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Air Cleaning Performance of Two Species of Potted Plants and
Different Substrates
Tatiana Armijos Moya ( T.E.ArmijosMoya@tudelft.nl )
Delft University of Technology: Technische Universiteit Delft https://orcid.org/0000-0003-0017-0598
Pieter de Visser
Wageningen UR PRI: Wageningen University and Research Wageningen Plant Research
Marc Ottele
Delft University of Technology: Technische Universiteit Delft
Andy van den Dobbelsteen
Delft University of Technology: Technische Universiteit Delft
Philomena M. Bluyssen
Delft University of Technology: Technische Universiteit Delft
Research Article
Keywords: Phytoremediation, Botanical bioltration, Indoor air quality, Plant monitoring, Clean air delivery rate, Formaldehyde
DOI: https://doi.org/10.21203/rs.3.rs-314387/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License
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Abstract
Potted plants have been reported to uptake VOCs and help ‘cleaning’ the air. This paper presents the results of a laboratory study in
which two species of plants (Peace Lily and Boston Fern) and three kinds of substrates (expanded clay, soil and activated carbon)
were tested and monitored on their capacity to deplete formaldehyde and CO2 in a glass chamber. Formaldehyde and CO2 were
selected as indicators to evaluate the bio-ltration ecacy of 28 different test conditions; relative humidity (RH) and temperature (T)
were monitored during the experiments. To evaluate the ecacy of every test the Clean Air Delivery Rate (CADR) was calculated.
Overall, soil had the best performance in removing formaldehyde (~ 0.07–0.16 m3/h), while plants, in particular, were more effective
in reducing CO2 concentrations (Peace lily 0.01m3/h) (Boston fern 0.02-0.03m3/h). On average, plants (~ 0.03 m3/h) were as
effective as dry expanded clay (0.02–0.04 m3/h) in depleting formaldehyde from the chamber. Regarding air cleaning performance,
Boston ferns presented the best performance among the plant species, and the best performing substrate was the soil.
1 Introduction
Studies have shown that poor Indoor Air Quality (IAQ) affects human health in a long-term exposure (WHO, 2010). In the INDEX
project (Kotzias et al. 2005) several chemicals, their concentration levels and their toxicity information were analysed and evaluated
in indoor environments. It was concluded that Volatile Organic Compounds (VOCs), such as benzene, toluene and xylene, together
with aldehydes should be considered as priority pollutants regarding their health effects. Several studies related with IAQ have
indicated that VOCs are emitted by indoor sources such as building materials, furnishings and cleaning products (Bluyssen et al.
1997; Bluyssen et al. 1996; Brown et al. 1994; Campagnolo et al. 2017; Sofuoglu et al. 2011). In 1998, Yu and Crump published a
review on VOC-emissions from newly built houses (Yu and Crump 1998). They stated that building material emissions are the
sources of VOCs in the indoor environment, especially most during the rst six months after construction. Among the indoor
pollutants, VOCs are ubiquitous and have harmful effects on human health such as asthma, wheezing, allergic rhinitis, and eczema.
VOCs are frequently classied according to their boiling point (Bluyssen 2009): very volatile organic compounds (VVOCs), such as
formaldehyde; VOCs, such as solvents and terpenes; Semi VOCs (SVOCs), such as pesticides; and Particulate Organic Matter (POM),
such as biocides. Regarding IAQ, VOCs and VVOCs are the pollutants most frequently found indoors (Wolkoff 2003). Some of them
are toxic and carcinogenic, such as formaldehyde; and in general, exposure to formaldehyde is higher indoors than outdoors (IARC
2006; Nielsen and Wolkoff 2010; Salthammer et al. 2010). Formaldehyde (CH2O) is a highly reactive aldehyde. It is a ubiquitous
pollutant and it is a component of different chemical and industrial products (Salthammer et al. 2010). Because of its occurrence
indoors and the evident impact on human health, the study presented focused on the reduction of indoor formaldehyde
concentrations.
1.1 Sources of formaldehyde
Formaldehyde is released directly into the indoor air from various types of sources. People are exposed to environmental
formaldehyde from adhesives, lubricants, wall coverings, rubber, water-based paints, cosmetics, electronic equipment, and glued
wood-based products. For instance, formaldehyde is known to be emitted considerably by chipboard, MDF, plywood and other wood-
based products containing resins (Bluyssen et al. 1996; Campagnolo et al. 2017). Next to these building materials, formaldehyde is
a component of tobacco smoke and of combustion gases from heating stoves and gas appliances. It is used as a disinfectant and
as a preservative in biological laboratories. It is also used in the fabric and clothing industry.
Major sources of formaldehyde in non-smoking environments are building materials and consumer products. This applies to new
materials and products and can last several months, especially in conditions with high relative humidity (RH) and high indoor
temperatures (Haghighat and De Bellis 1998; Knoeppel 1990; Salthammer et al. 2010). Formaldehyde is also one of the main
components for resins, which are contained in various products, mainly in wood products. Furthermore, it should be noted that
secondary formation of formaldehyde occurs in air through the oxidation of VOCs. However, the inuence of these secondary
chemical processes to the ambient and indoor concentrations has still not been fully measured (Kaden 2010).
1.2 Health effects of formaldehyde
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In general, humans are mainly exposed to formaldehyde through inhalation. Since formaldehyde is soluble in water, it is rapidly
absorbed in the respiratory and gastrointestinal tracts and metabolized (WHO 2010). Predominant symptoms of formaldehyde
exposure in humans are irritation of the eyes, nose and throat, discomfort, sneezing, coughing, nausea, among others (WHO 2000).
The lowest concentration may cause sensory irritation of the eyes with humans, increasing eye blink frequency and conjunctival
redness (WHO 2010).
1.3 Formaldehyde guidelines and regulations
In the Netherlands, several formaldehyde measurement studies have been executed specially in homes and at schools, where there
were complaints which might have been caused by formaldehyde. Several complaints were connected with a concentration above
120 µg/m3. In Dutch schools the highest concentration measured was 2.5 mg/m3. In homes, the highest concentrations found were
between 0.75 and 1 mg/m3 (Knoeppel 1990). In 2011, Van Gemert reported that the odour thresholds for formaldehyde can
uctuate from 0.03 to 2.2 mg/m3 (Van Gemert 2011).
WHO 2010 reported that the lowest concentration to cause sensory irritation of the eyes in humans is 0.38 mg/m3 for four hours.
Besides, a formaldehyde concentration of 0.6 mg/m3 increases eye blink frequency and conjunctival redness. Regarding the
perception of odour of formaldehyde, some individuals reported sensory irritation, and formaldehyde may be perceived at
concentrations below 0.1 mg/m3. However, this is not considered to be an adverse health effect (Kaden 2010; WHO 2000, 2010).
1.4 Effects of plants on formaldehyde removal
It has been well established that potted-plants can help to phytoremediate a diverse range of indoor air pollutants. In particular, a
substantial body of literature has demonstrated the ability of the potted-plant system to remove VOCs from the indoor air. These
ndings have largely originated from laboratory-scale chamber experiments, with several studies drawing different conclusions
regarding the primary VOC removal mechanism, and removal ecacies (Armijos Moya et al. 2019; Dela Cruz et al. 2014; Irga et al.
2018; Soreanu et al. 2013). The process of VOC depletion found in most studies is through the microbial activity in the substrate
and rhizosphere, where bacteria absorb the VOCs and metabolise them as a nutrient source (Armijos Moya et al. 2019; Aydogan and
Montoya 2011; Irga et al. 2018; Wolverton et al. 1989).
In 2011, Aydogan and Montoya tested the formaldehyde removal eciency of the root area and aerial parts independently and
found that while the aerial parts of plants were capable of VOC removal, removal by the root area occurred at a substantially faster
rate (Aydogan and Montoya 2011). Other research has identied the potential for the microorganisms existing on and in leaves to
remove VOCs (Khaksar 2016; Sandhu et al. 2007). However, most recent research has acknowledged that the mechanisms of
removal are mainly located in the substrate, rather than the plant itself (Kim et al. 2008; Orwell et al. 2004; Wood et al. 2002).
Based on the studies mentioned, it is valid to assume that plants together with is substrate can have a positive removal effect on
the concentration of formaldehyde in indoor environments. However, the extent to which different plants remove formaldehyde is
not well known yet. This paper presents the results of a study on the uptake of formaldehyde and CO2 from selected potted plants
and substrates, with the objective of using the outcome of these experiments to select the best performing plant and substrate for
the construction of an indoor plant-based system (biowall).
2 Materials And Methods
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
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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:
10 µl formaldehyde + 90 µl demi-water = 100 µl (nal mixture)
10 µl of the nal 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.
2.3 Preparation of the substrates
Three different growth media were chosen for the test: soil, activated carbon and expanded clay. The selected potting soil was
composed by peat, green compost, lime and fertilizers. The selection of the substrates was based on previous studies and because
they are common substrates available on the market (Aydogan and Montoya 2011; Wolverton et al. 1989). For every type of
substrate six tests were executed, three with a dry substrate and three with a wet substrate. The substrates were placed each in a
plastic container with a capacity of 1.1 litres (0.0011 m3) with 0.14 m diameter in the upper part, which was the exposed area of the
substrate.
2.4 Preparation of the plant samples
Two different plant species were tested: Spathiphyllum Wallisii Regel (Common name: Peace Lily) and Nephrolepis exaltata L.
(Common name: Boston Fern) (Fig.2
)
. Three plants of every species were chosen for the tests and they were selected with similar
characteristics of age and size (Peace lily: 0.35m height; Boston fern: 0.30 m height). The plants were selected based on
information gathered by previous studies, which demonstrated that the capability of these species in uptake of some VOCs was
good (Liu et al. 2007; Wolverton and Wolverton 1993; Wood et al. 2002). And they were also chosen because they can be used in
Living Wall Systems (LWSs) and/or green walls, besides, they are commonly used for indoor decoration. The plants were bought in
a house-plant shop in the Netherlands and were re-potted 25 days prior the experiments, to minimize the stress of the plant, in a 14
cm diameter plastic pot of 1.1 litre (0.0011 m3) of expanded clay growth medium. The expanded clay was selected as a growth
medium for the tests because it is the most common substrate used indoors and it is most suitable to be used in indoor living wall
systems. All the plants went through a 30 min acclimatization and adaptation process in the laboratory where they were exposed to
similar conditions, in order to minimize the stress of the plants prior the execution of the tests.
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 (Tables1 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
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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 sucient 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 eciency of the different test conditions the following formula was used (Irga et al. 2017):
With:η = Eciency [%]
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]
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Concentration [ppb] = 24.45 x concentration [µg/m3] ÷ molecular weight [g/mol]
To stablish the statistical signicance 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 signicant differences between the means of the tested variables. Additionally, a Pos-Hoc test was also required to
conrm 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.
3 Results
Figures 4 to 7 show the measured formaldehyde concentrations for the different test congurations. Figures8 presents the
measured CO2 concentrations when the selected potted plants were included. Figures9 presents the measured formaldehyde and
CO2 concentrations when the Boston ferns were included. In general, three measurements were executed for every test condition and
the gures present the mean values including standard errors (± SE). In Tables1 and 2, the CADRs of respectively formaldehyde and
CO2 depletion inside of the chamber for the different tests is presented. The CADRs were calculated using equations 1 and 2.
Tables3 and 4 present the statistical analysis of the CADR caused by the selected growth media and selected plants.
During the zero measurements of the setup, the sensor indicated the presence of around 30.7 µg/m3 (25 ppb) of formaldehyde in
the system. It is believed that this value was due to the calibration process. The zero measurement tests indicated that the
formaldehyde decreased slowly in the chamber (Figs.3–6), which could be the natural decay of the gas or because it was partially
adsorbed by the setup. When the plastic container was placed inside of the chamber the reduction slightly increased, which shows
that the formaldehyde was adsorbed by the container. These two values have to be taken in account when analysing the real effect
of the substrates and plants regarding formaldehyde depletion (Table1). Therefore, to calculate the CADR and establish the real air-
cleansing-impact of every test condition, the natural decay of the chamber (λn = 0.11 h− 1) and the decay rate of the plastic container
(λp = 0.15 h−1) were subtracted from the total decay rate (Tables1 and 2).
Figure 4 presents the depletion of formaldehyde when expanded clay was tested, under dry and wet conditions, indicating that wet
expanded clay was more effective on depleting formaldehyde than under dry conditions. Among all the conditions tested, soil was
the most effective element to reduce formaldehyde in the chamber, especially under wet conditions (Fig.5). Figure6 shows that
activated carbon under dry conditions was more ecient than under wet conditions in reducing formaldehyde in the chamber.
Regarding formaldehyde depletion, potted plants (0.03 m3/h) were as effective as dry activated carbon (0.03–0.04 m3/h), less
effective in general than soil (0.07–0.16 m3/h), less effective than wet expanded clay (0.04–0.16 m3/h) and as effective as dry
expanded clay (0.02–0.04 m3/h) (Table1). The selected plants (Boston Fern and Peace Lily) present similar performance regarding
formaldehyde removal (Fig.7).
With regards to CO2 levels, potted plants seemed to be the only test condition that reduced CO2, of which Boston fern was the most
effective (Table2). While in the case of activated carbon and soil, the levels of CO2 seemed to increase in the chamber.
Table1 shows that under dry conditions inside of the chamber, the selected soil adsorbed formaldehyde faster than the other
substrates, while the performance of the dry expanded clay was the lowest. The wet soil and expanded clay performed better than
the dry conditions tested. Furthermore, Table1 shows that the selected plants together with the substrate did not perform as well as
the wet substrates, but, in general, they performed better than the dry substrates with the exception of the dry soil. Regarding leaf
area, the selected plants had similar characteristics in size and number of leaves, therefore, for every species one plant was selected
and all its leaves were measured. Consequently, it was considered that the area of the other two plants of the selected species were
in the same area range. In general, the peace lilies (approx. 0.14 m2) had more leaf area than the Boston ferns (approx. 0.11 m2).
Table3 presents the statistical analysis of the CADR of formaldehyde depletion caused by the selected growth media. It shows that
soil has a better performance than the other samples. Regarding the data set of formaldehyde depletion, and once it was
established the statistically signicant differences between the means of the tested variables (P = 0.00) with ANOVA, the differences
between the variables were analyzed in Tables4 and 5. Table4 presents the statistical difference among the variables. It shows that
mainly wet soil has statistical differences with the other analyzed variables. Table5 indicates three homogeneous subsets among
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the variables in terms of formaldehyde depletion. Within a subset there is no signicance different while between subsets there is a
signicant difference. It is clear that Group 3 (wet soil, dry soil, wet expanded clay) is signicantly different from Group 1 (wet
activated carbon, dry activated carbon, dry expanded clay, peace lily, Boston fern).
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Table 1
CADR of formaldehyde depletion inside of the chamber.
Test
N. Test Condition RH* T* Time N(0) N(t) λeλnλp CADR η
(%) (°C) (h) (µg/m3) (µg/m3) (h)−1 (h)−1 (h)−1 (m3/h) (%)
1 Zero
measurement 1 (ZM_1) 53 24 2.38 481.48 0.09
2 Zero
measurement 2 (ZM_2) 59 24 1.52 524.47 0.13
3 Zero
measurement_Pot
1
(ZMP_1) 43 24 2.10 498.68 0.16
4 Zero
measurement_Pot
2
(ZMP_2) 58 24 1.52 515.87 0.14
5 Dry Expanded
Clay 1 (EC_D_1) 85 25 1.55 363.57 98.26 0.84 0.02 73
6 Dry Expanded
Clay 2 (EC_D_2) 83 24 1.13 335.32 70.01 1.38 0.04 79
7 Dry Expanded
Clay 3 (EC_D_3) 57 24 1.60 431.12 116.69 0.82 0.02 73
8 Wet Expanded
Clay 1 (EC_W_1) 93 26 1.10 308.30 1.23 5.02 0.16 100
9 Wet Expanded
Clay 2 (EC_W_2) 92 25 1.10 368.48 22.11 2.56 0.08 94
10 Wet Expanded
Clay 3 (EC_W_3) 95 24 1.65 174.41 17.20 1.40 0.04 90
11 Dry Soil 1 (S_D_1) 92 24 1.27 389.36 2.46 4.00 0.12 99
12 Dry Soil 2 (S_D_2) 93 24 1.50 336.55 4.91 2.82 0.08 99
13 Dry Soil 3 (S_D_3) 93 25 1.43 447.09 13.51 2.44 0.07 97
14 Wet Soil 1** (S_W_1) 91 25 1.07 197.75 1.00 4.96 0.16 99
15 Wet Soil 2 (S_W_2) 96 24 1.38 366.02 1.23 4.12 0.13 100
16 Wet Soil 3 (S_W_3) 93 24 1.48 381.99 1.23 3.87 0.12 100
17 Dry Activated
Carbon 1 (AC_D_1) 41 25 1.42 296.01 39.30 1.43 0.04 87
18 Dry Activated
Carbon 2 (AC_D_2) 43 24 1.52 297.24 45.45 1.24 0.03 85
19 Dry Activated
Carbon 3 (AC_D_3) 50 24 1.49 358.65 67.55 1.13 0.03 81
20 Wet Activated
Carbon 1 (AC_W_1) 95 25 1.57 383.22 126.51 0.71 0.01 67
21 Wet Activated
Carbon 2 (AC_W_2) 93 26 1.25 428.67 128.97 0.96 0.02 70
22 Wet Activated
Carbon 3 (AC_W_3) 91 24 0.75 356.20 1469.01 -1.89 -
23 Peace Lily 1 (SPA_1) 95 24 1.77 311.98 41.76 1.14 0.03 87
24 Peace Lily 2 (SPA_2) 95 24 1.67 367.25 44.22 1.27 0.03 88
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Test
N. Test Condition RH* T* Time N(0) N(t) λeλnλp CADR η
25 Peace Lily 3 (SPA_3) 94 24 1.72 348.83 46.67 1.17 0.03 87
26 Boston fern 1 (NEPH_1) 93 24 1.63 390.59 58.96 1.16 0.03 85
27 Boston fern 2 (NEPH_2) 94 24 1.58 413.93 67.55 1.14 0.03 84
28 Boston fern 3 (NEPH_3) 95 24 1.55 427.44 74.92 1.12 0.03 82
* Mean values
** The measured formaldehyde concentration was 0<, the value used for the calculation was N(t) = 1 (µg/m3)
Average values used for the calculations: λn = 0.11(h)−1; λp = 0.15(h)−1
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Table 2
CADR of CO2 depletion inside of the chamber.
Test
N. Test Condition RH* T* Time N(0) N(t) λeλnλp CADR η
(%) (°C) (h) (mg/m3) (mg/m3) (h)−1 (h)−1 (h)−1 (m3/h) (%)
1 Zero
measurement 1 (ZM_1) 53 24 2.38 756.00 0
2 Zero
measurement 2 (ZM_2) 59 24 1.52 887.40 0
3 Zero
measurement_Pot
1
(ZMP_1) 43 24 2.10 1024.21 0
4 Zero
measurement_Pot
2
(ZMP_2) 58 24 1.52 1054.81 0
5 Dry Expanded
Clay 1 (EC_D_1) 85 25 1.55 1368.01 0 -
6 Dry Expanded
Clay 2 (EC_D_2) 83 24 1.13 1297.81 1281.61 0.01 0.00 1
7 Dry Expanded
Clay 3 (EC_D_3) 57 24 1.60 1243.81 0 -
8 Wet Expanded
Clay 1 (EC_W_1) 93 26 1.10 1018.81 0 -
9 Wet Expanded
Clay 2 (EC_W_2) 92 25 1.10 1051.21 1031.41 0.02 0.00 2
10 Wet Expanded
Clay 3 (EC_W_3) 95 24 1.65 1351.81 1323.01 0.01 0.00 2
11 Dry Soil 1 (S_D_1) 92 24 1.27 977.40 -0.05 -
12 Dry Soil 2 (S_D_2) 93 24 1.50 1146.61 -0.04 -
13 Dry Soil 3 (S_D_3) 93 25 1.43 1099.81 -0.04 -
14 Wet Soil 1 (S_W_1) 91 25 1.07 851.40 -0.13 -
15 Wet Soil 2 (S_W_2) 96 24 1.38 932.40 -0.18 -
16 Wet Soil 3 (S_W_3) 93 24 1.48 981.00 -0.14 -
17 Dry Activated
Carbon 1 (AC_D_1) 41 25 1.42 2190.61 -0.21 -
18 Dry Activated
Carbon 2 (AC_D_2) 43 24 1.52 1002.61 -0.06 -
19 Dry Activated
Carbon 3 (AC_D_3) 50 24 1.49 1033.21 -0.01 -
20 Wet Activated
Carbon 1 (AC_W_1) 95 25 1.57 1432.81 -0.48 -
21 Wet Activated
Carbon 2 (AC_W_2) 93 26 1.25 1222.21 -0.09 -
22 Wet Activated
Carbon 3 (AC_W_3) 91 24 0.75 1272.61 -0.17 -
23 Peace Lily 1 (SPA_1) 95 24 1.77 1146.61 885.60 0.15 0.01 23
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Test
N. Test Condition RH* T* Time N(0) N(t) λeλnλp CADR η
24 Peace Lily 2 (SPA_2) 95 24 1.67 1288.81 925.20 0.20 0.01 28
25 Peace Lily 3 (SPA_3) 94 24 1.72 1337.41 963.00 0.19 0.01 28
26 Boston fern 1 (NEPH_1) 93 24 1.37 1002.61 351.00 0.77 0.03 65
27 Boston fern 2 (NEPH_2) 94 24 0.97 1202.41 718.20 0.53 0.02 40
28 Boston fern 3 (NEPH_3) 95 24 0.92 1126.81 718.20 0.49 0.02 36
* Mean values measured in the chamber
Table 3
Statistical analysis of the CADR of formaldehyde depletion caused by the selected growth media.
Dry expanded clay Wet expanded clay Dry soil Wet soil Dry activated carbon Wet activated carbon
Mean 0.02 0.09 0.09 0.13 0.03 0.02
SD* 0.01 0.06 0.03 0.02 0.01 0.01
SE** 0.01 0.04 0.02 0.01 0.00 0.00
* SD: Standard Deviation
** SE: Standard Error
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Table 4
Multiple Comparisons (Tukey HSD); Dependent Variable: CADR for formaldehyde.
(I) What is the variable? (J) What is the variable? Mean Difference (I-J) Std. Error Sig.
Dry Expanded Clay Wet Expanded Clay -0.067 0.021 0.093
Dry Soil -0.063 0.021 0.122
Wet Soil -0.110* 0.021 0.002
Dry Activated Carbon -0.007 0.021 1.000
Wet Activated Carbon 0.012 0.024 1.000
Peace Lily -0.003 0.021 1.000
Boston Fern -0.003 0.021 1.000
Wet Expanded Clay Dry Expanded Clay 0.067 0.021 0.093
Dry Soil 0.003 0.021 1.000
Wet Soil -0.043 0.021 0.488
Dry Activated Carbon 0.060 0.021 0.159
Wet Activated Carbon 0.078 0.024 0.070
Peace Lily 0.063 0.021 0.122
Boston Fern 0.063 0.021 0.122
Dry Soil Dry Expanded Clay 0.063 0.021 0.122
Wet Expanded Clay -0.003 0.021 1.000
Wet Soil -0.047 0.021 0.403
Dry Activated Carbon 0.057 0.021 0.205
Wet Activated Carbon 0.075 0.024 0.090
Peace Lily 0.060 0.021 0.159
Boston Fern 0.060 0.021 0.159
Wet Soil Dry Expanded Clay 0.110* 0.021 0.002
Wet Expanded Clay 0.04 0.021 0.488
Dry Soil 0.05 0.021 0.403
Dry Activated Carbon 0.103* 0.021 0.004
Wet Activated Carbon 0.122* 0.024 0.002
Peace Lily 0.107* 0.021 0.003
Boston Fern 0.107* 0.021 0.003
Dry Activated Carbon Dry Expanded Clay 0.01 0.021 1.000
Wet Expanded Clay -0.06 0.021 0.159
Dry Soil -0.06 0.021 0.205
Wet Soil -0.103* 0.021 0.004
Wet Activated Carbon 0.02 0.024 0.992
Page 13/24
(I) What is the variable? (J) What is the variable? Mean Difference (I-J) Std. Error Sig.
Peace Lily 0.00 0.021 1.000
Boston Fern 0.00 0.021 1.000
Wet Activated Carbon Dry Expanded Clay -0.01 0.024 1.000
Wet Expanded Clay -0.08 0.024 0.070
Dry Soil -0.08 0.024 0.090
Wet Soil -0.122* 0.024 0.002
Dry Activated Carbon -0.02 0.024 0.992
Peace Lily -0.02 0.024 0.998
Boston Fern -0.02 0.024 0.998
Peace Lily Dry Expanded Clay 0.00 0.021 1.000
Wet Expanded Clay -0.06 0.021 0.122
Dry Soil -0.06 0.021 0.159
Wet Soil -0.107* 0.021 0.003
Dry Activated Carbon 0.00 0.021 1.000
Wet Activated Carbon 0.02 0.024 0.998
Boston Fern 0.00 0.021 1.000
Boston Fern Dry Expanded Clay 0.00 0.021 1.000
Wet Expanded Clay -0.06 0.021 0.122
Dry Soil -0.06 0.021 0.159
Wet Soil -0.107* 0.021 0.003
Dry Activated Carbon -0.003 0.021 1.000
Wet Activated Carbon 0.015 0.024 0.998
Peace Lily 0.000 0.021 1.000
* The mean difference is signicant at the 0.05 level.
Page 14/24
Table 5
Homogeneous Subsets; Dependent Variable: CADR for formaldehyde.
What is the variable? N Subset for alpha = 0.05
1 2 3
Student-Newman-Keuls Wet Activated Carbon 2 0.015
Dry Expanded Clay 3 0.027 0.027
Peace Lily 3 0.030 0.030
Boston Fern 3 0.030 0.030
Dry Activated Carbon 3 0.033 0.033
Dry Soil 3 0.090 0.090
Wet Expanded Clay 3 0.093 0.093
Wet Soil 3 0.137
Sig. 0.914 0.072 0.116
Tukey HSD Wet Activated Carbon 2 0.015
Dry Expanded Clay 3 0.027 0.027
Peace Lily 3 0.030 0.030
Boston Fern 3 0.030 0.030
Dry Activated Carbon 3 0.033 0.033
Dry Soil 3 0.090 0.090 0.090
Wet Expanded Clay 3 0.093 0.093
Wet Soil 3 0.137
Sig. 0.056 0.110 0.437
Means for groups in homogeneous subsets are displayed.
Table 6
Statistical analysis of the CADR of formaldehyde and CO2
depletion caused by the selected plants.
Formaldehyde CO2
Peace lily Boston fern Peace lily Boston fern
Mean 0.03 0.03 0.01 0.02
SD* 0.00 0.00 0.00 0.00
SE** 0.00 0.00 0.00 0.00
* SD: Standard Deviation
** SE: Standard Error
Table6 presents the statistical analysis of the CADR of formaldehyde and CO2 depletion caused by the selected plants. Regarding
formaldehyde depletion, both species showed the same performance. Regarding CO2 depletion, Boston ferns showed a better
performance than Peace lilies. Regarding the data set of CO2 depletion, independent T-tests were executed to establish whether a
Page 15/24
statistically signicant difference occurred of the depletion of CO2 between the selected plants, the results showed that Boston ferns
depleted statistically signicantly more CO2 than the peace lilies (P = 0.02).
4 Discussion
This study provides data for the characterization of the removal of formaldehyde by three different substrates and two different
potted plants. Four series of zero measurements were executed to evaluate the setup. Two measurements of these series were
executed with a plastic pot to evaluate the effect of this element in the depletion of the formaldehyde inside of the chamber. As
expected, once the plastic pot was placed in the chamber the formaldehyde level was lower than the natural decay measured during
the zero-measurement evaluation. This value was used to calculate the CADR for every test condition as shown in Tables1 and 2.
4.1 Depletion of formaldehyde
Exploration of the potential of plants to purify air from pollutants started in the early 1980s (Armijos Moya et al. 2019; Wolverton et
al. 1984) and to date several plant species have been studied and identied for use in formaldehyde removal. However, previous
studies have tested extremely high concentrations of formaldehyde (over ~ 2000 µg/m3) (Dela Cruz et al. 2014), higher than the
concentrations that are usually found in common indoor environments (WHO 2010). This study, presents the results of the uptake of
formaldehyde with a concentration of 300 ppb (0.37 mg/m3), which is within the boundaries of the detection threshold of
formaldehyde indoors (0.03 mg/m3-0.6 mg/m3) (WHO 2010) and close to the guideline value based on sensory effects (0.1 mg/m3)
(WHO 2010). Furthermore, formaldehyde is soluble in water (WHO 2010), therefore, it may be depleted faster in wet environments
(Aydogan and Montoya 2011). In a study published in 2011, Aydogan and Montoya reported that activated carbon alone showed
the highest formaldehyde removal and the four plant species studied demonstrated similar abilities to remove formaldehyde
(Aydogan and Montoya 2011). During this set of experiments, the reduction of formaldehyde concentration inside of the chamber
was faster when wet substrates were present, the plant species have similar behaviour in formaldehyde removal (~ 0.03 m3/h).
However, activated carbon appeared to be a very unstable component. In none of the cases, activated carbon had an optimal
performance. Figure6 presents the results of the effect of dry activated (AC_D; n = 3), and wet activated carbon (AC_W; n = 2) on the
depletion of formaldehyde in the chamber. The third sample of wet activated carbon was excluded because instead of reducing the
formaldehyde concentration, the wet activated carbon released it into the chamber. The third sample of the wet activated carbon
came from a different package than the other samples. The packaging material most likely was polluted, which might have caused
the unstable behavior of the selected substrate.
Previous studies suggest that the depletion of formaldehyde also occurs due to photosynthesis and metabolism of the plant at
daytime (Teiri et al. 2018). A growing light was used during this test to ensure the optimal conditions of the plant.
Studies with potted plants in closed chambers continue to be useful for isolating factors that may enhance removal eciency and
contribute towards the improvement of plant-based systems (e.g. plant species and growth medium). Therefore, it is recommended
to use the lessons learned from this study in creating a plant-assisted botanical purier (“Biowalls” or active green walls), which
mechanically forces the air to pass through the leaves and the roots (Armijos Moya et al. 2019; Cummings and Waring 2019;
Darlington et al. 2000).
4.2 Depletion of CO2
For the evaluation of the reduction of CO2 levels inside of the chamber, it is important to mention that in general, plants regulate the
internal CO2 concentration through a partial stomatal closure when the CO2 concentration is too elevated to maintain adequate
internal CO2 and optimize water use eciency (Van de Geijn and Goudriaan 1996). Stomata are pores on leaf epidermis for both
water and CO2 uctuations that are controlled by two major factors: stomatal behaviour and density (Elliott-Kingston et al. 2016;
Wang et al. 2007). The fast speed opening and closing of the stomata can save energy and increase photosynthesis and water use
eciency (Grantz and Assmann 1991). Taking this in account, Table2 and Fig.8 present the depletion of CO2 inside the chamber
when the potted plants were present, and they show that even though the leave area of the Boston fern is lower than the peace lily,
the depletion of CO2 inside of the chamber was faster when the Boston fern was in the chamber. In order to ensure the optimal
behaviour of the plant during the experiments levels of CO2 were controlled (Elliott-Kingston et al. 2016; Van de Geijn and Goudriaan
Page 16/24
1996; Wang et al. 2007). Figure8 shows that in order to provide the optimal conditions for the plant it was necessary to inject CO2
inside of the chamber because the concentration was too low for the plants (IPCC 2014; NASA 2019). In each test condition,
activated carbon permanently released CO2 inside of the chamber, which, possibly could be compensated by the uptake of CO2 by
the plants.
4.3 Plants vs. growth media
Formaldehyde and CO2 were used as indicators of the effect of growth media and plants in reducing gaseous pollutants in a
controlled environment. Table1 shows that, in general, growth media were more effective in the depletion of formaldehyde inside of
the chamber than the plants. Regarding CO2 reduction inside of the chamber, as expected, Table2 shows that plants were more
effective than growth media: in most of the cases with only a growth medium present, CO2 was released instead of reduced inside
of the glass chamber. Figure9 presents the different behaviours of the potted plants regarding these two elements. Even though the
leave area of the Boston fern (approx. 0.11 m2) was smaller than the peace lily (approx. 0.14 m2), the Boston ferns reduced the
concentration of CO2 inside of the chamber faster than the peace lilies, which indicates that the stomatal conductance of the
Boston fern was higher than the peace lily, opening the hypothesis about the uptake of more gaseous pollutants by the stomata.
Regarding the depletion of formaldehyde, Tables4 and 5 show that wet soil, dry soil and expanded clay perform similarly and they
are more effective than the other variables tested (Table3).
As mentioned before formaldehyde is soluble in water (WHO 2010). However, this study shows that high levels of humidity seemed
to have no effect on the formaldehyde depletion inside of the chamber as in most of the test conditions the relative humidity level
was above 90%. Nonetheless, it is important to mention that in the case of the plants, high humidity levels may affect the depletion
of the CO2 and the formaldehyde inside of the chamber due to the fact that plants close their stomata at high humidity levels
(Elliott-Kingston et al. 2016; Wang et al. 2007). The temperature was quite stable during the experiments (Tables1 and 2), therefore,
it seemed to have no effect on the formaldehyde and CO2 depletion, but in general in the presence of wet growth media the
depletion of formaldehyde was faster. Regarding the effect of the growth media on the depletion of formaldehyde and CO2, it is
important to mention that when the substrate (wet or dry) was tested without the plant, the whole surface of the substrate was
exposed directly to formaldehyde and CO2. However, when the plants were included, the exposed surface of the selected substrate
was reduced and the results show that the depletion also was lower, which indicates that the ecacy of the growth media, in some
cases, was higher. This effect is produced by the microbial activity in the root zone, where bacteria absorb the gaseous pollutants
and metabolise them (Armijos Moya et al. 2019; Aydogan and Montoya 2011; Irga et al. 2018; Wolverton et al. 1989).
4.4 Potted plants and their effect in the indoor air quality
According to the ASHRAE standard 62.1–2016 the minimum ventilation rate in breathing zones in oce spaces is 0.3 l/s, m2 (1.08
m3/h for every one square meter of oor space) (ASHRAE-62.1 2016), likewise, the standard NEN-EN 15251 − 2007 the minimum
ventilation rate for new buildings and renovations is 0.35 l/s, m2 (1.26 m3/h for every one square meter of oor space) for very low
polluting buildings (NEN-EN15251 2007). Table1 presents that the CADR for formaldehyde depletion of the potted plants is 0.03
m3/h, therefore, it is necessary to have 42 plants for every square meter of oor space in order to meet the standards without any
additional ventilation system. Besides, Table2 presents that the CADR for CO2 depletion of the potted plants is 0.01 m3/h (Peace
lily
)
and 0.02 m3/h (Boston fern). Therefore, it is necessary to have > 100 plants for every square meter of oor space in order to
meet the standards without any additional ventilation system. Therefore, without any extra mechanical ventilation it is necessary an
indoor forest to meet the minimum standards for ventilation rates in breathing zones just with plants, however, in real situations less
plants will be required taking in account the size of the room and the ventilation system of every case.
4.5 Limitations
One of the limitations of this group of tests is the size of the chamber. Even though it has the requirements of a sealed glass
container with the necessary inlets, for future research it is recommended to execute the tests in a bigger sealed glass container to
prevent or reduce the stress of the plant, avoiding the closure of its stomata and reducing its metabolism.
As mentioned before, plant stress should be minimized, therefore, for future experiments the plant should be placed in the chamber
one day prior the execution of the test together with the activated growing light.
Page 17/24
5 Conclusions And Recommendations
A series of tests was performed to evaluate the effect of potted plants on reducing formaldehyde and CO2 levels in a controlled
glass chamber. The outcome of the tests showed some clear advantages and disadvantages of the different test conditions to
consider for the design of an indoor plant-based system.
In terms of air ‘cleaning’ of formaldehyde, the measurements and analysis showed that soil, in general, was most effective in
reducing formaldehyde concentrations in the chamber (~ 0.07–0.16 m3/h). Plants (~ 0.03 m3/h) were as effective as dry expanded
clay (0.02–0.04 m3/h). Wet and dry soil, wet expanded clay and dry activated carbon performed better than the selected plants in
formaldehyde depletion. In this study, it became clear that the substrate is an important ally in reducing gaseous pollutants, such as
formaldehyde.
Regarding CO2 reduction in the chamber, potted plants (Peace lilies − 0.01 m3/h) (Boston ferns 0.02–0.03 m3/h) were more effective
than the other tests. Specially, Boston fern which has a higher stomatal conductance than the peace lily, indicating the possibility of
allowing more gaseous pollutants to be absorbed in the long term.
Studies with potted plants in closed chambers showed to be useful for isolating factors that may enhance removal eciency and
contribute towards the improvement of plant-based systems (e.g. plant species and growth medium). However, the impact of one
potted plant on the cleaning of the indoor air, was insignicant. Therefore, several potted plants will be required to improve the IAQ
taking in account the specic characteristics of the place such as, size and the ventilation system.
It must be noted, however, that in this study the formaldehyde was introduced in a glass chamber in which the plant and its
substrate were located, hereby surrounding the plant and its substrate with formaldehyde. In a ‘normal’ indoor environment, usually
the source of formaldehyde may not be close to the plant system. For the plant-system to take-up the formaldehyde, the polluted air
needs to be transported to the vicinity of the plant. This could be realized, for example, by an active plant-substrate system, in which
the contaminated air is forced to go through the plant-leaves and through the substrate-roots. Further research with active plant-
based systems on the depletion of formaldehyde and other pollutants, is required.
6 Declarations
Ethical Approval: [No Applicable]
Consent to Participate: [No Applicable]
Consent to Publish: We conrm that the manuscript has been submitted solely to this journal and is not published, in press, or
submitted elsewhere.
Authors Contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis
were performed by Tatiana Armijos Moya. The rst draft of the manuscript was written by Tatiana Armijos Moya and all authors
commented and contributed on previous versions of the manuscript. All authors read and approved the nal manuscript. The
individual contribution of the authors is described as it follows:
Conceptualization: Tatiana Armijos Moya, Pieter de Visser, Marc Ottele, Andy van den Dobbelsteen and Philomena M. Bluyssen
Methodology: Tatiana Armijos Moya, Pieter de Visser and Philomena M. Bluyssen
Formal analysis and investigation: Tatiana Armijos Moya and Philomena M. Bluyssen
Writing - original draft preparation: Tatiana Armijos Moya
Writing - review and editing: Tatiana Armijos Moya, Pieter de Visser, Marc Ottele, Andy van den Dobbelsteen and Philomena M.
Bluyssen
Supervision: Pieter de Visser, Marc Ottele, Andy van den Dobbelsteen and Philomena M. Bluyssen
Funding: [No Applicable]
Competing Interests: We conrm that we do not have any potential or perceived conicts of interest
Page 18/24
Availability of data and materials:[No Applicable]
Acknowledgements
The work described in this paper is part of a PhD project, supported by the National Secretariat of Higher Education, Science,
Technology and Innovation of Ecuador (Senescyt) and Delft University of Technology. The authors would like to express their
gratitude to the Wageningen Plant Research Group at Wageningen University for their skilful help and support during the
performance of this set of experiments in their laboratory. Ton van der Zalm and Claire van Haren are thanked for their help during
the execution of the experiments.
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Figures
Figure 1
Schematic view of the experimental setup.
Figure 2
Page 21/24
Selected plants: a. Spathiphyllum Wallisii Regel (Common name: Peace Lily); and b. Nephrolepis exaltata L. (Common name:
Boston Fern) in the glass container.
Figure 3
Scan and calculation of the leaf area: a. Peace Lily; b. Boston Fern.
Figure 4
Measured formaldehyde concentration [(µg/m3)/h] when expanded clay samples were tested: zero measurement (ZM), zero
measurement with the plastic pot (ZM_P), dry expanded Clay (EC_D), wet expanded Clay (EC_W). Data means ± SE, n=3.
Page 22/24
Figure 5
Measured formaldehyde concentration [(µg/m3)/h] when soil samples were tested: zero measurement (ZM1), zero measurement
with the plastic pot (ZM_P), dry soil (SD), wet soil (SW). Data means ± SE, n=3.
Figure 6
Measured formaldehyde concentration [(µg/m3)/h] when activated carbon samples were tested: zero measurement (ZM), zero
measurement with the plastic pot (ZM_P), dry activated carbon (AC_D), wet activated carbon (AC_W). Data means ± SE, n=3 (AC_D)
and, n=2 (AC_W; the third test was excluded).
Page 23/24
Figure 7
Measured formaldehyde concentration [(µg/m3)/h] when plant samples were tested: zero measurement (ZM), zero measurement
with the plastic pot (ZM_P), Peace Lily (SPA), Boston Fern (NEPH). Data means ± SE, n=3.
Figure 8
Depletion of CO2 (mg/m3): for the three Boston Fern (NEPH_1, NEPH_2, and NEPH_3) and for the three Peace Lilies (SPA_1, SPA_2,
and SPA_3). Data means ± SE, n=3.
Page 24/24
Figure 9
Depletion of formaldehyde (NEPH_1, NEPH_2, and NEPH_3) vs depletion of CO2 (CO2_1, CO2_2, and CO2_3): for the three Boston
Ferns. Data means ± SE, n=3.
