Kinetics of manganese adsorption, desorption, and oxidation in coastal marine sediments
Limnol. Oceanogr., 58(3), 2013, 987-996 | DOI: 10.4319/lo.2013.58.3.0987
ABSTRACT: Ejection of excavated manganese (Mn)-laden particles from anoxic subsurface sediment onto an oxic sediment–water interface by infauna and by other physical disturbances triggers desorption and oxidation of reduced manganese species. These competing reactions determine whether adsorbed Mn(II) will be desorbed and escape into the water column or be oxidized to Mn(IV) and retained as an insoluble component of the sediment. Consequently, the net flux of Mn(II) to the water column depends on the relative rates of the two reactions. We determined the rates of Mn(II) adsorption, desorption, and oxidation by incubating slurries of natural anoxic sediment at 25°C and monitoring the concentration of manganese in the aqueous phase of the slurry. The kinetics of adsorption, desorption, and oxidation were fitted to first-order rate laws. The first-order rate constants for adsorption ( = 7.0 ± 3.4 h−1) and desorption ( = 12.7 ± 3.3 h−1) are more than two orders of magnitude greater than the first-order rate constant for oxidation ( = 0.020 ± 0.006 h−1). The combination of rapid desorption kinetics and slow oxidation kinetics allows dissolved manganese to be dispersed into the bottom water by turbulent diffusion and escape local reoxidation and retention at the sediment–water interface. Thus, advection of anoxic sediment to the sediment–water interface contributes to the overall flux of manganese from the sediment to the water column.