"We are taking something farmers usually throw away and turning it into a tool to protect water and public health," said lead author Ruogu Tang from the Department of Animal and Food Sciences at the University of Delaware. "By carefully tuning how we make biochar, we can trap both dissolved pollutants like ammonia and particulate contaminants like microplastics in a single filtration step."
Ammonia and micro or nanoplastics are very different contaminants but now frequently appear together in rivers, lakes, and wastewater around the world. Ammonia from fertilizer runoff, livestock operations, and industry can harm fish at low concentrations and drive algal blooms, while microplastics have been detected in drinking water, seafood, and human tissues. Existing treatment technologies often target one contaminant type at a time and can be expensive, hard to regenerate, or prone to generating secondary waste.
In this work, the team produced biochar by heating corn cobs, cocoa husks, walnut shells, bamboo, and poultry litter under low oxygen conditions at temperatures between 350 and 700 degrees Celsius for up to two and a half hours. The pyrolysis process created highly porous, carbon rich solids whose surface chemistry and internal pore networks depended on the original feedstock and specific heating conditions. Woody materials such as bamboo and walnut produced biochars with high surface areas, while corn cob biochar offered a balance of carbon content, pore structure, and functional groups that made it a strong candidate for detailed water filtration tests.
Among the corn cob samples, biochar made at 700 degrees Celsius for 2.5 hours, labeled CCB700, showed the highest carbon content, well developed pore structure, and favorable surface charge. These properties supported its use as an efficient adsorbent in simple gravity driven filter configurations.
To measure performance, researchers packed ground biochar into funnel style filter units and passed water containing ammonia or fluorescent polystyrene micro and nanoplastics through the biochar layer. At an initial ammonia concentration of 10 parts per million, a 30 gram bed of CCB700 removed 63.95 percent of ammonia in a single pass and still delivered more than 60 percent removal at lower loadings. At very low ammonia levels near 1 part per million, all corn cob biochars removed more than 65 percent of ammonia, but performance declined for every material at 100 parts per million as adsorption sites became saturated.
For plastic particles, high temperature corn cob biochars prepared at 550 and 700 degrees Celsius consistently removed around 90 percent or more of polystyrene particles spanning sizes from 0.10 to 2.10 micrometers across concentrations up to 20 million particles per milliliter. Even the lower temperature biochar, CCB350, exceeded 90 percent removal for larger microplastics at higher loadings, although its performance dropped for the smallest nanoplastic sized particles and at the highest particle concentrations.
Microscopy and surface analyses clarified the capture mechanisms. Scanning electron microscope images showed that microplastics became trapped on biochar surfaces and within pores, while pore volume measurements confirmed that internal pores filled with particles during filtration. Changes in surface charge and infrared spectra indicated that dissolved ammonia, present mainly as ammonium ions, binds through electrostatic attraction and interactions with oxygen containing functional groups on the biochar surface.
For real world use, the team evaluated potential release of hazardous organic compounds, focusing on 16 priority polycyclic aromatic hydrocarbons regulated by the US Environmental Protection Agency. In 24 hour leaching tests, they detected no release of these PAHs, and measured PAH levels in the biochars remained below the European Biochar Certificate safety limit, suggesting low risk of introducing new toxic compounds during treatment.
The researchers also demonstrated that corn cob biochar filters can be regenerated and reused. After treating solutions containing 10 parts per million ammonia, used biochar was dried, re pyrolyzed under the same conditions, and tested again through three regeneration cycles. CCB700 maintained more than 55 percent ammonia removal in the third cycle, with only modest declines from its initial performance, and lower temperature biochars also retained substantial adsorption capacity after repeated use.
"Biochar gives us a way to link water purification with climate smart agriculture," said senior author Juzhong Tan. "By converting agricultural residues into reusable filters, we can cut waste, store carbon in a stable form, and tackle emerging contaminants in one integrated approach."
Because biochar can be produced from locally available crop residues and animal wastes, the authors see potential for decentralized treatment in rural communities, livestock operations, and small scale systems. With further optimization of reactor design, filter configuration, and regeneration strategies, biochar based media could complement or partly replace more costly commercial adsorbents for removing both conventional pollutants and emerging contaminants such as micro and nanoplastics. The work underscores how engineering the structure and chemistry of a simple carbon material can provide tools for protecting water quality while promoting circular and climate conscious use of agricultural resources.
Research Report: Biochar: from agricultural waste byproducts to novel adsorbents for ammonia and micro/nanoplastics (MNPs)
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