European Green Cities/Site Development
[Reproduced]: Phytoremediation of Air Pollutants
Author: Prerita Agarwal1, Mangaldeep Sarkar2, Binayak Chakraborty2 and Tirthankar Banerjee1
1. Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi, India
2. Navsari Agricultural University, Waghai, India
The presence of any physical, chemical, or biological compound that adversely modifies the natural characteristics of the atmosphere is considered as an air pollutant. Air pollution has been recognized as a major global health issue, predominantly in middle to low income
countries. There are numerous evidences of the possible adverse impacts of air pollutants
on human health, most notably, chronic obstructive pulmonary diseases, acute respiratory
infections, lung cancer, enhanced mortality, and increased hospital admissions (Kumar
et al., 2015a; WHO, 2014a; Banerjee et al., 2017). Furthermore, air pollutants have also
been associated with premature mortality, mostly in children (50% of deaths under age
five, WHO, 2014a), kidney disorders, dry-eye syndrome, peptic ulcers, intestinal disorder,
and other acute health problems (Kumar et al., 2015a). The scenario is widespread, as a recent study conducted by the World Health Organization (WHO) confirms that 92% of the
world’s population lives in areas with poor air quality that frequently exceed the WHO limit
(WHO, 2016). Globally, air pollution is responsible for one in eight premature deaths (7 million in 2012), mainly associated with heart diseases and strokes. The majority of these deaths are accredited to indoor air pollution (61%, 4.3 million in 2012; WHO, 2014a), originating from the use of conventional fuels on low-cost energy intensive cooking stoves. The current air pollution emission control technologies are not sufficient to meet either environmental challenges or the strict emission norms. Under such circumstances, a possible solution would be to minimize emissions and enhance the number of natural sinks. Although a gradual improvement in automotive, industrial combustion technology,
and emission control systems have the potential to reduce the pollution level, there is still an urgent need to find alternative, ecofriendly, and sustainable practices for the removal of increasing air pollutants.
Phytoremediation is used to mitigate environmental pollutants and is widely applied to
treat soil and water pollutants. However, phytoremediation has also been explored for its
potential to clean ambient air by virtue of plant’s gas-exchange mechanism with ambient
air. Autotrophic plants require intensive gas exchange for their life-supporting processes
during which gaseous contaminants can be adsorbed/absorbed by them (Gawronski et al., 2017). In phytoremediation, plants, with their allied microorganisms, take up pollutants
from ambient air and subsequently degrade or detoxify it through various mechanisms. This has been proven to be an effective plant-based, environmentally friendly, and sustainable process to reduce air pollutants effectively from both indoor and outdoor environments (Weyens et al., 2015). Therefore, even with many existing technologies, phytoremediation
stands out for its self-maintaining, soil stabilizing, cost-effective processes, and for meeting
greater ethical and public approval (Doty et al., 2007). Further, compared to other mechanical methods, this is a cheaper, aesthetically pleasing, environmentally friendly, and sustainable process that can be used for a wide range of both organic and inorganic contaminants.
There are numerous reviews on plants’ potential to absorb specific pollutants under
varying environmental conditions. Many studies further explore their broader applications,
like the use of higher plants along with their microbiome to remove airborne pollutants,
especially in outdoor environments. Plants have the added advantage of having enormous, biologically active surface areas, which help harness various air pollutants, either directly through absorption/adsorption processes or by wet/dry deposition. Studies have revealed that ornamental plants have the ability to absorb and/or transport organic pollutants to microorganisms living in the rhizosphere (Wolverton and Wolverton, 1996) and phyllosphere
(Sorkhoh et al., 2011). Further, plantmicrobe interactions assume an important role during
phytoremediation by aiding in plant growth and further degrading, detoxifying, or sequestrating specific pollutants (Weyens et al., 2009a,b). There are evidences of exploiting plantmicrobe interactions to enhance the removal of pollutants. Additionally, the photosynthetic system of C3, C4, crassulacean acid metabolism (CAM) and facultative CAM plants have been explored under diverse conditions for the removal of air pollutants. C3 plants are capable of exchanging higher CO2, especially during the day, while C4 plants have a higher intensity of gaseous exchange. In comparison, CAM plants (constitutively) or facultative CAM plants (especially after exposure to stress, like
drought) exchange gases during the night (Winter and Holtum, 2014), which makes them
extremely useful in the phytoremediation of air pollutants, especially in indoor conditions. The waxy leaf surfaces and trichomes are evaluated for their potential to remediate air pollutants. Leaf surfaces are colonized by microorganisms (Vorholt, 2012), creating a phyllomicrobiome, and have been reported to cause degradation of various organic pollutants (De Kempeneer et al., 2004). Even soil microbes contribute to the removal of gaseous air pollutants, especially organic pollutants, and its performance has been reported to be enhanced when placed in combination with plants (Wood et al., 2002; Xu et al., 2011). The entire plantsoilmicrobe system, therefore, necessitate extensive investigation for the exploitation of its full remediation efficiency against air pollutants. Furthermore, few constraints, like slow removal rate and the selection of plant species for remediating a group of pollutants under diverse environmental conditions, need to be explored. This chapter discusses the potential of various plant species for remediating air pollutants from indoor and outdoor environments, deliberates on environmental and plant physiological factors that are limiting, and, in conclusion, provides a few user recommendations for efficient phytoremediation of air pollutants.
Plants mainly intake environmental components either from soil or water, but the focal
point of entry for air pollutants are the aerial parts. The stomata and cuticles present on leaves serve as the main entry point for any air pollutant. However, intake and subsequent
assimilation within plant cells are specific to the physicochemical properties of the pollutant, particular plant species, and environmental factors. Schreck et al. (2012) reported that after the deposition of metal-enriched particles on the leaf surface, the points of entry could be the cuticle and stomata. Alternatively, the process of absorbing lipophilic semivolatile compounds is primarily achieved through leaf surface adsorption, where atmospheric resistance serves as a major limiting factor (McLachlan et al., 1998). After uptake, a compound can either be sequestered inside the plant, detoxified, or metabolized to yield CO2 and H2O. The entire process of phytoremediation involves several separate, yet complementary, processes that vary considerably based on the nature of the pollutant and the physiology of the plant. Principal processes of phytoremediation are illustrated in Fig. 7.2, which include phytostabilization, phytofiltration, rhizodegradation, and phytoextraction. Phytostabilization is the process by which pollutants are initially sequestered for further metabolism, while phytofiltration involves the screening of airborne particulates by means of leaf surfaces. Rhizodegradation refers to plantmicrobe interactions within the rhizosphere for metabolizing pollutants. Phytoextraction involves the process of extracting contaminants from soil and translocating them to shoots, while phytovolatilization refers to the release of volatile pollutants into the air by means of stomata.
Phytoremediation of Outdoor Air Pollution
- Phytoremediation of Airborne Particulates
Plants are capable of removing considerable amounts of airborne particulates, especially in
urban areas, by adsorbing particulates on their foliage and/or stabilizing them in waxes (Beckett et al., 1998, 2000). Nowak et al. (2014) showed that trees in urban settings are able toreduce significant amounts of PM10, generating a monetary value of hundreds of millions of dollars.
- Phytoremediation of VOCs
VOCs are important constituents of the atmosphere, mainly as the precursor of ground-level ozone and secondary organic aerosols (Singh et al., 2017b). In most environments, OH radicals act as the dominant VOC sink, while VOCs may also nucleate to ultrafine particulates through the gradual process of oxidation (Singh et al., 2017b).
- Others Gases
Other than these compounds, common and important gaseous air pollutants include SO2,
CO2, CO, NOx, and O3. Stomata, wax, and cuticle are the main entry points of these pollutants at a marginal level into the plant body. Plants metabolize CO either by oxidation to CO2 or reduction into amino acid.
Due to the complexities in the nature and sources of air pollutants, it is often challenging
to formulate appropriate methodologies for their control. To further complicate the scenario,
the prevalence of particular pollutants varies considerably in different microenvironments,
which necessitates location-specific remediation either for individual, or groups of pollutants. Therefore, plantsoilmicrobe systems pose a promising potential for improving air
quality, either by metabolizing, sequestering, or degrading specific air pollutants. Together
with the plethora of varieties of microorganisms and plant species, there is huge potential
to purify both indoor and outdoor environments. However, on a commercial scale, the phytoremediation of air pollutants is still an emerging concept. Although, the scientific community and the general population are well aware of multiple advantages of planting
and growing a tree, the potential and suitability of individual species for specific pollutants entail uncertainties. Even the number of requirements and group of plant species required for removing multiple air pollutants, are limited. Another unexplored area is modeling the phytostabilization process, especially in terms of feasibility, cost, and the safe disposal of pollutants. Adding to the complexity, the nature of indoor and outdoor environments is considerably different, which ideally warrants different kinds of plant species and agronomic practices for harnessing the full potential of plants. An outdoor plant is required to tackle the synergistic effect of multiple air pollutants, environmental stress, and be adaptable to varying climate conditions; an indoor plant is expected to be frequently exposed to certain pollutants having high concentration levels. This indicates that the selection of the plantsoilmicrobe system should be quite different for each environment.
Another constraint in popularizing phytoremediation is the slow removal process, which effectively allows the pollutant to accumulate over the confined area.
Therefore, the phytoremediation of air pollutants needs more research, especially with
regard to harnessing the entire plant soilmicrobe interaction. Superior tools and
techniques need to be developed with a new vision to link urban forestry with city planning, especially for the rapidly urbanizing cities of the developing world.