Oral Bioavailability - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 304

three of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in
two-dimensions. The fourth vertex is not shared with another tetrahedron and all of
the tetrahedra 'point' in the same direction, i.e. all of the unshared vertices are on
the same side of the sheet.
Oxides of iron, manganese and aluminium are often referred to as hydrous oxides
and like clays are principally derived from weathering reactions of rock minerals.
Although different in chemical structure, these two classes of minerals are very fine
grained (<2 m) and hence have a very large reactive surface area and similar modes

of action in binding contaminants and hence controlling their bioaccessibility. These
modes are:

μ

• cation and anion exchange;

• specific adsorption.

For ion exchange, contaminant ions are bound electrostatically to the clay or
oxide surface sites with an opposite charge. As already discussed in the organic car-
bon section, organic matter can also act as ion exchangers. A measure for the ability
of the soil to attract and retain cations is known as the cation exchange capacity
(CEC). In general, oxides contribute little to the CEC when soil pH is <7, under
these conditions, the main contribution comes from organic matter and clays. Anion
exchange occurs where negatively charged ions are attracted to positively charged
sites. The highest anion exchange for oxides occurs at low pH. Cation exchange is
reversible, diffusion controlled and stoichiometric and has an order of selectivity
based on the size, concentration and charge of the ion. Electrostatically bound con-
taminants are displaced relatively easily from the soil matrix in the presence of the
low pH conditions of the human stomach.
Specific adsorption involves the exchange of cations and anions with surface lig-
ands on solids to form partly covalent bonds with lattice ions. As with ion exchange,
the process is highly dependant on pH, charge and ionic radius. In contrast to
ion exchange, however, contaminants bound by this mechanism are far less labile.
Brummer ( 1986 ) showed that the sorptive capacities of iron oxides and aluminium
oxides were up to 26 times higher than their ionic complexes at pH 7.6.
Many studies have confirmed the importance of clays and oxides on the bioac-
cessibility of contaminants, (e.g. Ahnstrom and Parker 2001 ; Boonfueng et al. 2005 ;
Bowell 1994 ;Caveetal. 2007 ; Chen et al. 2002 ; Esser et al. 1991 ; Foster et al. 1998 ;
Garcia-Sanchez et al. 2002 ; Lin et al. 1998 ; Manceau et al. 2000 ; Matera et al. 2003 ;
Palumbo-Roe et al. 2005 ; Smith et al. 1998 ; Somez and Pierzynski 2005 ;Stewart
et al. 2003a ; Sultan 2007 ; Violante and Pigna 2002 ; Violante et al. 2006 ; Yang et al.
2005 ; Zagury 2007 ). It is clear that iron oxides are most commonly reported as hosts
for sorbed arsenic (Camm et al. 2004 ; Cances et al. 2005 ;Caveetal. 2007 ; Palumbo-
Roe et al. 2005 ; Wragg et al. 2007 ). Depending on the form of the iron oxide present
in the soil they can be both sources of bioaccessible and non-bioaccessible arsenic.
Figure 7.8 shows how progressive ageing of iron oxides from amorphous forms
through to more crystalline forms increases the thermodynamic stability and, hence,
specifically adsorbed contaminants, notably arsenic, are less easily mobilised. For
other metals the picture is less clear and the geochemical host of the contaminant
under study is very dependant on the history of the soil.