PNNL

Abstract

Microbially mediated ammonium oxidation is a major process affecting nitrogen transformation and cycling in natural environments. The dissolved ammonium concentration is also affected by ion exchange in geologic media, such as in soils and groundwater systems. This study investigated whether ion exchange process can affect microbially mediated aerobic oxidation of ammonium. Sediments from the Columbia River hyporheic zone at US Department Energy's Hanford site Washington State were used as an example for this study. Experiments were conducted using the sediments in synthetic groundwater and river water with different ionic strength and chemical compositions. Results indicated that ammonium sorption through ion exchange decreased the rate of ammonium oxidation, apparently resulting from the influence of the ion exchange on dissolved ammonium concentration, thus decreasing the bioavailability of ammonium for microbial oxidation. However, with the decrease of dissolved ammonium concentration, the sorbed ammonium released back to aqueous phase, and became bioavailable so that all the ammonium in the suspensions were oxidized. Low ionic strength enhanced ammonium sorption, and thus decreased the rate of ammonium oxidation. Our results implied a dynamic change in ammonium oxidation kinetics in an environment such as at HZ where river water and groundwater exchanges frequently that can change ionic strength, and thus ammonium sorption and desorption.