A model of the oxidation of iron and cadmium sulfide in sediments

A model is presented for the temporal and vertical variation of acid-volatile sulfide (AVS), simultaneously extracted cadmium (SEMcd), and cadmium interstitial water concentration in sediments. The model is based 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 SEMcd, namely iron sulfide (FeS) and cadmium sulfide (CdS), and sorbed + pore-water cadmium (CdT). In addition, particulate organic carbon (POC), iron oxyhydroxide, and dissolved oxygen are explicitly modeled. The oxidation, displacement, and partitioning reactions that are sources and sinks of the state variables are included. The model is applied to three experimental data sets. Two of these employed marine sediments with typical organic carbon and AVS concentrations. The third employed a freshwater sediment with a high POC and low AVS. The calibration produced similar partitioning parameters on an organic carbon basis for both sediments. However, the oxidation and partitioning behavior of the FeS in the low AVS freshwater sediment was distinctly different from the high AVS cases. The model successfully reproduces the major features of the experimental data: the progressive decrease in AVS and SEMcd, and an increase in the SEMcd to AVS ratio in the surface layers. It is less successful in reproducing the pore-water cadmium concentrations.