Page 213 - Partition & Adsorption of Organic Contaminants in Environmental Systems

204 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

800

40°C Dieldrin only
Dieldrin Vapor Density (ng/L) 400
600

200
30°C
Dieldrin only

0
0 40 80 120 160 200
Soil Water Content (g/kg)
Figure 7.42 Influence of soil water content on the vapor density of dieldrin applied
to Gila silt loam at 100mg of dieldrin per kilogram of soil. [Data from Spencer et al.
(1969). Reproduced with permission.]

vapor concentrations, which became equal to the saturation vapor concentra-
tions of the pure compounds and stayed unchanged with water content up to
the soil’s field capacity (17%). Results with dieldrin at 30 and 40°C are shown
in Figure 7.42.
The fact that applied dieldrin on Gila soil at 100mg/kg soil and lindane at
50mg/kg soil show subsaturation vapor concentrations when the soil-water
content is low but display saturation vapor concentrations when the soil is wet
is illustrative of the distinct sorption mechanisms of soil minerals and organic
matter. The low vapor concentrations with the relatively dry soil are ascribed
to strong mineral adsorption,which overrides the effect of partition with SOM.
Upon wetting, water displaces most pesticide from soil minerals by adsorptive
competition, and, as a result the amount of pesticide in soil becomes more than
enough to saturate the SOM, and consequently, the vapor concentrations
become saturated. At water saturation, one notes that 100mg of dieldrin per
kilogram of Gila soil (with f om = 0.006) corresponds to 17g of dieldrin per kilo-
gram of SOM, and this loading is far greater than the solubility of solid diel-
drin in SOM at 25°C; that is, S om = 1.5g of dieldrin per kilogram of SOM,
calculated by Eq. (7.10) with S w = 0.20mg/L (Weil et al., 1974) and K om = 7400
(Briggs, 1981). Similarly, at water saturation, a loading of 50mg of lindane per
kilogram of Gila soil corresponds to 8.3g of lindane per kilogram of SOM,
which is more than the calculated S om = 3.0g of lindane per kilogram of SOM
for solid lindane at 25°C by Eq. (7.10) with S w = 7.8mg/L (Weil et al., 1974)