Metal Mitigation

Study uses biochar to reduce soil toxicity

The western U.S. is home to approximately 160,000 abandoned mining sites with nearby contaminated soils. Jim Ippolito, associate professor of soil science at Colorado State University, collaborated with a team of scientists from the U.S. Department of Agriculture's Agricultural Research Service, the University of Idaho and two private entities to explore the use of biochar to prevent toxic metal accumulation in soil. SWS Managing Editor Amy McIntosh asked Ippolito about the research.

Amy McIntosh: How does soil near abandoned mine sites become contaminated?

Jim Ippolito: Historic precious metal mining creates mine spoils, or waste rock that does not contain appreciable quantities of the precious metal(s) of interest. These spoils are typically disposed of on or near mining sites. Many of these spoils contain sulfide-bearing rocks, and once exposed to the atmosphere and water, these rocks begin to change from a reduced to an oxidized form. When this occurs, acidity in the form of sulfuric acid is typically generated. This acidity can quickly dissolve rocks and increase the available concentration of undesirable metals in the environment.

When acidity is generated and the availability of undesirable heavy metals is increased in the environment, these metals can accumulate to the point where they become toxic to life. In fact, some contaminated areas are simply barren landscapes.

McIntosh: What is biochar and how can it be used to treat the contaminated soils?

Ippolito: Biochar is a byproduct produced via pyrolysis of various materials from agricultural (e.g., wheat straw, rice husk, corn stover, animal manures), forestry (e.g., lodgepole pine beetle-killed trees), and municipal (sewage sludge) settings. Pyrolysis is basically burning materials in the lack of oxygen. Biochar can offer many benefits similar to other “classic” amendments used on acidic mine land sites, including supplying plant nutrients, adding organic carbon and helping improve soil carbon relations, acting as a liming agent to reduce acidity and raise soil pH, and promoting heavy metal sequestration.

Past research from my group, as well as others globally, has shown that biochars can be designed in such a way as to meet demands for treating metal-contaminated soil. For example, some biochars may sorb metals via organic functional groups, while other biochars may sorb metals though binding to the ash material present in biochar. Our findings using biochars in mine-affected soils over the past several years point us in the direction that biochar ash content is very important for heavy metal sorption. Furthermore, following metal binding to biochar ash materials, those metal phases should be stable as long as soil pH is maintained in an elevated condition. However, this typically is the goal for classic acidic mine land reclamation via the use of liming materials.

McIntosh: What is the state of your research?

Ippolito: We are currently screening several dozen biochars for heavy metal sorption and retention. We are using heavy metal-contaminated mine land soils, collecting leachate from those soils, and then mixing that leachate with various biochars to observe heavy metal sorption. We follow this step with attempts to strip heavy metals from biochars, and keep those particular biochars in our toolbox to utilize on heavy metal-contaminated acidic sites. We are also in the process of utilizing “the best” biochars on two different mine land sites within the U.S.

McIntosh: What do you hope to accomplish with your study?

Ippolito: What we’re trying to learn is the mechanism by which these various biochars sorb heavy metals and reduce metal availability to plants, animals and microorganisms, and reduce metal fate/transport in the environment. Ultimately, we would like to be able to select and create biochars from local waste materials that are designed to specifically sorb, retain and reduce heavy metal bioavailability in the environment within close proximity to waste material origin. We really want to be able to solve problems via locations solutions. 

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