(2000), who demonstrated a similar pattern in Cr desorption in their column studies, and those of James (1994), who also reported a similar Cr release pattern from alkaline soils.
Although the surface soil contained [is greater than] 62 g Cr/kg, only 5.12 mg/kg was desorbed into water in the first step of desorption (Fig.
The decrease in Cr(VI) desorption in the presence of [Ca.sup.2+] may be attributed to either the decrease in pH and/or the changes in the thickness of the diffuse double layer and possibly the reduction of Cr(VI) to Cr(III) under high organic and reduced pH conditions.
Since both chromate and phosphate are adsorbed by similar specific adsorption mechanisms (Bartlett and James 1988), the effect of Ca and Na on Cr(VI) desorption was expected.
Phosphate at a concentration of 3.2 mmol/L in both electrolytes desorbed nearly 85% more Cr(VI) than the phosphate-free system, and significant correlation of phosphate adsorption and chromate desorption was observed in the contaminated surface soil (Fig.
However, the concentrations of native Cd desorbed from the soils during 5 desorption equilibrations represented small proportions only of the intercept concentrations (Table 5).
Added Cd desorption data were plotted as cumulative Cd desorbed during 10 successive desorption events ([micro]g/g) from Cd initially sorbed from an addition of 2 [micro]g Cd/g.
(1984), and Jarvis and Jones (1980) who found Cd desorption to be very sensitive to soil pH.
Increasing the time available for sorption decreased the subsequent desorption of Cd in all 4 soils examined (Table 7).
A second explanation offered for the reduction in desorption with time may he in the finding that there is slow redistribution of ions to more strongly bound or less accessible sorption sites (Mann and Ritchie 1993b).
The desorption of silver ions from the 4 soil materials as a function of time is shown in Fig.
20 h of desorption, 23-35% of Ag was desorbed from ferrihydrite, and 2-5% of Ag was desorbed from charcoal regardless of the adsorption reaction time.
The micropores and intraparticle spaces in ferrihydrite are therefore much more likely to be accessible to Ag+ for diffusion-limited adsorption and desorption processes than are the corresponding intraparticle spaces in domains of goethite crystals.
Several models, including the first-order kinetic model, the Elovich equation, the parabolic diffusion equation, and the heterogeneous diffusion model have been used by many researchers to describe adsorption and desorption kinetics of ions from soils and soil minerals (Sparks 1988; Kithome et al.
Previous discussion seems to suggest that desorption from goethite and ferrihydrite is controlled by the diffusion process.