Introduction
Air pollution is a major environmental issue that poses significant threats to both human health and ecosystems, and its severity has recently been highlighted. Among various air pollutants, nitrogen dioxide (NO2) is classified as a representative gaseous component within the nitrogen oxides ( NOx) group. It is primarily formed when nitrogen monoxide (NO), emitted from the combustion processes of mobile pollution sources, power plants, and industrial facilities, is oxidized in the atmosphere. NO2 not only contributes to the formation of ozone (O3) through photochemical reactions in the atmosphere but also reacts with atmospheric moisture to produce nitric acid (HNO3), a precursor of acid rain. The resulting nitrate (NO3−) can exist as aerosol particles, which are among the major secondary pollutants that make up fine particulate matter (PM2.5) (Kim, 2017). In fact, it has been reported that approximately two-thirds of PM2.5 in the Seoul Metropolitan Area consist of secondary particles formed through the chemical reactions of such precursor substances (Ministry of Environment, 2016). Consequently, extensive research is being conducted on the management of NOx emissions—a representative group of gaseous precursors—to mitigate the formation of PM2.5.
Among various NOx management strategies, nature-based solutions (NbS) that utilize plants are being explored as environmentally friendly reduction alternatives. Several studies have reported that plants can absorb gaseous pollutants such as NO2, SO2, and O3 through the stomata on their leaves (Nowak et al., 2014), while pollutants like volatile organic compounds (VOCs) can be degraded and removed by microorganisms inhabiting the rhizosphere— the soil region surrounding plant roots (Irga et al., 2017). However, significant variations in pollutant removal efficiency has been observed, depending on factors such as plant species, leaf surface area, growth conditions, and air residence time (Irga et al., 2017). These findings underscore the limitations of relying solely on plants to effectively mitigate airborne pollutants.
One potential strategy for overcoming the limitations of plant-based pollutant removal is to integrate hydroponics, which offer not only the physiological benefits of plants but also facilitate the hydration reactions of NO2. Nitrogen dioxide (NO2) can be converted into nitric acid (HNO3) and nitrous acid (HNO2) through hydration, ultimately stabilizing in the form of nitrate (NO3−). Since this reaction is activated in a relatively high-moisture environment, hydroponics can provide favorable conditions that promote the hydration of NO2. In hydroponic cultivation, crops preferentially absorb nitrogen in the form of NO3−, which is commonly supplied in nutrient solutions (Andriolo et al., 2006). Moreover, it has been reported that plants can take up NO3− through their roots and utilize it as a key nitrogen source for synthesizing essential biomolecules such as proteins, chlorophyll, and nucleic acids (RNA and DNA; Hu and Sun, 2010).
Drawing on previous studies, the integration of NbS with hydroponic systems can be considered as a sustainable approach for the mitigation of NO2, a major atmospheric pollutant. This integration can create a favorable environment for the capture and absorption of NO3−, a key nitrogen source for plants, thereby promoting its oxidation. This approach is expected to offer a multifaceted solution that contributes to both the stabilization of atmospheric NO2 and the enhancement of plant growth.
Therefore, this study aimed to propose a hydroponic cultivation model integrated with NO2, that can simultaneously improve atmospheric environment and promote plant growth. To this end, the model was conceptualized as a hydroponic vegetated biosystem. Within the hydration reaction environment provided by this system, foundational research was conducted to evaluate its potential to capture and absorb NO2—a gaseous precursor of PM2.5— and to determine whether the resulting NO3−, an essential nitrogen source for plants, could be detected.
Research Methods
Experimental design
This study was conducted using an indoor air quality (IAQ) chamber connected to an irrigation system proposed in a previous study (Kim et al., 2020), in order to obtain empirical data on the NO2 hydration reaction environment, the capture and absorption of NO2, and the detection of NO3− within a hydroponic vegetated biosystem. The hydration reaction environment was defined as comprising the filter and irrigation sections, which react with the inflow of NO2 into the chamber. Capture and absorption efficiency were evaluated by monitoring time-series changes in NO2 concentration using the single-pass removal efficiency (SPRE) method (Irga et al., 2017). The presence of NO3− was determined by analyzing changes in NO3− concentration in the irrigation water using a UV-Vis spectrophotometer. The detailed experimental design is presented below.
Hydration reaction environment: IAQ chamber and vegetated biofilter
In this study, an IAQ chamber capable of accommodating a vegetated biofilter module (1 m2) was employed to monitor time-series changes in NO2 concentration associated with the vegetated biofilter (Fig. 1). The chamber, with a volume of approximately 5.6 m3, was connected to a building HVAC air handling unit (AHU), which enabled control of airflow via an air conditioning fan and regulation of outdoor and indoor air circulation through a damper. The irrigation system for plant growth was designed as a drip irrigation setup, in which perforated pipes were connected to an inverter pump to ensure uniform-pressure water distribution, with the flow rate set at 2 L min−1. In addition, a plant growth LED control system was integrated to provide the artificial light environment necessary for photosynthesis (Fig. 2).
The vegetated biofilter was designed as a modular component of a vertical hydroponic cultivation system. It was constructed in a multilayered configuration, incorporating three coarse filtration layers, each with a thickness of 25 mm. Each module contained 24 planting spaces, which were planted with three species of indoor air-purifying plants—Dracaena fragrans ‘Massangeana’, Epipremnum aureum, and Ficus elastica ‘Robusta’—whose growth stability and air purification efficiency had been validated in a previous study (Kim et al., 2020). To ensure successful establishment within the hydroponic vegetated biosystem, the plants were pre-treated by removing soil from their roots and wrapping them in hydroponic sponges to facilitate adequate water uptake.
Capture and absorption effects: Time-Series monitoring of NO2 concentrations
To assess the NO2 reduction performance of the vegetated biofilter, time-series concentration monitoring using the SPRE method was conducted in an IAQ chamber connected to an AHU. The removal efficiency was quantified by measuring the amount of NO2 eliminated during its passage through the biofilter. Additionally, to examine the effect of irrigation on the time-series reduction of NO2, the experimental conditions were categorized into irrigated (2 L/min) and non-irrigated settings.
For real-time monitoring of NO2 concentrations, a NOx analyzer (Serinus 40, Ecoem, USA) was connected to the IAQ chamber, with data recorded at 2-second intervals. After installing two vegetated biofilter modules inside the chamber, it was hermetically sealed using toggle clamps. A high concentration of NO2 (497.8 ppm) was injected via a gas regulation system coupled with a mass flow controller (MFC) to ensure precise volumetric dosing of the gaseous pollutant. To establish the background concentration, the chamber (volume: approximately 5.6 m3) was conditioned in accordance with the Standards for Maintaining Indoor Air Quality (Article 4) by injecting a high-concentration of NO2 for 40 seconds, thereby raising the internal concentration to 0.6 ppm—six times the NO2 guideline limit (0.1 ppm) applied to multi-use facilities. Following gas injection, air in the IAQ chamber was circulated through the connected AHU while maintaining an airflow velocity of 0.2 m/s—a condition set to minimize stress on plant growth (Kim et al., 2022). A time-series analysis of NO2 concentrations in the chamber was conducted by measuring values upstream and downstream of the vegetated biofilter modules. The removal efficiency was determined by comparing the concentrations before and after filtration. Monitoring was conducted in three replicated trials, and the pollutant removal performance of the vegetated biofilters was calculated using the following method (Irga et al., 2017) (Eq. 1).
Cin = Integrated decay curve of mean NO2 concentration prior to vegetation biofilter passage
Cout = Integrated decay curve of mean NO2 concentration following to vegetation biofilter passage
η = SPRE
Eq 1. Single Pass Removal Efficiency Equation
Nitrate (NO3−) detection: Analysis of NO3− concentration in irrigation water
To determine whether the NO2 introduced into the IAQ chamber was converted to NO3− through a hydration reaction (Fig. 3), a UV-Vis spectrophotometer (OPIZEN Alpha, KLAB) was employed to measure NO3− concentrations by analyzing the absorbance of the samples across a range of wavelengths. Samples were collected through time-series monitoring of NO2 reduction efficiency. Specifically, two samples were taken at 30-minute intervals: one from the irrigation tank water before the experiment, and the other from the remaining irrigation water inside the chamber after the experiment. Each collected sample was dispensed into a 100 mL tube using an auto-pipette, along with the NO3− analysis reagent. The mixture was homogenized using a vortex mixer and allowed to react for 10 minutes. Following the reaction, the solution was transferred to a 50 mm quartz cuvette, and its absorbance was measured at a wavelength of 340 nm. The final concentration for each sample was obtained as the mean of three replicate measurements.
Results and Discussion
Evaluation of NO2 reduction performance of vegetated biofilters
Time-series monitoring of NO2
To assess the effect of irrigation on the NO2 reduction performance of vegetated biofilters, time-series monitoring of NO2 concentrations was conducted, focusing on temporal changes in concentrations upstream and downstream of the filters. Under non-irrigated conditions, the upstream NO2 concentration peaked at 1 minute and 14 seconds (0.846 ppm), while the downstream concentration reached its maximum at 25 seconds (0.606 ppm) (Fig. 4). In contrast, under irrigated conditions, the upstream concentration peaked later, at 1 minute and 42 seconds (0.652 ppm), and the downstream concentration peaked at 1 minute and 12 seconds (0.327 ppm) (Fig. 5). Under non-irrigated conditions, the NO2 concentration increased rapidly, reaching a relatively high peak, and exhibited an irregular fluctuation pattern with high variability. In contrast, under irrigated conditions, the peak concentration was lower, and the time required to reach the peak was longer. The concentration profile in this case developed in a more gradual and stable manner. Notably, the irrigated condition showed a clear distinction between upstream and downstream concentrations, along with a narrower error range, indicating greater consistency and stability in concentration variations. According to previous studies examining the factors influencing NO2 removal, increased removal efficiency is generally associated with a significant extension of residence time (Mok et al, 2001). The face velocity applied to the vegetated biofilters was set at 0.2 m/s, a value identified in previous studies as optimal for VOC removal efficiency in active vegetated biofilters. This velocity was adopted to minimize adverse effects on plant growth while maximizing pollutant removal efficiency per unit time (Dat, 2021). It has also been reported that, under optimal airflow conditions, NO2 transport increases when the wetted state of the medium enhances flow distribution (Abdo et al., 2019). The provision of appropriate airflow and irrigation conditions in the present experimental setup can therefore be interpreted as a factor that facilitated the reduction of NO2 concentrations. This finding suggests that irrigation and airflow conditions in hydroponic vegetated biofilters exert a significant effect on NO2 concentration dynamics, by increasing its residence time and enhancing the stability of the removal process.
Single-Pass Removal Efficiency (SPRE)
The effect of irrigation on NO2 removal performance was evaluated by comparing the single-pass removal efficiency (SPRE) of the vegetated biofilters under irrigated and non-irrigated conditions through time-series monitoring (Fig. 6). Under non-irrigated conditions, the average NO2 removal efficiency was 26.17±12.04%, whereas under irrigated conditions, it increased to 49.96±17.79%, representing a 119.65% improvement relative to the non-irrigated state. This statistically significant difference indicates that irrigation affects NO2 removal efficiency of vegetated biofilters. The removal efficiency of NO2 is primarily attributed to its interaction with moisture. When NO2 comes into contact with water, it can be removed via hydration reactions that convert it into nitrous acid (HNO2) and nitric acid (HNO3), processes that are known to proceed more actively under conditions of higher relative humidity (Jacob, 2000). Accordingly, under irrigated conditions, the relatively elevated humidity likely facilitated these hydration reactions, resulting in a significant enhancement of NO2 removal efficiency. The results of this experiment suggest that, unlike conventional wet scrubbers, NO2 can be effectively removed through the combined use of vegetation and irrigation water without the addition of chemical agents. This approach enables the reduction of NO2 without the generation of secondary pollutants commonly associated with chemical reduction methods, thereby warranting further investigation as a sustainable environmental remediation strategy.
Variation in NO3− concentration in irrigation water
Under conditions where irrigation water was supplied to the vegetated biofilters, temporal changes in NO3− concentration of the irrigation water were analyzed following the introduction of NO2 gas, to evaluate whether NO2 was converted to NO3− via hydration reactions. At the start of the experiment, the NO3− concentration was measured at 0.995 ppm, representing the lowest recorded value. After 30 minutes, it increased to 1.201 ppm, indicating a rise of approximate increase of 20.7%. After 60 minutes, the concentration further increased to 1.402 ppm, reflecting an additional 16.74% increase and an overall increase of 40.9% relative to the initial value. This gradual increase in NO3− concentration was consistent with previous time-series monitoring results and the SPRE analysis. Under irrigated conditions, SPRE increased by approximately 119.65% compared to the non-irrigated conditions. Additionally, the NO2 concentration exhibited a relatively stable decreasing trend, with up to a 55% reduction in variability (based on the standard deviation) relative to the non-irrigated condition (Fig. 7). This phenomenon can be attributed to hydration reactions, in which NO2 interacts with water to form HNO2 and HNO3, which subsequently stabilize as NO3−—a mechanism consistent with those reported in previous studies (Jacob, 2000; Mok et al., 2001).
However, the increase in NO3− concentration observed in this experiment remained relatively limited, staying below 0.5 ppm with an approximate rise of 40%. This limited increase may be attributed to several interfering factors, such as NO3− uptake by plant roots (Hu and Sun, 2010), insufficient residence time under the given experimental conditions, suboptimal environmental parameters (Mok et al., 2001), or the adsorption and subsequent loss of intermediate products (Nowak et al., 2014). Therefore, to more objectively elucidate the NO2 removal mechanism via hydration reactions in association with plants, further experiments involving extended durations, optimized reaction times, and improved environmental control are warranted.
Conclusion
This study examined the time-series variation in nitrogen dioxide (NO2) concentration within a hydroponic vegetated biofilter system and investigated the conversion of NO2 into nitrate (NO3−) through the irrigation process, in order to evaluate the system’s pollutant removal efficiency. The experimental results demonstrated that the maximum NO2 concentration under irrigated conditions was lower than that observed under non-irrigated conditions. Additionally, the time required to reach the peak concentration was delayed by approximately 47 seconds compared to non-irrigated conditions, and the overall variation in concentration exhibited a 55% reduction in standard deviation, indicating a more stable removal performance. Furthermore, under irrigated conditions, NO2 concentration fluctuation was more constrained, resulting in an approximately 0.2 ppm smaller concentration difference between the upstream and downstream sections of the vegetated biofilter modules, relative to non-irrigated conditions. The single-pass removal efficiency (SPRE) of the vegetated biofilters increased by approximately 119.65% under irrigated conditions, confirming the significant positive effect of irrigation on NO2 removal.
In contrast, the concentration of NO3− in the irrigation water gradually increased over time, exhibiting an approximately 40.9% rise compared to the initial level at the end of the 60-minute experiment. This trend suggests that NO2 was dissolved into the water and subsequently underwent a hydration reaction, resulting in its conversion from nitrite (NO2−) to nitrate (NO3−). These findings imply that the vegetated biofilters may contribute to the physical and chemical stabilization of atmospheric NO2 through nitrate formation. Accordingly, based on the findings of this study, the potential application of vegetated biofilters as a nitrogen source for plant growth may be further explored.
However, the relatively limited increase in NO3− concentration observed in this study highlights the need for further investigation. To address this, future research should involve long-term experiments under elevated NO2 concentrations, taking into account environmental factors such as wind speed, temperature, and humidity. Additionally, the reactivity of NO3−, particularly in the context of hydration promotion and plant growth, warrants further examination.
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