Effective electrochemical ligand parameter

Table 2.7 Data for effective electrochemical ligand parameters of several different plant species. Values were calculated using...

Highly negative values of k in addition imply the possibility to retain or even enrich elements which will form but rather labile complexes given the effective electrochemical ligand parameter of the plant species and (Eq. 2.11). These include Sr, Ba or Mn and the REEs (except of Sm, Tb) if E (L) j is close to zero in the latter case, all of which are known to be hyperaccumulated in some plants, e.g. Ba and Mn in Brazil nuts (Emsley 2001) and - among our test set of plant species - Mn gets substantially enriched in blueberries (both leaves (to which data reported here (Markert 1996) pertain) and fruits), k thus is a kind of measure for amplification of differences in the sequence of transport within some plant, from sequestration in/ by root exudates to deposition in the tips of leaves. 

In both kinds of Vaccinium there is potential for thorough amplihcation of small differences in complex stabilities within the plant and thus for hyperaccumulation this mainly refers to Vaccinium myrtillus as its effective electrochemical ligand parameter is fairly low (-0.25 V). Grasses and trees (both deciduous ones and coniferes) vary around k 0, except for Deschampsia flexuosa. 

The corresponding equilibrium state of distributions of various metal ions usually differs from those of the soil substrate, hence entails some fractionation during transport as described before. So, fractionation can be compared to that effected by some single ligand, and Eq. 2.4 and its inversion are then used to define some effective electrochemical ligand parameter which describes the capability of some plant organ to fractionate among metals. 

Fig. 2.15 Element fractionations and effective electrochemical ligand parameters for photosynthetic organs of five plant species frequently occurring in forest understorey regions (fmm left to right) Vaccinium myrtillus (blueberry), Lolium perenne (ryegrass), Deschampsia flexuosa, Molinia caemlea and Vaccinium vitis-idaea (red whortleberry). For other domestic plants, E (L) values may be higher or lower than the average of the above values, e.g. -0.19 V for each Lolium perenne and Betula alba. Very low effective electrochemical hgand parameters (oak, dandehon, plants in tropical...

Table 2.16 Effective electrochemical ligand parameters, kinds of metal ions on which the calculation is based (there are no omissions for reasons of mismatch ) and biochemical effects in Daphnia magna (Franzle and Markert 2006a). For a denticity of 2, consistent values were obtained in aU cases. The rightmost column is for illustration only and not meant to suggest that these ligands are directly involved in the above (column 1) effects of metal administration. The values were derived from regression analysis of toxicological activities or BCF values (Weltje 2003) after rearrangement of Eq. 2.1 for the electrochemical ligand parameter...

Kind of biological/ biochemical activity Denticity [ - ] Effective electrochemical ligand parameter [V] Metal ions used for regression Biorelevant ligand types (examples)... 

Given hapticity (denticity) and electrochemical ligand parameter are identical, a certain ligand - say, the anion of diacetyldioxime - would exert the same discriminating effect on some set of metal ions as the kind of biomass, except for some amplification term (see below). This matches the way of metal transport and accumulation in and among different plant organs as described by Clemens et al. (2002), fractionation being due to different values of sensitivity and intrinsic bond stability. 

The activation of a ligand upon coordination is determined not only by the electronic properties of the binding metal site, but also by those of the ligand itself, namely its electron-donor/acceptor character. Both have a prominent effect on the redox potential of the complexes which has been the subject of parametrization methods (see also Chapter 2.19) by Pickett75 and Lever76-79 who have defined an electrochemical ligand parameter (PL or El, respectively) which is a measure of the net 7r-electron acceptor minus cr-donor ability of a ligand. 

More recent approaches to the effects of the ligands on the redox activity of metal complexes are based upon the assumption that the electrode potential of a redox change involving a metal complex is determined by the additivity of the electronic contribution of all the ligands linked to the metal centre, or to the overall balance between the c-donor and the 7r-acceptor capability of each ligand.3 In particular two ligand electrochemical parameters have gained popularity ... 

The second-order reaction with adsorption of the ligand (2.210) signifies the most complex cathodic stripping mechanism, which combines the voltammetric features of the reactions (2.205) and (2.208) [137]. For the electrochemically reversible case, the effect of the ligand concentration and its adsorption strength is identical as for reaction (2.205) and (2.208), respectively. A representative theoretical voltammo-gram of a quasireversible electrode reaction is shown in Fig. 2.86d. The dimensionless response is controlled by the electrode kinetic parameter m, the adsorption... 

The adsorption of organic ligands onto metal oxides and the parameters that have the greatest effect on adsorption were also studied (Stone et al., 1993). The extent of adsorption was measured by determining the loss of the compound of interest from solution. The physical and chemical forces that control adsorption into two general categories were classified as either specific or nonspecific adsorptions. Specific adsorption involves the physical and chemical interaction of the adsorbent and adsorbate. Under specific adsorption, the chemical nature of the sites influences the adsorptive capacity. Nonspecific adsorption does not depend on the chemical nature of the sites but on characteristics such as surface charge density (Stone et al., 1993). The interactions of specific adsorption can be explained in two ways. The first approach uses activity coefficients to relate the electrochemical activity at the oxide/water interface to its electrochemical activity in bulk solution (Stone et al., 1993). This approach is useful in situations... 

---