Environmental Fate and Transport of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 31

Photodegradation

Volatilization

Precipitation

Evapotranspiration

Up take by plants

Evaporation p

Run off

Biodegradation

Infiltration

Abiotic degradation

Absorption
Adsorption

Hydrolysis

Vadose zone

Leaching through
soil profile

Groundwater

Lithosphere

FIGURE 2.1
Overview of the fate and transport pathways of pesticides in the environment.

field, available for pesticidal activity or other movement (Aharonson et al. 1987). Although abi-
otic degradation plays a role in many cases, biodegradation of pesticides by microorganisms
is usually the most important and dominant transformation process (Torstensson 1987, 1990).

2.3.1.1 Abiotic Degradation

In certain conditions, the abiotic pathway for pesticides can be the key mechanism of
degradation. For example, under low microbial activity conditions, such as deep soils or
deep subsurface conditions, biodegradation is very limited (Lovley and Chapelle 1995),
and abiotic transformation may become the dominant pathway. Hydrolysis, oxidation,
and photolysis are the major abiotic transformation or degradation processes (Wolfe et al.
1990). Some studies list photolysis and hydrolysis as two top pathways of pesticide abiotic
degradation.
Oxidation : Oxidation is a reaction by which pesticides are oxidized. Oxidation of pes-
ticides is affected by various environmental factors, including the amount of oxygen,
metal ion concentration, natural organic matter content, and pH of the media. In the upper
vadose zone, oxidation takes place primarily due to the abundance of oxygen (Kookana
et al. 1998), whereas it becomes negligible once the subsurface depth gets further. Presence
of metal ions can also catalyze the oxidation reactions. Nowack and Stone (2000, 2002)
described numerous instances where phosphonate pesticide molecules chelate metal ions
and are subsequently degraded in aqueous solution. Similarly, studies have shown that
mineral phases of manganese (IV) (e.g., manganite, birnessite) may be involved in the deg-
radation of xenobiotics, including chlorophenols and triclosan (Barrett and McBride 2005).