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Wetlands act as nature's kidneys: They trap sediments, absorb excess nutrients and turn pollutants into less harmful substances. Now, the list of pollutants wetland plants can remove includes per- and polyfluoroalkyl substances (PFAS). From a greenhouse study, researchers in Environmental Science & Technology that moisture-loving yellow flag irises and fungi on their roots are a promising combination for PFAS removal. As part of a constructed wetland, this pair could effectively treat contaminated wastewater.

"Our study shows that a type of fungus (Rhizophagus irregularis) boosts wetlands' ability to remove PFAS and greatly reduces the environmental risks from 'forever chemicals' left in the outflowing water," said Bo Hu, a corresponding author of the research. "These results are key for developing stronger wetland-based cleanup methods and could inspire new technologies for removing PFAS."

Symbiotic relationships between plants and underground microbes, such as a group of fungi named arbuscular mycorrhizal fungi (AMF), are vital for wetland ecosystems. As fungi colonize roots, they break down nutrients in exchange for beneficial carbohydrates from the plants.

Previously, Bo Hu and colleagues found more benefits of this relationship: AMF helped wetland plants tolerate the presence of PFAS. PFAS are long-lasting compounds that pose potential health risks to people, animals and plants. So, Bo Hu, Feng Zhao and additional researchers wanted to study how well wetland plants, specifically yellow flag iris (Iris pseudacorus L.), remove PFAS in the presence and absence of one symbiotic fungus (R. irregularis). They aimed to develop guidance for constructed wetlands as a natural water treatment strategy.

Inside greenhouses, the researchers built small, wetland-like systems with yellow flag irises in tall plastic tubes. The flowers were planted in a sand-soil microbe mixture either with the fungus or without it for the control treatment. They watered the miniature wetlands with a solution that mimicked wastewater, and some were also watered with one of four individual PFAS at realistic concentrations.

Plant health declined when exposed to PFAS, with less growth and more signs of physiological distress (e.g., lower activity of antioxidant enzymes), compared to irises grown without PFAS exposure. In contrast, the researchers observed that adding the fungus improved growth for plants that were both exposed and not exposed to PFAS. For those wetland systems watered with the PFAS-containing solutions, the AMF-treated plants:

• Removed 10–13% more of the individual PFAS than those with the control treatment, incorporating more long-chain PFAS than short-chain PFAS in their shoots and roots.
• Boosted breakdown of PFAS into smaller compounds that had lower toxicity than their parent compounds, which the researchers suggest is because the fungi stimulate nearby microbial activity.

They also tested the water draining out of the wetland tubes exposed to PFAS. All the outflow samples contained PFAS, but those from the fungal tubes had 17–28% less total PFAS compared to samples from the bacterial tubes. These results indicate that adding AMF, specifically R. irregularis, in constructed wetlands could improve their removal of PFAS, say the researchers.

Their next steps are to test the constructed wetlands in more realistic scenarios, moving from the contained greenhouse environment to the natural world and using actual PFAS-contaminated wastewater.