Study shows that aquatic plant can remove excess manganese in soils affected by mining

The cattail, an aquatic vegetation approximately 2,5 meters high, is capable of removing up to 34 times more manganese from contaminated soils than other plants found in similar environments. Compared to hibiscus and reeds, for example, cattail accumulated, respectively, ten and 13 times more manganese, thus demonstrating its potential to sustainably recover areas affected by iron ore waste.

This is one of the results obtained in research published No. Journal of Cleaning Production by scientists from the Luiz de Queiroz College of Agriculture at the University of São Paulo (Esalq-USP) and collaborators.

The article revealed that the Typha domingensis, the scientific name for cattail, is highly efficient in the phytoremediation of manganese, a potentially toxic micronutrient with great ecological risk. The plant presented concentrations of 6.858 milligrams per kilogram (mg/kg) of manganese in the aerial part while other species accumulate, on average, 200 mg/kg.

The field research was carried out in the Rio Doce estuary, district of Regência (ES), a place heavily impacted by the deposition of waste released after the biggest environmental disaster recorded in Brazil – the collapse of the Fundão Dam, in November 2015, in Mariana (MG).

At the time, the disaster affected 41 cities in Minas Gerais and Espírito Santo, causing the death of 19 people, and the iron ore tailings reached the estuary around two weeks later. It is estimated that environmental degradation reached at least 240,8 hectares of Atlantic Forest and resulted in 14 tons of dead fish. Several actions have been taken since then to try to reduce the damage, but contamination in the estuary still persists.

The study showed that the cattail extraction capacity in the Rio Doce estuary reached 147 tons of ore, which represents the removal of 75,7 tons per hectare (t/ha).

Another study carried out in the same location and published in January this year had already demonstrated the cattail's ability to remove large amounts of iron from the environment when compared to hibiscus (Hibiscus tiliaceus), which measures 4 to 10 meters and has yellow flowers (read more here).

“We have been working in Rio Doce since 2015. We have managed to reach a level of understanding of the geochemical dynamics of several metals found in the tailings, such as iron, manganese and other potentially toxic elements. This gives us the opportunity to advance more appropriate strategies for remediating these contaminated areas. The accumulation of this knowledge allows not only to advance in the recovery of degraded regions but also in the search for agromining strategies, contributing to more sustainable mineral exploration”, explains to Agência FAPESP professor Tiago Osório Ferreira, professor in the Department of Soil Science at Esalq -USP and work advisor.

Using the phytoremediation technique, it is possible to reduce the impact on areas affected by minerals by removing parts of plants that accumulate these components. Agromining consists of the use of agronomic strategies to cultivate plants capable of extracting metals and then, using the biomass of this vegetation, concentrate that component, reducing the environmental impact. Currently, this technique is little used around the world, with some work underway in Australia, for example.

The research received support from FAPESP through five projects (19/14800-5, 18/04259-2, 21/00221-3, 19/19987-6 e 18/08408-2) and is part of the doctorate of Amanda Duim Ferreira, first author of the article.

“We already knew from the work of other researchers that manganese is a problem in the region, with contamination of water, soil and fish. When carrying out the study aimed at analyzing the area impacted by the iron-rich waste, we imagine that the cattails and reeds [Eleocharis acutangula] would accumulate more manganese than hibiscus, another tree species present at the site. But the results showed that the cattail accumulates 13 times more manganese in the aerial part than the other two species. In the roots, and through the iron plaque mechanism [physiological adaptation of the plant that leads to the precipitation of iron oxide and the formation of plaques], the result had less impact”, says Duim Ferreira, who was also the first author of the work published in January in Journal of Hazardous Materials.

According to the current study, manganese accumulation in cattail roots and iron plates was 18 mg/kg and 55 mg/kg, respectively.

Plants adapted to flooded environments capture oxygen from the atmosphere through the aerial part, taking it to the roots through porous spaces (aerenchyma). This oxygenation maintains the root system, responsible for fixation, as well as the absorption of water and mineral salts.

The absorption of organic matter by plants favors the dissolution of manganese oxides and the release of protons can trigger the dissolution of manganese carbonate. On the other hand, aquatic plants can also oxidize their rhizospheres (the region where the soil and roots come into contact) due to internal oxygen transport. This process can decrease the bioavailability of manganese.

Method

To carry out the research, soil physicochemical parameters (rhizospheric pH, soil pH and redox potential) and total organic carbon content were determined, in addition to the extraction of manganese in places naturally vegetated by the three plant species. The geochemical fractionation of the metal was also carried out in the soils studied.

The concentration of manganese was determined in each compartment of the plant – roots, shoots and iron plates. Bioconcentration and translocation factors were established, thus evaluating the vegetation's ability to act as a manganese hyperaccumulator and its potential use in phytoremediation programs.

“These results open up a range of possibilities for the use of phytoremediation. Knowing these absorption mechanisms, it is possible to cultivate cattails using different known agronomic strategies to obtain the best results. We left the scope of phytoremediation for agromining. That’s what we’re working on”, adds the professor, who coordinates the Soil Geochemistry Study and Research Group at Esalq.

Now Duim Ferreira, who is doing an internship in North Carolina (United States), is working on researching agronomic techniques for planting cattails in order to take advantage of the plant's phytoremediation potential.

“We are applying several agronomic techniques based on what we already know about the planting period, most suitable time, number of cuts per year to increase the cattail's biomass production potential and removal of manganese and iron”, says the researcher.

According to her, the cattail continues to accumulate large amounts of metals even over the years – collections have already been carried out in the estuary in 2019, August 2021 and February this year.

The article Screening for natural manganese scavengers: Divergent phytoremediation potentials of wetland plants can be read here.