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Published in J. Environ. Qual. 33:605-611 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORT

Organic Compounds in the Environment

Surface Retention and Photochemical Reactivity of the Diphenylether Herbicide Oxyfluorfen

Laura Scranoa, Sabino A. Bufo*,a, Tommaso R. I. Cataldia and Triantafyllos A. Albanisb

a Dipartimento di Chimica, Università della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy
b Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece

* Corresponding author (bufo{at}unibas.it).

Received for publication January 24, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The photochemical behavior of oxyfluorfen [2-chloro-1-(3-etoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene] on two Greek soils was investigated. Soils were sampled from Nea Malgara and Preveza regions, characterized by a different organic matter content. Soils were spiked with the diphenyl-ether herbicide and irradiation experiments were performed either in the laboratory with a solar simulator (xenon lamp) or outside, under natural sunlight irradiation; other soil samples were kept in the dark to control the retention reaction. Kinetic parameters of both retention and photochemical reactions were calculated using zero-, first- and second- (Langmuir–Hinshelwood) order equations, and best fit was checked through statistical analysis. The soil behaviors were qualitatively similar but quantitatively different, with the soil sampled from the Nea Malgara region much more sorbent as compared with Preveza soil. All studied reactions followed second-order kinetics and photochemical reactions were influenced by retaining capability of the soils. The contributions of the photochemical processes to the global dissipation rates were also calculated. Two main metabolites were identified as 2-chloro-1-(3-ethoxy-4-hydroxyphenoxy)-4-(trifluoromethyl)benzene and 2-chloro-1-(3-hydroxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
OXYFLUORFEN 1, as illustrated in Fig. 1 , is a selective pre- and postemergence diphenyl-ether herbicide used to control certain annual broadleaf and grassy weeds in vegetables, fruit, cotton, ornamentals, and on noncropped areas such as rail and highway right-of-ways (Kidd and James, 1991). It is a contact herbicide and light is required for it to affect target plants (EXTOXNET, 2002). Oxyfluorfen is moderately persistent in most soil environments, with a representative field half-life of about 30 to 40 d (USEPA, 1992; Weed Science Society of America, 1994). Such a herbicide is lightly subject to microbial degradation or hydrolysis (Adityachaudhury et al., 1994). In laboratory studies, Wauchope et al. (1992) found that soil half-life of oxyfluorfen was six months, indicating very low rates of microbial degradation.



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  		Fig. 1. Illustration of scheme showing the structure of oxyfluorfen and photo-derivative products identified in liquid phase. From Scrano et al. (1999a).

 
Oxyfluorfen is very well-sorbed to most soils, and binding is highest in soils with high organic matter and clay content. Once oxyfluorfen is adsorbed onto soil particles, it is not readily removed (Weed Science Society of America, 1994; Wauchope et al., 1992). Rapidly decomposed by light (Edwards et al., 1991; Scrano et al., 1999a) oxyfluorfen is nearly insoluble in water and exhibits a certain tendency to adsorb to soil, being also sorbed to suspended particles and sediments (USEPA, 1984).

The photochemical behavior of oxyfluorfen in liquid phase has been investigated by Scrano et al. (1999a) using a high pressure mercury arc and a solar simulator. Kinetic parameters and quantum yields were determined. Identification of the photoproducts was performed, suggesting that photolytic degradation occurs through the first excited singlet state of the molecule, which may undergo both to homolytic and heterolytic cleavage of the ethyl-oxygen bond in the side chain of oxyfluorfen (Fig. 1). Previously, Brodsky et al. (1992) identified a number of photoproducts following degradation of oxyfluorfen under lamp irradiation in a methanolic solution, and suggested that the degradation pathway was mainly due to ether bond cleavage, dechlorination, and photocyclization. Ying and Williams (1999) also studied the photodegradation of oxyfluorfen on soil surface and in Milli-Q water under sunlight. They limited to water the determination of the herbicide photoproducts, suggesting that loss of the nitro group, dechlorination, and cyclization were the predominant processes in the aqueous environment.

Data available on the fate of oxyfluorfen in the environment do not include the determination of photoproducts on sorbed phase (soil), and photodegradation in soil is only given as a disappearance possibility as well as evaporation and formation of bound residues (USEPA, 1992; Weed Science Society of America, 1994). To fill this lack of information on an important degradation pathway in the environment, we report here the photochemical degradation of oxyfluorfen on two Greek soils.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Chemicals
All solvents (pesticide-grade), reagents (analytical-grade), and filters (disposable sterilized packet) were purchased from Fluka (Buchs, Switzerland) and Sigma-Aldrich (St. Louis, MO). Ultrapure water was obtained with a Millipore (Billerica, MA) Milli-Q system. Oxyfluorfen (CAS RN 42874-03-3; purity = 98%, molecular weight = 361.7, vapor pressure = 0.026 mPa at 25°C, water solubility 0.1 mg L–1, log Kow = 4.47) was purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany).

Preparation of Sorbed Phase
Two soils from Greece were sampled: a sandy clay loam soil from Preveza region, in Southern Epirus, and a silt loam soil from Nea Malgara region. Physical and chemical properties of selected soils are shown in Table 1. Soils were sieved (2 mm) and sterilized before their use to avoid microbiological degradation. According to Cambon et al. (1998) soils with the addition of sodium azide (0.05%) were incubated for 24 h (FTC 90E refrigerated incubator; Velp Scientifica, Milan, Italy).


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  		Table 1. Physicochemical properties of selected soils.

 
Stock solution of the herbicide was prepared by dissolving 7.2 mg of oxyfluorfen in 98% methanol (50 mL). To reach complete herbicide dissolution, the solution was sonicated at 40°C for five minutes, and then was stored in the dark at 4°C. Aqueous solution (160 mL), previously spiked with 5 mL of the methanolic solution, was added drop by drop to 240 g of soil sample with gentle stirring. The soil paste thus obtained was spread on three glass TLC plates (20 x 20 cm) to obtain a 1-mm-thick soil layer. The plates were air-dried and equilibrated in the darkness at room temperature for one day. The first and second plates were used for irradiation experiments, the third one as control in dark conditions. All samples were protected with quartz cover plates to avoid herbicide volatilization. Each experiment was performed in triplicate. Other plates were prepared and kept in the dark to perform recovery tests.

Irradiation Experiments
The UV spectra of oxyfluorfen in methanol were recorded on a Cary 2300 spectrophotometer (Varian, Palo Alto, CA). Photochemical reactions were performed by using a solar simulator (Suntest CPS+; Heraeus Industrietechnik GmbH, Hanau, Germany), equipped with a xenon arc lamp (1.1 kW) that was protected with a quartz plate (total passing wavelength: 300 nm < {lambda} < 800 nm). During xenon irradiation samples were maintained at 20°C, using a conditioned airflow. Other experiments were performed under natural solar light irradiation, on both Preveza and Malgara soils, protecting TLC plates with a quartz chamber. Incident solar radiation was detected with a radiometer (Eppley Lab., Newport, RI). In the experiment period (October 2000), the average total daily radiations, measured in the wavelength range of 285 to 800 nm, was 438 W m–2, with mean sunshine of 8 h from sunrise to sunset and mean temperature of 18.4°C.

Analytical Procedure
The reaction kinetics were followed by scraping off soil strips of 1 cm from the TLC plates, as described in Konstantinou et al. (2000). The collected material (approximately 2 g of soil) was put into a test-tube and weighed, and methanol (10 mL) was added. The mixture was stirred (vortex) for 1 min and sonicated for 10 min. The solution was separated and the procedure was repeated two times by using 5 mL of methanol, respectively. The resulting methanol solution (20 mL) was centrifuged (10 min, 3000 x g) and concentrated by fluxing nitrogen; the final volume of the solution was adjusted to 5 mL. The samples were filtered and kept in the darkness at 4°C.

Analyses were performed on a Hewlett-Packard (Palo Alto, CA) 1090 liquid chromatograph equipped with a diode array detector (fixed at 225 nm), and a C18 5-µm packed column (25-cm-long, 3.2-mm-i.d.; Dionex [Sunnyvale, CA] Omnipac) plus guard column. The mobile phase used for all experiences was 70% acetonitrile and 30% water (H3PO4, pH 3) with a flow rate 0.6 mL min–1. Retention time of oxyfluorfen was 6.3 min (k' = 2.6). The calibration plot was performed in the concentration range 0.015 to 30 mg L–1 giving a linear correlation coefficient (r) of >0.99975. The detection limit of the analytical method for the determination of oxyfluorfen was 0.011 mg L–1.

The extracts were also analyzed for the presence of metabolites arisen from the photodegradation of oxyfluorfen as identified previously in photodegradation experiments in liquid phase by Scrano et al. (1999a). Methanol extracts were concentrated under a mild vacuum in a rotary evaporator, nitrogen-fluxed to dryness, and redissolved in 5 mL of chloroform. Analyses were performed using an HP 5971 mass selective detector on an HP 5890 gas chromatograph (OV-1 capillary column between 70 and 250°C, 12°C min–1) as described in Scrano et al. (1999a). Metabolites were identified by comparing recorded mass spectrometry spectra with those collected in our spectral data bank.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Photochemical Properties
The UV spectrum of oxyfluorfen methanolic solution exhibited absorption maxima at 271 nm ({epsilon} = 6.4 x 103 mol–1 L cm–1) and 325 ({epsilon} = 5.4 x 103 mol–1 L cm–1); its absorption spectrum is partially overlapped to the filtered emissions of the xenon-Suntest irradiation system (total passing wavelength: 300 nm < {lambda} < 800 nm) and the UV region 280 to 400 nm of sunlight irradiation, as filtered by the terrestrial atmosphere.

Herbicide Retention
The soil samples were spiked with 3.0 mg oxyfluorfen per kg of soil and retention capability was evaluated by performing a series of herbicide extractions. We found that the concentrations of oxyfluorfen extracted using methanol at t = 0 (C0 values in Table 2) were comparatively lower than the quantity of herbicide added to soils. Specifically, while the amount of oxyfluorfen extracted from Preveza soil was equal to 95.2% of that one added, the soil of Nea Malgara region exhibited a higher retention capability as the extracted amount was only 19.8%.


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  		Table 2. Kinetic parameters (second-order reaction) of oxyfluorfen.{dagger}

 
To improve the extraction efficiency, two different solvents were furthermore applied: ethyl acetate and acetone. But no significant augments of the herbicide recoveries were obtained. Oxyfluorfen recoveries from Preveza soil increased to 95.9 and 97.1% using ethyl acetate and acetone, respectively, and rose up to 20.8 and 21.6% in the case of Nea Malgara soil. According to Albanis et al. (2002) and Konstantinou et al. (2000), we used methanol because this solvent gave the cleanest chromatograms with respect to other extracting solvents. In fact, many chromatographic peaks, probably due to naturally occurring organic compounds in the soil, were found in the other tested solvents, especially from Nea Malgara soil.

Owing to the low value of the vapor pressure (26 x 10–6 Pa at 25°C), as compared with most pesticides, volatilization phenomena may explain the disappearance of a very limited amount of the herbicide during the preparation of treated soil samples, taking also into account the drying time of soil layers on glass plates. In our opinion, retention on sorbed phases has to be considered as the major factor in the dark of "non-extractability" of oxyfluorfen from soil (Scrano et al., 1996, 1999c). Indeed, it should be mentioned that the retention of oxyfluorfen on soil, with formation of "bound residues," has been previously well ascertained (USEPA, 1992; Wauchope et al., 1992). Organic matter has been generally suggested as the major factor of herbicide retention together with mineral colloids in soil (Schafmaier et al., 1997; Scrano et al., 1996, 1997). Hence, it is not unexpected that the soil from Nea Malgara, which shows a higher content of organic matter and a lower percentage of sandy particles, retained a great amount of oxyfluorfen. Moreover, observing the soil behavior of samples kept in the dark (Fig. 2) , we note that they are characterized by a continuous reactivity during the experimental time, from the first extraction (t = 0) up to 120 h, giving depletion curves that can be described by second-order kinetics (Tables 2 and 3). Therefore, the retention of oxyfluorfen has been reinforcing in time as the herbicide molecules were trying for the best arrangement into the sorption sites of soils (Mingelgrin and Prost, 1989; Ramamurthy, 1991). Indeed, we found that the extracted amounts of oxyfluorfen from Preveza and Nea Malgara soils after 120 h were lowered by approximately 54 and 11%, respectively.



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  		Fig. 2. Reduction of oxyfluorfen extractable in methanol from treated soils kept in the dark, as percent of added herbicide (3 mg kg–1). Error bars represent the standard deviations of three replicate samples.

 

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  		Table 3. Sum of least squares ({Sigma}LSq) of zero-, first-, and second-order (Langmuir–Hinshelwood) equations.{dagger}

 
Photodegradation versus Retention
Owing to the coexistence of retention and photochemical degradation during the irradiation experiments, the contribution of photolysis to the disappearance of oxyfluorfen cannot be directly quantified. Curves representing the disappearance of oxyfluorfen in the dark and under solar or xenon-Suntest irradiation on Preveza and Nea Malgara soils are shown in Fig. 3 and 4 , respectively.



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  		Fig. 3. Reaction rate of oxyfluorfen retained on Preveza soil as percent of disappeared herbicide with respect to the added quantity (3 mg kg–1).

 


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  		Fig. 4. Reaction rate of oxyfluorfen retained on Nea Malgara soil as percent of disappeared herbicide with respect to the added quantity (3 mg kg–1).

 
Following 120 h of herbicide–soil contact, the quantity of disappeared oxyfluorfen from Preveza soils was increased from 4.8 (t = 0) to 45.8% in darkness conditions, and up to 51.6 and 63.1% under solar and xenon irradiation, respectively. In the case of soils sampled from Nea Malgara region the augments were from 80.2 to 88.9% in the dark, and up to 95.8 and 98.8% under outdoor and Suntest illumination, respectively. In previous works (Albanis et al., 2002; Scrano et al., 1999a, 1999b, 2002) the contribution of photochemical degradation Dt(photo) was evaluated at each experimental time as the difference between Dt(light), the quantity of herbicide disappeared at time t under each irradiation experiment (total reaction, i.e., photolysis + retention), and Dt(dark), the amount retained in the dark at the same experimental time. Although such an approach is not fully rigorous since it does not take into account the synergistic effects of photolysis and retention (e.g., concentration depletion during reactions), it may provide a good approximation to the target generally set for this kind of research work. Adopting this method of evaluation we found that the maximum contribution of sunlight disappearance from Preveza soil (5.8%) was not significantly different compared with Nea Malgara soil (6.9%). Under xenon irradiation this difference appeared augmented, as the contribution of light was 17.3 and 9.8% for the disappearance of oxyfluorfen from Preveza and Nea Malgara samples, respectively. It is speculated that the higher contribution found in the xenon experiment may be related to the power of the Suntest equipment (approximately 1000 W m–2), which is twofold higher compared with solar measured irradiation (438 W m–2). The protective effect of retention, mainly exerted by organic and mineral colloids of soil, can be better evidenced when the higher energized irradiation from the xenon lamp is used rather than the natural light.

Kinetics
The knowledge of reaction order is essential for finding the correct integrated rate equation. By trying to fit data of various integrated rate equations it is possible to verify the reaction order. Kinetic parameters were calculated using integrated equations describing zero-, first-, and second-order reactions. According to Snedecor and Cochran (1989), the best fit was checked using the least square method of estimation (Table 3). Apparently, all measured reaction rates of the retained herbicide, either in the darkness or under irradiation conditions, were best fitted by a Langmuir–Hinshelwood type equation (Table 3), which describes a second-order reaction as:

[1]
where Ct is the amount (mg) of herbicide extracted at time t per kilogram of soil, and k is the rate (or kinetic) constant. The rationale behind such a finding may be found in the chemical and photochemical reaction rates in soil, which are both affected by sorption state. Interestingly, the amount of reactant disappeared at each time t is affected by its concentration in soil and also by the number of molecules that have reached the most effective steric arrangement on retention sites. In turn, this number depends again by the reactant concentration in soil (Mingelgrin and Prost, 1989; Ramamurthy, 1991). According to Scrano et al. (1994), Eq. [1] can also be written as:

[2]
where Dt is the quantity (mg) of disappeared (retained and/or degraded) herbicide per kilogram of soil, and Dmax is the maximum amount of herbicide that could disappear at the end of the process (i.e., if the reaction would be carried to completion). Integrating Eq. [2] and solving for Dt yields:

[3]
where {tau} = half-life = 1/(Dmaxk).

The terms Dmax and {tau} can be calculated developing the function obtained through the linearization of Eq. [3] as:

[4]

Parameters of the second-order kinetics are summarized in Table 2. Note that in the Eq. [3] Dt is equal to zero when t = 0. For this reason values of Dmax, as reported in Table 2, do not include the amount of oxyfluorfen disappeared at t = 0 (values of D0 referred in the same table). Meanwhile, reaction curves of Fig. 3 and 4 have been plotted beginning from D0 values on the y axis, and adding these to Dt values.

In the second-order reactions, the kinetic constants (and half-life times) depend on the reactant concentration, so they are not directly comparable. Yet, it is possible to evaluate the contribution of photolysis to the global reaction from the values of Dmax in Table 2 and integrated areas subtended to the reaction curves of Fig. 3 and 4, calculated in the interval 0 to 120 h from the Eq. [5]:

[5]

Owing to the higher value of the initial retention D0, the reactions on Nea Malgara soil (residue retention in the dark and retention + photodegradation under irradiation conditions) appear to involve a minor quantity of herbicide with respect to Preveza soil during the overall 120 h (see Dmax values in Table 2). On the other hand, under solar and xenon irradiations the increments of integrated areas with respect to the retention reaction (in the dark) were more evident in the case of Nea Malgara soil (92 and 136%, respectively) than Preveza soil (13 and 44%). For the same reason given above, kinetics under xenon-Suntest lamp were always more effective than solar irradiation. According to Da Silva et al. (2001) the contributions of the photochemical processes to the global dissipation rate were also assessed by comparing the initial degradation rates calculated for each set of conditions. The first derivative of Eq. [3] allows the calculation of dissipation rate, as:

[6]

Setting t = 0 in the Eq. [6], it is possible to obtain the initial dissipation rate:

[7]

Values of V0 are shown in Table 2. In agreement with the method of integrated areas shown above, the contributions of the photochemical processes to the global dissipation rates were higher in the case of Nea Malgara soil (64 and 78%, under solar and xenon irradiations, respectively) than Preveza soil (7 and 42%). It is noticeable that photochemical degradation is relatively well acting in the Nea Malgara organic soil. We speculate that organic matter might retain the herbicide mostly by a mechanism of solubilization and repartition rather than adsorption, which is a retention mechanism typically ascribed to mineral colloids; after that it becomes a sort of reservoir that can supply herbicide photolysis with retained molecules.

Though a direct comparison of kinetics of different order is not correct, it can be stressed that Ying and Williams (1999) asserted that the reaction of oxyfluorfen under sunlight follows first-order kinetics, and found a higher half-life value (5.19 d) when the herbicide was spiked on a sandy soil and a value of 5 h, closer to our findings, in aqueous solution. Apparently, the methanol extraction performed by these authors was not affected by the presence of bound residues. They postulated the reduced photolysis of the herbicide in soil, which is due to light attenuation exerted by organic and inorganic fractions of soil, and trapping phenomena of the chemical in the interior of soil particles. Using first-order kinetics, half-lives ranging from 24 to 28 h were calculated by Scrano et al. (1999a) in different organic solvents under xenon-lamp irradiation. They speculated the photolysis reaction arises with a double step pathway: the first step, faster, takes place during the first hour of oxyfluorfen irradiation; the second, slower, shows a longer duration (up to 50–60 h). The two steps were characterized by the formation of a different set of by-products.

Degradation Products
The presence of metabolites was ascertained during the first four hours of irradiation and, after that time, their gas chromatography–mass spectrometry (GC–MS) signals were not detected (S/N < 3). Two by-products were identified as the 2-chloro-1-(3-ethoxy-4-hydroxyphenoxy)-4-(trifluoromethyl) benzene (Compound 2) and the 2-chloro-1-(3-hydroxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene (Compound 4), and their structures are shown in Fig. 1. These substances, together with Compound 3 [2-chloro-1-(3-ethoxy-4-aminophenoxy)-4-(trifluoromethyl)benzene] in Fig. 1, were previously found as main photoproducts yielding under 1 h of irradiation in acetonitrile (Scrano et al., 1999a). In this solvent Metabolite 3 disappeared at longer photoreaction times and other by-products were detected. Unfortunately, the quantification of these metabolites was not attempted, as pure standards were not available. In Table 4 we report the relative peak area of the corresponding by-product formed at various irradiation times, expressed as percent of oxyfluorfen area. These metabolites were only detected in irradiated samples, and no other derivative substance was found in the sample kept in the dark. Figures 5 and 6 illustrate the evolution of Metabolites 2 and 4 during the first four hours; Compound 4 arising first from the photochemical reaction of oxyfluorfen. As expected, xenon-Suntest irradiation was also more effective than sunlight with respect to metabolites yielding. In fact, relative intensities of by-products in the same soil were higher under xenon irradiation than solar. At the same time, Nea Malgara soil exerted a more intense retention effect on Compounds 2 and 4 as well as oxyfluorfen. On the contrary, these metabolites were more easily extractable from Preveza soil.


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  		Table 4. Relative gas chromatography–mass spectrometry (GC–MS) peak area of each by-product, expressed as percent of oxyfluorfen area found in the same soil sample at each time observed.

 


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  		Fig. 5. Evolution of Metabolites 2 and 4 extracted from Preveza soil. Data have been reported as percent of their gas chromatography–mass spectrometry (GC–MS) peak area, calculated with respect to oxyfluorfen area in the same soil sample scraped from the TLC plate.

 


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  		Fig. 6. Evolution of Metabolites 2 and 4 extracted from Nea Malgara soil. Data have been reported as percent of their gas chromatography–mass spectrometry (GC–MS) peak area, calculated with respect to oxyfluorfen area in the same soil sample scraped from the TLC plate.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In the present study it has been demonstrated that soil type significantly affects the behavior of oxyfluorfen. The higher retention capability of Nea Malgara soil is to great extent attributable to the higher content of organic matter with respect to Preveza soil. All measured reaction rates (in darkness and under irradiation) of the retained herbicide can better described by a second-order equation, and half-life times are higher for the organic soil sampled from Nea Malgara region. The two soils, although different, apparently show a similar net loss of herbicide due to sunlight exposure. However, the contribution of the photochemical processes to the global dissipation rate is higher in the case of the organic soil than the sandy soil. The photodegradation of oxyfluorfen gives rise to two main oxidized metabolites.


   ACKNOWLEDGMENTS
 
This work was supported by the Italian Department of Research and Education (MUIR).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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This Issue in Journal of Environmental Quality

JEQ 2004 33: 413-418. [Full Text]  




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