A PLUG FLOW-THROUGH REACTOR FOR STUDYING BIOGEOCHEMICAL REACTIONS IN UNDISTURBED AQUATIC SEDIMENTS

A slow flow, plug-through reactor was developed for measuring equilibr ium and kinetic parameters of biogeochemical reactions on intact secti ons of sediment cores. The experimental approach was designed to prese rve the structural, geochemical and microbiological integrity of the s ediment sections and, hence, retrieve reaction parameters that apply t o in-situ conditions. Inert tracer breakthrough experiments were perfo rmed on a variety of unconsolidated surface sediments from lacustrine, estuarine and marine depositional environments. The sediments studied cover wide ranges of composition, porosity (46-83%) and mean grain si ze (10(-4)-10(-2) cm). Longitudinal dispersion coefficients were deter mined from the breakthrough curves of Br-. The curves were also used t o check for early breakthrough or trailing, that is, features indicati ve of non-ideal flow conditions. Sediment plugs that exhibited these f eatures were eliminated from further experiments. Dimensionless equili brium adsorption coefficients (K) of NH4+, were calculated from measur ed retardation times between the breakthrough of NH4+ and Br-. The val ues of K at 5 degrees C vary between 0.3 and 2.3, with the highest val ue obtained in a fine-grained marine sediment, the lowest in a coarse- grained lake sediment. The values for the marine and estuarine sedimen ts agree with values reported in the literature. The dependencies of K on ionic strength (range 0.2-0.7 m) and temperature (range 5-25 degre es C) in an estuarine sediment confirm that the main sorption mechanis m for NH4+ is ion exchange. The reactor was used in recirculation mode to measure steady-state rates of dissimilatory SO42- reduction in a s alt-marsh sediment. Recirculation homogenizes solute concentrations wi thin the reactor, hence facilitating the derivation of reaction rate e xpressions that depend on solution composition. The rate of microbial SO(4)(2-)reduction was found to be nearly independent of the dissolved SO42- concentration in the range of 2.2-1 mM. Fitting of the experime ntal rates to a Monod relationship resulted in a maximum estimate of t he half-saturation concentration, K-s, of 240 mu M. This value is comp arable to those reported for a pure culture of SO42--reducing bacteria , but is significantly smaller than the only other K-s value reported in the literature for SO42- utilization in a natural marine sediment. (C) 1998 Elsevier Science Ltd. All rights reserved.