2.1.1 Copper
Generally copper (Cu) content in soils ranges from 2 to 250 mg/kg (Sparks, 1995). For uncontaminated soils Cu content in different soil types usually varies 1 to 50 mg/kg (Kabata-Pendias, 2001). The mean values vary from 13 to 24 mg/kg. In contaminated soils at old mining areas, close to the smelters, the content of Cu appears to be up to 300-800 and up to 2000-4000 mg/kg in urban orchards,
sludged, irrigated, or fertilized farmland (Kabata-Pendias, 2001).
Cu usually occurs in soils adsorbed by organic matter, Fe and Mn oxides (hematite, goethite, birnessite), amorphous Fe and Al hydroxides and clay minerals.
Occlusion, coprecipitation, and substitution are involved in nonspecific adsorption of Cu. Among all soil components, the ability of soil organic compounds to bind Cu is well recognized. Stevenson et al. (1981) stated that maximum amount of Cu2+ that can be bound to humic and fulvic acids is close to the content of acidic functional groups. In general, this corresponds to the sorption of 48 to 160 mg Cu per gram of humic acid. According to Bloom et al. (1979), humic acids strongly immobilize Cu2+
ion in direct coordination with functional oxygen-atoms of organic substances.
Schilling and Cooper (2004) found that carboxyl and hydroxyl functional groups were most significant in Cu binding in the soil, with the aid of chemical modification and
13C CP-MAS NMR spectroscopy.
Cu is abundant as free and complexed ions in soil solutions of all types of soils. Concentration of Cu in soil solutions varies from 3 to 135 àg/L (0.047 to 2.1 àM), depending on soil types and the experimental techniques (Kabata-Pendias, 2001). Complexation of Cu2+ with soluble organic substances may govern the bioavailability and the migration of this metal in soils and surface waters. CuCO3 is reported to be the major inorganic soluble form of Cu in neutral and alkaline soil solutions (Sanders and Bloomfield, 1980).
Cu is one of the 3d transition metals and the Cu2+ ion has the [Ar]3d9 electron configuration. The first hydrolysis constant (pK1) of Cu2+ ion is 8.00 and the water exchange ratio is 1×9 1/s (Tatara et al., 1997; Sekaly et al., 2003).
2.1.2 Zinc
Zinc (Zn) is a common element and occurs naturally in soils as an element or as a mineral. Zn content in uncontaminated surface soils in the US and other countries ranges from 17 to 125 mg/kg (Kabata-Pendias, 2001). In the highly contaminated sites, Zn concentrations can be much higher, which are mainly present as Zn minerals (Roberts et al., 2002). Zinc concentrations in soil solutions range from
The reactivity of Zn with different soil components is inconclusive. Some researchers have reported that Zn binding by clay minerals and hydrous Fe and Al oxides, are likely to be the most important factors controlling Zn solubility, while complexation with SOM and precipitation may be less important (Zyrin et al., 1976;
Kabata-Pendias et al., 1995; Adb-Elfattah et al., 1981). Organic complexing and precipitation of Zn as hydroxide, carbonate, and sulfide are reported to be of much less importance (Kabata-Pendias, 2001). However, recent researches have demonstrated that SOM is the major sorbent controlling Zn reactions in soils (Weng et al., 2001;
Tipping et al., 2003). It is likely that the importance of different soil components for Zn binding may vary at different conditions.
Zn2+ ion has the [Ar]3d10 electron configuration. The pK1 of Zn2+ ion is 8.96 and the water exchange ratio is 7×107 1/s (Tatara et al., 1997; Sekaly et al., 2003).
2.1.3 Other Metals
Cadmium (Cd) and nickel (Ni) are another two common trace metals which are environmentally relevant. As one of the most eco-toxic metals in soils, Cd concentrations lie between 0.06 and 1.1 mg/kg (Kabata-Pendias, 2001). The sorption of Cd by soil components has been extensively studied and leads to some
generalizations: Cd activity is strongly affected by pH in all soils; in acid soils, the organic matter and oxides may largely control Cd solubility, and in alkaline soils, precipitation of Cd compounds is likely to account for Cd equilibrium.
Ni content in soils ranges from 0.2 to 450 mg/kg and in the US ranges from <5 to 150 mg/kg (Kabata-Pendias, 2001). The highest Ni contents are always in clay and loamy soils. In soils, Ni mainly occurs in organically bound forms and soil Ni carried in the oxides of Fe and Mn is also important and available for plants. Ni concentrations in soil solutions vary from 3 to 25 àg/L (Kabata-Pendias, 2001).
Ni is the 3d transition metal and Ni2+ has the [Ar]3d8 electron
configuration. The first hydrolysis constant of Ni2+ ion pK1 = 9.86 and the water exchange rate equals 3×104 1/s (Tatara et al., 1997; Sekaly et al., 2003). In the periodic table Cd is just below the Zn and Cd2+ has the [Ar]4d10 electron configuration. The first hydrolysis constant of Cd2+ ion pK1 = 10.3 and water exchange ratio equals 3×108 1/s (Tatara et al., 1997; Sekaly et al., 2003).
It is possible that the trace metal kinetics parameters can be related to the basic cation characteristics. Sekaly et al. (2003) tried to correlate the rate constants of trace metals (Co, Cu, Ni and Zn) dissociation from humic substances to the basic cation properties. They found that the lability of the metal complexes follows the reverse order of the ligand field stabilization energy (LFSE), except for Cu. The rate of the water exchange falls into the similar order of LFSEs (weak field). Furthermore, the experimentally observed fastest dissociation rate coefficients of Co, Cu, Ni and Zn complexes shows the similar trend, with the exception of Cu. Likewise, Fasfous et al.
(2004) also observed, except for Cu, the dissociation constants for metal complexes follow the similar trend predicted from the LFSEs (weak field) and the inert
complexes are those with large LFSEs. Although more research is needed, a general model for different trace metals may be possible based on the cation properties.