Environmental Fate and Transport of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 33

mechanisms by which pesticides can be photodegraded in soil and water (Zeep and Cline
1977; Dureja and Chattopadhyay 1995; Romero et al. 1995; Cheng and Hwang 1996; Pirisi
et al. 1996; Conceiçao et al. 2000; Konstantinou et al. 2001; Frank et al. 2002; Graebing et al.
2003). The occurrence and intensity of the process depend on the depth of pesticide loca-
tion, presence of catalysts, exposure to radiation, soil pH and aeration, and physicochemi-
cal properties of the pesticide molecule. Hebert and Miller (1990) demonstrated that the
vertical depth of direct photolysis is restricted to the top 0.2-0.3 mm in the soil, whereas
indirect photolysis could go as far as 0.7 mm. According to Konstantinou et al. (2001), the
presence of both humic acids and metal oxides could accelerate the pesticide photolysis
because humic acids act as photosensitizers by generating reactive oxidative species such
as singlet oxygen ( 1 O 2 ), hydroxyl radicals (⋅OH), hydrogen peroxide (H 2 O 2 ), and peroxy
radicals (ROO⋅), and metal oxides such as ZnO, Fe 2 O 3 , and MnO are able to absorb solar
radiation and trigger the photochemical reactions. Readers are referred to Burrows et al.
(2002) for a more detailed review on the photoreactivity of pesticides.

2.3.1.2 Biodegradation (Biotic Degradation)

The biodegradation process is usually characterized by half-life (T 1/2 ), the time for half of
the initial amount of a pesticide to be biodegraded. Theoretically, pesticide degradation
follows exponential decline and can be described as (Beulke et al. 2000) (Equation 2.2)

C = C exp kt ( − ) (2.2)

( ) 0

where C (t) is the pesticide concentration at time t (mg/kg soil), C 0 is the concentration at
time 0 (mg/kg soil), k is the degradation rate (per day), and t is the time (days). Therefore,
in Equation 2.3, T 1/2 is written as

T = . 693 k (2.3)

1 2

In real systems, however, the half-life of a pesticide has also been shown to be dependent
on temperature, soil and water content, and soil organic carbon content (Walker 1974).
Effects of temperature on the pesticide degradation can be described using the Arrhenius
equation (Beulke et al. 2000) (Equation 2.4):

( )

H

( ) =

A exp E RT (2.4)

1 a

T

and (Equation 2.5)

( ) (

)

H = H exp E T T − RT T (2.5)

) )

( (

a 1 2 1 2

T2 T1

where H (T) is the half-life at temperature T (days), T is the temperature (K), A 1 is a coeffi-
cient (days), E a is the activation energy (J/mol), R is the gas constant (8.314 J/mol K), H(T1) (T1) is
the half-life at temperature T 1 (days) and H (T2) is the half-life at temperature T 2 (days).
The effects of soil moisture can be quantified in Equation 2.6:

− B

H = AM (2.6)

( ) M