sustainability Article Removal of Volatile Organic Compounds by Means of a Felt-Based Living Wall Using Different Plant Species Gina Patricia Suárez-Cáceres and Luis Pérez-Urrestarazu * Urban Greening and Biosystems Engineering Research Group, ETSIA, Universidad de Sevilla, 41013 Seville, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-95-448-6420 Abstract: Poor indoor quality affects people’s health and well-being. Phytoremediation is one way in which this problem can be tackled, with living walls being a viable option for places with limited space. The aim of this study was to evaluate the efficiency of five plant species in a living wall to remove Volatile Organic Compounds (VOCs) and to identify whether the type of pollutant has any influence. An enclosed chamber was used to add the contaminants n-hexane and formaldehyde independently. Total VOCs were measured for three days in two scenarios: (1) empty chamber, and (2) chamber with living wall. Five living walls were prepared, each with three plants of the same species: Spathiphyllum wallisii, Philodendron hederaceum, Ficus pumila, Tradescantia pallida, and Chlorophytum comosum. There was no correlation between leaf area/fresh weight/dry weight and the contaminant reduction. In general, all five species were more efficient in reducing TVOCs when exposed to formaldehyde than to n-hexane. Chlorophytum comosum was the most efficient species in reducing the concentration of TVOCs for both contaminants, Spathiphyllum wallisii being the least efficient by far. Citation: Suárez-Cáceres, G.P.; Pérez-Urrestarazu, L. Removal of Keywords: green wall; indoor air quality; TVOCs; Spathiphyllum wallisii; Philodendron hederaceum; Volatile Organic Compounds by Ficus pumila; Tradescantia pallida; Chlorophytum comosum Means of a Felt-Based Living Wall Using Different Plant Species. Sustainability 2021, 13, 6393. https://doi.org/10.3390/su13116393 1. Introduction Academic Editor: People tend to spend most of their time indoors [1]. For that reason, maintaining an Giouli Mihalakakou adequate Indoor Air Quality (IAQ) is essential, as poor levels can affect people’s health (Sick Building Syndrome) [2]. Buildings are now increasingly being constructed in an Received: 17 May 2021 airtight manner in order to maximise their energy efficiency [3], preventing adequate Accepted: 2 June 2021 ventilation of indoor spaces [4]. IAQ can be affected by polluted air coming from outside Published: 4 June 2021 the building, but there are also many indoor contaminants [5]. Volatile Organic Compounds (VOC) are mostly anthropogenic contaminants with Publisher’s Note: MDPI stays neutral known effects on health, going from irritation of the eyes and respiratory tract to more with regard to jurisdictional claims in serious illnesses such as liver and kidney damage or cancer [6]. VOCs are widely emitted published maps and institutional affil- from products commonly used in construction (e.g., paints, solvents, varnishes, etc.) and iations. many others employed on a day-to-day basis (detergents, products, air fresheners, cleaning, and personal care products, etc.) [7]. Formaldehyde is one of the most common indoor VOCs [8–10] because it originates from indoor sources in composite wood products [11]. Another common contaminant Copyright: © 2021 by the authors. is n-hexane, which is classified as an alkane [12] and has adverse effects on the central Licensee MDPI, Basel, Switzerland. nervous system [13] but has been less researched as an indoor contaminant. This article is an open access article There are many different methods proposed to improve IAQ. One of them is the use of distributed under the terms and plants, known as phytoremediation [14], which offers a solution to the energy consumption conditions of the Creative Commons of other air purification technologies. Its efficiency depends on many different factors such Attribution (CC BY) license (https:// as the type of system, which can be passive (potted plants) or active (filter plants, activated creativecommons.org/licenses/by/ carbon) [15], temperature, light intensity, growing media, or VOC (identity, concentration, 4.0/). Sustainability 2021, 13, 6393. https://doi.org/10.3390/su13116393 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, x FOR PEER REVIEW 2 of 10 plants, activated carbon) [15], temperature, light intensity, growing media, or VOC (iden- tity, concentration, potential mixture effects) [16]. However, the type of plant or species used seems to be one of the main key factors influencing VOC removal efficiency [17]. There is often limited space indoors to provide the amount of vegetation needed to improve air quality. Thus, living walls can be considered as a viable solution [18]. They also contribute to improving the aesthetical component and offer psychological benefits associated with indoor vegetation [19]. In fact, some studies [20] point to living walls being more efficient than potted plants in removing indoor contaminants. The aim of this work was to evaluate the efficiency of five species planted in living wall modules to remove VOCs for IAQ improvement and to assess if the type of contam- inant had any influence. Sustainability 2021, 13, 6393 2 of 10 2. Materials and Methods 2.1. Preparations of the Tests and Environmental Conditions potentialA sealed mixture glass effects) chamber [16]. (0.8 However, m long; the 0.4 type m wide; of plant 0.4 or m species high) used[19] was seems used to bewhere one ofcontaminants the main key were factors released influencing in two VOC scenario removals: with efficiency the chamber [17]. empty and with a small livingThere wall isinside often (Figure limited 1). space indoors to provide the amount of vegetation needed to improveThe airliving quality. walls Thus,used for living the wallstests consisted can be considered of a felt-based as a viablemodule solution (Fytotextile [18].® They, Ter- alsoapia contributeUrbana, S.L., to improvingSeville, Spain) the aesthetical[21], 0.49 m component wide by 0.36 and m offer high. psychological The inner geotextile benefits associatedlayer was sawn withindoor to the vegetationexterior polyamide [19]. In fact, layer some forming studies a grid [20] pointof 2 by to 3 living pockets. walls Finally, being morea waterproof efficient layer than pottedwas added plants at inthe removing back of th indoore living contaminants. wall module in order to mimic the exactThe configuration aim of this workof the was commercial to evaluate system the efficiency (Figure of 2). five The species air temperature planted inliving inside wall the moduleschamber towas remove monitored VOCs by for a IAQ HOBO improvement Pro Temp-HR and to U23-001 assess if sensor the type (Onset of contaminant Computer hadCorp., any Bourne, influence. MA, USA). The temperature range in which the tests were carried out was 15.7 °C to 26.8 °C. 2. MaterialsInside the and chamber, Methods the air was mixed by means of a small portable fan to achieve a 2.1.uniform Preparations distribution of the Testsof the and contaminant. Environmental A ConditionsCF-UT01 LED Grow photosynthetic lamp (PandaA sealedGrow, glassShenzhen, chamber China) (0.8 with m long; a light 0.4 cycle m wide; of 15 0.4 h mwas high) used, [19 positioned] was used right where on contaminantsthe chamber at were an angle released of 14° intwo to the scenarios: vegetati withon with the chamberrespect to empty the horizontal and with position a small livingand with wall an inside average (Figure illuminance1). of 6828 Lux [19]. FigureFigure 1.1. SealedSealed glassglass chamberchamber wherewhere n-hexanen-hexane andand formaldehydeformaldehyde teststests werewere performedperformed forfor eacheach plantplant species.species. ((11)) GasGas detector;detector; ((22).). HOBOHOBO sensor;sensor; ((33)) PortablePortable fan;fan; ((44)) Felt-basedFelt-based modulemodule withwith threethree plantsplants perper species;species; ((55)) GrowGrow photosyntheticphotosynthetic lamp.lamp. The living walls used for the tests consisted of a felt-based module (Fytotextile®, Terapia Urbana, S.L., Seville, Spain) [21], 0.49 m wide by 0.36 m high. The inner geotextile layer was sawn to the exterior polyamide layer forming a grid of 2 by 3 pockets. Finally, a waterproof layer was added at the back of the living wall module in order to mimic the exact configuration of the commercial system (Figure2). The air temperature inside the chamber was monitored by a HOBO Pro Temp-HR U23-001 sensor (Onset Computer Corp., Bourne, MA, USA). The temperature range in which the tests were carried out was 15.7 ◦C to 26.8 ◦C. Inside the chamber, the air was mixed by means of a small portable fan to achieve a uniform distribution of the contaminant. A CF-UT01 LED Grow photosynthetic lamp (Panda Grow, Shenzhen, China) with a light cycle of 15 h was used, positioned right on the chamber at an angle of 14◦ to the vegetation with respect to the horizontal position and with an average illuminance of 6828 Lux [19]. SustainabilitySustainability 20212021,,13 13,, 6393x FOR PEER REVIEW 33 of 1010 Figure 2. Felt-based module design and distribution of the three plants for each species. Figure 2. Felt-based module design and distribution of the three plants for each species. 2.2. Selected Plants 2.2. Selected Plants Five small modules were prepared specifically for the tests, each planted with a differentFive species:small modules were prepared specifically for the tests, each planted with a dif- ferent species: 1. Spathiphyllum wallisii is commonly used as an ornamental houseplant due to its 1. Spathiphyllumavailability. It wallisii can withstand is commonly poor used growing as an conditions ornament [al22 houseplant] and its ability due to to its capture avail- ability.atmospheric It can particles withstand has poor been growing studied [conditions23]. [22] and its ability to capture at- 2. mosphericPhilodendron particles hederaceum has beenis an studied ornamental [23]. foliage climber plant commonly used in 2. Philodendronindoor environments. hederaceum It is has an theornamental potential foliage to absorb climber harmful plant gases commonly and clean used the in airin- doorinside environments.