Open pit mines and other mining activities often leave contaminated mine water behind. But peatlands could help to reduce pollution and thus make rivers and groundwater cleaner. Because when bogs are rewetted with this water, the peat soil develops a cleaning effect that reduces the iron and sulphate pollution of the water by up to 80 percent, as tests have shown.
The mining of lignite is not only a problem with regard to the climate, the mine water produced during mining can also damage water bodies and the environment. Because these liquids are highly acidic due to the sulfuric acid they contain, they also contain high concentrations of environmentally harmful sulphate and iron compounds. Although there are processes for cleaning mine water, they are expensive and often only partially effective, as demonstrated, for example, by the heavily polluted former mining landscape in Lusatia. Rivers and groundwater there are still heavily polluted.
Purifying reaction in peat soil
Lydia Roesel from the Humboldt University in Berlin and her colleagues have now investigated a new, nature-based approach to cleaning mine water. Their idea: peat soils could possibly help to reduce the pollution of such waters. Because the low-oxygen, acidic environment in the peat favors chemical reactions that convert iron compounds and sulfates back into an insoluble mineral. “Under oxygen-free conditions, pyrite should ideally be formed again in water-saturated peat soils and iron and sulfur should be removed at the same time in order to prevent renewed pyrite oxidation,” explains Roesel.
So far, however, it was unclear whether this process actually takes place in peat soils – for example, if drained peatlands were rewetted by sprinkling mine water. Roesel and her team have therefore reproduced this process in the laboratory experiment. To do this, they filled a cylindrical container with a grating underneath and closed the peat soil with mine water, which was acidic and heavily contaminated with iron and sulphates. The researchers then examined the seepage water escaping below to determine whether and how the chemical parameters had changed.
80 percent of the pollutants removed
In fact, evaluations showed that the peat soil had removed a large part of the pollution from the mine water during this rewetting. The acidity of the water dropped from pH four to pH six. The purified water was thus almost neutral. The initially high levels of more than 250 milligrams of iron per liter and over 770 milligrams of sulfate per liter fell by an average of 87 and 78 percent, respectively. The longer the pit water remained in the moorland, the better the cleaning effect.
“The results imply that the microbial degradation of sulphate and a subsequent precipitation of iron sulphides was the most important mechanism in reducing the pollution,” explains co-author Dominik Zak from the University of Aarhus in Denmark. According to the research team, rewetting bogs could be an effective measure to reduce pollution from acidic mine water. Specifically, they calculated that, for example, the sulphate pollution in the dirtiest section of the Spree could be reduced by around 20 percent simply by rewetting the moors in their catchment area – they cover around 6067 hectares.
“Our results show once again that the rewetting of peatlands is an important measure for protecting our environment. Peatlands stabilize the global carbon balance, they keep the water in the landscape and also have an important cleaning function,” says Roesel. Although the results of laboratory experiments cannot simply be transferred to large-scale field conditions, the scientists also concede this. Nevertheless, they can provide clues and help prepare larger-scale field studies.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB); Specialist article: Journal of Environmental Management, doi: 10.1016/j.jenvman.2022.114808