Nitrate controls methyl mercury production in a streambed bioreactor

Area of Science:

• Environmental Science
• Environmental Chemistry
• Biogeochemistry

Background:

• Organic carbon bioreactors are cost-effective for treating environmental contaminants.
• These bioreactors can have unintended consequences, such as methyl mercury (MeHg) production.
• Sulfate-reducing conditions, often occurring after nitrate removal, are known to stimulate MeHg production.

Purpose of the Study:

• To investigate the production of methyl mercury (MeHg) in a wood chip bioreactor designed for nitrate (NO₃) removal.
• To determine the conditions under which the bioreactor acts as a source or sink for MeHg.
• To assess the impact of bioreactor operation on total mercury (Hg) removal.

Main Methods:

• A 40 m³ wood chip bioreactor treating agricultural drainage in southern Ontario was monitored over one seasonal cycle.
• Effluent concentrations of methyl mercury (MeHg), total mercury (Hg), nitrate (NO₃), and sulfate (SO₄²⁻) were measured.
• Bioreactor performance was evaluated under varying NO₃ and SO₄²⁻ reducing conditions.
• Mercury flux was calculated to determine if the bioreactor was a source or sink.

Main Results:

• The bioreactor effectively removed 30–100% of nitrate (NO₃-N).
• When sulfate-reducing conditions were present, MeHg concentrations increased, and the bioreactor became a significant MeHg source (mean flux of 15.2 μg m⁻² yr⁻¹).
• Under conditions favoring only nitrate reduction, the bioreactor acted as a net MeHg sink (mean flux of -5.1 μg m⁻² yr⁻¹).
• Total mercury (Hg) was consistently removed, correlated with total suspended sediment removal via filtration.

Conclusions:

• Organic carbon bioreactors can be a substantial source of MeHg when sulfate-reducing conditions develop.
• Maintaining nitrate-nitrogen (NO₃-N) concentrations above 0.5 mg L effectively suppresses MeHg production.
• Bioreactor design and operation should consider potential MeHg production under specific redox conditions.