Oral Bioavailability - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 303

In the UK, a number of studies have examined the bioaccessibility of elevated
concentrations of arsenic in soils developed over Jurassic ironstones (Cave et al.
2003 , 2007 ; Palumbo-Roe et al. 2005 ; Wragg 2005 ; Wragg et al. 2007 ). In this
instance, there was a highly significant correlation between the total arsenic content
and the total iron content, but there was no significant correlation between the bioac-
cessible arsenic and the total iron content. Palumbo-Roe et al. ( 2005 ) concluded that
in these soils the bioaccessible arsenic is mainly contained within calcium iron car-
bonate (sideritic) assemblages and only partially in iron aluminosilicates, probably
berthierine, and iron oxyhydroxide phases, probably goethite. It is suggested that
the bulk of the non-bioaccessible arsenic is bound up with less reactive more highly
crystalline (see Fig. 7.8 ) iron oxide phases.
These studies highlight the role of two very important mineral classes which
are ubiquitous in soils and have been shown to be key controls on bioaccessibility.
These are:

• clays;

• oxides of iron, manganese and aluminium.

Clay minerals are produced through hydrolysis weathering reactions which is the
reaction between hydrogen ions and an aluminosilicate mineral (such as feldspar)
to form soluble cations, silicic acid and a clay mineral. They are characterised by
two-dimensional sheets of corner sharing SiO 4 and AlO 4 tetrahedra. These tetrahe-
dral sheets have the chemical composition (AlSi) 3 O 4 , and each tetrahedron shares

Fig. 7.8 Diagrammatic representation of the ageing processes of Fe oxides in soils