129
4
Ionic Interactions
4.1 Introduction
There has been an increasing concern for the geochemical fate of trace metals in the marine
environment. Many baseline studies have been made to determine the total concentration
of these metals in the solution phase, the gas phase, and the solid phase (i.e., on and in
living and nonliving material). Although these studies have been useful in indicating the
fate of trace metals in the marine environment, little progress has been made in determining the mechanisms involved (physical, chemical, and biological) in the transfer of
metals between phases. Part of the reason for this lack of understanding of trace metals in
the marine environment comes from the difficulties in detecting experimentally the true
activity (both thermodynamic and biochemical) of the trace metals; because of their low
concentrations, conventional electrode methods of determining thermodynamic activity
are difficult to apply. Although it is possible to detect the activity of some metals, we are at
present forced to use models to estimate the thermodynamic activity of most trace metals
to make comparisons with biological studies.
The importance of knowing the true biological activity (rather than total concentration)
of metal ions in seawater has been suggested by a number of workers. For example, we
have found that the growth depression by copper of the bacterium Vibrio alginolyticus was
affected by complex formation. This variation in copper toxicity is due to the presence of
differing concentrations of free copper because of the formation of complexes with ligands
such as fulvic acids.
Synthetic ligands (like NTA [nitriloacetic acid]) and natural organic ligands can have
similar effects in the marine environment. Three possible consequences may follow the
addition of such a ligand: (a) Metals may become solubilized and transported from regions
of high concentration to uncontaminated regions; (b) because of overcomplexation, necessary metals may not be available for the growth of marine organisms; (c) reducing the
natural metal toxicity may cause increased numbers of pathogenic organisms to occur.
The important role that free or complexed metal ions plays in productivity has also been
pointed out by a number of workers. For example, Barber and Ryther (1969) reported that
phytoplankton blooms in newly upwelled waters off the Peru coast are conditioned by the
organics in seawater. They suggested that the organic compounds are necessary to make
the trace metals available for growth. However, Steeman- Nielson and Wium- Anderson
(1970) suggested that the organic conditioning is due to the reduction in copper toxicity (caused by complexation). Although it is not currently possible to state with certainty
which of the suggested roles of organic ligands (or both) is true, we can state that it is the
form of the metal (not the total amount) that is important in determining its biological and
geochemical activity. The determination of the form of a given element is called speciation.
4
Ionic Interactions
4.1 Introduction
There has been an increasing concern for the geochemical fate of trace metals in the marine
environment. Many baseline studies have been made to determine the total concentration
of these metals in the solution phase, the gas phase, and the solid phase (i.e., on and in
living and nonliving material). Although these studies have been useful in indicating the
fate of trace metals in the marine environment, little progress has been made in determining the mechanisms involved (physical, chemical, and biological) in the transfer of
metals between phases. Part of the reason for this lack of understanding of trace metals in
the marine environment comes from the difficulties in detecting experimentally the true
activity (both thermodynamic and biochemical) of the trace metals; because of their low
concentrations, conventional electrode methods of determining thermodynamic activity
are difficult to apply. Although it is possible to detect the activity of some metals, we are at
present forced to use models to estimate the thermodynamic activity of most trace metals
to make comparisons with biological studies.
The importance of knowing the true biological activity (rather than total concentration)
of metal ions in seawater has been suggested by a number of workers. For example, we
have found that the growth depression by copper of the bacterium Vibrio alginolyticus was
affected by complex formation. This variation in copper toxicity is due to the presence of
differing concentrations of free copper because of the formation of complexes with ligands
such as fulvic acids.
Synthetic ligands (like NTA [nitriloacetic acid]) and natural organic ligands can have
similar effects in the marine environment. Three possible consequences may follow the
addition of such a ligand: (a) Metals may become solubilized and transported from regions
of high concentration to uncontaminated regions; (b) because of overcomplexation, necessary metals may not be available for the growth of marine organisms; (c) reducing the
natural metal toxicity may cause increased numbers of pathogenic organisms to occur.
The important role that free or complexed metal ions plays in productivity has also been
pointed out by a number of workers. For example, Barber and Ryther (1969) reported that
phytoplankton blooms in newly upwelled waters off the Peru coast are conditioned by the
organics in seawater. They suggested that the organic compounds are necessary to make
the trace metals available for growth. However, Steeman- Nielson and Wium- Anderson
(1970) suggested that the organic conditioning is due to the reduction in copper toxicity (caused by complexation). Although it is not currently possible to state with certainty
which of the suggested roles of organic ligands (or both) is true, we can state that it is the
form of the metal (not the total amount) that is important in determining its biological and
geochemical activity. The determination of the form of a given element is called speciation.