Page 181 - Soil Degradation, Conservation and Remediation

170 6 Soil Pollution

Fig. 6.14 Transformations of applied pesticides
at pH values to their pKa value. Acidic and anionic pesticides, such as phenoxyacetic
acids (2,4-D and 2,4,5-T) and esters, asulam and dicamba, can interact with
soil organic matter by H-bonding at pH values below their pKa in nonionized forms
through their −COOH, −COOR, and identical groups (Senesi et al. 1984 ).
Chemical reactions between pesticides and their metabolites often lead to the
formation of strong bonds (chemisorption), which increase in the persistence of
the residues in the soil, but decrease their bioavailability and toxicity (Dec and
Bollag 1997 ). It also reduces leaching and transport properties. The nature of the
binding forces involved and the types of mechanisms operating in the adsorption
processes of pesticides onto the soil include ionic, hydrogen, and covalent bonding;
charge transfer or electron donor–acceptor mechanisms; van der Walls forces; ligand
exchange; and hydrophobic bonding or partitioning (Gevao et al. 2000 ).

6.2.7 Atmospheric Deposition

Atmospheric deposition refers to substances that are deposited on land from the air.
It includes gas, smoke, ashes, and particulates and deposited to soil. The oxides

of sulfur and nitrogen (SO x , NO x ), chlorides, fluorides, ammonium, etc. are the principal
gas component of aerosols. Aerosols contain dust, smoke, ashes, sea salt, water droplets,
and some organic materials. Volcanic eruption and burning of biomass and fossil
fuel produce aerosol that contains sulfate compounds. The occurrence of aerosols is
the highest in the northern hemisphere where industrial activity is concentrated.
Suspended dust particles in air are carried to hundreds of kilometers by the wind
and settle on soil. Atmospheric deposition may contain persistent organic pollutants
as PCBs, and PAHs and heavy metals.
Atmospheric deposition occurs through three different processes: (1) wet deposition,
the deposition of material with precipitation; (2) dry deposition, the direct
deposition of atmospheric particles and gas to vegetation, soils, and surface water;
and (3) cloud deposition, the deposition of nonprecipitating droplets of clouds and
fogs to terrestrial surfaces (Fowler 1980 ; Lovett and Kinsman 1990 ). Wet and dry