TRANSITION METALS AND HEAVY METAL
SPECIATION
J. Donat and C. Dryden,
Old Dominion University, Norfolk, VA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The transition metals and heavy metals (those with
atomic weights greater than 20) enter the ocean via
river runoff, wind-blown dust, diffusion from sediments, hydrothermal inputs resulting from reactions
of sea water with newly formed ocean crust at midocean seafloor spreading centers, and from anthropogenic activities. Some of these metals (e.g.,
manganese, iron, cobalt, nickel, copper and zinc) are
extremely important micronutrients needed by
phytoplankton for various metabolic functions.
Several trace metals that are nonconservative with
short oceanic residence time (e.g., manganese and
aluminum, though the latter is not a heavy metal) are
valuable as tracers for circulation and mixing in the
ocean. Micronutrient metals, as well as metals like
mercury, lead, and silver, which have no biochemical
role, can be toxic very low concentrations.
Until recently, marine chemists and chemical
oceanographers, using sample collection and analytical techniques of the time, could not accurately
measure the naturally low concentrations of these
metals in unpolluted sea water because of sample
contamination problems and lack of instrumental
sensitivity. Development of modern techniques for
collection, storage, and analysis of uncontaminated
samples, plus the development of highly sensitive
analytical techniques and instrumentation, have only
recently enabled marine trace metal chemists to determine accurate concentrations of these elements in
sea water, furthering our understanding of their distributions and chemical behavior in the oceans.
These procedural, analytical, and instrumental advancements led to the discoveries that the concentrations of many of these metals were orders of
magnitude lower than previously believed, and that
the depth distributions (‘vertical profiles’) of transition and heavy metal concentrations result from
biological, physical, and geochemical processes in
the oceans.
We now have a basic understanding of the concentrations and distributions of nearly all the naturally occurring elements in sea water. However, it has
become increasingly clear that this information alone
is insufficient for providing a complete understanding
of the biological and geochemical interactions of
these metals in the sea. Metals in sea water can exist
in different physical forms (dissolved, colloidal,
particulate) and chemical forms (ions, inorganic
complexes, organic complexes, organometallic compounds) and in different oxidation states (collectively
termed ‘species’) within a given chemical form.
Knowing the distribution of a metal’s total concentration among these various forms (‘speciation’) is
critically important because the different forms can
have very different biological and geochemical behaviors, and thus different fates and transport.
Before considering the speciation of the transition
and heavy metals, we first present a brief overview of
the concentrations and distributions of these elements.
Overview: Transition Metal and Heavy
Metal Concentrations and
Distributions
Concentrations of the transition metals and heavy
metals vary both horizontally and vertically through
the world’s oceans. Table 1 lists the ranges in the
oceanic concentrations of the transition metals and
heavy metals. For a representation of the North Pacific depth profiles of the elements in the periodic
table, including the transition metals and heavy
metals (see Elemental Distribution: Overview). The
relative rates of supply and removal of the elements
determine their horizontal and vertical distributions.
These elements are supplied to the oceans primarily
by riverine input, atmospheric precipitation, hydrothermal venting, and anthropogenic activities, and
they are removed by adsorption onto sinking particles (‘scavenging’) or by incorporation into sinking
biologically produced material by active uptake by
phytoplankton. On the basis of their vertical profiles,
these elements can be classified into one of the following categories: (1) conservative type, (2) scavenged type, (3) nutrient (recycled) type, and (4)
mixed type. Figure 1 shows the shapes of the vertical
profiles for the conservative, scavenged, and nutrient
(recycled) categories and lists the elements that display them.
72
SPECIATION
J. Donat and C. Dryden,
Old Dominion University, Norfolk, VA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The transition metals and heavy metals (those with
atomic weights greater than 20) enter the ocean via
river runoff, wind-blown dust, diffusion from sediments, hydrothermal inputs resulting from reactions
of sea water with newly formed ocean crust at midocean seafloor spreading centers, and from anthropogenic activities. Some of these metals (e.g.,
manganese, iron, cobalt, nickel, copper and zinc) are
extremely important micronutrients needed by
phytoplankton for various metabolic functions.
Several trace metals that are nonconservative with
short oceanic residence time (e.g., manganese and
aluminum, though the latter is not a heavy metal) are
valuable as tracers for circulation and mixing in the
ocean. Micronutrient metals, as well as metals like
mercury, lead, and silver, which have no biochemical
role, can be toxic very low concentrations.
Until recently, marine chemists and chemical
oceanographers, using sample collection and analytical techniques of the time, could not accurately
measure the naturally low concentrations of these
metals in unpolluted sea water because of sample
contamination problems and lack of instrumental
sensitivity. Development of modern techniques for
collection, storage, and analysis of uncontaminated
samples, plus the development of highly sensitive
analytical techniques and instrumentation, have only
recently enabled marine trace metal chemists to determine accurate concentrations of these elements in
sea water, furthering our understanding of their distributions and chemical behavior in the oceans.
These procedural, analytical, and instrumental advancements led to the discoveries that the concentrations of many of these metals were orders of
magnitude lower than previously believed, and that
the depth distributions (‘vertical profiles’) of transition and heavy metal concentrations result from
biological, physical, and geochemical processes in
the oceans.
We now have a basic understanding of the concentrations and distributions of nearly all the naturally occurring elements in sea water. However, it has
become increasingly clear that this information alone
is insufficient for providing a complete understanding
of the biological and geochemical interactions of
these metals in the sea. Metals in sea water can exist
in different physical forms (dissolved, colloidal,
particulate) and chemical forms (ions, inorganic
complexes, organic complexes, organometallic compounds) and in different oxidation states (collectively
termed ‘species’) within a given chemical form.
Knowing the distribution of a metal’s total concentration among these various forms (‘speciation’) is
critically important because the different forms can
have very different biological and geochemical behaviors, and thus different fates and transport.
Before considering the speciation of the transition
and heavy metals, we first present a brief overview of
the concentrations and distributions of these elements.
Overview: Transition Metal and Heavy
Metal Concentrations and
Distributions
Concentrations of the transition metals and heavy
metals vary both horizontally and vertically through
the world’s oceans. Table 1 lists the ranges in the
oceanic concentrations of the transition metals and
heavy metals. For a representation of the North Pacific depth profiles of the elements in the periodic
table, including the transition metals and heavy
metals (see Elemental Distribution: Overview). The
relative rates of supply and removal of the elements
determine their horizontal and vertical distributions.
These elements are supplied to the oceans primarily
by riverine input, atmospheric precipitation, hydrothermal venting, and anthropogenic activities, and
they are removed by adsorption onto sinking particles (‘scavenging’) or by incorporation into sinking
biologically produced material by active uptake by
phytoplankton. On the basis of their vertical profiles,
these elements can be classified into one of the following categories: (1) conservative type, (2) scavenged type, (3) nutrient (recycled) type, and (4)
mixed type. Figure 1 shows the shapes of the vertical
profiles for the conservative, scavenged, and nutrient
(recycled) categories and lists the elements that display them.
72