ð15Þ
Kd values and the kinetic parameters related to the desorption process
(Table 2) are used to interpret the changes in r and consequently
the different kinetic regimes (P1 and P2) for the three
cations.
3.4.1.1. Cd. Cd is characterized by fast desorption kinetics (short Tc
and high klab), small reservoir of metal in the solid phase (Kd; cf. Table
2) and high r-values (over the first 1 h of the reaction time). The
initial peak in r (Fig. 4) is associated with the fast kinetics of
desorption; the soil solution is then readily replenished and Csoln
is nearly equal to Clab (Bierry) or larger than Clab (Grimault). After
1 h of reaction, r declines due to the small size of available reservoir.
This case is characterized by kinetic profile P2 (cf. Fig. 1b) in
soil/EDTA/mixture. Kinetics of resupply is presumably not limiting
in Cd transfer to Chelex and the depletion in Csoln is associated with
the limited capacity of soil reservoir to replenish the solution.
3.4.1.2. Pb. Pb is characterized by slow desorption kinetics
(Tc > 130, klab < 0.16), larger reservoir of metal in solid phase (Kd;
cf. Table 2) and lower r-value compared to Cd. The ratio (r) rises
to attain a plateau at its maximum value i.e. 0.2 in Grimault and
0.45 in Bierry (Fig. 4). Comparing these values with those of Cd
in both soils, it could be considered that Csoln is markedly depleted
with respect to the potential concentration in labile pool (Clab) due
to the slow kinetic of desorption. However, a low level of resupply
is sustained due to the large reservoir of Pb in solid phase and no