of pollutants from the nearness of the electrode surface towards the bulk of the wastewater, suggesting a mediated electro-oxidation (Panizza and Cerisola, 2005; Souza et al., 2014). However, the kinetic constant decreases to 8.5  104 min1 at 150 mA cm2. This fact can be associated to waste reactions that address the oxidation of the main relative oxidant BDD (OH), e.g., its oxidation to O2, its dimerization to H2O2, followed by generation of a weaker hydroperoxyl radical HO, and the formation of other weaker oxidants such as peroxodisulfate (S2O2 peroxodiphosphate 8 ), (P2O4 8 ) and ozone (Brillas et al., 2010). Consequently, the efficiency decreases with increasing current densities. The mineralization current efficiency was determined in according with literature (Comninellis and Pulgarin, 1991; Emmanuel et al., 2004). The values were 47.9%, 40.2% and 16.5% at 50, 100 and 150 mA cm2 respectively. This parameter was not calculated for 10 mA cm2 because the difference in TOC removal is nil. The process is mass-transport controlled, where the presence of low analyte mass in the solution and/or the continuous persistent products generation, i.e., species that react slowly with OH, are oxidized effectively at low current densities. Contrary to the current efficiency, power consumption increases as the current density is raised, from 0.43 kW h L1 at 10 mA cm2 to 17.40 kW h L1 at 150 mA cm2. To analyze the BDD anode oxidation ability, the pH and temperature effect on imazapyr degradation was evaluated. A series of electrolysis were carried out using imazapyr solutions at 100 mg L1 in pH range 3–10 at 50 mA cm2 and 25.0 °C for 9 h.

F.L. Souza et al. / Chemosphere 117 (2014) 596–603

Next, the same concentration level of imazapyr at pH 3.0 was electrolyzed at 50 mA cm2 for 9 h at different temperatures of 25, 35 and 45 °C. Fig. 3 shows the imazapyr and TOC reduction as a function of applied charge per volume unit of electrolyzed solution (Qap) for different values of pH and temperature. As can be seen in Fig. 3a, imazapyr is slightly better removed in acid medium than in alkaline solutions, attaining efficiencies of about 72% in pH 3 and 58% at pH 10. As demonstrated by Carrier et al. (2006), the imazapyr molecule exhibits five distinct chemical species under acidbase medium with dissociation constants varying between 1.88 and 10.8. At pH 3, 7 and 10 the predominant herbicide forms are positive, neutral and anionic, respectively (Carrier et al. (2006)). In addition, peroxosulphates are formed during diamond anode electrolysis in a medium rich in sulphate. Thus, considering the electrostatic interactions, some repulsion effects between imazapyr and peroxosulphate could take place in pH 10 as both species are negatively charged. On the other hand, since imazapyr is in the cationic form, the attractive interactions in pH 3 may allow the herbicide to be easily degraded. Fig. 3c demonstrates a poor mineralization for all solutions, although small differences are noticed, 26%, 24% and 20% at pH 3, 7 and 10, respectively, when a charge of 4.88 kA h L1 was applied. Although the concentrations of OH in these cases are almost insensitive to pH variations (El-Ghenymy et al., 2013), a similar pattern to imazapyr degradation is obtained as a function of pH. The mineralization results indicate that a faster reaction occurs between the intermediates and OH radicals produced on the anode surface in acid medium. Fig. 3b shows that a temperature increase from 25 to 45 °C, at pH 3.0 and current density at 50 mA cm2, accelerates the degradation process and decreases the Q values for a total degradation. Complete herbicide degradation is obtained at 45 °C, while the

599

degradation at 35 °C and 25 °C only reached 86% and 72%, respectively. These results demonstrate that rising the temperature could favor the mass transfer to the BDD anode due to increase of the specie’s coefficient diffusion. Furthermore, an increase in temperature also leads to an increase in the mediated oxidation rates. Specifically in this case, it accelerates the oxidation rate of organics with peroxodisulfate, rising the degradation rate (Panizza and Cerisola, 2008). Based on our results, the oxidation process is limited, at least partially, by the organic mass transfer to the BDD surface and mediated oxidation. Conversely, the low TOC reduction (see Fig. 2d) at all different temperatures show that the reaction products are hardly oxidized. The influence of initial imazapyr concentration on the degradation process was clarified through electrolysis at pH 3.0, 50 mA cm2 and 45 °C. Fig. 4 illustrates the relative imazapyr concentration and TOC- Q plots. Complete herbicide degradation was achieved in all assays. Assuming a pseudo-first-order reaction kinetics, the constant values were 0.004 min1 (R2 = 0.95) for 100 mg L1, 0.013 min1 (R2 = 0.90) for 14 mg L1 and 0.018 min1 (R2 = 0.89) for 6 mg L1). Fig. 4a shows a quick reduction as the initial concentration of imazapyr decreases. Consequently, the specific charge consumed to remove gradually increases, 1.9, 2.5 and 4.9 kA h L1 for 6.4, 14 and 100 mg L1 of imazapyr, respectively. Fig. 4b shows that a TOC mineralization of only 30% was achieved. This phenomenon could be explained by the fact that the solution is one hundred times more concentrated, requiring more time to be mineralized. The current efficiency calculated was 18.7%, 4.9% and 3.6% for 100, 14.0 and 6.4 mg L1 of imazapyr, respectively. This tendency may be attributed to the fact that less organic compounds are reaching the electrode through mass transfer mechanism in order to react with OH. Moreover, other factors should be considered, as the short lifetime of hydroxyl radicals

Fig. 3. Normalized (a), (b) imazapyr concentration and (c), (d) TOC as a function of the applied charge per volume unit of electrolyzed solution (Qap) for different pH (j) 3; (d) 7 and (N) 10 at 25 °C (continuous line). Temperatures: (h) 25 °C, (s) 35 °C and (4) 45 °C at pH 3 (dashed line). Conditions: 0.1 M K2SO4.

600

F.L. Souza et al. / Chemosphere 117 (2014) 596–603

Fig. 4. Normalized (a) imazapyr concentration and (b) TOC reduction as a function of the applied charge per volume unit of electrolyzed solution (Qap) for different initial concentrations: (j) 100 mg L1 (d) 14.0 mg L1 and (N) 6.4 mg L1. Conditions: 50 mA cm2; 0.1 M K2SO4; pH 3 and T 45 °C.

combined with the typical diffusion rate of chemical species limits the radical action of the electrode surface (Souza et al., 2013) and hardly oxidizable intermediates can be formed during the electrolysis as there is less imazapyr in the starting solution. Such results indicate that the imazapyr degradation at pH 3.0 may be more effective in concentrated solutions. In addition, an electrochemical degradation of 100 mg L1 imazapyr analytical standard was carried out in order to investigate the process efficiency and intermediates and by-products obtained. The previously optimized conditions were used in this assay, 50 mA cm2, pH = 3 and T = 45 °C. Fig. 5a shows a complete imazapyr degradation using about 3.8 A h L1. This reaction was faster than electrolysis with a commercial formulation. For the commercial formulation a higher charge was necessary to achieve a complete degradation. The mineralization process was lower than the decrease in the analytical standard concentration. This could demonstrate that the electrochemical degradation process was accompanied by intermediates

and/or by-products formation. This was confirmed by monitoring the main aromatics intermediates found during the process, as shown in Fig. 5b. Six intermediates were monitored using LC–UV by methods described in Section 2.3. These compounds were formed achieving maximum concentration in about 3 A h L1. Afterwards, in less than 9 h these compounds were completely removed from the reaction medium. Thus, this treatment was able to produce a severe oxidation condition. However, TOC of 40% remained in the reaction medium due to the short chain carboxylic acids formed that are not oxidized as other aromatic intermediates and imazapyr. Balance mass was carried thought analysis carboxylic acids (demonstrated below). The results show that carbon 107% remained is under form carboxylic acids. However, this value could present some statistical deviations related to each acid analyzed resulting to more 100%. The carboxylic acids formed during the electrochemical imazapyr oxidation were verified by ion-exclusion chromatography from the assays with commercial formulation and the analytical standard solutions. Fig. SM-1 in the Supplementary material indicates that formic acid is the most largely accumulated product in standard solutions follow by butyric, acetic and malic acids. The citric, formic, malic and latic acids were the most observed at the end of the commercial formulation. Different acids such as lactic, isobutyric and caproic acid were only observed at the end of commercial formulation electrolysis. This fact suggests that these by-products could not be generated from imazapyr degradation, since they had not been found in standard solution after electrolysis. Probably, these by-products are generated from excipients and other organic compounds at a higher concentration than imazapyr’s one. Such result also attests the sample complexity, due to the presence of many different compounds. Ion chromatography was used to verify charged products formed during the electrolysis, particularly, nitrate or nitrite ions obtained from the nitrogen heteroatom. Fig. SM-2 in the Supplementary material illustrates that most of the organic nitrogen is in the form of nitrate. No nitrite ions were detected in the standard solution electrolysis. Nitrate and ammonium ions were continuously formed. The initial concentration of nitrate is 2.33 mg dm3 , and attains 17.04 mg L1 at end of the electrolysis. Moreover, the ammonium concentration increases from 0.33 mg L1 to 7.19 mg L1. A similar behavior was observed in the commercial solution for the ammonium, which concentration increased from 3.68 mg L1 to 7.38 mg L1. Conversely, the nitrate concentration in the commercial solution was considerably higher, starting from 1.98 mg L1 and finishing at 50.24 mg L1. 3.3. Toxicity test results

Fig. 5. (a) Normalized (j) imazapyr concentration and (b) (d) TOC reduction as a function of the applied charge per volume unit of electrolyzed solution (Qap) for standard imazapyr solution. Conditions: 50 mA cm2; 0.1 M K2SO4; pH 3 and T 45 °C. (b) Main intermediates detected by LC–UV. Retention time: (j) 2.6 min, (d) 2.9 min, (N) 3.1 min, (.) 3.4 min, (J) 3.7 min and (I) 6.9 min.

Fig. SM-3 in the Supplementary material shows the inhibition as a function of the concentration by different dilutions with an exposure time of 30 min. This exposure time was selected since it presents less interference (see more details below). As can be seen, the inhibition values rise along with the increasing concentration of all samples. In the sample containing only the electrolyte (without imazapyr), negative inhibition values were found, resulting in an inhibition of 20%. According to Ma et al. (2013) coexisting salts can affect the toxicity tests and exhibit a very strong stimulating effect on the accurate toxicity test, mainly at lower exposure times. However, after carrying out electrolysis at 50 mA cm2, the inhibition reached about 100%. In this case, the formation of peroxosulphates from sulphates could happen (Cañizares et al., 2009), such species could likewise be responsible for increasing the medium toxicity. For the sample containing 100 mg L1 of imazapyr, an inhibition of about 80% was observed after the electrolysis. In the end of electrolysis little difference in the inhibition (which decreases about 10%) was found. EC50 values for electrolysis samples were

F.L. Souza et al. / Chemosphere 117 (2014) 596–603

601

Fig. 6. HOMO shape of the imazapyr molecule in (a) cationic (b) neutral and (c) anionic forms.

calculated as the imazapyr effective concentration that caused a decrease of 50% on the bioluminescence of V. fischeri. In order to investigate the incubation time influence on the toxicity, EC50, two different time intervals were studied, 15 and 30 min. With 100 mg L1 samples, the EC50 values were 16.9% and 21.9% using incubation times of 15 and 30 min, respectively. After the electrol-

ysis, the EC50 values were 33.4% and 24.5% using incubation of 15 and 30 min. According to Vasseur et al. (1986) cited by Boluda et al. (2002), samples with toxicity units (TU) (reaction time of 15 min) equal to or greater than 10 (EC50 < 10.0%) can already be considered toxic when V. fischeri is used (Boluda et al., 2002). Hence, these results suggest that imazapyr is a toxic compound at applied

Fig. 7. Electrooxidation imazapyr pathway proposed using diamond anode.

602

F.L. Souza et al. / Chemosphere 117 (2014) 596–603

concentrations. However, after the electrolysis, a decrease of 10% was observed in the inhibition and about 50% in the EC50 values (30 min exposure time). 3.4. Electrooxidation mechanism and Identification of intermediates In order to identify the intermediates formed during the herbicide degradation, an imazapyr standard solution of 100 mg L1 was analyzed after the electrooxidation process by LC–MS/MS. In addition, a theoretical study was performed to better elucidate the degradation pathway. 3.4.1. Theoretical study By considering that the Singly Occupied Molecular Orbital (SOMO) of the OH radical (an electrophilic species) is more likely to react with the imazapyr’s Highest Occupied Molecular Orbital (HOMO), calculations were carried out in order to provide further information about the sites of the imazapyr molecule that are more susceptible to the OH attack. All calculations were done in the GAUSSIAN 03 package (Frisch et al., 2004) Geometry optimizations were carried out by using a hybrid Density Functional Theory (DFT) method, B3LYP (Becke, 1993; Devlin, 1994), along with the 6-311++G(d,p) basis set. A visualization of molecular orbitals in the optimized structures was performed through the MOLDEN program (Noordik, 2000). Fig. 6 shows the HOMO shape of imazapyr in its cationic, neutral and anionic forms. The anionic structure was obtained by removing the proton from the carboxylic acid group, while the cationic species is the one formed by the addition of a proton to one of the nitrogen atoms of the imidazolinone ring. As can be seen, the HOMO in the cationic and neutral forms lies mainly over the imidazolinone ring, while its position is localized over the pyridine ring and the carboxylic acid group in the anionic form. These features indicate that a rupture in the imidazolinone ring is more likely to happen due to the OH attack in acid conditions than when the reaction occurs in a basic environment Further calculations (see Fig. SM-4) provide evidences that the carboxylic acid substitution in basic conditions may be followed by an OH addition in ortho and para positions with respect to the carboxylic acid group. On the other hand, the elimination of alkyl groups bound to the opened imidazolinone ring should be preferred in acid conditions. Therefore, with the LC–MS/MS analysis and the theoretical calculations, two degradation pathways are proposed for imazapyr electrochemical oxidation using diamond anodes, as shown in Fig. 7. In the first one, pathway A, the hydroxyl radical preferentially attacks the imidazolinone ring, which leads to the formation of 2-(Z)-amino [(1,2-dimethylpropyl)imino] methyl nicotinic acid, m/z = 235. Afterwards, the above molecule is oxidized, leading to the successive loss of methyl groups that allows the formation of 2-[(Z)-imino(ethylimino)methyl]nicotinic acid, m/z = 191, and 2-[amino(imino)methyl]nicotinic acid, m/z = 165, to finally yield carbon dioxide. Such pathway is sustained by the HOMO localization in the cationic forms of imazapyr that should be preponderant in acid solution. In pathway B, the attack of the hydroxyl radical occurs over the carboxylic acid where the HOMO is localized, forming the product (C12H15N3O2), m/z = 233, by decarbonylisation of the carboxyl group. The next products, m/z = 249 and m/z = 265, may be attributed to the consecutive hydroxylation of the aromatic ring. The product m/z = 239 corresponds to C11H17N3O3 that is formed by the protonic rearrangement in the imidazole ring through CO loss. Another possible attack in the methyl group can lead to the product m/z = 199. Although this pathway does not seem to be preferable in acid and neutral conditions, by what was implied in the theoretical study, some other factors must demonstrate an

important role in the description of the reaction routes. For instance, Carrier et al. (2006) studied the imazapyr degradation reaction pathway by the TiO2 photocatalysis on basis of the LC/ MS results, molecular orbital calculations of frontier electron densities and partial charges at the neutral form of the herbicide molecule. According to the authors, besides hydroxyl radicals, the h+ holes also take part in the oxidation process. These holes cause the imazapyr oxidation and lead to decarboxylation of the pyridinic cycle. This was attributed to imazapyr carboxylic group adsorption at natural pH (Carrier et al., 2006). The results obtained by Carrier et al. 2006 led to an aliphatic chain hydroxylation, followed by an imidazole ring opening that causes a rupture of the two bonding cycles. Hence, one can infer that the reaction pathways proposed in the present study are followed by the formation of carboxylic acids in a predominant mixture of butyric, malic, acetic and formic acid, which are then converted into CO2. 4. Conclusions The following conclusions are drawn from this study: (i) Electrochemical oxidation with BBD/Ti anode can be successfully used for treating aqueous wastes containing imazapyr. Parameters such as current density, initial concentration, pH and temperature affect the process efficiency. (ii) The efficiency of the process decreases with the increasing value of the current density applied and with the pH. However, it increases with the temperature. The optimum conditions for imazapyr degradation were 50 mA cm2, pH = 3.0 and 45 °C. (iii) The efficiency of the electrochemical process is strongly dependent on the imazapyr concentration. The treatment can achieve a complete degradation in the concentration range of 4.6–100.0 mg L1 of imazapyr. (iv) Theoretical studies and LC–MS/MS corroborate the existence of two possible reaction pathways for imazapyr degradation. In summary, OH species attack the imidazole ring in pathway A, while the hydroxyl radical attacks the carboxylic acid forming hydroxylated products in pathway B. Simultaneously, the reduction of oxidized nitrogen to ammonia occurs at the cathode. The final products of oxidation are formic, acetic and butyric acids. Afterwards, these acids must be converted into carbon dioxide when higher charge is applied.

Acknowledgements The authors are grateful to Brazilian research funding agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de nível Superior (CAPES) and Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP) for the financial support and scholarships. The authors wish to thank CiTecBio/IQSC/USP. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2014.09.051. References Ager, J.W., 1995. Fano interference of the Raman phonon in heavily boron doped diamond films grown by chemical vapor deposition. Appl. Phys. Lett., 616–618.

F.L. Souza et al. / Chemosphere 117 (2014) 596–603 Ahmed, S., Rasul, M.G., Brown, R., Hashib, M.A., 2011. Influence of parameters on the heterogeneous photocatalytic degradation of pesticides and phenolic contaminants in wastewater: a short review. J. Environ. Manage. 92, 311–330. Alister, C., Kogan, M., 2005. Efficacy of imidazolinone herbicides applied to imidazolinone-resistant maize and their carryover effect on rotational crops. Crop Prot. 24, 375–379. Alves, S.A., Ferreira, T.C., Sabatini, N.S., Trientini, A.C., Migliorini, F.L., Baldan, M.R., Ferreira, N.G., Lanza, M.R., 2012. A comparative study of the electrochemical oxidation of the herbicide tebuthiuron using boron-doped diamond electrodes. Chemosphere 88, 155–160. Becke, A.D., 1993. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648. Boluda, R., Quintanilla, J.F., Bonilla, J.A., Saez, E., Gamón, M., 2002. Application of the Microtox test and pollution indices to the study of water toxicity in the Albufera Natura Park (Valencia, Spain). Chemosphere 46, 355–369. Brillas, E., Garcia-Segura, S., Skoumal, M., Arias, C., 2010. Electrochemical incineration of diclofenac in neutral aqueous medium by anodic oxidation using Pt and boron-doped diamond anodes. Chemosphere 79, 605–612. Caldas, E.D., 2011. Pesticide poisoning in Brazil reference module in earth systems and environmental sciences. Enc. Environ. Health, 419–427. Cañizares, P., Sáez, C., Sánchez-Carretero, A., Rodrigo, M.A., 2009. Synthesis of novel oxidants by electrochemical technology. J. Appl. Electrochem. 39, 2143–2149. Carrier, M., Perol, N., Herrmann, J.-M., Bordes, C., Horikoshi, S., Paisse, J.O., Baudot, R., Guillard, C., 2006. Kinetics and reactional pathway of Imazapyr photocatalytic degradation Influence of pH and metallic ions. Appl. Catal. B: Environ. 65, 11–20. Cavalcanti, E.B., Garcia-Segura, S., Centellas, F., Brillas, E., 2013. Electrochemical incineration of omeprazole in neutral aqueous medium using a platinum or boron-doped diamond anode: degradation kinetics and oxidation products. Water Res. 47, 1803–1815. Cohen, Z.Z., Eiden, C., Lober, M.N., 1986. Evolution of Pesticide in Ground Water. A.S.S. American Chemical Society, Washington, DC. Comninellis, C., Pulgarin, J., 1991. Anodic oxidation of phenol for wastewater treatment. J. Appl. Electrochem. 21, 703–708. Cong, Y., Wu, Z., 2007. Electrocatalytic generation of radical intermediates over lead dioxide electrode doped with fluoride. J. Phys. Chem. C 111, 3442–3446. Devlin, P.J.S.F.J., 1994. Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields. J. Phys. Chem. 98, 11623–11627. Elezovic, I., Datta, A., Vrbnicanin, S., Glamoclija, D., Simic, M., Malidza, G., Knezevic, S.Z., 2012. Yield and yield components of imidazolinone-resistant sunflower (Helianthus annuus L.) are influenced by pre-emergence herbicide and time of post-emergence weed removal. Field Crops Res. 128, 137–146. El-Ghenymy, A., Garrido, J.A., Rodríguez, R.M., Cabot, P.L., Centellas, F., Arias, C., Brillas, E., 2013. Degradation of sulfanilamide in acidic medium by anodic oxidation with a boron-doped diamond anode. J. Electroanal. Chem. 689, 149– 157. Emmanuel, E., Keck, G., Blanchard, J.M., Vermande, P., Perrodin, Y., 2004. Toxicological effects of disinfections using sodium hypochlorite on aquatic organisms and its contribution to AOX formation in hospital wastewater. Environ. Int. 30, 891–900. Frisch, M.J., Trucks, G.W., Schlegel, H.B.; Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Montgomery, J.A., Vreven, T., Kudin, K.N., Burant J.C., Millam, J.M., Iyengar, S.S., Tomasi, J., Barone, V. Mennucci, B., Cossi, M., Scalmani, G., Rega, N., Petersson, G.A., Nakatsuji, H., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Klene, M., Li, X., Knox, J.E., Hratchian, H.P., Cross, J.B., Bakken, V., Adamo, C., Jaramillo, J. R., Gomperts, R., Stratmann, R.E., Yazyey, O., Austin, A.J., Cammi, R., Pomelli, C., Ochterski, J.W., Ayala, P.Y., Morokuma, K., Voth, G.A., Salvador, P., Dannenberg, J.J., Zakrzewski, V.G., Dapprich, S., Daniels, A.D., Strain, M.C., Farkas, O., Malick, D.K., Rabuck, A.D., Raghavachari, K., Foresman, J.B., Ortiz, J.V., Cui, Q., Baboul, A.G., Clifford, S., Cioslowski, J., Stefanov, B.B., Liu, G., Liashenko, A., Piskorz, P., Komaromi, I., Martin, R.L., Fox, D.J., Keith, T., Laham, Al-M.A., Peng, C.Y., Nanayakkara, A., Challacombe, M., Gill, P.M.W., Johnson, B., Chen, W., Wong, M.W., Gonzalez, C., Pople, J.A., 2004. GAUSSIAN 03, Revision D.02, Gaussian Inc., Wallingford, CT, 2004.

603

Harir, M., Gaspar, A., Kanawati, B., Fekete, A., Frommberger, M., Martens, D., Kettrup, A., El Azzouzi, M., Schmitt-Kopplin, P., 2008. Photocatalytic reactions of imazamox at TiO2, H2O2 and TiO2/H2O2 in water interfaces: kinetic and photoproducts study. Appl. Catal. B: Environ. 84, 524–532. Kaloumenos, N.S., Capote, N., Aguado, A., Eleftherohorinos, I.G., 2013. Red rice (Oryza sativa) cross-resistance to imidazolinone herbicides used in resistant rice cultivars grown in northern Greece. Pest. Biochem. Phys. 105, 177–183. Ma, X.Y., Wang, X.C., Ngo, H.H., Guo, H.H., Wu, M.N., Wang, N., 2013. Reverse osmosis pretreatment method for toxicity assessmentof domestic wastewater using Vibrio qinghaiensis sp.-Q67. Ecotox. Environ. Safe. 97, 248–254. Marselli, B., Garcia-Gomez, J., Michaud, P.A., Rodrigo, M.A., Comninellis, C., 2003. Electrogeneration of hydroxyl radicals on boron-doped diamond electrodes. J. Electrochem. Soc. 150, D79. May, P.W., Ludlow, W.J., Hannaway, M., Heard, P.J., Smith, J.A., Rosser, K.N., 2008. Raman and conductivity studies of boron-doped microcrystalline diamond, facetted nanocrystalline diamond and cauliflower diamond films. Diam. Rel. Mater. 17, 105–117. Migliorini, F.L., Braga, N.A., Alves, S.A., Lanza, M.R., Baldan, M.R., Ferreira, N.G., 2011. Anodic oxidation of wastewater containing the Reactive Orange 16 Dye using heavily boron-doped diamond electrodes. J. Hazard. Mater. 192, 1683–1689. Noordik, G.S.a.J.H., 2000. Molden: a pre- and post-processing program for molecular and electronic structures. J. Comp. Aided Mol. Des. 14, 123–134. Panizza, M., Cerisola, G., 2005. Application of diamond electrodes to electrochemical processes. Electrochim. Acta 51, 191–199. Panizza, M., Cerisola, G., 2008. Removal of colour and COD from wastewater containing acid blue 22 by electrochemical oxidation. J. Hazard. Mater. 153, 83– 88. Panizza, M., Cerisola, G., 2009. Direct and mediated anodic. Chem. Rev. 109, 6541– 6569. Pedlowski, M.A., Canela, M.C., da Costa Terra, M.A., Ramos de Faria, R.M., 2012. Modes of pesticides utilization by Brazilian smallholders and their implications for human health and the environment. Crop Prot. 31, 113–118. Pizarro, P., Guillard, C., Perol, N., Herrmann, J.M., 2005. Photocatalytic degradation of imazapyr in water: Comparison of activities of different supported and unsupported TiO2-based catalysts. Catal. Today 101, 211–218. Quivet, E., Faure, R., Georges, J., Paisse, J.O., Lanteri, P., 2006. Influence of metal salts on the photodegradation of imazapyr, an imidazolinone pesticide. Pest Manage. Sci. 62, 407–413. Rahman, S., 2013. Pesticide consumption and productivity and the potential of IPM in Bangladesh. Sci. Total Environ. 445–446, 48–56. Ramezani, M.K., Oliver, D.P., Kookana, R.S., Lao, W., Gill, G., Preston, C., 2010. Faster degradation of herbicidally-active enantiomer of imidazolinones in soils. Chemosphere 79, 1040–1045. Rodrigo, M.A., Cañizares, P., Sánchez-Carretero, A., Sáez, C., 2010. Use of conductivediamond electrochemical oxidation for wastewater treatment. Catal. Today 151, 173–177. Sales Solano, A.M., Costa de Araújo, C.K., Vieira de Melo, J., Peralta-Hernandez, J.M., Ribeiro da Silva, D., Martínez-Huitle, C.A., 2013. Decontamination of real textile industrial effluent by strong oxidant species electrogenerated on diamond electrode: Viability and disadvantages of this electrochemical technology. Appl. Catal. B: Environ. 130–131, 112–120. 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. Souza, F.L., Sáez, C., Cañizares, P., Motheo, A.J., Rodrigo, M.A., 2013. Sonoelectrolysis of wastewaters polluted with dimethyl phthalate. Ind. Eng. Chem. Res. 52, 9674–9682. Souza, F.L., Sáez, C., Cañizares, P., de Motheo, A., Rodrigo, M., 2014. Electrochemical removal of dimethyl phthalate with diamond anodes. J. Chem. Technol. Biotechnol. 89, 282–289. Vasseur, P., Ferard, J.F., Rast, C., Weingertner, P., 1986. Test Mictrotox et le controle de la qualité des eaux. J. Fr. dHidrologie. 17, 153–162. Vidales, M.J.M., Sáez, C., Cañizares, P., Rodrigo, M.A., 2012. Metoprolol abatement from wastewaters by electrochemical oxidation with boron doped diamond anodes. J. Chem. Technol. Biotechnol. 87, 225–231. Zhang, R.J., 1996. Characterization of heavily boron-doped diamond films. Diam. Relat. Mat. 5, 1288–1294.