Microscale Biogeochemical Controls on Manganese Oxyhydroxide Biomineral Formation and Associated Trace Metal Sequestration in ARD Biofilms
Abstract
Identifying the processes controlling reactive metal transport is a necessary prerequisite to the design of effective, mitigative, strategies for contaminated aqueous environments, such as acid rock drainage (ARD). Our research investigates the biogeochemical processes affecting trace metal fate in shallow tailings-associated seepage streams from a northern Ontario ARD environment (Onaping mine, Falconbridge Ltd., Sudbury, ON, Canada). Monthly, from June-Sept 2001, in situ characterization of biofilm geochemical parameters and quantification of biofilm-associated metal concentrations, by sequential extraction, was conducted on a diel scale. Results indicate that significant (p<0.05) seasonal accumulation of Mn, Ni, Co and Cr occurred within the biofilms (e.g. Ni, June: 3.60 mmol/kg; September: 25.7 mmol/kg). As much as 75%\ of the total biofilm concentration of these elements was associated with the amorphous oxyhydroxide fraction. Further, trace metal concentrations were strongly and positively correlated to Mn concentrations in that fraction (R2 > 0.89), implying an important role for Mn oxyhydroxides as a sorbent phase in this system. On a diel basis, Mn concentrations in the amorphous oxyhydroxide fraction decreased significantly in the afternoon compared to morning or late evening values. The magnitude of the loss of Mn was correlated to shifts in the relative depth of the oxic/anoxic boundary. Fine-scale profiling of biofilm pH and O2, using microelectrodes, reflected photosynthesis and respiration; the oxic/anoxic boundary deepened and pH increased within the biofilm during daylight hours. Due to the low pH conditions of the biofilms (3.5-4.5) Mn oxyhydroxide formation is necessarily microbially-catalyzed. Therefore, although the exact mechanisms controlling Mn cycling in this fraction have yet to be elucidated, likely processes include microbially mediated Mn oxidation during non-photosynthetically active hours and abiotic dissolution during photosynthetically active, daylight hours. Trace metal concentrations in the amorphous fraction showed element-specific diel variations. While Cr concentrations followed the same diel pattern as Mn, Ni and Co concentrations did not cycle on a diel basis, resulting in enriched Ni/Mn and Co/Mn ratios in the late afternoon. This enrichment is attributed to rapid resorption of these elements to the remaining pool of Mn oxyhydroxides whose sorptive capacity is increased as a result of elevated pH values in the biofilm in the afternoon. Further understanding of the microbial processes controlling Mn oxidation in the biofilms and the nature of the Mn-trace metal associations can be gained only with high resolution approaches. Thus, ongoing research investigates: 1) the extent of biofilm microbial diversity and potential Mn oxidizing species by molecular techniques (FISH); and 2) the reactive Mn oxyhydroxide phases (high resolution-TEM and associated EDX) and (XAS), in samples collected from June-August 2002. Results from the integration of these techniques, providing a mechanistic understanding of processes controlling biofilm Mn oxyhydroxide biomineral formation and associated metal sequestration will be presented.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.B22E..06H
- Keywords:
-
- 0330 Geochemical cycles;
- 0400 BIOGEOSCIENCES;
- 1065 Trace elements (3670)