Selenium Redox Reactions and Transport between Ponded Waters and Sediments

Understanding the cycling of trace elements such as selenium in the environment requires information on mass transfer and reactions occurring across boundaries. Selenium partitioning and redox transformations in ponded laboratory sediments was compared with a model based on the transport of Se(VI) and Se(IV) between surface waters and sediment pore waters. Waters containing Se(VI) were ponded over initially nonseleniferous sediment columns. X-ray absorption spectroscopy demonstrated Se reduction to Se(0) in these sediments. Synchrotron X-ray microprobe analyses revealed very high Se concentrations within mil limeters of the sediment surface. Concentrations of Se(VI) and Se(IV) in sediment pore waters were used as time-dependent boundary conditions to model concentrations of these species in overlying pool waters. Two parameters, the mass transfer coefficient and pool Se(VI) apparent reduction rate constant, were optimized. The successful application of mass transfer coefficients in this study resulted from Se reduction rates within very shallow boundary zones being proportional to Se concentration gradients acting across these zones, rather than solely representing diffusion across these zones. The large gradients in Se concentrations observed in these sediment columns indicate that experimental methods with spatial resolution finer than 1 mm are needed before mechanistic diffusion−redox analyses can be successfully applied to these types of systems.