Degradation of Benthiocarb Herbicide in Soil*

J. Pesticide Sci. 1, 49-57 (1976) Degradation of Benthiocarb Herbicide in Soil* Kanjl ISHIKAWA,** Yasuo NAKAMURAand Shozo KUWATSUKA Life Science Research Institute, Kumiai Chem. Ind. Co., Ltd., Kikugawa-cho, Ogasa-gun, Shizuoka-ken 439, Japan (Received October 10, 1975) Laboratory experiments were conducted on the persistence and the degradation products of benthiocarb (4-chrolobenzyl N, N diethylthiolcarbamate) in soil, using 14C-labeled and unlabeled benthiocarb. The radioactive substances extracted with organic solvents from soil consisted mainly of parent benthiocarb with small amounts of degradation products. About 20 radioactive spots were detected by thin layer chromatography. Desethyl benthiocarb, benthiocarb sulfoxide, 4-chlorobenzoic acid, 2-hydroxy Benthiocarb and 4- chlorobenzyl methyl sulfone were identified as relatively major products, and 4-chlorobenzyl methyl sulfoxide and 4-chlorobenzyl alcohol, as minor ones. The degradation rates of benthiocarb and six related compounds in soil were also compared. was synthesized by International Minerals & INTRODUCTION Chemical Corp. and supplied by Kumiai Benthiocarb, 4-chlorobenzyl N, N diethyl- Chemical Ind. Co., Ltd. The specific radio- thiolcarbamate, alone or in combination with activity was 3.21 m Ci/mmole and radioactive simetryne, prometryne and CNP, is now purity was about 990. extensively used to control weeds in paddy 2. Chemicals and upland fields. The herbicide is applied Benthiocarb, the 4-chlorobenzyl esters of mainly to the surface water of paddy fields N, NVdimethylthiolcarbamic, N, NVdiisopro- or to the soil of upland fields. In the fields, pylthiolcarbamic, N, N diethylcarbamic and a part of the herbicide is degraded photo- N, N diethyldithiocarbamic acids, and the l- chemically by sunlight, a part is absorbed and 3-chlorobenzyl esters of N, NVdiethyl- by the plants, while the major part penetrates thiolcarbamic acid were described previous- into the soil. Investigations on the fate and ly." 2-Hydroxy benthiocarb (mp 110-111C), behavior of the herbicide in soil are impor- 4-chlorobenzyl methyl sulf oxide (mp 101- tant. Studies on the metabolism of benthio- 102C) and its sulfone (mp 121-122C), bis(4- carb in mice" and in plants,Z and its photo- chlorobenzyl) sulfoxide (mp 164-164.5C) and degradation and volatility in aqueous solu- its sulfone (mp 241-242.5C), 4-chlorobenzyl tion3 have been reported. This paper deals methyl sulfide (bp 110-112C/10 mmHg), and with studies on the degradation of benthio- benthiocarb sulfoxide (bp>140C/008mmHg) carb in soil, with emphasis on the degradation were supplied by the Chemical Institute of pathways. The degradation rates of related Kumiai Chemical Ind. Co., Ltd. Other re- compounds are also compared. ference compounds were described in the MATERIALS AND METHODS previous paper." 3. Soil Sample 1. Radioactive Benthiocarb 14C-Benthiocarb labeled at the benzene ring Soil was taken from furrow slices of the paddy field at the Pref ectural Agricultural * Studies on the Metabolism of Benthiocarb. Part Experiment Station in Anjo, Aichi Prefecture III. See Ref. 1). ** Present address: Laboratory of Soil Science, in winter. The soil was crushed and passed Faculty of Agriculture, Nagoya University, through a 2 mm sieve. The physico-chemical Furo-cho, Chikusa-ku, Nagoya, 464, Japan. properties of the soil sample were as follows: 50 日本農薬学会誌第1巻第1号昭和51年2月 clay mineral: kaolin; texture: sandy clay 6. Extraction and Separation of Radioactive loam; clay content: 23.1%; pH (H20): 5.83; Compounds total carbon: 1.93%; C.E.C.: 13.6 me/100 g; The incubated soil was transferred into a free iron: 1.25%; available phosphorus: centrifuge tube. 100 ml of acetone was added 10.3mg%; phosphate absorption coefficient: (in unflooded soil, 40 ml of water was also 504mg (P205). added), the tube was capped, shaken 400 4. Soil Conditioning (pre-incubation) times per min for 30 min, and centrifuged (A) Flooded conditions: Fifty grams (oven- at 3000 rpm for 10 min. The supernatant was dried weight basis) of the soil sample in a collected. The sedimented soil was extracted 300 ml reagent bottle was flooded with water repeatedly with a mixture of 20 ml of water up to 1 cm deep. The soil was incubated at and 100 ml of acetone by the same procedure. 30C in the dark for 2 weeks. (B) Upland The supernatant solutions were combined conditions: Fifty grams of the soil sample and concentrated under reduced pressure to was placed in a 100 ml Erlenmeyer flask. distil off acetone. The residual water was Water was added to adjust the soil moisture extracted three times, each with 70 ml of to 600 of the maximum water holding n-hexane. These extract solutions were capacity and the soil was incubated in the radioassayed. The residual soil was air-dried same way as the flooded sample. and a small portion was oxidized to 14C- 5. Application of 14C-benthiocarb and Incuba- carbon dioxide using a Sample Oxidizer tion (Packard. Tri-Carb model 306) and the 14C-Benthiocarb (25.8 x 105 dpm) dissolved carbon dioxide evolved was trapped in in a small amount of acetone (0.5 ml) was scintillation liquid and radioassayed. mixed well with the pre-incubated soil in a 7. Radioassay flask at a concentration of 10 ppm on dry A liquid scintillation spectrometer was used soil basis. The soil was incubated under to measure the radioactivity. Samples were the same conditions as above, for 10, 20, 40 counted in 10 ml of dioxane liquid scintil- and 80 days. The moisture content was lator.l The amount of quenching was maintained at the initial level by the addi- determined by the use of an external standard tion of water once a week to replace the and comparison with a standardized quench- water lost by evaporation. ing curve. The radioactivity is expressed as Table 1 Rf values of benthiocarb and the reference compounds. A: benzene-ethyl acetate (10:1), B: chloroform-diisopropyl ether-n-hexane-acetic acid (10:10:10:1), C: benzene-ethyl alcohol-acetic acid (40:1:2), D: n-hexane-diisopropyl ether-diethylamine (20:20:1), E: benzene-methyl alcohol-formic acid (100: 20: 1), F: n-hexane Journal of Pesticide Science 1 (1), February 1976 51 dpm. Table 2 Relative retention times of compounds 8. Thin layer chromatography related to benthiocarb. n-Hexane extracts from the soil were dried with anhydrous sodium sulfate and concentrated. An aliquot of each concentrate was spotted on silicagel plates (Merck 60 F254, precoated, 0.25 mm thick, 20 x 20 cm) and an aliquot of the acetone solution containing reference compounds was spotted over this. Plates were developed mainly in two di- mensions with benzene-ethyl acetate (10:1) for the first development and chloroform-n- hexane-diisopropyl ether-acetic acid (10: 10: 10: 1) for the second one. Four other solvent systems, benzene-ethyl alcohol-acetic acid (40: 1: 2), n-hexane-diisopropyl ether-diethylamine (20:20:1), benzene-methyl alcohol- formic acid (100: 20: 1) and n-hexane were used, if necessary. Rf values of the refer- A Hitachi 063 gas chromatograph and 3 mm (i.d.) x 200 cm stainless steel column packed with 3% ence compounds are shown in Table 1. Silicone SE-30 on 60-80 mesh Chromosorb W Spots of the reference compounds were (AW. DMCS.) were used. The relative retention detected by irradiation of ultraviolet rays times were calculated from the retention time of (254 nm), and the radioactive ones, by radio- benzyl N, N-diisopropyldithiocarbamate. autograms on X-ray films. 9. Degradation of Benthiocarb and its Related pressure to a final volume of 1 ml. An ali- Compounds quot of the concentrate was injected into a Benthiocarb, 4-chlorobenzyl esters of N, N gas chromatograph. The gas chromatograph dimethylthiolcarbamic, N, Ndiisopropylthiol- and the operating conditions were described carbamic, N, Ndiethylcarbamic and N, N previously. Relative retention times of the diethyldithiocarbamic acids, and the 2- and compounds are shown in Table 2. The re- 3-chlorobenzyl esters of N, Ndiethylthiol- coveries of these compounds 30 min after carbamic acid were degraded in Anjo soil their addition into the soil were in the range under flooded conditions. The soil sample of 1000. was pre-incubated under flooded conditions for 2 weeks and each of these compounds RESULTS was mixed well with the soil at a concent- The rates of change in the levels of radio- ration of 30 ppm on dry soil basis. The activity present in acetone extracts of in- mixtures were incubated at 30C in the dark cubated Anjo soil sample and in the soil for 2, 4, 8, 16, 32 and 64 days. The analy- residue are shown in Fig. 1. The amount tical procedures employed were as follows: of radioactivity released from the soil sys- The incubated soil was transferred into a tems are also shown in the figure. One hour centrifuge tube, and 1 ml of 15N sodium after the addition of the radioactive chemical, hydroxide and 250 ml of n-hexane were about 1000 of the radioactivity was recovered added. The tube was capped air-tight, from the soil under both flooded and upland shaken 400 times per min for 25 min, and conditions using the acetone-water extrac- centrifuged at 3000 rpm for 10 min. Into 50m1 tant. However, the extractable radioactivity of the supernatant n-hexane solution, 1 or gradually decreased with time, and the 3 ml of benzyl N, Ndiisopropyldithiocarba- amount of radioactivity in the soil residue, mate in n-hexane (100 ppm) was added as as well as the amount released from soil, the internal standard substance, and the increased. The rate of change was lower solution was concentrated under reduced under flooded conditions than under upland 52 日本農薬学会誌第1巻第1号昭和51年2月 Flooded conditions Upland conditions Fig.

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