Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 734

correlated with metal concentrations in plants and many studies have been carried
out since (e.g., Brun et al. 2001 ; Fang et al. 2007 ; Mench et al. 1997 ; Perez-de-Mora
et al. 2006 ). A slightly different approach is to use CaCl 2 as the extractant but, in an
effort to mimic pore water conditions, to tailor the concentration of solution used on
a soil by soil basis. Thus, to simulate the metal concentrations in the pore water of
a soil, the first step would be to determine the ionic strength of the pore water. For
example, for clayish soils ionic strengths are typically around 2.5 mM (Schröder
et al. 2005 ), implying that 0.01 M CaCl 2 will overpredict the pore water concentra-
tions. As indicated by Schröder et al., 2.5 mM CaCl 2 as extractant would be a better
choice in this example to mimic pore water.
DTPA extractions are perhaps the most used and abused procedures to measure
bioavailability. Lindsay and Norvell ( 1978 ) proposed a method (shake 10 g < 2 mm
air dry soil at a rate of 120 cycles min − 1 on a horizontal shaker with a stroke of
8 cm in a solution comprising 0.005 M DTPA, 0.01 M CaCl 2 and 0.1 M TEA
buffered to pH 7.30 using HCl, then filter through Whatman No. 42 filter paper
and analyse) for determining deficiencies of iron, manganese, zinc and copper in
soils. The method was adopted as a mean to assess plant bioavailable metals at
contaminated sites. O'Connor ( 1988 ) documents the main problems with using
this method for soils containing high concentrations of metals, the most signif-
icant being that the chelating capacity of the solution (10 mmol kg − 1 of soil)
can be exceeded quite readily. However, Norvell ( 1984 ) proposed using a mod-
ified method with a 5:1 extractant to soil ratio for acid and metal-contaminated
soils.
EDTA based extractions have been used for longer than the DTPA based extrac-
tions, with references dating back to the 1950s (Cheng and Bray 1953 ;Viro 1955a,
b ). Unlike the DTPA extraction of Lindsay and Norvell ( 1978 ) there is no single
usual extraction with standardized concentration (e.g., in the range 0.1-0.01 M) and
standardized pH value. There are many documented cases of EDTA extractions cor-
relating well with plant metal concentrations (e.g., Cajuste and Laird 2000 ; Hooda
et al. 1997 ; Michaud et al. 2007 ).
A rather different approach worthy of mention is diffusive gradient in thin films
(DGT). The DGT methodology was originally developed for assessing water chem-
istry (Davison and Zhang 1994 ), but has more recently been applied to measure
bioavailability in soils, e.g., Zhang et al. ( 2001 ). The technique involves applying
an arrangement of a filter, layer of diffusive gel and layer of resin to a moist soil
surface. Over time, contaminants diffuse through the gel and are adsorbed by the
resin. This approach samples both contaminants in solution and those held loosely
on exchange sites which come into solution to replace contaminants that are sorbed
by the resin. Zhang et al. ( 2001 ) report good correlations between copper uptake by
Lepidium heterophyllum and the concentration of Cu measured using DGT. Good
correlations between plant and DGT concentrations have also been reported for cop-
per in other studies (e.g. Song et al. 2004 ) and zinc (e.g. Cornu and Denaix 2006 ;
Sonmez and Pierzynski 2005 ). Cornu and Denaix ( 2006 ), however, found a weak
correlation between plant and DGT concentrations, for cadmium. The use of DGT
for organisms is debated, for instance Koster et al. ( 2005 ), suggest that the DGT