Modelling evaluation of key cadmium transformation processes in acid paddy soil under alternating redox conditions

The intermittent flooding conditions in acid paddy field can substantially affect the phytoavailability of cadmium (Cd), but the underlying mechanisms controlling Cd transformation are still poorly understood, and furthermore the corresponding models for predict the Cd phytoavailability during pre-harvest drainage phase are still lacked. In this study, Cd-spiked paddy soil was incubated under anaerobic condition for 40 days and then aerobic condition for 15 days. The transformation of Cd speciation and the redox cycling of Fe/N/S was investigated. During anaerobic incubation, the dissolved and exchangeable Cd rapidly decreased, which were mainly attributed to enhanced adsorption on the surface of organic matter with the increasing soil pH. Moreover, the reductive dissolution of Fe oxides induced the release of soil colloids, which result in increasing the surface sites to immobilize Cd. In addition, the formation of Cd sulfide contributed to slight decrease the availability of Cd. However, when oxygen was re-introduced into the system, the rapid decrease of soil pH and surface sites resulted in the release of adsorption on soil surfaces, and oxidative dissolution of Cd sulfide also contributed to the increase of Cd availability. The kinetic models were established based on the key elementary reactions for soil biogeochemical processes on the transformation of Cd fractions. The kinetic models were applied to calculate the potential available Cd in acid paddy soils samples (pH ≤ 6.0) from the field, and moreover, the dissolved Cd was estimated from the surface complexation model (SCM) by incorporating the changes of soil pH. The concentration of model-predicted dissolved Cd during drainage phase had a stronger correlation with the Cd content in rice grain than that of chemical extraction methods. Our results demonstrated that it was feasible to use the kinetic models and SCM to predict Cd phytoavailability during drainage phase of acid paddy field.