In surface waters

Advances in analysing ultra-low metal levels in dissolved form in natural waters have also favoured the development of new speciation methods designed to distinguish between complexed and free metal ions. This led to an improved understanding of why metals are bioavailable, how they affect biota and how biota, in turn, respond to metal speciation. The obtained higher precision indicated that metals in natural waters may affect biota at levels much lower than previously assumed. In general, the thermodynamic stability of metal complexes, as defined by their stability constants, is considered to be critical for metal bioavailability. New hyphenated analytical techniques could show that also kinetically stable metal complexes can form even under thermodynamically unstable conditions. That the content of free metal ions is rather low in natural waters, is supposed to be due to the formation of kinetically stable organic and inorganic complexes. Recent studies show that it is the rate of exchange of a ligand between the dissolved free metal ion and a particular complex, which determines if a metal complex is stable or not. In fact, it was recently shown that stable metal-sulphide complexes, characterized by strong binding constants (preventing them from oxidation and dissociation), may be persistent in oxic surface waters.

Although the free or hydrated metal ion is still assumed as the most bioavailable form, new research confirms that also labile metal complexes (inorganic and weak organo-metal complexes with stability constants < may contribute to the supply of metal ions to the cell surface, depending on the rate of the flux-determining step (i. e. diffusion/transport through the supply medium, or uptake at the cell surface).

A state-of-the-art description of adsorption phenomena between dissolved metals and particulates is today based on aqueous speciation and surface adsorption models. They generate distribution coefficients (kd-values), and include the concept of additivity of discrete surface sites in binding free and/or complexed metals to solid surfaces. New results emphasize that multiple solid phases must be taken into account when modelling metal uptake processes, and that surface and solution complexation equilibria can explain much of the observed variations for kd in natural surface waters.

The development of highly sensitive and element-specific detectors, like electrothermal vaporization atomic absorption and inductively coupled plasma atomic emission, mass spectroscopic instrumentation, and of electrochemical methods, like ASV, and hence the tremendous lowering of the analytical detection limit (down to ppt levels) for total concentrations of almost all metals, has concomitantly supported developments to further improve the selectivity and reproducibility of chemical and instrumental separation methods (like exchange resin, dialysis membrane, competitive chelation or chromatographic techniques) to identify and quantify particular metal species in complex natural waters (see Reuther 1999 and Allen 2002). As a first approach the distribution of metal compounds in aqueous phases can be defined according to their size as dissolved (< 1 nm), colloidal (1 nm - 0.2 ^m) and particulate metals (> 0.2 ^m). Resulting partitioning or distribution coefficients (kd) provide an estimate of the binding affinities and dynamics prevailing between metals and key water components and factors.

Recently, Town and Filella (2000) compiled the available data on the complexation (e. g. conditional stability constants, effective ligand concentrations) of some trace metals (Cu, Zn, Pb, Cd) in sea, estuarine and freshwaters that have been published during the past 25 years, providing a comprehensive resource and reference book for scientists, who work in the field of dissolved metal ion speciation in natural waters.

To illustrate the situation ruling the partitioning and complexation of metals in natural waters, relevant speciation and surface reactions have been depicted in Table 5.3 for aqueous chromium, as an example.

Table 5.3. Aqueous and surface complexation reactions relevant for chromium speciation (from Nikoloaidis et al., 1999)

Reaction

log K

reference*

H2O + OH- « H+

14.00

Zachara et al., 1989

H+ + CrO42- « HCrO4

6.51

2H+ + CrO42- « H2CrO4

5.56

Na+ + CrO42- « NaCrO4

0.70

"

Cr3+ + OH- « CrOH2+

-10.0

Morel and Hering,

1993

Cr3+ + 2OH- « Cr(OH)2+

-18.3

"

Cr3+ + 3OH- « G"(OH)3

-24.0

"

Cr3+ + 4OH- « Cr(OH)4-

-28.6

"

Cr3+ + Org « Cr3+-Org

15.0

calibrated

=ROH « =RO- + H+

-4.5

soil titration

=ROH + Cr3+ « =ROH-Cr3+ + 2H+

-0.1

calibrated

=FeOH + H+ « =FeOH2+

4.2

Zachara et al., 1989

=FeOH « =FeO- + H+

-10.8

"

=FeOH + Cr3+ « FeOCrOH+ + 2H+

2.26

Dzombak and Morel,

1990

=FeOH + H+ + CrO42- « (=FeOH2+-CrO42-)

9.8

Zachara et al., 1989

=FeOH + 2H+ CrO42- « (=FeOH2+-HCrO42-)0

19.4

* references given in Nikoloaidis et al. (1999)

* references given in Nikoloaidis et al. (1999)

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