Phytoremediation of air pollutants: A review

Abstract

Industrialization and urbanization are vital for the economic development of a nation. In fact, these are the indicators of prosperity and progress today. However, unplanned industrialization and urbanization may lead to multifaceted environmental problems. Hence, in a developing nation like India, future industrialization and urbanization should be carefully planned to avoid any irreparable damage to the environment. As far as possible, air, being the lifeline, should be protected from the evils of pollution, as its quality depletion beyond a threshold limit may lead to serious health hazards to both man and livestock. Even, a possibility of change in the genetic make-up cannot be ruled out in case of spill over of nuclear materials. Despite this fact, a progressing country can avoid neither industrialization nor urbanization for its economic growth, which is triggered even at the cost of ecological imbalance. Therefore, the best way out appears today is to be an all-out effort to use such measures (engineering or biological), which can help in mitigation of ambient pollution. In this endeavour, various laws and policies have been framed to direct the industries and vehicle manufacturers to adopt new technologies to reduce the emission of pollutants from both stationary and mobile sources. In order to curb the menace of air pollution, mechanical collectors, fabric collectors, wet scrubbers, electromagnetic precipitators and fume incineration are often used to attenuate pollutant emission at source. But, as these devices are of mechanical nature, the possibility of their occasional failure cannot be ruled out. Moreover, engineering devices are very expensive, demanding a big budget which is itself a major problem in many cash-strapped industries. Alternatively, less efficient systems are, at times, used by many medium and small-scale industries, leading to the discharge of an alarming proportion of pollutants in the industrial pockets of our country. This situation demands our adequate attention for immediate remedial recourse. In fact, there is no device available, either mechanical or chemical, which can completely check the emission of pollutants at the source. Once the pollutants are released to the atmosphere, only the plants are the hope, which can mop up the pollutants by adsorbing, absorbing and metabolizing them from the atmosphere. Therefore, the plants role in the air pollution abatement has been increasingly recognized in recent years. Phytoremediation is an emerging eco-friendly technology, dealing with degradation, mitigation or stabilization of air, soil or water pollutants by plants (Macek et al. 2000; Garbisu et al. 2002 and Lasat et al. 2002). The main advantages of phytoremediation technology include; (1) it is an aesthetically pleasing and solar energy-driven cleanup technology (2) chances of environmental degradation, because of in situ application, are very less (3) a variety of environmental contaminants may be treated simultaneously and (4) it is a cost-effective technology, as cost involved in phytoremediation technology is 60-80% less than the conventional physico-chemical or mechanical technologies (Schnoor 1997). Despite of several advantages, this technology has also got some limitations like (1) it is a time consuming approach, as it may take several growing seasons to cleanup a site (2) plants after phytoremediation got loaded with toxic heavy metals or persistent chemicals that may pose a risk to wildlife or contaminate a food chain, and (3) accumulation of organic or inorganic compounds may lead to the formation of number of cytotoxic intermediates into plants or animals. Therefore, to prove the applicability of phytoremediation, there is a need to analyse the mass balance and metabolic fate of pollutants in the plant system (Morikawa and Erkin 2003). Most common air pollutants in the urban environment are sulfur oxides (SOx), nitrogen oxides (NOx), carbon monoxide (CO), suspended particulate matter (SPM) hydrocarbons (HC) and ozone (O3) (DAmato 1999). Out of which, SPM is of the greatest concern, as it contributes 50% to total air pollution (Fuller 1974) and causes respiratory disorders in human beings on prolonged exposure (Freer-Smith et al. 2004). Joshi (1998) carried out the monitoring of respirable suspended particulate matter (RSPM) and total suspended particulate matter (TSPM) in the core city area of Indore (Madhya Pradesh, India) and found higher RSPM and TSPM levels at about all the selected road intersections as compared to the prescribed standards of CPCB, New Delhi. NO2 and SO2 are the gases that contribute to acidic deposition in terrestrial ecosystem as dry deposited gases or in dissolved form in precipitation (Cox 2003). In aerosol form, they also impact visibility. NO2 is of particular nature, as it is a precursor of the formation of the photochemical oxidants, which directly impact health of human beings. Gaseous pollutants, such as SO2, NO2 and O3, have pernicious effects of varying magnitude on wheat, mustard, mung and palak plants, depending upon individual pollutant concentration, in combination, plant species and seasons (Rhode et al. 2002; Agrawal et al. 2003). © 2007 Springer-Verlag Berlin Heidelberg.