Oral Bioavailability - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 300

the uncertainties on the measurement there is a linear relationship between bioac-
cessibility and bioavailability (or its relative counterpart). Monte-Carlo simulations
show that for bioaccessibility to be predictive of bioavailability the straight line
relationship must meet the following criteria:

• the soils used in the validation exercise must cover greater than 70% of the

0-100% of the bioavailability/bioaccessibility range;

• the between laboratory reproducibility of the bioaccessibility and bioavail-

ability measurements must have a relative standard deviation of less

than 20%;

• the r square value of the straight line should be greater than 0.7.

7.2 Influence of Soil Properties on Oral Bioaccessibility

The bioavailability of any contaminant bound to the soil depends upon the soil type,
properties of the soil, the contaminant and the manner by which the contaminant has
entered the soil (Selinus 2005 ).

7.2.1 pH

Soils generally have pH values (measured in water) from 4 to 8.5, due to buffering
by aluminium at the lower end and calcium carbonate (CaCO 3 ) at the upper end of
the range. In general, most divalent cationic forms of contaminants are less strongly
absorbed, and therefore more bioaccessible, in acidic soils than they are at neutral
or alkaline soils as demonstrated for lead by Yang et al. ( 2003 ) and for cadmium
by Tang et al. ( 2006 ). In a study testing five Chinese sites (Tang et al. 2007 ), the
bioaccessibility of both spiked and endogenous arsenic increased with increasing
pH. A similar effect was observed by Yang et al. ( 2005 ), based on a study of 36
US soils. pH also affects other parameters such as the solubility of organic carbon,
and the sorptive capacity of iron oxides and aluminium oxides and clays which are
discussed in the next sections.

7.2.2 Soil Organic Matter

The soil organic matter content can vary widely among soils, from <1% to >70%.
The organic matter is divided into non-humic and humic. The former consists
of unaltered biochemicals which have not been degraded since they entered the
soil, through production by living organisms. The latter are formed by secondary
synthesis reactions involving micro-organisms.
The mechanisms that control the bioaccessibility of contaminants in the presence
of soil organic matter have been described as (Selinus 2005 ):