Researchers find a new use for biochar: filtering microplastics from farm soils

Biochar enriches farm soils and can lock away tons of carbon each year. Now among these environmental attributes, researchers have found another beneficial use for this material: it could help sift out tons of invasive microplastic particles from farm soils.

Scientists on a new study showed that when they ran microplastic samples through tubes of biochar—a product made by burning biomass, often farm waste, in the absence of oxygen—the biochar was able to filter out up to 92.6% of the minuscule plastic particles.

The presence of microplastics in waters and soils is a growing challenge: these pollutants are also uniquely intertwined with modern farming practices. Most industrial farms today are riddled with plastics, which may be used in fertilizers and mulches that are directly forked into the soil; in irrigation pipes; to store crops; or to retain soil moisture and control weeds via plastic sheeting that is layered over the land.

But this plastic inevitably breaks down, shedding microparticles into the water and soil: some studies suggest that microplastics may even be bioaccumulating in crops, and entering our diets. They’re also present in the water that we drink.

Previous research has indicated the potential of biochar to capture microparticles, but only a limited amount of research has explored how well it can actually remove microplastics from water and soils. That’s where the new study comes in.

The University of Mississippi-led research team began by gathering samples of run-off from a farm in Mississippi after a major rainfall event. To enable them to trace and track the plastic particles in the water, they stained them with a type of dye that makes them fluoresce under specific wavelengths of light.

Returning to the lab with the jars of collected rainwater, the researchers made their first striking discovery on the sheer scale of microplastics on farmland: their samples contained an average 237 pieces of microplastic per liter of water, with some samples showing as many as 600. Most of the plastic particles were polyethylene, the most widely-used plastic which accounts for one-third of the global plastic market, and is widespread on farmlands. In fact all the most common plastic types in the samples were associated with farm uses, such as mulching, irrigation, and sheeting.

Next, the researchers set out to see how well they could strain out this material. In the lab, they made columns filled with biochar, experimenting with two types: char made from sugarcane, and from pinewood. The idea behind their experiment is that the process of pyrolyzing organic matter turns it into a more porous material, with an increased surface area, qualities that can help trap and attract tiny particles. Prior research suggests that electrostatic interactions between biochar and other substances may also play a role in attracting microparticles to its surface.

Watching the fluorescent particles closely as they streamed the water samples through these columns, the researchers found that the biochar was able to remove a striking 86% to 92% of the particles from each one.

Of the two types of char, the pinewood material was better at trapping the plastic compared to the sugarcane, where the particles filtered deeper into the column of char. The researchers think this might be due to the larger, rougher, and more porous nature of the woody material formed under pyrolysis, which appears to be better at capturing the particles compared to the finer-grained sugarcane.

However, overall across both types of char, more than 90% of the plastic was trapped above the 20 centimeter mark. If replicated in the field, this would halt most of the plastic pollutants flowing off farmland before its onward journey into soils, streams, and eventually the sea.

The use of plastic in agriculture has become so intensive and widespread that it now even has its own name: ‘plasticulture’. That should be an indication of the scale of the microplastic challenge, and the urgent need for solutions. The research is still preliminary, but already the researchers have some ideas, such as fitting filtration socks to stormwater drains where microplastics could be captured with the help of biochar.

This marks the next stage of research, which they say has already begun in earnest: “Scaled-up field studies are underway and preliminary data show a marked decrease in microplastics.”

Olubusoye et. al. “Removal of microplastics from agricultural runoff using biochar: a column feasibility study.” Frontiers in Environmental Science. 2024.