Phosphorus runoff from farmland significantly contributes to water pollution, fueling harmful algae blooms and damaging aquatic ecosystems.
In response to this growing concern, researchers at the University of Illinois Urbana-Champaign developed “designer” biochar pellets that can capture dissolved phosphorus from subsurface tile drainage systems—used extensively on Midwestern farms. Crafted from byproducts like sawdust and lime sludge, these pellets provide a cost-effective and sustainable way to manage phosphorus, a critical nutrient and pollutant.
“Our goal was to create an affordable, eco-friendly material that can trap phosphorus efficiently,” explains Hongxu Zhou, the study’s lead author and a recent doctoral graduate in agricultural and biological engineering. Previous phosphorus capture materials have been either inefficient or costly to dispose of, motivating Zhou and his team to develop a more accessible and effective solution.
Unlike standard sorbents, biochar pellets capture phosphorus and can be reused on fields as a slow-release fertilizer once their capacity is reached.
The researchers tested their pellets in real-world conditions on tile-drained fields in Fulton County, Illinois, monitoring performance over two years. Fields fitted with subsurface drainage tiles—standard in Midwest corn and soybean production—were chosen for the trials. The drainage water flowed through phosphorus removal structures filled with biochar pellets of two different sizes, which allowed the team to test each size’s efficacy.
In the first year, the team tested larger pellets (2-3 cm) and observed phosphorus removal efficiencies between 1.3% and 12%. For the second year, they switched to smaller pellets (1 cm or less), achieving a significantly higher removal rate of 38-41%. This increase was expected as smaller particles increase the contact area for phosphorus attachment, enhancing the pellets’ overall binding capacity.
The smaller biochar pellets maintained phosphorus retention without clogging the drainage system or washing away, which had been a concern with powdered forms. “Powdered biochar would be too easily washed out in field conditions,” said co-author Wei Zheng, a principal scientist at the Illinois Sustainable Technology Center. “The pellets had to be designed for durability in real drainage systems while sacrificing minimal efficiency.”
A detailed cost analysis revealed that producing these biochar pellets costs approximately $413 per ton, less than half the market price of traditional phosphorus filters like granular activated carbon, which can range from $800 to $2,500 per ton. When installed in drainage systems, the total cost of phosphorus removal is about $359 per kilogram, with optimal performance when the biochar is replaced every two years. The researchers found this replacement cycle maximized efficiency while balancing costs for farmers.
The pellets’ environmental impact is also significant. A life-cycle assessment (LCA) found that each kilogram of phosphorus removed using biochar could offset 12 to 200 kg of carbon dioxide equivalent, a major benefit for greenhouse gas reduction. Returning the spent biochar to fields as fertilizer contributes additional benefits by supplying phosphorus and improving soil health.
While farmers are not yet required to remove phosphorus from drainage water, many conservation-minded farmers are eager to minimize nutrient losses. Many farms already have bioreactors to remove nitrate from tile drains, and adding biochar to these systems enables the simultaneous removal of nitrate and phosphorus. According to associate professor and study co-author Rabin Bhattarai, this integration could significantly reduce the need for additional fertilizers, directly lowering operational costs for farmers.
“Being able to capture and reuse phosphorus on the same fields is very attractive,” notes Bhattarai, emphasizing the simplicity of spreading used biochar pellets back onto fields to release phosphorus gradually, thereby recycling it within the same agricultural system.
