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

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.

 

Central hypothesis of the project

Changes in inflow water quality will lead to changes in As and Sb speciation in the peat, which will in turn lead to increased As and Sb mobility and thus leaching of accumulated contaminants.

 

Project goals and objectives

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

 

Project tasks

The project is subdivided into 3 work packages:

1: In situ As and Sb concentrations and speciation in the TPs

2: Simulation of As and Sb binding and leaching in flow-through columns

3: Environmental impact of leaching of As and Sb from TPs

 

Project news

WP1:

Sampling of peat and porewater concentrations was done in summer 2019. The results were compared to results obtained in earlier years. The As peak is still rather close to the inlet, but a shift towards the outlet seems to have occurred. Sampling will be continued in 2020 to confirm this trend.

Figure: Arsenic concentrations in TPA peat in different sampling years. Arsenic is mainly found near the TP inlet and is bound in the upper peat layers. A shift of the arsenic peak towards the outlet of TPA is observable.

 

WP2:

Column experiment have been started as part of the diploma thesis of Vera Luostarinen. Columns were constructed with options to sample inlet, outlet and porewater of the columns. The columns will first be loaded with higher concentrations of As or Sb. This loading phase is ongoing. After loading, columns will be supplied with low As/Sb water to induce leaching. The distribution of As and Sb in peat and porewater is followed in the different stages.

Figure: Column experiments to assess arsenic and antimony leaching. A) Schematic overview of the column experiment. B) Example column. Each column has one inflow tube at the bottom, one outflow tube at the top and three porewater samplers distributed evenly along the lenght of the column. C) Columns during the loading phase.

 

 

 

Last updated: 17.4.2020