Desorption kinetics of Cd, Zn, and Ni measured in soils by DGT

DGT (diffusive gradients in thin films) was used to measure the distribution and rates of exchange of Zn, Cd, and Ni between solid phase and solution in five different soils. Soil texture ranged from sandy loam to clay, pH ranged from 4.9 to 7.1, and organic carbon content ranged from 0.8% to 5.8%. DGT devices continuously remove metal to a Chelex gel layer after passage through a well-defined diffusion layer. The magnitude of the induced remobilization flux from the solid phase is related to the pool size of labile metal and the exchange kinetics between dissolved and sorbed metal. DGT devices were deployed over a series of times (4 h to 3 weeks), and the DIFS model (DGT induced fluxes in soils) was used to derive distribution coefficients for labile metal (Kdl) and the rate at which the soil system can supply metal from solid phase to solution, expressed as a response time. Response times for Zn and Cd were short generally (<8 min). They were so short in some soils (<1 min) that no distinction could be made between supply of metal being controlled by diffusion or the rate of release. Generally longer response times for Ni (5-20 min) were consistent with its slow desorption. The major factor influencing Kdl for Zn and Cd was pH, but association with humic substances in the solid phase also appeared to be important. The systematic decline, with increasing pH, in both the pool size of Ni available to the DGT device and the rate constant for its release is consistent with a part of the soil Ni pool being unavailable within a time scale of 1-20 min. This kinetic limitation is likely to limit the availability of Ni to plants.