Degradation of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 74

hydrolysis. Microbial degradation of benomyl, carbendazim, and carbofuran involves
hydroxylation at positions 4 and 5.
3.4.5 Pyrethroid Pesticides
The microbial degradation of pyrethroid pesticides is very slow. Degradation of pyre-
throids is brought about by a system of esterase and oxidase enzymes. Ester hydrolyzes the
ester linkage between the acid and the alcohol moieties. Hydroxylation occurs at 2′ or 5′ or
6′ position of the alcohol moiety. The aerobic degradation is faster than the anaerobic one.
3.4.6 Miscellaneous
Simazine : Simazine is not degraded easily by microbes. Strong et al. (2002) found that
Arthrobacter aurescens strain is capable of degrading s-triazines, including simazine. The
simazine during degradation undergoes dehalogenation, dealkylation, and hydroxylation
without cleavage of triazine ring. Kodama et al. (2001) found that Penicillium steckii , a fun-
gal strain, can also degrade simazine.
Pyriproxyfen : Pyriproxyfen undergoes degradation via biological catalysis and serves as
a carbon source for soil microorganisms. Hydroxylation followed by degradation to phe-
nol occurs during microbial decay.
Phenylurea : These pesticides undergo stepwise microbial degradation by demethylation,
demethoxylation, deamination, and decarboxylation and form phenol in the presence of
Pseudomonas striata , Fusarium solani , and Penicillium sp.
Butachlor : This chloroacetanilide on microbial degradation by Fusarium solani and F.
oxysporum undergoes dechlorination, hydroxylation, dehydrogenation, C-dealkylation,
N-dealkylation, and debutoxymethylation to form a number of products.
Phenylethyl propionate : Phenylethyl propionate undergoes microbial biotransformation to
form 2-phenyl ethanol (Hu and Coats 2008).
3.5 Pesticide Degradation and Environmental Pollution
Pesticides have been extensively applied to the agricultural activities as well as environmental
sanitation implementation due to their chemical characteristics of toxicity, bioaccumulation, and
persistence. The residual pesticides have become the contamination sources and pose a serious
threat to the soil and groundwater. This contamination becomes more serious when persistent
pesticides are involved. The abundant use of less-persistent pesticides such as organophosphate
and carbamate pesticides and their residuals in the environment have also posed a great threat
to the environment. Some studies also have reported a relationship between the amounts of
pesticides used and their concentration in groundwater and surface water.
A number of researchers have reported that surface water and groundwater all over the
world are contaminated by pesticides and their metabolites. Though the use of DDT and
benzene hexachloride (BHC) has been terminated globally in the last decade, still a number
of groundwater and surface water samples contain DDT, dichlorodiphenyldichloroethylene
(DDE), and dichlorodiphenyldichloroethane (DDD) in addition to benzene hexachloride ben-
zene hexachloride (BHC) above permissible limit, particularly in developing countries. The
pesticides and their metabolites from domestic, industrial, and agricultural effluents enter the
food chain through groundwater or surface water. A number of studies have reported that
the concentration of pesticides and their metabolites in vegetables is above permissible limits.