44
W. Stumm and H. Bilinski
form under conditions typically encountered in natural waters. Concentrations of
dissolved organic matter in waters range from 0.2 to 2 mg C per liter (occasionally up to
10 mg C per liter in polluted waters). This corresponds to a total sum of ca. 10"
7
*
5 to
10~
5 ·
5 moles per liter of ligands. Because there is probably a large number of organic
species present, many potential chelate formers must be typically present at
concentrations of 1CT
6 M or lower. The functional groups usually encountered in
dissolved organic substances, e.g., —COOH, —NH 2 , -OH, —SH show little specificity
toward individual metal ions. Hence the coordination tendency of most ligands will be
satisfied with Ca
2 + and Mg
2 + which are present at concentrations at least thousand fold
larger than those of the trace metals or of chelate forming substances. Although EDTA is
a strong complex former for Fe
3+ , a 1CT
6 M solution of pH = 8 in the presence of 2 X
1CT
3 M Ca
+2 cannot prevent a 1CT
5 M Fe(III) solution from precipitating as Fe(OH) 3 or as
FeOOH(ll).
Representative numerical examples have been given by Stumm and Morgan (9), Morel
and Morgan (15) and Duursma (14). Duursma (14) has reported investigations in which
the competition between chelation and sorption is used to determine the influence of
chelation. The results were compared with those computed from stability constants. For
example the addition of leucine at a concentration representative of that found in sea
water(2xltf
8 M ; l ^ g Γ
1 C) to a water of pH = 8.2 ([Ca
2+ ] = 1.3xlCT
3 M; [Mg
2+
] =
4.5 x 10"
3 M) that contained as trace metals Co(II) (1.5 x 1CT
8 M) and Zn(II) (1.7 x
IGT
6 M) would lead, according to the calculation, to a negligible formation ofCo-leucine
chelates (7.4 x 10"
10 M, = 5% of Co(II) and Zn-leucine chelates (8 x ΐ σ
1 0 Μ, = l%of
Zn (II)) because 98% of the leucine are bound to Ca
2 + and Mg
2 +
. Duursma demonstrated an
excellent agreement between calculation and experiment on the sorption. In order to
produce any effect of leucine on the sorption of Zn(II) and Co(II), concentrations of
leucine at least 10
4 times the natural concentrations had to be added.
HYPOTHETICAL MULTIMETAL, MULTILIGAND SYSTEMS
In order to obtain a more comprehensive picture of the possible chelating influence of
natural organic compounds in natural waters, Stumm and Morgan (9) and Morel and
Morgan (15) have considered systems that contain besides major and minor ions, typically
found in natural waters, a variety of organic substances individually present at small
concentrations (C = 1CT
6 M). Table 3 gives the computation of an equilibrium mixture of
nine metal ions and nine ligands, each one added at a fixed total concentration for a pH =
8.0, a temperature of 25°C and a constant ionic strength (4). The system of Table 3 is
quasi-realistic and serves primarily to illustrate the complexity of chemical forms found in
a natural water. According to this computed scheme a few trace metal chelates may
plausibly be expected in waters, especially in habitats where organic substances become
enriched. At the same time these calculations show that free trace metal aquo ions, or
their hydroxo or carbonato complexes may be present as predominant species even if
complex forming organic matter is present.
THE EFFECT OF ORGANIC COMPLEX FORMATION ON BIOLOGICAL ACTIVITY
Trace metals are essential for the growth and metabolism of organisms; they are also
W. Stumm and H. Bilinski
form under conditions typically encountered in natural waters. Concentrations of
dissolved organic matter in waters range from 0.2 to 2 mg C per liter (occasionally up to
10 mg C per liter in polluted waters). This corresponds to a total sum of ca. 10"
7
*
5 to
10~
5 ·
5 moles per liter of ligands. Because there is probably a large number of organic
species present, many potential chelate formers must be typically present at
concentrations of 1CT
6 M or lower. The functional groups usually encountered in
dissolved organic substances, e.g., —COOH, —NH 2 , -OH, —SH show little specificity
toward individual metal ions. Hence the coordination tendency of most ligands will be
satisfied with Ca
2 + and Mg
2 + which are present at concentrations at least thousand fold
larger than those of the trace metals or of chelate forming substances. Although EDTA is
a strong complex former for Fe
3+ , a 1CT
6 M solution of pH = 8 in the presence of 2 X
1CT
3 M Ca
+2 cannot prevent a 1CT
5 M Fe(III) solution from precipitating as Fe(OH) 3 or as
FeOOH(ll).
Representative numerical examples have been given by Stumm and Morgan (9), Morel
and Morgan (15) and Duursma (14). Duursma (14) has reported investigations in which
the competition between chelation and sorption is used to determine the influence of
chelation. The results were compared with those computed from stability constants. For
example the addition of leucine at a concentration representative of that found in sea
water(2xltf
8 M ; l ^ g Γ
1 C) to a water of pH = 8.2 ([Ca
2+ ] = 1.3xlCT
3 M; [Mg
2+
] =
4.5 x 10"
3 M) that contained as trace metals Co(II) (1.5 x 1CT
8 M) and Zn(II) (1.7 x
IGT
6 M) would lead, according to the calculation, to a negligible formation ofCo-leucine
chelates (7.4 x 10"
10 M, = 5% of Co(II) and Zn-leucine chelates (8 x ΐ σ
1 0 Μ, = l%of
Zn (II)) because 98% of the leucine are bound to Ca
2 + and Mg
2 +
. Duursma demonstrated an
excellent agreement between calculation and experiment on the sorption. In order to
produce any effect of leucine on the sorption of Zn(II) and Co(II), concentrations of
leucine at least 10
4 times the natural concentrations had to be added.
HYPOTHETICAL MULTIMETAL, MULTILIGAND SYSTEMS
In order to obtain a more comprehensive picture of the possible chelating influence of
natural organic compounds in natural waters, Stumm and Morgan (9) and Morel and
Morgan (15) have considered systems that contain besides major and minor ions, typically
found in natural waters, a variety of organic substances individually present at small
concentrations (C = 1CT
6 M). Table 3 gives the computation of an equilibrium mixture of
nine metal ions and nine ligands, each one added at a fixed total concentration for a pH =
8.0, a temperature of 25°C and a constant ionic strength (4). The system of Table 3 is
quasi-realistic and serves primarily to illustrate the complexity of chemical forms found in
a natural water. According to this computed scheme a few trace metal chelates may
plausibly be expected in waters, especially in habitats where organic substances become
enriched. At the same time these calculations show that free trace metal aquo ions, or
their hydroxo or carbonato complexes may be present as predominant species even if
complex forming organic matter is present.
THE EFFECT OF ORGANIC COMPLEX FORMATION ON BIOLOGICAL ACTIVITY
Trace metals are essential for the growth and metabolism of organisms; they are also