Environmental Engineering Reference
In-Depth Information
Table 2.4
Metal oxides, oxyhydroxides and hydroxides found commonly in soils
Name
Chemical formula
Name
Chemical formula
Anatase
TiO
2
Hematite
α
-Fe
2
O
3
Birnessite
Na
0.7
Ca
0.3
Mn
7
O
14
.2H
2
O Ilmenite
FeTiO
3
Boehmite
γ
-AlOOH
Lepidocrocite
γ
-FeOOH
Ferrihydrite
Fe
10
O
15
.9H
2
O
Lithiophorite
(Al,Li)MnO
2
(OH)
2
γ
γ
Gibbsite
-Al(OH)
3
Maghemite
-Fe
2
O
3
α
Goethite
-FeOOH
Magnetite
Fe
3
O
4
Sposito (
1989
)
iron soil colloids becomes increasingly strong with increasing acidity (Loeppert
et al.
2003
).
Finally, the organic matter fraction of soils also contributes a major portion of the
soil's CEC by virtue of the negatively charged endings of these organic molecules,
resulting from the dissociation of hydroxyls from -COOH-groups, while non-polar
organic contaminants can bind to the soil organic matter. Dissolved organic car-
bon can complex certain metals such as copper and make them mobile (Römkens
1994
).
The degree of adsorption is a function of the specific surface area of the soil as
well as of the nature of that surface area, for example the mineralogy of any coatings
that differ from the particle itself, and the presence of micropores in that coating.
It has been known for a long time that heavy metals are strongly adsorbed by
sesquioxides (McKenzie
1980
) and by soil minerals with strong cation exchange
or anion exchange capacity. Many metals like barium, cadmium, manganese, stron-
tium, and zinc occur exclusively or generally as cations in the soil solution and
hence are affected by the soil CEC, but others like arsenic, antimony, selenium and
vanadium usually occur in the soil as negatively charged oxy-anions and are sub-
ject to the soil anion exchange capacity. The strength of adsorption is affected by
the pH of the soil, with higher pH increasing the amount sorbed by hematite and
goethite as well as birnessite. Trivedi and Axe (
2007
) relate the sorption of metals
to the intraparticle diffusion in hydrous metal oxides. Diffusion is a time-dependent
process and in experiments with nickel and zinc it appeared that slow intraparticle
diffusion of the metal ions was the active process. After 4 hours of contact time the
metal ions exhibited a similar local structure to a system aged for 110 days. There
is, however, still very much to be understood of these processes that decrease the
sorption and increase availability of metals in soils. Figures
2.7
and
2.8
represent
adsorption of metals on iron and manganese oxides
Sorption of metals also takes place in nature over the long term. For example,
ferruginous nodules formed in soils by pedogenic processes often have elevated
concentrations of trace elements, such as arsenic, cobalt, copper, chromium, nickel,
vanadium and zinc. Geologic weathering processes that cause an accumulation
of iron oxide-hydroxide similarly are accompanied by enrichment in these met-
als. Some examples of this are provided in the next section, because they can be
considered to be inherited, as opposed to anthropogenic.
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