Cd-organic complexes

Nimmo, M. and Fones, G. (1997) The potential pool of Co, Ni, Cu, Pb and Cd organic complexing ligands in coastal and urban rainwaters. Atmos. Environ., 31, 693-702. 

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. 

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. 

Hirsh and Banin (1990) reported that an increase in Cl concentration decreased Cd sorption due to formation of the CdCl+ ion pair. Enhanced sorption in the presence of HC03 was observed due to the formation of the CdHC03+ ion pair. Mahler et al. (1980) found that in calcareous soils treated with sewage sludge spiked with CdS04, Cd complexes were mainly Cd sulfate and carbonate complexes, and the formation of Cd-Cl complexes increased in both soils as total Cd increased. In sludge-amended California soils with pH 7-8, both Cd and Zn organic complexes and Cd phosphate complexes increase with P levels, but free Cd2+ and Zn2+ decrease with P levels (Fig. 3.4) (Villarroel et al., 1993). 

Dissolved organic molecules have many acidic functions (hydroxol and carbonic groups) to complex trace elements and their compounds to form soluble chelates. This is one of the reasons why solubility and bioavailability of trace elements in the rhizosphere are higher than bulk soils. At the same time, many organic acids also directly dissolve trace elements and their compounds in soils. Plant-produced phytosiderophores facilitate elements, such as Fe and Zn, uptake by plants (Zhang et al., 1991 Romheld, 1991 Hopkins et al., 1998). However, Shenker et al. (2001) did not find significant uptake of the Cd-phytosiderophores complex by plant roots. 

The formation of CD inclusion complexes is favored in aqueous media, and hence most of the early chiral resolutions were carried out in the reversed-phase mode. However with the development of various derivatives of CDs, normal and polar organic mobile phases could also be used. The selectivities of these modes increase in the following order normal > polar organic > reversed phase. This is due to certain advantages associated with the normal and polar organic phase modes. Now that all three mobile phase modes can be used, optimization on these phases can be achieved by varying a number of parameters such as composition of the mobile phase, use of organic modifiers, mobile phase pH, flow rate, and temperature. The details of the optimization of the chiral resolution on these CSPs are presented next. 

Mackey, D.3., 1983b. Metal-organic complexes in seawater- an investigation of naturally occurring complexes of Cu, Zn, Fe, Mg, Ni, Cr, Mn and Cd using high performance liquid chromatography with atomic fluorescence detection. Mar. Chem., 13 169-180. 

Values for the thermodynamic stability constants of inoiganic complexes (with HCO, CO3 , S0 , Cl and 0H ) and organic complexes (except for Cu) of trace metals and major ions were chosen principally from the compilation by Zirino and Yamamoto [ 10] Tor Cd, Whitfield and Turner [11 ]for Pb, Van den Berg [9 ]for Cu, and Truesdell and Jones [12] for major ions. The formation of polynuclear complexes has generally been discounted (except for Pb). The activity coefficients of all species have been calculated using the Davies equation [l3 ]. The stability constants of humic complexes of Cu ( Cu-L been calculated in this work. 

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