Oral Bioavailability - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 301

• the adsorption of cations on negatively charged sites (ion exchange);

• the mobility and protection of some metal ions from adsorption through the

formation of chelates with low molecular weight; and

• the retention of many contaminants in the higher molecular weight solid forms

of humus.

Contaminants showing particularly high affinities to soil organic matter include
cobalt, copper, mercury, nickel and lead (Adriano 2001 ). A number of studies have
systematically investigated soil properties to determine those that exert the greatest
controls on the bioaccessibility of arsenic (Cave et al. 2007 ; Tang et al. 2006 ; Yang
et al. 2005 ). Although these studies cannot claim to have covered all soil types and
soil properties, none of the outcomes showed that organic carbon was significantly
influencing the bioaccessibility of arsenic.
Besides the direct complexation of contaminants by organic carbon there are also
two other important secondary factors which affect contaminant bioaccessibility:

• redox conditions;

• organic matter competition for sorption of contaminants on oxides and clay.

Several studies (e.g. Baker et al. 2003 ; Chen et al. 2003 ; Lindsay 1991 ;Rose
et al. 1990 ) show that organic matter has an abiotic and biotic role in the reduction
of Fe-oxides, causing dissolution of the host oxide and release of adsorbed contam-
inants. In addition, organic matter can compete with contaminants for adsorption
sites causing displacement from the oxide matrix into more available forms (Dixit
and Hering 2003 ). An example of this (Wragg 2005 ) is shown in soils containing
arsenic derived from natural underlying Jurassic ironstone. A small but statistically
significant increase in the bioaccessibility of arsenic in garden soils compared to
rural soils was found, which was attributed to gardening practices including the
addition of organic matter to improve soil fertility. Stewart et al. ( 2003b )showed
that the bioaccessibility of chromium(VI) was significantly influenced by reduc-
tion processes catalyzed by soil organic carbon. Other studies show how organic
matter mediates the adsorption of contaminant to clay minerals (Cornu et al. 1999 ;
Lin and Puls 2000 ; Luo et al. 2006 ; Manning and Goldberg 1996 ). In addition,
although further discussion is beyond the scope of this chapter, soil organic mat-
ter is known to sequester organic contaminants therefore playing a key role in the
potential reduction of the bioaccessibility of any organic compounds present (Ruby
et al. 2002 ).

7.2.3 Mineral Constituents

The inorganic constituents of soils usually make up the majority of the mass of the
soil and it is the interaction of contaminants with the surfaces of these materials that
is a major control on bioaccessibility. Davis et al. ( 1996b ) studied the mineralogical