A Yale-led team says one of the country's most difficult pollution problems may also have a surprisingly practical solution.
What's happening?
U.S. farms receive millions of dry tons of biosolids every year, according to ScienceAlert. The material, a nutrient-rich byproduct of municipal sewage treatment, is spread on fields as fertilizer.
Those applications have long been promoted as a substitute for petrochemical fertilizers. The problem is that much of the waste contains PFAS, the toxic "forever chemicals" that can remain in the environment for generations.
ScienceAlert reported that a 2001 National Sewage Sludge Survey from the U.S. Environmental Protection Agency found the yearly biosolids total produced in the United States contains an estimated 6,061 to 7,606 pounds of PFAS, and about half of it ends up on agricultural soils.
The new PNAS study describes a cheaper strategy for dealing with PFAS in that soil.
"This issue — the contamination of agricultural land with PFAS — disproportionately hurts small farmers and organic farmers, who used fertilizers with PFAS before the full extent of the problem was known," said environmental scientist Jake Thompson of Yale University.
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Using the method repeatedly for as long as 20 years would cost about $29,000 per hectare, the researchers estimated, versus roughly $800,000 to $1.6 million per hectare for commonly used remediation approaches, as ScienceAlert reported.
Why does it matter?
PFAS are known as forever chemicals for a reason. Their carbon-fluorine bonds are extremely strong, which means they do not readily break down. As a result, they can build up in soil over time, move into nearby waterways, and in some cases be absorbed by crops.
Contamination on farmland affects the safety and affordability of food, place additional strain on already struggling farms, and create enormous cleanup costs that ripple through local economies. A less costly remediation method could help communities preserve farmland rather than abandon it.
As ScienceAlert noted, research has associated PFAS exposure with problems including some cancers, cardiovascular risk, multiple sclerosis, and fertility issues, although many of those findings are still being examined.
As Thompson put it, "Though this study is US-focused, based on the available data, PFAS are a challenge globally."
What's being done?
Researchers start by spreading crushed alkaline rock on the land — basalt, for example, and possibly limestone — to make the soil more basic. With the pH increased, certain PFAS compounds become easier to move through the soil, which helps selected plants take them up.
From there, the plan calls for growing hemp and perennial grasses that absorb PFOS, then harvesting the plants and turning the material into biochar through pyrolysis, a high-heat process carried out without oxygen.
The team wrote, "Harvested biomass converted to biochar and reapplied to the field can also serve to immobilize residual PFAS, thereby reducing their transfer into forage crops."
As ScienceAlert noted, scientists still have major questions about PFAS during pyrolysis, and one recent study indicates temperatures of at least 1,472 degrees Fahrenheit may be required to reduce PFAS enough.
Unlike methods that dig up contaminated soil for burning, this one could leave the ground healthier. It also might, as ScienceAlert reported, pull about 11.6 million tons of carbon dioxide from the atmosphere each year if adopted across the country.
"This is a real pathway to actually give farmers in this situation some agency over their land," Thompson said, and "leave the soil in better condition than it was before."
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