Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 327

property of metals which controls environmental fate is oxidation state, as this may
affect charge e.g. chromium (Cr) - Cr 3+ present as this trivalent cation in most soils,
and Cr 6+ , present as an oxyanions as CrO 4 2 − or HCrO 4 − .

8.2.1 Cationic Metals

Elements in this group include cadmium (Cd), cobalt (Co), chromium (Cr), copper
(Cu), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn). These elements may have
different valence states, but are generally present in cationic form in soil, either
as free metals in solution (M n+ ), metals complexed to inorganic or organic lig-
ands (ML) or as solid phases (e.g. precipitated as minerals, or sorbed to negatively
charged surfaces) (McLaughlin 2002 - Table 8.1 ). The distribution coefficient ( K d )
is a measure of the relative distribution of elements between the solid phase and pore
water (water held in pore space between soil particles), and is an important property
to consider in assessing the potential availability of metals to plants:

K d = M solid / M pore water (8.1)

kg dw -1 ) and M pore water is the

where M solid = concentration in the solid phase (mg ·

L -1 ).
K d values for cationic metals vary widely across soils, and are also dependent
on metal properties such as valence state, e.g. Co 3+ has much higher K d values in
soils than Co 2+ . Metal partitioning in soils can be due to both sorption/desorption

concentration in the pore water (mg ·

Table 8.1 Physical and chemical properties of the cationic metals

Dominant
species in
soil

Dominant species in soil solution

Element Symbol Valence pH 3.5-6.0 pH 6.0-8.5

Cd 2+ Cd 2+ ,CdCl + ,

CdSO 4 0

Cd 2+ ,CdCl + ,
CdSO 4 0

Cadmium Cd 2

Cr 3+ ,CrO 4 2- Cr 3+ , CrOH 2+ Cr(OH) 4 -

Chromium Cr 2,3,6

Co 2+

Cobalt Co 2,3 - -

Cu 2+ Cu 2+ , Cu-org. Copper Cu 1,2 Cu-hydroxy

species,
CuCO 3 0 ,
Cu-org.

Pb 2+ Pb 2+ ,PbSO 4 0 ,

Pb-org.

Lead Pb 2,4 Pb-hydroxy and

carbonate
species, Pb-org.

Hg 2+ ,
(CH 3 ) 2 Hg

Mercury Hg 1,2 - -

Ni 2+ Ni 2+ ,NiSO 4 0 ,

Ni-org.

Ni 2+ ,NiHCO 3 + ,
NiCO 3

Nickel Ni 2,3

Zn 2+ Zn 2+ ,ZnSO 4 0 ,

Zn-org.

Zn 2+ , Zn-hydroxy
and carbonate
species, Zn-org.

Zinc Zn 2

Adapted from McLaughlin ( 2002 )