An Emerging Bioremediation Technique - Essay Example

Phytoremediation - an emerging bioremediation technique. Is it proving to be effective? Introduction The increasing population has put immense pressure on the limited resources of the earth such as the land, water and other natural resources. In other words human beings are responsible for all the major changes on the face of the earth. The unplanned developments have resulted in increased degradation of natural ecosystems and are eroding the life supporting systems. Soils often receive a wide range of contaminants and it mainly comes from industrial activities, sewage sludge disposal, metal processing, and energy production. Decontamination of these sites is often expensive and intrusive to the ecosystem. In general, it is observed that heavy metals are widely-used in industry and these can be a major source of heavy metals leaching into the environment (Water Framework Directive, 2008). Phytoremediation is a bioremediation technique which uses plants and plant processes to remove, degrade, and make the hazardous materials less toxic. Scientists have predicted that this emerging technology has good scope in the field of pollution control. This technique may offer a cost-effective, non-intrusive, and safe alternative to conventional soil cleanup techniques. Basically, phytoremediation technique makes use of the ability of certain tree, shrub, and grass species to remove, degrade, or immobilize harmful chemicals from the soil. If we trace back the history of phytoremediation, the science of phytoremediation came from the study of heavy metal tolerance in plants in the late 1980s. To be more precise it is the discovery of hyperaccumulator plants that resulted in the progress of phytoremediation research. It was found that hyperaccumulator plants which contain levels of heavy metals would be highly toxic to other plants. However, these plants could be easily used to decontaminate soil and extract metals from the soil and, in the process, clean up soil for other less tolerant plants. It was also found that certain plants could also be used to degrade organic contaminants by captivating them from the soil and metabolizing them into less harmful chemicals (uga.edu, N.D.). Phytoremediations also helps in treating water contaminated with metals and/or organic contaminants such as crude oil, solvents, and polyaromatic hydrocarbons (PAHs) (Zynda, 2001). It is not only the plants but also the microorganisms that live in the rhizosphere play a major role in degrading organic chemicals. In general, these organisms use these chemicals as a carbon source in their metabolism. Often times, these plants can recover the condition of the soil, giving it structure and stability and changing hydrology by enhancing water retention and preventing erosion. Scientists have proven beyond doubt that plants and the microbes associated with them can greatly alter an ecosystem. There are different types of phytoremediations that are used and each one has its own significance. For instance, there are plants that accumulate heavy metals in specific organs; there are plants that can perform voltilization from leaf surfaces; some plants can alter the form or availability of an organic chemical in the soil or within the plant; there are also some plants that can actively exclude chemicals from plant tissues and keeping them out of the food chain (uga.edu, N.D.). Not all plants can take part in the process of phytoremediation. It mainly involves the use of vascular plants, algae, and fungi to eliminate and manage waste or encourage breaking down of contaminants by microorganisms present in the soil or in the root nodules. Scientists have noted that there are diverse wastes that can be controlled by using phytoremediation such as xenobiotic organic chemicals, sewage, salts, nutrients, heavy metals, metalloids, and air pollutants (McCutcheon and Schnoor, 2003). It is a blessing to the environment that these plants can help clean up many kinds of pollutants including metals, pesticides, explosives, and oil. The plants also help prevent wind, rain, and groundwater from carrying pollution away from sites to other areas (EPA, 2001). One of the most important aspects to be noted here is that roots play a major role in phytoremediation. Plants eliminate harmful chemicals from the ground when their roots absorb water and nutrients from the contaminated soil, streams, and groundwater. In simple terms the plants can only clean up chemicals as deep as their roots can grow. For instance, a tree roots grow deeper than small shrubs, therefore trees are much useful to decontaminate the deeper ground. The plants have unique mechanism to manage the chemicals once they are inside the plant. Some plants can efficiently store these chemicals in the roots, stems, or leaves; some have the capacity to change the chemicals into less harmful chemicals within the plant; or there are also plants that can change these chemicals into gases that are released into the air as the plant transpires. There are also instances when the chemicals can stick or sorb to plant roots or they can be changed into less harmful chemicals by bugs or microbes that live near plant roots. (Please see A Citizen’s Guide to Bioremediation [EPA 542-F-01-001].) In this process the plants are allowed to grow and absorb chemicals. Later, these plants are harvested and destroyed, or sometimes they are also recycled if metals stored in the plants can be reused. Another important benefit of phytoremediation is that it helps keep harmful chemicals from moving from a polluted site to other areas as it can limit the amount of chemicals that can be carried away by the wind or by runoff (EPA, 2001). There are a range of processes intervened by plants that are useful in treating environmental problems. The following are a few of such techniques: Phytoextraction is a process by which plants uptake and concentrate substances from the environment into the plant body and are then harvested. This process has gained popularity around the world only in the last few decades. Besides it has been tried more often for removing heavy metals than for any other organics. The plants use their extensive root system to absorb the contaminants and they either store them in the root biomass and/or transfer them up into the stems and/or leaves. There are several examples of phytoextraction from soils: “Sunflower or the Helianthus annuus absorb Arsenic from the soil. Chinese Brake fern stores arsenic in its leaves. Cadmium and zinc are absorbed by alpine pennycress (Thlaspi caerulescens). Indian Mustard (Brassica juncea), Ragweed (Ambrosia artemisiifolia), Hemp Dogbane (Apocynum cannabinum), or Poplar trees are useful in the absorption of lead” (Meagher, 2000 qtd. in. Wikipedia 2008). Phytostabilization is another sub-technique of phytoremediation. In this process the plants are used to reduce the mobility of substances in the environment. The best example is the prevention of leaching of substances from the soil using plants. “Phytostabilization mainly focuses on long-term stabilization and suppression of the pollutant. In general the presence of plants can decrease wind erosion, prevent water erosion, lock the pollutants by adsorption or build up, and also offer a zone around the roots where the pollutant can precipitate and stabilize. An example function of phytostabilization is using a good vegetative cover to stabilize and contain mine tailings”(Mendez and Maier, 2008 qtd. in. Wikipedia 2008). Phytotransformation is yet another technique that is a part of phytoremediation. It is the chemical modification of environmental substances as a direct result of plant metabolism, often resulting in their inactivation, degradation (phytodegradation) or immobilization (phytostabilization). This technique is useful “in the case of organic pollutants, like pesticides, explosives, solvents, industrial chemicals, and other xenobiotic substances, certain plants, such as Cannas, turn into these substances non-toxic by their metabolism”. These complex and recalcitrant compounds cannot be broken down to basic molecules such as water, carbondioxide etc by plant molecules, and therefore the term phytotransformation represents a change in chemical structure without complete breakdown of the compound. In fact the term "Green Liver Model" (Boyd and Martens, 1998 qtd. in. Wikipedia 2008) is used to explain phytotransformation, because the plants behave like to the human liver when dealing with these xenobiotic compounds. Phytostimulation is a phytoremediation technique that is used for the enhancement of soil microbial activity for the degradation of contaminants, typically by organisms that associate with roots. Since the root nodules are involved in this process it is also known as rhizosphere degradation (Wikipedia 2008). Additionally, phytoremediation is an aesthetically pleasant mechanism that can decrease remedial costs, restore habitat, and clean up contamination in place rather than accumulating it in place or transporting the problem to another site (Zynda, 2001). One of the major limitations with this technique is that it works best at sites with low to medium amounts of pollution (EPA, 2001). Besides, the results vary from site to site and it is important to conduct site-specific research to select the best process. This approach is not commonly suitable for highly contaminated soils that pose threat to the ecological health. Further research is required in this field to gain benefits out of this process. It is essential to understand various plant–chemical interactions and learn to apply them safely in remediation programs is a challenging aspect (uga.edu, N.D.). Land Contamination and Phytoremediation in UK The intensification of agricultural production on the one hand together with the steps taken to reduce point source pollution from factories and sewage works on the other means that, as the Policy Commission on Food and Farming January 2002 noted, agriculture is now a major polluter in UK. Nitrate levels in many English ground and surface waters, are increasing. Nitrate pollution is of concern as it has to be removed before water can be supplied to consumers, and it can harm the organisms in the ecosystem. Estimates suggest that “over 70% of nitrate and 50% of phosphates enters water from agricultural land in UK. Up to a half of Englands bathing waters are affected by short-term contamination by agricultural pollution, mainly by organic matters from livestock manure being washed off farm land after rain. The majority of silt loads to English rivers and lakes are coming from heavy soil erosion from agricultural land”. This alters the composition of gravel sediments, reducing water clarity and results in serious problems for fish, plants and insects. Pesticides are contaminating drinking water sources, requiring expensive additional treatment at water works to remove pesticides before this water can be supplied to consumers (Department for Environment, Food and Rural Affairs, 2004). “The term land contamination encompasses a wide range of situations where land is contaminated in different way. In a few cases where definite criteria are met, a site might be determined contaminated land which has a specific legal definition set out in Part IIA of the Environmental Protection Act”. And it is reported that all through the UK, there are thousands of sites that have been contaminated by prior use.  Time and again this is linked with industrial processes or activities that have now stopped, but where waste products or remaining residues present a hazard to the general environment.  In UK there is increasing pressure to reuse land which is affected by contamination rather than develop greenfield sites such as parks or woodland (Environmental Agency, 2008a). According to a survey carried out by the Environment Agency (the UK Soil and Herbage Survey (UKSHS)) that was the pioneering survey of soils and plants throughout the UK, the analysed soil and vegetation from 122 rural, 28 urban and 50 industrial sites were all contaminated. “The survey tested for concentrations of dioxins, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and a range of metals across the UK” (Environmental Agency, 2008b). Decontamination of these sites is expensive and time consuming. Phytoremediation is one of the best ways in which UK’s contaminated land can be recovered. The main benefits of this technique are “low cost; in-situ application, which reduces the site disruption and the transport of contaminated material; negligible or no air emissions as in the case of other decontamination processes; use of dense-growing species such as turfgrasses that enhances the aesthetics and which offer a "cap" that can limit exposure to soil during treatment; use of naturally available energy (i.e., sunlight) by the plants when compared to other techniques; and last but not the least very less use of fertilizer and irrigation” (GTI, 2004). However, the major barrier to the implementation of phytoremediation in UK is that it is new and not fully developed. There is little regulatory knowledge with phytoremediation and it has to be well thought-out on a site by site basis. Furthermore, the intrinsic characteristics of phytoremediation limit the size of the niche that it occupies in the site remediation market. Additionally there are also few more limitations to phytoremediation. For instance it takes a long time and repeated treatments when compared to other treatments. Since it is dependent on plants it is directly linked with climate. It also requires the addition of nutrients for the growth of plants that adds to the expenses of this technique. It is also possible that the high metal concentration can be toxic to the plants. The livestock cannot be allowed into such fields for grazing as these plants can be immensely toxic to the animals. However, these limitations are less numerous than the benefits of the technique and hence it is important for further research to be carried out. “Momentum for the use of phytoremediation in UK and other parts of the world as a cleanup technique is building. This is especially true in application niches where other technologies are less suitable or are absent”. This technique can be combined with other bioremediation techniques to give better results (UNEP, N.D.). In conclusion, it can be said the phytoremediation technique is one of the best ways in which plants can be utilised to decontaminate the land that are polluted with heavy metals and other pollutants. Further research is definitely required to enhance its application base. It is cost-effective, non-intrusive, and safe alternative to conventional soil cleanup techniques for the contaminated land in UK. References Boyd R.S. and Martens S.N. (1998) The significance of metal hyperaccumulation for biotic interactions, Chemoecology 8 (1998) pp.1–7. Department for Environment, Food and Rural Affairs (DEFRA), (2004). NITRATES - Reducing Water Pollution from Agriculture. [Online] Available from [Accesses on 19 April 2008]. EPA, (2001) A Citizen’s Guide to Phytoremediation, [Online] Available from [Accesses on 19 April 2008]. Environmental Agency, (2008a) Land Quality: Land Contamination. [Online] Available from [Accesses on 20 April 2008]. Environmental Agency, (2008b) Science and Research: Survey of Soil Pollution. [Online] Available from [Accesses on 19 April 2008]. GTI, (2004) Phytoremediation Using Molecular Techniques. [Online] Available from [Accesses on 21 April 2008]. McCutcheon, S.C. and Schnoor, J.L. (2003) Phytoremediation: Transformation and Control of Contaminants, ecomed publishers. Meagher, RB (2000). "Phytoremediation of toxic elemental and organic pollutants". Current Opinion In Plant Biology 3 (2): 153-162. Mendez MO, Maier RM (2008). "Phytostabilization of mine tailings in arid and semiarid environments—an emerging remediation technology". Environ Health Perspect 116 (3): 278–83. UNEP, (N.D.) Phytotechnologies. [Online] Available from [Accesses on 19 April 2008]. uga.edu, (N.D.) Phytoremediation research. [Online] Available from [Accesses on 19 April 2008]. Water Framework Directive, (2005). Source of Pollution – Overview. [Online] Available from [Accesses on 18 April 2008]. Wikipedia (2008) Phytoremediation. [Online] Available from [Accesses on 19 April 2008]. Zynda, T. (2001) Phytoremediation. [Online] Available from [Accesses on 20 April 2008]. Read More