Cd speciation in soil solutions

Figure 3.2. Changes of Cd speciation in soil solutions of a typical Israeli calcareous soil with pH 4-9 (after Hirsh and Banin, 1990, with permission from Soil Sci. Soc. Am)...

Dissolved Organic Matter-Solution Speciation Few studies report actual measurement of Cd + speciation in soil solutions. Many authors report Cd speciation based on measured total dissolved Cd concentration and computation of free Cd2+ using chemical equilibrium models. The accuracy of the... 

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... 

Organic complexed Cd is not important in arid soil solution. Hirsh and Banin (1990) observed 5-10% of Cd bound to organic ligands in Israeli arid soil solution. Emmerich et al. (1982) found that organic-Cd complexes constituted 1-4% of Cd in California arid soil solution. However, Villarroel et al. (1993) reported that in a California sludge-treated soil, Cd was mainly present in both free ion and organic complex forms (each accounted for 32-40% and 30-45% of total Cd in soil solution, respectively), followed by the chloride complexes (8-20%), S04-complex (3-10%), and P04-Cd complex (1.5-7.7%). The nitrate Cd complexes were the lowest. Cadmium activities and speciation is not significantly affected by P and N treatments. 

The existence of an element in different chemical forms in the gaseous, solid or aqueous solution phase provides the conceptual basis for speciation in soils. More particularly, a chemical species in soil refers either to a specific molecular arrangement of the atoms of an element or, quite often, to the result of an operational process of detection and quantitation aimed at elucidating chemical forms (Bernhard et at., 1986, pp. 7-14). In principle, the former definition should be the outcome of the latter, methodological definition. In practice, this connection is difficult to achieve in natural systems (Bernhard et al., 1986) (see Chapter 1 for a definition of speciation). Understanding speciation is important in assessing the availability of plant nutrients, plant uptake of potentially toxic elements (e.g. Al, Cd), and the movement of both nutrient and toxic substances into waterways or other parts of an ecosystem (Da Silva et al., 1991). 

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. 

The speciation of Cd in soil solutions of soils from Northern France that had been contaminated by effluent from Pb-Zn metallurgical plant was calculated... 

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

In arid soil solution, Cd is predominately found in free Cd2+. However, when pH >7.5, Cd(OH)+ and Cd(OH)2° account for only a small percentage of the speciation (Lindsay, 1979). CdHC03+ is significant near pH 8.0. 

Figure 3.4. Effects of phosphate levels on Cd and Zn solution speciation in California soils that received sludge application (data extracted from Villarroel et al., 1993)...

Figure 3.5. Effects of Cl concentrations (NaCl or CaCl2) on Cd solution speciation in a California soil (data extracted from Bingham et al., 1983)...

Inorganic speciation in solution can also affect the mobility of metal ions (Doner, 1978). The formation of an ion-pair with Cl can more than double the mobility of Cd in the presence of 200molm 3NaCl. At the same chloride concentration, however, the mobilities of Cu2+ and Ni2+ are only increased slightly (5-10%), presumably because of very weak complexation with Cl. This mechanism could increase the leaching of Cd from saline soils but it may not be effective in non-saline soils because the ratio of the total concentrations of Cd Cl must be >1 106 before >50% of total Cd is complexed by Cl (estimated using the computer model TITRATOR (Cabaniss, 1987), which considered the chloro and hydroxy complexes of Cd at pH 5.0 and a total Cd concentration of 0.1 mmolm-3 equilibrium constants were taken from Lindsay (1979)). 

Figure 7.12. Temporal evolution of the speciation of Cd (closed symbols) and Zn (open symbols) in the soil solution collected in pots grown with radish (Raphanus sativus). Hie proportion of free metal ion (free ) which is close to 100% before plant growth is considerably decreased as a consequence of the complexation of Cd and Zn by root exudates. (Adapted from Hamon et al., 1995,)...

Candelaria L.M., Chang A.C. Cadmium activities, solution speciation, and solid phase distribution of Cd in cadmium nitrate and sewage sludge-treated soil systems. Soil Sci 1997 162 (10) 722-732. 

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