University of São Paulo: Biochar produced at high temperatures neutralizes lead-contaminated sediment

The experiment was carried out with sediment samples from a lead and silver ore processing area in the municipality of Apiaí, in the interior of São Paulo, which were contaminated with arsenic (As), present in the soil in the form of anion (negative charge). ), and with lead (Pb), present in the form of cation (positive charge). The remediation was carried out through the application of biochar produced from sugarcane straw, pyrolyzed at temperatures of 350 °C, 550 °C and 750 °C.

The researchers then followed how the As and Pb present in the remediated sediment behaved in time and space. For this, they monitored the mobility potential of the elements every 45 days, over six months. This process used, in isolation, four chemical extractors: water, magnesium chloride (MgCl2), dilute nitric acid (HNO3) and Mehlich-3 (a combination of acids, salts and complexing agent).

The objective of this variety of extractors was to make possible an understanding not only of the EPT content in the sediment, but also the specific potentials of different species of the elements present in the material and that can be released from the sediment to the environment. In this way, it is possible to understand how easily the contaminants can be released from the sediment to the environment and how many of them are strongly retained and, therefore, do not represent such a great risk of exposure.

Matheus Bortolanza Soares is an agronomist and doctoral candidate in the Postgraduate Program in Soils and Plant Nutrition at the Luiz de Queiroz Higher School of Agriculture. Image: Personal Archive
Matheus Bortolanza Soares is an agronomist and doctoral candidate in the Postgraduate Program in Soils and Plant Nutrition at the Luiz de Queiroz Higher School of Agriculture – Photo: Personal Archive
“Each of the extractors attacks specific parts of the sediment and gives an idea of ​​the binding energy between the contaminants and the sediment: the water removes the readily available content of the contaminants, the magnesium chloride extracts the content of the contaminants that can be released through ion exchange, dilute nitric acid that can be released with changes in pH (reactivity) and Mehlich-3, which is associated with organic matter”, explains the researcher.

The researchers observed that biochar produced at 750 °C caused a large decrease in lead availability. Bortolanza points out that this was probably due to the fact that the higher temperature generates a more porous biochar with a higher pH, which facilitates the trapping of lead.

On the other hand, arsenic had its availability increased due to the addition of biochars, but this increase was smaller in biochars produced at higher pyrolysis temperatures.

According to the researchers, this is still not ideal, which would be a remediation strategy capable of reducing the availability of both contaminants. However, it is very difficult to find an isolated technique capable of doing this, since As and Pb have opposite charges in the soil solution and react in opposite ways to the same conditioner. Even so, they believe that biochar has a lot of potential to be used in combination with other remediation techniques, such as phytoremediation.

“This is important information, as it can guide which management practices should be used and inform which type of biochar is most appropriate according to the combination of potentially toxic elements in question”, comments Luís Reynaldo Ferracciú Alleoni, professor at the Department of Science at the Solo at Esalq and doctoral advisor at Bortolanza.

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