Physiological Investigations of the Plants Involved in Air Biofiltration: Study Case

Physiological Investigations of the Plants Involved in Air Biofiltration: Study Case

1. Introduction

Addressing plant physiology–related challenges play an important role in botanical biofilter development for air treatment. In this study, a versatile aerial plant (Tillandsia xerographica) with high potential in air biofiltration is undertaken for investigation. It is already known that botanical biofiltration can improve the air quality both indoors and outdoors and can reduce the health risks associated with the exposure to the gaseous contaminants.

In contrast with the terrestrial plants (i.e., soil/substrate-based) that are usually involved in air biofiltration studies, where the aerial part along with the roots and their rhizosphere microorganisms act symbiotically for the gaseous pollutants removal during the air biofiltration process [1], the leaves of T. xerographica (a soil/substrate-free plant) play a central role in the plant architecture and gas uptake, as its roots are poorly developed—a common aspect for the Tillandsia group—thus, because the roots of Tillandsia do not have an anatomical structure adapted to absorb water from substrate, this epiphyte presents certain characteristics, such as the peltate trichomes (scales) formed by a stem directly connected to the internal tissues of the leaf, a multicellular shield, and the peripheral wing [2]. The trichomes are located on the epidermis and are specialized in the absorption of water and nutrients from the atmosphere. The indumentum of intricate trichomes is also involved in plant temperature regulation, light reflection, and the distribution of water by capillarity on the plant surface [3,4], permitting the plants development in significant stressful environments [2,5,6], as are the xeric microhabitats. It was included in aerophytes category [7], being able to colonize trees and shrub canopies as well as rocks and other supports [8]. The plants reproduction process is by seeds (sexual) or by production of sprouts (vegetative, asexual). Due to its ornamental qualities, it is used as an indoor and outdoor decorative plant.
Despite the plants widespread popularity in Mexico, Guatemala, El Salvador, Honduras, and a part of USA, where it often grows in the upper branches of tall trees, the species is considered threatened because of commercial exploitation. Consequently, the plant is intensively cultivated in nurseries [9].
Like most of the species within the Neotropical genus Tillandsia (subfamily Tillandsioideae within family Bromeliaceae), Tillandsia xerographica seems to exhibit [10,11] a crassulacean acid metabolism (CAM). This photosynthetic pathway, restricted to the genus Tillandsia (especially the epiphytic life-form), has a wide physiological plasticity, high water use efficiency, and a significant advantage in conditions of limited (atmospheric) moisture. Basically, the CAM mechanism allows the nocturnal fixation of CO2 in malic acid facilitated by phosphoenolpyruvate carboxylase, followed, in the light period, by its release, via decarboxylation and assimilation in the Calvin cycle [12,13,14]. Thus, in the light period there is an increase in the internal CO2 concentration causing the stomata closure, minimizing water loss through transpiration and optimizing water use efficiency [15].
Light intensity, environmental temperature, water, and nutrient availability are among the main environmental factors influencing the CO2 assimilation in the CAM plant species [16]. Martin et al. [17] highlighted that the epiphytic CAM bromeliads as Tillandsia ionantha can adapt to both high and low light levels but are more efficient in the utilization of lower irradiances, allowing colonization of the shaded microhabitats. Also, the capacity for dissipation of excess light energy was found to be beneficial for the plants developed in full sun and exposed to drought stress [17], while a study on Tillandsia usneoides [18] highlighted that under constant illumination the CO2 exchange was similar to day/night conditions. A comparison of the relationships between CO2 gas exchange and leaf trichomes cover for the 12 species of Tillandsia [5] found a positive correlation between the maximum rates of night CO2 uptake and leaf trichome cover, but also a low-level diurnal CO2 loss by diffusion, probably via trichome stalks which have a low resistance compared to the leaf cuticle.
A great number of studies have highlighted that plants can remove a wide spectrum of air pollutants, with direct effect on human health and energy consumption [8,19,20]. Among that, a constant objective was represented by a reduction in CO2 footprint [21,22], which can be achieved by using plants photosynthetic capabilities in biofilters. Although this particular species was never investigated in respect to CO2 exchange patterns, the capacity of removing CO2 in absence of light might make from Tillandsia xerographica a very promising solution (perhaps in combination with C3 plant species) for the selection of the appropriate plant species for CO2 removal in biofilters. Exploring this process aspect will contribute to a better understanding of the removal mechanisms and of biofilter functionality, depending on specific environmental conditions. For instance, coupling some C3 and CAM plants grown on a solid substrate resulted in a lower CO2 emission during some VOCs (volatile organic compounds) biofiltration [23,24]. From an implementation point of view, the use of substrate-free aerial plants such as T. xerographica (not limited to) is an affordable option, as they are residue free and are subject to easy transport and maintenance.
The plants ability to take up and transport a certain contaminant from the air to different plant parts, to then metabolize or store it in vacuoles or cell walls, depends on plant morphology/structure and contaminant characteristics, along with plant capacity to cope with any induced oxidative stress or effect that can influence their photosynthesis and respiration through various enzymatic and non-enzymatic mechanisms such as antioxidant systems, heat shock proteins, and phytochelatins [25,26,27,28]. In the case of soil-based plants, there is a continuous interaction between plants and their rhizosphere microorganisms through chemical and physical signals that favor the contaminant removal; particularly, microorganisms can influence plant physiology by producing hormones, enzymes, or toxins [29,30,31]. As introduced earlier, the environmental conditions (e.g., abiotic factors such as temperature, relative humidity, light, water availability, carbon dioxide and/or other pollutants, etc.) can influence the metabolism and plant performance, thus an important attention should be paid to the plant physiological behavior in respect to these aspects, for assuring an appropriate plant response and enhanced process performance [32]. Thus, physiological investigations of the plants involved in air biofiltration are important to understand the factors that influence their performance.

The performance of plants in air biofiltration processes strongly depend on the plant’s physiology, taking into consideration the above mentioned aspects. The aim of this study is to evaluate the behavior change of Tillandsia xerographica during air biofiltration, by monitoring CO2 concentration in the processed air as a response to a change in the environmental conditions. Aspects related to illumination conditions, gas contact time, irrigation, relative humidity, and temperature are particularly addressed. The obtained results are presented and discussed, allowing the pointing out of the plant physiology under different factors influencing and depicting the plant metabolism based on the light-dependent CO2 capture.

2. Materials and Methods

A transparent column-biofilter (7 L volume), made from plexiglass (Poly(methyl methacrylate)), was equipped with three Tillandsia xerographica plants (commercially available, 3–4 years old, with high axis of 21–24 cm considering an elliptical plant shape without extension of leaves). The plants were previously immersed in regular distilled water for about half an hour and then the excess water was removed from their surface before use. The biofilter (Figure 1) was continuously operated for about three weeks (period I–XI, Figure 2) during the summer period with ambient air (conditioned air and fresh air mixture), being most of the time (period IV–IX, Figure 2) exposed to moderate natural illumination under day/night regime. Prior to the first day of continuous biofilter monitoring, the biofilter was subject to an acclimatization period under dynamic regime (0.25 L/min air flowrate) and continuous (artificial) illumination (12 kLux light intensity) for about four days, which allowed the Tillandsia individuals to adapt in the microclimate of the column-biofilter. Carbon dioxide concentration, relative humidity, and temperature in inlet and outlet gas were continuously monitored, along with light intensity (usually, 1 min sampling frequency) by using specific sensors (NDIR sensor for CO2, capacitive polymer sensor for relative humidity, NTC thermistor for temperature, silicon photodiode sensor for light intensity) coupled to the LabQuest 2 stand-alone interface (Vernier, Beaverton, OR, USA), used to collect sensor data, acting as a data logger as well, with its built-in graphics and analysis application (integral/derivative function, etc.). The high-resolution touchscreen of this device facilitated fast and easy collection, analysis and sharing of the acquired data via wireless connectivity.
An AQ-Expert multifunctional indoor air monitor (E-Instruments, Langhorne, PA, USA), an Extech Thermometer and Humidity Meter RH101, and an Extech LT300 digital Lux meter (Nashua, NH, USA) were additionally used for regular control of these measured parameters. No other relevant gaseous contaminants or products were detected in the processed air. According to the the flowsheet diagram of the experimental setup presented in Figure 1, the processed air was fed at the bottom of the bioreactor by using an air pump at a controlled flow rate. The loading rate of the biofilter over the course of the monitoring period ranged between 38 ÷ 198 g CO2/(m3·d) (depending on the air flowrate and CO2 concentration in inlet air), considering the biomass packing bed volume (consisting in the studied plants) of about 6 L including void spaces. In addition, the influence of plant irrigation was investigated at a certain moment by feeding regular distilled water (100 mL, with electrical conductivity of 3 µS/cm and pH 6.5) at the top of the bioreactor (except this, no plant irrigation was performed over the course of the test period). Although in this study, distilled water was used as it is not expected to induce any possible influence on the physiological behavior of this plant type, on longer term it might be necessary considering a small nutrient supply [33]. The treated air was evacuated at the top of the reactor, while the wastewater was collected and evacuated at the bottom. Also, in addition to the natural light, which contains all of the visible spectra, the influence of the artificial lighting was occasionally investigated by illumination of the bioreactor with two LED-array lamps in a visible spectrum (characterized by a spectral range between 440 and 720 nm and a maximum absorption peak located at 580 nm) oppositely located at a distance from the reactor (in this case, the experimental installation was covered by an aluminum folium). The operating ranges of all these parameters are specified in Figure 2. A similar control reactor was also parallelly running, in order to check the process time to time (discontinuously), with a roughly similar response. The day/night cycles, respectively, the natural light/dark cycles, were approximately equal over the course of the test period ranging between day 4.5 and day 16.5, containing alternative cycles of about 15 h of daily light and 9 h of dark with a small tendency of day time decreasing. Also, it can be observed from the Figure 2, the period of artificial illumination during the monitored period (days 0–1, 2.5–4 and 16.5–19).

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