NO_x precipitation and valorization driven by photocatalysis and adsorption over red soil

Nitrogen oxides (NO_x) emissions can cause air pollution that is harmful to human health, even producing serious ecological problems. Whether it is diluted in the air or not, the management and valorization of NO_x from industrial emissions have been constrained by technology and finance. This study shows that red soil can be used as a photocatalyst to convert NO_x into soil nitrate nitrogen (NO₃^--N) in the soil. The addition of zinc oxide (ZnO) and titanium dioxide (TiO₂) onto the soil surface improves the photocatalytic precipitation efficiency of 1 ppm NO, approaching a removal efficiency of 77 % under ultraviolet (UV) light. The efficiency of red soil in precipitating NO_x through adsorption exceeded that of photocatalysis at 100 ppm NO_x (e.g. 16.02 % versus 7.70 % in 0.1-mm soil). Pot experiment reveals that the precipitated NO₃^--N promoted biomass of water spinach (Ipomoea aquatica Forsk). Additionally, adding ZnO or TiO₂ also affects mineral nutrition. This demonstration of converting air pollutants into available nitrogen (N) for plant growth not only provides a new perspective on treatment and valorization for NO_x but also sheds light on the transport of N in the air-soil-plant path.