The term phytoremediation coined by Dr Ilya Raskin, Rutgers University, New Jersey, USA, refers to the cleanup of pollutants primarily mediated by photosynthetic plants. Cleanup is defined as the destruction, inactivation or immobilisation of the pollutants and/or its conversion into a harmless form. The process also called ‘green remediation’ or ‘botanical bioremediation’, involves the use of living green plants for in situ risk reduction and/or removal of contaminants from soil, water, sediments and air.
A subset of phytoremediation – the concentrations of the contaminants in the soils of concern is reduced to an acceptable level through the action of plants, their associated microflora and agronomic practices. The process of phytodecontamination is achieved by phytoextraction, phytodegradation, rhizofilteration, phytovolatilisation, and rhizo (sphere) degradation (Fig 1).
Phytoextraction is the process where plant roots take up the metal contaminant from the soil to translocate them into their tissues. The degree of accumulation varies with several factors, but can be as high as 2 per cent of the plants’ (above ground) dry weight. Once the plants grow to an optimal size they are harvested and disposed off safely. This process is repeated several times to reduce contamination to acceptable levels. In some cases it is possible to recycle the metals through the process of phytomining. Metal compounds that have been successfully phytoextracted include zinc, copper, and nickel – also promising research on lead and chromium absorbing plants is underway. Hyperaccumulator plant species are used on many sites due to their tolerance of relatively extreme levels of pollution. Phytoextraction may be classified into two types.
Natural phytoextraction: It is usually conducted through planting selected species in the contaminated soil. These plants are grown under normal farming conditions to reach the optimal size, harvested and disposed off appropriately. The plants (such as Pteris vittata) are highly specialised, occur naturally and can tolerate highly elevated concentrations of metals that would be toxic to other plants. Typically, these plants are small, have a shallow root system and grow relatively slowly.
Induced phytoextraction: In non hyperaccumulators plants such as Thlaspi perfoliatum, factors limiting their potential for phytoextraction include small root uptake and little root-shoot translocation of metals. Methods that use metal-mobilising agents have been proposed specifically to overcome these limitations. Following this approach, a high biomass crop is grown on the contaminated soil requiring remediation. Throughout the growth period, amendments are added to the soil to increase availability of metals to the plants. The most commonly used agents for induced phytoextraction are: ethylene diamine tetra acetic acid (EDTA), diethyl triamine penta acetic (DTPA), cyclohexylene dinitrilo tetra acetic acid (CDTA) and citric acid etc.
Phytodegradation is the breakdown of organic contaminants by internal and external metabolic processes driven by the plant. Some contaminants can be absorbed by the plants and broken down by their enzymes. These smaller pollutant molecules may then be used as metabolites by the plant as it grows, thus becoming incorporated into the plant tissues. Plant enzymes that breakdown ammunition wastes, chlorinated solvents such as trichloroethane (TCE) have been identified.
Phytovolatilisation refers to the process through which plants uptake water soluble contaminants and release them into the atmosphere as they transpire water. As the water travels along the plant’s vascular system from the roots to the leaves, the contaminant may be modified whereby it evaporates or volatilises into the air surrounding the plant. Phytovolatilisation is relevant in the remediation of soils rich in mercury, selenium and to some extent in arsenic. The mercury ion is transformed into less toxic elemental mercury and selenium is lost to the atmosphere in the form of dimethylselenide (DMSe) (Fig 2.). It is also applicable for the removal of organic contaminants. For example, poplar trees have been shown to volatilise 90 per cent of the TCE they take up.
Rhizofiltration is similar in concept to phytoextraction but is concerned with the remediation of contaminated groundwater rather than the remediation of polluted soils. Plants (such as Helianthus annuus used for rhizoliltration are not planted directly but are acclimated to the pollutant first. Plants are hydroponically grown in clean water rather than the soil until a large root system develops. Once a large root system is in place, the water supply is substituted for a polluted water supply to acclimatise the plant. Then they are planted in the polluted area where the roots uptake the polluted water and the contaminants along with it. As the roots become saturated they are harvested and disposed of safely.
Rhizodegradation (also called enhanced rhizosphere biodegradation, phytostimulation, and plant assisted bioremediation) is the breakdown of organic contaminants in the soil by soil dwelling microbes which is enhanced by the rhizosphere’s presence. Plant root (switchgrass, ryegrass) exudates such as sugars, alcohols, and organic acids act as carbohydrate sources for the soil microflora and enhance microbial growth and activity. The plant roots also loosen the soil and transport water to the rhizosphere thus additionally enhancing microbial activity.
Phytostabilisation is the process in which plants (Festuca rubra L, Agrostis tenuis) are used to immobilise soil and water contaminants. It mainly focuses on sequestering pollutants in soil near the roots rather than in the plant tissues itself. Pollutants become less bioavailable and livestock, wildlife, and human exposure is reduced. The contaminants are absorbed and accumulated by roots, adsorbed onto the roots, or precipitated in the rhizosphere. This reduces or even prevents the contaminants migrating into the groundwater or air as well as the bioavailability of the contaminant which prevent its spread through the food chain. This technique can also be used to re-establish a plant community on sites that have been denuded due to the high levels of metal contamination. Once a community of tolerant species has been established, the potential for wind erosion (and thus spread of the pollutant) and the leaching of the soil contaminants is also reduced. Phytostabilisation involves three processes which include humification, lignification and irreversible binding.
Phytoremediation is a well known process for cleaning contaminated soils across the globe. Despite sizeable research, the use of this technology is limited. Phytoremediation has been primarily used in bauxite and coal mine spoils in Madhya Pradesh; lime stone mine spoils in outer Himalayas; rock-phosphate mine spoils in Musoorie; lignite mine spoils in Tamil Nadu; mica, copper, tungsten, marble, dolomite, and limestone mine spoils in Rajasthan; iron ore wastes in Odisha and haematite, magnetite, manganese spoils in Karnataka. This technology, however, requires more attention to extend it to heavy metal polluted areas.