Geoscience Reference
In-Depth Information
for a montmorillonite clay is given in Fig.
8.47
a (Calvet
1989
). Lehman and
Harter (
1984
) studied the kinetics of copper desorption from soils when various
ligands (Na
2
-oxalate, Na
3
-citrate, and Na
4
-EDTA) were added to copper-con-
taminated samples, in stoichiometric amounts relevant to the Cu addition (i.e., for
a charge ratio of 1:1 Cu/ligand). Figure
8.47
b shows the time-dependent release of
Cu adsorbed on soil in solutions where the three organic cations are present. It is
clear that each ligand in the subsurface environment affects Cu desorption dif-
ferently; Cu release is enhanced in the presence of the EDTA ligand compared to
citrate and oxalate. It also appears that Cu readsorbed onto the surface, in the case
of Cu-oxalate. An additional factor to consider is the competition among metallic
cations for sorption sites. Metals bound through surface complexation can be
displaced by other cations.
Sequential disposal of heavy metals on land surfaces may control the pathway
of their retention and release in the soil-subsurface system. An example of Pb and
Cd retention on and release from soil material is given by Appel et al. (
2008
). The
authors discuss behavior of these heavy metals when added sequentially or con-
currently to three tropical soils: oxisol, ultisol, and mollisol. The retention/release
patterns of monometal sequential addition of Pb [ Cd or Cd [ Pb in the three
soils are presented in Fig.
8.48
.
The differences in sorbed and exchanged Pb, when Pb was added to the soils
before Cd as compared to when it was applied after Cd, were insignificant despite
the fact that differences were observed between the soils. In contrast, the trend for
Cd was different. In the case of oxisol—characterized by a high surface area—a
significant decrease in the adsorbed Cd occurred when Cd was added to the soil
after Pb application. Lead, which is much less exchangeable than Cd, inhibited Cd
sorption. The authors attribute this behavior mainly to chemical characteristics of
the heavy metals. Due to its relatively high electronegativity, low pK
h
, and small
hydrated radius, Pb was sorbed in preference to Cd. Cadmium was retained on the
sorption sites of the soil-subsurface mineral phase to a greater extent than when it
was added prior to Pb. Appel et al. (
2008
) suggested that at a contaminated site
receiving successive amounts of Pb and Cd at different times, Cd may be more
mobile when introduced into soil after Pb.
Retention of organic contaminants on subsurface solid-phase constituents in
general is not completely reversible, so that release isotherms differ from retention
isotherms. As a consequence, the extent of sorption depends on the nature of the
sorbent. Subsurface constituents as well as the types of bonding mechanisms
between contaminants and the solid phase are factors that control the release of
adsorbed organic contaminants. Saltzman et al. (
1972
) demonstrated the influence
of soil OM on the extent of hysteresis. Adsorption isotherms of parathion showed
hysteresis (or apparent hysteresis) in its adsorption and desorption in a water
solution. In contrast, smaller differences between the two processes were observed
when the soils were pretreated with hydrogen peroxide (oxidized subsamples) to
reduce initial OM content. The parathion content of the natural soils was greater
(8 % in Golan soil, 24 % in Meron soil, and 31 % in Bet-Guvrin soil) than the
oxidized subsamples. Natural peat retained two to three times more parathion than
