Environ. Sci. Technol. 1995, 29, 431-438
Degradation of Organophosphoms Pesticides and Their Transformation Products in Estuarine Waters S I L V I A LACORTE,' SYLVAIN B . L A R T I G E S , * PHILIPPE G A R R I G U E S , * A N D DAMIA BARCELO*#+ Department of Environmental Chemistry, CID-CSIC, cuordi Girona, 18-26 08034 Barcelona, Spain, a n d URA 348 CNRS, University of Bordeaux I, 33405 Talence, France
The degradation of 10 organophosphorus pesticides in natural estuarine waters was studied. Estuarine water samples were spiked with organophosphorus pesticides at 50 pg/L level and were placed into 2-L Pyrex flasks being exposed outdoor to ambient sunlight and temperature. A sample of 10-75 mL of water was collected every week for analysis during a period of 5-6 weeks from January to March. The analytical determinations were performed by solidphase extraction (SPE) with c 1 8 Empore disks followed by GC-NPD and GC-MS with El and by on-line SPE using PLRP-s exchangeable cartridges (Prospekt) followed by LC-DAD and LC-thermospray MS in PI mode. Five organophosphorus pesticides were stable for less than 1 week (disulfoton, fenamiphos, fenthion, malathion, and temephos), others had a half-life of ca. 1 week (chlorpyrifos-methyl, methidathion, and diazinon), and the rest showed a half-life of ca. 10 days (isofenphos and pyridafenthion). The half-life of three pesticide transformation products: disulfoton sulfoxide, disulfoton sulfone, and fenthion sulfoxide varied from 7 to 12 days.
Introduction Organophosphorus pesticides are widely used as insecticides for different types of cultivation, e.g., rice (1-31,and for elimination of crustaceans (4) , fruit flies (3, and mosquitoes (6, 7). Residue levels of organophosphorus pesticides in environmental matrices have been reported. Among them are fenitrothion (1-4, pyridafenthion (31, malathion (3, fenthion ( 3 ,temephos (s), and chlorpyrifos (8). The stability of organophosphorus pesticides in water is a matter of controversy, with different opinions from the National Pesticide Survey (NPS) and the Commission of the European Communities (CEC). In this respect, it is worthwhileto mention that some pesticides that have been withdrawn from the NPS list because they suffer 100%loss when stored at 4 "C during 14 days in biologically inhibited well water (9) are, however, included in the CEC 76/464/ EEC council directive list of pesticides to be monitored in the aquatic environment (10) as, for example, parathion, azinphos-methyl, fenitrothion, demeton, fenthion, and malathion. The NPS has also observed 100%loss of many other organophosphorus pesticides stored in the same conditions as indicated above, among them diazinon, disulfoton (and its TPs sulfone and sulfoxide), phosmet, and others. Such a disagreement between NPS and the CEC requires the need for investigating to what extent organophosphorus pesticides are stable under natural environmental conditions. So far, several works concerning the persistence of organophosphorus pesticides in the aquatic environment have been carried out. Most of these works were carried out "in vitro" usually using sterile water (biologically inhibited) placed in the dark and at concentration levels higher than environmental ones (e.g.,0.1-1 mg/L). In this respect, hydrolysis due to pHvariation of 6.1-7.4 enhanced the degradation of malathion, chorpyrifos-methyl, chlorpyrifos-ethyl, and parathion (11). Under these conditions, half-lives of 11 and 13 days have been found for malathion and chlorpyrifos-methyl, respectively. Other studies have shown that hydrolysis was the main degradation process for chlorpyrifos-ethyl in estuarine waters kept in flasks in the dark and in sediment/water slurry systems (12) with half-lives varying from 14 to 24 days (13). Neely and Blau (14) predicted by computer simulation that after 25 days, hydrolysis, metabolism, soil, plants, and water accounted for 76%, 11.4%,11%,0.5%,and 0.8% of the total chlorpyrifos applied, respectively. Other pesticides such as fenthion degraded faster in river water (15) than in the presence of sediment (difference in half-life was from 7 to 23 days), biodegradation being a relevant process (12). However, the results obtained under laboratory conditions and under the different restrictions mentioned above (sterile water, dark, 4 "C, higher mg/L spiking level) may not be extrapolated into the field. This leads to the unavoidable requirement to carry out degradation studies of pesticides using real environmental waters, climate conditions, and levels usually detected in the environment * To whom correspondence should be addressed. + CID-CSIC.
* University of Bordeaux. 0013-936)(/95/0929-0431$09.00/0 Q 1995 American Chemical Society
VOL. 29, NO. 2,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
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followingpesticide application. The present study, which follows previous work on the rapid degradation of fenitrothion in estuarine waters of the Ebre Delta (Tarragona, Catalonia) ( 4 ) was executed by sampling water in the irrigation channels of the Ebre Delta. In this area, many organophosphorus pesticides are being applied in the rice crop fields by plane, helicopter, or manual spraying from spring to autumn. Although in the winter period few organophosphorus pesticides are applied in the rice crop fields, they are used for artichoke cultivation (1,2)and to eradicate the American crab (Procumburus cZurkic3 ( 4 ) . When the sampling of the water was carried out, during January 1994, no pesticides were applied on the channels, and the blanks showed nondetectable levels of organophosphorus pesticides (below 0.01 pglL). The estuarine water samples were transported to our laboratory (150 km from the area) and were spiked with different organophosphorus pesticides (and also a formulation) at concentration levels of 5OpglL. In this study, we have used fenamiphos and chlorpyrifos,which are included in a recent priority lists of pesticides within the EEC (16). Chlorpyrifos is probably the most applied organophosphorus pesticide in Central America (8). Other organophosphorus pesticides from this study, e.g., diazinon, malathion, fenitrothion, and pyridafenthion, are usually found in different monitoring programs in Greece (13 ,Italy (181, France (291, and Spain (1-4). The specific objectives of this study were (i) to evaluate the decay of 10 organophosphorus pesticides and one formulation in estuarine waters under similar conditions as the Ebre Delta area; (ii) to monitor the presence and concentrations of various TPs; and (iii) to calculate halflives for the different organophosphorus pesticides and their TPs. To achieve such purposes, novel techniques have been employed. Apart from the use of off-line solid-phase extraction (SPE) with C 18 Empore disks combined with GC-NPD and GC-MS (4),an automated method involving on-line SPE-LC-DAD and LC-MS has been employed. This is the first time that automated on-line SPE techniques are being used to study degradation of pesticide in estuarine waters, although they are currently used in our laboratory for monitoring pesticides residues in surface waters (2022). The main advantage of this automated technique is that with only 100 mL ofwater the limits of detection (LOD) are below O.lpg/L (21),so degradation experiments can be done with only 1-2 L ofwater and can be performed during variousweeks. In addition to that, the combined techniques permit validation and confirmation of the results obtained by GC-NPD and GC-MS together with a better precision (21). The determination of the more polar TPs, not GCamenable, is feasible, and it can be confirmed by means of on-line SPE-LC-MS (20).
Experimental Section ChedcalsandReagents. Pureanalyticalpesticidestandards chlorpy-rifos-methyl, diazinon, disulfoton, fenamiphos, fenthion, isofenphos, malathion, methidathion, pyridafenthion, temephos, disulfoton, disulfoton sulfone, disulfoton sulfoxide, fenamiphos sulfone, fenamiphos sulfoxide, fenthion-oxon, malaoxon, and temephos sulfoxide were obtained from Promochem (Wesel, Germany). The formulated pesticide was Ofunackwith 40% of pyridafenthion (Inagra). Pesticide-grade ethyl acetate and gradient HPLC 432
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 2, 1995
grade acetonitrile, methanol, and water (Baker analyzed) were purchased from J. T. Baker (Deventer, The Netherlands). Experimental Setup. Estuarine water (pH = 7.8, salinity of 20 g/L)was taken directly from the Ebre River CTarragona, Catalonia) andfilteredthrough 0.45flm (Millipore,Bedford, MA) to eliminate the suspended particles. Water samples were spiked with each single pesticide (10 pure standards and a formulation) by adding the standard pesticide in cyclohexane in a Pyrex borosilicate glass bottle and evaporating the solvent under nitrogen. Afterwards, each bottle was filled with 2 L of Ebre River water to produce a nominal pesticide concentration of 50 pglL. The bottle content was homogenized during 15 min by careful agitation. The bottles were placed capped on a terrace roof in Barcelona (Catalonia) on January 19 until March 2, 1994. Rubber serum stoppers were wrapped in aluminum foil-sealed bottles. A water aliquot of 10 mL (by triplicate) was sampled at the beginning of the experiment (time 0). Every week 10 mL ( x 3) was analyzed by SPE followed by GC-NPD and 75 mL ( x 3) was analyzed by on-line SPELC-DAD. Water samples from week 4 were collected for confirmatory analysis with both GC-MS and on-line SPELC-TSP-MS. After sampling,the bottles were immediately closed in order to avoid any evaporation of the water samples. During this period, the weather was sunny and the daily temperatures ranged from 7 f 2 "C (minimal temperature) to 15 f 3 "C (maximal temperature). The temperature inside the bottle is difficult to estimate during all the time the experiment lasted since it changes with the sunlight distribution (diurnally and nightly) and also the intensity of the sun varies during the day. Clouds were not present during most of the days of the experiments, so fluctuations in the sunlight intensities were not expected. Chromatographic Analysis. GC-NPD. A total of 10 mL of each water sample was solid-phase extracted with Cle Empore extraction disks 0. T. Baker, Deventer, The Netherlands) using 2 x 10 mL of ethyl acetate as the eluting solvent. The fractions were evaporated to dryness, and the residues were dissolved with 100 pL of ethyl acetate. Azinphos-methylwas used as the internal standard solution for quantitation by GC-NPD. A Carlo-Erba GC (HRGC 5300 Mega Series, Carlo Erba, Milan) equipped with a nitrogen-phosphorus detector (NPD-40,Carlo Erba) was used. A DB-1701 column (30 m x 0.25 mm i.d.) fused phenyl-cyanopropylmethylchemically bonded to silica U&W Scientific, Folsom, CA) was programmed from 70 to 90 "C at 30 "Clmin, from 90 to 180 "C at 10 W m i n , and from 180 to 280 "C at 8 "C/min. This final temperature was held for 10 min. The injector and the detector were set at 280 "C. Hydrogen was used as the carrier gas at 160 H a and helium was used as the makeup gas at 100 kPa. The detector gases were hydrogen and air at 80 and 90 kPa, respectively. One microliter of the water extract was injected in a splitless mode with the valve opened for 35 s. GC-MS-EI. The same GC column as indicated above was connected to a Fisons MD800 (Manchester, U.K.). The chromatographic conditions were identical to those described for GC-NPD. The transfer line and the ion source were maintained at 280 and 200 "C, respectively. Mass spectra were obtained at 70 eV with full scan. The oncolumn injection mode was used with the injector held at room temperature. The DB-1701 column head pressure was 14 psi, and helium was used as the carrier gas.
On-Line SPE-LC-DAD. On-line SPE coupled to LCDAD was performed with the fully automated preconcentration system (Prospekt, Spark Holland, Emmen, The Netherlands) . Samples were preconcentrated on a 10 mm x 2 mm i.d. disposable precolumn prepackaged with 1525pm styrene-divinylbenzene copolymer (PLRP-s) (Spark Holland). The precolumns were conditioned via a solventdelivery unit (SDU) from Spark Holland with 10 mL of acetonitrile, 10 mL of methanol, and 10 mL of water at a flow rate of 2 mL/min. A sample of 10 mL of water (time 0) or 75 mL (from time 2 weeks) was percolated through the precolumn at a flow rate of 3 mL/min. Desorption was carried out by coupling the precolumn on-line with the analytical column and starting the gradient. This method has been used previously and is described in detail in ref 21. The LC analysis was performed with a Waters 600-MS solvent delivery unit equippedwith a Waters 996 photodiode array detector (Waters, Millipore, Bedford, MA). A Superspher cartridge column (25 cm x 4 mm i.d.) packed with 4 mm C8, from Merck, (Darmstad, Germany) was used. The gradient conditions were carried out with acetonitrilewater with the following gradient: from 15%of acetonitrile and 85% of water to 30% acetonitrile and 70% of water in 15 min, and from these conditions to 100% of acetonitrile in 15min, which was kept isocratic for 5 min, back to initial conditions in 5 min and flow rate set at 1 mL/min. On-Line SPE-LC-MS. The eluent was delivered by two Model 510 high-pressure pumps coupled to a Model 680 automated gradient controller (Waters Chromatography Division, MiUipore, Bedford, MA) and a Model 7125 injection valve furnished with a 20-pL loop from Rheodyne (Cotati, CA). The general scheme of the system used for carrying out the on-line preconcentration of pesticides from water samples was similar to that described elsewhere (20). A column holder of 10 x 2 mm containing CISwas fit in a MUST column switching device from Spark Holland (AS Emmen, The Netherlands) and connected to an SSI Model 300 LC pump from Scientific Systems Inc. (State College, PA), which delivered the water samples containing the pesticides. A 100-mLsample of each spiked-water sample collected from week 4 was percolated through the precolumn at a flow rate of 3 mllmin. Following the preconcentration step, the MUST valve was switched and the components were desorbed and separated in an analytical column. The precolumn was cleaned with 15 mL of methanol and 15 mL of acetonitrile and was reused. With this method, one precolumn was enough to analyze 12 water samples. The LC column was the same as for LCDAD analysis. LC eluents were acetonitrile-water with 0.05 M ammonium formate with the following gradient: from 15%of acetonitrile and85% ofwater to 30%acetonitrile and 70% of water in 15 min, and from these conditions to 90% of acetonitrile and 10% of water with ammonium formate in 15 min, which is kept isocratic for 10 min, back to initial conditions in 5 min and flow rate set at 1mLlmin. A Hewlett-Packard Model 5988A LC-TSP-MS quadrupole mass spectrometer and a Hewlett-Packard Model 35741B instrument for data acquisition and processing was employed. The TSP temperatures were as follows: stem, 90 "C; tip, 200 O C ; and ion source, 250 "C. In all the experiments, the filament-on ionization mode was used.
Results and Discussion General Remarks. Although GC-based techniques (1-4) are commonly used for the determination of organophosphorus pesticides in environmental samples, LC has been recommended for thermally labile and polar organophosphorus pesticides (21, 23, 24). The decomposition of temephos and its TPs under GC conditions has been reported ( 2 3 , and an LC method based on Empore disk extraction and LC-DAD has been developed for fenamiphos for its determination in water samples at the 0.1 pglL level (23). Other authors (24) prefer to use off-line SPE and LC for the determination of variety of pesticides in environmental waters, including many organophosphorus pesticides, e.g., mevinphos and fenitrothion at the 0.1 pg/L level. The use of automated on-line SPE methods, e.g., Prospekt (21),is probably the most powerful approach for determining pesticides in water samples using SPE techniques. It allows the achievement of LOD below 0.1 pglL using 150mL ofwater. In addition, it yields good precision and robustness of the methods. Recently reported results from our group demonstrated that Prospekt is a useful system for interlaboratory studies. In such studies, groundwater samples containing organophosphorus pesticides were analyzed at levels varying from 0.02 to 0.2 pg/L, our results being comparable using conventional analytical techniques, e.g., GC-NPD (21). This system is linear over a concentration range of 0.1- 1.5pglL and has been used for the first time to follow the degradation of pesticides in natural waters. LC-based techniques (Prospekt-DAD and on-line SPELC-MS) will also permit the determination of the more thermallylabile and/or polar organophosphorus pesticides and the identification of the polar TPs, e.g., sulfoxide metabolites (26). This is in-line with one of our aims of this work to investigate the formation of the major TPs. LC-based techniques are more powerful, and the combination of DAD and MS should facilitate such unequivocal identification. GC-NPD and GC-MS are the most common ways of determining organophosphorus pesticides in degraded water samples and, in addition to LC, will serve as validation techniques. QualitativeInfonnation. The use of GC-NPD and LCDAD techniques allowed the identification of both the parental compounds and the potential TPs by means of standard and spectral comparison, respectively. These techniques were used for quantification purposes. Since not all the metabolites were available, all samples were injected by both GC-MS and on-line SPE-LC-MS for confirmatoryanalysis and with the perspectiveof searching new TPs. GC analysis permitted the identification of all the parental compounds except for temephos and pyridafenthion, which were in too little amounts. TPs of fenthion and disulfoton were detected. In this sense, fenthion sulfoxide was identified with GC-NPD and has been positively confirmed by GC-EI-MS (Figure 1 ) and by LC-TSP-MS. After 1week, disulfoton disappeared from water samples but disulfoton sulfoxide and sulfone could be detected by GC-NPD and were confirmed by GC-EIMS. The characteristic ions obtained under GC-MS with E1 are not shown here and followed previous results from our groups (1-4, 19). By on-line SPE-LC-TSP-MS, disulfoton sulfoxide was also detected. VOL. 29, NO. 2, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1433
1 .OO
MA
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0 - . .. . . . , ' . _ , . . .
, .
.
, . . . . , . . . . , . . . . , . . . ,
l
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.
RETENTION TIME (Min.)
1:
100
i
(18.9681 1
I
11i8 II
I
294
m/ z FIGURE 1. (A) Total ion current (TIC) obtained using GC-MS-El and (B) El spectrum of fenthion sulfoxide of an estuarine water extract collected after 3 weeks of exposure. Concentrationof fenthion and fenthion sulfoxide were 34and 9pe/L respectively. MA azinphos-methyl (internal stenderd). Typical ions of fenthion sulfoxide were at @ l o 9 and 125 (charecteristic of organophosphorus pesticides, see ref 3) and at 279 and 294 corresponding to [M - CH&+ and [MY+, respectively.
With on-line SPE-LC-TSP-MS in positive mode (PI), the characteristic fragments ions were [M HI+, [M NH4]+,and [M CH&N]+ as expected for these types of compounds (23,26). Advantages of on-line SPE-LC-MS over GC-MS are clear for the detection of polar pesticide TPs at low microgram per liter level. For example, chlorpynfoswas detected in all samples (untilthe fifthweek) when using GC analysis,but apparently no TPs were formed. By using on-line SPE-LC-DAD, additional peaks were
+
+
434 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 2,1995
+
detected, and they could be classified as possible TPs of chlorpyrifos by UV spectral similarities to the parent compound. Samples corresponding to the fourth week were analyzed by on-line SPE-LC-TSP-MS in PI, and chlorpynfos-oxonwas detectedwith [M NH4]+. The same water sample was also analyzed under the NI mode, and chlorpynfos was detected with fragmentation involving chlorine losses. 3,5,6-Trichloro-2-pyridinol was also formed with losses of chlorine atoms and proton abstraction similar
+
I
10
0
1
325
I
36
20
341
2
358
2
0
20
I
I
40
RETENTION TIME (Min.) FIGURE 2. (A) On-line SPE-LC-DAD at 230 nm and (B) on-line SPE-LC-TSP-MS using PI mode of an estuarine water extract collected after 4 weeks of exposure. Compounds 1 and 2 correspond to pyridafenthion-oxon and pyridafenthion at a concentration level of 5 and 20 pg/L, respectively. Selected ion chromatograms 01 pyridalenthion-oxon at mlz 325 ([M HI+) and of pyridafenthion at m/z 341 and 358 respectively. corresponding to [M HI+ and [M "el+,
+
+
+
to the parent compound. The presence of this compound agrees with other studies on photolysis in water using the suntest apparatus (26)and in different solid surfaces (27). Pyridafenthion and its oxon were detected by on-line SPE-LC-DAD and confirmed with TSP-MS, both giving [M H]+ as the characteristic ion (Figure 2 ). Temephos behaved similarly with the formation of [M NH4]+ for both the parental compound and its oxon. The determination of these two compounds and the detection of their these toxic TPs 4 weeks after natural exposure is of great environmental importance since they are widely used to eliminate mosquitoes in the area of the Ebre Delta. Similarly, diazinon and its oxon were also detected. Fenamiphos could be detected under LC-TSP-MS at [M H]+ after 4 weeks of natural exposure. Fenamiphos sulfoxide is the main TP formed under conditions of photodegradation, and by select ion monitoring, it could be found at mlz 320 and 337 corresponding to [M + HI+ and [M NH4]+, respectively. The importance of this compound is obvious since it has been included in the NPS-U.S. EPA list as a potential contaminant (9).
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Quality Assurance. Waters were spiked at a level of 50 pg/L, which is lower than the applied levels used for pesticide treatment in the environment (around 200 pglL) ( 4 ) . Samples were quantified immediately after spiking to avoid further degradation of the pesticides due to poor analytical care. The average percent recoveries and relative standard deviation are shown in Table 1. GC-NPD. Quantitation with NPD was carried out by height measurements using azinphos-methylas the internal standard, with a calibration curve constructed at a concentration ratio between organophosphorus and the internal standard of 0.1, 1, and 10. Limits of detection (LOD) and quantitation (LOQ) (signal to noise ratio greater than or equal to 3 and 10,respectively) varied from 5 to 10 nglL for LOD and from 15 to 30 nglL for LOQ. LC-DAD. Calibration curves were constructed for fenitrothion, fenamiphos, pyridafenthion, and temephos over a concentration range of 0.1- 1.5pglL, and correlation coefficients were around 0.99, except for fenamiphos which has a worse correlation coefficient (0.94) because it is easily VOL. 29, NO. 2. 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
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TABLE 1
TABLE 2
Mean Percent Rucoveries and Coefficient of Variation (CV) (n = 5) of OqaRepkespherus Pesticides and Selected Transformation Products in 50 m l of Estuarine Water Fortified at 1 pg/l Using GC-NPD and Prospekt-LC-DAW
Half=Lives (rln) in Days of Organopbesphwus Pesticides and Selected Tnnsbmathn Products in Estuarine Watm Using GC-NPD and Prospekt- LC-DADa
compound
chlorpyrifos-methyl diazinon disulfoton disulfoton sulfoxide disulfoton sulfone fenamiphos fenthion fenthion sulfoxide isofenphos malathion methidathion pyridafenthion temephos a N D = not determined.
GC-NPD rec (%) CV 94 77 89 14 71 ND 77 71 77 90 71 104 ND
5 7 6 8 8 ND 8 8 9 5 7 2 ND
Prospekt-LC- DAD
rec (YO) 90 88 91 80 78 91 ND ND 89 92 77 98 89
cv 8 7 7 7 10 8 ND ND 9 7 9 5 8
See problems for fenthion in ref 21, for fenamiphos in ref 16, and for temephos in ref 25.
degraded in a water solution (21).A linear range from 0.1 to 1.5pglLwas achieved, thus quantitation was carried out at this level. The precision of the method was also studied with a relative standard deviation of the peak areas below lo%, which confirm the good precision that can be attained with coupled-mode analysis. Using on-line SPE-LC-DAD, the LOD varied from 20 to 100 ng1L (with a signal to noise ratio of 3) depending on their absorption maxima and the presence of interferences (21).These LODs were obtained by percolating 75 mL of each water sample. However, these limits can be lowered by percolating a sample volume close to the breakthrough volume (Le., higher than 200 mL for disulfoton, fenamiphos, fenitrothion, pyridafenthion, and temephos), where still 100% recovery is achieved. Nevertheless, when carrying out studies on photodegradation and identification of TPs, it should be kept in mind that breakthrough volumes of the polar TPs are much lower than those of the parental compound (20-221,and therefore, the water volume percolated should be fixed in a way that both TPs and the parental compound could be determined. For this reason, the recovery values indicated in Table 1 have been calculated preconcentrating 50 mL of water. DegradationKinetics. The disappearance ofthe different compounds and the estimated values calculated with GCNPD and LC-DAD during a period of 5-6 weeks are shown in Table 2. The loss of compounds is described by linear regression of In of the area versus time. Initial values are based assuming that the initial concentration at time 0 was of 100%. From this table, it can be observed that the values of t l / z reported using both techniques differ between 6 and 30% depending on the compound studied but that the rate constants did not differ significally ( p > 0.05). A concordance between both results can be admitted since the precision reported for each technique is ca. 10% and considering that at this concentration level the precision reported in interlaboratory studies was of the same order as using cleaner groundwater samples (21). Since degradation in water follows first-order kinetics as reported by different authors (11, I,?), half-lives were calculated as reported in a previous paper from our group ( 4 ) by the 436
ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 29, NO. 2, 1995
GC-NPD compound
chlorpyrifos-methyl diazinon disulfoton sulfoxide disulfoton sulfone fenamiphos fenthion fenthion sulfoxide isofenphos malathion methidathion Ofunack pyridafenthion temephos N D = not determined.
tin
R*
11.1 10.6 10.4 8.19 ND 4.6 6.9 11.9 4.4 9.9 12.0 11.5 ND
0.93 0.86 0.97 0.98 ND 0.95 0.91 0.89 0.96 0.96 0.97 0.98 ND
Prospekt-LC-DAD
hn
a2
1.4 8.2 12.3 ND 1.80 ND ND 9.8 4.9 6.5 10.2 10.8
0.94 0.93 0.99 ND 0.89 ND ND 0.90 0.97 0.89 0.96 0.98 0.99
5.0
Correlation coefficients ( R 2 )of the plot of In of the concentration versus time.
formula t1/2= In 21k, where kis the first-order rate constant. Correlation coefficients for the linear curve, calculated within a time range from 0 to 35 days with GC-NPD and LC-DAD, and in some cases from 0 to 28 days with on-line SPE- LC-DAD, except for disulfoton sulfoxide and fenamiphos (0-14 days), are reported. The lowest values obtained by on-line SPE-LC-DAD are for fenamiphos and methidathion and, therefore, fail to predict t1/2accurately. The problem was solved for methidathion since GC-NPD gave much more precise values ( R 2 = 0.961, and the t112 reported is probably more reliable. For fenamiphos, such low R might be due to the instability of this compound in water, which is also reflected in the calibration curves. Lowest R values are obtained for diazinon and isofenphos with GC-NPD, but this variation can be explained by the error made by extracting only 10 mL of water as compared to 75 mL by on-line SPE. The overall R 2 values are acceptable if one considers that water samples were from the Ebre River, spiked at a low concentration level and kept under natural degradation conditions, and the water was not acidified. All these parameters will favor degradation and more variability within analysis. With the attempt to quantify and study the kinetics of the TPs, degradation kinetics of disulfoton sulfoxide, disulfoton sulfone, and fenthion sulfoxide are also reported in Table 2. t112 evaluated for disulfoton sulfoxide and sulfone and fenthion sulfoxide are 10.4, 8.2, and 6.9 days, respectively. These values are greater than those reported for their parent compound, especiallyin the case of disulfoton which disappeared within the first week. Disulfoton sulfoxide and sulfone were quantified by GC-NPD at their maximum concentrations, with 37 (time 7 days) and 4 pg/L (time 0, just after spiking), respectively. Since waters were spiked at a level of 50pg/L disulfoton, we can assume that degradation of disulfoton to its sulfoxide and its sulfone seems to be complete. In general, when looking at the values of Table 2, we can indicate that the t112 values reported here are comparable or lower than those reported in the literature. Major differencesare due to the experimental conditions that have been used, such as natural photolysis conditions, temperature, pH of the water, and presence of microorganisms.
For instance, in the case of malathion, half-lives of 1.65 days (pH = 8.16, salinity = 24 g/L, T = 28 "C)(28)and less than 1 week in river water are reported, Freed et al. (11) also observed a high variation on the half-life of malathion according to temperature (from20 to 37 "C it changed from 11 to 1.3 days) and with pH (from pH 7 to pH 6.1 half-life increased by a factor of 10). Our value is within the reported range and approaches natural environmental situations. Half-life for chlorpyrifos has been reported to be 13 ( 1 1 ) and 4.8 days (29)at 20-21 "C. The main difference was the pH of the water, being 7.4 (11) or 8-8.5 (29). At our pH of 7.8, half lives were 7-11 days. Half lives for diazinon and methidathion at 21 "C and at pH 8-8.5 appeared to be 14 and 20 days, respectively (29)- At pH 7.4 and at the same temperature, the half-life for diazinon was 185 days (30). We obtained shorter half lives for both compounds, suggesting that the natural environmental conditions used here enhance degradation. The half-life of fenthion reported here is similar to the 4 days obtained when water samples contained a sediment with microorganisms (12) and lower than in river water (15). The reported values for temephos, with a half-life of 5 days, showed higher half-life than the values of 1 day obtained in natural waters after pesticide application in Florida between June and August (6). Our studies have been performed between January and March, and consequently the ambient temperature is somewhat cooler. The presence of temephos-oxon (confirmed by on-line SPELC-MS with a base peak at mlz 468 corresponding to [M NH4]+)was detected in our experiments. For isofenpos and pyridafenthion, no data in the literature are available concerning pesticide degradation in water. Isofenphos is a compound quite studied in soil (31,321,and its half-life is quite similar to that of chlorpyrifos. In our experiments, this compound exhibits a half-life also similar to chlorpyrifos. The formation of the oxo derivative has been confirmed by on-line SPE-LC-MS with two main ions at mlz 330 and 346 corresponding to [M HI+ and [M NH4]+,respectively. Pyridafenthion was studied in soil samples (33) and exhibited a stability much lower than chlorpyrifos and higher than fenitrothion, microbial degradation being a relevant mechanism of degradation. The half-life obtained in estuarine water is even somewhat higher than the values for chlorpyrifos, suggesting that pyridafenthion is a quite stable compound. Its oxon derivative has been confirmed by on-line SPE-LC-MS, see Figure 2. Pyridafenthion was studied in its pure and formulated form. The use of such a formulation was interesting from the application point of view since this compound has replaced fenitrothion in many rice applications in the area of the Ebre Delta (31, and no degradation studies have been performed up until now. No differences were noticed between half-lives and the correlation coefficients of the pure compound and the formulation (Table 2). Pyridafenthion-oxon was detected by UV spectra (similar to its parental compound) at time 2 weeks. Confirmation was carried out by on-line SPE-LC-TSPMS. This oxo TP could be followed during the 6 weeks the experiment lasted, with a residual concentration of 2.9pgl
+
+
+
L.
Fenamiphos degraded very fast and its sulfoxide was formed. This is not surprising since this compound degraded very rapidly under controlled photolysis conditions (26'). Fenamiphos sulfoxide has been confirmed by on-line SPE-LC-MS by its two characteristic ions at mlz
+
+
HI+ and [M NH41+, respectively. This TP has been reported either in photolysis experiments in water (26) or in soil degradation studies (34, The rapid degradation of fenamiphos indicates that its inclusion in the NPS (9) can be somewhat doubtful. Although it clearly indicates that at the NPS conditions fenamiphos does not suffer degradation, when applied in the field it degrades very fast. This is quite surprising since other compounds that have been removed from the NPS list (e.g.,malathion, fenthion, and diazinon) exhibit ahigher half-life under the estuarine water conditions of this paper. The instability of disulfoton sulfoxide observed in the NPS with a 100%loss of the analyte during 14 days at 4 "C using biologically inhibited water (9) contradicts what was found in natural environmental conditions for this compound, which is formed and had a half-life of 10 days. A similar case occurs with fenthion sulfoxide,which is formed during the present study when the degradation of fenthion occurs. 320 and 337 corresponding to [M
Conclusions Even though the results obtained in the present experiments cannot be fully extrapolated to the field, a realistic approximation of what takes place under real environmental conditions is given. Many experiments performed in the literature use either a high spikingconcentration or distilled water, which do not reflect the environmental conditions. The present study has reported useful information on the behavior of organophosphorus pesticides in estuarine waters. No differenceswere found between the degradation studies conducted for pyridafenthionand its Corresponding formulation, Ofunack. The present results indicated that some compounds excluded in the NPS list, e.g., malathion, diazinon, and fenthion, exhibit a longer half-life than other organophosphorus pesticides included in the list, e.g., fenamiphos. Further studies are needed to establish a priority pesticide list for monitoring purposes that considers the various stability parameters depending on the environmental water type used. The combined approach used in this paper is very powerful and permits the determination of pesticides and their TPs at very lowlevels. Until now, most of the common degradation studies performed reached values of micrograms per liter, whereas now it is possible to quantify levels of 0.01 pg/L . Practically similar LODs are obtained using off-line SPE with GC-NPD or GC-MS and on-line SPELC-DAD or MS, thus on-line techniques reported for the first time in this paper in degradation studies of pesticides will be of great help for a better understanding of the environmental fate of pesticides in water. They are useful for the identification of polar TPs.
Acknowledgments R. Alonso is thanked for laboratory assistance in the LCMS. This work has been supported by the Environment R&D Program 1991-1994 (Commission of the European Communities) (Contract EV5V-CT94-0524) and from PLANICYT (AMB94-0950-CE).Research Agreement 79781CF from the International Atomic Energy Agency (IAEA) is also acknowledged. S.L. gratefully acknowledges financial support from CICYT (Grant AMB92-0218). S.B.L. thanks the EERO for its short-term fellowship (JVLlmhl93-7474). VOL. 29. NO. 2,1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Received for review May 10, 1994. Revised manuscript received August 9, 1994. Accepted October 19, 1994.@ ES940285D @
Abstract published inAdvanceACSAbstracts, November 15,1994.
