A Study on Hydration Reactions Involved in the Reduction of Nitrogen Oxides (NOx) by Hydroponic Vegetated Biofilters

Background and objective: Nitrogen dioxide (NO2), a prominent component of nitrogen oxides (NOx), is recognized as amajor air pollutant. This gaseous pollutant significantly contributes to the secondary formation of ultrafine particulatematter (PM2.5) through atmospheric chemical reactions. While nature-based solutions (NbS), such as the use of plants,have been explored as environmentally friendly mitigation strategies, the effectiveness of relying solely on the plantphysiological processes remains limited in effectively removing these pollutants. This study aimed to conduct foundationalresearch on the pollutant reduction efficacy of a hydroponic vegetated biofilter, which leverages both plant physiologicalaction and the induced hydration reaction of NO2. Methods: This study investigated the potential for NO2 removal through hydration reactions and its subsequent conversionto nitrate (NO3⁻) using a hydroponic vegetated biofilter. Time-series concentrations of NO2 were measured under bothirrigated and non-irrigated conditions within an indoor air quality (IAQ) chamber. Concurrently, changes in NO3⁻concentrations in the irrigation water were analyzed using a UV-Vis spectrophotometer. The NO2 removal efficiency wascalculated using the single-pass removal efficiency (SPRE) method. Results: Under irrigated conditions, the NO2 removal efficiency averaged 52.54%, compared to 23.92% under nonirrigatedconditions, representing a 119.65% improvement in removal performance with irrigation. Furthermore, theconcentration of NO3⁻ in the irrigation water increased by 40.9% relative to pre-experiment levels, confirming the conversionof NO2 to NO3⁻ through a hydration reaction. Conclusion: Hydroponic vegetated biofilter were anticipated to offer multifaceted benefits, effectively removingatmospheric NO2 while simultaneously providing NO3⁻, an essential nitrogen source for plant growth. This study successfullydemonstrated the potential of plant-based NbS technology as a viable approach for tangible approach for improvingatmospheric environmental quality.