Reducing Indoor Air Pollutants Through Biotechnology - Biotechnologies and Biomimetics for Civil Engineering - page 184

2005 ). The development of phytoremediation systems for mitigating contaminated
air is somewhat less developed. Whilst effective IAQ improvement has been
recorded (Darlington et al. 2000 ; Wood et al. 2006 ; Wang and Zhang 2011 ), there
are relatively few fully tested, commercial systems for integration into common
building schemes, although the systems that have been developed have consid-
erable potential, and will be discussed in a later section.
The most widely researched system for botanical air cleaning is by passive
filtration using potted plants. Research over the last three decades has demon-
strated that passive biofiltration with indoor plants can significantly reduce con-
centrations of most types of urban air pollutants (Wolverton et al. 1989 ; Coward
et al. 1996 ; Lee and Sim 1999 ; Yoneyama et al. 2002 ; Orwell et al. 2004 ; Wood
et al. 2006 ; Yoo et al. 2006 ; Kim et al. 2008 ; Irga et al. 2013 ). The primary
advantages of passive phytoremediation over active biofilters are cost and flexi-
bility of installation, as in many cases no specific infrastructure changes are
required to install potted plants in most indoor spaces (Soreanu et al. 2013 ).
The biologically active component of botanical air filtration systems for
hydrocarbon VOC biodegradation is the microorganisms associated with the roots
of the plants or the growing substrate of the plants, whether it be potting mix or a
hydroponic or hydroculture material. Whilst older studies hypothesised on the
VOC bio-degradative capacity of plant-associated bacteria (e.g. Wood et al. 2002 ),
Zhang et al. ( 2013 ) provided empirical evidence for this effect by demonstrating
toluene removal by bacteria isolated from the rhizosphere of potted indoor plants.
Although some VOC removal has been recorded across plant leaves with the
rhizosphere having been excluded (Ugrekhelidze et al. 1997 ; De Kempener et al.
2004 ), the quantities removed were trivial relative to the amounts removed by
whole potted plants, or even potting mix without plants present (e.g. Wood et al.
2002 ; Orwell et al. 2004 ). Reported formaldehyde removal rates by plant tissues
alone were found to be so low as to be insignificant for IAQ improvement purposes
(Schmitz et al. 2000 ), whilst substantial formaldehyde removal has been detected
for individual bacterial species associated with an active biofilter (Wang and
Zhang 2011 ), and also for indoor plants when the substrate was also present (Xu
et al. 2011 ). The removal of several other contaminants, however, such as CO 2 (Oh
et al. 2011 , Torpy et al. 2014 ), SO x ,NO x (Elkiey and Ormrod 1981 ; Esguerra et al.
1983 ) and ozone (Elkiey and Ormrod 1981 ) appear to be mostly or wholly plant
mediated, and are taken up directly through the stomates (gas exchange pores) of
the green shoots, which in most species are open only during daylight hours, and
not in the dark. Particulate matter (Lohr and Pearson-Mims 1996 ) is effectively
reduced by living plants by deposition through the extensive boundary layer area
present around leafy tissue.
It is generally thought that, with respect to VOC removal, the value of the
potted plant in passive systems is mainly to provide a supply of nutrients to the
microbial community as root exudates (e.g. Orwell et al. 2004 , 2006 ), although
this has not been directly tested. Further, the biochemical pathways by which
VOCs are degraded and what bacterial or fungal taxa are directly involved have
not been determined. However, although the biology of the process by which