ChapterS
Speciation of Metals in Natural Waters
F. Millero . D. Pierrot
8.1
Introduction
The reactions of many trace metals in natural waters are affected by their speciation
or form. This will affect the biological uptake (Anderson and Morel 1982) and toxicity
(Sunda and Ferguson 1983) as well as the solubility (Liu and Millero 1999). For example,
Fe(II) and Mn(II) are biologically available for marine organisms, while Fe(III) and
Mn(IV) are not normally available. Although the form of an element in natural waters
can be four phases (solid, gas, colloid and dissolved), we will only consider the form
of a metal in the dissolved state. The definition of a dissolved metal is defined by the
filter size used to separate solid and colloidal phases from the soluble form. In the past,
this separation was made with a 0.45 Ilm filter, and in more recent work smaller size
filters are used (-0.2 Ilm). The speciation of metals is controlled by ionic interactions of the metals with inorganic (Cr, OW, CO~-, etc.) and organic (fulvic and
humic acids) ligands. The dissolved forms of a metal like Fe in sea water can include:
• Free ions: Fe 2 +, Fe 3 +
• Inorganically Complexed: Fe(OH)+, Fe(OH}z, FeC03 and Fe(C03)~-' FeCf+, FeCI;,
FeSO;, Fe(S04)~-' Fe(OH)2+, Fe(OH);, Fe(OHh, FeCOr, and Fe(C03)~• Organically Complexed: FeL, where L can be a wide range of unknown natural ligands
(fulvic and humic acids, siderifores, etc.)
The speciation can also affect the rates of redox reactions (Millero 1990a,b) in natural water. Cu+ is rapidly oxidized with O2 or H20 2, while CuCI and CuC12 are nearly
inert to the oxidation (Sharma and Millero 1988,1989). CuHS- is rapidly oxidized with
02' while Zn(HSh is not oxidized (Vasquez et al.1989). Cu 2 + is rapidly reduced by H20 2,
while CuC03 or Cu-EDTA is not reduced (Millero et al. 1991, 1992).
Some of the factors that can control the equilibrium speciation in natural waters
are:
1. The oxidation potential (Eh)
2. The pH
3. The inorganic ligands (OW, CO~-)
4. The organic ligands (humics)
The ionic strength, temperature and pressure are also important. The importance
of Eh on the speciation of Fe is shown in Fig. 8.1. At low pH and Eh, iron is predomi-
Speciation of Metals in Natural Waters
F. Millero . D. Pierrot
8.1
Introduction
The reactions of many trace metals in natural waters are affected by their speciation
or form. This will affect the biological uptake (Anderson and Morel 1982) and toxicity
(Sunda and Ferguson 1983) as well as the solubility (Liu and Millero 1999). For example,
Fe(II) and Mn(II) are biologically available for marine organisms, while Fe(III) and
Mn(IV) are not normally available. Although the form of an element in natural waters
can be four phases (solid, gas, colloid and dissolved), we will only consider the form
of a metal in the dissolved state. The definition of a dissolved metal is defined by the
filter size used to separate solid and colloidal phases from the soluble form. In the past,
this separation was made with a 0.45 Ilm filter, and in more recent work smaller size
filters are used (-0.2 Ilm). The speciation of metals is controlled by ionic interactions of the metals with inorganic (Cr, OW, CO~-, etc.) and organic (fulvic and
humic acids) ligands. The dissolved forms of a metal like Fe in sea water can include:
• Free ions: Fe 2 +, Fe 3 +
• Inorganically Complexed: Fe(OH)+, Fe(OH}z, FeC03 and Fe(C03)~-' FeCf+, FeCI;,
FeSO;, Fe(S04)~-' Fe(OH)2+, Fe(OH);, Fe(OHh, FeCOr, and Fe(C03)~• Organically Complexed: FeL, where L can be a wide range of unknown natural ligands
(fulvic and humic acids, siderifores, etc.)
The speciation can also affect the rates of redox reactions (Millero 1990a,b) in natural water. Cu+ is rapidly oxidized with O2 or H20 2, while CuCI and CuC12 are nearly
inert to the oxidation (Sharma and Millero 1988,1989). CuHS- is rapidly oxidized with
02' while Zn(HSh is not oxidized (Vasquez et al.1989). Cu 2 + is rapidly reduced by H20 2,
while CuC03 or Cu-EDTA is not reduced (Millero et al. 1991, 1992).
Some of the factors that can control the equilibrium speciation in natural waters
are:
1. The oxidation potential (Eh)
2. The pH
3. The inorganic ligands (OW, CO~-)
4. The organic ligands (humics)
The ionic strength, temperature and pressure are also important. The importance
of Eh on the speciation of Fe is shown in Fig. 8.1. At low pH and Eh, iron is predomi-