Filtering Water-Borne Sulfates and Heavy Metals through Biochar and Bacteria

  2017-18 Smith Partners Sustainability Fellow Gloria Thomas at the University of Minnesota

2017-18 Smith Partners Sustainability Fellow Gloria Thomas at the University of Minnesota

While legislators, state agencies, and environmental and mining advocates continue to debate the water quality impacts of mining, a team at the University of Minnesota is exploring whether a combination of biochar -- produced from biomass -- and sulfate-reducing bacteria can act as a filter to remove sulfate and toxic metals from mine water.

Gloria Thomas, 2017-18 Smith Partners Sustainability Fellow and PhD candidate in the Water Resources Science Program at the University of Minnesota, has joined a team of researchers at UMN who are in the lab phase of fine-tuning this bio-filtration technique.

Relatively inexpensive and using natural components, the biochar-bacteria remediation technique could be applied to reduce sulfate levels in water to protect wild rice and to meet Minnesota regulatory mandates.

A Bio-Based Remediation System

The two-component remediation system consists of biochar, biomass that is burned at high heat in a low oxygen environment to produce fine charred particles more porous and sorbent than charcoal, and bacteria that do not require oxygen and that take in and process sulfates.

In this low-cost, passive treatment system, biochar adsorbs heavy metals from the water, holding these metals on the surface of the biochar, while sulfate-reducing bacteria use sulfate to fuel their activity and convert the sulfate to sulfide. Sulfide then precipitates with metal ions into a solid that is less reactive in the environment than sulfide itself. Bacteria in waters with high sulfate concentrations naturally convert sulfate to sulfide, but elevated sulfide concentrations can be toxic to many life forms in the environment, including wild rice. The biochar-bacteria system circumvents this problem.

“Sulfide is harmful to animals and plants that breathe oxygen because it disrupts their metabolism,” explained Thomas. “Sulfide can cause irritation and inflammation of the respiratory systems and lead to lung edema if high concentrations are inhaled. Sulfide gas affects wild rice by preventing plants from taking up nutrients through their roots and inhibits the plants’ ability to take in oxygen.  If you walked into a marsh or field and noticed a pungent smell of rotten eggs, it would be from sulfide gas.”

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Applications Beyond Sulfates

Thomas notes that field testing of the biochar and sulfate-reducing bacteria research is still to be conducted, and that future research is needed to make sure that the solid precipitate resulting from the heavy metal and sulfide gas combination does not negatively impact environmental systems. “Biochar is used already in bio-filtration systems to treat stormwater to remove nitrate, phosphate, and heavy metals,” noted Thomas. “Heavy metals can be removed by attaching to biochar. One study has shown that microbial pathogens, such as certain strains of E. coli, can be removed with the use of biochar filters.” 

The properties of biochar, such as high surface area and its porous structure, make biochar a good surface for microbial communities to grow on.

Biochar is an effective addition to bioremediation systems that contain various types of bacteria. Research is being done to study how biochar affects microbial community structure and activities, and how these communities can be used for applications such as wastewater treatment or other forms of bioremediation.

Thomas hopes her research will lead to a variety of remedial applications, and she is eager to contribute to broader public understanding of this work. "It would be very exciting if our research can help local communities and watersheds find cost-effective strategies to improve water quality," she noted. "This is perhaps one more tool at our disposal."