Graphitized Biochar Turns Paddy Soil Into a Cleaner, More Reactive Environment

Paddy soils are living chemical reactors. Under flooded and drained conditions, microbes, minerals, oxygen, and organic matter constantly exchange electrons, shaping how pollutants move, transform, or disappear. A new study published in Biochar reports that a specially engineered form of biochar can redirect these electron flows, helping paddy soils generate more hydroxyl radicals, one of nature’s most powerful oxidants, and break down the antibiotic sulfamethoxazole more efficiently.

The research team prepared graphitized biochar, or G-biochar, using flash Joule heating, a rapid electrical heating method that reorganizes the carbon structure of conventional biochar. Unlike ordinary biochar, which often acts mainly as an electron storage material, G-biochar behaved more like an electrical conductor in soil. Its enhanced graphitized framework increased electrical conductivity by 2.64 times, allowing electrons to move more easily between Fe(III)-reducing bacteria and iron minerals.

“We found that graphitized biochar does not simply store electrons. It helps guide them to where they are needed in the soil system,” said corresponding author Xiangdong Zhu. “This geoconductor function creates a more efficient electron transfer pathway between microorganisms and iron minerals, which then supports the production of reactive oxidants.”

In paddy soils, microbial Fe(III) reduction plays a central role in producing active Fe(II) species. These Fe(II) species can react with oxygen during redox fluctuations and promote the formation of hydroxyl radicals. Hydroxyl radicals are highly reactive molecules that can oxidize many organic pollutants, including antibiotics that enter agricultural soils through manure, wastewater, and irrigation.

The study found that G-biochar increased active Fe(II) generation by 18.9% compared with untreated soil. This improvement was linked to the enrichment of Fe(III)-reducing bacteria, including Bacillus, Anaeromyxobacter, Citrifermentans, and Flavisolibacter. The conductive G-biochar appeared to support these microbial communities by easing electron transfer, creating a self-reinforcing cycle in which more active bacteria supplied more electrons for iron reduction.