Fenton oxidation of biochar improves retention of cattle slurry nitrogen

Nitrogen (N) losses during fertilization with livestock slurry, mainly in the form of ammonia (NH₃ ), can cause environmental problems and reduce fertilizer efficiency. Leonardite, which is characterized by oxygen-rich functional groups and low pH, has been found to decrease losses of slurry N. However, leonardite, as a byproduct of open-cast lignite mining, is not a renewable resource. The objective of this study was to modify biochar by chemical surface oxidation in order to find a sustainable but similarly effective substitute for leonardite. Biochar was produced from spruce sawdust in a pyrolysis oven at a maximum temperature of 610 °C. Then the biochar was oxidized using the Fenton reaction, with a ratio of Fe²⁺ /H₂ O₂ of 1:1,000, as a source of highly reactive HO· radicals to introduce oxygen-rich functional groups to the biochar surface. The ammonium (NH₄ ⁺ ) adsorption capacity of biochar, oxidized biochar, and leonardite was tested in ammonium sulfate [(NH₄ )₂ SO₄ ] solution, pH-adjusted (NH₄ )₂ SO₄ solution, and cattle slurry. The results showed that biochar had the highest total NH₄ ⁺ adsorption of 1.4 mg N g^-1 in (NH₄ )₂ SO₄ solution, whereas oxidized biochar had the highest reversible NH₄ ⁺ adsorption of 0.8 mg N g^-1 . In the pH-adjusted ammonium solution, all materials reduced NH₃ emissions by ≥90%, and oxidized biochar reduced NH₃ emissions by 99.99%. In contrast, leonardite reduced NH₃ emissions the most in cattle slurry, and oxidation of biochar increased the reduction in NH₃ emissions from 22 to 67% compared with non-oxidized biochar. In conclusion, biochar oxidized by means of the Fenton reaction greatly decreased NH₃ emissions by increased adsorption of NH₄ ⁺ in cattle slurry compared with non-oxidized biochar, indicating the great potential of oxidized biochar for reducing N losses during slurry application.