The behavior, transport and fate of an organic chemical in environment are controlled by the properties of the chemical and the environmental conditions. The structure of the organic chemical determines its physical, chemical and biological properties.
The environmental processes that control an organic chemical’s behavior and fate of the pesticides can be classified into three types in figure 5 according to Cheng (1990):
o transformation processes which change its chemical structure
o transport processes which move it away from its initial point of introduction to the environment and throughout the surface water system
o retention
2.2.1 Transformation processes
The transformation of a pesticide results in changes in its chemical structure. Producing one or more new chemicals would lead to the disappearance of the original pesticide.
These new chemicals can be organic or inorganic molecules and ions.
Microbial breakdown is the breakdown of chemicals by microorganisms such as fungi and bacteria. Biodegradation or microbial breakdown is the only transformation processes able to completely mineralize the pesticide (Alexander, 1981). Microbial breakdown tends to increase when: temperature is high, soil pH is favorable, soil moisture and oxygen are adequate and soil fertility is good.
Chemical breakdown is the breakdown of pesticides by chemical reactions in the soil and can cause structural changes in an organic chemical. The rate of change and type of chemical reaction that occur are influenced by the binding of pesticides to the soil, soil temperatures, pH levels and moisture.
Photodegradation is the breakdown of pesticides by sunlight. All pesticides are susceptible to photodegradation to some extent. The rate of breakdown is influenced by the intensity and spectrum of sunlight, length to exposure and the properties of the pesticides. Immediately after application, pesticides will undergo biotic and abiotic processes that lead to their, more or less complete degradation. The major physical transformation is the photodecomposition by ultraviolet radiation. The majority of the pesticides have the maximum absorption between 200 and 400 nm.
2.2.2 Transport processes
In fact the environmental behavior of a pesticide is affected by the natural affinity of the chemical of one of four environmental compartments (Calamari and Barg, 1993): solid matter (mineral matter and particulate organic carbon), liquid (solubility in surface and soil water), gaseous form (volatilization), and biota. This behavior is often referred to as
«Partitioning» and involves, respectively, the determination of the soil sorption coefficient (KOC), solubility (SW), Henry’s Constant (H), and the n-octanol/water partition coefficient (KOW). These parameters are well known and enable us to predict the environmental fate of the pesticide.
Pesticides, like other chemicals can be affected just as they can affect the environment depending on the type of pesticide and the conditions of the environment. Generally, the changes can be classified as physical and non-physical reactions that determine the fate of pesticide. Usually pesticides are transferred from one environment to another without any change if the factors causing the movement are physical, such as drift, volatilization, …. . However, through non physical processes such as photochemical, microbial, and chemical and metabolism, pesticides will be degraded.
2.2.3 Retention
The term «retention» is most frequently equal with adsorption or simply sorption. And the term adsorption reffered to attraction of a chemical on the surface for a longer or shorter period of time depending on the affinity of the chemical to the surface. As an example, Hamalker and Thompson, (1972) have paied especial attention to the retention processes on pesticides in soil system.
Thus, a set of such physical and biochemical processes is determined the final environment fate of a pesticide.
Figure 5- Behavior and fate of a pesticide in the environment.
Processes
Pesticide Input Volatilization, Sorption Transfer Transformation Microbial, chemical and photo degradation
Retention
Air Water
Soil Biota Pesticide output
Processes
Pesticide Input Volatilization, Sorption Transfer Transformation Microbial, chemical and photo degradation
Retention
Air Water
Soil Biota Pesticide output
Table III- Transfer and degradation processes controlling the movement and fate of organic chemicals like pesticide.
(Source: Marathon-Agricultural and environmental consulting, Inc.1992. Video cassettes- Fate of pesticides in the Environment, box 6969, Las Cruces, NM 88006).
Vryzas et al. (2007) and Arias-Estévez et al. (2008), have reported that the mobility of pesticides in soil, and hence their bioavailability and transfer to other environmental compartments, depend on a variety of complex dynamic of physicochemical and biological processes, including sorption-desorption, volatilization, chemical and biological degradation, uptake by plants, runoff and leaching. However, catchment’s
Process Consequence Factors
Transfer (processes that relocate organic chemicals without altering their structure Physical drift Movement of organic chemical due
to wind action
Wind speed, size of droplet
Volatilization Loss of organic chemical due to evaporation from soil, plant or aquatic ecosystems
Vapour pressure, wind speed, temperature
Adsorption Removal of organic chemical by interacting with plant , soils, and sediment
Clay and organic matter content, clay type, moisture
Absorption Uptake of organic chemical by
plant roots or animal ingestion Cell membrane transport, contact time
Leaching Translocation of organic chemical either laterally of downward through soils.
Water content, macropores, soil texture, clay and organic matter Erosion Movement of organic chemical by
water or wind action Rainfall, wind speed, size of clay and organic matter particles with adsorbed organic chemicals
Degradation (processes that alter the organic chemical structure) Photochemical Breakdown of organic chemicals
due to the absorption of sunlight Structure of organic chemical intensity and duration of sunlight exposure
Microbial Degradation of organic chemicals
by microorganisms Environmental factors (pH, moisture, temperature) nutrient status
Chemical Alteration of organic chemical by chemical processes such as hydrolysis, and redox reaction
High and low pH, same factors as for microbial degradation
Metabolism Chemical transformation of organic chemical after being absorbed by plants or animals
Ability to be absorbed, organism metabolism, interactions within the organism
variables influencing runoff include the gradient of the land on which pesticides have been sprayed, crop type, organic carbon in the soil, the size of the cropped area and the vegetation type and density of buffer strips that lie between agricultural land and water body. It is also dependent on the application and physicochemical properties (such as solubility) of the pesticides.
Himel et al. (1990) pointed out the essential characteristics to study the fate and transport of pesticides in environment are:
o in physical level
o saturation vapor pressure
o solubility in water (in mg.L-1 in special temperature ) o distribution coefficient (kd)
o in chemical level
o ionic states (cationic, anionic, basic and acidic) o hydrophilic and hydrophobic characters
o chemical, photochemical and biological reactivity
2.3 Most important factors playing a major role in the fate of pesticides in