Bioremediation

Bioremediation is one of nature’s prudent ways to purify the polluted environment and that degraded by the anthropogenic activities. Although the term ‘bioremediation’ may be recent, but the process is not new as its origin relates to the origin of life when the first organism was stressed by certain compounds in less harmful forms by adopting certain detoxifying mechanisms in order to overcome the stress. The present day bioremediation technologies are based on the processes and potentials of almost all types of life forms, viz., plants (Phytoremediation), microorganisms (microbial remediation) and animals (zooremediation).

Phytoremediation

The term phytoremediation (“phyto” meaning plant, and the Latin suffix “remedium” meaning to clean or restore) actually refers to a diverse collection of plant-based technologies that use either naturally occurring or genetically engineered plants for cleaning contaminated environments. Phytoremediation is an emerging technology that employs the use of plants for the cleanup of contaminated environments. Fundamental and applied research have unequivocally demonstrated that selected plant species possess the genetic potential to remove, degrade, metabolize or immobilize a wide range of contaminants potential for phytoremediation depends upon the interaction among soil, contaminants, microbes, and plants. This complex interaction affected by various factors, such as climatic conditions, soil properties, and site hydrogeology argues against generalization, and in favour of site – specific phytoremediation practices. Phytoremediation can be applied to both organic and inorganic pollutants, present in solid substrates (e.g. soil), liquid substrates (e.g. water) and the air.

Phytoremediation Processes

Depending on the underlying processes, applicability, and type of contaminant, phyto-remediation can be broadly categorised as:

• Phytodegradation: use of plants to uptake, store and degrade contaminants within its tissue
• Phytostimulation or rhizodegradation: use of rhizospheric associations between plants and symbiotic soil microbes to degrade contaminants.
• Phytovolatilisation: use of a plant's ability to uptake contaminants from the growth matrix and subsequently transform and volatilise contaminants into the atmosphere.
• Phytoextraction: use plants to absorb, translocate and store toxic contaminants from a soil matrix into their root and shoot tissue.
• Rhizofiltration: use of roots to uptake also store contaminants from an aqueous growth matrix.
• Phytostabilisation: plant-mediated immobilisation or binding of contami-nants into the soil matrix, thereby reducing their bioavailability.

Source:http://www.unep.or.jp/Ietc/Publications/Freshwater/FMS2/2.asp

A review of phytoremediation technology: heavy metals uptake by plants

Figure: Various mechanisms involved in the phytoremediation of heavy metals

Source:http://iopscience.iop.org/article/10.1088/1755-1315/142/1/012023

Several crop plants used to heavy metals phytoremediation studies.

Plants	Contaminated areas	Heavy Metals
Allium schoenoprasum L.(Chive)	Soil	Ni, Co, Cd
Brassica juncea L. (Indian mustard)	Soil and water	Cd, Cu, Zn, Pb
Brassica napusL. (canola)	Soil	Cd, Cu, Zn, Pb
Cajanus Cajan(L.) Milsp.(pigeon pea)	Soil	As, Cd
Cicer aeritinum L. (chickpea)	Soil	Cd, Pb, Cr, Cu
Cucumis sativus L. (cucumber)	Water	Pb
Eichhornia crassipes L. (water hyacinth)	Water	As, Cr, Zn, Cs, Co
Jatropha curcasL. (purging nut, physic nut)	Soil	Fe, Al, Cu, Mn, Cr, As,Zn, Hg
Lantana camara L. (lantana)	Soil	Pb
Lens culinaris Medic. (lentil)	Soil	Pb
Lepidium sativum L. (cress)	Soil	As, Cd, Fe, Pb, Hg
Lactuca sativa L. (lettuce)	Soil	Cu, Fe, Mn, Zn, Ni, Cd,Pb, Co, As
Medicago sativa L. (alfalfa)	Soil	Cd
Oryza sativa L. (rice)	Soil	Cu, Cd
Pistia stratiotes L. (water lettuce)	Water	Cr, Cd, As
Pisum sativum L. (pea)	Soil	Pb, Cu, Zn, Fe, Cd, Ni,As, Cr
Rapanus sativus L. (radish)	Soil	As, Cd, Fe, Pb, Cu
Spinacia oleracea L. (spinach)	Soil	Cd, Cu, Fe, Ni, Pb, Zn,Cr
Solanum nigrum L. (black nightshade)	Soil	Cd
Sorghum bicolor L. (sorghum)	Soil	Cd, Cu, Zn, Fe
Zea mays L. (corn)	Soil	Cd, Pb, Zn, Cu

Source:http://iopscience.iop.org/article/10.1088/1755-1315/142/1/012023

Phytoextraction

Phytoextraction is the most commonly recognized of all phytoremediation technologies, the terms “phytoremediation” and “phytoextraction” are sometimes incorrectly used as synonyms, but phytoremediation is a concept while phytoextraction is a specific cleanup technology. The phytoextraction process involves the use of plants to facilitate the removal of metal contaminants from a soil matrix. In practice metal-accumulating plants are seeded or transplanted into metal-polluted soil and are cultivated using established agricultural practices. The roots of established plants absorb metal elements from the soil and translocate them to the above-ground shoot where they accumulate. This technology is suitable for the remediation of large areas of land that are contaminated at shallow depths with low to moderate levels of metal contaminants. As a plant-based technology, the success of phytoextraction is inherently dependent upon several plant characteristics including the ability to accumulate large quantities of biomass rapidly and the ability to accumulate large quantities of environmentally important metals in the shoot tissue. The selection of heavy metal tolerant species is a reliable tool to achieve success in phytoremediation. 163 plants taxa belonging to 45 families are found to be metal tolerant and are capable of growing on elevated concentrations of toxic metals.

Hyperaccumulator Species

Interest in phytoextraction has grown significantly following the identification of metal hyperaccumulator plant species. Hyperaccumulators are conventionally defined as species capable of accumulating metals at levels 100-fold greater than those typically measured in common non-accumulator plants.

Plant species	Metal	Leaf content (ppm)	References
Thalaspi caerulescens	Zn:Cd	39,600:1,800	Reeves & Brooks (1983) Baker & Walker (1990)
Ipomea alpina	Cu	12,300	Baker & Walker (1990)
Haumaniastrum robertii	Co	10,200	Brooks (1977)
Astragalus racemosus	Se	14,900	Beath et. al. (1937)
Sebertia acuminata	Ni	25% by wt. dried sap	Jeffre et. al. (1976)
Pteris vittata	As	22,630	Ma et. al. (2001)

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Metal hyperaccumulator species and their bioaccumulation potential

Phytodegradation/Biodegradation

Biodegradation is the process by which organic substances are broken down by the enzymes produced by living organisms.Biodegradable matter is generally organic material such as plant and animal matter and other substances originating from living organisms, or artificial materials that are similar enough to plant and animal matter to be put to use by microorganisms. Plant and microorganisms stimulate to rapidly degrade hazardous organic contaminants to environmentally safe levels in soils, subsurface materials, water, sludge, and residues. Bacteria, protozoa, blue-green algae are the main drivers of the biological treatment processes for industrial effluents and sewage streams.

Rhizofiltration

Pollutants in industrial-process water and in ground water are most commonly removed by precipitation or flocculation, followed by sedimentation and disposal of the resulting sludge. A promising alternative to this conventional clean-up method is rhizofiltration, a phytoremediative technique designed for the removal of pollutants in aquatic environments. The process involves raising plants hydroponically and transplanting them into polluted waters where plants absorb and concentrate the pollutants in their roots and shoots. For rhizofitration the plants should be able to accumulate and tolerate significant amounts of the target pollutant in conjunction with easy handling, low maintenance cost, and a minimum of secondary waste requiring disposal. Several aquatic species have the ability to remove pollutants from water e.g., water hyacinth (Eichhornia crassipes), pennywort (Hydrocotyle umbellata) and duck weed (Lemna minor ). Terrestrial plants are thought to be more suitable for rhizofiltration because they produce fibrous root systems. Sunflower (Helianthus annus) and Indian mustard (Brassica juncea ) are most promising terrestrial candidates for metal removal in water.

Phytostabilisation

Phytostabilisation, also known as phytorestoration, is a plant-based remediation technique that stabilizes wastes and prevents exposure pathways via wind and water erosion; provide hydraulic control, which suppresses the vertical migration of contaminants into groundwater; and physically and chemically immobilizes contaminants by root sorption and by chemical fixation with various soil amendments. The plants chosen for phytostabilisation should be poor translocators of contaminants to above ground plant tissues that could be consumed by humans or animals. Two cultivars of Agrostis tenuis and one of Festuca rubra are now commercially available for phytostabilisation of Pb, Zn and Cu contaminated soils. Phytostabilisation is the process within the roots and root zone of plants, which immobilises contaminants by preventing their migration by such processes as accumulation and absorption into the root, adsorption onto the root, and precipitation within the root zone.

Phytovolatilization

Some contaminants like As, Hg and Se may exist as gaseous species in environment. Plants are capable of absorbing elemental forms of these metals from the soil, biologically converting them to gaseous species within the plant, and releasing them into the atmosphere. Members of family Brassicaceae are capable of releasing up to 40 g Se ha-1 day-1 (Prasad and Freitas 2003). Some aquatic plants, such as cattail (Typha latifolia) are also good for Se phytoremediation. Arabidopsis thaliana and tobacco (Nicotiana tabacum ) have been genetically modified to absorb elemental Hg and methyl mercury from the soil and release volatile Hg from the leaves into the atmosphere.