Relationship between denitrification and redox potential in two sediment-water systems

Abstract

The process of denitrification was observed in vitro for two sediment-water systems. Overlying water was adjusted to a pH of 7.0, a temperature of 20ºC and a nitrate concentration of 10.0 mg L^-1 N-NO₃^-. Dissolved oxygen was maintained at a concentration above 6.0 mg L^-1 O₂ and incubations were carried out in the dark. These conditions insured that reduction of nitrate would result only from denitrification occurring in the sediments.

The average rate of the denitrification, calculated from the amount of nitrate removed during about one year of incubation, was 50 mg N-NO₃^- m^-2 day^-1 for lake sediments and 171 mg N-NO₃^- m^-2 day^-1 for river sediments. Continuous measurements of the redox potential at various depths in the sediments allowed determination of the zone of denitrification and estimation of its migration into the sediment. Denitrification in the sediments occurred in close proximity to the oxygenated surface of sediments.

Study of the kinetics of the denitrification process showed that the reactions for both sediment types were first-order. The exponential character of the reactions was presumably due to depletion of organic matter and could in part be a consequence of the diffusion-limited transport of nitrate into sediments. Diffusion of nitrate into the sediments was a major factor controlling denitrification rate.