Soil contamination by heavy metals such as cadmium, zinc and arsenic at harmful levels can occur for a number of reasons, depending on anthropogenic activity, in other words, human action. The causes range from mining, industrial activities and agriculture to the incorrect disposal of residues, including household waste.

There are many consequences: environmental impacts to the ecosystems with loss of flora and fauna; soil infertility; contamination of food chain; contamination of groundwater; and harm to human health, which can cause many pathologies, such as allergies, diseases of the nervous system or kidneys and even cancer. 

Fortunately, there are several techniques that can be used in soil decontamination by these materials. However, not all are so sustainable, cheap or have fast or effective action, depending on the heavy metal.

But there is good news: a study conducted by the Federal Rural University of the Amazon (UFRA), led by agronomist Cândido Ferreira Neto, presents promising results that are both low-cost and sustainable. The research explores the combined use of biochar and phytoremediation plants—species capable of absorbing or degrading toxic substances.

The study, conducted by the Grupo de Pesquisa de Estudos da Biodiversidade de Plantas Superiores (EBPS) [Research Group for Studies of the Biodiversity of Superior Plants] and funded by the Fundação Amazônia de Amparo a Estudos e Pesquisas (Fapespa) [Amazon Foundation for Support for Studies and Research], uses native species of the region as a solution to the problem.

PTE

According to agronomist Gilson Matos, also a professor at Ufra, the technical term for heavy metals is “potentially toxic elements” (PTE). "If we think about gold, it is a heavy metal, but it is not toxic to animals, while other metallic elements, if they are in low concentrations, are nutritious for plants. So, PTEs cover some without biological function and that are toxic even at low concentration, such as mercury, cadmium, chromium, arsenic, lead; or elements that even have a biological function, but are harmful if they are in excess, such as iron, copper, zinc, aluminum and cobalt," he explains.

According to the expert, they are elements that already exist naturally in the environment, as part of the minerals that make up the soil, but that can become toxic due to anthropogenic actions. "Some examples are the inadequate disposal of liquid and solid residue in landfills, disordered industrial activity, excessive burning of fossil fuels and the inappropriate use of fertilizers or agricultural pesticides. A Ufra study showed high levels of PTE in northeastern Pará, mainly correlated to inadequate basic sanitation of housing, in a critical situation," says Matos.

The professor points out that, to know if the potentially toxic element is above its natural levels, with the risk of harming the ecosystem, it is necessary to do soil collection and laboratory analysis. "The results are then compared with reference values for each element, which in Brazil are defined by a resolution of the Conselho Nacional de Meio Ambiente – Conama” [National Council of the Environment] says the agronomist. "The recovery of contaminated soils is totally feasible, but its efficiency varies according to the type of soil, nature of the contaminant and future use of the area," he adds.

Use of biochar helps in soil regeneration

Using tests with different heavy metals, different biochar compositions and different phytoremediation plants, the study led by Cândido Neto focuses on discovering the best options for each case, in the search for solutions for decontamination. The two fronts of work involve the use of biochar to recover the soil and plants to reforest it.

The biochar used is mainly obtained from açaí seeds, but also from other residues, such as cupuaçu shells, Brazil nut shells or pruning residue. To produce the active ingredient, the seed is placed in a piece of equipment called a muffle, which subjects the material to high temperatures and the absence of oxygen, in a process called pyrolysis. Each type of residue requires a specific temperature to be transformed into biochar, maintaining properties that will help nourish the soil.

After passing through the muffle furnace, the seed is transformed into powder, which is poured into the tested soil. “Biochar is a soil conditioner. When applied to the site, in the correct quantity, made from the correct residue, it will have both a physical and chemical response to improve the quality of the soil, with a high retention power of heavy metals. So, the metal that was available, free in the soil, is retained by the biochar, where it will remain trapped for years or decades. So, the amount of metal in that location is drastically reduced”, highlights Cândido Neto.

BENEFITS

According to the expert, the Amazon soil is naturally poor, with very low pH and low fertility. “We found that, with the application of biochar, there is a significant improvement in the soil, with faster growth in the production of seedlings, both of forest and fruit species,” he points out.

In this sense, the material reduces the need for soil fertilization. “The residue from the açaí seed, for example, has several essential nutrients for plants. We were amazed, because its chemical quality is very good. This means less expense for the producer and less use of chemical fertilizers in the soil, which help to cause the greenhouse effect,” emphasizes the agronomist.

In addition to promoting the retention of heavy metals and improving soil fertility, the use of biochar has another advantage for sustainability: it provides a destination for açaí seed residue. “Tons and tons of this residue are thrown away, which attracts diseases and releases leachate, with consequences for our health and carbon released into the atmosphere. However, after being transformed into biochar, we stabilize this carbon and contribute to reducing greenhouse gas emissions,” emphasizes the researcher.

Phytoremediators are planting options

The other key component in the soil decontamination process, following the application of biochar, is the use of phytoremediation plants. “With the drastic reduction of heavy metals in the area, it becomes much easier to work the soil by introducing species that can tolerate its conditions. These are Amazonian plant species that are not only tolerant to heavy metals but also capable of removing them from the soil while continuing to grow normally, without any disruption to their metabolism. The strategy, therefore, is to use both biochar and phytoremediation plants in combination”, explains Cândido Neto.

According to the researcher, tests are carried out to determine which species tolerate heavy metals, remaining alive, and in what concentrations. “There are plants that will remove the metal from the soil, accumulating it in their roots and stems, but will die. There are those that are tolerant to one metal but not another, or that tolerate up to a certain concentration or period of time. So, we test the variables to indicate the appropriate plants for decontaminating degraded soils, depending on the metals and concentrations,” says the agronomist.

So far, the species paricá, pau pretinho, mahogany, ucuúba and palheteira have been studied, but new plants are already being considered, such as andira-uxi and pau-brasil. The seedlings are produced in the so-called nursery, at Ufra, and then taken to the greenhouse, to be planted in the contaminated soils and with the biochar already applied.

FUTURE

In addition to studies on decontamination, the research also focuses on plant production: how biochar can help agricultural productivity. Tests have already been carried out with corn, jambu and, more recently, beans. The idea is to move beyond the laboratories and use the results to improve the production of family farmers.

“We are carrying out a series of tests at Ufra, so that we can reach out to producers and tell them which species the biochar will work well with, in terms of fertility and soil pH correction. In addition, the material also retains water in the soil, which helps with humidity during dry periods,” says Cândido.

According to the agronomist, tests will begin in July with family farmers in Santa Izabel. “We are already starting to make these contacts, to establish this partnership with them in the production of vegetables, such as lettuce and coriander,” he says.

The next step is to take the techniques to degraded areas. “We have already studied contamination by cadmium, chromium and copper, so we want to place biochar and phytoremediation plants in areas contaminated by these types of metals. Much is said about contamination due to mining, but little emphasis is given to landfills, which also have high levels of heavy metals, because batteries are disposed of improperly in these places, which causes a huge release of cadmium into the soil. Therefore, we will seek partnerships with government agencies or private companies to conduct tests in these locations, to plant there and see how the species behave,” he says.

“There is a population around that landfill in contact with the river flowing there. They eat the fish from there, or grow their vegetables nearby and they end up ingesting this contamination. So, we want to recover these areas, restoring biodiversity and human health in the area,” concludes the researcher.

INSTITUTIONAL PARTNERSHIP
The production of Liberal Amazon is one of the initiatives of the Technical Cooperation Agreement between the Liberal Group and the Federal University of Pará. The articles involving research from UFPA are revised by professionals from the academy. The translation of the content is also provided by the agreement, through the research project ET-Multi: Translation Studies: multifaces and multisemiotics.