Iron-fortified hemp biochar, derived from agricultural waste, has been shown to significantly reduce the transfer of “forever chemicals” from contaminated soil to food crops, according to a groundbreaking study on radishes grown in PFAS-polluted soil. This discovery could have profound implications for agricultural practices in contaminated areas.
Per and polyfluoroalkyl substances (PFAS) are persistent industrial chemicals known for their ability to infiltrate soil, water, and air, accumulating in both crops and humans. In a recent greenhouse study, researchers explored whether biochar made from hemp plants, enhanced with iron, could effectively trap PFAS, preventing their absorption into edible radish bulbs. The study revealed that iron-fortified hemp biochar significantly reduced PFAS levels in radish tissues compared to untreated soil and even plain biochar.
Understanding the Research
The research team collected PFAS-contaminated sandy loam soil from a former firefighting training site in Connecticut. This location had high concentrations of PFOS and related PFAS due to the long-term use of aqueous film-forming foams. To create the biochar, hemp stems and leaves were subjected to pyrolysis at temperatures ranging from 500 to 800 degrees Celsius. Some batches were “fortified” by soaking the biomass in an iron sulfate solution before pyrolysis, resulting in iron-rich sorption sites.
After analyzing the biochar’s surface area, pore structure, and mineral content, researchers mixed selected biochars into the contaminated soil at low application rates of 2 or 5 percent by weight. The mixtures were incubated for 90 days to allow PFAS interaction with the sorbents. Radish seedlings were then cultivated for four weeks in both amended and unamended soils, with PFAS levels measured in soil leachates, shoots, and edible bulbs using high-sensitivity liquid chromatography–mass spectrometry.
Key Findings and Implications
The study site soil contained approximately 576 nanograms of total PFAS per gram, with PFOS accounting for around 60 percent of the total burden. Biochar produced at the lowest temperature (500 degrees Celsius) exhibited the highest specific surface area and more oxygen-containing functional groups, enhancing PFAS retention compared to biochar made at higher temperatures. Iron fortification further increased surface area and pore volume, introducing iron oxide and hydroxide sites that attract anionic PFAS molecules.
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 reduced PFAS bioaccumulation by approximately 25.7 percent, with significant reductions observed for several short-chain sulfonic and carboxylic acids.
Mechanisms Behind the Success
Analyses indicated that increasing the pyrolysis temperature reduced the biochar’s surface area and pore volume, limiting PFAS sorption. Conversely, iron fortification enhanced porosity and created additional positively charged and hydrophilic sites, supporting 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.
This combination of physical and chemical mechanisms enables iron-fortified hemp biochar to effectively trap PFAS in soil pore spaces, reducing the freely dissolved fraction available for plant uptake.
Broader Environmental and Health Implications
The study underscores the potential for root vegetables like radishes to accumulate substantial amounts of short-chain PFAS when grown in contaminated fields, raising food safety concerns in affected agricultural regions. By demonstrating that a relatively low dose of iron-enriched biochar made from agricultural residue can improve soil properties and reduce PFAS transfer into edible tissues, the research suggests a practical soil management strategy to mitigate PFAS exposure through diet.
Future research is recommended to investigate 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.
As the world grapples with the pervasive issue of PFAS contamination, innovations like iron-fortified hemp biochar offer promising solutions for safer agricultural practices and enhanced food security.
