Kinetics of Cd desorption

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

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. 

Desorption kinetics of Cd by LMMOL from the soils described by the parabolic diffusion model q = a + where q is the amount of Cd desorbed... 

The kinetic data on desorption of Cd by phosphate, as related to the amount of Cd released during the initial 30-minute reaction period and the overall diffusion coefficients obtained from the desorption kinetics of Cd by mol NH4CI from the soils, reflect well the phytoavailable Cd in the two soils, as shown by the Cd availability index and the grain Cd content of two durum wheat cultivars, Kyle and Arcola, grown on the two soils (Table 5.6). 

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. 

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 bioavailability and toxicity of Cd in soil and related environments are governed by the speciation and dynamics of Cd. Therefore, desorption kinetics of Cd... 

The cationic forms of Cd, Pb Cr, and Ni were shown to be immobilized in clinoptilolite structure by two mechanisms ion exchange and chemisorption [05M1]. In case of lead and chromium, chemisorption predominates. The contributions of both mechanisms in case of Cd and Ni were equal. The long term kinetics of Cd sorption and desorption by Ca-exchanged clinoptilohte was studied by using isotope exchange technique while maintaining pH at circumneutral values [lOAl]. 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

Tien (1987) studied the kinetics of heavy metal sorption-desorption on sludge using the stirred-flow reactor method of Carski and Sparks (1985). Sorption-desorption reactions were rapid with an equilibrium reached in 30 min. The sorption-desorption reactions were reversible. The sorption rate coefficients were of the order Hg > Pb > Cd > Cu > Zn > Co > Ni, while the desorption rate coefficients were of the order Cd > Cu > Hg >... 

The kinetics of several well-known electrochemical reactions have been studied in the presence of an ultrasonic field by Altukhov et al. [142], The anodic polarization curves of Ag, Cu, Fe, Cd, and Zn in various solutions of HC1 and H2S04 and their salts were measured in an ultrasonic field at various intensities. The effect of the ultrasonic field on the reaction kinetics was found to be dependent on the mechanism of metal anodic dissolution, especially on the effect of this field on the rate-determining step of the reaction. The results showed that the limiting factor of the anodic dissolving of Cu and Ag is the diffusion of reaction products, while in the case of Fe it is the desorption of anions of solution from the anode surface, and at Cd the limiting factor is the rate of destruction of the crystal lattice. Similar results were obtained by Elliot et al. [ 143] who studied reaction geometry in the oxidation and reduction of an alkaline silver electrode. 

Desorption kinetics, in the presence and absence of 0.1 mol L monoammonium phosphate during Cd adsorption by the soils, described by the parabolic diffusion model q = a + where q is the amount of Cd desorbed in time t (hours), a is a constant, and kj is overall diffusion... 

Thus, there are two kinetic paths for the hydrogen evolution. The first path consists of charge transfer (CT) followed by chemical desorption (CD) path CT-CD. The second path consists of charge transfer (CT) followed by electrochemical desorption (ED) path CT-ED. Within each path, either of the consecutive steps can be slow and thus can be the rate-determining step (RDS). Each of these paths has two pKJSsible mechanisms. 

Note, that the variation in the concentration of the MO at the initial part of the kinetic from [MO] = 10 4M to [MO] = 0.8-10 4M - A[MO] = 0.2-10 4M corresponds to the MO amount initially adsorbed at the colloidal particle surface [K]-MOad = 2-10 7 M 100 = 2-10 5 M. In this case, one may see from the experimental data obtained (see, e.g., Fig. 2.23) that at the initial step of the reaction, the reaction quantum yield decreases only two-fold. Therefore, the desorption rate of the low-molecular components can not be much lower than the rate of the reaction proceeding on the CdS particle. Since the reaction quantum yield does not change with the varying the light intensity, the desorption of the low-molecular components is a much faster than the redox transformations at the photocatalyst surface. 

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