Title: Chemical Speciation of Environmentally Significant Heavy
Metals and Inorganic Ligands
Chairman: S. Sjoberg
Members: P. Brown,
R.H. Byrne, K.
Powell, G. Hefter,
T. Gajda and H.
To provide scientists involved in chemical modelling of trace metals
in environmental systems with access to the best possible (critically
evaluated) equilibrium data for the reactions of these metals with major
The importance of considering chemical speciation (concentration
of individual chemical entities) within a coherent framework is increasingly
being recognised. Detailed understanding of the bioavailability and
toxicity of heavy metals as well as their transport and sedimentation
in natural aquifers requires knowledge of their speciation. The optimisation
of many industrial processes, e.g. hydrometallurgy and pulp and paper
processes, relies heavily on the understanding of chemical speciation
in often-complicated multicomponent/multiphase systems.
Chemical speciation modelling based on the assumption that the system
is at equilibrium is frequently utilised. The validity of an assumption
like this should of course always be scrutinised. However, it should
be born in mind that the analytical techniques required for measuring
trace metal (and trace metal complex) concentrations are still to a
great extent missing. Thus, quite often the only option that remains
is the equilibrium approach to speciation.
The numerical modelling of equilibrium systems requires adequate, critically
evaluated databases of numerical constants for equilibrium reactions.
Access to the invaluable IUPAC
Stability Constants Database has significantly improved the
possibilities of doing literature searches for relevant thermodynamic
data of metal complexes in solution. This database now contains more
than 80000 records with over 18000 metal-ligand combinations. It has
become obvious that the inexperienced user of a non-critical database
like this runs into problems when trying to identify the most reliable
data. Therefore, critical evaluation of data is necessary.
Within the Analytical Chemistry Division's Commission on Equilibrium
Data one major responsibility has been to provide the scientific community
with critically evaluated speciation schemes and corresponding stability
constants for metal complexes in solution. Considering the great number
of metals of the Periodic Table and the increasing vast number of ligands
found in our environment, this task seems insurmountable. However, the
need for reliable thermodynamic data describing metal complexation as
a function of temperature, pressure and ionic strength is urgent. This
is true in many but not all metal - ligand systems. To satisfy the most
urgent needs, priorities on selecting metal - ligand systems must be
The present proposal puts forward a programme that identifies the most
important metal ion - ligand systems from an environmental point of
view. The pollutants Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II), together
with the inorganic ligands Cl-, OH-, CO32-,
PO43-and SO42-, have not
yet been critically evaluated and should have highest priority. This
work is seen as Part I of a series that may subsequently consider (a)
data applicable to body fluids, (b) data applicable to sea water, (c)
data applicable to high component concentrations and high ionic strengths
as may exist in industrial processes.
Besides an evaluation of soluble metal complexes in these systems,
it is recommended that solubility products for pertinent solid phases
be evaluated also.
As of August 2002 the hydrolysis of Cu(II), Hg(II), and Pb(II) have
been completed with written drafts. This is also the case with respect
to the chloride systems of Hg((II) and Cd(II). The different group members
have now agreed upon the following time schedule for the completion
of the critical evaluations: The different Hg(II) systems, followed
by Cu(II) should be completed by the end of September 2002, Pb(II) (end
of December 2002), Cd(II) (end of March 2003) and Zn(II) (end of June
The first paper dealing with the different Hg(II) systems is now completed.
It has been evaluated by external referees and is now ready to be printed
in PAC. A closely related paper entitled Chemical Speciation
of Hg(II) with Environmental Inorganic Ligands has been published
as a full paper in Australian Journal of Chemistry, 2004, 57,993-1000.
In addition, the Cu(II) and Pb(II) systems are close to completion
with written reports (drafts). Main parts of the Zn(II) and Cd(II) systems
remain to be done.
In March 2004, S. Sjöberg has given an invited lecture titled
Chemical Speciation of Environmentally Significant Heavy metals and
Inorganic Ligands at SOPRO 2004, Karlsruhe,
A total of 6 articles are in preparation or will be prepared as output
of this project. End of 2005 is the projected completion date.
> Nov 2004 report update (pdf
file - 13KB)
> April 2005 - Chemical speciation of environmentally significant
heavy metals with inorganic ligands. Part 1: The Hg2+
Cl, OH, CO32,
SO42, and PO43
aqueous systems (IUPAC Technical Report) published in Pure
Appl. Chem. 77(4),
> May 2006 report update (pdf
file - 20KB)
> May 2007 - Chemical speciation of environmentally significant
heavy metals with inorganic ligands. Part 2: The Cu2+
OH, Cl, CO32,
SO42, and PO43
systems (IUPAC Technical Report) published in Pure
79(5), 895-950, 2007
> July 2007 report update (pdf
file - 9KB)
Last update: 16 October 2007