Oral Bioavailability - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 305

7.2.4 Solid Phase Speciation and Bioaccessibility
Adsorption of contaminants to different solid phases has been shown to be a key
factor in determining the bioaccessibility. It is, therefore, very important that analyt-
ical methodologies are available that can be used to measure the physico-chemical
forms of contaminants in the soil. Such methods may then provide information on
the potential environmental redistribution of contaminants under different soil con-
ditions, and ultimately be used as additional lines of evidence to support in-vitro
bioaccessibility testing in the assessment of human health risks from soil ingestion.
Spectroscopic methods such as x-ray absorption fine structure (XAFS) and
x-ray absorption near edge structure (XANES), that directly measure the oxida-
tion state and chemical bonds holding the contaminant in the soil, have been used
very successfully, (e.g. Cances et al. 2005 ; Cutler et al. 2001 ; Manceau et al. 2000 ;
Peak et al. 2006 ; Welter et al. 1999 ). However, these methods require the use of a
synchrotron source and can be expensive and time consuming. A relatively simple
and well-adopted method to assess metal pools of differential relative lability in
soils is the use of sequential extraction with reagents of increasing dissolution
strength. Each reagent should target a specific solid phase associated with the con-
taminant. Many of these extraction schemes have been described in the literature
and have been reviewed in Filgueiras et al. ( 2002 ). In many instances, the steps
with low dissolution strength are equated to the bioaccessible fraction or are used
along side dedicated bioaccessibility tests to help interpret the geochemical source
of the bioaccessible fraction (Datta et al. 2006 ; Denys et al. 2007 ; Jimoh et al.
2005 ; Liu and Zhao 2007a , b ; Marschner et al. 2006 ; Palumbo-Roe et al. 2005 ;
Reeder et al. 2006 ; Schaider et al. 2007 ; Siebielec et al. 2006 ; Tang et al. 2004 ,
2006 , 2007 ).
Cave et al. ( 2004 ), amongst other workers, has highlighted major shortcomings
with traditional sequential extraction methods and developed a new procedure called
Chemometric Identification of Substrates and Element Distributions (CISED). This
procedure uses increasing strengths of simple mineral acids as the extractant, fol-
lowed by chemometric data processing of the resulting multi-element data obtained
from the extract analysis. This method has been shown to work well for a num-
ber of contaminants in the NIST 2710 reference soil compared to more traditional
sequential extraction schemes and has subsequently been applied very successfully
in a number of studies to identify the source of bioaccessible contaminants in soils
(Cave et al. 2003 ; Palumbo-Roe and Klinck 2007 ; Palumbo-Roe et al. 2005 ; Wragg
2005 ; Wragg et al. 2007 ).
In addition to the solid phase distribution of contaminants, their chemical form
(speciation) can affect bioaccessibility and also the toxicity. In human Exposure
Assessment, if a bioaccessibility factor is applied to the total soil concentration,
the soil speciation needs to be understood. For redox sensitive contaminants such
as arsenic, antimony or chromium, which have different toxicity levels accord-
ing to their redox state, this verification is particularly important. Denys et al.
( 2009 ) investigated whether the speciation of antimony changed during the human