In a study by Schlüter (1989) on rotational crops with [14C]teflubenzuron uniformly labelled in the aniline ring, soil was treated with 0.5 kg ai/ha and aged under anaerobic conditions. At 30, 120 and 360 days after the soil treatment, head lettuce, carrots and wheat were planted or sown as rotational crops and grown to maturity. The 14C residues, as mg/kg teflubenzuron equivalents, are shown in Table 2.
Table 2. 14C levels in rotational crops after treatment with [14C]teflubenzuron, expressed as mg/kg teflubenzuron equivalents (Schlüter, 1989).
|
Crop |
14C as teflubenzuron, mg/kg |
||
|
Soil aged 30 days |
Soil aged 120 days |
Soil aged 360 days |
|
|
head lettuce |
0.007 |
0.006 |
0.002 |
|
carrots, peeled |
0.013 |
0.006 |
0.002 |
|
peel |
0.08 |
0.053 |
0.017 |
|
whole root |
0.026 |
0.013 |
0.005 |
|
wheat straw |
0.24 |
0.088 |
0.035 |
|
wheat grain |
0.005 |
0.003 |
0.002 |
The comparatively high residue in wheat straw is partly due to the high degree of dryness of the material at harvest. Relatively high values were also found in carrots; the cause in this case is the direct contact with the treated soil. Peeled carrots, and all other fresh plant material, contained equivalents of <0.01 mg/kg. The results further show that, in all samples, the radioactive residues decrease significantly with increasing soil ageing periods. This decrease corresponds to the observed decrease in extractable radioactivity in soil on increasing the ageing period. Unextractable soil residues, on the other hand, increase markedly throughout the entire experimental period (up to 70% of the radioactivity originally applied). These observations indicate that the unextractable residues in soil are very probably not taken up by the rotational crops and that the radioactive residues found in the plants originate only from the extractable soil residues. Since at all times the concentration of the unchanged parent compound in the extractable soil residues is much higher than those of the individual degradation products it is probable that teflubenzuron itself is taken up by the plants.
In so far as the low residues made analysis possible, it could be shown that the plants contained numerous, mainly polar, compounds in very small amounts (<0.05 mg/kg). Neither teflubenzuron nor its known soil degradation products (3,5-dichloro-2,4-difluorophenylurea and 3,5-dichloro-2,4-difluoroaniline) could be detected in the plants (<0.01 mg/kg).
Environmental fate in water and water/sediment systems
Laboratory studies
Hydrolysis. Teflubenzuron (unlabelled) was incubated with buffers at pH 5, 7 and 9 at room temperature. It was stable at pH 5 but was hydrolysed at pH 7 and 9 with half-lives of 8 months and 8 days respectively. In additional studies with suspensions the products identified by HPLC were 2,6-difluorobenzoic acid, 2,6-difluorobenzamide, and 3,5-dichloro-2,4-difluoroaniline, showing that all three N-CO bonds are cleaved when teflubenzuron is hydrolysed (Hawkins et al., 1988b; Heupt, 1983).
Photolysis. The photodegradation of [14C]teflubenzuron in aqueous solution has been investigated by Hawkins et al. (1988c). [14C]teflubenzuron (uniformly labelled in the aniline ring) in ethanol was added to acetate buffer (0.1 M, pH 5) to a concentration 0.1 mg/l (10% ethanol) and irradiated with artificial sunlight from a xenon arc lamp for periods up 15 days. Control samples were maintained in the dark under similar conditions of temperature and ventilation.
In the exposed solutions, teflubenzuron was degraded with a half-life of about 10 days. Only one breakdown product exceeded 10% of the applied radioactivity, accounting for about 32% after 15 days, when teflubenzuron accounted for about 45%. About 5% of the applied radioactivity was recorded as volatile breakdown products. No volatile products were formed in the control incubations and after 15 days most of the applied radioactivity chromatographed with teflubenzuron.
The main product of photolysis chromatographed with the reference compound 1-(3,5-dichloro-2,4-difluorophenyl)-5-fluoro-3H-dihydroquinazoline-2,4-dione. The degradation product was isolated and its identification confirmed by mass spectrometry.
Sunlight has little effect on the aqueous stability of teflubenzuron. It may be adsorbed onto organic matter, thus reducing its concentration in field water. In highly polluted water, the chemical appeared to be lost because of microbial action as well as adsorption onto organic matter (Schaefer et al., 1988).
Water/sediment systems. The biodegradation of [14C]teflubenzuron labelled in the aniline ring was determined in two water/sediment systems in The Netherlands (Muttzall, 1987). Ditch water and one sediment sample were collected from an unpolluted ditch surrounding the premises of "TNO-Zuidpolder", Delft. A second sediment sample was collected from the river "Kromme Rijn" near Odijk (Province of Utrecht). This river has been contaminated with biocides for many years and is considered to be polluted. The concentrations of the test substance were 1 and 0.02 mg/l of labelled and unlabelled teflubenzuron.
In the "Kromme Rijn" sediment system, very little of the teflubenzuron was biodegraded to CO2: after 12 weeks (t12) only 0.9% of the initial radioactivity was detected as 14CO2. The radioactivity in the aqueous phase increased from 12% at t0 to 37% at t12 with the test concentration of 0.02 mg/l. In the test with 1 mg/l, the aqueous radioactivity reached only about 20% at the end of the test. While bound residues increased steadily from 4% at to 18% at t12 the radioactivity in the extracts of the solids decreased from 80% to 33%.
In the "TNO" sediment system only 0.6% of the initial 14C was detected as 14CO2 after 12 weeks. The biodegradation was similar to that in the "Kromme Rijn" system: the radioactivity in the aqueous phase rose from 8% at the beginning to 12% at the end of the test. Bound residues increased from 4% to 32% after 12 weeks, during which time the radioactivity in the extracts of the solids fell from 85% to 48%.
Analysis of the aqueous phase and the extracts of the solids by thin-layer chromatography showed that the amount of teflubenzuron decreased from 87% to 35% after 12 weeks in the Kromme Rijn sediment system: its half-life was calculated to be 6 weeks. Two major degradation products were found, one with an Rf value of 0.23, which accounted for 6% of the initial radioactivity, and one with an Rf value of 0.08, which was identified as 3,5-dichloro-2,4-difluorophenylurea. The amount of this compound increased from 3% at the beginning of the test to 14% at t12.
In the TNO sediment system, the amount of teflubenzuron fell from 86% at t0 to 40% at t12, giving a half-life of 7 weeks. In this system the same 2 major products were found, 5% of the radioactivity being present as the compound with an Rf value of 0.23. The product with the Rf of 0.08 (the phenylurea) increased from 1% at t0 to 23% at t12.
