Modeling exchangeable NH4+ adsorption in marine sediments: Process and controls of adsorption1,2

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Abstract
The process and controls of ammonium adsorption in marine sediments have been studied using data from adsorption experiments with sediments from Saanich Inlet, British Columbia, and with standard reference clays. The average NH4+ adsorption constant determined experimentally for a Saanich Inlet core of 5.3 ± 1.8 liters·mol−1 was not significantly different from the constant measured in situ of 6.1 liters·mol−1.Equations similar to a Langmuir‐type isotherm are derived which express ammonium adsorption in terms of equilibrium exchange expressions (selectivity coefficients) and activities of the major cations. An ammonium adsorption constant calculated from the model for a montmorillonite clay of 12.6 ± 1.7 liters·mol−1 agrees very well with the experimentally measured values of 11.7 ± 0.3 for a montmorillonite reference clay and 11.8 ± 1.2 for a montmorillonite‐rich pelagic sediment. The proposed model is also general enough to describe the adsorption of any minor component in seawater which undergoes ion exchange.Comparison of ammonium adsorption constants before and after the extraction of organic matter indicates that in organic‐rich sediments a “clay‐humic complex” may be controlling ammonium adsorption. However, in organic‐poor sediments, the clay mineralogy will tend to dictate ammonium adsorption behavior.