Mercury methylation occurs in aquatic bottom sediments

1
Nathan Johnson The University of Texas at Austin,
Texas
Mercury Methylation Beneath a Sediment Cap
Background
Objectives
Experimental Plan
Analytical Methods
Preliminary Results
Conventional Microelectrodes

• Characterize changes in biogeochemical conditions
  after the placement of a sediment cap.
• Understand how those changes affect mercury
  methylation.
• Develop a model to simulate biogeochemical
  changes after cap placement and the concomitant
  effects on mercury methylation

• Small laboratory microcosms are used to simulate
  conditions at the bottom of a river.

• Data from bulk Anacostia River samples
  equilibrated for gt3 months in laboratory
  microcosm
• Oxygen disappears in top 2-3 mm

• Mercury methylation occurs in aquatic bottom
  sediments
• Mercury methylation is tied to the activity of
  sulfate reducing bacteria

• Currently using amperometric electrode for O2
  measurements and potentiometric for sulfide and
  redox
• Relatively easy to employ.
• Oxygen and Sulfide electrodes are small, but
  redox is too large for detailed profiling.

• Microelectrodes are used to measure profiles of
  important redox-active species.

Voltammetry

• Redox potential drops to levels characteristic of
  sulfate reduction, -150 mV (Eh) at depths
  of 2-4 cm

• The location and extent of sulfate reducing
  activity is defined by biogeochemical processes
  in sediment

Questions to be answered

• Potential to measure O2, Fe2, Mn2, S2-
  simultaneously with µM detection limits.
• Difficult to employ.
• Electrodes small enough (100 µm) to profile
  without disturbing sediment.

Planned Tasks

• Will sulfate reducing activity and mercury
  methylation increase under a cap?
• Will methylmercury migrate through the cap?
• Will demethylation processes stabilize
  methylmercury concentrations or will
  methylmercury build up under a cap?
• Can these questions be answered a priori with
  site-specific data and a model?

• Measure profiles of O2, Fe2, HS-, and redox
  potential at quasi-steady state conditions with
  oxygen in overlying water.
• Place 1-2 cm cap on sediment and monitor changes
  in redox profiles.
• Core (slice) sediment cell and measure mercury
  and methylmercury profiles.
• Measure methylation and demethylation with
  radiolabeled mercury and methylmercury.
• Possibly use molecular techniques to characterize
  microbial consortia.

• Capping is being considered as a management
  strategy for containment of heavily contaminated
  sediments, esp. for chlorinated organics
• In addition to sequestering contaminants,
  sediment caps change the biogeochemistry of
  underlying sediment
• These biogeochemical changes could have a
  significant impact on the location and extent of
  mercury methylation

DET / DGT

• High sulfide levels (10-4M, 3mg/L) observed at
  2-3 cm

• Diffusion Equilibrium Thin-Film and Diffusion
  Gradient Thin-Film are some other technologies
  that people have used for profiling. We will
  look into these technologies as well.

Contact Info njohnson_at_mail.utexas.edu (512)
471-6682