Simulating the dynamics of sulfur species and zinc in wetland sediments

In situ measurements comparing vertical SO42− profiles in vegetated and non-vegetated sediments showed that SO42− concentrations in vegetated sediments increased significantly at the beginning of the growing season and then gradually decreased during the rest of the growing season. Throughout the growing season, SO42− concentrations remained higher in the vegetated sediments than in the sediments without plants. The higher SO42− concentrations in the vegetated sediments indicate that oxygen release from roots and evapotranspiration-induced advection by plants play an important role in the dynamics of sulfur species in sediments. Since the total pool of solid-phase sulfide is relatively large compared to the mass of SO42− in the sediments, the gradual decrease of SO42− concentrations may result from limitation of the solid-phase sulfide that is in direct contact with or very close to the roots and rhizomes. This would mean that the main pool of solid-phase sulfide and associated trace metals are not affected by the oxygen release from roots, and the associated trace metals will not become bioavailable during the growing season.A mathematical model was developed to gain a better understanding of how oxygen release from roots is linked to the dynamics of solid-phase sulfide, associated trace metals and dissolved SO42− concentrations in sediments throughout the growing season. The model consists of one-dimensional mass balance equations formulated for the reactive transport of organic carbon, terminal electron acceptors and their corresponding reduced species in the sediments. It accounts for the combined effects of oxygen release by roots and evapotranspiration as well as physical and biological reactions, and includes formulations for the oxidative dissolution of solid species, such as FeS(s) and ZnS(s), as well as the formation of these species. Model simulations indicate that only 0.1% of the total solid-phase sulfide in the sediment is available to be oxidized by the oxygen released from roots.The oxidation of solid-phase sulfide associated with oxygen release from roots not only produces oxidized sulfur species (i.e. SO42−, S0), it also releases the associated metals to the sediment pore-water and perhaps the water column. Simulation results indicate that this oxidation of AVS (acid-volatile sulfide) does not result in the flux of trace metals into the water column.