Kinetics of zinc and cadmium release in freshly contaminated soils

The kinetics of metal release from the solid phase to solution was measured on two sets of 14 freshly contaminated soils with diverse properties. From measurements of metal concentrations in extracted soil pore water, the amount accumulated from the soil by diffusive gradients in thin-film (DGT) devices, and the distribution coefficient for labile metal, Kdl, estimated by isotopic exchange, we calculated the response time, Tc, of the soil-solution system to the removal of metal by DGT and the rate constant for release from the solid phase, k(-1). Resupply was so fast for Zn that Tc (and k(-1)) could be measured only in three of the soils, with either a silty or a sandy loam texture and low to intermediate pH (4.84-5.66). In only six clay soils was resupply of Cd too fast to measure. The generally slower release rates of Cd compared to Zn may reflect the 100-fold lower concentration of Cd, which allowed a greater proportion of it to occupy stronger binding sites with slower release rates. The rate constants derived indicate that supply from the solid phase to solution will not limit uptake of Cd or Zn by plants in clay soils, but it could be a factor in sandy or silty soils with a low pH. These findings suggest that risk assessment of clay soils could be undertaken using measurements of metals in soil solution. However, devices such as DGT, which respond to the kinetics of supply, are necessary to assess available metal in low pH, sandy, and silty soils.