T R A C E M E T A L S I N W A T E R S
TRACE METALS IN NATURAL WATERS;
DIFFICULTIES OF INTERPRETATION ARISING FROM
OUR IGNORANCE ON THEIR SPECIATION
WERNER STUMM and HALKA BILINSKI
Institute for Water Resources and Water Pollution Control, Swiss Federal
Institute of Technology, Zurich, Switzerland
Trace metals in natural waters appear often not to behave as predicted by the known
chemistry of the metals in question. Waters apparently are often supersaturated with
respect to many inorganic compounds. A large fraction of metal species may not be
dialysable or extractable with suitable reagents. The addition to water or the exudation of
organic substances into the water from algae may increase or decrease biological activity,
presumably because of increased or decreased rate of metal ion uptake. Obviously, the
interaction between inorganic and organic substances must account for many of the
observed phenomena, but quantitative consideration of organic complex formation or
chelation frequently cannot resolve logically the discrepancy between observed behavior
and theory.
Many metal ions are released into the environment as a result of civilizatory activities.
Whenever the rate of release exceeds the natural rate of cycling, contamination or adverse
effects on the ecosystem may result. It appears established (1,2) that on the global scale,
the flux (e.g. rate of extraction) of Ag, Au, Cd, Cr, Cu, Hg, Pb, Sb, Sn, Tl and Zn exceeds
the natural rate of cycling (e.g. rate of oceanic sedimentation). Bertine and Goldberg (3)
have shown that the combustion of fossil fuels potentially can mobilize many elements
(preferentially As, Hg, Cd, Sn, Sb, Pb, Zn, Tl, Ag and Bi) into the atmosphere at rates, in
general, less than but comparable to those of flow through natural waters during the
weathering cycle.
The primary source of lead from society is emission from combustion engines.
Morozumi, Chow and Patterson (4) have shown in North Greenland Glaciers 500 times
greater Pb levels than the prehistoric values. Surface ocean concentrations in waters of the
northern hemisphere today are about 0.07 μg/l compared with estimated values of 0.01
to 0.02 Mg/1 at a time prior to the introduction of Pb as an anti-knock chemical (5).
In European rivers, concentrations of metal ions have been increasing progressively.
Table 1 gives a few data on metal ion concentrations of water and of suspended matter in
the Rhine River. The poUutional effect of these loads upon organisms is not yet fully
appreciated. Some of the metal ions associated with the suspended material may locally
become released to the water. Martin et al. (7) show that this occurs for example to a
substantial extent in estuaries. The mode of distribution of metal ions between water and
suspended matter is also not understood sufficiently.
The main objective of this discussion is to show that our ignorance on the speciation
of trace metals obfuscates the elucidation of their role in natural waters. Information on
the type of species encountered under different chemical conditions (e.g., type of
complexes, their stability and rate of complex formation) is a prerequisite to a better
understanding of the distribution and functions of trace elements in natural waters.
39
TRACE METALS IN NATURAL WATERS;
DIFFICULTIES OF INTERPRETATION ARISING FROM
OUR IGNORANCE ON THEIR SPECIATION
WERNER STUMM and HALKA BILINSKI
Institute for Water Resources and Water Pollution Control, Swiss Federal
Institute of Technology, Zurich, Switzerland
Trace metals in natural waters appear often not to behave as predicted by the known
chemistry of the metals in question. Waters apparently are often supersaturated with
respect to many inorganic compounds. A large fraction of metal species may not be
dialysable or extractable with suitable reagents. The addition to water or the exudation of
organic substances into the water from algae may increase or decrease biological activity,
presumably because of increased or decreased rate of metal ion uptake. Obviously, the
interaction between inorganic and organic substances must account for many of the
observed phenomena, but quantitative consideration of organic complex formation or
chelation frequently cannot resolve logically the discrepancy between observed behavior
and theory.
Many metal ions are released into the environment as a result of civilizatory activities.
Whenever the rate of release exceeds the natural rate of cycling, contamination or adverse
effects on the ecosystem may result. It appears established (1,2) that on the global scale,
the flux (e.g. rate of extraction) of Ag, Au, Cd, Cr, Cu, Hg, Pb, Sb, Sn, Tl and Zn exceeds
the natural rate of cycling (e.g. rate of oceanic sedimentation). Bertine and Goldberg (3)
have shown that the combustion of fossil fuels potentially can mobilize many elements
(preferentially As, Hg, Cd, Sn, Sb, Pb, Zn, Tl, Ag and Bi) into the atmosphere at rates, in
general, less than but comparable to those of flow through natural waters during the
weathering cycle.
The primary source of lead from society is emission from combustion engines.
Morozumi, Chow and Patterson (4) have shown in North Greenland Glaciers 500 times
greater Pb levels than the prehistoric values. Surface ocean concentrations in waters of the
northern hemisphere today are about 0.07 μg/l compared with estimated values of 0.01
to 0.02 Mg/1 at a time prior to the introduction of Pb as an anti-knock chemical (5).
In European rivers, concentrations of metal ions have been increasing progressively.
Table 1 gives a few data on metal ion concentrations of water and of suspended matter in
the Rhine River. The poUutional effect of these loads upon organisms is not yet fully
appreciated. Some of the metal ions associated with the suspended material may locally
become released to the water. Martin et al. (7) show that this occurs for example to a
substantial extent in estuaries. The mode of distribution of metal ions between water and
suspended matter is also not understood sufficiently.
The main objective of this discussion is to show that our ignorance on the speciation
of trace metals obfuscates the elucidation of their role in natural waters. Information on
the type of species encountered under different chemical conditions (e.g., type of
complexes, their stability and rate of complex formation) is a prerequisite to a better
understanding of the distribution and functions of trace elements in natural waters.
39