Temporal evolution of redox processes and free Cd dynamics in a metal-contaminated soil after rewetting

Soil testing procedures to address metals bioavailability currently use air-dried soil rewetted almost until saturation. Such practices may influence the redox state of soil and the related dynamics of metals. To assess this potential impact, a metal-contaminated soil was air-dried and rewetted to 90% water holding capacity. We monitored over a 21-day incubation period the temporal changes of soil redox potential and solution Cd concentration (either total or free). Other physico-chemical parameters were followed notably pH, ionic strength (I) and the concentrations of NO(3)(-), Mn, Fe and SO(4)(2-) in solution. Soil redox potential showed the progressive establishment of strong reducing conditions in soil, in agreement with the temporal changes of NO(3)(-), Mn, Fe and SO(4)(2-) concentrations. It decreased by 13 pe units over the culture period leading to sulphate-reducing conditions (pe<-3) within only 21days. Solution Cd concentration increased transitorily over the first 100-150h of incubation (2-fold increase) in relation with the parallel increase in the concentration of competing cations for adsorption (Ca, Mg). It steeply decreased over the last 300h of incubation (30-fold decrease) as a result of Cd precipitation as Cd sulphides. This biphasic evolution of Cd dynamics was related to the temporal changes of Cd resupply from the solid phase. Using the technique of DGT we described the kinetics of Cd resupply over time and needed to invoke the existence of two pools of Cd.