Phytoremediation

Contaminants derived from anthropogenic activities can adversely affect wildlife and impact human health (Figure 1). Even low levels of contaminants in the environment pose a risk due to potential accumulation at higher trophic levels, a process called biomagnification. Organic contaminants are usually xenobiotic to plants, but inorganic contaminants such as metals are commonly found in low concentration in the soil. Metals such as cobalt (Co), copper (Cu), iron (Fe), molybdenum (Mo), manganese (Mn) and zinc (Zn) are critical for plant growth, and are classified as essential micronutrients. Other metals that are commonly found as contaminants, and are non-essential for plants, include arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), nickel (Ni), lead (Pb), selenium (Se), uranium (U), vanadium (V), and wolfram (W). Metals can have toxic effects on plants, even at low concentrations (Figure 2). Metal uptake by plants generally takes place by specific transporters (channel proteins) or H⁺ coupled carrier proteins located on the cell membrane of the root. For example, the Fe regulated transporter (IRT1) allows the uptake of Fe. Uptake of other metals also occurs via IRT1 transporters, especially if very low concentrations of Fe exist in the soil. Inadvertent uptake of non-essential metals also takes place via other cell membrane transporters. For example, uptake of As (as AsO₃) occurs inadvertently via phosphorus-transporters. Organic molecules enter plant roots via simple diffusion.

Phytoremediation basically refers to the use of plants and associated soil microbes to reduce the concentrations or toxic effects of contaminants in the environment. Phytoremediation is widely accepted as a cost-effective environmental restoration technology. Phytoremediation is an alternative to engineering procedures that are usually more destructive to the soil. Phytoremediation of contaminated sites should ideally not exceed one decade to reach acceptable levels of contaminants in the environment. Phytoremediation is, however, limited to the root-zone of plants. Also, this technology has limited application where the concentrations of contaminants are toxic to plants. Phytoremediation technologies are available for various environments and types of contaminants. These involve different processes such as in situ stabilization or degradation and removal (i.e., volatilization or extraction) of contaminants (Figure 3A; Table 1).

Technology	Action on Contaminants	Main Type of Contaminants	Vegetation
Phytostabilization	Retained in situ	Organics and metals	Cover maintained
Phytodegradation	Attenuated in situ	Organics	Cover maintained
Phytovolatilization	Removed	Organics and metals	Cover maintained
Phytoextraction	Removed	Metals	Harvested repeatedly
Table 1: Comparison between phytoremediation technologies

Phytostabilization aims to retain contaminants in the soil and prevent further dispersal. Contaminants can be stabilized in the roots or within the rhizosphere (Figure 3B). Revegetation of mine tailings is a common practice to prevent further dispersal of contaminants (Figure 4). Mine tailings have been stabilized using commercially available varieties of metal tolerant grasses such as Agrostis tenuis cv. Goginan and cv. Parys and Festuca rubra cv. Merlin. Metal tolerance of plants is generally increased by symbiotic, root-colonizing, arbuscular mycorrhizal fungi (AMF), through metal sequestration in the AMF hyphae. In addition, excretion of the glycoprotein glomulin by AMF hyphae can complex metals in the soil. Populations of AMF that are adapted to metal contaminated soils have great potential in phytostabilization. Soil microbes can decrease toxic effects of contaminants in the soil. For example, exudates (peptides) from the bacterium Pseudomonas putida can decrease Cd toxicity in plants. Plants can also convert contaminants into less toxic forms, or decrease their bioavailability. Natural chelates released by the roots of certain plants can form complexes with metals in the rhizosphere. These include siderophores, organic acids and phenolics. Metals such as the toxic Cr(lll) can be converted to the much less toxic Cr(Vl) by enzymes found on the roots of wetland plants. In addition, plants, and their associated soil microbes, can release chemicals that act as biosurfactants in the soil that increase the uptake of contaminants. Contaminants can be stabilized in natural and constructed wetlands through a process called phytofiltration. This involves rhizofiltration where metals are precipitated within the rhizosphere. Metal-plaque forms typically on the roots of wetland plants through the release of oxygen via the aerenchyma of roots. Iron oxides precipitate along with other metals into the metal plaque. Metal plaque on roots acts as a reservoir for active iron (Fe²⁺), which in turn increases the tolerance of plants to other toxic metals.

Phytodegradation involves the degradation of organic contaminants directly, through the release of enzymes from roots, or through metabolic activities within plant tissues (Figure 3B). In phytodegradation organic contaminants are taken up by roots and metabolized in plant tissues to less toxic substances. Phytodegradation of hydrophobic organic contaminants have been particularly successful. Poplar trees (Populus spp.) have been used successfully in phytodegradation of toxic and recalcitrant organic compounds. Rhizodegradation involves attenuation of organic contaminants into less toxic substances within the rhizosphere through biodegradation of soil microbes. This process is facilitated by root exudates (organic molecules) that sustain populations of soil microbes. To enhance this process a specific inocula of bacteria can be added to contaminated soils. Bacterial inocula contain strains that have desired metabolic activity to degrade targeted contaminants. Inoculating plants with genetically engineered strains of bacteria that degrade a specific contaminant has shown promising results. Also, populations of soil bacteria can be amplified using a process called biostimulation. This involves, for example, manipulating the nutrient and pH levels of the soil to increase bacterial populations.

Phytovolatilization involves the uptake of contaminants by plant roots and its conversion to a gaseous state, and release into the atmosphere. This process is driven by the evapotranspiration of plants (Figure 3B). Plants that have high evapotranspiration rate are sought after in phytovolatilization. Organic contaminants, especially volatile organic compounds (VOCs) are passively volatilized by plants. For example, hybrid poplar trees have been used to volatilize trichloroethylene (TCE) by converting it to chlorinated acetates and CO₂. Metals such as Se can be volatilized by plants through conversion into dimethylselenide [Se(CH₃)₂]. Genetic engineering has been used to allow plants to volatilize specific contaminants. For example, the ability of the tuliptree (Liriodendron tulipifera) to volatilize methyl-Hg from the soil into the atmosphere (as Hg⁰) was improved by inserting genes of modified E. coli that encode the enzyme mercuric ion reductase (merA).

Phytoextraction uses the ability of plants to accumulate contaminants in the aboveground, harvestable biomass (Figure 3B). This process involves repeated harvesting of the biomass in order to lower the concentration of contaminants in the soil. Phytoextraction is either a continuous process (using metal hyperaccumulating plants, or fast growing plants), or an induced process (using chemicals to increase the bioavailability of metals in the soil). Continuous phytoextraction is based on the ability of certain plants to gradually accumulate contaminants (mainly metals) into their biomass. Certain plants can hyperaccumulate metals without any toxic effects. These plants are adapted to naturally occurring, metalliferous soils. More than 400 plant species can hyperaccumulate various metals. However, most plants can only hyperaccumulate one specific metal.

Hyperaccumulating plants can contain more than 1% of a metal in their dry biomass. For example, the hyperaccumulating plant Berkheya coddii was found to contain as much as 3.8% of Ni in the dry, aboveground biomass, when grown in contaminated soil. It is possible to extract metals from the harvested biomass in a process termed phytomining. The underlying mechanism of hyperaccumulation of metals in plants is the overexpression of genes that regulate cell membrane transporters. These include the Cu-transporter (COPT1) and Zn-transporter (ZNT1). The main limitations on the use of hyperaccumulating plants in phytoextraction are slow growth and low biomass production. The effectiveness of phytoextraction is a function of a plant's biomass production and the content of contaminants in the harvested biomass. Therefore, fast-growing crops that accumulate metals have a great potential in phytoextraction. The use of crops in phytoextraction can be improved by manipulation of their associated soil microbes. Inoculation of plant growth promoting bacteria (PGPR) and AMF can increase plant biomass. The AMF-plant symbiosis usually results in reduced accumulation of metals in the aboveground biomass of plants. Therefore, suppressing AMF activity, by using specific soil fungicides, has resulted in increased metal accumulation in plants. The role of AMF in regulating metal uptake by plants appears to vary depending on numerous factors, such as AMF populations, plant species, nutrient availability, and metal content in the soil. Also, this regulation of AMF is usually metal-specific; where the uptake of essential metals is generally increased, but the uptake of non-essential metals is inhibited. However, exceptions have been found where AMF increases uptake of Ni, Pb, and As in plants. Induced phytoextraction involves the use of fast-growing crops and chemical manipulation of the soil. Low bioavailability of metals in the soil is a limiting factor in phytoextraction. The bioavailability of metals can be increased by the use of synthetic chelates such as ethylenediaminetetracetic acid (EDTA) or acidifying chemicals (e.g., NH₄SO₄). The use of synthetic chelates increases the absorption of metals to the root and the translocation of metals from the roots to the foliage (Figure 3). The timing of chelate application is critical, and should ideally take place at the peak of biomass production. The effectiveness of using EDTA was demonstrated by growing corn (Zea mays) in Pb-contaminated soil treated with 10 mmol kg^-1 EDTA. This resulted in a high accumulation of Pb (1.6% of shoot dry weight), and facilitated the translocation of Pb from the roots to the foliage. Some drawbacks of using synthetic chelates in phytoremediation are the result of increased solubility of the metals within the soil. In turn this increases the risk of metal migration through the soil profile and into the groundwater. However, a possible solution is to treat contaminated soil ex situ in a confined site with an impervious surface. Also, periodic application of low doses of synthetic chelates reduces the risk of metal migration.

Contaminants in the environment pose a global problem for wildlife and human health. Phytoremediation is a recently developed technology that offers a cost-effective solution by using plants, and associated soil microbes, to reduce the content, or toxic effects, of contaminants in the environment. Phytoremediation technologies include: (a) phytostabilization, where contaminants are retained in the soil, (b) phytodegradation, where organic contaminants are converted to less harmful substances, (c) phytovolatilization, where contaminants are converted inside plants to a gaseous state and released into the atmosphere via the evapotranspiration process, and, (d) phytoextraction, where plants are used to accumulate contaminants in the aboveground, harvestable biomass.