The botanical biofiltration of VOCs with active airflow: is removal efficiency related to chemical properties?

Abstract

Botanical biofiltration using active green walls is showing increasing promise as a viable method for the filtration of volatile organic compounds (VOCs) from ambient air; however there is a high level of heterogeneity reported amongst VOC removal efficiencies, and the reasons for these observations have yet to be explained. Comparisons of removal efficiencies amongst studies is also difficult due to the use of many different VOCs, and systems that have been tested under different conditions. The current work describes a procedure to determine whether some of these differences may be related to the chemical properties of the VOCs themselves. This work used an active green wall system to test the single pass removal efficiency (SPRE) of nine different VOCs (acetone, benzene, cyclohexane, ethanol, ethyl acetate, hexane, isopentane, isopropanol and toluene) and explored which chemical properties were meaningful predictor variables of their biofiltration efficiencies. Ethanol was removed most efficiently (average SPRE of 96.34% ± 1.61), while benzene was least efficiently removed (average SPRE of 19.76% ± 2.93). Multiple stepwise linear regression was used to determine that the dipole moment and molecular mass were significant predictors of VOC SPRE, in combination accounting for 54.6% of the variability in SPREs amongst VOCs. The octanol water partition coefficient, proton affinity, Henry's law constant and vapour pressure were not significant predictors of SPRE. The most influential predictor variable was the dipole moment, alone accounting for 49.8% of the SPRE variability. The model thus allows for an estimation of VOC removal efficiency based on a VOC's chemical properties, and supports the idea that system optimization could be achieved through methods that promote both VOC partitioning into the biofilter's aqueous phase, and substrate development to enhance adsorption.

An Assessment of the Suitability of Active Green Walls for NO2 Reduction in Green Buildings Using a Closed-Loop Flow Reactor

“…As the mixture of VOC species and quantity of emitted VOCs varies amongst different plant species [59], it is possible that differences in biogenic VOC emissions between the S. wallisii and S. podophyllum biofilters may have contributed to the interaction between light type and botanical biofilter species, influencing the rate of NO 2 decay. Additionally, botanical biofilters are also capable of filtering out a range of VOCs [33] and the degree to which various VOCs are filtered is dependent on the plant species present within the biofilter [35]. These traits may have also differentially influenced the VOC concentration profile within the flow reactor and therefore, it is possible that the VOC profile associated with each plant species may have had ramifications for the NO 2 decay rate constants of each of the botanical biofilters.…”

“…Each PVC pipe contained a coconut husk-based growth substrate packed to a depth of 85 mm to represent a realistic active green wall substrate depth that would be sufficient to support plant growth, as has been tested in previous research [34,43]. Coconut husk is a favourable substrate for use as a growth substrate in botanical biofilters as it has not been associated with bioaerosol emissions [44], and has a demonstrated capacity to filter VOCs [33] and PM [30]. The substrate was retained within the pipe by loose weave high-density polyethylene (HDPE) cloth at each end of the pipe.…”

“…These species were Spathiphyllum wallisii (peace lily) and Syngonium podophyllum (arrowhead vine). These species are both common indoor houseplants and green wall species, and both have been tested for their capacity to phytoremediate a range of VOCs [33,[45][46][47][48][49][50]. All tested plants were grown in their biofilters in a glasshouse (Sydney, Australia) for~8 weeks prior to testing.…”

“…Whilst pollutant reduction by potted plants has been well described, for in situ use, such systems will be severely limited in their efficacy [29]. Several recent studies have reported that green wall systems, in particular active green walls, have a high capacity to phytoremediate several air pollutants including particulate matter (PM) [30,31] and VOCs [32,33]. Regarding green wall VOC removal, biodegradation of VOCs by the rhizospheric bacteria along with substrate adsorption are considered as the primary sinks for VOC removal [24,34], however, plant-associated effects also play a role in VOC removal [35].…”

“…Previous research testing the pollutant removal capabilities of this technology has been limited to VOCs [33,[37][38][39], CO 2 [40], and PM [30,31] and thus the use of active green wall technology for the remediation of other criteria air pollutants including NO 2 remains untested. In addition to these previously tested functions, a potential reduction of the ambient NO x concentration through botanical biofiltration is an important consideration that could further improve air quality and reduce occupant exposure to these pollutants.…”