Role of phytoremediation in treating heavy metal pollution

In the previous article, the role of genetic engineering in improving bioremediation were discussed. With rapid industrialization, heavy metal contamination and organic pollutants have increasingly affected soil and water bodies. These threaten the ecosystem, surface and ground waters, food and human health. Phytoremediation is a method which involves growing plants in a contaminated matrix to remove environmental contaminants. It is the direct use of living green plants for the removal and degradation of contaminants in the soil as well as water bodies (Hassani et al. 2014). There are other conventional methods for soil and water remediation such as soil washing, vapour extraction and incineration. However, phytoremediation is a low cost, solar energy driven cleanup technique. It can also replace mechanical methods of clean-up.

Role of phytoremediation in treating heavy metal pollution

Heavy metal contamination of soil is a major issue in the current scenario. There are a number of pollutant metals including iron, zinc, chromium, copper, arsenic, mercury and cadmium. They compromise the quality of soil. These metals are not biodegradable and may not be broken down by chemical oxidation (Vaziri et al. 2013). Thus, the level of these contaminants can be reduced by using biological organisms including microbes and plants. Conventional remediation methods for heavy metals are expensive and destructive. Plants remediating contaminated soil or water offer cost-effective and safe method of biodegradation with limited substrate demand (Tangahu et al. 2011). Therefore, phytoremediation offers higher advantages than bacteria. It enables heavy metals to be isolated from soil or water in a faster and cheaper way.

Freely available metals are taken up by roots of plants and transported to the stems and leaves (Chekroun & Baghour 2013). Phytoremediation takes into consideration various soil properties of the specific site. It is an emerging technology in both organic and inorganic pollutants. Not only does it generate less secondary wastes, but it also has the ability to leave soils in a usable condition following the treatment. The table below shows different plant species with naturally occurring remediation of heavy metal.

Plant species with naturally occurring remediation of heavy metal

Types of phytoremediation

There are five types of phytoremediation techniques. They depend upon the primary mechanism of action, contaminant type and applicability of technique.

1. Rhizofiltration: A water remediation technique which involves the uptake of contaminants by plant roots (Abubakar et al. 2014). Surface water rhizofilteration involves growing plants directly in the contaminated water body. Apart from removal through absorption, metals are also removed from the groundwater. This is done through precipitation caused by exudates (Szczygłowska et al. 2011).
2. Phytotransformation: Applicable to both soil and water. This technique involves the degradation of contaminants through plant metabolism. Groundwater can be remediated ‘in situ’ using phytotransformation by deep-rooted plants. It can also be done ‘ex situ’ by pumping water into troughs or constructed wetlands containing appropriate plants.
3. Phyto-stimulation or plant-assisted bioremediation: Also used for both soil and water. This involves the stimulation of microbial biodegradation through activities of plants in the root zone. The plants provide carbonaceous material from liquids released from roots. Also, oxygen released from the roots increases the oxygen content in the bioremediation area. These additions to the soil as a result of plant activity increase the rates of microbial activity. Ultimately they increase the rates of contaminant degradation.
4. Phytoextraction: This process involves the removal of metals, radionuclides and certain organic compounds by direct uptake into plant tissue.
5. Phytostabilization: It is the use of certain plant species to absorb contaminants, reducing their bioavailablity. Contaminants are generally metals. It also reduces the potential for human exposure to these contaminants.

Mechanisms of heavy metal uptake

Plants showcase varied methods of heavy metal uptake. It ranges from hyperaccumulation, vacuole sequestration and volatization of toxic metals like arsenic and mercury. There are four  major processes in remediation of metals from the soil to the shoots (Yang et al. 2005).

1. Bioactivation of metals in the rhizosphere through root–microbe interaction: Plant may alter its membrane permeability, change metal binding capacity of cell walls or exude more chelating substances
2. Enhanced uptake by metal transporters in the plasma membranes: Proteins like heavy metal ATPase, natural-resistance-associated macrophage protein family and cation diffusion facilitator. They are involved in transporting heavy metals across membranes.
3. Detoxification of metals by distributing to the apoplasts: Involves binding to cell walls and chelation of metals in the cytoplasm with various ligands. These include phytochelatins, metallothioneins, metal-binding proteins (Chekroun & Baghour 2013).
4. Sequestration of metals into the vacuole by tonoplast-located transporters: They can concentrate metal in their aerial parts, to levels far exceeding than soil. Hyperaccumulators are plants that can absorb high levels of contaminants concentrated either in their roots, shoots and leaves.

Bioavailability of compounds

However, the rate of uptake of pollutants by plants also depends upon the bioavailability of the compounds. Bioavailability refers to the free form of a pollutant in soil or water and are capable of interacting with organisms. This takes place either by crossing into cell membrane or binding to the membrane. The bioavalability of metals increases in soil through several means. One way plants achieve it is by secreting phytosidophores into the rhizosphere to chelate and solubilise metals that are soil bound (Olaniran et al. 2013).

Limitations of phytoremediation

Although phytoremediation offers advantages for large scale cleaning up of contaminated areas, it also suffers from several limitations. Firstly it is possible only in sites with shallow contamination areas within rooting zone of remediative plants. Harvested plant biomass from phytoextraction may be classified as hazardous waste and is difficult to dispose of. Also, introducing a new plant to a certain area as a non-native species affects the local biodiversity. Lastly, the consumption and irresponsible disposal of contaminated biomass causes harm to the environment.

Future prospects

To date, the utilities of phytoremediation have not been exploited completely. There are various technological and economic barriers that need to be addressed with it. Many hyperaccumulator plants still have to be investigated. Also, optimization of the process of heavy metal uptake and proper disposal of biomass produced is needed. So far, most of the phytoremediation experiments have taken place in a lab or small field scale. While these results are promising, scientists are ready to admit that solution culture is quite different from that of soil. This is because, in real soil, many metals are tied up in insoluble forms. Phytoremediation is a fast developing field and is a sustainable and inexpensive process. It is fast emerging as a viable alternative to conventional remediation methods. The next article explores a specific enzyme, merA, responsible for reduction of mercuric compounds in soil.

References

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