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 a major air pollutant. This gaseous pollutant significantly contributes to the secondary formation of ultrafine particulate matter (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 plant physiological processes remains limited in effectively removing these pollutants. This study aimed to conduct foundational research on the pollutant reduction efficacy of a hydroponic vegetated biofilter, which leverages both plant physiological action and the induced hydration reaction of NO2.Methods: This study investigated the potential for NO2 removal through hydration reactions and its subsequent conversion to nitrate (NO3⁻) using a hydroponic vegetated biofilter. Time-series concentrations of NO2 were measured under both irrigated 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 was calculated using the single-pass removal efficiency (SPRE) method.Results: Under irrigated conditions, the NO2 removal efficiency averaged 52.54%, compared to 23.92% under non-irrigated conditions, representing a 119.65% improvement in removal performance with irrigation. Furthermore, the concentration of NO3⁻ in the irrigation water increased by 40.9% relative to pre-experiment levels, confirming the conversion of NO2 to NO3⁻ through a hydration reaction.Conclusion: Hydroponic vegetated biofilter were anticipated to offer multifaceted benefits, effectively removing atmospheric NO2 while simultaneously providing NO3⁻, an essential nitrogen source for plant growth. This study successfully demonstrated the potential of plant-based NbS technology as a viable approach for tangible approach for improving atmospheric environmental quality.