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Pollution

Persistent pollutant broken down by sludge microbe

Pilot demonstrations at sites contaminated with dioxane could happen as early as next year

by Cici Zhang
August 2, 2018 | APPEARED IN VOLUME 96, ISSUE 32

 

09632-scicon1-azoarcus.jpg
Credit: Environ. Sci. Technol. Lett.
The bacterium characterized in this study, seen here in a micrograph, potentially uses an enzyme to oxidize dioxane, facilitating its breakdown.

From activated sludge collected at a local wastewater treatment plant, Mengyan Li and colleagues at New Jersey Institute of Technology have isolated a bacterium that degrades the pollutant 1,4-dioxane (Environ. Sci. Technol. Lett. 2018, DOI: 10.1021/acs.estlett.8b00312). EPA considers dioxane a likely human carcinogen, and last August, the Environmental Working Group, an advocacy organization, detected dangerous levels of the compound in the drinking water of 27 U.S. states.

Chemists use dioxane to stabilize industrial chlorinated solvents, and the manufacturing of certain surfactants generates dioxane as a by-product. As dioxane resists conventional water treatment methods and can cost billions of dollars nationally to remediate, researchers have been searching for biological methods to degrade this compound in contaminated subsurface water, which is a common drinking water source.

The new dioxane-eating microbe isn’t the first to be identified. A major hurdle to applying this kind of strategy has been that dioxane is usually mixed with 1,1-dichloroethylene (1,1-DCE), a derivative of the solvent products that commonly contain dioxane as a stabilizer. But 1,1-DCE inhibits bacteria from removing dioxane because the compound and its oxidized products are toxic to the cells. Fortunately for humans, the bacterium isolated in this study, Azoarcus sp. DD4, degrades both dioxane and 1,1-DCE at the same time. The microbe uses propane as its primary food source, which is critical, Li explains, because if microbe candidates feed only on target pollutants, they would need high concentrations in the field to sustain growth, and dioxane is present typically at parts-per-billion levels.

In the current study, Li’s team found that the DD4 bacterium degraded dioxane and 1,1-DCE in both lab media and contaminated groundwater samples over time and continued to reduce their levels to low enough concentration ranges to satisfy the remediation goals set by several U.S. states. Various states including California and New Jersey have stringent cleanup standards, though currently, no federal-level regulation exists. When the researchers further examined the bacterium, they found that it harbors a gene that encodes an enzyme categorized as a soluble di-iron monooxygenase (SDIMO). Li suspects it could be involved in the first step of the pollutant oxidation process in DD4 (shown).

David Freedman, an environmental engineer at Clemson University who studies biodegradation, says, “Li and colleagues have offered a potentially game-changing solution.” He adds that another feature that makes DD4 favorable is that it doesn’t form clumps to impede its movement through aquifers. David Adamson, who studies dioxane fate at GSI Environmental, says “the findings are highly promising and provide a much clearer path for cleaning up sites contaminated with 1,4-dioxane and chlorinated solvents.” Li says the team is planning on pilot field demonstrations which could happen as early as next year.

CORRECTION: The reaction scheme in this story was updated on August 2, 2018, to correct the product structure in the proposed dioxane degradation. The text of the story was updated on August 2, 2018, to account for the variety of dioxane concentrations at different sites.

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