Southern cattail is highly effective for remediating areas affected by iron mine tailings

Typha domingensis or southern cattail is a reedy marsh plant that inhabits fresh to slightly brackish water and grows to about 2.5 m (8 ft) tall.

The analysis performed by the researchers found that the amount of manganese in the plant’s shoots equates to 6,858 milligrams per kilogram (mg/kg), compared to an average of 200 mg/kg for other species.

Fieldwork was carried out in Regência, at the mouth of the Doce River in Espírito Santo State, an area inundated by part of the 50 million cubic meters of iron mining waste released when the Fundão tailings dam was constructed in November 2015 in the Mariana locality, Minas Gerais collapsed (a neighboring state of Espírito Santo), causing the worst environmental disaster in Brazilian history.

The disaster affected 41 towns in Minas Gerais and Espírito Santo, and 19 people died. The toxic waste reached the estuary about two weeks later and has contaminated an estimated 240.8 hectares of Atlantic Forest and killed 14 tons of fish. Since then, many projects and programs have been started to mitigate the damage, but the estuary is still contaminated.

The study showed that the plants in the estuary extracted 147 tons of manganese, or 75.7 tons per hectare (t/ha).

A previous study conducted in the same area showed that the same plants removed greater amounts of iron from contaminated soil and water than Hibiscus tilaceusa 4–10 m tall tree with yellow flowers, often called beach hibiscus.

“We have been working on Doce since 2015 and have gained a good understanding of the geochemical dynamics of several metals contained in the tailings, such as iron, manganese and other potentially toxic elements,” Tiago Osório Ferreira, senior author of both articles, said in a media statement. “This gives us the opportunity to make progress in planning more effective remediation strategies for these contaminated areas. The knowledge built up in these studies will not only be used to rehabilitate degraded areas, but also to formulate agromining strategies as a contribution to more sustainable mining.”

Phytoremediation involves the use of plants to extract and remove elemental pollutants or reduce their bioavailability in soil. Agromining involves growing plants capable of storing metals in their roots and leaves in order to extract those metals for industrial use.

“We knew from research done by others that manganese was a problem in the region, contaminating water, soil and fish. When we went to the area affected by iron ore tailings, we expected to find that T.domingensis and Eleocharis acutangula [acute spikerush] accumulated more manganese than H.tiliaceusa species of tree found in the area, but the results showed that T.domingensis can accumulate 13 times more manganese in its aerial parts than the other two species. The process had less impact over root iron plaque [a form of physiological adaptation whereby iron oxide precipitates to form iron plaque on the surface of the plant’s roots]said Amanda Duim Ferreira, first author of both articles.

According to the latest study T.domingensis accumulates 18 mg/kg in roots and 55 mg/kg in iron plaque.

Plants adapted to wetlands take up oxygen from the atmosphere through their aerial parts and transmit it to the roots through porous tissues called aerenchyma. This supply of oxygen maintains the root system, which is responsible for anchoring and absorbing water and mineral salts.

The uptake of organic matter by plants favors the dissolution of manganese oxide, and the release of protons can trigger the dissolution of manganese carbonate. On the other hand, aquatic plants can also oxidize their rhizosphere (the zone around plant roots where soil biology and chemistry is influenced by the roots) due to internal oxygen transport to the roots to avoid hypoxia. This process can reduce the bioavailability of manganese.

The measurement

Researchers measured soil physicochemical parameters (rhizosphere pH, total soil pH and redox potential) and total organic carbon content. They also extracted manganese from the soil under sites naturally covered by the three plant species and performed geochemical fractionation of manganese in the soils studied.

The manganese content was measured in the roots, the aerial part and the iron plaques of the plants. Bioconcentration and translocation factors were determined to assess their ability to act as manganese hyperaccumulators and their potential use in phytoremediation programs.

“These discoveries open up a number of possibilities for the use of phytoremediation. Knowing absorption mechanisms can be a basis for breeding T. domingensis via different strategies to achieve optimal results. We have moved from phytoremediation to agromining as we continue this research,” said Osório Ferreira.