Adsorption versus bioavailability

Aqueous speciation of trace metals in freshwaters deserves thorough investigation, in particular regarding the question of the rate-determining step in uptake processes. In this context, it is assumed for trace metals that the free M2+ or total labile M(II) activity represent the biologically most relevant parameters. According to Jansen et al. (1998) who intensively studied the equilibrium speciation and the labilities of Zn complexes in European river waters by means of DPV/SV (Differential Pulse Voltametry / Stripping Voltametry), complex species which are labile on the effective timescale of biouptake processes are also bioavailable by definition (compare section 5.1.2). However, if such complexes really contribute to the supply of Zn(II) to a cell surface, which selectively takes up the complexed metal, depends on the flux-determining step, e.g. on the transport through the supplying medium or uptake of Zn2+ at the surface. In general, complexation between a free (hydrated) metal ion (M) and a ligand (L) can be described as:

where kA and kD are the association and dissociation rate constant of the complex ML, respectively. Complexation equilibrium is then defined by the stability constant:

where CML, CM and CL are the bulk concentration of the complex, the free metal and free ligand, respectively.

We know that rates of metal complexation reactions in aqueous systems are controlled by the rate of removal of a coordinated water molecule from the inner hydration shell of the respective metal ion. The resulting dehydration rate constant kW is directly related to the association rate constant kA of the overall complexation reaction (in eq. 5.1). The corresponding kW value for Zn(II) has been found to be 107 and 108s-1, and the kA value in the order of 108 M-1 s- . For complexes with known diffusion and stability coefficients (k), it is now possible to make theoretical predictions about their labilities for a given effective time scale (t) and diffusion layer thickness (5).

It is generally assumed that complexation of metal ions by organic ligands may lower their affinity for surfaces and enhance their mobility, which may for example ease their passage through wastewater treatment plants. In addition, complexation is supposed to diminish metal toxicity. Today we can distinguish by means of various electrochemical and competitive ligand exchange techniques between labile metal species (e. g. free metal cations, inorganic complexes, and weak metal-organo complexes with conditional stability constants Kcond < 10 ), moderately strong complexes (e. g. metal complexes with humic material and Kcond of 105-1012), and strong complexes (e. g. by polydentate ligands with Kcond > 1012).

Voltametric experiments done by Jansen et al. (1998) suggest the existence of labile Zn(II) complexes with stability constants in the range of 106 4 and 107 M-1 in many river waters. These values are substantially higher than those found for commercial humic acid complexes (1050 - 1055M-1) at comparable pH and metal-to-ligand ratios. However, theoretical reconstruction of the labilities based on association/dissociation rate constants confirmed the experimental results. In the investigated river water samples it was found that 30 % of Zn was present as free metal ion and 70 % as voltametrically labile Zn(II) complexes. Concerning the bioavailability of Zn in these waters, one has to consider the rate-determining step. In case that transport of Zn(II) in solution may limit its biouptake rate, the coupled diffusion of free and complexed Zn will be flux determining. If so, the flux of Zn(II) will be reduced by about 50 %, if 70 % of Zn(II) is complexed, according to Jansen et al. (1998). However, if the actual uptake of Zn at the water-cell interface is considered as flux-determining, the flux is directly related to the concentration of the free Zn2+ ion (compare also with section 5.4.7.1 and Figure 5.1 below).

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