**5. Phytoremediation of major pollutants**

#### **5.1 Airborne particulates**

By adsorbing particles leaf surface or fixing them in waxes, plants effectively remove substantial amounts of airborne particulates from the atmosphere, particularly in metropolitan areas [77]. Urban areas with trees can significantly reduce PM10 level [73]. In order to decrease the spread of air pollution from industrial areas, an

#### *Phytoremediation toward Air Pollutants: Latest Status and Current Developments DOI: http://dx.doi.org/10.5772/intechopen.111559*

8 m wide green belt may be installed which could able to minimize dust fall over two to three times [78]. The trees such as *Ficus* spp., *Mangifera* spp., and *Azadirachta* spp. in urban roadsides can effectively control particulate matter emitted from vehicles. C3 and C4 plants intake several gaseous pollutants at larger quantity during daytime and CAM plants intake gases at night time through stomatal openings [79]. PCBs enter into plants through the cuticle and metabolized through cytochrome P-450 in some of the plant species such as pine and eucalyptus [80]. Generally, metals such as Cr, V, Ni, Pb, and Fe accumulated in plants from soil, but they are not translocated to aerial plant parts. Therefore, accumulation of such metals in aerial parts of the plants is majorly absorbed from atmospheric air [81]. High density organic compounds and metals can penetrate wax layer and enter into epidermis of the cell through the process of diffusion and sequestered in vacuoles or cell wall [82]. The ability of some bacteria to convert reactive oxygen species into less harmful forms through their antioxidant properties. This ability of bacteria in turn contributes to the bio-remediation of PM by plants as PM have shown to develop ROS which is harmful to plants [83–86].

### **5.2 Volatile organic compounds**

Phytoremediation can effectively remove VOCs like formaldehyde, xylene, toluene, benzene, and ethylbenzene from the environment [87]. In order to protect from pathogens, animals and other environmental stresses plants used low molecular weight VOCs [84]. Generally, plant uptake VOCs via leaf stomata, while few plants uptake VOCs through cuticle [88], and their further translocation is done by phloem to designated plant organs [89]. In plants system, VOCs are degraded, stored or removed through various process, and get volatilized into atmosphere through diffusion from trunks, stems, roots, and leaves of plants [90, 91]. Phytoremediation efficiency of plants is determined by properties of VOCs, as lipophilic VOCs are absorbed through cuticle, whereas hydrophilic VOCs are absorbed through leaf stomata [13]. VOCs also get deposited in soil and plant rhizosphere due to leaf fall and runoff. Microbes present in plants aid metabolization of these organic compounds into less toxic forms such as carbon dioxide, water, and cellular biomass [13]. Spider plant (*Chlorophytum comosum* L.) detoxifies low concentrations of formaldehyde into amino acid, sugars lipids, and cell wall components. Soybean plant (*Glycine max* L.) converts formaldehyde into serine and phosphatidylcholine. Additionally, microbes can modify plant VOC emissions by activate an immunological response [92]. An experiment was conducted using the green wall system for degradation of VOCs and showed proteobacteria as the dominant species. *Nevskiaceae* and *Patuli bacteraceae* were VOC utilizing bacteria in the irrigation water of the green wall system. *Burkholderiales* were part of bio-wall root bacterial communities where VOC degradation was also reported [93]. Bacteria associated with the rhizosphere of the plants also play a crucial role in the degradation of these air pollutants, and one such bacteria *Rhodococcus erhythopolis* U23A isolated from the roots of *Arabidopsis thaliana* was able to break down polychlorinated biphenyls [94]. The flavanones were found to be the inducers of the polychlorinated biphenyls pathway. According to Barac et al. [94], the introduction of a plasmid expressing a toluene-degrading enzyme reduced phytotoxicity and toluene evapotranspiration by 50–70% through the leaves. Sandhu et al. [95] provided direct evidence that endophytic bacteria in the phyllosphere degrade volatile organic compounds. De Kempeneer et al. [96] proved that phyllosphere micro-biota undertakes toluene cleanup via toluene-degrading bacteria. Kempeneer et al. [96] demonstrated that phyllosphere micro-biota significantly degrades the toluene

compound. As per the reports, formaldehyde has varied from 0.14 to 0.61 mg/m3 in remodeled residences; however, benzene, toluene, and xylenes were found in 124, 258, and 189.7 g/m3 concentrations, respectively [97]. Sriprapat et al. [98] highlighted that *Helianthus annuus* associated with EnL3 strain significantly removed benzene from the environment. The study also reported that a total of sixty-two proteins was up and down-regulated in leaves, while thirty-five proteins were up or down-regulated in roots of *H. annuus*. Additionally, toluene has been removed by the association of *F. verschaffeltii* and *H. carnosa* indoor foliage plants with rhizospheric bacteria [99, 100]. To summarize these, it is clear that the above ground and below ground plant-associated microorganisms play a crucial role in the mineralization of VOCs through their degradation potential.

#### **5.3 Gaseous pollutants**

Gaseous air pollutants include oxides of sulfur, carbon, and nitrogen as well as ozone [101]. Plants metabolize CO by oxidation into CO2 or get reduced into amino acids. The tendency of plant canopy to act as NO2 sinks and assimilation of NO2 has also been demonstrated. Plants can assimilate ammonia (NH4) from the air and soil [102]. Abatement of pollutants such as carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen dioxide (NO2), and ozone (O3) can be accomplished by the implementation of phytoremediation. Through the process of photosynthesis, plants remove carbon dioxide from the atmosphere and store it for any given period of time or convert it to humus [13]. This process of storing carbon dioxide in plants for an extended period of time is known as carbon sequestration [103]. The "reductive sulfur cycle" is the process by which sulfur dioxide is broken down after it is diffused through the stomata of plants. Plant's root uptake by-products of the sulfur cycle including sulfur containing amino acids necessary for their growth. The adsorption of nitrogen dioxide through stomata, leaf, and root surfaces is the first step in the nitrogen toxicity abatement process. Nitrogen dioxide has the potential to be used as an alternative fertilizer and to supply essential nutrients to plants but, prolonged exposure to high concentrations of nitrogen within the plants could be toxic [13]. Ozone reacts with the waxes of cuticle, ions, salts, and biogenic and anthropogenic volatile organic compounds. Stomata are the means through which plants take in ozone into their systems. In another study, plant and microbially-assisted bio-remediation systems accrued the air pollutant by 0.24 to 9.53 folds than individual plant systems [104]. Moreover, ozone was efficaciously reduced or removed in an uninterrupted system of *Z. zamiifolia* combined with *B. cereus* ERBP. This endophytic bacterium has the potential to protect the plant against ozone stress [105] and has been developed an effective microbially-assisted bio-remediation strategy against formaldehyde pollutants by adhering rhizosphere microbes with *T. zebrina*, A*. vera*, and *V. radiata*. As per the report [49], the phenolic pollutant has been eliminated by vetiver grass by using *A. xylosoxidans*. The association of F3B has the potential to endure plants against toluene stress and enhanced the chlorophyll content of leaves. As per the report [106], chloromethane (volatile halo-carbon pollutant) was removed/ degraded by *Hyphomicrobium* sp. (isolated from the leaf of *A. thaliana*). NO2 may also be accumulated in plants in the form of nitrate and nitrite. Later, it can be reduced by nitrate and nitrite reductase enzyme for the generation of ammonia, which is further assimilated to glutamate through the ammonium assimilation pathway (GS- GOGAT) [107, 108]. As per the recent report by Lee et al. [16], the absorbed pollutants are stored in the inter-cellular spaces of leaves. Further, it can react with the inner-leaf membranes or water film and after that it can be degraded or excreted into the environment [16].

*Phytoremediation toward Air Pollutants: Latest Status and Current Developments DOI: http://dx.doi.org/10.5772/intechopen.111559*

#### **5.4 Removal of polyaromatic hydrocarbons (PAHs)**

Various bacterial groups like *Haemophilus* spp., *Paenibacillus* spp., *Pseudomonas* spp., *Mycobacterium* spp., *and Rhodococcus* spp. are reported to degrade and utilize PAHs as a sole source of energy and carbon [109]. Yutthammo et al. [110] reported that about 1–10% of phyllosphere bacteria have potential to degrade PAHs (acenaphthylene, acenaphthene, phenanthrene, and fluorine) from the environment. However, the removal of PAH was achieved by the mutualistic association between microbes and *Epipremnum aureum* plant [111]. A lot of changes take place in the soil around the roots which leads to more aeration that supports the growth of autochthonous PAH degrading microbial communities, which allows the efficient mineralization of the PAH present in the rhizosphere soil [112]. Thus, even deeper layers support the process of aerobic degradation [113]. Myco-augmentation, phytoremediation and natural attenuation can be used individually for the bioremediation process. However, using these techniques in combination can increase bioremediation efficiency to several folds. Thus, the synergism between the microbes and plants can be exploited for the bioremediation not only for PAH but also other compounds as it is more effective than simple phytoremediation [114]. There have been few studies that has used the combination of bacteria and plants for the bioremediation of PAH [49], but rarely there has been a combination of plant and white-rot fungi used for the process of PAH bioremediation. The maize plant was associated with *Crucibulumleave* (fungi) in a comparative study where phytoremediation process was enhanced. This combination was highly effective in PAH degradation to 5–6 folds compared to phytoremediation alone. This could be possible due to the increased surface area of fungal hyphae which could assist bacterial transport through the soil and alteration of root exudates possibly increasing the bioavailability of the compounds and increasing the degradation of the PAH [115]. For the removal of hydrocarbons from soils, the most researched plants are prairie grasses because of their vast fibrous root systems [116].

#### **5.5 Indoor air pollution**

Green houseplants can act as a biological filter to purify the indoor air [76]. Plants considerably deplete CO2, VOCs, PMs, organic carbon, nitrate, sulphate, ammonia, and carbonate levels in indoor environments [117]. It was reported that *Dracaena deremensis*, *Dracaena marginata*, and *Spathiphyllum* spp. efficiently remove benzene, toluene, ethylbenzene, and xylenes in indoor environment [118]. Eight ornamental indoor plants namely *Chlorophytum comosum*, *Clitoria ternatea*, *Dracaena sanderiana*, *Euphorbia milli*, *Epipremnum aureum*, *Hedera helix*, *Syngonium podophyllum and Sansevieria trifasciata* were studied for the removal efficiency of benzene in indoor air pollutants. It was found that *C. Comosum* was most efficient in removing benzene from air and water pollutants. Green walls are recent innovation, formed of a pre-vegetated frame that are attached to a wall or other interior structure [87, 117]. An updated version called active living wall integrates the building heating, cooling, and ventilation systems [119]. A green wall system regulates temperature and also filters the air inside buildings [120]. The plants remove CO and CO2 and assist in removing particulate matter from air [121, 122]. Using urban indoor vegetation is one strategies for accomplishing this since it can be drastically reducing air pollution. Green walls are either partially or entirely covered with greenery, incorporating a growing medium. It is well-known that the incorporation of green walls and other forms of living infrastructure into an indoor environment has the potential to contribute to an improvement in air quality, through the reduction

in amount of volatile organic compounds (VOC), inorganic gaseous compounds, and carbon dioxide (CO2), as well as the retention of particulate matter (PM) [123]. Study revealed that potted plants are capable of removing significant levels of gaseous VOCs in sealed chambers, lowering VOCs by 10–90% in 24 hours [124]. It was demonstrated that the removal of organic contaminants is accomplished more effectively in areas of the root soil that are in contact with air [125]. The *Chamaedorea elegans* and *Opuntia microdasys* plants have significantly reduced the concentrations of formaldehyde and BTEX in controlled environmental chambers, respectively. In another study, the *Chlorophytum comosum* L. plants have significantly accumulated indoor PM10, PM2.5, and PM0.2. The researchers also observed that the plant wax is helpful in the accumulation or attachment of PM on their leaf's surfaces [126]. As per the literature, associated microorganisms can significantly enhance the remediation properties of plants. In one experiment, the rhizosphere microbes associated with the *Aloe vera*, *Tradescantia zebrine*, and *Vigna radiata* plant species have improved or enhanced the indoor formaldehyde neutralization efficiency by 2–3 times. In addition, the combined system of the *Ophiopogon japonicus* plant associated with *Staphylococcus epidermidis* and *Pseudomonas spp.,* bacterium has been degrading the phenol pollutants with 1000 g/L per day capacity [118]. In commercial buildings and urban areas, vertical gardens are gaining more attention because vertical alignment can offer a space-efficient method of exposing more plant biomass to contaminated air [127]. In these gardens, the significance of plant selection on the green walls demonstrated the varying capacity of pollutant degradation. As per the report, the especially fern species have high removal efficiency toward the particulate matter of size range PM0.3–0.5; 45.78% and PM5–10; 92.46%. Whereas, the plant species with fibrous roots have greater degradation efficacy toward air pollutants compared to plants with tap root systems. Moreover, for vertical gardens and green walls, the growing medium, vermicompost, perlite, coco-peat, and so forth significantly influence the plant microbe's associations and pollutant degradation mechanism. In *Apteniacordifoli*, the addition of granular activated carbon into coconut husk-based substrates could increase the VOC removal ability of the green walls [128]. Mikkonen et al. [129] investigated the filtration efficiencies for seven volatile organic chemicals as well as the microbial dynamics in simulated green wall systems. The results highlighted that Golden pothos plants had a minor effect on VOCs filtration and bacterial diversity. In another report by Gonzalez-Martin [92], indoor green walls contributed significantly to the ambient fungal load, but concentrations remained well below WHO safety standards. In this sense, the botanical air filtration approach is a promising way for reducing indoor air pollution, but a greater knowledge of the underlying mechanics is still required.

## **6. Biotechnological advances**

Generally, biotechnological tools provide researchers the ability to change the gene expression regulation at certain specified sites which speeds up the discovery of new information regarding the functional genomics of plants [128]. In *Noccaeac aerulescens*, *Arabidopsis spp.* (hyperaccumulator of Cd and Zn*), Hirschfeld spp.* (tolerant to Pb toxicity), *Pterisvittata*, and *Brassicaspp* have genomes sequenced for model phytoremediators [129]. These phytoremediators have been found to be effective at removing heavy metals from the environment. Similarly expressing the *MerC* gene in *Arabidopsis* and Tobacco plants led to an increase in the accumulation of the Hg metal, but it also rendered the plant hypersensitive to the effects of mercury (Hg). Several

#### *Phytoremediation toward Air Pollutants: Latest Status and Current Developments DOI: http://dx.doi.org/10.5772/intechopen.111559*

additional types of organic contaminants such as polycyclic aromatic hydrocarbons and polychlorinated biphenyls are also include in this method [92]. Editing the genes in rice and *Arabidopsis* for the production of naphthalene dioxygenase resulted in the recent development of genes that express tolerance against naphthalene and phenanthrene [130]. Some of the genetically modified plants for phytoremediation of organic pollutants include tobacco (gene *hCYP2E1*) for benzene, *Atropa belladonna* (gene *rCYP2E1*), and Poplar (gene *rCYP2E1*) for TCE [131]. An experiment was carried out on the *tou* cluster, which encodes for the enzyme Tolueneo-Xylene Monooxygenase (ToMO) which can degrade the aromatic hydrocarbons from *Pseudomonas stutzeri*. It was cloned and expressed in Antarctic *Pseudoalteromonas* algae [88]. Genetically modified plants like *P. angustifolia*, *N. tabacum*, and *S. cucubalis* have been shown to accumulate more heavy metals pollutants than their wild counterparts by over expressing-glutamylcysteine-synthetase [132]. The γ-ECS *B. juncea* transgenic seedlings (*E. coli* gshl gene insertion) showed greater tolerance toward cadmium, phytochelatins, glutathione, and non-protein thiols than wild type [132]. The expression of genes including *AtNramps*, *AtPcrs*, *CAD1* (*A. thaliana*), *gshI*, *gshII* (*D. innoxia*), *CAX-2*, *NtCBP4* (*N. tabacum*; *A. thaliana*), *Ferretin* (*soybean*), *merA* (bacteria), and *PCS* cDNA clone (*B. juncea*) has been shown enhanced heavy metal tolerance and accumulation [133, 134]. However, in transgenic *A. thaliana*, the combined expression of SRSIp/ArsC and ACT 2p/γ-ECS presented increased tolerance to arsenic (Ar). Additionally, plants accrued 4 to 17-fold shoot biomass and accumulated 2 to 3-fold more AR compared to wild plants [135, 136]. For enhanced phytoremediation, metabolic pathways have been expressed by introducing *MerA, MerB, ars C,* and *γ-ECS* genes in *Arabidopsis* plants and resulted in enhanced accumulation of mercury, arsenate, and arsenite pollutants [137]. The over-expression of 1- amino cyclopropane-1-carboxylic acid deaminase in plants resulted in a higher accumulation of pollutants [75, 137]. Presently, the researchers aim to work on the genetic modification of common ornamental and houseplants for the improvement of indoor air quality. In this context, pothos ivy or devil's ivy has been modified by genetic engineering approach for the removal of chloroform and benzene by the expression of the Mammalian Cytochrome P450 *2e1* gene [138].
