Iron fortified hemp biochar made from agricultural waste can significantly cut 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.
Plain language overview
Per and polyfluoroalkyl substances (PFAS) are extremely persistent industrial chemicals that can move through soil, water and air and build up in crops and people. In this greenhouse study, researchers tested whether biochar made from hemp plants, and enhanced with iron, could lock PFAS in place and keep them out of edible radish bulbs. They found that iron fortified hemp biochar lowered PFAS levels in radish tissues and reduced overall plant uptake compared with unamended soil and with plain biochar.
What the researchers did
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. They produced biochar from hemp stems and leaves at different temperatures between 500 and 800 degrees Celsius, with some batches "fortified" by soaking the biomass in an iron sulfate solution before pyrolysis to create iron rich sorption sites.
After characterizing surface area, pore structure, and mineral content, the researchers mixed selected biochars into the contaminated soil at low application rates of 2 or 5 percent 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.
Key findings
Soil at the field site contained about 576 nanograms of total PFAS per gram, dominated by PFOS which contributed roughly 60 percent of the total burden. Biochar made at the lowest temperature (500 degrees Celsius) had the highest specific surface area and more oxygen containing functional groups, which favored PFAS retention compared with material made at higher temperatures. Fortifying biochar with iron further increased surface area and pore volume and introduced iron oxide and hydroxide sites that can attract anionic PFAS molecules.
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. When the soil was amended with iron fortified hemp biochar produced at 500 degrees, total PFAS in whole radish plants dropped by about 37 percent compared with unamended soil, and by nearly 46 percent relative to plants grown with non fortified biochar. In the edible bulb, iron fortified biochar cut PFAS bioaccumulation by about 25.7 percent and produced especially large reductions for several short chain sulfonic and carboxylic acids.
Why iron fortified hemp biochar works
Analyses showed that increasing pyrolysis temperature shrank the biochar's surface area and pore volume and reduced the abundance of reactive surface functional groups, all of which limited PFAS sorption. In contrast, iron fortification boosted porosity and created additional positively charged and hydrophilic sites that support electrostatic attraction, ligand exchange, hydrogen bonding and complex formation with PFAS head groups while maintaining a hydrophobic carbon backbone that interacts with the fluorinated chains. The authors conclude that this combination of physical and chemical mechanisms allows iron fortified hemp biochar to hold PFAS more strongly in soil pore spaces, lowering the freely dissolved fraction available for plant uptake.
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 edible tissues, 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.
