A MODEL OF THE OXIDATION OF IRON AND CADMIUM-SULFIDE IN SEDIMENTS

A model is presented for the temporal and vertical variation of acid-v olatile sulfide (AVS), simultaneously extracted cadmium (SEM(Cd)), and cadmium interstitial water concentration in sediments. The model is b ased on the one-dimensional advective-dispersive mass balance equation . Transport by particle and pore-water mixing is considered. The state variables include the components of AVS and SEM(Cd), namely iron sulf ide (FeS) and cadmium sulfide (CdS), and sorbed + pore-water cadmium ( Cd-T). In addition, particulate organic carbon (POC), iron oxyhydroxid e, and dissolved oxygen are explicitly modeled. The oxidation, displac ement, and partitioning reactions that are sources and sinks of the st ate variables are included. The model is applied to three experimental data sets. Two of these employed marine sediments with typical organi c carbon and AVS concentrations. The third employed a freshwater sedim ent with a high POC and low AVS. The calibration produced similar part itioning parameters on an organic carbon basis for both sediments. How ever, the oxidation and partitioning behavior of the FeS in the low AV S freshwater sediment was distinctly different from the high AVS cases . The model successfully reproduces the major features of the experime ntal data: the progressive decrease in AVS and SEM(Cd), and an increas e in the SEM(Cd) to AVS ratio in the surface layers. It is less succes sful in reproducing the pore-water cadmium concentrations.