Geoscience Reference
In-Depth Information
Speciation is important for understanding the behavior of toxic heavy metals,
because different chemical species may behave differently in the subsurface
environment (including subsurface water). Subsurface water contains a broad
spectrum of dissolved and colloidally dispersed natural products, so it is possible
that the same molecule exists in one or more species. For example, in evaluating
the presence of heavy metals in subsurface water, one should recognize that the
subsurface environment has a direct influence on heavy metal speciation and, thus,
on the environmental impact. By complexation with the various components of
water, the initial properties of a contaminant can be changed. A free metal ion in
an aqueous solution is more biologically active than an ion adsorbed on suspended
particles or complexed to other species.
Natural inorganic ligands of heavy metals in subsurface water, which are
present in a concentration of about 1 mM, include nitrite, sulfate, chloride, car-
bonate, and bicarbonate. These potential ligands generally are efficient only under
special conditions. For example, in an alkaline environment, carbonate and
bicarbonate can be significant complexors of transition metals like Cu
2+
or the
uranyl ion, UO
2
2+
, and cadmium may be complexed with Cl
-
or SO
4
2-
to form
CdCl
+
or CdSO
4
. Additional inorganic ligands, at a micromolecular level, include
phosphate and fluoride, which have high affinities for Fe
3+
and Al
3+
(Sauve and
Parker
2005
).
Dissolved organic chemicals acting as potential ligands may be found in the
subsurface (including the root zone). This group is very diverse and comprises
many compounds like sugars, organic acids, phenols, and lipids. Not all natural
organic acids found in subsurface water are able to function as ligands for heavy
metals. The common amino acids, such as a root exhudate, are too weak to
enhance metal complexation. Low-molecular-weight organic acids (e.g., citric
acid), however, exhibit affinity for metals due to their carboxylic functionality.
While the aliphatic organic acids are very active in the speciation of metals, in
most environmental situations, the ability of phenolic acids to form ligands is of
minor significance. On the other hand, the presence of high-molecular-weight
compounds in natural waters, such as fulvic or humic acids, leads to significant
heavy metal speciation; this is an effect explained by the affinity of carboxyl
groups.
By complexation with various components of the natural subsurface solution,
the initial properties of a toxic molecule can be changed. These transformations
involve adsorption on the subsurface solid phase, transport into the saturated or
partially saturated zone, and contaminant half-life in the subsurface environment.
It is operationally difficult to distinguish between dissolved and colloidally
dispersed substances. For example, colloidal metal-ion precipitates occasionally
have particle sizes smaller than 100 A, sufficiently small to pass through a
membrane filter, and organic substances can exist as a stable colloidal suspension.
Information on the types of species encountered under different chemical condi-
tions (type of complexes, their stabilities, rate of formation) is a prerequisite to
better understanding of the transformation in properties of toxic chemicals in a
water body.
