Phytoremediation: The future of Environmental Remediation

Introduction

Aberdeen Proving Ground, Harford County, Maryland.  Between the years of 1940 and 1970, this location held some of the largest chemical dumping pits in the Eastern United States.  According to the EPA, munitions, industrial chemicals and even chemical warfare agents were some of the contaminants deposited in this site named J-Field.  Chemicals such as Trichloroethene (TCE) and 1,1,2,2-tetrachloroethane were some of the most prominent pollutants found in the soils around the dumping site.  Not known at the time was that these carcinogenic agents were being dumped on a surface, that had an underground water table only 2 feet below the surface.  Luckily, the groundwater below the site lay within a confined aquifer and did not have the potential to contaminate any potential human drinking water sources, yet, it couldn’t just be left in the ground for years to come.  It’s estimated that within 30 years the contaminant levels can be reduced over 85% due to a new remediation technology, phytoremediation (EPA).  

It has long been known that plants are capable of absorbing various metals and nonmetals through their roots and storing, or bioaccumulating them, in said roots, stems and leaves, respectively.  However, it wasn’t until recently that this became a viable method of remediation for various soil environments.  This new method is called Phytoremediation.  Phytoremediation is the use of genetically modified plants (GMPs) and associated soil microbes to reduce the concentrations or toxic effects of contaminants in the environment (Ali et al. 2013, p. 869). Phytoremediation using GMPs is a viable method of remediation compared to the various forms of mechanical remediation due to its versatility, cost effectiveness and environmental preservation.  More than just environmentally safe, Phytoremediation is environmentally beneficial (Glass, 2000). The range of environmental benefits include, improved biodiversity, soil protection, carbon sequestration, watershed management, diverse sources of energy and aesthetics (Dickinson et al. 2009), stability and sustainability (Adams et al. 2013).

Phytoremediation works in a variety of ways to remove pollutants from the environment.  Some plants can absorb pollutants from the earth through their roots and reconstitute the atoms to form an environmentally neutral or safe compound.  Plants like these have no need to be harvested and can remediate soils for the entire span of their life (Kumar et al. 1995). However, the most common form of Phytoremediation is called Phytoextraction.  Phytoextraction works in a very simple way.  Just as plants can absorb nutrients and water through their roots, they can absorb metals, explosives, solvents, and petroleum products alike.  Once these contaminants are absorbed they are stored and accumulated in the plant.  These contaminants can accumulate in the roots, stems and leaves (Kumar et al. 1995).  Different plant species have varying storage capabilities which can in turn be altered to accumulate more with the use of genetic modification.

Mechanical Remediation: Causing Problems While Solving Them

As of 2009 the Environmental Protection Agency (EPA) has become aware of the growing problem where modern remediation methods are causing much more of a mess than they are cleaning up one.  This pushed the EPA to pass the Superfund Green Remediation Strategy.  The plan currently recommends reducing energy use and greenhouse gas emissions during the remediation of hazardous waste dump sites (Adams et al. 2013).  However, this strategy does not include possible extensions of the remediation hierarchy to include phytoremediation.

The reason why we should focus on reducing mechanical remediation is because when contaminated sites are remediated using mechanical methods, they create more environmental problems. The mechanisms of these physical remediation processes vastly destroy the land by removing many layers of sediments as well as exposing possible environmental hazards. Furthermore, the machines used all burn fossil fuels, adding to air pollution.  One of the most common mechanical remediation techniques that can have negative impacts on the environment is excavation. Excavation is a remediation technique that is used to move contaminated soil into regulated landfills. It is one of the most common methods used to remediate superfund sites because it is a quick solution to clean contaminants from  “hot spots,” where they may be found in  very high concentrations. This technique requires the use of heavy machinery, such as excavators to collect and remove contaminated soil, and trucks to carry these excavated materials to an off site disposal area. Although very straightforward, this technique can have a major impacts on the environment. For example, excavation of land can lead to soil erosion, high rate of fuel consumption, and deterioration of soil through off site disposal of excavated materials (EPA, Green Remediation, 2008). This can cause adverse effects on human health, as well as the ecosystem.

In addition, dredging, a form of excavation operation that is mostly carried out to remove contaminated sediments underwater causes threats to underwater ecosystems. The biggest threat that can occur during dredging is the release of sediments in water, which can alter its physical characteristics  hence, creating turbid plume (Capello et al., 2013). This cloudy movement of sediments is frequently caused by the use of dredgehead, work boats, and control measures such as silt screen (EPA, Dredging and Excavation, pp. 6-21). An example of the habitat that can have a serious impact due to this resuspension of fine sediments caused by dredging is Posidonia oceanica meadows, a seagrass endemic to the Mediterranean sea (Capello et al. 2013).

Another widely used mechanical remediation method is landfill capping. The main purpose of this technique is to isolate the contaminated soil from the environment, mainly to prevent the surface water from infiltrating the contaminated sites. This process is carried out by sealing the contamination using natural and/or synthetic liners. One thing to keep in mind is that this process neither removes the contaminants, nor detoxifies them, and the biggest drawback that it has is that if the capping is not done properly, there is a chance for the contaminants to leak out into the soil and pollute groundwater. Therefore, polluting groundwater will become very hazardous to human health.

Genetically Modified Plants in Phytoremediation

Phytoremediation is comparatively a safer, better alternative to mechanical remediation, yet the use of plants as a remediation method has its limitations. Plants, being living organisms, have limitations to the amount of contaminants that they can accumulate in their tissues without suffering injury. Plants like humans, are not invincible.  Another drawback to phytoremediation is that it is often a slower method compared to physicochemical processes and may need to be considered as a long term remediation process (Cunningham, Berti & Huang ,1995).  Natural hyperaccumulators often accumulate only specific contaminants. Again, most of them grow slowly with small biomass (Cunningham et al. 1993). Metal hyperaccumulation especially is a relatively rare phenomenon that occurs only in a few terrestrial plants. About 400 species of natural hyperaccumulators have so far been identified which represent less than 0.2% of all angiosperms (Baker et al. 2000)

Due to the apparent limitations of natural hyperaccumulators such as their rare occurrence, slow growth and limited accumulation capacities, the use of genetically modified plants (GMPs) has become increasingly important. Due to the sexual incompatibility between parents, conventional breeding methods are not considered viable to naturally increase metal extraction (Lasat, 2002). Through biotechnology, plants producing large amounts of biomass with the ability to hyperaccumulate high concentrations of toxic metals and either translocate them from roots to shoots or degrade them have been developed. For example, Rugh et al. (1998) modified yellow poplar trees with two bacterial genes namely mer A and mer B to hyperaccumulate and detoxify mercury from contaminated soils (Datta & Sarkar, 2003).  The function of mer B in transgenic poplar trees is to catalyze the release of mercuric ion from methyl-mercury which is then converted to elemental mercury by mer A. Elemental mercury is less toxic and more volatile than mercuric ion and is released into the atmosphere ( Datta & Sarkar, 2003).  Pilon-Smits et al. (1999) over-expressed APT-sulfurylase (APS) gene in Indian mustard. The transgenic plants had four-fold higher APS activity and accumulated three times more selenium than wild type plants. Dhankar et al (2002) reported a genetic based strategy to remediate arsenic from contaminated soils whereby two bacterial genes were over-expressed in Arabidopsis. One was E. coli arsC gene encoding arsenate reductase, which reduces arsenate to arsenite coupled to a light-induced soybean rubisco promoter. The second one was E. coli ?-ECS coupled to a strong constitutive actin promoter. The transgenic plants showed substantially greater arsenic (As) tolerance.  When grown in arsenic contaminated soils, these plants accumulated 4-to 17-fold greater fresh weight and accumulated 2-to 3-folds than wild type plants.

However widespread use of genetically modified plants in phytoremediation often meets resistance from environmental advocacy groups such as Green Peace, who claim that environmental impact of GMPs has not been conclusively assessed. Another reason why widespread use of GMPs  has been slow is due to the fact that many countries still do not have legislative guidelines on the development and use of this technology. There is also fear that pollen from transgenic plants could contaminate non-target species with unknown consequences. The risk of transgenic plants contaminating non-target plants can be reduced by inserting terminator genes into genetically modified plants. Plants equipped with a terminator gene grow just like other plants with one crucial exception, nothing can be grown from the seed they produce.

Qualitative and Quantitative Cost Reductions:

Phytoremediation is a cost effective approach compared to Mechanical remediation methods. Cost effectiveness in this context refers to the actual cost in terms of money and resources employed while using a particular method of remediation, as well as the cost in terms of environmental impact. The combined monetary cost and environmental impact of using phytoremediation is negligible compared to mechanical methods such as excavation and land-filling. Phytoextraction is a cost-effective, sustainable, risk managing technology with costs similar to farming costs that minimize energy, water and material use while protecting land and ecosystems. As stated earlier, the use of heavy machinery for excavation and land-filling leads to combustion of huge volumes of fossil fuels that contribute to environmental pollution and global warming.

In recent years, there has been a global campaign to reduce the use of fossil fuels as an environmental preservation measure and promote reliance of new, sustainable and environmentally friendly sources of energy. Indeed what makes phytoremediation and phytoextraction even more interesting is how plants can be used when their remediation cycle is complete.  Utilization of the obtained biomass of an expired phytoextraction cycle can even turn it into a profit making operation by using the large biomass as energy.  Many plants used as phytoremediators and phytoextraction of heavy metals are incinerated after they are harvested, leaving behind the heavy metal pollutants.  This incineration process can be used as energy production as well.

Compared to other forms of mechanical bioremediation such as activated carbon absorption, ultraviolet oxidation and electron beam destruction, phytoremediation costs much less per volume of soil to clean up. Imagine an empty 2 liter bottle of soda filled with soil. Next, imagine this soil is contaminated with trinitrotoluene (TNT) from left over mine tailings. Using activated carbon absorption, it would cost $2 to remediate the 2 liter bottle of all TNT.  Using ultraviolet oxidation and electron beam destruction would cost even more at $6 and $10 respectively for every 2 liters remediated.  These cost grow exponentially when considering entire properties saturated with TNT.  However, phytoremediation cost much less at only $1.28 for 2 liters of sediment (Meagher et al. 2005).  The same can be said about lead (Pb) contamination.  Lead (Pb) is one of the most common pollutants on the planet, and can be found in almost every soil on the planet dated after the industrial revolution. This also makes it one of the most difficult contaminants to remediate.  In order to reduce a properties lead levels in soil from 1.4 g/kg to 0.4 g/kg over a ten year period would cost only $27,000.  This may seem like a large amount of money, but this is compared to $790,000 when soil leaching and $1,620,000, for the most popular remediation method in use, excavation and land filling (Meagher et al. 2005).

The numbers truly speak for themselves, as Cunningham, Berti & Huang (1995) claim that on average, Physicochemical remediation costs are $10-100 per cubic meter of soil for volatile or water soluble pollutants remediated in situ (in the ground). It costs $60-300 per cubic meter for compounds handled by landfilling or low temperature thermal treatment, and  $70-200 per cubic meter for materials requiring special landfill arrangements or high temperature thermal treatments. The incineration of contaminated soils can cost up to $100 per cubic meter, yet certain materials (e.g. radionuclides) require even more intensive management techniques that can cost well beyond $1000-3000 per cubic meter of soil. In contrast, phytoremediation costs range between US $200 and $10000 per hectare.  This represents a treatment cost of US $0.02-1.00 per cubic meter per year, which is far much less compared to the costs associated with physico-chemical technologies (Cunningham et al.1993).

In the United States, the number of Superfund Sites listed in the National Priorities List according to the EPA (Environmental Protection Agency’s) Superfund Green Remediation Strategy stood at 1,289 in April 2011. The EU, in its Communication on Thematic Strategy for soil protection, does suggest that a future Soil Framework Directive should allow member states to select which remediation strategies they consider most cost-effective (Biomass and Bioenergy 39, 2012, pp.454-455). The estimated costs for cleanup of all contaminated sites is $209 billion for a total of 294000 sites. The cost of conventional remediation technologies excluding excavation run as high as $252 per cubic meter of soil in the United States.  At an average cost of $200- $10000 per hectare (US $0.02- US $1.00 per cubic meter) together with its environmental conservation and energy production advantages, phytoremediation is a strategy that deserves serious attention.

 

Proposals

In recent years, there has been a global campaign to reduce the use of fossil fuels as an environmental preservation measure, and to promote reliance of new, sustainable and environmentally friendly sources of energy. Indeed what makes phytoremediation and phytoextraction even more interesting is how plants can be used when their remediation cycle is complete.  Utilization of the obtained biomass of an expired phytoextraction cycle can even turn it into a profit making operation by using the large biomass as energy.  Many plants used as phytoremediators and phytoextraction of heavy metals are incinerated after they are harvested, leaving behind the heavy metal pollutants.  This incineration process can be used as energy production as well.

In the long run, phytoremediation definitely has environmental and economical advantages over mechanical remediation; however, there are still a number of biased opinions against it. Some of these opinions are concerns coming from government, investors, consultants and general public who want a feasible, economically effective and immediate remediation of pollutants. Due to these growing concerns coming from various members of the community, we propose that the U.S.  Environmental Protection Agency (EPA) include regulations and standards that incorporate phytoremediation methods. We would also like to propose that EPA develop a protocol which includes guidelines on how to use phytoremediation, and present a comparison of performance and cost effectiveness of various remediation techniques to prove the usefulness of phytoremediation. This will give the general public and parties involved in remediation process  a better understanding of phytoremediation and its benefits. We also propose that the EPA develop strong regulatory frameworks on the production of genetically modified species  to be used in the remediation process. Since GM species tend to be invasive, we highly recommend that EPA legislations incorporate mechanisms that ensure GMPs do not contaminate non-target species such as the use of terminator genes to prevent cross pollination. The  EPA should design procedures for proper extraction and disposal of the harvested plants, and proper fencing of extraction site to restrict animals from grazing. This will ensure safety for the ecosystem.

Conclusions

The increasing amount of pollution in the environment has direct and indirect effects on our lives and remediating environmental pollutants should be our priority. We should, however, be concerned about the methods that we use, lest we create even greater problems. Although there are many different chemical and mechanical remediation options available, these processes are only short term fixes to the environmental problems that we are facing. Therefore, we need to adopt a long-term process to completely eliminate these pollutants from the environment. Not only has it been proven to be environmentally and economically beneficial, but also as a reliable and safer alternative to other remediation options. Therefore, with extensive research in this field and proper guidelines and regulations from the EPA, phytoremediation through GMP species will have a promising future in the push to a cleaner, greener world.

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