2.1 Experimental setup
The setup, schematically presented in Fig. 1, consisted mainly of a dynamic chamber. The dynamic chamber was made out of glass with an inner diameter of 28 cm, height of 60 cm and volume (V) of 0.033 m3. The glass chamber had three air entrances that were sealed during the tests. The gas stream of 300 ppb concentration of formaldehyde was released in the chamber by heating the formaldehyde solution.
The actual formaldehyde concentration was determined by a formaldehyde sensor (DART-sensor 11 mm, calibrated, ppb-level, lower detection limit of < 30 ppb, response time (T90) < 30 s, resolution 10 ppb). Two axial fans were placed into the glass chamber to distribute the air evenly within the chamber. The sensor performed a measurement every minute. During the tests a LED growing lamp was activated (1500 µmolm− 2s− 1 – 1900 µmolm− 2s− 1), and the temperature, relative humidity and CO2 levels were also monitored. CO2 levels were monitored with VAISALA CO2 probe GMP252 (ppm-level). Furthermore, the glass container was sealed with a solvent free, plastic, self-adhesive sealant, kneading material, based on synthetic rubber during the tests.
2.2 Chemicals
The formaldehyde solution used for these experiments was: Solution Sigma F8775, 25 ml (36.5–38% formaldehyde in H2O). The formaldehyde solution was mixed with demi-water in order to generate 300 ppb within the chamber. The mixture was executed by technicians in the laboratories of the University of Wageningen, as follows:
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10 µl formaldehyde + 90 µl demi-water = 100 µl (final mixture)
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10 µl of the final mixture generated 300 ppb of formaldehyde, within the chamber.
It is important to report that the formaldehyde solution contained 10–15% of methanol, as stabiliser to prevent polymerisation. The DART-sensor is also sensitive to methanol. So, by introducing formaldehyde, a small amount of methanol was introduced as well. The response of the DART-sensor to this amount of methanol therefore also needed to be tested.
2.5 Procedure
Two zero-measurement evaluations were performed to establish the conditions of the set-up in the glass container in which the depletion of the formaldehyde took place: one at the beginning of the test series and one at the end. Similarly, two extra zero-measurement evaluations were performed with a plastic container that had the same characteristics of the containers that were used during every test.
The measurements were executed for 1-1.5 hours until the formaldehyde was depleted or stabilized in the chamber. Gas concentrations were measured in ppb in the case of formaldehyde and in ppm in the case of CO2. For further analysis the concentrations of these gases were expressed as micrograms per cubic meter (µg/m3) and milligrams per cubic meter, respectively. For each test, ~ 368.48 µg/m3 (~ 300 ppb) of formaldehyde was released in the chamber to generate every time exactly the same condition.
Each set of experiments was conducted three times, in order to evaluate consistently each condition tested (Tables 1 and 2). For each test, the glass container was wiped with a wet paper towel after each measurement. The plastic container with the substrate or plant sample was placed in the centre of the glass chamber. Depending on the height of the plant a stainless-steel base was placed at the bottom (stainless steel is an inert material).
A small plate connected to a heat source was placed in the lower hole and 10 µl of formaldehyde solution was placed on the plate with a pipette. After a drop of formaldehyde solution was placed on the plate, the hole was closed, and the heat source was activated in order to realise the solution in the air. This was the beginning of the test. During the tests with the Boston ferns, it was necessary to inject some CO2 when the level was lower than ~ 410 ppm (~ 738 mg/m3) which is the global atmospheric CO2 concentration (average outdoor concentration) (IPCC, 2014; NASA, 2019) and is sufficient for the plants to grow although some studies have shown that the optimal CO2 concentration is around 900 ppm (Zheng et al. 2018).
To calculate the amount of formaldehyde depleted inside of the chamber the following formula was used (Irga et al. 2017):
With: λ = Decay rate [h− 1]
N(t) = Amount of pollutant after time t [µg/m3] or [mg/m3]
N(0) = Initial amount of pollutant at t = 0h [µg/m3] or [mg/m3]
To calculate the rates of contaminant reduction in the test chamber the Clean Air Delivery Rate (CADR) was calculated (ANSI/AHAM-AC-1-2013, 2015; EPA., 2008):
With: λe = Total decay rate [h− 1]
λn = Natural decay rate which is the reduction of the contaminant due to natural phenomena in the test chamber [h− 1]
λp = Decay rate when the plastic pot was placed in the chamber [h− 1]
V = Volume of the chamber [m3], 0.033 [m3]
To calculate the removal efficiency of the different test conditions the following formula was used (Irga et al. 2017):
N(t) = Amount of pollutant after time t [µg/m3] or [mg/m3]
N(0) = Initial amount of pollutant at t = 0h [µg/m3] or [mg/m3]
A portable leaf area meter was used to scan and calculate the leave area of the plant species. Since the three plants of every species had similar characteristics, one plant of every species was selected to be measured (Fig. 3).
Conversions for chemicals in air were made assuming an air pressure of 1 atmosphere and an air temperature of 25 degrees Celsius. The conversion factor was based on the molecular weight of the chemical and is different for each chemical in this case the molecular weight of formaldehyde is 30.031 g/mol and of the carbon dioxide (CO2) is 44.01 g/mol:
Concentration [mg/m3] = 0.0409 x concentration [ppm] x molecular weight [g/mol]
Concentration [ppm] = 24.45 x concentration [mg/m3] ÷ molecular weight [g/mol]
Concentration [µg/m3] = 0.0409 x concentration [ppb] x molecular weight [g/mol]
Concentration [ppb] = 24.45 x concentration [µg/m3] ÷ molecular weight [g/mol]
To stablish the statistical significance of the results, several Independent T-Tests were executed and the mean values and standard errors (± S.E.) were included. Finally, the one-way analysis of variance (ANOVA) was chosen to determine whether there are any statistically significant differences between the means of the tested variables. Additionally, a Pos-Hoc test was also required to confirm where the differences occurred. Based on the nature of this data set, Tukey HSD and the Student-Newman-Keuls were performed to execute a multiple comparison among the groups and to determine homogeneous sets.