Cd-DOM complexes

TABLE 11.1. Speciation of Cd in Soil Solution of Kapinnie Soil Using Various log K Values for the Cd-DOM Complexes... 

The speciation of Cd in soil solutions obtained from soils incubated with 10 mg Cd kg soil was conducted using the M1NTEQA2 computer model (Krishnamurti and Naidu, 2003). The Cd-DOM complexes were found to be the most dominant species with small amounts (average < 5%) of free Cd"+ species (Table 11.4). Recent reports (Almas et al., 2000 Sauve et al., 2000) have also shown the Cd-organic complexes to be the dominant Cd species in most soil solutions, using differential pulse anodic stripping voltammetry for measuring Cd2+ activity. 

Speciation of Cd in soil solution using different log K values for Cd-DOM complexes (from Krishnamurti and Naidu, 2003)... 

Few studies report actual measurement of Cd + speciation in soil solutions. Many authors report Cd speciation based on the measured total dissolved Cd concentration and computation of free Cd using chemical equilibrium models. The accuracy of the speciation depends on the correctness of Cd-complex stability constants (Turner, 1995). For example, the proportion of free Cd (as Cd ) in solution varies from 16 to 82% depending on the log K values for Cd-(DOM) dissolved organic matter complexes (Table 4). The stability constants of Cd-humics are particularly uncertain. The reported high values of pCd + ranged... 

Kinetic studies with EDTA, as a well-defined model ligand for strong complexes forming natural ligands, on the chelation of Cd, Pb and Zn have revealed the typical formation rate constants and the mechanism operative in the formation of stable complexes of heavy metals with DOM-... 

Al(III) appears both to decrease and increase DOM fluorescence when forming a complex with fulvic acid (FA), isolated from soil (Ryan et al., 1996), river (Elkins and Nelson, 2002), and marine (da Silva and Machado, 1996) environments. The effect is presented in Figure 7.4 (redrawn from Elkins and Nelson, 2002), in which the change to relative fluorescence intensity of a river FA after addition of Al(ni) is compared to additions of Cd(II) and of Cu(II). Immediately after addition of the Al(lll) metal, fluorescence (ex = 344/an = 424) increased and reached a stable intensity at 5.0 x 10 M Al(IIl). By contrast, additions of up to 2 X 10 M Cd(II) did not change river FA fluorescence intensity. The addition of Cu(II) to the river FA solution produced fluorescence quenching, as shown in Figure 3B. 

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