Scientists tackle farm nutrient pollution with sustainable, affordable designer biochar pellets

"Phosphorus removal structures have been developed to capture dissolved phosphorus from tile drainage systems, but current phosphorus sorption materials are either inefficient or they are industrial waste products that aren't easy to dispose of. This motivated us to develop an eco-friendly and acceptable material to remove phosphorus from tile drainage systems," said study author Hongxu Zhou, who completed the study as a doctoral student in the Department of Agricultural and Biological Engineering (ABE), part of the College of Agricultural, Consumer and Environmental Sciences and The Grainger College of Engineering at U. of I.

Zhou and his co-authors used sawdust and lime sludge, byproducts from milling and drinking water treatment plants, respectively. They mixed the two ingredients, formed the mixture into pellets, and slow-burned them under low-oxygen conditions to create a "designer" biochar with significantly higher phosphorus-binding capacity compared to lime sludge or biochar alone. Importantly, once these pellets bind all the phosphorus they can hold, they can be spread onto fields where the captured nutrient is slowly released over time.

Leveraging designer biochar's many sustainable properties, the team tested pellets in working field conditions for the first time, monitoring phosphorus removal in Fulton County, Illinois, fields for two years. Like the majority of Midwestern corn and soybean fields, the experimental fields were fitted with subsurface drainage pipes. This drainage water flowed through phosphorus removal structures filled with designer biochar pellets of two different sizes. The team tested 2-3 centimeter biochar pellets during the first year of the experiment, then replaced them with 1 cm pellets for the second year.

Both pellet sizes removed phosphorus, but the 1-centimeter pellets performed much better, reaching 38 to 41% phosphorus removal efficiency, compared with 1.3 to 12% efficiency for the larger pellets.

The result was not a surprise for study co-author Wei Zheng, who said smaller particle sizes allow more contact time for phosphorus to stick on designer biochar. Zheng, a principal research scientist at the Illinois Sustainable Technology Center (ISTC), part of the Prairie Research Institute at U. of I., has done previous laboratory studies showing a powdered form of designer biochar is highly efficient for phosphorus removal. But powdered materials wouldn't work in the field.

"If we put powder-form biochar in the field, it would easily wash away," Zhou said. "This is why we have to make pellets. We have to sacrifice some efficiency to ensure the system will work under field conditions."

After showing the pellets are effective in real-world scenarios, the research team performed techno-economic and life-cycle analyses to evaluate the economic breakdown for farmers and the overall sustainability of the system.

The cost to produce designer biochar pellets was estimated at $413 per ton, less than half the market cost of alternatives such as granular activated carbon ($800-$2,500 per ton). The team also estimated the total cost of phosphorus removal using the system, arriving at an average cost of $359 per kilogram removed. This figure varied according to inflation and depending on the frequency of replacing pellets -- two years appeared to be the most cost-effective scenario.

The life cycle analysis showed the system -- including returning spent biochar pellets to crop fields and avoiding additional phosphorus and other inputs -- could save 12 to 200 kilograms of carbon dioxide-equivalent per kilogram of phosphorus removed. Zhou says the benefits go beyond nutrient loss reduction and carbon sequestration to include energy production, reduction of eutrophication, and improving soils.

"At the moment, there's no regulation that requires farmers to remove phosphorus from drainage water. But we know there are many conservation conscious farmers who want to reduce nitrate and phosphorus losses from their fields," said co-author Rabin Bhattarai, associate professor in ABE. "If they're already installing a woodchip bioreactor to remove nitrate, all they'd have to do is add the pellets to the control structure to remove the phosphorus at the same time. And there's something very attractive about being able to reuse the pellets on the fields."