Cd desorption

TABLE 5.6. Kinetics of Cd Desorption from Soils by mol L NH4C1 and Phosphate... 

The kinetics of Cd release, as influenced by the LMMOLs, play an important role in plant Cd uptake. The kinetic rate constant of Cd release, as obtained from desorption kinetics of Cd by LMMOLs and the amount of Cd released by renewal of LMMOLs from the soil, followed the same trend as the cadmium availability index and Cd grain content of durum wheat grown on the soils (Table 5.7). These reports highlight the significance of Cd desorption kinetics in understanding Cd dynamics and phytoavailability. 

Research on cadmium pollution has received increasing international attention inasmuch as cadmium is toxic and can cause severe human health problems such as kidney disorder and itai-itai disease (Webb, 1979 Alloways, 1995). The influence of residence time and organic acids on Cd desorption from pure goethite has been studied (Glover et al., 2002). However, pure iron oxides rarefy... 

The objective of the present study is to examine the desorption kinetics of Cd following its adsorption on iron oxides. To simulate the effects of organic ligands in soil rhizosphere environment and chloride-bearing fertilizer, Cd desorption caused by citrate, acetate and chloride was investigated. 

The rate coefficients of Cd desorption from the iron oxides by various extractants are presented in Table 3. The data show that in all of the systems studied, the rate coefficient of the fast reaction was much greater than that of the slow reaction. The rate of Cd release by different extractants for the fast reaction from all the systems studied was in the order Cl > citrate > acetate > NO (Table 3), which was generally consistent with the stability constants of Cd-extractant ligand... 

The degree of fitting of various equations to the fast reaction (5 min to 2 h) of Cd desorption from the Fe oxide formed at the initial citrate/Fe(II) MR of 0.1 by chloride and citrate... 

In the chloride and nitrate extractant systems, the iron oxides formed at the initial citrate/Fe(II) MRs of 0,0.001, and 0.01 had similar rate fast Cd desorption reaction rates (Table 3). However, the iron oxide formed at the MR of 0.1 had a significantly slower rate of fast Cd desorption reaction (Table 3), although it had the greatest specific surface area (Table 1). UnUke the iron oxides formed at the initial citrate/Fe(II) MRs of 0,0.001 and 0.01, the surface area of the iron oxide formed at the MR of 0.1 is predominantly composed of micropore surface instead of mesopore surface (Liu and Huang, 1999a). This further confirmed that the fast reaction of Cd desorption could predominantly occur on the mesopore surface of the iron oxides. 

Fig. 6. Plotting of Cd desorption by chloride or citrate from Fe oxides formed at initial citrate/Fe(II) MRs of (a) 0 and (b) 0.1 based on overall parabolic diffusion equation.

In all the extractant systems, the rate of slow reaction of Cd desorption from the iron oxides formed at various initial citrate/Fe(II) MRs followed the order... 

Rate coefficients (cmol kg of Cd desorption in overall diffusion equation ... 

The desorption kinetics can be divided into the fast and slow reactions. For the fast reaction, the rate of Cd desorption is generally in accord with the stability constants of Cd-extractant ligand complexes chloride > citrate > acetate > nitrate. For the slow reaction, the rate of Cd desorption is apparently influenced by the size of extractant molecules citrate > acetate > chloride > nitrate. A longer induction period for an extractant with a larger molecule is required for Cd desorption from the micropore surface. Therefore, stability constants of Cd-extractant ligand complexes, and steric factor of the molecular size of the extractants in relation to the pore size of the oxides merit attention in understanding the kinetics of Cd desorption. 

---