Fate of Accumulated Arsenic and Antimony in Peatlands used for Treatment of Mining-Affected Wastewaters

Lost Arsenic

Project description

Natural peatlands are used as passive purification systems for mine wastewaters. They are well-suited for purification purposes as they retard the flow of water and provide a large filtration network with many adsorptive surfaces on plant roots or soil particles. Contaminant removal in peatlands is based on a variety of chemical, physical and biological processes such as adsorption, chemical or biological oxidation or reduction, precipitation and sedimentation, or plant uptake. The efficiency of contaminant removal is dependent on many factors, including inflow water quality, wetland hydrology, system pH and temperature, the nature of the predominating purification processes, and the presence of other components (metals, salts etc.).

In past years, research by our group has focused on the removal of contaminants (including As and Sb) in two treatment peatlands (TPs) used at a mine in Finnish Lapland to polish pre-treated process water (TP A) and drainage water (TP B). The TPs have initially retained As and Sb rather well, and high concentrations of As and Sb are accumulated in the peat close to the water inlet. In the beginning of 2017, the mine has taken up additional treatment to remove sulfate from their process waters. This has led to changes in inflow water quality of TP A, as concentrations of sulfate and other contaminants (including As and Sb) are now lower. Changes in inflow water quality will likely affect long-term retention of contaminants in the peatlands, as less contaminated inflow water might lead to leaching of stored contaminants, as shown in recent laboratory incubations.

Project goal: The project aims to resolve the fate of the initially retained arsenic (As) and antimony (Sb) from treatment peatlands under the present conditions and after possible future discontinuation of their use. Concerning the fate of the stored As and Sb, the following scenarios are feasible:

1) Initially stored As and Sb are released from their original location near the water inlet, the concentration peak in the peat travels down the peatland and all initially retained contaminants eventually end up in the river.
2) Initially stored As and Sb from surface peat is relocated to deeper peat layers.
3) Concentrations of As and Sb even out throughout the peatland.
4) Part of the stored As and Sb is converted to volatile methylated forms and leave the peatlands to the atmosphere.

Thus, the project addresses the following objectives:
1. Assessment of the horizontal spatial distribution in As and Sb concentrations in surface peat over time
2. Assessment of the depths gradients of As and Sb concentrations over time
3. Assessment of porewater As and Sb speciation in the treatment peatlands
4. Simulation of As and Sb retention, leaching and speciation in a laboratory experiment
5. Monitoring of the effect of potential As and Sb leaching on their concentrations in the Seurujoki river
6. Assessment of the impact of changes in outflow water composition on microbial communities in the Seurujoki river

Methods: Environmental Impact Assessment, Microbial Communities, Elemental Analysis, Peat sampling, Porewater sampling, Flow-through columns

 

Further details and project news can be found here.

 

Project coordinator

University of Oulu

Project results

Further details and project news can be found here.