Cation Exchange Capacity
The most important factor regulating
Cd bioavailability is the cation exchange capacity (CEC) of the soil. Clay
particles, called micelles, are negatively charged and reversably bind
(adsorb) positively charged particles (cations) to their surface. Cations
such as Cd may be exchanged for H+ on the micelle surface. Cations adsorbed
to micelles are not available for plant uptake or groundwater-promoted
migration. Conversely, cations not adsorbed to the micelle are available
for uptake (Moore, et al. 1995). Different metal cations adsorb to micelles
with different affinity, and adsorption among clay soils varies by as much
as 21% (Atanassova, 1999). Copper (Cu) binds more tightly than zinc (Zn)
which binds more tightly than nickel (Ni) or Cd (Atanassova, 1999). Because
of this, the mineral composition of the soil may increase or decrease depending
on the presence of other metals. For example, a soil high in Cu would have
more bioavailable Cd since Cu inhibits Cd-adsorption to micelles (Atanassova,
1999).
Furthermore, cation exchange
capacity varies among soil types, regulated by the relative amount of clay
present. Since only clay binds cations, soils high in sand or silt have
lower CEC. Sandy soils have greater cation availability than other soils,
but cations are often leached out by water quicker than they can be utilized
by plants. For this reason, clay is often added to soils to prevent cation
mobilization and/or uptake (Vangronsveld, 1990). In order for phytoextraction
of Cd to proceed efficiently, the CEC of the soil must be low enough that
significant amounts of Cd are bioavailable, yet high enough that Cd is
not so bioavailable that it leads to toxic reactions in the plant. Several
strategies for altering Cd bioavailability via manipulation of soil CEC
are available.
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