Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 729

where f oc is the fractional organic matter content of the soil and C w and C solid phase
are the concentrations of the contaminant in the pore water and soil solid phase,
respectively.
Based on additional work of Sablji´cetal.( 1995 ), 19 quantitative structure-
activity relationships (QSARs) for a variety of contaminant classes are listed in
the EU Technical Guidance Document (TGD - European Commission 2003 )for
Risk Assessment of chemicals in the form of Eq. ( 16.2 ) (Table 16.2 ). Low polarity
organic contaminants, however, may, on top of equilibrium partitioning, also bind
to solid phases through adsorption mechanisms which result in greater (non-linear)
binding coefficients. Thermally or diagenetically altered forms of carbonaceous
materials such as coal, kerogen from shales, soot, and charcoal, have a particularly
high binding affinity and nonlinear adsorption behaviour, with carbon-normalized
Freundlich sorption coefficients that are as much as 50-250 times higher than
typically reported K oc values (Binger et al. 1999 ; Bucheli and Gustaffson 2000 ;
Grathwohl 1990 ).
Hydrophobic partitioning is less important for polar and ionisable contaminants.
These contaminants are involved in more diverse binding mechanisms that con-
tribute to contaminant retention, including ion bonding or ligand exchange, covalent
binding to the soil molecular structure, ion-dipole and dipole-dipole interactions,
charge transfer, hydrogen bonding and hydrophobic bonding (Van der Waals forces;
Von Oepen et al. 1991 ). The chemically most active component of the soil is
the colloidal fraction which consists of organic matter and inorganic clay miner-
als (Stevenson 1994 ). Both components display a negative electrical charge at the
surface, resulting in weak binding forces and typically reversible sorption, as sorp-
tion often is restricted to a limited number of binding sites at the surface layer.
The effect of the negative electrical charge at the surface can be measured by
the cationic exchange capacity, which on average is 50 meq/100 g for clays and
290 meq/100 g for humic acids (Krogh 2000 ). Electrical forces involving charge
transfer ( 4kJ/mol)(Von

Oepen et al. 1991 ) so that they dominate when present. Thus, a different degree
of sorption of anions, cations and neutral molecules can be expected, with cations
showing the highest potential for sorption, due to electrical attraction. Although
lipophilic interactions are weaker than the other interactions mentioned, they are
the most important for the majority of organic contaminants.
A typical example of this general sorption behaviour is provided by Vasudevan
et al. ( 2002 ). These authors studied the sorption of two ionic contaminants
(2,4-D and quinmerac) and one neutral chemical (norflurazon) onto iron oxide-
rich, variable charged soils. It was found that sorption of 2,4-D and quinmerac
was strongly influenced by mineralogy, particularly soil iron and aluminium oxides,
whereas sorption of the neutral norflurazon was only related to total soil carbon.
An appreciable fraction of the mass sorbed in stirred-flow studies was easily des-
orbed by deionised water, and desorption of ionic contaminants was initially more
rapid than sorption. This sorption-desorption behaviour, although contrary to des-
orption hysteresis commonly observed in batch studies, suggests that the reversibly
sorbed fraction is weakly bound to the soil surface. 2,4-D sorption to iron oxide-rich

∼ 40 kJ/mol) are stronger than hydrophobic bonding ( ∼