Superfund Research Program

High-Temperature Biochar for Arsenic Remediation

Release Date: 11/01/2023

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Adding biochar produced at a high temperature may be an effective way to immobilize arsenic in sediment, according to researchers partially funded by the NIEHS Superfund Research Program (SRP). The study, led by Owen Duckworth, Ph.D., of the University of North Carolina at Chapel Hill SRP Center, in partnership with researchers from the Luiz de Queiroz College of Agriculture, University of São Paulo, Brazil, also provided further insight into the conditions that influenced the effectiveness of biochar for soil remediation.

Biochar is a substance formed by heating organic matter, such as plant material, at high temperatures in the partial or total absence of oxygen. The material can be mixed into soil to adsorb and bind to certain contaminants like arsenic — a naturally occurring element linked to a variety of health issues, including cancer, cardiovascular disease, and respiratory problems.

However, biochar can degrade over time, releasing organic carbon, which can convert arsenic into a water-soluble, more easily dispersed form. Although biochar is often used to remediate arsenic-contaminated soil, not much is known about how to improve its efficacy.

Testing the Biochar Mixtures

The researchers hypothesized that heating the biochar to different temperatures would affect the chemical composition of the material. According to the team, changing the composition of biochar may affect how much organic carbon is released and impact the biochar’s ability to remediate arsenic.

To test their hypothesis, the team produced biochar by heating sugarcane straw at three temperatures: 350 °C, 550 °C, and 750 °C.

The biochars were mixed with soil contaminated with arsenic and allowed to age for 30 days. Each biochar and soil mixture, along with a control of only soil, went through two cycles of flooding with water for 30 days before draining for 10 days. Finally, the researchers collected water and soil samples after each flooding cycle to analyze for arsenic and organic carbon content.

Results showed that heating biochar to higher temperatures made the material less likely to release organic carbon. The lack of organic carbon means less arsenic will be converted into its mobile form, the authors explained. Higher temperatures also created more pores in the biochar particles, increasing the surface area available to adsorb arsenic.

Two box plots showing arsenic concentrations during the 1st and 2nd flooding cycles. The first flood does not show great diversity across the control and three biochar temperatures (350, 550, and 750). The second cycle showed that biochar significantly decresed the arsenic concentrations in the soil, all three biochar temperatures had less arsenic than the lowest noted concentration in the control.
Adding biochar decreased the concentration of dissolved arsenic in water compared to the soil alone after flooding cycles. According to the scientists, the biochar formed at 350 °C was generally more efficient when immobilizing arsenic. (Image adapted from Soares et al., 2023)

Biochar Affects Redox Fluctuations

The research team also found that biochar significantly decreased the amount of dissolved arsenic in water compared to the soil without biochar after the second flooding cycle, but not the first cycle. They hypothesized that biochar may help regulate reduction and oxidation potential in soil, which can affect if arsenic remains dissolved in water or precipitates out. Reduction and oxidation (redox) potential measures the ability of a chemical species to gain or lose — reduce or oxidize, respectively — an electron under certain conditions.

To test this, they used probes in soil and water to measure the redox state for each mixture and found that soils amended with high temperature biochar became less reduced during the second flooding cycles. The redox state may change the physical and chemical properties of the biochar, causing it to immobilize arsenic more efficiently, the scientists explained.

Line chart showing the redox potential over time, shwoing that levels decreased during the 1st and 2nd flooind stage, rose during the drying stage (between the two floods) across all four experiments.
The biochar-added soil and control soil became more reduced during the first flooding stage and more oxidized during the drying stage. In the second flooding stage, the four trials became less oxidized than during the first flooding stage, shown by the more gradual decline in the redox potential. (Image adapted from Soares et al., 2023)

According to the authors, this study demonstrates that biochar, particularly biochar formed at higher temperatures, is an effective soil additive for arsenic remediation. They recommend that more studies be carried out to examine how the movement of electrons affects biochar efficacy and the environmental conditions necessary for electron movement, such as the presence of water or pH of the soil.

For More Information Contact:

Owen Duckworth
North Carolina State University
Department of Crop & Soil Sciences
Williams Hall 3208, Box 7620-7619
Raleigh, North Carolina 27695
Phone: 919-513-1577

To learn more about this research, please refer to the following sources:

  • Soares MB, Duckworth O, Styblo M, Cable PH, Alleoni LR. 2023. Pyrolysis temperature and biochar redox activity on arsenic availability and speciation in a sediment. J Hazard Mater 460:132308. doi:10.1016/j.jhazmat.2023.132308 PMID:37639794

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