General considerations

During the last three decades, evidence has become overwhelming that it is the particular chemical form or association (species) of a metal and not its total concentration, that controls its geochemical (mobility, reactivitiy) and biological behaviour (bioavailability, toxicity) in natural, contaminated, or man-made systems. This growing insight has inspired the development and application of new sophisticated chemical methods, including advanced sampling and instrumental analytical techniques, modelling approaches, designed to tackle questions regarding speciation and bioavailability of biologically relevant trace metals. Table 5.1 provides a more generalized overview about current main speciation techniques.

Table 5.1. Overview of currently used speciation approaches (modified after Turner and Whitfield, 1982)

methods

dissolved < 0.45 um

particulate > 0.45 um

metal forms

free ions completed colloidal

adsorbed inorganic

organic

operational

UV photolysis!

size fractionation ASV with chemical pretreatment

new spectroscopic mctnoas

chelex

extraction procedures

modelling

equilibrium models biotic ligand modellin

effective ligand models

pure phases

electrochemical

ÏSE

ASV

exhaustive electrolysis

biological

phyto-plankton

particle feeders

Metals in water, sediment and soil occur both as dissolved, colloidal and particulate species depending on the particular environmental conditions. Dissolved metal species include the free unhydrated or hydrated metal ion (Mn+), as well as dissolved organic and inorganic complexes. Colloidally bound metals are mainly associated with amorphous Fe and Mn oxyhydroxides, or humic substances. Particulate metal forms can occur as discrete chemical forms (precipitates), adsorbed onto solid surfaces of hydrous oxides and organic substances, or as intermediates (coprecipitates) (see Reuther, 1999).

Despite obvious progress in developing a variety of laboratory-based speciation techniques, suitable in-situ field methods are still not existing to analyse the original species composition and availability of a metal for chemical and biological reactions in a distinct sample matrix (see Hirner et al., 2000, and Adriano, 2001). Main reasons why these techniques are still hardly used for official monitoring and regulatory purposes, and why most controlling bodies still seem to ignore the obvious advantages of metal speciation over just determining total concentrations, are the existing deficiencies in the selectivity of methods, in analytical precision, reproducibility and accuracy, and not least the high equipment costs involved. In the following, we will show that any serious attempt to assess the risk associated with the occurrence of trace metals will nevertheless need site-specific data about the true chemical form, in which a particular metal exists.

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