Environmental Fate and Transport of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 34

where H (M) is the half-life at moisture M (days), A is the half-life at a moisture content of
1 kg H 2 O 100/kg soil dry matter (days), M is the moisture content (kg H 2 O 100/kg soil), and
B is a coefficient (unitless) (Beulke et al. 2000).

2.3.2 Sorption and Desorption

Adsorption is a process by which an aqueous molecule is attracted and retained onto
the surface of a solid. Although adsorption can be either a chemical (e.g., electrostatic
interaction) or a physical (e.g., van der Waals forces) process, adsorption of pesticide
molecules on soil usually takes place as a result of coulombic attraction between the
positively charged pesticide molecules and the negatively charged soil particles or
organic matter.
Distribution coefficient (K d ) is an important parameter used to quantify the adsorption
of pesticide molecules to soils. It is defined as the ratio of the sorbed phase concentration
to the solution phase concentration at equilibrium (Equation 2.7).

K = C C (2.7)

d a d

where K d is the distribution coefficient of a pesticide molecule between soil and water
(V/M); C a is the amount of pesticide adsorbed per unit of adsorbent mass (M/M); and C d is
the concentration of pesticide dissolved (M/V). K d is directly related to the K oc value of the
pesticide and the organic matter (OM) and clay content of the soil.
A number of methods have been proposed to determine the distribution coefficient
(Karickhoff and Brown 1978; Veith et al. 1979). Karickhoff et al. (1979) corroborated a lin-
ear correlation between the distribution coefficient and the soil's organic carbon content
(Equation 2.8).

(2.8)

K = K OC ⋅ 100

d oc

where K oc is soil organic partition coefficient and OC is the organic carbon content (%).
Several other studies have also shown that the values of K d are directly related to the
concentration of organic matter in soil (O'Connor and Connolly 1980; Voice et al. 1983;
Gschwend and Wu 1985) and pesticide chemical structure (Lohninger 1994).
There are a variety of factors affecting the adsorption process, including physicochemi-
cal properties of soils and the pesticide molecules, such as soil pH, surface charge, surface
area and size of the soil particles, chemical functions, solubility, polarity, and octanol-
water partition coefficient (K ow ) of the pesticide molecules (Senesi 1992; Pignatello and
Xing 1996). Soil pH affects not only the solubility and reactivity of the pesticide molecules
but also soil ecological functions such as microbial activities. For different pesticides, the
effects of pH on sorption are different. For example, Andrade et al. (2010) concluded that
soils with low organic matter content and/or high pH showed less ametryn sorption rates.
Using a model, Lohninger (1994) summarized that adsorption increases with pH and
organic matter content but decreases with ionic strength.
Adsorption plays an important role in determining the fate and transport of a pesti-
cide as it reduces the pesticide's bioavailability and mobility and, consequently, its envi-
ronmental and health impacts. One of the most widely used experimental techniques for
evaluating the adsorption potential of a pesticide by soil is to determine its adsorption
isotherm. Adsorption isotherm is a graph between the mass of sorbate per unit dry mass
of sorbent (S) and the concentration (C) of the sorbate.