D I S C U S S I O N
M.C. Goldberg, USA.
Metal complexes, even chelates, may be as soluble at 10"
7 as opposed to the K.sp of less than 10"
7 .
However, trace metals can be solubilized by humic acids to concentrations far in excess of 10"
7 , even
to a few percent. These are not absorbed nor are they necessarily ionized forms of trace metals. They
are, however, very soluble. They are found in large concentration, i.e. the humic acids may be as
soluble as 5% in a saline solution.
Reply
I do not doubt that humic acids form complexes with metal ions and that these complexes are of
importance in natural water systems, but I doubt that the complexes formed are soluble. Adding
"humic acids" to systems of not very soluble metal ions may increase the concentration of metal that
passes a membrane filter, but it may not increase the concentration of (conceptually) soluble metal
ions; in other words the "humic acid" may enhance the dispersion and stability of insoluble colloids.
T. Helfgott, USA.
Is it not likely that particulate and colloidal metal sulfides constitute a significant amount of
metals found in the wet environment as other than "free metal ions"?
Reply
In anerobic environments, the presence of HS" precludes the existence of "free" d -metal cations
in concentrations larger than ca. 10" wl. The smaller the solubility product of the metal sulfide, the
stronger is the tendency to form soluble associates. Hence soluble thio complexes such for example as
AgSH aq. Ag(SH) 2 aq, or Ag 2 S3H 2 " , aq, at concentrations of typically 10"
7 to 10*
8 M may occur.
In addition colloidal metal sulfides may exist at least temporarily.
A. Jernelov, Denmark.
The mining of mercury yields 9 - 10,000 tons annually. Man also releases some from fossil fuel
and from mining other metals, e.g. sulphur ores. These sources total not more than another 10,000
tons annually. The total outfall of mercury with rainwater has been calculated to be several orders of
magnitude larger. On the other hand Goldberg et al have demonstrated a fivefold increase in mercury
content in Greenland ice-cover over the last decades. This is likely to be caused by human activities
but not through direct release. The most likely causes are changes in agricultural techniques with
improved ploughing and increased use of nutrients causing increased evaporation from soil. The figures
of 2.4 ppb mercury in filtered water from the Rhine seem far too high. Even in very polluted waters
in Sweden we normally find "mercury in water" in concentrations of 0.03-0.13 ppb.
Reply
I concur with the opinions expressed by Jernelöv and Goldberg (see chapters by A. Jernelov and
E.D. Goldberg, in "The Changing Chemistry of the Oceans", p. 161 and p.267, respectively, D. Dyrssen
and D. Jagner eds., Almqvist and Wiksell, Stockholm 1972).
J.B. Sprague, Canada.
We have shown that the maximum lethal toxicity of copper and zinc to fish was at pH 6.8 or
thereabouts, in very soft water. The metals were less toxic at high pH values. But the metals were also
less toxic at lower pH values. What is the explanation for this unusual effect at low pH?
Reply
There are various possible reasons for the pH-dependence of metal ion toxicity; for example, the
non-charged CuC0 3 (aq) and ZnCOa(aq). species, that prevail in soft water at neutral pH values, could
be more toxic than the charged aquo-metal ions or the dicarbonato complexes that predominate at
lower or higher pH values, respectively. The observations of Steemann Nielsen et al. (Physiologica
Plantarum 22, 1121 (1969)) are similar to your observations.
J. Shapiro, USA.
Despite the authors' comments that the work of many others in the field is based on analytical
artifacts, some of us agree that there is little evidence of truly dissolved organic complexes existing in
natural waters. In my work I have stated that iron is held only in apparent 'solution' as a protected
colloid, and my laboratory experiments using organics extracted from natural waters, not model
organic compounds, have corroborated this. On the other hand, because of the present and past
inability of chemists to guide biologists and to provide them with appropriate methods for
51
M.C. Goldberg, USA.
Metal complexes, even chelates, may be as soluble at 10"
7 as opposed to the K.sp of less than 10"
7 .
However, trace metals can be solubilized by humic acids to concentrations far in excess of 10"
7 , even
to a few percent. These are not absorbed nor are they necessarily ionized forms of trace metals. They
are, however, very soluble. They are found in large concentration, i.e. the humic acids may be as
soluble as 5% in a saline solution.
Reply
I do not doubt that humic acids form complexes with metal ions and that these complexes are of
importance in natural water systems, but I doubt that the complexes formed are soluble. Adding
"humic acids" to systems of not very soluble metal ions may increase the concentration of metal that
passes a membrane filter, but it may not increase the concentration of (conceptually) soluble metal
ions; in other words the "humic acid" may enhance the dispersion and stability of insoluble colloids.
T. Helfgott, USA.
Is it not likely that particulate and colloidal metal sulfides constitute a significant amount of
metals found in the wet environment as other than "free metal ions"?
Reply
In anerobic environments, the presence of HS" precludes the existence of "free" d -metal cations
in concentrations larger than ca. 10" wl. The smaller the solubility product of the metal sulfide, the
stronger is the tendency to form soluble associates. Hence soluble thio complexes such for example as
AgSH aq. Ag(SH) 2 aq, or Ag 2 S3H 2 " , aq, at concentrations of typically 10"
7 to 10*
8 M may occur.
In addition colloidal metal sulfides may exist at least temporarily.
A. Jernelov, Denmark.
The mining of mercury yields 9 - 10,000 tons annually. Man also releases some from fossil fuel
and from mining other metals, e.g. sulphur ores. These sources total not more than another 10,000
tons annually. The total outfall of mercury with rainwater has been calculated to be several orders of
magnitude larger. On the other hand Goldberg et al have demonstrated a fivefold increase in mercury
content in Greenland ice-cover over the last decades. This is likely to be caused by human activities
but not through direct release. The most likely causes are changes in agricultural techniques with
improved ploughing and increased use of nutrients causing increased evaporation from soil. The figures
of 2.4 ppb mercury in filtered water from the Rhine seem far too high. Even in very polluted waters
in Sweden we normally find "mercury in water" in concentrations of 0.03-0.13 ppb.
Reply
I concur with the opinions expressed by Jernelöv and Goldberg (see chapters by A. Jernelov and
E.D. Goldberg, in "The Changing Chemistry of the Oceans", p. 161 and p.267, respectively, D. Dyrssen
and D. Jagner eds., Almqvist and Wiksell, Stockholm 1972).
J.B. Sprague, Canada.
We have shown that the maximum lethal toxicity of copper and zinc to fish was at pH 6.8 or
thereabouts, in very soft water. The metals were less toxic at high pH values. But the metals were also
less toxic at lower pH values. What is the explanation for this unusual effect at low pH?
Reply
There are various possible reasons for the pH-dependence of metal ion toxicity; for example, the
non-charged CuC0 3 (aq) and ZnCOa(aq). species, that prevail in soft water at neutral pH values, could
be more toxic than the charged aquo-metal ions or the dicarbonato complexes that predominate at
lower or higher pH values, respectively. The observations of Steemann Nielsen et al. (Physiologica
Plantarum 22, 1121 (1969)) are similar to your observations.
J. Shapiro, USA.
Despite the authors' comments that the work of many others in the field is based on analytical
artifacts, some of us agree that there is little evidence of truly dissolved organic complexes existing in
natural waters. In my work I have stated that iron is held only in apparent 'solution' as a protected
colloid, and my laboratory experiments using organics extracted from natural waters, not model
organic compounds, have corroborated this. On the other hand, because of the present and past
inability of chemists to guide biologists and to provide them with appropriate methods for
51