Introduction - Environmental Analytical Chemistry
3
a. sampling, avoiding contamination
b. separation by matrix in order to remove interference
c. cleanup
d. selective preconcentration
e. accuracy and precision of the instrumental detection method
The development of these procedures and the use of chromatographic techniques
(particularly HPLC) in the preconcentration and separation steps, together with the
availability of very sensitive instrumental techniques, such as Atomic Absorption Spectroscopywith graphite furnace (AAS-ETA) or Inductively-Coupled Plasma Mass Spectrometry (ICP-MS, Alves et al. 1993), means that it is now possible to determine trace
metal concentrations in ng rl rather than in flg r 1 , i.e. a significant achievement. However, efforts have still to be made to obtain better reproducibility of results and to set
up faster analytical methods. As regards sensitivity, low accuracy risk and low cost
requirements, the Cathodic and Anodic Stripping Voltammetry, (CSV and ASV, respectively) are the most versatile analytical methods for direct determination of the total
concentration of some metals (Van Den Berg 1989). To carry out simultaneous detection of many trace elements, the PIXE (Photo Inductive X-ray Emission) analysis can
be also used.
In spite of their low concentration, metals are generally extremely reactive and generally widely dispersed in nature in association with or as part of a variety of solid
phases. The natural pathways of heavy metals are largely controlled by two major processes: incorporation to or into various solid phases and chemical complexation in
solution. Different factors regulate these processes in the marine environment: pH, pE,
the oxidation state of the metal and its acidity hardness. These parameters act to respectively restrict and promote the solubilization of the metal, but, owing to pH and
pE values and to remarkable concentrations of complexing organic matter in sediments, immobilisation to the various solid phases generally predominates in the marine environment, so that sediments behave as a "reservoir sink" for metals in the environment (Bloom et al. 1999). Kinetics studies can give useful information about the
absorption-release mechanism and contribute to defining the equilibrium distribution of metals between different phases (Millero 1996).
Chemical Speciation. Bare-metal ions cannot exist in water. Their basic unit is the
hydrated aquo-ion which behaves like Lewis acid and, by a series of proton transfer
(hydrolysis) reactions, a sequence of hydroxo- and possibly oxo-complexes may be produced (Stumm and Morgan 1996; Morel 1983). In complexation studies of metal ions
in aqueous solutions, hydrolytic species cannot be neglected (Baes and Mesmer 1976).
Moreover, when dealing with a multicomponent solution, such as sea water, other possible interactions of metals with inorganic (chloride, sulfate, carbonate, phosphate) and
organic low and high molecular weight ligands must be taken into account (Stumm
and Brauner 1975). Then, a network of interactions is established and metal species
are present in different chemical forms (Chemical Speciation), each of them often showing different trends in bioavailability, accumulation, toxicity, absorption and! or release,
etc. (Sadiq 1992; Tessier and Turner 1995).
Some metals, such as lead, tin, mercury, arsenic and antimony, are often present in
sea waters as organic derivatives deriving both from anthropic inputs and from the
3
a. sampling, avoiding contamination
b. separation by matrix in order to remove interference
c. cleanup
d. selective preconcentration
e. accuracy and precision of the instrumental detection method
The development of these procedures and the use of chromatographic techniques
(particularly HPLC) in the preconcentration and separation steps, together with the
availability of very sensitive instrumental techniques, such as Atomic Absorption Spectroscopywith graphite furnace (AAS-ETA) or Inductively-Coupled Plasma Mass Spectrometry (ICP-MS, Alves et al. 1993), means that it is now possible to determine trace
metal concentrations in ng rl rather than in flg r 1 , i.e. a significant achievement. However, efforts have still to be made to obtain better reproducibility of results and to set
up faster analytical methods. As regards sensitivity, low accuracy risk and low cost
requirements, the Cathodic and Anodic Stripping Voltammetry, (CSV and ASV, respectively) are the most versatile analytical methods for direct determination of the total
concentration of some metals (Van Den Berg 1989). To carry out simultaneous detection of many trace elements, the PIXE (Photo Inductive X-ray Emission) analysis can
be also used.
In spite of their low concentration, metals are generally extremely reactive and generally widely dispersed in nature in association with or as part of a variety of solid
phases. The natural pathways of heavy metals are largely controlled by two major processes: incorporation to or into various solid phases and chemical complexation in
solution. Different factors regulate these processes in the marine environment: pH, pE,
the oxidation state of the metal and its acidity hardness. These parameters act to respectively restrict and promote the solubilization of the metal, but, owing to pH and
pE values and to remarkable concentrations of complexing organic matter in sediments, immobilisation to the various solid phases generally predominates in the marine environment, so that sediments behave as a "reservoir sink" for metals in the environment (Bloom et al. 1999). Kinetics studies can give useful information about the
absorption-release mechanism and contribute to defining the equilibrium distribution of metals between different phases (Millero 1996).
Chemical Speciation. Bare-metal ions cannot exist in water. Their basic unit is the
hydrated aquo-ion which behaves like Lewis acid and, by a series of proton transfer
(hydrolysis) reactions, a sequence of hydroxo- and possibly oxo-complexes may be produced (Stumm and Morgan 1996; Morel 1983). In complexation studies of metal ions
in aqueous solutions, hydrolytic species cannot be neglected (Baes and Mesmer 1976).
Moreover, when dealing with a multicomponent solution, such as sea water, other possible interactions of metals with inorganic (chloride, sulfate, carbonate, phosphate) and
organic low and high molecular weight ligands must be taken into account (Stumm
and Brauner 1975). Then, a network of interactions is established and metal species
are present in different chemical forms (Chemical Speciation), each of them often showing different trends in bioavailability, accumulation, toxicity, absorption and! or release,
etc. (Sadiq 1992; Tessier and Turner 1995).
Some metals, such as lead, tin, mercury, arsenic and antimony, are often present in
sea waters as organic derivatives deriving both from anthropic inputs and from the