Open-air mines and other mining activities often leave polluted mine water behind. But peatlands can help reduce pollution and thus make rivers and groundwater cleaner. Because when bogs are rehydrated with this water, the peat soil develops a cleaning effect that reduces water contamination with iron and sulfate by up to 80 percent, tests show.
Not only is lignite mining a problem with respect to the climate, the mine water generated during mining can also damage bodies of water and the environment. Since these liquids are highly acidic due to their sulfuric acid content, they also contain high concentrations of environmentally harmful sulfate and iron compounds. Although there are operations to clean mine water, they are expensive and often only partially effective, as evidenced, for example, by the heavily polluted former mining landscapes of Lusatia. The rivers and groundwater there are still heavily polluted.
Purification reaction in peat soil
Lydia Russell at Humboldt University in Berlin and colleagues have now investigated a new nature-based approach to cleaning mine water. Their idea: peat soils can help reduce the pollution of this water. Because the acidic, low-oxygen environment of peat favors chemical reactions that convert iron and sulfate compounds into an insoluble mineral. “Under oxygen-free conditions, pyrite should ideally form again in water-saturated peat soils and the iron and sulfur should be removed at the same time in order to prevent regeneration of pyrite oxidation,” Russell explains.
However, it is not yet clear whether this process actually occurs on peat soils – for example, if dried peatlands are rehydrated by spraying mine water. So Russell and her team reproduced this process in a lab experiment. To do this, they filled a cylindrical container with a net under it and closed it with swamp soil and added mine water on top, which was acidic and heavily contaminated with iron and sulfur. The researchers then examined the leaky water below to determine if and how the chemical agents had changed.
80% of contaminants removed
In fact, evaluations have shown that peat soils have removed a significant portion of the contamination from the mine water during this re-hydration process. The acidity of the water decreased from four to six pH. Thus the purified water was almost neutral. Initial high concentrations of more than 250 milligrams of iron per liter and more than 770 milligrams of sulfate per liter decreased with averages of 87 and 78 percent, respectively. The longer the pit water stays in the swamp, the better the cleaning effect.
“The results indicate that microbial decomposition of sulfate and subsequent precipitation of iron sulfides was the most important mechanism in reducing contamination,” explains co-author Dominic Zack of Aarhus University in Denmark. According to the research team, swamp re-wetting can be an effective measure to reduce pollution from acidic mine water. Specifically, they calculated that, for example, sulfate pollution in the dirtiest section of the Spree could be reduced by about 20 percent simply by re-wetting the swamps in their catchment area – they cover about 6,067 hectares.
“Our results show once again that re-wetting peatlands is an important measure to protect our environment. Peatlands stabilize the global carbon balance, conserve water in landscapes and also have an important cleaning function,” says Russell. Although the results of laboratory experiments cannot simply be transferred to large-scale field conditions, scientists acknowledge this, too. However, they can provide evidence and help prepare large-scale field studies.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB); Specialized Article: Journal of Environmental Management, doi: 10.1016/j.jenvman.2022.114808
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