Ammonium adsorption in nearshore anoxic sediments1

Abstract

The distributions of dissolved, exchangeable, and fixed ammonium were measured in sediment cores from Long Island Sound, Florida Bay, and Pettaquamscutt River, Rhode Island, and in laboratory experiments to determine the importance of ammonium adsorption in anoxic sediments. Apparently, a “dynamic equilibrium” exists between dissolved, exchangeable, and fixed ammonium in sediments. The concentration of exchangeable ammonium increased linearly with increasing concentrations of dissolved ammonium; exchangeable ammonium adsorption was rapid, reversible, and predominantly associated with the organic matter rather than the clay minerals. The concentration of fixed ammonium also increased with increasing concentrations of dissolved ammonium, but this change, due to diagenesis, is small compared to the total fixed ammonium in sediments and is also smaller than the corresponding increase in exchangeable ammonium. The ammonium adsorption coefficient for Long Island Sound sediment was between one and two: of the ammonium produced by organic matter decomposition, as much or twice as much is associated with the sediment as is dissolved in the interstitial water. Therefore, ammonium adsorption by sediments is an important process in the diagenesis of nitrogen in nearshore anoxic sediments.