Fenton oxidation of biochar improves retention of cattle slurry nitrogen

Abstract

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+/H2O2 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)2SO4] solution, pH-adjusted (NH4)2SO4 solution, and cattle slurry. The results showed that biochar had the highest total NH4+ adsorption of 1.4 mg N g−1 in (NH4)2SO4 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.