1. The study monitored the degradation of two major metabolites (endosulfan diol and endosulfan sulfate) of the insecticide endosulfan in soil under sterile and non-sterile conditions. 2. It found that both metabolites degraded to CO2, demonstrating they are not ultimate degradation products of endosulfan in soil. Endosulfan diol degraded to a greater extent than endosulfan sulfate. 3. Of the originally applied endosulfan diol, 6.5% was mineralized to CO2 over the 9 week study period under non-sterile conditions, approximately 20 times greater than for endosulfan sulfate.

1. International Journal of Environmental Studies,
Vol. 62, No. 2, April 2005, 235–248
Natural attenuation of metabolites of a chlorinated
pesticide in soil
T.F. GUERIN*
40 Forman Street, Glenwood NSW 2768, Australia
GENV041064.sgm
Taylor and Francis Ltd
(Received in final form 7 June 2004)
40
Dr Tu[racute]laughF.GUERIN
March
000000
62 Forman StreetGlenwood
Taylor & Francis
2005 2005
Original Article Group LtdNSW
0020-7233 print/1029-0400 online
International Journal of Environmental Studies
10.1080/0020723042000257745 2768Australiaturlough.guerin@shell.com
Monitoring the fate of metabolites (of contaminants) provides a further line of evidence that contami-
nants can be naturally attenuated in soil. Endosulfan diol and endosulfan sulfate, the two major degra-
dation products of the widely used insecticide endosulfan, were aerobically degraded in soil under
sterile and non-sterile conditions. The trial demonstrated that endosulfan diol and endosulfan sulfate
are both degraded to CO2, demonstrating that they are not ultimate degradation products of the parent
endosulfan compound in soil. Endosulfan diol was transformed to 14CO2 to a greater extent than
endosulfan sulfate. Of the originally applied endosulfan diol, 6.5% was mineralized over the 9 week
trial under non-sterile conditions. This was approximately 20 times greater than that for endosulfan
sulfate.
Keywords: Metabolites; Soil; Attenuation; Endosulfan diol; Endosulfan sulfate
1. Introduction
Insecticides pose a challenge for remediation technologists and practitioners. While there are
reports on the remediation of insecticide contaminated sites and their remediation, these are
few in comparison with the detailed studies on hydrocarbons and industrial chemicals.
Furthermore, much of the contamination associated with insecticides and other agricultural
chemicals is diffuse in its distribution, requiring remediation strategies to encompass wide
areas of land. Endosulfan is one such insecticide and several reports have described the dissi-
pation of the environmentally important insecticide, endosulfan, when applied to soil. There
are, however, few studies in the literature which have reported the soil degradation (and
specifically biodegradation) of the two major degradation products of endosulfan I and II,
endosulfan sulfate and endosulfan diol [1–9,14].
Endosulfan has been in use for many years in most countries for the control of insects in a
wide range of crops. Under common field conditions, endosulfan acts selectively on arthro-
pods as a contact and stomach poison. At higher temperatures, it may also act as a respiratory
poison via the gaseous phase. It is commonly used to control populations of chewing and suck-
ing insects, as well as mites of the families Eriophyidae and Tarsonemidae and a host of other
plant pests. One of the most important applications of endosulfan in Australian agriculture,
particularly in cotton growing, has been its use in the Insecticide Resistance Strategy (IRS). It
*Email: turlough.guerin@shell.com
International Journal of Environmental Studies
ISSN 0020-7233 print: ISSN 1029-0400 online © 2005 Taylor & Francis Group Ltd
http://www.tandf.co.uk/journals
DOI: 10.1080/0020723042000257745

2. 236 T. F. Guerin
Table 1. Common names and trade names of technical formulations of endosulfan
Benzoepin Malix
Beosit Niagara 5462
BIO 5462 OM 570
Chlorthiepin Phaser
Endocel SD 4314
Endosan Thimul
Endosulfan 35EC Thiodan
Endosulphan Thionex
Endotaff Thiotox
ENT 23979 Tiovel
Hidan Tionex
HOE 2671
was chosen because of its selective nature in control of insect pests. Endosulfan, though highly
toxic to moths and various mites that attack crop plants, does not adversely affect the survival
of insect parasite and predator populations to any significant extent. One benefit of the IRS in
Australia has been that its use has reduced substantially over the period of 1991 to 2001 with
application rates falling from 2.5 kg/ha to <1kg/ha [7].
Endosulfan is applied to crops in a number of different ways in various formulations. Some
of the names of the various technical formulations are presented in table 1. Endosulfan, which
is comprised of two isomers, endosulfan I (or α) and endosulfan II (or β), has five well char-
acterized degradation products. These are endosulfan sulfate, endosulfan diol, endosulfan
ether, endosulfan hydroxyether and endosulfan lactone [10]. These compounds are different
to the parent isomers in terms of their chemical, physical and physico-chemical properties.
The degradation products are considerably more water soluble than the parent isomers, with
the exception of endosulfan sulfate. Endosulfan is related to the classic cyclodienes, aldrin and
dieldrin, indicated by the presence of a hexachloronorbornene ring in its structure. This chlo-
rinated ring structure generally attributes to cyclodienes, low water solubility, high volatility
and recalcitrance in the environment. However, unlike the classic cyclodienes such as aldrin
and dieldrin, endosulfan is relatively labile or unstable in the environment. This characteristic
can be attributed to the presence of a cyclic sulfite diester group in its structure which makes
the molecule highly reactive. Specifically, this group imparts to the parent endosulfan
isomers, susceptibility to hydrolysis in water and in alkaline solutions and the possibility of
further breakdown in the environment, particularly through biological degradation [14].
Endosulfan, if not used with care and vigilance may cause environmental problems. Drift
of endosulfan to wooded areas occupied by wildlife, fish-bearing waters and other land areas
not intended for treatment are the main problems associated with the current use of endosul-
fan. There are other areas of concern with regard to endosulfan use, namely the problems of
run-off or wash-off by rain from treated areas. Further problems arise where applications are
made too often or in excess of the recommended amounts, or where they are made at the
wrong times of the growing season. Operator carelessness is one of the major problems asso-
ciated with the pollution of the environment with endosulfan and related chemicals. Table 2
shows how endosulfan sulfate concentrations in river water tends to increase throughout a
cotton growing season where endosulfan has been used. The endosulfan compounds in the
river have come from run-off from nearby cotton growing areas.
The parent isomers of endosulfan are toxic to a wide range of organisms [14]. There is,
however, only limited information in the literature regarding the toxicity of the endosulfan

3. Endosulfan metabolites 237
Table 2. Endosulfan and endosulfan sulfate in river water during cotton season
Sampling datea Endosulfan (µg/L) Endosulfan sulphate (µg/L)
15/10 ND 0.002
23/11 0.011 0.071
14/1 0.01 0.139
4/2 0.004 0.071
25/2 0.022 0.199
25/3 ND 0.047
23/4 ND 0.015
20/5 ND NDb
Notes: a Samples of water were taken from a river adjacent to a cotton crop (Otton 1991 cited
in Guerin 1993). b ND means concentrations <0.01 µ/L.
degradation products. From the data available, it appears that insects, birds and mammals are
much less sensitive to endosulfan than fish (table 3). It is apparent that the degradation prod-
ucts endosulfan diol, endosulfan ether, endosulfan hydroxyether and endosulfan lactone are
considerably less toxic than the parent compounds. From the limited number of toxicity stud-
ies conducted, it appears that the degradation products have considerably lower toxicities,
with one exception, endosulfan sulfate. Endosulfan sulfate is reported to be even more toxic
to mammals, than either of the parent isomers. However, with regard to fish, the sulfate and
parent isomers have a similar toxicity (table 3). Endosulfan sulfate, on the other hand, has a
similar toxicity to the parent compounds and is reported to be equally (or even more) toxic to
mammals, than either of the parent isomers (table 4). In this regard, it is considered to be a co-
contaminant of endosulfan and is often reported (as a residue) along with the parent isomers.
The aim of the current laboratory study was to determine the fate of endosulfan sulfate and
endosulfan diol in an unamended clay soil; representative of typical cotton growing soils in
Australia that contain these compounds. A further hypothesis tested was that endosulfan diol
and endosulfan sulfate could be mineralized to 14CO2. This work follows on from a related
study on the natural attenuation of the parent compound, endosulfan [11].
2. Materials and methods
2.1. Chemicals and soil preparation
[5a,9a -14C]-endosulfan sulfate (specific activity of 252.3 MBq/g) and [2,3-14C]-endosulfan
diol (specific activity of 1794.5 MBq/g) (Hoechst AG Melbourne and Frankfurt) was
Table 3. Acute toxicity of endosulfan and its degradation productsa
Toxicity LD50 (mg.kg−1)
Compound Insects Fish Birds Mammals
Endosulfan I 5.5 0.001–0.01 26–1000 9.4–40
Endosulfan II 9.0 0.001–0.01 26–1000 177
Endosulfan sulfate 9.5 0.001–0.01 – 8–76
Endosulfan diol >500 1–10 – >1500
Note: a Anonymous [7], Guerin [14] and references cited therein. Values for fish are LC50 for 24–96 tests.

4. 238
Table 4. The toxicological consequences of microbial cometabolism of endosulfana
Organism grouping Parent compound Toxicity (mg.kg−1) Degradation product Toxicity (mg.kg−1) Change in Toxicity
Mammalian Endosulfan I (II) 9.4 (177)b Endosulfan diol >1500 Decrease (from both isomers) 160 (8.5)b×
Endosulfan sulfate 76 Increase (from Endosulfan I) 8.1×
Decrease (from Endosulfan II) 2.3×
Fish Endosulfan I (II) 0.001 Endosulfan diol 0.1 Decrease (from both isomers) 100×
(0.001)b Endosulfan sulfate 0.001 No Decrease (from either isomers) 1×
T. F. Guerin
Notes: a Mammalian data based on acute oral toxicity (LD50) to rats. Toxicity to fish is LC50. b The bracketed values refer to endosulfan II. Endosulfan sulfate is 2.2–2.3 times more toxic than either of the
parent compounds.

5. Endosulfan metabolites 239
Figure 1. Summary degradation scheme for the endosulfan compounds under (a) low oxygen conditions and (b)
oxygenated conditions (asterix indicates position of 14C label).
dissolved in 100 % methanol (1000 mg/ml). Both compounds were radiolabeled in non-
chlorinated ring carbons (figure 1). The solution was added to subsamples of soil (1–2 g) to
give starting concentrations of 2 mg/kg (equivalent to 0.78 and 3.36 MBq/g soil, for endosul-
fan sulfate and endosulfan diol, respectively). The radiopurity of the compounds were 99%.
Chemical properties of endosulfan and the major metabolites are listed in table 5. The soil
was collected from a cotton farming area in northern New South Wales, Australia (table 6).
The soil were then prepared as previously described, using 250 ml glass jars [11]. One set of
sterile (autoclaved 3× and NaN3 added at 1% w/w) controls was used. At the beginning and
Figure 2. Extraction

6. 240 T. F. Guerin
Table 5. Liquid-phase physico-chemical properties of endosulfan and its major degradation products
Compound Solubility in water (S) ppmc Log Kow v.p. (Pa)a,c Hb
Endosulfan I 0.51 3.6 4 × 10−4 0.72
Endosulfan II 0.45 3.83 8.0 × 10−5 0.04
Endosulfan sulfate 0.48 3.66 3.7 × 10−5 0.03
Endosulfan diol 300 3.68 2.3 × 10−6 0.00013
Notes: a Vapor pressure in units of Pa. b Henry’s constant (H) = v.p./S in units of Pa.m3.mol−1, calculated from the v.p. & S data
reported in this table. c Values reported are from the PhysProp and DATALOG Databases from Syracuse Research Corporation where
available [22]. Values for endosulfan diol are from elsewhere [14, 23]
end of the trial, microbiological plate counts were conducted on soil extract agar, in both the
sterile and non-sterile treatments. The gravimetric moisture content (θg) of the soil in each of
the duplicate incubation vessels, was maintained at θg = 0.3 g/g during the course of the
experiment.
Figure 2. Extractiondegradation scheme for the endosulfan compounds under (a) low oxygen conditions and (b) oxygenated conditions (asterix indicates position of14C label).
1. Summary
2.2. Analytical procedures
Glass vials containing 1 ml of 2 M NaOH were placed on the soil surface inside the flasks.
These were removed and replaced with fresh solutions at each of the sampling times. The
amount of 14CO2 captured was determined by adding the alkali solution into a liquid scintilla-
tion (LS) fluid, trade name Hionic Fluor® (Canberra Packard, Australia) and counted in a LS
counter (LSC) as described in the following section. The detailed methodology for detecting
14
CO2 has been described elsewhere [12].
After any soil-bound 14CO2 was checked for using the method described [12], soil samples
(1–2 g) were placed into 250 ml ground glass sealed flasks and analysed as previously
described [11]. The soil was extracted on an orbital shaker at 160 rpm for 2–3 h with 40 ml
hexane:acetone (3:1). The extraction regime is illustrated in figure 1. The radioactivity in
each of the three phases (HAE, ME and WRE) (1–2 ml in 8–9 ml Packard 299®) was deter-
mined by counting the samples for 10 min using a United Technologies 4000 series liquid
scintillation counter. Background counts and counts from each treatment and control were
determined (recorded as counts per minute). The corresponding disintegrations per minute
(DPM) were then determined from the quench correction curve. Endosulfan degradation
products in the HAE, ME and WRE phases, were analysed using the methods previously
described [13,14]. Total heterotrophic populations were determined by a plating technique.
The solid growth medium was a soil extract yeast mannitol agar previously described [15].
Representative soil samples were taken at weeks 0 and 9 from the sterile and non-sterile
Table 6. Characteristics of the soils used in the studya
Fraction (%)
Soil sample Moistureb (g.g−1) OC(%) OM(%) Cc Sd FSe CSf
Cotton farming soil 0.09 1.09 1.91 62.4 20.4 14.3 2.9
a b c d
Notes: OC = organic carbon, OM = organic matter. Moisture content of the soils at the time of sampling. C = clay fraction. S =
sand fraction. e FS = fine sand fraction. f CS = coarse sand fraction.

7. Endosulfan metabolites 241
microcosms and serial dilutions in 0.9% saline were prepared from soil-water extracts. Plates
were incubated at 30°C for 3–5 days prior to counting.
3. Results and discussion
3.1. Dissipation of endosulfan sulfate and endosulfan diol from soil
The radioactivity in the HAE phase, which contained the majority of the original radiolabel,
decreased in soils treated with both endosulfan sulfate and endosulfan diol (figure 3(a) and
3(b)). This decrease in the radioactivity in the HAE phase was significantly faster (p < 0.05)
14
Figure 3. Decrease in radioactivity in the HAE from cotton farming soil with applied (A) C-endosulfan sulfate
and (B) 14C-endosulfan diol.

8. 242 T. F. Guerin
in the non-sterile treatments, indicating that there was a relatively important contribution to
the dissipation by the indigenous soil microflora. After 9 weeks of incubation in the nonster-
ile treatments, these losses were 50 and 38% of the originally applied endosulfan sulfate and
endosulfan diol, respectively. The corresponding values in the sterilized treatments were
approximately 5 and 10%, respectively. The relatively large differences between the steril-
ized and non-sterilized treatments suggested that endosulfan sulfate and endosulfan diol were
largely biodegraded under these conditions.
Figure 3. Decrease in radioactivity in the HAE from cotton farming soil with applied (A) 14C-endosulfan sulfate and (B) 14C-endosulfan diol.
In the endosulfan sulfate treated soil, the ME radioactivity remained constant in the steril-
ized treatments (figure 4(a)). In the non-sterile soil, there was a substantial decrease in the
14
Figure 4. Changes in the distribution of radioactivity in the ME from cotton farming soil with applied (A) C-
endosulfan sulfate and (B) 14C-endosulfan diol.

9. Endosulfan metabolites 243
Table 7. Endosulfan degradation product mass balance study in cotton farming soil
Sterilized Non-sterilized
Treatment Extract/Phase 0 weeks 9 weeks 0 weeks 9 weeks
Endosulfan diol a HAE 72.15 62.64 70.89 44.20
ME 5.06 6.33 4.43 15.83
WRE 1.52 2.49 1.14 3.88
14
CO2 0 1.67 0 6.56
Unrecoveredc 21.27 26.87 23.54 29.54
Endosulfan sulfate b HAE 44.4 37.2 47.22 23.8
ME 22.2 22.2 21.1 13.3
WRE 0.22 2.0 0.11 0.5
14
CO2 0 0.12 0 0.32
Unrecoveredc 33.2 38.4 31.5 62.0
Notes: a The amount of radioactivity added to the soil as endosulfan diol was 3.36 MBq/g soil (w/w) (dry weight); standard errors (%)
between replicates were 3.5, 14.6 and 13.2 for HAE, ME and WRE, respectively. b The amount of radioactivity added to the soil as
endosulfan sulfate was 0.78 MBq/g soil (w/w) (dry weight); the average standard errors (%) between replicates were 14.3, 8.5 and 33
for HAE, ME and WRE, respectively. c The unrecovered fraction was calculated by difference.
radioactivity, suggesting that microorganisms had contributed to the decrease. However, in
the endosulfan diol treated soil, the ME radioactivity under non-sterile conditions increased
during the course of the trial. This suggested a microbiological contribution to the formation
of methanol-soluble forms of endosulfan diol (figure 4(b)).
Figure 4. Changes in the distribution of radioactivity in the ME from cotton farming soil with applied (A) 14C-endosulfan sulfate and (B) 14C-endosulfan diol.
The radioactivity in the WRE phase from the soils treated with endosulfan sulfate, did not
increase in either the sterilized or non-sterilized treatments (table 7). However, in the
endosulfan diol treated soil, the radioactivity in the WRE phase increased significantly in the
microbiologically active treatments (p < 0.05).
In the treatments containing endosulfan diol in non-sterile soil, ME phase assays indicated
that microorganisms increased the extent of conversion of endosulfan diol to less hydropho-
bic forms. Endosulfan sulfate, conversely, did not undergo further transformation to metha-
nol and water soluble forms in either the sterile or non-sterile treatments.
Large amounts (62%) of endosulfan sulfate were converted to forms which were not
recoverable in the current study. This was particularly observed in the non-sterile treatments.
This suggested that this compound was adsorbed or incorporated into the soil matrix over the
trial period (table 7). Further analysis of the ME and WRE revealed that 40–50% of the orig-
inally applied radioactivity from endosulfan sulfate and endosulfan diol was converted to
unidentified forms, and some limited evidence was obtained that these compounds were
acidic [14]. In the endosulfan sulfate treated soil, a small proportion of the endosulfan sulfate
(<10%) was converted to endosulfan diol over the 9 week period. From the studies recently
reviewed [7], the extractability of endosulfan compounds are shown to typically decline
during incubation in non-sterile soils, indicating that endosulfan and its residues become
increasingly bound to the soil with time. It is recommended that any further work in this area
should address the identity of these unrecovered forms of endosulfan, including the nature of
their interaction with the soil.
Previous laboratory and field studies have reported the formation (and subsequent dissipa-
tion) of endosulfan sulfate, in soil and water [7]. Endosulfan sulfate is typically found in soils
and sediments with the parent isomers, typically 10–50 days after application of the parent

10. 244 T. F. Guerin
isomers to soil or a crop. Endosulfan diol, on the other hand, is not always reported in routine
studies of endosulfan in soil. This is probably due to the relative difficulty in extracting and
analysing this compound compared with the parent isomers or endosulfan sulfate [14]. In
previous studies there has been only limited evidence that endosulfan sulfate and diol are
further metabolized to CO2 [7]. Martens [19] has demonstrated that 18% of soil applied
endosulfan diol can be converted to 14CO2 over a period of a year.
Studies on the specific contribution of microorganisms to the degradation of endosulfan
are limited. Miles and Moy [16] have reported that endosulfan sulfate and endosulfan diol
had half-lives of 14 and 11 weeks respectively, in a mixed liquid culture of soil micro-
organisms isolated from a sandy loam. In the same study, endosulfan hydroxyether, endosul-
fan ether and endosulfan lactone demonstrated half-lives of 8 weeks, 6 weeks, and 5.5 h
respectively. Katayama and Matsumura [17] claim that the common soil fungus, Tricho-
derma harzianum, can degrade the parent isomers, as well as endosulfan sulfate and endosul-
fan diol, also in liquid culture. These researchers have provided evidence that both parent
isomers are first converted to endosulfan sulfate and then subsequently to endosulfan diol
under aerobic conditions. A recent study has clearly demonstrated that the parent isomers of
endosulfan can be biodegraded under conditions of low oxygen such as in water logged soils
or sediments [18].
3.2. Mineralization of endosulfan diol and endosulfan sulfate
Mineralization was highest in the nonsterilized soil treated with endosulfan diol. There were
significant differences (p < 0.05) observed in 14CO2 release between the sterilized and non-
sterilized treatments when the radioactive degradation products of endosulfan were added to
the soil (figure 5). Although the absolute amounts of 14CO2 released from the endosulfan
sulfate treated soils were low, the rate of mineralization was approximately twice that found
in the biologically active (non-sterile) treatment compared with the sterile treatment. In this
biologically active treatment, 6.5% of the 14C-endosulfan diol applied, was converted to
14
CO2 over the trial period. These results demonstrated that endosulfan diol and endosulfan
sulfate can be mineralized, at least to an extent, by indigenous soil micro-organisms.
Figure 5. Release of radioactive carbon dioxide from the cotton farming soil with applied (A) 14C-endosulfan sulfate and (B) 14C-endosulfan diol.
Studies have been conducted on endosulfan diol, also radiolabelled in the ring at the two
non-chlorinated positions, with the aim of determining the mineralization rate of the bicyclic
ring. Endosulfan diol was added to a loamy soil (pH 5.3) at a level of 1 mg/kg and incubated
at 22°C for a year, with evolved 14CO2 monitored on a fortnightly basis. The main degrada-
tion products found in the soil were endosulfan lactone and a polar unknown. Small quanti-
ties of the hydroxyether and ether were also detected. Evolution of 14CO2 reached
approximately 18% over the year. At the end of the study, the soil contained ∼40% each of
extractable and residual radioactivity [19].
3.3. Mechanisms for dissipation of endosulfan sulfate and endosulfan diol
Although there are reports describing the formation of endosulfan diol and endosulfan sulfate
from the degradation of the parent endosulfan compounds in soils and water, there have been
very few studies that describe the subsequent fate of these compounds. It is now well estab-
lished that in soils treated with technical grade endosulfan, there is a formation and subse-
quent (and often gradual) disappearance of the degradation product endosulfan sulfate [14]
and references cited therein. The mechanisms for dissipation of endosulfan diol and endosul-
fan sulfate in soil are, however, largely unknown, though micro-organisms are likely to play

11. Endosulfan metabolites 245
14
Figure 5. Release of radioactive carbon dioxide from the cotton farming soil with applied (A) C-endosulfan
sulfate and (B) 14C-endosulfan diol.
a role. Losses of endosulfan sulfate due to leaching from a soil profile are unlikely because of
its reported high binding affinity to soil and sediment particles [20,21] and from leaching
studies conducted on the parent compounds [7]. Volatilization is unlikely to be a route due to
the increased water solubility of these compounds (table 4) compared with the parent
compounds (although there is no readily available data on the volatility of either degradation
product). There is no direct evidence in the literature to indicate that endosulfan sulfate is
degraded either biologically or chemically. It is possible, however, that endosulfan sulfate
may undergo hydrolysis, under certain environmental conditions, to form endosulfan diol
(Anonymous 1998). Katayama and Matsumura [17] have proposed that the common soil
fungus, T. harzianum, can hydrolyse endosulfan sulfate to endosulfan diol under laboratory

12. 246 T. F. Guerin
conditions in liquid culture, but there is relatively little known about this reaction. One other
study, using mixed aerobic cultures from an agricultural soil, has suggested that endosulfan
may be completely dissipated to forms that are not detected by ECD gas chromatography
[16]. This does not mean that the parent isomers of endosulfan are readily degraded to the
diol and sulfate forms since this study did not control nonbiological losses.
Endosulfan diol does not contain the cyclic sulfite structure and therefore is not as readily
subject to hydrolysis as are the parent isomers. Endosulfan diol is also more mobile in soil
than endosulfan sulfate. Endosulfan sulfate, which contains a cyclic sulfate structure, has
been reported to be significantly more recalcitrant than the parent isomers [7,10,16] predom-
inantly due to it being less susceptible to hydrolysis. This attribute may account for its slower
rate of loss under sterile conditions, compared to endosulfan I, in the current trial.
The slower rates of loss of endosulfan sulfate and endosulfan diol under sterile conditions
reflects their low volatility and very low susceptibility to alkaline hydrolysis. This is unlike
the faster rates of degradation due to chemical losses and volatilization previously observed
with the principal parent isomer, endosulfan I [7].
3.4. Monitoring of microbial populations
At the beginning of the trial, in the nonsterile treatments, the number of heterotrophic micro-
organisms (per gram of oven dry soil) were ∼108. This value did not significantly change
during the course of the incubations. In the sterile treatments, the number of heterotrophs was
<102 (per gram of oven dry soil). This also was the case at both the beginning and end of the
trial period, indicating that microbial numbers were kept suppressed in the sterile treatments.
4. Conclusion
It is apparent that both chemical and biological mechanisms cause the further dissipation of
endosulfan sulfate and diol in soil. This study has demonstrated that microbial processes are
responsible for their degradation. Biodegradation is therefore put forward as the main mecha-
nism for the dissipation of these major degradation products in the soil studied. Analyses on
the HAE, ME and WRE indicate that water soluble degradation products are formed and the
extent of this formation varied between 10–30% of the originally applied endosulfan diol and
endosulfan sulfate. Further research would be required to identify the nature of the biodegra-
dation products of both endosulfan sulfate and endosulfan diol.
There were marked differences between rates of dissipation of endosulfan sulfate and
endosulfan diol in sterilized and non-sterilized cotton farming soil (table 8). These results
demonstrated that micro-organisms contribute to loss of these degradation products in the
soil studied. This result suggests that both of these degradation products are likely to be
further biodegraded in the soil profile, after they are formed from the parent compounds.
Based on the findings here, the degradation products of endosulfan will have half-lives of
60–85 days in biologically active soils, whereas in sterile soils, these values increase to
240–260 days.
Endosulfan diol underwent substantial mineralization. Endosulfan sulfate also released
14
CO2, but at lower rates when compared with endosulfan diol.
The major degradation products of the chlorinated insecticide endosulfan, endosulfan diol
and endosulfan sulfate, can therefore be further degraded in laboratory scale studies, in soils
in which they commonly occur. This indicates that these compounds are unlikely to be the

13. Endosulfan metabolites 247
Table 8. The estimated half lives of endosulfan diol and endosulfan sulfate in the cotton farming soil a
Half life (days)
Compound Treatment Current study Other studies
Endosulfan diol Sterile 255 –b
Non-sterile 83 –
Endosulfan sulfate Sterile 240 –
Non-sterile 60 100–150c
Notes: a Determined from the decrease in radioactivity in the HAE, using an exponential decay equation. b Not Reported. c NRA
(1998) [7].
ultimate degradation products of endosulfan, and that they are intermediates only in the over-
all degradation of endosulfan in soil.
References
[1] Stewart, D.K.R. and Cairns, K.G., 1974, Endosulfan persistence in soil and uptake by potato tubers. Journal of
Agricultural and Food Chemistry 22(6), 984–986.
[2] Doelman, P., Loonen, H. and Vos, A., 1988, Ecotoxicologisch onderzoek in met endosulfan verontreinigde
grond: toxiciteit en sanering (RIN [88/39]) (unpublished report).
[3] Doelman, P., 1990, Microbial degradation of hexachlorocyclohexane isomers in mineral soil and of endosulfan
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