Initially batch desorption experiments were conducted on the surface (0-15 cm) and subsurface (50-90 cm) soils to assess the kinetics of Cr release, and to determine a suitable solution:soil ratio for detailed studies.
The effect of cations, [Ca.sup.2+] and [Na.sup.+] (Ca[Cl.sub.2], NaCl), and phosphate on desorption of Cr was investigated.
This was expected because the desorption experiments were carried out under batch conditions and there was a greater sink for soluble Cr with increasing solution:soil ratio.
Given that 80% of the desorbable Cr was released within 2 h of equilibration of the surface and subsurface soils, we opted to conduct all sequential desorption studies for 2 h duration per equilibration.
Subsampling for analysis was carried out immediately after this moisture addition and samples were subjected to desorption in the same manner as described above.
All sorption and desorption work was carried out in duplicate and all data presented represent means values of the duplicate determinations.
Native Cd desorption data are plotted as cumulative amounts of Cd desorbed ([micro]g Cd/kg) with 5 successive equilibrations in 0.01 M Ca[(NO.sub.3]).sub.2].
Effect of pH on cumulative desorption of native cadmium Soil pH ([micro] g (% of total Cd) Cd/kg soil) Te Kowhai silt loam 5.1 78.56 22.1 5.3 43.41 14.7 6.1 2.33 7.7 Te Kowhai humic silt loam 5.1 26.38 6.4 5.4 17.65 4.1 6.2 0.00 0.0 Te Rapa peaty silt loam 4.9 61.26 9.7 5.3 38.89 6.3 5.9 2.70 0.5 Motumaoho silty peat 5.0 26.15 2.7 5.3 17.74 1.9 5.9 0.00 0.0 Kaipaki peat 4.9 30.33 2.7 5.1 13.43 1.4 5.8 0.00 0.0 Waiotira silt loam 5.1 37.35 11.6 5.7 32.12 8.8 6.0 1.50 4.7
After shaking for 4 days at 20[degrees]C, the samples were immediately used for desorption or allowed to stand in a 35[degrees]C constant temperature room for up to 4 months with periodic shaking.
Desorption was carried out by continuous peristaltic pumping of 0.01 M Ca[(N[O.sub.3]).sub.2] solution through the filter loaded with adsorbent particles at a flow rate of 1.5 mL/min.
It has been shown that kinetics of Cd and Co desorption from single soil components (Backes et al.
where [C.sub.M] is the concentration of metal ion sorbed to the soil particles at time t, [C.sub.1], and [C.sub.2] are the initial concentrations (t = 0) of metal ion bound to sites with first-order desorption rate constants [k.sub.1] and [k.sub.2], respectively.