Study evaluates low-cost, integrated approach to remediate PFAS in agricultural soils

Researchers from Yale University have modeled a multi-step method to remediate agricultural soils contaminated with per- and polyfluoroalkyl substances (PFAS). Published in the Proceedings of the National Academy of Sciences (PNAS), the study outlines an integrated framework combining crushed alkaline rock, plant uptake, thermal processing, and biochar application to extract target "forever chemicals" while sequestering atmospheric carbon.

The contamination of farmland stems largely from the application of biosolids—treated municipal wastewater sludge—used for decades as a nutrient-rich fertilizer. While rich in nitrogen and phosphorus, biosolids often contain persistent compounds, including perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA). Standard municipal wastewater treatment does not eliminate these substances. Consequently, they accumulate in soil, migrate into groundwater, and enter agricultural supply chains, prompting regulatory action and land-use restrictions.

Established remediation methods, such as large-scale soil excavation followed by off-site thermal treatment, are estimated to cost between $800,000 and $1.6 million per hectare in the United States. The researchers evaluated an in situ strategy designed to reduce both costs and carbon footprints through a sequential, four-step process.

First, crushed alkaline rock such as basalt is applied to elevate soil pH to approximately 7, which increases the mobility and bioavailability of compounds like PFOS, making them easier for plant roots to absorb. Next, accumulator crops like hemp and red fescue are cultivated to extract contaminants from the root zone; stochastic modeling indicates that managing soil pH in this manner could shorten removal timelines by more than a decade compared to unassisted processes. The harvested biomass then undergoes pyrolysis—a high-temperature treatment between 500 °C and 700 °C in an oxygen-deprived environment—to destroy PFOA and PFOS in plant tissues. Finally, the resulting carbon-rich biochar is reapplied to the field to act as a sorbent, binding residual soil contaminants and reducing estimated groundwater leaching by up to 95%.

Economic calculations suggest a median annual treatment cost of $1,460 per hectare. Across a 20-year cycle, total remediation expenses are estimated at just over $29,000 per hectare, which includes accounting for farmers' lost crop revenue during the cleanup period.

The model also incorporates enhanced weathering and carbon dioxide removal (CDR). Weathering basalt reacts with CO2, while biochar locks captured carbon into the soil. If deployed across approximately one million hectares of impacted U.S. cropland, the authors estimate the framework could sequester roughly 10.5 to 11 million metric tons of CO2 annually. Revenues from carbon removal offset a significant portion of the total remediation cost.

The study's authors emphasize that the findings reflect computer simulations constrained by existing experimental data. Real-world performance remains to be tested through multi-year field trials that account for site-specific soil parameters, complex PFAS mixtures, crop yields, and the energy required to mine, transport, and pyrolyze materials.

Source:

PNAS