Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 329

Table 8.2 Range of K d values for cationic metals and anionic metals/metalloids in soils

K d
(mean)

log 10
K d Element SD CV Median Min Max N

Cationic metals
Cd 2,869 12,246 4.27 390 0.44 1,92,000 3.46 830

Cu 4,799 9,875 2.06 2,120 6.8 82,850 3.68 452

Hg 8,946 5,641 0.63 7,500 4286 16,500 3.95 4

Ni 16,761 45,350 2.71 2,333 8.9 2,56,842 4.22 139

Pb 1,71,214 3,04,089 1.78 1,02,410 60.56 23,04,762 5.23 204

Zn 11,615 30,693 2.64 1,731 1.4 3,20,000 4.07 302

Anionic metals/metalloids
As 13,119 65,086 4.96 1,825 1.6 5,30,000 4.12 66

B 160 96 0.60 136 61 389 2.20 12

Mo 36 19 0.52 38 14 52 1.55 4

Se 43,937 1,19,534 2.72 15 1.6 6,00,000 4.64 63

Sauvé et al. ( 2000 )

between metal loading to soil and metal K d (Hendrickson and Corey 1981 ), highly
contaminated soils are likely to have lower metal retention than soils contaminated
by lower metal loads.
While K d values are normally determined experimentally in the laboratory, for
many metals and metalloids approximate K d values can be estimated from empirical
relationships derived using total metal concentrations in soil, and the main factors
affecting partitioning, usually pH, and soil clay/oxide or organic matter content
(Anderson and Christensen 1988 ; Buchter et al. 1989 ; Sauvé et al. 2000 ). These
models do not require detailed input data (as do more mechanistic models of metal
partitioning). Therefore, they can be a useful first screening tool in first tier Risk
Assessments to assess potential metal availability at a contaminated site, using very
simple soil analytical data - total metal concentrations and pH and/or clay/organic
matter content.
Cationic metals may also react with soluble natural or anthropogenic ligands (e.g.
chloride, dissolved organic matter or synthetic chelates such as ethylenediaminete-
traacetate - EDTA) to form solution complexes in soil pore waters (Fig. 8.1 ). This
may reduce or even reverse the charge on the cationic metal and hence markedly
reduce the K d value, increasing metal availability, but not necessarily increasing
metal uptake as this is governed by the free metal ion concentration in the pore
water and other factors (see Section 8.4.1 ).

8.2.2 Anionic Metals/Metalloids

Several of the metals and metalloids may be found in anionic form in soils, due
to their ability to combine with oxygen to form oxyanions (Table 8.3 ). These ions
may not be held as strongly by soil due to the net negative charge in most soils,