Trace Metals in Natural Waters
43
It is usually very difficult to determine experimentally solubilities and to establish the
prevalent metal species in the concentration range 1CT
6 to 1CT
8 M. Solubility information
applicable to the neutral pH range of natural waters is often obtained by extrapolating
from measurements in pH regions where solubilities are higher. New analytical methods
have now become available to elucidate the type of metal ion species encountered in
waters and to establish solubilities in the 1CT
6 to 10"
7 M concentration range. Anodic
stripping voltammetry, square wave and pulse polarography are convenient methods to
analyze trace metals, to identify various species and to distinguish the various physical
and chemical forms (ionic, complexes, particulate) of metal ions actually present in
natural waters (10). Fig. 3 illustrates from results in the authors' laboratory the
application of anodic stripping voltammetry ("inverse polarography") to establish the
factors that establish the solubility of Pb(II). It is shown that the method permits the
identification of Pb(C0 3 ) and the prevalent species under pH and concentration
conditions typically encountered in natural waters.
ADSORBABILITY OF TRACE METALS DEPENDS ON THEIR SPECIATION
As Table 1 illustrates, a dominant fraction of the metal ions in aquatic environments is
associated with suspended and colloidal material. The adsorption of metal ions from
aqueous solutions is not controlled primarily by the concentration of the free (aquo)
metal ions. Hydroxo, sulfato, carbonato and other complex ions or molecules tend to be
sorbed much stronger at interfaces than free metal ions. While there is still confusion as to
the mechanism of this enhanced adsorption, a few qualitative reasons can be given (11):
The replacement of an aquo group by a ligand in the coordination sheath of a metal atom
may render the complex more hydrophobic by reducing the interaction between the
central metal ion and the remaining aquo groups; furthermore, hydrogen bridges may
more readily join interfaces with the ligand groups. The strong pH dependence of the
sorbability of trace metals finds its explanation in the pH-dependent variation of metal
species. For example, Pb(C0 3 ) is much stronger sorbed on negatively charged silica than
Pb
2+ . A decrease in pH reduces [C0 3
2 ~ ] , shifts the equilibrium from Pb(C0 3 ) to Pb
2+
and causes the desorption of Pb(II).
INORGANIC - ORGANIC ASSOCIATION IN NATURAL WATERS
Are there soluble organic complexes in natural waters? Two schools of thought are
exemplified by the following quotations: 1, "Complexes of organic materials with
dissolved metals are abundant in natural waters", (12); 2, "It is likely that chelates are of
only minor importance in oceanic waters" (13). There is circumstantial evidence but no
direct proof for the existence of soluble chelates in natural waters (Table 2). No soluble
chelates have been detected by any direct method or isolated from natural waters (3).
Authors who mention chelation of trace metals as an explanation of biological or
chemical phenomena in waters frequently base their claim on the well established fact
that organic substances commonly found in waters, e.g., amino-acids, glycolic acid
saccharides, fulvic and humic acids indeed are able to form, under suitable conditions,
soluble metal complexes; but it does not necessarily follow that these complexes also
43
It is usually very difficult to determine experimentally solubilities and to establish the
prevalent metal species in the concentration range 1CT
6 to 1CT
8 M. Solubility information
applicable to the neutral pH range of natural waters is often obtained by extrapolating
from measurements in pH regions where solubilities are higher. New analytical methods
have now become available to elucidate the type of metal ion species encountered in
waters and to establish solubilities in the 1CT
6 to 10"
7 M concentration range. Anodic
stripping voltammetry, square wave and pulse polarography are convenient methods to
analyze trace metals, to identify various species and to distinguish the various physical
and chemical forms (ionic, complexes, particulate) of metal ions actually present in
natural waters (10). Fig. 3 illustrates from results in the authors' laboratory the
application of anodic stripping voltammetry ("inverse polarography") to establish the
factors that establish the solubility of Pb(II). It is shown that the method permits the
identification of Pb(C0 3 ) and the prevalent species under pH and concentration
conditions typically encountered in natural waters.
ADSORBABILITY OF TRACE METALS DEPENDS ON THEIR SPECIATION
As Table 1 illustrates, a dominant fraction of the metal ions in aquatic environments is
associated with suspended and colloidal material. The adsorption of metal ions from
aqueous solutions is not controlled primarily by the concentration of the free (aquo)
metal ions. Hydroxo, sulfato, carbonato and other complex ions or molecules tend to be
sorbed much stronger at interfaces than free metal ions. While there is still confusion as to
the mechanism of this enhanced adsorption, a few qualitative reasons can be given (11):
The replacement of an aquo group by a ligand in the coordination sheath of a metal atom
may render the complex more hydrophobic by reducing the interaction between the
central metal ion and the remaining aquo groups; furthermore, hydrogen bridges may
more readily join interfaces with the ligand groups. The strong pH dependence of the
sorbability of trace metals finds its explanation in the pH-dependent variation of metal
species. For example, Pb(C0 3 ) is much stronger sorbed on negatively charged silica than
Pb
2+ . A decrease in pH reduces [C0 3
2 ~ ] , shifts the equilibrium from Pb(C0 3 ) to Pb
2+
and causes the desorption of Pb(II).
INORGANIC - ORGANIC ASSOCIATION IN NATURAL WATERS
Are there soluble organic complexes in natural waters? Two schools of thought are
exemplified by the following quotations: 1, "Complexes of organic materials with
dissolved metals are abundant in natural waters", (12); 2, "It is likely that chelates are of
only minor importance in oceanic waters" (13). There is circumstantial evidence but no
direct proof for the existence of soluble chelates in natural waters (Table 2). No soluble
chelates have been detected by any direct method or isolated from natural waters (3).
Authors who mention chelation of trace metals as an explanation of biological or
chemical phenomena in waters frequently base their claim on the well established fact
that organic substances commonly found in waters, e.g., amino-acids, glycolic acid
saccharides, fulvic and humic acids indeed are able to form, under suitable conditions,
soluble metal complexes; but it does not necessarily follow that these complexes also