Iron-fortified hemp biochar made from agricultural waste can significantly reduce the amount of forever chemicals that move from contaminated soil into food crops, according to a new study on radishes grown in PFAS-polluted soil.
The team collected PFAS-contaminated sandy loam soil from a former firefighting training area in Connecticut, where long-term use of aqueous film-forming foams had left high concentrations of PFOS and related PFAS in food crops.
“PFAS do not simply disappear once they reach farmland, and our results show that they can move efficiently from soil into the foods we grow,” said lead author Trung Huu Bui.
“Iron fortified hemp biochar offers a promising way to trap these contaminants in the soil and reduce their entry into the food chain without sacrificing plant growth.”
How biochars reduced PFAS levels in food crops
The researchers produced biochar from hemp stems and leaves at different temperatures between 500 and 800°C, with some batches “fortified” by soaking the biomass in an iron sulfate solution before pyrolysis to create iron-rich sorption sites.
After characterising surface area, pore structure, and mineral content, the researchers mixed selected biochars into the contaminated soil at low application rates of 2 or 5% by weight and incubated the mixtures for 90 days to allow PFAS to interact with the sorbents.
Radish seedlings were then grown for four weeks in amended and unamended soils, and PFAS were measured in soil leachates, shoots and edible bulbs using high-sensitivity liquid chromatography-mass spectrometry.
How temperature affected PFAS retention
Soil at the field site contained about 576 nanograms of total PFAS per gram, dominated by PFOS, which contributed roughly 60% of the total burden.
Biochar made at the lowest temperatures had the highest specific surface area and more oxygen-containing functional groups, which favoured PFAS retention compared with material made at higher temperatures.
Fortifying biochar with iron further increased surface area and pore volume, introducing iron oxide and hydroxide sites that can attract anionic PFAS molecules to food crops.
Across all treatments, radishes grown in the contaminated soil without amendments showed strong accumulation of short-chain PFAS, with bioaccumulation factors above 1 and particularly high values for short-chain carboxylic and sulfonic acids.
Public health and environmental significance
The study highlights that even root vegetables like radish can accumulate substantial amounts of short-chain PFAS when grown in contaminated fields, raising concerns for food safety in affected farming regions.
By demonstrating that a relatively low dose of iron-enriched biochar made from an agricultural residue can both improve soil properties and reduce PFAS transfer into food crops, the work points to a practical soil management strategy for reducing PFAS exposure through diet.
The authors note that future research should examine long-term field performance, potential effects on soil microbes and PFAS transformation, and whether similar approaches can protect other crop species and soils with different PFAS mixtures.
