11 Role of microbes and plants in remediation – Phytoremediation

Dr. Yogalakshmi K N

Objectives:

• To understand the concept of phytoremediation
• To identify suitable plant for phytoremediation
• To understand the mechanism of phytoremediation
• To differentiate the merits and demerits of phytoremediation
• To study the different strategies used in phytoremediation

1. Introduction

Soil, air and water are contaminated from a variety of pollutants, affecting the environment and the health of living beings. Most contaminants enter in the environment from various domestic, industrial and commercial activities, chemical and oil spills, sewage systems, wastewater treatment plants and non-point sources such as roads, storm drains and parking lots. For decades these activities have polluted the environment resulting in health hazards. To protect the environment and the public from the health hazards, it is essential to clean the contaminated sites with a reliable and eco-friendly technology that does not pose secondary pollution to the environment. Remediation using microorganisms and plants is the best technology for removal and degradation of pollutants from the environment. The plant-assisted remediation which involves interaction of plant roots and associated rhizosphere microorganisms for the remediation of soil and water contaminated with high levels of metals, pesticides, solvents, radionuclides, explosives, nutrients, crude oil, organic compounds is termed as “phytoremediation”. Phytoremediation is an emerging, economical and environment friendly technique.

2. Phytoremediation

The use of living plants for cleaning of contaminated soil, air and water is known as “phytoremediation” and the term has been used since 1991. The term “phytoremediation” consist of Greek word phyto (plant) and Latin word remedium (to remove evil). Plants ability to remove contaminants from the environment is known from last 300 years. Over the time, the use of plants for construction of wetlands on contaminated sites or planting of trees to overcome the air pollution has evolved. From last 20 years this technology has flourished and recognized as an eco-friendly bioremediation technology. In United States this technology has been implemented as a remedy of choice at 18 superfund sites (USEPA, 2000). In this approach hyper accumulator plants concentrate the contaminants from the contaminated media and metabolize them to less harmful compounds. Phytoremediation is a promising approach for remediation of organic pollutants and toxic heavy metals. As heavy metals and radionuclide are immutable, the focus of the phytoremediation strategy is to hyper-accumulate them above ground in plant biomass.

2.1 Mechanism involved: The mechanism involved in phytoremediation include extraction and translocation of metals to above ground plant tissues, harvesting, and transformation of the toxic metal species to lesser toxic form or sequestering the metal in the roots to restrict the leaching and mobility of metals to the surroundings. On the other hand organic pollutants can be potentially completely mineralized into carbon dioxide, chlorine, nitrate and ammonia by the phytoremediation plant (Cunningham et al., 1996). Organic pollutants such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and linear halogenated hydrocarbons are the potentially significant targets for phytoremediation. Many of these pollutants are toxic, teratogenic and carcinogenic.

Plants use several biophysical and biochemical mechanisms such as adsorption, transport, translocation, hyperaccumulation, transformation and mineralization for remediation of the pollutants. For example, inorganic pollutants are taken up by the plants through the system of nutrient transport. Similarly, organic pollutants are degraded endogenously or sequestered in vacuoles. Henceforth the host plant is not affected by the toxic xenobiotic compounds. In some cases the application of genetic engineering for over expression of the concerned genes or transgenic expression of genes from bacteria or animal is required to improve or accelerate these natural phytoremediation processes.

From last two decades, with the advent of molecular, physiological and biotechnological strategies, phytoremediation is the approach of interest for remediation of contaminated soils and stagnant water. Contaminants such as heavy metals, pesticides, explosives, solvents and crude oil and its derivatives have been mitigated in phytoremediation projects worldwide. In addition, several studies confirmed the potential of plants for environmental cleanup. Table 1 enlists the plants popularly used in phytoremediation. Many plants such as Indian mustard (Brassica juncea), poplar tree (Populus deltoids), Indian grass (Sorghastrum nutans), Sunflower (Helianthus annuus), Willow (Salix sp.) have confirmed to be successful as hyper accumulator of toxic substances in many studies.

2.2 Characteristics of plant used in phytoremediation: An ideal plant for phytoremediation should possess the following properties:

• Able to extract or degrade the contaminants of concern should adapt to local climates
• should possess high biomass
• Should have well developed deep root structure Compatibility with soil where it is grown
• Should have high growth rate Easy to plant and maintain
• Able to take large amount of water through roots

2.3 Advantages and limitations of phytoremediation: Phytoremediation process is associated with a number of advantages. The advantages are listed below:

• It is eco-friendly and natural method of remediation
• It is a cost effective technique.
• It can be applied both as insitu and exsitu remediation
• The plants can be easily monitored
• Can be applied to large area
• can treat variety of organic, inorganic pollutants and heavy metals
• Valuable metals can be recovered and re-used by “phyto mining”
• easy to implement and accepted by public
• minimizes spread of pollutant through air and other medium.
• It is aesthetically pleasing

The phytoremediation process also suffers from many disadvantages such as

• The remediation is limited till the reachability of roots. Roots of certain plants are not deep.
• Time consuming process as phytoremediation takes years for removal of pollutants.
• Contaminants may be toxic to the remediating plant.
• Not effective for sites with higher concentration of pollutants. The process is influenced by climate and soil of that area.
• Sometimes phytoremediation mobilizes the contaminants which then enter into water.
• Bio-accumulated contaminants like metals require safe disposal or recovery, otherwise they might reach to upper level consumers through food chain.

3. Different strategies of phytoremediation:

Plants use several ways or approaches to clean or remediate the contaminated environment. These include phytoextraction, phytofiltration, phytostabilization, phytodegradation and phytovolatization (Figure 1).

3.1 Phytoextraction

Phytoextraction also known as phytoaccumulation or phytoabsorption is the uptake of contaminants from the contaminated media (soil, water) by host plant roots, translocation and accumulation in the aboveground plant biomass. Phytoextraction is mainly used for the remediation of polluted soils.

Phytoextraction can be categorized in two groups: continuous phytoextraction and chelate-induced phytoextraction. In continuous phytoextraction hyper accumulator plants are used, whereas high biomass producing plants and chelating substances are used in chelate-induced phytoextraction. Continuous phytoextraction involves seeding or transplanting of pollutant accumulating plant species into the polluted site and are cultivated through the established agricultural practices. The pollutant in the soil is absorbed by roots of the plant, translocated and accumulated in the aerial part. Hyperaccumulator plant can accumulate 100 times more metal than a common plant. A hyperaccumulator plant is able to grow faster, have deep and profuse root system and large biomass production. Zinc, copper and nickel are the best metal candidates phytoextracted by hyper accumulators. According to a study, approximately 450 plants species of angiosperm belonging to the families Asteraceae, Brassicaceae, Caryophyllaceae, Cunouniaceae, Cyperaceae, Euphobiaceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae and Violaceae are known to uptake and absorb large amount of heavy metals (Padmavathiamma and Li, 2007). These hyper accumulator species account for less than 0.2 percent of the total known plant species. Approximately 25% discovered hyper accumulators are from family Brassicaceae (Rascio and Navari-Izzo, 2011). Research is continued to search for new hyper accumulator plant species. The process involves repeated cycle of planting and harvesting of plants to remove the pollutants from the site. The time period required for the process depends on the target contaminant, plant selected and their efficiency in pollutant removal. The remediation can differ from 1 to 20 years.

When the pollutant present in the soil is in non-available form and cannot be up taken by the remediating plant, then chelate-induced phytoextraction is applied. To make the pollutant bio-available various chelating or acidifying compounds are added to the soil, thus improving the biosorption and accumulation ability of non-hyperaccumulating plant. Ethylenediaminetetraacetic acid (EDTA), nitrilo triacetic acid, ethylene diamine disuccinate (EDDS), and low-molecular-weight organic acids (LMWOA) are some common chelators used for phytoextraction of contaminants. EDTA is powerful and extensively studied and popularly used chelating agent. Recently, the interest is shifted towards use of biodegradable chelating agent such as EDDS. EDDS is a naturally occurring isomer of EDTA. It can be easily degraded into less harmful bio-products.

The advantages of phytoextraction are as follows:

• Contaminant can be completely removed from the soil.
• Contaminant can be recycled from the accumulated plant biomass.

The disadvantages of the process include rate of metal uptake by roots and the cellular tolerance to the specific metal

3.2 Phytosequestration

Phytosequestration is an approach in which plants sequester certain contaminants in the root zone. This process involves several plant physiological mechanisms. Phytochemicals secreted in the root zone can be immobilize or precipitate the target pollutant. The transport proteins of the root stabilize the target contaminants. The contaminants can also be sequestered by vacuoles in the roots of the host plant.

3.3 Phytofiltration

Source: https://en.wikipedia.org/wiki/Phytoremediation

Figure 1: Different phytoremediation processes

Phytofiltration is an approach of phytoremediation that removes contaminants from the polluted water. Pollutants are either absorbed or adsorbed by the plant or plant material. This method is used to remove contamination of the groundwater. Phytofiltration can be categorized into rhizofiltration (use of host plant roots), blastofiltration (use of seedling) and caulofiltration (use of plat shoots). Aquatic plants, semi-aquatic plants and dried plant material have been practised in different water purification systems (Dushenkov et al., 1995). However, in some recent studies, hydroponically cultivated terrestrial plants roots are more successful in removal of heavy metals from the contaminated water system due to their profuse fibrous roots with root hairs resulting in large surface area. Commercial application of rhizofiltration is reduced by slow growth rate and pollutant binding capability of the tested phytoremediation plant. Therefore, an ideal plant for this approach should have rapid and profusely growing roots system with pollutant removal ability. Some varieties of sunflower have been screened hydroponically as most proficient plants for rhizofiltration of heavy metals from the polluted water (Dushenkov et al., 1995). Brassica juncea roots are also reported to be very effective in rhizofiltration. Rhizofiltration technology has been boot up from the introduction of ‘feeder layer’ concept. Feeder layer is a layer of artificial soil built up several centimetres deep in the water system to anchor the plant. Regular application of fertilizer to the feeder layer develops the extensive root network which is responsible for nutrient supply to the plant. A large portion of the root, responsible for metal removal, grows on the bottom of the layer and in the water below. As a result no further contamination of water occurs by addition of the fertilizers (i.e) both process remediation and fertilization are separated.

Rhizofiltration is improved by the introduction of blastofiltration (‘blasto’ meaning ‘seedling’ in Greek). It is discovered that juvenile plant seedlings growing in aerated water are more efficient than roots in removing metals from water (Salt, 1997). Blastofiltration is the second generation plant-based water remediation technology. It is more efficient than rhizofiltration as the surface to volume ratio is increased after germination and several germinating seedlings absorb or adsorb a large quantity of metal ions. Seedling cultures can be germinated and grown in light or darkness.

Various aquatic plants studied for phytofiltration are Eichhornia crassipes, Lemna minor L., Hydrocotyle umbellata L., Micranthemum umbrosum, Callitriche stagnalis S., P. pectinatus L.

Potamogeton natans L., Azolla caroliniana, and Ranunculus trichophyllus. Terrestrial plants that show good potential of phytofiltration are sunflower, Indian mustard, rye, tobacco, spinach and corn. Sunflower removes lead, uranium, caesium and strontium in hydroponic solutions (Mo et al., 1989; Dushenkov et al., 1997). Whereas, Indian mustard roots are effective in removal of cadmium, chromium, copper, nickel, lead and zinc (Mo et al., 1989). Polygonum amphibium, L. Minor, E. Crassipes, Oenathe javanica and Lepironia articulate are appropriate for polluted water phytoremediation because they are hyper accumulator of N and P, Cd, Hg and Pb respectively, (Wang, 2002).

3.4 Phytostabilization

Phytostabilization or phytoimmobilization is the reduction of mobility or bioavailability of contaminants in the contaminated soil through the use of certain plant species. It is also known as in place inactivation and primarily used for the remediation of contaminated soil, sediment and sludge. This is the technique of stabilizing the pollutants in the contaminated media and thus prevents their entry in groundwater and food chain. This technique is widely used for remediation of metal contamination in the soils through sorption by plant roots, precipitation, complexation and reduction in metal valence in the rhizosphere. Toxicity of the metal varies according to the valence of the metal. Plants secrete special redox enzymes to convert more toxic metals into less toxic metal form. Reduction of chromium and mercury is widely studied and confirmed to be less toxic and mobile. It is useful remediation approach for remediation of metals such as lead, arsenic, cadmium, chromium, copper and zinc. However, phytostabilization is not the permanent remediation solution as the metals remain in the soil; it is the only management strategy. This approach can also be used for re-establishment of vegetation cover at contaminated sites where native plants fail to survive. Metal-tolerant plant species are used to restore the vegetation. Advantages of phytostabilization are as follows:

• Disposal of plant biomass is not required
• Very effective if quick immobilization is required to preserve soil and water surface.
• Contaminated sites can be prevented from further soil erosion.

However, this technology is also limited by disadvantages like: contaminants remain in the soil and application of extensive fertilizers required.

3.5 Phytovolatilization

In phytovolatilization, the pollutants can be up taken by the plants from the contaminated soil and released into the atmosphere after being converted into volatile form (USEPA, 2000). This technique is mostly used for organic contaminants such as trichloroethene and some of the heavy metals such as arsenic, mercury and selenium. This technology can be applied to contaminated soil, sediments and water. However, it is the most controversial technique of the phytoremediation as the pollutants are not completely removed; only transferred from one media to another.

3.6 Phytodegradation

Phytodegradation is the breakdown of organic contaminants with the help of certain enzymes like dehalogenase and oxygenase secreted by the plant. This process is independent of rhizospheric microorganisms. Plants absorb and accumulate the organic pollutants from the contaminated environment and detoxify them through various metabolic processes and therefore, plants can be called as “Green Liver” of biosphere. However, phytodegradation is limited to the remediation of only biodegradable contaminants i.e. organic contaminants including synthetic insecticides and herbicides. Genetically modified plants can also be used for phytodegradation of contaminants in some studies (Doty et al., 2007).

3.7 Rhizodegradation

Remediation of pollutants in the rhizosphere cooperatively by both microorganisms and plants is known as rhizodegradation. Rhizosphere is the area surrounding the roots of the plants and it extends to about 1 millimetre. The bio degradation of the contaminants in the rhizosphere is enhanced because of increase in microbial density and activity. Microbial activity in the rhizosphere can be increased about 10-100 times more by the exudates secreted by the plant roots containing carbohydrates, amino acids and flavonoids etc. Nutrient rich exudates secreted by the plant roots could be utilized as carbon and nitrogen source by soil microbes and hence microbial activity is enhanced in rhizosphere. In addition to energy source plant roots also release certain enzymes capable to degrade organic pollutants in the soil.

3.8 Phytodesalination

It is an emerging technique of phytoremediation. It involves the use of halophytes for removal of salts from the salt affected sites to make soil suitable for normal plant growth. It has been reported that halophytes are better adapted to cope heavy metal stress as compared to glycophytic plants (Manousaki and Kalogerakis, 2011). Halopytes Suaeda maritime and Sesuvium portulacastrum have been reported to remove 507 and 474 kg NaCl from 1 hectare saline soil in a time period of four months. In one another study obligate halophyte S. Portulacastrum is reported to accumulate about 1 t/hectare of sodium in aboveground biomass, cultivated in a saline soil. This treatment enhances the growth of a glycophytic plant Hordeum vulgare cultivated in the same soil (Rabhi et al., 2010). Therefore, these helophytes could be used to remediate saline soils and soil could be used for crop cultivation after repeated cultivation and harvesting these halophytes (Ravindran et al., 2007).

All the phytoremediation strategy is summarized in Table 2.

Summary

• At the end of this module we have studied about
• definition and concept of phytoremediation
• Mechanism of pollutant removal by phytoremediation Merits and demerits of phytoremediation
• Plants used in phytoremediation
• Strategies used in phytoremediation

References

1. Cunningham, S.D., Anderson, T.A., Schwab, P. and Hsu, F.C. (1996). Phytoremediation of soils contaminated with organic pollutants. Advances in Agronomy 56: 55-114.
2. Dushenkov, S., Vasudev, D., Kapulnik, Y., Gleba, D., Fleisher, D., Ting, K.C., et al. (1997). Phytoremediation: A novel approach to an old problem. in DL Wise (ed.), Global Environmental Biotechnology. Elsevier, Amsterdam, 563-572.
3. Dushenkov, V., Kumar, P.B.A.N., Motto, H. and Raskin, I. (1995). Rhizofiltration: the use of plants to remove heavy metals from aqueous streams. Environmental Science and Technology 29: 1239-1245.
4. Mo, S.C., Choi, D.S. and Robinson, J.W. (1989). Uptake of mercury from aqueous solution by duckweed: The effect of pH, copper, and humic acid. Journal of Environmental Health 24, 135-146.
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6. Rascio, N. and Navari-Izzo, F. (2011). Heavy metal hyperaccumulating plants: how and why do they do it? And what makes them so interesting? Plant Science 180: 169-181.
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11. Rabhi M, Ferchichi S, Jouini J, Hamrouni MH, Koyro HW, Ranieri A, Abdelly C, Smaoui A. (2010). Phytodesalination of a salt-affected soil with the halophyte Sesuvium portulacastrum L. to arrange in advance the requirements for the successful growth of a glycophytic crop. Bioresource Technology. 101(17):6822-8