Chapter 9
Organic Complexation of Metals in Sea Water
C.M.G. van den Berg
9.1
Introduction
Metals dissolve in water by ionisation, and their solubility is usually restricted by their
solubility product with one of the major anions in sea water, such as with hydroxide,
carbonate or phosphate. Their solubility is increased by the formation of soluble complexes with the major anions. The concentrations of the vast majority of the metals
in the oceans are usually much less than their solubility because they are scavenged
from solution by interaction with settling particles, including microorganisms, faecal pellets and inorganic hydroxides or oxides of poorly soluble metals. This interaction, and therefore the residence time of the elements, could be modelled using chemical equilibria if we knew the chemical speciation of the metals, and the interaction of
the various species with the particles including their availability for uptake by the
microorganisms. For some metals it is now known that their chemical speciation
is dominated by organic complexation. This aspect will be discussed in detail in this
chapter.
It is relatively easy to calculate the inorganic speciation of metals in sea water, as
most reactions have been investigated and stability constants exist. However, recently
it has been discovered that the metal speciation is more complicated and that several
metals occur in species which react differently from the known inorganic species. By
destruction of all organic matter in sea water, it has been demonstrated that these
species are of organic nature, and it is now known that copper, zinc and iron exist
predominantly bound by organic matter, whereas nickel and cobalt may be only partially associated with organic matter. Even metals which are expected to form hydroxide species (titanium, and aluminium) are known to occur at least in part as non-reactive species although for these elements the nature of these species has not yet been
elucidated (van den Berg et al. 1994).
Using electroanalytical techniques it has been demonstrated that the organic species of copper, zinc, iron, nickel and cobalt are chemically reversible, meaning that they
are complexes between the metal ions and organic ligands which can dissociate when
the free metal ion concentration is lowered, and the concentration of organic complexes is increased when either the ligand or the metal concentration is raised.
Organic complexing ligands occur in all natural waters including sea water. The
concentrations of these ligands are much less than those of the major ions in sea water but usually greater than those of the trace metals. Their concentrations have been
determined using titrations with metal ions with detection of the reactive metal species using various techniques, mostly using voltammetry.
Organic Complexation of Metals in Sea Water
C.M.G. van den Berg
9.1
Introduction
Metals dissolve in water by ionisation, and their solubility is usually restricted by their
solubility product with one of the major anions in sea water, such as with hydroxide,
carbonate or phosphate. Their solubility is increased by the formation of soluble complexes with the major anions. The concentrations of the vast majority of the metals
in the oceans are usually much less than their solubility because they are scavenged
from solution by interaction with settling particles, including microorganisms, faecal pellets and inorganic hydroxides or oxides of poorly soluble metals. This interaction, and therefore the residence time of the elements, could be modelled using chemical equilibria if we knew the chemical speciation of the metals, and the interaction of
the various species with the particles including their availability for uptake by the
microorganisms. For some metals it is now known that their chemical speciation
is dominated by organic complexation. This aspect will be discussed in detail in this
chapter.
It is relatively easy to calculate the inorganic speciation of metals in sea water, as
most reactions have been investigated and stability constants exist. However, recently
it has been discovered that the metal speciation is more complicated and that several
metals occur in species which react differently from the known inorganic species. By
destruction of all organic matter in sea water, it has been demonstrated that these
species are of organic nature, and it is now known that copper, zinc and iron exist
predominantly bound by organic matter, whereas nickel and cobalt may be only partially associated with organic matter. Even metals which are expected to form hydroxide species (titanium, and aluminium) are known to occur at least in part as non-reactive species although for these elements the nature of these species has not yet been
elucidated (van den Berg et al. 1994).
Using electroanalytical techniques it has been demonstrated that the organic species of copper, zinc, iron, nickel and cobalt are chemically reversible, meaning that they
are complexes between the metal ions and organic ligands which can dissociate when
the free metal ion concentration is lowered, and the concentration of organic complexes is increased when either the ligand or the metal concentration is raised.
Organic complexing ligands occur in all natural waters including sea water. The
concentrations of these ligands are much less than those of the major ions in sea water but usually greater than those of the trace metals. Their concentrations have been
determined using titrations with metal ions with detection of the reactive metal species using various techniques, mostly using voltammetry.