Photocatalytic nitrogen-oxide conversion in red soil

Gaseous nitrogen oxides (NOx) produced by biological reactions in the soil not only affects air quality and atmospheric ozone concentration but also cause global warming and acid rain. Soil NOx emissions mainly come from soil microbial processes, but there is little evidence that the photocatalytic activities of inorganic minerals in the soil would affect the NOx transformation. This work studied the effect of soil types, red soil particle sizes, application of urea, and the organic matter content on the NO degradation efficiency and NO2 production, demonstrated that NOx exchange in soils by photochemical reactions and the degradation activity of NO varied widely amongst different soil types. The red soil has an obvious photocatalytic oxidation effect on 1 ppm NO in the air (removal efficiency reaches 24%), and the selectivity for ion species is close to 100%. Soil oxidizes extraneous NO under visible light, it can also oxidize the intrinsic nitrogen-containing compounds into NO2 to contribute emissions in the atmosphere. The ferric oxide could be important for the color of red soil, but its mixture with other minerals is more important for photocatalysis. This nitrogen conversion mechanism provides an alternative pathway for soil NO2 emissions and a new perspective for the nitrogen cycle.